Subject: RE: [pim] PIM-SM v2 deployment issues - 1 minute latency?

From: "Shekhar, Ravi" <[email-masked]>
Date: Fri, 2 Nov 2001 14:20:34 -0500

> > 1. Are there anybody out there notice that when sender 
> starts before the
> > receiver. The receiver will have to wait 1 minute to 
> receive data after
> > it joins the group.
> 
> This is a bug in the implementation of the protocol in your switches.
> Either the last-hop router is failing to trigger a (*,G) join 
> on receipt
> of the IGMP host report, or the RP is failing to convert that (*,G) 
> join into an (S,G) join toward the source.  This is not a PIM protocol
> problem. 

Hi John,
  In case the RP has sent a register-stop to the sender-DR, the protocol 
  itself doesnt provide any mechanism to cancel that register-stop sent 
  earlier, in which case it could take as much as 60 seconds before RP
starts
  getting register-messages again.

  What you are suggesting requires that RP triggers a switch to the shortest

  path <S, G> on its own. But that isnt something mandated by the old PIM-SM

  draft (I havent read the new one to comment on it). Am I missing
something?

  Regards.  
  - Ravi Shekhar.

---

From: John Zwiebel <[email-masked]>
Date: Fri, 02 Nov 2001 11:46:11 -0800

>   getting register-messages again.

When the RP gets the first register it is suppose to create and maintain
(S,G) state with a NULL olist.

>   draft (I havent read the new one to comment on it). Am I missing

Any router that has (S,G) state that receives a (*,G) join on an 
interface that is not in the (S,G) olist should add that interface
to the (S,G) olist.  (It is an inherited olist, which may be important
in some implementations but which is not yet recognized by all yet,
and may not be -- remember Cisco's X-flag).

This would change the upstream interface to "JOIN-DESIRED" == TRUE.
meaning, _any_ router, not just the RP, should trigger a join toward
the source.

FWIW, the RP can be thought of as a "virtual receiver".  The line
between the source and the RP (called the X-line for want of a better
term) is a branch of the shortest path tree.  Any time the RP has
a (*,G) with a non-NULL olist it is suppose to be sending JOINs toward
the source (in general).  So, yes the RP is suppose to join to the
shortest-path on its own.

I don't want to repeat what is suppose to happen because of (S,G,R) 
prunes here because that is, in some ways, implementation dependent.
It also gets complicated by the 'inherited/immediate' olist questions.

So, yes the RP is always suppose to be able to join the SPT on its own.

FWIW, the important part of the PIM register packet isn't that it 
includes the encapsulated first data packet.  Its that this message
informs the RP of a source that is sending so the RP can join toward
the source.  The data is encapsulated so receivers have the _potential_
of getting the first packet.  In practice, this has proven to be
extremely problematic.  What good is it to receive the first packet but
lose 2-n as the last hop router switches to the SPT?  (This is a 
implementation problem not necessarily a PIM protocol problem, and 
probably a point I shouldn't have brought up in this thread since its
orthogonal.).

z