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

Tom Herbert <tom@herbertland.com> Tue, 12 March 2019 23:05 UTC

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From: Tom Herbert <tom@herbertland.com>
Date: Tue, 12 Mar 2019 16:04:48 -0700
Message-ID: <CALx6S37NrQ+QPARM0-+Fe9ZR2aWCTJc3Zsot7yiSdnf9-sxhag@mail.gmail.com>
To: "C. M. Heard" <heard@pobox.com>
Cc: Joe Touch <touch@strayalpha.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 3:56 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).

> >  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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