netdev - Re: TCP_MAXSEG vs TCP/generic segmentation offload (tso/gso)

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Message-ID: <nn1v69qsjd.fsf@stalhein.lysator.liu.se>
Date:	Thu, 25 Nov 2010 17:25:42 +0100
From:	nisse@...ator.liu.se (Niels Möller)
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: TCP_MAXSEG vs TCP/generic segmentation offload (tso/gso)

Eric Dumazet <eric.dumazet@...il.com> writes:

> I believe TCP_MAXSEG is working fine, but GRO/GSO dont care at all :
> They coalesce frames whatever their size is.

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.

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.

Regards, and thanks for your patience,
/Niels

-- 
Niels Möller. PGP-encrypted email is preferred. Keyid C0B98E26.
Internet email is subject to wholesale government surveillance.
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