faqs.org - Internet FAQ Archives

RFC 2816 - A Framework for Integrated Services Over Shared and S


Or Display the document by number




Network Working Group                                         A. Ghanwani
Request for Comments: 2816                                Nortel Networks
Category: Informational                                           W. Pace
                                                                      IBM
                                                            V. Srinivasan
                                                    CoSine Communications
                                                                 A. Smith
                                                         Extreme Networks
                                                                M. Seaman
                                                                  Telseon
                                                                 May 2000

                  A Framework for Integrated Services
           Over Shared and Switched IEEE 802 LAN Technologies

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This memo describes a framework for supporting IETF Integrated
   Services on shared and switched LAN infrastructure.  It includes
   background material on the capabilities of IEEE 802 like networks
   with regard to parameters that affect Integrated Services such as
   access latency, delay variation and queuing support in LAN switches.
   It discusses aspects of IETF's Integrated Services model that cannot
   easily be accommodated in different LAN environments.  It outlines a
   functional model for supporting the Resource Reservation Protocol
   (RSVP) in such LAN environments.  Details of extensions to RSVP for
   use over LANs are described in an accompanying memo [14].  Mappings
   of the various Integrated Services onto IEEE 802 LANs are described
   in another memo [13].

Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Document Outline . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Frame Forwarding in IEEE 802 Networks  . . . . . . . . . .  5
       4.1. General IEEE 802 Service Model  . . . . . . . . . . .  5
       4.2. Ethernet/IEEE 802.3 . . . . . . . . . . . . . . . . .  7
       4.3. Token Ring/IEEE 802.5 . . . . . . . . . . . . . . . .  8
       4.4. Fiber Distributed Data Interface  . . . . . . . . . . 10
       4.5. Demand Priority/IEEE 802.12 . . . . . . . . . . . . . 10
   5.  Requirements and Goals . . . . . . . . . . . . . . . . . . 11
       5.1. Requirements  . . . . . . . . . . . . . . . . . . . . 11
       5.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . 13
       5.3. Non-goals . . . . . . . . . . . . . . . . . . . . . . 14
       5.4. Assumptions . . . . . . . . . . . . . . . . . . . . . 14
   6.  Basic Architecture . . . . . . . . . . . . . . . . . . . . 15
       6.1. Components  . . . . . . . . . . . . . . . . . . . . . 15
             6.1.1. Requester Module  . . . . . . . . . . . . . . 15
             6.1.2. Bandwidth Allocator . . . . . . . . . . . . . 16
             6.1.3. Communication Protocols . . . . . . . . . . . 16
       6.2. Centralized vs.  Distributed Implementations  . . . . 17
   7.  Model of the Bandwidth Manager in a Network  . . . . . . . 18
       7.1. End Station Model . . . . . . . . . . . . . . . . . . 19
             7.1.1. Layer 3 Client Model  . . . . . . . . . . . . 19
             7.1.2. Requests to Layer 2 ISSLL . . . . . . . . . . 19
             7.1.3. At the Layer 3 Sender . . . . . . . . . . . . 20
             7.1.4. At the Layer 3 Receiver . . . . . . . . . . . 21
       7.2. Switch Model  . . . . . . . . . . . . . . . . . . . . 22
             7.2.1. Centralized Bandwidth Allocator . . . . . . . 22
             7.2.2. Distributed Bandwidth Allocator . . . . . . . 23
       7.3. Admission Control . . . . . . . . . . . . . . . . . . 25
       7.4. QoS Signaling . . . . . . . . . . . . . . . . . . . . 26
             7.4.1. Client Service Definitions  . . . . . . . . . 26
             7.4.2. Switch Service Definitions  . . . . . . . . . 27
   8.  Implementation Issues  . . . . . . . . . . . . . . . . . . 28
       8.1. Switch Characteristics  . . . . . . . . . . . . . . . 29
       8.2. Queuing . . . . . . . . . . . . . . . . . . . . . . . 30
       8.3. Mapping of Services to Link Level Priority  . . . . . 31
       8.4. Re-mapping of Non-conforming Aggregated Flows . . . . 31
       8.5. Override of Incoming User Priority  . . . . . . . . . 32
       8.6. Different Reservation Styles  . . . . . . . . . . . . 32
       8.7. Receiver Heterogeneity  . . . . . . . . . . . . . . . 33
   9.  Network Topology Scenarios   . . . . . . . . . . . . . . . 35
       9.1. Full Duplex Switched Networks . . . . . . . . . . . . 36
       9.2. Shared Media Ethernet Networks  . . . . . . . . . . . 37
       9.3. Half Duplex Switched Ethernet Networks  . . . . . . . 38
       9.4. Half Duplex Switched and Shared Token Ring Networks . 39

       9.5. Half Duplex and Shared Demand Priority Networks . . . 40
   10. Justification  . . . . . . . . . . . . . . . . . . . . . . 42
   11. Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 43
   References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
   Security Considerations  . . . . . . . . . . . . . . . . . . . 45
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 46
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . 47

1. Introduction

   The Internet has traditionally provided support for best effort
   traffic only.  However, with the recent advances in link layer
   technology, and with numerous emerging real time applications such as
   video conferencing and Internet telephony, there has been much
   interest for developing mechanisms which enable real time services
   over the Internet.  A framework for meeting these new requirements
   was set out in RFC 1633 [8] and this has driven the specification of
   various classes of network service by the Integrated Services working
   group of the IETF, such as Controlled Load and Guaranteed Service
   [6,7].  Each of these service classes is designed to provide certain
   Quality of Service (QoS) to traffic conforming to a specified set of
   parameters.  Applications are expected to choose one of these classes
   according to their QoS requirements.  One mechanism for end stations
   to utilize such services in an IP network is provided by a QoS
   signaling protocol, the Resource Reservation Protocol (RSVP) [5]
   developed by the RSVP working group of the IETF. The IEEE under its
   Project 802 has defined standards for many different local area
   network technologies.  These all typically offer the same MAC layer
   datagram service [1] to higher layer protocols such as IP although
   they often provide different dynamic behavior characteristics -- it
   is these that are important when considering their ability to support
   real time services.  Later in this memo we describe some of the
   relevant characteristics of the different MAC layer LAN technologies.
   In addition, IEEE 802 has defined standards for bridging multiple LAN
   segments together using devices known as "MAC Bridges" or "Switches"
   [2].  Recent work has also defined traffic classes, multicast
   filtering, and virtual LAN capabilities for these devices [3,4].
   Such LAN technologies often constitute the last hop(s) between users
   and the Internet as well as being a primary building block for entire
   campus networks.  It is therefore necessary to provide standardized
   mechanisms for using these technologies to support end-to-end real
   time services.  In order to do this, there must be some mechanism for
   resource management at the data link layer.  Resource management in
   this context encompasses the functions of admission control,
   scheduling, traffic policing, etc.  The ISSLL (Integrated Services

   over Specific Link Layers) working group in the IETF was chartered
   with the purpose of exploring and standardizing such mechanisms for
   various link layer technologies.

2. Document Outline

   This document is concerned with specifying a framework for providing
   Integrated Services over shared and switched LAN technologies such as
   Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, FDDI, etc.  We begin in
   Section 4 with a discussion of the capabilities of various IEEE 802
   MAC layer technologies.  Section 5 lists the requirements and goals
   for a mechanism capable of providing Integrated Services in a LAN.
   The resource management functions outlined in Section 5 are provided
   by an entity referred to as a Bandwidth Manager (BM). The
   architectural model of the BM is described in Section 6 and its
   various components are discussed in Section 7.  Some implementation
   issues with respect to link layer support for Integrated Services are
   examined in Section 8.  Section 9 discusses a taxonomy of topologies
   for the LAN technologies under consideration with an emphasis on the
   capabilities of each which can be leveraged for enabling Integrated
   Services.  This framework makes no assumptions about the topology at
   the link layer.  The framework is intended to be as exhaustive as
   possible; this means that it is possible that all the functions
   discussed may not be supportable by a particular topology or
   technology, but this should not preclude the usage of this model for
   it.

3. Definitions

   The following is a list of terms used in this and other ISSLL
   documents.

   -  Link Layer or Layer 2 or L2:  Data link layer technologies such as
      Ethernet/IEEE 802.3 and Token Ring/IEEE 802.5 are referred to as
      Layer 2 or L2.

   -  Link Layer Domain or Layer 2 Domain or L2 Domain:  Refers to a set
      of nodes and links interconnected without passing through a L3
      forwarding function.  One or more IP subnets can be overlaid on a
      L2 domain.

   -  Layer 2 or L2 Devices:  Devices that only implement Layer 2
      functionality as Layer 2 or L2 devices.  These include IEEE 802.1D
      [2] bridges or switches.

   -  Internetwork Layer or Layer 3 or L3:  Refers to Layer 3 of the ISO
      OSI model.  This memo is primarily concerned with networks that
      use the Internet Protocol (IP) at this layer.

   -  Layer 3 Device or L3 Device or End Station:  These include hosts
      and routers that use L3 and higher layer protocols or application
      programs that need to make resource reservations.

   -  Segment:  A physical L2 segment that is shared by one or more
      senders.  Examples of segments include:  (a) a shared Ethernet or
      Token Ring wire resolving contention for media access using CSMA
      or token passing; (b) a half duplex link between two stations or
      switches; (c) one direction of a switched full duplex link.

   -  Managed Segment:  A managed segment is a segment with a DSBM
      (designated subnet bandwidth manager, see [14]) present and
      responsible for exercising admission control over requests for
      resource reservation.  A managed segment includes those
      interconnected parts of a shared LAN that are not separated by
      DSBMs.

   -  Traffic Class:  Refers to an aggregation of data flows which are
      given similar service within a switched network.

   -  Subnet:  Used in this memo to indicate a group of L3 devices
      sharing a common L3 network address prefix along with the set of
      segments making up the L2 domain in which they are located.

   -  Bridge/Switch:  A Layer 2 forwarding device as defined by IEEE
      802.1D [2].  The terms bridge and switch are used synonymously in
      this memo.

4. Frame Forwarding in IEEE 802 Networks

4.1. General IEEE 802 Service Model

   The user_priority is a value associated with the transmission and
   reception of all frames in the IEEE 802 service model.  It is
   supplied by the sender that is using the MAC service and is provided
   along with the data to a receiver using the MAC service.  It may or
   may not be actually carried over the network.  Token Ring/IEEE 802.5
   carries this value encoded in its FC octet while basic Ethernet/IEEE
   802.3 does not carry it.  IEEE 802.12 may or may not carry it
   depending on the frame format in use.  When the frame format in use
   is IEEE 802.5, the user_priority is carried explicitly.  When IEEE
   802.3 frame format is used, only the two levels of priority
   (high/low) that are used to determine access priority can be
   recovered.  This is based on the value of priority encoded in the
   start delimiter of the IEEE 802.3 frame.

   NOTE: The original IEEE 802.1D standard [2] contains the
   specifications for the operation of MAC bridges.  This has recently
   been extended to include support for traffic classes and dynamic
   multicast filtering [3].  In this document, the reader should be
   aware that references to the IEEE 802.1D standard refer to [3],
   unless explicitly noted otherwise.

   IEEE 802.1D [3] defines a consistent way for carrying the value of
   the user_priority over a bridged network consisting of Ethernet,
   Token Ring, Demand Priority, FDDI or other MAC layer media using an
   extended frame format.  The usage of user_priority is summarized
   below.  We refer the interested reader to the IEEE 802.1D
   specification for further information.

   If the user_priority is carried explicitly in packets, its utility is
   as a simple label enabling packets within a data stream in different
   classes to be discriminated easily by downstream nodes without having
   to parse the packet in more detail.

   Apart from making the job of desktop or wiring closet switches
   easier, an explicit field means they do not have to change hardware
   or software as the rules for classifying packets evolve; e.g.  based
   on new protocols or new policies.  More sophisticated Layer 3
   switches, perhaps deployed in the core of a network, may be able to
   provide added value by performing packet classification more
   accurately and, hence, utilizing network resources more efficiently
   and providing better isolation between flows.  This appears to be a
   good economic choice since there are likely to be very many more
   desktop/wiring closet switches in a network than switches requiring
   Layer 3 functionality.

   The IEEE 802 specifications make no assumptions about how
   user_priority is to be used by end stations or by the network.
   Although IEEE 802.1D defines static priority queuing as the default
   mode of operation of switches that implement multiple queues, the
   user_priority is really a priority only in a loose sense since it
   depends on the number of traffic classes actually implemented by a
   switch.  The user_priority is defined as a 3 bit quantity with a
   value of 7 representing the highest priority and a value of 0 as the
   lowest.  The general switch algorithm is as follows.  Packets are
   queued within a particular traffic class based on the received
   user_priority, the value of which is either obtained directly from
   the packet if an IEEE 802.1Q header or IEEE 802.5 network is used, or
   is assigned according to some local policy.  The queue is selected
   based on a mapping from user_priority (0 through 7) onto the number
   of available traffic classes.  A switch may implement one or more
   traffic classes.  The advertised IntServ parameters and the switch's
   admission control behavior may be used to determine the mapping from

   user_priority to traffic classes within the switch.  A switch is not
   precluded from implementing other scheduling algorithms such as
   weighted fair queuing and round robin.

   IEEE 802.1D makes no recommendations about how a sender should select
   the value for user_priority.  One of the primary purposes of this
   document is to propose such usage rules, and to discuss the
   communication of the semantics of these values between switches and
   end stations.  In the remainder of this document we use the term
   traffic class synonymously with user_priority.

4.2. Ethernet/IEEE 802.3

   There is no explicit traffic class or user_priority field carried in
   Ethernet packets.  This means that user_priority must be regenerated
   at a downstream receiver or switch according to some defaults or by
   parsing further into higher layer protocol fields in the packet.
   Alternatively, IEEE 802.1Q encapsulation [4] may be used which
   provides an explicit user_priority field on top of the basic MAC
   frame format.

   For the different IP packet encapsulations used over Ethernet/IEEE
   802.3, it will be necessary to adjust any admission control
   calculations according to the framing and padding requirements as
   shown in Table 1.  Here, "ip_len" refers to the length of the IP
   packet including its headers.

                    Table 1: Ethernet encapsulations

   ---------------------------------------------------------------
   Encapsulation                          Framing Overhead  IP MTU
                                             bytes/pkt       bytes
   ---------------------------------------------------------------
   IP EtherType (ip_len<=46 bytes)             64-ip_len    1500
                (1500>=ip_len>=46 bytes)         18         1500

   IP EtherType over 802.1D/Q (ip_len<=42)     64-ip_len    1500*
                (1500>=ip_len>=42 bytes)         22         1500*

   IP EtherType over LLC/SNAP (ip_len<=40)     64-ip_len    1492
                (1500>=ip_len>=40 bytes)         24         1492
   ---------------------------------------------------------------

   *Note that the packet length of an Ethernet frame using the IEEE
   802.1Q specification exceeds the current IEEE 802.3 maximum packet
   length values by 4 bytes.  The change of maximum MTU size for IEEE
   802.1Q frames is being accommodated by IEEE 802.3ac [21].

4.3. Token Ring/IEEE 802.5

   The Token Ring standard [6] provides a priority mechanism that can be
   used to control both the queuing of packets for transmission and the
   access of packets to the shared media.  The priority mechanisms are
   implemented using bits within the Access Control (AC) and the Frame
   Control (FC) fields of a LLC frame.  The first three bits of the AC
   field, the Token Priority bits, together with the last three bits of
   the AC field, the Reservation bits, regulate which stations get
   access to the ring.  The last three bits of the FC field of a LLC
   frame, the User Priority bits, are obtained from the higher layer in
   the user_priority parameter when it requests transmission of a
   packet.  This parameter also establishes the Access Priority used by
   the MAC. The user_priority value is conveyed end-to-end by the User
   Priority bits in the FC field and is typically preserved through
   Token Ring bridges of all types.  In all cases, 0 is the lowest
   priority.

   Token Ring also uses a concept of Reserved Priority which relates to
   the value of priority which a station uses to reserve the token for
   its next transmission on the ring.  When a free token is circulating,
   only a station having an Access Priority greater than or equal to the
   Reserved Priority in the token will be allowed to seize the token for
   transmission.  Readers are referred to [14] for further discussion of
   this topic.

   A Token Ring station is theoretically capable of separately queuing
   each of the eight levels of requested user_priority and then
   transmitting frames in order of priority.  A station sets Reservation
   bits according to the user_priority of frames that are queued for
   transmission in the highest priority queue.  This allows the access
   mechanism to ensure that the frame with the highest priority
   throughout the entire ring will be transmitted before any lower
   priority frame.  Annex I to the IEEE 802.5 Token Ring standard
   recommends that stations send/relay frames as follows.

          Table 2: Recommended use of Token Ring User Priority

            -------------------------------------
            Application             User Priority
            -------------------------------------
            Non-time-critical data      0
                  -                     1
                  -                     2
                  -                     3
            LAN management              4
            Time-sensitive data         5
            Real-time-critical data     6
            MAC frames                  7
            -------------------------------------

   To reduce frame jitter associated with high priority traffic, the
   annex also recommends that only one frame be transmitted per token
   and that the maximum information field size be 4399 octets whenever
   delay sensitive traffic is traversing the ring.  Most existing
   implementations of Token Ring bridges forward all LLC frames with a
   default access priority of 4.  Annex I recommends that bridges
   forward LLC frames that have a user_priority greater than 4 with a
   reservation equal to the user_priority (although IEEE 802.1D [3]
   permits network management override this behavior).  The capabilities
   provided by the Token Ring architecture, such User Priority and
   Reserved Priority, can provide effective support for Integrated
   Services flows that require QoS guarantees.

   For the different IP packet encapsulations used over Token Ring/IEEE
   802.5, it will be necessary to adjust any admission control
   calculations according to the framing requirements as shown in Table
   3.

                   Table 3: Token Ring encapsulations

   ---------------------------------------------------------------
   Encapsulation                          Framing Overhead  IP MTU
                                             bytes/pkt       bytes
   ---------------------------------------------------------------
   IP EtherType over 802.1D/Q                    29          4370*
   IP EtherType over LLC/SNAP                    25          4370*
   ---------------------------------------------------------------

   *The suggested MTU from RFC 1042 [13] is 4464 bytes but there are
   issues related to discovering the maximum supported MTU between any
   two points both within and between Token Ring subnets.  The MTU
   reported here is consistent with the IEEE 802.5 Annex I
   recommendation.

4.4. Fiber Distributed Data Interface

   The Fiber Distributed Data Interface (FDDI) standard [16] provides a
   priority mechanism that can be used to control both the queuing of
   packets for transmission and the access of packets to the shared
   media.  The priority mechanisms are implemented using similar
   mechanisms to Token Ring described above.  The standard also makes
   provision for "Synchronous" data traffic with strict media access and
   delay guarantees.  This mode of operation is not discussed further
   here and represents area within the scope of the ISSLL working group
   that requires further work.  In the remainder of this document, for
   the discussion of QoS mechanisms, FDDI is treated as a 100 Mbps Token
   Ring technology using a service interface compatible with IEEE 802
   networks.

4.5. Demand Priority/IEEE 802.12

   IEEE 802.12 [19] is a standard for a shared 100 Mbps LAN. Data
   packets are transmitted using either the IEEE 802.3 or IEEE 802.5
   frame format.  The MAC protocol is called Demand Priority.  Its main
   characteristics with respect to QoS are the support of two service
   priority levels, normal priority and high priority, and the order of
   service for each of these.  Data packets from all network nodes (end
   hosts and bridges/switches) are served using a simple round robin
   algorithm.

   If the IEEE 802.3 frame format is used for data transmission then the
   user_priority is encoded in the starting delimiter of the IEEE 802.12
   data packet.  If the IEEE 802.5 frame format is used then the
   user_priority is additionally encoded in the YYY bits of the FC field
   in the IEEE 802.5 packet header (see also Section 4.3).  Furthermore,
   the IEEE 802.1Q encapsulation with its own user_priority field may
   also be applied in IEEE 802.12 networks.  In all cases, switches are
   able to recover any user_priority supplied by a sender.

   The same rules apply for IEEE 802.12 user_priority mapping in a
   bridge as with other media types.  The only additional information is
   that normal priority is used by default for user_priority values 0
   through 4 inclusive, and high priority is used for user_priority
   levels 5 through 7.  This ensures that the default Token Ring
   user_priority level of 4 for IEEE 802.5 bridges is mapped to normal
   priority on IEEE 802.12 segments.

   The medium access in IEEE 802.12 LANs is deterministic.  The Demand
   Priority mechanism ensures that, once the normal priority service has
   been preempted, all high priority packets have strict priority over
   packets with normal priority.  In the event that a normal priority
   packet has been waiting at the head of line of a MAC transmit queue

   for a time period longer than PACKET_PROMOTION (200 - 300 ms) [19],
   its priority is automatically promoted to high priority.  Thus, even
   normal priority packets have a maximum guaranteed access time to the
   medium.

   Integrated Services can be built on top of the IEEE 802.12 medium
   access mechanism.  When combined with admission control and bandwidth
   enforcement mechanisms, delay guarantees as required for a Guaranteed
   Service can be provided without any changes to the existing IEEE
   802.12 MAC protocol.

   Since the IEEE 802.12 standard supports the IEEE 802.3 and IEEE 802.5
   frame formats, the same framing overhead as reported in Sections 4.2
   and 4.3 must be considered in the admission control computations for
   IEEE 802.12 links.

5. Requirements and Goals

   This section discusses the requirements and goals which should drive
   the design of an architecture for supporting Integrated Services over
   LAN technologies.  The requirements refer to functions and features
   which must be supported, while goals refer to functions and features
   which are desirable, but are not an absolute necessity.  Many of the
   requirements and goals are driven by the functionality supported by
   Integrated Services and RSVP.

5.1. Requirements

   -  Resource Reservation:  The mechanism must be capable of reserving
      resources on a single segment or multiple segments and at
      bridges/switches connecting them.  It must be able to provide
      reservations for both unicast and multicast sessions.  It should
      be possible to change the level of reservation while the session
      is in progress.

   -  Admission Control:  The mechanism must be able to estimate the
      level of resources necessary to meet the QoS requested by the
      session in order to decide whether or not the session can be
      admitted.  For the purpose of management, it is useful to provide
      the ability to respond to queries about availability of resources.
      It must be able to make admission control decisions for different
      types of services such as Guaranteed Service, Controlled Load,
      etc.

   -  Flow Separation and Scheduling:  It is necessary to provide a
      mechanism for traffic flow separation so that real time flows can
      be given preferential treatment over best effort flows.  Packets
      of real time flows can then be isolated and scheduled according to
      their service requirements.

   -  Policing/Shaping:  Traffic must be shaped and/or policed by end
      stations (workstations, routers) to ensure conformance to
      negotiated traffic parameters.  Shaping is the recommended
      behavior for traffic sources.  A router initiating an ISSLL
      session must have implemented traffic control mechanisms according
      to the IntServ requirements which would ensure that all flows sent
      by the router are in conformance.  The ISSLL mechanisms at the
      link layer rely heavily on the correct implementation of
      policing/shaping mechanisms at higher layers by devices capable of
      doing so.  This is necessary because bridges and switches are not
      typically capable of maintaining per flow state which would be
      required to check flows for conformance.  Policing is left as an
      option for bridges and switches, which if implemented, may be used
      to enforce tighter control over traffic flows.  This issue is
      further discussed in Section 8.

   -  Soft State:  The mechanism must maintain soft state information
      about the reservations.  This means that state information must
      periodically be refreshed if the reservation is to be maintained;
      otherwise the state information and corresponding reservations
      will expire after some pre-specified interval.

   -  Centralized or Distributed Implementation:  In the case of a
      centralized implementation, a single entity manages the resources
      of the entire subnet.  This approach has the advantage of being
      easier to deploy since bridges and switches may not need to be
      upgraded with additional functionality.  However, this approach
      scales poorly with geographical size of the subnet and the number
      of end stations attached.  In a fully distributed implementation,
      each segment will have a local entity managing its resources.
      This approach has better scalability than the former.  However, it
      requires that all bridges and switches in the network support new
      mechanisms.  It is also possible to have a semi-distributed
      implementation where there is more than one entity, each managing
      the resources of a subset of segments and bridges/switches within
      the subnet.  Ideally, implementation should be flexible; i.e.  a
      centralized approach may be used for small subnets and a
      distributed approach can be used for larger subnets.  Examples of
      centralized and distributed implementations are discussed in
      Section 6.

   -  Scalability:  The mechanism and protocols should have a low
      overhead and should scale to the largest receiver groups likely to
      occur within a single link layer domain.

   -  Fault Tolerance and Recovery:  The mechanism must be able to
      function in the presence of failures; i.e.  there should not be a
      single point of failure.  For instance, in a centralized
      implementation, some mechanism must be specified for back-up and
      recovery in the event of failure.

   -  Interaction with Existing Resource Management Controls:  The
      interaction with existing infrastructure for resource management
      needs to be specified.  For example, FDDI has a resource
      management mechanism called the "Synchronous Bandwidth Manager".
      The mechanism must be designed so that it takes advantage of, and
      specifies the interaction with, existing controls where available.

5.2. Goals

   -  Independence from higher layer protocols:  The mechanism should,
      as far as possible, be independent of higher layer protocols such
      as RSVP and IP. Independence from RSVP is desirable so that it can
      interwork with other reservation protocols such as ST2 [10].
      Independence from IP is desirable so that it can interwork with
      other network layer protocols such as IPX, NetBIOS, etc.

   -  Receiver heterogeneity:  this refers to multicast communication
      where different receivers request different levels of service.
      For example, in a multicast group with many receivers, it is
      possible that one of the receivers desires a lower delay bound
      than the others.  A better delay bound may be provided by
      increasing the amount of resources reserved along the path to that
      receiver while leaving the reservations for the other receivers
      unchanged.  In its most complex form, receiver heterogeneity
      implies the ability to simultaneously provide various levels of
      service as requested by different receivers.  In its simplest
      form, receiver heterogeneity will allow a scenario where some of
      the receivers use best effort service and those requiring service
      guarantees make a reservation.  Receiver heterogeneity, especially
      for the reserved/best effort scenario, is a very desirable
      function.  More details on supporting receiver heterogeneity are
      provided in Section 8.

   -  Support for different filter styles:  It is desirable to provide
      support for the different filter styles defined by RSVP such as
      fixed filter, shared explicit and wildcard.  Some of the issues
      with respect to supporting such filter styles in the link layer
      domain are examined in Section 8.

   -  Path Selection:  In source routed LAN technologies such as Token
      Ring/IEEE 802.5, it may be useful for the mechanism to incorporate
      the function of path selection.  Using an appropriate path
      selection mechanism may optimize utilization of network resources.

5.3. Non-goals

   This document describes service mappings onto existing IEEE and ANSI
   defined standard MAC layers and uses standard MAC layer services as
   in IEEE 802.1 bridging.  It does not attempt to make use of or
   describe the capabilities of other proprietary or standard MAC layer
   protocols although it should be noted that published work regarding
   MAC layers suitable for QoS mappings exists.  These are outside the
   scope of the ISSLL working group charter.

5.4. Assumptions

   This framework assumes that typical subnetworks that are concerned
   about QoS will be "switch rich"; i.e. most communication between end
   stations using integrated services support is expected to pass
   through at least one switch.  The mechanisms and protocols described
   will be trivially extensible to communicating systems on the same
   shared medium, but it is important not to allow problem
   generalization which may complicate the targeted practical
   application to switch rich LAN topologies.  There have also been
   developments in the area of MAC enhancements to ensure delay
   deterministic access on network links e.g.  IEEE 802.12 [19] and also
   proprietary schemes.

   Although we illustrate most examples for this model using RSVP as the
   upper layer QoS signaling protocol, there are actually no real
   dependencies on this protocol.  RSVP could be replaced by some other
   dynamic protocol, or the requests could be made by network management
   or other policy entities.  The SBM signaling protocol [14], which is
   based upon RSVP, is designed to work seamlessly in the architecture
   described in this memo.

   There may be a heterogeneous mix of switches with different
   capabilities, all compliant with IEEE 802.1D [2,3], but implementing
   varied queuing and forwarding mechanisms ranging from simple systems
   with two queues per port and static priority scheduling, to more
   complex systems with multiple queues using WFQ or other algorithms.

   The problem is decomposed into smaller independent parts which may
   lead to sub-optimal use of the network resources but we contend that
   such benefits are often equivalent to very small improvement in
   network efficiency in a LAN environment.  Therefore, it is a goal
   that the switches in a network operate using a much simpler set of

   information than the RSVP engine in a router.  In particular, it is
   assumed that such switches do not need to implement per flow queuing
   and policing (although they are not precluded from doing so).

   A fundamental assumption of the IntServ model is that flows are
   isolated from each other throughout their transit across a network.
   Intermediate queuing nodes are expected to shape or police the
   traffic to ensure conformance to the negotiated traffic flow
   specification.  In the architecture proposed here for mapping to
   Layer 2, we diverge from that assumption in the interest of
   simplicity.  The policing/shaping functions are assumed to be
   implemented in end stations.  In some LAN environments, it is
   reasonable to assume that end stations are trusted to adhere to their
   negotiated contracts at the inputs to the network, and that we can
   afford to over-allocate resources during admission control to
   compensate for the inevitable packet jitter/bunching introduced by
   the switched network itself.  This divergence has some implications
   on the types of receiver heterogeneity that can be supported and the
   statistical multiplexing gains that may be exploited, especially for
   Controlled Load flows.  This is discussed in Section 8.7 of this
   document.

6. Basic Architecture

   The functional requirements described in Section 5 will be performed
   by an entity which we refer to as the Bandwidth Manager (BM). The BM
   is responsible for providing mechanisms for an application or higher
   layer protocol to request QoS from the network.  For architectural
   purposes, the BM consists of the following components.

6.1. Components

6.1.1. Requester Module

   The Requester Module (RM) resides in every end station in the subnet.
   One of its functions is to provide an interface between applications
   or higher layer protocols such as RSVP, ST2, SNMP, etc.  and the BM.
   An application can invoke the various functions of the BM by using
   the primitives for communication with the RM and providing it with
   the appropriate parameters.  To initiate a reservation, in the link
   layer domain, the following parameters must be passed to the RM: the
   service desired (Guaranteed Service or Controlled Load), the traffic
   descriptors contained in the TSpec, and an RSpec specifying the
   amount of resources to be reserved [9].  More information on these
   parameters may be found in the relevant Integrated Services documents
   [6,7,8,9].  When RSVP is used for signaling at the network layer,
   this information is available and needs to be extracted from the RSVP
   PATH and RSVP RESV messages (See [5] for details).  In addition to

   these parameters, the network layer addresses of the end points must
   be specified.  The RM must then translate the network layer addresses
   to link layer addresses and convert the request into an appropriate
   format which is understood by other components of the BM responsible
   admission control.  The RM is also responsible for returning the
   status of requests processed by the BM to the invoking application or
   higher layer protocol.

6.1.2. Bandwidth Allocator

   The Bandwidth Allocator (BA) is responsible for performing admission
   control and maintaining state about the allocation of resources in
   the subnet.  An end station can request various services, e.g.
   bandwidth reservation, modification of an existing reservation,
   queries about resource availability, etc.  These requests are
   processed by the BA. The communication between the end station and
   the BA takes place through the RM. The location of the BA will depend
   largely on the implementation method.  In a centralized
   implementation, the BA may reside on a single station in the subnet.
   In a distributed implementation, the functions of the BA may be
   distributed in all the end stations and bridges/switches as
   necessary.  The BA is also responsible for deciding how to label
   flows, e.g.  based on the admission control decision, the BA may
   indicate to the RM that packets belonging to a particular flow be
   tagged with some priority value which maps to the appropriate traffic
   class.

6.1.3. Communication Protocols

   The protocols for communication between the various components of the
   BM system must be specified.  These include the following:

   -  Communication between the higher layer protocols and the RM:  The
      BM must define primitives for the application to initiate
      reservations, query the BA about available resources, change
      change or delete reservations, etc.  These primitives could be
      implemented as an API for an application to invoke functions of
      the BM via the RM.

   -  Communication between the RM and the BA: A signaling mechanism
      must be defined for the communication between the RM and the BA.
      This protocol will specify the messages which must be exchanged
      between the RM and the BA in order to service various requests by
      the higher layer entity.

   -  Communication between peer BAs:  If there is more than one BA in
      the subnet, a means must be specified for inter-BA communication.
      Specifically, the BAs must be able to decide among themselves

      about which BA would be responsible for which segments and bridges
      or switches.  Further, if a request is made for resource
      reservation along the domain of multiple BAs, the BAs must be able
      to handle such a scenario correctly.  Inter-BA communication will
      also be responsible for back-up and recovery in the event of
      failure.

6.2. Centralized vs.  Distributed Implementations

   Example scenarios are provided showing the location of the components
   of the bandwidth manager in centralized and fully distributed
   implementations.  Note that in either case, the RM must be present in
   all end stations that need to make reservations.  Essentially,
   centralized or distributed refers to the implementation of the BA,
   the component responsible for resource reservation and admission
   control.  In the figures below, "App" refers to the application
   making use of the BM. It could either be a user application, or a
   higher layer protocol process such as RSVP.

                                +---------+
                            .-->|  BA     |<--.
                           /    +---------+    \
                          / .-->| Layer 2 |<--. \
                         / /    +---------+    \ \
                        / /                     \ \
                       / /                       \ \
   +---------+        / /                         \ \       +---------+
   |  App    |<----- /-/---------------------------\-\----->|  App    |
   +---------+      / /                             \ \     +---------+
   |  RM     |<----. /                               \ .--->|  RM     |
   +---------+      / +---------+        +---------+  \     +---------+
   | Layer 2 |<------>| Layer 2 |<------>| Layer 2 |<------>| Layer 2 |
   +---------+        +---------+        +---------+        +---------+

   RSVP Host/         Intermediate       Intermediate       RSVP Host/
      Router          Bridge/Switch      Bridge/Switch         Router

     Figure 1: Bandwidth Manager with centralized Bandwidth Allocator

   Figure 1 shows a centralized implementation where a single BA is
   responsible for admission control decisions for the entire subnet.
   Every end station contains a RM. Intermediate bridges and switches in
   the network need not have any functions of the BM since they will not
   be actively participating in admission control.  The RM at the end
   station requesting a reservation initiates communication with its BA.
   For larger subnets, a single BA may not be able to handle the
   reservations for the entire subnet.  In that case it would be
   necessary to deploy multiple BAs, each managing the resources of a

   non-overlapping subset of segments.  In a centralized implementation,
   the BA must have some knowledge of the Layer 2 topology of the subnet
   e.g., link layer spanning tree information, in order to be able to
   reserve resources on appropriate segments.  Without this topology
   information, the BM would have to reserve resources on all segments
   for all flows which, in a switched network, would lead to very
   inefficient utilization of resources.

   +---------+                                              +---------+
   |  App    |<-------------------------------------------->|  App    |
   +---------+        +---------+        +---------+        +---------+
   |  RM/BA  |<------>|  BA     |<------>|  BA     |<------>|  RM/BA  |
   +---------+        +---------+        +---------+        +---------+
   | Layer 2 |<------>| Layer 2 |<------>| Layer 2 |<------>| Layer 2 |
   +---------+        +---------+        +---------+        +---------+

   RSVP Host/         Intermediate       Intermediate       RSVP Host/
      Router          Bridge/Switch      Bridge/Switch         Router

                  Figure 2: Bandwidth Manager with fully
                     distributed Bandwidth Allocator

   Figure 2 depicts the scenario of a fully distributed bandwidth
   manager.  In this case, all devices in the subnet have BM
   functionality.  All the end hosts are still required to have a RM. In
   addition, all stations actively participate in admission control.
   With this approach, each BA would need only local topology
   information since it is responsible for the resources on segments
   that are directly connected to it.  This local topology information,
   such as a list of ports active on the spanning tree and which unicast
   addresses are reachable from which ports, is readily available in
   today's switches.  Note that in the figures above, the arrows between
   peer layers are used to indicate logical connectivity.

7. Model of the Bandwidth Manager in a Network

   In this section we describe how the model above fits with the
   existing IETF Integrated Services model of IP hosts and routers.
   First, we describe Layer 3 host and router implementations.  Next, we
   describe how the model is applied in Layer 2 switches.  Throughout we
   indicate any differences between centralized and distributed
   implementations.  Occasional references are made to terminology from
   the Subnet Bandwidth Manager specification [14].

7.1. End Station Model

7.1.1. Layer 3 Client Model

   We assume the same client model as IntServ and RSVP where we use the
   term "client" to mean the entity handling QoS in the Layer 3 device
   at each end of a Layer 2 Domain.  In this model, the sending client
   is responsible for local admission control and packet scheduling onto
   its link in accordance with the negotiated service.  As with the
   IntServ model, this involves per flow scheduling with possible
   traffic shaping/policing in every such originating node.

   For now, we assume that the client runs an RSVP process which
   presents a session establishment interface to applications, provides
   signaling over the network, programs a scheduler and classifier in
   the driver, and interfaces to a policy control module.  In
   particular, RSVP also interfaces to a local admission control module
   which is the focus of this section.

   The following figure, reproduced from the RSVP specification, depicts
   the RSVP process in sending hosts.

                     +-----------------------------+
                     | +-------+  +-------+        |   RSVP
                     | |Appli- |  | RSVP  <------------------->
                     | | cation<-->       |        |
                     | |       |  |process| +-----+|
                     | +-+-----+  |       +->Polcy||
                     |   |        +--+--+-+ |Cntrl||
                     |   |data       |  |   +-----+|
                     |===|===========|==|==========|
                     |   |  +--------+  |   +-----+|
                     |   |  |        |  +--->Admis||
                     | +-V--V-+  +---V----+ |Cntrl||
                     | |Class-|  | Packet | +-----+|
                     | | ifier|==>Schedulr|===================>
                     | +------+  +--------+        |    data
                     +-----------------------------+

                    Figure 3: RSVP in Sending Hosts

7.1.2. Requests to Layer 2 ISSLL

   The local admission control entity within a client is responsible for
   mapping Layer 3 session establishment requests into Layer 2
   semantics.

   The upper layer entity makes a request, in generalized terms to ISSLL
   of the form:

      "May I reserve for traffic with <traffic characteristic> with
      <performance requirements> from <here> to <there> and how should I
      label it?"

   where

   <traffic characteristic> = Sender Tspec (e.g. bandwidth, burstiness,
   MTU)
   <performance requirements> = FlowSpec (e.g. latency, jitter bounds)
   <here> = IP address(es)
   <there> = IP address(es) - may be multicast

7.1.3. At the Layer 3 Sender

   The ISSLL functionality in the sender is illustrated in Figure 4.

   The functions of the Requester Module may be summarized as follows:

   -  Maps the endpoints of the conversation to Layer 2 addresses in the
      LAN, so that the client can determine what traffic is going where.
      This function probably makes reference to the ARP protocol cache
      for unicast or performs an algorithmic mapping for multicast
      destinations.

   -  Communicates with any local Bandwidth Allocator module for local
      admission control decisions.

   -  Formats a SBM request to the network with the mapped addresses and
      flow/filter specs.

   -  Receives a response from the network and reports the admission
      control decision to the higher layer entity, along with any
      negotiated modifications to the session parameters.

   -  Saves any returned user_priority to be associated with this
      session in a "802 header" table.  This will be used when
      constructing the Layer 2 headers for future data packets belonging
      to this session.  This table might, for example, be indexed by the
      RSVP flow identifier.

                    from IP     from RSVP
                  +----|------------|------------+
                  | +--V----+   +---V---+        |
                  | | Addr  <--->       |        | SBM signaling
                  | |mapping|   |Request|<----------------------->
                  | +---+---+   |Module |        |
                  |     |       |       |        |
                  | +---+---+   |       |        |
                  | |  802  <--->       |        |
                  | | header|   +-+-+-+-+        |
                  | +--+----+    /  | |          |
                  |    |        /   | |  +-----+ |
                  |    | +-----+    | +->|Band-| |
                  |    | |          |    |width| |
                  | +--V-V-+  +-----V--+ |Alloc| |
                  | |Class-|  | Packet | +-----+ |
                  | | ifier|==>Schedulr|=========================>
                  | +------+  +--------+         |  data
                  +------------------------------+

                Figure 4: ISSLL in a Sending End Station

   The Bandwidth Allocator (BA) component is only present when a
   distributed BA model is implemented.  When present, its function is
   basically to apply local admission control for the outgoing link
   bandwidth and driver's queuing resources.

7.1.4. At the Layer 3 Receiver

   The ISSLL functionality in the receiver is simpler and is illustrated
   in Figure 5.

   The functions of the Requester Module may be summarized as follows:

   -  Handles any received SBM protocol indications.

   -  Communicates with any local BA for local admission control
      decisions.

   -  Passes indications up to RSVP if OK.

   -  Accepts confirmations from RSVP and relays them back via SBM
      signaling towards the requester.

                          to RSVP       to IP
                            ^            ^
                       +----|------------|------+
                       | +--+----+       |      |
         SBM signaling | |Request|   +---+---+  |
         <-------------> |Module |   | Strip |  |
                       | +--+---++   |802 hdr|  |
                       |    |    \   +---^---+  |
                       | +--v----+\      |      |
                       | | Band- | \     |      |
                       | |  width|  \    |      |
                       | | Alloc |   .   |      |
                       | +-------+   |   |      |
                       | +------+   +v---+----+ |
         data          | |Class-|   | Packet  | |
         <==============>| ifier|==>|Scheduler| |
                       | +------+   +---------+ |
                       +------------------------+

               Figure 5: ISSLL in a Receiving End Station

   -  May program a receive classifier and scheduler, if used, to
      identify traffic classes of received packets and accord them
      appropriate treatment e.g., reservation of buffers for particular
      traffic classes.

   -  Programs the receiver to strip away link layer header information
      from received packets.

   The Bandwidth Allocator, present only in a distributed implementation
   applies local admission control to see if a request can be supported
   with appropriate local receive resources.

7.2. Switch Model

7.2.1. Centralized Bandwidth Allocator

   Where a centralized Bandwidth Allocator model is implemented,
   switches do not take part in the admission control process.
   Admission control is implemented by a centralized BA, e.g., a "Subnet
   Bandwidth Manager" (SBM) as described in [14].  This centralized BA
   may actually be co-located with a switch but its functions would not
   necessarily then be closely tied with the switch's forwarding
   functions as is the case with the distributed BA described below.

7.2.2. Distributed Bandwidth Allocator

   The model of Layer 2 switch behavior described here uses the
   terminology of the SBM protocol as an example of an admission control
   protocol.  The model is equally applicable when other mechanisms,
   e.g.  static configuration or network management, are in use for
   admission control.  We define the following entities within the
   switch:

   -  Local Admission Control Module:  One of these on each port
      accounts for the available bandwidth on the link attached to that
      port.  For half duplex links, this involves taking account of the
      resources allocated to both transmit and receive flows.  For full
      duplex links, the input port accountant's task is trivial.

   -  Input SBM Module:  One instance on each port performs the
      "network" side of the signaling protocol for peering with clients
      or other switches.  It also holds knowledge about the mappings of
      IntServ classes to user_priority.

   -  SBM Propagation Module:  Relays requests that have passed
      admission control at the input port to the relevant output ports'
      SBM modules.  This will require access to the switch's forwarding
      table (Layer-2 "routing table" cf.  RSVP model) and port spanning
      tree state.

   -  Output SBM Module:  Forwards requests to the next Layer 2 or Layer
      3 hop.

   -  Classifier, Queue and Scheduler Module:  The functions of this
      module are basically as described by the Forwarding Process of
      IEEE 802.1D (see Section 3.7 of [3]).  The Classifier module
      identifies the relevant QoS information from incoming packets and
      uses this, together with the normal bridge forwarding database, to
      decide at which output port and traffic class to enqueue the
      packet.  Different types of switches will use different techniques
      for flow identification (see Section 8.1).  In IEEE 802.1D
      switches this information is the regenerated user_priority
      parameter which has already been decoded by the receiving MAC
      service and potentially remapped by the forwarding process (see
      Section 3.7.3 of [3]).  This does not preclude more sophisticated
      classification rules such as the classification of individual
      IntServ flows.  The Queue and Scheduler implement the

      output queues for ports and provide the algorithm for servicing
      the queues for transmission onto the output link in order to
      provide the promised IntServ service.  Switches will implement one
      or more output queues per port and all will implement at least a
      basic static priority dequeuing algorithm as their default, in
      accordance with IEEE 802.1D.

   -  Ingress Traffic Class Mapping and Policing Module:  Its functions
      are as described in IEEE 802.1D Section 3.7.  This optional module
      may police the data within traffic classes for conformance to the
      negotiated parameters, and may discard packets or re-map the
      user_priority.  The default behavior is to pass things through
      unchanged.

   -  Egress Traffic Class Mapping Module:  Its functions are as
      described in IEEE 802.1D Section 3.7.  This optional module may
      perform re-mapping of traffic classes on a per output port basis.
      The default behavior is to pass things through unchanged.

   Figure 6 shows all of the modules in an ISSLL enabled switch.  The
   ISSLL model is a superset of the IEEE 802.1D bridge model.

                     +-------------------------------+
    SBM signaling    | +-----+   +------+   +------+ | SBM signaling
   <------------------>| IN  |<->| SBM  |<->| OUT  |<---------------->
                     | | SBM |   | prop.|   | SBM  | |
                     | +-++--+   +---^--+   /----+-+ |
                     |  / |          |     /     |   |
       ______________| /  |          |     |     |   +-------------+
      | \             /+--V--+       |     |  +--V--+            / |
      |   \      ____/ |Local|       |     |  |Local|          /   |
      |     \   /      |Admis|       |     |  |Admis|        /     |
      |       \/       |Cntrl|       |     |  |Cntrl|      /       |
      | +-----V+\      +-----+       |     |  +-----+    /+-----+  |
      | |traff |  \              +---+--+ +V-------+   /  |egrss|  |
      | |class |    \            |Filter| |Queue & | /    |traff|  |
      | |map & |=====|==========>|Data- |=| Packet |=|===>|class|  |
      | |police|     |           |  base| |Schedule| |    |map  |  |
      | +------+     |           +------+ +--------+ |    +-+---+  |
      +----^---------+-------------------------------+------|------+
   data in |                                                |data out
   ========+                                                +========>

                      Figure 6: ISSLL in a Switch

7.3. Admission Control

   On receipt of an admission control request, a switch performs the
   following actions, again using SBM as an example.  The behavior is
   different depending on whether the "Designated SBM" for this segment
   is within this switch or not.  See [14] for a more detailed
   specification of the DSBM/SBM actions.

   -  If the ingress SBM is the "Designated SBM" for this link, it
      either translates any received user_priority or selects a Layer 2
      traffic class which appears compatible with the request and whose
      use does not violate any administrative policies in force.  In
      effect, it matches the requested service with the available
      traffic classes and chooses the "best" one.  It ensures that, if
      this reservation is successful, the value of user_priority
      corresponding to that traffic class is passed back to the client.

   -  The ingress DSBM observes the current state of allocation of
      resources on the input port/link and then determines whether the
      new resource allocation from the mapped traffic class can be
      accommodated.  The request is passed to the reservation propagator
      if accepted.

   -  If the ingress SBM is not the "Designated SBM" for this link then
      it directly passes the request on to the reservation propagator.

   -  The reservation propagator relays the request to the bandwidth
      accountants on each of the switch's outbound links to which this
      reservation would apply.  This implies an interface to
      routing/forwarding database.

   -  The egress bandwidth accountant observes the current state of
      allocation of queuing resources on its outbound port and bandwidth
      on the link itself and determines whether the new allocation can
      be accommodated.  Note that this is only a local decision at this
      switch hop; further Layer 2 hops through the network may veto the
      request as it passes along.

   -  The request, if accepted by this switch, is propagated on each
      output link selected.  Any user_priority described in the
      forwarded request must be translated according to any egress
      mapping table.

   -  If accepted, the switch must notify the client of the
      user_priority to be used for packets belonging to that flow.
      Again, this is an optimistic approach assuming that admission
      control succeeds; downstream switches may refuse the request.

   -  If this switch wishes to reject the request, it can do so by
      notifying the client that originated the request by means of its
      Layer 2 address.

7.4. QoS Signaling

   The mechanisms described in this document make use of a signaling
   protocol for devices to communicate their admission control requests
   across the network.  The service definitions to be provided by such a
   protocol e.g.  [14] are described below.  We illustrate the
   primitives and information that need to be exchanged with such a
   signaling protocol entity.  In all of the examples, appropriate
   delete/cleanup mechanisms will also have to be provided for tearing
   down established sessions.

7.4.1. Client Service Definitions

   The following interfaces can be identified from Figures 4 and 5.

   -  SBM <-> Address Mapping

      This is a simple lookup function which may require ARP protocol
      interactions or an algorithmic mapping.  The Layer 2 addresses are
      needed by SBM for inclusion in its signaling messages to avoid
      requiring that switches participating in the signaling have Layer
      3 information to perform the mapping.

      l2_addr = map_address( ip_addr )

   -  SBM <-> Session/Link Layer Header

      This is for notifying the transmit path of how to add Layer 2
      header information, e.g.  user_priority values to the traffic of
      each outgoing flow.  The transmit path will provide the
      user_priority value when it requests a MAC layer transmit
      operation for each packet.  The user_priority is one of the
      parameters passed in the packet transmit primitive defined by the
      IEEE 802 service model.

      bind_l2_header( flow_id, user_priority )

   -  SBM <-> Classifier/Scheduler

      This is for notifying transmit classifier/scheduler of any
      additional Layer 2 information associated with scheduling the
      transmission of a packet flow.  This primitive may be unused in
      some implementations or it may be used, for example, to provide
      information to a transmit scheduler that is performing per traffic

      class scheduling in addition to the per flow scheduling required
      by IntServ; the Layer 2 header may be a pattern (in addition to
      the FilterSpec) to be used to identify the flow's traffic.

      bind_l2schedulerinfo( flow_id, , l2_header, traffic_class )

   -  SBM <-> Local Admission Control

      This is used for applying local admission control for a session
      e.g.  is there enough transmit bandwidth still uncommitted for
      this new session?  Are there sufficient receive buffers?  This
      should commit the necessary resources if it succeeds.  It will be
      necessary to release these resources at a later stage if the
      admission control fails at a subsequent node.  This call would be
      made, for example, by a segment's Designated SBM.

      status = admit_l2session( flow_id, Tspec, FlowSpec )

   -  SBM <-> RSVP

      This is outlined above in Section 7.1.2 and fully described in
      [14].

   -  Management Interfaces

      Some or all of the modules described by this model will also
      require configuration management.  It is expected that details of
      the manageable objects will be specified by future work in the
      ISSLL WG.

7.4.2. Switch Service Definitions

   The following interfaces are identified from Figure 6.

   -  SBM <-> Classifier

      This is for notifying the receive classifier of how to match
      incoming Layer 2 information with the associated traffic class.
      It may in some cases consist of a set of read only default
      mappings.

      bind_l2classifierinfo( flow_id, l2_header, traffic_class )

   -  SBM <-> Queue and Packet Scheduler

      This is for notifying transmit scheduler of additional Layer 2
      information associated with a given traffic class.  It may be
      unused in some cases (see discussion in previous section).

      bind_l2schedulerinfo( flow_id, l2_header, traffic_class )

   -  SBM <-> Local Admission Control

      Same as for the host discussed above.

   -  SBM <-> Traffic Class Map and Police

      Optional configuration of any user_priority remapping that might
      be implemented on ingress to and egress from the ports of a
      switch.  For IEEE 802.1D switches, it is likely that these
      mappings will have to be consistent across all ports.

      bind_l2ingressprimap( inport, in_user_pri, internal_priority )
      bind_l2egressprimap( outport, internal_priority, out_user_pri )

      Optional configuration of any Layer 2 policing function to be
      applied on a per class basis to traffic matching the Layer 2
      header.  If the switch is capable of per flow policing then
      existing IntServ/RSVP models will provide a service definition for
      that configuration.

      bind_l2policing( flow_id, l2_header, Tspec, FlowSpec )

   -  SBM <-> Filtering Database

      SBM propagation rules need access to the Layer 2 forwarding
      database to determine where to forward SBM messages.  This is
      analogous to RSRR interface in Layer 3 RSVP.

      output_portlist = lookup_l2dest( l2_addr )

   -  Management Interfaces

      Some or all of the modules described by this model will also
      require configuration management.  It is expected that details of
      the manageable objects will be specified by future work in the
      ISSLL working group.

8. Implementation Issues

   As stated earlier, the Integrated Services working group has defined
   various service classes offering varying degrees of QoS guarantees.
   Initial effort will concentrate on enabling the Controlled Load [6]
   and Guaranteed Service classes [7].  The Controlled Load service
   provides a loose guarantee, informally stated as "the same as best
   effort would be on an unloaded network".  The Guaranteed Service
   provides an upper bound on the transit delay of any packet.  The

   extent to which these services can be supported at the link layer
   will depend on many factors including the topology and technology
   used.  Some of the mapping issues are discussed below in light of the
   emerging link layer standards and the functions supported by higher
   layer protocols.  Considering the limitations of some of the
   topologies, it may not be possible to satisfy all the requirements
   for Integrated Services on a given topology.  In such cases, it is
   useful to consider providing support for an approximation of the
   service which may suffice in most practical instances.  For example,
   it may not be feasible to provide policing/shaping at each network
   element (bridge/switch) as required by the Controlled Load
   specification.  But if this task is left to the end stations, a
   reasonably good approximation to the service can be obtained.

8.1. Switch Characteristics

   There are many LAN bridges/switches with varied capabilities for
   supporting QoS. We discuss below the various kinds of devices that
   that one may expect to find in a LAN environment.

   The most basic bridge is one which conforms to the IEEE 802.1D
   specification of 1993 [2].  This device has a single queue per output
   port, and uses the spanning tree algorithm to eliminate topology
   loops.  Networks constructed from this kind of device cannot be
   expected to provide service guarantees of any kind because of the
   complete lack of traffic isolation.

   The next level of bridges/switches are those which conform to the
   more recently revised IEEE 802.1D specification [3].  They include
   support for queuing up to eight traffic classes separately. The level
   of traffic isolation provided is coarse because all flows
   corresponding to a particular traffic class are aggregated.  Further,
   it is likely that more than one priority will map to a traffic class
   depending on the number of queues implemented in the switch.  It
   would be difficult for such a device to offer protection against
   misbehaving flows.  The scope of multicast traffic may be limited by
   using GMRP to only those segments which are on the path to interested
   receivers.

   A next step above these devices are bridges/switches which implement
   optional parts of the IEEE 802.1D specification such as mapping the
   received user_priority to some internal set of canonical values on a
   per-input-port basis.  It may also support the mapping of these
   internal canonical values onto transmitted user_priority on a per-
   output-port basis.  With these extra capabilities, network
   administrators can perform mapping of traffic classes between
   specific pairs of ports, and in doing so gain more control over
   admission of traffic into the protected classes.

   Other entirely optional features that some bridges/switches may
   support include classification of IntServ flows using fields in the
   network layer header, per-flow policing and/or reshaping which is
   essential for supporting Guaranteed Service, and more sophisticated
   scheduling algorithms such as variants of weighted fair queuing to
   limit the bandwidth consumed by a traffic class.  Note that it is
   advantageous to perform flow isolation and for all network elements
   to police each flow in order to support the Controlled Load and
   Guaranteed Service.

8.2. Queuing

   Connectionless packet networks in general, and LANs in particular,
   work today because of scaling choices in network provisioning.
   Typically, excess bandwidth and buffering is provisioned in the
   network to absorb the traffic sourced by higher layer protocols,
   often sufficient to cause their transmission windows to run out on a
   statistical basis, so that network overloads are rare and transient
   and the expected loading is very low.

   With the advent of time-critical traffic such over-provisioning has
   become far less easy to achieve.  Time-critical frames may be queued
   for annoyingly long periods of time behind temporary bursts of file
   transfer traffic, particularly at network bottleneck points, e.g.  at
   the 100 Mbps to 10 Mbps transition that might occur between the riser
   to the wiring closet and the final link to the user from a desktop
   switch.  In this case, however, if it is known a priori (either by
   application design, on the basis of statistics, or by administrative
   control) that time-critical traffic is a small fraction of the total
   bandwidth, it suffices to give it strict priority over the non-time-
   critical traffic.  The worst case delay experienced by the time-
   critical traffic is roughly the maximum transmission time of a
   maximum length non-time-critical frame -- less than a millisecond for
   10 Mbps Ethernet, and well below the end to end delay budget based on
   human perception times.

   When more than one priority service is to be offered by a network
   element e.g.  one which supports both Controlled Load as well as
   Guaranteed Service, the requirements for the scheduling discipline
   become more complex.  In order to provide the required isolation
   between the service classes, it will probably be necessary to queue
   them separately.  There is then an issue of how to service the queues
   which requires a combination of admission control and more
   intelligent queuing disciplines.  As with the service specifications
   themselves, the specification of queuing algorithms is beyond the
   scope of this document.

8.3. Mapping of Services to Link Level Priority

   The number of traffic classes supported and access methods of the
   technology under consideration will determine how many and what
   services may be supported.  Native Token Ring/IEEE 802.5, for
   instance, supports eight priority levels which may be mapped to one
   or more traffic classes.  Ethernet/IEEE 802.3 has no support for
   signaling priorities within frames.  However, the IEEE 802 standards
   committee has recently developed a new standard for bridges/switches
   related to multimedia traffic expediting and dynamic multicast
   filtering [3].  A packet format for carrying a user_priority field on
   all IEEE 802 LAN media types is now defined in [4].  These standards
   allow for up to eight traffic classes on all media.  The
   user_priority bits carried in the frame are mapped to a particular
   traffic class within a bridge/switch.  The user_priority is signaled
   on an end-to-end basis, unless overridden by bridge/switch
   management.  The traffic class that is used by a flow should depend
   on the quality of service desired and whether the reservation is
   successful or not.  Therefore, a sender should use the user_priority
   value which maps to the best effort traffic class until told
   otherwise by the BM. The BM will, upon successful completion of
   resource reservation, specify the value of user_priority to be used
   by the sender for that session's data.  An accompanying memo [13]
   addresses the issue of mapping the various Integrated Services to
   appropriate traffic classes.

8.4. Re-mapping of Non-conforming Aggregated Flows

   One other topic under discussion in the IntServ context is how to
   handle the traffic for data flows from sources that exceed their
   negotiated traffic contract with the network.  An approach that shows
   some promise is to treat such traffic with "somewhat less than best
   effort" service in order to protect traffic that is normally given
   "best effort" service from having to back off.  Best effort traffic
   is often adaptive, using TCP or other congestion control algorithms,
   and it would be unfair to penalize those flows due to badly behaved
   traffic from reserved flows which are often set up by non-adaptive
   applications.

   A possible solution might be to assign normal best effort traffic to
   one user_priority and to label excess non-conforming traffic as a
   lower user_priority although the re-ordering problems that might
   arise from doing this may make this solution undesirable,
   particularly if the flows are using TCP. For this reason the
   controlled load service recommends dropping excess traffic, rather
   than re-mapping to a lower priority.  This is further discussed
   below.

8.5. Override of Incoming User Priority

   In some cases, a network administrator may not trust the
   user_priority values contained in packets from a source and may wish
   to map these into some more suitable set of values.  Alternatively,
   due perhaps to equipment limitations or transition periods, the
   user_priority values may need to be re-mapped as the data flows
   to/from different regions of a network.

   Some switches may implement such a function on input that maps
   received user_priority to some internal set of values.  This function
   is provided by a table known in IEEE 802.1D as the User Priority
   Regeneration Table (Table 3-1 in [3]).  These values can then be
   mapped using an output table described above onto outgoing
   user_priority values.  These same mappings must also be used when
   applying admission control to requests that use the user_priority
   values (see e.g.  [14]).  More sophisticated approaches are also
   possible where a device polices traffic flows and adjusts their
   onward user_priority based on their conformance to the admitted
   traffic flow specifications.

8.6. Different Reservation Styles

   In the figure above, SW is a bridge/switch in the link layer domain.
   S1, S2, S3, R1 and R2 are end stations which are members of a group
   associated with the same RSVP flow.  S1, S2 and S3 are upstream end
   stations.  R1 and R2 are the downstream end stations which receive
   traffic from all the senders.  RSVP allows receivers R1 and R2 to
   specify reservations which can apply to:  (a) one specific sender
   only (fixed filter); (b) any of two or more explicitly specified
   senders (shared explicit filter); and (c) any sender in the group
   (shared wildcard filter).  Support for the fixed filter style is
   straightforward; a separate reservation is made for the traffic from
   each of the senders.  However, support for the other two filter
   styles has implications regarding policing; i.e.  the merged flow
   from the different senders must be policed so that they conform to
   traffic parameters specified in the filter's RSpec.  This scenario is
   further complicated if the services requested by R1 and R2 are
   different.  Therefore, in the absence of policing within
   bridges/switches, it may be possible to support only fixed filter
   reservations at the link layer.

              +-----+       +-----+       +-----+
              | S1  |       | S2  |       | S3  |
              +-----+       +-----+       +-----+
                 |             |             |
                 |             v             |
                 |          +-----+          |
                 +--------->| SW  |<---------+
                            +-----+
                             |   |
                        +----+   +----+
                        |             |
                        v             V
                     +-----+       +-----+
                     | R1  |       | R2  |
                     +-----+       +-----+

                Figure 7: Illustration of filter styles

8.7. Receiver Heterogeneity

   At Layer 3, the IntServ model allows heterogeneous receivers for
   multicast flows where different branches of a tree can have different
   types of reservations for a given multicast destination.  It also
   supports the notion that trees may have some branches with reserved
   flows and some using best effort service.  If we were to treat a
   Layer 2 subnet as a single network element as defined in [8], then
   all of the branches of the distribution tree that lie within the
   subnet could be assumed to require the same QoS treatment and be
   treated as an atomic unit as regards admission control, etc.  With
   this assumption, the model and protocols already defined by IntServ
   and RSVP already provide sufficient support for multicast
   heterogeneity.  Note, however, that an admission control request may
   well be rejected because just one link in the subnet is
   oversubscribed leading to rejection of the reservation request for
   the entire subnet.

   As an example, consider Figure 8, SW is a Layer 2 device
   (bridge/switch) participating in resource reservation, S is the
   upstream source end station and R1 and R2 are downstream end station
   receivers.  R1 would like to make a reservation for the flow while R2
   would like to receive the flow using best effort service.  S sends
   RSVP PATH messages which are multicast to both R1 and R2.  R1 sends
   an RSVP RESV message to S requesting the reservation of resources.

                           +-----+
                           |  S  |
                           +-----+
                              |
                              v
              +-----+      +-----+      +-----+
              | R1  |<-----| SW  |----->| R2  |
              +-----+      +-----+      +-----+

              Figure 8: Example of receiver heterogeneity

   If the reservation is successful at Layer 2, the frames addressed to
   the group will be categorized in the traffic class corresponding to
   the service requested by R1.  At SW, there must be some mechanism
   which forwards the packet providing service corresponding to the
   reserved traffic class at the interface to R1 while using the best
   effort traffic class at the interface to R2.  This may involve
   changing the contents of the frame itself, or ignoring the frame
   priority at the interface to R2.

   Another possibility for supporting heterogeneous receivers would be
   to have separate groups with distinct MAC addresses, one for each
   class of service.  By default, a receiver would join the "best
   effort" group where the flow is classified as best effort.  If the
   receiver makes a reservation successfully, it can be transferred to
   the group for the class of service desired.  The dynamic multicast
   filtering capabilities of bridges and switches implementing the IEEE
   802.1D standard would be a very useful feature in such a scenario.  A
   given flow would be transmitted only on those segments which are on
   the path between the sender and the receivers of that flow.  The
   obvious disadvantage of such an approach is that the sender needs to
   send out multiple copies of the same packet corresponding to each
   class of service desired thus potentially duplicating the traffic on
   a portion of the distribution tree.

   The above approaches would provide very sub-optimal utilization of
   resources given the expected size and complexity of the Layer 2
   subnets.  Therefore, it is desirable to enable switches to apply QoS
   differently on different egress branches of a tree that divide at
   that switch.

   IEEE 802.1D specifies a basic model for multicast whereby a switch
   makes multicast forwarding decisions based on the destination
   address.  This would produce a list of output ports to which the
   packet should be forwarded.  In its default mode, such a switch would
   use the user_priority value in received packets, or a value
   regenerated on a per input port basis in the absence of an explicit
   value, to enqueue the packets at each output port.  Any IEEE 802.1D
   switch which supports multiple traffic classes can support this
   operation.

   If a switch selects per port output queues based only on the incoming
   user_priority, as described by IEEE 802.1D, it must treat all
   branches of all multicast sessions within that user_priority class
   with the same queuing mechanism.  Receiver heterogeneity is then not
   possible and this could well lead to the failure of an admission
   control request for the whole multicast session due to a single link
   being oversubscribed.  Note that in the Layer 2 case as distinct from
   the Layer 3 case with RSVP/IntServ, the option of having some
   receivers getting the session with the requested QoS and some getting
   it best effort does not exist as basic IEEE 802.1 switches are unable
   to re-map the user_priority on a per link basis.  This could become
   an issue with heavy use of dynamic multicast sessions.  If a switch
   were to implement a separate user_priority mapping at each output
   port, then, in some cases, reservations can use a different traffic
   class on different paths that branch at such a switch in order to
   provide multiple receivers with different QoS. This is possible if
   all flows within a traffic class at the ingress to a switch egress in
   the same traffic class on a port.  For example, traffic may be
   forwarded using user_priority 4 on one branch where receivers have
   performed admission control and as user_priority 0 on ones where they
   have not.  We assume that per user_priority queuing without taking
   account of input or output ports is the minimum standard
   functionality for switches in a LAN environment (IEEE 802.1D) but
   that more functional Layer 2 or even Layer 3 switches (i.e.  routers)
   can be used if even more flexible forms of heterogeneity are
   considered necessary to achieve more efficient resource utilization.
   The behavior of Layer 3 switches in this context is already well
   standardized by the IETF.

9. Network Topology Scenarios

   The extent to which service guarantees can be provided by a network
   depend to a large degree on the ability to provide the key functions
   of flow identification and scheduling in addition to admission
   control and policing.  This section discusses some of the
   capabilities of the LAN technologies under consideration and provides
   a taxonomy of possible topologies emphasizing the capabilities of
   each with regard to supporting the above functions.  For the

   technologies considered here, the basic topology of a LAN may be
   shared, switched half duplex or switched full duplex.  In the shared
   topology, multiple senders share a single segment.  Contention for
   media access is resolved using protocols such as CSMA/CD in Ethernet
   and token passing in Token Ring and FDDI. Switched half duplex, is
   essentially a shared topology with the restriction that there are
   only two transmitters contending for resources on any segment.
   Finally, in a switched full duplex topology, a full bandwidth path is
   available to the transmitter at each end of the link at all times.
   Therefore, in this topology, there is no need for any access control
   mechanism such as CSMA/CD or token passing as there is no contention
   between the transmitters.  Obviously, this topology provides the best
   QoS capabilities.  Another important element in the discussion of
   topologies is the presence or absence of support for multiple traffic
   classes.  These were discussed earlier in Section 4.1.  Depending on
   the basic topology used and the ability to support traffic classes,
   we identify six scenarios as follows:

   1. Shared topology without traffic classes.
   2. Shared topology with traffic classes.
   3. Switched half duplex topology without traffic classes.
   4. Switched half duplex topology with traffic classes.
   5. Switched full duplex topology without traffic classes.
   6. Switched full duplex topology with traffic classes.

   There is also the possibility of hybrid topologies where two or more
   of the above coexist.  For instance, it is possible that within a
   single subnet, there are some switches which support traffic classes
   and some which do not.  If the flow in question traverses both kinds
   of switches in the network, the least common denominator will
   prevail.  In other words, as far as that flow is concerned, the
   network is of the type corresponding to the least capable topology
   that is traversed.  In the following sections, we present these
   scenarios in further detail for some of the different IEEE 802
   network types with discussion of their abilities to support the
   IntServ services.

9.1. Full Duplex Switched Networks

   On a full duplex switched LAN, the MAC protocol is unimportant as as
   access is concerned, but must be factored into the characterization
   parameters advertised by the device since the access latency is equal
   to the time required to transmit the largest packet.  Approximate
   values for the characteristics on various media are provided in the
   following tables.  These delays should be also be considered in the
   context of the speed of light delay which is approximately 400 ns for
   typical 100 m UTP links and 7 us for typical 2 km multimode fiber
   links.

           Table 4: Full duplex switched media access latency

        --------------------------------------------------
        Type               Speed      Max Pkt   Max Access
                                       Length      Latency
        --------------------------------------------------
        Ethernet         10 Mbps       1.2 ms       1.2 ms
                        100 Mbps       120 us       120 us
                          1 Gbps        12 us        12 us
        Token Ring        4 Mbps         9 ms         9 ms
                         16 Mbps         9 ms         9 ms
        FDDI            100 Mbps       360 us       8.4 ms
        Demand Priority 100 Mbps       120 us       120 us
        --------------------------------------------------

   Full duplex switched network topologies offer good QoS capabilities
   for both Controlled Load and Guaranteed Service when supported by
   suitable queuing strategies in the switches.

9.2. Shared Media Ethernet Networks

   Thus far, we have not discussed the difficulty of dealing with
   allocation on a single shared CSMA/CD segment.  As soon as any
   CSMA/CD algorithm is introduced the ability to provide any form of
   Guaranteed Service is seriously compromised in the absence of any
   tight coupling between the multiple senders on the link.  There are a
   number of reasons for not offering a better solution to this problem.

   Firstly, we do not believe this is a truly solvable problem as it
   would require changes to the MAC protocol.  IEEE 802.1 has examined
   research showing disappointing simulation results for performance
   guarantees on shared CSMA/CD Ethernet without MAC enhancements.
   There have been proposals for enhancements to the MAC layer
   protocols, e.g.  BLAM and enhanced flow control in IEEE 802.3.
   However, any solution involving an enhanced software MAC running
   above the traditional IEEE 802.3 MAC, or other proprietary MAC
   protocols, is outside the scope of the ISSLL working group and this
   document.  Secondly, we are not convinced that it is really an
   interesting problem.  While there will be end stations on shared
   segments for some time to come, the number of deployed switches is
   steadily increasing relative to the number of stations on shared
   segments.  This trend is proceeding to the point where it may be
   satisfactory to have a solution which assumes that any network
   communication requiring resource reservations will take place through
   at least one switch or router.  Put another way, the easiest upgrade
   to existing Layer 2 infrastructure for QoS support is the
   installation of segment switching.  Only when this has been done is
   it worthwhile to investigate more complex solutions involving

   admission control.  Thirdly, the core of campus networks typically
   consists of solutions based on switches rather than on repeated
   segments.  There may be special circumstances in the future, e.g.
   Gigabit buffered repeaters, but the characteristics of these devices
   are different from existing CSMA/CD repeaters anyway.

             Table 5: Shared Ethernet media access latency

        --------------------------------------------------
        Type             Speed        Max Pkt   Max Access
                                       Length      Latency
        --------------------------------------------------
        Ethernet       10 Mbps         1.2 ms    unbounded
                      100 Mbps         120 us    unbounded
                        1 Gbps          12 us    unbounded
        --------------------------------------------------

9.3. Half Duplex Switched Ethernet Networks

   Many of the same arguments for sub optimal support of Guaranteed
   Service on shared media Ethernet also apply to half duplex switched
   Ethernet.  In essence, this topology is a medium that is shared
   between at least two senders contending for packet transmission.
   Unless these are tightly coupled and cooperative, there is always the
   chance that the best effort traffic of one will interfere with the
   reserved traffic of the other.  Dealing with such a coupling would
   require some form of modification to the MAC protocol.

   Not withstanding the above argument, half duplex switched topologies
   do seem to offer the chance to provide Controlled Load service.  With
   the knowledge that there are exactly two potential senders that are
   both using prioritization for their Controlled Load traffic over best
   effort flows, and with admission control having been done for those
   flows based on that knowledge, the media access characteristics while
   not deterministic are somewhat predictable.  This is probably a close
   enough useful approximation to the Controlled Load service.

      Table 6: Half duplex switched Ethernet media access latency

        ------------------------------------------
        Type        Speed     Max Pkt   Max Access
                              Length       Latency
        ------------------------------------------
        Ethernet   10 Mbps     1.2 ms    unbounded
                  100 Mbps     120 us    unbounded
                    1 Gbps      12 us    unbounded
        ------------------------------------------

9.4. Half Duplex Switched and Shared Token Ring Networks

   In a shared Token Ring network, the network access time for high
   priority traffic at any station is bounded and is given by
   (N+1)*THTmax, where N is the number of stations sending high priority
   traffic and THTmax is the maximum token holding time [14].  This
   assumes that network adapters have priority queues so that
   reservation of the token is done for traffic with the highest
   priority currently queued in the adapter.  It is easy to see that
   access times can be improved by reducing N or THTmax.  The
   recommended default for THTmax is 10 ms [6].  N is an integer from 2
   to 256 for a shared ring and 2 for a switched half duplex topology.
   A similar analysis applies for FDDI.

             Table 7: Half duplex switched and shared Token
                       Ring media access latency
        ----------------------------------------------------
        Type        Speed               Max Pkt   Max Access
                                         Length      Latency
        ----------------------------------------------------
        Token Ring  4/16 Mbps shared       9 ms      2570 ms
                    4/16 Mbps switched     9 ms        30 ms
        FDDI         100 Mbps            360 us         8 ms
        ----------------------------------------------------

   Given that access time is bounded, it is possible to provide an upper
   bound for end-to-end delays as required by Guaranteed Service
   assuming that traffic of this class uses the highest priority
   allowable for user traffic.  The actual number of stations that send
   traffic mapped into the same traffic class as Guaranteed Service may
   vary over time but, from an admission control standpoint, this value
   is needed a priori.  The admission control entity must therefore use
   a fixed value for N, which may be the total number of stations on the
   ring or some lower value if it is desired to keep the offered delay
   guarantees smaller.  If the value of N used is lower than the total
   number of stations on the ring, admission control must ensure that
   the number of stations sending high priority traffic never exceeds

   this number.  This approach allows admission control to estimate
   worst case access delays assuming that all of the N stations are
   sending high priority data even though, in most cases, this will mean
   that delays are significantly overestimated.

   Assuming that Controlled Load flows use a traffic class lower than
   that used by Guaranteed Service, no upper bound on access latency can
   be provided for Controlled Load flows.  However, Controlled Load
   flows will receive better service than best effort flows.

   Note that on many existing shared Token Rings, bridges transmit
   frames using an Access Priority (see Section 4.3) value of 4
   irrespective of the user_priority carried in the frame control field
   of the frame.  Therefore, existing bridges would need to be
   reconfigured or modified before the above access time bounds can
   actually be used.

9.5. Half Duplex and Shared Demand Priority Networks

   In IEEE 802.12 networks, communication between end nodes and hubs and
   between the hubs themselves is based on the exchange of link control
   signals.  These signals are used to control access to the shared
   medium.  If a hub, for example, receives a high priority request
   while another hub is in the process of serving normal priority
   requests, then the service of the latter hub can effectively be
   preempted in order to serve the high priority request first.  After
   the network has processed all high priority requests, it resumes the
   normal priority service at the point in the network at which it was
   interrupted.

   The network access time for high priority packets is basically the
   time needed to preempt normal priority network service.  This access
   time is bounded and it depends on the physical layer and on the
   topology of the shared network.  The physical layer has a significant
   impact when operating in half duplex mode as, e.g.  when used across
   unshielded twisted pair cabling (UTP) links, because link control
   signals cannot be exchanged while a packet is transmitted over the
   link.  Therefore the network topology has to be considered since, in
   larger shared networks, the link control signals must potentially
   traverse several links and hubs before they can reach the hub which
   has the network control function.  This may delay the preemption of
   the normal priority service and hence increase the upper bound that
   may be guaranteed.

   Upper bounds on the high priority access time are given below for a
   UTP physical layer and a cable length of 100 m between all end nodes
   and hubs using a maximum propagation delay of 570 ns as defined in

   [19].  These values consider the worst case signaling overhead and
   assume the transmission of maximum sized normal priority data packets
   while the normal priority service is being preempted.

      Table 8: Half duplex switched Demand Priority UTP access latency

        ------------------------------------------------------------
        Type            Speed                    Max Pkt  Max Access
                                                  Length     Latency
        ------------------------------------------------------------
        Demand Priority 100 Mbps, 802.3 pkt, UTP  120 us      254 us
                                  802.5 pkt, UTP  360 us      733 us
        ------------------------------------------------------------

   Shared IEEE 802.12 topologies can be classified using the hub
   cascading level "N".  The simplest topology is the single hub network
   (N = 1).  For a UTP physical layer, a maximum cascading level of N =
   5 is supported by the standard.  Large shared networks with many
   hundreds of nodes may be built with a level 2 topology.  The
   bandwidth manager could be informed about the actual cascading level
   by network management mechanisms and can use this information in its
   admission control algorithms.

   In contrast to UTP, the fiber optic physical layer operates in dual
   simplex mode.  Upper bounds for the high priority access time are
   given below for 2 km multimode fiber links with a propagation delay
   of 10 us.

   For shared media with distances of up to 2 km between all end nodes
   and hubs, the IEEE 802.12 standard allows a maximum cascading level
   of 2.  Higher levels of cascaded topologies are supported but require
   a reduction of the distances [15].

   The bounded access delay and deterministic network access allow the
   support of service commitments required for Guaranteed Service and
   Controlled Load, even on shared media topologies.  The support of
   just two priority levels in 802.12, however, limits the number of
   services that can simultaneously be implemented across the network.

           Table 9: Shared Demand Priority UTP access latency

     ----------------------------------------------------------------
     Type            Speed              Max Pkt  Max Access  Topology
                                         Length     Latency
     ----------------------------------------------------------------
     Demand Priority 100 Mbps, 802.3 pkt 120 us      262 us     N = 1
                                         120 us      554 us     N = 2
                                         120 us      878 us     N = 3
                                         120 us     1.24 ms     N = 4
                                         120 us     1.63 ms     N = 5

     Demand Priority 100 Mbps, 802.5 pkt 360 us      722 us     N = 1
                                         360 us     1.41 ms     N = 2
                                         360 us     2.32 ms     N = 3
                                         360 us     3.16 ms     N = 4
                                         360 us     4.03 ms     N = 5
     -----------------------------------------------------------------

             Table 10: Half duplex switched Demand Priority
                          fiber access latency
     -------------------------------------------------------------
     Type            Speed                     Max Pkt  Max Access
                                                Length     Latency
     -------------------------------------------------------------
     Demand Priority 100 Mbps, 802.3 pkt, fiber 120 us      139 us
                               802.5 pkt, fiber 360 us      379 us
     -------------------------------------------------------------

         Table 11: Shared Demand Priority fiber access latency

     ---------------------------------------------------------------
     Type            Speed              Max Pkt  Max Access Topology
                                         Length    Latency
     ---------------------------------------------------------------
     Demand Priority 100 Mbps, 802.3 pkt 120 us     160 us     N = 1
                                         120 us     202 us     N = 2

     Demand Priority 100 Mbps, 802.5 pkt 360 us     400 us     N = 1
                                         360 us     682 us     N = 2
     ---------------------------------------------------------------

10. Justification

   An obvious concern is the complexity of this model.  It essentially
   does what RSVP already does at Layer 3, so why do we think we can do
   better by reinventing the solution to this problem at Layer 2?

   The key is that there are a number of simple Layer 2 scenarios that
   cover a considerable portion of the real QoS problems that will
   occur.  A solution that covers the majority of problems at
   significantly lower cost is beneficial.  Full RSVP/IntServ with per
   flow queuing in strategically positioned high function switches or
   routers may be needed to completely resolve all issues, but devices
   implementing the architecture described in herein will allow for a
   significantly simpler network.

11. Summary

   This document has specified a framework for providing Integrated
   Services over shared and switched LAN technologies.  The ability to
   provide QoS guarantees necessitates some form of admission control
   and resource management.  The requirements and goals of a resource
   management scheme for subnets have been identified and discussed.  We
   refer to the entire resource management scheme as a Bandwidth
   Manager.  Architectural considerations were discussed and examples
   were provided to illustrate possible implementations of a Bandwidth
   Manager.  Some of the issues involved in mapping the services from
   higher layers to the link layer have also been discussed.
   Accompanying memos from the ISSLL working group address service
   mapping issues [13] and provide a protocol specification for the
   Bandwidth Manager protocol [14] based on the requirements and goals
   discussed in this document.

References

   [1]  IEEE Standards for Local and Metropolitan Area Networks:
        Overview and Architecture, ANSI/IEEE Std 802, 1990.

   [2]  ISO/IEC 10038 Information technology - Telecommunications and
        information exchange between systems - Local area networks -
        Media Access Control (MAC) Bridges, (also ANSI/IEEE Std 802.1D-
        1993), 1993.

   [3]  ISO/IEC 15802-3 Information technology - Telecommunications and
        information exchange between systems - Local and metropolitan
        area networks - Common specifications - Part 3: Media Access
        Control (MAC) bridges (also ANSI/IEEE Std 802.1D-1998), 1998.

   [4]  IEEE Standards for Local and Metropolitan Area Networks:
        Virtual Bridged Local Area Networks, IEEE Std 802.1Q-1998, 1998.

   [5]  Braden, B., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
        "Resource Reservation Protocol (RSVP) - Version 1 Functional
        Specification", RFC 2205, September 1997.

   [6]  Wroclawski, J., "Specification of the Controlled Load Network
        Element Service", RFC 2211, September 1997.

   [7]  Shenker, S., Partridge, C. and R. Guerin, "Specification of
        Guaranteed Quality of Service", RFC 2212, September 1997.

   [8]  Braden, R., Clark, D. and S. Shenker, "Integrated Services in
        the Internet Architecture: An Overview", RFC 1633, June 1994.

   [9]  Wroclawski, J., "The Use of RSVP with IETF Integrated Services",
        RFC 2210, September 1997.

   [10] Shenker, S. and J. Wroclawski, "Network Element Service
        Specification Template", RFC 2216, September 1997.

   [11] Shenker, S. and J. Wroclawski, "General Characterization
        Parameters for Integrated Service Network Elements", RFC 2215,
        September 1997.

   [12] Delgrossi, L. and L. Berger (Editors), "Internet Stream Protocol
        Version 2 (ST2)  Protocol Specification - Version ST2+", RFC
        1819, August 1995.

   [13] Seaman, M., Smith, A. and E. Crawley, "Integrated Service
        Mappings on IEEE 802 Networks", RFC 2815, May 2000.

   [14] Yavatkar, R., Hoffman, D., Bernet, Y. and F. Baker,  "SBM Subnet
        Bandwidth Manager): Protocol for RSVP-based Admission Control
        Over IEEE 802-style Networks", RFC 2814, May 2000.

   [15] ISO/IEC 8802-3 Information technology - Telecommunications and
        information exchange between systems - Local and metropolitan
        area networks - Common specifications - Part 3:  Carrier Sense
        Multiple Access with Collision Detection (CSMA/CD) Access Method
        and Physical Layer Specifications, (also ANSI/IEEE Std 802.3-
        1996), 1996.

   [15] ISO/IEC 8802-5 Information technology - Telecommunications and
        information exchange between systems - Local and metropolitan
        area networks - Common specifications - Part 5: Token Ring
        Access Method and Physical Layer Specifications, (also ANSI/IEEE
        Std 802.5-1995), 1995.

   [17] Postel, J. and J. Reynolds, "A Standard for the Transmission of
        IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042, February
        1988.

   [18] C. Bisdikian, B. V. Patel, F. Schaffa, and M Willebeek-LeMair,
        The Use of Priorities on Token Ring Networks for Multimedia
        Traffic, IEEE Network, Nov/Dec 1995.

   [19] IEEE Standards for Local and Metropolitan Area Networks:  Demand
        Priority Access Method, Physical Layer and Repeater
        Specification for 100 Mb/s Operation, IEEE Std 802.12-1995.

   [20] Fiber Distributed Data Interface MAC, ANSI Std. X3.139-1987.

   [21] ISO/IEC 15802-3 Information technology - Telecommunications and
        information exchange between systems - Local and metropolitan
        area networks - Specific requirements - Supplement to Carrier
        Sense Multiple Access with Collision Detection (CSMA/CD) Access
        Method and Physical Layer Specifications - Frame Extensions for
        Virtual Bridged Local Area Network (VLAN) Tagging on 802.3
        Networks, IEEE Std 802.3ac-1998 (Supplement to IEEE 802.3 1998
        Edition), 1998.

Security Considerations

   Implementation of the model described in this memo creates no known
   new avenues for malicious attack on the network infrastructure.
   However, readers are referred to Section 2.8 of the RSVP
   specification [5] for a discussion of the impact of the use of
   admission control signaling protocols on network security.

Acknowledgements

   Much of the work presented in this document has benefited greatly
   from discussion held at the meetings of the Integrated Services over
   Specific Link Layers (ISSLL) working group.  We would like to
   acknowledge contributions from the many participants via discussion
   at these meetings and on the mailing list.  We would especially like
   to thank Eric Crawley, Don Hoffman and Raj Yavatkar for contributions
   via previous Internet drafts, and Peter Kim for contributing the text
   about Demand Priority networks.

Authors' Addresses

   Anoop Ghanwani
   Nortel Networks
   600 Technology Park Dr
   Billerica, MA 01821, USA

   Phone: +1-978-288-4514
   EMail: aghanwan@nortelnetworks.com

   Wayne Pace
   IBM Corporation
   P. O. Box 12195
   Research Triangle Park, NC 27709, USA

   Phone: +1-919-254-4930
   EMail: pacew@us.ibm.com

   Vijay Srinivasan
   CoSine Communications
   1200 Bridge Parkway
   Redwood City, CA 94065, USA

   Phone: +1-650-628-4892
   EMail: vijay@cosinecom.com

   Andrew Smith
   Extreme Networks
   3585 Monroe St
   Santa Clara, CA 95051, USA

   Phone: +1-408-579-2821
   EMail: andrew@extremenetworks.com

   Mick Seaman
   Telseon
   480 S. California Ave
   Palo Alto, CA 94306
   USA

   Email: mick@telseon.com

Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

User Contributions:

Comment about this RFC, ask questions, or add new information about this topic:

CAPTCHA