Subject: Re: (S,G) join message direction

From: "Jyotheeswara Rao Kurma" <[email-masked]>
Date: Wed, 11 Apr 2001 09:51:13 +0500

On Tue, 10 Apr 2001 15:05:09 -0400 "Matthew Koivisto" wrote:

> Hi, I've been having this conversation with some other people and it 
has
> been brought to my attention that i should bring it to the mailing 
list.
> 
> Say there is already a spt tree established between S and R, so no 
packets
> are getting recieved by R from the RP. (we're in phase 3). Now say 
another
> node (R2), who is topolgocially close to R1, wishes to join that spt
> directly (he does not wat to go through all the phases), his DR sends 
a
> (S,G) join message. Now does that (S,G) join message travel towards
> the RP?
> or does it travel towards S? The draft is not clear as to where that 
(S,G)
> join travels too. (see page 10, 3rd paragraph).

  (S,G) joins are always towards S only. There is no point in going to 
RP.
   As the receiver wants to join SP tree, Join will go towards S from 
DR 
  of  the Receiver R2, that you are mentioning.

> Now it makes perfect sence
> to me that the message will go towards S, but this isn't the case is 
it?
> 
> Additionally, it just occured to me that durring phase 3 of PIM-SM no
> packets are being recieved form the RP, so if a new source starts to 
send
> packets on the group, the RP tree has been removed and R will not 
recieve
> those new packets right? Hence the reason to not goto phase 3.

   Till the PIM -Router (at which  the RP tree and SP Tree diverge), 
sends
   (S,G) RP bit Prune message towards RP, multicast packets will travel
on both RP ( Shared ) tree, as well as SP Tree. So there is no question 
of 
loosing packets. Once the redundant (duplicate) packets statrs arriving,
then that (S,G) Rp bit prune message will be sent towards RP.

Thanks,
Jyotheesh

> Thanks,
> Matt

---

From: Hitoshi Asaeda <[email-masked]>
Date: Wed, 11 Apr 2001 17:53:34 +0900

>   (S,G) joins are always towards S only. There is no point in going to 

No, I don't think so.

I think original question is a little ambiguous, though.
I thought "topologically close to R1" didn't mean R1 and R2 were on a
same LAN. If my assumption is correct, when the group range is *not in
a SSM range*, R2's DR which is not R1's DR would try to send (*,G)
join toward the RP, even if R1 sends (S,G) join (i.e., IGMPv3's
source-specific join). This is because other nodes on an R2's LAN may
want to join the group sent by another sender. R1's (S,G) join effects
"source address filtering" in this case. But the DR would send (*,G)
join.

Of course, if the group range is in a SSM range, then each DR sends
(S,G) join.
--
Hitoshi Asaeda