Subject: duplication of multicast packets FDDI -> 10Base-T

From: Marc Gumbold <[email-masked]>
Date: Wed, 7 May 1997 22:02:24 +0200 (MET DST)

Dear experts,

I asked fairly the same question already on September 12 last
year. But as the problem still persists, and I could find no
fix so far, I dare to present it again to this audience:

Scenario:

A Sun SPARCstation 20/512, running Solaris 2.5.1, is connected to
three subnets, and does IP packet forwarding to/from all of them:

  1. FDDI subnet, via nf0 interface
  2. Ethernet (10Base-T) subnet, via le0 interface
  3. Ethernet (10Base-T) subnet, via le1 interface

The SPARCstation runs mrouted [version 3.8,prune,genid,mtrace].
Forwarding of IP multicast packets is OK, besides the following
phenomenon:

** Every IP multicast packet sent from a host on the FDDI subnet
** results in *two* *identical* IP multicast packets on any of the
** two Ethernets (in case a host listening to the corresponding
** group is there, of course).

The two packets are closely spaced (0.7 - 1 ms), and are not
due to fragmentation.

The effect is not temporary, so as due to route changes, but
constant.

By "snoop"ing on the three interfaces, and sending IP multicast
traffic from the three subnets, the following observations can be 
made:

The duplicates appear on both Ethernets, when the sender is 
on the FDDI. The duplicates do not appear on the FDDI network.

If the sender is in one of the Ethernets, no duplicates at
all appear, neither on the other Ethernet, nor on the FDDI.

"Normal" unicast IP packets sent from the FDDI do *not* result
in any duplicates. Only multicast packets do.

From these facts it is quite clear that the culprit must
be the FDDI side of the routing SPARCstation, most probably
the IP layer. FDDI software version is 3.0 (or 3.1).

I am absolutely lost. I scanned the mbone list archives already
but found no hint whatsoever. Of course, the duplicates can not
only seen by snoop, but by any application reading from the
socket layer. I'm otherwise sucessfully experimenting with a
reliable multicast file transfer protocol, and it is quite
annoying to have a double load on the Ethernets compared to the
originating traffic in the FDDI.

So, again, any clues? 

Please respond directly to me, via [email-masked] or the header
address; I'm not sure if I'm currently subscribed to the mbone list.

Thank you,
Marc