[Tofoo] VXLAN (UDP tunnel protocols) and non-zero checksums

Tom Herbert <therbert@google.com> Wed, 30 April 2014 19:02 UTC

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From: Tom Herbert <therbert@google.com>
To: "nvo3@ietf.org" <nvo3@ietf.org>, "tofoo@ietf.org" <tofoo@ietf.org>, mallik_mahalingam@yahoo.com, ddutt.ietf@hobbesdutt.com
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Subject: [Tofoo] VXLAN (UDP tunnel protocols) and non-zero checksums
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Hi,

I noticed that the VXLAN draft allows an implementation to potentially
ignore a non-zero invalid UDP checksum.

From: http://tools.ietf.org/html/draft-mahalingam-dutt-dcops-vxlan-09

"When a decapsulating endpoint receives a packet with a non-zero
checksum it MAY choose to verify the checksum"

However, from RFC 1122:

"If a UDP datagram is received with a checksum that is non-zero and
invalid, UDP MUST silently discard the datagram."

It doesn't seem like 1122 allows checksum verification to be optional,
so these would seem to be a conflict. Presumably, ignoring the RX csum
is included for performance but since the sender can already send zero
checksums in UDP (they are optional in IPv4 and allowed for IPv6
tunnels in RFC 6935) I'm not sure this is necessary. Besides that, the
UDP checksum is potentially the only thing that protection of the vni
against corruption end to end so allowing a receiver to ignore a bad
checksum seems very risky.

As a comparison, RFC 3931 (L2TP) has the following wording:

"Thus, UDP checksums MAY be disabled in order to reduce the associated
packet processing burden at the L2TP endpoints."

This is somewhat ambiguous, but seems like the correct interpretation
should be that zero checksums may be sent with L2TP/UDP, but on
receive non-zero checksums should still be validated.

Are these interpretations correct? Is there there a need to clarify
the requirement for UDP tunnel protocols and checksums?

Thanks,
Tom