Why is the multicast stream included in a PIM Register message?

[Mitrixsen]

Hello, everyone

When a router forwards a REGISTER message to the RP, it will include the multicast stream.

If a register-stop message is received, the further registers are sent without the multicast stream, only the IP header

My question is, why is the multicast stream even included in the first REGISTER message? The RP doesn't really do anything with this information, does it? All it needs is to know the source IP and the destination GROUP address so it can create an S,G entry and figure out whether there are any interested receivers.

Thank you.

David

[Harold Ritter · Accepted solution]

Hi @Mitrixsen ,

> The RP doesn't really do anything with this information, does it?

Yes, it does. The data is actually forwarded down the shared tree if there are active receivers. Otherwise, a receiver stop is immediately sent to the DR.

You might want to read RFC7761, section 3.2 to better understand why it is worth for DR to encapsulate the actual multicast message in the register message, but in short it avoids the active receivers incurring an extra delay in receiving the multicast traffic while the RP joins the source tree.

https://datatracker.ietf.org/doc/rfc7761/