IP Fragmentation

Glen Kent glen.kent at gmail.com
Wed Aug 20 11:13:44 CDT 2008


Do transit routers in the wild actually get to do IP fragmentation
these days? I was wondering if routers actually do it or not, because
the source usually discovers the path MTU and sends its data with the
least supported MTU. Is this true?

Even if this is, then this would break for multicast IP. The source
cannot determine which receivers would get interested in the traffic
and what capacities the links connecting them would support. So, a
source would send IP packets with some size, and theres a chance that
one of the routers *may* have to fragment those IP packets before
passing it on to the next router.

I would wager that the vendors and operators would want to avoid IP
fragmentation since thats usually done in SW (unless you've got a very
powerful ASIC or your box is NP based).


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