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Versions: 04 06 07 08 09 10 RFC 2814

  Internet Engineering Task Force               Raj Yavatkar, Intel
  INTERNET-DRAFT                                Don Hoffman, Sun Microsystems
                                                Yoram Bernet, Microsoft
                                                Fred Baker, Cisco
  
                                                July 20, 1997
                                                Expires: January 30, 1998
  
                      SBM (Subnet Bandwidth Manager):
       A Proposal for Admission Control over IEEE 802-style networks
  
                            Status of this Memo
  
  This document is an Internet-Draft.  Internet-Drafts are working
  documents of the Internet Engineering Task Force (IETF), its areas,
  and its working groups.  Note that other groups may also distribute
  working documents as Internet-Drafts.
  
  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time.  It is inappropriate to use Internet-Drafts as reference
  material or to cite them other than as ``work in progress.''
  
  To learn the current status of any Internet-Draft, please check the
  ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
  Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au
  (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West
  Coast).
  
  This document is a product of the ISSLL (IS802) subgroup of the Integrated
  Services working group of the Internet Engineering Task Force.  Comments are
  solicited and should be addressed to the working group's mailing list at
  issll@mercury.lcs.mit.edu, and/or the author(s).
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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                                  Abstract
  
  This document outlines a signaling method and protocol for RSVP-based
  admission control over IEEE 802-style LANs.  The proposed method is designed
  to work both with the current generation of IEEE 802 LANs and new work being
  defined within the IEEE 802 committee.
  
  
  
  
                               What's Changed
  
  *    This draft obsoletes its previous version, draft-yavatkar-sbm-
       ethernet-03.txt.
  
  *    Incorporated detailed message processing rules in to this document.
  
  *    Added additional MAC-level information in several SBM-specific
       objects to facilitate efficient implementation in switches.
  
  *    Defined well-known multicast addresses for SBM-encapsulated PATH
       messages.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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  1. Introduction
  
  
  The IETF working groups such as Integrated Services and RSVP have been
  chartered to develop extensions to the IP architecture and service model
  which allow applications to request specific qualities or levels  of
  service from an internetwork in addition to the current IP best-effort
  service. The work at these working groups has led to the definition of
  RSVP (ReSource reServation Protocol), a new resource reservation setup
  protocol, and definition of new service classes to be supported by
  Integrated Services routers. The specifications produced by these work-
  ing groups are largely independent of underlying networking technolo-
  gies.
  
  A separate working group, ISSLL (Integrated Services over Specific Link
  Layers), is chartered to define the mapping of RSVP and Integrated Ser-
  vices specifications onto specific subnetwork technologies. 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.
  
  In particular, the IS802 subgroup of the ISSLL working group has
  addressed the following three aspects of mapping the RSVP and Integrated
  Services specifications over IEEE-802-style LAN technologies:
  
  
  *    Specification of a framework [9] for providing Integrated Services
       over shared and switched IEEE-802-style LAN technologies.
  
  *    Definition of service mappings [10] that describes the limitations
       and ways of supporting Controlled Load [4] and Guaranteed Services
       [5] using the inherent capabilities of the relevant IEEE 802 LAN
       technologies.
  
  *    Specification of a signaling mechanism to map an internet-level
       setup protocol such as RSVP onto IEEE 802 LAN technologies.
  
  
  This document deals with the third of these aspects, and describes a
  signaling method which uses the existing RSVP protocol to allow admis-
  sion control over IEEE 802-style LANs.  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.
  
  
  
  2. Goals and Assumptions
  
  
  
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  Our proposal is  based on the following architectural goals and assump-
  tions:
  
  
  
  I.   Even though the current trend is towards increased use of switched
       LAN topologies consisting of newer switches that support the prior-
       ity 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, our proposal specifies a signal-
       ing mechanism for managing bandwidth over both legacy and newer LAN
       topologies and takes advantage of the additional functionality
       (such as an explicit support for different traffic classes or
       integrated service classes) as it becomes available in the new gen-
       eration of  switches, hubs, or bridges. As a result, our proposal
       would allow for a range of LAN bandwidth 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 assume that the
       IEEE 802 working groups will specify and standardize the traffic
       control mechanisms needed at the link layer. In addition, we assume
       that the Layer 3 end-systems (e.g., a host or a router) will exer-
       cise 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 admis-
       sion control.
  
       We believe that the LAN-based admission control when combined with
       per-flow policing  at end-systems and traffic control and priority
       queuing at link layer should realize some approximation of Con-
       trolled Load (and even Guaranteed) services  over IEEE 802-style
       LANs.
  
  
  III. In the absence of any link-layer traffic control or priority queu-
       ing mechanisms in the underlying LAN (such as a shared LAN seg-
       ment), the proposed 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-
  
  
  
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       compliant switches. However, we assume that the proposed signaling
       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 Services traffic flows utilize the proposed
       admission control procedure to request reservation of resources
       before sending any traffic, the proposed mechanism will restrict
       the total amount of traffic generated by Integrated Services flows
       within the bounds desired by a LAN administrator. Second, the
       best-effort traffic generated by the TCP/IP-based traffic sources
       is generally rate-adaptive (using a TCP-style "slow start" conges-
       tion 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 combination of admission control and rate adaptation should
       avoid persistent traffic congestion. This does not, however,
       guarantee that non-Integrated-Services traffic will not interfere
       with the Integrated Services traffic in the absence of traffic con-
       trol 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, 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) to reserve resources on a LAN
       segment.
  
  
  
  *    Appendix A describes the details of the DSBM election algorithm
  
  
  
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       used for electing a designated SBM on a LAN segment when more than
       one SBM is present.  It also describes how DSBM clients discover
       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.
  
  
  
  4. Overview
  
  
  4.1. Definitions
  
  
  -    Link Layer/Layer 2 or L2: We refer to link layer technologies such
       as IEEE 802.3/Ethernet as L2 or layer 2.
  
  -    Layer 2 or L2 domain: An L2 domain is a set of nodes and links
       interconnected without passing through a L3 (Layer 3 or IP) for-
       warding function. An IP subnet corresponds to a L2 domain.
  
  -    Layer2 or L2 devices: We refer to devices that only implement
       Layer2 functionality as Layer2 or L2 devices and these include
       802.1D bridges or switches.
  
  
  -    Layer 3 or L3 devices: these include hosts and routers that use L3
       and higher layer protocols or application programs that need to
       make resource reservations.
  
  
  -    Segment: A L2 segment that is shared by one or more senders. Thus,
       a segment includes a "yellow" wire (the term "yellow wire" is used
       to refer to a physical segment shared by many L2 or L3 devices.
       Such devices typically resolve contention for media access using
       CSMA or token passing), a switched half duplex link, or a switched
       full-duplex link.
  
  
  -    DSBM: Designated SBM (DSBM) is a L2 or L3 device that manages
       resources on a L2 segment. At most one DSBM exists for each L2 seg-
       ment.
  
  
  -    SBM: the SBM is a L2 or L3 device that is capable of managing
       resources on a segment. As an SBM, it is not actually managing
  
  
  
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       resources. When more than one SBM exists on a segment, one of the
       SBMs is elected to be a DSBM.
  
  
  -    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 intercon-
       nected parts of a shared LAN that are not separated by DSBMs.
  
  
  -    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 segment.
  
  
  
  -    DSBM clients: These are devices that transmit traffic onto a
       managed segment and use the services of a DSBM for the managed seg-
       ment for admission control over a LAN segment. Only the L3 devices
       such as hosts and routers are expected to act as sources of traffic
       that requires resource reservations, and, therefore, DSBM clients
       are L3 devices.
  
  
  -    SBM transparent devices: An "SBM transparent" device is unaware of
       SBMs or DSBMs (though it may or may not be RSVP aware) and, there-
       fore, does not participate in the SBM-based admission control pro-
       cedure over a managed segment. Such a device uses standard forward-
       ing rules appropriate for the device and is  transparent with
       respect to SBM.  An example of such a L2 device is a legacy switch
       that does not participate in resource reservation. In addition, an
       L3 device may also be SBM transparent. For example, such an L3 dev-
       ice may participate in a L3 resource reservation protocol (RSVP)
       and use standard forwarding rules for RSVP messages appropriate for
       the device and is, thus, transparent with respect to the SBM pro-
       cedures.
  
  
  -    Layer 3 and layer 2 addresses: We refer to layer 3 addresses of
       L3/L2 devices as "L3 addresses" and layer2 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
       and 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
  
  
  
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       previous hop device will be referred to as the RSVP_HOP_L2 address.
  
  
  
  4.2. Outline of the SBM-based Admission Control Procedure
  
  
  We assume that a 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 segment. For example, many
  SBM-capable devices may be attached to a yellow wire whereas two SBM-
  capable switches may share a half-duplex switched segment. In that case,
  a single DSBM is elected for the segment. A fail-safe procedure for
  dynamically electing the DSBM is described in Appendix A.  The presence
  of a DSBM makes the segment a "managed segment". Sometimes, two or more
  L2 segments may be interconnected  by SBM transparent devices. In that
  case, a single DSBM will manage the resources for those segments treat-
  ing the collection of such segments as a 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 yellow wire 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 maximum bandwidth that can be
       reserved on each managed segment under its control. 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 pro-
       cedure 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
  
  
  
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       domain  so that it can communicate with the DSBM for admission con-
       trol purposes.
  
  
  
  3.   DSBM-based Admission Control: To request reservation of resources
       (LAN bandwidth in a L2 domain), DSBM clients (RSVP-capable L3 dev-
       ices such as  hosts and routers) follow the following steps:
  
  
    a)   When a DSBM client  sends or forwards a PATH message over an
         interface attached to a  managed segment, it sends the PATH mes-
         sage to its DSBM instead of sending it to the RSVP session desti-
         nation address (as is done in conventional RSVP processing).
         After processing (and possibly updating an ADSPEC), the DSBM will
         forward the PATH message toward its destination address. As part
         of its processing, the DSBM builds and maintains a PATH state for
         the session and notes the previous hop that sent it the PATH mes-
         sage.
  
         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.
  
         Router R1, which is a DSBM client, forwards the  PATH message
         from the sender to its DSBM. The DSBM processes the PATH message
         and forwards the PATH message towards the RSVP session address
         (Detailed message processing and forwarding rules are described
         in Section 5).  In the process, the DSBM builds the PATH state,
         remembers the router R1 as the previous hop for the session, puts
         its own IP address in the PHOP (RSVP_HOP) object, and effectively
         inserts itself as an intermediate node between the sender (or R1
         in Figure 1) and the  receiver (host A) on the managed segment.
  
    b)   When an application on host A wishes to make a reservation for
         the RSVP session, host A follows the standard RSVP message pro-
         cessing rules and sends a RSVP RESV message to the previous hop
         address (the DSBMs address) obtained from the PHOP object in the
         previously received PATH message.
  
  
    c)   The DSBM processes the RSVP RESV message based on the bandwidth
         available and returns an ResvErr message to the requester (host
         A) if the request cannot be granted. The admission control algo-
         rithm at DSBM ensures that sufficient bandwidth is available on
  
  
  
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         managed segments  between the NHOP (requester) and the PHOP
         (sender/router) before accepting a request. 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 the conventional RSVP processing.
  
  
    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 propagate hop-by-hop in
         reverse through the intermediate DSBMs and eventually reach the
         original forwarder (router R1) on the L2 domain if admission con-
         trol at all DSBMs succeeds.
  
  
  
  
  4.2.2. Enhancements  to the conventional RSVP operation
  
  
  The addition of a DSBM for admission control over managed segments
  results in some additions to the RSVP message processing rules at a DSBM
  client. In the following, we first motivate and summarize the additions
  and a detailed description of the message processing and forwarding
  rules at (D)SBMs and DSBM clients is provided in Section 5:
  
  
  -    Normal RSVP forwarding rules  apply at a DSBM client when it is not
       forwarding an outgoing PATH message over a managed segment. How-
       ever, outgoing PATH messages on a managed segment are sent to the
       DSBM for the corresponding managed segment.
  
  
  -    In conventional RSVP processing over point-to-point links, RSVP
       nodes (hosts/routers) use NHOP and PHOP objects to keep track of
       the next hop (downstream node in the path of data packets in a
       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 destination address based on the L3 routing (packet
       forwarding) tables.
  
       For example, consider the L2 domain in Figure 1. Assume that both
  
  
  
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       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 L2 domain consisting of
       segments using IEEE 802 LAN technologies, 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, typi-
       cally, L2 devices (such as a switch) will serve as the DSBM for the
       managed segments in a  switched topology. When R1 forwards the PATH
       message to its DSBM (an L2 device), the DSBM will 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 additional
       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).
  
  
  
  -    When a DSBM client (a host or a router acting as the originator of
       a PATH message) sends out a PATH message to its DSBM, it must
       include LAN_NHOP information in the message. In the case of a uni-
       cast destination, the LAN_NHOP address specifies the destination
       address (if the destination exists within the 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 acting 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 mes-
       sage.  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 reason-
       able to expect these devices to have an ARP capability to determine
       the  MAC address (we call it L2ADDR for Layer 2 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)
  
  
  
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       and the corresponding MAC address (LAN_NHOP_L2 address ) for the
       next L3 hop over the L2 domain. The exact format of the LAN_NHOP
       information and relevant objects is described later in Appendix B.
  
  
  -    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 modified
       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 for-
       wards 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_CONFM, 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 inter-
       mediate 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 helps the L2 devices along the path
       in forwarding the PATH message toward the next L3 hop.
  
       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 intermedi-
       ate node builds the path state, it remembers the previous hop
       (stores the PHOP IP address available in the RSVP_HOP object of an
       incoming message) 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 L2 devices to act as DSBMs (and,
  
  
  
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       therefore, intermediate hops in an L2 domain) along the path
       between a sender (PHOP) and receiver (NHOP). Thus, when a RESV mes-
       sage 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
       complements 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 add 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.
  
  
  -    When an RSVP session address is a multicast address and an SBM,
       DSBM, and DSBM clients share the same yellow wire, it is possible
       for an 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 for-
       wards 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
       reflecting 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
       duplicates 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.
  
  
  -    The model proposed by the Integrated Services working group
       requires isolation of traffic flows from each other during their
  
  
  
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       transit across a network. The motivation for traffic flow separa-
       tion is to provide Integrated Services flows protection from mis-
       behaving 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 service classes. However, IEEE 802.1p
       defines (see [10, 11] for further details) a way of assigning dif-
       ferent "user priority" values to packets from different flows so
       that packets in different service classes can be discriminated by
       L2 devices. The priority value assigned to each packet will be car-
       ried in the frame header using the new, extended frame format
       defined by IEEE 802.1Q [12]. IEEE, however, makes no recommenda-
       tions about how a sender or network should use the priority values.
       The IS802 subgroup of the ISSLL working group makes recommendations
       on the usage of the user priority values as described in [10].
  
       Under the Integrated Services model, L3 devices that transmit
       traffic flows onto a IEEE 802 segment are expected to perform per-
       flow policing to ensure that the flows do not exceed their traffic
       specification as specified during admission control. In addition,
       L3 devices may label the frames in such flows with a user-priority
       value to identify their service class.
  
       For the purpose of this discussion, we will refer to the priority
       value carried in the extended frame header as a "traffic class" of
       a packet. Under the ISSLL model, the L3 devices, that send traffic
       and that use the SBM protocol, are not expected to select the
       traffic class of outgoing packets. Instead, we assume that once a
       sender sends a  PATH message, downstream switches (L2 devices) will
       insert a new traffic class object (TCLASS object) in the PATH mes-
       sage that travels to the next L3 device (L3 NHOP for the PATH mes-
       sage). The L3 device that receives the PATH message is expected to
       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, it is expected to pass the user_priority value
       in the TCLASS object to its local packet classifier (traffic con-
       trol)  so that subsequent, outgoing data packets for this RSVP flow
       will have the user priority value included in the extended MAC
       header.
  
       The format of the TCLASS object is specified in Appendix B.  In
       summary, use of TCLASS objects requires following additions to the
       conventional RSVP message processing at DSBMs, SBMs, and DSBM
       clients:
  
  
  
  
  
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    *    If an SBM receives a PATH or RESV 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 should forward
         the message without changing the contents of the TCLASS object.
  
  
    *    When a DSBM client (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 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 so that outgoing
         data packets sent subsequently over the interface will contain
         the appropriate value in their MAC-layer frame header.
  
  
    *    When a DSBM receives a PATH message with a  TCLASS object, it
         must forward it unchanged. When a DSBM receives a  RESV message
         with a TCLASS object, it may use the traffic class information
         for its own admission control for the managed segment. If admis-
         sion control succeeds, it must forward the TCLASS object in the
         RESV message.
  
  
  
    *    When an L3 device receives a PATH message over a managed segment
         in one L2 domain and it needs to forward the PATH 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) before forwarding the PATH message.  If the outgoing
         interface is on a separate L2 domain, these objects may be regen-
         erated according to the processing rules applicable to that
         interface.
  
  
  
  
  5. Detailed Message Processing Rules
  
  
  
  
  
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  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 interfaces may
       act in different capacities on each interface. For example, a dev-
       ice could be an SBM on interface A, and a DSBM on interface B.
  
  
  *    The SBMs functionality is a superset of the DSBM Client's func-
       tionality. Further, the DSBMs functionality is a superset of the
       SBM's functionality. For the purpose of this discussion, we iden-
       tify a device by its highest level of functionality. So - even
       though an SBM includes the functionality of a DSBM Client, it is
       referred to as an SBM only (not as an SBM and a DSBM Client).
  
  
  *    Layer 3 devices can be DSBM clients, SBMs, DSBMs or none of the
       above ("SBM transparent"). DSBM clients are always L3 devices.
  
  
  *    Layer 2 devices can be SBMs,  DSBMs or none of the above (SBM tran-
       sparent). There is no marginal value in a layer 2 device being a
       DSBM client vs. SBM transparent. Therefore, layer 2 devices will
       never be DSBM clients.
  
  
       5.2. Use Of a Reserved IP Multicast Address
  
  
  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
  L3 hop in the path. The RSVP PATH messages are typically addressed to
  their destination address (RSVP session address which can be either an
  IP unicast or multicast address). However, in the case of a managed seg-
  ment, the RSVP PATH messages are sent to the DSBM's unicast IP address
  (we refer to this address as the "DSBMAddress").  When a L2 device acts
  as the DSBM, the device is expected to receive the PATH message and hand
  it over to the local DSBM agent (Since the PATH message is directed to
  the L2 device, there is no need for the L2 device to snoop for L3 RSVP
  messages).
  
  DSBM clients discover the unicast IP address of the DSBM using a method
  described as part of the election algorithm discussed in APPENDIX A.  To
  facilitate an  election of a DSBM and to allow dynamic DSBM-client bind-
  ing, the election algorithm relies on a reserved IP multicast address.
  
  
  
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  In addition, the reserved multicast address is also used for easy for-
  warding of PATH messages from a DSBM to its clients.
  
  For example, when a DSBM forwards a PATH message towards the next hop,
  it need not be aware of the identity of multiple DSBM clients or SBMs on
  the path towards the L3 NHOP. In addition, L2 devices (SBMs) on the path
  should be able to easily detect and capture PATH messages without
  requiring any snooping for RSVP messages. To facilitate forwarding of
  PATH messages from a DSBM to other SBMs and DSBM clients as well as to
  enable easy detection and capture of PATH messages at L2 devices, we
  require that a PATH message forwarded by a DSBM be addressed using a
  logical address rather than a physical address.
  
  We make use of a reserved IP multicast address for this purpose.  We
  require that the PATH messages forwarded from a DSBM to its clients or
  to other SBMs be addressed using a reserved IP multicast address. Thus,
  SBMs  on a L2 device can simply subscribe to the reserved IP multicast
  address on the interfaces corresponding to managed segments to easily
  receive PATH messages.
  
  On the other hand, RSVP Resv messages need not be multicast as they are
  expected to be unicast to the previous hop address stored as part of the
  PATH state at each intermediate hop.
  
  
  
  We define use of one reserved IP multicast address called the "AllSBM
  Address". The address is chosen from the range of local multicast
  addresses, such that:
  
  
  *    It is not passed through layer 3 devices.
  
  
  *    It is passed transparently through layer 2 devices which are SBM
       transparent.
  
  
  *    It is configured in the permanent database of layer 2 devices which
       are SBMs or DSBMs, such that messages directed to this address are
       forwarded to the SBM management entity in these devices. This obvi-
       ates the need for these devices to explicitly snoop for SBM related
       control packets.
  
  
  *    The reserved address is 224.0.1.59 (AllSBMAddress).
  
  The two addresses, AllSBMAddress and DSBMAddress, are used as described
  
  
  
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  in the following table:
  
  
  Type        DSBMAddress                             AllSBM Address
              (Unicast IP addr of DSBM)         (reserved IP mcast addr)
  
  DSBM        * Sends PATH messages           * Monitors this address to
  Client        to this address                 detect the presence of 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
                address 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 com-
  puted algorithmically using a reserved L2 address block (the high order
  24-bits are 00:00:5e). The Assigned Numbers RFC [RFC 1700] gives addi-
  tional details.
  
  5.3. Layer 3 to Layer 2 Address Mapping
  
  
  As stated earlier, L3 devices that act as DSBMs or DSBM clients 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).
  
  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
  
  
  
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  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 Appendix B.
  
  
  5.5. 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 L3 device forwards a RSVP message onto a L2 domain based on the
  IEEE 802 LAN technology, it will only use RAW IP encapsulation.
  
  (NOTE -- THIS IS SUBJECT TO VERIFICATION THAT RSVP V1 WILL DROP UDP
  ENCAP.  Otherwise, we have an inter-operability issue here.)
  
  
  5.6. 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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  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 yellow wire (shared segment). The sample network contains the follow-
  ing 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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  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 devices
  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.  Exceptions 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 car-
       ries the address of this device, silently discard the message. (see
       multicast exception discussion below).
  
  
  *    Layer 3 devices use the RSVP session address and perform a routing
       lookup to determine the forwarding interface(s).
  
  
  *    Layer 2 devices use the LAN_NHOP_L2 address in the LAN_NHOP infor-
       mation and MAC forwarding tables to determine the forwarding
       interface(s).  (see multicast exception discussion below).
  
  
  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 objects (if present), and forward to the standard
         unicast or multicast destination.  All layer 2 device's inter-
         faces are considered to be on managed segments.
  
         (Please note that the RSVP Class Numbers for these new objects
         are chosen so that if an RSVP message includes these objects, the
         nodes that are not aware of SBM will ignore and silently discard
         such objects.)
  
  
  
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    *    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 is expected to use the map_addr() function described
         earlier to obtain an L2 address corresponding to an IP address.
  
  
    *    If the device is the DSBM for the segment to which the forwarding
         interface is attached, 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.
  
         If the device is the DSBM for the segment to which the forwarding
         interface is attached, copy the IP address of the forwarding
         interface (layer 2 devices 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 device is a layer 3 device and an SBM for the segment 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 an SBM for the segment to
         which the forwarding interface is attached, it is *not* required
         to retrieve and store the PHOP info. If it does not do so, it
         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 L2 device (which is a SBM) chooses to overwrite the RSVP
         HOP and RSVP_HOP_L2 objects with the IP and L2 addresses of  its
         forwarding interface, it will receive RESV messages. In this
         case, it must store the PHOP address info received in the
  
  
  
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         standard RSVP_HOP field and RSVP_HOP_L2 objects  of the incident
         PATH message.
  
  
    *    Copy the IP address of the forwarding interface into the
         LAN_LOOPBACK object, unless the device is a DSBM reflecting a
         PATH message for a multicast session, back onto the incident
         interface. (See multicast exception discussion below).
  
  
    *    If the device is the DSBM for the segment to which the forwarding
         interface is attached, send the PATH message to the AllSBMAd-
         dress.
  
  
    *    If the device is an SBM or a DSBM Client on the segment to which
         the forwarding interface is attached, send the PATH message to
         the DSBMAddress.
  
  
  
      5.6.1. Multicast Exception
  
      Rule #1 states that standard forwarding rules should be used to
      determine the interfaces on which the PATH message should be for-
      warded.  In the case of multicast messages, standard forwarding
      rules dictate that the message should not be forwarded on the inter-
      face 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.  If there are members of the multicast group
      address (specified by the addresses in the LAN_NHOP object), on the
      segment from which the message was received, the message should be
      forwarded back onto the interface from which it was received. The
      message is reflected back onto the incoming interface, using the
      AllSBMAddress.
  
      Since it is possible for a DSBM to reflect a multicast 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 devices (except DSBMs
      reflecting a multicast PATH message) overwrite the LAN_LOOPBACK
      object in the PATH message with the IP address of the outgoing
      interface. DSBMs which are reflecting a multicast PATH message,
      leave the LAN_LOOPBACK object unchanged.  Thus, devices will always
      be able to recognize a reflected multicast message by the presence
      of their own address in the LAN_LOOPBACK object. These messages
      should be silently discarded.
  
  
  
  
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      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)
  
  
  
  
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                                   IP Header
      Source address              IP address of H1
      Destn address               DSBM's IP address (S1's IP address)
  
                                   MAC Header
      Destn address               DSBM (S1's) MAC address over segment A
  
  
  
      *    R1 does a routing lookup on the RSVP session address, to deter-
           mine 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 seg-
           ment, 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 L3L2 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 car-
           ries the MAC address) in the message.
  
  
      *    Since R1 is not the DSBM for seg A, it sends the PATH message
           to the DSBM's (S1's)  IP Address.
  
  
  
      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
  
  
  
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      Destn address               AllSBMIPaddr (224.0.1.59, since S1 is the
                                  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.
  
  
      *    Since S1 is the DSBM for seg B, it 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                   DSBM address (R2's IP address on F)
  
                                   MAC Header
      Destn address                   DSBM (R2's) MAC address
  
  
  
  
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      *    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 there-
           fore sends the PATH message to R2's IP address on segment F.
  
  
      *    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 such 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 deter-
           mine 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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      *    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 for-
           wards 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 dev-
      ices track multicast group membership on each port individually.
      Layer 2 devices which do not do so, will merely generate extra mul-
      ticast traffic. This is the case for L2 devices which do not imple-
      ment 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 uni-
      cast 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 unicast
      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 DSBMAddress (R2's IP
      address), to be picked up by R2. The PATH message will not have been
      seen by H3 (one of the m/c receivers), since the PATH message was
      directly sent to the DSBMAddress. 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
      Destn address           AllSBMIP address (since R2 is the DSBM for seg F)
  
  
  
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                                   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 forward-
      ing rules here by sending a multicast  message back onto the inter-
      face 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
  
  
      When an L3 device receives TCLASS objects from different senders
      (different PATH messages) in the same RSVP session and needs to
  
  
  
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      combine them for sending back a single RESV message (as in a wild-
      card style reservation), the device should use the lowest priority
      value among the values received in TCLASS objects of the PATH mes-
      sages.
  
      Similarly, when a L2 device needs to merge RESVs from different next
      hops at a merge point, it should merge the TCLASS values in the
      incoming RESVs to the lowest priority value among those received.
  
  
      5.10. Operation of SBM Transparent Devices
  
      We previously defined SBM Transparent Devices. Since no SBM tran-
      sparent devices were illustrated in the example provided, we will
      describe the operation of these in the following paragraph.
  
      SBM transparent devices are unaware of the entire SBM/DSBM protocol.
      They do not intercept messages addressed to the ALLSBMAddress, but
      instead, pass them through. As a result, they do not divide the DSBM
      election scope, they do not explicitly participate in routing of
      PATH or RESV messages, and they do not participate in admission con-
      trol. 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 no knowledge of the segment's topology. In
      this case, the network administrator is expected to configure the
      DSBM for the 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 segment. A
      liberal policy would configure the DSBM for the highest capacity
      route through the managed segment. A network administrator 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 is an SBM, but did not win the elec-
      tion to act as DSBM on any segment. One might ask what purpose such
      a device serves. SBMs actually provide the important service of
      dividing the election scope and reducing the size and complexity of
      managed segments. For example, if S3 were SBM transparent, seg B and
  
  
  
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      seg F would not be separate segments. Instead, they would constitute
      a single managed segment, managed by a single DSBM. As it is, seg B
      and seg F are each managed by separate DSBMs.  Each of these seg-
      ments have a trivial topology and a well defined capacity. As a
      result, the DSBMs for these segments do not need to perform admis-
      sion control based on approximations (as would be the case if S3
      were SBM transparent).
  
      Note that,  SBM devices 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 are not expected to pro-
      cess RESV messages that do pass through them, other than to forward
      them towards their destination address, using standard forwarding
      rules.
  
      SBM devices 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 environ-
      ment 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 signaling
      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.  This proposal does not
      attempt to provide an admission control solution for such an
      environment. We believe that the SBM-based approach is part of an
      end2end signaling approach to establish resource reservations and
      does not attempt to provide a solution for SNMP-based configuration
      scenario.
  
      As stated earlier, the SBM-based approach can, however, co-exist
  
  
  
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      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 com-
      mon 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 physical
      segments and the DSBM for the managed segment may not be as effi-
      cient in allocating resources as it would if all L2 devices were
      SBM-aware.
  
  
  
      6.3. An L2 domain on which some senders are not DSBM clients.
  
      We believe that all senders that are sending RSVP/int-serv based
      traffic flows onto a managed segment must be SBM-aware and partici-
      pate in the SBM protocol. As stated above, those senders that rely
      on another reservation mechanism must avoid over-commitment of
      resources through a shared bandwidth allocator or static partition-
      ing of resources. All other senders (senders that are not sending
      streams subject to admission control) must be elastic applications
      that send best-effort traffic and use TCP-like congestion avoidance
      mechanism.
  
      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. Also, such devices will not be aware
      of DSBMs and will, therefore, not address them directly.
  
  
      6.4. A non-SBM router that interconnects two DSBM-managed L2
      domains.
  
  
      This is not a problem as SBM-messages have local scope and do not
      pass between the two domains.
  
  
      7. Guidelines for Implementors
  
  
  
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      In the following, we provide guidelines for implementors on dif-
      ferent aspects of the implementation of the SBM-based admission con-
      trol 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 intercon-
      nected by a single standard, legacy 10 Mbps or 100 Mbps hub ("yellow
      wire"), DSBM devices should assume the default - the bandwidth of
      their interface, as the total allocatable bandwidth. In the case of
      L2 devices interconnected by a more modern, but still blocking, sin-
      gle switch, the DSBM should be configured with an estimate of the
      switch's backplane capacity.   Given the total allocatable
      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 physical segments and the implementors
      must bear in mind efficiency implications of the use DSBM in dif-
      ferent 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 L2 devices. Such an L2
      network is still a single 'segment' and can be collectively managed
      by a single DSBM. In this case, the configuration compromises the
      efficiency with which the DSBM can allocate resources. This is
      because the single DSBM is required to make admission control deci-
      sions for all reservation requests within the L2 topology, with no
      knowledge of the actual physical segments affected by the reserva-
      tion.
  
      We can realize improvements in the efficiency of resource allocation
      by subdividing the complex segment into a number of managed seg-
      ments, each managed by their own DSBM. In this case, each DSBM
      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 reserva-
      tions in the managed segment. In the ultimate configuration, each
      physical segment is a managed segment and is managed by its own
  
  
  
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      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
      some security issues, but they are no different than the ones raised
      by use of RSVP and Integrated Services; the need to control and
      authenticate access to enhanced qualities of service.  This require-
      ment is discussed further in [1], [4], and [5]. [2] describes the
      mechanism used to protect the integrity of RSVP messages carrying
      the information described here.
  
      In the case of the DSBM election over a segment shared by two or
      more entities, it is also necessary to authenticate DSBM candidates
      and a mechanism based on a shared secret among the DSBM candidates
      may be used.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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      9. References
  
      [1] R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin, "Resource
      ReSerVation Protocol (RSVP) -- Version 1 Functional Specification ",
      Internet Draft <draft-ietf-rsvp-spec-16{.txt,.ps}>, June 1997.
  
      [2] F. Baker., "RSVP Cryptographic Authentication", Internet Draft
      <draft-ietf-rsvp-md5-02.txt>, June 1996.
  
      [3] F. Baker, J. Krawczyk, "RSVP Management Information Base",
      Internet Draft <draft-ietf-rsvp-mib-08.txt>, June 1997.
  
      [4] J. Wroclawski, "Specification of the Controlled-Load Network
      Element Service", Internet Draft <draft-ietf-intserv-ctrl-load-svc-
      05.txt>, May 1997.
  
      [5] S. Shenker, C. Partridge, R. Guerin, "Specification of
      Guaranteed Quality of Service", Internet Draft <draft-ietf-intserv-
      guaranteed-svc-08.txt>, July 1997
  
      [6] S. Shenker, J. Wroclawski, "General Characterization Parameters
      for Integrated Service Network Elements", Internet Draft <draft-
      ietf-intserv-charac-03.txt>, July 1997.
  
      [7] J. Wroclawski, "The Use of RSVP with IETF Integrated Services",
      Internet Draft <draft-ietf-intserv-rsvp-use-02.txt>, July 1997.
  
      [8] F. Baker, J. Krawczyk, "Integrated Services Management Informa-
      tion Base", Internet Draft <draft-ietf-intserv-mib-07.txt>, June
      1997.
  
      [9] A. Ghanwani, W. Pace, V. Srinivasan, "A Framework for Providing
      Integrated Services Over Shared and Switched LAN Technologies",
      Internet Draft <draft-ietf-issll-is802-framework-02.txt>, May 1997.
  
      [10] M. Seaman, A. Smith, E. Crawley, "Integrated Services over IEEE
      802.1D/802.1p Networks", Internet Draft <draft-ietf-issll-802-
      01.txt>, June 1997.
  
      [11] "Supplement to MAC Bridges: Traffic Class Expediting and
      Dynamic Multicast Filtering",  May 1997, IEEE P802.1p/D6
  
      [12] "Draft Standard for Virtual Bridged Local Area Networks", May
      1997, IEEE P802.1Q/D6
  
  
  
  
  
  
  
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                                   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 yellow
      wire 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 proto-
      col for electing and discovering DSBM is called the "DSBM election
      protocol" and is described in the rest of this document.
  
      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 DSBMRefreshIn-
      terval 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 election. The DSBM clients
      always listen for periodic DSBM advertisements. 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 advertisement before resuming any communication with its
      DSBM. During the period when a DSBM is not present, a DSBM client
      may forward outgoing PATH messages using the standard RSVP forward-
      ing 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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      When an SBM first starts up, it listens for incoming DSBM advertise-
      ments for some period to check whether a DSBM already exists in its
      L2 domain. If one already exists (and no new election is in pro-
      gress), 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 AllSBMAddress.
  
      If no DSBM exists, the SBM initiates the election of a DSBM by send-
      ing out a DSBM_WILLING message that lists its IP address as a candi-
      date DSBM and its "SBM priority". Each SBM is assigned a priority
      to determine its relative precedence. When more than one SBM candi-
      date 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 protocol
      start in Section A.4.
  
  
      A.2.1 Summary of the Election Algorithm
  
  
  
      For the purpose of the algorithm, an SBM is in one of the four
      states (SteadyState, DetectDSBM, ElectDSBM, I_AM_DSBM).
  
      An 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 SteadyState 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 an SBM (call it X) enters the ElectDSBM state, it sets a timer
      (called ElectionIntervalTimer that is typically set to a value at
      least equal to the DeadIntervalTimer value) to wait for the election
      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 (includ-
      ing itself). Whenever it receives another DSBM_WILLING message, 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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      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 send-
      ing out the I_AM_DSBM advertisement) and enters the I_AM_DSBM state;
      otherwise, it decides to wait for the best candidate to declare
      itself the winner. To wait, X re-initializes its ElectDSBM state and
      continues to wait for another round of election (each round lasts
      for an ElectionTimerInterval duration).
  
      An SBM is in SteadyState 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 DSBMDeadInterval timer 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 an 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 can-
      didacy).
  
      In the I_AM_DSBM state, the DSBM sends out I_AM_DSBM advertisements
      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 procedure. 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 (DSBMDeadInterval timer fires), all the SBMs enter
      the ElectDSBM state and start the election process.
  
      At the end of the ElectionInterval, the elected DSBM sends out
      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 address of the DSBM
           and its SBM priority (a configuration parameter -- priority
           specified by a network administrator). The priority value is
  
  
  
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           used to choose among candidate SBMs during the election algo-
           rithm. 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.
  
  
      2.   refresh interval -- contains the value of the refresh interval
           in seconds.  Value zero indicates the parameter has been omit-
           ted in the message.  Receivers may substitute their own default
           value in this case.
  
  
      3.   SBMDeadInterval -- contains the value of the SBMDeadInterval in
           seconds. If the value is omitted (or value zero is specified),
           a default value (from initial configuration) should be used.
  
  
  
  
  
      A.5. DSBM_WILLING Messages
  
  
      When an 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 address, if
           it wishes to be the DSBM. Also, contains the  priority of the
           SBM whose address is given above.
  
  
  
  
  
      A.6. SBM State Variables
  
  
      For each network interface, an 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.
  
  
  
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           b) OwnAddrInfo -- SBM's own IP address for the interface and
           its own priority (a configuration parameter).
  
  
           c) DSBM RefreshInterval in seconds. When the DSBM is not yet
           elected, it is set to a default value specified as a configura-
           tion 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 an SBM spends in the DetectDSBM state (see
           below).
  
  
           g) ElectionInterval in seconds -- a configuration parameter
           that decides how long an SBM spends in the ElectDSBM state when
           it has declared its candidacy.
  
  
      Figure 3 shows the state transition diagram for the election proto-
      col and the various states are described below. A complete descrip-
      tion of the state machine is provided in Section A.10.
  
  
      A.7. DSBM Election States
  
  
           DOWN -- SBM is not operational.
  
  
           DetectDSBM -- typically, the initial state of an SBM when it
           starts up. In this state, it checks to see whether a DSBM
           already exists in its domain.
  
  
  
           SteadyState -- SBM is in this state when no election is in pro-
           gress and it is not the DSBM. In this state, SBM passively mon-
           itors the state of the DSBM.
  
  
           ElectDSBM -- SBM is in this state when a DSBM election is in
  
  
  
  draft-ietf-issll-is802-sbm-04.txt                              [Page 41]


  INTERNET-DRAFT       SBM (Subnet Bandwidth Manager)           July, 1997
  
  
           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 an SBM wishes to declare its candidacy to
           be the DSBM.
  
  
           I_AM_DSBM message received -- SBM received a DSBM advertisement
           from the DSBM in its L2 domain.
  
  
           SBMDeadInterval Timeout -- The SBMDeadInterval timer has fired.
           This means that the SBM did not receive even one DSBM adver-
           tisement during this period and indicates possible failure of
           the DSBM.
  
  
           RefreshInterval Timeout -- The RefreshInterval timer has fired.
           In the I_AM_DSBM 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 ElectionInterval timer has
           fired. This means that the SBM has waited long enough after
           declaring its candidacy to determine whether or not it suc-
           ceeded.
  
  
  
  
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      A.9. State Transition Diagram (Figure 3)
  
  
                                   +-----------+
               +--<--------------<-|DetectDSBM |---->------+
               |                   +-----------+           |
               |                                           |
               |                                           |
               |                                           |
               |     +-------------+       +---------+     |
               +->---| SteadyState |--<>---|ElectDSBM|--<--+
                     +-------------+       +---------+
                          |                        |
                          |                        |
                          |                        |
                          |        +-----------+   |
                          +<<- +---|  IAMDSBM  |-<-+
                               |   +-----------+
                               |
                               |   +-----------+
                               +>>-| SHUTDOWN  |
                                   +-----------+
  
  
      A.10. Election State Machine
  
  
      Based on the events and states described above, the state changes at
      an 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 implementation
      (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 address information. 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
          choice. FALSE otherwise.
  
  
  
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      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 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:  SteadyState
      Event:      I_AM_DSBM message received
      Action:     set LocalDSBMAddrInfo = IncomingDSBMAddrInfo
                  start DeadDSBMInterval timer
                  goto SteadyState
  
      State:      DetectDSBM
  
  
  
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      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:      SteadyState
      Event:      SBMDeadInterval timer fired.
      New State:  ElectDSBM
      Action:     start ElectionIntervalTimer
                  set LocalDSBMAddrInfo = OwnAddrInfo
                  SendDSBMWiliingMessage()
  
      State:      SteadyState
      Event:      I_AM_DSBM message received.
      New State:  SteadyState
      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:      SteadyState
      Event:      DSBM_WILLING Message is received
      New State:  Depends on action (ElectDSBM or SteadyState)
      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 */
                  }
                  /* else, ignore it */
  
  
  
  draft-ietf-issll-is802-sbm-04.txt                              [Page 46]


  INTERNET-DRAFT       SBM (Subnet Bandwidth Manager)           July, 1997
  
  
                  continue in current state
  
      State:      ElectDSBM
      Event:      ElectionIntervalTimer Fired
      New State:  depends on action (I_AM_DSBM or SteadyState)
      Action:     If (LocalDSBMAddrInfo == OwnAddrInfo) {
                      /* I won */
                      send I_AM_DSBM message
                      start RefreshIntervalTimer
                      goto I_AM_DSBM state
                  } else {   /* someone else won, so wait for it to declare
                               itself to be the DSBM */
                      set LocalDSBMAddressInfo = OwnAddrInfo
                      start ElectionIntervalTimer
                      continue in current state
                  }
  
      State:      ElectDSBM
      Event:      I_AM_DSBM message received
      New State:  SteadyState
      Action:     set LocalDSBMAddrInfo = IncomingDSBMAddrInfo
                  Cancel any active timers
                  start DeadDSBMInterval timer
                  goto SteadyState
  
      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
                  }
                  continue in current state
  
      State:      ElectDSBM
      Event:      RefreshIntervalTimer fired
      New State:  ElectDSBM
      Action:     SendDSBMWillingMessage()
  
      State:      I_AM_DSBM
      Event:      DSBM_WILLING message received
      New State:  I_AM_DSBM
      Action:     send I_AM_DSBM message  /* reassert myself */
                  restart RefreshIntervalTimer
  
      State:      I_AM_DSBM
      Event:      RefreshIntervalTimer fired
      New State:  I_AM_DSBM
  
  
  
  draft-ietf-issll-is802-sbm-04.txt                              [Page 47]


  INTERNET-DRAFT       SBM (Subnet Bandwidth Manager)           July, 1997
  
  
      Action:     send I_AM_DSBM message
                  restart RefreshIntervalTimer
  
      State:      I_AM_DSBM
      Event:      I_AM_DSBM message received
      New State:  depends on action (I_AM_DSBM 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 DSBMDeadInterval timer
                      goto SteadyState
                }
  
      State:      I_AM_DSBM
      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 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 (DeadIntervalTimer, 2*DeadIntervalTi-
      mer). The ElectionIntervalTimer should be set at least to the value
      of DeadIntervalTimer 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
  
  
  
  
  
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      Network administrators are expected to configure each SBM-capable
      device with its "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 indicates that the device is not
      eligible to be a DSBM.
  
      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 election to
      its local segment. As described in Section B.1 below, all messages
      related to the DSBM election are sent to a special multicast 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 desti-
      nation address are not forwarded and instead directed 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 responsible
      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
  
  
  
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  INTERNET-DRAFT       SBM (Subnet Bandwidth Manager)           July, 1997
  
  
      DSBM will not receive any DSBM advertisement from its peer and
      assume itself to be the DSBM for traffic traversing in both direc-
      tions over the managed segment.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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  INTERNET-DRAFT       SBM (Subnet Bandwidth Manager)           July, 1997
  
  
                                   APPENDIX B
                       Message Encapsulation and Formats
  
  
      To minimize changes to existing RSVP implementations and to ensure
      quick deployment of an SBM in conjunction with RSVP, all communica-
      tion to and from a DSBM will be performed using messages constructed
      using the current rules for RSVP message formats and raw IP encapsu-
      lation. For more details on the RSVP message formats, refer to the
      RSVP specification (draft-ietf-rsvp-spec-15.ps).  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 accom-
      modate 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 seg-
      ment by listening to I_AM_DSBM advertisements and records the
      DSBMAddress (unicast IP address of the DSBM) for the purpose of com-
      munication with the DSBM over the managed segment.
  
      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 message 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 continue
      to follow the formatting rules specified in the RSVP specification.
      In addition, an RSVP implementation must also recognize new object
      classes that are described below.
  
      B.3.1. Object Formats
  
  
      All objects are defined using the format specified in the RSVP
  
  
  
  draft-ietf-issll-is802-sbm-04.txt                              [Page 51]


  INTERNET-DRAFT       SBM (Subnet Bandwidth Manager)           July, 1997
  
  
      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. LAN_NHOP, RSVP_HOP_L2, and LAN_LOOPBACK Objects
  
  
      LAN_NHOP, LAN_LOOPBACK,  and RSVP_HOP_L2 objects are identified as
      separate object classes and the value of Class_Num for the objects
      is chosen so that non-SBM aware RSVP nodes will ignore the objects
      without forwarding them or generating an error message.
  
  
      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 destina-
      tion MAC address fields. The same wire order applied to both Ether-
      net and Token Ring. Since the bit transmission order of Ethernet and
      Token Ring data differ - Ethernet octets are transmitted least sig-
      nificant 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 canonical 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 transmitted, the next least significant
      bit the second bit, and so on to 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 formal
      encoding rules for addresses on media which do not transmit bit
      serially are derived from the canonical format octet values.
  
      This document requires that the MAC address for links based on the
      IEEE 802 LAN technologies be encoded in the canonical format as a
      sequence of 6 octets. In the following, we define the object formats
      for objects that contain L2 addresses that are based on the canoni-
      cal representation.
  
  
  
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      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, CType=1 represents the IEEE Canonical Address
      format.
  
               0              1             2                 3
      +---------------+---------------+---------------+---------------+
      |       Length                  |   161         |CType(addrtype)|
      +---------------+---------------+---------------+---------------+
      |                  Variable length Opaque data                  |
      +---------------+---------------+---------------+---------------+
  
      CType = 1 (IEEE Canonical Address format)
  
      When CType=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.  |   /////       |     ////      |
      +---------------+---------------+---------------+---------------+
  
      //// -- 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         |CType(addrtype)|
  
  
  
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  INTERNET-DRAFT       SBM (Subnet Bandwidth Manager)           July, 1997
  
  
      +---------------+---------------+---------------+---------------+
      |                  Variable length Opaque data                  |
      +---------------+---------------+---------------+---------------+
  
      CType = 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
  
  
      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
  
      +---------------+---------------+---------------+---------------+
  
  
  
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      |       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 Message Format
  
  
      As specified in the RSVP specification, an RSVP_PATH message con-
      tains the RSVP Common Header and the relevant RSVP objects. For the
      RSVP Common Header, refer to the RSVP specification (draft-ietf-
      rsvp-spec-15.ps). Enhancements to an RSVP_PATH message include addi-
      tional objects as specified below.
  
      <RSVP_PATH> ::= <RSVP Common Header> [<INTEGRITY>]
                      <RSVP_HOP_L2> <LAN_NHOP>
                      <LAN_LOOPBACK> [<TCLASS>]  <SESSION><RSVP_HOP>
                      <TIME_VALUES> [<POLICY DATA>] <sender descriptor>
  
      If the INTEGRITY object is present, it must immediately follow the
      RSVP common header. L2-specific objects must always precede the SES-
      SION object.
  
      B.5. RSVP RESV Message Format
  
  
  
  
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      As specified in the RSVP specification, an RSVP_RESV message con-
      tains the RSVP Common Header and relevant RSVP objects. In addition,
      it may contain an optional TCLASS object as described earlier.
  
  
      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 discovering
      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 permissi-
      ble choice of object types. These rules are specified using Backus-
      Naur Form (BNF) augmented with square brackets surrounding optional
      sub-sequences. The BNF implies an order for the objects in a mes-
      sage. 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>
                                 <SBM PRIORITY>
  
  
      I_AM_DSBM Message
  
  
      <I_AM_DSBM> ::= <SBM Common Header> <DSBM IP ADDRESS>
                                 <SBM PRIORITY> <DSBM Timer Intervals>
  
  
  
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      All I_AM_DSBM messages are multicast to the well known DSBM_GROUP
      address.  The default priority of an 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
      +---------------+---------------+---------------+---------------+
      |                                                               |
      +                                                               +
      |                                                               |
      +                       IPv6 DSBM IP Address                    +
      |                                                               |
      +                                                               +
      |                                                               |
      +---------------+---------------+---------------+---------------+
  
      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
  
      +---------------+---------------+---------------+----------------+
  
  
  
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      |   ////        |   ////        | DeadInterval  |Refresh Interval|
      +---------------+---------------+---------------+----------------+
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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                                ACKNOWLEDGEMENTS
  
      Authors are grateful to 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 draft.
  
      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
              Sun Microsystems, Inc.
              2550 Garcia Avenue
              Mountain View, California 94043-1100
              USA
              phone: +1 503-297-1580
              email: don.hoffman@eng.sun.com
  
              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
  
  
  
  
  
  
  
  
  
  
  
  
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