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Re: [lwip-users] buffering outgoing packets

From: address@hidden
Subject: Re: [lwip-users] buffering outgoing packets
Date: Fri, 30 Apr 2010 19:12:44 +0200
User-agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; de; rv: Gecko/20100317 Thunderbird/3.0.4

Kieran Mansley wrote:
transmit is only an issue if there is IP fragmentation.  Is that
correct?  If so, then is it true that I would not have problems if I
have a known MTU and never send a "PDU" larger than the MTU?  Or in
other words, if I never send a UDP datagram that doesn't fit in the
MTU, and ensure that the TCP maximum segment size fits within the MTU?
MTU is a property of the local network.  IP fragmentation could happen
at any point in the path between the communicating devices, so I don't
think that would help.

I think it would: as we don't do path MTU discovery in lwIP we cannot find out whith which MTU to send and thus are always sending with our configured MTU. Therefore, fragmentation won't take place as long as you don't try to send a UDP or ICMP packet bigger than fits the MTU - TCP doesn't create such packets anyway.
In fact, it sounds like maybe I should just disable support for IP
fragmentation and reassembly altogether.
I don't fully understand the problem, but Simon mentions a workaround in
the bug tracker.  Disabling IP fragmentation sounds like it would work
but perhaps not necessary to go that far.
As I stated above, I think there's no need for IP_FRAG when you know your application that well - it's what I did, too, although rather to save code size...

However, I wouldn't disable IP_REASSEMBLY as you never know which host sends fragmented packets for you, so while you know you never will create such packets, you might still have to receive them and be able to reassemble them.


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