netdev - Re: [RFC PATCH 1/9] ipv4/GRO: Allow multiple frames to use the same IP ID

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Date:	Wed, 23 Mar 2016 18:43:40 -0700
From:	Jesse Gross <jesse@...nel.org>
To:	Alexander Duyck <aduyck@...antis.com>
Cc:	Edward Cree <ecree@...arflare.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Alexander Duyck <alexander.duyck@...il.com>,
	Tom Herbert <tom@...bertland.com>
Subject: Re: [RFC PATCH 1/9] ipv4/GRO: Allow multiple frames to use the same
 IP ID

On Fri, Mar 18, 2016 at 4:24 PM, Alexander Duyck <aduyck@...antis.com> wrote:
> In RFC 6864 it is stated that we can essentially ignore the IPv4 ID field
> if we have not and will not use fragmentation.  Such a frame is defined
> as having the DF flag set to 1, and the MF and frag_offset as 0.  Currently
> for GRO we were requiring that the inner header always have an increasing
> IPv4 ID, but we are ignoring the outer value.
>
> This patch is a first step in trying to reverse some of that.  Specifically
> what this patch does is allow us to coalesce frames that have a static IPv4
> ID value.  So for example if we had a series of frames where the DF flag
> was set we would allow the same IPv4 ID value to be used for all the frames
> belonging to that flow.  This would become the standard behavior for TCP so
> it would support either a fixed IPv4 ID value, or one in which the value
> increments.
>
> Signed-off-by: Alexander Duyck <aduyck@...antis.com>

One thing that is a bit odd is that the TSO output procedure stays the
same. So that means that if we get a stream of packets with the DF bit
set and a constant IP ID, aggregate them with GRO, and the retransmit
with GSO/TSO then we'll get packets with IDs that increment for each
burst and then start back again to the original value. I guess it
doesn't matter in practice since the IDs are supposed to be ignored
but it does seem a little strange - especially because the new packets
will now be violating the rules of the same GRO implementation that
produced them.