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

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Message-ID: <D5C1322C3E673F459512FB59E0DDC3290509D672@orsmsx414.amr.corp.intel.com>
Date:	Wed, 7 May 2008 15:23:06 -0700
From:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
To:	"Ben Hutchings" <bhutchings@...arflare.com>,
	"prodyut hazarika" <prodyuth@...il.com>
Cc:	<netdev@...r.kernel.org>
Subject: RE: State of TCP segmentation offload to NIC in linux

> > > 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?
> 
> It sets skb_shinfo(skb)->gso_size to the desired segment size.
> 
> > 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.
>  
> It's checked using sk_can_gso() (defined in <net/sock.h>) 
> which calls net_gso_ok() (defined in <linux/netdevice.h>).

To add: the flag that sk_can_gso() checks is set on the socket creation,
based on the underlying netdevice's TSO capabilities.  There are a bunch
of flags in the sk struct that get set on TCP socket creation that are
then used elsewhere in the TCP stack.

Cheers,
-PJ
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