Subject: Duplicate IP Multicast packets

From: Ajay Kachrani USG <[email-masked]>
Date: Wed, 8 May 1996 15:21:27 -0500 (EDT)

Can someone answer this?

> Bill,
> 
>   It there any way that a host could possible receive a duplicate multicast
> packet?  Especially, if a packet could get duplicated if there are two
> parallel routing paths between a sender and receiver.
> 
> We're  trying to deal with some error condition handling in this
> IPv6 over ATM stuff we're doing.  We're trying to decide if it's best to
> handle certain types of transitions by assuring there are no duplicate
> packets delivered (which could result on none getting delivered if
> there is an error), or if some duplicates can be tolerated for a brief
> period of time (in which case there are will be no loss of data).
> 
> Any thoughts on which, a host could best tolerate?
> 
> Thanks much,
> 
> --Ajay
>

---

From: Bill Fenner <[email-masked]>
Date: Wed, 8 May 1996 14:43:11 PDT

> parallel routing paths between a sender and receiver.

Duplicate multicast packets are typically only generated by either
misbehaving network equipment or transiently during topology changes.
DVMRP tends towards black-holing during topology changes instead of
duplication but there are instances where duplicates can occur.

Duplicating multicast packets can be very dangerous, especially if it
occurs during topology changes which can include forwarding loops or if
it may occur on several different hops.  This can cause immense explosion
in the number of packets.

The safest thing to do is to blackhole packets if you're not sure.  As
long as the event that causes you to blackhole packets doesn't occur too
often, this will probably result in acceptable behavior.

  Bill

---

From: "Danny J. Mitzel" <[email-masked]>
Date: Wed, 08 May 1996 15:34:15 -0700

>> > parallel routing paths between a sender and receiver.

1. multicast being based on UDP/IP would lead me to believe that there
are no guarantees against duplicates.

2. in the case of parallel paths to a net, the current mrouted (3.8) has
problems that lead to duplicate packets.  I believe this is not inherent
in dvmrp and is one of the fixes in upcoming mrouted.

3. other multicast protocols, such as PIM, can result in multiple packets
on parallel paths.  in the case of PIM this would trigger PIM-ASSERT
processing to perform a designated router election to prune the duplicates.

danny <[email-masked]>