netdev - Re: State of TCP segmentation offload to NIC in linux

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Message-ID: <49c0ff980805071426h54073ddcw89ebe37a3d4e95b4@mail.gmail.com>
Date:	Wed, 7 May 2008 14:26:50 -0700
From:	"prodyut hazarika" <prodyuth@...il.com>
To:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
Cc:	netdev@...r.kernel.org
Subject: Re: State of TCP segmentation offload to NIC in linux

Thanks for your reply, Peter. I would appreciate if you could clarify
little more.

> The IP layer does not need to be bypassed.  The TCP layer will set the
> proper headers for TSO to work properly and continue sending the skb
> down the stack.  IP will not fragment a packet on transmit.

Which flag/header does the TCP layer set to indicate to IP that it
should not fragment a 64K segment, but rather offload the whole
segment to NIC? I tried to look at the TCP code in linux stack, but
could not figure the flag or the logic that sets the flag/header you
mentioned. The NIC driver must indicate that it is capable of doing
TSO by setting NETIF_F_TSO flag, but NETIF_F_TSO capability is not
checked in IP/TCP layer.

I would really appreciate if you could point me to the function in TCP
code in linux.

Thanks for your time,
Prodyut
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