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RFC 3940 - Negative-acknowledgment (NACK)-Oriented Reliable Mult

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Network Working Group                                         B. Adamson
Request for Comments: 3940                                           NRL
Category: Experimental                                        C. Bormann
                                                 Universitaet Bremen TZI
                                                              M. Handley
                                                               J. Macker
                                                           November 2004

                Negative-acknowledgment (NACK)-Oriented
                   Reliable Multicast (NORM) Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).


   This document describes the messages and procedures of the Negative-
   acknowledgment (NACK) Oriented Reliable Multicast (NORM) protocol.
   This protocol is designed to 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 repair and other IETF
   reliable multicast transport (RMT) building blocks in its design.

Table of Contents

   1.  Introduction and Applicability. . . . . . . . . . . . . . . .   3
       1.1. NORM Delivery Service Model. . . . . . . . . . . . . . .   4
       1.2. NORM Scalability . . . . . . . . . . . . . . . . . . . .   6
       1.3. Environmental Requirements and Considerations. . . . . .   7
   2.  Architecture Definition . . . . . . . . . . . . . . . . . . .   7
       2.1. Protocol Operation Overview. . . . . . . . . . . . . . .   9
       2.2. Protocol Building Blocks . . . . . . . . . . . . . . . .  10
       2.3. Design Tradeoffs . . . . . . . . . . . . . . . . . . . .  11
   3.  Conformance Statement . . . . . . . . . . . . . . . . . . . .  12
   4.  Message Formats . . . . . . . . . . . . . . . . . . . . . . .  13
       4.1. NORM Common Message Header and Extensions. . . . . . . .  14
       4.2. Sender Messages. . . . . . . . . . . . . . . . . . . . .  16
            4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . .  16
            4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . .  24
            4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . .  26
       4.3. Receiver Messages. . . . . . . . . . . . . . . . . . . .  43
            4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . .  43
            4.3.2. NORM_ACK Message. . . . . . . . . . . . . . . . .  50
       4.4. General Purpose Messages . . . . . . . . . . . . . . . .  52
            4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . .  52
   5.  Detailed Protocol Operation . . . . . . . . . . . . . . . . .  52
       5.1. Sender Initialization and Transmission . . . . . . . . .  54
            5.1.1. Object Segmentation Algorithm . . . . . . . . . .  55
       5.2. Receiver Initialization and Reception. . . . . . . . . .  57
       5.3. Receiver NACK Procedure. . . . . . . . . . . . . . . . .  57
       5.4. Sender NACK Processing and Response. . . . . . . . . . .  59
            5.4.1. Sender Repair State Aggregation . . . . . . . . .  60
            5.4.2. Sender FEC Repair Transmission Strategy . . . . .  61
            5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . .  62
            5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation. . . . . .  62
       5.5. Additional Protocol Mechanisms . . . . . . . . . . . . .  63
            5.5.1. Greatest Round-trip Time Collection . . . . . . .  63
            5.5.2. NORM Congestion Control Operation . . . . . . . .  64
            5.5.3. NORM Positive Acknowledgment Procedure. . . . . .  72
            5.5.4. Group Size Estimate . . . . . . . . . . . . . . .  74
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  75
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  75
   8.  Suggested Use . . . . . . . . . . . . . . . . . . . . . . . .  75
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  76
   10. References. . . . . . . . . . . . . . . . . . . . . . . . . .  76
       10.1. Normative References. . . . . . . . . . . . . . . . . .  76
       10.2. Informative References. . . . . . . . . . . . . . . . .  77
   11. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  79
       Full Copyright Statement. . . . . . . . . . . . . . . . . . .  80

1.  Introduction and Applicability

   The Negative-acknowledgment (NACK) Oriented Reliable Multicast (NORM)
   protocol is designed to provide reliable transport of data from one
   or more sender(s) 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 designed to be self-adapting to a wide
   range of dynamic network conditions with little or no pre-
   configuration.  The protocol is purposely designed to be tolerant of
   inaccurate timing estimations or lossy conditions that may occur in
   many networks including mobile and wireless.  The protocol is also
   designed to exhibit convergence and efficient operation even in
   situations of heavy packet loss and large queuing or transmission

   This document is a product of the IETF RMT WG and follows the
   guidelines provided in RFC 3269 [1].  The key words "MUST", "MUST
   "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in BCP 14, RFC 2119 [2].

Statement of Intent

   This memo contains part of the definitions necessary to fully specify
   a Reliable Multicast Transport protocol in accordance with RFC 2357.
   As per RFC 2357, the use of any reliable multicast protocol in the
   Internet requires an adequate congestion control scheme.

   While waiting for such a scheme to be available, or for an existing
   scheme to be proven adequate, the Reliable Multicast Transport
   working group (RMT) publishes this Request for Comments in the
   "Experimental" category.

   It is the intent of RMT to re-submit this specification as an IETF
   Proposed Standard as soon as the above condition is met.

1.1.  NORM 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 that has been chosen via means outside
   the context of the given NormSession.  Other IETF data format and
   protocol standards exist that may be applied to describe and convey
   the required "a priori" information for a specific NormSession (e.g.,
   Session Description Protocol (SDP) [7], Session Announcement Protocol
   (SAP) [8], 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
   independent of one another and receivers will maintain state as
   necessary for each sender.  However, in future versions of NORM, it
   is possible that some aspects of protocol operation (e.g., round-trip
   time collection) may 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 should be
   allocated 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
   designed to be 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 in its message headers.  Note
   the NORM_INFO out-of-band data mechanism could be leveraged by the
   application for this purpose if desired, or identification could
   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.  Each sender
   maintains its NormTransportId assignments independently so that
   individual NormObjects may be uniquely identified during transport
   with the concatenation of the sender session-unique identifier
   (NormNodeId) and the assigned NormTransportId.  The NormTransportIds
   are assigned from a large, but fixed, numeric space in increasing
   order and may be reassigned during long-lived sessions.  The NORM
   protocol provides mechanisms so that the sender application may
   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 that reliable multicast application variants may
   construct 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.

1.2.  NORM Scalability

   Group communication scalability requirements lead to adaptation of
   negative acknowledgment (NACK) based protocol schemes when feedback
   for reliability is required [9].  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 [10].  FEC-based repair can be used to
   greatly reduce the quantity of reliable multicast repair requests and
   repair transmissions [11] 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 [12].  NORM dynamically measures the
   group's roundtrip 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 "advertises" the feedback state to the group to
   facilitate feedback suppression.  In typical Internet environments,
   it is expected that 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 [13].
   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
   that NORM may scale to larger group sizes.  With respect to computer
   resource usage, the NORM protocol does _not_ require that state be
   kept on all receivers in the group.  NORM senders maintain state only
   for receivers providing explicit congestion control feedback.  NORM
   receivers must maintain state for each active sender.  This may
   constrain the number of simultaneous senders in some uses of NORM.

1.3.  Environmental Requirements and Considerations

   All of the environmental requirements and considerations that apply
   to the RMT NORM Building Block [4] and the RMT FEC Building Block [5]
   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 RFC 1112 [3], but SHALL also be capable of
   scalable operation in asymmetric topologies such as Source Specific
   Multicast (SSM) [14] where there may only be unicast routing service
   from the receivers to the sender(s).

   NORM is compatible with IPv4 and IPv6.  Additionally, NORM may be
   used with networks employing Network Address Translation (NAT)
   providing 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 possible multiple
   senders and to distinguish feedback information from different
   receivers.  There are two reserved NormNodeId values.  A value of
   0x00000000 is considered an invalid NormNodeId value and a value of
   0xffffffff is a "wildcard" NormNodeId.  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 coordination.

   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
   provide by TCP for unicast data transport.  The format of the stream
   content is application-defined and may 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, an 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 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 must receive a sufficient number of symbols to reconstruct
   (via FEC decoding) the original user data for the given block.  In
   this document, the terms "symbol" and "segment" are used

   Transmitted NormObjects are temporarily yet uniquely identified
   within the NormSession context using the given sender's NormNodeId,
   NormInstanceId, and a temporary NormObjectTransportId.  Depending
   upon the implementation, individual NORM senders may manage their
   NormInstanceIds independently, or a common NormInstanceId may be
   agreed upon for all participating nodes within a session if needed as
   a session identifier.  NORM NormObjectTransportId 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 NormObjectTransportId field can wrap and
   previously-used identifiers may be re-used.  Note that globally
   unique identification of transported data content is not provided by
   NORM and, if required, must be managed by the NORM application.  The
   individual segments or symbols of the NormObject are further
   identified with FEC payload identifiers which include coding block
   and symbol identifiers.  These are discussed in detail later in this

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 optionally
   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 option may be sensible for certain
   network conditions and can allow for robust, asymmetric multicast
   (e.g., unidirectional routing, satellite, cable) [15] 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.  NORM_INFO may
   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, round trip time 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.
   These procedures are: 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.).

   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 available for use.  All sender and receiver 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 may be 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.

2.2.  Protocol Building Blocks

   The operation of the NORM protocol is based primarily upon the
   concepts presented in the Nack-Oriented Reliable Multicast (NORM)
   Building Block document [4].  This includes the basic NORM
   architecture and the data transmission, repair, and feedback
   strategies discussed in that document.  Additional reliable multicast
   building blocks are applied in creating the full NORM protocol
   instantiation [16].  NORM also makes use of Forward Error Correction
   encoding techniques for repair messaging and optional transmission
   robustness as described in [10].  NORM uses the FEC Payload ID as

   specified by the FEC Building Block Document [5].  Additionally, for
   congestion control, this document includes a baseline congestion
   control mechanism (NORM-CC) based on the TCP-Friendly Multicast
   Congestion Control (TFMCC) scheme described in [19].

2.3.  Design Tradeoffs

   While the various features of NORM are designed to 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 tradeoffs involved
   in reliable multicast transport design and this requires 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 be designed to
   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 tradeoffs 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 Protocol Instantiation document, in conjunction with the RMT
   Building Block documents of [4] and [5], completely specifies a
   working reliable multicast transport protocol that conforms to the
   requirements described in RFC 2357 [17].

   This document specifies the following message types and mechanisms
   which 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 may be 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 which 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

   As mentioned in Section 2.1, there are two primary classes of NORM
   messages: 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.  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 designed to be compatible with the 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                           |

                     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 which may be 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 header extensions that may be applied.  The presence of
   header extensions are 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 as a monotonically increasing number incremented with each
   NORM message transmitted to a given destination address.  A
   "sequence" field number space SHOULD be maintained for messages sent
   to the NormSession group address.  This value can be monitored by
   receiving nodes to detect packet losses in the transmission from a
   sender and used in estimating raw packet loss for congestion control
   purposes.  Note that this value is NOT used in the NORM protocol to
   detect missing reliable data content and does NOT identify the
   application data or FEC payload that may be attached.  With message
   authentication, the "sequence" field may also be leveraged for
   protection from message "replay" attacks, particularly of NORM_NACK
   or other feedback messages.  In this case, the receiver node should
   maintain a monotonically increasing "sequence" field space for each
   destination to which it transmits (this may be multiple destinations
   when unicast feedback is used).  The size of this field is intended
   to be sufficient to allow detection of a reasonable range of packet
   loss within the delay-bandwidth product of expected network

   The "source_id" field is a 32-bit value identifying the node that
   sent the message.  A participant's NORM node identifier (NormNodeId)
   can be set according to application needs but unique identifiers must
   be assigned within a single NormSession.  In some cases, use of the
   host IP address or a hash of it can suffice, but alternative
   methodologies for assignment and potential collision resolution of
   node identifiers within a multicast session need to be considered.
   For example, the "source identifier" mechanism defined in the Real-
   Time Protocol (RTP) specification [18] may be applicable to use for
   NORM node identifiers.  At this point in time, the protocol makes no
   assumptions about how these unique identifiers are actually assigned.

   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.  These formats
   are 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 <=127   |      hel      |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                    Header Extension Content                   |
   |                              ...                              |

              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   |
           NORM Fixed Length (32-bit) Header Extension Format

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

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 expected to be the predominant type
   transmitted by NORM senders.  These messages are used to encapsulate
   segmented data content for objects of type NORM_OBJECT_DATA,
   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_reserved*       |          payload_len*         |
   |                        payload_offset*                        |
   |                          payload_data*                        |
   |                              ...                              |

                        NORM_DATA Message Format

   *NOTE:  The "payload_reserved", "payload_len" and "payload_offset"
   fields are present only for objects of type NORM_OBJECT_STREAM.  The
   "payload_len" and "payload_offset" 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 algorithm described
   in Section 5.1.1.  The "payload_reserved" field is kept for
   anticipated future NORM stream control functions.  When systematic
   FEC codes (e.g., "fec_id" = 129) are used, the "payload_len" and
   "payload_offset" fields contain actual length and offset values for
   the encapsulated application data segment for those NORM_DATA
   messages containing source data symbols.  In NORM_DATA messages that
   contain parity information, these fields are not actual length or

   offset values, but instead are values computed from FEC encoding the
   "payload_len" and "payload_offset" fields of the _source_ data
   symbols of the corresponding applicable coding block.

   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 (32-bit words) plus the size of the
   "fec_payload_id" field.  The "fec_payload_id" field size depends upon
   the FEC encoding used for the referenced NormObject.  The "fec_id"
   field is used to indicate the FEC coding type.  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"

   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.

   The "grtt" field contains a non-linear quantized representation of
   the sender's current estimate of group round-trip time (GRTT) (this
   is also referred to as R_max in [19]).  This value is used to control
   timing of the NACK repair process and other aspects of protocol
   operation as described in this document.  The algorithm for encoding
   and decoding this field is described in the RMT NORM Building Block
   document [4].

   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
   which is multiplied by the sender GRTT to determine the maximum
   backoff timeout.  The "backoff" field informs the receiver set of the
   sender's backoff factor parameter "Ksender".  Recommended values and
   their use 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.  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 examples, 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" = 0x4) 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 regarding how the receiver should
   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 an 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_MSG_START | 0x40  | Marks the first segment of application  |
|                    |       | messages embedded in                    |
|                    |       | NORM_OBJECT_STREAMs.                    |

   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" (previously untransmitted) 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.  Note that receivers may 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
   should 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_FLAG_MSG_START can be optionally used to mark the first data
   segments of application-layer messages transported within the NORM
   stream.  This allows NORM receiver applications to "synchronize" with
   NORM senders and to be able to properly interpret application layer
   data when joining a NORM session already in progress.  In practice,
   the NORM implementation MAY set this flag for the segment transmitted
   following an explicit "flush" of the stream by the application.

   The "fec_id" field corresponds to the FEC Encoding Identifier
   described in the FEC Building Block document [5].  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 the NORM_OBJECT_STREAM
   requires systematic FEC codes for most efficient performance.

   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 may
   be repeated, but it is presumed that the 16-bit field size provides
   an adequate enough 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 FEC Building Block document [5] and any subsequent
   extensions of that document.  As an example, the format of the
   "fec_payload_id" format small block, systematic codes ("fec_id" =
   129) 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       |

   Small Block, Systematic Code ("fec_id" = 129) "fec_payload_id" Format

   The FEC payload identifier "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 given by the IETF FEC Building Block document
   [5].  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" may 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.  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 may be a user data or an 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 (as described in the
   FEC Building Block document [5]) is required to properly receive and
   decode NORM transport objects.  This information MAY be provided as
   out-of-band session information.  However, in some cases, it may be
   useful for the sender to include this information "in band" to
   facilitate receiver operation with minimal preconfiguration.  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 exact format of the extension depends upon the FEC
   code in use, but in general it SHOULD contain any required details on
   the FEC code in use (e.g., FEC Instance ID, etc.) and the byte size
   of the associated NormObject (For the NORM_OBJECT_STREAM type, this
   size corresponds to the stream buffer size maintained by the NORM
   sender).  As an example, the format of the EXT_FTI for small block
   systematic codes ("fec_id" = 129) 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_length (msb)      |
   |                      object_length (lsb)                      |
   |       fec_instance_id         |          segment_size         |
   |       fec_max_block_len       |         fec_num_parity        |

   FEC Object Transmission Information Header Extension (EXT_FTI) for
   Small Block Systematic Codes ("fec_id" = 129)

   The header extension type "het" field value for this header extension
   is 64.  The header extension length "hel" depends upon the format of
   the FTI for FEC code type identified by the "fec_id" field.  In this
   example (for "fec_id" = 129), the "hel" field value is 4.

   The 48-bit "object_length" field indicates the total size 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 may 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_length"

   field is used by the sender to indicate 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.

   The "fec_instance_id" corresponds to the "FEC Instance ID" described
   in the FEC Building Block document [5].  In this case, the
   "fec_instance_id" SHALL be 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 [5].  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.

   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
   for FEC Object Transmission Information for Small Block Systematic
   Codes in the FEC Building Block document [5].  For example, Reed-
   Solomon codes may 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 "payload_reserved", "payload_len" and "payload_offset" fields are
   present ONLY for transport objects of type NORM_OBJECT_STREAM.  These
   fields indicate the size and relative position (within the stream) of
   the application content represented by the message payload.  For
   senders employing systematic FEC encoding, these fields contain
   _actual_ length and offset values (in bytes) for the payload of
   messages which contain original data source symbols.  For NORM_DATA
   messages containing calculated parity content, these fields will
   actually contain values computed by FEC encoding of the "payload_len"
   and "payload_offset" values of the NORM_DATA data segments of the
   corresponding FEC coding block.  Thus, the "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 NOT present when the "flags" portion of the NORM_DATA
   message indicate the transport object if of type NORM_OBJECT_FILE or
   NORM_OBJECT_DATA.  In this case, the length and offset information
   can be calculated from the "fec_payload_id" using the methodology
   described in Section 5.1.1.  Note that for long-lived streams, the
   "payload_offset" field can 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 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 may possibly vary on a per-object basis.  The
   NormSegmentSize is expected to be configurable by the sender
   application prior to session participation as needed for network
   topology maximum transmission unit (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 algorithm described in
   Section 5.1.1.  For NORM_OBJECT_STREAM objects, the length and offset
   is obtained from the segment's corresponding "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 may use the NORM_INFO
   content to make a decision as whether to participate in reliable
   reception of the associated object.  Each NormObject can have an
   independent unit of NORM_INFO associated with it.  NORM_DATA messages
   contain a flag to indicate the availability of NORM_INFO for a given
   NormObject.  NORM receivers may 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                          |
   |                              ...                              |

                        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 "hdr_len" field when no header extensions are present is 4.

   The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
   "object_transport_id" fields carry the same information and serve the
   same purpose as with 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

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

   The NORM_INFO "payload_data" field contains sender application-
   defined content which 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 may have dynamic
   content attached.  Any attached content will be limited to maximum
   length of the sender NormSegmentSize to retain the atomic nature of
   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 |
   |     flavor    |                                               |
   +-+-+-+-+-+-+-+-+        NORM_CMD Content                       +
   |                              ...                              |

                        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 "flavor" field.

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

|       Command        |Flavor Value |            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 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 which may need to      |
|                      |             | temporarily preempt 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 may indicate a temporary or permanent end of data
   transmission, but the sender is still willing to respond to repair
   requests.  This command is repeated once per 2*GRTT 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.  If a NORM_NACK message interrupts the flush process, the
   sender will 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 2*GRTT*NORM_ROBUST_FACTOR and will be discussed more
   later.  With a sufficient NORM_ROBUST_FACTOR value, data content is
   delivered with a high assurance of reliability.  The penalty of a
   large NORM_ROBUST_FACTOR value is potentially excess sender
   NORM_CMD(FLUSH) transmissions and a longer timeout for receivers to
   self-initiate the terminal NACK process.

   For finite-size 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 may occur at any time, including in the
   middle of an FEC coding block if systematic FEC codes are employed.
   In this case, the sender will not yet be able to provide FEC parity
   content as repair for the concurrent coding block and will be limited
   to explicitly repairing stream 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 on-going series of intermittent, relatively small messaging
   content 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 |
   |   flavor = 1  |    fec_id     |      object_transport_id      |
   |                         fec_payload_id                        |
   |                              ...                              |
   |                acking_node_list (if applicable)               |
   |                              ...                              |

                     NORM_CMD(FLUSH) Message Format

   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" which 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 an 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.  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 |
   |   flavor = 2  |                    reserved                   |

                      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 which 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 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, it is expected 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    |          sequence             |
   |                           source_id                           |
   |          instance_id          |     grtt      |backoff| gsize |
   |  flavor = 3   |     fec_id    |      object_transport_id      |
   |                         fec_payload_id                        |
   |                              ...                              |
   |                        invalid_object_list                    |
   |                              ...                              |

                    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 which initiated the squelch transmission.

   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 is 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 may not be capable of listing the
   entire set of invalid objects in the repair window.  In this case,

   the sender SHALL ensure that the list begins with a NormObjectId that
   is greater than or equal to the lowest ordinal invalid NormObjectId
   from the NACK message(s) that prompted the NORM_CMD(SQUELCH)
   generation.  The NormObjectIds in the "invalid_object_list" MUST be
   greater than the "object_transport_id" marking the beginning of the
   sender's repair window.  This insures 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" based 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) messages contains fields to enable sender-to-
   receiver group greatest round-trip time (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 [19] is described in
   Section 5.5.2 of this document.  The NORM_CMD(CC) message is usually
   transmitted as part of NORM-CC congestion control 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 private network with congestion
   control operation disabled, the NORM_CMD(CC) message is then used for
   GRTT measurement only and may optionally 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 |
   |   flavor = 4  |    reserved   |          cc_sequence          |
   |                         send_time_sec                         |
   |                        send_time_usec                         |
   |               header extensions (if applicable)               |
   |                              ...                              |
   |                  cc_node_list (if applicable)                 |
   |                              ...                              |

                      NORM_CMD(CC) Message Format

   The NORM common message header and standard NORM_CMD fields serve
   their usual purposes.

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

   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 [19].  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           |

            NORM-CC Rate Header Extension Format (EXT_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 exponent (order of magnitude) information in the least
   significant portion.  The 12-bit mantissa portion of the field is
   scaled such that a floating point value of 0.0 corresponds to 0 and a
   floating point value of 10.0 corresponds to 4096.  Thus:

   send_rate = (((int)(Value_mantissa * 4096.0 / 10.0 + 0.5)) << 4) |

   For example, to represent a transmission rate of 256kbps (3.2e+04
   bytes per second), the lower 4 bits of the 16-bit field contain a
   value of 0x04 to represent the exponent while the upper 12 bits
   contain a value of 0x51f as determined from the equation given above:

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) * 10.0 / 4096.0 *
        power(10.0, (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" may be attached as
   the payload of the NORM_CMD(CC) message.  The presence of this header
   extension also implies that NORM receivers should 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 may be
   configurable for more timely feedback to the group.  The list length
   can be inferred from the length of the NORM_CMD(CC) message.

   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            |

              Congestion Control Node List Item Format

   The "cc_node_id" is the NormNodeId of the receiver which 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 NORM Building Block
   document [4].  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 "echoing" 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 [13].

      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 |
   |  flavor = 5   |     flags     |            reserved           |
   |               header extensions (if applicable)               |
   |                              ...                              |
   |                       repair_adv_payload                      |
   |                              ...                              |

                  NORM_CMD(REPAIR_ADV) Message Format

   The "instance_id", "grtt", "backoff", "gsize", and "flavor" 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 provide 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 should limit their NACK response to generating NACK
   content only up through the maximum ordinal transmission position
   (objectId::fecPayloadId) included in the "repair_adv_content".

   When congestion control operation is enabled, a header extension may
   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 may be used with 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) [20]) 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:

   |     het = 3   |    hel = 3    |          cc_sequence          |
   |    cc_flags   |     cc_rtt    |            cc_loss            |
   |            cc_rate            |          cc_reserved          |

           NORM-CC Feedback Header Extension (EXT_CC) Format

   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 may 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
   should 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 should be set only when no feedback has been
   received from non-CLR or non-PLR receivers.  And the
   NORM_FLAG_CC_LEAVE should 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"

   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" = 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 receivers 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 |
   |  flavor = 6   |    reserved   |    ack_type   |    ack_id     |
   |                       acking_node_list                        |
   |                              ...                              |

                    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 sender.

   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 may implement application-defined,
   positively-acknowledged commands that are able to leverage internal
   transmission and round-trip timing information available to the NORM
   protocol implementation.

   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 may 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 acknowledge
   (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.  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
   may include initiation of data transmission, other
   NORM_CMD(APPLICATION) messages, or even application-defined,
   positively-acknowledge commands from other NormSession participants.
   The transmission of these commands will preempt data transmission
   when they are scheduled and may 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
   requires 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 |
   |  flavor = 7   |                    reserved                   |
   |                   Application-Defined Content                 |
   |                              ...                              |

                  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

4.3.  Receiver Messages

   The NORM message types generated by participating receivers consist
   of 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                         |
   |                              ...                              |

                        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 which contain
   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 differential from  when the receiver received the NORM_CMD(CC)

   to when the NORM_NACK is transmitted to calculate the value in the
   "grtt_response" field.  This is the
   "receive_to_response_differential" value used in the following

   "grtt_response" = NORM_CMD(CC) "send_time" +

   The receiver SHALL set the "grtt_response" to a ZERO value, to
   indicate that it has not yet received a NORM_CMD(CC) message from the
   indicated sender and that the sender should 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 receivers current
   state with respect to congestion control operation.  Note that
   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_content" 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 requires from the
   sender in order 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 may be concatenated within the "nack_payload" field
   of a NORM_NACK message.  Note that 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                     |
   |                             ...                               |

                       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 that the
   "repair_request_items" list consists of pairs of repair request items
   that correspond to inclusive ranges of repair needs.  And the
   NORM_NACK_ERASURES "form" indicates that the repair request items are
   to be treated individually and that 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 are required as repair.     |
|NORM_NACK_BLOCK   | 0x02  | Indicates the listed block(s) or range  |
|                  |       | of blocks in entirety are required as   |
|                  |       | repair.                                 |
|NORM_NACK_INFO    | 0x04  | Indicates that NORM_INFO is required as |
|                  |       | repair for the listed object(s).        |
|NORM_NACK_OBJECT  | 0x08  | Indicates the listed object(s) or range |
|                  |       | of objects in entirety are required 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
   requires transmissions sufficient to repair the indicated block(s) in
   their entirety.  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 may be set in
   combination with the NORM_NACK_BLOCK or NORM_NACK_SEGMENT flags, or
   may 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                         |
   |                              ...                              |

                    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 required 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 may 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 2,5,8

    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.  As
   mentioned in the NORM_CMD(ACK_REQ) message description, the
   acknowledgment type NORM_ACK_CC is provided for this purpose.  The
   generation of NORM_ACK(CC) messages for round-trip timing estimation
   and congestion-control operation is described in Sections 5.5.1 and
   5.5.2, respectively.  However, some multicast applications may
   benefit from some limited form of positive acknowledgment for certain
   functions.  A simple, scalable positive acknowledgment scheme is
   defined in Section 5.5.3 that can be leveraged by protocol
   implementations when appropriate.  The NORM_CMD(FLUSH) may 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 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)                 |
   |                              ...                              |

                        NORM_ACK Message Format

   The NORM common message header fields serve their usual purposes.

   The "server_id", "instance_id",  and "grtt_response" fields serve the
   same purpose as the corresponding fields in NORM_NACK messages.  And
   header extensions may 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 that the sender can
   verify that a NORM_ACK message received 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                         |
   |                              ...                              |

                  NORM_ACK_FLUSH "ack_payload" Format

   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 the NormSegmentSize of the sender referenced by the

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 could 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 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
   protocol operation is given here:

   1) The sender periodically transmits NORM_CMD(CC) messages as needed
      to initialize and collect roundtrip 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.  NORM_INFO messages may optionally precede the
      transmission of data content for NORM transport objects.

   3) As receivers detect missing content from the sender, they initiate
      repair requests with NORM_NACK messages.  Note the receivers track
      the sender's most recent objectId::fecPayloadId transmit position
      and NACK _only_ for content ordinally prior to that 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

   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.  Previously untransmitted FEC parity
      content 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 expected to be 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 require 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
   operating in the general Internet.  Even if congestion control
   operation is disabled at the sender, it may 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 may wish for the sender to also proceed
   with data transmission immediately.  In other cases, the sender may
   wish to defer data transmission until it has received some feedback
   or request from the receiver set indicating that 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, 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 that 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.  The
   segmentation algorithm is described in Section 5.1.1.  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.  Receivers may
   respond to these NORM_CMD(FLUSH) messages with additional repair
   requests.  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 function
   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
   that appropriate NACKs can be constructed to request repair of
   missing data.  NORM FEC coding blocks are comprised of multi-byte
   symbols which are transmitted in the payload of NORM_DATA messages.
   Each NORM_DATA message contains one source or encoding symbol and the
   NormSegmentSize sender parameter defines the maximum symbol size in
   bytes.  The FEC encoding type and associated parameters govern the
   source block size (number of source symbols per coding block).  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 Transmission Information for each object.  The algorithm
   given below is used to compute 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 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.

   The NORM block segmentation algorithm is defined as follows.  For a
   transport object of a given length (L_obj) in bytes, a first number
   of FEC source blocks (N_large) is delineated of a larger block size
   (B_large), and a second number of source blocks (N_small) is
   delineated of a smaller block size (B_small).  Given the maximum FEC
   source block size (B_max) and the sender's NormSegmentSize, the block
   segmentation for a given NORM transport object is determined as


   B_max = Maximum source block length (i.e., maximum number of source
           symbols per source block)

   L_sym = Encoding symbol length in bytes (i.e., NormSegmentSize)

   L_obj = Object length in bytes


   N_total = The total number of source blocks into which the transport
             object is partitioned.

   N_large = Number of larger source blocks (first set of blocks)

   B_large = Size (in encoding symbols) of the larger source blocks

   N_small = Number of smaller source blocks (second set of blocks)

   B_small = Size (in encoding symbols) of the smaller source blocks

   L_final = Length (in bytes) of the last source symbol of the last
             source block (All other symbols are of length L_sym).


   1) The total number of source symbols in the transport object is
      computed as:  S_total = L_obj/L_sym [rounded up to the nearest

   2) The transport object is partitioned into N_total source blocks,
      where:  N_total = S_total/B_max [rounded up to the nearest

   3) The average length of a source block is computed as:  B_ave =
      S_total/N_total (this may be non-integer)

   4) The size of the first set of larger blocks is computed as:
      B_large = B_ave [rounded up to the nearest integer] (Note it will
      always be the case that B_large <= B_max)

   5) The size of the second set of smaller blocks is computed as:
      B_small = B_ave [rounded down to the nearest integer] (Note if
      B_ave is an integer B_small = B_large; otherwise B_small = B_large
      - 1)

   6) The fractional part of B_ave is computed as:  B_fraction = B_ave -

   7) The number of larger source blocks is computed as:  N_large =
      B_fraction * N_total (Note N_large is an integer in the range 0
      through N_total - 1)

   8) The number of smaller source blocks is computed as:  N_small =
      N_total - N_large

   9) Each of the first N_large source blocks consists of B_large source
      symbols.  Each of the remaining N_small source blocks consists of
      B_small source symbols.  All symbols are L_sym bytes in length
      except for the final source symbol of the final source block which
      is of length (in bytes):
      L_final = L_obj - (N_large*B_large + N_small*B_small - 1) * L_sym

5.2.  Receiver Initialization and Reception

   The NORM protocol is designed such that receivers may join and leave
   the group at will.  However, some applications may be constrained
   such that receivers need to be members of the group prior to start of
   data transmission.  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 wish to 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" may be appropriate for different applications and/or
   scenarios.  For general purpose operation, 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 that the join policy constrain
   receivers to start reliable reception at the current FEC coding block
   for which non-repair content is received.

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, and upon receipt of a NORM_CMD(FLUSH) message.

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


        T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)

   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 (Ksender-
   1)*GRTTsender.  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
      objectId::fecPayloadId) exceeds the earliest repair position of
      the receiver.

   2) The repair state accumulated from NORM_NACK and
      NORM_CMD(REPAIR_ADV) messages do 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 reinitiate 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 [4] is:

                   T_rcvrHoldoff =(Ksender+2)*GRTTsender

   The NORM_NACK message contains repair request content beginning with
   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 that 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 that the sender may have provided 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
   required 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
   requires no 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 principle goal of the sender is to make forward progress in the
   transmission of data its application has enqueued.  However, the
   sender must 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 [4], the period of time during which the sender
   aggregates NORM_NACK messages is equal to:

                    T_sndrAggregate = (Ksender+1)*GRTT

   where "Ksender" is the same backoff scaling value used by the
   receivers, and "GRTT" is the sender's current estimate of the group's
   greatest round-trip time.

   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 [4], the value of
   this sender "holdoff" period is:

                         T_sndrHoldoff = (1*GRTT)

   If additional NORM_NACK messages are received during this sender
   "holdoff" period, the sender will immediately incorporate these "late
   messages" into its pending transmission state 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 begun (upon
   arrival of a NACK) in concert with the pending repair and new data
   transmission.  Recall that 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 the
   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.  Only after exhausting its
   supply of "fresh" (unsent) parity segments for a given coding block
   should the sender resort to explicit transmission of the receiver
   set's repair needs.  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 reliable 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 that limit new receivers from joining the reliable
   transmission when only repair transmissions have been received.
   Additionally, the sender SHOULD additionally flag NORM_DATA
   transmissions sent as explicit repair with the NORM_FLAG_EXPLICIT

   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 be
   required 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 additional
   NORM_NACK suppression/aggregation as it conducts this repair state
   accumulation for NORM repair cycles.  The detail of this type of
   operation are beyond the scope of this document, but this information
   is provided for possible future consideration.

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.  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.  Lower ordinal invalid "object_transport_ids" should be
   included only while the NORM_CMD(SQUELCH) payload is less than 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.  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 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 are
   subject to the sender transmit rate limit and NormSegmentSize
   limitation.  When the NORM_CMD(REPAIR_ADV) message is of maximum
   size, 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 may also
   need to provide information so that dynamic congestion control
   feedback can be suppressed as needed 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 that help NORM
   to adapt to network conditions and play fairly with other coexistent

5.5.1.  Greatest Round-trip Time Collection

   For NORM receivers to appropriately scale backoff timeouts and the
   senders to use proper corresponding timeouts, the participants must
   agree on a common timeout basis.  Each NORM sender monitors the
   round-trip time of active receivers and determines the group greatest
   round-trip time (GRTT).  The sender advertises this GRTT estimate in
   every message it transmits so that receivers have this value
   available for scaling their timers.  To measure the current GRTT, the
   sender periodically sends NORM_CMD(CC) messages that contain 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
   weights and smoothes the values for a conservative estimate of the
   GRTT.  The algorithm and methodology are described in the NORM
   Building Block document [4] in the section entitled "One-to-Many
   Sender GRTT Measurement".  A conservative estimate helps 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 that the NORM-CC Rate header extension may also be
   used proactively solicit RTT feedback from the receiver group per
   congestion control operation even though the sender may not be
   conducting congestion control rate adjustment.  NORM operation
   without congestion control should be considered only in closed

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 described in
   [19].  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 (e.g., PGMCC [20]).  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 that the NORM protocol message set may
   alternatively be used to support a window-based multicast congestion
   control scheme such as PGMCC.  The details of that 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 that correspond to the lowest result transmission rate is
   identified as the "current limiting receiver" (CLR).

   As described in [21], a steady-state sender transmission rate, to be
   "friendly" with competing TCP flows can be calculated as:

Rsender = --------------------------------------------------------------
          tRTT * (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).

tRTT = The RTT estimate of the current "current limiting receiver"

   p = The 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:

   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 that current
      "worst path" in the group multicast topology.

   The format of the NORM_CMD(CC) message is describe 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 may be determined administratively or possibly
   algorithmically based on 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 details of PLR
   selection are not discussed in this document.  NORM_CMD(CC) Transmission

   The NORM_CMD(CC) message is transmitted periodically by the sender
   along with its normal data transmission.  Note that the repeated
   transmission of NORM_CMD(CC) messages may be initiated some time
   before transmission of user data content at session startup.  This
   may 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 GRTT of 0.5 seconds is recommended when
      no feedback has yet been received from the group.

   2) If a CLR has been identified (based on previous receiver
      feedback), the interval is the RTT between the sender and CLR.

   3) Additionally, if the interval of nominal data message transmission
      is greater than the GRTT or RTT_clr interval, the NORM_CMD(CC)
      interval is set to this greater value.  This ensures that 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 may 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 that 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
   may allow the sender to be more responsive to congestion control
   dynamics.  The length of the list may 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 are prioritized with following rules:

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

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

   There are "cc_flag" values in addition to NORM_FLAG_CC_CLR that are
   used for other congestion control functions.  The NORM_FLAG_CC_PLR
   flag value is used to mark additional receivers from that the sender
   would like to have immediate, non-suppressed feedback.  These may be
   receivers that the sender algorithmically identified as potential
   future CLRs or that 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 that the NORM sender has been pre-configured
   with a set of PLR nodes, feedback from those receivers may not yet
   have been collected and thus the "cc_rtt" and "cc_rate" fields do not
   contain valid values when this flag is not set.  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 that are 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 that 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*GRTTsender, GSIZEsender)

   The "RandomBackoff()" algorithm provides a truncated exponentially
   distributed random number and is described in the NORM Building Block
   document [4].  The same backoff factor K = Ksender MAY be used as
   with NORM_NACK suppression.  However, in cases where the application
   purposefully specifies a very small Ksender backoff factor to
   minimize the NACK repair process latency (trading off group size
   scalability), it may still be desirable to maintain a larger backoff
   factor for congestion control feedback, since there may often be a
   larger volume of congestion control feedback than NACKs in many cases
   and congestion control feedback latency may be tolerable where
   reliable delivery latency is not.  As previously noted, a backoff
   factor value of K = 4 is generally recommended for ASM operation and
   K = 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 conditions:

   1) The receiver generates another feedback message (NORM_NACK or
      other NORM_ACK) before the congestion control feedback timeout

   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.  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) may wish to compete as
      a receiver with no prior RTT measurement after some expiration

   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
   the "cc_sequence" value from that command in the applicable 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 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*GRTT)

   Thus, non-CLR receivers are constrained to providing explicit
   congestion control feedback once per K*GRTT intervals.  Note,
   however, that 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 [19], 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 either determined 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 can use the "cc_rtt"
   value provided in the NORM-CC Feedback header extension as the
   receiver's previous RTT measurement (RTT_rcvrPrev) to smooth
   according to:

             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 that
   if a PLR is "promoted" to CLR status, the smoothed estimate can be

   There are some additional periods besides steady-state operation that
   need 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:

   Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.

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

   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 rate.  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 that the sender does not have
   explicit knowledge that 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

   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 observer "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 that are 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), which is 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 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 for receivers that should 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 that generates
   NORM_CMD(FLUSH) or NORM_CMD(ACK_REQ) messages in periodic "rounds".
   For NORM_ACK_FLUSH requests, the NORM_CMD(FLUSH) contain 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 which is
   similarly "echoed" in response so that the sender may match the
   response to the appropriate request.

   In response to the NORM_CMD(ACK_REQ), the listed receivers randomly
   spread NORM_ACK messages uniformly in time over a window of (1*GRTT).
   These NORM_ACK messages are typically unicast to the sender.  (Note
   that 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 in that:

   1) Only a single NORM_CMD(ACK_REQ) message is generated once per
      (2*GRTT), 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 may be
   required 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 required 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

   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 is used in scaling random backoff timer ranges.
   The use of the group size estimate within the NORM protocol does not
   require 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 possible that group size may be algorithmically approximated
   from the volume of congestion control feedback messages which follow
   the exponentially weighted random backoff.  However, the
   specification of such an algorithm is currently beyond the scope of
   this document.

6.  Security Considerations

   The same security considerations that apply to the NORM, and FEC
   Building Blocks also apply to the NORM protocol.  In addition to
   vulnerabilities that any IP and IP multicast protocol implementation
   may be generally subject to, the NACK-based feedback of NORM may be
   exploited by replay attacks which force the NORM sender to
   unnecessarily transmit repair information.  This MAY be addressed by
   network layer IP security implementations that guard against this
   potential security exploitation.  It is RECOMMENDED that such IP
   security mechanisms be used when available.  Another possible
   approach is for NORM senders to use the "sequence" field from the
   NORM Common Message Header to detect replay attacks.  This can be
   accomplished if the NORM packets are cryptographically protected and
   the sender is willing to maintain state on receivers which are
   NACKing.  A cache of receiver state may provide some protection
   against replay attacks.  Note that the "sequence" field of NORM
   messages should be incremented with independent values for different
   destinations (e.g., group-addressed versus unicast-addressed messages
   versus "receiver" messages).  Thus, the congestion control loss
   estimation function of the "sequence" field can be preserved for
   sender messages when receiver messages are unicast to the sender.
   The NORM protocol is compatible with the use of the IP security
   (IPsec) architecture described in [22].  It is important to note that
   while NORM does leverage FEC-based repair for scalability, this does
   not alone guarantee integrity of received data.  Application-level
   integrity-checking of data content is highly RECOMMENDED.

7.  IANA Considerations

   No information in this specification is currently subject to IANA
   registration.  However, several Header Extensions are defined within
   this document.  If/when additional Header Extensions are developed,
   the first RFC MUST establish an IANA registry for them, with a
   "Specification Required" policy [6] and all Header Extensions,
   including those in the present document, MUST be registered
   thereafter.  Additionally, building blocks components used by NORM
   may introduce additional IANA considerations.  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 [5].

8.  Suggested Use

   The present NORM protocol is seen as 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 misordering.  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 may
   be 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.,
   DBS/cable/PSTN hybrids) and efficient, reliable bulk data transfer
   will be an important capability for servicing large groups of
   subscribed receivers.

9.  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.

10.  References

10.1.  Normative References

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

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

   [3]  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
        1112, August 1989.

   [4]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
        "Negative-Acknowledgment (NACK)-Oriented Reliable Multicast
        (NORM) Building Blocks", RFC 3941, November 2004.

   [5]  Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
        J. Crowcroft, "Forward Error Correction (FEC) Building Block",
        RFC 3452, December 2002.

   [6]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

10.2.  Informative References

   [7]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

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

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

   [10] 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.

   [11] Macker, J. and B. Adamson, "The Multicast Dissemination Protocol
        (MDP) Toolkit", Proc. IEEE MILCOM 99, October 1999.

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

   [13] J. Macker, B. Adamson, "Quantitative Prediction of Nack Oriented
        Reliable Multicast (NORM) Feedback", Proc. IEEE MILCOM 2002,
        October 2002.

   [14] H.W. Holbrook, "A Channel Model for Multicast", Ph.D.
        Dissertation, Stanford University, Department of Computer
        Science, Stanford, California, August 2001.

   [15] D. Gossink, J. Macker, "Reliable Multicast and Integrated Parity
        Retransmission with Channel Estimation", IEEE GLOBECOMM 98',
        September 1998.

   [16] 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.

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

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

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

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

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

   [22] Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

11.  Authors' Addresses

   Brian Adamson
   Naval Research Laboratory
   Washington, DC, USA, 20375

   EMail: adamson@itd.nrl.navy.mil

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28334 Bremen, Germany

   EMail: cabo@tzi.org

   Mark Handley
   Department of Computer Science
   University College London
   Gower Street
   WC1E 6BT

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

   Joe Macker
   Naval Research Laboratory
   Washington, DC, USA, 20375

   EMail: macker@itd.nrl.navy.mil

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