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[lwip-users] Re: [lwip] IP fragmentation support questions


From: Adam Dunkels
Subject: [lwip-users] Re: [lwip] IP fragmentation support questions
Date: Thu, 09 Jan 2003 01:32:27 -0000

Hi!

On Monday 20 May 2002 13.40, you wrote:
> > packets (on Ethernet). Since TCP usually limits the MSS to 1500 most TCP
> > streams never include any fragmented packets. (Studies of packet traces
> > from wide area networks have shown that > 0.1% of all packets are
> > fragmented.)
>
> I assume ou mean "<" here

Yes :-)

> does this include PPP links?
>
> what about dial-up links with a MSS of 512 bytes?
> (PPP negotiation can reduce the MSS.)

Things are much better when the last hop is the limiting link, i.e., when the 
link nearest the device has the smallest MTU on the path. Since this fact 
usually is known to the TCP module, it can adjust its advertised MSS 
accordingly.

In the case of lwIP, the TCP_MSS #define could be set to a global variable 
instead, that would be updated by the PPP code that negotiates the MTU.

/adam
-- 
Adam Dunkels <address@hidden>
http://www.sics.se/~adam
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