RE: Gen-ART LC Review of draft-ietf-pim-sm-bsr-10.txt

[ Changed Nidhi's address ]

Eric,

On Wed, 14 Mar 2007 11:27:47 -0500, "Eric Gray \(LO/EUS\)" <eric.gray <at> ericsson.com> said:

> 	Thanks, I missed that.  But, now that I see how it is meant
> (in what context), I wonder that the same action is not expected
> if a C-PSR receives a Non-Preferred BSM from a P-BSR.  Or is it
> the case that the P-BSR state is so temporary that a C-BSR would
> treat a likely P-BSR as if it is already the elected BSR?

The event can only occur in C-BSR state, because that is the only
state in which the router considers a different router to be the
elected BSR.  When in P-BSR state, it considers itself to be the BSR
with highest weight but only originates a BSM when it transitions to
the E-BSR state.  When a BSM is received, the local router knows that
the originating router considered itself to be the elected BSR.

> 	When I read the event, I was not clear on the context - i.e.
> - "non-preferred" with respect to what?  If the "non-preferred" is
> with respect to the local C-BSR, then I think it should also be an

Yes, for this event, "non-preferred" is with respect to the local
C-BSR (this is explained in section 3.1.4).

> argument that the same transistion (C-BSR -> P-BSR) should apply 
> in receiving a non-preferred BSM from a current P-BSR.

There is no notion of a "current P-BSR" (see above), but I know what
you mean (the router receives a BSM from a router that is not the
currently elected BSR, which can happen, of course).  In that case,
the weight is compared with that of the elected BSR, not the one of
the local C-BSR, which is lower than the latter by definition.  

If the weight is lower, the new BSM is considered to be non-preferred
and is ignored.  This event is omitted from the state table because it
triggers no action.

If the weight is higher, the originator of this BSM becomes the
elected BSR.

In both cases, the local router remains in the C-BSR state.

> 	This is pretty murky, since we appear to be trying to guess
> the transition that should be occuring in a peer.  Does the elected
> BSR that sent the non-preferred BSM know that it is no-longer the
> elected BSR, or is that transition assumed to take place when it 
> receives a reciprocal (assumed symmetric) preferred BSM from the 
> local BSR?  

I don't think any assumptions are necessary.  At any point in time,
there is a strict ordering within the set of candidate BSRs according
to their weight.  The state machine must guarantee that the router
with highest weight eventually starts flooding its BSMs, which will
cause all other routers to enter C-BSR state and stay there.  

In this particular case, when the priority of the elected BSR is
lowered in such a manner that a different C-BSR now has higher weight,
that router will start flooding BSMs.  These BSMs will be considered
to be preferred by the former elected BSR which will then transition
to C-BSR and stop sending BSMs.

If you find a scenario, in which the C-BSR with highest weight remains
in the C-BSR state indefinitely, that would be a problem.

> Would the same apply if that BSR were still in the 
> temporary P-BSR state?

> 	Possibly these things are clear to people who've followed the
> discussion-history, but the answers to these questions don't jump 
> out at me...

Essentially, we convinced ourselves (and the WG), that the state
machine does indeed cover all cases, although not in a formal manner.

This has not been the subject of discussion for a long time.  There
also exist various implementations of this specification which did not
reveal any problems so far.

I hope my explanations above helped to make things clearer.

--
Alex