netdev - Re: [net-next PATCH 2/2] VXLAN: Support outer IPv4 Tx checksums by default

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Message-ID: <CAEh+42iPR4LRLpQzV7bw21Q0h45t0WoE4RCg7tv-2TGbKSY-Yw@mail.gmail.com>
Date:	Fri, 19 Feb 2016 13:36:34 -0800
From:	Jesse Gross <jesse@...nel.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Alexander Duyck <aduyck@...antis.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [net-next PATCH 2/2] VXLAN: Support outer IPv4 Tx checksums by default

On Fri, Feb 19, 2016 at 12:27 PM, Tom Herbert <tom@...bertland.com> wrote:
> I would also note RFC7348 specifies:
>
> UDP Checksum: It SHOULD be transmitted as zero. ...
>
> The RFC doesn't provide any rationale as to why this is a SHOULD
> (neither is there any discussion as to whether this pertains to IPv6
> which has stronger requirements for non-zero UDP checksum). I think
> there are two possibilities in the intent: 1) The authors assume that
> computing UDP checksums is a significant performance hit which is
> dis-proven by this patch 2) They are worried about devices that are
> unable to compute receive checksums, however this would be addressed
> by an allowance that devices can ignore non-zero UDP checksums for
> VXLAN ("When a decapsulating end point receives a packet with a
> non-zero checksum, it MAY choose to verify the checksum value.")

It's #2.

All of the performance concerns around checksums and tunneling stem
from devices implemented using switching ASICs. In those devices,
computing/verifying checksums is so slow (software path) that they are
effectively unable to do it.