Subject: Use of srcmask in prune/grafts

From: [email-masked]
Date: Thu, 20 Jul 2000 18:00:21 +0530

Hi all,
 While there is discussion about whether Netmask should be specified in IGMPv3 source 
filtering, I would like to seek clarification regarding its use in prune/graft messages in Mcast 
routing protocols(dvmrp/pim).

In DVMRPv3(as in draft-ietf-idmr-dvmrp-v3-09.txt) for instance, the multicast delivery tree is 
established per source.Then when the multicast datagram is forwarded down the tree, if a 
router finds that it has no members for the GROUP, it sends upstream prune.This prune 
contains source address and group address of the datagram. But what netmask is to be used 
in this prune?
If there is any change in the upstream/downstream dependency then it will be indicated in the 
flash route reports and the delivery tree for the (source-netmask) adjusted.  Iam unable to 
comprehend why  the netmask field given in the prune/graft messages? . I thought if IGMPv3 
uses netmask to filter source then it may be used in the prune.grafts.

Now, I think I have misunderstood something. Can anyone explain

Regards,
Vijay

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From: [email-masked]
Date: Sat, 22 Jul 2000 11:03:32 +0530

Hello Sir,
You wrote in your earlier mail,

DVMRPv3 applies prunes to routes; the netmask in a prune is used to
ensure that the right route is matched.

There is some mention about this in the draft, but I think it is not necessary to associate the 
prune with a route. Prune is used to cut off a downstream interface from the forwarding 
cache.It shouls not be for cutting of a branch of the per source delivery tree.  The delivery tree 
is constructed by exchange of route reports/ poison reports and the route entry can have a list 
of downstreams for that route,ie, per source-network(src-mask). But forwarding cache is built 
for multicast packets for every (S,G). If a router finds that there are no group members for 
(S,G) it can prune(S,G) and thereby the interface can be removed from the list of 
downstreams for that forwarding cache entry.This way it would'nt affect the per source delivery 
tree. The per source  delivery tree should not be affected  because of the prune. It should be 
affected by the route reports and poison reports. Hence if IGMP does not indicate Masks but 
only S,G as in IGMPv3 now, why use source masks in DVMRP prune/grafts.

If Iam wrong kindly clarify when a route specific prune will be done and how it will help.

Regards,
Vijay