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

[David Miller <davem@davemloft.net>]

From: Tom Herbert <tom@herbertland.com>
Date: Tue, 16 Feb 2016 12:50:23 -0800

> On Feb 16, 2016 12:40 PM, "David Miller" <davem@davemloft.net> wrote:
>>
>> From: Jesse Gross <jesse@kernel.org>
>> Date: Tue, 16 Feb 2016 12:11:57 -0800
>>
>> > On Tue, Feb 16, 2016 at 11:47 AM, David Miller <davem@davemloft.net>
> wrote:
>> >> From: Jesse Gross <jesse@kernel.org>
>> >> Date: Tue, 16 Feb 2016 10:22:38 -0800
>> >>
>> >>> On Thu, Feb 11, 2016 at 2:41 AM, Jiri Benc <jbenc@redhat.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.
>>
>> Oh yes, I'm mixing different parts of the conversation.  We must
>> accept on RX zero checksum fields even for ipv6 because of the way the
>> VXLAN RFC is worded, correct.
> 
> That MUST conflicts directly with RFC2460 (zero UDP csums must be dropped).
> We allow configuring to accept zero checksums per Rfc6935 and rfc6936. So
> there is no interoperability issue and by default we maintain IPv6 protocol
> compliance.

And practically speaking we disappear from the internet for VXLAN tunnel
endpoints implementing the VXLAN spec properly.

That's not going to help anyone at all.