diff options
Diffstat (limited to 'doc/rfc/rfc1584.txt')
| -rw-r--r-- | doc/rfc/rfc1584.txt | 5713 |
1 files changed, 5713 insertions, 0 deletions
diff --git a/doc/rfc/rfc1584.txt b/doc/rfc/rfc1584.txt new file mode 100644 index 0000000..8543c33 --- /dev/null +++ b/doc/rfc/rfc1584.txt @@ -0,0 +1,5713 @@ + + + + +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. + + + + + +Moy [Page 1] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + +Moy [Page 2] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + + + + + + + + + + +Moy [Page 3] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + +Moy [Page 4] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 5] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 6] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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]. + + + +Moy [Page 7] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 8] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + + + + + | 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 + + + + + +Moy [Page 9] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 10] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 11] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + + + +Moy [Page 12] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + + + **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. + + + + + + +Moy [Page 13] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + +Moy [Page 14] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + 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 + + + +Moy [Page 15] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 16] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 17] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + + 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 + + + +Moy [Page 18] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + +Moy [Page 19] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + .................................. + . + . + . | 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 + + + + + +Moy [Page 20] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + + + +Moy [Page 21] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 22] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + **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. + + + + + + + + + + + + + + + +Moy [Page 23] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + **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 + + + +Moy [Page 24] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + + + +Moy [Page 25] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + + +Moy [Page 26] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + +Moy [Page 27] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + (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 + + + +Moy [Page 28] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 29] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + 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 + + + +Moy [Page 30] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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) + + + +Moy [Page 31] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + + + +Moy [Page 32] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 33] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + +-------+ + |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. + + + + + +Moy [Page 34] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 35] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 36] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 37] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + + <-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 + + + +Moy [Page 38] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 39] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + +Moy [Page 40] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 41] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 42] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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]. + + + + +Moy [Page 43] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + +Moy [Page 44] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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). + + + + +Moy [Page 45] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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: + + + + + +Moy [Page 46] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + (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 + + + +Moy [Page 47] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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- + + + +Moy [Page 48] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 49] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 50] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + + + +Moy [Page 51] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + +Moy [Page 52] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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: + + + + + +Moy [Page 53] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + +Moy [Page 54] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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: + + + + +Moy [Page 55] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 56] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + + + +Moy [Page 57] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + +Moy [Page 58] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + +Moy [Page 59] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 60] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + +Moy [Page 61] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + +Moy [Page 62] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + (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 + + + +Moy [Page 63] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 64] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 65] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + +Moy [Page 66] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + (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. + + + + +Moy [Page 67] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 68] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 69] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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: + + + +Moy [Page 70] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + (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: + + + + + +Moy [Page 71] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 72] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + +Moy [Page 73] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + +Moy [Page 74] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 75] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + +Moy [Page 76] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 77] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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). + + + + + + +Moy [Page 78] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 79] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 80] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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 + + + +Moy [Page 81] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 82] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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. + + + + + + + + + + + + + + +Moy [Page 83] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + +Moy [Page 84] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + +Moy [Page 85] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + [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 + + + +Moy [Page 86] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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). + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 87] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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. + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 88] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + +Moy [Page 89] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 90] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + + + + + + +Moy [Page 91] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + +---+---+---+---+---+---+---+---+ + | * | * | * | * | 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). + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 92] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + +Moy [Page 93] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + (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. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 94] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + +Moy [Page 95] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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. + + + + + + + + + + + + + + + + + + + +Moy [Page 96] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 97] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + + + +Moy [Page 98] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + + 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) + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 99] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + + + +Moy [Page 100] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + + + +Moy [Page 101] + +RFC 1584 Multicast Extensions to OSPF March 1994 + + +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 + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Moy [Page 102] + |