[Xorp-users] (Most) Multicast packets arriving twice

[Mark Handley]

>	Any thoughts??

One useful piece of information you could gather is the TTL of the
dups.

Knowing what the TTL of a packet is and the TTL of its duplicate is
will provide quite a bit of information about the possible causes
because it will let you know what the difference is in the length of
the paths the two packets took.

A made up example:

echidna.cs.ucl.ac.uk: tcpdump -n -v net 224
tcpdump: listening on vr0
15:12:06.376554 128.16.64.4.51166 > 224.2.0.251.4567: [udp sum ok] udp 41 [ttl 16] (id 36064, len 69)
15:12:06.376678 128.16.64.4.51166 > 224.2.0.251.4567: [udp sum ok] udp 41 [ttl 15] (id 36064, len 69)

In this case you know that these are copies of the same packet because
they have the same IP ID (36064) and you can see that one copy went
one IP hop further than the other (TTL=16 vs TTL=15).

Cheers,
	Mark