netdev - Re: [net-next PATCH 0/2] GENEVE/VXLAN: Enable outer Tx checksum by default

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Date:	Tue, 23 Feb 2016 08:47:30 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Edward Cree <ecree@...arflare.com>
Cc:	Jesse Gross <jesse@...nel.org>, Alex Duyck <aduyck@...antis.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [net-next PATCH 0/2] GENEVE/VXLAN: Enable outer Tx checksum by default

On Tue, Feb 23, 2016 at 7:18 AM, Edward Cree <ecree@...arflare.com> wrote:
> On 23/02/16 03:31, Jesse Gross wrote:
>> The only issue that I see is that making TSO completely unaware of
>> outer headers will likely cause performance regressions in some cases.
>> Imagine if we have an incoming TCP stream with incrementing IP IDs
>> that we aggregate through GRO and forward. Today's TSO would be able
>> to recreate the stream by incrementing the ID as new segments are
>> created. However, if the outgoing NIC is truly only dealing with the
>> L4 header then it wouldn't be able to do this.
> Perhaps TSO should force setting the DF bit, so that the IP ID can be
> ignored.  After all, if your network is going to cause fragmentation and
> reassembly, your performance will probably be bad enough that TSO won't
> help you much.  (And TCP usually wants DF anyway so it can do PMTUD.)
> Arguably, as soon as we perform GRO on traffic to be forwarded, we've
> already violated the end-to-end principle (there are always imaginable
> situations in which a different packet stream comes out than went in),
> so it doesn't really matter if we go on to change the network layer
> parameters in this way - it's not really the same IP datagram any more
> so it's OK for its identification to change.
> And of course this problem doesn't present itself for IPv6 :)

Right, GRO should probably not coalesce packets with non-zero IP
identifiers due to the loss of information. Besides that, RFC6848 says
the IP identifier should only be set for fragmentation anyway so there
shouldn't be any issue and really no need for HW TSO (or LRO) to
support that.

We need to do increment IP identifier in UFO, but I only see one
device (neterion) that advertises NETIF_F_UFO-- honestly, removing
that feature might be another good simplification!

Tom

> --
> -Ed