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rfc:rfc2022

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.

Armitage Standards Track [Page 1] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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

Armitage Standards Track [Page 2] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

    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

Armitage Standards Track [Page 3] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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

Armitage Standards Track [Page 4] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 5] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 6] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 7] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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).

Armitage Standards Track [Page 8] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.)

Armitage Standards Track [Page 9] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 10] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 11] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.)

Armitage Standards Track [Page 12] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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)

Armitage Standards Track [Page 13] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 14] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 15] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.

Armitage Standards Track [Page 16] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.)

Armitage Standards Track [Page 17] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 18] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 19] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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   |
             +-+-+-+-+-+-+-+-+

Armitage Standards Track [Page 20] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. ATM Forum NSAPA format (x = 0).
  2. 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

Armitage Standards Track [Page 21] 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.

Armitage Standards Track [Page 22] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 23] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. The ATM address of a node joining or leaving a group.
  2. The layer 3 address of the group(s) being joined or left.
  3. 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.

Armitage Standards Track [Page 24] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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

Armitage Standards Track [Page 25] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. 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

Armitage Standards Track [Page 26] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 27] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

     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.

Armitage Standards Track [Page 28] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.

Armitage Standards Track [Page 29] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 30] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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).

Armitage Standards Track [Page 31] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 32] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. mar$op
  2. mar$flags.register
  3. mar$flags.sequence
  4. mar$pnum
  5. Source ATM address
  6. First <min,max> pair
 In addition, a valid copy MUST have the following field values:
  1. mar$flags.punched = 0
  2. mar$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.

Armitage Standards Track [Page 33] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.

Armitage Standards Track [Page 34] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 35] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. It fails to establish a point to point VC to the MARS.
  2. MARS_REQUESTs fail (section 5.1.1).
  3. MARS_JOIN/MARS_LEAVEs fail (section 5.2.2).
  4. 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

Armitage Standards Track [Page 36] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

      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

Armitage Standards Track [Page 37] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. mar$tpln field replaced by mar$redirf.
  2. mar$spln field reserved.
  3. mar$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.

Armitage Standards Track [Page 38] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. open a point to point VC to the first ATM address.
  2. 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.

Armitage Standards Track [Page 39] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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

Armitage Standards Track [Page 40] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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]

Armitage Standards Track [Page 41] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

                         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.

Armitage Standards Track [Page 42] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:
  1. Adds the node to ClusterControlVC.
  2. Allocates a new Cluster Member ID (CMI).
  3. Inserts the new CMI into the mar$cmi field of the MARS_JOIN.
  4. 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.

Armitage Standards Track [Page 43] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 44] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

       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.

Armitage Standards Track [Page 45] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 46] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 47] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 48] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 49] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 50] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

    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

Armitage Standards Track [Page 51] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 52] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 53] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 54] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 55] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 56] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 57] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 58] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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:

Armitage Standards Track [Page 59] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

    [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

Armitage Standards Track [Page 60] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 61] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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).

Armitage Standards Track [Page 62] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

    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

Armitage Standards Track [Page 63] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

    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.

Armitage Standards Track [Page 64] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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

Armitage Standards Track [Page 65] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.

Armitage Standards Track [Page 66] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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;

Armitage Standards Track [Page 67] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

       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++;
    }

Armitage Standards Track [Page 68] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.

Armitage Standards Track [Page 69] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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.

Armitage Standards Track [Page 70] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.

Armitage Standards Track [Page 71] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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;

Armitage Standards Track [Page 72] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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]

Armitage Standards Track [Page 73] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

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.

Armitage Standards Track [Page 74] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

           }
           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:
  1. 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.
       }

Armitage Standards Track [Page 75] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

       Return.
  1. MARS_MULTI
       Construct a MARS_MULTI for the specified map.
       If the param indicates TLV support then {
         Process the TLV as required.
       }
       Return.
  1. 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 */

Armitage Standards Track [Page 76] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

      If a message exists then {
        mar$flags.copy = 1.
        Send message as indicated.
      }
      Return.
  1. MARS_LEAVE
      If (mar$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$flags.copy = 1.
        Send message as indicated.
      }
      Return.
  1. MARS_MSERV
       If (mar$flags.register == 1) then { /* server register */
         Add the endpoint as a leaf node to ServerControlVC.

Armitage Standards Track [Page 77] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

         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$flags.copy = 1.
                    } /* new server map */
                } /* New MCS to add. */
             } /* source is registered */
       } /* not a register */
       If a message exists then {
         Send message as indicated.
       }
       Return.
  1. MARS_UNSERV
    If (mar$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 */

Armitage Standards Track [Page 78] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

          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.
  1. MARS_NAK
    Build command.
    Return.
  1. MARS_GROUPLIST_REQUEST
    If (mar$pnum != 1) then Return.
    Call MARS_GROUPLIST_REPLY with the range and output VC.
    Return.
  1. MARS_GROUPLIST_REPLY
    Build command for specified range.
    Indicate message to be sent on specified VC.
    Send message as indicated.
    Return.
  1. 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.

Armitage Standards Track [Page 79] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

 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

Armitage Standards Track [Page 80] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

      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.

Armitage Standards Track [Page 81] RFC 2022 Multicast over UNI 3.0/3.1 based ATM November 1996

    } /* no holes punched */
    Return.

Armitage Standards Track [Page 82]

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