UDP lossage (was: Types of packet modifications allowed for networks)

Matthew Kaufman matthew at eeph.com
Mon Jun 2 14:35:20 UTC 2008

I was reminded by the "packet modifications" thread that it seems that 
dropping (rather than fragmenting) large UDP packets has become quite 
the norm, which is unfortunate.

We're working on a (popular software) product that sends UDP datagrams 
(with DF cleared), and it is amazing how small they have to be to get 
through. Between the Cisco VPN software and the high-end NAT boxes that 
have broken hairpin behavior and broken consumer "routers", we're 
finding that whereas sizes in the mid 1400-byte range used to be safe, 
going much over 1200 bytes is now routinely a problem. Path MTU 
discovery (PLPMTUD) shouldn't need to be looking for and finding black 
holes when the DF flag is cleared, but that's what we're having to 
implement to work on today's Internet.

Operational relevance: 1) This software will be running on your 
networks, and your customers will be happier if you don't drop UDP 
datagrams that are of reasonable size, 2) Knowing that this is going on 
might help you debug problems customers are having with other 
applications if you didn't know already how bad it has gotten.

Matthew Kaufman
matthew at eeph.com

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