netdev - Re: [PATCH net-next] lwt: fix rx checksum setting for lwt devices tunneling over ipv6

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Date:	Tue, 16 Feb 2016 12:11:57 -0800
From:	Jesse Gross <jesse@...nel.org>
To:	David Miller <davem@...emloft.net>
Cc:	Jiri Benc <jbenc@...hat.com>, Paolo Abeni <pabeni@...hat.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Pravin B Shelar <pshelar@...ira.com>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: [PATCH net-next] lwt: fix rx checksum setting for lwt devices
 tunneling over ipv6

On Tue, Feb 16, 2016 at 11:47 AM, David Miller <davem@...emloft.net> wrote:
> From: Jesse Gross <jesse@...nel.org>
> Date: Tue, 16 Feb 2016 10:22:38 -0800
>
>> On Thu, Feb 11, 2016 at 2:41 AM, Jiri Benc <jbenc@...hat.com> wrote:
>> There's a bigger problem here, not really related to lightweight tunnels or OVS.
>>
>> The VXLAN RFC says (referring to the UDP checksum and not specific to IPv4/v6):
>> "It SHOULD be transmitted as zero. When a packet is received with a
>> UDP checksum of zero, it MUST be accepted for decapsulation."
>>
>> We can debate whether this is correct or whether it conflicts with RFC
>> 2460 but this is what essentially everyone is going to implement. With
>> the default settings of the flags in IPv6, we are violating both
>> statements. With the second one in particular, the result is that
>> Linux will not be able to communicate with any non-Linux VXLAN
>> endpoint over IPv6 with default settings.
>
> I do not see any such conflict here.
>
> It's a SHOULD, therefore a recommendation.  Likely they thought this
> would improve performance, and ironically it has the opposite effect.
>
> The text of the VXLAN RFC does not say that the checksum MUST be sent
> as zero, and it also does not say that receiving a non-zero checksum
> is violating the RFC.
>
> I therefore do not see the interoperability issue.  Maybe some
> deployed systems will run more slowly or hit a slot path (which is not
> our problem), but they absolutely should not drop such frames.

"When a packet is received with a UDP checksum of zero, it MUST be
accepted for decapsulation."

This is a requirement and directly in conflict with having
VXLAN_F_UDP_ZERO_CSUM6_RX set to false as the default.

As far as the use of checksums, saying that it improves performance is
only valid for a class of devices. It is not true as a blanket
statement, such as for switching ASICs. In these cases, the
performance drop in hitting a slow path is so big that it is
equivalent to dropping frames and creates a de-facto interoperability
issue.