Subject: Re: Use of srcmask in prune/grafts

From: Bill Fenner <[email-masked]>
Date: Thu, 20 Jul 2000 11:23:48 -0700

I struggled with the semantics of prunes-with-masks when trying to
figure out how it should work for DVMRP.  In particular, when route
aggregation occurs, you have to decide what the semantics of the
message are or you end up multicasting the message upstream towards
all of the component subnets.

I think the best thing to do with a prune with a mask is for the
prune to travel towards the source, allowing the mask to get longer
if aggregation happened on the way.  It instantiates prune state
where it travels, which will cause further traffic from sources that
fall under other pieces of the aggregate to trigger prunes.

This isn't very applicable to PIM-SM, though, since the only prunes
we're interested either:
- cancel Joins that we've sent, or
- are S,G,RPbit prunes that all travel towards the RP so don't have to
  deal with route aggregation.

Do S-prefix, G Joins in PIM-SM make sense in the face of MSDP, where
the source domain can tell you exactly what mask makes sense?  Obviously
the general case doesn't make sense since you have to "multicast" the
Join when it's bigger than the individual routes you have (e.g. a
Join for 0.0.0.0/0 turns into 75,000 individual Joins upstream?...)

(Should this discussion happen on the PIM mailing list?)

  Bill

---

From: "John M. Zwiebel" <[email-masked]>
Date: Thu, 20 Jul 2000 13:31:32 -0700

^ 
 ^ Do S-prefix, G Joins in PIM-SM make sense in the face of MSDP, where
 ^ the source domain can tell you exactly what mask makes sense?  Obviously
 ^ the general case doesn't make sense since you have to "multicast" the
 ^ Join when it's bigger than the individual routes you have (e.g. a
 ^ Join for 0.0.0.0/0 turns into 75,000 individual Joins upstream?...)

MSDP doesn't help, especially if you consider SSM and IGMPv3 where you may 
want to receive from one source on a subnet, but not the one right 
next to it.

 ^ 
 ^ (Should this discussion happen on the PIM mailing list?)
 ^ 

Yes, if I thought there was a chance of something coming out of it, but
sparse-mode aggregation just doesn't make sense to me -- although in the
"long run" perhaps something will have to happen.  AFAIC, that isn't going
to be until we discover how multicast really is used in the interdomain
environment.  (2cents)

------------------------------------------------------------------------------
	John Zwiebel                       Phone: 408-526-5303
	Cisco Systems Inc.                 
	IP Multicast Group