IP Fragmentation

John Lee john at internetassociatesllc.com
Wed Aug 20 13:10:32 CDT 2008


With the v4 networks that I have worked on in the past, they did not do end to end MTU discovery before sending packets. The TTL had to be set appropriately so that if you had low speed links, for example, the packet and response would get through in time. On our DS3 (T3) and OC-3c packet links we did 4k, 9k, and 16k packet sizes for video and file transfers.

At the other end of the spectrum are civilian and military systems with tactical links, both wired and radio, with low bit rates and header compression on IP and TCP packets. Speeds range from 300 -9,600 bps, 16k, 32k, 64k and Nx64k bps links that can do packet fragmentation and adding proprietary ECC codes for the radio links. Some systems strip the IP packet and use standard or non-standard link layer protocols across the mediums. Some of these systems are store and forward so that the computer/router that is connected to the low speed link will ack the packet for the high speed network connection and buffer it up until it can be sent on the lower speed system.

IMHO current IPv6 protocols ignore the lower end segment by specifying the lowest MTU for the circuit be the MTU for the entire circuit and not allow fragmentation. I do not see this as an efficient use of high speed network resources and local link management can handle fragmentation just fine.

John (ISDN) Lee

A slightly different History Channel.
From: Glen Kent [glen.kent at gmail.com]
Sent: Wednesday, August 20, 2008 12:13 PM
To: OPS Gurus
Subject: IP Fragmentation


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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