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Versions: 04 06 07 08 09 10 RFC 2814

   Internet Engineering Task Force               Raj Yavatkar, Intel
   INTERNET-DRAFT                                Don Hoffman, Teledesic
                                                 Yoram Bernet, Microsoft
                                                 Fred Baker, Cisco
                                                 Michael Speer, Sun Microsystems

                                                 October 1999

                      SBM (Subnet Bandwidth Manager):
   A Protocol for RSVP-based Admission Control over IEEE 802-style networks

                            Status of this Memo

   This document is an Internet-Draft and is in full conformance with all
   provisions of Section 10 of RFC2026.

   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 inappropriate to use Internet Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

                                  Abstract

   This document describes a signaling method and protocol for RSVP-based
   admission control over IEEE 802-style LANs. The protocol is designed
   to work both with the current generation of IEEE 802 LANs as well as with the
   recent work completed by the IEEE 802.1 committee.

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   1. Introduction

   New extensions to the Internet architecture and service models have
   been defined for an integrated services Internet [RFC-1633, RFC-2205,
   RFC-2210] so that applications can request specific qualities or lev-
   els of service from an internetwork in addition to the current IP
   best-effort service.  These extensions include RSVP, a resource reser-
   vation setup protocol, and definition of new service classes to be
   supported by Integrated Services routers. RSVP and service class
   definitions are largely independent of the underlying networking tech-
   nologies and it is necessary to define the mapping of RSVP and
   Integrated Services specifications onto specific subnetwork technolo-
   gies. For example, a definition of service mappings and reservation
   setup protocols is needed for specific link-layer technologies such as
   shared and switched IEEE-802-style LAN technologies.

   This document defines SBM, a signaling protocol for RSVP-based admis-
   sion control over IEEE 802-style networks. SBM provides a method for
   mapping an internet-level setup protocol such as RSVP onto IEEE 802-
   style networks.  In particular, it describes the operation of RSVP-
   enabled hosts/routers and link layer devices (switches, bridges) to
   support reservation of LAN resources for RSVP-enabled data flows.  A
   framework for providing Integrated Services over shared and switched
   IEEE-802-style LAN technologies and a definition of service mappings
   have been described in separate documents [RFC-FRAME, RFC-MAP].

   2. Goals and Assumptions

   The SBM (Subnet Bandwidth Manager) protocol and its use for admission
   control and bandwidth management in IEEE 802 level-2 networks is based
   on the following architectural goals and assumptions:

   I.   Even though the current trend is towards increased use of
        switched LAN topologies consisting of newer switches that support
        the priority queuing mechanisms specified by IEEE 802.1p, we
        assume that the LAN technologies will continue to be a mix of
        legacy shared/ switched LAN segments and newer switched segments
        based on IEEE 802.1p specification.  Therefore, we specify a sig-
        naling protocol for managing bandwidth over both legacy and newer
        LAN topologies and that takes advantage of the additional func-
        tionality (such as an explicit support for different traffic
        classes or integrated service classes) as it becomes available in
        the new generation of switches, hubs, or bridges. As a result,
        the SBM protocol would allow for a range of LAN bandwidth

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                       SBM (Subnet Bandwidth Manager)         October, 1999

        management solutions that vary from one that exercises purely
        administrative control (over the amount of bandwidth consumed by
        RSVP-enabled traffic flows) to one that requires cooperation (and
        enforcement) from all the end-systems or switches in a IEEE 802
        LAN.

   II.  This document specifies only a signaling method and protocol for
        LAN-based admission control over RSVP flows.  We do not define
        here any traffic control mechanisms for the link layer; the pro-
        tocol is designed to use any such mechanisms defined by IEEE 802.
        In addition, we assume that the Layer 3 end-systems (e.g., a host
        or a router) will exercise traffic control by policing Integrated
        Services traffic flows to ensure that each flow stays within its
        traffic specifications stipulated in an earlier reservation
        request submitted for admission control.  This then allows a sys-
        tem using SBM admission control combined with per-flow shaping at
        end-systems and IEEE-defined traffic control at link layer to
        realize some approximation of Controlled Load (and even
        Guaranteed) services over IEEE 802-style LANs.

   III. In the absence of any link-layer traffic control or priority
        queuing mechanisms in the underlying LAN (such as a shared LAN
        segment), the SBM-based admission control mechanism only limits
        the total amount of traffic load imposed by RSVP-enabled flows on
        a shared LAN.  In such an environment, no traffic flow separation
        mechanism exists to protect the RSVP-enabled flows from the
        best-effort traffic on the same shared media and that raises the
        question of the utility of such a mechanism outside a topology
        consisting only of 802.1p-compliant switches. However, we assume
        that the SBM-based admission control mechanism will still serve a
        useful purpose in a legacy, shared LAN topology for two reasons.
        First, assuming that all the nodes that generate Integrated Ser-
        vices traffic flows utilize the SBM-based admission control pro-
        cedure to request reservation of resources before sending any
        traffic, the mechanism will restrict the total amount of traffic
        generated by Integrated Services flows within the bounds desired
        by a LAN administrator (see discussion of the NonResvSendLimit
        parameter in Appendix C). Second, the best-effort traffic gen-
        erated by the TCP/IP-based traffic sources is generally rate-
        adaptive (using a TCP-style "slow start" congestion avoidance
        mechanism or a feedback-based rate adaptation mechanism used by
        audio/video streams based on RTP/RTCP protocols) and adapts to
        stay within the available network bandwidth. Thus, the combina-
        tion of admission control and rate adaptation should avoid per-
        sistent traffic congestion. This does not, however, guarantee
        that non-Integrated-Services traffic will not interfere with the

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                       SBM (Subnet Bandwidth Manager)         October, 1999

        Integrated Services traffic in the absence of traffic control
        support in the underlying LAN infrastructure.

   3. Organization of the rest of this document

   The rest of this document provides a detailed description of the SBM-
   based admission control procedure(s) for IEEE 802 LAN technologies.
   The document is organized as follows:

   *    Section 4 first defines the various terms used in the document
        and then provides an overview of the admission control procedure
        with an example of its application to a sample network.

   *    Section 5 describes the rules for processing and forwarding PATH
        (and PATH_TEAR) messages at DSBMs (Designated Subnet Bandwidth
        Managers), SBMs, and DSBM clients.

   *    Section 6 addresses the inter-operability issues when a DSBM may
        operate in the absence of RSVP signaling at Layer 3 or when
        another signaling protocol (such as SNMP) is used to reserve
        resources on a LAN segment.

   *    Appendix A describes the details of the DSBM election algorithm
        used for electing a designated SBM on a LAN segment when more
        than one SBM is present.  It also describes how DSBM clients dis-
        cover the presence of a DSBM on a managed segment.

   *    Appendix B specifies the formats of SBM-specific messages used
        and the formats of new RSVP objects needed for the SBM operation.

   *    Appendix C describes usage of the DSBM to distribute configura-
        tion information to senders on a managed segment.

   4. Overview

   4.1. Definitions

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   -    Link Layer or Layer 2 or L2: We refer to data-link layer techno-
        logies such as IEEE 802.3/Ethernet as L2 or layer 2.

   -    Link Layer Domain or Layer 2 domain or L2 domain: 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: We refer to devices that only implement
        Layer 2 functionality as Layer 2 or L2 devices. These include
        802.1D bridges or switches.

   -    Internetwork Layer or Layer 3 or L3: Layer 3 of the ISO 7 layer
        model. This document 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 L2 physical segment that is shared by one or more
        senders. Examples of segments include (a) a shared Ethernet or
        Token-Ring wire resolving contention for media access using CSMA
        or token passing ("shared L2 segment"), (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: An aggregation of data flows which are given simi-
        lar service within a switched network.

   -    User_priority: 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 ser-
        vice. It is provided along with the data to a receiver using the
        MAC service. It may or may not be actually carried over the

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        network: Token-Ring/802.5 carries this value (encoded in its FC
        octet), basic Ethernet/802.3 does not, 802.12 may or may not
        depending on the frame format in use. 802.1p defines a consistent
        way to carry this value over the bridged network on Ethernet,
        Token Ring, Demand-Priority, FDDI or other MAC-layer media using
        an extended frame format. The usage of user_priority is fully
        described in section 2.5 of 802.1D [IEEE8021D] and 802.1p
        [IEEE8021P] "Support of the Internal Layer Service by Specific
        MAC Procedures".

   -    Subnet: used in this memo to indicate a group of L3 devices shar-
        ing a common L3 network address prefix along with the set of seg-
        ments making up the L2 domain in which they are located.

   -    Bridge/Switch: a layer 2 forwarding device as defined by IEEE
        802.1D. The terms bridge and switch are used synonymously in this
        document.

   -    DSBM: Designated SBM (DSBM) is a protocol entity that resides in
        a L2 or L3 device and manages resources on a L2 segment. At most
        one DSBM exists for each L2 segment.

   -    SBM: the SBM is a protocol entity that resides in a L2 or L3 dev-
        ice and is capable of managing resources on a segment. However,
        only a DSBM manages the resources for a managed segment. When
        more than one SBM exists on a segment, one of the SBMs is elected
        to be the DSBM.

   -    Extended segment: An extended segment includes those parts of a
        network which are members of the same IP subnet and therefore are
        not separated by any layer 3 devices. Several managed segments,
        interconnected by layer 2 devices, constitute an extended seg-
        ment.

   -    Managed L2 domain: An L2 domain consisting of managed segments is
        referred to as a managed L2 domain to distinguish it from a L2
        domain with no DSBMs present for exercising admission control
        over resources at segments in the L2 domain.

   -    DSBM clients: These are entities that transmit traffic onto a

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                       SBM (Subnet Bandwidth Manager)         October, 1999

        managed segment and use the services of a DSBM for the managed
        segment for admission control over a LAN segment. Only the layer
        3 or higher layer entities on L3 devices such as hosts and
        routers are expected to send traffic that requires resource
        reservations, and, therefore, DSBM clients are L3 entities.

   -    SBM transparent devices: A "SBM transparent" device is unaware of
        SBMs or DSBMs (though it may or may not be RSVP aware) and,
        therefore, does not participate in the SBM-based admission con-
        trol procedure over a managed segment. Such a device uses stan-
        dard forwarding rules appropriate for the device and is tran-
        sparent with respect to SBM.  An example of such a L2 device is a
        legacy switch that does not participate in resource reservation.

   -    Layer 3 and layer 2 addresses: We refer to layer 3 addresses of
        L3/L2 devices as "L3 addresses" and layer 2 addresses as "L2
        addresses". This convention will be used in the rest of the docu-
        ment to distinguish between Layer 3 and layer 2 addresses used to
        refer to RSVP next hop (NHOP) and previous hop (PHOP) devices.
        For example, in conventional RSVP message processing, RSVP_HOP
        object in a PATH message carries the L3 address of the previous
        hop device. We will refer to the address contained in the
        RSVP_HOP object as the RSVP_HOP_L3 address and the corresponding
        MAC address of the previous hop device will be referred to as the
        RSVP_HOP_L2 address.

   4.2. Overview of the SBM-based Admission Control Procedure

   A protocol entity called "Designated SBM" (DSBM) exists for each
   managed segment and is responsible for admission control over the
   resource reservation requests originating from the DSBM clients in
   that segment.  Given a segment, one or more SBMs may exist on the seg-
   ment. For example, many SBM-capable devices may be attached to a
   shared L2 segment whereas two SBM-capable switches may share a half-
   duplex switched segment. In that case, a single DSBM is elected for
   the segment. The procedure for dynamically electing the DSBM is
   described in Appendix A. The only other approved method for specifying
   a DSBM for a managed segment is static configuration at SBM-capable
   devices.

   The presence of a DSBM makes the segment a "managed segment". Some-
   times, two or more L2 segments may be interconnected by SBM tran-
   sparent devices. In that case, a single DSBM will manage the resources
   for those segments treating the collection of such segments as a

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   single managed segment for the purpose of admission control.

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   4.2.1. Basic Algorithm

   Figure 1 - An Example of a Managed Segment.

          +-------+      +-----+     +------+    +-----+   +--------+
          |Router |      | Host|     | DSBM |    | Host|   | Router |
          | R2    |      | C   |     +------+    |  B  |   |  R3    |
          +-------+      +-----+     /           +-----+   +--------+
             |             |        /               |          |
             |             |       /                |          |
      ==============================================================LAN
                       |                                   |
                       |                                   |
                     +------+                          +-------+
                     | Host |                          | Router|
                     |  A   |                          |   R1  |
                     +------+                          +-------+

   Figure 1 shows an example of a managed segment in a L2 domain that
   interconnects a set of hosts and routers. For the purpose of this dis-
   cussion, we ignore the actual physical topology of the L2 domain
   (assume it is a shared L2 segment and a single managed segment
   represents the entire L2 domain). A single SBM device is designated to
   be the DSBM for the managed segment. We will provide examples of
   operation of the DSBM over switched and shared segments later in the
   document.

   The basic DSBM-based admission control procedure works as follows:

   1.   DSBM Initialization:  As part of its initial configuration, DSBM
        obtains information such as the limits on fraction of available
        resources that can be reserved on each managed segment under its
        control. For instance, bandwidth is one such resource. Even
        though methods such as auto-negotiation of link speeds and
        knowledge of link topology allow discovery of link capacity, the
        configuration may be necessary to limit the fraction of link
        capacity that can be reserved on a link.  Configuration is likely
        to be static with the current L2/L3 devices. Future work may
        allow for dynamic discovery of this information. This document
        does not specify the configuration mechanism.

   2.   DSBM Client Initialization:  For each interface attached, a DSBM
        client determines whether a DSBM exists on the interface. The

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        procedure for discovering and verifying the existence of the DSBM
        for an attached segment is described in Appendix A. If the client
        itself is capable of serving as the DSBM on the segment, it may
        choose to participate in the election to become the DSBM. At the
        start, a DSBM client first verifies that a DSBM exists in its L2
        domain so that it can communicate with the DSBM for admission
        control purposes.

        In the case of a full-duplex segment, an election may not be
        necessary as the SBM at each end will typically act as the DSBM
        for outgoing traffic in each direction.

   3.   DSBM-based Admission Control: To request reservation of resources
        (e.g., LAN bandwidth in a L2 domain), DSBM clients (RSVP-capable
        L3 devices such as hosts and routers) follow the following steps:

     a)   When a DSBM client sends or forwards a RSVP PATH message over
          an interface attached to a managed segment, it sends the PATH
          message to the segment's DSBM instead of sending it to the RSVP
          session destination address (as is done in conventional RSVP
          processing). After processing (and possibly updating an
          ADSPEC), the DSBM will forward the PATH message toward its des-
          tination address. As part of its processing, the DSBM builds
          and maintains a PATH state for the session and notes the previ-
          ous L2/L3 hop that sent it the PATH message.

          Let us consider the managed segment in Figure 1. Assume that a
          sender to a RSVP session (session address specifies the IP
          address of host A on the managed segment in Figure 1) resides
          outside the L2 domain of the managed segment and sends a PATH
          message that arrives at router R1 which is on the path towards
          host A.

          DSBM client on Router R1 forwards the PATH message from the
          sender to the DSBM. The DSBM processes the PATH message and
          forwards the PATH message towards the RSVP receiver (Detailed
          message processing and forwarding rules are described in Sec-
          tion 5).  In the process, the DSBM builds the PATH state,
          remembers the router R1 (its L2 and l3 addresses) as the previ-
          ous hop for the session, puts its own L2 and L3 addresses in
          the PHOP objects (see explanation later), and effectively
          inserts itself as an intermediate node between the sender (or
          R1 in Figure 1) and the receiver (host A) on the managed seg-
          ment.

     b)   When an application on host A wishes to make a reservation for

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                       SBM (Subnet Bandwidth Manager)         October, 1999

          the RSVP session, host A follows the standard RSVP message pro-
          cessing rules and sends a RSVP RESV message to the previous hop
          L2/L3 address (the DSBMs address) obtained from the PHOP
          object(s) in the previously received PATH message.

     c)   The DSBM processes the RSVP RESV message based on the bandwidth
          available and returns an RESV_ERR message to the requester (host
          A) if the request cannot be granted. If sufficient resources
          are available and the reservation request is granted, the DSBM
          forwards the RESV message towards the PHOP(s) based on its
          local PATH state for the session. The DSBM merges reservation
          requests for the same session as and when possible using the
          rules similar to those used in the conventional RSVP processing
          (except for an additional criterion described in Section 5.9).

     d)   If the L2 domain contains more than one managed segment, the
          requester (host A) and the forwarder (router R1) may be
          separated by more than one managed segment. In that case, the
          original PATH message would propagate through many DSBMs (one
          for each managed segment on the path from R1 to A) setting up
          PATH state at each DSBM. Therefore, the RESV message would pro-
          pagate hop-by-hop in reverse through the intermediate DSBMs and
          eventually reach the original forwarder (router R1) on the L2
          domain if admission control at all DSBMs succeeds.

   4.2.2. Enhancements to the conventional RSVP operation

   (D)SBMs and DSBM clients implement minor additions to the standard
   RSVP protocol. These are summarized in this section. A detailed
   description of the message processing and forwarding rules follows in
   section 5.

   4.2.2.1 Sending PATH Messages to the DSBM on a Managed Segment

   Normal RSVP forwarding rules apply at a DSBM client when it is not
   forwarding an outgoing PATH message over a managed segment. However,
   outgoing PATH messages on a managed segment are sent to the DSBM for
   the corresponding managed segment (Section 5.2 describes how the PATH
   messages are sent to the DSBM on a managed segment).

   4.2.2.2 The LAN_NHOP Objects

   In conventional RSVP processing over point-to-point links, RSVP nodes
   (hosts/routers) use RSVP_HOP object (NHOP and PHOP info) to keep track
   of the next hop (downstream node in the path of data packets in a

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   traffic flow) and the previous hop (upstream nodes with respect to the
   data flow) nodes on the path between a sender and a receiver.  Routers
   along the path of a PATH message forward the message towards the des-
   tination address based on the L3 routing (packet forwarding) tables.

   For example, consider the L2 domain in Figure 1. Assume that both the
   sender (some host X) and the receiver (some host Y) in a RSVP session
   reside outside the L2 domain shown in the Figure, but PATH messages
   from the sender to its receiver pass through the routers in the L2
   domain using it as a transit subnet. Assume that the PATH message from
   the sender X arrives at the router R1. R1 uses its local routing
   information to decide which next hop router (either router R2 or
   router R3) to use to forward the PATH message towards host Y. However,
   when the path traverses a managed L2 domain, we require the PATH and
   RESV messages to go through a DSBM for each managed segment. Such a L2
   domain may span many managed segments (and DSBMs) and, typically, SBM
   protocol entities on L2 devices (such as a switch) will serve as the
   DSBMs for the managed segments in a switched topology. When R1 for-
   wards the PATH message to the DSBM (an L2 device), the DSBM may not
   have the L3 routing information necessary to select the egress router
   (between R2 and R3) before forwarding the PATH message. To ensure
   correct operation and routing of RSVP messages, we must provide addi-
   tional forwarding information to DSBMs.

   For this purpose, we introduce new RSVP objects called LAN_NHOP
   address objects that keep track of the next L3 hop as the PATH message
   traverses an L2 domain between two L3 entities (RSVP PHOP and NHOP
   nodes).

   4.2.2.3 Including Both Layer-2 and Layer-3 Addresses in the LAN_NHOP

   When a DSBM client (a host or a router acting as the originator of a
   PATH message) sends out a PATH message to the DSBM, it must include
   LAN_NHOP information in the message. In the case of a unicast destina-
   tion, the LAN_NHOP address specifies the destination address (if the
   destination is local to its L2 domain) or the address of the next hop
   router towards the destination. In our example of an RSVP session
   involving the sender X and receiver Y with L2 domain in Figure 1 act-
   ing as the transit subnet, R1 is the ingress node that receives the
   PATH message.  R1 first determines that R2 is the next hop router (or
   the egress node in the L2 domain for the session address) and then
   inserts a LAN_NHOP object that specifies R2's IP address. When a DSBM
   receives a PATH message, it can now look at the address in the
   LAN_NHOP object and forward the PATH message towards the egress node
   after processing the PATH message.  However, we expect the L2 devices
   (such as switches) to act as DSBMs on the path within the L2 domain
   and it may not be reasonable to expect these devices to have an ARP
   capability to determine the MAC address (we call it L2ADDR for Layer 2

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   address) corresponding to the IP address in the LAN_NHOP object.

   Therefore, we require that the LAN_NHOP information (generated by the
   L3 device) include both the IP address (LAN_NHOP_L3 address) and the
   corresponding MAC address (LAN_NHOP_L2 address ) for the next L3 hop
   over the L2 domain.  The LAN_NHOP_L3 address is used by SBM protocol
   entities on L3 devices to forward the PATH message towards its desti-
   nation whereas the L2 address is used by the SBM protocol entities on
   L2 devices to determine how to forward the PATH message towards the L3
   NHOP (egress point from the L2 domain).  The exact format of the
   LAN_NHOP information and relevant objects is described later in Appen-
   dix B.

   4.2.2.4 Similarities to Standard RSVP Message Processing

   -    When a DSBM receives a RSVP PATH message, it processes the PATH
        message according to the PATH processing rules described in the
        RSVP specification. In particular, the DSBM retrieves the IP
        address of the previous hop from the RSVP_HOP object in the PATH
        message and stores the PHOP address in its PATH state.  It then
        forwards the PATH message with the PHOP (RSVP_HOP) object modi-
        fied to reflect its own IP address (RSVP_HOP_L3 address). Thus,
        the DSBM inserts itself as an intermediate hop in the chain of
        nodes in the path between two L3 nodes across the L2 domain.

   -    The PATH state in a DSBM is used for forwarding subsequent RESV
        messages as per the standard RSVP message processing rules.  When
        the DSBM receives a RESV message, it processes the message and
        forwards it to appropriate PHOP(s) based on its PATH state.

   -    Because a DSBM inserts itself as a hop between two RSVP nodes in
        the path of a RSVP flow, all RSVP related messages (such as PATH,
        PATH_TEAR, RESV, RESV_CONF, RESV_TEAR, and RESV_ERR) now flow
        through the DSBM.  In particular, a PATH_TEAR message is routed
        exactly through the intermediate DSBM(s) as its corresponding
        PATH message and the local PATH state is first cleaned up at each
        intermediate hop before the PATH_TEAR message gets forwarded.

   -    So far, we have described how the PATH message propagates through
        the L2 domain establishing PATH state at each DSBM along the
        managed segments in the path. The layer 2 address (LAN_NHOP_L2
        address) in the LAN_NHOP object should be used by the L2 devices
        along the path to decide how to forward the PATH message toward
        the next L3 hop.  Such devices will apply the standard IEEE

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                       SBM (Subnet Bandwidth Manager)         October, 1999

        802.1D forwarding rules (e.g., send it on a single port based on
        its filtering database, or flood it on all ports active in the
        spanning tree if the L2 address does not appear in the filtering
        database) to the LAN_NHOP_L2 address as are applied normally to
        data packets destined to the address.

        4.2.2.5 Including Both Layer-2 and Layer-3 Addresses in the
        RSVP_HOP Objects

   In the conventional RSVP message processing, the PATH state esta-
   blished along the nodes on a path is used to route the RESV message
   from a receiver to a sender in an RSVP session. As each intermediate
   node builds the path state, it remembers the previous hop (stores the
   PHOP IP address available in the RSVP_HOP object of an incoming mes-
   sage) that sent it the PATH message and, when the RESV message
   arrives, the intermediate node simply uses the stored PHOP address to
   forward the RESV after processing it successfully.

   In our case, we expect the SBM entities residing at L2 devices to act
   as DSBMs (and, therefore, intermediate RSVP hops in an L2 domain)
   along the path between a sender (PHOP) and receiver (NHOP). Thus, when
   a RESV message arrives at a DSBM, it must use the stored PHOP IP
   address to forward the RESV message to its previous hop. However, it
   may not be reasonable to expect the L2 devices to have an ARP cache or
   the ARP capability to map the PHOP IP address to its corresponding L2
   address before forwarding the RESV message.

   To obviate the need for such address mapping at L2 devices, we use a
   RSVP_HOP_L2 object in the PATH message. The RSVP_HOP_L2 object
   includes the Layer 2 address (L2ADDR) of the previous hop and comple-
   ments the L3 address information included in the RSVP_HOP object
   (RSVP_HOP_L3 address).

   When a L3 device constructs and forwards a PATH message over a managed
   segment, it includes its IP address (IP address of the interface over
   which PATH is sent) in the RSVP_HOP object and adds a RSVP_HOP_L2
   object that includes the corresponding L2 address for the interface.
   When a device in the L2 domain receives such a PATH message, it
   remembers the addresses in the RSVP_HOP and RSVP_HOP_L2 objects in its
   PATH state and then overwrites the RSVP_HOP and RSVP_HOP_L2 objects
   with its own addresses before forwarding the PATH message over a
   managed segment.

   The exact format of RSVP_HOP_L2 object is specified in Appendix B.

   4.2.2.6 Loop Detection

   When an RSVP session address is a multicast address and a SBM, DSBM,

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   and DSBM clients share the same L2 segment (a shared segment), it is
   possible for a SBM or a DSBM client to receive one or more copies of a
   PATH message that it forwarded earlier when a DSBM on the same wire
   forwards it (See Section 5.8 for an example of such a case). To facil-
   itate detection of such loops, we use a new RSVP object called the
   LAN_LOOPBACK object. DSBM clients or SBMs (but not the DSBMs reflect-
   ing a PATH message onto the interface over which it arrived earlier)
   must overwrite (or add if the PATH message does NOT already include a
   LAN_LOOPBACK object) the LAN_LOOPBACK object in the PATH message with
   their own unicast IP address.

   Now, a SBM or a DSBM client can easily detect and discard the dupli-
   cates by checking the contents of the LAN_LOOPBACK object (a duplicate
   PATH message will list a device's own interface address in the
   LAN_LOOPBACK object). Appendix B specifies the exact format of the
   LAN_LOOPBACK object.

   4.2.2.7 802.1p, User Priority and TCLASS

   The model proposed by the Integrated Services working group requires
   isolation of traffic flows from each other during their transit across
   a network. The motivation for traffic flow separation is to provide
   Integrated Services flows protection from misbehaving flows and other
   best-effort traffic that share the same path. The basic IEEE
   802.3/Ethernet networks do not provide any notion of traffic classes
   to discriminate among different flows that request different services.
   However, IEEE 802.1p defines a way for switches to differentiate among
   several "user_priority" values encoded in packets representing dif-
   ferent traffic classes (see [IEEE802Q, IEEE8021p] for further
   details). The user_priority values can be encoded either in native LAN
   packets (e.g., in IEEE 802.5's FC octet) or by using an encapsulation
   above the MAC layer (e.g., in the case of Ethernet, the user_priority
   value assigned to each packet will be carried in the frame header
   using the new, extended frame format defined by IEEE 802.1Q
   [IEEE8021Q]. IEEE, however, makes no recommendations about how a
   sender or network should use the user_priority values. An accompanying
   document makes recommendations on the usage of the user_priority
   values (see [RFC-MAP] for details).

   Under the Integrated Services model, L3 (or higher) entities that
   transmit traffic flows onto a L2 segment should perform per-flow pol-
   icing to ensure that the flows do not exceed their traffic specifica-
   tion as specified during admission control. In addition, L3 devices
   may label the frames in such flows with a user_priority value to iden-
   tify their service class.

   For the purpose of this discussion, we will refer to the user_priority
   value carried in the extended frame header as the "traffic class" of a

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   packet. Under the ISSLL model, the L3 entities, that send traffic and
   that use the SBM protocol, may select the appropriate traffic class of
   outgoing packets [RFC-MAP]. This selection may be overridden by DSBM
   devices, in the following manner. once a sender sends a PATH message,
   downstream DSBMs will insert a new traffic class object (TCLASS
   object) in the PATH message that travels to the next L3 device (L3
   NHOP for the PATH message). To some extent, the TCLASS object contents
   are treated like the ADSPEC object in the RSVP PATH messages.  The L3
   device that receives the PATH message must remove and store the TCLASS
   object as part of its PATH state for the session. Later, when the same
   L3 device needs to forward a RSVP RESV message towards the original
   sender, it must include the TCLASS object in the RESV message. When
   the RESV message arrives at the original sender, the sender must use
   the user_priority value from the TCLASS object to override its selec-
   tion for the traffic class marked in outgoing packets.

   The format of the TCLASS object is specified in Appendix B.  Note that
   TCLASS and other SBM-specific objects are carried in a RSVP message in
   addition to all the other, normal RSVP objects per RFC 2205.

   4.2.2.8 Processing the TCLASS Object

   In summary, use of TCLASS objects requires following additions to the
   conventional RSVP message processing at DSBMs, SBMs, and DSBM clients:

     *    When a DSBM receives a PATH message over a managed segment and
          the PATH message does not include a TCLASS object, the DSBM MAY
          add a TCLASS object to the PATH message before forwarding it.
          The DSBM determines the appropriate user_priority value for the
          TCLASS object. A mechanism for selecting the appropriate
          user_priority value is described in an accompanying document
          [RFC-MAP].

     *    When SBM or DSBM receives a PATH message with a TCLASS object
          over a managed segment in a L2 domain and needs to forward it
          over a managed segment in the same L2 domain, it will store it
          in its path state and typically forward the message without
          changing the contents of the TCLASS object.  However, if the
          DSBM/SBM cannot support the service class represented by the
          user_priority value specified by the TCLASS object in the PATH
          message, it may change the priority value in the TCLASS to a
          semantically "lower" service value to reflect its capability
          and store the changed TCLASS value in its path state.

          [NOTE: An accompanying document defines the int-serv mappings

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                       SBM (Subnet Bandwidth Manager)         October, 1999

          over IEEE 802 networks [RFC-MAP] provides a precise definition
          of user_priority values and describes how the user_priority
          values are compared to determine "lower" of the two values or
          the "lowest" among all the user_priority values.]

     *    When a DSBM receives a RESV message with a TCLASS object, it
          may use the traffic class information (in addition to the usual
          flowspec information in the RSVP message) for its own admission
          control for the managed segment.

          Note that this document does not specify the actual algorithm
          or policy used for admission control. At one extreme, a DSBM
          may use per-flow reservation request as specified by the
          flowspec for a fine grain admission control. At the other
          extreme, a DSBM may only consider the traffic class information
          for a very coarse-grain admission control based on some static
          allocation of link capacity for each traffic class. Any combi-
          nation of the options represented by these two extremes may
          also be used.

     *    When a DSBM (at an L2 or L3) device receives a RESV message
          without a TCLASS object and it needs to forward the RESV mes-
          sage over a managed segment within the same L2 domain, it
          should first check its path state and check whether it has
          stored a TCLASS value. If so, it should include the TCLASS
          object in the outgoing RESV message after performing its own
          admission control. If no TCLASS value is stored, it must for-
          ward the RESV message without inserting a TCLASS object.

     *    When a DSBM client (residing at an L3 device such as a host or
          an edge router) receives the TCLASS object in a PATH message
          that it accepts over an interface, it should store the TCLASS
          object as part of its PATH state for the interface. Later, when
          the client forwards a RESV message for the same session on the
          interface, the client must include the TCLASS object (unchanged
          from what was received in the previous PATH message) in the
          RESV message it forwards over the interface.

     *    When a DSBM client receives a TCLASS object in an incoming RESV
          message over a managed segment and local admission control
          succeeds for the session for the outgoing interface over the
          managed segment, the client must pass the user_priority value
          in the TCLASS object to its local packet classifier. This will
          ensure that the data packets in the admitted RSVP flow that are
          subsequently forwarded over the outgoing interface will contain

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                       SBM (Subnet Bandwidth Manager)         October, 1999

          the appropriate value encoded in their frame header.

     *    When an L3 device receives a PATH or RESV message over a
          managed segment in one L2 domain and it needs to forward the
          PATH/RESV message over an interface outside that domain, the L3
          device must remove the TCLASS object (along with LAN_NHOP,
          RSVP_HOP_L2, and LAN_LOOPBACK objects in the case of the PATH
          message) before forwarding the PATH/RESV message. If the outgo-
          ing interface is on a separate L2 domain, these objects may be
          regenerated according to the processing rules applicable to
          that interface.

   5. Detailed Message Processing Rules

   5.1. Additional Notes on Terminology

   *    An L2 device may have several interfaces with attached segments
        that are part of the same L2 domain. A switch in a L2 domain is
        an example of such a device. A device which has several inter-
        faces may contain a SBM protocol entity that acts in different
        capacities on each interface. For example, a SBM protocol entity
        could act as a SBM on interface A, and act as a DSBM on interface
        B.

   *    A SBM protocol entity on a layer 3 device can be a DSBM client,
        and SBM, a DSBM, or none of the above (SBM transparent). Non-
        transparent L3 devices can implement any combination of these
        roles simultaneously. DSBM clients always reside at L3 devices.

   *    A SBM protocol entity residing at a layer 2 device can be a SBM,
        a DSBM or none of the above (SBM transparent). A layer 2 device
        will never host a DSBM client.

   5.2. Use Of Reserved IP Multicast Addresses

   As stated earlier, we require that the DSBM clients forward the RSVP
   PATH messages to their DSBMs in a L2 domain before they reach the next

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   L3 hop in the path. RSVP PATH messages are addressed, according to
   RFC-2205, to their destination address (which can be either an IP uni-
   cast or multicast address).  When a L2 device hosts a DSBM, a simple-
   to-implement mechanism must be provided for the device to capture an
   incoming PATH message and hand it over to the local DSBM agent without
   requiring the L2 device to snoop for L3 RSVP messages.

   In addition, DSBM clients need to know how to address SBM messages to
   the DSBM. For the ease of operation and to allow dynamic DSBM-client
   binding, it should be possible to easily detect and address the exist-
   ing DSBM on a managed segment.

   To facilitate dynamic DSBM-client binding as well as to enable easy
   detection and capture of PATH messages at L2 devices, we require that
   a DSBM be addressed using a logical address rather than a physical
   address. We make use of reserved IP multicast address(es) for the pur-
   pose of communication with a DSBM.  In particular, we require that
   when a DSBM client or a SBM forwards a PATH message over a managed
   segment, it is addressed to a reserved IP multicast address. Thus, a
   DSBM on a L2 device needs to be configured in a way to make it easy to
   intercept the PATH message and forward it to the local SBM protocol
   entity. For example, this may involve simply adding a static entry in
   the device's filtering database (FDB) for the corresponding MAC multi-
   cast address to ensure the PATH messages get intercepted and are not
   forwarded further without the DSBM intervention.

   Similarly, a DSBM always sends the PATH messages over a managed seg-
   ment using a reserved IP multicast address and, thus, the SBMs or DSBM
   clients on the managed segments must simply be configured to intercept
   messages addressed to the reserved multicast address on the appropri-
   ate interfaces to easily receive PATH messages.

   RSVP RESV messages continue to be unicast to the previous hop address
   stored as part of the PATH state at each intermediate hop.

   We define use of two reserved IP multicast addresses. We call these
   the "AllSBM Address" and the "DSBMLogicalAddress". These are chosen
   from the range of local multicast addresses, such that:

   *    They are not passed through layer 3 devices.

   *    They are passed transparently through layer 2 devices which are
        SBM transparent.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   *    They are configured in the permanent database of layer 2 devices
        which host SBMs or DSBMs, such that they are directed to the SBM
        management entity in these devices. This obviates the need for
        these devices to explicitly snoop for SBM related control pack-
        ets.

   *    The two reserved addresses are 224.0.0.16 (DSBMLogicalAddress)
        and 224.0.0.17 (AllSBMAddress).

   These addresses are used as described in the following table:

   Type        DSBMLogicaladdress              AllSBMAddress

   DSBM        * Sends PATH messages           * Monitors this address to detect
   Client        to this address                 the presence of a DSBM
                                               * Monitors this address to
                                                 receive PATH messages
                                                 forwarded by the DSBM

   SBM         * Sends PATH messages           * Monitors and sends on this
                 to this address                 address to participate in
                                                 election of the DSBM
                                               * Monitors this address to
                                                 receive PATH messages
                                                 forwarded by the DSBM

   DSBM        * Monitors this address         * Monitors and sends on this
                 for PATH messages               to participate in election
                 directed to it                  of the DSBM
                                               * Sends PATH messages to this
                                                   address

   The L2 or MAC addresses corresponding to IP multicast addresses are
   computed algorithmically using a reserved L2 address block (the high
   order 24-bits are 00:00:5e). The Assigned Numbers RFC [RFC-1700] gives
   additional details.

   5.3. Layer 3 to Layer 2 Address Mapping

   As stated earlier, DSBMs or DSBM clients residing at a L3 device must
   include a LAN_NHOP_L2 address in the LAN_NHOP information so that L2
   devices along the path of a PATH message do not need to separately
   determine the mapping between the LAN_NHOP_L3 address in the LAN_NHOP
   object and its corresponding L2 address (for example, using ARP).

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   For the purpose of such mapping at L3 devices, we assume a mapping
   function called "map_address" that performs the necessary mapping:

                   L2ADDR object = map_addr(L3Addr)

   We do not specify how the function is implemented; the implementation
   may simply involve access to the local ARP cache entry or may require
   performing an ARP function.  The function returns a L2ADDR object that
   need not be interpreted by an L3 device and can be treated as an
   opaque object.  The format of the L2ADDR object is specified in Appen-
   dix B.

   5.4. Raw vs. UDP Encapsulation

   We assume that the DSBMs, DSBM clients, and SBMs use only raw IP for
   encapsulating RSVP messages that are forwarded onto a L2 domain.
   Thus, when a SBM protocol entity on a L3 device forwards a RSVP mes-
   sage onto a L2 segment, it will only use RAW IP encapsulation.

   5.5. The Forwarding Rules

   The message processing and forwarding rules will be described in the
   context of the sample network illustrated in Figure 2.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   Figure 2 - A sample network or L2 domain consisting of switched and
   shared L2 segments

    ..........
             .
   +------+  .    +------+  seg A  +------+  seg C  +------+  seg D  +------+
   |  H1  |_______|  R1  |_________|  S1  |_________|  S2  |_________|  H2  |
   |      |  .    |      |         |      |         |      |         |      |
   +------+  .    +------+         +------+         +------+         +------+
             .                        |                /
   1.0.0.0   .                        |               /
             .                        |___           /
             .                    seg B  |          / seg E
    ..........                           |         /
                        2.0.0.0          |        /
                                        +-----------+
                                        |    S3     |
                                        |           |
                                        +-----------+
                                             |
                                             |
                                             |
                                             |
                            seg F            |            .................
                    ------------------------------        .
                      |         |             |           .
                   +------+  +------+        +------+     .      +------+
                   |  H3  |  |  H4  |        |  R2  |____________|  H5  |
                   |      |  |      |        |      |     .      |      |
                   +------+  +------+        +------+     .      +------+
                                                          .
                                                          .     3.0.0.0
                                                          .................

   Figure 2 illustrates a sample network topology consisting of three IP
   subnets (1.0.0.0, 2.0.0.0, and 3.0.0.0) interconnected using two
   routers. The subnet 2.0.0.0 is an example of a L2 domain consisting of
   switches, hosts, and routers interconnected using switched segments
   and a shared L2 segment. The sample network contains the following
   devices:

   Device          Type                    SBM Type

   H1, H5      Host (layer 3)          SBM Transparent
   H2-H4       Host  (layer 3)         DSBM Client
   R1          Router (layer 3)        SBM
   R2          Router (layer 3)        DSBM for segment F

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                       SBM (Subnet Bandwidth Manager)         October, 1999

   S1          Switch (layer 2)        DSBM for segments A, B
   S2          Switch (layer 2)        DSBM for segments C, D, E
   S3          Switch (layer 2)        SBM

   The following paragraphs describe the rules, which each of these dev-
   ices should use to forward PATH messages (rules apply to PATH_TEAR
   messages as well). They are described in the context of the general
   network illustrated above. While the examples do not address every
   scenario, they do address most of the interesting scenarios. Excep-
   tions can be discussed separately.

   The forwarding rules are applied to received PATH messages (routers
   and switches) or originating PATH messages (hosts), as follows:

   1.   Determine the interface(s) on which to forward the PATH message
        using standard forwarding rules:

   *    If there is a LAN_LOOPBACK object in the PATH message, and it
        carries the address of this device, silently discard the message.
        (See the section below on "Additional notes on forwarding the
        PATH message onto a managed segment).

   *    Layer 3 devices use the RSVP session address and perform a rout-
        ing lookup to determine the forwarding interface(s).

   *    Layer 2 devices use the LAN_NHOP_L2 address in the LAN_NHOP
        information and MAC forwarding tables to determine the forwarding
        interface(s). (See the section below on "Additional notes on for-
        warding the PATH message onto a managed segment")

   2.   For each forwarding interface:

     *    If the device is a layer 3 device, determine whether the inter-
          face is on a managed segment managed by a DSBM, based on the
          presence or absence of I_AM_DSBM messages. If the interface is
          not on a managed segment, strip out RSVP_HOP_L2, LAN_NHOP,
          LAN_LOOPBACK, and TCLASS objects (if present), and forward to
          the unicast or multicast destination.

          (Note that the RSVP Class Numbers for these new objects are

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                       SBM (Subnet Bandwidth Manager)         October, 1999

          chosen so that if an RSVP message includes these objects, the
          nodes that are RSVP-aware, but do not participate in the SBM
          protocol, will ignore and silently discard such objects.)

     *    If the device is a layer 2 device or it is a layer 3 device
          *and* the interface is on a managed segment, proceed to rule
          #3.

   3.   Forward the PATH message onto the managed segment:

     *    If the device is a layer 3 device, insert LAN_NHOP address
          objects, a LAN_LOOPBACK, and a RSVP_HOP_L2 object into the PATH
          message. The LAN_NHOP objects carry the LAN_NHOP_L3 and
          LAN_NHOP_L2 addresses of the next layer 3 hop. The RSVP_HOP_L2
          object carries the device's own L2 address, and the
          LAN_LOOPBACK object contains the IP address of the outgoing
          interface.

          An L3 device should use the map_addr() function described ear-
          lier to obtain an L2 address corresponding to an IP address.

     *    If the device hosts the DSBM for the segment to which the for-
          warding interface is attached, do the following:

       -    Retrieve the PHOP information from the standard RSVP HOP
            object in the PATH message, and store it. This will be used
            to route RESV messages back through the L2 network. If the
            PATH message arrived over a managed segment, it will also
            contain the RSVP_HOP_L2 object; then retrieve and store also
            the previous hop's L2 address in the PATH state.

       -    Copy the IP address of the forwarding interface (layer 2 dev-
            ices must also have IP addresses) into the standard RSVP HOP
            object and the L2 address of the forwarding interface into
            the RSVP_HOP_L2 object.

       -    If the PATH message received does not contain the TCLASS
            object, insert a TCLASS object. The user_priority value
            inserted in the TCLASS object is based on service mappings
            internal to the device that are configured according to the
            guidelines listed in [RFC-MAP]. If the message already

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                       SBM (Subnet Bandwidth Manager)         October, 1999

            contains the TCLASS object, the user_priority value may be
            changed based again on the service mappings internal to the
            device.

     *    If the device is a layer 3 device and hosts a SBM for the seg-
          ment to which the forwarding interface is attached, it *is
          required* to retrieve and store the PHOP info.

          If the device is a layer 2 device and hosts a SBM for the seg-
          ment to which the forwarding interface is attached, it is *not*
          required to retrieve and store the PHOP info. If it does not do
          so, the SBM must leave the standard RSVP HOP object and the
          RSVP_HOP_L2 objects in the PATH message intact and it will not
          receive RESV messages.

          If the SBM on a L2 device chooses to overwrite the RSVP HOP and
          RSVP_HOP_L2 objects with the IP and L2 addresses of its for-
          warding interface, it will receive RESV messages. In this case,
          it must store the PHOP address info received in the standard
          RSVP_HOP field and RSVP_HOP_L2 objects of the incident PATH
          message.

          In both the cases mentioned above (L2 or L3 devices), the SBM
          must forward the TCLASS object in the received PATH message
          unchanged.

     *    Copy the IP address of the forwarding interface into the
          LAN_LOOPBACK object, unless the SBM protocol entity is a DSBM
          reflecting a PATH message back onto the incident interface.
          (See the section below on "Additional notes on forwarding a
          PATH message onto a managed segment").

     *    If the SBM protocol entity is the DSBM for the segment to which
          the forwarding interface is attached, it must send the PATH
          message to the AllSBMAddress.

     *    If the SBM protocol entity is a SBM or a DSBM Client on the
          segment to which the forwarding interface is attached, it must
          send the PATH message to the DSBMLogicalAddress.

       5.6.1. Additional notes on forwarding a PATH message onto a

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       managed segment

       Rule #1 states that normal IEEE 802.1D forwarding rules should be
       used to determine the interfaces on which the PATH message should
       be forwarded. In the case of data packets, standard forwarding
       rules at a L2 device dictate that the packet should not be for-
       warded on the interface from which it was received. However, in
       the case of a DSBM that receives a PATH message over a managed
       segment, the following exception applies:

         E1.  If the address in the LAN_NHOP object is a unicast address,
              consult the filtering database (FDB) to determine whether
              the destination address is listed on the same interface
              over which the message was received. If yes, follow the
              rule below on "reflecting a PATH message back onto an
              interface" described below; otherwise, proceed with the
              rest of the message processing as usual.

         E2.  If there are members of the multicast group address (speci-
              fied by the addresses in the LAN_NHOP object), on the seg-
              ment from which the message was received, the message
              should be forwarded back onto the interface from which it
              was received and follow the rule on "reflecting a PATH mes-
              sage back onto an interface" described below.

       *** Reflecting a PATH message back onto an interface ***

         Under the circumstances described above, when a DSBM reflects
         the PATH message back onto an interface over which it was
         received, it must address it using the AllSBMAddress.

         Since it is possible for a DSBM to reflect a PATH message back
         onto the interface from which it was received, precautions must
         be taken to avoid looping these messages indefinitely. The
         LAN_LOOPBACK object addresses this issue. All SBM protocol enti-
         ties (except DSBMs reflecting a PATH message) overwrite the
         LAN_LOOPBACK object in the PATH message with the IP address of
         the outgoing interface. DSBMs which are reflecting a PATH mes-
         sage, leave the LAN_LOOPBACK object unchanged. Thus, SBM proto-
         col entities will always be able to recognize a reflected multi-
         cast message by the presence of their own address in the
         LAN_LOOPBACK object. These messages should be silently dis-
         carded.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       5.7. Applying the Rules -- Unicast Session

       Let's see how the rules are applied in the general network illus-
       trated previously (see Figure 2).

       Assume that H1 is sending a PATH for a unicast session for which
       H5 is the receiver. The following PATH message is composed by H1:

                                 RSVP Contents
       RSVP session IP address     IP address of H5 (3.0.0.35)
       Sender Template             IP address of H1 (1.0.0.11)
       PHOP                        IP address of H1 (1.0.0.11)
       RSVP_HOP_L2                 n/a  (H1 is not sending onto a managed
                                       segment)
       LAN_NHOP                    n/a  (H1 is not sending onto a managed
                                       segment)
       LAN_LOOPBACK                n/a  (H1 is not sending onto a managed
                                       segment)

                                   IP Header
       Source address              IP address of H1 (1.0.0.11)
       Destn address               IP addr of H5 (3.0.0.35, assuming raw mode &
                                    router alert)

                                   MAC Header
       Destn address               The L2 addr corresponding to R1 (determined
                                    by map_addr() and routing tables at H1)

       Since H1 is not sending onto a managed segment, the PATH message
       is composed and forwarded according to standard RSVP processing
       rules.

       Upon receipt of the PATH message, R1 composes and forwards a PATH
       message as follows:

                                 RSVP Contents
       RSVP session IP address     IP address of H5
       Sender Template             IP address of H1
       PHOP                        IP address of R1 (2.0.0.1)
                                   (seed the return path for RESV messages)
       RSVP_HOP_L2                 L2 address of R1
       LAN_NHOP                    LAN_NHOP_L3 (2.0.0.2) and
                                   LAN_NHOP_L2 address of R2 (L2ADDR)
                                   (this is the next layer 3 hop)
       LAN_LOOPBACK                IP address of R1 (2.0.0.1)

                                   IP Header

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       Source address              IP address of H1
       Destn address               DSBMLogical IP address (224.0.0.16)

                                   MAC Header
       Destn address               DSBMLogical MAC address

       *    R1 does a routing lookup on the RSVP session address, to
            determine the IP address of the next layer 3 hop, R2.

       *    It determines that R2 is accessible via seg A and that seg A
            is managed by a DSBM, S1.

       *    Therefore, it concludes that it is sending onto a managed
            segment, and composes LAN_NHOP objects to carry the layer 3
            and layer 2 next hop addresses. To compose the LAN_NHOP
            L2ADDR object, it invokes the L3 to L2 address mapping function
            ("map_address") to find out the MAC address for the next hop
            L3 device, and then inserts a LAN_NHOP_L2ADDR object (that
            carries the MAC address) in the message.

       *    Since R1 is not the DSBM for seg A, it sends the PATH message
            to the DSBMLogicalAddress.

       Upon receipt of the PATH message, S1 composes and forwards a PATH
       message as follows:

                                 RSVP Contents
       RSVP session IP address     IP address of H5
       Sender Template             IP address of H1
       PHOP                        IP addr of S1 (seed the return path for RESV
                                   messages)
       RSVP_HOP_L2                 L2 address of S1
       LAN_NHOP                    LAN_NHOP_L3 (IP)  and LAN_NHOP_L2
                                       address of R2
                                   (layer 2 devices do not modify the LAN_NHOP)
       LAN_LOOPBACK                IP addr of S1

                                   IP Header
       Source address              IP address of H1
       Destn address               AllSBMIPaddr (224.0.0.17, since S1 is the

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                       SBM (Subnet Bandwidth Manager)         October, 1999

                                   DSBM for seg B).

                                   MAC Header
       Destn address               All SBM MAC address (since S1 is the DSBM for
                                   seg B).

       *    S1 looks at the LAN_NHOP address information to determine the
            L2 address towards which it should forward the PATH message.

       *    From the bridge forwarding tables, it determines that the L2
            address is reachable via seg B.

       *    S1 inserts the RSVP_HOP_L2 object and overwrites the RSVP HOP
            object (PHOP) with its own addresses.

       *    Since S1 is the DSBM for seg B, it addresses the PATH message
            to the AllSBMAddress.

            Upon receipt of the PATH message, S3 composes and forwards a
            PATH message as follows:

                                 RSVP Contents
       RSVP session IP addr            IP address of H5
       Sender Template                 IP address of H1
       PHOP                            IP addr of S3 (seed the return
                                           path for RESV messages)
       RSVP_HOP_L2                     L2 address of S3
       LAN_NHOP                        LAN_NHOP_L3 (IP) and
                                       LAN_NHOP_L2 (MAC) address of R2
                                       (L2 devices don't modify  LAN_NHOP)
       LAN_LOOPBACK                    IP address of S3

                                   IP Header
       Source address                  IP address of H1
       Destn address                   DSBMLogical IP addr (since S3 is
                                           not the DSBM for seg F)

                                   MAC Header
       Destn address                   DSBMLogical MAC address

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       *    S3 looks at the LAN_NHOP address information to determine the
            L2 address towards which it should forward the PATH message.

       *    From the bridge forwarding tables, it determines that the L2
            address is reachable via segment F.

       *    It has discovered that R2 is the DSBM for segment F. It
            therefore sends the PATH message to the DSBMLogicalAddress.

       *    Note that S3 may or may not choose to overwrite the PHOP
            objects with its own IP and L2 addresses. If it does so, it
            will receive RESV messages. In this case, it must also store
            the PHOP info received in the incident PATH message so that
            it is able to forward the RESV messages on the correct path.

       Upon receipt of the PATH message, R2 composes and forwards a PATH
       message as follows:

                                 RSVP Contents
       RSVP session IP addr    IP address of H5
       Sender Template         IP address of H1
       PHOP                    IP addr of R2 (seed the return path for RESV
                               messages)
       RSVP_HOP_L2             Removed by R2  (R2 is not sending onto a
                                   managed segment)
       LAN_NHOP                Removed by R2  (R2 is not sending onto a
                               managed segment)

                                   IP Header
       Source address          IP address of H1
       Destn address           IP address of H5, the RSVP session address

                                   MAC Header
       Destn address           L2 addr corresponding to H5, the next
                                   layer 3 hop

       *    R2 does a routing lookup on the RSVP session address, to
            determine the IP address of the next layer 3 hop, H5.

       *    It determines that H5 is accessible via a segment for which
            there is no DSBM (not a managed segment).

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       *    Therefore, it removes the LAN_NHOP and RSVP_HOP_L2 objects
            and places the RSVP session address in the destination
            address of the IP header. It places the L2 address of the
            next layer 3 hop, into the destination address of the MAC
            header and forwards the PATH message to H5.

       5.8. Applying the Rules - Multicast Session

       The rules described above also apply to multicast (m/c) sessions.
       For the purpose of this discussion, it is assumed that layer 2
       devices track multicast group membership on each port individu-
       ally. Layer 2 devices which do not do so, will merely generate
       extra multicast traffic. This is the case for L2 devices which do
       not implement multicast filtering or GARP/GMRP capability.

       Assume that H1 is sending a PATH for an m/c session for which H3
       and H5 are the receivers. The rules are applied as they are in the
       unicast case described previously, until the PATH message reaches
       R2, with the following exception. The RSVP session address and the
       LAN_NHOP carry the destination m/c addresses rather than the uni-
       cast addresses carried in the unicast example.

       Now let's look at the processing applied by R2 upon receipt of the
       PATH message. Recall that R2 is the DSBM for segment F. Therefore,
       S3 will have forwarded its PATH message to the DSBMLogicalAddress,
       to be picked up by R2. The PATH message will not have been seen by
       H3 (one of the m/c receivers), since it monitors only the
       AllSBMAddress, not the DSBMLogicalAddress for incoming PATH mes-
       sages.  We rely on R2 to reflect the PATH message back onto seg f,
       and to forward it to H5. R2 forwards the following PATH message
       onto seg f:

                                 RSVP Contents
       RSVP session addr       m/c session address
       Sender Template         IP address of H1
       PHOP                    IP addr of R2 (seed the return path for
                               RESV messages)
       RSVP_HOP_L2             L2 addr of R2
       LAN_NHOP                m/c session address and corresponding L2 address
       LAN_LOOPBACK            IP addr of S3 (DSBMs reflecting a PATH
                               message don't modify this object)

                                   IP Header
       Source address          IP address of H1

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       Destn address           AllSBMIP address (since R2 is the DSBM for seg F)

                                   MAC Header
       Destn address           AllSBMMAC address (since R2 is the
                                  DSBM for seg F)

       Since H3 is monitoring the All SBM Address, it will receive the
       PATH message reflected by R2. Note that R2 violated the standard
       forwarding rules here by sending an incoming message back onto the
       interface from which it was received. It protected against loops
       by leaving S3's address in the LAN_LOOPBACK object unchanged.

       R2 forwards the following PATH message on to H5:

                                 RSVP Contents
       RSVP session addr       m/c session address
       Sender Template         IP address of H1
       PHOP                    IP addr of R2 (seed the return path for RESV
                               messages)
       RSVP_HOP_L2             Removed by R2 (R2 is not sending onto a
                               managed segment)
       LAN_NHOP                Removed by R2 (R2 is not sending onto a
                               managed segment)
       LAN_LOOPBACK            Removed by R2 (R2 is not sending onto a
                               managed segment)

                                   IP Header
       Source address          IP address of H1
       Destn address           m/c session address

                                   MAC Header
       Destn address           MAC addr corresponding to the m/c
                               session address

       *    R2 determines that there is an m/c receiver accessible via a
            segment for which there is no DSBM. Therefore, it removes the
            LAN_NHOP and RSVP_HOP_L2 objects and places the RSVP session
            address in the destination address of the IP header. It
            places the corresponding L2 address into the destination
            address of the MAC header and multicasts the message towards
            H5.

       5.9. Merging Traffic Class objects

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       When a DSBM client receives TCLASS objects from different senders
       (different PATH messages) in the same RSVP session and needs to
       combine them for sending back a single RESV message (as in a
       wild-card style reservation), the DSBM client must choose an
       appropriate value that corresponds to the desired-delay traffic
       class. An accompanying document discusses the guidelines for
       traffic class selection based on desired service and the TSpec
       information [RFC-MAP].

       In addition, when a SBM or DSBM needs to merge RESVs from dif-
       ferent next hops at a merge point, it must decide how to handle
       the TCLASS values in the incoming RESVs if they do not match. Con-
       sider the case when a reservation is in place for a flow at a DSBM
       (or SBM) with a successful admission control done for the TCLASS
       requested in the first RESV for the flow. If another RESV (not the
       refresh of the previously admitted RESV) for the same flow arrives
       at the DSBM, the DSBM must first check the TCLASS value in the new
       RESV against the TCLASS value in the already installed RESV. If
       the two values are same, the RESV requests are merged and the new,
       merged RESV installed and forwarded using the normal rules of mes-
       sage processing. However, if the two values are not identical, the
       DSBM must generate and send  a RESV_ERR message towards the sender
       (NHOP) of the newer, RESV message. The RESV_ERR must specify the
       error code corresponding to the RSVP  "traffic control error"
       (RESV_ERR code 21) that indicates failure to merge two incompati-
       ble service requests (sub-code 01 for the RSVP traffic control
       error) [RFC-2205]. The RESV_ERR message may include additional
       objects to assist downstream nodes in recovering from this condi-
       tion.  The definition and usage of such objects is beyond the
       scope of this draft.

       5.10. Operation of SBM Transparent Devices

       SBM transparent devices are unaware of the entire SBM/DSBM proto-
       col. They do not intercept messages addressed to either of the SBM
       related local group addresses (the DSBMLogicalAddrss and the
       ALLSBMAddress), but instead, pass them through. As a result, they
       do not divide the DSBM election scope, they do not explicitly par-
       ticipate in routing of PATH or RESV messages, and they do not par-
       ticipate in admission control. They are entirely transparent with
       respect to SBM operation.

       According to the definitions provided, physical segments intercon-
       nected by SBM transparent devices are considered a single managed
       segment. Therefore, DSBMs must perform admission control on such
       managed segments, with limited knowledge of the segment's topol-
       ogy. In this case, the network administrator should configure the

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       DSBM for each managed segment, with some reasonable approximation
       of the segment's capacity. A conservative policy would configure
       the DSBM for the lowest capacity route through the managed seg-
       ment. A liberal policy would configure the DSBM for the highest
       capacity route through the managed segment. A network administra-
       tor will likely choose some value between the two, based on the
       level of guarantee required and some knowledge of likely traffic
       patterns.

       This document does not specify the configuration mechanism or the
       choice of a policy.

       5.11. Operation of SBMs Which are NOT DSBMs

       In the example illustrated, S3 hosts a SBM, but the SBM on S3 did
       not win the election to act as DSBM on any segment. One might ask
       what purpose such a SBM protocol entity serves. Such SBMs actually
       provide two useful functions.  First, the additional SBMs remain
       passive in the background for fault tolerance. They listen to the
       periodic announcements from the current DSBM for the managed seg-
       ment (Appendix A describes this in more detail) and step in to
       elect a new DSBM when the current DSBM fails or ceases to be
       operational for some reason.  Second, such SBMs also provide the
       important service of dividing the election scope and reducing the
       size and complexity of managed segments. For example, consider the
       sample topology in Figure 3 again. the device S3 contains an SBM
       that is not a DSBM for any f the segments, B, E, or F, attached to
       it. However, if the SBM protocol entity on S3 was not present,
       segments B and F would not be separate segments from the point of
       view of the SBM protocol. Instead, they would constitute a single
       managed segment, managed by a single DSBM. Because the SBM entity
       on S3 divides the election scope, seg B and seg F are each
       managed by separate DSBMs. Each of these segments have a trivial
       topology and a well defined capacity. As a result, the DSBMs for
       these segments do not need to perform admission control based on
       approximations (as would be the case if S3 were SBM transparent).

       Note that, SBM protocol entities which are not DSBMs, are not
       required to overwrite the PHOP in incident PATH messages with
       their own address. This is because it is not necessary for RESV
       messages to be routed through these devices. RESV messages are
       only required to be routed through the correct sequence of DSBMs.
       SBMs may not process RESV messages that do pass through them,
       other than to forward them towards their destination address,
       using standard forwarding rules.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       SBM protocol entities which are not DSBMs are required to
       overwrite the address in the LAN_LOOPBACK object with their own
       address, in order to avoid looping multicast messages. However, no
       state need be stored.

       6. Inter-Operability Considerations

       There are a few interesting inter-operability issues related to
       the deployment of a DSBM-based admission control method in an
       environment consisting of network nodes with and without RSVP
       capability.  In the following, we list some of these scenarios and
       explain how SBM-aware clients and nodes can operate in those
       scenarios:

       6.1. An L2 domain with no RSVP capability.

       It is possible to envisage L2 domains that do not use RSVP signal-
       ing for requesting resource reservations, but, instead, use some
       other (e.g., SNMP or static configuration) mechanism to reserve
       bandwidth at a particular network device such as a router. In that
       case, the question is how does a DSBM-based admission control
       method work and interoperate with the non-RSVP mechanism.  The
       SBM-based method does not attempt to provide an admission control
       solution for such an environment. The SBM-based approach is part
       of an end to end signaling approach to establish resource reserva-
       tions and does not attempt to provide a solution for SNMP-based
       configuration scenario.

       As stated earlier, the SBM-based approach can, however, co-exist
       with any other, non-RSVP bandwidth allocation mechanism as long as
       resources being reserved are either partitioned statically between
       the different mechanisms or are resolved dynamically through a
       common bandwidth allocator so that there is no over-commitment of
       the same resource.

       6.2. An L2 domain with SBM-transparent L2 Devices.

       This scenario has been addressed earlier in the document. The
       SBM-based method is designed to operate in such an environment.
       When SBM-transparent L2 devices interconnect SBM-aware devices,
       the resulting managed segment is a combination of one or more phy-
       sical segments and the DSBM for the managed segment may not be as
       efficient in allocating resources as it would if all L2 devices
       were SBM-aware.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       6.3. An L2 domain on which some RSVP-based senders are not DSBM
       clients.

       All senders that are sourcing RSVP-based traffic flows onto a
       managed segment MUST be SBM-aware and participate in the SBM pro-
       tocol. Use of the standard, non-SBM version of RSVP may result in
       over-allocation of resources, as such use bypasses the resource
       management function of the DSBM. All other senders (i.e., senders
       that are not sending streams subject to RSVP admission control)
       should be elastic applications that send traffic of lower priority
       than the RSVP traffic, and use TCP-like congestion avoidance
       mechanisms.

       All DSBMs, SBMs, or DSBM clients on a managed segment (a segment
       with a currently active DSBM) must not accept PATH messages from
       senders that are not SBM-aware. PATH messages from such devices
       can be easily detected by SBMs and DSBM clients as they would not
       be multicast to the ALLSBMAddress (in case of SBMs and DSBM
       clients) or the DSBMLogicalAddress (in case of DSBMs).

       6.4. A non-SBM router that interconnects two DSBM-managed L2
       domains.

       Multicast SBM messages (e.g., election and PATH messages) have
       local scope and are not intended to pass between the two domains.
       A correctly configured non-SBM router will not pass such messages
       between the domains. A broken router implementation that does so
       may cause incorrect operation of the SBM protocol and consequent
       over- or under-allocation of resources.

       6.5. Interoperability with RSVP clients that use UDP encapsulation
       and are not capable of receiving/sending RSVP messages using
       RAW_IP

       This document stipulates that DSBMs, DSBM clients, and SBMs use
       only raw IP for encapsulating RSVP messages that are forwarded
       onto a L2 domain. RFC-2205 (the RSVP Proposed Standard) includes
       support for both raw IP and UDP encapsulation. Thus, a RSVP node
       using only the UDP encapsulation will not be able to interoperate
       with the DSBM unless DSBM accepts and supports UDP encapsulated
       RSVP messages.

       7. Guidelines for Implementers

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       In the following, we provide guidelines for implementers on dif-
       ferent aspects of the implementation of the SBM-based admission
       control procedure including suggestions for DSBM initialization,
       etc.

       7.1. DSBM Initialization

       As stated earlier, DSBM initialization includes configuration of
       maximum bandwidth that can be reserved on a managed segment under
       its control.  We suggest the following guideline.

       In the case of a managed segment consisting of L2 devices inter-
       connected by a single shared segment, DSBM entities on such dev-
       ices should assume the bandwidth of the interface as the total
       link bandwidth. In the case of a DSBM located in a L2 switch, it
       might additionally need to be configured with an estimate of the
       device's switching capacity if that is less than the link
       bandwidth, and possibly with some estimate of the buffering
       resources of the switch (see [RFC-FRAME] for the architectural
       model assumed for L2 switches). Given the total link bandwidth,
       the DSBM may be further configured to limit the maximum amount of
       bandwidth for RSVP-enabled flows to ensure spare capacity for
       best-effort traffic.

       7.2. Operation of DSBMs in Different L2 Topologies

       Depending on a L2 topology, a DSBM may be called upon to manage
       resources for one or more segments and the implementers must bear
       in mind efficiency implications of the use of DSBM in different L2
       topologies.  Trivial L2 topologies consist of a single "physical
       segment". In this case, the 'managed segment' is equivalent to a
       single segment. Complex L2 topologies may consist of a number of
       'physical segments', separated by SBM-transparent L2 switches.
       Admission control on such an L2 extended segment can be performed
       from a single pool of resources, similar to a single shared seg-
       ment, from the point of view of a single DSBM.

       This configuration compromises the efficiency with which the DSBM
       can allocate resources. This is because the single DSBM is
       required to make admission control decisions for all reservation
       requests within the L2 topology, with no knowledge of the actual
       physical segments affected by the reservation.

       We can realize improvements in the efficiency of resource alloca-
       tion by subdividing the complex segment into a number of managed
       segments, each managed by their own DSBM. In this case, each DSBM

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       manages a managed segment having a relatively simple topology.
       Since managed segments are simpler, the DSBM can be configured
       with a more accurate estimate of the resources available for all
       reservations in the managed segment. In the ultimate configura-
       tion, each physical segment is a managed segment and is managed by
       its own DSBM. We make no assumption about the number of managed
       segments but state, simply, that in complex L2 topologies, the
       efficiency of resource allocation improves as the granularity of
       managed segments increases.

       8. Security Considerations

       The message formatting and usage rules described in this note
       raise security issues, identical to those raised by the use of
        RSVP and Integrated Services. It is necessary to control and
        authenticate access to enhanced qualities of service enabled by
        the technology described in this RFC. This requirement is
discussed       further in [RFC-2205], [RFC-2211], and [RFC-2212].

        [RFC-RSVPMD5] describes the mechanism used to protect the
integrity of    RSVP messages carrying the information described
here. A SBM     implementation should satisfy the requirements of
that RFC and provide
        the suggested mechanisms just as though it were a
conventional RSVP       implementation. It should further use the
same mechanisms to
        protect the additional, SBM-specific objects in a message.

       Finally, it is also necessary to authenticate DSBM candidates
       during the election process, and a mechanism based on a shared
       secret among the DSBM candidates may be used.  The mechanism
       defined in [RFC-RSVPMD5] should be used.

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       9. References

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

       [RFC-RSVPMD5] F. Baker., "RSVP Cryptographic Authentication", draft-
       ietf-rsvp-md5-05.txt, August 1997. [XXX- is this a RFC yet??]

       [RFC-2206] F. Baker, J. Krawczyk, "RSVP Management Information
       Base", RFC 2206, September 1997.

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

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

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

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

       [RFC-2213] F. Baker, J. Krawczyk, "Integrated Services Management
       Information Base", RFC 2213, September 1997.

       [RFC-FRAME] A. Ghanwani, W. Pace, V. Srinivasan, A.Smith,
       M.Seaman "A Framework for Providing Integrated Services Over
       Shared and Switched LAN Technologies", RFC-XXX, June, 1999.

       [RFC-MAP] M. Seaman, A. Smith, E. Crawley, "Integrated Service
       Mappings on IEEE 802 Networks", RFC-XXX, June 1999.

       [IEEE802Q] "IEEE Standards for Local and Metropolitan Area Net-
       works:  Virtual Bridged Local Area Networks", Draft Standard
       P802.1Q/D9, February 20, 1998.

       [IEEEP8021p] "Information technology - Telecommunications and
       information exchange between systems - Local and metropolitan area
       networks - Common specifications - Part 3:  Media Access Control
       (MAC) Bridges: Revision (Incorporating IEEE P802.1p:  Traffic
       Class Expediting and Dynamic Multicast Filtering)", ISO/IEC Final
       CD 15802-3 IEEE P802.1D/D15, November 24, 1997.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       [IEEE8021D] "MAC Bridges", ISO/IEC 10038, ANSI/IEEE Std 802.1D-
       1993.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

                                   Appendix A
                            DSBM Election Algorithm

       A.1. Introduction

       To simplify the rest of this discussion, we will assume that there
       is a single DSBM for the entire L2 domain (i.e., assume a shared
       L2 segment for the entire L2 domain). Later, we will discuss how a
       DSBM is elected for a half-duplex or full-duplex switched segment.

       To allow for quick recovery from the failure of a DSBM, we assume
       that additional SBMs may be active in a L2 domain for fault toler-
       ance. When more than one SBM is active in a L2 domain, the SBMs
       use an election algorithm to elect a DSBM for the L2 domain. After
       the DSBM is elected and is operational, other SBMs remain passive
       in the background to step in to elect a new DSBM when necessary.
       The protocol for electing and discovering DSBM is called the "DSBM
       election protocol" and is described in the rest of this Appendix.

       A.1.1. How a DSBM Client Detects a Managed Segment

       Once elected, a DSBM periodically multicasts an I_AM_DSBM message
       on the AllSBMAddress to indicate its presence. The message is sent
       every period (e.g., every 5 seconds) according to the
       RefreshInterval timer value (a configuration parameter).
       Absence of such a message over a certain time interval (called
       "DSBMDeadInterval"; another configuration parameter typically set
       to a multiple of RefreshInterval) indicates that the DSBM has
       failed or terminated and triggers another round of the DSBM elec-
       tion. The DSBM clients always listen for periodic DSBM advertise-
       ments. The advertisement includes the unicast IP address of the
       DSBM (DSBMAddress) and DSBM clients send their PATH/RESV (or
       other) messages to the DSBM. When a DSBM client detects the
       failure of a DSBM, it waits for a subsequent I_AM_DSBM advertise-
       ment before resuming any communication with the DSBM. During the
       period when a DSBM is not present, a DSBM client may forward out-
       going PATH messages using the standard RSVP forwarding rules.

       The exact message formats and addresses used for communication
       with (and among) SBM(s) are described in Appendix B.

       A.2. Overview of the DSBM Election Procedure

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       When a SBM first starts up, it listens for incoming DSBM adver-
       tisements for some period to check whether a DSBM already exists
       in its L2 domain. If one already exists (and no new election is in
       progress), the new SBM stays quiet in the background until an
       election of DSBM is necessary. All messages related to the DSBM
       election and DSBM advertisements are always sent to the AllSBMAd-
       dress.

       If no DSBM exists, the SBM initiates the election of a DSBM by
       sending out a DSBM_WILLING message that lists its IP address as a
       candidate DSBM and its "SBM priority". Each SBM is assigned a
       priority  to determine its relative precedence. When more than one
       SBM candidate exists, the SBM priority determines who gets to be
       the DSBM based on the relative priority of candidates. If there is
       a tie based on the priority value, the tie is  broken using the IP
       addresses of tied candidates (one with the higher IP address in
       the lexicographic order wins). The details of the election proto-
       col start in Section A.4.

       A.2.1 Summary of the Election Algorithm

       For the purpose of the algorithm, a SBM is in one of the four
       states (Idle, DetectDSBM, ElectDSBM, IAMDSBM).

       A SBM (call it X) starts up in the DetectDSBM state and waits for
       a ListenInterval for incoming I_AM_DSBM (DSBM advertisement) or
       DSBM_WILLING messages. If an I_AM_DSBM advertisement is received
       during this state, the SBM notes the current DSBM (its IP address
       and priority) and enters the Idle state. If a DSBM_WILLING message
       is received from another SBM (call it Y) during this state, then X
       enters the ElectDSBM state. Before entering the new state, X first
       checks to see whether it itself is a better candidate than Y and,
       if so, sends out a DSBM_WILLING message and then enters the
       ElectDSBM state.

       When a SBM (call it X) enters the ElectDSBM state, it sets a timer
       (called ElectionIntervalTimer, and typically set to a value at
       least equal to the DSBMDeadInterval value) to wait for the elec-
       tion to finish and to discover who is the best candidate. In this
       state, X keeps track of the best (or better) candidate seen so far
       (including itself). Whenever it receives another DSBM_WILLING mes-
       sage, it updates its notion of the best (or better) candidate
       based on the priority (and tie-breaking) criterion.  During the
       ElectionInterval, X sends out a DSBM_WILLING message every
       RefreshInterval to (re)assert its candidacy.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       At the end of the ElectionInterval, X checks whether it is the
       best candidate so far. If so, it declares itself to be the DSBM
       (by sending out the I_AM_DSBM advertisement) and enters the
       IAMDSBM state; otherwise, it decides to wait for the best candi-
       date to declare itself the winner. To wait, X re-initializes its
       ElectDSBM state and continues to wait for another round of elec-
       tion (each round lasts for an ElectionTimerInterval duration).

       A SBM is in Idle state when no election is in progress and the
       DSBM is already elected (and happens to be someone else).  In this
       state, it listens  for incoming I_AM_DSBM advertisements and uses
       a DSBMDeadIntervalTimer to detect the failure of DSBM. Every time
       the advertisement is received, the timer is restarted. If the
       timer fires, the SBM goes into the DetectDSBM state to prepare to
       elect the new DSBM. If a SBM receives a DSBM_WILLING message from
       the current DSBM in this state, the SBM enters the ElectDSBM state
       after sending  out a DSBM_WILLING message (to announce its own
       candidacy).

       In the IAMDSBM state, the DSBM sends out I_AM_DSBM advertise-
       ments every refresh interval. If the DSBM wishes to shut down
       (gracefully terminate), it sends out a DSBM_WILLING message (with
       SBM priority value set to zero) to initiate the election pro-
       cedure. The priority value zero effectively removes the outgoing
       DSBM from the election procedure and makes way for the election of
       a different DSBM.

       A.3. Recovering from DSBM Failure

       When a DSBM fails (DSBMDeadIntervalTimer fires), all the SBMs
       enter the ElectDSBM state and start the election process.

       At the end of the ElectionInterval, the elected DSBM sends out an
       I_AM_DSBM advertisement and the DSBM is then operational.

       A.4. DSBM Advertisements

       The I_AM_DSBM advertisement contains the following information:

       1.   DSBM address information -- contains the IP and L2 addresses
            of the DSBM and its SBM priority (a configuration parameter

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                       SBM (Subnet Bandwidth Manager)         October, 1999

            -- priority specified by a network administrator). The prior-
            ity value is used to choose among candidate SBMs during the
            election algorithm. Higher integer values indicate higher
            priority and the value is in the range 0..255. The value zero
            indicates that the SBM is not eligible to be the DSBM.  The
            IP address is required and used for breaking ties. The L2
            address is for the interface of the managed segment.

       2.   RegreshInterval -- contains the value of RefreshInterval
            in seconds.  Value zero indicates the parameter has been
            omitted in the message.  Receivers may substitute their own
            default value in this case.

       3.   DSBMDeadInterval -- contains the value of DSBMDeadInterval
            in seconds. If the value is omitted (or value zero is speci-
            fied), a default value (from initial configuration) should be
            used.

       4.   Miscellaneous configuration information to be advertised to
            senders on the managed segment. See Appendix C for further
            details.

       A.5. DSBM_WILLING Messages

       When a SBM wishes to declare its candidacy to be the DSBM  during
       an election phase, it sends out a DSBM_WILLING message. The
       DSBM_WILLING message contains the following information:

       1.   DSBM address information -- Contains the SBM's own addresses
            (IP and L2 address), if it wishes to be the DSBM. The IP
            address is required and used for breaking ties. The L2
            address is the address of the interface for the managed seg-
            ment in question.  Also, the DSBM address information
            includes the corresponding  priority of the SBM whose address
            is given above.

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       A.6. SBM State Variables

       For each network interface, a SBM maintains the following state
       variables related to the election of the DSBM for the L2 domain on
       that interface:

            a) LocalDSBMAddrInfo -- current DSBM's IP address (initially,
            0.0.0.0) and priority. All IP addresses are assumed to be in
            network byte order. In addition, current DSBM's L2 address is
            also stored as part of this state information.

            b) OwnAddrInfo -- SBM's own IP address and L2 address for the
            interface and its own priority (a configuration parameter).

            c) RefreshInterval in seconds. When the DSBM is not yet
            elected, it is set to a default value specified as a confi-
            guration parameter.

            d) DSBMDeadInterval in seconds. When the DSBM is not yet
            elected, it is initially set to  a default value specified as
            a configuration parameter.

            f) ListenInterval in seconds -- a configuration parameter
            that decides how long a SBM spends in the DetectDSBM state
            (see below).

            g) ElectionInterval in seconds -- a configuration parameter
            that decides how long a SBM spends in the ElectDSBM state
            when it has declared its candidacy.

       Figure 3 shows the state transition diagram for the election pro-
       tocol and the various states are described below. A complete
       description of the state machine is provided in Section A.10.

       A.7. DSBM Election States

            DOWN -- SBM is not operational.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

            DetectDSBM -- typically, the initial state of a SBM when it
            starts up. In this state, it checks to see whether a DSBM
            already exists in its domain.

            Idle -- SBM is in this state when no election is in progress
            and it is not the DSBM. In this state, SBM passively monitors
            the state of the DSBM.

            ElectDSBM -- SBM is in this state when a DSBM election is in
            progress.

            IAMDSBM -- SBM is in this state when it is the DSBM for the
            L2 domain.

       A.8. Events that cause state changes

            StartUp -- SBM starts operation.

            ListenInterval Timeout -- The ListenInterval timer has fired.
            This means that the SBM has monitored its domain to check for
            an existing DSBM or to check whether there are candidates
            (other than itself) willing to be the DSBM.

            DSBM_WILLING message received -- This means that the SBM
            received a DSBM_WILLING message from some other SBM. Such a
            message is sent when a SBM wishes to declare its candidacy to
            be the DSBM.

            I_AM_DSBM message received -- SBM received a DSBM advertise-
            ment from the DSBM in its L2 domain.

            DSBMDeadInterval Timeout -- The DSBMDeadIntervalTimer has
            fired. This means that the SBM did not receive even one DSBM
            advertisement during this period and indicates possible
            failure of the DSBM.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

            RefreshInterval Timeout -- The RefreshIntervalTimer has
            fired. In the IAMDSBM state, this means it is the time for
            sending out the next DSBM advertisement. In the ElectDSBM
            state, the event means that it is the time to send out
            another DSBM_WILLING message.

            ElectionInterval Timeout -- The ElectionIntervalTimer has
            fired. This means that the SBM has waited long enough after
            declaring its candidacy to determine whether or not it suc-
            ceeded.

                             CONTINUED ON NEXT PAGE

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       A.9. State Transition Diagram (Figure 3)

                                    +-----------+
                +--<--------------<-|DetectDSBM |---->------+
                |                   +-----------+           |
                |                                           |
                |                                           |
                |                                           |
                |     +-------------+       +---------+     |
                +->---|   Idle      |--<>---|ElectDSBM|--<--+
                      +-------------+       +---------+
                           |                        |
                           |                        |
                           |                        |
                           |        +-----------+   |
                           +<<- +---|  IAMDSBM  |-<-+
                                |   +-----------+
                                |
                                |   +-----------+
                                +>>-| SHUTDOWN  |
                                    +-----------+

       A.10. Election State Machine

       Based on the events and states described above, the state changes
       at a SBM are described below. Each state change is triggered by an
       event and is typically accompanied by a sequence of actions.  The
       state machine is described assuming a single threaded implementa-
       tion (to avoid race conditions between state changes and timer
       events) with no timer events occurring during the execution of the
       state machine.

       The following routines will be frequently used in the description
       of the state machine:

       ComparePrio(FirstAddrInfo, SecondAddrInfo)
         -- determines whether the entity represented by the first parameter
           is better than the second entity using the priority information
           and the IP address information in the two parameters.
           If any address is zero, that entity
           automatically loses; then first priorities are compared; higher
           priority candidate wins. If there is a tie based on
           the priority value, the tie is  broken using the IP
           addresses of tied candidates (one with the higher IP address in the
           lexicographic order wins). Returns TRUE if first entity is a better

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                       SBM (Subnet Bandwidth Manager)         October, 1999

           choice. FALSE otherwise.

       SendDSBMWilling Message()
       Begin
           Send out DSBM_WILLING message listing myself as a candidate for
           DSBM (copy OwnAddr and priority into appropriate fields)
           start RefreshIntervalTimer
           goto ElectDSBM state
       End

       AmIBetterDSBM(OtherAddrInfo)
       Begin
           if (ComparePrio(OwnAddrInfo, OtherAddrInfo))
               return TRUE

           change LocalDSBMInfo = OtherDSBMAddrInfo
           return FALSE
       End

       UpdateDSBMInfo()
       /* invoked in an assignment such as LocalDSBMInfo = OtherAddrInfo */
       Begin
           update LocalDSBMInfo such as  IP addr, DSBM L2 address,
           DSBM priority, RefreshIntervalTimer, DSBMDeadIntervalTimer
       End

       A.10.1 State Changes

       In the following, the action "continue" or "continue in current
       state" means an "exit" from the current action sequence without a
       state transition.

       State:      DOWN
       Event:      StartUp
       New State:  DetectDSBM
       Action:     Initialize the local state variables (LocalDSBMADDR and
                   LocalDSBMAddrInfo set to 0). Start the ListenIntervalTimer.

       State:      DetectDSBM
       New State:  Idle
       Event:      I_AM_DSBM message received
       Action:     set LocalDSBMAddrInfo = IncomingDSBMAddrInfo
                   start DeadDSBMInterval timer
                   goto Idle State

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       State:      DetectDSBM
       Event:      ListenIntervalTimer fired
       New State:  ElectDSBM
       Action:     Start ElectionIntervalTimer
                   SendDSBMWillingMessage();

       State:      DetectDSBM
       Event:      DSBM_WILLING message received
       New State:  ElectDSBM
       Action:     Cancel any active timers

                   Start ElectionIntervalTimer
                   /* am I a better choice than this dude? */
                   If (ComparePrio(OwnAddrInfo, IncomingDSBMInfo)) {
                       /* I am better */
                       SendDSBMWillingMessage()
                   } else {
                       Change LocalDSBMAddrInfo = IncomingDSBMAddrInfo
                       goto ElectDSBM state
                   }

       State:      Idle
       Event:      DSBMDeadIntervalTimer fired.
       New State:  ElectDSBM
       Action:     start ElectionIntervalTimer
                   set LocalDSBMAddrInfo = OwnAddrInfo
                   SendDSBMWiliingMessage()

       State:      Idle
       Event:      I_AM_DSBM message received.
       New State:  Idle
       Action:     /* first check whether anything has changed */
                   if (!ComparePrio(LocalDSBMAddrInfo, IncomingDSBMAddrInfo))
                       change LocalDSBMAddrInfo to reflect new info
                   endif
                   restart DSBMDeadIntervalTimer;
                   continue in current state;

       State:      Idle
       Event:      DSBM_WILLING Message is received
       New State:  Depends on action (ElectDSBM or Idle)
       Action:     /* check whether it is from the DSBM itself (shutdown) */
                   if (IncomingDSBMAddr == LocalDSBMAddr) {
                       cancel active timers
                       Set LocalDSBMAddrInfo = OwnAddrInfo
                       Start ElectionIntervalTimer
                       SendDSBMWillingMessage() /* goto ElectDSBM state */
                   }

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                       SBM (Subnet Bandwidth Manager)         October, 1999

                   /* else, ignore it */
                   continue in current state

       State:      ElectDSBM
       Event:      ElectionIntervalTimer Fired
       New State:  depends on action (IAMDSBM or Current State)
       Action:     If (LocalDSBMAddrInfo == OwnAddrInfo) {
                       /* I won */
                       send I_AM_DSBM message
                       start RefreshIntervalTimer
                       goto IAMDSBM state
                   } else {   /* someone else won, so wait for it to declare
                                itself to be the DSBM */
                       set LocalDSBMAddressInfo = OwnAddrInfo
                       start ElectionIntervalTimer
                       SendDSBMWillingMessage()
                       continue in current state
                   }

       State:      ElectDSBM
       Event:      I_AM_DSBM message received
       New State:  Idle
       Action:     set LocalDSBMAddrInfo = IncomingDSBMAddrInfo
                   Cancel any active timers
                   start DeadDSBMInterval timer
                   goto Idle State

       State:      ElectDSBM
       Event:      DSBM_WILLING message received
       New State:  ElectDSBM
       Action:     Check whether it's a loopback and if so, discard, continue;
                   if (!AmIBetterDSBM(IncomingDSBMAddrInfo)) {
                       Change LocalDSBMAddrInfo = IncomingDSBMAddrInfo
                       Cancel RefreshIntervalTimer
                   } else if (LocalDSBMAddrInfo == OwnAddrInfo) {
                       SendDSBMWillingMessage()
                   }
                   continue in current state

       State:      ElectDSBM
       Event:      RefreshIntervalTimer fired
       New State:  ElectDSBM
       Action:     /* continue to send DSBMWilling messages until
                     election interval ends */
                   SendDSBMWillingMessage()

       State:      IAMDSBM
       Event:      DSBM_WILLING message received

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       New State:  depends on action (IAMDSBM or SteadyState)
       Action:     /* check whether other guy is better */
                   If (ComparePrio(OwnAddrInfo, IncomingAddrInfo))  {
                   /* I am better */
                       send I_AM_DSBM message
                       restart RefreshIntervalTimer
                      continue in current state
                   } else {
                      Set LocalDSBMAddrInfo = IncomingAddrInfo
                      cancel active timers
                      start DSBMDeadIntervalTimer
                      goto SteadyState
                   }

       State:      IAMDSBM
       Event:      RefreshIntervalTimer fired
       New State:  IAMDSBM
       Action:     send I_AM_DSBM message
                   restart RefreshIntervalTimer

       State:      IAMDSBM
       Event:      I_AM_DSBM message received
       New State:  depends on action (IAMDSBM or Idle)
       Action:     /* check whether other guy is better */
                   If (ComparePrio(OwnAddrInfo, IncomingAddrInfo))  {
                       /* I am better */
                       send I_AM_DSBM message
                       restart RefreshIntervalTimer
                       continue in current state
                  } else {
                       Set LocalDSBMAddrInfo = IncomingAddrInfo
                       cancel active timers
                       start DSBMDeadIntervalTimer
                       goto Idle State
                 }

       State:      IAMDSBM
       Event:      Want to shut myself down
       New State:  DOWN
       Action:     send DSBM_WILLING message with My address filled in, but
                   priority set to zero
                   goto Down State

       A.10.2 Suggested Values of Interval Timers

       To avoid DSBM outages for long period, to ensure quick recovery

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       from DSBM failures, and to avoid timeout of PATH and RESV state at
       the edge devices, we suggest  the following values for various
       timers.

       Assuming that the RSVP implementations use a 30 second timeout for
       PATH and RESV refreshes, we suggest that the RefreshIntervalTimer
       should be set to about 5 seconds with DSBMDeadIntervalTimer set to
       15 seconds (K=3, K*RefreshInterval). The DetectDSBMTimer should be
       set to a random value between (DSBMDeadIntervalTimer, 2*DSBMDeadInterval-
       Timer). The ElectionIntervalTimer should be set at least to the
       value of DSBMDeadIntervalTimer to ensure that each SBM has a chance to
       have its DSBM_WILLING message (sent every RefreshInterval in
       ElectDSBM state) delivered to others.

       A.10.3. Guidelines for Choice of Values for SBM_PRIORITY

       Network administrators should configure SBM protocol entity at
       each SBM-capable device with the device's "SBM priority" for each
       of the interfaces attached to a managed segment. SBM_PRIORITY is
       an 8-bit, unsigned integer value (in the range 0-255) with higher
       integer values denoting higher priority. The value zero for an
       interface indicates that the SBM protocol entity on the device is
       not eligible to be a DSBM for the segment attached to the inter-
       face.

       A separate range of values is reserved for each type of SBM-
       capable device to reflect the relative priority among different
       classes of L2/L3 devices. L2 devices get higher priority followed
       by routers followed by hosts. The priority values in the range of
       128..255 are reserved for L2 devices, the values in the range of
       64..127 are reserved for routers, and values in the range of 1..63
       are reserved for hosts.

       A.11. DSBM Election over switched links

       The election algorithm works as described before in this case
       except each SBM-capable L2 device restricts the scope of the elec-
       tion to its local segment. As described in Section B.1 below, all
       messages related to the DSBM election are sent to a special multi-
       cast address (AllSBMAddress). AllSBMAddress (its corresponding MAC
       multicast address) is configured in the permanent database of
       SBM-capable, layer 2 devices so that all frames with AllSBMAddress
       as the destination address are not forwarded and instead directed

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       to the SBM management entity in those devices. Thus, a DSBM can be
       elected separately on each point-to-point segment in a switched
       topology. For example, in Figure 2, DSBM for "segment A" will be
       elected using the election algorithm between R1 and S1 and none of
       the election-related messages on this segment will be forwarded by
       S1 beyond "segment A". Similarly, a separate election will take
       place on each segment in this topology.

       When a switched segment is a half-duplex segment, two senders (one
       sender at each end of the link) share the link. In this case, one
       of the two senders will win the DSBM election and will be respon-
       sible for managing the segment.

       If a switched segment is full-duplex, exactly one sender sends on
       the link in each direction. In this case, either one or two DSBMs
       can exist on such a managed segment. If a sender at each end
       wishes to serve as a DSBM for that end, it can declare itself to
       be the DSBM by sending out an I_AM_DSBM advertisement and start
       managing the resources for the outgoing traffic over the segment.
       If one of the two senders does not wish itself to be the DSBM,
       then the other DSBM will not receive any DSBM advertisement from
       its peer and assume itself to be the DSBM for traffic traversing
       in both directions over the managed segment.

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                                   Appendix B
                       Message Encapsulation and Formats

       To minimize changes to the existing RSVP implementations and to
       ensure quick deployment of a SBM in conjunction with RSVP, all
       communication to and from a DSBM will be performed using messages
       constructed using the current rules for RSVP message formats and
       raw IP encapsulation. For more details on the RSVP message for-
       mats, refer to the RSVP specification (RFC 2205).  No changes to
       the RSVP message formats are proposed, but new message types and
       new L2-specific objects are added to the RSVP message formats to
       accommodate DSBM-related messages. These additions are described
       below.

       B.1 Message Addressing

       For the purpose of DSBM election and detection, AllSBMAddress is
       used as the destination address while sending out both
       DSBM_WILLING and I_AM_DSBM messages. A DSBM client first detects a
       managed segment by listening to I_AM_DSBM advertisements and
       records the DSBMAddress (unicast IP address of the DSBM).

       B.2. Message Sizes

       Each message must occupy exactly one IP datagram. If it exceeds
       the MTU, such a datagram will be fragmented by IP and reassembled
       at the recipient node. This has a consequence that a single mes-
       sage may not exceed the maximum IP datagram size, approximately
       64K bytes.

       B.3. RSVP-related Message Formats

       All RSVP messages directed to and from a DSBM may contain various
       RSVP objects defined in the RSVP specification and messages con-
       tinue to follow the formatting rules specified in the RSVP specif-
       ication. In addition, an RSVP implementation must also recognize
       new object classes that are described below.

       B.3.1. Object Formats

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                       SBM (Subnet Bandwidth Manager)         October, 1999

       All objects are defined using the format specified in the RSVP
       specification. Each object has a 32-bit header that contains
       length (of the object in bytes including the object header), the
       object class number, and a C-Type. All unused fields should be set
       to zero and ignored on receipt.

       B.3.2. SBM Specific Objects

       Note that the Class-Num values for the SBM specific objects
       (LAN_NHOP, LAN_LOOPBACK, and RSVP_HOP_L2) are chosen from the
       codespace 10XXXXXX. This coding assures that non-SBM aware RSVP
       nodes will ignore the objects without forwarding them or generat-
       ing an error message.

       Within the SBM specific codespace, note the following interpreta-
       tion of the third most significant bit of the Class-Num:

              a) Objects of the form 100XXXXX are to be silently dis-
              carded by SBM nodes that do not recognize them.

              b) Objects of the form 101XXXXX are to be silently for-
              warded by SBM nodes that do not recognize them.

      B.3.3. IEEE 802 Canonical Address Format

      The 48-bit MAC Addresses used by IEEE 802 were originally defined
      in terms of wire order transmission of bits in the source and des-
      tination MAC address fields. The same wire order applied to both
      Ethernet and Token Ring. Since the bit transmission order of Ether-
      net and Token Ring data differ - Ethernet octets are transmitted
      least significant bit first, Token Ring most significant first -
      the numeric values naturally associated with the same address on
      different 802 media differ. To facilitate the communication of
      address values in higher layer protocols which might span both
      token ring and Ethernet attached systems connected by bridges, it
      was necessary to define one reference format - the so called canon-
      ical format for these addresses. Formally the canonical format
      defines the value of the address, separate from the encoding rules
      used for transmission. It comprises a sequence of octets derived
      from the original wire order transmission bit order as follows. The
      least significant bit of the first octet is the first bit transmit-
      ted, the next least significant bit the second bit, and so on to

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                       SBM (Subnet Bandwidth Manager)         October, 1999

      the most significant bit of the first octet being the 8th bit
      transmitted; the least significant bit of the second octet is the
      9th bit transmitted, and so on to the most significant bit of the
      sixth octet of the canonical format being the last bit of the
      address transmitted.

      This canonical format corresponds to the natural value of the
      address octets for Ethernet. The actual transmission order or for-
      mal encoding rules for addresses on media which do not transmit bit
      serially are derived from the canonical format octet values.

      This document requires that all L2 addresses used in conjunction
      with the SBM protocol be encoded in the canonical format as a
      sequence of 6 octets. In the following, we define the object for-
      mats for objects that contain L2 addresses that are based on the
      canonical representation.

      B.3.4. RSVP_HOP_L2 object

      RSVP_HOP_L2 object uses object class = 161; it contains the L2
      address of the previous hop L3 device in the IEEE Canonical address
      format discussed above.

      RSVP_HOP_L2 object: class = 161, C-Type represents the addressing format
      used. In our case, C-Type=1 represents the IEEE Canonical Address
      format.

               0              1             2                 3
      +---------------+---------------+---------------+----------------+
      |       Length                  |   161         |C-Type(addrtype)|
      +---------------+---------------+---------------+----------------+
      |                  Variable length Opaque data                   |
      +---------------+---------------+---------------+----------------+

      C-Type = 1 (IEEE Canonical Address format)

      When C-Type=1, the object format is:

              0               1               2               3
      +---------------+---------------+---------------+---------------+
      |              12               |   161         |      1        |
      +---------------+---------------+---------------+---------------+
      |             Octets 0-3 of the MAC address                     |
      +---------------+---------------+---------------+---------------+
      |  Octets 4-5 of the MAC addr.  |   ///         |     ///       |
      +---------------+---------------+---------------+---------------+

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      /// -- unused (set to zero)

      B.3.5. LAN_NHOP object

      LAN_NHOP object represents two objects, namely, LAN_NHOP_L3 address
      object and LAN_NHOP_L2 address object.
           <LAN_NHOP object> ::= <LAN_NHOP_L2 object> <LAN_NHOP_L3 object>

      LAN_NHOP_L2 address object uses object class = 162 and uses the
      same format (but different class number) as the RSVP_HOP_L2 object.
      It provides the L2 or MAC address of the next hop L3 device.

              0               1               2               3
      +---------------+---------------+---------------+----------------+
      |       Length                  |   162         |C-Type(addrtype)|
      +---------------+---------------+---------------+----------------+
      |                  Variable length Opaque data                   |
      +---------------+---------------+---------------+----------------+

      C-Type = 1 (IEEE 802 Canonical Address Format as defined below)
      See the RSVP_HOP_L2 address object for more details.

      LAN_NHOP_L3 object uses object class = 163 and gives the L3 or IP
      address of the next hop L3 device.

      LAN_NHOP_L3 object: class = 163, C-Type specifies IPv4 or IPv6 address
      family used.

      IPv4 LAN_NHOP_L3 object: class =163, C-Type = 1
      +---------------+---------------+---------------+---------------+
      |       Length = 8              |   163         |       1       |
      +---------------+---------------+---------------+---------------+
      |               IPv4 NHOP address                               |
      +---------------------------------------------------------------+

      IPv6 LAN_NHOP_L3 object: class =163, C-Type = 2
      +---------------+---------------+---------------+---------------+
      |       Length = 20             |   163         |       2       |
      +---------------+---------------+---------------+---------------+
      |               IPv6 NHOP address (16 bytes)                    |
      +---------------------------------------------------------------+

      B.3.6. LAN_LOOPBACK Object

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      The LAN_LOOPBACK object gives the IP address of the outgoing inter-
      face for a PATH message and uses object class=164; both IPv4 and
      IPv6 formats are specified.

      IPv4 LAN_LOOPBACK object: class = 164, C-Type = 1

              0               1               2               3
      +---------------+---------------+---------------+---------------+
      |       Length                  |   164         |       1       |
      +---------------+---------------+---------------+---------------+
      |                  IPV4 address of an interface                 |
      +---------------+---------------+---------------+---------------+

      IPv6 LAN_LOOPBACK object: class = 164, C-Type = 2

      +---------------+---------------+---------------+---------------+
      |       Length                  |   164         |       2       |
      +---------------+---------------+---------------+---------------+
      |                                                               |
      +                                                               +
      |                                                               |
      +                  IPV6 address of an interface                 +
      |                                                               |
      +                                                               +
      |                                                               |
      +---------------+---------------+---------------+---------------+

      B.3.7. TCLASS Object

      TCLASS object (traffic class based on IEEE 802.1p) uses  object
      class = 165.

               0              1               2               3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Length                |   165         |       1       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    ///        |    ///        |  ///          | ///     | PV  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Only  3 bits in data contain the user_priority value (PV).

      B.4. RSVP PATH and PATH_TEAR Message Formats

      As specified in the RSVP specification, a PATH and PATH_TEAR mes-
      sages contain the RSVP Common Header and the relevant RSVP objects.

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                       SBM (Subnet Bandwidth Manager)         October, 1999

      For the RSVP Common Header, refer to the RSVP specification (RFC
      2205). Enhancements to an RSVP_PATH message include additional
      objects as specified below.

      <PATH Message> ::= <RSVP Common Header> [<INTEGRITY>]
                      <RSVP_HOP_L2> <LAN_NHOP>
                      <LAN_LOOPBACK> [<TCLASS>]  <SESSION><RSVP_HOP>
                      <TIME_VALUES> [<POLICY DATA>] <sender descriptor>

      <PATH_TEAR Message> ::= <RSVP Common Header> [<INTEGRITY>]
                      <LAN_LOOPBACK> <LAN_NHOP> <SESSION> <RSVP_HOP>
                      [<sender descriptor>]

      If the INTEGRITY object is present, it must immediately follow the
      RSVP common header. L2-specific objects must always precede the
      SESSION object.

      B.5. RSVP RESV Message Format

      As specified in the RSVP specification, an RSVP_RESV message con-
      tains the RSVP Common Header and relevant RSVP objects. In addi-
      tion, it may contain an optional TCLASS object as described ear-
      lier.

      B.6. Additional RSVP message types to handle SBM interactions

      New RSVP message types are introduced to allow interactions between
      a DSBM and an RSVP node (host/router) for the purpose of discover-
      ing and binding to a DSBM. New RSVP message types needed are as
      follows:

      RSVP Msg Type (8 bits)      Value
      DSBM_WILLING                66
      I_AM_DSBM                   67

      All SBM-specific messages are formatted as RSVP messages with an
      RSVP common header followed by SBM-specific objects.

      <SBMP_MESSAGE> ::= <SBMP common header> <SBM-specific objects>

      where <SBMP common header> ::= <RSVP common Header> [<INTEGRITY>]

      For each SBM message type, there is a set of rules for the permis-
      sible choice of object types. These rules are specified using

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                       SBM (Subnet Bandwidth Manager)         October, 1999

      Backus-Naur Form (BNF) augmented with square brackets surrounding
      optional sub-sequences. The BNF implies an order for the objects in
      a message. However, in many (but not all) cases, object order makes
      no logical difference. An implementation should create messages
      with the objects in the order shown here, but accept the objects in
      any permissible order. Any exceptions to this rule will be pointed
      out in the specific message formats.

      DSBM_WILLING Message

      <DSBM_WILLING message> ::= <SBM Common Header> <DSBM IP ADDRESS>
                                 <DSBM L2 address> <SBM PRIORITY>

      I_AM_DSBM Message

      <I_AM_DSBM> ::= <SBM Common Header> <DSBM IP ADDRESS> <DSBM L2 address>
                                 <SBM PRIORITY> <DSBM Timer Intervals>
                                 [<NON_RESV_SEND_LIMIT>]

      For compatibility reasons, receivers of the I_AM_DSBM message must
      be prepared to receive additional objects of the Unknown Class type
      [RFC-2205].

      All I_AM_DSBM messages are multicast to the well known AllSBMAd-
      dress.  The default priority of a SBM is 1 and higher priority
      values represent higher precedence. The priority value zero indi-
      cates that the SBM is not eligible to be the DSBM.

      Relevant Objects

      DSBM IP ADDRESS objects use object class = 42; IPv4 DSBM IP ADDRESS
      object uses <Class=42, C-Type=1> and IPv6 DSBM IP ADDRESS object
      uses <Class=42, C-Type=2>.

      IPv4 DSBM IP ADDRESS object: class = 42, C-Type =1
              0               1               2               3
      +---------------+---------------+---------------+---------------+
      |                       IPv4 DSBM IP Address                    |
      +---------------+---------------+---------------+---------------+

      IPv6 DSBM IP ADDRESS object: Class = 42, C-Type = 2

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                       SBM (Subnet Bandwidth Manager)         October, 1999

      +---------------+---------------+---------------+---------------+
      |                                                               |
      +                                                               +
      |                                                               |
      +                       IPv6 DSBM IP Address                    +
      |                                                               |
      +                                                               +
      |                                                               |
      +---------------+---------------+---------------+---------------+

      <DSBM L2 address> Object is the same as <RSVP_HOP_L2> object with C-Type
      =1 for IEEE Canonical Address format.

      <DSBM L2 address> ::= <RSVP_HOP_L2>

      a SBM  may omit this object by including a NULL L2 address object. For
      C-Type=1 (IEEE Canonical address format), such a version of the L2
      address object contains value zero in the six octet s corresponding to the
      MAC address (see section B.3.4 for the exact format).

      SBM_PRIORITY Object: class = 43, C-Type =1

              0               1               2               3
      +---------------+---------------+---------------+---------------+
      |   ///         |   ///         | ///           | SBM priority  |
      +---------------+---------------+---------------+---------------+

      TIMER INTERVAL VALUES.

      The two timer intervals, namely, DSBM Dead Interval and DSBM
      Refresh Interval, are specified as integer values each  in the
      range of 0..255 seconds. Both values are included in a  single
      "DSBM Timer Intervals" object described below.

      DSBM Timer Intervals Object: class = 44, C-Type =1

      +---------------+---------------+---------------+----------------+
      |   ///        |   ///          | DeadInterval  | RefreshInterval|
      +---------------+---------------+---------------+----------------+

      NON_RESV_SEND_LIMIT Object: class = 45, C-Type = 1

          0       1       2       3
      +---------------+---------------+---------------+----------------+
      | NonResvSendLimit(limit on traffic allowed to send without RESV)|
      |                                                                |
      +---------------+---------------+---------------+----------------+

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      <NonResvSendLimit> ::= <Intserv Sender_TSPEC object> (class=12, C-Type =2)

      The NON_RESV_SEND_LIMIT object specifies a per-flow limit on the
      profile of traffic which a sending host is allowed to send onto a
      managed segment without a valid RSVP reservation (see Appendix C
      for further details on the usage of this object). The object con-
      tains the NonResvSendLimit parameter.  This parameter is equivalent
      to the Intserv SENDER_TSPEC (see RFC 2210 for contents and encoding
      rules). The SENDER_TSPEC includes five parameters which describe a
      traffic profile (r, b, p, m and M). Sending hosts compare the
      SENDER_TSPEC describing a sender traffic flow to the SENDER_TSPEC
      advertised by the DSBM. If the SENDER_TSPEC of the traffic flow in
      question is less than or equal to the SENDER_TSPEC advertised by
      the DSBM, it is allowable to send traffic on the corresponding flow
      without a valid RSVP reservation in place. Otherwise it is not.

      The network administrator may configure the DSBM to disallow any
      sent traffic in the absence of an RSVP reservation by configuring a
      NonResvSendLimit in which r = 0, b = 0, p = 0, m = infinity and M =
      0. Similarly the network administrator may allow any traffic to be
      sent in the absence of an RSVP reservation by configuring a Non-
      ResvSendLimit in which r = infinity, b = infinity, p = infinity, m
      = 0 and M = infinity. Of course, any of these parameters may be set
      to values between zero and infinity to advertise finite per-flow
      limits.

      The NON_RESV_SEND_LIMIT object is optional. Senders on a managed
      segment should interpret the absence of the NON_RESV_SEND_LIMIT
      object as equivalent to an infinitely large SENDER_TSPEC (it is
      permissible to send any traffic profile in the absence of an RSVP
      reservation).

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                                  Appendix C
         The DSBM as a Source of Centralized Configuration Information

      There are certain configuration parameters which it may be useful
      to distribute to layer-3 senders on a managed segment. The DSBM may
      serve as a centralized management point from which such parameters
      can easily be distributed. In particular,  it is possible for the
      network administrator configuring a DSBM to cause certain confi-
      guration parameters to be distributed as objects appended to the
      I_AM_DSBM messages. The following configuration object is defined
      at this time. Others may be defined in the future. See Appendix B
      for further details regarding the NON_RESV_SEND_LIMIT object.

      C.1. NON_RESV_SEND_LIMIT

      As we QoS enable layer 2 segments, we expect an evolution from sub-
      nets comprised of traditional shared segments (with no means of
      traffic separation and no DSBM), to subnets comprised of dedicated
      segments switched by sophisticated switches (with both DSBM and
      802.1p traffic separation capability).

      A set of intermediate configurations consists of a group of QoS
      enabled hosts sending onto a traditional shared segment. A layer-3
      device (or a layer-2 device) acts as a DSBM for the shared segment,
      but cannot enforce traffic separation. In such a configuration, the
      DSBM can be configured to limit the number of reservations approved
      for senders on the segment, but cannot prevent them from sending.
      As a result, senders may congest the segment even though a network
      administrator has configured an appropriate limit for admission
      control in the DSBM.

      One solution to this problem which would give the network adminis-
      trator control over the segment, is to require applications (or
      operating systems on behalf of applications) not to send until they
      have obtained a reservation. This is problematic as most applica-
      tions are used to sending as soon as they wish to and expect to get
      whatever service quality the network is able to grant at that time.
      Furthermore, it may often be acceptable to allow certain applica-
      tions to send before a reservation is received. For example, on a
      segment comprised of a single 10 Mbps ethernet and 10 hosts, it may
      be acceptable to allow a 16 Kbps telephony stream to be transmitted
      but not a 3 Mbps video stream.

      A more pragmatic solution then, is to allow the network administra-
      tor to set a per-flow limit on the amount of non-adaptive traffic
      which a sender is allowed to generate on a managed segment in the
      absence of a valid reservation. This limit is advertised by the
      DSBM and received by sending hosts. An API on the sending host can

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                       SBM (Subnet Bandwidth Manager)         October, 1999

      then approve or deny an application's QoS request based on the
      resources requested.

      The NON_RESV_SEND_LIMIT object can be used to advertise a Flowspec
      which describes the shape of traffic that a sender is allowed to
      generate on a managed segment when its RSVP reservation requests
      have either not yet completed or have been rejected.

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                               ACKNOWLEDGEMENTS

      Authors are grateful to Eric Crawley (Argon), Russ Fenger (Intel),
      David Melman (Siemens), Ramesh Pabbati (Microsoft), Mick Seaman
      (3COM), Andrew Smith (Extreme Networks) for their constructive com-
      ments on the SBM design and the earlier versions of this document.

      6. Authors` Addresses

              Raj Yavatkar
              Intel Corporation
              2111 N.E. 25th Avenue,
              Hillsboro, OR 97124
              USA
              phone: +1 503-264-9077
              email: yavatkar@ibeam.intel.com

              Don Hoffman
              Teledesic Corporation
              2300 Carillon Point
              Kirkland, WA 98033
              USA
              phone: +1 425-602-0000

              Yoram Bernet
              Microsoft
              1 Microsoft Way
              Redmond, WA 98052
              USA
              phone: +1 206 936 9568
              email: yoramb@microsoft.com

              Fred Baker
              Cisco Systems
              519 Lado Drive
              Santa Barbara, California 93111
              USA
              phone: +1 408 526 4257
              email: fred@cisco.com

              Michael Speer
              Sun Microsystems, Inc
              901 San Antonio Road UMPK15-215
              Palo Alto, CA 94303
              phone: +1 650-786-6368
              email: speer@Eng.Sun.COM

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