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

Eric Dumazet
2010-11-25T15:18:31Z

Le jeudi 25 novembre 2010 à 16:09 +0100, Niels Möller a écrit :
> Eric Dumazet <eric.dumazet@gmail.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.