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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