Multicast Open Shortest Path First

Multicast Open Shortest Path First ( MOSPF ) can be used in computer networks to process package shipping to the multicast principle. It is in the MOSPF Multicast Extension to the intra -domain routing protocol OSPF ( Open Shortest Path First ). MOSPF allows multicast routing within an OSPF domain. Here, however, special attention should be put on the fact that this domain may / will not be arbitrarily large and that the routing calculation works efficiently and quickly.

Operation

In OSPF, each router maintains a complete data base which describes the overall network topology. By introducing a new link-state - record type - the Group Membership LSA - is it possible with MOSPF to store the location of all multicast nodes of a multicast group in the topology database. Since OSPF is a link-state protocol, this information is therefore available on all routers in the domain are available. Group Membership LSAs are generated by those routers, which concludes on a subnet ( LAN) through IGMP group membership. These LSAs remain, as well as conventional router and network LSAs only within one OSPF area, so do not be forwarded to other areas or even other Autonomous Systems.

In combination with the conventional router and network LSAs ( topology database ) can thus starting from that router, behind which a multicast source is located (source), for each source, an implicit Sourcebase Tree - a shortest path tree, engl. Shortest Path Tree ( SPT short ), which has the multicast source as root - are computed by the network. It is to no additional multicast routing protocol is required because the conventional unicast routing OSPF is shared.

On-Demand Calculation

Theoretically, the calculation of the SPT via RPF ( Reverse Path Forwarding ) and pruning carried out on each router before a multicast packet is sent (because the entire topology database in each router is available ). However, this can lead to performance problems because the router for each multicast source must calculate its own SPT. These calculations can also force routers that are equipped with comparatively powerful CPUs, to its knees.

Instead of this automatic calculation of all SPTs the SPTs are now calculated according to demand ( computation on demand): The SPT is only calculated when the first multicast packet to a multicast source arrives for a specific multicast group. Possible activities in this calculated SPT, which do not have multicast receivers are removed from the tree ( pruning ). Nevertheless, this method can also lead to a performance problem when behind a router are many different sources and many multicast receivers frequently log on and off. Such behavior in turn leads to constant recalculation of the SPT.

When it has come to a calculation of a SPT, this information is cached in a cache of the router and used for subsequent communication between the multicast source and the corresponding multicast group.

Inter- area routing

OSPF makes it possible to divide an OSPF domain ( for example, an autonomous system ) into a number of so-called areas. If this is done, no longer has any router on a full topology map of the OSPF domain. Instead, each router knows only its area ( s ), while the communication between areas of the ABRs ( Area Border Router) takes place. While this is advantageous for unicast OSPF; however, can be calculated thus in multicast OSPF no complete SPTs. This leads u.U. to inefficient multicast routing.

This MOSPF works between two areas, the Area Border routers must know about the multicast behavior of the network communication. Such routers are called Area Border Router Multicast ( MABR ). A MABR will summarize all multicast information and the corresponding LSAs into Area 0 send ( Note: Area 0 is not distributed these received information back to other areas, that is, this approach is only useful if the multicast sources are in Area 0 ). If multicast sources but also lie outside of Area 0, this problem is solved by setting a wildcard bit in the router -LSA for each MABR. This bit causes each MABR is a member of each multicast group, and hence the multicast traffic from a source sucks, does not lie in Area 0. Thus, the multicast traffic reaches the Area 0, and this can then forward according to group memberships known in Area 0 multicast traffic to the affected areas. However, this can lead to unwanted network traffic (traffic) result if no "real" multicast sources exist.

Alternatives

Swell

  • RFC 1584 Multicast Extensions to OSPF
  • Routing protocol
  • Abbreviation
Link-state routing protocol Internet Group Management Protocol Cache (computing)
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