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RE: [lwip-users] Jumbo frame questions


From: Kieran Mansley
Subject: RE: [lwip-users] Jumbo frame questions
Date: Tue, 05 Feb 2008 15:23:46 +0000

On Fri, 2008-02-01 at 12:33 -0500, Bill Auerbach wrote:
> Hi Simon,
> 
>  
> 
> I updated and retested – so it’s the latest as of yesterday.  I can’t
> say if it’s an lwIP issue or not.  The problem exists only when lwIP
> is configured for jumbo frames, the connection is to the PC configured
> for large frames, but the network cannot support large frames.  WinXP
> works in this configuration but the lwIP device connected does not.  I
> can change the Frame Size to 1512 and both the PC and lwIP device
> work.
> 
>  
> 
> Could it be that the netif MTU has to be adjusted – it’s set to the
> frame size – 28 (for headers).
> 

There may be a bug here, but I think it's just you're hoping the lwIP
has a feature that it doesn't have: path MTU discovery.  Normally if you
try and send a packet with a larger MTU than some piece of the network
can't handle it will either fragment it (if it's allowed) or drop it and
send an ICMP message back to the sender.  I expect that lwIP does
nothing with this ICMP message.  This could be construed a bug
(especially if we also set the DF - Don't Fragment - bit in the header)
but could also be seen as a property of lwIP being "lightweight".  

For the avoidance of confusion:
 MSS = largest TCP payload
 MTU = largest IP packet (i.e. MSS + IP and TCP headers).
 Frame size = largest ethernet frame (i.e. MTU + ethernet headers etc)
i.e. You may need to adjust your MSS settings as it looks like you're
giving it the values you want for the MTU or frame size.

Kieran





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