path: root/doc/rfc/rfc1584.txt

Network Working Group                                             J. Moy
Request for Comments: 1584                                 Proteon, Inc.
Category: Standards Track                                     March 1994


                      Multicast Extensions to OSPF



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

    This memo documents enhancements to the OSPF protocol enabling the
    routing of IP multicast datagrams. In this proposal, an IP multicast
    packet is routed based both on the packet's source and its multicast
    destination (commonly referred to as source/destination routing). As
    it is routed, the multicast packet follows a shortest path to each
    multicast destination. During packet forwarding, any commonality of
    paths is exploited; when multiple hosts belong to a single multicast
    group, a multicast packet will be replicated only when the paths to
    the separate hosts diverge.

    OSPF, a link-state routing protocol, provides a database describing
    the Autonomous System's topology. A new OSPF link state
    advertisement is added describing the location of multicast
    destinations. A multicast packet's path is then calculated by
    building a pruned shortest-path tree rooted at the packet's IP
    source. These trees are built on demand, and the results of the
    calculation are cached for use by subsequent packets.

    The multicast extensions are built on top of OSPF Version 2. The
    extensions have been implemented so that a multicast routing
    capability can be introduced piecemeal into an OSPF Version 2
    routing domain. Some of the OSPF Version 2 routers may run the
    multicast extensions, while others may continue to be restricted to
    the forwarding of regular IP traffic (unicasts).

    Please send comments to mospf@gated.cornell.edu.





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Table of Contents

    1       Introduction ........................................... 4
    1.1     Terminology ............................................ 5
    1.2     Acknowledgments ........................................ 6
    2       Multicast routing in MOSPF ............................. 6
    2.1     Routing characteristics ................................ 6
    2.2     Sample path of a multicast datagram .................... 8
    2.3     MOSPF forwarding mechanism ............................ 10
    2.3.1   IGMP interface: the local group database .............. 10
    2.3.2   A datagram's shortest-path tree ....................... 14
    2.3.3   Support for Non-broadcast networks .................... 16
    2.3.4   Details concerning forwarding cache entries ........... 16
    3       Inter-area multicasting ............................... 18
    3.1     Extent of group-membership-LSAs ....................... 19
    3.2     Building inter-area datagram shortest-path trees ...... 22
    4       Inter-AS multicasting ................................. 27
    4.1     Building inter-AS datagram shortest-path trees ........ 28
    4.2     Stub area behavior .................................... 30
    4.3     Inter-AS multicasting in a core Autonomous System ..... 31
    5       Modelling internal group membership ................... 31
    6       Additional capabilities ............................... 33
    6.1     Mixing with non-multicast routers ..................... 34
    6.2     TOS-based multicast ................................... 35
    6.3     Assigning multiple IP networks to a physical network .. 36
    6.4     Networks on Autonomous System boundaries .............. 37
    6.5     Recommended system configuration ...................... 38
    7       Basic implementation requirements ..................... 40
    8       Protocol data structures .............................. 40
    8.1     Additions to the OSPF area structure .................. 41
    8.2     Additions to the OSPF interface structure ............. 42
    8.3     Additions to the OSPF neighbor structure .............. 43
    8.4     The local group database .............................. 43
    8.5     The forwarding cache .................................. 44
    9       Interaction with the IGMP protocol .................... 45
    9.1     Sending IGMP Host Membership Queries .................. 46
    9.2     Receiving IGMP Host Membership Reports ................ 46
    9.3     Aging local group database entries .................... 47
    9.4     Receiving IGMP Host Membership Queries ................ 47
    10      Group-membership-LSAs ................................. 48
    10.1    Constructing group-membership-LSAs .................... 49
    10.2    Flooding group-membership-LSAs ........................ 52
    11      Detailed description of multicast datagram forwarding . 52
    11.1    Associating a MOSPF interface with a received datagram  55
    11.2    Locating the source network ........................... 55
    11.3    Forwarding locally originated multicasts .............. 57
    12      Construction of forwarding cache entries .............. 58
    12.1    The Vertex data structure ............................. 59



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    12.2    The SPF calculation ................................... 60
    12.2.1  Candidate list Initialization: Case SourceIntraArea ... 65
    12.2.2  Candidate list Initialization: Case SourceInterArea1 .. 66
    12.2.3  Candidate list Initialization: Case SourceInterArea2 .. 66
    12.2.4  Candidate list Initialization: Case SourceExternal .... 67
    12.2.5  Candidate list Initialization: Case SourceStubExternal  70
    12.2.6  Processing labelled vertices .......................... 70
    12.2.7  Merging datagram shortest-path trees .................. 71
    12.2.8  TOS considerations .................................... 72
    12.2.9  Comparison to the unicast SPF calculation ............. 74
    12.3    Adding local database entries to the forwarding cache   75
    13      Maintaining the forwarding cache ...................... 76
    14      Other additions to the OSPF specification ............. 77
    14.1    The Designated Router ................................. 77
    14.2    Sending Hello packets ................................. 78
    14.3    The Neighbor state machine ............................ 78
    14.4    Receiving Database Description packets ................ 78
    14.5    Sending Database Description packets .................. 79
    14.6    Originating Router-LSAs ............................... 79
    14.7    Originating Network-LSAs .............................. 79
    14.8    Originating Summary-link-LSAs ......................... 80
    14.9    Originating AS external-link-LSAs ..................... 80
    14.10   Next step in the flooding procedure ................... 81
    14.11   Virtual links ......................................... 81
    15      References ............................................ 83
            Footnotes ............................................. 84
    A       Data Formats .......................................... 88
    A.1     The Options field ..................................... 89
    A.2     Router-LSA ............................................ 91
    A.3     Group-membership-LSA .................................. 93
    B       Configurable Constants ................................ 95
    B.1     Global parameters ..................................... 95
    B.2     Router interface parameters ........................... 95
    C       Sample datagram shortest-path trees ................... 97
    C.1     An intra-area tree .................................... 98
    C.2     The effect of areas .................................. 100
    C.3     The effect of virtual links .......................... 101
            Security Considerations .............................. 102
            Author's Address ..................................... 102












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1.  Introduction

    This memo documents enhancements to OSPF Version 2 to support IP
    multicast routing. The enhancements have been added in a backward-
    compatible fashion; routers running the multicast additions will
    interoperate with non-multicast OSPF routers when forwarding regular
    (unicast) IP data traffic. The protocol resulting from the addition
    of the multicast enhancements to OSPF is herein referred to as the
    MOSPF protocol.

    IP multicasting is an extension of LAN multicasting to a TCP/IP
    internet. Multicasting support for TCP/IP hosts has been specified
    in [RFC 1112]. In that document, multicast groups are represented by
    IP class D addresses. Individual TCP/IP hosts join (and leave)
    multicast groups through the Internet Group Management Protocol
    (IGMP, also specified in [RFC 1112]). A host need not be a member of
    a multicast group in order to send datagrams to the group. Multicast
    datagrams are to be delivered to each member of the multicast group
    with the same "best-effort" delivery accorded regular (unicast) IP
    data traffic.

    MOSPF provides the ability to forward multicast datagrams from one
    IP network to another (i.e., through internet routers). MOSPF
    forwards a multicast datagram on the basis of both the datagram's
    source and destination (this is sometimes called source/destination
    routing). The OSPF link state database provides a complete
    description of the Autonomous System's topology. By adding a new
    type of link state advertisement, the group-membership-LSA, the
    location of all multicast group members is pinpointed in the
    database. The path of a multicast datagram can then be calculated by
    building a shortest-path tree rooted at the datagram's source. All
    branches not containing multicast members are pruned from the tree.
    These pruned shortest-path trees are initially built when the first
    datagram is received (i.e., on demand).  The results of the shortest
    path calculation are then cached for use by subsequent datagrams
    having the same source and destination.

    OSPF allows an Autonomous System to be split into areas. However,
    when this is done complete knowledge of the Autonomous System's
    topology is lost. When forwarding multicasts between areas, only
    incomplete shortest-path trees can be built. This may lead to some
    inefficiency in routing. An analogous situation exists when the
    source of the multicast datagram lies in another Autonomous System.
    In both cases (i.e., the source of the datagram belongs to a
    different OSPF area, or to a different Autonomous system) the
    neighborhood immediately surrounding the source is unknown. In these
    cases the source's neighborhood is approximated by OSPF summary link
    advertisements or by OSPF AS external link advertisements



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    respectively.

    Routers running MOSPF can be intermixed with non-multicast OSPF
    routers. Both types of routers can interoperate when forwarding
    regular (unicast) IP data traffic. Obviously, the forwarding extent
    of IP multicasts is limited by the number of MOSPF routers present
    in the Autonomous System (and their interconnection, if any). An
    ability to "tunnel" multicast datagrams through non-multicast
    routers is not provided. In MOSPF, just as in the base OSPF
    protocol, datagrams (multicast or unicast) are routed "as is" --
    they are not further encapsulated or decapsulated as they transit
    the Autonomous System.

    1.1.  Terminology

        This memo uses the terminology listed in section 1.2 of [OSPF].
        For this reason, terms such as "Network", "Autonomous System"
        and "link state advertisement" are assumed to be understood. In
        addition, the abbreviation LSA is used for "link state
        advertisement". For example, router links advertisements are
        referred to as router-LSAs and the new link state advertisement
        describing the location of members of a multicast group is
        referred to as a group-membership-LSA.

        [RFC 1112] discusses the data-link encapsulation of IP multicast
        datagrams. In contrast to the normal forwarding of IP unicast
        datagrams, on a broadcast network the mapping of an IP multicast
        destination to a data-link destination address is not done with
        the ARP protocol. Instead, static mappings have been defined
        from IP multicast destinations to data-link addresses. These
        mappings are dependent on network type; for some networks IP
        multicasts are algorithmically mapped to data-link multicast
        addresses, for other networks all IP multicast destinations are
        mapped onto the data-link broadcast address. This document
        loosely describes both of these possible mappings as data-link
        multicast.

        The following terms are also used throughout this document:

        o   Non-multicast router. A router running OSPF Version 2, but
            not the multicast extensions. These routers do not forward
            multicast datagrams, but can interoperate with MOSPF routers
            in the forwarding of unicast packets. Routers running the
            MOSPF protocol are referred to herein as either multicast-
            capable routers or MOSPF routers.

        o   Non-broadcast networks. A network supporting the attachment
            of more than two stations, but not supporting the delivery



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            of a single physical datagram to multiple destinations
            (i.e., not supporting data-link multicast). [OSPF] describes
            these networks as non-broadcast, multi-access networks. An
            example of a non-broadcast network is an X.25 PDN.

        o   Transit network. A network having two or more OSPF routers
            attached.  These networks can forward data traffic that is
            neither locally-originated nor locally-destined. In OSPF,
            with the exception of point-to-point networks and virtual
            links, the neighborhood of each transit network is described
            by a network links advertisement (network-LSA).

        o   Stub network. A network having only a single OSPF router
            attached. A network belonging to an OSPF system is either a
            transit or a stub network, but never both.

    1.2.  Acknowledgments

        The multicast extensions to OSPF are based on Link-State
        Multicast Routing algorithm presented in [Deering]. In addition,
        the [Deering] paper contains a section on Hierarchical Multicast
        Routing (providing the ideas for MOSPF's inter-area multicasting
        scheme) and several Distance Vector (also called Bellman-Ford)
        multicast algorithms. One of these Distance Vector multicast
        algorithms, Truncated Reverse Path Broadcasting, has been
        implemented in the Internet (see [RFC 1075]).

        The MOSPF protocol has been developed by the MOSPF Working Group
        of the Internet Engineering Task Force. Portions of this work
        have been supported by DARPA under NASA contract NAG 2-650.

2.  Multicast routing in MOSPF

    This section describes MOSPF's basic multicast routing algorithm.
    The basic algorithm, run inside a single OSPF area, covers the case
    where the source of the multicast datagram is inside the area
    itself. Within the area, the path of the datagram forms a tree
    rooted at the datagram source.

    2.1.  Routing characteristics

        As a multicast datagram is forwarded along its shortest-path
        tree, the datagram is delivered to each member of the
        destination multicast group. In MOSPF, the forwarding of the
        multicast datagram has the following properties:

        o   The path taken by a multicast datagram depends both on the
            datagram's source and its multicast destination. Called



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            source/destination routing, this is in contrast to most
            unicast datagram forwarding algorithms (like OSPF) that
            route based solely on destination.

        o   The path taken between the datagram's source and any
            particular destination group member is the least cost path
            available. Cost is expressed in terms of the OSPF link-state
            metric. For example, if the OSPF metric represents delay, a
            minimum delay path is chosen. OSPF metrics are configurable.
            A metric is assigned to each outbound router interface,
            representing the cost of sending a packet on that interface.
            The cost of a path is the sum of its constituent (outbound)
            router interfaces[1].

        o   MOSPF takes advantage of any commonality of least cost paths
            to destination group members. However, when members of the
            multicast group are spread out over multiple networks, the
            multicast datagram must at times be replicated. This
            replication is performed as few times as possible (at the
            tree branches), taking maximum advantage of common path
            segments.

        o   For a given multicast datagram, all routers calculate an
            identical shortest-path tree. There is a single path between
            the datagram's source and any particular destination group
            member. This means that, unlike OSPF's treatment of regular
            (unicast) IP data traffic, there is no provision for equal-
            cost multipath.

        o   On each packet hop, MOSPF normally forwards IP multicast
            datagrams as data-link multicasts. There are two exceptions.
            First, on non-broadcast networks, since there are no data-
            link multicast/broadcast services the datagram must be
            forwarded to specific MOSPF neighbors (see Section 2.3.3).
            Second, a MOSPF router can be configured to forward IP
            multicasts on specific networks as data-link unicasts, in
            order to avoid datagram replication in certain anomalous
            situations (see Section 6.4).

        While MOSPF optimizes the path to any given group member, it
        does not necessarily optimize the use of the internetwork as a
        whole. To do so, instead of calculating source-based shortest-
        path trees, something similar to a minimal spanning tree
        (containing only the group members) would need to be calculated.
        This type of minimal spanning tree is called a Steiner tree in
        the literature. For a comparison of shortest-path tree routing
        to routing using Steiner trees, see [Deering2] and [Bharath-
        Kumar].



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    2.2.  Sample path of a multicast datagram

        As an example of multicast datagram routing in MOSPF, consider
        the sample Autonomous System pictured in Figure 1. This figure
        has been taken from the OSPF specification (see [OSPF]). The
        larger rectangles represent routers, the smaller rectangles
        hosts. Oblongs and circles represent multi-access networks[2].
        Lines joining routers are point-to-point serial connections. A
        cost has been assigned to each outbound router interface.

        All routers in Figure 1 are assumed to be running MOSPF. A
        number of hosts have been added to the figure. The hosts
        labelled Ma have joined a particular multicast group (call it
        Group A) via the IGMP protocol.  These hosts are located on
        networks N2, N6 and N11. Similarly, using IGMP the hosts
        labelled Mb have joined a separate multicast group B; these
        hosts are located on networks N1, N2 and N3. Note that hosts can
        join multiple multicast groups; it is only for clarity of
        presentation that each host has joined at most one multicast
        group in this example.  Also, hosts H2 through H5 have been
        added to the figure to serve as sources for multicast datagrams.
        Again, the datagrams' sources have been made separate from the
        group members only for clarity of presentation.

        To illustrate the forwarding of a multicast datagram, suppose
        that Host H2 (attached to Network N4) sends a multicast datagram
        to multicast group B. This datagram originates as a data-link
        layer multicast on Network N4. Router RT3, being a multicast
        router, has "opened up" its interface data-link multicast
        filters. It therefore receives the multicast datagram, and
        attempts to forward it to the members of multicast group B
        (located on networks N1, N2 and N3). This is accomplished by
        sending a single copy of the datagram onto Network N3, again as
        a data-link multicast[3].  Upon receiving the multicast datagram
        from RT3, routers RT1 and RT2 will then multicast the datagram
        on their connected stub networks (N1 and N2 respectively).  Note
        that, since the datagram is sent onto Network N3 as a data-link
        multicast, Router RT4 will also receive a copy. However, it will
        not forward the datagram, since it does not lie on a shortest
        path between the source (Host H2) and any members of multicast
        group B.

        Note that the path of the multicast datagram depends on the
        datagram's source network. If the above multicast datagram was
        instead originated by Host H3, the path taken would be
        identical, since hosts H2 and H3 lie on the same network
        (Network N4). However, if the datagram was originated by Host
        H4, its path would be different. In this case, when Router RT3



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                 +
                 | 3+---+    +--+  +--+       N12      N14
               N1|--|RT1|\1  |Mb|  |H4|         \ N13 /
                _|  +---+ \  +--+ /+--+         8\ |8/8
               | +         \ _|__/                \|/
             +--+   +--+    /    \   1+---+8    8+---+6
             |Mb|   |Mb|   *  N3  *---|RT4|------|RT5|--------+
             +--+  /+--+    \____/    +---+      +---+        |
                  +         /   |                  |7         |
                  | 3+---+ /    |                  |          |
                N2|--|RT2|/1    |1                 |6         |
                __|  +---+    +---+8            6+---+        |
               |  +           |RT3|--------------|RT6|        |
             +--+    +--+     +---+     +--+     +---+        |
             |Ma|    |H3|_      |2     _|H2|     Ia|7         |
             +--+    +--+ \     |     / +--+       |          |
                           +---------+             |          |
                               N4                  |          |
                                                   |          |
                                                   |          |
                       N11                         |          |
                   +---------+                     |          |
                        |     \                    |          |    N12
                        |3     +--+                |          |6 2/
                      +---+    |Ma|                |        +---+/
                      |RT9|    +--+                |        |RT7|---N15
                      +---+                        |        +---+ 9
                        |1                   +     |          |1
                       _|__                  |   Ib|5       __|_   +--+
                      /    \      1+----+2   |  3+----+1   /    \--|Ma|
                     *  N9  *------|RT11|----|---|RT10|---*  N6  * +--+
                      \____/       +----+    |   +----+    \____/
                        |                    |                |
                        |1                   +                |1
             +--+   10+----+                N8              +---+
             |H1|-----|RT12|                                |RT8|
             +--+SLIP +----+                                +---+  +--+
                        |2                                    |4  _|H5|
                        |                                     |  / +--+
                   +---------+                            +--------+
                       N10                                    N7

                    Figure 1: A sample MOSPF configuration





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        receives the datagram, RT3 will drop the datagram instead of
        forwarding it (since RT3 is no longer on the shortest path to
        any member of Group B).

        Note that the path of the multicast datagram also depends on the
        destination multicast group. If Host H2 sends a multicast to
        Group A, the path taken is as follows. The datagram again starts
        as a multicast on Network N4. Router RT3 receives it, and
        creates two copies. One is sent onto Network N3 which is then
        forwarded onto Network N2 by RT2. The other copy is sent to
        Router RT10 (via RT6), where the datagram is again split,
        eventually to be delivered onto networks N6 and N11. Note that,
        although multiple copies of the datagram are produced, the
        datagram itself is not modified (except for its IP TTL) as it is
        forwarded. No encapsulation of the datagram is performed; the
        destination of the datagram is always listed as the multicast
        group A.

    2.3.  MOSPF forwarding mechanism

        Each MOSPF router in the path of a multicast datagram bases its
        forwarding decision on the contents of a data cache. This cache
        is called the forwarding cache. There is a separate forwarding
        cache entry for each source/destination combination[4].  Each
        cache entry indicates, for multicast datagrams having matching
        source and destination, which neighboring node (i.e., router or
        network) the datagram must be received from (called the upstream
        node) and which interfaces the datagram should then be forwarded
        out of (called the downstream interfaces).

        A forwarding cache entry is actually built from two component
        pieces.  The first of these components is called the local group
        database. This database, built by the IGMP protocol, indicates
        the group membership of the router's directly attached networks.
        The local group database enables the local delivery of multicast
        datagrams. The second component is the datagram's shortest path
        tree. This tree, built on demand, is rooted at a multicast
        datagram's source. The datagram's shortest path tree enables the
        delivery of multicast datagrams to distant (i.e., not directly
        attached) group members.

        2.3.1.  IGMP interface: the local group database

            The local group database keeps track of the group membership
            of the router's directly attached networks. Each entry in
            the local group database is a [group, attached network]
            pair, which indicates that the attached network has one or
            more IP hosts belonging to the IP multicast destination



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            group. This information is then used by the router when
            deciding which directly attached networks to forward a
            received IP multicast datagram onto, in order to complete
            delivery of the datagram to (local) group members.

            The local group database is built through the operation of
            the Internet Group Management Protocol (IGMP; see [RFC
            1112]). When a MOSPF router becomes Designated Router on an
            attached network (call the network N1), it starts sending
            periodic IGMP Host Membership Queries on the network. Hosts
            then respond with IGMP Host Membership Reports, one for each
            multicast group to which they belong. Upon receiving a Host
            Membership Report for a multicast group A, the router
            updates its local group database by adding/refreshing the
            entry [Group A, N1]. If at a later time Reports for Group A
            cease to be heard on the network, the entry is then deleted
            from the local group database.

            It is important to note that on any particular network, the
            sending of IGMP Host Membership Queries and the listening to
            IGMP Host Membership Reports is performed solely by the
            Designated Router. A MOSPF router ignores Host Membership
            Reports received on those networks where the router has not
            been elected Designated Router[5].  This means that at most
            one router performs these IGMP functions on any particular
            network, and ensures that the network appears in the local
            group database of at most one router. This prevents
            multicast datagrams from being replicated as they are
            delivered to local group members. As a result, each router
            in the Autonomous System has a different local group
            database. This is in contrast to the MOSPF link state
            database, and the datagram shortest-path trees (see Section
            2.3.2), all of which are identical in each router belonging
            to the Autonomous System.

            The existence of local group members must be communicated to
            the rest of the routers in the Autonomous System. This
            ensures that a remotely-originated multicast datagram will
            be forwarded to the router for distribution to its local
            group members. This communication is accomplished through
            the creation of a group-membership-LSA. Like other link
            state advertisements, the group-membership-LSA is flooded
            throughout the Autonomous System. The router originates a
            separate group-membership-LSA for each multicast group
            having one or more entries in the router's local group
            database. The router's group-membership-LSA (say for Group
            A) lists those local transit vertices (i.e., the router
            itself and/or any directly connected transit networks) that



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            should not be pruned from Group A's datagram shortest-path
            trees. The router lists itself in its group-membership-LSA
            for Group A if either 1) one or more of the router's
            attached stub networks contain Group A members or 2) the
            router itself is a member of Group A. The router lists a
            directly connected transit network in the group-membership-
            LSA for Group A if both 1) the router is Designated Router
            on the network and 2) the network contains one or more Group
            A members.

            Consider again the example pictured in Figure 1. If Router
            RT3 has been elected Designated Router for Network N3, then
            Table 1: lists the local group database for the routers
            RT1-RT4.

            In this case, each of the routers RT1, RT2 and RT3 will
            originate a group-membership-LSA for Group B. In addition,
            RT2 will also be originating a group-membership-LSA for
            Group A. RT1 and RT2's group-membership-LSAs will list
            solely the routers themselves (N1 and N2 are stub networks).
            RT3's group-membership-LSA will list the transit Network N3.

            Figure 2 displays the Autonomous System's link state
            database. A router/transit network is labelled with a
            multicast group if (and only if) it has been mentioned in a
            group-membership-LSA for the group When building the
            shortest-path tree for a particular multicast datagram, this
            labelling enables those branches without group members to be
            pruned from the tree. The process of building a multicast
            datagram's shortest path tree is discussed in Section 2.3.2.

            Note that none of the hosts in Figure 1 belonging to
            multicast groups A and B show up explicitly in the link
            state database (see Figure 2).  In fact, looking at the link
            state database you cannot even determine which stub networks


                 Router   local group database
                 _____________________________________
                 RT1      [Group B, N1]
                 RT2      [Group A, N2], [Group B, N2]
                 RT3      [Group B, N3]
                 RT4      None


                 Table 1: Sample local group databases





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                                **FROM**

                 |RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
                 |1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
              ----- ---------------------------------------------
              RT1|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT2|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT3|  |  |  |  |  |6 |  |  |  |  |  |  |0 |  |  |  |
              RT4|  |  |  |  |8 |  |  |  |  |  |  |  |0 |  |  |  |
              RT5|  |  |  |8 |  |6 |6 |  |  |  |  |  |  |  |  |  |
              RT6|  |  |8 |  |7 |  |  |  |  |5 |  |  |  |  |  |  |
              RT7|  |  |  |  |6 |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT8|  |  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT9|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          T  RT10|  |  |  |  |  |7 |  |  |  |  |  |  |  |0 |0 |  |
          O  RT11|  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |0 |
          *  RT12|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          *    N1|3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N2|  |3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N3|1 |1 |1 |1 |  |  |  |  |  |  |  |  |  |  |  |  |
               N4|  |  |2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N6|  |  |  |  |  |  |1 |1 |  |1 |  |  |  |  |  |  |
               N7|  |  |  |  |  |  |  |4 |  |  |  |  |  |  |  |  |
               N8|  |  |  |  |  |  |  |  |  |3 |2 |  |  |  |  |  |
               N9|  |  |  |  |  |  |  |  |1 |  |1 |1 |  |  |  |  |
              N10|  |  |  |  |  |  |  |  |  |  |  |2 |  |  |  |  |
              N11|  |  |  |  |  |  |  |  |3 |  |  |  |  |  |  |  |
              N12|  |  |  |  |8 |  |2 |  |  |  |  |  |  |  |  |  |
              N13|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N14|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N15|  |  |  |  |  |  |9 |  |  |  |  |  |  |  |  |  |
               H1|  |  |  |  |  |  |  |  |  |  |  |10|  |  |  |  |


                     Figure 2: The MOSPF database.

                 Networks and routers are represented by vertices.
                 An edge of cost X connects Vertex A to Vertex B iff
                 the intersection of Column A and Row B is marked
                 with an X. In addition, RT1, RT2 and N3 are labelled
                 with multicast group A and RT1, N6 and RT9 are
                 labelled with multicast group B.






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            contain multicast group members. The link state database
            simply indicates those routers/transit networks having
            attached group members. This is all that is necessary for
            successful forwarding of multicast datagrams.

        2.3.2.  A datagram's shortest-path tree

            While the local group database facilitates the local
            delivery of multicast datagrams, the datagram's shortest-
            path tree describes the intermediate hops taken by a
            multicast datagram as it travels from its source to the
            individual multicast group members. As mentioned above, the
            datagram's shortest-path tree is a pruned shortest-path tree
            rooted at the datagram's source. Two datagrams having
            differing [source net, multicast destination] pairs may
            have, and in fact probably will have, different pruned
            shortest-path trees.

            A datagram's shortest path tree is built "on demand"[6],
            i.e., when the first multicast datagram is received having a
            particular [source net, multicast destination] combination.
            To build the datagram's shortest-path tree, the following
            calculations are performed. First, the datagram's source IP
            network is located in the link state database. Then using
            the router-LSAs and network-LSAs in the link state database,
            a shortest-path tree is built having the source network as
            root. To complete the process, the branches that do not
            contain routers/transit networks that have been labelled
            with the particular multicast destination (via a group-
            membership-LSA) are pruned from the tree.

            As an example of the building of a datagram's shortest path
            tree, again consider the Autonomous System in Figure 1. The
            Autonomous System's link state database is pictured in
            Figure 2. When a router initially receives a multicast
            datagram sent by Host H2 to the multicast group A, the
            following steps are taken: Host H2 is first determined to be
            on Network N4. Then the shortest path tree rooted at net N4
            is calculated[7], pruning those branches that do not contain
            routers/transit networks that have been labelled with the
            multicast group A. This results in the pruned shortest-path
            tree pictured in Figure 3. Note that at this point all the
            leaves of the tree are routers/transit networks labelled
            with multicast group A (routers RT2 and RT9 and transit
            Network N6).

            In order to forward the multicast datagram, each router must
            find its own position in the datagram's shortest path tree.



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                                       o RT3 (N4, origin)
                                      / \
                                    1/   \8
                                    /     \
                           N3 (Mb) o       o RT6
                                  /         \
                                0/           \7
                                /             \
                   RT2 (Ma,Mb) o               o RT10
                                              / \
                                            3/   \1
                                            /     \
                                        N8 o       o N6 (Ma)
                                          /
                                        0/
                                        /
                                  RT11 o
                                      /
                                    1/
                                    /
                                N9 o
                                  /
                                0/
                                /
                      RT9 (Ma) o



                 Figure 3: Sample datagram's shortest-path tree,
                          source N4, destination Group A

            The router's (call it Router RTX) position in the datagram's
            pruned shortest-path tree consists of 1) RTX's parent in the
            tree (this will be the forwarding cache entry's upstream
            node) and 2) the list of RTX's interfaces that lead to
            downstream routers/transit networks that have been labelled
            with the datagram's destination (these will be added to the
            forwarding cache entry as downstream interfaces). Note that
            after calculating the datagram's shortest path tree, a
            router may find that it is itself not on the tree. This
            would be indicated by a forwarding cache entry having no
            upstream node or an empty list of downstream interfaces.

            As an example of a router describing its position on the
            datagram's shortest-path tree, consider Router RT10 in
            Figure 3. Router RT10's upstream node for the datagram is
            Router RT6, and there are two downstream interfaces: one



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            connecting to Network N6 and the other connecting to Network
            N8.

        2.3.3.  Support for Non-broadcast networks

            When forwarding multicast datagrams over non-broadcast
            networks, the datagram cannot be sent as a link-level
            multicast (since neither link-level multicast nor broadcast
            are supported on these networks), but must instead be
            forwarded separately to specific neighbors. To facilitate
            this, forwarding cache entries can also contain downstream
            neighbors as well as downstream interfaces.

            The IGMP protocol is not defined over non-broadcast
            networks. For this reason, there cannot be group members
            directly attached to non-broadcast networks, nor do non-
            broadcast networks ever appear in local group database
            entries.

            As an example, suppose that Network N3 in Figure 1 is an
            X.25 PDN.  Consider Router RT3's forwarding cache entry for
            datagrams having source Network N4 and multicast destination
            Group B. In place of having the interface to Network N3
            appear as the downstream interface in the matching
            forwarding cache entry, the neighboring routers RT1 and RT2
            would instead appear as separate downstream neighbors. In
            addition, in this case there could not be a Group B member
            directly attached to Network N3.

        2.3.4.  Details concerning forwarding cache entries

            Each of the downstream interface/neighbors in the cache
            entry is labelled with a TTL value. This value indicates the
            number of hops a datagram forwarded out of the interface (or
            forwarded to the neighbor) would have to travel before
            encountering a router/transit network requesting the
            multicast destination. The reason that a hop count is
            associated with each downstream interface/neighbor is so
            that IP multicast's expanding ring search procedure can be
            more efficiently implemented. By expanding ring search is
            meant the following. Hosts can restrict the frowarding
            extent of the IP multicast datagrams that they send by
            appropriate setting of the TTL value in the datagram's IP
            header.  Then, for example, to search for the nearest server
            the host can send multicasts first with TTL set to 1, then
            2, etc. By attaching a hop count to each downstream
            interface/neighbor in the forwarding cache, datagrams will
            not be forwarded unless they will ultimately reach a



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            multicast destination before their TTL expires[8].  This
            avoids wasting network bandwidth during an expanding ring
            search.

            As an example consider Router RT10's forwarding cache in
            Figure 3.  Router RT10's cache entry has two downstream
            interfaces. The first, connecting to Network N6, is labelled
            as having a group member one hop away (Network N6). The
            second, which connects to Network N8, is labelled as having
            a group member two hops away (Router RT9).

            Both the datagram shortest path tree and the local group
            database may contribute downstream interfaces to the
            forwarding cache entries. As an example, if a router has a
            local group database entry of [Group G, NX], then a
            forwarding cache entry for Group G, regardless of
            destination, will list the router interface to Network NX as
            a downstream interface. Such a downstream interface will
            always be labelled with a TTL of 1.

            As an example of forwarding cache entries, again consider
            the Autonomous System pictured in Figure 1. Suppose Host H2
            sends a multicast datagram to multicast group A. In that
            case, some routers will not even attempt to build a
            forwarding cache entry (e.g, router RT5) because they will
            never receive the multicast datagram in the first place.
            Other routers will receive the multicast datagram (since
            they are forwarded as link-level multicasts), but after
            building the pruned shortest path tree will notice that they
            themselves are not a part of the tree (routers RT1, RT4,
            RT7, RT8 and RT12). These latter routers will install an
            empty cache entry, indicating that they do not participate
            in the forwarding of the multicast datagram. A sample of the
            forwarding cache entries built by the other routers in the
            Autonomous System is pictured in Table 2.

            A MOSPF router must clear its entire forwarding cache when
            the Autonomous System's topology changes, because all the
            datagram shortest-path trees must be rebuilt. Likewise, when
            the location of a multicast group's membership changes
            (reflected by a change in group-membership-LSAs), all cache
            entries associated with the particular multicast destination
            group must be cleared. Other than these two cases,
            forwarding cache entries need not ever be deleted or
            otherwise modified; in particular, the forwarding cache
            entries do not have to be aged. However, forwarding cache
            entries can be freely deleted after some period of
            inactivity (i.e., garbage collected), if router memory



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              Router   Upstream     Downstream interfaces
                       node         (interface:hops)
              ___________________________________________
              RT10     Router RT6   (N6:1), (N8:2)
              RT11     Net N8       (N9:1)
              RT3      Net N4       (N3:1), (RT6:3)
              RT6      Router RT3   (RT10:2)
              RT2      Net N3       (N2:1)


               Table 2: Sample forwarding cache entries,
                 for source N4 and destination Group A.

            resources are in short supply.

3.  Inter-area multicasting

    Up to this point this memo has discussed multicast forwarding when
    the entire Autonomous System is a single OSPF area. The logic for
    when the multicast datagram's source and its destination group
    members belong to the same OSPF area is the same. This section
    explains the behavior of the MOSPF protocol when the datagram's
    source and (at least some of) its destination group members belong
    to different OSPF areas. This situation is called inter-area
    multicast.

    Inter-area multicast brings up the following issues, which are
    resolved in succeeding sections:

    o   Are the group-membership-LSAs specific to a single area? And if
        they are, how is group membership information conveyed from one
        area to the next?

    o   How are the datagram shortest-path trees built in the inter-area
        case, since complete information concerning the topology of the
        datagram source's neighborhood is not available to routers in
        other areas?

    o   In an area border router, multiple datagram shortest-path trees
        are built, one for each attached area. How are these separate
        datagram shortest-path trees combined into a single forwarding
        cache entry?

    It should be noted in the following that the basic protocol
    mechanisms in the inter-area case are the same as for the intra-area
    case.  Forwarding of multicasts is still defined by the contents of



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    the forwarding cache. The forwarding cache is still built from the
    same two components: the local group database and the datagram
    shortest-path trees. And while the calculation of the datagram
    shortest-path trees is different in the inter-area case (see Section
    3.2), the local group database is built exactly the same as in the
    intra-area case (i.e., MOSPF's interface with IGMP remains unchanged
    in the presence of areas). Finally, the forwarding algorithm
    described in Section 11 is the same for both the intra-area and
    inter-area cases.

    The following discussion uses the area configuration pictured in
    Figure 4 as an example. This figure, taken from the OSPF
    specification, shows an Autonomous System split into three areas
    (Area 1, Area 2 and Area 3). A single backbone area has been
    configured (everything outside of the shading). Since the backbone
    area must be contiguous, a single virtual link has been configured
    between the area border routers RT10 and RT11. Additionally, an area
    address range has been configured in Router RT11 so that Networks
    N9-N11 and Host H1 will be reported as a single route outside of
    Area 3 (via summary-link-LSAs).

    3.1.  Extent of group-membership-LSAs

        Group-membership-LSAs are specific to a single OSPF area. This
        means that, just as with OSPF router-LSAs, network-LSAs and
        summary-link-LSAs, a group-membership-LSA is flooded throughout
        a single area only[9].  A router attached to multiple areas
        (i.e., an area border router) may end up originating several
        group-membership-LSAs concerning a single multicast destination,
        one for each attached area.  However, as we will see below, the
        contents of these group-membership-LSAs will vary depending on
        their associated areas.

        Just as in OSPF, each MOSPF area has its own link state
        database. The MOSPF database is simply the OSPF link state
        database enhanced by the group-membership-LSAs. Consider again
        the area configuration pictured in Figure 4. The result of
        adding the group-membership-LSAs to the area databases yields
        the databases pictured in Figures 6 and 7.  Figure 6 shows Area
        1's MOSPF database. Figure 7 shows the backbone's MOSPF
        database. Superscripts indicate which transit vertices have been
        advertised as requesting particular multicast destinations. A
        superscript of "w" indicates that the router is advertising
        itself as a wild-card multicast receiver (see below). The dashed
        lines are OSPF summary-link-LSAs or AS external-link-LSAs. Note
        in Figure 7 that Router RT11 has condensed its routes to
        Networks N9-N11 and Host H1 into a single summary-link-LSA.




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           ..................................
           .     +                          .
           .     | 3+---+    +--+  +--+     . N12      N14
           .   N1|--|RT1|\1  |Mb|  |H4|     .   \ N13 /
           .    _|  +---+ \  +--+ /+--+     .   8\ |8/8
           .   | +         \ _|__/          .     \|/
           . +--+   +--+    /    \   1+---+8.   8+---+6
           . |Mb|   |Mb|   *  N3  *---|RT4|------|RT5|--------+
           . +--+  /+--+    \____/    +---+ .    +---+        |
           .      +         /   |           .      |7         |
           .      | 3+---+ /    |           .      |          |
           .    N2|--|RT2|/1    |1          .      |6         |
           .    __|  +---+    +---+8        .   6+---+        |
           .   |  +           |RT3|--------------|RT6|        |
           . +--+    +--+     +---+     +--+.    +---+        |
           . |Ma|    |H3|_      |2     _|H2|.    Ia|7         |
           . +--+    +--+ \     |     / +--+.      |          |
           .               +---------+      .      |          |
           .Area 1             N4           .      |          |
           ..................................      |          |
           ................................        |          |
           .           N11                .        |          |
           .       +---------+            .        |          |
           .            |     \           .        |          |    N12
           .            |3     +--+       .        |          |6 2/
           .          +---+    |Ma|       .        |        +---+/
           .          |RT9|    +--+       .        |        |RT7|---N15
           .          +---+               .......  |        +---+ 9
           .            |1                .. +  ...|..........|1........
           .           _|__               .. |   Ib|5       __|_   +--+.
           .          /    \      1+----+2.. |  3+----+1   /    \--|Ma|.
           .         *  N9  *------|RT11|----|---|RT10|---*  N6  * +--+.
           .          \____/       +----+ .. |   +----+    \____/      .
           .            |            !*******|*****!          |        .
           .            |1           Virtual + Link           |1       .
           . +--+   10+----+              ..N8              +---+      .
           . |H1|-----|RT12|              ..                |RT8|      .
           . +--+SLIP +----+              ..                +---+  +--+.
           .            |2                ..                  |4  _|H5|.
           .            |                 ..                  |  / +--+.
           .       +---------+            ..              +--------+   .
           .           N10          Area 3..Area 2            N7       .
           .............................................................

                    Figure 4: A sample MOSPF area configuration





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        Suppose an OSPF router has a local group database entry for
        [Group Y, Network X]. The router then originates a group-
        membership-LSA for Group Y into the area containing Network X.
        For example, in the area configuration pictured in Figure 4,
        Router RT1 originates a group-membership-LSA for Group B. This
        group-membership-LSA is flooded throughout Area 1, and no
        further. Likewise, assuming that Router RT3 has been elected
        Designated Router for Network N3, RT3 originates a group-
        membership-LSA into Area 1 listing the transit Network N3 as
        having group members. Note that in the link state database for
        Area 1 (Figure 6) both Router RT1 and Network N3 have
        accordingly been labelled with Group B.

        In OSPF, the area border routers forward routing information and
        data traffic between areas. In MOSPF. a subset of the area
        border routers, called the inter-area multicast forwarders,
        forward group membership information and multicast datagrams
        between areas. Whether a given OSPF area border router is also a
        MOSPF inter-area multicast forwarder is configuration dependent
        (see Section B.1). In Figure 4 we assume that all area border
        routers are also inter-area multicast forwarders.

        In order to convey group membership information between areas,
        inter-area multicast forwarders "summarize" their attached
        areas' group membership to the backbone. This is very similar
        functionality to the summary-link-LSAs that are generated in the
        base OSPF protocol.  An inter-area multicast forwarder
        calculates which groups have members in its attached non-
        backbone areas. Then, for each of these groups, the inter-area
        multicast forwarder injects a group-membership-LSA into the
        backbone area. For example, in Figure 4 there are two groups
        having members in Area 1: Group A and Group B. For that reason,
        both of Area 1's inter-area multicast forwarders (Routers RT3
        and RT4) inject group-membership-LSAs for these two groups into
        the backbone.  As a result both of these routers are labelled

                membership    +------------------+   datagrams
                    + > > > >>|     Backbone     |< < < < +
                    ^         +------------------+        ^
                    ^        /         |          \       ^
                    ^       /          |           \      ^
               +----^-----+/      +----------+      \+----^-----+
               |  Area 1  |       |  Area 2  |       |  Area 3  |
               +----------+       +----------+       +----------+

                    Figure 5: Inter-area routing architecture





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        with Groups A and B in the backbone link state database (see
        Figure 7).

        However, unlike the summarization of unicast destinations in the
        base OSPF protocol, the summarization of group membership in
        MOSPF is asymmetric. While a non-backbone area's group
        membership is summarized to the backbone, this information is
        not then readvertised into other non-backbone areas. Nor is the
        backbone's group membership summarized for the non-backbone
        areas. Going back to the example in Figure 4, while the presence
        of Area 3's group (Group A) is advertised to the backbone, this
        information is not then redistributed to Area 1. In other words,
        routers internal to Area 1 have no idea of Area 3's group
        membership.

        At this point, if no extra functionality was added to MOSPF,
        multicast traffic originating in Area 1 destined for Multicast
        Group A would never be forwarded to those Group A members in
        Area 3. To accomplish this, the notion of wild-card multicast
        receivers is introduced. A wild-card multicast receiver is a
        router to which all multicast traffic, regardless of multicast
        destination, should be forwarded. A router's wild-card multicast
        reception status is per-area. In non-backbone areas, all inter-
        area multicast forwarders[10] are wild-card multicast receivers.
        This ensures that all multicast traffic originating in a non-
        backbone area will be forwarded to its inter-area multicast
        forwarders, and hence to the backbone area. Since the backbone
        has complete knowledge of all areas' group membership, the
        datagram can then be forwarded to all group members. Note that
        in the backbone itself there is no need for wild-card multicast
        receivers[11].  As an example, note that Routers RT3 and RT4 are
        wild-card multicast receivers in Area 1 (see Figure 6), while
        there are none in the backbone (see Figure 7).

        This yields the inter-area routing architecture pictured in
        Figure 5.  All group membership is advertised by the non-
        backbone areas into the backbone. Likewise, all IP multicast
        traffic arising in the non-backbone areas is forwarded to the
        backbone. Since at this point group membership information meets
        the multicast datagram traffic, delivery of the multicast
        datagrams becomes possible.

    3.2.  Building inter-area datagram shortest-path trees

        When building datagram shortest-path trees in the presence of
        areas, it is often the case that the source of the datagram and
        (at least some of) the destination group members are in separate
        areas. Since detailed topological information concerning one



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                                  **FROM**

                             |RT|RT|RT|RT|RT|RT|
                             |1 |2 |3 |4 |5 |7 |N3|
                          ----- -------------------
                          RT1|  |  |  |  |  |  |0 |
                          RT2|  |  |  |  |  |  |0 |
                          RT3|  |  |  |  |  |  |0 |
                      *   RT4|  |  |  |  |  |  |0 |
                      *   RT5|  |  |14|8 |  |  |  |
                      T   RT7|  |  |20|14|  |  |  |
                      O    N1|3 |  |  |  |  |  |  |
                      *    N2|  |3 |  |  |  |  |  |
                      *    N3|1 |1 |1 |1 |  |  |  |
                           N4|  |  |2 |  |  |  |  |
                        Ia,Ib|  |  |15|22|  |  |  |
                           N6|  |  |16|15|  |  |  |
                           N7|  |  |20|19|  |  |  |
                           N8|  |  |18|18|  |  |  |
                    N9-N11,H1|  |  |19|16|  |  |  |
                          N12|  |  |  |  |8 |2 |  |
                          N13|  |  |  |  |8 |  |  |
                          N14|  |  |  |  |8 |  |  |
                          N15|  |  |  |  |  |9 |  |


                     Figure 6: Area 1's MOSPF database.

             Networks and routers are represented by vertices.
             An edge of cost X connects Vertex A to Vertex B iff
             the intersection of Column A and Row B is marked
             with an X. In addition, RT1, RT2 and N3 are labelled
             with multicast group A, RT1 is labelled with multicast
             group B, and both RT3 and RT4 are labelled as
             wild-card multicast receivers.















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                                 **FROM**

                           |RT|RT|RT|RT|RT|RT|RT
                           |3 |4 |5 |6 |7 |10|11|
                        ------------------------
                        RT3|  |  |  |6 |  |  |  |
                        RT4|  |  |8 |  |  |  |  |
                        RT5|  |8 |  |6 |6 |  |  |
                        RT6|8 |  |7 |  |  |5 |  |
                        RT7|  |  |6 |  |  |  |  |
                    *  RT10|  |  |  |7 |  |  |2 |
                    *  RT11|  |  |  |  |  |3 |  |
                    T    N1|4 |4 |  |  |  |  |  |
                    O    N2|4 |4 |  |  |  |  |  |
                    *    N3|1 |1 |  |  |  |  |  |
                    *    N4|2 |3 |  |  |  |  |  |
                         Ia|  |  |  |  |  |5 |  |
                         Ib|  |  |  |7 |  |  |  |
                         N6|  |  |  |  |1 |1 |3 |
                         N7|  |  |  |  |5 |5 |7 |
                         N8|  |  |  |  |4 |3 |2 |
                  N9-N11,H1|  |  |  |  |  |  |1 |
                        N12|  |  |8 |  |2 |  |  |
                        N13|  |  |8 |  |  |  |  |
                        N14|  |  |8 |  |  |  |  |
                        N15|  |  |  |  |9 |  |  |


                 Figure 7: The backbone's MOSPF database.

             Networks and routers are represented by vertices.
             An edge of cost X connects Vertex A to Vertex B iff
             the intersection of Column A and Row B is marked
             with an X. In addition, RT3 and RT4 are labelled
             with both multicast groups A and B, and RT7, RT10,
             and RT11 are labelled with multicast group A.

        OSPF area is not distributed to other OSPF areas (the flooding
        of router-LSAs, network-LSAs and group-membership-LSAs is
        restricted to a single OSPF area only), the building of complete
        datagram shortest-path trees is often impossible in the inter-
        area case. To compensate, approximations are made through the
        use of wild-card multicast receivers and OSPF summary-link-LSAs.

        When it first receives a datagram for a particular [source net,
        destination group] pair, a router calculates a separate datagram
        shortest-path tree for each of the router's attached areas. Each
        datagram shortest-path tree is built solely from LSAs belonging



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        to the particular area's link state database. Suppose that a
        router is calculating a datagram shortest-path tree for Area A.
        It is useful then to separate out two cases.

        The first case, Case 1: The source of the datagram belongs to
        Area A has already been described in Section 2.3.2. However, in
        the presence of OSPF areas, during tree pruning care must be
        taken so that the branches leading to other areas remain, since
        it is unknown whether there are group members in these (remote)
        areas. For this reason, only those branches having no group
        members nor wild-card multicast receivers are pruned when
        producing the datagram shortest-path tree.

        As an example, suppose in Figure 4 that Host H2 sends a
        multicast datagram to destination Group A. Then the datagram's
        shortest-path tree for Area 1, built identically by all routers
        in Area 1 that receive the datagram, is shown in Figure 8. Note
        that both inter-area multicast forwarders (RT3 and RT4) are on
        the datagram's shortest-path tree, ensuring the delivery of the
        datagram to the backbone and from there to Areas 2 and 3.

        o   Case 2: The source of the datagram belongs to an area other
            than Area A. In this case, when building the datagram
            shortest-path tree for Area A, the immediate neighborhood of
            the datagram's source is unknown. However, there are
            summary-link-LSAs in the Area A link state database
            indicating the cost of the paths between each of Area A's
            inter-area multicast forwarders and the datagram source.
            These summary links are used to approximate the neighborhood
            of the datagram's source; the tree begins with links
            directly connecting the source to each of the inter-area
            multicast forwarders. These links point in the reverse

                                      o RT3 (W, origin=N4)
                                      |
                                     1|
                                      |
                              N3 (Mb) o
                                     / \
                                   0/   \0
                                   /     \
                      RT2 (Ma,Mb) o       o RT4 (W)


                    Figure 8: Datagram's shortest-path tree,
                      Area 1, source N4, destination Group A





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            direction (towards instead of away from the datagram source)
            from the links considered in Case 1 above. All additional
            links added to the tree also point in the reverse direction.
            The final datagram shortest-path tree is then produced by,
            as before, pruning all branches having no group-members nor
            wild-card multicast receivers.

            As an example, suppose again that Host H2 in Figure 4 sends
            a multicast datagram to destination Group A. The datagram's
            shortest-path tree for the backbone is shown in Figure 9.
            The neighborhood around the source (Network N4) has been
            approximated by the summary links advertised by routers RT3
            and RT4. Note that all links in Figure 9's datagram
            shortest-path tree have arrows pointing in the reverse
            direction, towards Network N4 instead of away from it.

        The reverse costs used for the entire tree in Case 2 are forced
        because summary-link-LSAs only specify the cost towards the
        datagram source. In the presence of asymmetric link costs, this
        may lead to less efficient routes when forwarding multicasts

                                     o N4
                                    / \
                                  2/   \3
                                  /     \
                     RT3 (Ma,Mb) o       o RT4 (Ma,Mb)
                                /         \
                              6/           \8
                              /             \
                         RT6 o               o RT5
                             |               |
                            5|               |6
                             |               |
                   RT10 (Ma) o               o RT7 (Ma)
                             |
                            2|
                             |
                   RT11 (Ma) o



               Figure 9: Datagram shortest-path tree: Backbone,
                  source N4, destination Group A. Note that
                  reverse costs (i.e., toward origin) are
                             used throughout.






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        between areas.

        Those routers attached to multiple areas must calculate multiple
        trees and then merge them into a single forwarding cache entry.
        As shown in Section 2.3.2, when connected to a single area the
        router's position on the datagram shortest-path tree determines
        (in large part) its forwarding cache entry. When attached to
        multiple areas, and hence calculating multiple datagram
        shortest-path trees, each tree contributes to the forwarding
        cache entry's list of downstream interfaces/neighbors. However,
        only one of the areas' datagram shortest-path trees will
        determine the forwarding cache entry's upstream node. When one
        of the attached areas contains the datagram source, that area
        will determine the upstream node. Otherwise, the tiebreaking
        rules of Section 12.2.7 are invoked.

        Consider again the example of Host H2 in Figure 4 sending a
        multicast datagram to destination Group A. Router RT3 will
        calculate two datagram shortest-path trees, one for Area 1 and
        one for the backbone.  Since the source of the datagram (Host
        H2) belongs to Area 1, the Area 1 datagram shortest-path tree
        determines RT3's upstream node: Network N4. Router RT3
        calculates two downstream interfaces for the datagram: the
        interface to Network N3 (which comes from Area 1's datagram
        shortest-path tree) and the serial line to Router RT6 (which
        comes from the backbone's datagram shortest-path tree). As for
        Router RT10, it calculates two trees, determining its upstream
        node from the backbone tree and its two downstream interfaces
        from the Area 2 tree.  Finally, Router RT11 calculates three
        trees, determining its upstream node from the Area 2 tree and
        its downstream interface from the Area 3 tree.

4.  Inter-AS multicasting

    This section explains how MOSPF deals with the forwarding of
    multicast datagrams between Autonomous Systems. Certain AS boundary
    routers in a MOSPF system will be configured as inter-AS multicast
    forwarders. It is assumed that these routers will also be running an
    inter-AS multicast routing protocol. This specification does not
    dictate the operation of such an inter-AS multicast routing
    protocol. However, the following interactions between MOSPF and the
    inter-AS routing protocol are assumed:

    (1) MOSPF guarantees that the inter-AS multicast forwarders will
        receive all multicast datagrams; but it is up to each router so
        designated to determine whether the datagram should be forwarded
        to other Autonomous Systems. This determination will probably be
        made via the inter-AS routing protocol.



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    (2) MOSPF assumes that the inter-AS routing protocol is forwarding
        multicast datagrams in an RPF (reverse path forwarding; see
        [Deering] for an explanation of this terminology) fashion. In
        other words, it is assumed that a multicast datagram whose
        source (call it X) lies outside the MOSPF domain will enter the
        MOSPF domain at those points that are advertising (into OSPF)
        the best routes back to X. MOSPF calculates the path of the
        datagram through the MOSPF domain based on this assumption.

    MOSPF designates an inter-AS multicast forwarder as a wild-card
    multicast receiver in all of its attached areas. As in the inter-
    area case, this ensures that the routers remain on all pruned
    shortest-path trees and thereby receive all multicast datagrams,
    regardless of destination.

    As an example, suppose that in Figure 1 both RT5 and RT7 were
    configured as inter-AS multicast forwarders. Then the link state
    database would look like the one pictured in Figure 2, with the
    addition of a) wild-card status for RT5 and RT7 (they would appear
    with superscripts of "w") and b) the external links originated by
    RT5 and RT7 being labelled as multicast-capable[12].

    As another example, consider the area configuration in Figure 4.
    Again suppose RT5 and RT7 are configured as inter-AS multicast
    forwarders. Then in Area 1's link state database (Figure 6), the
    external links originated by RT5 and RT7 would again be labelled as
    multicast-capable. However, note that in Area 1's database RT5 and
    RT7 are not labelled as wild-card multicast receivers. This is
    unnecessary; since Area 1's inter-area multicast forwarders (RT3 and
    RT4) are wild-cards, all multicast datagrams will be forwarded to
    the backbone. And in the backbone's link state database (Figure 7),
    RT5 and RT7 will be labelled as wild-cards.

    4.1.  Building inter-AS datagram shortest-path trees.

        When multicast datagrams are to be forwarded between Autonomous
        Systems, the datagram shortest-path tree is built as follows.
        Remember that the router builds a separate tree for each area to
        which it is attached; these trees are then merged into a single
        forwarding cache entry. Suppose that the router is building the
        tree for Area A. We break up the tree building into three cases.
        This first two cases have already been described earlier in this
        memo: Case 1 (the source of the datagram belongs to Area A)
        having been described in Section 2.3.2 and Case 2 (the source of
        the datagram belongs to another OSPF area) having been described
        in Section 3.2. The only modification to these cases is that
        inter-AS multicast forwarders, as well as group members and
        inter-area multicast forwarders, must remain on the pruned



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        trees.  The new case is as follows:

        o   Case 3: The source of the datagram belongs to another
            Autonomous System. The immediate neighborhood of the source
            is then unknown. In this case the multicast-capable AS
            external links are used to approximate the neighborhood of
            the source; the tree begins with links directly attaching
            the source to one or more inter-AS multicast forwarders. The
            approximating AS external links point in the reverse
            direction (i.e., towards the source), just as with the
            approximating summary links in Case 2. Also, as in Case 2,
            all links included in the tree must point in the reverse
            direction. The final datagram shortest-path tree is then
            produced (as always) by pruning those branches having no
            group members nor wild-card multicast receivers.

            As an example, suppose that a host on Network N12 (see
            Figure 4) originates a multicast datagram for Destination
            Group B. Assume that all external costs pictured are OSPF
            external type 1 metrics. Then any routers in Area 1
            receiving the datagram would build the datagram shortest-
            path tree pictured in Figure 10. Note that all links in the
            tree point in the reverse direction, towards the source. The
            tree indicates that the routers expect the datagram to enter
            the Autonomous System at Router RT7, and then to enter the
            area at Router RT4.

            Note that in those cases where the "best" inter-AS multicast
            forwarder is not directly attached to the area, the
            neighborhood of the source is actually approximated by the
            concatenation of a summary link and a multicast-capable AS
            external link. This is in fact the case in Figure 10.

        In Case 3 (datagram source in another AS) the requirement that
        all tree links point in the reverse direction (towards the
        source) accommodates the fact that summary links and AS external
        links already point in the reverse direction. This also leads to
        the requirement that the inter-AS multicast routing protocol
        operate in a reverse path forwarding fashion (see condition 2 of
        Section 4). Note that Reverse path forwarding can lead to sub-
        optimal routing when costs are configured asymmetrically. And it
        can even lead to non-delivery of multicast datagrams in the case
        of asymmetric reachability.

        Inter-AS multicast forwarders may end up calculating a
        forwarding cache entry's upstream node as being external to the
        AS. As an example, Router RT7 in Figure 10 will end up
        calculating an external router (via its external link to Network



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                                     o N12
                                     |
                                    2|
                                     |
                                     o RT7
                                     |
                                   14|
                                     |
                                     o RT4 (W)
                                     |
                                    0|
                                     |
                                     o N3 (Mb)
                                    /|\
                                   / | \
                                 1/  | 1\
                                 /  1|   \
                                /    |    \
                      RT1 (Mb) o     |     o RT3 (W)
                                     o
                                RT2 (Ma,Mb)


               Figure 10: Datagram shortest-path tree: Area 1,
                 source N12, destination Group B. Note that
                  reverse costs (i.e., toward origin) are
                             used throughout.

        N12) as the upstream node for the datagram. This means that RT7
        must receive the datagram from a router in another AS before
        injecting the datagram into the MOSPF system.

    4.2.  Stub area behavior

        AS external links are not imported into stub areas. Suppose that
        the source of a particular datagram lies outside of the
        Autonomous System, and that the datagram is forwarded into a
        stub area. In the stub area's datagram shortest-path tree the
        neighborhood of the datagram's source cannot be approximated by
        AS external links. Instead the neighborhood of the source is
        approximated by the default summary links (see Section 3.6 of
        [OSPF]) that are originated by the stub area's intra-area
        multicast forwarders.

        Except for this small change to the construction of a stub
        area's datagram shortest-path trees, all other MOSPF algorithms
        (e.g., merging with other areas' datagram shortest-path trees to



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        form the forwarding cache) function the same for stub areas as
        they do for non-stub areas.

    4.3.  Inter-AS multicasting in a core Autonomous System

        It may be the case that the MOSPF routing domain connects
        together many different Autonomous Systems, thereby serving as a
        "core Autonomous System" (e.g, the old NSFNet backbone). In this
        case, it could very well be that the majority of the MOSPF
        routers are also inter-AS multicast forwarders. Having each
        inter-AS multicast forwarder then declare itself a wild-card
        multicast receiver could very well waste considerable network
        bandwidth. However, as an alternative to declaring themselves
        wild-card multicast receivers, the inter-AS multicast routers
        could instead explicitly advertise all groups that they were
        interested in forwarding (to other "client" Autonomous Systems)
        in group-membership-LSAs. These advertised groups would have to
        be learned through an inter-AS multicast routing protocol (or
        possibly even statically configured).

        This in essence allows the clients of the core Autonomous System
        to advertise their group membership into the core. However,
        since any client MOSPF domains will still have their inter-AS
        multicast forwarders configured as wild-card multicast
        receivers, this advertisement will be asymmetric: the core will
        not advertise its or others' group membership to the clients.
        The achieves the same inter-AS multicast routing architecture
        that MOSPF uses for inter-area multicast routing (see Figure 5).

5.  Modelling internal group membership

    A MOSPF router may itself contain multicast applications. A typical
    example of this is a UNIX workstation that doubles as a multicast
    router. This section concerns two alternative ways of representing
    the group membership of the MOSPF router's internal applications.
    Both representations have advantages. For maximum flexibility, the
    MOSPF forwarding algorithm (see Section 11) has been specified so
    that either representation can be used in a MOSPF router (and in
    fact, both representations can be used at once, depending on the
    application).

    The first representation is based on the paradigm presented in RFC
    1112. In this case, an application joins a multicast group on one or
    more specific physical interfaces. The application then receives a
    multicast datagram if and only if it is received on one of the
    specified interfaces. If a datagram is received on multiple
    specified interfaces, the application receives multiple copies.
    Figure 11 shows this algorithm as it is implemented in (modified)



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    BSD UNIX kernels.  The figure shows the processing of a multicast
    datagram, starting with its reception on a particular interface.
    First copies of the datagram are given to those applications that
    have joined on the receiving interface. Then the forwarding decision
    (pictured as a box containing a question mark) is made, and the
    packet is (possibly) forwarded out certain interfaces. If these
    interfaces are not capable of receiving their own multicasts, a copy
    of the datagram must be internally looped back to appropriately
    joined applications.

    The advantages to the RFC 1112 representation are as follows:

    o   It is the standard for the way an IP host joins multicast
        groups. It is simplest to use the same membership model for
        hosts and routers; most would consider an IP router to be a
        special case of an IP host anyway.

    o   It is the way group membership has been implemented in BSD UNIX.
        Existing multicast applications are written to join multicast
        groups on specific interfaces.

    o   The possibility of receiving multiple datagram copies may
        improve fault tolerance. If the datagram is dropped due to an

                            +-------+
                            |receive|
                            +-------+
                                |
                                |---> To application
                                |
                      +-------------------+
                      |forwarding decision|
                      +-------------------+
                                |
                               / \
                              /---\----> To application
                             /     \------> To application
                            /       \
                           /         \
                     +--------+  +--------+
                     |transmit|  |transmit|
                     +--------+  +--------+


              Figure 11: RFC 1112 representation of internal
                          group membership





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        error on the path to some interface, another interface may still
        receive a copy.

    o   The ability to specify a particular receiving interface may
        improve the accuracy of IP multicast's expanding ring search
        mechanism (see Section 2.3.4).

    o   Membership in the non-routable multicast groups (224.0.0.1 -
        224.0.0.255) must be on a per-interface basis. An OSPF router
        always belongs to 224.0.0.5 (AllSPFRouters) on its OSPF
        interfaces, and may belong to 224.0.0.6 (AllDRouters) on one or
        more of its OSPF interfaces.

    The second representation is MOSPF-specific. In this case, an
    application joins a multicast group on an interface-independent
    basis.  In other words, group membership is associated with the
    router as a whole, not separately on each interface. The application
    then receives a copy of a multicast datagram if and only if the
    datagram would actually be forwarded by the MOSPF router. Figure 12
    shows how this algorithm would be implemented. The datagram is
    received on a particular interface. If the datagram is validated for
    forwarding (i.e., the receiving interface connects to the matching
    forwarding cache entry's upstream node), a copy of the datagram is
    also given to appropriately joined applications. Note that this
    model of group membership is not as general as the RFC 1112 model,
    in that it can only be implemented in MOSPF routers and not in
    arbitrary IP hosts.  However, it has the following advantages:

    o   The application does not need to have knowledge of the router
        interfaces. It does not need to know what kind or how many
        interfaces there are; this will be taken care of by the MOSPF
        protocol itself.

    o   As long as any interface is operational, the application will
        continue to receive multicast datagrams. This happens
        automatically, without the application modifying its group
        membership.

    o   The application receives only one copy of the datagram. Using
        the RFC1112 representation, whenever an application joins on
        more than one interface (which must be done if the application
        does not want to rely on a single interface), multiple datagram
        copies will be received during normal operation.

6.  Additional capabilities

    This section describes the MOSPF configuration options that allow
    routers of differing capabilities to be mixed together in the same



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                            +-------+
                            |receive|
                            +-------+
                                |
                                |
                                |
                      +-------------------+
                      |forwarding decision|---> to application
                      +-------------------+
                                |
                               / \
                              /   \
                             /     \
                            /       \
                           /         \
                     +--------+  +--------+
                     |transmit|  |transmit|
                     +--------+  +--------+


              Figure 12: MOSPF-specific representation of internal
                             group membership

    routing domain. Note that these options handle special circumstances
    that may not be encountered in normal operation. Default values for
    the configuration settings are specified in Appendix B.

    6.1.  Mixing with non-multicast routers

        MOSPF routers can be mixed freely with routers that are running
        only the base OSPF algorithm (called non-multicast routers in
        the following). This allows MOSPF to be deployed in a piecemeal
        fashion, thereby speeding deployment and allowing
        experimentation with multicast routing on a limited scale.

        When a MOSPF router builds a datagram shortest-path tree, it
        omits all non-multicast routers. For example, in Figure 1, if
        Router RT6 was not a multicast router, the datagram shortest-
        path tree in Figure 3 would be built with a more circuitous
        branch through Router RT5, instead of through Router RT6. In
        addition, non-multicast routers do not participate in the
        flooding of the new group-membership-LSAs. This adheres to the
        general principle that a router should not have to handle those
        link state advertisements whose format (or contents) the router
        does not understand.





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        Mixing MOSPF routers with non-multicast routers creates a number
        of potential problems. Certain mixings of MOSPF and non-
        multicast routers can cause multicast datagrams to take
        suboptimal paths, or in other cases can lead to the non-delivery
        of multicast datagrams. In addition, mixing MOSPF routers and
        non-multicast routers can cause the paths of multicast datagrams
        to diverge radically from the path of unicast datagrams. Such
        divergences can make routing problems harder to debug.

        In particular, the following specific difficulties may arise
        when mixing MOSPF routers with non-multicast routers:

        o   Even though there is unicast connectivity to a destination,
            there may not be multicast connectivity. For example, if
            Router RT10 in Figure 1 becomes a non-multicast router, the
            group member connected to Network N11 will no longer be able
            to receive multicasts sourced by Host H2.  But the two hosts
            will be able to exchange unicasts (e.g., ICMP pings).

        o   When the Designated Router for a multi-access network is a
            non-multicast router, the network will not be used for
            forwarding multicast datagrams. For example, if in Figure 1
            Router RT4 is Designated Router for Network N3, and RT4 is
            non-multicast, Network N3 will not be used to forward IP
            multicasts. This would mean that multicast datagrams
            originated by Hosts H2 and H3 would not be forwarded beyond
            their local network (N4), even though it seems that the
            needed multicast connectivity exists.

        o   When forwarding multicast datagrams between areas, mixing of
            MOSPF routers and non-multicast routers in the source area
            may cause unexpected loss of multicast connectivity. This is
            because in the inter-area routing of multicast datagrams the
            neighborhood of the datagram's source is approximated by
            OSPF summary links, and OSPF summary-link-LSAs do not carry
            indications/guarantees of the summarized path's multicast
            routing capability.

    6.2.  TOS-based multicast

        MOSPF allows a separate datagram shortest-path tree to be built
        for each IP Type of Service. This means that the path of a
        multicast datagram can vary depending on the datagram's TOS
        classification, as well as its source and destination.

        For each router interface, OSPF allows a separate metric to be
        configured for each IP TOS. When building the shortest path tree
        for TOS X, the cost of a path is the sum of the component



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        interfaces' TOS X metrics. Note that OSPF requires that a TOS 0
        metric be specified for each interface. However, as a form of
        data compression, metrics need only be specified for non-zero
        TOS if they are different than the TOS 0 metric.

        Additionally, OSPF routers can be configured to ignore TOS when
        forwarding packets. Such routers, called TOS-incapable, build
        only the TOS 0 portion of the routing table. TOS-incapable
        routers can be mixed freely with TOS-capable routers when
        forwarding unicast packets. The way this is handled for unicast
        packets is that the unicast is forwarded along the TOS 0 route
        whenever the TOS X route does not exist. However, MOSPF must
        treat this situation somewhat differently, since each router
        must build the exact same tree rooted at the datagram's source.

        Like OSPF, MOSPF allows TOS-based routing to be optional. TOS-
        capable and TOS-incapable multicast routers can be mixed freely
        in the routing domain. TOS-incapable routers will only ever
        build TOS 0 datagram shortest-path trees. TOS-capable routers
        will first build TOS 0 datagram shortest-path trees. If these
        trees contain only TOS-capable routers, datagram shortest-path
        trees are then built separately for non-zero TOS values.
        Otherwise, the TOS 0 datagram shortest-path tree is used to
        forward all traffic, regardless of its TOS designation.  Using
        this logic, all routers in essence continue to utilize identical
        datagram shortest-path trees. See Section 12.2.8 for more
        details.

    6.3.  Assigning multiple IP networks to a physical network

        Assigning multiple IP networks/subnets to a single physical
        network causes some confusion in MOSPF. This is because the
        underlying OSPF protocol treats these IP networks/subnets as
        entirely separate entities, originating separate network-LSAs
        for each and forming separate adjacencies for each, while IGMP
        recognizes only the single underlying physical network. Adding
        to the problem is the fact that when a multicast datagram is
        received from such a multiply-addressed physical wire, there is
        no good way to choose the datagram's upstream node (which must
        be done in order to make the forwarding decision; see Section 11
        for details). As a result, unless this situation is dealt with
        through configuration, unwanted replication of multicast
        datagrams may occur when they are forwarded over multiply-
        addressed wires.

        As a remedy, MOSPF allows multicast forwarding to be disabled on
        certain IP networks/subnets. When multicast forwarding is
        disabled on the wire's "extra" subnets (i.e., all but one), the



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        extra subnets will not appear in datagram shortest-path trees,
        nor will they appear in local group database or forwarding cache
        entries. As a result, the possibility of unwanted datagram
        replication is eliminated. The actual disabling of multicast
        forwarding on a subnet is done through setting the
        IPMulticastForwarding parameter to disabled on all router
        interfaces connecting to the subnet (see Section B.2).

    6.4.  Networks on Autonomous System boundaries

        Another complication can arise on IP networks/subnets that lie
        on the boundary of a MOSPF Autonomous System. Similar to the
        unicast situation where these networks may be running multiple
        IGPs (Interior Gateway Protocols), these networks may also be
        running multiple multicast routing protocols. It may then become
        impossible for a MOSPF router to determine whether a multicast
        datagram is being forwarded along the datagram shortest-path
        tree, or whether it has been inadvertently received from the
        other Autonomous System. Guessing wrong can lead to either
        unwanted replication or non-delivery of the multicast datagram.
        In addition, in order to prevent receiving duplicate multicast
        datagrams, group members on these boundary networks will
        probably want to declare their membership to one Autonomous
        System and not another.

        For example, consider the two Autonomous Systems pictured in
        Figure 13. Network X is on the boundary of both ASes. One
        possible multicast datagram path is shown; the datagram
        originates in a third Autonomous System, and is then delivered
        to both AS #1 and AS #2 separately. The paths through the two
        Autonomous Systems may end up having certain boundary networks
        as common segments. In Figure 13, Network X is common to both
        paths. In this case, if both Autonomous Systems were running
        (separate copies of) MOSPF, the same datagram would appear twice
        on Network X as a data-link multicast. This would cause
        duplicate datagrams to be received by any group members on
        Network X or downstream from Network X.

        MOSPF has two mechanisms to eliminate this replication of
        multicast datagrams. First, a system administrator can configure
        certain networks to forward multicast datagrams as data-link
        unicasts instead of data-link multicasts. This is done by
        setting the IPMulticastForwarding parameter to data-link unicast
        on those router interfaces attaching to the network (see Section
        B.2). As an example, in Figure 13 the routers in AS #2 could be
        configured so that Router C would send the multicast datagram
        out onto Network X as a data-link unicast addressed directly to
        Router D. Router D would accept this data-link unicast, but



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                              <-Datagram path->*
                             *                 *
                             *                 *
                             *            .....*.........
                    .........*.....   |   .    *    AS #2
                    AS #1    *    .   |*****+---+
                            +---+*****|*----|RTC|
                            |RTA|----*|*  . +---+
                            +---+ .  *|*  .
                                  .  *|*  .
                                  .  *|*  . +---+
                            +---+ .  *|*----|RTD|
                            |RTB|----*|*****+---+
                            +---+*****|   .....*..........
                    .........*....    |        *
                             *        |        *
                             *    Network X    *
                             *

                     Figure 13: Networks on AS boundaries

        would reject any data-link multicast forwarded by Router A. This
        would eliminate replication of multicast datagrams downstream
        from Network X. In addition, if the IPMulticastForwarding
        parameter is set to data-link unicast on Network X, group
        membership will not be monitored on the network. This will
        prevent group members attached directly to Network X from
        receiving multiple datagram copies, since group membership on
        the boundary network will be monitored from only one AS (AS #1
        in our example).

        It should be noted that forwarding IP multicasts as data-link
        unicasts has some disadvantages when three or more MOSPF routers
        are attached to the network. First of all, it is more work for a
        router to send multiple unicasts than a single multicast.
        Second, the multiple unicasts consume more network bandwidth
        than a single multicast. And last, it increases the delay for
        some group members since multiple unicasts also take longer to
        send than a single multicast.

    6.5.   Recommended system configuration

        In order to make MOSPF's selection of routes more predictable,
        it is recommended that all routers in any particular OSPF area
        have the same multicast and TOS capabilities.Keeping areas
        homogeneous ensures that IP multicast packets will follow
        relatively the same path as IP unicasts. In contrast, while



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        heterogeneous areas will function, and will probably be
        necessary at least during the initial introduction of multicast
        routing, such areas may produce seemingly sub-optimal and
        unexpected routes. For example, see Section 6.1 above for a
        detailed description of the possible pitfalls when mixing
        multicast and non-multicast routers.

        As for the other options presented above, to achieve the most
        predictable results it is recommended that a router interface's
        IPMulticastForwarding parameter be set to a value other than
        data-link multicast only when either a) multiple IP networks
        have been assigned to a single physical wire or b) multiple
        multicast routing protocols are running on the attached network.






































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7.  Basic implementation requirements

    An implementation of MOSPF requires the following pieces of system
    support. Note that this support is in addition to that required for
    the base OSPF implementation as outlined in Section 4.4 of [OSPF].

    o   Promiscuous multicast reception. In a multicast router, it is
        necessary to receive all IP multicasts at the data-link level.
        On those interfaces where IP multicast datagrams are
        encapsulated by a wide range of data-link multicast destination
        addresses (e.g, ethernet and FDDI), this is most easily
        accomplished by disabling any hardware filtering of multicast
        destinations (i.e., by "opening up" the interface's multicast
        filter).

    o   Data-link multicast/broadcast detection. To avoid unwanted
        replication of multicast datagrams in certain exceptional
        conditions, it is necessary for the multicast router to
        determine whether a datagram was received as a data-link
        multicast/broadcast or as a data-link unicast, for later use by
        the MOSPF forwarding mechanism.  See Section 6.4 for more
        details.

    o   An implementation of IGMP. MOSPF uses the Internet Group
        Management Protocol (IGMP, documented in [RFC 1112]) to monitor
        multicast group membership. See Section 9 for details.

8.  Protocol data structures

    The MOSPF protocol is described herein in terms of its operation on
    various protocol data structures. These data structures are included
    for explanatory uses only, and are not intended to constrain a MOSPF
    implementation. Besides the data structures listed below, this
    specification will also reference the various data structures (e.g.,
    OSPF interfaces and neighbors) defined in [OSPF].

    In a MOSPF router, the following items are added to the list of
    global OSPF data structures described in Section 5 of [OSPF]:

    o   Local group database. This database describes the group
        membership on all attached networks for which the router is
        either Designated Router or Backup Designated Router. This in
        turn determines the group-membership-LSAs that the router will
        originate, and the local delivery of multicast datagrams (see
        Sections 2.3.1 and 10).

    o   Forwarding cache. Each entry in the forwarding cache describes
        the path of a multicast datagram having a particular [source



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        net, multicast destination, TOS] combination. These cache
        entries are calculated when building the datagram shortest-path
        trees. See Sections 2.3.4 and 11 for more details.

    o   Multicast routing capability. Indicates whether the router is
        running the multicast extensions defined in this memo. A router
        running the multicast extensions must still run the base OSPF
        algorithm as set forth in [OSPF]. Such a router will continue to
        interoperate with non-multicast-capable OSPF routers when
        forwarding IP unicast traffic.

    o   Inter-area multicast forwarder. Indicates whether the router
        will forward IP multicasts from one OSPF area to another. Such a
        router declares itself a wild-card multicast receiver in its
        non-backbone area router-LSAs (see Section 14.6), and also
        summarizes its attached areas' group membership to the backbone
        in group-membership-LSAs. When building inter-area datagram
        shortest-path trees, it is these routers that appear immediately
        adjacent to the datagram source at the root of the tree (see
        Section 3.2). Not all multicast-capable area border routers need
        be configured as inter-area multicast forwarders. However,
        whenever both ends of a virtual link are multicast-capable, they
        must both be configured as inter-area multicast forwarders (see
        Section 14.11).

    o   Inter-AS multicast forwarder. Indicates whether the router will
        forward IP multicasts between Autonomous Systems. Such a router
        declares itself a wild-card multicast receiver in its router-
        LSAs (see Section 14.6). These routers are also assumed to be
        running some kind of inter-AS multicast protocol. They mark all
        external routes that they import into the OSPF domain as to
        whether they provide multicast connectivity (see Section 14.9).
        When building inter-AS multicast datagram trees, it is these
        routers that appear immediately adjacent to the datagram source
        at the root of the tree.

    8.1.  Additions to the OSPF area structure

        The OSPF area data structure is described in Section 6 of
        [OSPF]. In a MOSPF router, the following item is added to the
        OSPF area structure:

        o   List of group-membership-LSAs. These link state
            advertisements describe the location of the area's multicast
            group members.  Group-membership-LSAs are flooded throughout
            a single area only. Area border routers also summarize their
            attached areas' membership by originating group-membership-
            LSAs into the backbone area. For more information, see



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            Sections 3.1 and 10.

    8.2.  Additions to the OSPF interface structure

        The OSPF interface structure is described in Section 9 of
        [OSPF]. In a MOSPF router, the following items are added to the
        OSPF interface structure. Note that the IPMulticastForwarding
        parameter is really a description of the attached network. As
        such, it should be configured identically on all routers
        attached to a common network; otherwise incorrect routing of
        multicast datagrams may result[13].

        o   IPMulticastForwarding. This configurable parameter indicates
            whether IP multicasts should be forwarded over the attached
            network, and if so, how the forwarding should be done. The
            parameter can assume one of three possible values: disabled,
            data-link multicast and data-link unicast. When set to
            disabled, IP multicast datagrams will not be forwarded out
            the interface. When set to data-link multicast, IP multicast
            datagrams will be forwarded as data-link multicasts. When
            set to data-link unicast, IP multicast datagrams will be
            forwarded as data-link unicasts. The default value for this
            parameter is data-link multicast. The other two settings are
            for use in the special circumstances described in Sections
            6.3 and 6.4. When set to disabled or to data-link unicast,
            IGMP group membership is not monitored on the attached
            network.

        o   IGMPPollingInterval. When the router is actively monitoring
            group membership on the attached network, it periodically
            sends IGMP Host Membership Queries. IGMPPollingInterval is a
            configurable parameter indicating the number of seconds
            between IGMP Host Membership Queries.  The router actively
            monitors group membership on the attached network when both
            a) the interface's IPMulticastForwarding parameter is set to
            data-link multicast and b) the router has been elected
            Designated Router on the attached network. See Section 9 for
            details.

        o   IGMPTimeout. This configurable parameter indicates the
            length of time (in seconds) that a local group database
            entry associated with this interface will persist without
            another matching IGMP Host Membership Report being received.
            See Section 9 for details.

        o   IGMP polling timer. The firing of this interval timer causes
            an IGMP Host Membership Query to be sent out the interface.
            The length of this timer is the configurable parameter



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            IGMPPollingInterval. See Section 9 for details.

    8.3.  Additions to the OSPF neighbor structure

        The OSPF neighbor structure is defined in Section 10 of [OSPF].
        In a MOSPF router, the following items are added to the OSPF
        neighbor structure:

        o   Neighbor Options. This field was already defined in the OSPF
            specification. However, in MOSPF there is a new option which
            indicates the neighbor's multicast capability. This new
            option is learned in the Database Exchange process through
            reception of the neighbor's Database Description packets,
            and determines whether group-membership-LSAs are flooded to
            the neighbor. See the items concerning flooding in Section
            14 for a more detailed explanation.

    8.4.  The local group database

        The local group database has already been introduced in Section
        2.3.1.  The current section attempts a more precise definition.
        The local group database tracks the group membership of the
        router's directly attached networks. Database entries are
        created and maintained by the IGMP protocol. Database entries
        can cause group-membership-LSAs to be originated, which in turn
        enable the pruning of datagram shortest-path trees. The local
        group database also dictates the router's responsibility for the
        delivery of multicast datagrams to directly attached group
        members.

        Each entry in the local group database has three components: the
        multicast group, the attached network and the entry's age. A
        database entry is indexed by the first two components: multicast
        group and attached network. A database lookup function is
        assumed to exist, so that given a [multicast group, attached
        network] pair, the matching database entry (if any) can be
        discovered. A database entry for [Group A, Network N1] exists if
        and only if there are Group A members currently located on
        Network N1.

        The three components of a local group database entry are defined
        as follows:

        o   MulticastGroup. The multicast group whose members are being
            tracked by this entry. Each multicast group is represented
            as a class D IP address. For the semantics of multicast
            group membership, see [RFC 1112].




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        o   AttachedNetwork. Each database entry is concerned with the
            group members belonging to a single attached network. To get
            a complete picture of the local group membership (when for
            example building a group-membership-LSA), it may be
            necessary to consult multiple database entries, one for each
            attached network. Note that a router is only required to
            maintain entries for those attached networks on which the
            router has been elected Designated Router or Backup
            Designated Router (see Section 9).

        o   Age. Indicates the number of seconds since an IGMP Host
            Membership Report for multicast Group A has been seen on
            Network N1. If the age field hits Network N1's configured
            IGMPTimeout value, the local group database entry is removed
            (i.e., the entry has "aged out"). See Sections 9.2 and 9.3
            for more information.

    8.5.  The forwarding cache

        The forwarding cache has already been defined in Section 2.3.
        The current section attempts a more precise definition. Each
        entry in the forwarding cache indicates how a multicast datagram
        having a particular [source network, destination multicast
        group, IP TOS] will be forwarded. A forwarding cache entry is
        built on demand from the local group database and the datagram's
        shortest-path tree. For more details, consult Sections 2.3.4 and
        12.

        Each entry in the forwarding cache has six components: the
        multicast datagram's source network, the destination multicast
        group, the IP TOS, the upstream node, the list of downstream
        interfaces and (possibly) a list of downstream neighbors. A
        forwarding cache entry is indexed by source network, destination
        multicast group and IP TOS. A lookup function is assumed to
        exist, so that given a multicast datagram with a particular [IP
        source, destination multicast group, IP TOS], a matching cache
        entry (if any) can be found.

        The six components of a forwarding cache entry are defined as
        follows:

        o   Source network. The datagram's source network is described
            by a network/subnet/supernet number and its corresponding
            mask. The source network for a datagram is discovered via a
            routing table/database lookup of the datagram's IP source
            address, as described in Section 11.2.





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        o   Destination multicast group. The destination group to which
            matching datagrams are being forwarded. For the semantics of
            multicast group membership, see [RFC 1112].

        o   IP TOS. The IP Type of Service specified by matching
            datagrams. Note that this means that the path of the
            multicast datagram depends on its TOS classification.

        o   Upstream node. The attached network/neighboring router from
            which the datagram must be received. If received from a
            different attached network/neighboring router, the matching
            datagram is dropped instead of forwarded. This prevents
            unwanted replication of multicast datagrams. It is possible
            that the upstream node is unspecified (i.e., set to NULL).
            In this case, matching datagrams will always be dropped, no
            matter where they are received from. It is also possible
            that the upstream node is specified as the placeholder
            EXTERNAL. This means that the datagram must be received on a
            non-MOSPF interface in order to be forwarded.

        o   List of downstream interfaces. These are the router
            interfaces that the matching datagram should be forwarded
            out of (assuming that the datagram was received from
            upstream node). Each interface is also listed with a TTL
            value. The TTL value is the minimum number of hops necessary
            to reach the closest (in terms of router hops) group member.
            This allows the router to drop datagrams that have no chance
            of reaching a destination group member.

        o   List of downstream neighbors. When the datagram is to be
            forwarded out a non-broadcast multi-access network, or if
            the interface's IPMulticastForwarding parameter is set to
            data-link unicast, the datagram must be forwarded separately
            to each downstream neighbor (see Sections 2.3.3 and 6.4). As
            done for downstream interfaces, each downstream neighbor is
            specified together with the smallest TTL that will actually
            reach a group member.

9.  Interaction with the IGMP protocol

    MOSPF uses the IGMP protocol (see [RFC 1112]) to monitor multicast
    group membership. In short, the Designated Router on a network
    periodically sends IGMP Host Membership Queries (see Section 9.1),
    which in turn elicit IGMP Host Membership Reports from the network's
    multicast group members. These Host Membership Reports are then
    recorded in the Designated Router's and Backup Designated Router's
    local group databases (see Section 9.2).




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    9.1.  Sending IGMP Host Membership Queries

        Only the network's Designated Router sends Host Membership
        Queries.  This minimizes the amount of group membership
        information on the network, both in terms of queries and
        responses.

        When a MOSPF router becomes Designated Router on a network, it
        checks to see that the network's IPMulticastForwarding parameter
        is set to data-link multicast (see Section B.2). If so, it
        starts the interface's IGMP polling timer. Then, whenever the
        timer fires (every IGMPPollingInterval seconds), the MOSPF
        router sends a Host Membership Query out the interface. The
        destination of the query is the IP address 224.0.0.1. For the
        format of the query, see [RFC 1112].  If/when the MOSPF router
        ceases to be the network's Designated Router, the IGMP polling
        timer is disabled and no more Hosts Membership Queries are sent.

        Unusual behavior can result when multiple IP networks are
        assigned to a single physical network. MOSPF treats each such IP
        network separately, electing (possibly) a different Designated
        Router for each network.  However, IGMP operates on a physical
        network basis only: when a Host Membership Query is sent, all
        group members on the physical network respond, regardless of
        their IP addresses. So unless the IPMulticastForwarding
        parameter is set to a value other than data-link multicast on
        all but one of the physical network's IP networks, excess
        multicast membership reporting will result.

    9.2.  Receiving IGMP Host Membership Reports

        Received Host Membership Reports are processed by both the
        network's Designated Router and Backup Designated Router. It is
        the Designated Router's responsibility to distribute the
        network's group membership information throughout the routing
        domain, by originating group-membership-LSAs (see Section 10).
        The Backup Designated Router processes Reports so that it too
        has a complete picture of the network's group membership,
        enabling a quick cutover upon Designated Router failure.

        An IGMP Host Membership Report concerns membership in a single
        IP multicast group (call it Group A). The Report is sent to the
        Group A address so that other group members may see the Report
        and avoid sending duplicates (see [RFC 1112] for details). When
        an IGMP Host Membership Report, sent on Network N[14], is
        received by a MOSPF router, the following steps are executed:





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        (1) If the router is neither the Designated Router nor the
            Backup Designated Router on the network, the Report is
            discarded and processing stops.

        (2) If the Report concerns a multicast group in the range
            224.0.0.1 - 224.0.0.255, the Report is discarded and
            processing stops. This range of multicast groups are for
            local use (single hop) only, and datagrams sent to these
            destinations are never forwarded by multicast routers.

        (3) Locate the entry for [Group A, Network N] in the local group
            database.  If no such entry exists, create one. In any case,
            set the age of the entry to 0. Note that even if multiple
            hosts attached to Network N report membership in the same
            group, only a single local group database entry will be
            formed. See Section 8.4 for more details concerning the
            local group database.

        (4) If the router is the network's Designated Router, and a
            local group database entry was created in the previous step,
            it may be necessary to originate a new group-membership-LSA.
            See Section 10 for details.

    9.3.  Aging local group database entries

        Every local database entry has an age field. Suppose that there
        is a database entry for [Group A, Network N1]. The age field
        then indicates the length of time (in seconds) since the last
        Host Membership Report for Group A was received on Network N1.
        If the age of the entry reaches Network N1's configured
        IGMPTimeout value (see Section B.2), the entry is considered
        invalid and is removed from the database.

        Note that when a router, after having been either Network N1's
        Designated Router or Backup Designated Router, but now being
        neither, will (after IGMPTimeout seconds) automatically age out
        all of its local group database entries associated with Network
        N1. For this reason, it is not necessary to purge local group
        database entries on OSPF interface state changes.

    9.4.  Receiving IGMP Host Membership Queries

        If a MOSPF router has internal multicast applications, and if
        the applications have bound themselves to certain interfaces
        (using the RFC 1112 representation described in Section 5), then
        the MOSPF router responds to received Host Membership Queries by
        issuing Host Membership Reports. Identical to the operation of
        any IP host supporting multicast applications, the exact



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        procedure for issuing these Host Membership Reports is specified
        in [RFC 1112]. Note that in this case, if the router has been
        elected Designated Router on a network, it must receive its own
        Host Membership Reports and Host Membership Queries.

        If instead all of its applications have joined groups in an
        interface-independent fashion (using the MOSPF-specific
        representation described in Section 5), the MOSPF router does
        not respond to Host Membership Queries. Instead, the MOSPF
        router communicates this membership information by originating
        appropriate group-membership-LSAs (see Section 10.1).

10.  Group-membership-LSAs

    Group-membership-LSAs provide the means of distributing membership
    information throughout the MOSPF routing domain. Group-membership-
    LSAs are specific to a single OSPF area (see Section 3.1). Each
    group-membership-LSA concerns a single multicast group. Essentially,
    the group-membership-LSA lists those networks which are directly
    connected to the LSA's originator and which contain one or more
    group members. For more details on how the group-membership-LSA
    augments the OSPF link state database, see Section 2.3.1.

    The creation of group-membership-LSAs is discussed in Section 10.1.
    The format of the group-membership-LSA is described in Section A.3.
    A router will originate a group membership-LSA for multicast group A
    when one or more of the following conditions hold:

    (1) The router is Designated Router on a network (call it Network
        X), the interface to Network X has its IPMulticastForwarding
        parameter set to data-link multicast (see Section B.2), and
        Network X contains one or more members of Group A.

    (2) The router is an inter-area multicast forwarder (see Section
        B.1), and one or more of the router's attached non-backbone
        areas contain Group A members. In this case, the router will
        originate a group-membership-LSA for Group A into the backbone.
        This is the way group membership is conveyed between areas (see
        Section 3.1).

    (3) The router itself has applications that are requesting
        membership in Group A, in an interface-independent fashion (see
        Section 5).

    As for all other types of OSPF link state advertisements (e.g,
    router-LSAs, network-LSAs, etc.), group-membership-LSAs are aged as
    they are held in a router's link state database. To prevent valid
    advertisements from "aging out", a router must refresh its self-



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    originated group-membership-LSAs every LSRefreshTime interval, by
    incrementing their LS sequence numbers and reissuing them. In
    addition, when an event occurs that would alter one of the router's
    self-originated group-membership-LSAs, a new instance of the LSA is
    issued with an updated (i.e., incremented by 1) LS sequence number.
    Note however that a router is not allowed to originate two new
    instances of the same advertisement within MinLSInterval seconds.
    For that reason, occasionally advertisement originations will need
    to be deferred. Also, an event may occur that makes it inappropriate
    for the router to continue to originate a particular LSA. In that
    case, the router flushes the advertisement from the routing domain
    by "premature aging". For more information concerning the
    maintenance of LSAs, see Sections 12, 12.4, 14 and 14.1 of [OSPF].

    When one of the following events occurs, it may be necessary for a
    router to (re)issue one or more group-membership-LSAs:

    (1) One of the router's interfaces changes state. For example, the
        router may have become Designated Router on a particular
        network, causing the router to start advertising the network's
        group membership to the rest of the MOSPF system in group-
        membership-LSAs.

    (2) The router receives an IGMP Host Membership Report, causing a
        new local group database entry to be formed (see Section 9.2).

    (3) One of the router's local group database entries "ages out",
        because it is no longer being refreshed by received IGMP Host
        Membership Reports (see Section 9.3).

    (4) The router is an inter-area multicast forwarder, and the group
        membership of one of the router's attached non-backbone areas
        changes.  This is detected by the reception of a new, or the
        flushing of an old, group-membership-LSA into/from the non-
        backbone area's link state database.

    (5) The group membership of one of the router's internal
        applications changes.

    10.1.  Constructing group-membership-LSAs

        This section details how to build a group-membership-LSA. The
        format of a group-membership-LSA is described in Section A.3.
        Each group-membership-LSA concerns a single multicast group. The
        body of the advertisement is a list of the local transit nodes
        (the router itself and directly attached transit networks) that
        contain group members. Section 10 listed the conditions
        requiring the (re)origination of a group-membership-LSA. Note



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        that if the router is an area border router, it may be necessary
        to originate a separate group-membership-LSA for each attached
        area.

        The following defines the contents of a group-membership-LSA, as
        originated by Router X into Area A. It is assumed that the
        group-membership-LSA is to report membership in multicast group
        G:

        o   The advertisement fields that are not type-specific (LS age,
            LS sequence number, LS checksum and length) are set
            according to Section 12.1 of [OSPF].

        o   The Options field of a group-membership-LSA is not processed
            on receipt. However, for consistency, the Option field in
            these advertisements should have its MC-bit set, T-bit
            clear, and the E-bit should match the configuration of Area
            A (i.e., set if and only if Area A is not a stub area). The
            rest of the Options field is set to 0.

        o   The Link State ID is set to the group whose membership is
            being reported (Group G).

        o   The Advertising Router is set to the OSPF Router ID of the
            router originating the advertisement (Router X).

        o   The body of the advertisement is a list of local transit
            vertices that should be labelled with Group G membership
            (see Section 2.3.1). This list may include the advertising
            router itself, and any of the transit networks that are
            directly attached to said router. The following steps
            determine which of these transit vertices are actually
            included in the group-membership-LSA. Note that any
            particular vertex should be listed at most once, even though
            the following may indicate multiple reasons for a particular
            vertex to be listed. Also note that if no transit vertices
            are listed by the advertisement, the advertisement should
            not be (re)originated; if an instance of the advertisement
            already exists, it should then be flushed from the link
            state database using the premature aging procedure specified
            in Section 14.1 of [OSPF].

            a.  Consider those entries in the local group database that
                describe Group G membership (see Section 8.4). Consider
                each such entry in turn. Each entry references one of
                Router X's attached networks (call it Network N). If
                either Network N does not belong to Area A, or if Router
                X is not Network N's Designated Router[15], Network N



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                should not be added to the group-membership-LSA, and the
                next local group database entry should be examined.
                Otherwise, if N is a stub network (e.g., Router X is the
                only OSPF router attached to N), Router X adds itself to
                the advertisement by adding a vertex with Vertex type
                set to 1 (router) and Vertex ID set to Router X's OSPF
                Router ID. Otherwise, N is a transit network. In this
                case, Network N should be added to the advertisement by
                adding a vertex with Vertex type set to 2 (network) and
                Vertex ID set to the IP address of Network N's
                Designated Router (i.e., Router X's IP interface address
                on Network N).

            b.  If Router X itself has applications requesting Group G
                membership on an interface-independent basis (see
                Section 5), it should add itself to the advertisement by
                adding a vertex with Vertex type set to 1 (router) and
                Vertex ID set to Router X's OSPF Router ID.

            c.  If Router X is an inter-area multicast forwarder (see
                Section 3.1), Area A is the backbone area (Area ID
                0.0.0.0), and at least one of Router X's attached non-
                backbone areas has Group G members (indicated by the
                presence of one or more advertisements in the areas'
                link state databases having Link State ID set to Group G
                and LS age set to a value other than MaxAge[16]), then
                Router X should add itself to the advertisement by
                adding a vertex with Vertex type set to 1 (router) and
                Vertex ID set to Router X's OSPF Router ID.

        Consider as an example the network configuration in Figure 4.
        Suppose that Router RT2 has been elected Designated Router for
        Network N3.  Router RT2 would then originate (into Area 1) the
        following group-membership-LSA for Group B:

          ; RT2's group-membership-LSA for Group B

          LS age = 0                     ;always true on origination
          Options = (E-bit|MC-bit)
          LS type = 6                    ;group-membership-LSA
          Link State ID = Group B
          Advertising Router = RT2's Router ID
                 Vertex type = 1         ;RT2 itself (for stub N2)
                 Vertex ID = RT2's Router ID
                 Vertex type = 2         ;Network N3 (since RT2 is DR)
                 Vertex ID = RT2's IP interface address on N3





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    10.2.  Flooding group-membership-LSAs

        When MOSPF routers and non-multicast OSPF routers are mixed
        together in a routing domain, the group-membership-LSAs are not
        flooded to the non-multicast routers[17].  As a general design
        principle, optional OSPF advertisements are only flooded to
        those routers that understand them.

        A MOSPF router learns of its neighbor's multicast-capability at
        the beginning of the "Database Exchange Process" (see Section
        10.6 of [OSPF], receiving Database Description packets from a
        neighbor in state Exstart). A neighbor is multicast-capable if
        and only if it sets the MC-bit in the Options field of its
        Database Description packets.  Then, in the next step of the
        Database Exchange process, group-membership-LSAs are included in
        the Database summary list sent to the neighbor (see Sections 7.2
        and 10.3 of [OSPF]) if and only if the neighbor is multicast-
        capable.

        When flooding group-membership-LSAs to adjacent neighbors, a
        MOSPF router looks at the neighbor's multicast-capability.
        Group-membership-LSAs are only flooded to multicast-capable
        neighbors. To be more precise, in Section 13.3 of [OSPF],
        group-membership-LSAs are only placed on the Link state
        retransmission lists of multicast-capable neighbors[18].  Note
        however that when sending Link State Update packets as
        multicasts, a non-multicast neighbor may (inadvertently) receive
        group-membership-LSAs. The non-multicast router will then simply
        discard the LSA (see Section 13 of [OSPF], receiving LSAs having
        unknown LS types).

11.  Detailed description of multicast datagram forwarding

    This section describes in detail the way MOSPF forwards a multicast
    datagram. The forwarding process has already been informally
    presented in Section 2.2. However, there are several obscure
    configuration options (e.g., the IPMulticastForwarding interface
    parameter) that have been presented elsewhere in this document,
    which may influence the forwarding process. This section gathers
    together all the influencing factors into a single algorithm.

    It is assumed in the following that the datagram under consideration
    has actually be received on one of the router's interfaces. Locally
    generated datagrams (i.e., originated by one of the router's
    internal applications) are handled instead by the algorithm in
    Section 11.3.





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    Assume that the datagram's IP destination is Group G. The forwarding
    process then consists of the following steps:

    (1) Upon reception of the datagram, the MOSPF router notes the
        following parameters. These parameters are examined in later
        steps, to determine whether the datagram should be forwarded.

        a.  The receiving MOSPF interface associated with the datagram.
            Based on the receiving physical interface, the receiving
            MOSPF interface is selected by the algorithm in Section
            11.1.

        b.  Whether the datagram was received as a link-level
            multicast/broadcast or as a link-level unicast. This
            information is used later in Step 7 to help determine
            whether the datagram should be forwarded.

    (2) A copy of the datagram should be passed to each internal
        application that has joined Group G on the receiving MOSPF
        interface (see Section 5).

    (3) If the datagram's IP source address matches the receiving MOSPF
        interface's IP address, the datagram should not be forwarded
        further, and should instead be discarded, completing the
        forwarding process.  This keeps the router's own locally
        originated datagrams from being mistakenly replicated, in those
        cases where the receiving MOSPF interface receives its own
        multicast transmissions.

    (4) If Group G falls into the range 224.0.0.1 through 224.0.0.255
        inclusive, the datagram should not be forwarded further. This
        range of addresses has been dedicated for use on a local network
        segment only.

    (5) Associate a source network (SourceNet) with the multicast
        datagram, as described in Section 11.2. If SourceNet cannot be
        determined (i.e., there is no available unicast route back to
        the datagram source), the datagram should not be forwarded
        further.

    (6) Look up the forwarding cache entry (see Section 8.5) matching
        the datagram's [SourceNet, Group G, TOS] combination. If the
        cache entry does not yet exist, one is built by the calculation
        in Section 12. In order for the datagram to be forwarded, the
        contents of the forwarding cache entry must be further verified
        against the received datagram's characteristics as follows:





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        a.  If the forwarding cache entry's upstream node is unspecified
            (i.e., NULL), then the datagram should not be forwarded
            further.

        b.  Otherwise, suppose that the forwarding cache entry's
            upstream node is set to EXTERNAL. In this case, the datagram
            is forwarded further if and only if the receiving MOSPF
            interface is set to NULL (i.e., if and only if the datagram
            was received on a non-MOSPF interface).

        c.  Otherwise, if the datagram's receiving MOSPF interface does
            not attach to the forwarding cache entry's upstream node,
            the datagram should not be forwarded further.

    (7) If the receiving MOSPF interface's IPMulticastForwarding
        parameter is set to data-link unicast, the datagram should be
        forwarded further only if it was received as a data-link
        unicast.

    (8) At this point the datagram is eligible for further forwarding.
        Before forwarding, the router checks to see whether it has any
        internal applications that have joined Group G on an interface-
        independent basis. If so, a copy of the datagram should be
        passed to each such requesting application process.

    (9) Examine each of the downstream interfaces listed in the
        forwarding cache entry. If the TTL in the datagram is greater
        than or equal to the TTL specified for the downstream interface,
        a copy of the datagram should be forwarded out the downstream
        interface. Before forwarding the datagram copy, the copy's TTL
        should be decremented by 1. On most interfaces, the datagram is
        forwarded as a data-link multicast/broadcast. The exact data-
        link encapsulation is dependent on the attached network's type:

        o   On ethernet and IEEE 802.3 networks, the datagram is
            forwarded as a data-link multicast. The destination data-
            link multicast address is selected as an algorithmic
            translation of the IP multicast destination. See [RFC 1112]
            for details.

        o   On FDDI networks, the datagram is forwarded as a data-link
            multicast.  The destination data-link multicast address is
            selected as an algorithmic translation of the IP multicast
            destination. See [RFC 1390] for details.

        o   On SMDS networks, the datagram is forwarded using the same
            SMDS address that is used by IP broadcast datagrams. See
            [RFC 1209] for details.



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        o   On networks that support broadcast, but not multicast (e.g.,
            the Experimental Ethernet), the datagram is forwarded as a
            data-link broadcast. See [RFC 1112] for details.

        o   On point-to-point networks, the datagram is forwarded in the
            same way that unicast datagrams are forwarded. See [RFC
            1112] for details.

    (10)
        Examine each of the downstream neighbors listed in the
        forwarding cache entry. If the TTL in the datagram is greater
        than or equal to the TTL specified for the downstream neighbor,
        a copy of the datagram should be forwarded to the downstream
        neighbor (as a data-link unicast). Before forwarding the
        datagram copy, the copy's TTL should be decremented by 1.

    ICMP error messages are never generated in response to received IP
    multicasts. In particular, ICMP destination unreachables and ICMP
    TTL expired messages are not generated by the above procedure if the
    router refuses to forward a multicast datagram.

    11.1.  Associating a MOSPF interface with a received datagram

        A MOSPF interface must be associated with a received multicast
        datagram before it is forwarded (see Step 1a of Section 11), and
        with received IGMP Host Membership Reports before they are
        processed (see Section 9.2).

        When there is only a single IP network assigned to the physical
        interface that received the datagram, the choice of receiving
        MOSPF interface is clear. When there are multiple logical IP
        networks attached to the receiving physical interface, the
        receiving MOSPF interface is selected as follows. Examine all of
        the MOSPF interfaces associated with the receiving physical
        interface. Discard those interfaces whose IPMulticastForwarding
        parameter has been set to disabled. The receiving MOSPF
        interface is then the remaining interface having the highest IP
        interface address (or NULL if there are no remaining
        interfaces)[19].

    11.2.  Locating the source network

        MOSPF forwarding cache entries are indexed by the datagram's
        source IP network/subnet/supernet. For this reason, whenever an
        IP multicast datagram is received, the IP network belonging to
        the datagram's IP source address must be found. This is
        accomplished by the following algorithm:




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        Look up the OSPF TOS 0 routing table entry[20] corresponding to
        the datagram's IP source address, as described in Section 11.1
        of [OSPF].  If this routing table entry describes an OSPF
        intra-area or inter-area route, the source network is set to be
        the network defined by the routing table entry's Destination ID
        and Address Mask (see Section 11 of [OSPF]). Otherwise (i.e.,
        the routing table entry specifies an external route, or there is
        no matching routing table entry), the list of matching AS
        external-link-LSAs is examined. A matching AS external-link-LSA
        is one that describes a network which contains the datagram's IP
        source address. The list of matching AS external-link-LSAs is
        pruned in the following steps to determine the source network:

        (1) Those AS external-link-LSAs with MC-bit clear (see Section
            A.1), or with LS age set to MaxAge, or which have been
            originated by unreachable AS boundary routers are discarded.

        (2) AS external-link-LSAs specifying Type 1 external metrics are
            always preferred over those specifying Type 2 external
            metrics.

        (3) If there are still multiple AS external-link-LSAs remaining,
            those specifying the best matching (i.e., most specific)
            network are selected. The source network is then set to the
            network/subnet/supernet (possibly even the default route)
            described by the best matching AS external-link-LSAs. Note
            that AS external-link-LSAs specifying a cost of LSInfinity
            are eligible for this best match, as long as their MC-bit is
            set.[21]

        It is possible that two different MOSPF routers may calculate
        the same multicast datagram's source network differently. For
        example, consider the network configuration shown in Figure 4.
        When calculating the source network for a datagram whose source
        is Network N10 and destination is Group Ma, Router RT11 would
        calculate the source network as Network N10 itself, while Router
        RT10 would calculate the source network as the aggregate of
        Networks N9-N11 and Host H1 (advertised in a single summary-
        link-LSA by Router RT11). However, despite the possibility of
        routers selecting different source networks, all routers will
        still agree on the datagram's shortest-path tree.

        External sources are treated differently in the above
        calculation since it is likely that the Internet will have
        separate multicast and unicast topologies for some time to come.
        When the multicast and unicast topologies do merge, the MC-bit
        will be set on all AS external-link-LSAs and the above use of
        the LSInfinity metric (to indicate a route that is to be used



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        for multicast traffic, but not unicast traffic), will no longer
        be necessary. At that time, the determination of source network
        for external sources will revert to the same simple routing
        table lookup that is used for internal sources.

        As an example of the logic for external sources, suppose a
        multicast datagram is received having the IP source address
        10.1.1.1. Suppose also that the three AS external-link-LSAs
        shown in Table 3 are in the router's OSPF database. The OSPF
        routing table lookup would yield the network 10.1.1.0 with a
        mask of 255.255.255.0, however the above calculation would
        choose a source network of 10.1.0.0 with a mask of 255.255.0.0,
        despite the fact that its matching LSA has a cost of LSInfinity.

    11.3.  Forwarding locally originated multicasts

        This section describes how a MOSPF router forwards a multicast
        datagram that has been originated by one of the router's own
        internal applications. The process begins with one of the
        router's internal applications formatting and addressing the
        datagram. Forwarding the locally originated multicast then
        consists of the following steps:

        (1) Find the router interface whose IP address matches the
            datagram's source address. Multicast the datagram out that
            interface, according to the Host extensions for IP
            multicasting specified in [RFC 1112].

        (2) If the router interface found in the previous step has been
            configured for MOSPF, and if its IPMulticastForwarding
            parameter is not equal to disabled, then set the receiving
            MOSPF interface to that interface.  Otherwise, set the
            receiving MOSPF interface to NULL.

        (3) Execute the MOSPF forwarding process described in Section
            11, beginning with its Step 4.


         Network    Mask            Cost                 MC-bit
         ______________________________________________________
         10.1.1.0   255.255.255.0   Type 1: 10           clear
         10.1.0.0   255.255.0.0     Type 2: LSInfinity   set
         10.0.0.0   255.0.0.0       Type 2: 1            set


                 Table 3: Sample AS external-link-LSAs





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        The above algorithm amounts to the router always multicasting
        the datagram out the source interface, and the executing the
        basic forwarding algorithm (in Section 11) as if the datagram
        had actually been received on the source interface. In those
        cases where the router receives its own multicast transmissions,
        unwanted replication is prevented by Step 3 of Section 11. In
        fact, this specification has purposely presented the forwarding
        algorithm (both for received and for locally originated
        datagrams) so that the correct forwarding actions are taken
        independent of whether the router receives its own multicast
        transmissions.

12.  Construction of forwarding cache entries

    This section details the building of a MOSPF forwarding cache entry.
    A high level discussion of this construction has already been
    presented in Sections 2.3, 2.3.1, 2.3.2, 3.2, and 4.1. Forwarding
    cache entries are built on demand, when a multicast datagram is
    received and no matching forwarding cache entry is found (see Step 6
    of Section 11).  The parameters passed to the forwarding cache entry
    build process are: the datagram's source network (see Section 11.2)
    and its destination group address. These two parameters are called
    SourceNet and Group G in the following algorithm. The main steps in
    the build process are the following:

    (1) Allocate the forwarding cache entry. Initialize its Source
        network to SourceNet, its Destination multicast group to Group G
        and its IP TOS field to match the multicast datagram's TOS.
        Initialize its upstream node and list of downstream interfaces
        to NULL.

    (2) For each Area A to which the calculating router is attached:

        a.  Calculate Area A's datagram shortest-path tree. This
            calculation is described in Section 12.2 below. In many ways
            it is similar to the calculation of OSPF's intra-area
            routes, described in Section 16.1 of [OSPF]. The main
            differences between the multicast datagram shortest-path
            tree calculation and OSPF's intra-area unicast calculation
            are listed in Section 12.2.9 below. As a product of each
            area's datagram shortest-path tree, the forwarding cache
            entry's list of outgoing interfaces is (possibly) updated.

            Area A's datagram shortest-path tree is dependent on the
            datagram's IP TOS. Section 12.2 describes the TOS 0 datagram
            shortest-path tree. The modifications necessary for non-zero
            TOS values are detailed in Section 12.2.8.




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        b.  Possibly set the forwarding cache entry's upstream node.
            Only one of the calculating router's attached areas will
            determine the forwarding cache entry's upstream node. This
            area is called the datagram's RootArea. The RootArea is
            initially set to NULL. After completing Area A's datagram
            shortest-path tree, the calculation in Section 12.2.7 will
            determine whether Area A is the datagram's RootArea.

    (3) Update the forwarding cache entry's list of outgoing interfaces,
        according to the contents of the local group database. This
        ensures multicast delivery to group members residing on the
        calculating router's directly attached networks. This process is
        described in Section 12.3.

    These main steps are described in more detail below. The detailed
    description begins with an explanation of the major data structure
    used by the datagram shortest-path tree calculation: The Vertex data
    structure.

    12.1.  The Vertex data structure

        A datagram shortest-path tree is built by the Dijkstra or SPF
        algorithm. The algorithm is stated herein using graph-oriented
        language: vertices and links. Vertices are the area's routers
        and transit networks, and links are the router interfaces and
        point-to-point lines that connect them. Each vertex has the
        following state information attached to it. Basically, this
        information indicates the current best path from the SourceNet
        to the vertex, and the position of the vertex relative to the
        calculating router. Note that a separate datagram shortest-path
        tree is built for each area, and that the vertices described
        below are also specific to a single area (called Area A).

        o   Vertex type. Set to 1 for routers, 2 for transit networks.
            Note that this coding matches the coding for vertices listed
            in the group-membership-LSA (see Section A.3).

        o   Vertex ID. A 32-bit identifier for the vertex. For routers,
            set to the router's OSPF Router ID. For transit networks,
            set the IP address of the network's Designated Router. Note
            that this coding matches the coding for vertices listed in
            the group-membership-LSA (see Section A.3).

        o   LSA. The link state advertisement describing the vertex'
            immediate neighborhood. Can be discovered by performing a
            database lookup in Area A's link state database (see Section
            12.2 of [OSPF]), with LS type set to Vertex type and Link
            State ID set to Vertex ID.



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        o   Parent. In the current best path from SourceNet to the
            vertex, the router/transit network immediately preceding the
            vertex. Note that the parent can change as better and better
            paths are found, up until the vertex is installed on the
            shortest-path tree.

        o   IncomingLinkType. This parameter is set to the type of link
            that led to Vertex's inclusion on the shortest-path tree.
            Listed in order of decreasing preference[22], the possible
            types are: ILVirtual (virtual links), ILDirect (vertex is
            directly attached to SourceNet), ILNormal (either router-
            to-router or router-to-network links), ILSummary (OSPF
            summary links), ILExternal (OSPF AS external links), or
            ILNone (the vertex is not on the shortest-path tree).

        o   AssociatedInterface/Neighbor. If the current best path from
            SourceNet to the vertex goes through the calculating router,
            this parameter indicates the calculating router's interface
            (or neighbor) which leads to the vertex.

        o   Cost. The cost, in terms of the OSPF link state metric, of
            the current best path from SourceNet to the vertex. Note
            that if the cost of the path is a combination of both
            external type 2 and internal OSPF metrics, that the vertex'
            cost parameter reflects both cost components. Remember that
            the type 2 cost component is always more significant than
            the type 1 component.

        o   TTL. If the current best path from SourceNet to vertex goes
            through the calculating router, TTL is set to the number of
            routers between the calculating router and the vertex. This
            includes the calculating router, but does not include the
            vertex itself.

    12.2.  The SPF calculation

        This section details the construction of datagram shortest-path
        trees.  Such a tree describes the path of a multicast datagram
        as it traverses an OSPF area. For a given datagram, each router
        in an OSPF area builds an identical tree. A router connected to
        multiple areas builds a separate datagram shortest-path tree for
        each area.

        The datagram shortest-path tree is built by the Dijkstra or SPF
        algorithm, which is the same algorithm used to discover OSPF's
        intra-area unicast routes (see Section 16.1 of [OSPF]). The
        algorithm is stated herein and in [OSPF] using graph-oriented
        language: vertices and links. Vertices are the area's routers



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        and transit networks, and links are the router interfaces and
        point-to-point lines that connect them. Basically, the algorithm
        manipulates two lists of vertices: the candidate list and the
        forming shortest-path tree. The candidate list consists of those
        vertices to which paths have been discovered, but for which the
        optimality of the discovered paths is yet unknown. At each cycle
        of the algorithm, the vertex closest to the tree's root, yet
        still remaining on the candidate list, is moved from the
        candidate list to the shortest-path tree. Then the neighbors of
        the just processed vertex are examined for possible addition
        to/modification of the candidate list. The algorithm terminates
        when the candidate list is empty.

        The datagram shortest-path tree for Area A is constructed in the
        following steps. The datagram's SourceNet and its destination
        group G are inputs to the calculation (see Step 6 of Section
        11). The datagram shortest-path tree also depends on the IP Type
        of service specified in the datagrams' IP Header. However, a
        discussion of TOS is deferred until Section 12.2.8; all
        calculations and costs in the current section concern TOS 0
        only. Call the router performing the calculation Router RTX. At
        each step (and in the subordinate Sections 12.2.1 through
        12.2.8) LSAs from Area A's link state database are examined. In
        all cases, any LSA having LS age equal to MaxAge is ignored. The
        main body of the calculation is in Steps 4 and 5, which are
        repeated until the candidate list becomes empty:

        (1) Initialize the algorithm's data structures. Clear the
            shortest-path tree.  Initialize the state of each vertex in
            Area A (i.e., the area's routers and transit networks) to:
            Parent set to NULL, IncomingLinkType set to ILNone and
            AssociatedInterface/Neighbor set to NULL.

        (2) Initialize the candidate list. One or more vertices are
            initially placed on the candidate list, depending on the
            location of SourceNet with respect to Area A and Router RTX.
            This breaks down into the following cases (which are named
            for later reference):

            o   Case SourceIntraArea: SourceNet belongs to Area A. In
                this case, the candidate list is initialized as in
                Section 12.2.1.

            o   Case SourceInterArea1: SourceNet belongs to an OSPF area
                that is not directly attached to Router RTX. In this
                case, the candidate list is initialized as in Section
                12.2.2.




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            o   Case SourceInterArea2: SourceNet does not belong to Area
                A, but it still belongs to an OSPF area that is directly
                attached to Router RTX.  In this case, the candidate
                list is initialized as in Section 12.2.3.

            o   Case SourceExternal: SourceNet is external to the OSPF
                routing domain, and Area A is not an OSPF stub area. In
                this case, the candidate list is initialized as in
                Section 12.2.4.

            o   Case SourceStubExternal: SourceNet is external to the
                OSPF routing domain, and Area A is an OSPF stub area. In
                this case, the candidate list is initialized as in
                Section 12.2.5.

            Two different routers in Area A may select different
            initialization cases above. For example, consider the
            network configuration shown in Figure 4. When calculating
            the Area 3 datagram shortest-path tree for a datagram whose
            source is Network N7 (e.g., from Host H5) and destination is
            Group Ma, Router RT11 would initialize the candidate list
            using Case SourceInterArea2 while Router RT9 would use Case
            SourceInterArea1. Likewise, if Area 3 were configured as an
            OSPF stub area and the datagram source was the external
            Network N12, Router RT11 would use Case SourceStubExternal
            while Router RT9 would use Case SourceInterArea1! However,
            despite the possibility of routers selecting different
            cases, all routers in an area will still initialize the
            candidate list (and in fact, run the rest of the SPF
            calculation) identically.

        (3) If the candidate list is empty, the algorithm terminates.

        (4) Move the closest candidate vertex to the shortest-path tree.
            Select the vertex on the candidate list that is closest to
            SourceNet (i.e., has the smallest Cost value). If there are
            multiple possibilities, select transit networks over
            routers. If there are still multiple possibilities
            remaining, select the vertex having the highest Vertex ID.
            Call the chosen vertex Vertex V. Remove Vertex V from the
            candidate list, and install it on the shortest-path tree.

            Next, determine whether Vertex V has been labelled with the
            Destination multicast Group G. If so, it may cause the
            forwarding cache entry's list of outgoing
            interfaces/neighbors to be updated. See Section 12.2.6 for
            details.




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        (5) Examine Vertex V's neighbors for possible inclusion in the
            candidate list. Consider Vertex V's LSA. Each link in the
            LSA describes a connection to a neighboring router/network.
            If the link connects to a stub network, examine the next
            link in the LSA. Otherwise, the link (Link L) connects to a
            neighboring transit node. Call this node Vertex W. Perform
            the following steps on Vertex W:

            a.  If W is already on the shortest-path tree, or if W's LSA
                does not contain a link back to vertex V, or if W's LSA
                has LS age of MaxAge, or if W is not multicast-capable
                (indicated by the MC-bit in the LSA's Options field),
                examine the next link in V's LSA.

            b.  Otherwise determine the cost to associate with the link
                from V to W.  If SourceNet belongs to Area A (Case
                SourceIntraArea in Step 2), use the cost listed for Link
                L in V's LSA. Otherwise, use the link's reverse cost:
                Examine W's LSA, and find the cost listed for the link
                connecting back to V. Actually, when V and W are both
                routers, there may be multiple links between them. In
                this case, use the smallest cost listed in W's LSA for
                any of the links connecting back to V and having the
                same Type (as specified in the Router-LSA; must be
                either: point-to-point connection or virtual link) as
                Link L[23].

            c.  Calculate the cost from SourceNet to W, when using Link
                L. It is the sum of the cost of SourceNet to V (i.e.,
                V's Cost parameter) plus the link cost calculated in
                Step 5b. Let this sum be Cost C. If W is not yet on the
                candidate list, install W on the candidate list,
                modifying its parameters as specified below (Step 5d).
                Otherwise, W is on the candidate list already. In this
                case, if:

                o   C is less than W's current Cost, update W's
                    parameters on the candidate list as specified below
                    (Step 5d).

                o   C is equal to W's current Cost, then the following
                    tiebreakers are invoked. The type of Link L is
                    compared to W's current IncomingLinkType, and
                    whichever link has the preferred type is chosen (the
                    preference order of link types is listed in Section
                    12.1's definition of IncomingLinkType). If the link
                    types are the same, then a link whose Parent is a
                    transit network is preferred over one whose Parent



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                    is a router. If the links are still equivalent, the
                    link whose Parent has the higher Vertex ID is
                    chosen. Whenever Link L is chosen, W's parameters
                    are modified as below (Step 5d). Whenever the
                    previously discovered link is chosen, the next link
                    in V's LSA is examined instead.

                o   C is greater than W's current Cost, examine the next
                    link in V's LSA.

            d.  At this point, a better candidate path has been found to
                Vertex W, using Link L. Modify Vertex W's parameters
                accordingly. W's Parent is set to Vertex V. W's
                IncomingLinkType is set to ILVirtual if Link L is a
                virtual link, otherwise IncomingLinkType is set to
                ILNormal. W's Cost parameter is set to C. W's TTL and
                AssociatedInterface/Neighbor parameters are set
                according to one of the following cases:

                o   Vertex V is the calculating router itself. In this
                    case, W's TTL parameter is set to 1. If Link L is a
                    virtual link, W's AssociatedInterface/Neighbor is
                    set to NULL. Otherwise, W's
                    AssociatedInterface/Neighbor is set to the non-
                    virtual interface connecting the calculating router
                    to W which has the smallest cost value. Note that,
                    in the reverse cost (inter-area and inter-AS
                    multicast) cases, this may not be the interface
                    corresponding to Link L. However, since W is only
                    concerned with the node it is receiving the datagram
                    from (the upstream node; see Section 11), and not
                    with the particular interface the datagram is
                    received on, the calculating router is free to pick
                    the sending interface when there are multiple
                    connecting links.

                o   Vertex V is upstream of the calculating router
                    (i.e., V's AssociatedInterface/Neighbor is equal to
                    NULL). In this case, Vertex W's TTL parameter is set
                    to 0, and its AssociatedInterface/Neighbor is set to
                    NULL.

                o   V is a transit network, and is directly downstream
                    from the calculating router (i.e., V's
                    AssociatedInterface/Neighbor is non-NULL and V's TTL
                    is set to 1). W is then one of the calculating
                    router's neighbors. In this case, W's TTL parameter
                    is also set to 1. If network V has been configured



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                    for data-link unicasting (see Section B.2) or if V
                    is a non-broadcast network, W's
                    AssociatedInterface/Neighbor is set to W itself (a
                    neighbor of the calculating router). Otherwise, W's
                    AssociatedInterface/Neighbor is set to the
                    calculating router's interface to Network V.

                o   Vertex V is downstream from the calculating router
                    (i.e., V's AssociatedInterface/Neighbor is non-
                    NULL), and either a) V is a router or b) V's TTL
                    parameter is greater than 1. In these cases, W's
                    AssociatedInterface/Neighbor parameter is copied
                    directly from V.  If V is a router, W's TTL
                    parameter is set to V's TTL parameter incremented by
                    one. If V is a transit network, W's TTL parameter is
                    set directly to V's TTL parameter.

        (6) If the candidate list is non-empty, go to Step 4. Otherwise,
            the algorithm terminates.

        After the datagram shortest-path tree for Area A is complete,
        the calculating router (RTX) must decide whether Area A, out of
        all of RTX's attached areas, determines the forwarding cache
        entry's upstream node. This determination is described in
        Section 12.2.7.

        Examples of the above SPF calculation, with particular emphasis
        on the tiebreaking rules, are given in Appendix C.

        12.2.1.  Candidate list Initialization: Case SourceIntraArea

            In this case, SourceNet belongs to Area A.  The candidate
            list is then initialized as follows. Start with the LSA
            listed as Link State Origin in the matching OSPF routing
            table entry.  If this LSA is not multicast-capable (i.e, its
            Options field has the MC-bit clear) the candidate list
            should be set to NULL. Otherwise, the vertex identified by
            the LSA is installed on the candidate list, setting its
            vertex parameters as follows: IncomingLinkType set to
            ILDirect, Cost set to 0, Parent to NULL and
            AssociatedInterface/Neighbor to NULL.

            As a consequence of this initialization, note that if
            SourceNet is a stub network, then the datagram shortest-path
            tree will not actually be rooted at the datagram source, but
            will instead be rooted at the MOSPF router that attaches the
            stub network to the rest of the MOSPF system. For example,
            consider the network configuration shown in Figure 4. When



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            calculating the Area 2 datagram shortest-path tree for a
            datagram whose source is Network N7 (e.g., from Host H5) and
            destination is Group Ma, Router RT11 (and all other routers
            attached to Area 2) will begin with the candidate list set
            to Router RT8. As another example, the datagram shortest-
            path tree pictured in Figure 3 is really rooted at Router
            RT3 instead of Network N4.

        12.2.2.  Candidate list Initialization: Case SourceInterArea1

            In this case, SourceNet belongs to an OSPF area that is not
            directly attached to the calculating router (RTX).  The
            candidate list is then initialized as follows. Examine the
            Area A summary-link-LSAs advertising SourceNet. For each
            such summary-link-LSA: if both a) the MC-bit is set in the
            LSA's Options field and b) the advertised cost is not equal
            to LSInfinity, then the vertex representing the LSA's
            advertising area border router is added to the candidate
            list. An added vertex' state is initialized as:
            IncomingLinkType set to ILSummary, Cost to whatever is
            advertised in the LSA, Parent to NULL and
            AssociatedInterface/Neighbor to NULL.

            For example, consider the network configuration shown in
            Figure 4.  When calculating the Area 1 datagram shortest-
            path tree for a datagram whose source is Network N7 (e.g.,
            from Host H5) and destination is Group Ma, Router RT2 would
            initialize the candidate list to contain the two area border
            routers RT3 (with a cost of 20) and RT4 (with a cost of 19).
            See Figure 6 for more details.

        12.2.3.  Candidate list Initialization: Case SourceInterArea2

            In this case, SourceNet belongs to an OSPF area other than
            Area A, but one that is still directly attached to the
            calculating router (RTX).  The candidate list is then
            initialized in the following two steps:

            (1) Find the Area A summary-link-LSA that best matches
                SourceNet, excluding those summary-link-LSAs specifying
                cost LSInfinity or having unreachable Advertising
                Routers[24].  A matching summary-link-LSA is one that
                advertises a range of addresses containing SourceNet;
                the best matching is as usual the most specific match.
                Let SourceRange be the network described by the best
                matching summary-link-LSA.





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            (2) Similar to the logic in the SourceInterArea1 case,
                examine all the Area A summary-link-LSAs which advertise
                SourceRange. For each such summary-link-LSA: if both a)
                the MC-bit is set in the LSA's Options field, b) the
                advertised cost is not equal to LSInfinity and c) the
                Advertising Router is reachable, then the vertex
                representing the LSA's Advertising Router is added to
                the candidate list. An added vertex' state is
                initialized as: IncomingLinkType set to ILSummary, Cost
                to whatever is advertised in the LSA, Parent to NULL and
                AssociatedInterface/Neighbor to NULL.

            The reason why SourceRange is used, instead of simply using
            SourceNet (as was done in case SourceInterArea1), is that
            routing information may have been collapsed at area
            boundaries. In order for Area A's area border routers and
            its internal routers to construct the same Area A datagram
            shortest-path tree, they must both start at SourceRange -
            Area A's internal routers know nothing about SourceNet. Note
            that SourceRange is not discovered simply by looking at the
            calculating router's configured set of area address ranges,
            in order to avoid dependence on the configured area address
            ranges being synchronized across all area border routers.

            For example, consider the network configuration shown in
            Figure 4.  When calculating the Area 2 datagram shortest-
            path tree for a datagram whose source is Network N11 and
            destination is Group Ma, Router RT11 would calculate
            SourceRange to be the collection: Networks N9-N11 and Host
            H1. It would then initialize the candidate list to contain
            itself (RT11) only, with an associated Cost of 1 (since RT11
            is advertising Networks N9-N11 and Host H1 in a summary-
            link-LSA with a cost of 1).

        12.2.4.  Candidate list Initialization: Case SourceExternal

            In this case, SourceNet is external to the OSPF routing
            domain, and Area A is not an OSPF stub area.  The candidate
            list is then initialized as follows. Note that an attempt
            may be made to add a Vertex W to the candidate list when W
            already belongs to the candidate list. When this happens,
            W's vertex parameters are updated if the Cost parameter it
            would be added with is better[25] (closer to SourceNet) than
            its previous value. When the costs are the same, W's
            parameters are still modified if the IncomingLinkType it
            would be added with is better (see IncomingLinkType's
            definition in Section 12.1) than its previous value.




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            For each AS external-link-LSA advertising SourceNet, the
            following steps are performed:

            o   If the AS external-link-LSA's MC-bit is clear or if its
                advertising router is not reachable, then the AS
                external-link-LSA is not used. AS external-link-LSAs
                having their MC-bit set and advertising a cost of
                LSInfinity can be used; these LSAs describe paths that
                can be used for multicast, but not unicast, data traffic
                (see Section 11.2).

            o   If the AS external-link-LSA's Forwarding address field
                is 0.0.0.0, the following vertices are added to the
                candidate list. If the Advertising AS boundary router
                (call it ASBR) belongs to Area A, the vertex
                representing the AS boundary router is added to the
                candidate list using parameters: IncomingLinkType set to
                ILExternal, Cost to whatever is advertised in the LSA,
                Parent to NULL and AssociatedInterface/Neighbor to NULL.
                Then, regardless of whether ASBR belongs to Area A, all
                Area A area border routers that are advertising
                reachable multicast-capable (MC-bit set) type 4
                summary-link-LSAs for ASBR are added to the candidate
                list. Each such area border router is added with the
                parameters: IncomingLinkType set to ILSummary, Cost to
                the sum of whatever is advertised in the type 4
                summary-link-LSA plus the value in the original AS
                external-link-LSA, Parent to NULL and
                AssociatedInterface/Neighbor to NULL.

            o   If the AS external-link-LSA's Forwarding address field
                is non-zero, the Forwarding address is looked up in the
                OSPF routing table. Then processing breaks into one of
                the following cases:

                o   The Forwarding address is not usable. In this case,
                    nothing is added to the candidate list. The
                    Forwarding address is not usable if either it has no
                    matching routing table entry, or if the matching
                    routing table entry is neither of type intra-area
                    nor of type inter-area.

                o   The Forwarding address belongs to Area A[26]: the
                    Forwarding address' matching routing table entry has
                    Path-type of intra-area and its Associated area is
                    Area A. In this case, the vertex represented by the
                    matching routing table entry's Link State Origin
                    field is added to the candidate list (assuming that



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                    the vertex is multicast-capable). The vertex is
                    added with the parameters: IncomingLinkType set to
                    ILExternal, Cost to whatever was advertised in the
                    original AS external-link-LSA, Parent to NULL and
                    AssociatedInterface/Neighbor to NULL.

                o   The Forwarding address belongs to an area that is
                    not attached to Router RTX[27]: the Forwarding
                    address' matching routing table entry has Path-type
                    of inter-area. Call the network represented by the
                    matching routing table entry ForwardNet. For each
                    reachable multicast-capable summary-link-LSA (in
                    Area A) advertising ForwardNet, add the LSA's
                    advertising area border router to the candidate list
                    using parameters: IncomingLinkType set to ILSummary,
                    Cost to the sum of whatever is advertised in the
                    summary-link-LSA plus the value in the original AS
                    external-link-LSA, Parent to NULL and
                    AssociatedInterface/Neighbor to NULL.

                o   The Forwarding address belongs to another one of
                    Router RTX's attached areas[28]: the Forwarding
                    address' matching routing table entry has Path-type
                    of intra-area and its associated Area is other than
                    Area A.  Call the network represented by the
                    matching routing table entry ForwardNet. First find
                    the Area A summary-link-LSA that best matches
                    ForwardNet, excluding those summary-link-LSAs
                    specifying cost LSInfinity or having unreachable
                    Advertising Routers. Let ForwardRange be the network
                    described by the best matching summary-link-LSA.
                    Then, for each reachable multicast-capable summary-
                    link-LSA (in Area A) advertising ForwardRange, add
                    the LSA's advertising area border router to the
                    candidate list using parameters: IncomingLinkType
                    set to ILSummary, Cost to the sum of whatever is
                    advertised in the summary-link-LSA plus the value in
                    the original AS external-link-LSA, Parent to NULL
                    and AssociatedInterface/Neighbor to NULL.

            The above calculation can be restated as follows. Each of
            Area A's inter-area multicast forwarders and inter-AS
            multicast forwarders are examined. Those that have
            multicast-capable paths to SourceNet (represented as either
            a multicast-capable AS external link or the concatenation of
            a Type 4 summary link and a multicast-capable AS external
            link) are added to the candidate list as router vertices.
            (It is possible that, when considering a router that is both



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            an inter-area multicast forwarder and an inter-AS multicast
            forwarder, two equal cost paths exist to SourceNet, one an
            AS external link and the other a concatenation of a Type 4
            summary link and an AS external link. In this case, the
            concatenation of the Type 4 summary link and the AS external
            link is preferred). The added vertex' state is set as
            follows: IncomingLinkType set to ILSummary if the path is
            represented as a concatenation of a Type 4 summary link and
            an AS external link, IncomingLinkType set to ILExternal
            otherwise, Cost set to the cost of the shortest path from
            vertex to SourceNet, Parent set to NULL and
            AssociatedInterface/Neighbor set to NULL.

            For example, consider the network configuration shown in
            Figure 4.  When calculating the Area 2 datagram shortest-
            path tree for a datagram whose source is Network N14 and
            destination is Group Ma, the candidate list would be
            initialized to the two routers RT7 at a cost of 14 and RT10
            at a cost of 19. This assumes that the external costs
            pictured in Figure 4 are external type 1s.

        12.2.5.  Candidate list Initialization: Case
            SourceStubExternal

            In this case, SourceNet is external to the OSPF routing
            domain, and Area A is an OSPF stub area.  The candidate list
            is then initialized similarly to case SourceInterArea1. The
            Area A summary-link-LSAs advertising DefaultDestination are
            examined. For each such summary-link-LSA having both its
            MC-bit set and its advertised cost not equal to LSInfinity,
            the vertex representing the LSA's advertising area border
            router is added to the candidate list. An added vertex'
            state is initialized as: IncomingLinkType set to ILSummary,
            Cost to whatever is advertised in the LSA, Parent to NULL
            and AssociatedInterface/Neighbor to NULL.

            The most likely outcome of the above is that all of stub
            Area A's inter-area multicast forwarders will be installed
            on the candidate list, with appropriate costs.

        12.2.6.  Processing labelled vertices

            When encountered during the SPF calculation, vertices
            labelled with the destination multicast group (Group G) may
            cause the forwarding cache entry's list of downstream
            interfaces/neighbors to be modified.  A Vertex V in Area A
            is labelled with Group G if and only if at least one of the
            following holds:



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            (1) V is a router, and its router-LSA indicates that it is a
                wild-card multicast receiver (i.e., bit W in its
                router-LSA is set). This may be true when V is an
                inter-area or inter-AS multicast forwarder.

            (2) V is listed in the body of a group membership-LSA. In
                particular, find the originator of Vertex V's LSA; call
                it Router Y. Then find the group-membership-LSA in Area
                A's link state database which has Link State ID = Group
                G and Advertising Router = Router Y (see Section A.3).
                If this group-membership-LSA exists, and if Vertex V is
                listed in the body of the LSA (see Sections 10 and A.3),
                then Vertex V is labelled with Group G.

            When Vertex V is added to the shortest-path tree in Step 4
            of Section 12.2, and if Vertex V is both downstream from the
            calculating router (i.e., Vertex V's
            AssociatedInterface/Neighbor is non-NULL) and labelled with
            Group G, then Vertex V's AssociatedInterface/Neighbor is
            added to the forwarding cache entry's list of downstream
            interfaces/neighbors. In addition, Vertex V's TTL value is
            attached to the added downstream interface/neighbor. If the
            particular interface/neighbor had already been added to the
            list of downstream interfaces/neighbors, the list is simply
            modified by setting the downstream interface/neighbor's TTL
            value to the minimum of its existing TTL value and Vertex
            V's TTL value.

        12.2.7.  Merging datagram shortest-path trees

            After the datagram shortest-path tree for Area A is
            complete, the calculating router (RTX) must decide whether
            Area A, out of all of its attached areas, determines the
            forwarding cache entry's upstream node.  This is done by
            examining RTX's position on the Area A datagram shortest-
            path tree, which is in turn described by RTX's Area A Vertex
            data structure. If RTX's Vertex parameter IncomingLinkType
            is either ILNone (RTX is not on the tree), ILVirtual or
            ILSummary, then some area other than Area A will determine
            the upstream node. Otherwise, Area A might possibly
            determine the upstream node (i.e., may be selected the
            RootArea), depending on the following tiebreakers[29]:

            o   If RootArea has not been set, then set RootArea to Area
                A. Otherwise, compare the present RootArea to Area A in
                the following:





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            o   Choose the area that is "nearest to the source". Nearest
                to the source depends on each area's candidate list
                initialization case, as it occurs in Step 2 of Section
                12.2. The initialization cases, listed in order of
                decreasing preference (or nearest to farthest) are:
                SourceIntraArea, SourceInterArea1, SourceExternal and
                SourceStubExternal. Areas whose candidate list
                initialization falls into case SourceInterArea2 are
                never used as the RootArea. As an example, consider the
                network configuration shown in Figure 4. When
                calculating the datagram shortest-path tree for a
                datagram whose source is Network N7 (e.g., from Host H5)
                and destination is Group Ma, Router RT11 would set its
                RootArea to Area 2 (Case SourceIntraArea) instead of
                Area 3 (Case SourceInterArea2) or the backbone Area 0
                (Case SourceInterArea).

            o   If there are still two equally good areas, and one of
                them is the backbone, set RootArea to the backbone (Area
                0).

            o   If there are still two equally good areas, set RootArea
                to the area whose datagram shortest-path tree provides
                the shortest path from SourceNet to RTX. This is a
                comparison of RTX's Vertex parameter Cost in the two
                areas.

            o   If there are still two equally good areas, set RootArea
                to one with the highest OSPF Area ID.

            If the above has set the RootArea to be Area A, the
            forwarding cache entry's upstream node must be set
            accordingly. This setting depends on the IncomingLinkType in
            RTX's Area A Vertex structure. If IncomingLinkType is equal
            to ILDirect, the upstream node is set to the appropriate
            directly-connected stub network. If equal to ILNormal, the
            upstream node is set to the Parent field in RTX's Area A
            Vertex structure. If equal to ILExternal, the upstream node
            is set to the placeholder EXTERNAL.

        12.2.8.  TOS considerations

            The previous sections 12.2 through 12.2.7 described the
            construction of a TOS 0 (default TOS) datagram shortest-path
            tree. However, in a TOS-capable router, a separate tree may
            be built for each TOS. If a TOS-capable router receives a
            multicast datagram that specifies a non-zero TOS X, it first
            builds the TOS 0 datagram shortest-path tree.  Then, if all



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            the routers on the pruned tree are TOS-capable, a separate
            TOS X datagram shortest-path tree is calculated[30].
            Otherwise, the TOS 0 tree is used for all datagrams,
            regardless of their specified TOS.

            To determine whether there are any TOS-incapable routers on
            the pruned TOS 0 tree, the following additions are made to
            Section 12.2's tree calculation:

            o   A new piece of state information is added to each
                vertex: TOS-capable path. This indicates whether the
                present path from SourceNet to vertex, as represented on
                the datagram shortest-path tree, contains only TOS-
                capable routers.

            o   The TOS-capable path parameter is calculated when the
                vertex is first added to the candidate list and
                recalculated when/if the vertex' position on the
                candidate list is modified (see Section 12.2's Step 2
                and Step 5d). The parameter is set to TRUE if both the
                vertex itself is TOS-capable and the vertex' parent has
                its TOS-capable path parameter set to TRUE; otherwise,
                TOS-capable path is set to FALSE.

            o   All routers on the TOS 0 datagram shortest-path tree are
                TOS-capable if and only if, whenever a vertex labelled
                with Group G is added to the shortest-path tree (Section
                12.2.6), the value of the vertex' TOS-capable path
                parameter is TRUE.

            The source of the multicast datagram is always located using
            a TOS 0 routing table lookup, regardless of the datagram's
            TOS classification (see Section 11.2). If the calculating
            router is not capable of TOS-based routing, it calculates
            only TOS 0 datagram shortest-path trees, and uses them to
            route datagrams independent of TOS value.  Otherwise, when
            calculating the TOS X datagram shortest-path tree, the
            algorithm in Section 12.2 is used, with the modifications
            listed below.

            o   When calculating RangeNet and ForwardRange in Sections
                12.2.3 and 12.2.4 respectively, only summary-link-LSAs
                having TOS 0 cost of LSInfinity are excluded (no change
                from the TOS 0 case). However, when adding vertices to
                the candidate list in Sections 12.2.2 through 12.2.5,
                the TOS X cost of the summary links and/or AS external
                links (and not the TOS 0 cost) are reflected in the
                added vertices' Cost parameter.



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            o   In Step 5 of Section 12.2, the TOS X cost of Link L (in
                the appropriate direction) is used, not the TOS 0 cost.

            o   Non-TOS-routers are not added to the candidate list, and
                are thus excluded from the trees.

        12.2.9.  Comparison to the unicast SPF calculation

            There are many similarities between the construction of a
            multicast datagram's shortest-path trees in Section 12.2 and
            OSPF's intra-area route calculation for unicast traffic
            (Section 16.1 of [OSPF]). Both have been described in terms
            of Dijkstra's algorithm. However, there are some
            differences. The major differences are listed below:

            o   In the multicast case, the datagram SPF calculation is
                rooted at the datagram's source. In the unicast case,
                each router is the root of its own unicast intra-area
                SPF calculation.

            o   In the multicast case, the datagram shortest-path tree
                is a true tree; i.e., between any two nodes on the tree
                there is one path. However, due to the provision for
                equal-cost multipath in [OSPF], the unicast SPF
                calculation may add additional links to the shortest-
                path tree.

            o   In order to avoid unwanted replication of multicast
                datagrams, MOSPF ensures that, for any given datagram,
                each router builds the exact same datagram shortest-path
                tree. This forces two differences from the unicast SPF
                calculation. First, it eliminates the possibility of
                equal-cost multipath. Secondly, when the MOSPF system
                contains multiple alternate paths, the algorithm must
                ensure that each MOSPF router deterministically chooses
                the same alternative. For this reason, tie-breaking
                mechanisms have been specified in Steps 2, 4 and 5b of
                Section 12.2.

            o   The calculation of datagram shortest path trees takes
                into account only those links that connect transit nodes
                (i.e, router to router or router to transit network
                links). The unicast SPF calculation in Section 16.1 of
                [OSPF] must additionally examine links to stub networks,
                although this is done after all the transit links are
                examined.





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            o   While both the multicast and unicast trees select
                shortest paths on the basis of the OSPF metric, the
                datagram shortest-path trees also keep track of the TTL
                values between the root (datagram source) and all
                destinations (group members). This enables more
                efficient implementation of IP multicast's "expanding
                ring search" (see Section 2.3.4).

            o   In the multicast case, the algorithm is sometimes forced
                to use the link state cost for the reverse direction
                (i.e, the cost towards, instead of away from, the
                source). This is because the costs of OSPF summary-
                link-LSAs and AS external-link-LSAs, which sometime form
                the base of the multicast datagram shortest-path trees,
                are specified in the reverse direction (from the
                multicast perspective).

            o   There are potentially many more datagram shortest-path
                trees that need to be calculated (one for each source
                net, destination group and TOS combination), than the
                limited number of unicast SPF trees (one per each TOS).
                This is the main reason that the datagram shortest-path
                trees are calculated on demand; it is hoped that this
                will spread the cost of the SPF calculations over
                time[31].

            o   The way that the two algorithms handle TOS is different.
                In the multicast case, if a TOS-incapable node is
                encountered during the calculation of the TOS 0 datagram
                shortest-path tree, the TOS 0 datagram shortest-path
                tree is used instead of trying to build the TOS X tree
                (see Section 12.2.8). In the unicast case, the TOS X
                tree is always used, only falling back on the TOS 0
                paths when a TOS X path does not exist.

    12.3.  Adding local database entries to the forwarding cache

        After the datagram shortest-path trees have been built for each
        attached area, the forwarding cache has an upstream node and a
        list of downstream interfaces. In order to ensure the delivery
        of the multicast datagram to group members on directly attached
        networks, the local group database (Section 8.4) must then be
        scanned for possible addition to the list of downstream
        interfaces. All local group database entries having Group G as
        MulticastGroup are examined.  Suppose [Group G, Network N] is
        one such entry. If the calculating router (RTX) is Network N's
        Designated Router, then RTX's Network N interface is added to
        the list of outgoing interfaces, with a TTL of 1. If the Network



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        N interface was already present in the list of outgoing
        interfaces, its TTL is simply set to 1.

        For example, consider the network configuration shown in Figure
        4 when calculating the forwarding cache entry for a datagram
        whose source is Network N4 (e.g., from Host H2) and destination
        is Group Mb. After calculating the datagram shortest-path tree
        for Area 1, Router RT2 would have set it upstream node to
        Network N3 and its list of downstream interfaces to NULL. But
        then looking at its local group database, it would add its
        Network N2 interface with a TTL of 1 to its list of downstream
        interfaces.

13.  Maintaining the forwarding cache

    A MOSPF router may, for resource reasons, limit the size of its
    forwarding cache. At any time cache entries can be purged to make
    room for newer entries, since the purged entries can always be
    rebuilt when necessary. This memo does not specify an algorithm to
    select which entries to purge. However, care should be taken to
    ensure that any particular entry is not continually rebuilt and then
    purged again (i.e., thrashing should be avoided).

    The building of the forwarding cache has been previously described
    in Section 12. There are events that force one or more forwarding
    cache entries to be deleted; these events are described below. Note
    that deleted cache entries will be rebuilt on an as-needed basis.

    o   When the internal topology of the MOSPF system changes, all
        forwarding cache entries must be deleted. This is because
        internal topology changes may invalidate the previously
        calculated datagram shortest-path trees. Since the multicast
        routing calculation depends on the result of the unicast routing
        calculations, the forwarding cache should be cleared after the
        unicast routing table is rebuilt.  Internal topology changes are
        indicated when both a) a new instance of either a router-LSA or
        a network-LSA is received and b) the contents of the new
        advertisement (other than the LS age, LS sequence number and LS
        checksum fields) are different from the previous instance. This
        covers routers and links going up or down, routers that change
        from being multicast-incapable to being multicast-capable, etc.

    o   When a Type 3 summary-link-LSA (network summary) changes, those
        forwarding cache entries specifying datagram sources belonging
        to the range of addresses described by the updated summary-
        link-LSA must be deleted. See Sections 12.2.3 and 12.2.5.





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    o   Suppose that the content of an AS external-link-LSA changes. If
        the AS external-link-LSA describes an external network N, then
        all forwarding cache entries specifying an external source
        network that is contained in N or that contains N (i.e.,
        external sources that are a subset or a superset of N) must be
        deleted.

    o   When membership in a multicast group changes, all forwarding
        cache entries for the particular group must be deleted. Group
        membership changes are indicated when either a) the content of a
        group-membership-LSA changes or b) an entry in the local group
        database (see Section 8.4) changes.

    o   When the cost to an AS boundary router or to a forwarding
        address specified by one or more AS external-link-LSAs changes,
        all forwarding cache entries specifying an external network as
        datagram source must be deleted. In this case, potentially all
        inter-AS datagram shortest-path trees have been invalidated. The
        forwarding cache entries should be deleted after the new best
        cost to the AS boundary router/forwarding address has been
        calculated.

14.  Other additions to the OSPF specification

    MOSPF requires some modifications to the base OSPF protocol. All
    these modifications are backward-compatible. A router running MOSPF
    will still interoperate with an OSPF router when forwarding unicast
    traffic. Most of the modifications have been described earlier in
    this document. This section collects together those changes which
    have yet to be mentioned, organizing them by the affected Section of
    [OSPF].

    14.1.  The Designated Router

        This functionality is described in Section 7.3 of [OSPF]. In
        OSPF, a network's Designated Router has two specialized roles.
        First, it originates the network's network-LSA. Second, it
        controls the flooding on the network, in that all of the routers
        on the network synchronize with the Designated Router (and the
        Backup Designated Router) only.  For these reasons[32], when one
        or more of the network's routers are running MOSPF, the
        Designated Router should be running MOSPF also.  This can be
        ensured by assigning all non-multicast routers the Router
        Priority of 0.

        In MOSPF, the Designated Router also has the additional
        responsibility of monitoring the network's multicast group
        membership. This is done by periodically sending Host Membership



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        Queries, and receiving Host Membership Reports in response (see
        Section 9). This is yet another reason why the Designated Router
        must be multicast-capable.

    14.2.  Sending Hello packets

        This functionality is described in Section 9.5 of [OSPF]. A
        MOSPF router sets the MC-bit in the Options field of its Hello
        packets. This indicates that the router is multicast-capable; it
        does not necessarily indicate the state of the sending
        interface's IPMulticastForwarding parameter (see Section B.2).
        Setting the MC-bit in Hellos is done strictly for informational
        purposes. Neighbors receiving the router's Hello packets do not
        act on the state of the MC-bit. A neighbor's multicast-
        capability is learned instead during the Database Exchange
        Process (see Section 14.4).

    14.3.  The Neighbor state machine

        This functionality is described in Section 10.3 of [OSPF]. When
        a neighbor enters state Exchange, the neighbor Database summary
        list is initialized (see the OSPF neighbor FSM entry for State:
        ExStart and Event: NegotiationDone). This list describes of the
        portion of the router's link state database that needs to be
        synchronized with the neighbor.  Group-membership-LSAs are
        included in the neighbor Database summary list if and only if
        the neighbor is multicast-capable. The neighbor's multicast
        capability is learned by examining the neighbor's Database
        Description packets (see Section 14.4).

    14.4.  Receiving Database Description packets

        This functionality is described in Section 10.6 of [OSPF]. A
        neighbor's multicast-capability is learned through received
        Database Description packets. When the Database Description
        packet is received that transitions the neighbor from ExStart to
        Exchange, the state of the MC-bit in the packet's Options field
        is examined. The neighbor is multicast-capable if and only if
        the MC-bit is set.

        The neighbor's multicast capability controls whether group-
        membership-LSAs are summarized to the neighbor during the
        Database Exchange process (see Section 14.3), and whether
        group-membership-LSAs are flooded to the neighbor during the
        flooding process (see Section 10.2).






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    14.5.  Sending Database Description packets

        This functionality is described in Section 10.8 of [OSPF]. A
        MOSPF router sets the MC-bit in the Options field of its
        Database Description packets. This indicates to its adjacent
        neighbors that the router is multicast-capable; it does not
        necessarily indicate the state of the sending interface's
        IPMulticastForwarding parameter (see Section B.2).

        When a router goes from being multicast-capable to multicast-
        incapable, or vice-versa, it must indicate this fact to its
        adjacent neighbors by restarting the Database Description
        process (i.e., rolling back the state of all adjacent neighbors
        to Exstart).

    14.6.  Originating Router-LSAs

        This functionality is described in Section 12.4.1 of [OSPF]. A
        MOSPF router sets the MC-bit in the Options field of its
        router-LSA. This allows the router to be included in datagram
        shortest-path trees (see Step 5a of Section 12.2).

        In addition, MOSPF has introduced a new flag in the router-LSA's
        rtype field: the W-bit. When the W-bit is set, the router is
        included on all datagram shortest-path trees, regardless of
        multicast group (see Section 12.2.6). Such a router is called a
        wild-card multicast receiver. The router sets the W-bit when it
        wishes to receive all multicast datagrams, regardless of
        destination. This will sometimes be true of inter-area multicast
        forwarders (see Section 3.1), and inter-AS multicast forwarders
        (see Section 4).

        A router must originate a new instance of its router-LSA
        whenever an event occurs that would invalidate the LSA's current
        contents. In particular, if the router's multicast capability or
        its ability to function as either an inter-area or inter-AS
        multicast forwarder changes, its router-LSA must be
        reoriginated.

    14.7.  Originating Network-LSAs

        This functionality is described in Section 12.4.2 of [OSPF]. In
        OSPF, a transit network's network-LSA is originated by the
        network's Designated Router. The Designated Router sets the MC-
        bit in the Options field of the network-LSA if and only if both
        a) the Designated Router is multicast-capable (i.e., running
        MOSPF) and b) the Designated Router's interface's
        IPMulticastForwarding parameter has been set to a value other



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        than disabled (see Section B.2). When the network-LSA has the
        MC-bit set, the network can be included in datagram shortest-
        path trees (see Section 12.2.6).

        It is intended that all routers attached to a common network
        agree on the network's IPMulticastForwarding capability.
        However, this agreement is not enforced. When there are
        disagreements, incorrect routing of multicast datagrams can
        result.

    14.8.  Originating Summary-link-LSAs

        This functionality is described in Section 12.4.3 of [OSPF].
        Inter-area multicast forwarders always set the MC-bit in the
        Options field of their summary-link-LSAs, regardless of whether
        the path described by the summary-link-LSA is actually
        multicast-capable. Indeed, it is possible that there is no
        multicast-capable path to the described destination. All other
        area border routers (ones that are not inter-area multicast
        forwarders) clear the MC-bit in the Options field of their
        summary-link-LSAs.

        If its MC-bit is clear, the summary-link-LSA will not be used
        when initializing the candidate list in Sections 12.2.2, 12.2.3
        and 12.2.5.

    14.9.  Originating AS external-link-LSAs

        This functionality is described in Section 12.4.4 of [OSPF].
        Unlike in summary-link-LSAs, an inter-AS multicast forwarder
        should clear the MC-bit in the Options field of one of its AS
        external-link-LSAs if it is known that there is no multicast-
        capable path from the described destination to the router
        itself. This knowledge may possibly be obtained, for example,
        from an inter-AS multicast routing algorithm (see Section 4).
        If the inter-AS multicast forwarder is unsure of whether a
        multicast-capable path exists between the described destination
        and the router itself, the MC-bit should be set in the AS
        external-link-LSA.  All other AS boundary routers (ones that are
        not inter-AS multicast forwarders) clear the MC-bit in the
        Options field of their AS external-link-LSAs.

        If its MC-bit is clear, the AS external-link-LSA will not be
        used when initializing the candidate list in Section 12.2.4.

        When multicast connectivity to an external destination exists,
        but no unicast connectivity, an AS external-link-LSA can be
        originated having its MC-bit set and specifying a cost of



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        LSInfinity. Such an AS external-link-LSA will still be used by
        the multicast routing calculation (see Section 12.2.4). As a
        result, when a MOSPF router wishes to stop advertising an AS
        external destination, it must use the premature aging procedure
        specified in Section 14.1 of [OSPF], rather than simply setting
        the AS external-link-LSA's cost to LSInfinity.

    14.10.  Next step in the flooding procedure

        This functionality is described in Section 13.3 of [OSPF].
        Group-membership-LSAs are specific to a OSPF single area, and
        are flooded to multicast-capable routers only. When flooding a
        group-membership-LSA, Section 13.3 of the OSPF specification is
        modified as follows: 1) The list of interfaces examined during
        flooding (called the eligible interfaces in Section 13.3 of
        [OSPF]) is the set of all interfaces attaching to Area A (the
        area that the group-membership-LSA is received from), just as
        for router-LSAs, network-LSAs and summary-link-LSAs. 2) When
        examining each interface, a group-membership-LSA is added to a
        neighbor's link state retransmission list if and only if both a)
        Step 1d of [OSPF]'s Section 13.3 is reached for the neighbor and
        b) the neighbor is multicast-capable. The neighbor's multicast
        capability is discovered during the Database Exchange process
        (see Section 14.4).

        Note that, since on broadcast networks Link State Update packets
        are sent initially as multicasts, non-multicast routers may
        receive group-membership-LSAs. However, non-multicast routers
        will simply drop the group-membership-LSAs, for reasons of
        unrecognized LS type (see Step 2 of [OSPF]'s Section 13). Link
        State acknowledgments for group-membership-LSAs are not expected
        from non-multicast routers, and group-membership-LSAs will never
        be retransmitted to non-multicast routers, since the LSAs are
        not added to these routers' link state retransmission lists (see
        above paragraph).

        For more information on flooding group-membership-LSAs, see
        Section 10.2.

    14.11.  Virtual links

        This functionality is described in Section 15 of [OSPF]. When a
        MOSPF router (i.e., multicast-capable router) is both an area
        border router and an endpoint of a virtual link whose other
        endpoint is also multicast capable, the router must then also be
        an inter-area multicast forwarder. This is necessary to ensure
        that multicast datagrams will flow through the virtual link's
        transit area, from one endpoint to the other. When the



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        backbone's datagram shortest-path tree is constructed in Section
        12.1, it is assumed that virtual links are capable of forwarding
        multicast datagrams whenever both endpoints are multicast-
        capable.















































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15.  References

    [Bharath-Kumar] Bharath-Kumar, K. and J. Jaffe, "Routing to Multiple
                    Destinations in Computer Networks", IEEE
                    Transactions on Communications, COM-31[3], March
                    1983.

    [Deering]       Deering, S., "Multicast Routing in Internetworks and
                    Extended LANs", SIGCOMM Summer 1988 Proceedings,
                    August 1988.

    [Deering2]      Deering, S., "Multicast Routing in a Datagram
                    Internetwork", Stanford Technical Report, STAN-CS-
                    92-1415, Department of Computer Science, Stanford
                    University, December 1991.

    [OSPF]          Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc.,
                    March 1994.

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

    [RFC 1112]      Deering, S., "Host Extensions for IP Multicasting",
                    STD 5, RFC 1112, Stanford University, May 1988.

    [RFC 1209]      Piscitello, D., and J. Lawrence, "Transmission of IP
                    Datagrams over the SMDS Service", RFC 1209, Bell
                    Communications Research, March 1991.

    [RFC 1340]      Reynolds, J. and J. Postel, "Assigned Numbers", STD
                    2, RFC 1340, USC/Information Sciences Institute,
                    July 1992.

    [RFC 1390]      Katz, D., "Transmission of IP and ARP over FDDI
                    Networks", STD 36, RFC 1390, cisco Systems, Inc.,
                    January 1993.














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Footnotes

    [1]Actually, OSPF allows a separate link cost to be configured for
    each TOS. MOSPF then potentially calculates separate paths for each
    TOS. For details, see Section 6.2.

    [2]We also assume in this section that the pictured multi-access
    networks provide data-link multicast/broadcast services.

    [3]Note that if N3 were a non-broadcast network, Router RT3 would
    send separate copies of the datagram to routers RT1 and RT2. Since
    the IGMP protocol is not defined on non-broadcast networks, there
    could in this case be no Group B member attached to Network N3.
    However the multicast datagram would still be delivered to the Group
    B members attached to networks N1 and N2.

    [4]Actually, in MOSPF there is a separate forwarding cache entry for
    each combination of source, destination and TOS. For a discussion of
    TOS-based multicast routing, see Section 6.2.

    [5]The discussion in this section omits mention of the Backup
    Designated Router's role in the IGMP protocol. While the Backup
    Designated Router does not send IGMP Host Membership Queries, it
    does listen to IGMP Host Membership Reports, building "shadow" local
    group database entries in the process. These entries do not lead to
    group-membership-LSAs, nor do they influence delivery of multicast
    datagrams, but are merely maintained to ease the transition from
    Backup Designated Router to Designated Router, should the Designated
    Router fail. See Sections 2.3.4, 9 and 10 for details.

    [6]One might imagine building all possible datagram shortest-path
    trees up front. However, this might be expensive, both in router CPU
    time and in router memory. It is hoped that building the datagram
    shortest-path trees on demand and caching the results will ease
    demands on router resources by spreading out the calculations over a
    longer period of time.

    [7]It is possible that, due to the existence of alternate paths,
    several different shortest-path trees are available. MOSPF depends
    on all routers constructing the exact same shortest path tree. For
    that reason, tie-breaking schemes have been implemented during tree
    construction to ensure that identical trees result. See Section 12
    for more details.

    [8]Note that the expanding ring search yields the nearest server in
    terms of hop count, but not necessarily in terms of the OSPF metric.

    [9]This means that in MOSPF, just as in OSPF, the only kind of link



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    state advertisement that can be flooded between areas is the AS
    external-link-LSA.

    [10]A router indicates that it is a wild-card multicast receiver by
    setting the appropriate flag in its router-LSA. See Section 14.6 for
    details.

    [11]This is not quite true. As we shall see, any inter-AS multicast
    forwarders belonging to the backbone are designated as wild-card
    multicast receivers. See Section 4.

    [12]It is possible that through the operation of an inter-AS
    multicast routing protocol, Router RT7 knows that it does not have
    multicast connectivity to Network N15 (even though it has unicast
    connectivity). In this case, RT7 would not advertise the external
    link to N15 as being multicast capable.

    [13]Synchronization of the IPMulticastForwarding interface parameter
    is not enforced by the MOSPF protocol, since it is not included in
    the contents of a MOSPF router's Hello packets.

    [14]Actually, when multiple IP networks have been assigned to the
    same physical network, the first thing that needs to be done is to
    associate an IP network with the received Host Membership Report.
    This is done in the same way that a receiving interface is
    associated with a received multicast datagram; see Section 11.1.

    [15]For this reason when a transit network has both MOSPF routers
    and non-multicast OSPF routers attached, care should be taken to
    ensure that a MOSPF router is elected Designated Router. This can be
    accomplished through proper setting of the routers' configured
    Router Priority.

    [16]Note that just because these advertisements exist in the link
    state database, it does not mean that the Group G members are
    reachable.  Reachability does not enter into the building of the
    transit vertex list, in order to simplify the calculation. This is a
    trade-off. As a result, some multicast datagrams may be forwarded
    further than necessary, when the described Group G members actually
    are unreachable.

    [17]Since the Designated Router controls flooding on the network,
    this is another reason to ensure that a MOSPF router is elected as
    Designated Router.

    [18]In other words, group-membership-LSAs will never be
    retransmitted to non-multicast routers.




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    [19]This last step will not be necessary if the configuration
    guidelines presented in Section 6.5 are followed.

    [20]The TOS 0 routing table entry is examined regardless of the TOS
    specified by the multicast datagram.

    [21]It is assumed that a MOSPF router that wants to stop advertising
    a route to an external destination will use the premature aging
    procedure specified in Section 14.1 of [OSPF], rather than setting
    the AS external-link-LSA's cost to LSInfinity.

    [22]This preference ordering is used in Step 5c of Section 12.2.

    [23]No attempt is made to match the links' two halves. See Step 5d.

    [24]However, a summary-link-LSA is eligible for matching even if the
    MC-bit in its Options field is clear.

    [25]Costs may have both a Type 2 and a Type 1 component; the Type 2
    component is always most significant.

    [26]This case mirrors the SourceIntraArea candidate list
    initialization in Section 12.2.1.

    [27]This case mirrors the SourceInterArea1 candidate list
    initialization in Section 12.2.2.

    [28]This case mirrors the SourceInterArea2 candidate list
    initialization in Section 12.2.3.

    [29]Note that selecting the upstream node in this manner enforces
    the inter-area routing architecture outlined in Section 3.1. Namely,
    the multicast datagram is forwarded from the source area, over the
    backbone and then into the non-backbone areas. This is similar to
    the "hub and spoke" architecture for unicast forwarding described in
    Section 3.2 of [OSPF].

    [30]This procedure seems backwards. One would expect that the TOS X
    datagram tree would be built first. However, the SPF calculation
    must ensure that all routers participating in the forwarding of that
    datagram, both TOS-capable and non-TOS-capable, build the same tree.
    Since it is known that the non-TOS-capable routers will use the TOS
    0 tree, the only safe way to use the TOS X tree is when you are
    guaranteed that the non-TOS-capable routers will decline to forward
    the datagram. This guarantee is clearly met when there are only
    TOS-capable routers on the TOS 0 datagram tree.

    [31]Indeed, there will also be those cases where the router, not



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    being on a particular datagram shortest-path tree, will never have
    to calculate the particular tree, since the router will not receive
    the datagram in the first place.

    [32]Group-membership-LSAs are not processed by non-multicast routers
    (see Section 10.2). Also, if the Designated Router was not running
    the multicast extensions, multicast datagrams would not be forwarded
    over the network because its network-LSA would have its MC-bit clear
    (see Step 5a in Section 12.2).










































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A. Data Formats

    This section documents the format of MOSPF protocol packets and link
    state advertisements (LSAs). All changes and additions made to the
    OSPF Version 2 data formats have been made in a backward-compatible
    manner. In other words, multicast routers running MOSPF can
    interoperate with (non-multicast) OSPF Version 2 routers when
    forwarding regular (unicast) IP data traffic.

    The MOSPF packet formats are the same as for OSPF Version 2
    (described in Appendix A of [OSPF]). One additional option has been
    added to the Options field that appears in OSPF Hello packets,
    Database Description packets and all link state advertisements. This
    new option indicates a router's/network's multicast capability, and
    is documented in Section A.1.  The presence of this new option is
    ignored by all non-multicast routers.

    To support MOSPF, one of OSPF's link state advertisements has been
    modified, and a new link state advertisement has been added. The
    format of the router-LSA has been modified (see Section A.2) to
    include a new flag indicating whether the router is a wild-card
    multicast receiver. A new link state advertisement, called the
    group-membership-LSA, has been added to pinpoint multicast group
    members in the link state database. This new advertisement is
    neither flooded nor processed by non-multicast routers. The group-
    membership-LSA is documented in Section A.3.

























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A.1 The Options field

    The OSPF Options field is present in OSPF Hello packets, Database
    Description packets and all link state advertisements. The Options
    field enables OSPF routers to support (or not support) optional
    capabilities, and to communicate their capability level to other
    OSPF routers. Through this mechanism routers of differing
    capabilities can be mixed within an OSPF routing domain.

    When used in Hello packets, the Options field allows a router to
    reject a neighbor because of a capability mismatch. Alternatively,
    when capabilities are exchanged in Database Description packets a
    router can choose not to forward certain LSA types to a neighbor
    because of its reduced functionality. Lastly, listing capabilities
    in LSAs allows routers to route traffic around reduced functionality
    routers, by excluding them from parts of the routing table
    calculation.

    Three capabilities are currently defined. For each capability, the
    effect of the capability's appearance (or lack of appearance) in
    Hello packets, Database Description packets and link state
    advertisements is specified below. For example, the
    ExternalRoutingCapability (below called the E-bit) has meaning only
    in OSPF Hello packets.

                     +---+---+---+---+---+---+---+---+
                     | * | * | * | * | * |MC | E | T |
                     +---+---+---+---+---+---+---+-+-+

                          The OSPF Options field


    o   T-bit. This describes the router's TOS capability. If the T-bit
        is reset, then the router supports only a single TOS (TOS 0).
        Such a router is also said to be incapable of TOS-routing. The
        absence of the T-bit in a router links advertisement causes the
        router to be skipped when building a non-zero TOS shortest-path
        tree. In other words, routers incapable of TOS routing will be
        avoided as much as possible when forwarding data traffic
        requesting a non-zero TOS. The absence of the T-bit in a summary
        link advertisement or an AS external link advertisement
        indicates that the advertisement is describing a TOS 0 route
        only (and not routes for non-zero TOS).

    o   E-bit. AS external link advertisements are not flooded
        into/through OSPF stub areas. The E-bit ensures that all members
        of a stub area agree on that area's configuration. The E-bit is
        meaningful only in OSPF Hello packets. When the E-bit is reset



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        in the Hello packet sent out a particular interface, it means
        that the router will neither send nor receive AS external link
        state advertisements on that interface (in other words, the
        interface connects to a stub area). Two routers will not become
        neighbors unless they agree on the state of the E-bit.

    o   MC-bit. The MC-bit describes the multicast capability of the
        various pieces of the OSPF routing domain. When calculating the
        path of multicast datagrams, only those link state
        advertisements having their MC-bit set are used. In addition, a
        router uses the MC-bit in its Database Description packets to
        tell adjacent neighbors whether the router will participate in
        the flooding of the new group-membership-LSAs.






































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A.2 Router-LSA

    An OSPF router originates a router-LSA into each of its attached
    areas. The router-LSA describes the state and cost of the router's
    interfaces to the area. The contents of the router-LSA are described
    in detail in Section A.4.2 of [OSPF]. There are flags in the
    router-LSA that indicate whether the router is either a) an area
    border router or b) an AS boundary router or c) the endpoint of a
    virtual link. One more flag has been added to the router-LSA for
    MOSPF; it is called bit W below. This flag indicates whether the
    router wishes to receive all multicast datagrams regardless of
    destination (i.e., is a wild-card multicast receiver).

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       1       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    rtype      |        0      |            # links            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
       |                          Link ID                              | P
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ E
       |                         Link Data                             | R
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     # TOS     |        TOS 0 metric           | #
     + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L
     # |      TOS      |        0      |            metric             | I
     T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
     O |                              ...                              | K
     S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S
     | |      TOS      |        0      |            metric             | |
     + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
       |                              ...                              |

                                The router LSA








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                     +---+---+---+---+---+---+---+---+
                     | * | * | * | * | W | V | E | B |
                     +---+---+---+---+---+---+---+-+-+

                                The rtype field

    The following defines the flags found in the rtype field. Each flag
    classifies the router by function:

    o   bit B. When set, the router is an area border router (B is for
        border). These routers forward unicast data traffic between OSPF
        areas.

    o   bit E. When set, the router is an AS boundary router (E is for
        external). These routers forward unicast data traffic between
        Autonomous Systems.

    o   bit V. When set, the router is an endpoint of an active virtual
        link (V is for virtual) which uses the described area as its
        Transit area.

    o   bit W. When set, the router is a wild-card multicast receiver.
        These routers receive all multicast datagrams, regardless of
        destination.  Inter-area multicast forwarders and inter-AS
        multicast forwarders are sometimes wild-card multicast receivers
        (see Sections 3 and 4).

























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A.3 Group-membership-LSA

    Group-membership-LSAs are the Type 6 link state advertisements.
    Group-membership-LSAs are specific to a particular OSPF area. They
    are never flooded beyond their area of origination. A router's
    group-membership-LSA for Area A indicates its directly attached
    networks which belong to Area A and contain members of a particular
    multicast group. A router originates a group-membership-LSA for
    multicast group D when the following conditions are met for at least
    one directly attached network: 1) the router has been elected
    Designated Router for the network and 2) at least one host on the
    network has joined Group D via the IGMP protocol.

    A router may also originate a group-membership-LSA for Group D if
    the router itself has internal applications belonging to Group D. In
    addition, area border routers originate group-membership-LSAs into
    the backbone area when there are group members in the router's
    attached non-backbone areas. See Section 10 for more information
    concerning the origination of group-membership-LSAs.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       6       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Link State ID = Destination Group                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Vertex type                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Vertex ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |

                           The group-membership-LSA


    The group-membership-LSA consists of the standard 20-byte link state
    header (see Section A.4.1 of [OSPF]) followed by a list of transit
    vertices to label with the multicast destination. The
    advertisement's Link State ID is set to the destination multicast
    group address. There is no metric associated with the advertisement.
    Each transit vertex is specified by its Vertex type and Vertex ID



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    (see Section 12.1 for an explanation of this terminology):

    o   Vertex type. Set equal to 1 for a router, and 2 for a transit
        network.  Note that the only router that may be included in the
        list is the Advertising Router itself.

    o   Vertex ID. For router vertices, this field indicates the
        router's OSPF Router ID. For transit network vertices, this
        field indicates the IP address of the network's Designated
        Router. Note that the link state advertisement associated with
        the transit vertex is the LSA whose LS type = Vertex type, Link
        State ID = Vertex ID and Advertising Router = the group-
        membership-LSA's Advertising Router.






































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B. Configurable Constants

    This section documents the configurable parameters used by OSPF's
    multicast routing extensions. These parameters are in addition to
    the configurable constants used by the base OSPF protocol
    (documented in Appendix C of [OSPF]). An implementation of MOSPF
    must provide the ability to set these parameters, either through
    network management or some other means.

    B.1 Global parameters

        The following parameters apply to the router as a whole.

        o   Multicast capability. An indication of whether the router is
            running MOSPF. If the router is running MOSPF, it will
            perform the algorithms as set forth in this specification.
            Otherwise, the router is still able to run the basic OSPF
            algorithm (as set forth in [OSPF]), and will be able to
            interoperate with multicast capable routers (see Section
            6.1) when forwarding regular (unicast) IP data traffic.

        o   Inter-area multicast forwarder. This parameter indicates
            whether the router will forward multicast datagrams between
            OSPF areas. Such a router summarizes group membership
            information to the backbone, and acts as a wild-card
            multicast receiver in all its attached non-backbone areas
            (see Section 3.1). Not all multicast-capable area border
            routers need be configured as inter-area multicast
            forwarders.  However, whenever both ends of a virtual link
            are multicast-capable, they must both be configured as
            inter-area multicast forwarders (see Section 14.11). By
            default, all multicast-capable area border routers are
            configured as inter-area multicast forwarders.

        o   Inter-AS multicast forwarder. This parameter indicates
            whether the router forwards multicast datagrams between
            Autonomous Systems. Such a router acts as a wild-card
            multicast receiver in all attached areas (see Section 4). It
            is also assumed that an inter-AS multicast forwarder runs
            some kind of inter-AS multicast routing algorithm.

    B.2 Router interface parameters

        The following parameters can be configured separately for each
        of the router's OSPF interfaces. Remember that an OSPF interface
        is the connection between the router and one of its attached IP
        networks.  Note that the IPMulticastForwarding parameter is
        really a description of the attached network. As such, it should



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        be configured identically on all routers attached to a common
        network; otherwise incorrect routing of multicast datagrams may
        result.

        o   IPMulticastForwarding. This configurable parameter indicates
            whether IP multicasts should be forwarded over the attached
            network, and if so, how the forwarding should be done. The
            parameter can assume one of three possible values: disabled,
            data-link multicast and data-link unicast. When set to
            disabled, IP multicast datagrams will not be forwarded out
            the interface. When set to data-link multicast, IP multicast
            datagrams will be forwarded as data-link multicasts. When
            set to data-link unicast, IP multicast datagrams will be
            forwarded as data-link unicasts. The default value for this
            parameter is data-link multicast. The other two settings are
            for use in the special circumstances described in Sections
            6.3 and 6.4. When set to disabled or to data-link unicast,
            IGMP group membership is not monitored on the attached
            network.

        o   IGMPPollingInterval. The number of seconds between IGMP Host
            Membership Queries sent out this interface. A multicast-
            capable router sends IGMP Host Membership Queries only when
            it has been elected Designated Router for the attached
            network. See [RFC 1112] for a discussion of this parameter's
            value.

        o   IGMP timeout. If no IGMP Host Membership Reports have been
            heard on an attached network for a particular multicast
            group A after this period of time, the entry [Group A,
            attached network] is deleted from the router's local group
            database. See Section 9 for more information.



















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C. Sample datagram shortest-path trees

    In MOSPF, all routers must calculate exactly the same datagram
    shortest-path trees. In order to ensure this in internetworks having
    redundant links, a number of tie-breakers were defined in the MOSPF
    routing table calculation (see Steps 4 and 5c of Section 12.2, and
    Sections 12.2.4 and 12.2.7). This section illustrates the use of
    these tie-breakers on a sample topology.

    Three different examples are given. All examples use the same
    physical topology and the same set of OSPF interface costs (see the
    left side of Figure 14). The source of the datagram is always Host
    H1 on the network at the top of the figure (192.9.1.0), and the
    destination group members are the two hosts labelled with Group Ma
    at the bottom of the figure. The first case shows an example of
    intra-area multicast, while the remaining two cases show the
    influence of OSPF areas on the path of a multicast datagram.


































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C.1 An intra-area tree

    The datagram shortest-path tree resulting from the intra-area case
    is shown on the right of Figure 14. The root of the tree is the
    source network (192.9.1.0), and the leaves are the two routers (RT4
    and RT3) directly attached to the stub networks containing Group Ma
    members.

    There are equal-cost paths available to both group members. For the
    group member on the left, the path could go either through network
    10.1.0.0 or through network 10.2.0.0. By the tie-breaking rules, the
    path through 10.2.0.0 is chosen since it has the larger IP network
    number (see Step 5c of Section 12.2).

    For the group member on the right, the path could go either over
    Network 10.2.0.0 or over the serial line connecting routers RT2 and
    RT3. The path over Network 10.2.0.0 is chosen after executing two
    tie-breaking rules. First, Network 10.2.0.0 is placed on the
    shortest-path tree before Router RT3 since networks are always
    chosen over routers (see Step 4 of Section 12.2). Then, given a

                                 +--+
                                 |H1|
                                 +--+
                    Net 192.9.1.0  |
                         +------------------+
                            |            |
        +----------+        |1           |1
        |  Network |     8+---+        +---+            o 192.9.1.0
        | 10.1.0.0 |------|RT1|        |RT2|            |
        +----------+      +---+        +---+           0|
             |              |8          8|              |
            8|         +----------+      |8             o RT1
           +---+10     | Network  |  10+---+            |
           |RT4|-------| 10.2.0.0 |----|RT3|           8|
           +---+       +----------+    +---+            |
             |3                          |3             o 10.2.0.0
             |                           |             / \
        +---------+                  +-------+       0/   \0
             |                           |           /     \
           +--+                        +--+         o       o
           |Ma|                        |Ma|        RT4      RT3
           +--+                        +--+


                        Figure 14: An intra-area tree





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    choice of either Network 10.2.0.0 or Router RT2 for RT3's parent on
    the tree, Net 10.2.0.0 is again preferred since it is a network (see
    Step 5c of Section 12.2)
















































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C.2 The effect of areas

    In Figure 15 below, the previous diagram has been modified by the
    inclusion of OSPF areas. The datagram source is now part of the OSPF
    backbone (Area 0), while the rest of the topology is in Area 1. In
    this case, since the datagram source and the group members belong to
    different areas, reverse costs are used when building the tree (see
    Step 5b of Section 12.2). This actually eliminates the equal cost
    paths from the diagram, and leads to the Area 1 datagram shortest-
    path tree on the right of Figure 15.











                                 +--+
                                 |H1|
                                 +--+
                    Net 192.9.1.0  |
                         +------------------+
      ..................... |            |
      . +----------+      . |1           |1            192.9.1.0
      . |  Network |     8+---+        +---+                o
      . | 10.1.0.0 |------|RT1|........|RT2|...            / \
      . +----------+      +---+        +---+  .          1/   \1
      .      |              |8          8|    .          /     \
      .     8|         +----------+      |8   .         o RT1   o RT2
      .    +---+10     | Network  |  10+---+  .         |        \
      .    |RT4|-------| 10.2.0.0 |----|RT3|  .        0|         \8
      .    +---+       +----------+    +---+  .         |          \
      .      |3                          |3   .         o 10.1.0.0  o
      .      |                           |    .         |          RT3
      . +---------+                  +-------+.        8|
      .      |                           |    .         |
      .    +--+                        +--+   .         o
      .    |Ma|                        |Ma|   .        RT4
      .    +--+     Area 1             +--+   .
      .........................................

                        Figure 15: The effect of areas





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C.3 The effect of virtual links

    In Figure 16 below, Network 10.1.0.0 has been configured as a
    separate area (Area 1), while everything else belongs to the OSPF
    backbone (Area 0). In addition, a virtual link has been configured
    through Area 1, enhancing the backbone connectivity. In this case,
    both the source and the group members belong to the same area, so
    forward costs are used. However, since virtual links are preferred
    over regular links (see Step 5c of Section 12.2), the backbone
    datagram shortest-path tree uses Network 10.1.0.0 instead of
    10.2.0.0 on the path to the left group member. This leads to the
    tree on the right of Figure 16.









                                 +--+
                                 |H1|
                                 +--+
                    Net 192.9.1.0  |
      ................   +------------------+
      . +----------+ .     /1            |
      . |  Network |8.    /              |1
      . | 10.1.0.0 |-+---+             +---+            o 192.9.1.0
      . +----------+*|RT1|             |RT2|            |
      .     8|*******+---+             +---+           0|
      .Area1 |*VL    .    \8            8|              |
      .....+---+...... +----------+      |8             o RT1
           |RT4|10     | Network  |  10+---+           / \
           +---+-------| 10.2.0.0 |----|RT3|          /8  \8
             |         +----------+    +---+         /     \
             |3                          |3         o 10.1  o 10.2.0.0
             |                           |          |       |
        +---------+                  +-------+      |0      |0
             |                           |          |       |
           +--+                        +--+         o       o
           |Ma|                        |Ma|        RT4      RT3
           +--+                        +--+


                   Figure 16: The effect of virtual links





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Security Considerations

    Security issues are not discussed in this memo.

Author's Address

    John Moy
    Proteon, Inc.
    9 Technology Drive
    Westborough, MA 01581
    Phone: (508) 898-2800
    Email: jmoy@proteon.com







































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