Subject: Question on IGMP/RPF ?

From: Jesus Arango <[email-masked]>
Date: Thu, 14 Aug 1997 16:21:03 -0500

Hi, I have some questions related to IGMP and RPF protocols. Although I
understand the basics I still have some doubts that have to do with the
relation between the two:

1. Are prunes and grafts part of the IGMP specification or are they
implemented inside specific RPF protocols?

2. Is IGMP only used on interfaces of broadcast networks such as
Ethernet, or is it also used on poit-to-point links?

3. Supose that a router in tree (S,G) is connected to his parent node
through an ethernet, which also has hosts (See figure). The child router
has a member for G in the lowest subnetwork. My question is: does the
child router (R2) have to send and IGMP report to the parent router (R1)
or through the RPF protocol. All I know is that some kind of report has
to be sent to R1 in order to tell it that R2 wants to receive datagrams
for group G. How is this done if we have point-to-point links?

             R1
             |
     -----------------
                 |
                R2
                 |
          --------------
             |
             H1 (Member)

I think you guys get the point of my confussion, So if you have any
thing to make it clearer I would apreciate it.

- Jesus

---

From: Bill Fenner <[email-masked]>
Date: Fri, 15 Aug 1997 10:48:10 PDT

Jesus Arango <[email-masked]> wrote:
>Hi, I have some questions related to IGMP and RPF protocols.

I think you mean to say "multicast routing protocols", not "RPF
protocols".  RPF describes a characteristic of some multicast routing
protocols but there are other types as well.

>implemented inside specific RPF protocols?

Prunes and grafts are part of the multicast routing protocol.  See,
e.g., draft-ietf-idmr-dvmrp-v3-04.txt, or RFC2117.

>Ethernet, or is it also used on poit-to-point links?

It is also used on point-to-point links.

>or through the RPF protocol.

Routers never use IGMP reports to report group memberships on other
networks to other routers.  The exact mechanism depends on the
multicast routing protocol, for example with DVMRP the first packet
from any given source to a group is broadcasted to all routers in the
network, which then prune back the delivery tree if there are no local
group members.  With PIM-SM, the routers send Join messages towards the
RP to build up the tree.  The only case in which routers send IGMP
membership reports is when they are themselves members of a group.
(e.g. a router that runs NTP might want to join the well-known NTP
multicast group, NTP.MCAST.NET).

  Bill