path: root/doc/rfc/rfc2022.txt

Network Working Group                                        G. Armitage
Request for Comments: 2022                                      Bellcore
Category: Standards Track                                  November 1996


       Support for Multicast over UNI 3.0/3.1 based ATM Networks.

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited

Abstract

   Mapping the connectionless IP multicast service over the connection
   oriented ATM services provided by UNI 3.0/3.1 is a non-trivial task.
   This memo describes a mechanism to support the multicast needs of
   Layer 3 protocols in general, and describes its application to IP
   multicasting in particular.

   ATM based IP hosts and routers use a Multicast Address Resolution
   Server (MARS) to support RFC 1112 style Level 2 IP multicast over the
   ATM Forum's UNI 3.0/3.1 point to multipoint connection service.
   Clusters of endpoints share a MARS and use it to track and
   disseminate information identifying the nodes listed as receivers for
   given multicast groups. This allows endpoints to establish and manage
   point to multipoint VCs when transmitting to the group.

   The MARS behaviour allows Layer 3 multicasting to be supported using
   either meshes of VCs or ATM level multicast servers. This choice may
   be made on a per-group basis, and is transparent to the endpoints.

















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Table of Contents

   1. Introduction.................................................   4
    1.1 The Multicast Address Resolution Server (MARS).............   5
    1.2 The ATM level multicast Cluster............................   5
    1.3 Document overview..........................................   6
    1.4 Conventions................................................   7
   2. The IP multicast service model...............................   7
   3. UNI 3.0/3.1 support for intra-cluster multicasting...........   8
    3.1 VC meshes..................................................   9
    3.2 Multicast Servers..........................................   9
    3.3 Tradeoffs..................................................  10
    3.4 Interaction with local UNI 3.0/3.1 signalling entity.......  11
   4. Overview of the MARS.........................................  12
    4.1 Architecture...............................................  12
    4.2 Control message format.....................................  12
    4.3 Fixed header fields in MARS control messages...............  13
      4.3.1 Hardware type..........................................  14
      4.3.2 Protocol type..........................................  14
      4.3.3 Checksum...............................................  15
      4.3.4 Extensions Offset......................................  15
      4.3.5 Operation code.........................................  16
      4.3.6 Reserved...............................................  16
   5. Endpoint (MARS client) interface behaviour...................  16
    5.1 Transmit side behaviour....................................  17
      5.1.1 Retrieving Group Membership from the MARS..............  18
      5.1.2 MARS_REQUEST, MARS_MULTI, and MARS_NAK messages........  20
      5.1.3 Establishing the outgoing multipoint VC................  22
      5.1.4 Monitoring updates on ClusterControlVC.................  24
        5.1.4.1 Updating the active VCs............................  24
        5.1.4.2 Tracking the Cluster Sequence Number...............  25
      5.1.5 Revalidating a VC's leaf nodes.........................  26
        5.1.5.1 When leaf node drops itself........................  27
        5.1.5.2 When a jump is detected in the CSN.................  27
      5.1.6 'Migrating' the outgoing multipoint VC.................  27
    5.2. Receive side behaviour....................................  29
      5.2.1 Format of the MARS_JOIN and MARS_LEAVE Messages........  30
        5.2.1.1 Important IPv4 default values......................  32
      5.2.2 Retransmission of MARS_JOIN and MARS_LEAVE messages....  33
      5.2.3 Cluster member registration and deregistration.........  34
    5.3 Support for Layer 3 group management.......................  34
    5.4 Support for redundant/backup MARS entities.................  36
      5.4.1 First response to MARS problems........................  36
      5.4.2 Connecting to a backup MARS............................  37
      5.4.3 Dynamic backup lists, and soft redirects...............  37
    5.5 Data path LLC/SNAP encapsulations..........................  40
      5.5.1 Type #1 encapsulation..................................  40
      5.5.2 Type #2 encapsulation..................................  41



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      5.5.3 A Type #1 example......................................  42
   6. The MARS in greater detail...................................  42
    6.1 Basic interface to Cluster members.........................  43
      6.1.1 Response to MARS_REQUEST...............................  43
      6.1.2 Response to MARS_JOIN and MARS_LEAVE...................  43
      6.1.3 Generating MARS_REDIRECT_MAP...........................  45
      6.1.4 Cluster Sequence Numbers...............................  45
    6.2 MARS interface to Multicast Servers (MCSs).................  46
      6.2.1 MARS_REQUESTs for MCS supported groups.................  47
      6.2.2 MARS_MSERV and MARS_UNSERV messages....................  47
      6.2.3 Registering a Multicast Server (MCS)...................  49
      6.2.4 Modified response to MARS_JOIN and MARS_LEAVE..........  49
      6.2.5 Sequence numbers for ServerControlVC traffic...........  51
    6.3 Why global sequence numbers?...............................  52
    6.4 Redundant/Backup MARS Architectures........................  52
   7. How an MCS utilises a MARS...................................  53
    7.1 Association with a particular Layer 3 group................  53
    7.2 Termination of incoming VCs................................  54
    7.3 Management of outgoing VC..................................  54
    7.4 Use of a backup MARS.......................................  54
   8. Support for IP multicast routers.............................  54
    8.1 Forwarding into a Cluster..................................  55
    8.2 Joining in 'promiscuous' mode..............................  55
    8.3 Forwarding across the cluster..............................  56
    8.4 Joining in 'semi-promiscous' mode..........................  56
    8.5 An alternative to IGMP Queries.............................  57
    8.6 CMIs across multiple interfaces............................  58
   9. Multiprotocol applications of the MARS and MARS clients......  59
   10. Supplementary parameter processing..........................  60
    10.1 Interpreting the mar$extoff field.........................  60
    10.2 The format of TLVs........................................  60
    10.3 Processing MARS messages with TLVs........................  62
    10.4 Initial set of TLV elements...............................  62
   11. Key Decisions and open issues...............................  62
   Security Considerations.........................................  65
   Acknowledgments.................................................  65
   Author's Address................................................  65
   References......................................................  66
   Appendix A. Hole punching algorithms............................  67
   Appendix B. Minimising the impact of IGMP in IPv4 environments..  69
   Appendix C. Further comments on 'Clusters'......................  71
   Appendix D. TLV list parsing algorithm..........................  72
   Appendix E. Summary of timer values.............................  73
   Appendix F. Pseudo code for MARS operation......................  74







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1.  Introduction.

   Multicasting is the process whereby a source host or protocol entity
   sends a packet to multiple destinations simultaneously using a
   single, local 'transmit' operation. The more familiar cases of
   Unicasting and Broadcasting may be considered to be special cases of
   Multicasting (with the packet delivered to one destination, or 'all'
   destinations, respectively).

   Most network layer models, like the one described in RFC 1112 [1] for
   IP multicasting, assume sources may send their packets to abstract
   'multicast group addresses'.  Link layer support for such an
   abstraction is assumed to exist, and is provided by technologies such
   as Ethernet.

   ATM is being utilized as a new link layer technology to support a
   variety of protocols, including IP. With RFC 1483 [2] the IETF
   defined a multiprotocol mechanism for encapsulating and transmitting
   packets using AAL5 over ATM Virtual Channels (VCs). However, the ATM
   Forum's currently published signalling specifications (UNI 3.0 [8]
   and UNI 3.1 [4]) does not provide the multicast address abstraction.
   Unicast connections are supported by point to point, bidirectional
   VCs. Multicasting is supported through point to multipoint
   unidirectional VCs. The key limitation is that the sender must have
   prior knowledge of each intended recipient, and explicitly establish
   a VC with itself as the root node and the recipients as the leaf
   nodes.

   This document has two broad goals:

      Define a group address registration and membership distribution
      mechanism that allows UNI 3.0/3.1 based networks to support the
      multicast service of protocols such as IP.

      Define specific endpoint behaviours for managing point to
      multipoint VCs to achieve multicasting of layer 3 packets.

   As the IETF is currently in the forefront of using wide area
   multicasting this document's descriptions will often focus on IP
   service model of RFC 1112.  A final chapter will note the
   multiprotocol application of the architecture.

   This document avoids discussion of one highly non-trivial aspect of
   using ATM - the specification of QoS for VCs being established in
   response to higher layer needs. Research in this area is still very
   formative [7], and so it is assumed that future documents will
   clarify the mapping of QoS requirements to VC establishment. The
   default at this time is that VCs are established with a request for



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   Unspecified Bit Rate (UBR) service, as typified by the IETF's use of
   VCs for unicast IP, described in RFC 1755 [6].

1.1  The Multicast Address Resolution Server (MARS).

   The Multicast Address Resolution Server (MARS) is an extended analog
   of the ATM ARP Server introduced in RFC 1577 [3].  It acts as a
   registry, associating layer 3 multicast group identifiers with the
   ATM interfaces representing the group's members.  MARS messages
   support the distribution of multicast group membership information
   between MARS and endpoints (hosts or routers).  Endpoint address
   resolution entities query the MARS when a layer 3 address needs to be
   resolved to the set of ATM endpoints making up the group at any one
   time. Endpoints keep the MARS informed when they need to join or
   leave particular layer 3 groups.  To provide for asynchronous
   notification of group membership changes the MARS manages a point to
   multipoint VC out to all endpoints desiring multicast support

   Valid arguments can be made for two different approaches to ATM level
   multicasting of layer 3 packets - through meshes of point to
   multipoint VCs, or ATM level multicast servers (MCS). The MARS
   architecture allows either VC meshes or MCSs to be used on a per-
   group basis.

1.2  The ATM level multicast Cluster.

   Each MARS manages a 'cluster' of ATM-attached endpoints. A Cluster is
   defined as

      The set of ATM interfaces choosing to participate in direct ATM
      connections to achieve multicasting of AAL_SDUs between
      themselves.

   In practice, a Cluster is the set of endpoints that choose to use the
   same MARS to register their memberships and receive their updates
   from.

   By implication of this definition, traffic between interfaces
   belonging to different Clusters passes through an inter-cluster
   device. (In the IP world an inter-cluster device would be an IP
   multicast router with logical interfaces into each Cluster.) This
   document explicitly avoids specifying the nature of inter-cluster
   (layer 3) routing protocols.

   The mapping of clusters to other constrained sets of endpoints (such
   as unicast Logical IP Subnets) is left to each network administrator.
   However, for the purposes of conformance with this document network
   administrators MUST ensure that each Logical IP Subnet (LIS) is



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   served by a separate MARS, creating a one-to-one mapping between
   cluster and unicast LIS.  IP multicast routers then interconnect each
   LIS as they do with conventional subnets. (Relaxation of this
   restriction MAY only occur after future research on the interaction
   between existing layer 3 multicast routing protocols and unicast
   subnet boundaries.)

   The term 'Cluster Member' will be used in this document to refer to
   an endpoint that is currently using a MARS for multicast support.
   Thus potential scope of a cluster may be the entire membership of a
   LIS, while the actual scope of a cluster depends on which endpoints
   are actually cluster members at any given time.

1.3  Document overview.

   This document assumes an understanding of concepts explained in
   greater detail in RFC 1112, RFC 1577, UNI 3.0/3.1, and RFC 1755 [6].

   Section 2 provides an overview of IP multicast and what RFC 1112
   required from Ethernet.

   Section 3 describes in more detail the multicast support services
   offered by UNI 3.0/3.1, and outlines the differences between VC
   meshes and multicast servers (MCSs) as mechanisms for distributing
   packets to multiple destinations.

   Section 4 provides an overview of the MARS and its relationship to
   ATM endpoints. This section also discusses the encapsulation and
   structure of MARS control messages.

   Section 5 substantially defines the entire cluster member endpoint
   behaviour, on both receive and transmit sides. This includes both
   normal operation and error recovery.

   Section 6 summarises the required behaviour of a MARS.

   Section 7 looks at how a multicast server (MCS) interacts with a
   MARS.

   Section 8 discusses how IP multicast routers may make novel use of
   promiscuous and semi-promiscuous group joins. Also discussed is a
   mechanism designed to reduce the amount of IGMP traffic issued by
   routers.

   Section 9 discusses how this document applies in the more general
   (non-IP) case.





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   Section 10 summarises the key proposals, and identifies areas for
   future research that are generated by this MARS architecture.

   The appendices provide discussion on issues that arise out of the
   implementation of this document. Appendix A discusses MARS and
   endpoint algorithms for parsing MARS messages. Appendix B describes
   the particular problems introduced by the current IGMP paradigms, and
   possible interim work-arounds.  Appendix C discusses the 'cluster'
   concept in further detail, while Appendix D briefly outlines an
   algorithm for parsing TLV lists.  Appendix E summarises various timer
   values used in this document, and Appendix F provides example
   pseudo-code for a MARS entity.

1.4  Conventions.

   In this document the following coding and packet representation rules
   are used:

      All multi-octet parameters are encoded in big-endian form (i.e.
      the most significant octet comes first).

      In all multi-bit parameters bit numbering begins at 0 for the
      least significant bit when stored in memory (i.e. the n'th bit has
      weight of 2^n).

      A bit that is 'set', 'on', or 'one' holds the value 1.

      A bit that is 'reset', 'off', 'clear', or 'zero' holds the value
      0.

2.  Summary of the IP multicast service model.

   Under IP version 4 (IPv4), addresses in the range between 224.0.0.0
   and 239.255.255.255 (224.0.0.0/4) are termed 'Class D' or 'multicast
   group' addresses. These abstractly represent all the IP hosts in the
   Internet (or some constrained subset of the Internet) who have
   decided to 'join' the specified group.

   RFC1112 requires that a multicast-capable IP interface must support
   the transmission of IP packets to an IP multicast group address,
   whether or not the node considers itself a 'member' of that group.
   Consequently, group membership is effectively irrelevant to the
   transmit side of the link layer interfaces. When Ethernet is used as
   the link layer (the example used in RFC1112), no address resolution
   is required to transmit packets. An algorithmic mapping from IP
   multicast address to Ethernet multicast address is performed locally
   before the packet is sent out the local interface in the same 'send
   and forget' manner as a unicast IP packet.



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   Joining and Leaving an IP multicast group is more explicit on the
   receive side - with the primitives JoinLocalGroup and LeaveLocalGroup
   affecting what groups the local link layer interface should accept
   packets from. When the IP layer wants to receive packets from a
   group, it issues JoinLocalGroup. When it no longer wants to receive
   packets, it issues LeaveLocalGroup. A key point to note is that
   changing state is a local issue, it has no effect on other hosts
   attached to the Ethernet.

   IGMP is defined in RFC 1112 to support IP multicast routers attached
   to a given subnet. Hosts issue IGMP Report messages when they perform
   a JoinLocalGroup, or in response to an IP multicast router sending an
   IGMP Query. By periodically transmitting queries IP multicast routers
   are able to identify what IP multicast groups have non-zero
   membership on a given subnet.

   A specific IP multicast address, 224.0.0.1, is allocated for the
   transmission of IGMP Query messages. Host IP layers issue a
   JoinLocalGroup for 224.0.0.1 when they intend to participate in IP
   multicasting, and issue a LeaveLocalGroup for 224.0.0.1 when they've
   ceased participating in IP multicasting.

   Each host keeps a list of IP multicast groups it has been
   JoinLocalGroup'd to. When a router issues an IGMP Query on 224.0.0.1
   each host begins to send IGMP Reports for each group it is a member
   of. IGMP Reports are sent to the group address, not 224.0.0.1, "so
   that other members of the same group on the same network can overhear
   the Report" and not bother sending one of their own. IP multicast
   routers conclude that a group has no members on the subnet when IGMP
   Queries no longer elicit associated replies.

3. UNI 3.0/3.1 support for intra-cluster multicasting.

   For the purposes of the MARS protocol, both UNI 3.0 and UNI 3.1
   provide equivalent support for multicasting. Differences between UNI
   3.0 and UNI 3.1 in required signalling elements are covered in RFC
   1755.

   This document will describe its operation in terms of 'generic'
   functions that should be available to clients of a UNI 3.0/3.1
   signalling entity in a given ATM endpoint. The ATM model broadly
   describes an 'AAL User' as any entity that establishes and manages
   VCs and underlying AAL services to exchange data. An IP over ATM
   interface is a form of 'AAL User' (although the default LLC/SNAP
   encapsulation mode specified in RFC1755 really requires that an 'LLC
   entity' is the AAL User, which in turn supports the IP/ATM
   interface).




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   The most fundamental limitations of UNI 3.0/3.1's multicast support
   are:

      Only point to multipoint, unidirectional VCs may be established.

      Only the root (source) node of a given VC may add or remove leaf
      nodes.

   Leaf nodes are identified by their unicast ATM addresses.  UNI
   3.0/3.1 defines two ATM address formats - native E.164 and NSAP
   (although it must be stressed that the NSAP address is so called
   because it uses the NSAP format - an ATM endpoint is NOT a Network
   layer termination point).  In UNI 3.0/3.1 an 'ATM Number' is the
   primary identification of an ATM endpoint, and it may use either
   format. Under some circumstances an ATM endpoint must be identified
   by both a native E.164 address (identifying the attachment point of a
   private network to a public network), and an NSAP address ('ATM
   Subaddress') identifying the final endpoint within the private
   network. For the rest of this document the term will be used to mean
   either a single 'ATM Number' or an 'ATM Number' combined with an 'ATM
   Subaddress'.

3.1 VC meshes.

   The most fundamental approach to intra-cluster multicasting is the
   multicast VC mesh.  Each source establishes its own independent point
   to multipoint VC (a single multicast tree) to the set of leaf nodes
   (destinations) that it has been told are members of the group it
   wishes to send packets to.

   Interfaces that are both senders and group members (leaf nodes) to a
   given group will originate one point to multipoint VC, and terminate
   one VC for every other active sender to the group. This criss-
   crossing of VCs across the ATM network gives rise to the name 'VC
   mesh'.

3.2 Multicast Servers.

   An alternative model has each source establish a VC to an
   intermediate node - the multicast server (MCS). The multicast server
   itself establishes and manages a point to multipoint VC out to the
   actual desired destinations.

   The MCS reassembles AAL_SDUs arriving on all the incoming VCs, and
   then queues them for transmission on its single outgoing point to
   multipoint VC. (Reassembly of incoming AAL_SDUs is required at the
   multicast server as AAL5 does not support cell level multiplexing of
   different AAL_SDUs on a single outgoing VC.)



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   The leaf nodes of the multicast server's point to multipoint VC must
   be established prior to packet transmission, and the multicast server
   requires an external mechanism to identify them. A side-effect of
   this method is that ATM interfaces that are both sources and group
   members will receive copies of their own packets back from the MCS
   (An alternative method is for the multicast server to explicitly
   retransmit packets on individual VCs between itself and group
   members. A benefit of this second approach is that the multicast
   server can ensure that sources do not receive copies of their own
   packets.)

   The simplest MCS pays no attention to the contents of each AAL_SDU.
   It is purely an AAL/ATM level device. More complex MCS architectures
   (where a single endpoint serves multiple layer 3 groups) are
   possible, but are beyond the scope of this document. More detailed
   discussion is provided in section 7.

3.3 Tradeoffs.

   Arguments over the relative merits of VC meshes and multicast servers
   have raged for some time. Ultimately the choice depends on the
   relative trade-offs a system administrator must make between
   throughput, latency, congestion, and resource consumption. Even
   criteria such as latency can mean different things to different
   people - is it end to end packet time, or the time it takes for a
   group to settle after a membership change? The final choice depends
   on the characteristics of the applications generating the multicast
   traffic.

   If we focussed on the data path we might prefer the VC mesh because
   it lacks the obvious single congestion point of an MCS.  Throughput
   is likely to be higher, and end to end latency lower, because the
   mesh lacks the intermediate AAL_SDU reassembly that must occur in
   MCSs. The underlying ATM signalling system also has greater
   opportunity to ensure optimal branching points at ATM switches along
   the multicast trees originating on each source.

   However, resource consumption will be higher. Every group member's
   ATM interface must terminate a VC per sender (consuming on-board
   memory for state information, instance of an AAL service, and
   buffering in accordance with the vendors particular architecture). On
   the contrary, with a multicast server only 2 VCs (one out, one in)
   are required, independent of the number of senders. The allocation of
   VC related resources is also lower within the ATM cloud when using a
   multicast server. These points may be considered to have merit in
   environments where VCs across the UNI or within the ATM cloud are
   valuable (e.g. the ATM provider charges on a per VC basis), or AAL
   contexts are limited in the ATM interfaces of endpoints.



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   If we focus on the signalling load then MCSs have the advantage when
   faced with dynamic sets of receivers. Every time the membership of a
   multicast group changes (a leaf node needs to be added or dropped),
   only a single point to multipoint VC needs to be modified when using
   an MCS. This generates a single signalling event across the MCS's
   UNI. However, when membership change occurs in a VC mesh, signalling
   events occur at the UNIs of every traffic source - the transient
   signalling load scales with the number of sources. This has obvious
   ramifications if you define latency as the time for a group's
   connectivity to stabilise after change (especially as the number of
   senders increases).

   Finally, as noted above, MCSs introduce a 'reflected packet' problem,
   which requires additional per-AAL_SDU information to be carried in
   order for layer 3 sources to detect their own AAL_SDUs coming back.

   The MARS architecture allows system administrators to utilize either
   approach on a group by group basis.

3.4 Interaction with local UNI 3.0/3.1 signalling entity.

   The following generic signalling functions are presumed to be
   available to local AAL Users:

   L_CALL_RQ     - Establish a unicast VC to a specific endpoint.
   L_MULTI_RQ    - Establish multicast VC to a specific endpoint.
   L_MULTI_ADD   - Add new leaf node to previously established VC.
   L_MULTI_DROP  - Remove specific leaf node from established VC.
   L_RELEASE     - Release unicast VC, or all Leaves of a multicast VC.

   The signalling exchanges and local information passed between AAL
   User and UNI 3.0/3.1 signalling entity with these functions are
   outside the scope of this document.

   The following indications are assumed to be available to AAL Users,
   generated by the local UNI 3.0/3.1 signalling entity:

   L_ACK          - Succesful completion of a local request.
   L_REMOTE_CALL  - A new VC has been established to the AAL User.
   ERR_L_RQFAILED - A remote ATM endpoint rejected an L_CALL_RQ,
                    L_MULTI_RQ, or L_MULTI_ADD.
   ERR_L_DROP     - A remote ATM endpoint dropped off an existing VC.
   ERR_L_RELEASE  - An existing VC was terminated.

   The signalling exchanges and local information passed between AAL
   User and UNI 3.0/3.1 signalling entity with these functions are
   outside the scope of this document.




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4.  Overview of the MARS.

   The MARS may reside within any ATM endpoint that is directly
   addressable by the endpoints it is serving. Endpoints wishing to join
   a multicast cluster must be configured with the ATM address of the
   node on which the cluster's MARS resides.  (Section 5.4 describes how
   backup MARSs may be added to support the activities of a cluster.
   References to 'the MARS' in following sections will be assumed to
   mean the acting MARS for the cluster.)

4.1  Architecture.

   Architecturally the MARS is an evolution of the RFC 1577 ARP Server.
   Whilst the ARP Server keeps a table of {IP,ATM} address pairs for all
   IP endpoints in an LIS, the MARS keeps extended tables of {layer 3
   address, ATM.1, ATM.2, ..... ATM.n} mappings. It can either be
   configured with certain mappings, or dynamically 'learn' mappings.
   The format of the {layer 3 address} field is generally not
   interpreted by the MARS.

   A single ATM node may support multiple logical MARSs, each of which
   support a separate cluster. The restriction is that each MARS has a
   unique ATM address (e.g. a different SEL field in the NSAP address of
   the node on which the multiple MARSs reside).  By definition a single
   instance of a MARS may not support more than one cluster.

   The MARS distributes group membership update information to cluster
   members over a point to multipoint VC known as the ClusterControlVC.
   Additionally, when Multicast Servers (MCSs) are being used it also
   establishes a separate point to multipoint VC out to registered MCSs,
   known as the ServerControlVC.  All cluster members are leaf nodes of
   ClusterControlVC. All registered multicast servers are leaf nodes of
   ServerControlVC (described further in section 6).

   The MARS does NOT take part in the actual multicasting of layer 3
   data packets.

4.2  Control message format.

   By default all MARS control messages MUST be LLC/SNAP encapsulated
   using the following codepoints:

      [0xAA-AA-03][0x00-00-5E][0x00-03][MARS control message]
          (LLC)       (OUI)     (PID)

   (This is a PID from the IANA OUI.)





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   MARS control messages are made up of 4 major components:

      [Fixed header][Mandatory fields][Addresses][Supplementary TLVs]

   [Fixed header] contains fields indicating the operation being
   performed and the layer 3 protocol being referred to (e.g IPv4, IPv6,
   AppleTalk, etc). The fixed header also carries checksum information,
   and hooks to allow this basic control message structure to be re-used
   by other query/response protocols.

   The [Mandatory fields] section carries fixed width parameters that
   depend on the operation type indicated in [Fixed header].

   The following [Addresses] area carries variable length fields for
   source and target addresses - both hardware (e.g. ATM) and layer 3
   (e.g. IPv4). These provide the fundamental information that the
   registrations, queries, and updates use and operate on. For the MARS
   protocol fields in [Fixed header] indicate how to interpret the
   contents of [Addresses].

   [Supplementary TLVs] represents an optional list of TLV (type,
   length, value) encoded information elements that may be appended to
   provide supplementary information.  This feature is described in
   further detail in section 10.

   MARS messages contain variable length address fields. In all cases
   null addresses SHALL be encoded as zero length, and have no space
   allocated in the message.

   (Unique LLC/SNAP encapsulation of MARS control messages means MARS
   and ARP Server functionality may be implemented within a common
   entity, and share a client-server VC, if the implementor so chooses.
   Note that the LLC/SNAP codepoint for MARS is different to the
   codepoint used for ATMARP.)

4.3  Fixed header fields in MARS control messages.

   The [Fixed header] has the following format:

      Data:
       mar$afn      16 bits  Address Family (0x000F).
       mar$pro      56 bits  Protocol Identification.
       mar$hdrrsv   24 bits  Reserved. Unused by MARS control protocol.
       mar$chksum   16 bits  Checksum across entire MARS message.
       mar$extoff   16 bits  Extensions Offset.
       mar$op       16 bits  Operation code.
       mar$shtl      8 bits  Type & length of source ATM number. (r)
       mar$sstl      8 bits  Type & length of source ATM subaddress. (q)



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   mar$shtl and mar$sstl provide information regarding the source's
   hardware (ATM) address. In the MARS protocol these fields are always
   present, as every MARS message carries a non-null source ATM address.
   In all cases the source ATM address is the first variable length
   field in the [Addresses] section.

   The other fields in [Fixed header] are described in the following
   subsections.

4.3.1  Hardware type.

   mar$afn defines the type of link layer addresses being carried. The
   value of 0x000F SHALL be used by MARS messages generated in
   accordance with this document. The encoding of ATM addresses and
   subaddresses when mar$afn = 0x000F is described in section 5.1.2.
   Encodings when mar$afn != 0x000F are outside the scope of this
   document.

4.3.2  Protocol type.

   The mar$pro field is made up of two subfields:

      mar$pro.type 16 bits  Protocol type.
      mar$pro.snap 40 bits  Optional SNAP extension to protocol type.

   The mar$pro.type field is a 16 bit unsigned integer representing the
   following number space:

      0x0000 to 0x00FF  Protocols defined by the equivalent NLPIDs.
      0x0100 to 0x03FF  Reserved for future use by the IETF.
      0x0400 to 0x04FF  Allocated for use by the ATM Forum.
      0x0500 to 0x05FF  Experimental/Local use.
      0x0600 to 0xFFFF  Protocols defined by the equivalent Ethertypes.

   (based on the observations that valid Ethertypes are never smaller
   than 0x600, and NLPIDs never larger than 0xFF.)

   The NLPID value of 0x80 is used to indicate a SNAP encoded extension
   is being used to encode the protocol type. When mar$pro.type == 0x80
   the SNAP extension is encoded in the mar$pro.snap field.  This is
   termed the 'long form' protocol ID.

   If mar$pro.type != 0x80 then the mar$pro.snap field MUST be zero on
   transmit and ignored on receive. The mar$pro.type field itself
   identifies the protocol being referred to. This is termed the 'short
   form' protocol ID.





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   In all cases, where a protocol has an assigned number in the
   mar$pro.type space (excluding 0x80) the short form MUST be used when
   transmitting MARS messages. Additionally, where a protocol has valid
   short and long forms of identification, receivers MAY choose to
   recognise the long form.

   mar$pro.type values other than 0x80 MAY have 'long forms' defined in
   future documents.

   For the remainder of this document references to mar$pro SHALL be
   interpreted to mean mar$pro.type, or mar$pro.type in combination with
   mar$pro.snap as appropriate.

   The use of different protocol types is described further in section
   9.

4.3.3 Checksum.

   The mar$chksum field carries a standard IP checksum calculated across
   the entire MARS control message (excluding the LLC/SNAP header). The
   field is set to zero before performing the checksum calculation.

   As the entire LLC/SNAP encapsulated MARS message is protected by the
   32 bit CRC of the AAL5 transport, implementors MAY choose to ignore
   the checksum facility. If no checksum is calculated these bits MUST
   be reset before transmission. If no checksum is performed on
   reception, this field MUST be ignored. If a receiver is capable of
   validating a checksum it MUST only perform the validation when the
   received mar$chksum field is non-zero. Messages arriving with
   mar$chksum of 0 are always considered valid.

4.3.4 Extensions Offset.

   The mar$extoff field identifies the existence and location of an
   optional supplementary parameters list. Its use is described in
   section 10.















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4.3.5 Operation code.

   The mar$op field is further subdivided into two 8 bit fields -
   mar$op.version (leading octet) and mar$op.type (trailing octet).
   Together they indicate the nature of the control message, and the
   context within which its [Mandatory fields], [Addresses], and
   [Supplementary TLVs] should be interpreted.

      mar$op.version
         0               MARS protocol defined in this document.
         0x01 - 0xEF     Reserved for future use by the IETF.
         0xF0 - 0xFE     Allocated for use by the ATM Forum.
         0xFF            Experimental/Local use.

      mar$op.type
         Value indicates operation being performed, within context of
         the control protocol version indicated by mar$op.version.

   For the rest of this document references to the mar$op value SHALL be
   taken to mean mar$op.type, with mar$op.version = 0x00. The values
   used in this document are summarised in section 11.

   (Note this number space is independent of the ATMARP operation code
   number space.)

4.3.6 Reserved.

   mar$hdrrsv may be subdivided and assigned specific meanings for other
   control protocols indicated by mar$op.version != 0.

5.  Endpoint (MARS client) interface behaviour.

   An endpoint is best thought of as a 'shim' or 'convergence' layer,
   sitting between a layer 3 protocol's link layer interface and the
   underlying UNI 3.0/3.1 service. An endpoint in this context can exist
   in a host or a router - any entity that requires a generic 'layer 3
   over ATM' interface to support layer 3 multicast.  It is broken into
   two key subsections - one for the transmit side, and one for the
   receive side.

   Multiple logical ATM interfaces may be supported by a single physical
   ATM interface (for example, using different SEL values in the NSAP
   formatted address assigned to the physical ATM interface). Therefore
   implementors MUST allow for multiple independent 'layer 3 over ATM'
   interfaces too, each with its own configured MARS (or table of MARSs,
   as discussed in section 5.4), and ability to be attached to the same
   or different clusters.




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   The initial signalling path between a MARS client (managing an
   endpoint) and its associated MARS is a transient point to point,
   bidirectional VC.  This VC is established by the MARS client, and is
   used to send queries to, and receive replies from, the MARS. It has
   an associated idle timer, and is dismantled if not used for a
   configurable period of time. The minimum suggested value for this
   time is 1 minute, and the RECOMMENDED default is 20 minutes.  (Where
   the MARS and ARP Server are co-resident, this VC may be used for both
   ATM ARP traffic and MARS control traffic.)

   The remaining signalling path is ClusterControlVC, to which the MARS
   client is added as a leaf node when it registers (described in
   section 5.2.3).

   The majority of this document covers the distribution of information
   allowing endpoints to establish and manage outgoing point to
   multipoint VCs - the forwarding paths for multicast traffic to
   particular multicast groups. The actual format of the AAL_SDUs sent
   on these VCs is almost completely outside the scope of this
   specification.  However, endpoints are not expected to know whether
   their forwarding path leads directly to a multicast group's members
   or to an MCS (described in section 3). This requires additional per-
   packet encapsulation (described in section 5.5) to aid in the the
   detection of reflected AAL_SDUs.

5.1  Transmit side behaviour.

   The following description will often be in terms of an IPv4/ATM
   interface that is capable of transmitting packets to a Class D
   address at any time, without prior warning. It should be trivial for
   an implementor to generalise this behaviour to the requirements of
   another layer 3 data protocol.

   When a local Layer 3 entity passes down a packet for transmission,
   the endpoint first ascertains whether an outbound path to the
   destination multicast group already exists. If it does not, the MARS
   is queried for a set of ATM endpoints that represent an appropriate
   forwarding path. (The ATM endpoints may represent the actual group
   members within the cluster, or a set of one or more MCSs. The
   endpoint does not distinguish between either case. Section 6.2
   describes the MARS behaviour that leads to MCSs being supplied as the
   forwarding path for a multicast group.)









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   The query is executed by issuing a MARS_REQUEST.  The reply from the
   MARS may take one of two forms:

      MARS_MULTI - Sequence of MARS_MULTI messages returning the set of
                   ATM endpoints that are to be leaf nodes of an
                   outgoing point to multipoint VC (the forwarding
                   path).

      MARS_NAK - No mapping found, group is empty.

   The formats of these messages are described in section 5.1.2.

   Outgoing VCs are established with a request for Unspecified Bit Rate
   (UBR) service, as typified by the IETF's use of VCs for unicast IP,
   described in RFC 1755 [6].  Future documents may vary this approach
   and allow the specification of different ATM traffic parameters from
   locally configured information or parameters obtained through some
   external means.

5.1.1   Retrieving Group Membership from the MARS.

   If the MARS had no mapping for the desired Class D address a MARS_NAK
   will be returned. In this case the IP packet MUST be discarded
   silently. If a match is found in the MARS's tables it proceeds to
   return addresses ATM.1 through ATM.n in a sequence of one or more
   MARS_MULTIs.  A simple mechanism is used to detect and recover from
   loss of MARS_MULTI messages.

   (If the client learns that there is no other group member in the
   cluster - the MARS returns a MARS_NAK or returns a MARS_MULTI with
   the client as the only member - it MUST delay sending out a new
   MARS_REQUEST for that group for a period no less than 5 seconds and
   no more than 10 seconds.)

   Each MARS_MULTI carries a boolean field x, and a 15 bit integer field
   y - expressed as MARS_MULTI(x,y). Field y acts as a sequence number,
   starting at 1 and incrementing for each MARS_MULTI sent.  Field x
   acts as an 'end of reply' marker. When x == 1 the MARS response is
   considered complete.

   In addition, each MARS_MULTI may carry multiple ATM addresses from
   the set {ATM.1, ATM.2, .... ATM.n}. A MARS MUST minimise the number
   of MARS_MULTIs transmitted by placing as many group members'
   addresses in a single MARS_MULTI as possible. The limit on the length
   of an individual MARS_MULTI message MUST be the MTU of the underlying
   VC.





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   For example, assume n ATM addresses must be returned, each MARS_MULTI
   is limited to only p ATM addresses, and p << n. This would require a
   sequence of k MARS_MULTI messages (where k = (n/p)+1, using integer
   arithmetic), transmitted as follows:

      MARS_MULTI(0,1) carries back {ATM.1 ... ATM.p}
      MARS_MULTI(0,2) carries back {ATM.(p+1) ... ATM.(2p)}
            [.......]
      MARS_MULTI(1,k) carries back { ... ATM.n}

   If k == 1 then only MARS_MULTI(1,1) is sent.

   Typical failure mode will be losing one or more of MARS_MULTI(0,1)
   through MARS_MULTI(0,k-1). This is detected when y jumps by more than
   one between consecutive MARS_MULTI's. An alternative failure mode is
   losing MARS_MULTI(1,k).  A timer MUST be implemented to flag the
   failure of the last MARS_MULTI to arrive. A default value of 10
   seconds is RECOMMENDED.

   If a 'sequence jump' is detected, the host MUST wait for the
   MARS_MULTI(1,k), discard all results, and repeat the MARS_REQUEST.

   If a timeout occurs, the host MUST discard all results, and repeat
   the MARS_REQUEST.

   A final failure mode involves the MARS Sequence Number (described in
   section 5.1.4.2 and carried in each part of a multi-part MARS_MULTI).
   If its value changes during the reception of a multi-part MARS_MULTI
   the host MUST wait for the MARS_MULTI(1,k), discard all results, and
   repeat the MARS_REQUEST.

   (Corruption of cell contents will lead to loss of a MARS_MULTI
   through AAL5 CPCS_PDU reassembly failure, which will be detected
   through the mechanisms described above.)

   If the MARS is managing a cluster of endpoints spread across
   different but directly accessible ATM networks it will not be able to
   return all the group members in a single MARS_MULTI. The MARS_MULTI
   message format allows for either E.164, ISO NSAP, or (E.164 + NSAP)
   to be returned as ATM addresses. However, each MARS_MULTI message may
   only return ATM addresses of the same type and length. The returned
   addresses MUST be grouped according to type (E.164, ISO NSAP, or
   both) and returned in a sequence of separate MARS_MULTI parts.








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5.1.2   MARS_REQUEST, MARS_MULTI, and MARS_NAK messages.

   MARS_REQUEST is shown below. It is indicated by an 'operation type
   value' (mar$op) of 1.

   The multicast address being resolved is placed into the the target
   protocol address field (mar$tpa), and the target hardware address is
   set to null (mar$thtl and mar$tstl both zero).

   In IPv4 environments the protocol type (mar$pro) is 0x800 and the
   target protocol address length (mar$tpln) MUST be set to 4. The
   source fields MUST contain the ATM number and subaddress of the
   client issuing the MARS_REQUEST (the subaddress MAY be null).

      Data:
       mar$afn      16 bits  Address Family (0x000F).
       mar$pro      56 bits  Protocol Identification.
       mar$hdrrsv   24 bits  Reserved. Unused by MARS control protocol.
       mar$chksum   16 bits  Checksum across entire MARS message.
       mar$extoff   16 bits  Extensions Offset.
       mar$op       16 bits  Operation code (MARS_REQUEST = 1)
       mar$shtl      8 bits  Type & length of source ATM number. (r)
       mar$sstl      8 bits  Type & length of source ATM subaddress. (q)
       mar$spln      8 bits  Length of source protocol address (s)
       mar$thtl      8 bits  Type & length of target ATM number (x)
       mar$tstl      8 bits  Type & length of target ATM subaddress (y)
       mar$tpln      8 bits  Length of target group address (z)
       mar$pad      64 bits  Padding (aligns mar$sha with MARS_MULTI).
       mar$sha      roctets  source ATM number
       mar$ssa      qoctets  source ATM subaddress
       mar$spa      soctets  source protocol address
       mar$tpa      zoctets  target multicast group address
       mar$tha      xoctets  target ATM number
       mar$tsa      yoctets  target ATM subaddress

   Following the RFC1577 approach, the mar$shtl, mar$sstl, mar$thtl and
   mar$tstl fields are coded as follows:

                7 6 5 4 3 2 1 0
               +-+-+-+-+-+-+-+-+
               |0|x|  length   |
               +-+-+-+-+-+-+-+-+









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   The most significant bit is reserved and MUST be set to zero.  The
   second most significant bit (x) is a flag indicating whether the ATM
   address being referred to is in:

      - ATM Forum NSAPA format (x = 0).
      - Native E.164 format (x = 1).

   The bottom 6 bits is an unsigned integer value indicating the length
   of the associated ATM address in octets. If this value is zero the
   flag x is ignored.

   The mar$spln and mar$tpln fields are unsigned 8 bit integers, giving
   the length in octets of the source and target protocol address fields
   respectively.

   MARS packets use true variable length fields. A null (non-existant)
   address MUST be coded as zero length, and no space allocated for it
   in the message body.

   MARS_NAK is the MARS_REQUEST returned with operation type value of 6.
   All other fields are left unchanged from the MARS_REQUEST (e.g. do
   not transpose the source and target information. In all cases MARS
   clients use the source address fields to identify their own messages
   coming back).

   The MARS_MULTI message is identified by an mar$op value of 2. The
   message format is:

      Data:
       mar$afn      16 bits  Address Family (0x000F).
       mar$pro      56 bits  Protocol Identification.
       mar$hdrrsv   24 bits  Reserved. Unused by MARS control protocol.
       mar$chksum   16 bits  Checksum across entire MARS message.
       mar$extoff   16 bits  Extensions Offset.
       mar$op       16 bits  Operation code (MARS_MULTI = 2).
       mar$shtl      8 bits  Type & length of source ATM number. (r)
       mar$sstl      8 bits  Type & length of source ATM subaddress. (q)
       mar$spln      8 bits  Length of source protocol address (s)
       mar$thtl      8 bits  Type & length of target ATM number (x)
       mar$tstl      8 bits  Type & length of target ATM subaddress (y)
       mar$tpln      8 bits  Length of target group address (z)
       mar$tnum     16 bits  Number of target ATM addresses returned (N)
       mar$seqxy    16 bits  Boolean flag x and sequence number y.
       mar$msn      32 bits  MARS Sequence Number.
       mar$sha      roctets  source ATM number
       mar$ssa      qoctets  source ATM subaddress
       mar$spa      soctets  source protocol address
       mar$tpa      zoctets  target multicast group address



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RFC 2022          Multicast over UNI 3.0/3.1 based ATM     November 1996


       mar$tha.1    xoctets  target ATM number 1
       mar$tsa.1    yoctets  target ATM subaddress 1
       mar$tha.2    xoctets  target ATM number 2
       mar$tsa.2    yoctets  target ATM subaddress 2
                 [.......]
       mar$tha.N    xoctets  target ATM number N
       mar$tsa.N    yoctets  target ATM subaddress N

   The source protocol and ATM address fields are copied directly from
   the MARS_REQUEST that this MARS_MULTI is in response to (not the MARS
   itself).

   mar$seqxy is coded with flag x in the leading bit, and sequence
   number y coded as an unsigned integer in the remaining 15 bits.

          |  1st octet    |   2nd octet   |
           7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |x|                 y           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   mar$tnum is an unsigned integer indicating how many pairs of
   {mar$tha,mar$tsa} (i.e. how many group member's ATM addresses) are
   present in the message. mar$msn is an unsigned 32 bit number filled
   in by the MARS before transmitting each MARS_MULTI. Its use is
   described further in section 5.1.4.

   As an example, assume we have a multicast cluster using 4 byte
   protocol addresses, 20 byte ATM numbers, and 0 byte ATM subaddresses.
   For n group members in a single MARS_MULTI we require a (60 + 20n)
   byte message. If we assume the default MTU of 9180 bytes, we can
   return a maximum of 456 group member's addresses in a single
   MARS_MULTI.

5.1.3   Establishing the outgoing multipoint VC.

   Following the completion of the MARS_MULTI reply the endpoint may
   establish a new point to multipoint VC, or reuse an existing one.

   If establishing a new VC, an L_MULTI_RQ is issued for ATM.1, followed
   by an L_MULTI_ADD for every member of the set {ATM.2, ....ATM.n}
   (assuming the set is non-null). The packet is then transmitted over
   the newly created VC just as it would be for a unicast VC.

   After transmitting the packet, the local interface holds the VC open
   and marks it as the active path out of the host for any subsequent IP
   packets being sent to that Class D address.




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   When establishing a new multicast VC it is possible that one or more
   L_MULTI_RQ or L_MULTI_ADD may fail.  The UNI 3.0/3.1 failure cause
   must be returned in the ERR_L_RQFAILED signal from the local
   signalling entity to the AAL User. If the failure cause is not 49
   (Quality of Service unavailable), 51 (user cell rate not available -
   UNI 3.0), 37 (user cell rate not available - UNI 3.1), or 41
   (Temporary failure), the endpoint's ATM address is dropped from the
   set {ATM.1, ATM.2, ..., ATM.n} returned by the MARS.  Otherwise, the
   L_MULTI_RQ or L_MULTI_ADD should be reissued after a random delay of
   5 to 10 seconds.  If the request fails again, another request should
   be issued after twice the previous delay has elapsed.  This process
   should be continued until the call succeeds or the multipoint VC gets
   released.

   If the initial L_MULTI_RQ fails for ATM.1, and n is greater than 1
   (i.e. the returned set of ATM addresses contains 2 or more addresses)
   a new L_MULTI_RQ should be immediately issued for the next ATM
   address in the set. This procedure is repeated until an L_MULTI_RQ
   succeeds, as no L_MULTI_ADDs may be issued until an initial outgoing
   VC is established.

   Each ATM address for which an L_MULTI_RQ failed with cause 49, 51,
   37, or 41 MUST be tagged rather than deleted. An L_MULTI_ADD is
   issued for these tagged addresses using the random delay procedure
   outlined above.

   The VC MAY be considered 'up' before failed L_MULTI_ADDs have been
   successfully re-issued. An endpoint MAY implement a concurrent
   mechanism that allows data to start flowing out the new VC even while
   failed L_MULTI_ADDs are being re-tried. (The alternative of waiting
   for each leaf node to accept the connection could lead to significant
   delays in transmitting the first packet.)

   Each VC MUST have a configurable inactivity timer associated with it.
   If the timer expires, an L_RELEASE is issued for that VC, and the
   Class D address is no longer considered to have an active path out of
   the local host. The timer SHOULD be no less than 1 minute, and a
   default of 20 minutes is RECOMMENDED. Choice of specific timer
   periods is beyond the scope of this document.

   VC consumption may also be reduced by endpoints noting when a new
   group's set of {ATM.1, ....ATM.n} matches that of a pre-existing VC
   out to another group. With careful local management, and assuming the
   QoS of the existing VC is sufficient for both groups, a new pt to mpt
   VC may not be necessary.  Under certain circumstances endpoints may
   decide that it is sufficient to re-use an existing VC whose set of
   leaf nodes is a superset of the new group's membership (in which case
   some endpoints will receive multicast traffic for a layer 3 group



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   they haven't joined, and must filter them above the ATM interface).
   Algorithms for performing this type of optimization are not discussed
   here, and are not required for conformance with this document.

5.1.4   Tracking subsequent group updates.

   Once a new VC has been established, the transmit side of the cluster
   member's interface needs to monitor subsequent group changes - adding
   or dropping leaf nodes as appropriate. This is achieved by watching
   for MARS_JOIN and MARS_LEAVE messages from the MARS itself. These
   messages are described in detail in section 5.2 - at this point it is
   sufficient to note that they carry:

      - The ATM address of a node joining or leaving a group.
      - The layer 3 address of the group(s) being joined or left.
      - A Cluster Sequence Number (CSN) from the MARS.

   MARS_JOIN and MARS_LEAVE messages arrive at each cluster member
   across ClusterControlVC. MARS_JOIN or MARS_LEAVE messages that simply
   confirm information already held by the cluster member are used to
   track the Cluster Sequence Number, but are otherwise ignored.

5.1.4.1   Updating the active VCs.

   If a MARS_JOIN is seen that refers to (or encompasses) a group for
   which the transmit side already has a VC open, the new member's ATM
   address is extracted and an L_MULTI_ADD issued locally. This ensures
   that endpoints already sending to a given group will immediately add
   the new member to their list of recipients.

   If a MARS_LEAVE is seen that refers to (or encompasses) a group for
   which the transmit side already has a VC open, the old member's ATM
   address is extracted and an L_MULTI_DROP issued locally. This ensures
   that endpoints already sending to a given group will immediately drop
   the old member from their list of recipients. When the last leaf of a
   VC is dropped, the VC is closed completely and the affected group no
   longer has a path out of the local endpoint (the next outbound packet
   to that group's address will trigger the creation of a new VC, as
   described in sections 5.1.1 to 5.1.3).

   The transmit side of the interface MUST NOT shut down an active VC to
   a group for which the receive side has just executed a
   LeaveLocalGroup.  (This behaviour is consistent with the model of
   hosts transmitting to groups regardless of their own membership
   status.)

   If a MARS_JOIN or MARS_LEAVE arrives with mar$pnum == 0 it carries no
   <min,max> pairs, and is only used for tracking the CSN.



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5.1.4.2   Tracking the Cluster Sequence Number.

   It is important that endpoints do not miss group membership updates
   issued by the MARS over ClusterControlVC. However, this will happen
   from time to time. The Cluster Sequence Number is carried as an
   unsigned 32 bit value in the mar$msn field of many MARS messages
   (except for MARS_REQUEST and MARS_NAK).  It increments once for every
   transmission the MARS makes on ClusterControlVC, regardless of
   whether the transmission represents a change in the MARS database or
   not. By tracking this counter, cluster members can determine whether
   they have missed a previous message on ClusterControlVC, and possibly
   a membership change. This is then used to trigger revalidation
   (described in section 5.1.5).

   The current CSN is copied into the mar$msn field of MARS messages
   being sent to cluster members, whether out ClusterControlVC or on a
   point to point VC.

   Calculations on the sequence numbers MUST be performed as unsigned 32
   bit arithmetic.

   Every cluster member keeps its own 32 bit Host Sequence Number (HSN)
   to track the MARS's sequence number. Whenever a message is received
   that carries an mar$msn field the following processing is performed:

         Seq.diff = mar$msn - HSN

         mar$msn -> HSN
         {...process MARS message as appropriate...}

         if ((Seq.diff != 1) && (Seq.diff != 0))
            then {...revalidate group membership information...}

   The basic result is that the cluster member attempts to keep locked
   in step with membership changes noted by the MARS. If it ever detects
   that a membership change occurred (in any group) without it noticing,
   it re-validates the membership of all groups it currently has
   multicast VCs open to.

   The mar$msn value in an individual MARS_MULTI is not used to update
   the HSN until all parts of the MARS_MULTI (if more than 1) have
   arrived. (If the mar$msn changes the MARS_MULTI is discarded, as
   described in section 5.1.1.)

   The MARS is free to choose an initial value of CSN. When a new
   cluster member starts up it should initialise HSN to zero. When the
   cluster member sends the MARS_JOIN to register (described later), the
   HSN will be correctly updated to the current CSN value when the



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   endpoint receives the copy of its MARS_JOIN back from the MARS.

5.1.5   Revalidating a VC's leaf nodes.

   Certain events may inform a cluster member that it has incorrect
   information about the sets of leaf nodes it should be sending to.  If
   an error occurs on a VC associated with a particular group, the
   cluster member initiates revalidation procedures for that specific
   group. If a jump is detected in the Cluster Sequence Number, this
   initiates revalidation of all groups to which the cluster member
   currently has open point to multipoint VCs.

   Each open and active multipoint VC has a flag associated with it
   called 'VC_revalidate'. This flag is checked everytime a packet is
   queued for transmission on that VC. If the flag is false, the packet
   is transmitted and no further action is required.

   However, if the VC_revalidate flag is true then the packet is
   transmitted and a new sequence of events is started locally.

   Revalidation begins with re-issuing a MARS_REQUEST for the group
   being revalidated.  The returned set of members {NewATM.1, NewATM.2,
   .... NewATM.n} is compared with the set already held locally.
   L_MULTI_DROPs are issued on the group's VC for each node that appears
   in the original set of members but not in the revalidated set of
   members. L_MULTI_ADDs are issued on the group's VC for each node that
   appears in the revalidated set of members but not in the original set
   of members. The VC_revalidate flag is reset when revalidation
   concludes for the given group. Implementation specific mechanisms
   will be needed to flag the 'revalidation in progress' state.

   The key difference between constructing a VC (section 5.1.3) and
   revalidating a VC is that packet transmission continues on the open
   VC while it is being revalidated. This minimises the disruption to
   existing traffic.

   The algorithm for initiating revalidation is:

      - When a packet arrives for transmission on a given group,
        the groups membership is revalidated if VC_revalidate == TRUE.
        Revalidation resets VC_revalidate.
      - When an event occurs that demands revalidation, every
        group has its VC_revalidate flag set TRUE at a random time
        between 1 and 10 seconds.

   Benefit: Revalidation of active groups occurs quickly, and
   essentially idle groups are revalidated as needed. Randomly
   distributed setting of VC_revalidate flag improves chances of



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   staggered revalidation requests from senders when a sequence number
   jump is detected.

5.1.5.1   When leaf node drops itself.

   During the life of a multipoint VC an ERR_L_DROP may be received
   indicating that a leaf node has terminated its participation at the
   ATM level. The ATM endpoint associated with the ERR_L_DROP MUST be
   removed from the locally held set {ATM.1, ATM.2, .... ATM.n}
   associated with the VC.

   After a random period of time between 1 and 10 seconds the
   VC_revalidate flag associated with that VC MUST be set true.

   If an ERR_L_RELEASE is received then the entire set {ATM.1, ATM.2,
   .... ATM.n} is cleared and the VC is considered to be completely shut
   down. Further packet transmission to the group served by this VC will
   result in a new VC being established as described in section 5.1.3.

5.1.5.2   When a jump is detected in the CSN.

   Section 5.1.4.2 describes how a CSN jump is detected. If a CSN jump
   is detected upon receipt of a MARS_JOIN or a MARS_LEAVE then every
   outgoing multicast VC MUST have its VC_revalidate flag set true at
   some random interval between 1 and 10 seconds from when the CSN jump
   was detected.

   The only exception to this rule is if a sequence number jump is
   detected during the establishment of a new group's VC (i.e. a
   MARS_MULTI reply was correctly received, but its mar$msn indicated
   that some previous MARS traffic had been missed on ClusterControlVC).
   In this case every open VC, EXCEPT the one just established, MUST
   have its VC_revalidate flag set true at some random interval between
   1 and 10 seconds from when the CSN jump was detected.  (The VC being
   established at the time is considered already validated.)

5.1.6  'Migrating' the outgoing multipoint VC

   In addition to the group tracking described in section 5.1.4, the
   transmit side of a cluster member must respond to 'migration'
   requests by the MARS. This is triggered by the reception of a
   MARS_MIGRATE message from ClusterControlVC. The MARS_MIGRATE message
   is shown below, with an mar$op code of 13.

      Data:
       mar$afn      16 bits  Address Family (0x000F).
       mar$pro      56 bits  Protocol Identification.
       mar$hdrrsv   24 bits  Reserved. Unused by MARS control protocol.



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       mar$chksum   16 bits  Checksum across entire MARS message.
       mar$extoff   16 bits  Extensions Offset.
       mar$op       16 bits  Operation code (MARS_MIGRATE = 13).
       mar$shtl      8 bits  Type & length of source ATM number. (r)
       mar$sstl      8 bits  Type & length of source ATM subaddress. (q)
       mar$spln      8 bits  Length of source protocol address (s)
       mar$thtl      8 bits  Type & length of target ATM number (x)
       mar$tstl      8 bits  Type & length of target ATM subaddress (y)
       mar$tpln      8 bits  Length of target group address (z)
       mar$tnum     16 bits  Number of target ATM addresses returned (N)
       mar$resv     16 bits  Reserved.
       mar$msn      32 bits  MARS Sequence Number.
       mar$sha      roctets  source ATM number
       mar$ssa      qoctets  source ATM subaddress
       mar$spa      soctets  source protocol address
       mar$tpa      zoctets  target multicast group address
       mar$tha.1    xoctets  target ATM number 1
       mar$tsa.1    yoctets  target ATM subaddress 1
       mar$tha.2    xoctets  target ATM number 2
       mar$tsa.2    yoctets  target ATM subaddress 2
                                 [.......]
       mar$tha.N    xoctets  target ATM number N
       mar$tsa.N    yoctets  target ATM subaddress N

   A migration is requested when the MARS determines that it no longer
   wants cluster members forwarding their packets directly to the ATM
   addresses it had previously specified (through MARS_REQUESTs or
   MARS_JOINs). When a MARS_MIGRATE is received each cluster member MUST
   perform the following steps:

      Close down any existing outgoing VC associated with the group
      carried in the mar$tpa field (L_RELEASE), or dissociate the group
      from any outgoing VC it may have been sharing (as described in
      section 5.1.3).

      Establish a new outgoing VC for the specified group, using the
      algorithm described in section 5.1.3 and taking the set of ATM
      addresses supplied in the MARS_MIGRATE as the group's new set of
      members {ATM.1, .... ATM.n}.

   The MARS_MIGRATE carries the new set of members {ATM.1, .... ATM.n}
   in a single message, in similar manner to a single part MARS_MULTI.
   As with other messages from the MARS, the Cluster Sequence Number
   carried in mar$msn is checked as described in section 5.1.4.2.







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5.2.   Receive side behaviour.

   A cluster member is a 'group member' (in the sense that it receives
   packets directed at a given multicast group) when its ATM address
   appears in the MARS's table entry for the group's multicast address.
   A key function within each cluster is the distribution of group
   membership information from the MARS to cluster members.

   An endpoint may wish to 'join a group' in response to a local, higher
   level request for membership of a group, or because the endpoint
   supports a layer 3 multicast forwarding engine that requires the
   ability to 'see' intra-cluster traffic in order to forward it.

   Two messages support these requirements - MARS_JOIN and MARS_LEAVE.
   These are sent to the MARS by endpoints when the local layer 3/ATM
   interface is requested to join or leave a multicast group. The MARS
   propagates these messages back out over ClusterControlVC, to ensure
   the knowledge of the group's membership change is distributed in a
   timely fashion to other cluster members.

   Certain models of layer 3 endpoints (e.g. IP multicast routers)
   expect to be able to receive packet traffic 'promiscuously' across
   all groups.  This functionality may be emulated by allowing routers
   to request that the MARS returns them as 'wild card' members of all
   Class D addresses.  However, a problem inherent in the current ATM
   model is that a completely promiscuous router may exhaust the local
   reassembly resources in its ATM interface. MARS_JOIN supports a
   generalisation to the notion of 'wild card' entries, enabling routers
   to limit themselves to 'blocks' of the Class D address space. Use of
   this facility is described in greater detail in Section 8.

   A block can be as small as 1 (a single group) or as large as the
   entire multicast address space (e.g. default IPv4 'promiscuous'
   behaviour).  A block is defined as all addresses between, and
   inclusive of, a <min,max> address pair. A MARS_JOIN or MARS_LEAVE may
   carry multiple <min,max> pairs.

   Cluster members MUST provide ONLY a single <min,max> pair in each
   JOIN/LEAVE message they issue. However, they MUST be able to process
   multiple <min,max> pairs in JOIN/LEAVE messages when performing VC
   management as described in section 5.1.4 (the interpretation being
   that the join/leave operation applies to all addresses in the range
   from <min> to <max> inclusive, for every <min,max> pair).

   In RFC1112 environments a MARS_JOIN for a single group is triggered
   by a JoinLocalGroup signal from the IP layer. A MARS_LEAVE for a
   single group is triggered by a LeaveLocalGroup signal from the IP
   layer.



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   Cluster members with special requirements (e.g. multicast routers)
   may issue MARS_JOINs and MARS_LEAVEs specifying a single block of 2
   or more multicast group addresses. However, a cluster member SHALL
   NOT issue such a multi-group block join for an address range fully or
   partially overlapped by multi-group block join(s) that the cluster
   member has previously issued and not yet retracted. A cluster member
   MAY issue combinations of single group MARS_JOINs that overlap with a
   multi-group block MARS_JOIN.

   An endpoint MUST register with a MARS in order to become a member of
   a cluster and be added as a leaf to ClusterControlVC.  Registration
   is covered in section 5.2.3.

   Finally, the endpoint MUST be capable of terminating unidirectional
   VCs (i.e. act as a leaf node of a UNI 3.0/3.1 point to multipoint VC,
   with zero bandwidth assigned on the return path). RFC 1755 describes
   the signalling information required to terminate VCs carrying
   LLC/SNAP encapsulated traffic (discussed further in section 5.5).

5.2.1 Format of the MARS_JOIN and MARS_LEAVE Messages.

   The MARS_JOIN message is indicated by an operation type value of 4.
   MARS_LEAVE has the same format and operation type value of 5. The
   message format is:

      Data:
       mar$afn      16 bits  Address Family (0x000F).
       mar$pro      56 bits  Protocol Identification.
       mar$hdrrsv   24 bits  Reserved. Unused by MARS control protocol.
       mar$chksum   16 bits  Checksum across entire MARS message.
       mar$extoff   16 bits  Extensions Offset.
       mar$op       16 bits  Operation code (MARS_JOIN or MARS_LEAVE).
       mar$shtl      8 bits  Type & length of source ATM number. (r)
       mar$sstl      8 bits  Type & length of source ATM subaddress. (q)
       mar$spln      8 bits  Length of source protocol address (s)
       mar$tpln      8 bits  Length of group address (z)
       mar$pnum     16 bits  Number of group address pairs (N)
       mar$flags    16 bits  layer3grp, copy, and register flags.
       mar$cmi      16 bits  Cluster Member ID
       mar$msn      32 bits  MARS Sequence Number.
       mar$sha      roctets  source ATM number.
       mar$ssa      qoctets  source ATM subaddress.
       mar$spa      soctets  source protocol address
       mar$min.1    zoctets  Minimum multicast group address - pair.1
       mar$max.1    zoctets  Maximum multicast group address - pair.1
                 [.......]
       mar$min.N    zoctets  Minimum multicast group address - pair.N
       mar$max.N    zoctets  Maximum multicast group address - pair.N



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   mar$spln indicates the number of bytes in the source endpoint's
   protocol address, and is interpreted in the context of the protocol
   indicated by the mar$pro field. (e.g. in IPv4 environments mar$pro
   will be 0x800, mar$spln is 4, and mar$tpln is 4.)

   The mar$flags field contains three flags:

      Bit 15  - mar$flags.layer3grp.
      Bit 14  - mar$flags.copy.
      Bit 13  - mar$flags.register.
      Bit 12  - mar$flags.punched.
      Bit 0-7 - mar$flags.sequence.

   Bits 8 to 11 are reserved and MUST be zero.

   mar$flags.sequence is set by cluster members, and MUST always be
   passed on unmodified by the MARS when retransmitting MARS_JOIN or
   MARS_LEAVE messages. It is source specific, and MUST be ignored by
   other cluster members. Its use is described in section 5.2.2.

   mar$flags.punched MUST be zero when the MARS_JOIN or MARS_LEAVE is
   transmitted to the MARS. Its use is described in section 5.2.2 and
   section 6.2.4.

   mar$flags.copy MUST be set to 0 when the message is being sent from a
   MARS client, and MUST be set to 1 when the message is being sent from
   a MARS. (This flag is intended to support integrating the MARS
   function with one of the MARS clients in your cluster. The
   destination of an incoming MARS_JOIN can be determined from its
   value.)

   mar$flags.layer3grp allows the MARS to provide the group membership
   information described further in section 5.3. The rules for its use
   are:

      mar$flags.layer3grp MUST be set when the cluster member is issuing
      the MARS_JOIN as the result of a layer 3 multicast group being
      explicitly joined. (e.g. as a result of a JoinHostGroup operation
      in an RFC1112 compliant host).

      mar$flags.layer3grp MUST be reset in each MARS_JOIN if the
      MARS_JOIN is simply the local ip/atm interface registering to
      receive traffic on that group for its own reasons.

      mar$flags.layer3grp is ignored and MUST be treated as reset by the
      MARS for any MARS_JOIN that specifies a block covering more than a
      single group (e.g. a block join from a router ensuring their
      forwarding engines 'see' all traffic).



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   mar$flags.register indicates whether the MARS_JOIN or MARS_LEAVE is
   being used to register or deregister a cluster member (described in
   section 5.2.3). When used to join or leave specific groups the
   mar$register flag MUST be zero.

   mar$pnum indicates how many <min,max> pairs are included in the
   message. This field MUST be 1 when the message is sent from a cluster
   member. A MARS MAY return a MARS_JOIN or MARS_LEAVE with any mar$pnum
   value, including zero.  This will be explained futher in section
   6.2.4.

   The mar$cmi field MUST be zeroed by cluster members, and is used by
   the MARS during cluster member registration, described in section
   5.2.3.

   mar$msn MUST be zero when transmitted by an endpoint. It is set to
   the current value of the Cluster Sequence Number by the MARS when the
   MARS_JOIN or MARS_LEAVE is retransmitted. Its use has been described
   in section 5.1.4.

   To simplify construction and parsing of MARS_JOIN and MARS_LEAVE
   messages, the following restrictions are imposed on the <min,max>
   pairs:

      Assume max(N) is the <max> field from the Nth <min,max> pair.
      Assume min(N) is the <min> field from the Nth <min,max> pair.
      Assume a join/leave message arrives with K <min,max> pairs.
      The following must hold:
         max(N) < min(N+1) for 1 <= N < K
         max(N) >= min(N) for 1 <= N <= K

   In plain language, the set must specify an ascending sequence of
   address blocks. The definition of "greater" or "less than" may be
   protocol specific. In IPv4 environments the addresses are treated as
   32 bit, unsigned binary values (most significant byte first).

5.2.1.1 Important IPv4 default values.

   The JoinLocalGroup and LeaveLocalGroup operations are only valid for
   a single group. For any arbitrary group address X the associated
   MARS_JOIN or MARS_LEAVE MUST specify a single pair <X, X>.
   mar$flags.layer3grp MUST be set under these circumstances.

   A router choosing to behave strictly in accordance with RFC1112 MUST
   specify the entire Class D space. The associated MARS_JOIN or
   MARS_LEAVE MUST specify a single pair <224.0.0.0, 239.255.255.255>.
   Whenever a router issues a MARS_JOIN only in order to forward IP
   traffic it MUST reset mar$flags.layer3grp.



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   The use of alternative <min, max> values by multicast routers is
   discussed in Section 8.

5.2.2   Retransmission of MARS_JOIN and MARS_LEAVE messages.

   Transient problems may result in the loss of messages between the
   MARS and cluster members

   A simple algorithm is used to solve this problem. Cluster members
   retransmit each MARS_JOIN and MARS_LEAVE message at regular intervals
   until they receive a copy back again, either on ClusterControlVC or
   the VC on which they are sending the message.  At this point the
   local endpoint can be certain that the MARS received and processed
   it.

   The interval should be no shorter than 5 seconds, and a default value
   of 10 seconds is recommended. After 5 retransmissions the attempt
   should be flagged locally as a failure. This MUST be considered as a
   MARS failure, and triggers the MARS reconnection described in section
   5.4.

   A 'copy' is defined as a received message with the following fields
   matching a previously transmitted MARS_JOIN/LEAVE:

      - mar$op
      - mar$flags.register
      - mar$flags.sequence
      - mar$pnum
      - Source ATM address
      - First <min,max> pair

   In addition, a valid copy MUST have the following field values:

      - mar$flags.punched = 0
      - mar$flags.copy = 1

   The mar$flags.sequence field is never modified or checked by a MARS.
   Implementors MAY choose to utilize locally significant sequence
   number schemes, which MAY differ from one cluster member to the next.
   In the absence of such schemes the default value for
   mar$flags.sequence MUST be zero.

   Careful implementations MAY have more than one unacknowledged
   MARS_JOIN/LEAVE outstanding at a time.







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5.2.3   Cluster member registration and deregistration.

   To become a cluster member an endpoint must register with the MARS.
   This achieves two things - the endpoint is added as a leaf node of
   ClusterControlVC, and the endpoint is assigned a 16 bit Cluster
   Member Identifier (CMI). The CMI uniquely identifies each endpoint
   that is attached to the cluster.

   Registration with the MARS occurs when an endpoint issues a MARS_JOIN
   with the mar$flags.register flag set to one (bit 13 of the mar$flags
   field).

   The cluster member MUST include its source ATM address, and MAY
   choose to specify a null source protocol address when registering.

   No protocol specific group addresses are included in a registration
   MARS_JOIN.

   The cluster member retransmits this MARS_JOIN in accordance with
   section 5.2.2 until it confirms that the MARS has received it.

   When the registration MARS_JOIN is returned it contains a non-zero
   value in mar$cmi. This value MUST be noted by the cluster member, and
   used whenever circumstances require the cluster member's CMI.

   An endpoint may also choose to de-register, using a MARS_LEAVE with
   mar$flags.register set. This would result in the MARS dropping the
   endpoint from ClusterControlVC, removing all references to the member
   in the mapping database, and freeing up its CMI.

   As for registration, a deregistration request MUST include the
   correct source ATM address for the cluster member, but MAY choose to
   specify a null source protocol address.

   The cluster member retransmits this MARS_LEAVE in accordance with
   section 5.2.2 until it confirms that the MARS has received it.

5.3   Support for Layer 3 group management.

   Whilst the intention of this specification is to be independent of
   layer 3 issues, an attempt is being made to assist the operation of
   layer 3 multicast routing protocols that need to ascertain if any
   groups have members within a cluster.

   One example is IP, where IGMP is used (as described in section 2)
   simply to determine whether any other cluster members are listening
   to a group because they have higher layer applications that want to
   receive a group's traffic.



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   Routers may choose to query the MARS for this information, rather
   than multicasting IGMP queries to 224.0.0.1 and incurring the
   associated cost of setting up a VC to all systems in the cluster.

   The query is issued by sending a MARS_GROUPLIST_REQUEST to the MARS.
   MARS_GROUPLIST_REQUEST is built from a MARS_JOIN, but it has an
   operation code of 10. The first <min,max> pair will be used by the
   MARS to identify the range of groups in which the querying cluster
   member is interested. Any additional <min,max> pairs will be ignored.
   A request with mar$pnum = 0 will be ignored.

   The response from the MARS is a MARS_GROUPLIST_REPLY, carrying a list
   of the multicast groups within the specified <min,max> block that
   have Layer 3 members.  A group is noted in this list if one or more
   of the MARS_JOINs that generated its mapping entry in the MARS
   contained a set mar$flags.layer3grp flag.

   MARS_GROUPLIST_REPLYs are transmitted back to the querying cluster
   member on the VC used to send the MARS_GROUPLIST_REQUEST.

   MARS_GROUPLIST_REPLY is derived from the MARS_MULTI but with mar$op =
   11. It may have multiple parts if needed, and is received in a
   similar manner to a MARS_MULTI.

      Data:
       mar$afn      16 bits  Address Family (0x000F).
       mar$pro      56 bits  Protocol Identification.
       mar$hdrrsv   24 bits  Reserved. Unused by MARS control protocol.
       mar$chksum   16 bits  Checksum across entire MARS message.
       mar$extoff   16 bits  Extensions Offset.
       mar$op       16 bits  Operation code (MARS_GROUPLIST_REPLY).
       mar$shtl      8 bits  Type & length of source ATM number. (r)
       mar$sstl      8 bits  Type & length of source ATM subaddress. (q)
       mar$spln      8 bits  Length of source protocol address (s)
       mar$thtl      8 bits  Unused - set to zero.
       mar$tstl      8 bits  Unused - set to zero.
       mar$tpln      8 bits  Length of target group address (z)
       mar$tnum     16 bits  Number of group addresses returned (N).
       mar$seqxy    16 bits  Boolean flag x and sequence number y.
       mar$msn      32 bits  MARS Sequence Number.
       mar$sha      roctets  source ATM number.
       mar$ssa      qoctets  source ATM subaddress.
       mar$spa      soctets  source protocol address
       mar$mgrp.1   zoctets  Group address 1
                 [.......]
       mar$mgrp.N   zoctets  Group address N

   mar$seqxy is coded as for the MARS_MULTI - multiple



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   MARS_GROUPLIST_REPLY components are transmitted and received using
   the same algorithm as described in section 5.1.1 for MARS_MULTI. The
   only difference is that protocol addresses are being returned rather
   than ATM addresses.

   As for MARS_MULTIs, if an error occurs in the reception of a multi
   part MARS_GROUPLIST_REPLY the whole thing MUST be discarded and the
   MARS_GROUPLIST_REQUEST re-issued. (This includes the mar$msn value
   being constant.)

   Note that the ability to generate MARS_GROUPLIST_REQUEST messages,
   and receive MARS_GROUPLIST_REPLY messages, is not required for
   general host interface implementations. It is optional for interfaces
   being implemented to support layer 3 multicast forwarding engines.
   However, this functionality MUST be supported by the MARS.

5.4   Support for redundant/backup MARS entities.

   Endpoints are assumed to have been configured with the ATM address of
   at least one MARS. Endpoints MAY choose to maintain a table of ATM
   addresses, representing alternative MARSs that will be contacted in
   the event that normal operation with the original MARS is deemed to
   have failed. It is assumed that this table orders the ATM addresses
   in descending order of preference.

   An endpoint will typically decide there are problems with the MARS
   when:

      - It fails to establish a point to point VC to the MARS.
      - MARS_REQUESTs fail (section 5.1.1).
      - MARS_JOIN/MARS_LEAVEs fail (section 5.2.2).
      - It has not received a MARS_REDIRECT_MAP in the last 4 minutes
      (section 5.4.3).

   (If it is able to discern which connection represents
   ClusterControlVC, it may also use connection failures on this VC to
   indicate problems with the MARS).

5.4.1   First response to MARS problems.

   The first response is to assume a transient problem with the MARS
   being used at the time. The cluster member should wait a random
   period of time between 1 and 10 seconds before attempting to re-
   connect and re-register with the MARS. If the registration MARS_JOIN
   is successful then:

        The cluster member MUST then proceed to rejoin every group that
        its local higher layer protocol(s) have joined. It is



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        recommended that a random delay between 1 and 10 seconds be
        inserted before attempting each MARS_JOIN.

        The cluster member MUST initiate the revalidation of every
        multicast group it was sending to (as though a sequence number
        jump had been detected, section 5.1.5).

        The rejoin and revalidation procedure must not disrupt the
        cluster member's use of multipoint VCs that were already open at
        the time of the MARS failure.

   If re-registration with the current MARS fails, and there are no
   backup MARS addresses configured, the cluster member MUST wait for at
   least 1 minute before repeating the re-registration procedure. It is
   RECOMMENDED that the cluster member signals an error condition in
   some locally significant fashion.

   This procedure may repeat until network administrators manually
   intervene or the current MARS returns to normal operation.

5.4.2   Connecting to a backup MARS.

   If the re-registration with the current MARS fails, and other MARS
   addresses have been configured, the next MARS address on the list is
   chosen to be the current MARS, and the cluster member immediately
   restarts the re-registration procedure described in section 5.4.1. If
   this is succesful the cluster member will resume normal operation
   using the new MARS. It is RECOMMENDED that the cluster member signals
   a warning of this condition in some locally significant fashion.

   If the attempt at re-registration with the new MARS fails, the
   cluster member MUST wait for at least 1 minute before choosing the
   next MARS address in the table and repeating the procedure. If the
   end of the table has been reached, the cluster member starts again at
   the top of the table (which should be the original MARS that the
   cluster member started with).

   In the worst case scenario this will result in cluster members
   looping through their table of possible MARS addresses until network
   administrators manually intervene.

5.4.3   Dynamic backup lists, and soft redirects.

   To support some level of autoconfiguration, a MARS message is defined
   that allows the current MARS to broadcast on ClusterControlVC a table
   of backup MARS addresses. When this message is received, cluster
   members that maintain a list of backup MARS addresses MUST insert
   this information at the top of their locally held list (i.e. the



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   information provided by the MARS has a higher preference than
   addresses that may have been manually configured into the cluster
   member).

   The message is MARS_REDIRECT_MAP. It is based on the MARS_MULTI
   message, with the following changes:

      - mar$tpln field replaced by mar$redirf.
      - mar$spln field reserved.
      - mar$tpa and mar$spa eliminated.

   MARS_REDIRECT_MAP has an operation type code of 12 decimal.

      Data:
       mar$afn      16 bits  Address Family (0x000F).
       mar$pro      56 bits  Protocol Identification.
       mar$hdrrsv   24 bits  Reserved. Unused by MARS control protocol.
       mar$chksum   16 bits  Checksum across entire MARS message.
       mar$extoff   16 bits  Extensions Offset.
       mar$op       16 bits  Operation code (MARS_REDIRECT_MAP).
       mar$shtl      8 bits  Type & length of source ATM number. (r)
       mar$sstl      8 bits  Type & length of source ATM subaddress. (q)
       mar$spln      8 bits  Length of source protocol address (s)
       mar$thtl      8 bits  Type & length of target ATM number (x)
       mar$tstl      8 bits  Type & length of target ATM subaddress (y)
       mar$redirf    8 bits  Flag controlling client redirect behaviour.
       mar$tnum     16 bits  Number of MARS addresses returned (N).
       mar$seqxy    16 bits  Boolean flag x and sequence number y.
       mar$msn      32 bits  MARS Sequence Number.
       mar$sha      roctets  source ATM number
       mar$ssa      qoctets  source ATM subaddress
       mar$tha.1    xoctets  ATM number for MARS 1
       mar$tsa.1    yoctets  ATM subaddress for MARS 1
       mar$tha.2    xoctets  ATM number for MARS 2
       mar$tsa.2    yoctets  ATM subaddress for MARS 2
                 [.......]
       mar$tha.N    xoctets  ATM number for MARS N
       mar$tsa.N    yoctets  ATM subaddress for MARS N

   The source ATM address field(s) MUST identify the originating MARS.
   A multi-part MARS_REDIRECT_MAP may be transmitted and reassembled
   using the mar$seqxy field in the same manner as a multi-part
   MARS_MULTI (section 5.1.1). If a failure occurs during the reassembly
   of a multi-part MARS_REDIRECT_MAP (a part lost, reassembly timeout,
   or illegal MARS Sequence Number jump) the entire message MUST be
   discarded.





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   This message is transmitted regularly by the MARS (it MUST be
   transmitted at least every 2 minutes, it is RECOMMENDED that it is
   transmitted every 1 minute).

   The MARS_REDIRECT_MAP is also used to force cluster members to shift
   from one MARS to another. If the ATM address of the first MARS
   contained in a MARS_REDIRECT_MAP table is not the address of cluster
   member's current MARS the client MUST 'redirect' to the new MARS. The
   mar$redirf field controls how the redirection occurs.

   mar$redirf has the following format:

                7 6 5 4 3 2 1 0
               +-+-+-+-+-+-+-+-+
               |x|             |
               +-+-+-+-+-+-+-+-+

   If Bit 7 (the most significant bit) of mar$redirf is 1 then the
   cluster member MUST perform a 'hard' redirect. Having installed the
   new table of MARS addresses carried by the MARS_REDIRECT_MAP, the
   cluster member re-registers with the MARS now at the top of the table
   using the mechanism described in sections 5.4.1 and 5.4.2.

   If Bit 7 of mar$redirf is 0 then the cluster member MUST perform a
   'soft' redirect, beginning with the following actions:

      - open a point to point VC to the first ATM address.
      - attempt a registration (section 5.2.3).

   If the registration succeeds, the cluster member shuts down its point
   to point VC to the current MARS (if it had one open), and then
   proceeds to use the newly opened point to point VC as its connection
   to the 'current MARS'. The cluster member does NOT attempt to rejoin
   the groups it is a member of, or revalidate groups it is currently
   sending to.

   This is termed a 'soft redirect' because it avoids the extra
   rejoining and revalidation processing that occurs when a MARS failure
   is being recovered from. It assumes some external synchronisation
   mechanisms exist between the old and new MARS - mechanisms that are
   outside the scope of this specification.

   Some level of trust is required before initiating a soft redirect. A
   cluster member MUST check that the calling party at the other end of
   the VC on which the MARS_REDIRECT_MAP arrived (supposedly
   ClusterControlVC) is in fact the node it trusts as the current MARS.

   Additional applications of this function are for further study.



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5.5  Data path LLC/SNAP encapsulations.

   An extended encapsulation scheme is required to support the filtering
   of possible reflected packets (section 3.3).

   Two LLC/SNAP codepoints are allocated from the IANA OUI space. These
   support two different mechanisms for detecting reflected packets.
   They are called Type #1 and Type #2 multicast encapsulations.

   Type #1

      [0xAA-AA-03][0x00-00-5E][0x00-01][Type #1 Extended Layer 3 packet]
          LLC         OUI        PID

   Type #2

      [0xAA-AA-03][0x00-00-5E][0x00-04][Type #2 Extended Layer 3 packet]
          LLC         OUI        PID

   For conformance with this document MARS clients:

      MUST transmit data using Type #1 encapsulation.

      MUST be able to correctly receive traffic using Type #1 OR Type #2
      encapsulation.

      MUST NOT transmit using Type #2 encapsulation.

5.5.1 Type #1 encapsulation.

   The Type #1 Extended layer 3 packet carries within it a copy of the
   source's Cluster Member ID (CMI) and either the 'short form' or 'long
   form' of the protocol type as appropriate (section 4.3).

   When carrying packets belonging to protocols with valid short form
   representations the [Type #1 Extended Layer 3 packet] is encoded as:

      [pkt$cmi][pkt$pro][Original Layer 3 packet]
        2octet   2octet        N octet

   The first 2 octets (pkt$cmi) carry the CMI assigned when an endpoint
   registers with the MARS (section 5.2.3). The second 2 octets
   (pkt$pro) indicate the protocol type of the packet carried in the
   remainder of the payload. This is copied from the mar$pro field used
   in the MARS control messages.

   When carrying packets belonging to protocols that only have a long
   form representation (pkt$pro = 0x80) the overhead SHALL be further



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   extended to carry the 5 byte mar$pro.snap field (with padding for 32
   bit alignment). The encoded form SHALL be:

      [pkt$cmi][0x00-80][mar$pro.snap][padding][Original Layer 3 packet]
        2octet   2octet   5 octets   3 octets        N octet


   The CMI is copied into the pkt$cmi field of every outgoing Type #1
   packet.  When an endpoint interface receives an AAL_SDU with the
   LLC/SNAP codepoint indicating Type #1 encapsulation it compares the
   CMI field with its own Cluster Member ID for the indicated protocol.
   The packet is discarded silently if they match. Otherwise the packet
   is accepted for processing by the local protocol entity identified by
   the pkt$pro (and possibly SNAP) field(s).

   Where a protocol has valid short and long forms of identification,
   receivers MAY choose to additionally recognise the long form.

5.5.2 Type #2 encapsulation.

   Future developments may enable direct multicasting of AAL_SDUs beyond
   cluster boundaries. Expanding the set of possible sources in this way
   may cause the CMI to become an inadequate parameter with which to
   detect reflected packets.  A larger source identification field may
   be required.

   The Type #2 Extended layer 3 packet carries within it an 8 octet
   source ID field and either the 'short form' or 'long form' of the
   protocol type as appropriate (section 4.3).  The form and content of
   the source ID field is currently unspecified, and is not relevant to
   any MARS client built in conformance with this document. Received
   Type #2 encapsulated packets MUST always be accepted and passed up to
   the higher layer indicated by the protocol identifier.

   When carrying packets belonging to protocols with valid short form
   representations the [Type #2 Extended Layer 3 packet] is encoded as:

      [8 octet sourceID][mar$pro.type][Null pad][Original Layer 3
      packet]
                           2octets     2octets

   When carrying packets belonging to protocols that only have a long
   form representation (pkt$pro = 0x80) the overhead SHALL be further
   extended to carry the 5 byte mar$pro.snap field (with padding for 32
   bit alignment). The encoded form SHALL be:

      [8 octet sourceID][mar$pro.type][mar$pro.snap][Null pad][Layer 3
      packet]



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                           2octets      5octets      1octet

   (Note that in this case the padding after the SNAP field is 1 octet
   rather than the 3 octets used in Type #1.)

   Where a protocol has valid short and long forms of identification,
   receivers MAY choose to additionally recognise the long form.

   (Future documents may specify the contents of the source ID field.
   This will only be relevant to implementations sending Type #2
   encapsulated packets, as they are the only entities that need to be
   concerned about detecting reflected Type #2 packets.)

5.5.3 A Type #1 example.

   An IPv4 packet (fully identified by an Ethertype of 0x800, therefore
   requiring 'short form' protocol type encoding) would be transmitted
   as:

      [0xAA-AA-03][0x00-00-5E][0x00-01][pkt$cmi][0x800][IPv4 packet]

      The different LLC/SNAP codepoints for unicast and multicast packet
      transmission allows a single IPv4/ATM interface to support both by
      demuxing on the LLC/SNAP header.

6. The MARS in greater detail.

   Section 5 implies a lot about the MARS's basic behaviour as observed
   by cluster members. This section summarises the behaviour of the MARS
   for groups that are VC mesh based, and describes how a MARSs
   behaviour changes when an MCS is registered to support a group.

   The MARS is intended to be a multiprotocol entity - all its mapping
   tables, CMIs, and control VCs MUST be managed within the context of
   the mar$pro field in incoming MARS messages. For example, a MARS
   supports completely separate ClusterControlVCs for each layer 3
   protocol that it is registering members for. If a MARS receives
   messages with a mar$pro that it does not support, the message is
   dropped.

   In general the MARS treats protocol addresses as arbitrary byte
   strings. For example, the MARS will not apply IPv4 specific 'class'
   checks to addresses supplied under mar$pro = 0x800.  It is sufficient
   for the MARS to simply assume that endpoints know how to interpret
   the protocol addresses that they are establishing and releasing
   mappings for.





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   The MARS requires control messages to carry the originator's identity
   in the source ATM address field(s). Messages that arrive with an
   empty ATM Number field are silently discarded prior to any other
   processing by the MARS. (Only the ATM Number field needs to be
   checked. An empty ATM Number field combined with a non-empty ATM
   Subaddress field does not represent a valid ATM address.)

   (Some example pseudo-code for a MARS can be found in Appendix F.)

6.1 Basic interface to Cluster members.

   The following MARS messages are used or required by cluster members:

      1    MARS_REQUEST
      2    MARS_MULTI
      4    MARS_JOIN
      5    MARS_LEAVE
      6    MARS_NAK
      10   MARS_GROUPLIST_REQUEST
      11   MARS_GROUPLIST_REPLY
      12   MARS_REDIRECT_MAP

6.1.1  Response to MARS_REQUEST.

   Except as described in section 6.2, if a MARS_REQUEST arrives whose
   source ATM address does not match that of any registered Cluster
   member the message MUST be dropped and ignored.

6.1.2  Response to MARS_JOIN and MARS_LEAVE.

   When a registration MARS_JOIN arrives (described in section 5.2.3)
   the MARS performs the following actions:

      - Adds the node to ClusterControlVC.
      - Allocates a new Cluster Member ID (CMI).
      - Inserts the new CMI into the mar$cmi field of the MARS_JOIN.
      - Retransmits the MARS_JOIN back privately.

   If the node is already a registered member of the cluster associated
   with the specified protocol type then its existing CMI is simply
   copied into the MARS_JOIN, and the MARS_JOIN retransmitted back to
   the node.  A single node may register multiple times if it supports
   multiple layer 3 protocols. The CMIs allocated by the MARS for each
   such registration may or may not be the same.

   The retransmitted registration MARS_JOIN must NOT be sent on
   ClusterControlVC.  If a cluster member issues a deregistration
   MARS_LEAVE it too is retransmitted privately.



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   Non-registration MARS_JOIN and MARS_LEAVE messages are ignored if
   they arrive from a node that is not registered as a cluster member.

   MARS_JOIN or MARS_LEAVE messages MUST arrive at the MARS with
   mar$flags.copy set to 0, otherwise the message is silently ignored.

   All outgoing MARS_JOIN or MARS_LEAVE messages SHALL have
   mar$flags.copy set to 1, and mar$msn set to the current Cluster
   Sequence Number for ClusterControlVC (Section 5.1.4.2).

   mar$flags.layer3grp (section 5.3) MUST be treated as reset for
   MARS_JOINs specifying a single <min,max> pair covering more than a
   single group. If a MARS_JOIN/LEAVE is received that contains more
   than one <min,max> pair, the MARS MUST silently drop the message.

   If one or more MCSs have registered with the MARS, message processing
   continues as described in section 6.2.4.

   The MARS database is updated to add the node to any indicated
   group(s) that it was not already considered a member of, and message
   processing continues as follows:

   If a single group was being joined or left:

      mar$flags.punched is set to 0.

      If the joining (leaving) node was already (is still) considered a
      member of the specified group, the message is retransmitted
      privately back to the cluster member.  Otherwise the message is
      retransmitted on ClusterControlVC.

   If a single block covering 2 or more groups was being joined or left:

      A copy of the original MARS_JOIN/LEAVE is made. This copy then has
      its <min,max> block replaced with a 'hole punched' set of zero or
      more <min,max> pairs.  The 'hole punched' set of <min,max> pairs
      covers the entire address range specified by the original
      <min,max> pair, but excludes those addresses/groups which the
      joining (leaving) node is already (still) a member of due to a
      previous single group join.

      If no 'holes' were punched in the specified block, the original
      MARS_JOIN/LEAVE is retransmitted out on ClusterControlVC.
      Otherwise the following occurs:

         The original MARS_JOIN/LEAVE is transmitted back to the source
         cluster member unchanged, using the VC it arrived on. The
         mar$flags.punched field MUST be reset to 0 in this message.



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         If the hole-punched set contains 1 or more <min,max> pair, the
         copy of the original MARS_JOIN/LEAVE is transmitted on
         ClusterControlVC, carrying the new <min,max> list. The
         mar$flags.punched field MUST be set to 1 in this message.  (The
         mar$flags.punched field is set to ensure the hole-punched copy
         is ignored by the message's source when trying to match
         received MARS_JOIN/LEAVE messages with ones previously sent
         (section 5.2.2)).

   If the MARS receives a deregistration MARS_LEAVE (described in
   section 5.2.3) that member's ATM address MUST be removed from all
   groups for which it may have joined, dropped from ClusterControlVC,
   and the CMI released.

   If the MARS receives an ERR_L_RELEASE on ClusterControlVC indicating
   that a cluster member has disconnected, that member's ATM address
   MUST be removed from all groups for which it may have joined, and the
   CMI released.

6.1.3  Generating MARS_REDIRECT_MAP.

   A MARS_REDIRECT_MAP message (described in section 5.4.3) MUST be
   regularly transmitted on ClusterControlVC.  It is RECOMMENDED that
   this occur every 1 minute, and it MUST occur at least every 2
   minutes. If the MARS has no knowledge of other backup MARSs serving
   the cluster, it MUST include its own address as the only entry in the
   MARS_REDIRECT_MAP message (in addition to filling in the source
   address fields).

   The design and use of backup MARS entities is beyond the scope of
   this document, and will be covered in future work.

6.1.4  Cluster Sequence Numbers.

   The Cluster Sequence Number (CSN) is described in section 5.1.4, and
   is carried in the mar$msn field of MARS messages being sent to
   cluster members (either out ClusterControlVC or on an individual VC).
   The MARS increments the CSN after every transmission of a message on
   ClusterControlVC.  The current CSN is copied into the mar$msn field
   of MARS messages being sent to cluster members, whether out
   ClusterControlVC or on a private VC.

   A MARS should be carefully designed to minimise the possibility of
   the CSN jumping unnecessarily. Under normal operation only cluster
   members affected by transient link problems will miss CSN updates and
   be forced to revalidate. If the MARS itself glitches, it will be
   innundated with requests for a period as every cluster member
   attempts to revalidate.



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   Calculations on the CSN MUST be performed as unsigned 32 bit
   arithmetic.

   One implication of this mechanism is that the MARS should serialize
   its processing of 'simultaneous' MARS_REQUEST, MARS_JOIN and
   MARS_LEAVE messages. Join and Leave operations should be queued
   within the MARS along with MARS_REQUESTS, and not processed until all
   the reply packets of a preceeding MARS_REQUEST have been transmitted.
   The transmission of MARS_REDIRECT_MAP should also be similarly
   queued.

   (The regular transmission of MARS_REDIRECT_MAP serves a secondary
   purpose of allowing cluster members to track the CSN, even if they
   miss an earlier MARS_JOIN or MARS_LEAVE.)

6.2   MARS interface to Multicast Servers (MCS).

   When the MARS returns the actual addresses of group members, the
   endpoint behaviour described in section 5 results in all groups being
   supported by meshes of point to multipoint VCs. However, when MCSs
   register to support particular layer 3 multicast groups the MARS
   modifies its use of various MARS messages to fool endpoints into
   using the MCS instead.

   The following MARS messages are associated with interaction between
   the MARS and MCSs.

      3   MARS_MSERV
      7   MARS_UNSERV
      8   MARS_SJOIN
      9   MARS_SLEAVE

   The following MARS messages are treated in a slightly different
   manner when MCSs have registered to support certain group addresses:

      1   MARS_REQUEST
      4   MARS_JOIN
      5   MARS_LEAVE

   A MARS must keep two sets of mappings for each layer 3 group using
   MCS support.  The original {layer 3 address, ATM.1, ATM.2, ... ATM.n}
   mapping (now termed the 'host map', although it includes routers) is
   augmented by a parallel {layer 3 address, server.1, server.2, ....
   server.K} mapping (the 'server map'). It is assumed that no ATM
   addresses appear in both the server and host maps for the same
   multicast group. Typically K will be 1, but it will be larger if
   multiple MCSs are configured to support a given group.




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   The MARS also maintains a point to multipoint VC out to any MCSs
   registered with it, called ServerControlVC (section 6.2.3). This
   serves an analogous role to ClusterControlVC, allowing the MARS to
   update the MCSs with group membership changes as they occur. A MARS
   MUST also send its regular MARS_REDIRECT_MAP transmissions on both
   ServerControlVC and ClusterControlVC.

6.2.1   Response to a MARS_REQUEST if MCS is registered.

   When the MARS receives a MARS_REQUEST for an address that has both
   host and server maps it generates a response based on the identity of
   the request's source. If the requestor is a member of the server map
   for the requested group then the MARS returns the contents of the
   host map in a sequence of one or more MARS_MULTIs.  Otherwise, if the
   source is a valid cluster member, the MARS returns the contents of
   the server map in a sequence of one or more MARS_MULTIs.  If the
   source is neither a cluster member, nor a member of the server map
   for the group, the request is dropped and ignored.

   Servers use the host map to establish a basic distribution VC for the
   group. Cluster members will establish outgoing multipoint VCs to
   members of the group's server map, without being aware that their
   packets will not be going directly to the multicast group's members.

6.2.2   MARS_MSERV and MARS_UNSERV messages.

   MARS_MSERV and MARS_UNSERV are identical to the MARS_JOIN message.
   An MCS uses a MARS_MSERV with a <min,max> pair of <X,X> to specify
   the multicast group X that it is willing to support. A single group
   MARS_UNSERV indicates the group that the MCS is no longer willing to
   support.  The operation code for MARS_MSERV is 3 (decimal), and
   MARS_UNSERV is 7 (decimal).

   Both of these messages are sent to the MARS over a point to point VC
   (between MCS and MARS). After processing, they are retransmitted on
   ServerControlVC to allow other MCSs to note the new node.

   When registering or deregistering support for specific groups the
   mar$flags.register flag MUST be zero. (This flag is only one when the
   MCS is registering as a member of ServerControlVC, as described in
   section 6.2.3.)

   When an MCS issues a MARS_MSERV for a specific group the message MUST
   be dropped and ignored if the source has not already registered with
   the MARS as a multicast server (section 6.2.3).  Otherwise, the MARS
   adds the new ATM address to the server map for the specified group,
   possibly constructing a new server map if this is the first MCS for
   the group.



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   If a MARS_MSERV represents the first MCS to register for a particular
   group, and there exists a non null host map serving that particular
   group, the MARS issues a MARS_MIGRATE (section 5.1.6) on
   ClusterControlVC. The MARS's own identity is placed in the source
   protocol and hardware address fields of the MARS_MIGRATE.  The ATM
   address of the MCS is placed as the first and only target ATM
   address. The address of the affected group is placed in the target
   multicast group address field.

   If a MARS_MSERV is not the first MCS to register for a particular
   group the MARS simply changes its operation code to MARS_JOIN, and
   sends a copy of the message on ClusterControlVC.  This fools the
   cluster members into thinking a new leaf node has been added to the
   group specified. In the retransmitted MARS_JOIN mar$flags.layer3grp
   MUST be zero, mar$flags.copy MUST be one, and mar$flags.register MUST
   be zero.

   When an MCS issues a MARS_UNSERV the MARS removes its ATM address
   from the server maps for each specified group, deleting any server
   maps that end up being null after the operation.

   The operation code is then changed to MARS_LEAVE and the MARS sends a
   copy of the message on ClusterControlVC. This fools the cluster
   members into thinking a leaf node has been dropped from the group
   specified. In the retransmitted MARS_LEAVE mar$flags.layer3grp MUST
   be zero, mar$flags.copy MUST be one, and mar$flags.register MUST be
   zero.

   The MARS retransmits redundant MARS_MSERV and MARS_UNSERV messages
   directly back to the MCS generating them. MARS_MIGRATE messages are
   never repeated in response to redundant MARS_MSERVs.

   The last or only MCS for a group MAY choose to issue a MARS_UNSERV
   while the group still has members. When the MARS_UNSERV is processed
   by the MARS the 'server map' will be deleted. When the associated
   MARS_LEAVE is issued on ClusterControlVC, all cluster members with a
   VC open to the MCS for that group will close down the VC (in
   accordance with section 5.1.4, since the MCS was their only leaf
   node). When cluster members subsequently find they need to transmit
   packets to the group, they will begin again with the
   MARS_REQUEST/MARS_MULTI sequence to establish a new VC. Since the
   MARS will have deleted the server map, this will result in the host
   map being returned, and the group reverts to being supported by a VC
   mesh.

   The reverse process is achieved through the MARS_MIGRATE message when
   the first MCS registers to support a group.  This ensures that
   cluster members explicitly dismantle any VC mesh they may have had



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   up, and re-establish their multicast forwarding path with the MCS as
   its termination point.

6.2.3  Registering a Multicast Server (MCS).

   Section 5.2.3 describes how endpoints register as cluster members,
   and hence get added as leaf nodes to ClusterControlVC. The same
   approach is used to register endpoints that intend to provide MCS
   support.

   Registration with the MARS occurs when an endpoint issues a
   MARS_MSERV with mar$flags.register set to one.  Upon registration the
   endpoint is added as a leaf node to ServerControlVC, and the
   MARS_MSERV is returned to the MCS privately.

   The MCS retransmits this MARS_MSERV until it confirms that the MARS
   has received it (by receiving a copy back, in an analogous way to the
   mechanism described in section 5.2.2 for reliably transmitting
   MARS_JOINs).

   The mar$cmi field in MARS_MSERVs MUST be set to zero by both MCS and
   MARS.

   An MCS may also choose to de-register, using a MARS_UNSERV with
   mar$flags.register set to one. When this occurs the MARS MUST remove
   all references to that MCS in all servermaps associated with the
   protocol (mar$pro) specified in the MARS_UNSERV, and drop the MCS
   from ServerControlVC.

   Note that multiple logical MCSs may share the same physical ATM
   interface, provided that each MCS uses a separate ATM address (e.g. a
   different SEL field in the NSAP format address). In fact, an MCS may
   share the ATM interface of a node that is also a cluster member
   (either host or router), provided each logical entity has a different
   ATM address.

   A MARS MUST be capable of handling a multi-entry servermap. However,
   the possible use of multiple MCSs registering to support the same
   group is a subject for further study. In the absence of an MCS
   synchronisation protocol a system administrator MUST NOT allow more
   than one logical MCS to register for a given group.

6.2.4   Modified response to MARS_JOIN and MARS_LEAVE.

   The existence of MCSs supporting some groups but not others requires
   the MARS to modify its distribution of single and block join/leave
   updates to cluster members. The MARS also adds two new messages -
   MARS_SJOIN and MARS_SLEAVE - for communicating group changes to MCSs



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   over ServerControlVC.

   The MARS_SJOIN and MARS_SLEAVE messages are identical to MARS_JOIN,
   with operation codes 18 and 19 (decimal) respectively.

   When a cluster member issues MARS_JOIN or MARS_LEAVE for a single
   group, the MARS checks to see if the group has an associated server
   map. If the specified group does not have a server map processing
   continues as described in section 6.1.2.

   However, if a server map exists for the group a new set of actions
   are taken.

      If the joining (leaving) node was not already (is no longer)
      considered a member of the specified group, a copy of the
      MARS_JOIN/LEAVE is made with type MARS_SJOIN or MARS_SLEAVE as
      appropriate, and transmitted on ServerControlVC.  This allows the
      MCS(s) supporting the group to note the new member and update
      their data VCs.

      The original message is transmitted back to the source cluster
      member unchanged, using the VC it arrived on rather than
      ClusterControlVC.  The mar$flags.punched field MUST be reset to 0
      in this message.

   (Section 5.2.2 requires cluster members have a mechanism to confirm
   the reception of their message by the MARS. For mesh supported
   groups, using ClusterControlVC serves dual purpose of providing this
   confirmation and distributing group update information. When a group
   is MCS supported, there is no reason for all cluster members to
   process null join/leave messages on ClusterControlVC, so they are
   sent back on the private VC between cluster member and MARS.)

   Receipt of a block MARS_JOIN (e.g. from a router coming on-line) or
   MARS_LEAVE requires a more complex response. The single <min,max>
   block may simultaneously cover mesh supported and MCS supported
   groups.  However, cluster members only need to be informed of the
   mesh supported groups that the endpoint has joined. Only the MCSs
   need to know if the endpoint is joining any MCS supported groups.

   The solution is to modify the MARS_JOIN or MARS_LEAVE that is
   retransmitted on ClusterControlVC. The following action is taken:

      A copy of the MARS_JOIN/LEAVE is made with type MARS_SJOIN or
      MARS_SLEAVE as appropriate, with its <min,max> block replaced with
      a 'hole punched' set of zero or more <min,max> pairs.  The 'hole
      punched' set of <min,max> pairs covers the entire address range
      specified by the original <min,max> pair, but excludes those



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      addresses/groups which the joining (leaving) node is already
      (still) a member of due to a previous single group join.

      Before transmission on the ClusterControlVC, the original
      MARS_JOIN/LEAVE then has its <min,max> block replaced with a 'hole
      punched' set of zero or more <min,max> pairs.  The 'hole punched'
      set of <min,max> pairs covers the entire address range specified
      by the original <min,max> pair, but excludes those
      addresses/groups supported by MCSs or which the joining (leaving)
      node is already (still) a member of due to a previous single group
      join.

      If no 'holes' were punched in the specified block, the original
      MARS_JOIN/LEAVE is re-transmitted out on ClusterControlVC
      unchanged.  Otherwise the following occurs:

         The original MARS_JOIN/LEAVE is transmitted back to the source
         cluster member unchanged, using the VC it arrived on. The
         mar$flags.punched field MUST be reset to 0 in this message.

         If the hole-punched set contains 1 or more <min,max> pair, a
         copy of the original MARS_JOIN/LEAVE is transmitted on
         ClusterControlVC, carrying the new <min,max> list. The
         mar$flags.punched field MUST be set to 1 in this message.

      The mar$flags.punched field is set to ensure the hole-punched copy
      is ignored by the message's source when trying to match received
      MARS_JOIN/LEAVE messages with ones previously sent (section
      5.2.2).

   (Appendix A discusses some algorithms for 'hole punching'.)

   It is assumed that MCSs use the MARS_SJOINs and MARS_SLEAVEs to
   update their own VCs out to the actual group's members.

   mar$flags.layer3grp is copied over into the messages transmitted by
   the MARS. mar$flags.copy MUST be set to one.

6.2.5  Sequence numbers for ServerControlVC traffic.

   In an analogous fashion to the Cluster Sequence Number, the MARS
   keeps a Server Sequence Number (SSN) that is incremented after every
   transmission on ServerControlVC. The current value of the SSN is
   inserted into the mar$msn field of every message the MARS issues that
   it believes is destined for an MCS. This includes MARS_MULTIs that
   are being returned in response to a MARS_REQUEST from an MCS, and
   MARS_REDIRECT_MAP being sent on ServerControlVC.  The MARS must check
   the MARS_REQUESTs source, and if it is a registered MCS the SSN is



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   copied into the mar$msn field, otherwise the CSN is copied into the
   mar$msn field.

   MCSs are expected to track and use the SSNs in an analogous manner to
   the way endpoints use the CSN in section 5.1 (to trigger revalidation
   of group membership information).

   A MARS should be carefully designed to minimise the possibility of
   the SSN jumping unnecessarily. Under normal operation only MCSs that
   are affected by transient link problems will miss mar$msn updates and
   be forced to revalidate. If the MARS itself glitches it will be
   innundated with requests for a period as every MCS attempts to
   revalidate.

6.3 Why global sequence numbers?

   The CSN and SSN are global within the context of a given protocol
   (e.g. IPv4, mar$pro = 0x800).  They count ClusterControlVC and
   ServerControlVC activity without reference to the multicast group(s)
   involved.  This may be perceived as a limitation, because there is no
   way for cluster members or multicast servers to isolate exactly which
   multicast group they may have missed an update for. An alternative
   was to try and provide a per-group sequence number.

   Unfortunately per-group sequence numbers are not practical. The
   current mechanism allows sequence information to be piggy-backed onto
   MARS messages already in transit for other reasons. The ability to
   specify blocks of multicast addresses with a single MARS_JOIN or
   MARS_LEAVE means that a single message can refer to membership change
   for multiple groups simultaneously. A single mar$msn field cannot
   provide meaningful information about each group's sequence.  Multiple
   mar$msn fields would have been unwieldy.

   Any MARS or cluster member that supports different protocols MUST
   keep separate mapping tables and sequence numbers for each protocol.

6.4 Redundant/Backup MARS Architectures.

   If backup MARSs exist for a given cluster then mechanisms are needed
   to ensure consistency between their mapping tables and those of the
   active, current MARS.

   (Cluster members will consider backup MARSs to exist if they have
   been configured with a table of MARS addresses, or the regular
   MARS_REDIRECT_MAP messages contain a list of 2 or more addresses.)

   The definition of an MARS-synchronization protocol is beyond the
   current scope of this document, and is expected to be the subject of



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   further research work.  However, the following observations may be
   made:

      MARS_REDIRECT_MAP messages exist, enabling one MARS to force
      endpoints to move to another MARS (e.g. in the aftermath of a MARS
      failure, the chosen backup MARS will eventually wish to hand
      control of the cluster over to the main MARS when it is
      functioning properly again).

      Cluster members and MCSs do not need to start up with knowledge of
      more than one MARS, provided that MARS correctly issues
      MARS_REDIRECT_MAP messages with the full list of MARSs for that
      cluster.

   Any mechanism for synchronising backup MARSs (and coping with the
   aftermath of MARS failures) should be compatible with the cluster
   member behaviour described in this document.

7.   How an MCS utilises a MARS.

   When an MCS supports a multicast group it acts as a proxy cluster
   endpoint for the senders to the group. It also behaves in an
   analogous manner to a sender, managing a single outgoing point to
   multipoint VC to the real group members.

   Detailed description of possible MCS architectures are beyond the
   scope of this document. This section will outline the main issues.

7.1   Association with a particular Layer 3 group.

   When an MCS issues a MARS_MSERV it forces all senders to the
   specified layer 3 group to terminate their VCs on the supplied source
   ATM address.

   The simplest MCS architecture involves taking incoming AAL_SDUs and
   simply flipping them back out a single point to multipoint VC. Such
   an MCS cannot support more than one group at once, as it has no way
   to differentiate between traffic destined for different groups.
   Using this architecture, a physical node would provide MCS support
   for multiple groups by creating multiple logical instances of the
   MCS, each with different ATM Addresses (e.g. a different SEL value in
   the node's NSAPA).

   A slightly more complex approach would be to add minimal layer 3
   specific processing into the MCS. This would look inside the received
   AAL_SDUs and determine which layer 3 group they are destined for. A
   single instance of such an MCS might register its ATM Address with
   the MARS for multiple layer 3 groups, and manage multiple independent



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   outgoing point to multipoint VCs (one for each group).

   When an MCS starts up it MUST register with the MARS as described in
   section 6.2.3, identifying the protocol it supports with the mar$pro
   field of the MARS_MSERV. This also applies to logical MCSs, even if
   they share the same physical ATM interface. This is important so that
   the MARS can react to the loss of an MCS when it drops off
   ServerControlVC. (One consequence is that 'simple' MCS architectures
   end up with one ServerControlVC member per group.  MCSs with layer 3
   specific processing may support multiple groups while still only
   registering as one member of ServerControlVC.)

   An MCS MUST NOT share the same ATM address as a cluster member,
   although it may share the same physical ATM interface.

7.2   Termination of incoming VCs.

   An MCS MUST terminate unidirectional VCs in the same manner as a
   cluster member.  (e.g. terminate on an LLC entity when LLC/SNAP
   encapsulation is used, as described in RFC 1755 for unicast
   endpoints.)

7.3   Management of outgoing VC.

   An MCS MUST establish and manage its outgoing point to multipoint VC
   as a cluster member does (section 5.1).

   MARS_REQUEST is used by the MCS to establish the initial leaf nodes
   for the MCS's outgoing point to multipoint VC. After the VC is
   established, the MCS reacts to MARS_SJOINs and MARS_SLEAVEs in the
   same way a cluster member reacts to MARS_JOINs and MARS_LEAVEs.

   The MCS tracks the Server Sequence Number from the mar$msn fields of
   messages from the MARS, and revalidates its outgoing point to
   multipoint VC(s) when a sequence number jump occurs.

7.4   Use of a backup MARS.

   The MCS uses the same approach to backup MARSs as a cluster member
   (section 5.4), tracking MARS_REDIRECT_MAP messages on
   ServerControlVC.

8.   Support for IP multicast routers.

   Multicast routers are required for the propagation of multicast
   traffic beyond the constraints of a single cluster (inter-cluster
   traffic).  (In a sense, they are multicast servers acting at the next
   higher layer, with clusters, rather than individual endpoints, as



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   their abstract sources and destinations.)

   Multicast routers typically participate in higher layer multicast
   routing algorithms and policies that are beyond the scope of this
   memo (e.g. DVMRP [5] in the IPv4 environment).

   It is assumed that the multicast routers will be implemented over the
   same sort of IP/ATM interface that a multicast host would use.  Their
   IP/ATM interfaces will register with the MARS as cluster members,
   joining and leaving multicast groups as necessary. As noted in
   section 5, multiple logical 'endpoints' may be implemented over a
   single physical ATM interface. Routers use this approach to provide
   interfaces into each of the clusters they will be routing between.

   The rest of this section will assume a simple IPv4 scenario where the
   scope of a cluster has been limited to a particular LIS that is part
   of an overlaid IP network. Not all members of the LIS are necessarily
   registered cluster members (you may have unicast-only hosts in the
   LIS).

8.1    Forwarding into a Cluster.

   If the multicast router needs to transmit a packet to a group within
   the cluster its IP/ATM interface opens a VC in the same manner as a
   normal host would. Once a VC is open, the router watches for
   MARS_JOIN and MARS_LEAVE messages and responds to them as a normal
   host would.

   The multicast router's transmit side MUST implement inactivity timers
   to shut down idle outgoing VCs, as for normal hosts.

   As with normal host, the multicast router does not need to be a
   member of a group it is sending to.

8.2    Joining in 'promiscuous' mode.

   Once registered and initialised, the simplest model of IPv4 multicast
   router operation is for it to issue a MARS_JOIN encompassing the
   entire Class D address space.  In effect it becomes 'promiscuous', as
   it will be a leaf node to all present and future multipoint VCs
   established to IPv4 groups on the cluster.

   How a router chooses which groups to propagate outside the cluster is
   beyond the scope of this document.

   Consistent with RFC 1112, IP multicast routers may retain the use of
   IGMP Query and IGMP Report messages to ascertain group membership.
   However, certain optimisations are possible, and are described in



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   section 8.5.

8.3    Forwarding across the cluster.

   Under some circumstances the cluster may simply be another hop
   between IP subnets that have participants in a multicast group.

      [LAN.1] ----- IPmcR.1 -- [cluster/LIS] -- IPmcR.2 ----- [LAN.2]

   LAN.1 and LAN.2 are subnets (such as Ethernet) with attached hosts
   that are members of group X.

   IPmcR.1 and IPmcR.2 are multicast routers with interfaces to the LIS.

   A traditional solution would be to treat the LIS as a unicast subnet,
   and use tunneling routers. However, this would not allow hosts on the
   LIS to participate in the cross-LIS traffic.

   Assume IPmcR.1 is receiving packets promiscuously on its LAN.1
   interface. Assume further it is configured to propagate multicast
   traffic to all attached interfaces. In this case that means the LIS.

   When a packet for group X arrives on its LAN.1 interface, IPmcR.1
   simply sends the packet to group X on the LIS interface as a normal
   host would (Issuing MARS_REQUEST for group X, creating the VC,
   sending the packet).

   Assuming IPmcR.2 initialised itself with the MARS as a member of the
   entire Class D space, it will have been returned as a member of X
   even if no other nodes on the LIS were members. All packets for group
   X received on IPmcR.2's LIS interface may be retransmitted on LAN.2.

   If IPmcR.1 is similarly initialised the reverse process will apply
   for multicast traffic from LAN.2 to LAN.1, for any multicast group.
   The benefit of this scenario is that cluster members within the LIS
   may also join and leave group X at anytime.

8.4   Joining in 'semi-promiscuous' mode.

   Both unicast and multicast IP routers have a common problem -
   limitations on the number of AAL contexts available at their ATM
   interfaces.  Being 'promiscuous' in the RFC 1112 sense means that for
   every M hosts sending to N groups, a multicast router's ATM interface
   will have M*N incoming reassembly engines tied up.

   It is not hard to envisage situations where a number of multicast
   groups are active within the LIS but are not required to be
   propagated beyond the LIS itself. An example might be a distributed



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   simulation system specifically designed to use the high speed IP/ATM
   environment. There may be no practical way its traffic could be
   utilised on 'the other side' of the multicast router, yet under the
   conventional scheme the router would have to be a leaf to each
   participating host anyway.

   As this problem occurs below the IP layer, it is worth noting that
   'scoping' mechanisms at the IP multicast routing level do not provide
   a solution. An IP level scope would still result in the router's ATM
   interface receiving traffic on the scoped groups, only to drop it.

   In this situation the network administrator might configure their
   multicast routers to exclude sections of the Class D address space
   when issuing MARS_JOIN(s). Multicast groups that will never be
   propagated beyond the cluster will not have the router listed as a
   member, and the router will never have to receive (and simply ignore)
   traffic from those groups.

   Another scenario involves the product M*N exceeding the capacity of a
   single router's interface (especially if the same interface must also
   support a unicast IP router service).

   A network administrator may choose to add a second node, to function
   as a parallel IP multicast router. Each router would be configured to
   be 'promiscuous' over separate parts of the Class D address space,
   thus exposing themselves to only part of the VC load. This sharing
   would be completely transparent to IP hosts within the LIS.

   Restricted promiscuous mode does not break RFC 1112's use of IGMP
   Report messages. If the router is configured to serve a given block
   of Class D addresses, it will receive the IGMP Report.  If the router
   is not configured to support a given block, then the existence of an
   IGMP Report for a group in that block is irrelevant to the router.
   All routers are able to track membership changes through the
   MARS_JOIN and MARS_LEAVE traffic anyway. (Section 8.5 discusses a
   better alternative to IGMP within a cluster.)

   Mechanisms and reasons for establishing these modes of operation are
   beyond the scope of this document.

8.5   An alternative to IGMP Queries.

   An unfortunate aspect of IGMP is that it assumes multicasting of IP
   packets is a cheap and trivial event at the link layer. As a
   consequence, regular IGMP Queries are multicasted by routers to group
   224.0.0.1. These queries are intended to trigger IGMP Replies by
   cluster members that have layer 3 members of particular groups.




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   The MARS_GROUPLIST_REQUEST and MARS_GROUPLIST_REPLY messages were
   designed to allow routers to avoid actually transmitting IGMP Queries
   out into a cluster.

   Whenever the router's forwarding engine wishes to transmit an IGMP
   query, a MARS_GROUPLIST_REQUEST can be sent to the MARS instead. The
   resulting MARS_GROUPLIST_REPLY(s) (described in section 5.3) from the
   MARS carry all the information that the router would have ascertained
   from IGMP replies.

   It is RECOMMENDED that multicast routers utilise this MARS service to
   minimise IGMP traffic within the cluster.

   By default a MARS_GROUPLIST_REQUEST SHOULD specify the entire address
   space (e.g. <224.0.0.0, 239.255.255.255> in an IPv4 environment).
   However, routers serving part of the address space (as described in
   section 8.4) MAY choose to issue MARS_GROUPLIST_REQUESTs that specify
   only the subset of the address space they are serving.

   (On the surface it would also seem useful for multicast routers to
   track MARS_JOINs and MARS_LEAVEs that arrive with mar$flags.layer3grp
   set. These might be used in lieu of IGMP Reports, to provide the
   router with timely indication that a new layer 3 group member exists
   within the cluster. However, this only works on VC mesh supported
   groups, and is therefore NOT recommended).

   Appendix B discusses less elegant mechanisms for reducing the impact
   of IGMP traffic within a cluster, on the assumption that the IP/ATM
   interfaces to the cluster are being used by un-optimised IP
   multicasting code.

8.6   CMIs across multiple interfaces.

   The Cluster Member ID is only unique within the Cluster managed by a
   given MARS. On the surface this might appear to leave us with a
   problem when a multicast router is routing between two or more
   Clusters using a single physical ATM interface.  The router will
   register with two or more MARSs, and thereby acquire two or more
   independent CMI's. Given that each MARS has no reason to synchronise
   their CMI allocations, it is possible for a host in one cluster to
   have the same CMI has the router's interface to another Cluster. How
   does the router distinguish between its own reflected packets, and
   packets from that other host?

   The answer lies in the fact that routers (and hosts) actually
   implement logical IP/ATM interfaces over a single physical ATM
   interface. Each logical interface will have a unique ATM Address (eg.
   an NSAP with different SELector fields, one for each logical



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   interface).

   Each logical IP/ATM interface is configured with the address of a
   single MARS, attaches to only one cluster, and so has only one CMI to
   worry about. Each of the MARSs that the router is registered with
   will have been given a different ATM Address (corresponding to the
   different logical IP/ATM interfaces) in each registration MARS_JOIN.

   When hosts in a cluster add the router as a leaf node, they'll
   specify the ATM Address of the appropriate logical IP/ATM interface
   on the router in the L_MULTI_ADD message. Thus, each logical IP/ATM
   interface will only have to check and filter on CMIs assigned by its
   own MARS.

   In essence the cluster differentiation is achieved by ensuring that
   logical IP/ATM interfaces are assigned different ATM Addresses.

9.    Multiprotocol applications of the MARS and MARS clients.

   A deliberate attempt has been made to describe the MARS and
   associated mechanisms in a manner independent of a specific higher
   layer protocol being run over the ATM cloud. The immediate
   application of this document will be in an IPv4 environment, and this
   is reflected by the focus of key examples.  However, the mar$pro.type
   and mar$pro.snap fields in every MARS control message allow any
   higher layer protocol that has a 'short form' or 'long form' of
   protocol identification (section 4.3) to be supported by a MARS.

   Every MARS MUST implement entirely separate logical mapping tables
   and support. Every cluster member must interpret messages from the
   MARS in the context of the protocol type that the MARS message refers
   to.

   Every MARS and MARS client MUST treat Cluster Member IDs in the
   context of the protocol type carried in the MARS message or data
   packet containing the CMI.

   For example, IPv6 has been allocated an Ethertype of 0x86DD.  This
   means the 'short form' of protocol identification must be used in the
   MARS control messages and the data path encapsulation (section 5.5).
   An IPv6 multicasting client sets the mar$pro.type field of every MARS
   message to 0x86DD.  When carrying IPv6 addresses the mar$spln and
   mar$tpln fields are either 0 (for null or non-existent information)
   or 16 (for the full IPv6 address).

   Following the rules in section 5.5, an IPv6 data packet is
   encapsulated as:




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      [0xAA-AA-03][0x00-00-5E][0x00-01][pkt$cmi][0x86DD][IPv6 packet]

   A host or endpoint interface that is using the same MARS to support
   multicasting needs of multiple protocols MUST not assume their CMI
   will be the same for each protocol.

10.    Supplementary parameter processing.

   The mar$extoff field in the [Fixed header] indicates whether
   supplementary parameters are being carried by a MARS control message.
   This mechanism is intended to enable the addition of new
   functionality to the MARS protocol in later documents.

   Supplementary parameters are conveyed as a list of TLV (type, length,
   value) encoded information elements.  The TLV(s) begin on the first
   32 bit boundary following the [Addresses] field in the MARS control
   message (e.g. after mar$tsa.N in a MARS_MULTI, after mar$max.N in a
   MARS_JOIN, etc).

10.1   Interpreting the mar$extoff field.

   If the mar$extoff field is non-zero it indicates that a list of one
   or more TLVs have been appended to the MARS message.  The first TLV
   is found by treating mar$extoff as an unsigned integer representing
   an offset (in octets) from the beginning of the MARS message (the MSB
   of the mar$afn field).

   As TLVs are 32 bit aligned the bottom 2 bits of mar$extoff are also
   reserved. A receiver MUST mask off these two bits before calculating
   the octet offset to the TLV list.  A sender MUST set these two bits
   to zero.

   If mar$extoff is zero no TLVs have been appended.

10.2   The format of TLVs.

   When they exist, TLVs begin on 32 bit boundaries, are multiples of 32
   bits in length, and form a sequential list terminated by a NULL TLV.

   The TLV structure is:

      [Type - 2 octets][Length - 2 octets][Value - n*4 octets]

   The Type subfield indicates how the contents of the Value subfield
   are to be interpreted.

   The Length subfield indicates the number of VALID octets in the Value
   subfield. Valid octets in the Value subfield start immediately after



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   the Length subfield.  The offset (in octets) from the start of this
   TLV to the start of the next TLV in the list is given by the
   following formula:

      offset = (length + 4 + ((4-(length & 3)) % 4))

   (where % is the modulus operator)

   The Value subfield is padded with 0, 1, 2, or 3 octets to ensure the
   next TLV is 32 bit aligned. The padded locations MUST be set to zero.

   (For example, a TLV that needed only 5 valid octets of information
   would be 12 octets long. The Length subfield would hold the value 5,
   and the Value subfield would be padded out to 8 bytes.  The 5 valid
   octets of information begin at the first octet of the Value
   subfield.)

   The Type subfield is formatted in the following way:

          |   1st octet   |   2nd octet   |
           7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | x |               y           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The most significant 2 bits (Type.x) determine how a recipient should
   behave when it doesn't recognise the TLV type indicated by the lower
   14 bits (Type.y). The required behaviours are:

      Type.x = 0   Skip the TLV, continue processing the list.
      Type.x = 1   Stop processing, silently drop the MARS message.
      Type.x = 2   Stop processing, drop message, give error indication.
      Type.x = 3   Reserved. (currently treat as x = 0)

   (The error indication generated when Type.x = 2 SHOULD be logged in
   some locally significant fashion. Consequential MARS message activity
   in response to such an error condition will be defined in future
   documents.)

   The TLV type space (Type.y) is further subdivided to encourage use
   outside the IETF.

      0                       Null TLV.
      0x0001 - 0x0FFF         Reserved for the IETF.
      0x1000 - 0x11FF         Allocated to the ATM Forum.
      0x1200 - 0x37FF         Reserved for the IETF.
      0x3800 - 0x3FFF         Experimental use.




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10.3   Processing MARS messages with TLVs.

   Supplementary parameters act as modifiers to the basic behaviour
   specified by the mar$op field of any given MARS message.

   If a MARS message arrives with a non-zero mar$extoff field its TLV
   list MUST be parsed before handling the MARS message in accordance
   with the mar$op value. Unrecognised TLVs MUST be handled as required
   by their Type.x value.

   How TLVs modify basic MARS operations will be mar$op and TLV
   specific.

10.4   Initial set of TLV elements.

   Conformance with this document only REQUIRES the recognition of one
   TLV, the Null TLV. This terminates a list of TLVs, and MUST be
   present if mar$extoff is non-zero in a MARS message. It MAY be the
   only TLV present.

   The Null TLV is coded as:

      [0x00-00][0x00-00]

   Future documents will describe the formats, contents, and
   interpretations of additional TLVs. The minimal parsing requirements
   imposed by this document are intended to allow conformant MARS and
   MARS client implementations to deal gracefully and predictably with
   future TLV developments.

11.    Key Decisions and open issues.

   The key decisions this document proposes:

      A Multicast Address Resolution Server (MARS) is proposed to co-
      ordinate and distribute mappings of ATM endpoint addresses to
      arbitrary higher layer 'multicast group addresses'. The specific
      case of IPv4 multicast is used as the example.

      The concept of 'clusters' is introduced to define the scope of a
      MARS's responsibility, and the set of ATM endpoints willing to
      participate in link level multicasting.

      A MARS is described with the functionality required to support
      intra-cluster multicasting using either VC meshes or ATM level
      multicast servers (MCSs).





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      LLC/SNAP encapsulation of MARS control messages allows MARS and
      ATMARP traffic to share VCs, and allows partially co-resident MARS
      and ATMARP entities.

      New message types:

         MARS_JOIN, MARS_LEAVE, MARS_REQUEST. Allow endpoints to join,
         leave, and request the current membership list of multicast
         groups.

         MARS_MULTI. Allows multiple ATM addresses to be returned by the
         MARS in response to a MARS_REQUEST.

         MARS_MSERV, MARS_UNSERV. Allow multicast servers to register
         and deregister themselves with the MARS.

         MARS_SJOIN, MARS_SLEAVE. Allow MARS to pass on group membership
         changes to multicast servers.

         MARS_GROUPLIST_REQUEST, MARS_GROUPLIST_REPLY.  Allow MARS to
         indicate which groups have actual layer 3 members. May be used
         to support IGMP in IPv4 environments, and similar functions in
         other environments.

         MARS_REDIRECT_MAP.  Allow MARS to specify a set of backup MARS
         addresses.

         MARS_MIGRATE.  Allows MARS to force cluster members to shift
         from VC mesh to MCS based forwarding tree in single operation.

      'wild card' MARS mapping table entries are possible, where a
      single ATM address is simultaneously associated with blocks of
      multicast group addresses.

   For the MARS protocol mar$op.version = 0. The complete set of MARS
   control messages and mar$op.type values is:

      1   MARS_REQUEST
      2   MARS_MULTI
      3   MARS_MSERV
      4   MARS_JOIN
      5   MARS_LEAVE
      6   MARS_NAK
      7   MARS_UNSERV
      8   MARS_SJOIN
      9   MARS_SLEAVE
      10  MARS_GROUPLIST_REQUEST
      11  MARS_GROUPLIST_REPLY



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      12  MARS_REDIRECT_MAP
      13  MARS_MIGRATE

   A number of issues are left open at this stage, and are likely to be
   the subject of on-going research and additional documents that build
   upon this one.

      The specified endpoint behaviour allows the use of
      redundant/backup MARSs within a cluster. However, no
      specifications yet exist on how these MARSs co-ordinate amongst
      themselves. (The default is to only have one MARS per cluster.)

      The specified endpoint behaviour and MARS service allows the use
      of multiple MCSs per group.  However, no specifications yet exist
      on how this may be used, or how these MCSs co-ordinate amongst
      themselves.  Until futher work is done on MCS co-ordination
      protocols the default is to only have one MCS per group.

      The MARS relies on the cluster member dropping off
      ClusterControlVC if the cluster member dies. It is not clear if
      additional mechanisms are needed to detect and delete 'dead'
      cluster members.

      Supporting layer 3 'broadcast' as a special case of multicasting
      (where the 'group' encompasses all cluster members) has not been
      explicitly discussed.

      Supporting layer 3 'unicast' as a special case of multicasting
      (where the 'group' is a single cluster member, identified by the
      cluster member's unicast protocol address) has not been explicitly
      discussed.

      The future development of ATM Group Addresses and Leaf Initiated
      Join to ATM Forum's UNI specification has not been addressed.
      (However, the problems identified in this document with respect to
      VC scarcity and impact on AAL contexts will not be fixed by such
      developments in the signalling protocol.)

      Possible modifications to the interpretation of the mar$hrdrsv and
      mar$afn fields in the Fixed header, based on different values for
      mar$op.version, are for further study.










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Security Considerations

   Security issues are not addressed in this document.

Acknowledgments

   The discussions within the IP over ATM Working Group have helped
   clarify the ideas expressed in this document. John Moy (Cascade
   Communications Corp.) initially suggested the idea of wild-card
   entries in the ARP Server.  Drew Perkins (Fore Systems) provided
   rigorous and useful critique of early proposed mechanisms for
   distributing and validating group membership information.  Susan
   Symington (and co-workers at MITRE Corp., Don Chirieleison, and Bill
   Barns) clearly articulated the need for multicast server support,
   proposed a solution, and challenged earlier block join/leave
   mechanisms. John Shirron (Fore Systems) provided useful improvements
   on my original revalidation procedures.

   Susan Symington and Bryan Gleeson (Adaptec) independently championed
   the need for the service provided by MARS_GROUPLIST_REQUEST/REPLY.
   The new encapsulation scheme arose from WG discussions, captured by
   Bryan Gleeson in an interim Work in Progress (with Keith McCloghrie
   (Cisco), Andy Malis (Ascom Nexion), and Andrew Smith (Bay Networks)
   as key contributors).  James Watt (Newbridge) and Joel Halpern
   (Newbridge) motivated the development of a more multiprotocol MARS
   control message format, evolving it away from its original ATMARP
   roots.  They also motivated the development of Type #1 and Type #2
   data path encapsulations.  Rajesh Talpade (Georgia Tech) helped
   clarify the need for the MARS_MIGRATE function.

   Maryann Maher (ISI) provided valuable sanity and implementation
   checking during the latter stages of the document's development.
   Finally, Jim Rubas (IBM) supplied the MARS pseudo-code in Appendix F
   and also provided detailed proof-reading in the latter stages of the
   document's development.

Author's Address

   Grenville Armitage
   Bellcore, 445 South Street
   Morristown, NJ, 07960
   USA

   EMail: gja@thumper.bellcore.com
   Phone: +1 201 829 2635






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References

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

   [2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaption
   Layer 5", RFC 1483, Telecom Finland, July 1993.

   [3] Laubach, M., "Classical IP and ARP over ATM", RFC 1577, Hewlett-
   Packard Laboratories, December 1993.

   [4] ATM Forum, "ATM User Network Interface (UNI) Specification
   Version 3.1", ISBN 0-13-393828-X, Prentice Hall, Englewood Cliffs,
   NJ, June 1995.

   [5] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
   Multicast Routing Protocol", RFC 1075, November 1988.

   [6] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E., and
   A.  Malis, "ATM Signaling Support for IP over ATM", RFC 1755,
   February 1995.

   [7] Borden, M., Crawley, E., Davie, B., and S. Batsell, "Integration
   of Real-time Services in an IP-ATM Network Architecture.", RFC 1821,
   August 1995.

   [8] ATM Forum, "ATM User-Network Interface Specification Version
   3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993.























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Appendix A.  Hole punching algorithms.

   Implementations are entirely free to comply with the body of this
   memo in any way they see fit. This appendix is purely for
   clarification.

   A MARS implementation might pre-construct a set of <min,max> pairs
   (P) that reflects the entire Class D space, excluding any addresses
   currently supported by multicast servers. The <min> field of the
   first pair MUST be 224.0.0.0, and the <max> field of the last pair
   must be 239.255.255.255. The first and last pair may be the same.
   This set is updated whenever a multicast server registers or
   deregisters.

   When the MARS must perform 'hole punching' it might consider the
   following algorithm:

      Assume the MARS_JOIN/LEAVE received by the MARS from the cluster
      member specified the block <Emin, Emax>.

      Assume Pmin(N) and Pmax(N) are the <min> and <max> fields from the
      Nth pair in the MARS's current set P.

      Assume set P has K pairs. Pmin(1) MUST equal 224.0.0.0, and
      Pmax(M) MUST equal 239.255.255.255. (If K == 1 then no hole
      punching is required).

      Execute pseudo-code:

         create copy of set P, call it set C.

         index1 = 1;
         while (Pmax(index1) <= Emin)
            index1++;

         index2 = K;
         while (Pmin(index2) >= Emax)
            index2--;

         if (index1 > index2)
            Exit, as the hole-punched set is null.

         if (Pmin(index1) < Emin)
            Cmin(index1) = Emin;

         if (Pmax(index2) > Emax)
            Cmax(index2) = Emax;




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         Set C is the required 'hole punched' set of address blocks.

   The resulting set C retains all the MARS's pre-constructed 'holes'
   covering the multicast servers, but will have been pruned to cover
   the section of the Class D space specified by the originating host's
   <Emin,Emax> values.

   The host end should keep a table, H, of open VCs in ascending order
   of Class D address.

      Assume H(x).addr is the Class address associated with VC.x.
      Assume H(x).addr < H(x+1).addr.

   The pseudo code for updating VCs based on an incoming JOIN/LEAVE
   might be:

      x = 1;
      N = 1;

      while (x < no.of VCs open)
      {
            while (H(x).addr > max(N))
            {
                  N++;
                  if (N > no. of pairs in JOIN/LEAVE)
                        return(0);
            }

            if ((H(x).addr <= max(N) &&
                        ((H(x).addr >= min(N))
                              perform_VC_update();
            x++;
      }


















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Appendix B.  Minimising the impact of IGMP in IPv4 environments.

   Implementing any part of this appendix is not required for
   conformance with this document.  It is provided solely to document
   issues that have been identified.

   The intent of section 5.1 is for cluster members to only have
   outgoing point to multipoint VCs when they are actually sending data
   to a particular multicast group. However, in most IPv4 environments
   the multicast routers attached to a cluster will periodically issue
   IGMP Queries to ascertain if particular groups have members.  The
   current IGMP specification attempts to avoid having every group
   member respond by insisting that each group member wait a random
   period, and responding if no other member has responded before them.
   The IGMP reply is sent to the multicast address of the group being
   queried.

   Unfortunately, as it stands the IGMP algorithm will be a nuisance for
   cluster members that are essentially passive receivers within a given
   multicast group. It is just as likely that a passive member, with no
   outgoing VC already established to the group, will decide to send an
   IGMP reply - causing a VC to be established where there was no need
   for one. This is not a fatal problem for small clusters, but will
   seriously impact on the ability of a cluster to scale.

   The most obvious solution is for routers to use the
   MARS_GROUPLIST_REQUEST and MARS_GROUPLIST_REPLY messages, as
   described in section 8.5. This would remove the regular IGMP Queries,
   resulting in cluster members only sending an IGMP Report when they
   first join a group.

   Alternative solutions do exist. One would be to modify the IGMP reply
   algorithm, for example:

      If the group member has VC open to the group proceed as per RFC
      1112 (picking a random reply delay between 0 and 10 seconds).

      If the group member does not have VC already open to the group,
      pick random reply delay between 10 and 20 seconds instead, and
      then proceed as per RFC 1112.

   If even one group member is sending to the group at the time the IGMP
   Query is issued then all the passive receivers will find the IGMP
   Reply has been transmitted before their delay expires, so no new VC
   is required. If all group members are passive at the time of the IGMP
   Query then a response will eventually arrive, but 10 seconds later
   than under conventional circumstances.




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   The preceding solution requires re-writing existing IGMP code, and
   implies the ability of the IGMP entity to ascertain the status of VCs
   on the underlying ATM interface. This is not likely to be available
   in the short term.

   One short term solution is to provide something like the preceding
   functionality with a 'hack' at the IP/ATM driver level within cluster
   members. Arrange for the IP/ATM driver to snoop inside IP packets
   looking for IGMP traffic. If an IGMP packet is accepted for
   transmission, the IP/ATM driver can buffer it locally if there is no
   VC already active to that group. A 10 second timer is started, and if
   an IGMP Reply for that group is received from elsewhere on the
   cluster the timer is reset. If the timer expires, the IP/ATM driver
   then establishes a VC to the group as it would for a normal IP
   multicast packet.

   Some network implementors may find it advantageous to configure a
   multicast server to support the group 224.0.0.1, rather than rely on
   a mesh. Given that IP multicast routers regularly send IGMP queries
   to this address, a mesh will mean that each router will permanently
   consume an AAL context within each cluster member. In clusters served
   by multiple routers the VC load within switches in the underlying ATM
   network will become a scaling problem.

   Finally, if a multicast server is used to support 224.0.0.1, another
   ATM driver level hack becomes a possible solution to IGMP Reply
   traffic.  The ATM driver may choose to grab all outgoing IGMP packets
   and send them out on the VC established for sending to 224.0.0.1,
   regardless of the Class D address the IGMP message was actually for.
   Given that all hosts and routers must be members of 224.0.0.1, the
   intended recipients will still receive the IGMP Replies. The negative
   impact is that all cluster members will receive the IGMP Replies.



















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Appendix C.   Further comments on 'Clusters'.

   The cluster concept was introduced in section 1 for two reasons.  The
   more well known term of Logical IP Subnet is both very IP specific,
   and constrained to unicast routing boundaries. As the architecture
   described in this document may be re-used in non-IP environments a
   more neutral term was needed. As the needs of multicasting are not
   always bound by the same scopes as unicasting, it was not immediately
   obvious that apriori limiting ourselves to LISs was beneficial in the
   long term.

   It must be stressed that Clusters are purely an administrative being.
   You choose their size (i.e. the number of endpoints that register
   with the same MARS) based on your multicasting needs, and the
   resource consumption you are willing to put up with. The larger the
   number of ATM attached hosts you require multicast support for, the
   more individual clusters you might choose to establish (along with
   multicast routers to provide inter-cluster traffic paths).

   Given that not all the hosts in any given LIS may require multicast
   support, it becomes conceivable that you might assign a single MARS
   to support hosts from across multiple LISs. In effect you have a
   cluster covering multiple LISs, and have achieved 'cut through'
   routing for multicast traffic. Under these circumstances increasing
   the geographical size of a cluster might be considered a good thing.

   However, practical considerations limit the size of clusters.  Having
   a cluster span multiple LISs may not always be a particular 'win'
   situation.  As the number of multicast capable hosts in your LISs
   increases it becomes more likely that you'll want to constrain a
   cluster's size and force multicast traffic to aggregate at multicast
   routers scattered across your ATM cloud.

   Finally, multi-LIS clusters require a degree of care when deploying
   IP multicast routers. Under the Classical IP model you need unicast
   routers on the edges of LISs. Under the MARS architecture you only
   need multicast routers at the edges of clusters. If your cluster
   spans multiple LISs, then the multicast routers will perceive
   themselves to have a single interface that is simultaneously attached
   to multiple unicast subnets. Whether this situation will work depends
   on the inter-domain multicast routing protocols you use, and your
   multicast router's ability to understand the new relationship between
   unicast and multicast topologies.

   In the absence of futher research in this area, networks deployed in
   conformance to this document MUST make their IP cluster and IP LIS
   coincide, so as to avoid these complications.




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Appendix D.  TLV list parsing algorithm.

   The following pseudo-code represents how the TLV list format
   described in section 10 could be handled by a MARS or MARS client.

      list = (mar$extoff & 0xFFFC);

      if (list == 0) exit;

      list = list + message_base;

      while (list->Type.y != 0)
            {
                  switch (list->Type.y)
                  {
                        default:
                          {
                           if (list->Type.x == 0) break;

                           if (list->Type.x == 1) exit;

                           if (list->Type.x == 2) log-error-and-exit;
                          }

                        [...other handling goes here..]

                  }

                  list += (list->Length + 4 + ((4-(list->Length & 3)) %
                  4));

            }

      return;

















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Appendix E.  Summary of timer values.

   This appendix summarises various timers or limits mentioned in the
   main body of the document. Values are specified in the following
   format:  [x, y, z] indicating a minimum value of x, a recommended
   value of y, and a maximum value of z. A '-' will indicate that a
   category has no value specified. Values in minutes are followed by
   'min', values in seconds are followed by 'sec'.

      Idle time for MARS - MARS client pt to pt VC:
                                        [1 min, 20 min, -]

      Idle time for multipoint VCs from client.
                                        [1 min, 20 min, -]

      Allowed time between MARS_MULTI components.
                                        [-, -, 10 sec]

      Initial random L_MULTI_RQ/ADD retransmit timer range.
                                        [5 sec, -, 10 sec]

      Random time to set VC_revalidate flag.
                                        [1 sec, -, 10 sec]

      MARS_JOIN/LEAVE retransmit interval.
                                        [5 sec, 10 sec, -]

      MARS_JOIN/LEAVE retransmit limit.
                                        [-, -, 5]

      Random time to re-register with MARS.
                                        [1 sec, -, 10 sec]

      Force wait if MARS re-registration is looping.
                                        [1 min, -, -]

      Transmission interval for MARS_REDIRECT_MAP.
                                        [1 min, 1 min, 2 min]

      Limit for client to miss MARS_REDIRECT_MAPs.
                                        [-, -, 4 min]










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Appendix F.  Pseudo code for MARS operation.

   Implementations are entirely free to comply with the body of this
   memo in any way they see fit. This appendix is purely for possible
   clarification.

   A MARS implementation might be built along the lines suggested in
   this pseudo-code.

   1. Main

    1.1 Initilization

         Define a server list as the list of leaf nodes
                                            on ServerControlVC.
         Define a cluster list as the list of leaf nodes
                                            on ClusterControlVC.
         Define a host map as the list of hosts that are
                                            members of a group.
         Define a server map as the list of hosts (MCSs)
                                            that are serving a group.
         Read config file.
         Allocate message queues.
         Allocate internal tables.
         Set up passive open VC connection.
         Set up redirect_map timer.
         Establish logging.

    1.2 Message Processing

         Forever {
           If the message has a TLV then {
             If TLV is unsupported then {
               process as defined in TLV type field.
             } /* unknown TLV */
           } /* TLV present */
           Place incoming message in the queue.
           For (all messages in the queue) {
             If the message is not a JOIN/LEAVE/MSERV/UNSERV with
               mar$flags.register == 1 then {
               If the message source is (not a member of server list) &&
                (not a member of cluster list) then {
                Drop the message silently.
              }
             }
             If (mar$pro.type is not supported) or
                (the ATM source address is missing) then {
                Continue.



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             }
             Determine type of message.
             If an ERR_L_RELEASE arrives on ClusterControlVC then {
               Remove the endpoints ATM address from all groups
               for which it has joined.
               Release the CMI.
               Continue.
             } /* error on CCVC */
             Call specific message handling routine.
             If redirect_map timer pops {
               Call MARS_REDIRECT_MAP message handling routine.
             } /* redirect timer pop */
           } /* all msgs in the queue */
         } /* forever loop */

   2. Message Handler

    2.1 Messages:

       - MARS_REQUEST

         Indicate no MARS_MULTI support of TLV.
         If the supported TLV is not NULL then {
           Indicate MARS_MULTI support of TLV.
           Process as required.
         } else { /* TLV NULL */
            Indicate message to be sent on Private VC.
            If the message source is a member of server list then {
              If the group has a non-null host map then {
                Call MARS_MULTI with the host map for the group.
              } else { /* no group */
                 Call MARS_NAK message routine.
              } /* no group */
            } else { /* source is cluster list */
               If the group has a non-null server map then {
                 Call MARS_MULTI with the server map for the group.
               } else { /* cluster member but no server map */
                  If the group has a non-null host map then {
                    Call MARS_MULTI with the host map for the group.
                  } else { /* no group */
                     Call MARS_NAK message routine.
                  } /* no group */
                 } /* cluster member but no server map */
              } /* source is a cluster list */
            } /* TLV NULL */
         If a message exists then {
           Send message as indicated.
         }



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         Return.

       - MARS_MULTI

         Construct a MARS_MULTI for the specified map.
         If the param indicates TLV support then {
           Process the TLV as required.
         }
         Return.

     - MARS_JOIN

        If (mar$flags.copy != 0) silently ignore the message.
        If more than a single <min,max> pair is specified then
        silently ignore the message.
        Indicate message to be sent on private VC.
        If (mar$flags.register == 1) then {
          If the node is already a registered member of the cluster
          associated with protocol type then { /*previous register*/
            Copy the existing CMI into the MARS_JOIN.
          } else { /* new register */
             Add the node to ClusterControlVC.
             Add the node to cluster list.
             mar$cmi = obtain CMI.
            } /* new register */
         } else { /* not a register */
           If the group is a duplicate of a previous MARS_JOIN then {
             mar$msn = current csn.
             Indicate message to be sent on Private VC.
           } else {
              Indicate no message to be sent.
              If the message source is in server map then {
                Drop the message silently.
              } else {
                 If the first <min,max> encompasses any group with
                                                a server map then {
                   Call the Modified JOIN/LEAVE Processing routine.
                 } else {
                    If the MARS_JOIN is for a multi group then {
                     Call the MultiGroup JOIN/LEAVE Processing Routine.
                    } else {
                       Indicate message to be sent on ClusterControlVC.
                    } /* not for a multi group */
                  } /* group not handled by server */
                 } /* msg src not in server map */
                Update internal tables.
              } /* not a duplicate */
             } /* not a register */



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        If a message exists then {
          mar$flags.copy = 1.
          Send message as indicated.
        }
        Return.

     - MARS_LEAVE

        If (mar$flags.copy != 0) silently ignore the message.
        If more than a single <min,max> pair is specified then
        silently ignore the message.
        Indicate message to be sent on ClusterControlVC.
        If (mar$flags.register == 1) then { /* deregistration */
          Update internal tables to remove the member's ATM addr
          from all groups it has joined.
          Drop the endpoint from ClusterControlVC.
          Drop the endpoint from cluster list.
          Release the CMI.
          Indicate message to be sent on Private VC.
        } else { /* not a deregistration */
           If the group is a duplicate of a previous MARS_LEAVE then {
             mar$msn = current csn.
             Indicate message to be sent on Private VC.
           } else {
              Indicate no message to be sent.
              If the first <min,max> encompasses any group with
                                             a server map then {
                Call the Modified JOIN/LEAVE Processing routine.
              } else {
                 If the MARS_LEAVE is for a multi group then {
                   Call the MultiGroup JOIN/LEAVE Processing Routine.
                 } else {
                    Indicate message to be sent on ClusterControlVC.
                 }
               }
           Update internal tables.
          } /* not a duplicate */
        } /* not a deregistration */
        If a message exists then {
          mar$flags.copy = 1.
          Send message as indicated.
        }
        Return.

    - MARS_MSERV

         If (mar$flags.register == 1) then { /* server register */
           Add the endpoint as a leaf node to ServerControlVC.



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           Add the endpoint to the server list.
           Indicate the message to be sent on Private VC.
           mar$cmi = 0.
         } else { /* not a register */
         If the source has not registered then {
                 Drop and ignore the message.
                 Indicate no message to be sent.
               } else {  /* source is registered */
                  If MCS is already member of indicated server map {
                     Indicate message to be sent on Private VC.
                     mar$flags.layer3grp = 0;
                     mar$flags.copy = 1.
                  } else { /* New MCS to add. */
                     Add the server ATM addr to server map for group.
                     Indicate message to be sent on ServerControlVC.
                     Send message as indicated.
                     Make a copy of the message.
                     Indicate message to be sent on ClusterControlVC.
                     If new server map was just created {
                          Construct MARS_MIGRATE, with MCS as target.
                      } else {
                          Change the op code to MARS_JOIN.
                          mar$flags.layer3grp = 0.
                          mar$flags.copy = 1.
                      } /* new server map */
                  } /* New MCS to add. */
               } /* source is registered */
         } /* not a register */

         If a message exists then {
           Send message as indicated.
         }
         Return.


    - MARS_UNSERV

      If (mar$flags.register == 1) then { /* deregister */
        Remove the ATM addr of the MCS from all server maps.
        If a server map becomes null then delete it.
        Remove the endpoint as a leaf of ServerControlVC.
        Remove the endpoint from server list.
        Indicate the message to be sent on Private VC.
      } else { /* not a deregister */
         If the source is not a member of server list then {
          Drop and ignore the message.
          Indicate no message to be sent.
         } else {  /* source is registered */



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            If MCS is not member of indicated server map {
               Indicate message to be sent on Private VC.
               mar$flags.layer3grp = 0;
               mar$flags.copy = 1.
             } else { /* MCS existed, must be removed. */
               Remove ATM addr of the MCS from indicated server map.
               If a server map is null then delete it.
               Indicate the message to be sent on ServerControlVC.
               Send message as indicated.
               Make a copy of the message.
               Change the op code to MARS_LEAVE.
               Indicate message (copy) to be sent on ClusterControlVC.
               mar$flags.layer3grp = 0;
               mar$flags.copy = 1.
             } /* MCS existed, must be removed. */
           } /* source is registered */
        } /* not a deregister */
      If a message exists then {
        Send message as indicated.
      }
      Return.

    - MARS_NAK

      Build command.
      Return.

    - MARS_GROUPLIST_REQUEST

      If (mar$pnum != 1) then Return.
      Call MARS_GROUPLIST_REPLY with the range and output VC.
      Return.

    - MARS_GROUPLIST_REPLY

      Build command for specified range.
      Indicate message to be sent on specified VC.
      Send message as indicated.
      Return.

    - MARS_REDIRECT_MAP

       Include the MARSs own address in the message.
       If there are backup MARSs then include their addresses.
       Indicate MARS_REDIRECT_MAP is to be sent on ClusterControlVC.
       Send message back as indicated.
       Return.




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   3. Send Message Handler

      If (the message is going out ClusterControlVC) &&
              (a new csn is required) then {
       mar$msn = obtain a CSN
      }
      If (the message is going out ServerControlVC) &&
              (a new ssn is required) then {
       mar$msn = obtain a SSN
      }
      Return.

   4.  Number Generator

   4.1 Cluster Sequence Number

       Generate the next sequence number.
       Return.

   4.2 Server Sequence Number

       Generate the next sequence number.
       Return.

   4.3 CMI

       CMIs are allocated uniquely per registered cluster member
       within the context of a particular layer 3 protocol type.
       A single node may register multiple times if it supports
       multiple layer 3 protocols.
       The CMIs allocated for each such registration may or may
       not be the same.
       Generate a CMI for this protocol.
       Return.

   5. Modified JOIN/LEAVE Processing

      This routine processes JOIN/LEAVE when a server map exists.

      Make a copy of the message.
      Change the type of the copy to MARS_SJOIN.
      If the message is a MARS_LEAVE then {
       Change the type of the copy to MARS_SLEAVE.
      }
      mar$flags.copy = 1 (copy).
      Hole punch the <min,max> group by excluding
        from the range those groups which the joining
        (leaving) node is already (still) a member of



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        due to it having previously issued a single group
        join.
      Indicate the message to be sent on ServerControlVC.
      If the message (copy) contains one or more <min,max> pair {
        Send message (copy) as indicated.
      }
      mar$flags.punched = 0 in the original message.
      Indicate the message to be sent on Private VC.
      Send message (original) as indicated.
      Hole punch the <min,max> group by excluding
        from the range those groups that are served by MCSs
        or which the joining (leaving) node is already
        (still) a member of due to it having previously
        issued a single group join.
      Indicate the (original) message to be sent on ClusterControlVC.
      If (number of holes punched > 0) then { /* punched holes */
        In original message do {
         mar$flags.punched = 1.
         old punched list <- new punched list.
        }
      } /* punched holes */
      mar$flags.copy = 1.
      Send message as indicated.
      Return.

   5.1 MultiGroup JOIN/LEAVE Processing

      This routine processes JOIN/LEAVE when a multi group exists.

      If (mar$flags.layer3grp) {
       Ignore this setting, consider it reset.
      }
      mar$flags.copy = 1.
      Make a copy of the message.
      From the copy hole punch the <min,max> group by
       excluding from the range those groups that this
       node has already joined or left.
      If (number of holes punched > 0) then {
       mar$flags.punch = 0 in original message.
       Indicate original message to be sent on Private VC.
       Send original message as indicated.
       mar$flags.punch = 1 in copy message.
       old group range <- new punched list.
       Indicate message to be sent on ClusterControlVC.
       Send copy of message as indicated.
      } else {
         Indicate message to be sent on ClusterControlVC.
         Send original message as indicated.



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      } /* no holes punched */
      Return.

















































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