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RFC 5740 - NACK-Oriented Reliable Multicast (NORM) Transport Pro

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Network Working Group                                         B. Adamson
Request for Comments: 5740                     Naval Research Laboratory
Obsoletes: 3940                                               C. Bormann
Category: Standards Track                        Universitaet Bremen TZI
                                                              M. Handley
                                               University College London
                                                               J. Macker
                                               Naval Research Laboratory
                                                           November 2009

       NACK-Oriented Reliable Multicast (NORM) Transport Protocol


   This document describes the messages and procedures of the Negative-
   ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) protocol.
   This protocol can provide end-to-end reliable transport of bulk data
   objects or streams over generic IP multicast routing and forwarding
   services.  NORM uses a selective, negative acknowledgment mechanism
   for transport reliability and offers additional protocol mechanisms
   to allow for operation with minimal a priori coordination among
   senders and receivers.  A congestion control scheme is specified to
   allow the NORM protocol to fairly share available network bandwidth
   with other transport protocols such as Transmission Control Protocol
   (TCP).  It is capable of operating with both reciprocal multicast
   routing among senders and receivers and with asymmetric connectivity
   (possibly a unicast return path) between the senders and receivers.
   The protocol offers a number of features to allow different types of
   applications or possibly other higher-level transport protocols to
   utilize its service in different ways.  The protocol leverages the
   use of FEC-based (forward error correction) repair and other IETF
   Reliable Multicast Transport (RMT) building blocks in its design.
   This document obsoletes RFC 3940.

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction and Applicability . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
     1.2.  NORM Data Delivery Service Model . . . . . . . . . . . . .  5
     1.3.  NORM Scalability . . . . . . . . . . . . . . . . . . . . .  7
     1.4.  Environmental Requirements and Considerations  . . . . . .  8
   2.  Architecture Definition  . . . . . . . . . . . . . . . . . . .  8
     2.1.  Protocol Operation Overview  . . . . . . . . . . . . . . . 10
     2.2.  Protocol Building Blocks . . . . . . . . . . . . . . . . . 12
     2.3.  Design Trade-Offs  . . . . . . . . . . . . . . . . . . . . 12
   3.  Conformance Statement  . . . . . . . . . . . . . . . . . . . . 13
   4.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 15
     4.1.  NORM Common Message Header and Extensions  . . . . . . . . 15
     4.2.  Sender Messages  . . . . . . . . . . . . . . . . . . . . . 18
       4.2.1.  NORM_DATA Message  . . . . . . . . . . . . . . . . . . 18
       4.2.2.  NORM_INFO Message  . . . . . . . . . . . . . . . . . . 28
       4.2.3.  NORM_CMD Messages  . . . . . . . . . . . . . . . . . . 29
     4.3.  Receiver Messages  . . . . . . . . . . . . . . . . . . . . 47
       4.3.1.  NORM_NACK Message  . . . . . . . . . . . . . . . . . . 47
       4.3.2.  NORM_ACK Message . . . . . . . . . . . . . . . . . . . 53
     4.4.  General Purpose Messages . . . . . . . . . . . . . . . . . 55
       4.4.1.  NORM_REPORT Message  . . . . . . . . . . . . . . . . . 55
   5.  Detailed Protocol Operation  . . . . . . . . . . . . . . . . . 55
     5.1.  Sender Initialization and Transmission . . . . . . . . . . 57
       5.1.1.  Object Segmentation Algorithm  . . . . . . . . . . . . 58

     5.2.  Receiver Initialization and Reception  . . . . . . . . . . 59
     5.3.  Receiver NACK Procedure  . . . . . . . . . . . . . . . . . 59
     5.4.  Sender NACK Processing and Response  . . . . . . . . . . . 62
       5.4.1.  Sender Repair State Aggregation  . . . . . . . . . . . 62
       5.4.2.  Sender FEC Repair Transmission Strategy  . . . . . . . 63
       5.4.3.  Sender NORM_CMD(SQUELCH) Generation  . . . . . . . . . 64
       5.4.4.  Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65
     5.5.  Additional Protocol Mechanisms . . . . . . . . . . . . . . 65
       5.5.1.  Group Round-Trip Time (GRTT) Collection  . . . . . . . 65
       5.5.2.  NORM Congestion Control Operation  . . . . . . . . . . 67
       5.5.3.  NORM Positive Acknowledgment Procedure . . . . . . . . 75
       5.5.4.  Group Size Estimate  . . . . . . . . . . . . . . . . . 77
   6.  Configurable Elements  . . . . . . . . . . . . . . . . . . . . 77
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 80
     7.1.  Baseline Secure NORM Operation . . . . . . . . . . . . . . 82
       7.1.1.  IPsec Approach . . . . . . . . . . . . . . . . . . . . 83
       7.1.2.  IPsec Requirements . . . . . . . . . . . . . . . . . . 85
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 86
     8.1.  Explicit IANA Assignment Guidelines  . . . . . . . . . . . 87
       8.1.1.  NORM Header Extension Types  . . . . . . . . . . . . . 87
       8.1.2.  NORM Stream Control Codes  . . . . . . . . . . . . . . 88
       8.1.3.  NORM_CMD Message Sub-Types . . . . . . . . . . . . . . 88
   9.  Suggested Use  . . . . . . . . . . . . . . . . . . . . . . . . 89
   10. Changes from RFC 3940  . . . . . . . . . . . . . . . . . . . . 90
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 91
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 91
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 91
     12.2. Informative References . . . . . . . . . . . . . . . . . . 92

1.  Introduction and Applicability

   The Negative-ACKnowledgment (NACK) Oriented Reliable Multicast (NORM)
   protocol can provide reliable transport of data from one or more
   senders to a group of receivers over an IP multicast network.  The
   primary design goals of NORM are to provide efficient, scalable, and
   robust bulk data (e.g., computer files, transmission of persistent
   data) transfer across possibly heterogeneous IP networks and
   topologies.  The NORM protocol design provides support for
   distributed multicast session participation with minimal coordination
   among senders and receivers.  NORM allows senders and receivers to
   dynamically join and leave multicast sessions at will with minimal
   overhead for control information and timing synchronization among
   participants.  To accommodate this capability, NORM protocol message
   headers contain some common information allowing receivers to easily
   synchronize to senders throughout the lifetime of a reliable
   multicast session.  NORM is self-adapting to a wide range of dynamic
   network conditions with little or no pre-configuration.  The protocol
   is tolerant of inaccurate timing estimations or lossy conditions that
   can occur in many networks including mobile and wireless.  The
   protocol can also converge and maintain efficient operation even in
   situations of heavy packet loss and large queuing or transmission
   delays.  This document obsoletes the Experimental RFC 3940

   This document is a product of the IETF RMT working group and follows
   the guidelines provided in the Author Guidelines for Reliable
   Multicast Transport (RMT) Building Blocks and Protocol Instantiation
   documents [RFC3269].

   Statement of Intent

   This memo contains the definitions necessary to fully specify a
   Reliable Multicast Transport protocol in accordance with the criteria
   of IETF Criteria for Evaluating Reliable Multicast Transport and
   Application Protocols [RFC2357].  The NORM specification described in
   this document was previously published in the Experimental Category
   [RFC3940].  It was the stated intent of the RMT working group to re-
   submit this specifications as an IETF Proposed Standard in due
   course.  This Proposed Standard specification is thus based on RFC
   3940 and has been updated according to accumulated experience and
   growing protocol maturity since the publication of RFC 3940.  Said
   experience applies both to this specification itself and to
   congestion control strategies related to the use of this
   specification.  The differences between RFC 3940 and this document
   are listed in Section 10.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  NORM Data Delivery Service Model

   A NORM protocol instance (NormSession) is defined within the context
   of participants communicating connectionless (e.g., Internet Protocol
   (IP) or User Datagram Protocol (UDP)) packets over a network using
   pre-determined addresses and host port numbers.  Generally, the
   participants exchange packets using an IP multicast group address,
   but unicast transport MAY also be established or applied as an
   adjunct to multicast delivery.  In the case of multicast, the
   participating NormNodes will communicate using a common IP multicast
   group address and port number chosen via means outside the context of
   the given NormSession.  Other existing IETF data format and protocol
   standards MAY be applied to describe and convey the necessary a
   priori information for a specific NormSession (e.g., Session
   Description Protocol (SDP) [RFC4566], Session Announcement Protocol
   (SAP) [RFC2974], etc.).

   The NORM protocol design is principally driven by the assumption of a
   single sender transmitting bulk data content to a group of receivers.
   However, the protocol MAY operate with multiple senders within the
   context of a single NormSession.  In initial implementations of this
   protocol, it is anticipated that multiple senders will transmit
   independently of one another and receivers will maintain state as
   necessary for each sender.  In future versions of NORM, it is
   possible some aspects of protocol operation (e.g., round-trip time
   collection) will provide for alternate modes allowing more efficient
   performance for applications requiring multiple senders.

   NORM provides for three types of bulk data content objects
   (NormObjects) to be reliably transported.  These types include:

   1.  static computer memory data content (NORM_OBJECT_DATA type),

   2.  computer storage files (NORM_OBJECT_FILE type), and

   3.  non-finite streams of continuous data content (NORM_OBJECT_STREAM

   The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is
   simply to provide a hint to receivers in NormSessions serving
   multiple types of content as to what type of storage to allocate for
   received content (i.e., memory or file storage).  Other than that

   distinction, the two are identical, providing for reliable transport
   of finite (but potentially very large) units of content.  These
   static data and file services are anticipated to be useful for
   multicast-based cache applications with the ability to reliably
   provide transmission of large quantities of static data.  Other types
   of static data/file delivery services might make use of these
   transport object types, too.  The use of the NORM_OBJECT_STREAM type
   is at the application's discretion and could be used to carry static
   data or file content also.  The NORM reliable stream service opens up
   additional possibilities such as serialized reliable messaging or
   other unbounded, perhaps dynamically produced content.  The
   NORM_OBJECT_STREAM provides for reliable transport analogous to that
   of the Transmission Control Protocol (TCP), although NORM receivers
   will be able to begin receiving stream content at any point in time.
   The applicability of this feature will depend upon the application.

   The NORM protocol also allows for a small amount of out-of-band data
   (sent as NORM_INFO messages) to be attached to the data content
   objects transmitted by the sender.  This readily available out-of-
   band data allows multicast receivers to quickly and efficiently
   determine the nature of the corresponding data, file, or stream bulk
   content being transmitted.  This allows application-level control of
   the receiver node's participation in the current transport activity.
   This also allows the protocol to be flexible with minimal pre-
   coordination among senders and receivers.  The NORM_INFO content is
   atomic in that its size MUST fit into the payload portion of a single
   NORM message.

   NORM does NOT provide for global or application-level identification
   of data content within its message headers.  Note the NORM_INFO out-
   of-band data mechanism can be leveraged by the application for this
   purpose if desired, or identification can alternatively be embedded
   within the data content.  NORM does identify transmitted content
   (NormObjects) with transport identifiers that are applicable only
   while the sender is transmitting and/or repairing the given object.
   These transport data content identifiers (NormTransportIds) are
   assigned in a monotonically increasing fashion by each NORM sender
   during the course of a NormSession.  Participants, including senders,
   in NORM protocol sessions are also identified with unique identifiers
   (NormNodeIds).  Each sender maintains its NormTransportId assignments
   independently and thus individual NormObjects can be uniquely
   identified during transport by concatenation of the session-unique
   sender identifier (NormNodeId) and the assigned NormTransportId.  The
   NormTransportIds are assigned from a large, but fixed, numeric space
   in increasing order and will be reassigned during long-lived
   sessions.  The NORM protocol provides mechanisms so the sender
   application can terminate transmission of data content and inform the
   group of this in an efficient manner.  Other similar protocol control

   mechanisms (e.g., session termination, receiver synchronization,
   etc.) are specified so reliable multicast application variants can
   realize different, complete bulk transfer communication models to
   meet their goals.

   To summarize, the NORM protocol provides reliable transport of
   different types of data content (including potentially mixed types).
   The senders enqueue and transmit bulk content in the form of static
   data or files and/or non-finite, ongoing stream types.  NORM senders
   provide for repair transmission of data and/or FEC content in
   response to NACK messages received from the receiver group.
   Mechanisms for out-of-band information and other transport control
   mechanisms are specified for use by applications to form complete
   reliable multicast solutions for different purposes.

1.3.  NORM Scalability

   Group communication scalability requirements lead to adaptation of
   NACK-based protocol schemes when feedback for reliability is needed
   [RmComparison].  NORM is a protocol centered around the use of
   selective NACKs to request repairs of missing data.  NORM provides
   for the use of packet-level forward error correction (FEC) techniques
   for efficient multicast repair and OPTIONAL proactive transmission
   robustness [RFC3453].  FEC-based repair can be used to greatly reduce
   the quantity of reliable multicast repair requests and repair
   transmissions [MdpToolkit] in a NACK-oriented protocol.  The
   principal factor in NORM scalability is the volume of feedback
   traffic generated by the receiver set to facilitate reliability and
   congestion control.  NORM uses probabilistic suppression of redundant
   feedback based on exponentially distributed random backoff timers.
   The performance of this type of suppression relative to other
   techniques is described in [McastFeedback].  NORM dynamically
   measures the group's round-trip timing status to set its suppression
   and other protocol timers.  This allows NORM to scale well while
   maintaining reliable data delivery transport with low latency
   relative to the network topology over which it is operating.

   Feedback messages can be either multicast to the group at large or
   sent via unicast routing to the sender.  In the case of unicast
   feedback, the sender relays the feedback state to the group to
   facilitate feedback suppression.  In typical Internet environments,
   the NORM protocol will readily scale to group sizes on the order of
   tens of thousands of receivers.  A study of the quantity of feedback
   for this type of protocol is described in [NormFeedback].  NORM is
   able to operate with a smaller amount of feedback than a single TCP
   connection, even with relatively large numbers of receivers.  Thus,
   depending upon the network topology, it is possible for NORM to scale
   to larger group sizes.  With respect to computer resource usage, the

   NORM protocol does not need state to be kept on all receivers in the
   group.  NORM senders maintain state only for receivers providing
   explicit congestion control feedback.  However, NORM receivers need
   to maintain state for each active sender.  This can constrain the
   number of simultaneous senders in some uses of NORM.

1.4.  Environmental Requirements and Considerations

   All of the environmental requirements and considerations that apply
   to the "Multicast Negative-Acknowledgment (NACK) Building Blocks"
   [RFC5401], "Forward Error Correction (FEC) Building Block" [RFC5052],
   and "TCP-Friendly Multicast Congestion Control (TFMCC) Protocol
   Specification" [RFC4654] also apply to the NORM protocol.

   The NORM protocol SHALL be capable of operating in an end-to-end
   fashion with no assistance from intermediate systems beyond basic IP
   multicast group management, routing, and forwarding services.  While
   the techniques utilized in NORM are principally applicable to flat,
   end-to-end IP multicast topologies, they could also be applied in the
   sub-levels of hierarchical (e.g., tree-based) multicast distribution
   if so desired.  NORM can make use of reciprocal (among senders and
   receivers) multicast communication under the Any-Source Multicast
   (ASM) model defined in "Host Extensions for IP Multicasting"
   [RFC1112], but it SHALL also be capable of scalable operation in
   asymmetric topologies such as Source-Specific Multicast (SSM)
   [RFC4607] where only unicast routing service is available from the
   receivers to the sender(s).

   NORM is compatible with IPv4 and IPv6.  Additionally, NORM can be
   used with networks employing Network Address Translation (NAT)
   provided that the NAT device supports IP multicast and/or can cache
   UDP traffic source port numbers for remapping feedback traffic from
   receivers to the sender(s).

2.  Architecture Definition

   A NormSession is comprised of participants (NormNodes) acting as
   senders and/or receivers.  NORM senders transmit data content in the
   form of NormObjects to the session destination address, and the NORM
   receivers attempt to reliably receive the transmitted content using
   negative acknowledgments to request repair.  Each NormNode within a
   NormSession is assumed to have a preselected unique 32-bit identifier
   (NormNodeId).  NormNodes MUST have uniquely assigned identifiers
   within a single NormSession to distinguish between multiple possible
   senders and to distinguish feedback information from different
   receivers.  There are two reserved NormNodeId values.  A value of
   0x00000000 is considered an invalid NormNodeId (NORM_NODE_NONE), and
   a value of 0xffffffff is a "wild card" NormNodeId (NORM_NODE_ANY).

   While the protocol does not preclude multiple sender nodes
   concurrently transmitting within the context of a single NORM session
   (i.e., many-to-many operation), any type of interactive coordination
   among NORM senders is assumed to be controlled by the application- or
   higher-protocol layer.  There are some OPTIONAL mechanisms specified
   in this document that can be leveraged for such application-layer

   As previously noted, NORM allows for reliable transmission of three
   different basic types of data content.  The first type is
   NORM_OBJECT_DATA, which is used for static, persistent blocks of data
   content maintained in the sender's application memory storage.  The
   second type is NORM_OBJECT_FILE, which corresponds to data stored in
   the sender's non-volatile file system.  The NORM_OBJECT_DATA and
   NORM_OBJECT_FILE types both represent NormObjects of finite but
   potentially very large size.  The third type of data content is
   NORM_OBJECT_STREAM, which corresponds to an ongoing transmission of
   undefined length.  This is analogous to the reliable stream service
   provided by TCP for unicast data transport.  The format of the stream
   content is application-defined and can be "byte" or "message"
   oriented.  The NORM protocol provides for "flushing" of the stream to
   expedite delivery or possibly enforce application message boundaries.
   NORM protocol implementations MAY offer either (or both) in-order
   delivery of the stream data to the receive application or out-of-
   order (more immediate) delivery of received segments of the stream to
   the receiver application.  In either case, NORM sender and receiver
   implementations provide buffering to facilitate repair of the stream
   as it is transported.

   All NormObjects are logically segmented into FEC coding blocks and
   symbols for transmission by the sender.  In NORM, a FEC encoding
   symbol directly corresponds to the payload of NORM_DATA messages or
   "segment".  Note that when systematic FEC codes are used, the payload
   of NORM_DATA messages sent for the first portion of a FEC encoding
   block are source symbols (actual segments of original user data),
   while the remaining symbols for the block consist of parity symbols
   generated by FEC encoding.  These parity symbols are generally sent
   in response to repair requests, but some number MAY be sent
   proactively at the end of each encoding block to increase the
   robustness of transmission.  When non-systematic FEC codes are used,
   all symbols sent consist of FEC encoding parity content.  In this
   case, the receiver needs to receive a sufficient number of symbols to
   reconstruct (via FEC decoding) the original user data for the given

   Transmitted NormObjects are temporarily, yet uniquely, identified
   within the NormSession context using the given sender's NormNodeId,
   NormInstanceId, and a temporary NormTransportId.  Depending upon the

   implementation, individual NORM senders can manage their
   NormInstanceIds independently, or a common NormInstanceId could be
   agreed upon for all participating nodes within a session, if needed,
   as a session identifier.  NORM NormTransportId data content
   identifiers are sender-assigned and applicable and valid only during
   a NormObject's actual transport (i.e., for as long as the sender is
   transmitting and providing repair of the indicated NormObject).  For
   a long-lived session, the NormTransportId field can wrap and
   previously used identifiers will be re-used.  Note that globally
   unique identification of transported data content is not provided by
   NORM and, if necessary, is expected to be managed by the NORM
   application.  The individual segments or symbols of the NormObject
   are further identified with FEC payload identifiers that include
   coding block and symbol identifiers.  These are discussed in detail
   later in this document.

2.1.  Protocol Operation Overview

   A NORM sender primarily generates messages of type NORM_DATA.  These
   messages carry original data segments or FEC symbols and repair
   segments/symbols for the bulk data/file or stream NormObjects being
   transferred.  By default, redundant FEC symbols are sent only in
   response to receiver repair requests (NACKs) and thus normally little
   or no additional transmission overhead is imposed due to FEC
   encoding.  However, the NORM implementation MAY be configured to
   proactively transmit some amount of redundant FEC symbols along with
   the original content to potentially enhance performance (e.g.,
   improved delay) at the cost of additional transmission overhead.
   This configuration is sensible for certain network conditions and can
   allow for robust, asymmetric multicast (e.g., unidirectional routing,
   satellite, cable) [FecHybrid] with reduced receiver feedback, or, in
   some cases, no feedback.

   A sender message of type NORM_INFO is also defined and is used to
   carry OPTIONAL out-of-band context information for a given transport
   object.  A single NORM_INFO message can be associated with a
   NormObject.  Because of its atomic nature, missing NORM_INFO messages
   can be NACKed and repaired with a slightly lower delay process than
   NORM's general FEC-encoded data content.  The NORM_INFO message can
   serve special purposes for some bulk transfer, reliable multicast
   applications where receivers join the group mid-stream and need to
   ascertain contextual information on the current content being
   transmitted.  The NACK process for NORM_INFO will be described later.
   When the NORM_INFO message type is used, its transmission SHOULD
   precede transmission of any NORM_DATA message for the associated

   The sender also generates messages of type NORM_CMD to assist in

   certain protocol operations such as congestion control, end-of-
   transmission flushing, group round-trip time (GRTT) estimation,
   receiver synchronization, and OPTIONAL positive acknowledgment
   requests or application-defined commands.  The transmission of
   NORM_CMD messages from the sender is accomplished by one of three
   different procedures: single, best-effort unreliable transmission of
   the command; repeated redundant transmissions of the command; and
   positively acknowledged commands.  The transmission technique used
   for a given command depends upon the function of the command.
   Several core commands are defined for basic protocol operation.
   Additionally, implementations MAY wish to consider providing the
   OPTIONAL application-defined commands that can take advantage of the
   transmission methodologies available for commands.  This allows for
   application-level session management mechanisms that can make use of
   information available to the underlying NORM protocol engine (e.g.,
   round-trip timing, transmission rate, etc.).  A notable distinction
   between NORM_DATA message and some NORM_CMD message transmissions is
   that typically a receiver will need to allocate resources to manage
   reliable reception when NORM_DATA messages are received.  However,
   some NORM_CMD messages are completely atomic and no specific
   reliability (buffering) state needs to be kept.  Thus, for session
   management or other purposes, it is possible that even participants
   acting principally as data receivers MAY transmit NORM_CMD messages.
   However, it is RECOMMENDED that this is not done within the context
   of the NORM multicast session unless congestion control is addressed.
   For example, many receiver nodes transmitting NORM_CMD messages
   simultaneously can cause congestion for the destination(s).

   All sender transmissions are subject to rate control governed by a
   peak transmission rate set for each participant by the application.
   This can be used to limit the quantity of multicast data transmitted
   by the group.  When NORM's congestion control algorithm is enabled,
   the rate for senders is automatically adjusted.  In some networks, it
   is desirable to establish minimum and maximum bounds for the rate
   adjustment depending upon the application even when dynamic
   congestion control is enabled.  However, in the case of the general
   Internet, congestion control policy SHALL be observed that is
   compatible with coexistent TCP flows.

   NORM receivers generate messages of type NORM_NACK or NORM_ACK in
   response to transmissions of data and commands from a sender.  The
   NORM_NACK messages are generated to request repair of detected data
   transmission losses.  Receivers generally detect losses by tracking
   the sequence of transmission from a sender.  Sequencing information
   is embedded in the transmitted data packets and end-of-transmission
   commands from the sender.  NORM_ACK messages are generated in
   response to certain commands transmitted by the sender.  In the
   general (and most scalable) protocol mode, NORM_ACK messages are sent

   only in response to congestion control commands from the sender.  The
   feedback volume of these congestion control NORM_ACK messages is
   controlled using the same timer-based probabilistic suppression
   techniques as for NORM_NACK messages to avoid feedback implosion.  In
   order to meet potential application requirements for positive
   acknowledgment from receivers, other NORM_ACK messages are defined
   and are available for use.

2.2.  Protocol Building Blocks

   The operation of the NORM protocol is based primarily upon the
   concepts presented in the Multicast NACK Building Block [RFC5401]
   document.  This includes the basic NORM architecture and the data
   transmission, repair, and feedback strategies discussed in that
   document.  The reliable multicast building block approach, as
   described in "Reliable Multicast Transport Building Blocks for One-
   to-Many Bulk-Data Transfer" [RFC3048], is applied in creating the
   full NORM protocol instantiation.  NORM also makes use of the parity-
   based encoding techniques for repair messaging and added transmission
   robustness as described in "The Use of Forward Error Correction (FEC)
   in Reliable Multicast" [RFC3453].  NORM uses the FEC Payload ID as
   specified by the FEC Building Block document [RFC5052].
   Additionally, for congestion control, this document fully specifies a
   baseline congestion control mechanism (NORM-CC) based on the TCP-
   Friendly Multicast Congestion Control (TFMCC) scheme [TfmccPaper],

2.3.  Design Trade-Offs

   While the various features of NORM provide some measure of general
   purpose utility, it is important to emphasize the understanding that
   "no one size fits all" in the reliable multicast transport arena.
   There are numerous engineering trade-offs involved in reliable
   multicast transport design and this necessitates an increased
   awareness of application and network architecture considerations.
   Performance requirements affecting design can include: group size,
   heterogeneity (e.g., capacity and/or delay), asymmetric delivery,
   data ordering, delivery delay, group dynamics, mobility, congestion
   control, and transport across low-capacity connections.  NORM
   contains various parameters to accommodate many of these differing
   requirements.  The NORM protocol and its mechanisms MAY be applied in
   multicast applications outside of bulk data transfer, but there is an
   assumed model of bulk transfer transport service that drives the
   trade-offs that determine the scalability and performance described
   in this document.

   The ability of NORM to provide reliable data delivery is also
   governed by any buffer constraints of the sender and receiver

   applications.  NORM protocol implementations SHOULD operate with the
   greatest efficiency and robustness possible within application-
   defined buffer constraints.  Buffer requirements for reliability, as
   always, are a function of the delay-bandwidth product of the network
   topology.  NORM performs best when allowed more buffering resources
   than typical point-to-point transport protocols.  This is because
   NORM feedback suppression is based upon randomly delayed
   transmissions from the receiver set, rather than immediately
   transmitted feedback.  There are definitive trade-offs between buffer
   utilization, group size scalability, and efficiency of performance.
   Large buffer sizes allow the NORM protocol to perform most
   efficiently in large delay-bandwidth topologies and allow for longer
   feedback suppression backoff timeouts.  This yields improved group
   size scalability.  NORM can operate with reduced buffering but at a
   cost of decreased efficiency (lower relative goodput) and reduced
   group size scalability.

3.  Conformance Statement

   This RMT Protocol Instantiation document, in conjunction with the
   "Multicast Negative-Acknowledgment (NACK) Building Blocks" [RFC5401]
   and "Forward Error Correction (FEC) Building Block" [RFC5052]
   Building Blocks, completely specifies a working reliable multicast
   transport protocol that conforms to the requirements described in RFC

   This document specifies the following message types and mechanisms
   that are REQUIRED in complying NORM protocol implementations:

   | Message Type         | Purpose                                    |
   | NORM_DATA            | Sender message for application data        |
   |                      | transmission.  Implementations MUST        |
   |                      | support at least one of the                |
   |                      | NORM_OBJECT_DATA, NORM_OBJECT_FILE, or     |
   |                      | NORM_OBJECT_STREAM delivery services.  The |
   |                      | use of the NORM FEC Object Transmission    |
   |                      | Information header extension is OPTIONAL   |
   |                      | with NORM_DATA messages.                   |
   | NORM_CMD(FLUSH)      | Sender command to excite receivers for     |
   |                      | repair requests in lieu of ongoing         |
   |                      | NORM_DATA transmissions.  Note the use of  |
   |                      | the NORM_CMD(FLUSH) for positive           |
   |                      | acknowledgment of data receipt is          |
   |                      | OPTIONAL.                                  |

   | NORM_CMD(SQUELCH)    | Sender command to advertise its current    |
   |                      | valid repair window in response to invalid |
   |                      | requests for repair.                       |
   | NORM_CMD(REPAIR_ADV) | Sender command to advertise current repair |
   |                      | (and congestion control state) to group    |
   |                      | when unicast feedback messages are         |
   |                      | detected.  Used to control/suppress        |
   |                      | excessive receiver feedback in asymmetric  |
   |                      | multicast topologies.                      |
   | NORM_CMD(CC)         | Sender command used in collection of       |
   |                      | round-trip timing and congestion control   |
   |                      | status from group (this is OPTIONAL if     |
   |                      | alternative congestion control mechanism   |
   |                      | and round-trip timing collection is used). |
   | NORM_NACK            | Receiver message used to request repair of |
   |                      | missing transmitted content.               |
   | NORM_ACK             | Receiver message used to proactively       |
   |                      | provide feedback for congestion control    |
   |                      | purposes.  Also used with the OPTIONAL     |
   |                      | NORM Positive Acknowledgment Process.      |

   This document also describes the following message types and
   associated mechanisms that are OPTIONAL for complying NORM protocol

   | Message Type          | Purpose                                   |
   | NORM_INFO             | Sender message for providing ancillary    |
   |                       | context information associated with NORM  |
   |                       | transport objects.  The use of the NORM   |
   |                       | FEC Object Transmission Information       |
   |                       | header extension is OPTIONAL with         |
   |                       | NORM_INFO messages.                       |
   | NORM_CMD(EOT)         | Sender command to indicate it has reached |
   |                       | end-of-transmission and will no longer    |
   |                       | respond to repair requests.               |
   | NORM_CMD(ACK_REQ)     | Sender command to support                 |
   |                       | application-defined, positively           |
   |                       | acknowledged commands sent outside of the |
   |                       | context of the bulk data content being    |
   |                       | transmitted.  The NORM Positive           |
   |                       | Acknowledgment Procedure associated with  |
   |                       | this message type is OPTIONAL.            |

   | NORM_CMD(APPLICATION) | Sender command containing                 |
   |                       | application-defined commands sent outside |
   |                       | of the context of the bulk data content   |
   |                       | being transmitted.                        |
   | NORM_REPORT           | Optional message type reserved for        |
   |                       | experimental implementations of the NORM  |
   |                       | protocol.                                 |

4.  Message Formats

   There are two primary classes of NORM messages (see Section 2.1):
   sender messages and receiver messages.  NORM_CMD, NORM_INFO, and
   NORM_DATA message types are generated by senders of data content, and
   NORM_NACK and NORM_ACK messages generated by receivers within a
   NormSession.  Sender messages SHALL be governed by congestion control
   for Internet use.  For session management or other purposes,
   receivers can also employ NORM_CMD message transmissions.  The
   principal rationale for distinguishing sender and receiver messages
   is that receivers will typically need to allocate resources to
   support reliable reception from sender(s) and NORM sender messages
   are subject to congestion control.  NORM receivers MAY employ the
   NORM_CMD message type for application-defined purposes, but it is
   RECOMMENDED that congestion control and feedback implosion issues be
   addressed.  Additionally, an auxiliary message type of NORM_REPORT is
   also provided for experimental purposes.  This section describes the
   message formats used by the NORM protocol.  These messages and their
   fields are referenced in the detailed functional description of the
   NORM protocol given in Section 5.  Individual NORM messages are
   compatible with the Maximum Transmission Unit (MTU) limitations of
   encapsulating Internet protocols including IPv4, IPv6, and UDP.  The
   current NORM protocol specification assumes UDP encapsulation and
   leverages the transport features of UDP.  The NORM messages are
   independent of network addresses and can be used in IPv4 and IPv6

4.1.  NORM Common Message Header and Extensions

   There are some common message fields contained in all NORM message
   types.  Additionally, a header extension mechanism is defined to
   expand the functionality of the NORM protocol without revision to
   this document.  All NORM protocol messages begin with a common header
   with information fields as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version|  type |    hdr_len    |          sequence             |
     |                           source_id                           |

                Figure 1: NORM Common Message Header Format

   The "version" field is a 4-bit value indicating the protocol version
   number.  NORM implementations SHOULD ignore received messages with
   version numbers different from their own.  This number is intended to
   indicate and distinguish upgrades of the protocol that are non-
   interoperable.  The NORM version number for this specification is 1.

   The message "type" field is a 4-bit value indicating the NORM
   protocol message type.  These types are defined as follows:

                  | Message          |       Value      |
                  | NORM_INFO        |         1        |
                  | NORM_DATA        |         2        |
                  | NORM_CMD         |         3        |
                  | NORM_NACK        |         4        |
                  | NORM_ACK         |         5        |
                  | NORM_REPORT      |         6        |

   The 8-bit "hdr_len" field indicates the number of 32-bit words that
   comprise the given message's header portion.  This is used to
   facilitate the addition of header extensions.  The presence of header
   extensions is implied when the "hdr_len" value is greater than the
   base value for the given message "type".

   The "sequence" field is a 16-bit value that is set by the message
   originator.  The "sequence" field serves two separate purposes,
   depending upon the message type:

   1.  NORM senders MUST set the "sequence" field of sender messages
       (NORM_INFO, NORM_DATA, and NORM_CMD) so that receivers can
       monitor the "sequence" value to maintain an estimate of packet
       loss that can be used for congestion control purposes (see
       Section 5.5.2 for a detailed description of NORM Congestion
       Control operation).  A monotonically increasing sequence number
       space MUST be maintained to mark NORM sender messages in this
       way.  Note that this "sequence" number is explicitly NOT used in

       NORM as part of its reliability procedures.  The NORM object and
       FEC payload identifiers are used to detect missing content for
       reliable transfer purposes.

   2.  NORM receivers SHOULD set the "sequence" field to support
       protection from message replay attacks of NORM_NACK or NORM_NACK
       messages.  Note that, depending upon configuration, NORM feedback
       messages are sent to the session multicast address or the unicast
       address(es) of the active NORM sender(s).  Thus, a separate,
       monotonically increasing sequence number space MUST be maintained
       for each destination address to which the NORM receiver is
       transmitting feedback messages.

   Note that these two separate purposes necessitate the maintenance of
   separate sequence spaces to support the functions described here.
   And, in the case of NORM receivers, additional sequence spaces are
   needed when feedback messages are sent to the sender unicast
   address(es) instead of the session address.

   The "source_id" field is a 32-bit value that uniquely identifies the
   node that sent the message within the context of a single
   NormSession.  This value is termed the NORM node identifier
   (NormNodeId) and unique NormNodeIds MUST be assigned within a single
   NormSession.  In some cases, use of the host IPv4 address or a hash
   of an address can suffice, but alternative methodologies for
   assignment and potential collision resolution of node identifiers
   within a multicast session SHOULD be considered.  For example, the
   techniques for managing the 32-bit "synchronization source" (SSRC)
   identifiers defined in the Real-Time Protocol (RTP) specification
   [RFC3550] are applicable for use with NORM node identifiers when an
   ASM traffic model is observed.  In most deployments of the NORM
   protocol to date, the NormNodeId assignments are administratively
   configured, and this form of NormNodeId assignment is RECOMMENDED for
   most purposes.  NORM sender NormNodeId values MUST be unique within
   an ASM session so that NORM receiver feedback can be properly
   demultiplexed by senders, and NORM receiver NormNodeId values MUST
   also be unique for congestion control operation or when the OPTIONAL
   positive acknowledgment mechanism is used.

   NORM Header Extensions

   When header extensions are applied, they follow the message type's
   base header and precede any payload portion.  There are two formats
   for header extensions, both of which begin with an 8-bit "het"
   (header extension type) field.  One format is provided for variable-
   length extensions with "het" values in the range from 0 through 127.
   The other format is for fixed-length (one 32-bit word) extensions
   with "het" values in the range from 128 through 255.

   For variable-length extensions, the value of the "hel" (header
   extension length) field is the length of the entire header extension,
   expressed in multiples of 32-bit words.  The "hel" field MUST be
   present for variable-length extensions ("het" between 0 and 127) and
   MUST NOT be present for fixed-length extensions ("het" between 128
   and 255).

   The formats of the variable-length and fixed-length header extensions
   are given, respectively, here:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   het <=127   |      hel      |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                    Header Extension Content                   |
     |                              ...                              |

          Figure 2: NORM Variable-Length Header Extension Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   het >=128   |    reserved   |    Header Extension Content   |

       Figure 3: NORM Fixed-Length (32-bit) Header Extension Format

   The "Header Extension Content" portion of the header extension is
   defined for each extension type.  Some header extensions are defined
   within this document for NORM baseline FEC and congestion control

4.2.  Sender Messages

   NORM sender messages include the NORM_DATA type, the NORM_INFO type,
   and the NORM_CMD type.  NORM_DATA and NORM_INFO messages contain
   application data content while NORM_CMD messages are used for various
   protocol control functions.

4.2.1.  NORM_DATA Message

   The NORM_DATA message is generally the predominant type transmitted
   by NORM senders.  These messages are used to encapsulate segmented
   data content for objects of type NORM_OBJECT_DATA, NORM_OBJECT_FILE,
   and NORM_OBJECT_STREAM.  NORM_DATA messages contain original or FEC-
   encoded application data content.

   The format of NORM_DATA messages is comprised of three logical
   portions: 1) a fixed-format NORM_DATA header portion, 2) a FEC
   Payload ID portion with a format dependent upon the FEC encoding
   used, and 3) a payload portion containing source or encoded
   application data content.  Note for objects of type
   NORM_OBJECT_STREAM, the payload portion contains additional fields
   used to appropriately recover stream content.  NORM implementations
   MAY also extend the NORM_DATA header to include a FEC Object
   Transmission Information (EXT_FTI) header extension.  This allows
   NORM receivers to automatically allocate resources and properly
   perform FEC decoding without the need for pre-configuration or out-
   of-band information.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=2|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |     flags     |    fec_id     |     object_transport_id       |
     |                         fec_payload_id                        |
     |                              ...                              |
     |                header_extensions (if applicable)              |
     |                              ...                              |
     |          payload_len*         |       payload_msg_start*      |
     |                        payload_offset*                        |
     |                          payload_data*                        |
     |                              ...                              |

                    Figure 4: NORM_DATA Message Format

   *IMPORTANT NOTE: The "payload_len", "payload_msg_start" and
   "payload_offset" fields are present only for objects of type
   NORM_OBJECT_STREAM.  These fields, as with the entire payload, are
   subject to any FEC encoding used.  Thus, when systematic FEC codes
   are used, these values can be directly interpreted only for packets
   containing source symbols while packets containing FEC parity content
   need decoding before these fields can be interpreted.

   The "version", "type", "hdr_len", "sequence", and "source_id" fields

   form the NORM common message header as described in Section 4.1.  The
   value of the NORM_DATA "type" field is 2.  The NORM_DATA base
   "hdr_len" value is 4 (i.e., four 32-bit words) plus the size of the
   "fec_payload_id" field.  The "fec_payload_id" field size depends upon
   the FEC encoding type referenced by the "fec_id" field.  For example,
   when small block, systematic codes are used, a "fec_id" value of 129
   is indicated, and the size of the "fec_payload_id" is two 32-bit
   words.  In this case the NORM_DATA base "hdr_len" value is 6.  The
   cumulative size of any header extensions applied is added into the
   "hdr_len" field.

   The "instance_id" field contains a value generated by the sender to
   uniquely identify its current instance of participation in the
   NormSession.  This allows receivers to detect when senders have
   perhaps left and rejoined a session in progress.  When a sender
   (identified by its "source_id") is detected to have a new
   "instance_id", the NORM receivers SHOULD drop their previous state on
   the sender and begin reception anew, or at least treat this
   "instance" as a new, separate sender.

   The "grtt" field contains a non-linear quantized representation of
   the sender's current estimate of group round-trip time (GRTT_sender)
   (this is also referred to as R_max in [TfmccPaper]).  This value is
   used to control timing of the NACK repair process and other aspects
   of protocol operation as described in this document.  Normally, the
   advertised "grtt" value will correspond to what the sender has
   measured based on feedback from the group, but, at low transmission
   rates, the advertised "grtt" SHALL be set to MAX(grttMeasured,
   NormSegmentSize/senderRate) where the NormSegmentSize is the sender's
   segment size in bytes and the senderRate is the sender's current
   transmission rate in bytes per second.  The algorithm for encoding
   and decoding this field is described in the Multicast NACK Building
   Block [RFC5401] document.

   The "backoff" field value is used by receivers to determine the
   maximum backoff timer value used in the timer-based NORM NACK
   feedback suppression.  This 4-bit field supports values from 0-15
   that are multiplied by GRTT_sender to determine the maximum backoff
   timeout.  The "backoff" field informs the receivers of the sender's
   backoff factor parameter (K_sender).  Recommended values and their
   uses are described in the NORM receiver NACK procedure description in
   Section 5.3.

   The "gsize" field contains a representation of the sender's current
   estimate of group size (GSIZE_sender).  This 4-bit field can roughly
   represent values from ten to 500 million where the most significant
   bit value of 0 or 1 represents a mantissa of 1 or 5, respectively,
   and the three least significant bits incremented by one represent a

   base-10 exponent (order of magnitude).  For example, a field value of
   "0x0" represents 1.0e+01 (10), a value of "0x8" represents 5.0e+01
   (50), a value of "0x1" represents 1.0e+02 (100), and a value of "0xf"
   represents 5.0e+08.  For NORM feedback suppression purposes, the
   group size does not need to be represented with a high degree of
   precision.  The group size MAY even be estimated somewhat
   conservatively (i.e., overestimated) to maintain low levels of
   feedback traffic.  A default group size estimate of 10,000 ("gsize" =
   0x3) is RECOMMENDED for general purpose reliable multicast
   applications using the NORM protocol.

   The "flags" field contains a number of different binary flags
   providing information and hints for the receiver to appropriately
   handle the identified object.  Defined flags in this field include:

   | Flag                 | Value | Purpose                            |
   | NORM_FLAG_REPAIR     |  0x01 | Indicates message is a repair      |
   |                      |       | transmission                       |
   | NORM_FLAG_EXPLICIT   |  0x02 | Indicates a repair segment         |
   |                      |       | intended to meet a specific        |
   |                      |       | receiver erasure, as compared to   |
   |                      |       | parity segments provided by the    |
   |                      |       | sender for general purpose (with   |
   |                      |       | respect to a FEC coding block)     |
   |                      |       | erasure filling.                   |
   | NORM_FLAG_INFO       |  0x04 | Indicates availability of          |
   |                      |       | NORM_INFO for object.              |
   | NORM_FLAG_UNRELIABLE |  0x08 | Indicates that repair              |
   |                      |       | transmissions for the specified    |
   |                      |       | object will be unavailable         |
   |                      |       | (one-shot, best-effort             |
   |                      |       | transmission).                     |
   | NORM_FLAG_FILE       |  0x10 | Indicates object is file-based     |
   |                      |       | data (hint to use disk storage for |
   |                      |       | reception).                        |
   | NORM_FLAG_STREAM     |  0x20 | Indicates object is of type        |
   |                      |       | NORM_OBJECT_STREAM.                |

   NORM_FLAG_REPAIR is set when the associated message is a repair
   transmission.  This information can be used by receivers to help
   observe a join policy where it is desired that newly joining
   receivers only begin participating in the NACK process upon receipt
   of new (non-repair) data content.  NORM_FLAG_EXPLICIT is used to mark
   repair messages sent when the data sender has exhausted its ability
   to provide "fresh" (not previously transmitted) parity segments as

   repair.  This flag could possibly be used by intermediate systems
   implementing functionality to control sub-casting of repair content
   to different legs of a reliable multicast topology with disparate
   repair needs.  NORM_FLAG_INFO is set only when OPTIONAL NORM_INFO
   content is actually available for the associated object.  Thus,
   receivers will NACK for retransmission of NORM_INFO only when it is
   available for a given object.  NORM_FLAG_UNRELIABLE is set when the
   sender wishes to transmit an object with only "best effort" delivery
   and will not supply repair transmissions for the object.  NORM
   receivers SHOULD NOT execute repair requests for objects marked with
   the NORM_FLAG_UNRELIABLE flag.  There are cases where receivers can
   inadvertently request repair of such objects when all segments (or
   info content) for those objects are not received (i.e., a gap in the
   "object_transport_id" sequence is noted).  In this case, the sender
   SHALL invoke the NORM_CMD(SQUELCH) process as described in
   Section 4.2.3.

   NORM_FLAG_FILE can be set as a hint from the sender that the
   associated object SHOULD be stored in non-volatile storage.
   NORM_FLAG_STREAM is set when the identified object is of type
   NORM_OBJECT_STREAM.  The presence of NORM_FLAG_STREAM overrides that
   of NORM_FLAG_FILE with respect to interpretation of object size and
   the format of NORM_DATA messages.

   The "fec_id" field corresponds to the FEC Encoding Identifier
   described in the FEC Building Block document [RFC5052].  The "fec_id"
   value implies the format of the "fec_payload_id" field and, coupled
   with FEC Object Transmission Information, the procedures to decode
   FEC-encoded content.  Small block, systematic codes ("fec_id" = 129)
   are expected to be used for most NORM purposes and systematic FEC
   codes are RECOMMENDED for the most efficient performance of
   NORM_OBJECT_STREAM transport.

   The "object_transport_id" field is a monotonically and incrementally
   increasing value assigned by the sender to NormObjects being
   transmitted.  Transmissions and repair requests related to that
   object use the same "object_transport_id" value.  For sessions of
   very long or indefinite duration, the "object_transport_id" field
   will wrap and be repeated, but it is presumed that the 16-bit field
   size provides a sufficient sequence space to avoid object confusion
   amongst receivers and sources (i.e., receivers SHOULD re-synchronize
   with a server when receiving object sequence identifiers sufficiently
   out-of-range with the current state kept for a given source).  During
   the course of its transmission within a NORM session, an object is
   uniquely identified by the concatenation of the sender "source_id"
   and the given "object_transport_id".  Note that NORM_INFO messages
   associated with the identified object carry the same
   "object_transport_id" value.

   The "fec_payload_id" identifies the attached NORM_DATA "payload"
   content.  The size and format of the "fec_payload_id" field depends
   upon the FEC type indicated by the "fec_id" field.  These formats are
   given in the descriptions of specific FEC schemes such as those
   described in the FEC Basic Schemes [RFC5445] specification or in
   other FEC Schemes.  As an example, the format of the "fec_payload_id"
   format for Small Block, Systematic codes ("fec_id" = 129) from the
   FEC Basic Schemes [RFC5445] specification is given here:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |                       source_block_number                     |
     |        source_block_len       |      encoding_symbol_id       |

        Figure 5: Example: FEC Payload Id Format for 'fec_id' = 129

   In this example, FEC payload identifier, the "source_block_number",
   "source_block_len", and "encoding_symbol_id" fields correspond to the
   "Source Block Number", "Source Block Length", and "Encoding Symbol
   ID" fields of the FEC Payload ID format for Small Block Systematic
   FEC Schemes identified by a "fec_id" value of 129 as specified by the
   FEC Basic Schemes [RFC5445] specification.  The "source_block_number"
   identifies the coding block's relative position with a NormObject.
   Note that, for NormObjects of type NORM_OBJECT_STREAM, the
   "source_block_number" will wrap for very long-lived sessions.  The
   "source_block_len" indicates the number of user data segments in the
   identified coding block.  Given the "source_block_len" information of
   how many symbols of application data are contained in the block, the
   receiver can determine whether the attached segment is data or parity
   content and treat it appropriately.  Applications MAY dynamically
   "shorten" code blocks when the pending information content is not
   predictable (e.g., real-time message streams).  In that case, the
   "source_block_len" value given for an "encoding_symbol_id" that
   contains FEC parity content SHALL take precedence over the
   "source_block_len" value provided for any packets containing source
   symbols.  Also, the "source_block_len" value given for an ordinally
   higher "encoding_symbol_id" SHALL take precedence over the
   "source_block_len" given for prior encoding symbols.  The reason for
   this is that the sender will only know the maximum source block
   length at the time it is transmitting source symbols, but then
   subsequently "shorten" the code and then provide that last source
   symbol and/or encoding symbols with FEC parity content.  The
   "encoding_symbol_id" identifies which specific symbol (segment)
   within the coding block the attached payload conveys.  Depending upon
   the value of the "encoding_symbol_id" and the associated
   "source_block_len" parameters for the block, the symbol (segment)

   referenced will be a user data or a FEC parity segment.  For
   systematic codes, encoding symbols numbered less than the
   source_block_len contain original application data while segments
   greater than or equal to source_block_len contain parity symbols
   calculated for the block.  The concatenation of object_transport_id::
   fec_payload_id can be viewed as a unique transport protocol data unit
   identifier for the attached segment with respect to the NORM sender's
   instance within a session.

   Additional FEC Object Transmission Information (FTI) (as described in
   the FEC Building Block [RFC5052]) document is needed to properly
   receive and decode NORM transport objects.  This information MAY be
   provided as out-of-band session information.  In some cases, it will
   be useful for the sender to include this information "in-band" to
   facilitate receiver operation with minimal pre-configuration.  For
   this purpose, the NORM FEC Object Transmission Information Header
   Extension (EXT_FTI) is defined.  This header extension MAY be applied
   to NORM_DATA and NORM_INFO messages to provide this necessary
   information.  The format of the EXT_FTI consists of two parts, a
   general part that contains the size of the associated transport
   object and a portion that depends upon the FEC scheme being used.
   The "fec_id" field in NORM_DATA and NORM_INFO messages identifies the
   FEC scheme.  The format of the EXT_FTI general part is given here.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |    het = 64   |    hel = 4    |       object_size (msb)       |
     |                       object_size (lsb)                       |
     |                  FEC scheme-specific content ...              |

         Figure 6: EXT_FTI Header Extension General Portion Format

   The header extension type "het" field value for the EXT_FTI header
   extension is 64.  The header extension length "hel" value depends
   upon the format of the FTI for encoding type identified by the
   "fec_id" field.

   The 48-bit "object_size" field indicates the total length of the
   object (in bytes) for the static object types of NORM_OBJECT_FILE and
   NORM_OBJECT_DATA.  This information is used by receivers to determine
   storage requirements and/or allocate storage for the received object.
   Receivers with insufficient storage capability might wish to forego
   reliable reception (i.e., not NACK for) of the indicated object.  In
   the case of objects of type NORM_OBJECT_STREAM, the "object_size"
   field is used by the sender to advertise the size of its stream

   buffer to the receiver group.  In turn, the receivers SHOULD use this
   information to allocate a stream buffer for reception of
   corresponding size.

   As noted, the format of the extension depends upon the FEC code in
   use, but in general, it contains any necessary details on the code in
   use (e.g., FEC Instance ID, etc.).  As an example, the format of the
   EXT_FTI for small block systematic codes ("fec_id" = 129) is given
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |    het = 64   |    hel = 4    |       object_size (msb)       |
     |                       object_size (lsb)                       |
     |       fec_instance_id         |          segment_size         |
     |       fec_max_block_len       |         fec_num_parity        |

   Figure 7: Example: EXT_FTI Header Extension Format for 'fec_id' = 129

   In this example (for "fec_id" = 129), the "hel" field value is 4.
   The size of the EXT_FTI header extension will possibly be different
   for other FEC schemes.

   The 48-bit "object_size" serves the purpose described previously.

   The "fec_instance_id" corresponds to the "FEC Instance ID" described
   in the FEC Building Block [RFC5052] document.  In this case, the
   "fec_instance_id" is a value corresponding to the particular type of
   Small Block Systematic Code being used (e.g., Reed-Solomon GF(2^8),
   Reed-Solomon GF(2^16), etc).  The standardized assignment of FEC
   Instance ID values is described in RFC 5052.

   The "segment_size" field indicates the sender's current setting for
   maximum message payload content (in bytes).  This allows receivers to
   allocate appropriate buffering resources and to determine other
   information in order to properly process received data messaging.
   Typically, FEC parity symbol segments will be of this size.

   The "fec_max_block_len" indicates the current maximum number of user
   data segments per FEC coding block to be used by the sender during
   the session.  This allows receivers to allocate appropriate buffer
   space for buffering blocks transmitted by the sender.

   The "fec_num_parity" corresponds to the "maximum number of encoding

   symbols that can be generated for any source block" as described in
   FEC Object Transmission Information for Small Block Systematic Codes
   as described in the FEC Building Block [RFC5052] document.  For
   example, Reed-Solomon codes can be arbitrarily shortened to create
   different code variations for a given block length.  In the case of
   Reed-Solomon (GF(2^8) and GF(2^16)) codes, this value indicates the
   maximum number of parity segments available from the sender for the
   coding blocks.  This field MAY be interpreted differently for other
   systematic codes as they are defined.

   The payload portion of NORM_DATA messages includes source data or
   FEC-encoded application content.  The content of this payload depends
   upon the FEC scheme being employed, and support for streaming using
   the NORM_OBJECT_STREAM type, when applicable, necessitates some
   additional content in the payload.

   The "payload_len", "payload_msg_start", and "payload_offset" fields
   are present only for transport objects of type NORM_OBJECT_STREAM.
   These REQUIRED fields allow senders to arbitrarily vary the size of
   NORM_DATA payload segments for streams.  This allows applications to
   flush transmitted streams as needed to meet unique streaming
   requirements.  For objects of types NORM_OBJECT_FILE and
   NORM_OBJECT_DATA, these fields are unnecessary since the receiver can
   calculate the payload length and offset information from the
   "fec_payload_id" using the REQUIRED block partitioning algorithm
   described in the FEC Building Block [RFC5052] document.  When
   systematic FEC codes (e.g., "fec_id" = 129) are used, the
   "payload_len", "payload_msg_start", and "payload_offset" fields
   contain actual payload_data length, message start index (or stream
   control code), and byte offset values for the associated application
   stream data segment (the remainder of the "payload_data" field
   content) for those NORM_DATA messages containing source data symbols.
   In NORM_DATA messages that contain FEC parity content, these fields
   do not contain values that can be directly interpreted, but instead
   are values computed from FEC encoding the "payload_len",
   "payload_msg_start", and "payload_offset" fields for the source data
   segments of the corresponding coding block.  The actual
   "payload_msg_start", "payload_len" and, "payload_offset" values of
   missing data content can be determined upon decoding a FEC coding
   block.  Note that these fields do NOT contribute to the value of the
   NORM_DATA "hdr_len" field.  These fields are present only when the
   "flags" portion of the NORM_DATA message indicate the transport
   object is of type NORM_OBJECT_STREAM.

   The "payload_len" value, when non-zero, indicates the length (in
   bytes) of the source content contained in the associated
   "payload_data" field.  However, when the "payload_len" value is equal
   to ZERO, this indicates that the "payload_msg_start" field be

   alternatively interpreted as a "stream_control_code".  The only
   "stream_control_code" value defined is NORM_STREAM_END = 0.  The
   NORM_STREAM_END code indicates that the sender is terminating the
   transmission of stream content at the corresponding position in the
   stream and the receiver MUST NOT expect content (or request repair
   for any content) following that position in the stream.  Additional
   specifications MAY extend the functionality of the NORM stream
   transport mode by defining additional stream control codes.  These
   control codes are delivered to the recipient application reliably,
   in-order with respect to the streamed application data content.

   The "payload_msg_start" field serves one of two exclusive purposes.
   When the "payload_len" value is non-zero, the "payload_msg_start"
   field, when also set to a non-zero value, indicates that the
   associated "payload_data" content contains an application-defined
   message boundary (start-of-message).  When such a message boundary is
   indicated, the first byte of an application-defined message, with
   respect to the "payload_data" field, will be found at an offset of
   "payload_msg_start - 1" bytes.  Thus, if a NORM_DATA payload for a
   NORM_OBJECT_STREAM contains the start of an application message at
   the first byte of the "payload_data" field, the value of the
   "payload_msg_start" field will be '1'.  NORM implementations SHOULD
   provide sender stream applications with a capability to mark message
   boundaries in this manner.  Similarly, the NORM receiver
   implementation SHOULD enable the application to recover such message
   boundary information.  This enables NORM receivers to "synchronize"
   reliable reception of transmitted message stream content in a
   meaningful way (i.e., meaningful to the application) at any time,
   whether joining a session already in progress, or departing the
   session and returning.  Note that if the value of the
   "payload_msg_start" field is ZERO, no message boundary is present.
   The "payload_msg_start" value will always be less than or equal to
   the "payload_len" value except for the special case of "payload_len =
   0", which indicates the "payload_msg_start" field be instead
   interpreted as a "stream_control_code"

   The "payload_offset" field indicates the relative byte position (from
   the sender stream transmission start) of the source content contained
   in the "payload_data" field.  Note that for long-lived streams, the
   "payload_offset" field will wrap.

   The "payload_data" field contains the original application source or
   parity content for the symbol identified by the "fec_payload_id".
   The length of this field SHALL be limited to a maximum of the
   sender's NormSegmentSize bytes as given in the FTI for the object.
   Note the length of this field for messages containing parity content
   will always be of length NormSegmentSize.  When encoding data
   segments of varying sizes, the FEC encoder SHALL assume ZERO value

   padding for data segments with a length less than the
   NormSegmentSize.  It is RECOMMENDED that a sender's NormSegmentSize
   generally be constant for the duration of a given sender's term of
   participation in the session, but can possibly vary on a per-object
   basis.  The NormSegmentSize SHOULD be configurable by the sender
   application prior to session participation as needed for network
   topology MTU considerations.  For IPv6, MTU discovery MAY be possibly
   leveraged at session startup to perform this configuration.  The
   "payload_data" content MAY be delivered directly to the application
   for source symbols (when systematic FEC encoding is used) or upon
   decoding of the FEC block.  For NORM_OBJECT_FILE and
   NORM_OBJECT_STREAM objects, the data segment length and offset can be
   calculated using the block partitioning algorithm described in the
   FEC Building Block [RFC5052] document.  For NORM_OBJECT_STREAM
   objects, the length and offset is obtained from the segment's
   corresponding embedded "payload_len" and "payload_offset" fields.

4.2.2.  NORM_INFO Message

   The NORM_INFO message is used to convey OPTIONAL, application-
   defined, out-of-band context information for transmitted NormObjects.
   An example NORM_INFO use for bulk file transfer is to place MIME type
   information for the associated file, data, or stream object into the
   NORM_INFO payload.  Receivers could then use the NORM_INFO content to
   make a decision as to whether to participate in reliable reception of
   the associated object.  Each NormObject can have an independent unit
   of NORM_INFO with which it is associated.  NORM_DATA messages contain
   a flag to indicate the availability of NORM_INFO for a given
   NormObject.  NORM receivers will NACK for retransmission of NORM_INFO
   when they have not received it for a given NormObject.  The size of
   the NORM_INFO content is limited to that of a single NormSegmentSize
   for the given sender.  This atomic nature allows the NORM_INFO to be
   rapidly and efficiently repaired within the NORM reliable
   transmission process.

   When NORM_INFO content is available for a NormObject, the
   NORM_FLAG_INFO flag SHALL be set in NORM_DATA messages for the
   corresponding "object_transport_id" and the NORM_INFO message SHALL
   be transmitted as the first message for the NormObject.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=1|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |     flags     |     fec_id    |     object_transport_id       |
     |                header_extensions (if applicable)              |
     |                              ...                              |
     |                         payload_data                          |
     |                              ...                              |

                    Figure 8: NORM_INFO Message Format

   The "version", "type", "hdr_len", "sequence", and "source_id" fields
   form the NORM common message header as described in Section 4.1.  The
   value of the "hdr_len" field when no header extensions are present is

   The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
   "object_transport_id" fields carry the same information and serve the
   same purpose as NORM_DATA messages.  These values allow the receiver
   to prepare appropriate buffering, etc., for further transmissions
   from the sender when NORM_INFO is the first message received.

   As with NORM_DATA messages, the NORM FTI Header Extension (EXT_FTI)
   MAY be optionally applied to NORM_INFO messages.  To conserve
   protocol overhead, NORM implementations MAY apply the EXT_FTI when
   used to NORM_INFO messages only and not to NORM_DATA messages.

   The NORM_INFO "payload_data" field contains sender application-
   defined content that can be used by receiver applications for various
   purposes as described above.

4.2.3.  NORM_CMD Messages

   NORM_CMD messages are transmitted by senders to perform a number of
   different protocol functions.  This includes functions such as round-
   trip timing collection, congestion control functions, synchronization
   of sender/receiver repair "windows", and notification of sender
   status.  A core set of NORM_CMD messages is enumerated.
   Additionally, a range of command types remain available for potential

   application-specific use.  Some NORM_CMD types can have dynamic
   content attached.  Any attached content will be limited to the
   maximum length of the sender NormSegmentSize to retain the atomic
   nature of the commands.  All NORM_CMD messages begin with a common
   set of fields, after the usual NORM message common header.  The
   standard NORM_CMD fields are:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |    sub-type   |                                               |
     +-+-+-+-+-+-+-+-+        NORM_CMD Content                       +
     |                              ...                              |

                    Figure 9: NORM_CMD Standard Fields

   The "version", "type", "hdr_len", "sequence", and "source_id" fields
   form the NORM common message header as described in Section 4.1.  The
   value of the "hdr_len" field for NORM_CMD messages without header
   extensions present depends upon the "sub-type" field.

   The "instance_id", "grtt", "backoff", and "gsize" fields provide the
   same information and serve the same purpose as NORM_DATA and
   NORM_INFO messages.  The "sub-type" field indicates the type of
   command to follow.  The remainder of the NORM_CMD message is
   dependent upon the command sub-type.  NORM command sub-types include:

   | Command               | Sub-type | Purpose                        |
   | NORM_CMD(FLUSH)       |     1    | Used to indicate sender        |
   |                       |          | temporary end-of-transmission. |
   |                       |          | (Assists in robustly           |
   |                       |          | initiating outstanding repair  |
   |                       |          | requests from receivers).  May |
   |                       |          | also be optionally used to     |
   |                       |          | collect positive               |
   |                       |          | acknowledgment of reliable     |
   |                       |          | reception from a subset of     |
   |                       |          | receivers.                     |
   | NORM_CMD(EOT)         |     2    | Used to indicate sender        |
   |                       |          | permanent end-of-transmission. |

   | NORM_CMD(SQUELCH)     |     3    | Used to advertise sender's     |
   |                       |          | current repair window in       |
   |                       |          | response to out-of-range NACKs |
   |                       |          | from receivers.                |
   | NORM_CMD(CC)          |     4    | Used for GRTT measurement and  |
   |                       |          | collection of congestion       |
   |                       |          | control feedback.              |
   | NORM_CMD(REPAIR_ADV)  |     5    | Used to advertise sender's     |
   |                       |          | aggregated repair/feedback     |
   |                       |          | state for suppression of       |
   |                       |          | unicast feedback from          |
   |                       |          | receivers.                     |
   | NORM_CMD(ACK_REQ)     |     6    | Used to request                |
   |                       |          | application-defined positive   |
   |                       |          | acknowledgment from a list of  |
   |                       |          | receivers (OPTIONAL).          |
   | NORM_CMD(APPLICATION) |     7    | Used for application-defined   |
   |                       |          | purposes that need to          |
   |                       |          | temporarily preempt or         |
   |                       |          | supplement data transmission   |
   |                       |          | (OPTIONAL).                    |
   +-----------------------+----------+--------------------------------+  NORM_CMD(FLUSH) Message

   The NORM_CMD(FLUSH) command is sent when the sender reaches the end
   of all data content and pending repairs it has queued for
   transmission.  This can indicate either a temporary or permanent end-
   of-data transmission, but that the sender is still willing to respond
   to repair requests.  This command is repeated once per 2*GRTT_sender
   to excite the receiver set for any outstanding repair requests up to
   and including the transmission point indicated within the
   NORM_CMD(FLUSH) message.  The number of repeats is equal to
   NORM_ROBUST_FACTOR unless a list of receivers from which explicit
   positive acknowledgment is expected ("acking_node_list") is given.
   In that case, the "acking_node_list" is updated as acknowledgments
   are received and the NORM_CMD(FLUSH) is repeated according to the
   mechanism described in Section 5.5.3.  The greater the
   NORM_ROBUST_FACTOR, the greater the probability that all applicable
   receivers will be excited for acknowledgment or repair requests
   (NACKs) AND that the corresponding NACKs are delivered to the sender.
   A default value of NORM_ROBUST_FACTOR equal to 20 is RECOMMENDED.  If
   a NORM_NACK message interrupts the flush process, the sender SHALL
   re-initiate the flush process after any resulting repair
   transmissions are completed.

   Note that receivers also employ a timeout mechanism to self-initiate
   NACKing (if there are outstanding repair needs) when no messages of

   any type are received from a sender.  This inactivity timeout is
   related to the NORM_CMD(FLUSH) and NORM_ROBUST_FACTOR and is
   specified in Section 5.3.  Receivers SHALL self-initiate the NACK
   repair process when the inactivity timeout has expired for a specific
   sender and the receiver has pending repairs needed from that sender.
   With a sufficiently large NORM_ROBUST_FACTOR value, data content is
   delivered with a high assurance of reliability.  The penalty of a
   large NORM_ROBUST_FACTOR value is the potential transmission of
   excess NORM_CMD(FLUSH) messages and a longer inactivity timeout for
   receivers to self-initiate a terminal NACK process.

   For finite-sized transport objects such as NORM_OBJECT_DATA and
   NORM_OBJECT_FILE, the flush process (if there are no further pending
   objects) occurs at the end of these objects.  Thus, FEC repair
   information is always available for repairs in response to repair
   requests elicited by the flush command.  However, for
   NORM_OBJECT_STREAM, the flush can occur at any time, including in the
   middle of a FEC coding block if systematic FEC codes are employed.
   In this case, the sender will not yet be able to provide FEC parity
   content for the concurrent coding block and will be limited to
   explicitly repairing the stream with source data content for that
   block.  Applications that anticipate frequent flushing of stream
   content SHOULD be judicious in the selection of the FEC coding block
   size (i.e., do not use a very large coding block size if frequent
   flushing occurs).  For example, a reliable multicast application
   transmitting an ongoing series of intermittent, relatively small
   messages will need to trade-off using the NORM_OBJECT_DATA paradigm
   versus the NORM_OBJECT_STREAM paradigm with an appropriate FEC coding
   block size.  This is analogous to application trade-offs for other
   transport protocols such as the selection of different TCP modes of
   operation such as "no delay", etc.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |  sub-type = 1 |    fec_id     |      object_transport_id      |
     |                         fec_payload_id                        |
     |                              ...                              |
     |                acking_node_list (if applicable)               |
     |                              ...                              |

                 Figure 10: NORM_CMD(FLUSH) Message Format

   The "version", "type", "hdr_len", "sequence", and "source_id" fields
   form the NORM common message header as described in Section 4.1.  In
   addition to the NORM common message header and standard NORM_CMD
   fields, the NORM_CMD(FLUSH) message contains fields to identify the
   current status and logical transmit position of the sender.

   The "fec_id" field indicates the FEC type used for the flushing
   "object_transport_id" and implies the size and format of the
   "fec_payload_id" field.  Note the "hdr_len" value for the
   NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
   field when no header extensions are present.

   The "object_transport_id" and "fec_payload_id" fields indicate the
   sender's current logical "transmit position".  These fields are
   interpreted in the same manner as in the NORM_DATA message type.
   Upon receipt of the NORM_CMD(FLUSH), receivers are expected to check
   their completion state THROUGH (including) this transmission
   position.  If receivers have outstanding repair needs in this range,
   they SHALL initiate the NORM NACK Repair Process as described in
   Section 5.3.  If receivers have no outstanding repair needs, no
   response to the NORM_CMD(FLUSH) is generated.

   For NORM_OBJECT_STREAM objects using systematic FEC codes, receivers
   MUST request "explicit-only" repair of the identified
   "source_block_number" if the given "encoding_symbol_id" is less than
   the "source_block_len".  This condition indicates the sender has not
   yet completed encoding the corresponding FEC block and parity content
   is not yet available.  An "explicit-only" repair request consists of

   NACK content for the applicable "source_block_number" that does not
   include any requests for parity-based repair.  This allows NORM
   sender applications to "flush" an ongoing stream of transmission when
   needed, even if in the middle of a FEC block.  Once the sender
   resumes stream transmission and passes the end of the pending coding
   block, subsequent NACKs from receivers SHALL request parity-based
   repair as usual.  Note that the use of a systematic FEC code is
   assumed here.  Note that a sender has the option of arbitrarily
   shortening a given code block when such an application "flush"
   occurs.  In this case, the receiver will request explicit repair, but
   the sender MAY provide FEC-based repair (parity segments) in
   response.  These parity segments MUST contain the corrected
   "source_block_len" for the shortened block and that
   "source_block_len" associated with segments containing parity content
   SHALL override the previously advertised "source_block_len".
   Similarly, the "source_block_len" associated with the highest ordinal
   "encoding_symbol_id" SHALL take precedence over prior symbols when a
   difference (e.g., due to code shortening at the sender) occurs.
   Normal receiver NACK initiation and construction is discussed in
   detail in Section 5.3.

   The OPTIONAL "acking_node_list" field contains a list of NormNodeIds
   for receivers from which the sender is requesting explicit positive
   acknowledgment of reception up through the transmission point
   identified by the "object_transport_id" and "fec_payload_id" fields.
   The length of the list can be inferred from the length of the
   received NORM_CMD(FLUSH) message.  When the "acking_node_list" is
   present, the lightweight positive acknowledgment process described in
   Section 5.5.3 SHALL be observed.  NORM_CMD(EOT) Message

   The NORM_CMD(EOT) command is sent when the sender reaches permanent
   end-of-transmission with respect to the NormSession and will not
   respond to further repair requests.  This allows receivers to
   gracefully reach closure of operation with this sender (without
   requiring any timeout) and free any resources that are no longer
   needed.  The NORM_CMD(EOT) command SHOULD be sent with the same
   robust mechanism as used for NORM_CMD(FLUSH) commands to provide a
   high assurance of reception by the receiver set.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |  sub-type = 2 |                    reserved                   |

                  Figure 11: NORM_CMD(EOT) Message Format

   The value of the "hdr_len" field for NORM_CMD(EOT) messages without
   header extensions present is 4.  The "reserved" field is reserved for
   future use and MUST be set to an all ZERO value.  Receivers MUST
   ignore the "reserved" field.  NORM_CMD(SQUELCH) Message

   The NORM_CMD(SQUELCH) command is transmitted in response to outdated
   or invalid NORM_NACK content received by the sender.  Invalid
   NORM_NACK content consists of repair requests for NormObjects for
   which the sender is unable or unwilling to provide repair.  This
   includes repair requests for outdated objects, aborted objects, or
   those objects that the sender previously transmitted marked with the
   NORM_FLAG_UNRELIABLE flag.  This command indicates to receivers what
   content is available for repair, thus serving as a description of the
   sender's current "repair window".  Receivers SHALL NOT generate
   repair requests for content identified as invalid by a

   The NORM_CMD(SQUELCH) command is sent once per 2*GRTT_sender at the
   most.  The NORM_CMD(SQUELCH) advertises the current "repair window"
   of the sender by identifying the earliest (lowest) transmission point
   for which it will provide repair, along with an encoded list of
   objects from that point forward that are no longer valid for repair.
   This mechanism allows the sender application to cancel or abort
   transmission and/or repair of specific previously enqueued objects.
   The list also contains the identifiers for any objects within the
   repair window that were sent with the NORM_FLAG_UNRELIABLE flag set.
   In normal conditions, the NORM_CMD(SQUELCH) will be needed
   infrequently, and generally only to provide a reference repair window
   for receivers who have fallen "out-of-sync" with the sender due to
   extremely poor network conditions.

   The starting point of the invalid NormObject list begins with the

   lowest invalid NormTransportId greater than the current "repair
   window" start from the invalid NACK(s) that prompted the generation
   of the squelch.  The length of the list is limited by the sender's
   NormSegmentSize.  This allows the receivers to learn the status of
   the sender's applicable object repair window with minimal
   transmission of NORM_CMD(SQUELCH) commands.  The format of the
   NORM_CMD(SQUELCH) message is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     | sub-type = 3  |     fec_id    |      object_transport_id      |
     |                         fec_payload_id                        |
     |                              ...                              |
     |                        invalid_object_list                    |
     |                              ...                              |

                Figure 12: NORM_CMD(SQUELCH) Message Format

   In addition to the NORM common message header and standard NORM_CMD
   fields, the NORM_CMD(SQUELCH) message contains fields to identify the
   earliest logical transmit position of the sender's current repair
   window and an "invalid_object_list" beginning with the index of the
   logically earliest invalid repair request from the offending NACK
   message that initiated the NORM_CMD(SQUELCH) transmission.  The value
   of the "hdr_len" field when no extensions are present is 4 plus the
   size of the "fec_payload_id" field that is dependent upon the FEC
   scheme identified by the "fec_id" field.

   The "object_transport_id" and "fec_payload_id" fields are
   concatenated to indicate the beginning of the sender's current repair
   window (i.e., the logically earliest point in its transmission
   history for which the sender can provide repair).  The "fec_id" field
   implies the size and format of the "fec_payload_id" field.  This
   serves as an advertisement of a "synchronization" point for receivers
   to request repair.  Note, that while an "encoding_symbol_id" MAY be
   included in the "fec_payload_id" field, the sender's repair window
   SHOULD be aligned on FEC coding block boundaries and thus the
   "encoding_symbol_id" SHOULD be zero.

   The "invalid_object_list" is a list of 16-bit NormTransportIds that,
   although they are within the range of the sender's current repair
   window, are no longer available for repair from the sender.  For
   example, a sender application MAY dequeue an out-of-date object even
   though it is still within the repair window.  The total size of the
   "invalid_object_list" content can be determined from the packet's
   payload length and is limited to a maximum of the NormSegmentSize of
   the sender.  Thus, for very large repair windows, it is possible that
   a single NORM_CMD(SQUELCH) message cannot include the entire set of
   invalid objects in the repair window.  In this case, the sender SHALL
   ensure that the list begins with a NormTransportId that is greater
   than or equal to the lowest ordinal invalid NormTransportId from the
   NACK message(s) that prompted the NORM_CMD(SQUELCH) generation.  The
   NormTransportId in the "invalid_object_list" MUST be ordinally
   greater than the "object_transport_id" marking the beginning of the
   sender's repair window.  This ensures convergence of the squelch
   process, even if multiple invalid NACK/squelch iterations are
   required.  This explicit description of invalid content within the
   sender's current window allows the sender application (most notably
   for discrete object transport) to arbitrarily invalidate (i.e.,
   dequeue) portions of enqueued content (e.g., certain objects) for
   which it no longer wishes to provide reliable transport.  NORM_CMD(CC) Message

   The NORM_CMD(CC) message contains fields to enable sender-to-group
   GRTT measurement and to excite the group for congestion control
   feedback.  A baseline NORM congestion control scheme (NORM-CC), based
   on the TCP-Friendly Multicast Congestion Control (TFMCC) scheme of
   RFC 4654 is fully specified in Section 5.5.2 of this document.  The
   NORM_CMD(CC) message is usually transmitted as part of NORM-CC
   operation.  A NORM header extension is defined below to be used with
   the NORM_CMD(CC) message to support NORM-CC operation.  Different
   header extensions MAY be defined for the NORM_CMD(CC) (and/or other
   NORM messages as needed) to support alternative congestion control
   schemes in the future.  If NORM is operated in a network where
   resources are explicitly dedicated to the NORM session and therefore
   congestion control operation is disabled, the NORM_CMD(CC) message is
   then used solely for GRTT measurement and MAY be sent less frequently
   than with congestion control operation.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |            sequence           |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |  sub-type = 4 |    reserved   |          cc_sequence          |
     |                         send_time_sec                         |
     |                        send_time_usec                         |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                  cc_node_list (if applicable)                 |
     |                              ...                              |

                  Figure 13: NORM_CMD(CC) Message Format

   The NORM common message header and standard NORM_CMD fields serve
   their usual purposes.  The value of the "hdr_len" field when no
   header extensions are present is 6.

   The "reserved" field is for potential future use and MUST be set to
   ZERO in this version of the NORM protocol and its baseline NORM-CC
   congestion control scheme.  It is possible for alternative congestion
   control schemes to use the NORM_CMD(CC) message defined here and
   leverage the "reserved" field for scheme-specific purposes.

   The "cc_sequence" field is a sequence number applied by the sender.
   For NORM-CC operation, it is used to provide functionality equivalent
   to the "feedback round number" (fb_nr) described in RFC 4654.  The
   most recently received "cc_sequence" value is recorded by receivers
   and can be fed back to the sender in congestion control feedback
   generated by the receivers for that sender.  The "cc_sequence" number
   can also be used in NORM implementations to assess how recently a
   receiver has received NORM_CMD(CC) probes from the sender.  This can
   be useful instrumentation for complex or experimental multicast
   routing environments.

   The "send_time" field is a timestamp indicating the time that the
   NORM_CMD(CC) message was transmitted.  This consists of a 64-bit
   field containing 32-bits with the time in seconds ("send_time_sec")

   and 32-bits with the time in microseconds ("send_time_usec") since
   some reference time the source maintains (usually 00:00:00, 1 January
   1970).  The byte ordering of the fields is "Big Endian" network
   order.  Receivers use this timestamp adjusted by the amount of delay
   from the time they received the NORM_CMD(CC) message to the time of
   their response as the "grtt_response" portion of NORM_ACK and
   NORM_NACK messages generated.  This allows the sender to evaluate
   round-trip times to different receivers for congestion control and
   other (e.g., GRTT determination) purposes.

   To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
   a NORM-CC Rate header extension (EXT_RATE) is defined to inform the
   group of the sender's current transmission rate.  This is used along
   with the loss detection "sequence" field of all NORM sender messages
   and the NORM_CMD(CC) GRTT collection process to support NORM-CC
   congestion control operation.  The format of this header extension is
   as follows:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |    het = 128  |    reserved   |           send_rate           |

   The "send_rate" field indicates the sender's current transmission
   rate in bytes per second.  The 16-bit "send_rate" field consists of
   12 bits of mantissa in the most significant portion and 4 bits of
   base 10 integer exponent (E) information in the least significant
   portion.  The 12-bit mantissa portion of the field is scaled such
   that a base 10 mantissa (M) floating point value of 0.0 corresponds
   to 0 and a value of 10.0 corresponds to 4096 in the upper 12 bits of
   the 16-bit "send_rate" field.  Thus:

          send_rate = (((int)(M * 4096.0 / 10.0 + 0.5)) << 4) | E;

   For example, to represent a transmission rate of 256 kbit/s (3.2e+04
   bytes per second), the lower 4 bits of the 16-bit field contain a
   value of 0x04 to represent the exponent (E) while the upper 12 bits
   contain a value of 0x51f (M) as determined from the equation given
        send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;
                  = (0x51f << 4) | 0x4
                  = 0x51f4

   To decode the "send_rate" field, the following equation can be used:

   value = (send_rate >> 4) * (10/4096) * power(10, (send_rate & x000f))

   Note the maximum transmission rate that can be represented by this

   scheme is approximately 9.99e+15 bytes per second.

   When this extension is present, a "cc_node_list" might be attached as
   the payload of the NORM_CMD(CC) message.  The presence of this header
   extension also implies that NORM receivers MUST respond according to
   the procedures described in Section 5.5.2.

   The "cc_node_list" consists of a list of NormNodeIds and their
   associated congestion control status.  This includes the current
   limiting receiver (CLR) node, any potential limiting receiver (PLR)
   nodes that have been identified, and some number of receivers for
   which congestion control status is being provided, most notably
   including the receivers' current RTT measurement.  The maximum length
   of the "cc_node_list" provides for at least the CLR and one other
   receiver, but can be increased for more timely feedback to the group.
   The list length can be inferred from the length of the NORM_CMD(CC)

   Each item in the "cc_node_list" is in the following format:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |                          cc_node_id                           |
     |    cc_flags   |     cc_rtt    |            cc_rate            |

   The "cc_node_id" is the NormNodeId of the receiver the item

   The "cc_flags" field contains flags indicating the congestion control
   status of the indicated receiver.  The following flags are defined:

   | Flag               | Value | Purpose                              |
   | NORM_FLAG_CC_CLR   |  0x01 | Receiver is the current limiting     |
   |                    |       | receiver (CLR).                      |
   | NORM_FLAG_CC_PLR   |  0x02 | Receiver is a potential limiting     |
   |                    |       | receiver (PLR).                      |
   | NORM_FLAG_CC_RTT   |  0x04 | Receiver has measured RTT with       |
   |                    |       | respect to sender.                   |

   | NORM_FLAG_CC_START |  0x08 | Sender/receiver is in "slow start"   |
   |                    |       | phase of congestion control          |
   |                    |       | operation (i.e., the receiver has    |
   |                    |       | not yet detected any packet loss and |
   |                    |       | the "cc_rate" field is the           |
   |                    |       | receiver's actual measured receive   |
   |                    |       | rate).                               |
   | NORM_FLAG_CC_LEAVE |  0x10 | Receiver is imminently leaving the   |
   |                    |       | session and its feedback SHOULD not  |
   |                    |       | be considered in congestion control  |
   |                    |       | operation.                           |

   The "cc_rtt" contains a quantized representation of the RTT as
   measured by the sender with respect to the indicated receiver.  This
   field is valid only if the NORM_FLAG_CC_RTT flag is set in the
   "cc_flags" field.  This one-byte field is a quantized representation
   of the RTT using the algorithm described in the Multicast NACK
   Building Block [RFC5401] document.

   The "cc_rate" field contains a representation of the receiver's
   current calculated (during steady-state congestion control operation)
   or twice its measured (during the slow start phase) congestion
   control rate.  This field is encoded and decoded using the same
   technique as described for the NORM_CMD(CC) "send_rate" field.  NORM_CMD(REPAIR_ADV) Message

   The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
   its aggregated repair state from NORM_NACK messages accumulated
   during a repair cycle and/or congestion control feedback received.
   This message is sent only when the sender has received NORM_NACK
   and/or NORM_ACK(CC) (when congestion control is enabled) messages via
   unicast transmission instead of multicast.  By relaying this
   information to the receiver set, suppression of feedback can be
   achieved even when receivers are unicasting that feedback instead of
   multicasting it among the group [NormFeedback].

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     | sub-type = 5  |     flags     |            reserved           |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                       repair_adv_payload                      |
     |                              ...                              |

              Figure 14: NORM_CMD(REPAIR_ADV) Message Format

   The "instance_id", "grtt", "backoff", "gsize", and "sub-type" fields
   serve the same purpose as in other NORM_CMD messages.  The value of
   the "hdr_len" field when no extensions are present is 4.

   The "flags" field provides information on the NORM_CMD(REPAIR_ADV)
   content.  There is currently one NORM_CMD(REPAIR_ADV) flag defined:

                     NORM_REPAIR_ADV_FLAG_LIMIT = 0x01

   This flag is set by the sender when it is unable to fit its full
   current repair state into a single NormSegmentSize.  If this flag is
   set, receivers SHALL limit their NACK response to generating NACK
   content only up through the maximum ordinal transmission position
   (objectTransportId::fecPayloadId) included in the

   When congestion control operation is enabled, a header extension
   SHOULD be applied to the NORM_CMD(REPAIR_ADV) representing the most
   limiting (in terms of congestion control feedback suppression)
   congestion control response.  This allows the NORM_CMD(REPAIR_ADV)
   message to suppress receiver congestion control responses as well as
   NACK feedback messages.  The field is defined as a header extension
   so that alternative congestion control schemes can be used for NORM
   without revision to this document.  A NORM-CC Feedback Header
   Extension (EXT_CC) is defined to encapsulate congestion control
   feedback within NORM_NACK, NORM_ACK, and NORM_CMD(REPAIR_ADV)
   messages.  If another congestion control technique (e.g., Pragmatic
   General Multicast Congestion Control (PGMCC) [PgmccPaper]) is used

   within a NORM implementation, an additional header extension MAY need
   to be defined to encapsulate any required feedback content.  The
   NORM-CC Feedback Header Extension format is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |     het = 3   |    hel = 3    |          cc_sequence          |
     |    cc_flags   |     cc_rtt    |            cc_loss            |
     |            cc_rate            |          cc_reserved          |

   The "cc_sequence" field contains the current greatest "cc_sequence"
   value receivers have received in NORM_CMD(CC) messages from the
   sender.  This information assists the sender in congestion control
   operation by providing an indicator of how current ("fresh") the
   receiver's round-trip measurement reference time is and whether the
   receiver has been successfully receiving recent congestion control
   probes.  For example, if it is apparent the receiver has not been
   receiving recent congestion control probes (and thus possibly other
   messages from the sender), the sender SHOULD choose to take
   congestion avoidance measures.  For NORM_CMD(REPAIR_ADV) messages,
   the sender SHALL set the "cc_sequence" field value to the value set
   in the last NORM_CMD(CC) message sent.

   The "cc_flags" field contains bits representing the receiver's state
   with respect to congestion control operation.  The possible values
   for the "cc_flags" field are those specified for the NORM_CMD(CC)
   message node list item flags.  These fields are used by receivers in
   controlling (suppressing as necessary) their congestion control
   feedback.  For NORM_CMD(REPAIR_ADV) messages, the NORM_FLAG_CC_RTT
   SHALL be set only when all feedback messages received by the sender
   have the flag set.  Similarly, the NORM_FLAG_CC_CLR or
   NORM_FLAG_CC_PLR SHALL be set only when no feedback has been received
   from non-CLR or non-PLR receivers.  And the NORM_FLAG_CC_LEAVE SHALL
   be set only when all feedback messages the sender has received have
   this flag set.  These heuristics for setting the flags in
   NORM_CMD(REPAIR_ADV) ensure the most effective suppression of
   receivers providing unicast feedback messages.

   The "cc_rtt" field SHALL be set to a default maximum value, and the
   NORM_FLAG_CC_RTT flag SHALL be cleared when no receiver has yet
   received RTT measurement information.  When a receiver has received
   RTT measurement information, it SHALL set the "cc_rtt" value
   accordingly and set the NORM_FLAG_CC_RTT flag in the "cc_flags"
   field.  For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the
   "cc_rtt" field value to the largest non-CLR/non-PLR RTT it has

   measured from receivers for the current feedback round.

   The "cc_loss" field represents the receiver's current packet loss
   fraction estimate for the indicated source.  The loss fraction is a
   value from 0.0 to 1.0 corresponding to a range of zero to 100 percent
   packet loss.  The 16-bit "cc_loss" value is calculated by the
   following formula:

             "cc_loss" = floor(decimal_loss_fraction * 65535.0)

   For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
   field value to the largest non-CLR/non-PLR loss estimate it has
   received from receivers for the current feedback round.

   The "cc_rate" field represents the receiver's current local
   congestion control rate.  During "slow start", when the receiver has
   detected no loss, this value is set to twice the actual rate it has
   measured from the corresponding sender and the NORM_FLAG_CC_START is
   set in the "cc_flags" field.  Otherwise, the receiver calculates a
   congestion control rate based on its loss measurement and RTT
   measurement information (even if default) for the "cc_rate" field.
   For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
   field value to the lowest non-CLR/non-PLR "cc_rate" report it has
   received from receivers for the current feedback round.

   The "cc_reserved" field is reserved for future NORM protocol use.
   Currently, senders SHALL set this field to ZERO, and receivers SHALL
   ignore the content of this field.

   The "repair_adv_payload" is in exactly the same form as the
   "nack_content" of NORM_NACK messages and can be processed by
   receivers for suppression purposes in the same manner, with the
   exception of the condition when the NORM_REPAIR_ADV_FLAG_LIMIT is
   set.  NORM_CMD(ACK_REQ) Message

   The NORM_CMD(ACK_REQ) message is used by the sender to request
   acknowledgment from a specified list of receivers.  This message is
   used in providing a lightweight positive acknowledgment mechanism
   that is OPTIONAL for use by the reliable multicast application.  A
   range of acknowledgment request types is provided for use at the
   application's discretion.  Provision for application-defined,
   positively acknowledged commands allows the application to
   automatically take advantage of transmission and round-trip timing
   information available to the NORM protocol.  The details of the NORM
   Positive Acknowledgment Process including transmission of the
   NORM_CMD(ACK_REQ) messages and the receiver response (NORM_ACK) are

   described in Section 5.5.3.  The format of the NORM_CMD(ACK_REQ)
   message is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     | sub-type = 6  |    reserved   |    ack_type   |    ack_id     |
     |                       acking_node_list                        |
     |                              ...                              |

                Figure 15: NORM_CMD(ACK_REQ) Message Format

   The NORM common message header and standard NORM_CMD fields serve
   their usual purposes.  The value of the "hdr_len" field for
   NORM_CMD(ACK_REQ) messages with no header extension present is 4.

   The "ack_type" field indicates the type of acknowledgment being
   requested and thus implies rules for how the receiver will treat this
   request.  The following "ack_type" values are defined and are also
   used in NORM_ACK messages described later:

   | ACK Type              | Value      | Purpose                      |
   | NORM_ACK(CC)          | 1          | Used to identify NORM_ACK    |
   |                       |            | messages sent in response to |
   |                       |            | NORM_CMD(CC) messages.       |
   | NORM_ACK(FLUSH)       | 2          | Used to identify NORM_ACK    |
   |                       |            | messages sent in response to |
   |                       |            | NORM_CMD(FLUSH) messages.    |
   | NORM_ACK(RESERVED)    | 3-15       | Reserved for possible future |
   |                       |            | NORM protocol use.           |
   | NORM_ACK(APPLICATION) | 16-255     | Used at application's        |
   |                       |            | discretion.                  |

   The NORM_ACK(CC) value is provided for use only in NORM_ACKs
   generated in response to the NORM_CMD(CC) messages used in congestion
   control operation.  Similarly, the NORM_ACK(FLUSH) is provided for
   use only in NORM_ACKs generated in response to applicable
   NORM_CMD(FLUSH) messages.  NORM_CMD(ACK_REQ) messages with "ack_type"

   of NORM_ACK(CC) or NORM_ACK(FLUSH) SHALL NOT be generated by the

   The NORM_ACK(RESERVED) range of "ack_type" values is provided for
   possible future NORM protocol use.

   The NORM_ACK(APPLICATION) range of "ack_type" values is provided so
   that NORM applications can implement application-defined, positively
   acknowledged commands that are able to leverage internal transmission
   and round-trip timing information available to the NORM protocol

   The "ack_id" provides a sequenced identifier for the given
   NORM_CMD(ACK_REQ) message.  This "ack_id" is returned in NORM_ACK
   messages generated by the receivers so that the sender can associate
   the response with its corresponding request.

   The "reserved" field is reserved for possible future protocol use and
   SHALL be set to ZERO by senders and ignored by receivers.

   The "acking_node_list" field contains the NormNodeIds of the current
   NORM receivers that are desired to provide positive acknowledgment
   (NORM_ACK) to this request.  The packet payload length implies the
   length of the "acking_node_list", and its length is limited to the
   sender NormSegmentSize.  The individual NormNodeId items are listed
   in network (Big Endian) byte order.  If a receiver's NormNodeId is
   included in the "acking_node_list", it SHALL schedule transmission of
   a NORM_ACK message as described in Section 5.5.3.  NORM_CMD(APPLICATION) Message

   This command allows the NORM application to robustly transmit
   application-defined commands.  The command message preempts any
   ongoing data transmission and is repeated up to NORM_ROBUST_FACTOR
   times at a rate of once per 2*GRTT_sender.  This rate of repetition
   allows the application to observe any response (if that is the
   application's purpose for the command) before it is repeated.
   Possible responses can include initiation of data transmission, other
   NORM_CMD(APPLICATION) messages, or even application-defined,
   positively acknowledged commands from other NormSession participants.
   The transmission of these commands will preempt data transmission
   when they are scheduled and can be multiplexed with ongoing data
   transmission.  This type of robustly transmitted command allows NORM
   applications to define a complete set of session control mechanisms
   with less state than the transfer of FEC-encoded reliable content
   needs while taking advantage of NORM transmission and round-trip
   timing information.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     | sub-type = 7  |                    reserved                   |
     |                   Application-Defined Content                 |
     |                              ...                              |

              Figure 16: NORM_CMD(APPLICATION) Message Format

   The NORM common message header and NORM_CMD fields are interpreted as
   previously described.  The value of the NORM_CMD(APPLICATION)
   "hdr_len" field when no header extensions are present is 4.

   The "Application-Defined Content" area contains information in a
   format at the discretion of the application.  The size of this
   payload SHALL be limited to a maximum of the sender's NormSegmentSize
   setting.  Upon reception, the NORM protocol implementation SHALL
   deliver the content to the receiver application.  Note that any
   detection of duplicate reception of a NORM_CMD(APPLICATION) message
   is the responsibility of the application.

4.3.  Receiver Messages

   The NORM message types generated by participating receivers consist
   of the NORM_NACK and NORM_ACK message types.  NORM_NACK messages are
   sent to request repair of missing data content from sender
   transmission, and NORM_ACK messages are generated in response to
   certain sender commands including NORM_CMD(CC) and NORM_CMD(ACK_REQ).

4.3.1.  NORM_NACK Message

   The principal purpose of NORM_NACK messages is for receivers to
   request repair of sender content via selective, negative
   acknowledgment upon detection of incomplete data.  NORM_NACK messages
   will be transmitted according to the rules of NORM_NACK generation
   and suppression described in Section 5.3.  NORM_NACK messages also
   contain additional fields to provide feedback to the sender(s) for
   purposes of round-trip timing collection and congestion control.

   The payload of NORM_NACK messages contains one or more repair

   requests for different objects or portions of those objects.  The
   NORM_NACK message format is as follows:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=4|    hdr_len    |            sequence           |
     |                           source_id                           |
     |                           server_id                           |
     |           instance_id         |            reserved           |
     |                       grtt_response_sec                       |
     |                       grtt_response_usec                      |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                          nack_payload                         |
     |                              ...                              |

                    Figure 17: NORM_NACK Message Format

   The NORM common message header fields serve their usual purposes.
   The value of the "hdr_len" field for NORM_NACK messages without
   header extensions present is 6.

   The "server_id" field identifies the NORM sender to which the
   NORM_NACK message is destined.

   The "instance_id" field contains the current session identifier given
   by the sender identified by the "server_id" field in its sender
   messages.  The sender SHOULD ignore feedback messages containing an
   invalid "instance_id" value.

   The "grtt_response" fields contain an adjusted version of the
   timestamp from the most recently received NORM_CMD(CC) message for
   the indicated NORM sender.  The format of the "grtt_response" is the
   same as the "send_time" field of the NORM_CMD(CC).  The
   "grtt_response" value is relative to the "send_time" the source
   provided with a corresponding NORM_CMD(CC) command.  The receiver
   adjusts the source's NORM_CMD(CC) "send_time" timestamp by adding the
   time delta from when the receiver received the NORM_CMD(CC) to when
   the NORM_NACK is transmitted in response to calculate the value in
   the "grtt_response" field.  This is the "receive_to_response_delta"

   value used in the following formula:
     grtt_response = NORM_CMD(CC) send_time + receive_to_response_delta

   The receiver SHALL set the "grtt_response" to a ZERO value, to
   indicate it has not yet received a NORM_CMD(CC) message from the
   indicated sender, and the sender MUST ignore the "grtt_response" in
   this message.

   For NORM-CC operation, the NORM-CC Feedback Header Extension, as
   described in the NORM_CMD(REPAIR_ADV} message description, is added
   to NORM_NACK messages to provide feedback on the receiver's current
   state with respect to congestion control operation.  Alternative
   header extensions for congestion control feedback MAY be defined for
   alternative congestion control schemes for NORM use in the future.

   The "reserved" field is for potential future NORM use and SHALL be
   set to ZERO for this version of the protocol.

   The "nack_payload" of the NORM_NACK message specifies the repair
   needs of the receiver with respect to the NORM sender indicated by
   the "server_id" field.  The receiver constructs repair requests based
   on the NORM_DATA and/or NORM_INFO segments it needs from the sender
   to complete reliable reception up to the sender's transmission
   position at the moment the receiver initiates the NACK procedure as
   described in Section 5.3.  A single NORM Repair Request consists of a
   list of items, ranges, and/or FEC coding block erasure counts for
   needed NORM_DATA and/or NORM_INFO content.  Multiple repair requests
   can be concatenated within the "nack_payload" field of a NORM_NACK
   message.  A single NORM Repair Request can possibly include multiple
   "items", "ranges", or "erasure_counts".  In turn, the "nack_payload"
   field MAY contain multiple repair requests.  A single NORM Repair
   Request has the following format:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |      form     |     flags     |             length            |
     |                      repair_request_items                     |
     |                             ...                               |

                   Figure 18: NORM Repair Request Format

   The "form" field indicates the type of repair request items given in
   the "repair_request_items" list.  Possible values for the "form"
   field include:

                      | Form               | Value |
                      | NORM_NACK_ITEMS    |   1   |
                      | NORM_NACK_RANGES   |   2   |
                      | NORM_NACK_ERASURES |   3   |

   A "form" value of NORM_NACK_ITEMS indicates each repair request item
   in the "repair_request_items" list is to be treated as an individual
   request.  A value of NORM_NACK_RANGES indicates the
   "repair_request_items" list consists of pairs of repair request items
   corresponding to the inclusive ranges of repair needs.  The
   NORM_NACK_ERASURES "form" indicates the repair request items are to
   be treated individually and the "encoding_symbol_id" portion of the
   "fec_payload_id" field of the repair request item (see below) is to
   be interpreted as an erasure count for the FEC coding block
   identified by the repair request item's "source_block_number".

   The "flags" field is currently used to indicate the level of data
   content for which the repair request items apply (i.e., an individual
   segment, entire FEC coding block, or entire transport object).
   Possible flag values include:

   | Flag              |  Value | Purpose                              |
   | NORM_NACK_SEGMENT |  0x01  | Indicates the listed segment(s) or   |
   |                   |        | range of segments needed as repair.  |
   | NORM_NACK_BLOCK   |  0x02  | Indicates the listed block(s) or     |
   |                   |        | range of blocks in entirety that are |
   |                   |        | needed as repair.                    |
   | NORM_NACK_INFO    |  0x04  | Indicates NORM_INFO is needed as     |
   |                   |        | repair for the listed object(s).     |
   | NORM_NACK_OBJECT  |  0x08  | Indicates the listed object(s) or    |
   |                   |        | range of objects in entirety are     |
   |                   |        | needed as repair.                    |

   When the NORM_NACK_SEGMENT flag is set, the "object_transport_id" and
   "fec_payload_id" fields are used to determine which sets or ranges of
   individual NORM_DATA segments are needed to repair content at the
   receiver.  When the NORM_NACK_BLOCK flag is set, this indicates the
   receiver is completely missing the indicated coding block(s), and
   that transmissions sufficient to repair the indicated block(s) in
   their entirety are needed.  When the NORM_NACK_INFO flag is set, this
   indicates the receiver is missing the NORM_INFO segment for the
   indicated "object_transport_id".  Note the NORM_NACK_INFO can be set

   in combination with the NORM_NACK_BLOCK or NORM_NACK_SEGMENT flags,
   or can be set alone.  When the NORM_NACK_OBJECT flag is set, this
   indicates the receiver is missing the entire NormTransportObject
   referenced by the "object_transport_id".  This also implicitly
   requests any available NORM_INFO for the NormObject, if applicable.
   The "fec_payload_id" field is ignored when the flag NORM_NACK_OBJECT
   is set.

   The "length" field value is the length in bytes of the
   "repair_request_items" field.

   The "repair_request_items" field consists of a list of individual or
   range pairs of transport data unit identifiers in the following
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |     fec_id    |   reserved    |      object_transport_id      |
     |                        fec_payload_id                         |
     |                              ...                              |

                Figure 19: NORM Repair Request Item Format

   The "fec_id" indicates the FEC type and can be used to determine the
   format of the "fec_payload_id" field.  The "reserved" field is kept
   for possible future use and SHALL be set to a ZERO value and ignored
   by NORM nodes processing NACK content.

   The "object_transport_id" corresponds to the NormObject for which
   repair is being requested, and the "fec_payload_id" identifies the
   specific FEC coding block and/or segment being requested.  When the
   NORM_NACK_OBJECT flag is set, the value of the "fec_payload_id" field
   is ignored.  When the NORM_NACK_BLOCK flag is set, only the FEC code
   block identifier portion of the "fec_payload_id" is to be

   The format of the "fec_payload_id" field depends upon the "fec_id"
   field value.

   When the receiver's repair needs dictate that different forms (mixed
   ranges and/or individual items) or types (mixed specific segments
   and/or blocks or objects in entirety) are needed to complete reliable
   transmission, multiple NORM Repair Requests with different "form" and
   or "flags" values can be concatenated within a single NORM_NACK
   message.  Additionally, NORM receivers SHALL construct NORM_NACK
   messages with their repair requests in ordinal order with respect to

   "object_transport_id" and "fec_payload_id" values.  The
   "nack_payload" size SHALL NOT exceed the NormSegmentSize for the
   sender to which the NORM_NACK is destined.

   NORM_NACK Content Examples:

   In these examples, a small block, systematic FEC code ("fec_id" =
   129) is assumed with a user data block length of 32 segments.  In
   Example 1, a list of individual NORM_NACK_ITEMS repair requests is
   given.  In Example 2, a list of NORM_NACK_RANGES requests AND a
   single NORM_NACK_ITEMS request are concatenated to illustrate the
   possible content of a NORM_NACK message.  Note that FEC coding block
   erasure counts could also be provided in each case.  However, the
   erasure counts are not really necessary since the sender can easily
   determine the erasure count while processing the NACK content.
   However, the erasure count option can be useful for operation with
   other FEC codes or for intermediate system purposes.

    Example 1: NORM_NACK "nack_payload" for: Object 12, Coding Block 3,
                           Segments 2, 5, and 8
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   form = 1    | flags = 0x01  |       length  = 36            |
     |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
     |                    source_block_number = 3                    |
     |    source_block_length = 32   |    encoding_symbol_id = 2     |
     |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
     |                    source_block_number = 3                    |
     |    source_block_length = 32   |    encoding_symbol_id = 5     |
     |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
     |                    source_block_number = 3                    |
     |    source_block_length = 32   |    encoding_symbol_id = 8     |

    Example 2: NORM_NACK "nack_payload" for: Object 18, Coding Block 6,
   Segments 5, 6, 7, 8, 9, 10; and Object 19 NORM_INFO and Coding Block
                               1, Segment 3
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   form = 2    | flags = 0x01  |       length  = 24            |
     |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
     |                    source_block_number = 6                    |
     |    source_block_length = 32   |    encoding_symbol_id = 5     |
     |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
     |                    source_block_number = 6                    |
     |    source_block_length = 32   |    encoding_symbol_id = 10    |
     |   form = 1    | flags = 0x05  |       length  = 12            |
     |  fec_id = 129 |   reserved    |    object_transport_id = 19   |
     |                    source_block_number = 1                    |
     |    source_block_length = 32   |    encoding_symbol_id = 3     |

4.3.2.  NORM_ACK Message

   The NORM_ACK message is intended to be used primarily as part of NORM
   congestion control operation and round-trip timing measurement.  The
   acknowledgment type NORM_ACK(CC) is provided for this purpose as
   described in the NORM_CMD(ACK_REQ) message description.  The
   generation of NORM_ACK(CC) messages for round-trip timing estimation
   and congestion control operation is described in Section 5.5.1 and
   Section 5.5.2, respectively.  However, some multicast applications
   can benefit from some limited form of positive acknowledgment for
   certain functions.  A simple, scalable positive acknowledgment scheme
   is defined in Section 5.5.3, which can be leveraged by protocol
   implementations when appropriate.  The NORM_CMD(FLUSH) can also be
   used for OPTIONAL collection of positive acknowledgment of reliable
   reception to a certain "watermark" transmission point from specific
   receivers using this mechanism.  The NORM_ACK type NORM_ACK(FLUSH) is
   provided for this purpose and the format of the "nack_payload" for
   this acknowledgment type is given below.  Beyond that, a range of
   application-defined "ack_type" values is provided for use at the NORM

   application's discretion.  Implementations making use of application-
   defined positive acknowledgments MAY also make use of the
   "nack_payload" as needed, observing the constraint that the
   "nack_payload" field size be limited to a maximum of the
   NormSegmentSize for the sender to which the NORM_ACK is destined.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=5|    hdr_len    |          sequence             |
     |                           source_id                           |
     |                           server_id                           |
     |           instance_id         |    ack_type  |     ack_id     |
     |                       grtt_response_sec                       |
     |                       grtt_response_usec                      |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                   ack_payload (if applicable)                 |
     |                              ...                              |

                    Figure 20: NORM_ACK Message Format

   The NORM common message header fields serve their usual purposes.
   The value of the "hdr_len" field when no header extensions are
   present is 6.

   The "server_id", "instance_id", and "grtt_response" fields serve the
   same purpose as the corresponding fields in NORM_NACK messages.
   Header extensions can be applied to support congestion control
   feedback or other functions in the same manner.

   The "ack_type" field indicates the nature of the NORM_ACK message.
   This directly corresponds to the "ack_type" field of the
   NORM_CMD(ACK_REQ) message to which this acknowledgment applies.

   The "ack_id" field serves as a sequence number so the sender can
   verify a received NORM_ACK message actually applies to a current
   acknowledgment request.  The "ack_id" field is not used in the case
   of the NORM_ACK(CC) and NORM_ACK(FLUSH) acknowledgment types.

   The "ack_payload" format is a function of the "ack_type".  The

   NORM_ACK(CC) message has no attached content.  Only the NORM_ACK
   header applies.  In the case of NORM_ACK(FLUSH), a specific
   "ack_payload" format is defined:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |     fec_id    |   reserved    |      object_transport_id      |
     |                        fec_payload_id                         |
     |                              ...                              |

   The "object_transport_id" and "fec_payload_id" are used by the
   receiver to acknowledge applicable NORM_CMD(FLUSH) messages
   transmitted by the sender identified by the "server_id" field.

   The "ack_payload" of NORM_ACK messages for application-defined
   "ack_type" values is specific to the application but is limited in
   size to a maximum of the NormSegmentSize of the sender referenced by
   the "server_id".

4.4.  General Purpose Messages

   Some additional message formats are defined for general purpose in
   NORM multicast sessions whether the participant is acting as a sender
   and/or receiver within the group.

4.4.1.  NORM_REPORT Message

   This is an OPTIONAL message generated by NORM participants.  This
   message can be used for periodic performance reports from receivers
   in experimental NORM implementations.  The format of this message is
   currently undefined.  Experimental NORM implementations MAY define
   NORM_REPORT formats as needed for test purposes.  These report
   messages SHOULD be disabled for interoperability testing between
   different compliant NORM implementations.

5.  Detailed Protocol Operation

   This section describes the detailed interactions of senders and
   receivers participating in a NORM session.  A simple synopsis of the
   protocol operation is given here:

   1.  The sender periodically transmits NORM_CMD(CC) messages as needed
       to initialize and collect round-trip timing and congestion
       control feedback from the receiver set.

   2.  The sender transmits an ordinal set of NormObjects segmented in
       the form of NORM_DATA messages labeled with NormTransportIds and
       logically identified with FEC encoding block numbers and symbol
       identifiers.  When applicable, NORM_INFO messages MAY optionally
       precede the transmission of data content for NORM transport

   3.  As receivers detect missing content from the sender, they
       initiate repair requests with NORM_NACK messages.  The receivers
       track the sender's most recent objectTransportId::fecPayloadId
       transmit position and NACK only for content that is ordinally
       prior to that current transmit position.  The receivers schedule
       random backoff timeouts before generating NORM_NACK messages and
       wait an appropriate amount of time before repeating the NORM_NACK
       if their repair request is not satisfied.

   4.  The sender aggregates repair requests from the receivers and
       logically "rewinds" its transmit position to send appropriate
       repair messages.  The sender sends repairs for the earliest
       ordinal transmit position first and maintains this ordinal repair
       transmission sequence.  FEC parity content not previously
       transmitted for the applicable FEC coding block is used for
       repair transmissions to the greatest extent possible.  If the
       sender exhausts its available FEC parity content on multiple
       repair cycles for the same coding block, it resorts to an
       explicit repair strategy (possibly using parity content) to
       complete repairs.  (The use of explicit repair is an exception in
       general protocol operation, but the possibility does exist for
       extreme conditions).  The sender immediately assumes transmission
       of new content once it has sent pending repairs.

   5.  The sender transmits NORM_CMD(FLUSH) messages when it reaches the
       end of enqueued transmit content and pending repairs.  Receivers
       respond to the NORM_CMD(FLUSH) messages with NORM_NACK
       transmissions (following the same suppression backoff timeout
       strategy as for data) if they need further repair.

   6.  The sender transmissions are subject to rate control limits
       determined by congestion control mechanisms.  In the baseline
       NORM-CC operation, each sender in a NormSession maintains its own
       independent congestion control state.  Receivers provide
       congestion control feedback in NORM_NACK and NORM_ACK messages.
       NORM_ACK feedback for congestion control purposes is governed
       using a suppression mechanism similar to that for NORM_NACK

   While this overall concept is relatively simple, there are details to
   each of these aspects that need to be addressed for successful,

   efficient, robust, and scalable NORM protocol operation.

5.1.  Sender Initialization and Transmission

   Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
   messages to collect round-trip timing and other information from the
   potential group.  If NORM-CC congestion control operation is enabled,
   the NORM-CC Rate header extension MUST be included in these messages.
   Congestion control operation SHALL be observed at all times when not
   operating using dedicated resources, like in the general Internet.
   Even if congestion control operation is disabled at the sender, it
   can be desirable to use the NORM_CMD(CC) messaging to collect
   feedback from the group using the baseline NORM-CC feedback
   mechanisms.  This proactive feedback collection can be used to
   establish a GRTT estimate prior to data transmission and potential
   NACK operation.

   In some cases, applications might need the sender to also proceed
   with data transmission immediately.  In other cases, the sender might
   wish to defer data transmission until it has received some feedback
   or request from the receiver set indicating receivers are indeed
   present.  Note, in some applications (e.g., web push), this
   indication MAY come out-of-band with respect to the multicast session
   via other means.  As noted, the periodic transmission of NORM_CMD(CC)
   messages MAY precede actual data transmission in order to have an
   initial GRTT estimate.

   With inclusion of the OPTIONAL NORM FEC Object Transmission
   Information Header Extension (EXT_FTI), the NORM protocol sender
   message headers can contain all information necessary to prepare
   receivers for subsequent reliable reception.  This includes FEC
   coding parameters, the sender NormSegmentSize, and other information.
   If this header extension is not used, it is presumed receivers have
   received the FEC Object Transmission Information via other means.
   Additionally, applications MAY leverage the use of NORM_INFO messages
   associated with the session data objects in the session to provide
   application-specific context information for the session and data
   being transmitted.  These mechanisms allow for operation with minimal
   pre-coordination among the senders and receivers.

   The NORM sender begins segmenting application-enqueued data into
   NORM_DATA segments and transmitting it to the group.  For objects of
   type NORM_OBJECT_DATA and NORM_OBJECT_FILE, the segmentation
   algorithm described in FEC Building Block [RFC5052] is RECOMMENDED.
   For objects of type NORM_OBJECT_STREAM, segmentation will typically
   be into uniform FEC coding block sizes, with individual segment sizes
   controlled by the application.  In most cases, the application and
   NORM implementation SHOULD strive to produce full-sized

   (NormSegmentSize) segments when possible.  The rate of transmission
   is controlled via congestion control mechanisms or is a fixed rate if
   desired for closed network operations.  The receivers participating
   in the multicast group provide feedback to the sender as needed.
   When the sender reaches the end of data it has enqueued for
   transmission or any pending repairs, it transmits a series of
   NORM_CMD(FLUSH) messages at a rate of one per 2*GRTT_sender.  Similar
   to the end of each transmitted FEC coding block during transmission,
   receivers SHALL respond to these NORM_CMD(FLUSH) messages with
   additional repair requests as needed.  A protocol parameter
   NORM_ROBUST_FACTOR determines the number of flush messages sent.  If
   receivers request repair, the repair is provided, and flushing occurs
   again at the end of repair transmission.  The sender MAY attach an
   OPTIONAL "acking_node_list" to NORM_CMD(FLUSH) containing the
   NormNodeIds for receivers from which it expects explicit positive
   acknowledgment of reception.  The NORM_CMD(FLUSH) message MAY be also
   used for this OPTIONAL purpose any time prior to the end of data
   enqueued for transmission with the NORM_CMD(FLUSH) messages
   multiplexed with ongoing data transmissions.  The OPTIONAL NORM
   positive acknowledgment procedure is described in Section 5.5.3.

5.1.1.  Object Segmentation Algorithm

   NORM senders and receivers MUST use a common algorithm for logically
   segmenting transport data into FEC encoding blocks and symbols so
   appropriate NACKs can be constructed to request repair of missing
   data.  NORM FEC coding blocks are comprised of multi-byte symbols
   (segments) transmitted in the payload of NORM_DATA messages.  Each
   NORM_DATA message will contain one or more source or encoding symbols
   identified by the "fec_payload_id" field, and the NormSegmentSize
   sender parameter defines the maximum size (in bytes) of the
   "payload_data" field containing the content (a "segment").  The FEC
   encoding type and associated parameters govern the source block size
   (number of source symbols per coding block, etc.).  NORM senders and
   receivers use these FEC parameters, along with the NormSegmentSize
   and transport object size to compute the source block structure for
   transport objects.  These parameters are provided in the FEC Object
   Transmission Information for each object.  The block partitioning
   algorithm described in the FEC Building Block [RFC5052] document is
   RECOMMENDED for use in computing a source block structure such that
   all source blocks are as close to being equal length as possible.
   This helps avoid the performance disadvantages of "short" FEC blocks.
   Note that this algorithm applies only to the statically sized
   NORM_OBJECT_DATA and NORM_OBJECT_FILE transport object types where
   the object size is fixed and predetermined.  For NORM_OBJECT_STREAM
   objects, the object is segmented according to the maximum source
   block length given in the FEC Transmission Information, unless the
   FEC Payload ID indicates an alternative size for a given block.

5.2.  Receiver Initialization and Reception

   For typical operation, NORM receivers will join a specified multicast
   group and listen on a specific port number for sender transmissions.
   As the NORM receiver receives NORM_DATA messages, it will establish
   buffering state and provide content to its application as appropriate
   for the given data type.  The NORM protocol allows receivers to join
   and leave the group at will, although some applications might need
   receivers to be members of the group prior to start of data
   transmission.  Thus, different NORM applications MAY use different
   policies to constrain the impact of new receivers joining the group
   in the middle of a session.  For example, a useful implementation
   policy is for new receivers joining the group to limit or avoid
   repair requests for transport objects already in progress.  The NORM
   sender implementation MAY impose additional constraints to limit the
   ability of receivers to disrupt reliable multicast performance by
   joining, leaving, and rejoining the group often.  Different receiver
   "join policies" might be appropriate for different applications
   and/or scenarios.  For general purpose operation, a default policy
   where receivers are allowed to request repair only for coding blocks
   with a NormTransportId and FEC coding block number greater than or
   equal to the first non-repair NORM_DATA or NORM_INFO message received
   upon joining the group is RECOMMENDED.  For objects of type
   NORM_OBJECT_STREAM, it is RECOMMENDED the join policy constrain
   receivers to begin reliable reception at the current FEC coding block
   for which non-repair content is received.

   In some deployments, different multicast receivers might have
   differing quality of network connectivity.  Some receivers may suffer
   significantly poorer performance with very limited goodput due to low
   connection rate or substantial packet loss.  Similar to the "join
   policies" described above, a NORM sender implementation MAY choose to
   enforce different "service policies" to perhaps exclude exceptionally
   poorly performing (or otherwise badly behaving) receivers from the
   group.  The sender implementation could choose to ignore NACKs from
   such receivers and/or force advancement of its logical "repair
   window" (i.e., enforcing a minimal level of service) and use the
   NORM_CMD(SQUELCH) message to advise those poor performers of its
   advance.  Note in some cases, the application may need to support the
   "weakest member" regardless of the time needed to achieve reliable
   delivery.  When implemented, the protocol instantiation SHOULD expose
   controls to the set of "join" and/or "service" policies available to
   support the needs of different applications.

5.3.  Receiver NACK Procedure

   When the receiver detects it is missing data from a sender's NORM
   transmissions, it initiates its NACKing procedure.  The NACKing

   procedure SHALL be initiated only at FEC coding block boundaries,
   NormObject boundaries, upon receipt of a NORM_CMD(FLUSH) message, or
   upon an "inactivity" timeout when NORM_DATA or NORM_INFO
   transmissions are no longer received from a previously active sender.
   The RECOMMENDED value of such an inactivity timeout is:

            T_inactivity = NORM_ROBUST_FACTOR * 2 * GRTT_sender

   where the GRTT_sender value corresponds to the GRTT estimate
   advertised in the "grtt" field of NORM sender messages.  A minimum
   T_inactivity value of 1 second is RECOMMENDED.  The NORM receiver
   SHOULD reset this inactivity timer and repeat NACK initiation upon
   timeout for up to NORM_ROBUST_FACTOR times or more depending upon the
   application's need for persistence by its receivers.  It is also
   important receivers rescale the T_inactivity timeout as the sender's
   advertised GRTT changes.

   The NACKing procedure begins with a random backoff timeout.  The
   duration of the backoff timeout is chosen using the "RandomBackoff"
   algorithm described in the Multicast NACK Building Block [RFC5401]
   document using (K_sender*GRTT_sender) for the maxTime parameter and
   the sender advertised group size (GSIZE_sender) as the groupSize
   parameter.  NORM senders provide values for GRTT_sender, K_sender and
   GSIZE_sender via the "grtt", "backoff", and "gsize" fields of
   transmitted messages.  The GRTT_sender value is determined by the
   sender based on feedback it has received from the group while the
   K_sender and GSIZE_sender values can be determined by application
   requirements and expectations or ancillary information.  The backoff
   factor K_sender MUST be greater than one to provide for effective
   feedback suppression.  A value of K_sender = 4 is RECOMMENDED for the
   Any Source Multicast (ASM) model, while a value of K_sender = 6 is
   RECOMMENDED for Single Source Multicast (SSM) operation.

       T_backoff = RandomBackoff(K_sender*GRTT_sender, GSIZE_sender)

   To avoid the possibility of NACK implosion in the case of sender or
   network failure during SSM operation, the receiver SHALL
   automatically suppress its NACK and immediately enter the "holdoff"
   period described below when T_backoff is greater than (K_sender-
   1)*GRTT_sender.  Otherwise, the backoff period is entered and the
   receiver MUST accumulate external pending repair state from NORM_NACK
   messages and NORM_CMD(REPAIR_ADV) messages received.  At the end of
   the backoff time, the receiver SHALL generate a NORM_NACK message
   only if the following conditions are met:

   1.  The sender's current transmit position (in terms of
       objectTransportId::fecPayloadId) exceeds the earliest repair
       position of the receiver.

   2.  The repair state accumulated from NORM_NACK and
       NORM_CMD(REPAIR_ADV) messages does not equal or supersede the
       receiver's repair needs up to the sender transmission position at
       the time the NACK procedure (backoff timeout) was initiated.

   If these conditions are met, the receiver immediately generates a
   NORM_NACK message when the backoff timeout expires.  Otherwise, the
   receiver's NACK is considered to be "suppressed" and the message is
   not sent.  At this time, the receiver begins a "holdoff" period
   during which it constrains itself to not re-initiate the NACKing
   process.  The purpose of this timeout is to allow the sender worst-
   case time to respond to the repair needs before the receiver requests
   repair again.  The value of this "holdoff" timeout (T_rcvrHoldoff) as
   described in [RFC5401] is:
                  T_rcvrHoldoff =(K_sender+2)*GRTT_sender

   The NORM_NACK message contains repair request content beginning with
   the lowest ordinal repair position of the receiver up through the
   coding block prior to the most recently heard ordinal transmission
   position for the sender.  If the size of the NORM_NACK content
   exceeds the sender's NormSegmentSize, the NACK content is truncated
   so the receiver only generates a single NORM_NACK message per NACK
   cycle for a given sender.  In summary, a single NACK message is
   generated containing the receiver's lowest ordinal repair needs.

   For each partially received FEC coding block requiring repair, the
   receiver SHALL, on its FIRST repair attempt for the block, request
   the parity portion of the FEC coding block beginning with the lowest
   ordinal parity "encoding_symbol_id" (i.e., "encoding_symbol_id" =
   "source_block_len") and request the number of FEC symbols
   corresponding to its data segment erasure count for the block.  On
   subsequent repair cycles for the same coding block, the receiver
   SHALL request only those repair symbols from the first set it has not
   yet received up to the remaining erasure count for that applicable
   coding block.  Note the sender might have transmitted other
   different, additional parity segments for other receivers that could
   also be used to satisfy the local receiver's erasure-filling needs.
   In the case where the erasure count for a partially received FEC
   coding block exceeds the maximum number of parity symbols available
   from the sender for the block (as indicated by the NORM_DATA
   "fec_num_parity" field), the receiver SHALL request all available
   parity segments plus the ordinally highest missing data segments
   needed to satisfy its total erasure needs for the block.  The goal of
   this strategy is for the overall receiver set to request a lowest

   common denominator set of repair symbols for a given FEC coding
   block.  This allows the sender to construct the most efficient repair
   transmission segment set and enables effective NACK suppression among
   the receivers even with uncorrelated packet loss.  This approach also
   does not demand synchronization among the receiver set in their
   repair requests for the sender.

   For FEC coding blocks or NormObjects missed in their entirety, the
   NORM receiver constructs repair requests with NORM_NACK_BLOCK or
   NORM_NACK_OBJECT flags set as appropriate.  The request for
   retransmission of NORM_INFO is accomplished by setting the
   NORM_NACK_INFO flag in a corresponding repair request.

5.4.  Sender NACK Processing and Response

   The principal goal of the sender is to make forward progress in the
   transmission of data its application has enqueued.  However, the
   sender will need to occasionally "rewind" its logical transmission
   point to satisfy the repair needs of receivers who have NACKed.
   Aggregation of multiple NACKs is used to determine an optimal repair
   strategy when a NACK event occurs.  Since receivers initiate the NACK
   process on coding block or object boundaries, there is some loose
   degree of synchronization of the repair process even when receivers
   experience uncorrelated data loss.

5.4.1.  Sender Repair State Aggregation

   When a sender is in its normal state of transmitting new data and
   receives a NACK, it begins a procedure to accumulate NACK repair
   state from NORM_NACK messages before beginning repair transmissions.
   Note that this period of aggregating repair state does NOT interfere
   with its ongoing transmission of new data.

   As described in [RFC5401], the period of time during which the sender
   aggregates NORM_NACK messages is equal to:

               T_sndrAggregate = (K_sender + 1) * GRTT_sender

   where K_sender is the backoff scaling value advertised to the
   receivers, and GRTT_sender is the sender's current estimate of the
   group's greatest round-trip time.  Note, for NORM unicast sessions,
   the T_sndrAggregate time can be set to ZERO since there is only one
   receiver.  Similarly, the K_sender value SHOULD be set to ZERO for
   NORM unicast sessions to minimize repair latency.

   When this period ends, the sender "rewinds" by incorporating the
   accumulated repair state into its pending transmission state and
   begins transmitting repair messages.  After pending repair

   transmissions are completed, the sender continues with new
   transmissions of any enqueued data.  Also, at this point in time, the
   sender begins a "holdoff" timeout during which time the sender
   constrains itself from initiating a new repair aggregation cycle,
   even if NORM_NACK messages arrive.  As described in [RFC5401], the
   value of this sender "holdoff" period is:

                     T_sndrHoldoff = (1 * GRTT_sender)

   If additional NORM_NACK messages are received during this sender
   "holdoff" period, the sender will immediately incorporate these late-
   arriving messages into its pending transmission state if, and only
   if, the NACK content is ordinally greater than the sender's current
   transmission position.  This "holdoff" time allows worst-case time
   for the sender to propagate its current transmission sequence
   position to the group, thus avoiding redundant repair transmissions.
   After the holdoff timeout expires, a new NACK accumulation period can
   be started (upon arrival of a NACK) in concert with the pending
   repair and new data transmission.  Recall receivers are not to
   initiate the NACK repair process until the sender's logical
   transmission position exceeds the lowest ordinal position of their
   repair needs.  With the new NACK aggregation period, the sender
   repeats the same process of incorporating accumulated repair state
   into its transmission plan and subsequently "rewinding" to transmit
   the lowest ordinal repair data when the aggregation period expires.
   Again, this is conducted in concert with ongoing new data and/or
   pending repair transmissions.

5.4.2.  Sender FEC Repair Transmission Strategy

   The NORM sender SHOULD leverage transmission of FEC parity content
   for repair to the greatest extent possible.  Recall that receivers
   use a strategy to request a lowest common denominator of explicit
   repair (including parity content) in the formation of their NORM_NACK
   messages.  Before falling back to explicitly satisfying different
   receivers' repair needs, the sender can make use of the general
   erasure-filling capability of FEC-generated parity segments.  The
   sender can determine the maximum erasure-filling needs for individual
   FEC coding blocks from the NORM_NACK messages received during the
   repair aggregation period.  Then, if the sender has a sufficient
   number (less than or equal to the maximum erasure count) of
   previously unsent parity segments available for the applicable coding
   blocks, the sender can transmit these in lieu of the specific packets
   the receiver set has requested.  The sender SHOULD NOT resort to
   explicit transmission of the receiver set's repair needs until after
   exhausting its supply of "fresh" (unsent) parity segments for a given
   coding block.  In general, if a sufficiently powerful FEC code is
   used, the need for explicit repair will be an exception, and the

   fulfillment of reliable multicast can be accomplished quite
   efficiently.  However, the ability to resort to explicit repair
   allows the protocol to be continue to operate under even very extreme

   NORM_DATA messages sent as repair transmissions SHALL be flagged with
   the NORM_FLAG_REPAIR flag.  This allows receivers to obey any
   policies limiting new receivers from joining the reliable
   transmission when only repair transmissions have been received.
   Additionally, the sender SHOULD flag NORM_DATA transmissions sent as
   explicit repair with the NORM_FLAG_EXPLICIT flag.

   Although NORM end system receivers do not make use of the
   NORM_FLAG_EXPLICIT flag, this message transmission status could be
   leveraged by intermediate systems wishing to "assist" NORM protocol
   performance.  If such systems are properly positioned with respect to
   reciprocal reverse-path multicast routing, they need to sub-cast only
   a sufficient count of non-explicit parity repairs to satisfy a
   multicast routing sub-tree's erasure-filling needs for a given FEC
   coding block.  When the sender has resorted to explicit repair, then
   the intermediate systems SHOULD sub-cast all of the explicit repair
   packets to those portions of the routing tree still requiring repair
   for a given coding block.  Note the intermediate systems will need to
   conduct repair state accumulation for sub-routes in a manner similar
   to the sender's repair state accumulation in order to have sufficient
   information to perform the sub-casting.  Additionally, the
   intermediate systems could perform NORM_NACK suppression/aggregation
   as it conducts this repair state accumulation for NORM repair cycles.
   The details of this type of operation are beyond the scope of this
   document, but this information is provided for possible future

5.4.3.  Sender NORM_CMD(SQUELCH) Generation

   If the sender receives a NORM_NACK message for repair of data it is
   no longer supporting, the sender generates a NORM_CMD(SQUELCH)
   message to advertise its repair window and squelch any receivers from
   additional NACKing of invalid data.  The transmission rate of
   NORM_CMD(SQUELCH) messages is limited to once per 2*GRTT_sender.  The
   "invalid_object_list" (if applicable) of the NORM_CMD(SQUELCH)
   message SHALL begin with the lowest "object_transport_id" from the
   invalid NORM_NACK messages received since the last NORM_CMD(SQUELCH)
   transmission.  The list includes as many lower ordinal invalid
   "object_transport_ids" that can fit for the NORM_CMD(SQUELCH) payload
   size to less than or equal to the sender's NormSegmentSize parameter.

5.4.4.  Sender NORM_CMD(REPAIR_ADV) Generation

   When a NORM sender receives NORM_NACK messages from receivers via
   unicast transmission, it uses NORM_CMD(REPAIR_ADV) messages to
   advertise its accumulated repair state to the receiver set since the
   receiver set is not directly sharing their repair needs via multicast
   communication.  A NORM sender implementation MAY use a separate port
   number from the NormSession port number as the source port for its
   transmissions.  Thus, NORM receivers can direct any unicast feedback
   messages to this separate sender port number, distinct from the NORM
   session (or destination) port number.  Then, the NORM sender
   implementation can discriminate unicast feedback messages from
   multicast feedback messages when there is a mix of multicast and
   unicast feedback receivers.  The NORM_CMD(REPAIR_ADV) message is
   multicast to the receiver set by the sender.  The payload portion of
   this message has content in the same format as the NORM_NACK receiver
   message payload.  Receivers are then able to perform feedback
   suppression in the same manner as with NORM_NACK messages directly
   received from other receivers.  Note that the sender does not merely
   retransmit NACK content it receives, but instead transmits a
   representation of its aggregated repair state.  The transmission of
   NORM_CMD(REPAIR_ADV) messages is subject to the sender transmit rate
   limit and NormSegmentSize limitation.  When the NORM_CMD(REPAIR_ADV)
   message is of maximum size (as indicated by the flag
   NORM_REPAIR_ADV_FLAG_LIMIT), receivers SHALL consider the maximum
   ordinal transmission position value embedded in the message as the
   senders current transmission position and implicitly suppress
   requests for ordinally higher repair.  For congestion control
   operation, the sender will also need to provide any information
   needed so dynamic congestion control feedback can be suppressed among
   receivers.  This document specifies the NORM-CC Feedback Header
   Extension that is applied for baseline NORM-CC operation.  If other
   congestion control mechanisms are used within a NORM implementation,
   other header extensions MAY be defined.  Whatever content format is
   used for this purpose SHOULD ensure that maximum possible suppression
   state is conveyed to the receiver set.

5.5.  Additional Protocol Mechanisms

   In addition to the principal function of data content transmission
   and repair, there are some other protocol mechanisms to help NORM to
   adapt to network conditions and play fairly with other coexistent

5.5.1.  Group Round-Trip Time (GRTT) Collection

   For NORM receivers to appropriately scale backoff timeouts and the
   senders to use proper corresponding timeouts, the participants need

   to use a common timeout basis.  Each NORM sender monitors the round-
   trip time of active receivers and determines the greatest group
   round-trip time.  The sender advertises this GRTT estimate in every
   message it transmits so receivers have this value available for
   scaling their timers.  To measure the current GRTT, the sender
   periodically sends NORM_CMD(CC) messages containing a locally
   generated timestamp.  Receivers are expected to record this timestamp
   along with the time the NORM_CMD(CC) message is received.  Then, when
   the receivers generate feedback messages to the sender, an adjusted
   version of the sender timestamp is embedded in the feedback message
   (NORM_NACK or NORM_ACK).  The adjustment adds the amount of time the
   receiver held the timestamp before generating its response.  Upon
   receipt of this adjusted timestamp, the sender is able to calculate
   the round-trip time to that receiver.

   The round-trip time for each receiver is fed into an algorithm that
   assigns weights and smoothes the values for a conservative estimate
   of the GRTT.  The algorithm and methodology are described in the
   Multicast NACK Building Block [RFC5401] document in the section
   entitled "One-to-Many Sender GRTT Measurement".  A conservative
   estimate helps guarantee feedback suppression at a small cost in
   overall protocol repair delay.  The sender's current estimate of GRTT
   is advertised in the "grtt" field found in all NORM sender messages.
   The advertised GRTT is also limited to a minimum of the nominal
   inter-packet transmission time given the sender's current
   transmission rate and system clock granularity.  The reason for this
   additional limit is to keep the receiver somewhat event-driven by
   making sure the sender has had adequate time to generate any response
   to repair requests from receivers given transmit rate limitations due
   to congestion control or configuration.

   When the NORM-CC Rate header extension is present in NORM_CMD(CC)
   messages, the receivers respond to NORM_CMD(CC) messages as described
   in Section 5.5.2, "NORM Congestion Control Operation".  The
   NORM_CMD(CC) messages are periodically generated by the sender as
   described for congestion control operation.  This provides for
   proactive, but controlled, feedback from the group in the form of
   NORM_ACK messages.  This provides for GRTT feedback even if no
   NORM_NACK messages are being sent.  If operating without congestion
   control in a closed network, the NORM_CMD(CC) messages MAY be sent
   periodically without the NORM-CC Rate header extension.  In this
   case, receivers will only provide GRTT measurement feedback when
   NORM_NACK messages are generated since no NORM_ACK messages are
   generated.  In this case, the NORM_CMD(CC) messages MAY be sent less
   frequently, perhaps as little as once per minute, to conserve network
   capacity.  Note the NORM-CC Rate header extension MAY also be used to
   proactively solicit RTT feedback from the receiver group per
   congestion control operation even when the sender is not conducting

   congestion control rate adjustment.  NORM operation without
   congestion control SHOULD be considered only in closed networks.

5.5.2.  NORM Congestion Control Operation

   This section describes baseline congestion control operation for the
   NORM protocol (NORM-CC).  The supporting NORM message formats and
   approach described here are an adaptation of the equation-based TCP-
   Friendly Multicast Congestion Control (TFMCC) approach [RFC4654].
   This congestion control scheme is REQUIRED for operation within the
   general Internet unless the NORM implementation is adapted to use
   another IETF-sanctioned reliable multicast congestion control
   mechanism.  With this TFMCC-based approach, the transmissions of NORM
   senders are controlled in a rate-based manner as opposed to window-
   based congestion control algorithms as in TCP.  However, it is
   possible the NORM protocol message set MAY alternatively be used to
   support a window-based multicast congestion control scheme such as
   PGMCC.  The details of such an alternative MAY be described
   separately or in a future revision of this document.  In either case
   (rate-based TFMCC or window-based PGMCC), successful control of
   sender transmission depends upon collection of sender-to-receiver
   packet loss estimates and RTTs to identify the congestion control
   bottleneck path(s) within the multicast topology and adjust the
   sender rate accordingly.  The receiver with loss and RTT estimates
   corresponding to the lowest resulting calculated transmission rate is
   identified as the "current limiting receiver" (CLR).  In the case of
   a tie (where candidate CLRs are within 10% of the same calculated
   rate), the receiver with the largest RTT value SHOULD be designated
   as the CLR.

   As described in [TcpModel], a steady-state sender transmission rate,
   to be "friendly" with competing TCP flows, can be calculated as:
   Rsender = ----------------------------------------------------------
           T_rtt*(sqrt((2/3)*p) + 12*sqrt((3/8)*p) * p * (1 + 32*(p^2)))


   S = nominal transmitted packet size.  (In NORM, the "nominal" packet
   size can be determined by the sender as an exponentially weighted
   moving average (EWMA) of transmitted packet sizes to account for
   variable message sizes).

   T_rtt = RTT estimate of the current "current limiting receiver"

   p = loss event fraction of the CLR.

   To support congestion control feedback collection and operation, the
   NORM sender periodically transmits NORM_CMD(CC) command messages.
   NORM_CMD(CC) messages are multiplexed with NORM data and repair
   transmissions and serve several purposes, they:

   1.  Stimulate explicit feedback from the general receiver set to
       collect congestion control information.

   2.  Communicate state to the receiver set on the sender's current
       congestion control status including details of the CLR.

   3.  Initiate rapid (immediate) feedback from the CLR in order to
       closely track the dynamics of congestion control for the current
       worst path in the group multicast topology.

   The format of the NORM_CMD(CC) message is described in Section 4.2.3
   of this document.  The NORM_CMD(CC) message contains information to
   allow measurement of RTTs, to inform the group of the congestion
   control CLR, and to provide feedback of individual RTT measurements
   to the receivers in the group.  The NORM_CMD(CC) also provides for
   exciting feedback from OPTIONAL "potential limiting receiver" (PLR)
   nodes that might be determined administratively or possibly
   algorithmically based upon congestion control feedback.  PLR nodes
   are receivers that have been identified to have potential for
   (perhaps soon) becoming the CLR and thus immediate, up-to-date
   feedback is beneficial for congestion control performance.  The PLR
   list MAY be populated with a small number of receivers the sender
   identifies as approaching the CLR loss and delay conditions based on
   feedback from the group.  NORM_CMD(CC) Transmission

   The NORM_CMD(CC) message is transmitted periodically by the sender
   along with its normal data transmission.  Note the repeated
   transmission of NORM_CMD(CC) messages MAY be initiated some time
   before transmission of user data content at session startup.  This
   can be done to collect some estimation of the current state of the
   multicast topology with respect to group and individual RTT and
   congestion control state.

   A NORM_CMD(CC) message is immediately transmitted at sender startup.
   The interval of subsequent NORM_CMD(CC) message transmission is
   determined as follows:

   1.  By default, the interval is set according to the current sender
       GRTT estimate.  A startup initial value of GRTT_sender = 0.5
       seconds is RECOMMENDED when no feedback has yet been received
       from the group.

   2.  Until a CLR has been identified (based on previous receiver
       feedback) or when no data transmission is pending, the
       NORM_CMD(CC) interval is doubled up from its current interval to
       a maximum of once per 30 seconds.  This results in a low duty
       cycle for NORM_CMD(CC) probing when no CLR is identified or there
       is no pending data to transmit.

   3.  When a CLR has been identified (based on receiver feedback) and
       data transmission is pending, the probing interval is set to the
       RTT between the sender and the CLR (RTT_clr).

   4.  Additionally, when the data transmission rate is low with respect
       to the RTT_clr interval used for probing, the implementation
       SHOULD ensure no more than one NORM_CMD(CC) message is sent per
       NORM_DATA message when there is data pending transmission.  This
       ensures the transmission of this control message is not done to
       the exclusion of user data transmission.

   The NORM_CMD(CC) "cc_sequence" field is incremented with each
   transmission of a NORM_CMD(CC) command.  The greatest "cc_sequence"
   recently received by receivers is included in their feedback to the
   sender.  This allows the sender to determine the age of feedback to
   assist in congestion avoidance.

   The NORM-CC Rate Header Extension is applied to the NORM_CMD(CC)
   message and the sender advertises its current transmission rate in
   the "send_rate" field.  The rate information is used by receivers to
   initialize loss estimation during congestion control startup or

   The "cc_node_list" contains a list of entries identifying receivers
   and their current congestion control state (status "flags", "rtt",
   and "loss" estimates).  The list will be empty if the sender has not
   yet received any feedback from the group.  If the sender has received
   feedback, the list will minimally contain an entry identifying the
   CLR.  A NORM_FLAG_CC_CLR flag value is provided for the "cc_flags"
   field to identify the CLR entry.  It is RECOMMENDED the CLR entry be
   the first in the list for implementation efficiency.  Additional
   entries in the list are used to provide sender-measured individual
   RTT estimates to receivers in the group.  The number of additional
   entries in this list is dependent upon the percentage of control
   traffic the sender application is willing to send with respect to
   user data message transmissions.  More entries in the list will allow
   the sender to be more responsive to congestion control dynamics.  The
   length of the list can be dynamically determined according to the
   current transmission rate and scheduling of NORM_CMD(CC) messages.
   The maximum length of the list corresponds to the sender's
   NormSegmentSize parameter for the session.  The inclusion of

   additional entries in the list based on receiver feedback is
   prioritized with the following rules:

   1.  Receivers that have not yet been provided an RTT measurement get
       first priority.  Of these, those with the greatest loss fraction
       receive precedence for list inclusion.

   2.  Secondly, receivers that have previously been provided an RTT
       measurement are included with receivers yielding the lowest
       calculated congestion rate getting precedence.

   There are "cc_flag" values in addition to NORM_FLAG_CC_CLR used for
   other congestion control functions.  The NORM_FLAG_CC_PLR flag value
   is used to mark additional receivers from which the sender would like
   to have immediate, non-suppressed feedback.  These can be receivers
   the sender algorithmically identified as potential future CLRs or
   have been pre-configured as potential congestion control points in
   the network.  The NORM_FLAG_CC_RTT indicates the validity of the
   "cc_rtt" field for the associated receiver node.  Normally, this flag
   will be set since the receivers in the list will typically be
   receivers from which the sender has received feedback.  However, in
   the case the NORM sender has been pre-configured with a set of PLR
   nodes, feedback from those receivers might not have yet been
   collected and thus the "cc_rtt" field does not contain a valid value
   when this flag is not set.  Similarly, a value of ZERO for the
   "cc_rate" field here MUST be treated as an invalid value and be
   ignored for the purposes of feedback suppression, etc.  NORM_CMD(CC) Feedback Response

   Receivers explicitly respond to NORM_CMD(CC) messages in the form of
   a NORM_ACK(RTT) message.  The goal of the congestion control feedback
   is to determine the receivers with the lowest congestion control
   rates.  Receivers marked as CLR or PLR nodes in the NORM_CMD(CC)
   "cc_node_list" immediately provide feedback in the form of a NORM_ACK
   to this message.  When a NORM_CMD(CC) is received, non-CLR or non-PLR
   nodes initiate random feedback backoff timeouts similar to those used
   when the receiver initiates a repair cycle (see Section 5.3) in
   response to detection of data loss.  The backoff timeout for the
   congestion control response is generated as follows:

      T_backoff = RandomBackoff(K_backoff * GRTT_sender, GSIZE_sender)

   The RandomBackoff() algorithm provides a truncated exponentially
   distributed random number and is described in the Multicast NACK
   Building Block [RFC5401] document.  The same backoff factor,
   K_backoff = K_sender, as used with NORM_NACK suppression is generally
   RECOMMENDED.  However, in cases where the application purposefully

   specifies a very small K_sender backoff factor to minimize the NACK
   repair process latency (trading off group size scalability), it is
   RECOMMENDED a larger backoff factor for congestion control feedback
   be maintained, since there can be a larger volume of congestion
   control feedback than NACKs in many cases and some congestion control
   feedback latency might be tolerable where reliable delivery latency
   is not.  As previously noted, a backoff factor value of K_sender = 4
   is generally RECOMMENDED for ASM operation and K_sender = 6 for SSM
   operation.  A receiver SHALL cancel the backoff timeout and thus its
   pending transmission of a NORM_ACK(RTT) message under the following

   1.  The receiver generates another feedback message (NORM_NACK or
       other NORM_ACK) before the congestion control feedback timeout
       expires (these messages will convey the current congestion
       control feedback information).

   2.  A NORM_CMD(CC) or other receiver feedback with an ordinally
       greater "cc_sequence" field value is received before the
       congestion control feedback timeout expires (this is similar to
       the TFMCC feedback round number).

   3.  When the T_backoff is greater than 1*GRTT_sender.  This prevents
       NACK implosion in the event of sender or network failure.

   4.  "Suppressing" congestion control feedback is heard from another
       receiver (in a NORM_ACK or NORM_NACK) or via a
       NORM_CMD(REPAIR_ADV) message from the sender.  The local
       receiver's feedback is "suppressed" if the rate of the competing
       feedback (Rfb) is sufficiently close to or less than the local
       receiver's calculated rate (Rcalc).  The local receiver's
       feedback is canceled when Rcalc > (0.9 * Rfb).  Also, note
       receivers that have not yet received an RTT measurement from the
       sender are suppressed only by other receivers that have not yet
       measured RTT.  Additionally, receivers whose RTT estimate has
       aged considerably (i.e., they haven't been included in the
       NORM_CMD(CC) "cc_node_list" in a long time) might wish to compete
       as a receiver with no prior RTT measurement after some long-term
       expiration period.

   When the backoff timer expires, the receiver SHALL generate a
   NORM_ACK(RTT) message to provide feedback to the sender and group.
   This message MAY be multicast to the group for most effective
   suppression in ASM topologies or unicast to the sender depending upon
   how the NORM protocol is deployed and configured.

   Whenever any feedback is generated (including this NORM_ACK(RTT)
   message), receivers include an adjusted version of the sender

   timestamp from the most recently received NORM_CMD(CC) message and
   its "cc_sequence" value in the corresponding NORM_ACK or NORM_NACK
   message fields.  For NORM-CC operation, any generated feedback
   message SHALL also contain the NORM-CC Feedback header extension.
   The receiver provides its current "cc_rate" estimate, "cc_loss"
   estimate, "cc_rtt" if known, and any applicable "cc_flags" via this
   header extension.

   During slow start (when the receiver has not yet detected loss from
   the sender), the receiver uses a value equal to two times its
   measured rate from the sender in the "cc_rate" field.  For steady-
   state congestion control operation, the receiver "cc_rate" value is
   from the equation-based value using its current loss event estimate
   and sender<->receiver RTT information.  (The GRTT_sender is used when
   the receiver has not yet measured its individual RTT.)

   The "cc_loss" field value reflects the receiver's current loss event
   estimate with respect to the sender in question.

   When the receiver has a valid individual RTT measurement, it SHALL
   include this value in the "cc_rtt" field.  The NORM_FLAG_CC_RTT MUST
   be set when the "cc_rtt" field is valid.

   After a congestion control feedback message is generated or when the
   feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
   period during which it will restrain itself from providing congestion
   control feedback, even if NORM_CMD(CC) messages are received from the
   sender (unless the receive becomes marked as a CLR or PLR node).  The
   value of this holdoff timeout (T_ccHoldoff) period is:

                   T_ccHoldoff = (K_sender * GRTT_sender)

   Thus, non-CLR receivers are constrained to providing explicit
   congestion control feedback once per K_sender*GRTT_sender intervals.
   However, as the session progresses, different receivers will be
   responding to different NORM_CMD(CC) messages and there will be
   relatively continuous feedback of congestion control information
   while the sender is active.  Congestion Control Rate Adjustment

   During steady-state operation, the sender will directly adjust its
   transmission rate to the rate indicated by the feedback from its
   currently selected CLR.  As noted in [TfmccPaper], the estimation of
   parameters (loss and RTT) for the CLR will generally constrain the
   rate changes possible within acceptable bounds.  For rate increases,
   the sender SHALL observe a maximum rate of increase of one packet per
   RTT at all times during steady-state operation.

   The sender processes congestion control feedback from the receivers
   and selects the CLR based on the lowest rate receiver.  Receiver
   rates are determined either directly from the slow start "cc_rate"
   provided by the receiver in the NORM-CC Feedback header extension or
   by performing the equation-based calculation using individual RTT and
   loss estimates ("cc_loss") as feedback is received.

   The sender can calculate a current RTT for a receiver (RTT_rcvrNew)
   using the "grtt_response" timestamp included in feedback messages.
   When the "cc_rtt" value in a response is not valid, the sender simply
   uses this RTT_rcvrNew value as the receiver's current RTT (RTT_rcvr).
   For non-CLR and non-PLR receivers, the sender SHOULD use the "cc_rtt"
   provided in the NORM-CC Feedback header extension as the receiver's
   previous RTT measurement (RTT_rcvrPrev) averaged with the current
   measurement ("RTT_rcvrNew") as the receiver's RTT value:

             RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew

   For CLR receivers where feedback is received more regularly, the
   sender SHOULD maintain a more smoothed RTT estimate upon new feedback
   from the CLR where:

                 RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew

   RTT_clrNew is the new RTT calculated from the timestamp in the
   feedback message received from the CLR.  The RTT_clr is initialized
   to RTT_clrNew on the first feedback message received.  Note that the
   same procedure is observed by the sender for PLR receivers, and if a
   PLR is "promoted" to CLR status, the smoothed estimate can be

   There are some additional periods besides steady-state operation to
   be considered in NORM-CC operation.  These periods are:

   1.  during session startup,

   2.  when no feedback is received from the CLR, and

   3.  when the sender has a break in data transmission.

   During session startup, the congestion control operation SHALL
   observe a "slow-start" procedure to quickly approach its fair
   bandwidth share.  An initial sender startup rate is assumed where:

    Rinit = MIN(NormSegmentSize/GRTT_sender, NormSegmentSize) bytes/sec

   The rate is increased only when feedback is received from the
   receiver set.  The "slow start" phase proceeds until any receiver

   provides feedback indicating loss has occurred.  Rate increase during
   slow start is applied as:
                              Rnew = Rrecv_min

   where Rrecv_min is the minimum reported receiver rate in the
   "cc_rate" field of congestion control feedback messages received from
   the group.  Note during slow start, receivers use two times their
   measured rate from the sender in the "cc_rate" field of their
   feedback.  Rate increase adjustment is limited to once per GRTT
   during slow start.

   If the CLR or any receiver intends to leave the group, it will set
   the NORM_FLAG_CC_LEAVE in its congestion control feedback message as
   an indication the sender SHOULD NOT select it as the CLR.  When the
   CLR changes to a lower rate receiver, the sender SHOULD immediately
   adjust to the new lower rate.  The sender is limited to increasing
   its rate at one additional packet per RTT towards any new, higher CLR

   The sender SHOULD also track the age of the feedback it has received
   from the CLR by comparing its current "cc_sequence" value
   (Seq_sender) to the last "cc_sequence" value received from the CLR
   (Seq_clr).  As the age of the CLR feedback increases with no new
   feedback, the sender SHALL begin reducing its rate once per RTT_clr
   as a congestion avoidance measure.  The following algorithm is used
   to determine the decrease in sender rate (Rsender bytes/sec) as the
   CLR feedback, unexpectedly, excessively ages:

                   Age = Seq_sender - Seq_clr;
                   if (Age > 4) Rsender = Rsender * 0.5;

   This rate reduction is limited to the lower bound on NORM
   transmission rates.  After NORM_ROBUST_FACTOR consecutive
   NORM_CMD(CC) rounds without any feedback from the CLR, the sender
   SHOULD assume the CLR has left the group and pick the receiver with
   the next lowest rate as the new CLR.  Note this assumes the sender
   does not have explicit knowledge the CLR intentionally left the
   group.  If no receiver feedback is received, the sender MAY wish to
   withhold further transmissions of NORM_DATA segments and maintain
   NORM_CMD(CC) transmissions only until feedback is detected.  After
   such a CLR timeout, the sender will be transmitting with a minimal
   rate and SHOULD return to slow start as described here for a break in
   data transmission.

   When the sender has a break in its data transmission, it can continue
   to probe the group with NORM_CMD(CC) messages to maintain RTT
   collection from the group.  This will enable the sender to quickly
   determine an appropriate CLR upon data transmission restart.

   However, the sender SHOULD exponentially reduce its target rate to be
   used for transmission restart as time since the break elapses.  The
   target rate SHOULD be recalculated once per RTT_clr as:

                          Rsender = Rsender * 0.5;

   If the minimum NORM rate is reached, the sender SHOULD set the
   NORM_FLAG_START flag in its NORM_CMD(CC) messages upon restart and
   the group SHOULD observe slow-start congestion control procedures
   until any receiver experiences a new loss event.

5.5.3.  NORM Positive Acknowledgment Procedure

   NORM provides options for the source application to request positive
   acknowledgment (ACK) of NORM_CMD(FLUSH) and NORM_CMD(ACK_REQ)
   messages from members of the group.  There are some specific
   acknowledgment requests defined for the NORM protocol and a range of
   acknowledgment request types left to be defined by the application.
   One predefined acknowledgment type is the NORM_ACK(FLUSH) type.  This
   acknowledgment is used to determine if receivers have achieved
   completion of reliable reception up through a specific logical
   transmission point with respect to the sender's sequence of
   transmission.  The NORM_ACK(FLUSH) acknowledgment MAY be used to
   assist in application flow control when the sender has information on
   a portion of the receiver set.  Another predefined acknowledgment
   type is NORM_ACK(CC) used to explicitly provide congestion control
   feedback in response to NORM_CMD(CC) messages transmitted by the
   sender for NORM-CC operation.  Note the NORM_ACK(CC) response does
   NOT follow the positive acknowledgment procedure described here.  The
   NORM_CMD(ACK_REQ) and NORM_ACK messages contain an "ack_type" field
   to identify the type of acknowledgment requested and provided.  A
   range of "ack_type" values is provided for application-defined use.
   While the application is responsible for initiating the
   acknowledgment request and interprets application-defined "ack_type"
   values, the acknowledgment procedure SHOULD be conducted within the
   protocol implementation to take advantage of timing and transmission
   scheduling information available to the NORM transport.

   The NORM Positive Acknowledgment Procedure uses polling by the sender
   to query the receiver group for response.  Note this polling
   procedure is not intended to scale to very large receiver groups, but
   could be used in a large group setting to query a critical subset of
   the group.  Either the NORM_CMD(ACK_REQ), or when applicable, the
   NORM_CMD(FLUSH) message is used for polling and contains a list of
   NormNodeIds of the receivers expected to respond to the command.  The
   list of receivers providing acknowledgment is determined by the
   source application with a priori knowledge of participating nodes or
   via some other application-level mechanism.

   The ACK process is initiated by the sender generating NORM_CMD(FLUSH)
   or NORM_CMD(ACK_REQ) messages in periodic rounds.  For
   NORM_ACK(FLUSH) requests, the NORM_CMD(FLUSH) contains a
   "object_transport_id" and "fec_payload_id" denoting the watermark
   transmission point for which acknowledgment is requested.  This
   watermark transmission point is echoed in the corresponding fields of
   the NORM_ACK(FLUSH) message sent by the receiver in response.
   NORM_CMD(ACK_REQ) messages contain an "ack_id" field that is
   similarly echoed in response so the sender can match the response to
   the appropriate request.

   In response to the NORM_CMD(ACK_REQ), the listed receivers randomly,
   with a uniform distribution, transmit NORM_ACK messages over a time
   window of (1*GRTT_sender).  These NORM_ACK messages are typically
   unicast to the sender.  (Note NORM_ACK(CC) messages SHALL be
   multicast or unicast in the same manner as NORM_NACK messages.)

   The ACK process is self-limiting and avoids ACK implosion because:

   1.  Only a single NORM_CMD(ACK_REQ) message is generated once per
       (2*GRTT_sender), and

   2.  The size of the "acking_node_list" of NormNodeIds from which
       acknowledgment is requested is limited to a maximum of the sender
       NormSegmentSize setting per round of the positive acknowledgment

   Because the size of the included list is limited to the sender's
   NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds will
   sometimes be necessary to achieve responses from all receivers
   specified.  The content of the attached NormNodeId list will be
   dynamically updated as this process progresses and NORM_ACK responses
   are received from the specified receiver set.  As the sender receives
   valid responses (i.e., matching watermark point or "ack_id") from
   receivers, it SHALL eliminate those receivers from the subsequent
   NORM_CMD(ACK_REQ) message "acking_node_list" and add in any pending
   receiver NormNodeIds while keeping within the NormSegmentSize
   limitation of the list size.  Each receiver is queried a maximum
   number of times (NORM_ROBUST_FACTOR, by default).  Receivers not
   responding within this number of repeated requests are removed from
   the payload list to make room for other potential receivers pending
   acknowledgment.  The transmission of the NORM_CMD(ACK_REQ) is
   repeated until no further responses are needed or until the repeat
   threshold is exceeded for all pending receivers.  The transmission of
   NORM_CMD(ACK_REQ) or NORM_CMD(FLUSH) messages to conduct the positive
   acknowledgment process is multiplexed with ongoing sender data
   transmissions.  However, the NORM_CMD(FLUSH) positive acknowledgment
   process MAY be interrupted in response to negative acknowledgment

   repair requests (NACKs) received from receivers during the
   acknowledgment period.  The NORM_CMD(FLUSH) positive acknowledgment
   process is restarted for receivers pending acknowledgment once any
   the repairs have been transmitted.

   In the case of NORM_CMD(FLUSH) commands with an attached
   "acking_node_list", receivers will not ACK until they have received
   complete transmission of all data up to and including the given
   watermark transmission point.  All receivers SHALL interpret the
   watermark point provided in the request NACK for repairs if needed as
   for NORM_CMD(FLUSH) commands with no attached "acking_node_list".

5.5.4.  Group Size Estimate

   NORM sender messages contain a "gsize" field that is a representation
   of the group size and that is used in scaling random backoff timer
   ranges.  The use of the group size estimate within the NORM protocol
   does not demand a precise estimation and works reasonably well if the
   estimate is within an order of magnitude of the actual group size.
   By default, the NORM sender group size estimate MAY be
   administratively configured.  Also, given the expected scalability of
   the NORM protocol for general use, a default value of 10,000 is
   RECOMMENDED for use as the group size estimate.  It is also possible
   the group size MAY be algorithmically approximated from the volume of
   congestion control feedback messages based on the exponentially
   weighted random backoff.  However, the specification of such an
   algorithm is currently beyond the scope of this document.

6.  Configurable Elements

   The NORM protocol supports a modest number of configurable parameters
   that control operation.  Most of these need only be set at NORM
   sender(s) and the configuration information is communicated to the
   receiver set in NORM header and/or header extension fields.  A
   notable exception to this is the NORM_ROBUST_FACTOR that is presumed
   to be a common value preset among senders and receivers for a given
   NORM session.  The following table summarizes these configurable

   | Configurable       | Purpose                                      |
   | Element            |                                              |
   | Sender initial     | Sender's initial estimate of greatest group  |
   | GRTT Estimate      | round-trip time.  Affects timing of feedback |
   | (GRTT_sender)      | suppression and sender command transmissions |
   |                    | at sender startup.                           |
   | Backoff Factor     | Sender's scaling factor used for timer-based |
   | (K_sender)         | feedback suppression.                        |
   | Group Size         | Sender's rough estimate of receiver group    |
   | Estimate           | size used in generation of random feedback   |
   | (GSIZE_sender)     | backoff timeout.                             |
   | NORM_ROBUST_FACTOR | Integer factor determining how persistently  |
   |                    | (i.e., robust) senders transmit repeated     |
   |                    | control messages and receivers self-initiate |
   |                    | timeout-based NACKing in the absence of      |
   |                    | sender activity.                             |
   | FEC Type           | Sender FEC encoding type.                    |
   | ("fec_id")         |                                              |
   | Sender segment     | Maximum size (in bytes) of the payload       |
   | size               | portion of NORM_DATA and other messages.     |
   | (NormSegmentSize)  |                                              |
   | NormNodeId         | Unique identifiers pre-assigned to all NORM  |
   |                    | session participants.                        |

   The sender-controlled GRTT estimate (referred to as GRTT_sender in
   this document) is used to set and scale various timers associated
   with NORM protocol operation.  During steady-state operation, the
   sender probes the receiver set, adapts to the group round-trip timing
   state, and advertises its estimate to the receiver set in the "grtt"
   field of relevant NORM protocol messages.  However, an initial value
   must be assumed at sender startup.  A large initial estimate is
   conservative and safer with regard to preventing feedback implosion
   and starting up congestion control operation, but requires the sender
   and receivers to allocate more buffering resources for a given
   transmission rate (i.e., larger effective delay*bandwidth product) to
   maintain efficient operation.  A default initial value of GRTT_sender
   = 0.5 seconds is RECOMMENDED.

   The sender-controlled Backoff Factor (referred to a K_sender in this
   document) is used to scale protocol timers and contributes to the
   generation of the random backoff timeout value that facilitates
   timer-based feedback suppression.  The sender advertises its
   configured Backoff Factor to the receiver set in the "backoff" field
   of applicable NORM messages and thus no receiver configuration is
   necessary.  For ASM operation, a default value of K_sender = 4 is

   RECOMMENDED; for SSM operation, a default value of K_sender = 6 is

   The sender estimate of session Group Size (referred to as
   GSIZE_sender in this document) also plays a role in the random
   selection of feedback suppression timeout values.  The sender
   advertises its configured Group Size estimate to the receiver set in
   the "gsize" field of applicable NORM messages; thus, no receiver
   configuration is necessary.  Only a rough estimate (i.e., "order-of-
   magnitude") is needed for effective feedback suppression and a
   default value of GSIZE_sender = 10,000 is RECOMMENDED as a
   conservative estimate for most uses.

   The NORM_ROBUST_FACTOR is an integer parameter that determines how
   persistently NORM senders transmit control messages (NORM_CMD
   messages) such as end-of-transmission flushing, OPTIONAL positive
   acknowledgment requests, etc.  Additionally, the receivers use their
   knowledge of NORM_ROBUST_FACTOR to determine when to consider a NORM
   sender inactive and MAY use the factor in determining how
   persistently to self-initiate repeated NACK repair requests upon such
   timeouts.  This parameter is NOT communicated in NORM protocol
   message headers and is presumed to be preset to a consistent value
   among sender and receivers for a given NORM session.  A default value

   Another NORM sender configuration element is the FEC type used to
   encode NORM_DATA message content.  The FEC type is communicated from
   the sender to the receiver set in the "fec_id" field of relevant NORM
   message headers.  The "fec_id" value corresponds to an IANA-assigned
   value identifying the FEC encoding type as described in the FEC
   Building Block [RFC5052] document.  Typically, a sender SHOULD use a
   consistent FEC encoding for its participation in a session to
   simplify receiver state allocation and maintenance, but its
   implementations MAY vary the FEC encoding type on a per-object basis
   if necessary.

   The sender NormSegmentSize setting determines the maximum size of the
   payload portion of NORM_DATA and other messages that the sender
   transmits.  Additionally, the payload size of feedback messages from
   receivers to a given sender is limited to that sender's
   NormSegmentSize.  The NormSegmentSize SHOULD be configured to be
   compatible with expected network MTU limitations, given the added
   overhead of NORM, UDP, and IP protocol message headers.
   Additionally, MTU Discovery MAY be employed by the sender to
   determine an appropriate NormSegmentSize.  The NormSegmentSize for a
   given sender can be determined by receivers from the FEC Object
   Transmission Information (FTI) provided either in applied EXT_FTI
   header extensions or pre-configured session information.

   Although it is not technically a configurable element, the receivers
   MUST have FEC Object Transmission Information for transmitted
   NormObjects to properly buffer, decode, and reassemble the original
   content.  For loosely organized NORM protocol sessions, the sender
   MAY apply the EXT_FTI Header Extension to NORM_DATA and NORM_INFO (if
   applicable) messages so that receivers can get this information
   without prior coordination.  An implementation MAY also apply the
   EXT_FTI only to NORM_INFO messages for reduced overhead.  Finally,
   applications MAY also provide the FTI out-of-band prior to sender

   Each participant in a NORM protocol session MUST be configured with a
   unique NormNodeId value.  The NormNodeId value is used by receivers
   to identify the sender to which their NACK or other feedback messages
   are addressed, and senders use the NormNodeId to differentiate
   receivers for purposes of congestion control and OPTIONAL positive
   acknowledgment collection.  Assignment of unique NormNodeId values
   can be done via a priori coordination and/or use of a deconfliction
   mechanism external to the NORM protocol itself.  The values of
   NORM_NODE_NONE = 0x00000000 and NORM_NODE_ANY = 0xffffffff are
   reserved and MUST NOT be assigned to NORM participants.

7.  Security Considerations

   The same security considerations that apply to the Multicast NACK
   [RFC5401], TFMCC [RFC4654], and FEC [RFC5052] Building Blocks also
   apply to the NORM protocol.  In addition to the vulnerabilities to
   which any IP and IP multicast protocol implementation is subject,
   malicious hosts might engage in excessive NACKing in an attempt to
   prevent the NORM sender(s) from making forward progress in reliable
   transmission.  Receiver "join" and "service" policy enforcement as
   described in Section 5.2 can be applied if such activity is detected.
   The use of cryptographic peer authentication, integrity checks,
   and/or confidentiality mechanisms can be used to provide a more
   effective degree of protection from objectionable transmissions from
   unauthorized hosts.  But in some cases, even with authentication and
   integrity checks, the NACK-based feedback of NORM can be exploited by
   replay attacks forcing the NORM sender to unnecessarily transmit
   repair information.  This MAY be addressed in part with network-layer
   IP security implementations that guard against this potential
   security exploitation or alternatively with a security mechanism
   using the EXT_AUTH header extension for similar purposes.  Such
   security mechanisms SHOULD be deployed and used when available.  Use
   of security mechanisms will impose additional "a priori"
   configuration upon the NORM deployment depending upon the techniques

   The NORM protocol is compatible with the use of IP security (IPsec)

   [RFC4301], and the IPsec Encapsulating Security Payload (ESP)
   protocol or Authentication Header (AH) extension can be used to
   secure IP packets transmitted by NORM participants.  A baseline
   approach to secure NORM operation using IPsec is described below.
   Compliant implementations of this specification are REQUIRED to be
   compatible with IPsec usage as described in Section 7.1.  IPsec can
   be used to provide peer authentication, integrity protection, and/or
   encryption of packets containing NORM messages.

   Additionally, the EXT_AUTH header extension (HET = 1) is reserved for
   use by security mechanisms to provide alternatives to IPsec for the
   security of NORM messages.  The format of this header extension and
   its processing is outside the scope of this document and is to be
   communicated out-of-band as part of the session description.  It is
   possible an EXT_AUTH implementation MAY also provide for encryption
   of NORM message payloads as well as peer authentication and integrity
   protection.  The use of this approach as compared to IPsec can allow
   for header compression techniques to be applied jointly to IP and
   NORM protocol headers.  In cases where security analysis deems
   encryption of NORM protocol header content to be beneficial or
   necessary, the aforementioned use of IPsec ESP might be more
   appropriate.  Additionally, the EXT_AUTH header extension can be
   utilized when NORM is implemented in a network with Network Address
   Translation (NAT) systems that are incompatible with use of the IPsec
   AH extension.  If EXT_AUTH is present, whatever packet authentication
   or integrity checks that can be performed immediately upon reception
   of the packet MUST be performed before accepting the packet and
   performing any congestion-control-related action on it.  Some packet
   authentication schemes impose a delay of several seconds between when
   a packet is received and when the packet can be fully authenticated.
   Any appropriate congestion control related action MUST NOT be
   postponed by any such packet security mechanism (i.e., security
   mechanisms MUST NOT result in poor congestion control behavior).

   Consideration MUST also be given to the potential for replay-attacks
   that would transplant authenticated packets from one NORM session to
   another to disrupt service.  To avoid this potential, unique keys
   SHOULD be assigned on a per-session basis or NORM sender nodes SHOULD
   be configured to use unique "instance_id" identifiers managed as part
   of the security association for the sessions.

   Note NORM implementations can use the "sequence" field from the NORM
   common message header to detect replay attacks.  This can be
   accomplished if the NORM sender maintains state on actively NACKing
   receivers.  A cache of such receiver state can be used to provide
   protection against NACK replay attacks.  NORM receivers MUST also
   maintain similar state for protection against possible replay of
   other receiver messages in ASM operation as well.  For example, a

   receiver could be suppressed from providing NACK or congestion
   control feedback by replay of certain receiver messages.  For these
   reasons, authentication of NORM messages (e.g., via IPsec) SHOULD be
   applied for protection against similar attacks that use fabricated
   messages.  Also, encryption of messages to provide confidentiality of
   application data and protect privacy of users MAY also be applied
   using IPsec or similar mechanisms.

   When applicable security measures are used, automated key management
   mechanisms such as those described in the Group Domain of
   Interpretation (GDOI) [RFC3547], Multimedia Internet KEYing (MIKEY)
   [RFC3830], or Group Secure Association Key Management Protocol
   (GSAKMP) [RFC4535] specifications SHOULD be applied.

   While NORM does leverage FEC-based repair for scalability, this alone
   does not guarantee integrity of received data.  Application-level
   integrity-checking of received data content is highly RECOMMENDED.
   This recommendation also applies when the IPsec security approach
   described below is used for added assurance in data content integrity
   given the shared use of IPsec Security Association information among
   the group.

7.1.  Baseline Secure NORM Operation

   This section describes a baseline mode of secure NORM protocol
   operation based on application of the IPsec security protocol.  This
   approach is documented here to provide a baseline interoperable
   secure mode of operation.  This particular approach represents one
   possible trade-off in the level of assurance that can be achieved and
   the scalability of multicast group-size given current IPsec
   mechanisms and the state required to support them.  For example, this
   baseline approach specifies the use of a Security Association that is
   shared among the receiver set for feedback messages to the sender.
   This model requires that the receiver membership receiving the
   session keys is trusted and only provides protection from attacks
   that are external to the NORM group membership.  More stateful and
   complex IPsec approaches and key management schemes may be applied
   for higher levels of assurance, but those are beyond the scope of
   this transport protocol specification.  Additional approaches to NORM
   security, including other forms of IPsec application, MAY be
   specified in the future.  For example, the use of the EXT_AUTH header
   extension could enable NORM-specific authentication or security
   encapsulation headers similar to those of IPsec to be specified and
   inserted into the NORM protocol message headers.  This would allow
   header compression techniques to be applied to IP and NORM protocol
   headers when needed in a similar fashion to RTP [RFC3550] and as
   preserved in the specification for Secure Real Time Protocol (SRTP)

   The baseline approach described is applicable to NORM operation
   configured for SSM (or SSM-like) operation where there is a single
   sender and the receivers are providing unicast feedback.  This form
   of NORM operation allows for IPsec to be used with a manageable
   number of security associations (SA).

7.1.1.  IPsec Approach

   For NORM one-to-many SSM operation with unicast feedback from
   receivers, each node SHALL be configured with two transport mode
   IPsec security associations and corresponding Security Policy
   Database (SPD) entries.  One entry will be used for sender-to-group
   multicast packet authentication and optionally encryption while the
   other entry will be used to provide security for the unicast feedback
   messaging from the receiver(s) to the sender.  Note that this single
   SA for NORM receiver feedback messages is shared to protect traffic
   from possibly multiple receivers to the single sender.

   For each NormSession, the NORM sender SHALL use an IPsec SA
   configured for ESP protocol [RFC4303] operation with the option for
   data origin authentication enabled.  It is also RECOMMENDED this
   IPsec ESP SA be also configured to provide confidentiality protection
   for IP packets containing NORM protocol messages.  This is suggested
   to make the realization of complex replay attacks much more
   difficult.  The encryption key for this SA SHALL be preplaced at the
   sender and receiver(s) prior to NORM protocol operation.  Use of
   automated key management is RECOMMENDED as a rekey SHALL be REQUIRED
   prior to expiration of the sequence space for the SA.  This is
   necessary so receivers can use the built-in IPsec replay attack
   protection possible for an IPsec SA with a single source (the NORM
   sender).  Thus, the receivers SHALL enable replay attack protection
   for this SA used to secure NORM sender traffic.  An IPsec SPD entry
   MUST be configured to process outbound packets to the session
   (destination) address and UDP port number of the applicable

   The NORM receiver(s) MUST be configured with the SA and SPD entry to
   properly process the IPsec-secured packets from the sender.  The NORM
   receiver(s) SHALL also use a common, second IPsec SA (common Security
   Parameter Index (SPI) and encryption key) configured for ESP
   operation with the option for data origination authentication
   enabled.  Similar to the NORM sender, is RECOMMENDED this IPsec ESP
   SA be also configured to provide confidentiality protection for IP
   packets containing NORM protocol messages.  The receivers MUST have
   an IPsec SPD entry configured to process outbound NORM/UDP packets
   directed to the NORM sender source address and port number using this
   second SA.  To support NORM unicast feedback, the sender's
   transmission port number SHOULD be selected to be distinct from the

   multicast session port number to allow discrimination between unicast
   and multicast feedback messages when access to the IP destination
   address is not possible (e.g., a user-space NORM implementation).
   For processing of packets from receivers, the NORM sender SHALL be
   configured with this common, second SA (and the corresponding SPD
   entry needed) in order to properly process messages from the

   Multiple receivers using a common IPsec SA for traffic directed to
   the NORM sender (i.e., many-to-one) typically prevents the use of
   built-in IPsec replay attack protection by the NORM sender with
   current IPsec implementations.  Thus the built-in IPsec replay attack
   protection for this second SA at the sender MUST be disabled unless
   the particular IPsec implementation manages its replay protection on
   a per-source basis (which is not typical of existing IPsec
   implementations).  So, to support a fully secure mode of operation,
   the NORM sender implementation MUST provide replay attack protection
   based upon the "sequence" field of NORM protocol messages from
   receivers.  This can be accomplished with a high assurance of
   security, even with the limited size (16-bits) of this field,

   1.  NORM receiver NACK and non-CLR ACK feedback messages are sparse.

   2.  The more frequent NORM_ACK feedback from CLR or PLR nodes is only
       a small set of receivers for which the sender needs to keep more
       persistent replay attack state.

   3.  NORM_NACK feedback messages preceding the sender's current repair
       window do not significantly impact protocol operation (generation
       of NORM_CMD(SQUELCH) is limited) and could be in fact ignored.
       This means the sender can prune any replay attack state that
       precedes the current repair window.

   4.  NORM_ACK messages correspond to either a specific sender
       "ack_id", the sender "cc_sequence" for ACKs sent in response to
       NORM_CMD(CC), or the sender's current repair window in the case
       of ACKs sent in response to NORM_CMD(FLUSH).  Thus, the sender
       can prune any replay attack state for receivers that precede the
       current applicable sequence or repair window space.

   The use of ESP confidentiality for secure NORM protocol operation
   makes it more difficult for adversaries to conduct any form of replay
   attacks.  Additionally, a NORM sender implementation with access to
   the full ESP protocol header could also use the ESP sequence
   information to make replay attack protection even more robust by
   maintaining the per-source ESP sequence state that existing IPsec
   implementations typically do not provide.  The design of this

   baseline security approach for NORM intentionally places any more
   complex processing state or processing (e.g., replay attack
   protection given multiple receivers) at the NORM sender since NORM
   receiver implementations might often need to be less complex.

   This baseline approach can be used for NORM protocol sessions with
   multiple senders if the SA pairs described are established for each
   sender.  For small-sized groups, it is even possible many-to-many
   (ASM) IPsec configuration could be achieved where each participant
   uses a unique SA (with a unique SPI).  In this case, the sender(s)
   would maintain an SA for each other participant rather than a single,
   shared SA for receiver feedback messages.  This does not scale to
   larger group sizes given the complex set of SA and SPD entries each
   participant would need to maintain.

   It is anticipated in early deployments of this baseline approach to
   NORM security that key management will be conducted out-of-band with
   respect to NORM protocol operation.  In the case of one-to-many NORM
   operation, it is possible receivers will retrieve keying information
   from a central server as needed or otherwise conduct group key
   updates with a similar centralized approach.  Alternatively, it is
   possible with some key management schemes for rekey messages to be
   transmitted to the group as a message or transport object within the
   NORM reliable transfer session.  Similarly, for group-wise
   communication sessions, it is possible for potential group
   participants to request keying and/or rekeying as part of NORM
   communications.  Additional specification is necessary to define an
   in-band key management scheme for NORM sessions perhaps using the
   mechanisms of the automated group key management specifications cited
   in this document.  Additional specification outside of the scope of
   this document would be needed to provide an interoperable approach
   for key management in-band of a NORM reliable transport session.

7.1.2.  IPsec Requirements

   In order to implement this secure mode of NORM protocol operation,
   the following IPsec capabilities are REQUIRED.  Selectors

   The implementation MUST be able to use the source address,
   destination address, protocol (UDP), and UDP port numbers as
   selectors in the SPD.  Mode

   IPsec in transport mode MUST be supported.  The use of IPsec
   [RFC4301] processing for secure NORM traffic MUST be configured such

   that unauthenticated packets are not received by the NORM protocol
   implementation.  Key Management

   An automated key management scheme for group key distribution and
   rekeying such as GDOI [RFC3547], GSAKMP [RFC4535], or MIKEY [RFC3830]
   is RECOMMENDED for use.  Note it is possible for key update messages
   (e.g., the GDOI GROUPKEY-PUSH message) to be included as part of the
   NORM application reliable data transmission if appropriate interfaces
   are available between the NORM application and the key management
   daemon.  Relatively short-lived NORM sessions MAY be able to use
   Manual Keying with a single, preplaced key, particularly if Extended
   Sequence Numbering (ESN) [RFC4303] is available in the IPsec
   implementation used.  When manual keys are used, it is important that
   cryptographic algorithms suitable for manual key use are selected.  Security Policy

   Receivers MUST accept protocol messages only from the designated,
   authorized sender(s).  Appropriate key management will provide
   authentication, integrity and/or encryption keys only to receivers
   authorized to participate in a designated session.  The approach
   outlined here allows receiver sets to be controlled on a per-sender
   basis.  Authentication and Encryption

   Large NORM group sizes will necessitate some form of key management
   that does rely upon shared secrets.  The GDOI and GSAKMP protocols
   mentioned here allow for certificate-based authentication.  It is
   RECOMMENDED these certificates use IP addresses for authentication.  Availability

   The IPsec requirements profile outlined here is commonly available on
   many potential NORM hosts.  Configuration and operation of IPsec
   typically requires privileged user authorization.  Automated key
   management implementations are typically configured with the
   privileges necessary to affect system IPsec configuration.

8.  IANA Considerations

   Values of NORM Header Extension Types, Stream Control Codes, and
   NORM_CMD message sub-types are subject to IANA registration.  They
   are in the registry named "Reliable Multicast Transport (RMT) NORM
   Protocol Parameters" available from http://www.iana.org.

   Note the reliable multicast building block components used by this
   specification also have their respective IANA considerations, and
   those documents SHOULD be consulted accordingly.  In particular, the
   FEC Building Block used by NORM does REQUIRE IANA registration of the
   FEC codecs used.  The registration instructions for FEC codecs are
   provided in RFC 5052.  It is possible additional extensions of the
   NORM protocol might be specified in the future (e.g., additional NORM
   message types) and additional registries be established at that time
   with appropriate IETF standards action.

8.1.  Explicit IANA Assignment Guidelines

   This document introduces three registries for the NORM Header
   Extension Types, Stream Control Codes, and NORM_CMD Message sub-
   types.  This section describes explicit IANA assignment guidelines
   for each of these.

8.1.1.  NORM Header Extension Types

   This document defines a registry for NORM Header Extensions named
   "NORM Header Extension Types".

   The NORM Header Extension Type field is an 8-bit value.  The values
   of this field identify extended header content allowing the protocol
   functionality to be expanded to include additional features and
   operating modes.  The values that can be assigned within the "NORM
   Header Extensions" registry are numeric indexes in the range {0,
   255}, boundaries included.  Values in the range {0,127} indicate
   variable-length extended header fields while values in the range
   {128,255} indicate extensions of a fixed 4-byte length.  This
   specification registers the following NORM Header Extension Types:

                 | Value | Name     | Reference          |
                 | 1     | EXT_AUTH | This specification |
                 | 3     | EXT_CC   | This specification |
                 | 64    | EXT_FTI  | This specification |
                 | 128   | EXT_RATE | This specification |

   Requests for assignment of additional NORM Header Extension Type
   values are granted on a "Specification Required" basis as defined by
   IANA Guidelines [RFC5226].  Any such header extension specifications
   MUST include a description of protocol actions to be taken when the
   extension type is encountered by a protocol implementation not
   supporting that specific option.  For example, it is often possible
   for protocol implementations to ignore unknown header extensions.

8.1.2.  NORM Stream Control Codes

   This document defines a registry for NORM Stream Control Codes named
   "NORM Stream Control Codes".

   NORM Stream Control Codes are 16-bit values that can be inserted
   within a NORM_OBJECT_STREAM delivery object to convey sequenced, out-
   of-band (with respect to the stream data) control signaling
   applicable to the referenced stream object.  These control codes are
   to be delivered to the application or protocol implementation with
   reliable delivery, in-order with respect to the their inserted
   position within the stream.  This specification registers the
   following NORM Stream Control Code:

             | Value | Name            | Reference          |
             | 0     | NORM_STREAM_END | This specification |

   Additional NORM Stream Control Code value assignment requests are
   granted on a "Specification Required" basis as defined by IANA
   Guidelines [RFC5226].  The full 16-bit space outside of the value
   assigned in this specification are available for future assignment.
   In addition to describing the control code's expected interpretation,
   such specifications MUST include a description of protocol actions to
   be taken when the control code is encountered by a protocol
   implementation not supporting that specific option.

8.1.3.  NORM_CMD Message Sub-Types

   This document defines a registry for NORM_CMD message sub-types named
   "NORM Command Message Sub-types".

   The NORM_CMD message "sub-type" field is an 8-bit value with valid
   values in the range of 1-255.  Note the value 0 is reserved to
   indicate an invalid NORM_CMD message sub-type.  The current
   specification defines a number of NORM_CMD message sub-types senders
   can use to signal the receivers in various aspects of NORM protocol
   operation.  This specification registers the following NORM_CMD
   Message Sub-types:

          | Value | Name                  | Reference          |
          | 0     | reserved              | This specification |
          | 1     | NORM_CMD(FLUSH)       | This specification |
          | 2     | NORM_CMD(EOT)         | This specification |
          | 3     | NORM_CMD(SQUELCH)     | This specification |
          | 4     | NORM_CMD(CC)          | This specification |
          | 5     | NORM_CMD(REPAIR_ADV)  | This specification |
          | 6     | NORM_CMD(ACK_REQ)     | This specification |
          | 7     | NORM_CMD(APPLICATION) | This specification |

   Future specifications extending NORM MAY define additional NORM_CMD
   messages to enhance protocol functionality.  NORM_CMD message sub-
   type value assignment requests are granted on a "Specification
   Required" basis as defined by IANA Guidelines [RFC5226].  In addition
   to describing the command sub-type's expected interpretation,
   specifications MUST include a description of protocol actions to be
   taken when the command is encountered by a protocol implementation
   not supporting that specific option.

   This specification already defines an "application-defined" NORM_CMD
   message sub-type for use at the discretion of individual applications
   using NORM for transport.  These "application-defined" commands are
   suitable for many application-specific purposes and do not involve
   standards action.  In any case, such additional messages SHALL be
   subject to the same congestion control constraints as the existing
   NORM sender message set.

9.  Suggested Use

   The present NORM protocol is seen as a useful tool for the reliable
   data transfer over generic IP multicast services.  It is not the
   intention of the authors to suggest it is suitable for supporting all
   envisioned multicast reliability requirements.  NORM provides a
   simple and flexible framework for multicast applications with a
   degree of concern for network traffic implosion and protocol overhead
   efficiency.  NORM-like protocols have been successfully demonstrated
   within the MBone for bulk data dissemination applications, including
   weather satellite compressed imagery updates servicing a large group
   of receivers and a generic web content reliable "push" application.

   In addition, this framework approach has some design features making
   it attractive for bulk transfer in asymmetric and wireless
   internetwork applications.  NORM is capable of successfully operating
   independent of network structure and in environments with high packet
   loss, delay, and out-of-order delivery.  Hybrid proactive/reactive

   FEC-based repairing improve protocol performance in some multicast
   scenarios.  A sender-only repair approach often makes additional
   engineering sense in asymmetric networks.  NORM's unicast feedback
   capability is suitable for use in asymmetric networks or in networks
   where only unidirectional multicast routing/delivery service exists.
   Asymmetric architectures supporting multicast delivery are likely to
   make up an important portion of the future Internet structure (e.g.,
   direct broadcast satellite (DBS) or cable and public-switched
   telephone network (PSTN) hybrids, etc.) and efficient, reliable bulk
   data transfer will be an important capability for servicing large
   groups of subscribed receivers.

10.  Changes from RFC 3940

   This section lists the changes between the Experimental version of
   this specification, RFC 3940, and this version:

       replacing it with the "payload_msg_start" field in the FEC-
       encoded preamble of the NORM_OBJECT_STREAM NORM_DATA payload.

   2.  Definition of IANA registry for header extension and other

   3.  Removal of file blocking scheme description now specified in the
       FEC Building Block document [RFC5052].

   4.  Removal of restriction of NORM receiver feedback message rate to
       local NORM sender rate (this caused congestion control failures
       in high speed operation.  The extremely low feedback rate of the
       NORM protocol as compared to TCP avoids any resultant impact to
       the network as shown in [Mdpcc].)

   5.  Correction of errors in some message format descriptions.

   6.  Correction of inconsistency in specification of the inactivity

   7.  Addition of IPsec secure mode description with IPsec

   8.  Addition of the EXT_AUTH header extension definition.

   9.  Clarification of interpretation of "Source Block Length" when FEC
       codes are arbitrarily shortened by the sender.

11.  Acknowledgments

   (and these are not Negative)

   The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
   Toni Paila, Michael Luby, and Joerg Widmer for their valuable input
   and comments on this document.  The authors would also like to thank
   the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for
   their support in development of this specification, and Sally Floyd
   for her early input into this document.

12.  References

12.1.  Normative References

   [RFC1112]        Deering, S., "Host extensions for IP multicasting",
                    STD 5, RFC 1112, August 1989.

   [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                    Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4301]        Kent, S. and K. Seo, "Security Architecture for the
                    Internet Protocol", RFC 4301, December 2005.

   [RFC4303]        Kent, S., "IP Encapsulating Security Payload (ESP)",
                    RFC 4303, December 2005.

   [RFC4607]        Holbrook, H. and B. Cain, "Source-Specific Multicast
                    for IP", RFC 4607, August 2006.

   [RFC4654]        Widmer, J. and M. Handley, "TCP-Friendly Multicast
                    Congestion Control (TFMCC): Protocol Specification",
                    RFC 4654, August 2006.

   [RFC5052]        Watson, M., Luby, M., and L. Vicisano, "Forward
                    Error Correction (FEC) Building Block", RFC 5052,
                    August 2007.

   [RFC5226]        Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 5226, May 2008.

   [RFC5401]        Adamson, B., Bormann, C., Handley, M., and J.
                    Macker, "Multicast Negative-Acknowledgment (NACK)
                    Building Blocks", RFC 5401, November 2008.

12.2.  Informative References

   [FecHybrid]      Gossink, D. and J. Macker, "Reliable Multicast and
                    Integrated Parity Retransmission with Channel
                    Estimation", IEEE GLOBECOMM, 1998.

   [McastFeedback]  Nonnenmacher, J. and E. Biersack, "Optimal Multicast
                    Feedback", IEEE INFOCOM, p. 964, March/April 1998.

   [MdpToolkit]     Macker,  J. and B. Adamson, "The Multicast
                    Dissemination Protocol (MDP) Toolkit", Proc.
                    IEEE MILCOM, October 1999.

   [Mdpcc]          Adamson,  B. and J. Macker, "A TCP-Friendly, Rate-
                    based Mechanism for NACK-Oriented Reliable Multicast
                    Congestion Control", Proc. IEEE GLOBECOMM,
                    November 2001.

   [NormFeedback]   Adamson, B. and J. Macker, "Quantitative Prediction
                    of NACK-Oriented Reliable Multicast (NORM)
                    Feedback", IEEE MILCOM, October 2002.

   [PgmccPaper]     Rizzo, L., "pgmcc: A TCP-Friendly Single-Rate
                    Multicast Congestion Control Scheme", ACM SIGCOMM,
                    August 2000.

   [RFC2357]        Mankin, A., Romanov, A., Bradner, S., and V. Paxson,
                    "IETF Criteria for Evaluating Reliable Multicast
                    Transport and Application Protocols", RFC 2357,
                    June 1998.

   [RFC2974]        Handley, M., Perkins, C., and E. Whelan, "Session
                    Announcement Protocol", RFC 2974, October 2000.

   [RFC3048]        Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
                    Floyd, S., and M. Luby, "Reliable Multicast
                    Transport Building Blocks for One-to-Many Bulk-Data
                    Transfer", RFC 3048, January 2001.

   [RFC3269]        Kermode, R. and L. Vicisano, "Author Guidelines for
                    Reliable Multicast Transport (RMT) Building Blocks
                    and Protocol Instantiation documents", RFC 3269,
                    April 2002.

   [RFC3453]        Luby, M., Vicisano, L., Gemmell, J., Rizzo, L.,
                    Handley, M., and J. Crowcroft, "The Use of Forward
                    Error Correction (FEC) in Reliable Multicast",
                    RFC 3453, December 2002.

   [RFC3547]        Baugher, M., Weis, B., Hardjono, T., and H. Harney,
                    "The Group Domain of Interpretation", RFC 3547,
                    July 2003.

   [RFC3550]        Schulzrinne, H., Casner, S., Frederick, R., and V.
                    Jacobson, "RTP: A Transport Protocol for Real-Time
                    Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]        Baugher, M., McGrew, D., Naslund, M., Carrara, E.,
                    and K. Norrman, "The Secure Real-time Transport
                    Protocol (SRTP)", RFC 3711, March 2004.

   [RFC3830]        Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,
                    and K. Norrman, "MIKEY: Multimedia Internet KEYing",
                    RFC 3830, August 2004.

   [RFC3940]        Adamson, B., Bormann, C., Handley, M., and J.
                    Macker, "Negative-acknowledgment (NACK)-Oriented
                    Reliable Multicast (NORM) Protocol", RFC 3940,
                    November 2004.

   [RFC4535]        Harney, H., Meth, U., Colegrove, A., and G. Gross,
                    "GSAKMP: Group Secure Association Key Management
                    Protocol", RFC 4535, June 2006.

   [RFC4566]        Handley, M., Jacobson, V., and C. Perkins, "SDP:
                    Session Description Protocol", RFC 4566, July 2006.

   [RFC5445]        Watson, M., "Basic Forward Error Correction (FEC)
                    Schemes", RFC 5445, March 2009.

   [RmComparison]   Pingali, S., Towsley, D., and J. Kurose, "A
                    Comparison of Sender-Initiated and Receiver-
                    Initiated Reliable Multicast Protocols", Proc.
                    INFOCOMM, San Francisco CA, October 1993.

   [TcpModel]       Padhye,  J., Firoiu, V., Towsley, D., and J. Kurose,
                    "Modeling TCP Throughput: A Simple Model and its
                    Empirical Validation", ACM SIGCOMM, 1998.

   [TfmccPaper]     Widmer, J. and M. Handley, "Extending Equation-Based
                    Congestion Control to Multicast Applications",
                    ACM SIGCOMM, August 2001.

Authors' Addresses

   Brian Adamson
   Naval Research Laboratory
   Washington, DC  20375

   EMail: adamson@itd.nrl.navy.mil

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28334 Bremen

   EMail: cabo@tzi.org

   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT

   EMail: M.Handley@cs.ucl.ac.uk

   Joe Macker
   Naval Research Laboratory
   Washington, DC  20375

   EMail: macker@itd.nrl.navy.mil


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