Subject: [pim] PIM packet fragmentation

From: Christopher Thomas Brown <[email-masked]>
Date: Wed, 28 Jan 2004 14:34:22 -0500

After reading much of this discussion, it seems many people are still
confused on the subject of fragmentation.

PIM calls for specialized fragmentation in only two places: register messages
and bootstrap messages.

In a register message, one includes an encapsulating IP header and a PIM
header along with the IP data packet. This new header adds 28 bytes to
the packet. It is possible for a multicast data packet to be upto 65535
bytes in length. Adding the 28 bytes to such a packet is not possible
without fragmentation. The PIM specification provides for how such
fragmentation should be done.

In a bootstrap message, specialized fragmentation is performed for
mainly for reasons others have specified. That it is better to lose
a fragment for one group than to lose the whole message because one
IP fragment is lost. Additionally, it is possible to generation a
bootstrap message that is be larger than 65535 if one included enough
group ranges. So the specialized fragmentation helps here as well.

No other message includes any form of specialized fragmentation. However,
for Join/Prune messages it is prudent for reasons already mentioned to send
joins and prunes spread out over a number of messages so that IP
fragmentation does not occur. Additionally, one is limited by the maximum
packet size in the number of s,g,rpt prunes that can be included with a *,g
join.

Chris

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