netdev - Re: [PATCH net-next] gro: relax ID check in inet_gro

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Date:	Thu, 21 Mar 2013 12:31:22 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	bhutchings@...arflare.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org,
	dmitry@...adcom.com, eilong@...adcom.com, pshelar@...ira.com,
	hkchu@...gle.com, maze@...gle.com
Subject: Re: [PATCH net-next] gro: relax ID check in inet_gro_receive()

From: Ben Hutchings <bhutchings@...arflare.com>
Date: Thu, 21 Mar 2013 16:20:25 +0000

> On Thu, 2013-03-21 at 11:46 -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Wed, 20 Mar 2013 21:52:33 -0700
>> 
>> > GRE TSO support doesn't increment the ID in the inner IP header.
>> 
>> Is this a fundamental limitation of doing TSO over GRO or
>> were the Broadcom folks just being lazy with their firmware
>> implementation?
>> 
>> I really don't want to apply this patch, because ipv4 frames
>> even with DF set should have an incrementing ID field, in
>> order to accomodate various header compression schemes.
>> 
>> We go out of our way to do this for normal unencapsulated TCP stream
>> packets, rather than set the ID field to zero (which we did for some
>> time until the compression issue was pointed out to us).
> 
> Besides which, GRO has been reliably reversible until now.  (gso_size is
> available through packet sockets, even if tcpdump doesn't appear to use
> it yet.)  Ignoring IPv4 IDs will break that guarantee.

Right, even ignoring the header compression issues, our segmentation
offloads must be perfectly reversible.

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