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Internet Engineering Task Force                           Anoop Ghanwani
INTERNET DRAFT                                             J. Wayne Pace
                                                        Vijay Srinivasan
                                                                   (IBM)
                                                            Andrew Smith
                                                      (Extreme Networks)
                                                             Mick Seaman
                                                                  (3Com)
                                                                May 1998


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

                draft-ietf-issll-is802-framework-05.txt


Status of This Memo

   This document is an Internet-Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months, and may be updated, replaced, or obsoleted by other documents
   at any time.  It is not appropriate to use Internet Drafts as
   reference material, or to cite them other than as a ``working draft''
   or ``work in progress.''

   To view the entire list of current Internet-Drafts, please check
   the "1id-abstracts.txt" listing contained in the Internet-Drafts
   Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
   (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
   (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
   (US West Coast).



















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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 queueing 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].





































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                                Contents



Status of This Memo                                                    i

Abstract                                                              ii

 1. Introduction                                                       1

 2. Document Outline                                                   1

 3. Definitions                                                        2

 4. Frame Forwarding in IEEE 802 Networks                              3
     4.1. General IEEE 802 Service Model  . . . . . . . . . . . . .    3
     4.2. Ethernet/IEEE 802.3 . . . . . . . . . . . . . . . . . . .    5
     4.3. Token Ring/IEEE 802.5 . . . . . . . . . . . . . . . . . .    6
     4.4. Fiber Distributed Data Interface  . . . . . . . . . . . .    7
     4.5. Demand Priority/IEEE 802.12 . . . . . . . . . . . . . . .    8

 5. Requirements and Goals                                             9
     5.1. Requirements  . . . . . . . . . . . . . . . . . . . . . .    9
     5.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   11
     5.3. Non-goals . . . . . . . . . . . . . . . . . . . . . . . .   12
     5.4. Assumptions . . . . . . . . . . . . . . . . . . . . . . .   12

 6. Basic Architecture                                                13
     6.1. Components  . . . . . . . . . . . . . . . . . . . . . . .   13
           6.1.1. Requester Module  . . . . . . . . . . . . . . . .   13
           6.1.2. Bandwidth Allocator . . . . . . . . . . . . . . .   14
           6.1.3. Communication Protocols . . . . . . . . . . . . .   14
     6.2. Centralized vs.  Distributed Implementations  . . . . . .   15

 7. Model of the Bandwidth Manager in a Network                       17
     7.1. End Station Model . . . . . . . . . . . . . . . . . . . .   17
           7.1.1. Layer 3 Client Model  . . . . . . . . . . . . . .   17
           7.1.2. Requests to Layer 2 ISSLL . . . . . . . . . . . .   17
           7.1.3. At the Layer 3 Sender . . . . . . . . . . . . . .   18
           7.1.4. At the Layer 3 Receiver . . . . . . . . . . . . .   19
     7.2. Switch Model  . . . . . . . . . . . . . . . . . . . . . .   21
           7.2.1. Centralized Bandwidth Allocator . . . . . . . . .   21
           7.2.2. Distributed Bandwidth Allocator . . . . . . . . .   21
     7.3. Admission Control . . . . . . . . . . . . . . . . . . . .   22
     7.4. QoS Signaling . . . . . . . . . . . . . . . . . . . . . .   24
           7.4.1. Client Service Definitions  . . . . . . . . . . .   24



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           7.4.2. Switch Service Definitions  . . . . . . . . . . .   25

 8. Implementation Issues                                             27
     8.1. Switch Characteristics  . . . . . . . . . . . . . . . . .   27
     8.2. Queueing  . . . . . . . . . . . . . . . . . . . . . . . .   28
     8.3. Mapping of Services to Link Level Priority  . . . . . . .   29
     8.4. Re-mapping of Non-conforming Aggregated Flows . . . . . .   29
     8.5. Override of Incoming User Priority  . . . . . . . . . . .   30
     8.6. Different Reservation Styles  . . . . . . . . . . . . . .   30
     8.7. Receiver Heterogeneity  . . . . . . . . . . . . . . . . .   31

 9. Network Topology Scenarios                                        34
     9.1. Full Duplex Switched Networks . . . . . . . . . . . . . .   35
     9.2. Shared Media Ethernet Networks  . . . . . . . . . . . . .   35
     9.3. Half Duplex Switched Ethernet Networks  . . . . . . . . .   36
     9.4. Half Duplex Switched and Shared Token Ring Networks . . .   37
     9.5. Half Duplex and Shared Demand Priority Networks . . . . .   38

10. Justification                                                     41

11. Summary                                                           41






























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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



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   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 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




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       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
       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.

   IEEE 802.1D [3] (1) defines a consistent way carry this value over a
   bridged network consisting of Ethernet, Token Ring, Demand Priority,

----------------------------
1. 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.




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   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 queueing 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 P802.1Q 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 queueing and round robin.






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   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 P802.1Q 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.


                    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 draft IEEE P802.1Q specification exceeds the current
   IEEE 802.3 maximum packet length values by 4 bytes.  The change of
   maximum MTU size for IEEE P802.1Q frames is being accommodated by
   IEEE P802.3ac.








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4.3. Token Ring/IEEE 802.5

   The Token Ring standard [6] provides a priority mechanism that can
   be used to control both the queueing 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 an
   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
   the 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 queueing
   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.

   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 the draft 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.



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          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
            -------------------------------------



   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 what 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 queueing
   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



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   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 P802.1Q 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 abnormal situation 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.





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   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.





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    -  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



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       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




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       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"; 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 to complicate the targeted practical application which
   is 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 queueing 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



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   assumed that such switches do not need to implement per flow queueing
   and policing (although they may do so).

   A fundamental assumption of the IntServ model is that flows are
   isolated from each other throughout their transit across a network.
   Intermediate queueing nodes are expected 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



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   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, and
       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.




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    -  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 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 which desire 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.



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   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 model 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.








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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.


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 hop.  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 source.

   For now, we assume that the client runs an RSVP process which
   presents a session establishment interface to applications, signals
   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.


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 language.

   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,



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


                    Figure 3: RSVP in Sending Hosts



   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.






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                    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



    -  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.

   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 queueing resources.


7.1.4. At the Layer 3 Receiver

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

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




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                          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



    -  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.

    -  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.






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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



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       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 hold 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 dequeueing 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.


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.




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                    +-------------------------------+
   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



    -  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 queueing 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.





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    -  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 original client 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




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       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 potential 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 later stage.
       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.



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    -  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



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       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 IEEE 802.1D [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.  It will include
   support for queueing 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



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   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 gains more control over
   admission to 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 queueing 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. Queueing

   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 on
   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



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   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
   becomes 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 queueing disciplines.  As with the service specifications
   themselves, the specification of queueing 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



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   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, 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



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


                Figure 7: Illustration of filter styles



   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.


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



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   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.


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


              Figure 8: Example of receiver heterogeneity



   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.

   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



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   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 queueing 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 queueing 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.




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   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




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   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 far as access is concerned, but must be factored in to 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


           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
        --------------------------------------------------


   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.

   Full duplex switched network topologies offer good QoS capabilities
   for both Controlled Load and Guaranteed Service when supported by
   suitable queueing 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.




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   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 repeated
   segments for some time to come, the number of deployed switches is
   steadily increasing relative to the number of stations on repeated
   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, there 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



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   reserved traffic of the other.  Dealing with such a coupling would
   seem to 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. Using the default values gives

   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



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             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
        ----------------------------------------------------



   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.




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   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.





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           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
     ------------------------------------------------------------



   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.









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         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 queueing 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.




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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 Final CD 15802-3 Information technology - Tele-
     communications and information exchange between systems -
     Local and metropolitan area networks - Common specifications -
     Part 3: Media Access Control (MAC) bridges, (current draft
     available as IEEE P802.1D/D15).

[4]  IEEE Standards for Local and Metropolitan Area Networks: Draft
     Standard for Virtual Bridged Local Area Networks, P802.1Q/D8,
     January 1998.

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

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

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

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

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

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

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

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

[13] M. Seaman, A. Smith, and E. Crawley, Integrated Service Mappings on
     IEEE 802 Networks, work in progress, Internet Draft,



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     <draft-ietf-issll-is802-svc-mapping-01.txt>, November 1997.

[14] R. Yavatkar, D. Hoffman, Y. Bernet, and F. Baker,  SBM
     (Subnet Bandwidth Manager): Protocol for RSVP-based Admission
     Control over IEEE 802-style networks, work in progress,
     Internet Draft, <draft-ietf-issll-is802-bm-05.txt>,
     November 1997.

[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] J. Postel and J. Reynolds, A Standard for the Transmission of
     IP Datagrams over IEEE 802 Networks, 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.


Security Considerations

   Implementation of the model described in this memo creates no known
   new avenues for malicious attack on the network infrastructure
   although 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



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   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
        IBM Corporation
        P.O.Box 12195
        Research Triangle Park, NC 27709
        USA
        +1-919-254-0260
        anoop@raleigh.ibm.com

        J. Wayne Pace
        IBM Corporation
        P. O. Box 12195
        Research Triangle Park, NC 27709
        USA
        +1-919-254-4930
        pacew@raleigh.ibm.com

        Vijay Srinivasan
        IBM Corporation
        P. O. Box 12195
        Research Triangle Park, NC 27709
        USA
        +1-919-254-2730
        vijay@raleigh.ibm.com

        Andrew Smith
        Extreme Networks
        10460 Bandley Drive
        Cupertino CA 95014
        USA
        +1-408-863-2821
        andrew@extremenetworks.com

        Mick Seaman
        3Com Corporation
        5400 Bayfront Plaza
        Santa Clara CA 95052-8145
        USA
        +1-408-764-5000



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        mick_seaman@3com.com


















































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