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Subject: Re: TCP_MAXSEG vs TCP/generic segmentation offload (tso/gso)
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Niels =?ISO-8859-1?Q?M=F6ller?= <nisse@lysator.liu.se>
Cc: linux-kernel@vger.kernel.org, netdev <netdev@vger.kernel.org>
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Date: Thu, 25 Nov 2010 17:44:45 +0100
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Le jeudi 25 novembre 2010 à 17:25 +0100, Niels Möller a écrit :


> 
> I was under the impression that TSO (and maybe GSO) implied more
> cleverness in the network card; that the network card more or less gets
> to decide by itself how to divide a tcp stream into segments. And for
> example in the atl1c driver which I looked a bit into, this was what the
> REG_MTU register was for. Seems I have gotten this totally wrong.
> 

You were not totally wrong, but device does not use its own MTU to
perform the split : We give it the MSS of the flow.

You can have multiple flows in parallel, each with its own MSS, while
device has a single MTU.

> Maybe Documentation/networking/netdevices.txt could clarify how it
> works. Currently, it says
> 
> : Segmentation Offload (GSO, TSO) is an exception to this rule.  The
> : upper layer protocol may pass a large socket buffer to the device
> : transmit routine, and the device will break that up into separate
> : packets based on the current MTU.


MTU means : maximum transmission unit. But each layer has its own :)

In this context, TCP protocol, so MSS should be taken into account.

By default, MSS derives from device MTU  (ipv4 without options case :
MSS = MTU - 40), but user can change it with TCP_MAXSEG.


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