Subject: Strange effect of duplicate packets

From: Marc Gumbold <[email-masked]>
Date: Thu, 12 Sep 1996 15:47:21 -0100 (GMT)

Hi multicast experts,

(if you feel this message covers FAQ knowledge, please be so kind to
point me to the right document -- I did find however no pointer in the
mbone-faq)

I just again came across a strange effect with sending IP multicast
packets. I've seen that one before, but now I'm deeper into it:

In the following scenario,

  keaton                 hni                  pooh             max
  SS10                   SS20                 SS10             SS5
  Solaris 2.5            Solaris 2.4          Solaris 2.5      Solaris 2.5
     |                    |      |               |              |
  ---+--------------------+-     |               |              |
 /       FDDI Ring          \  ==+===============+==============+====
 \__________________________/             10Base-T segment

-- both FDDI Ring and 10Base-T segment are IP subnetworks, "hni" serves as
IP (m)router, version 3.8,prune,genid,mtrace --

I send small (i.e. 10 bytes payload) IP multicast packets via a UDP
socket from "keaton" to an IP hostgroup that both "pooh" and "max" are
subscribed to.

Packets travel happily across the FDDI ring via router "hni" to the
Ethernet segment. However both "pooh" and "max" happen to receive
_two_ packets from their UDP sockets per single packet sent by
"keaton".

I applied snoop (a software network sniffer bundled with Solaris) on
both IP subnetworks, and indeed I could see that while only one packet
travels the FDDI ring, two packets travel via the Ethernet segment.

As I have just provided for measuring interarrival times of packets in
my C code, I could see that the second packet follows very shortly
after the first packet. Interarrival times of the two consecutive
packets are 700-1000 microsecs (around 700 us on the SS5, around 1000
us on the SS10). Packet content is identical.

The effect is not due to fragmentation (identical content and length).
Also the effect is not limited to exactly this configuration. In earlier
setups I experienced the same effect with different machines and network
configurations.

Please reply to me personally ([email-masked]). If the problem is a real
one (as opposed to something absolutely brain-damaged I simply overlooked),
I'll be happy to summarize to the list.

I'd very much appreciate any help or pointer,
cheers,

Marc

---

From: Jim Bogard <[email-masked]>
Date: Thu, 12 Sep 1996 13:14:26 -0400 (EDT)

>   Solaris 2.5            Solaris 2.4          Solaris 2.5      Solaris 2.5
[problem description at end, if you missed it]

Have you tried snooping on hni at both interfaces?  If hni is the cause 
of the duplicates, you should be able to see it that way.  Also, as 10BT 
is point-to-point, unless there's two interfaces in pooh, there should be 
a hub or switch somewhere between pooh/max and hni.  This may be the 
culprit.

Actually, I think I'd put my money on a switch.  But what do I know?

J-.
----
Jim Bogard  JHU/APL BIX Group             "This is my banjo.  There are many
Backbone Network Engineer                   like it, but this one is mine."