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Re: [lwip-devel] IP fragmentation

From: Jonathan Larmour
Subject: Re: [lwip-devel] IP fragmentation
Date: Wed, 09 May 2007 10:23:15 +0100
User-agent: Mozilla Thunderbird 1.0.8-1.1.fc3.4.legacy (X11/20060515)

Goldschmidt Simon wrote:
I'm looking at the code to do the IP reassembly, and notice that it


that it is only able to handle a single fragmented payload at a time.


this really true, or am I seeing things again?  If so, the


is somewhat problematic in that if one of the fragments of an IP

datagram is

lost, then the ip_reass_tmr has to expire before the stack can handle a
fragment belonging to a new IP datagram.  By default, this timer is set


expire every seconds.

As far as I can see, you're right. What also disturbs me is that,
there is only one reassembly-buffer, you can only receive fragmented
from one host at a time. That's because I didn't enable IP_REASSEMBLY...
Or am I wrong and this would never happen??

This is correct and intentional - more frags to reassemble means more memory, it's as simple as that. You should try and avoid getting fragmented packets in the first place!

Of course if lwIP could implement path MTU discovery, it could keep up its side of the bargain.

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