Subject: [pim] Patch the share tree

From: "Loa, Kanchei" <[email-masked]>
Date: Tue, 18 Jan 2005 17:59:53 -0800

In draft-ietf-pim-sm-v2-new-11.txt [page 123]
o The combination of a (S,G) Prune and a (S,G,rpt) Join could possibly
  be used by a router to switch from receiving a particular source on
  the shortest-path tree back to receiving it on the shared tree
  (provided that the RPF neighbor for the shortest-path and shared trees
  is common). However Sparse-Mode PIM does not provide a mechanism for
  switching back to the shared tree.

After the initial transmission is established, the switch-over to the
Shortest Path Tree occurs once a certain number of bits (the
SPT-Threshold) have been sent down the Shared Tree. This threshold seems
to be almost always set to zero, so that the transition starts
immediately. However, after the multicast source S is silent for 210
seconds, (S,G) entry timeout and there is no mechanism for switching
back to the shared tree. S must wait for max. 60 seconds ((*,G) Join
refresh from the receiver) before its multicast packet could flow
through the shared tree to the receiver again.

In general, this 60 seconds no-sending-period is not a problem for a
generic multimedia source that leaving and join the group. But, in the
scenario where the multicast source only sends short bursts of near
real-time traffic spontaneously. Thereby, no-sending-period become an
issue once the source goes silent for more than 210 seconds, then must
send some critical data right after (S,G) timeout.

To prevent multicast route flipping, I could understand the rational
choice of not implementing a mechanism for switching back to the shared
tree due to SPT-Threshold. However, a (S,G) entry timeout event should
logically triggers a Join (S,G rpt) sent to the shared tree from the
router which initiate Prune (S,G,rpt) to undo the source blocking in the
shared tree. Is there any reason why this is not part of the PIM
specification? Do I miss something important?

Kanchei
-------------------------------
Kanchei Loa
Boeing Company
Email: [email-masked]
Phone: 714-7622559
Fax: 714-7622023

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