MSDP essentially connects PIM-SM domains beyond the borders of autonomous systems. MSDP in combination with MBGP can connect easily multicast-enabled ISP domains at private or public peering points by maintaining their individual RPs. MSDP is also a key enabler for anycast RPs. Anycast RPs add load sharing and redundancy to PIM-SM multicast networks. The MSDP peering connection is implemented in TCP, whereas PIM-SM takes care of the actual forwarding between domains.
BGPv4 Multicast Extensions (Multiprotocol BGP, RFC 2858)
The pragmatic way of interconnecting isolated multicast realms or islands is to use unicast tunnels (GRE or IP-IP) constituting the MBONE. With the advent of multiprotocol features for BGP, it is now possible to let BGP deal with separate sets of unicast and multicast network layer reachability information (NLRI). This is also the evolution we see today—fewer tunnels, more and more MBGP/MSDP-enabled border or peering routers, and MBGP/MSDP and PIM-SM replacing DVMRP for interautonomous system multicast routing, as sketched in Figure 14-4.
It is the general consensus that this task has to be carried out with a sparse-mode protocol. Zebra/Quagga supports MBGP hooks. I consider BGP peering points and exchanges the best place to exchange interdomain multicast signaling information. Eventually, multicast route server and looking glasses will become as popular as their unicast counterparts.
Multicast Transport Layer Protocols
These research endeavors deal with the issue of adding reliable transport mechanisms to intrinsically unreliable UDP multicasting transport to suit the needs of special multicast application layer requirements. For details, consult the resources at the IETF Reliable Multicast Transport (rmt) Working Group at http://www.ietf.org/html.charters/rmt-charter.html.
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