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Re: [tsvwg] Alternative version of the UDP FRAG option

"C. M. Heard" <heard@pobox.com> Tue, 12 March 2019 22:56 UTC

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From: "C. M. Heard" <heard@pobox.com>
Date: Tue, 12 Mar 2019 15:56:06 -0700
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To: Joe Touch <touch@strayalpha.com>
Cc: Raffaele Zullo <raffaele@erg.abdn.ac.uk>, "C. M. Heard" <heard@pobox.com>, tsvwg <tsvwg@ietf.org>
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Subject: Re: [tsvwg] Alternative version of the UDP FRAG option
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On Tue, Mar 12, 2019 at 3:40 PM Joe Touch wrote:
> I'm still not following the need for this alternative.
>
> FRAG+LITE works fine - even through broken middleboxes with the checksum
> bug - if used with CS=0, which is how it ought to be used.

Not all of us agree that this is how it ought to be used, since CS=0 affords
no protection for the UDP header itself.

>  The trailing checksum in the last fragment is over the reassembled total
> (or not if also zero) and there's no utility in checksumming the fragments
> themselves.

My position is that the level of protection that we should strive to
achieve is what is available with conventional IP fragmentation coupled
with the standard UDP checksum. That combination protects the entire UDP
datagram, including the UDP header. The trailing checksum the terminal
FRAG option, when used with CS=0, does not do that.

A second reason for preferring this proposal to FRAG as now presented
in the draft is that it eliminates the hazard of FRAG without LITE.

Mike Heard

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