CBT Join Messages can be Terminated before reaching Core

[hwei0]

Currently in all our course content, we only specify that when JOIN messages are sent by a host:

1. they are unicast to the core router
2. and each intermediate router receiving that request will enable the necessary link(s) to join that new host to the tree, and forward the message to the next-hop router towards the core.

Thus, our material (at least based on my interpretation) assumes that all JOIN messages will reach the core.

However, this is inconsistent with the the original CBT paper, which says on page 91:

The join continues its journey until it either reaches
the addressed core, or reaches a CBT-capable router
that is already part of the tree, as identified by the
group-id. At this point, the join’s journey is terminated
by the receiving router, which then decidesl whether to
acknowledge the join request.

Content affected (not comprehensive):

• Discussion 12 of SP25, problem 2.2(b) -- answer will vary depending on whether JOIN messages are terminated before reaching core.
• Lecture slide 72, 73 of SP25 Lecture 24 on Multicast -- implicitly states that all messages will reach core, and that "early termination" is not enabled.
• Textbook section on Building CBTs -- can modify to include this detail