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Re: [tsvwg] OCS option in draft-ietf-tsvwg-udp-options-07

Joe Touch <touch@strayalpha.com> Wed, 13 March 2019 03:39 UTC

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From: Joe Touch <touch@strayalpha.com>
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Date: Tue, 12 Mar 2019 23:39:22 -0400
Cc: Raffaele Zullo <raffaele@erg.abdn.ac.uk>, Gorry Fairhurst <gorry@erg.abdn.ac.uk>, tsvwg <tsvwg@ietf.org>
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To: "C. M. Heard" <heard@pobox.com>
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Subject: Re: [tsvwg] OCS option in draft-ietf-tsvwg-udp-options-07
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> On Mar 12, 2019, at 11:26 PM, C. M. Heard <heard@pobox.com> wrote:
> 
> On Tue, Mar 12, 2019 at 4:16 PM Joe Touch <touch@strayalpha.com> wrote:
>>> On 2019-03-12 16:01, Raffaele Zullo wrote:
>>> OK, but what happens if UDP Length (that was 8) is increased to
>>> 8 < UDP Length <= IPPayloadLength?
>>> Will part of the Options area be delivered to user as a regular UDP
>>> Payload?
>> 
>> If the reassembly checksum isn't zero, this would be caught (with high
>> probability) during reassembly.
> 
> If LITE+FRAG is being used, then the initial LITE option will be corrupted,
> as at least one octet will be skipped. With exceedingly high probability,
> the LITE option will not be processed. The remaining options won't be
> found, and what is there is unlikely to be parsed. Result: the
> (UDP Length - 8) octets that appear to be conventional UDP user data
> will be delivered, and the trailing octets will be discarded since they
> will not appear to be valid UDP options.
> 
> Reassembly won't even be invoked.
> 
> So, CS=0 does have its risks even for the length field.
> 
> Mike Heard

Fair enough. Now show us how to avoid that without recopying the payloads and/or recomputing the checksum twice. 

That’s the issue. A fix for a problem that current UDP already deals with and doesn’t come up vs so much extra work the result won’t be useful. 

Joe

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