network protocols handbook.pdf

85Protocols GuideTCP/IP - Network Layer ProtocolsProtocol NameMOSPF: Multicast Extensionsto OSPFProtocol DescriptionMulticast Extensions to OSPF (MOSPF) provides enhancementsto OSPF Version 2 to support IP multicast routing. Theenhancements have been added in a backward-compatiblefashion; routers running the multicast additions will interoperatewith non-multicast OSPF routers when forwarding regular(unicast) IP data traffic.MOSPF works by including multicast information in OSPF linkstate advertisements. An MOSPF router learns which multicastgroups are active on which LANs. MOSPF builds a distributiontree for each source/group pair and computes a tree for activesources sending to the group. The tree state is cached, andtrees must be recomputed when a link state change occurs orwhen the cache times out.MOSPF provides the ability to forward multicast datagrams fromone IP network to another through internet routers. MOSPF forwardsa multicast datagram on the basis of both the datagram’ssource and destination. The OSPF link state database providesa complete description of the Autonomous System’s topology.By adding a new type of link state advertisement, the groupmembership-LSA,the location of all multicast group membersis pinpointed in the database. The path of a multicast datagramcan then be calculated by building a shortest-path tree rootedat the datagram’s source. All branches not containing multicastmembers are pruned from the tree. These pruned shortest-pathtrees are initially built when the first datagram is received. Theresults of the shortest path calculation are then cached for useby subsequent datagrams having the same source and destination.MOSPF is used internal to a single Autonomous System. Whensupporting IP multicast over the entire Internet, MOSPF wouldhave to be used in concert with an inter-AS multicast routingprotocol such as DVMRP.Routers running MOSPF works only in internetworks that areusing MOSPF but can be intermixed with non-multicast OSPFrouters. Both types of routers can interoperate when forwardingregular (unicast) IP data traffic. In MOSPF, just as in the baseOSPF protocol, datagrams (multicast or unicast) are routed “asis”; they are not further encapsulated or decapsulated as theytransit the Autonomous System.Protocol StructureThe MOSPF packet formats are the same as for OSPF Version2. One additional option has been added to the Options field thatappears in OSPF Hello packets, Database Description packetsand all link state advertisements. This new option indicates arouter’s/network’s multicast capability. The presence of this newoption is ignored by all non-multicast routers.1 2 3 4 5 6 7 8bit* * * * * MC E T• T-bit – describes the router’s TOS capability.• E-bit – AS external link advertisements are not floodedinto/through OSPF sub areas. The E-bit ensuresthat all members of a stub area agree on that area’sconfiguration.• MC-bit – describes the multicast capability of thevarious pieces of the OSPF routing domain.To support MOSPF, one of OSPF’s link state advertisementshas been modified, and a new link state advertisement hasbeen added. The format of the router-LSA has been modifiedto include a new flag indicating whether the router is a wild-cardmulticast receiver.The rtype field in the router LSA:1 2 3 4 5 6 7 8bit* * * * W V E B• bit B - B is for border . When set, the router is an areaborder router. These routers forward unicast datatraffic between OSPF areas.• bit E - E is for external. When set, the router is anAS boundary router (). These routers forward unicastdata traffic between Autonomous Systems.• bit V - V is for virtual. When set, the router is an endpointof an active virtual link which uses the describedarea as its Transit area.• bit W - When set, the router is a wild-card multicastreceiver. These routers receive all multicast datagrams,regardless of destination. Inter-area multicastforwarders and inter-AS multicast forwarders aresometimes wild-card multicast receivers.A new link state advertisement, called the group-membership-LSA, has been added to pinpoint multicast group members inthe link state database. This new advertisement is neither floodednor processed by non-multicast routers.Related protocolsIP, TCP, OSPF, IGMPSponsor SourceMOSPF is defined by IETF (http://www.ietf.org) in RFC 1584.