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

Joe Touch <touch@strayalpha.com> Tue, 12 March 2019 23:14 UTC

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Date: Tue, 12 Mar 2019 16:13:59 -0700
From: Joe Touch <touch@strayalpha.com>
To: Tom Herbert <tom@herbertland.com>
Cc: "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 2019-03-12 16:04, Tom Herbert wrote:

> On Tue, Mar 12, 2019 at 3:56 PM C. M. Heard <heard@pobox.com> wrote: 
> 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.
 Also, UDP checksums are not optional in IPv6 (except in a very narrow
use case of tunnels which is not applicable here). 

By adding our own reassembly checksum, we're in the spirit of RFC 6936 -
notably the requirement in Section 5, item #5. 

> 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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