Re: [tsvwg] A review of draft-ietf-tsvwg-udp-options-12

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

On Mon, Jun 14, 2021 at 10:17 AM Joe Touch <[e-mail]> wrote:

> Yes, but we still need the fraglen field in the last frag too.
>

No, it is not necessary, if every option precedes the FRAG header.

See the format I posted this am.
>

I did read it. If I correctly understand what you wrote, there is an
underlying ASSUMPTION that options that apply per-fragment need to precede
the FRAG header and those that apply to the reassembled datagram have to
appear afterward. To be clear, I do not dispute that such a design will
work; it will. What I dispute is that it is necessary.

In the design that I presented earlier, I also made an implicit assumption
that whether an option applies to an individual fragment or to the
reassembled datagram can be determined from the Option Kind (or UKind or
ExID). Going down the list:

EOL - not applicable (FRAG consumes all trailing data)
NOP - per fragment
OCS - per fragment
ACS - per datagram
FRAG  - per fragment (obviously)
UNSAFE - depends on Ukind
TIME - per datagram
AE - per fragment (see below)
REQ, RES - per datagram
EXP - depends on UDP ExID

I will grant that defining AE as per-fragment or per-datagram is a design
choice. Either can be made to work, but if it's designed to be per-fragment
then the description in the draft of how it works needs no modifications --
in particular, this:

   >> Only the OCS and AE options depend on the contents of the option
   area. AE is always computed as if the AE hash and OCS checksum are
   zero; OCS is always computed as if the OCS checksum is zero and
   after the AE hash has been computed.


I'm still struggling to wrap my head around how zero copy interacts with
all this, but I hope to post a response to that within the next 12 hours.

Thanks and regards,

Mike Heard