[Xorp-users] Multicast in general, and perhaps xorp in particular

[Kees Jan Hermans]

Dear reader,

I'm a bit at my wits' end; I have until Friday to complete this demo, 
which is for a prototype of a networking device that must also do IP 
multicasting.  I have the following network topology:

host1----------device------------router-------------device------host2
192.168.2.20 anonymous 192.168.2.1 <-> 192.168.3.1 anonymous 192.168.3.20

The router is using xorp, which I think is a fantastic product, altough 
I'm not all that familiar with it, having discovered it only a few days 
ago.  That being said, getting routing done was a piece of cake, so 
nothing but compliments there.  I must add that I was a little bit 
surprised to discover that one had to make special arrangements for 
getting multicasting work out of the box; apparently it isn't in the 
design philosophy of xorp to have one segment subscribe to a multicast 
address, and route traffic to that segment's interface automatically 
when traffic to that multicast address is discovered on another 
interface.  I wonder why that is, but not for too long, because I got it 
to work (through a 'rp').  That is to say, if you take out the two 
'devices' in the diagram above, everything works flawlessly: host2 has 
VLC subscribe to a multicast address, and on host1 VLC sends packets to 
that address, which arrive and I can hear pretty music play on host2.  
So far, so good.

The devices in question, amongst other tasks, proxy for the IGMP and 
multicast traffic; they act as bridges effectively, cleaning and 
monitoring the network packets.  To do what they have to do, the devices 
cache the multicast subscriptions, and create IGMP packets themselves on 
a timely basis.  And when I have those devices in the middle (as drawn 
in the diagram above), it doesn't work.  By now I'm pretty sure that 
'my' IGMP packets are good, and equal to the packets sent by 'host2', 
however, when I send them, the subscriptions won't show up in the 'show 
igmp groups' list (they do if you take my devices out).  I've noticed a 
few things though, that I wonder could be related to this problem: 
namely that 'host2' sends IGMP subscriptions TWICE in quick succession 
(where I send them only once), and that there's some PIM traffic going 
on to address 224.0.0.13 (that I don't react to at all).  My questions are:

a) are my suspicions about the differences in behaviour any good ?
b) do I have to react to PIM traffic, and in effect create a dialog 
before subscription to a multicast address becomes effective in the router ?
c) is there a way to see in the xorp router what happens with a packet 
such as a IGMP report packet ?

Thanks for your time, and only too willing to also post configurations 
and packet data in hexadecimal,

Sincerely,

KJ Hermans