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

["C. M. Heard" <[e-mail]>]

On Tue, Mar 12, 2019 at 10:13 AM Raffaele Zullo wrote:
> On 2019-03-12 16:36, C. M. Heard wrote:
> > On Tue, Mar 12, 2019 at 3:09 AM Joe Touch wrote:
>
> > A revised OCS (that includes the pseudo-header correction) will allow
> > FRAG **without** LITE (as proposed in the draft) to traverse the
> > affected middleboxes with a non-zero UDP CS, but that method has the
> > following downsides:
> >
> > 1) A legacy host interprets FRAG without LITE as a complete UDP
> > datagram
> > 2) The same is true for an options-aware host if OCS fails
> > 3) There is duplicate checksum coverage (UDP checksum covers each
> > fragment,
> >    plus post-reassembly checksum contained in the terminal FRAG option)
>
> This is true, but
> 1 can be solved if FRAG Option has a 2 bytes Length and carries the
> fragment inside it.

Correct; your proposal for allowing an "escape" for two-byte
lengths would allow for this

> 2 if the fragment is inside FRAG and OCS is not valid, the fragment is
> just discarded

Correct

> 3 covering each fragment with OCS is basically the same of covering the
> reassembled packet

That was my assessment

> You actually already pointed out 3
> and already suggested The FRAG Option carrying fragment inside
> previously.

Yes but that suggestion missed the need for a two-byte length.

Mike