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Re: [tsvwg] Fwd: New Version Notification for draft-herbert-udp-space-hdr-00.txt

"C. M. Heard" <heard@pobox.com> Tue, 12 March 2019 19:22 UTC

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From: "C. M. Heard" <heard@pobox.com>
Date: Tue, 12 Mar 2019 12:21:49 -0700
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To: Tom Herbert <tom@herbertland.com>
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Subject: Re: [tsvwg] Fwd: New Version Notification for draft-herbert-udp-space-hdr-00.txt
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On Tue, Mar 12, 2019 at 7:23 AM Tom Herbert <tom@herbertland.com> wrote:
> I see. The same effect can by had by using a two byte pseudo header in
> the checksum that contains the difference between the IP length and
> the UDP length.

Yes. My sense from reading the draft is that the optimum approach would
be to incorporate this pseudo-header compensation for ***each*** section
separately. Basically, the total length for a section (including header
and pad bytes) would be included in the checksum. The pad bytes can be
ignored since they are required to be zero. This assumes that every
surplus octet is included in some section.

> So for the "cost" of a mandatory two byte checksum, we get:
>
> - Detection of non-standard uses of the surplus space
> - Detection of corrupted bits in the option space
> - Middleboxes can calculate UDP checksum based on IP length with no
> additional bits needed for adjustment value
> - Detection that the IP length is corrupted or changed inflight
> - Uniformity and reuse of checksum APIs in implementation with other
> IP protocols that contain a checksum (IP, TCP, UDP, GRE)
> - Compatibility with NIC checksum offload

Sounds correct to me. One cost: the LITE option is of no use under this
proposal, since the LITE data would be checksummed. That defeats the
purpose of LITE.

Mike Heard

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