Multicast Source Discovery Protocol (MSDP)

Another multicast border router protocol is the Multicast Source Discovery Protocol (MSDP). This particular protocol can only be used to connect domains that also use PIM-SM internally. You might find it helpful to review the section,“Protocol Independent Multicast-Sparse Mode (PIM-SM),” in Chapter 2, “Multicast Protocols,” before proceeding. This protocol also utilizes the BGP routing tables rather than building its own for unicast data.

MSDP routers keep a continuous connection open to their neighbors, or peers, using TCP. A KeepAlive timer is used to check whether KeepAlive messages arrive within specified intervals, though these messages are not sent unless no Source-Active (SA) messages have arrived within the KeepAlive requirements. If the interval is exceeded with no KeepAlive or SA messages having arrived, the peer tries to reset the TCP connection.

The developers of MSDP did not feel the need to reinvent the wheel. PIM-SM and MSDP utilize the already existing PIM-SM Rendezvous Point (RP) setup to speak to one another. Whenever a new source starts broadcasting and this information reaches an RP, the machine creates an SA message containing the following information:

• The IP address of the machine originating the broadcast

• The multicast channel address to where the data is being sent

• The IP address of the Rendezvous Point that handles this channel

This message is then sent to the MSDP border router. After the SA message is received, it is forwarded to all of the border router’s MSDP neighbors except for the machine it received the message from—this is called peer-RPF flooding.When each peer receives the SA packet, it checks to see which of its neighbors is the RPF peer for this particular multicast channel. The RPF peer is the MSDP router closest to the MSDP router that sent the original SA message, which we’ll call the SA originator.

The router double-checks any SA message against the RPF peer to avoid loopback problems. If the SA message came from a machine that is closer to the SA originator than this router is, but is not the RPF peer, the SA message is ignored. Other than this, the MSDP router sends the SA message out to all of its peers—namely, those that are not closer to the SA originator than it is.

This process continues throughout the entire Internet, as long as there are more MSDP peers to talk to. Another thing that each MSDP peer has to examine is whether or not it’s also a PIM-SM Rendezvous Point for its domain. If this machine serves both purposes, it checks to see if any of its own domain members have requested access to this new channel. Even one request causes this MSDP/PIM-SM RP to send a Join request to the channel’s RP. Doing this creates the odd situation where the tree for this particular channel reaches out of its local domain.

As you can see, this protocol is fairly straightforward because it’s able to rely on BGP for routing tables and PIM-SM for tracking group memberships. The Linux program that implements this protocol is the multicast version of gated.