Subject: Re: Multicast and satellites

From: IZUMIYAMA Hidetaka <[email-masked]>
Date: Fri, 29 Aug 1997 12:21:26 +0900

Scott,

As I said in UDLR WG on IETF38 and IETF39,
our implementaion can correctly use unidirectional
satellite link for IP Multicast.

Please refer the UDLR WG slides of IETF38 and IETF39.

	ftp://zenon.inria.fr/rodeo/udlr/slides/wide38.ps.gz
	ftp://zenon.inria.fr/rodeo/udlr/slides/wide39.ps.gz

We are using DVMRP as multicast routing protocol. 

-- izu --

Hidetaka Izumiyama

Satellite Internet Department
Japan Satellite Systems Inc.

---

From: [email-masked] (Scott Michel)
Date: Fri, 29 Aug 1997 10:05:58 -0700 (PDT)

> > 1) This is obviously incorrect behavior, but should IP be delivering the
> >    multicast packet to an application if the source IP address is one of
> >    the host's interface IP addresses?
> 
> I would expect that whether or not the packet was delivered back to the
> application would be a function of the "want to hear my own packets"
> switch that the application sets (with some APIs, at least).

You've missed what happens at the link layer -- even if you turn
IP_MULTICAST_LOOP off, the behavior still occurs. Why? Because each
outbound packet in the satellite network is tunnelled to the uplink (U)
and sprayed to all of the downlinks, including the multicast source.

This is a L2/L3 layer issue: Regardless of what you do at the application
layer, apart from listening on a raw socket so you can reclaim the IP
packet header, you will still hear your own multicast packets. It just
happens to be a feature of this particular kind of network. Presumably
this same situation will occur for the unidirectional cable modem
people as well.

Yes, I claim this is a bug, and that the IP stack should filter them
off for you, before it gets to the application. It sounds like it should
be an operational I-D recommendation if currently this is unspecified
behavior.

BTW: Before the WIDE people deluge me with their creative solution,
their solution doesn't scale (simply because it requires configuration.)
In our application domain, we're looking at several hundred nodes in
the middle of tactical situation. If you can look a Marine grunt straight
in the face and tell him he has to configure mrouted while bullets
are flying at him, I will develop a lot of respect for your intrepidity.

-scottm