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

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Message-ID: <1290698312.2858.341.camel@edumazet-laptop>
Date:	Thu, 25 Nov 2010 16:18:31 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Niels Möller <nisse@...ator.liu.se>
Cc:	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: TCP_MAXSEG vs TCP/generic segmentation offload (tso/gso)

Le jeudi 25 novembre 2010 à 16:09 +0100, Niels Möller a écrit :
> Eric Dumazet <eric.dumazet@...il.com> writes:
> 
> > So... there is no 'bug', unless you trust too much tcpdump output.
> 
> I really expected tcpdump -e to display the actual values in the link
> layer header, including the correct frame size. It's more than a bit
> confusing if that is not the case...
> 
> In the future, I will try to remember to always run tcpdump on a network
> node which (i) is different from the sending one, and (ii) has GRO
> disabled (and hence will discard packets if it has trouble processing
> them all, rather than coalesce them).
> 

Just disable GSO and TSO on sending machine, then tcpdump will show you
individual frames.


> What about the TCP_MAXSEG socket option, should that work? From a quick
> look at driver source code, I could only see the handling of the
> per-interface MTU, no per-socket segment size.

TCP_MAXSEG is certainly not handled in driver layer, but TCP layer.

/* If user gave his TCP_MAXSEG, record it to clamp */
if (tp->rx_opt.user_mss)
	tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;

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



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