Subject: [pim] I-D ACTION:draft-ietf-pim-sm-bsr-03.txt,.ps

From: [email-masked]
Date: Tue, 04 Mar 2003 07:29:15 -0500

A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the Protocol Independent Multicast Working Group of the IETF.

	Title		: Bootstrap Router (BSR) Mechanism for PIM Sparse Mode
	Author(s)	: B. Fenner et al.
	Filename	: draft-ietf-pim-sm-bsr-03.txt,.ps
	Pages		: 35
	Date		: 2003-3-3
	
This document specifies the Bootstrap Router (BSR) mechanism
for Protocol Independent Multicast - Sparse Mode (PIM-SM).
BSR is one way that a PIM-SM router can learn the set of
group-to-RP mappings required in order to function.

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From: Mark Handley <[email-masked]>
Date: Wed, 05 Mar 2003 10:23:47 -0800

>group-to-RP mappings required in order to function.

This is essentially a re-issue of the PIM BSR draft which had expired.
I appreciate that it needs more work to make it more readable, but I
want to draw your attention to one important change.

This change fixes a problem with Bootstrap Message flooding, where
BSMs could be blackholed.  Previously a BSM was not forwarded out of
the interface on which it was received.  In section 3, the text on
forwarding BSMs now reads:

     Forward BSM
          A bootstrap message that passes the Bootstrap Message
          Processing Checks is forwarded out of all interfaces with PIM
          neighbors (including the interface it is received on), except
          where this would cause the BSM to cross an admin-scope
          boundary for the scope zone indicated in the message.  The
          source IP address of the message is the forwarding router's IP
          address on the interface the message is being forwarded from,
          the destination address is ALL-PIM-ROUTERS, and the TTL of the
          message is set to 1.

          As an optimation, a router MAY choose not to forward a BSM out
          of the interface the message was received on if that interface
          is a point-to-point interface.  On interfaces with multiple
          PIM neighbors, a router MUST forward an accepted BSM onto the
          interface that BSM was received on, but if the number of PIM
          neighbors on that interface is large, it MAY delay forwarding
          a BSM onto that interface by a small randomized interval to
          prevent message implosion.

          Rationale: A BSM needs to be forwarded onto the interface the
          message was received on (in addition to the other interfaces)
          because the routers on a LAN may not have consistent routing
          information.  If three routers on a LAN are A, B, and C, and
          at router B RPF(BSR)==A and at router C RPF(BSR)==B, then
          router A originally forwards the BSM onto the LAN, but router
          C will only accept it when router B re-forwards the message
          onto the LAN.

Cheers,
	Mark
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