[tsvwg] Resolving UDP Options Issue #52

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

Greetings,

By way of background:

On Tue, Apr 2, 2024 at 2:23 PM Martin Duke wrote:

> (11.4) 2 fragments/3000 bytes as a minimum will work, of course, but it
> seems like an odd number. This seems to be related to the canonical 1500B
> MTU, but that would imply that 2 fragments would be 2926 for IPv4 and 2886
> in IPv6, if my math is correct. (To be clear, I can live with 3000)
>

Note that Section 11.6 currently requires that endpoints MUST support a
local MRDS size of at least 3,000 bytes.

I opened Issue #52
<https://github.com/tsvwg/draft-ietf-tsvwg-udp-options/issues/52> on April
22, 2024 to address this comment, with a proposal to change the limit from
3,000 bytes to 2,926 bytes. While trying to craft suitable text, discussion
with the author revealed the following problem: if an endpoint advertises
either the proposed value (2,926 byes) or the current one (3,000 bytes) in
the MRDS option, then* more than two fragments* will be required to send a
(pre-fragmentation or post-reassembly) packet of that size, even if the MTU
is 1,500 bytes (which is likely maximum outside data centers). And none of
the three values (2,886, 2,926, or 3,000) would work if we are using IPv6
and the MTU is the statutory minimum of 1,280 bytes. Indeed, in the latter
case, I get a maximum reassembled UDP packet size of 2,446 bytes if only
two fragments are supported. That seems to me to be rather small.

I would like to propose instead that we specify that an implementation must
support a minimum of 3 fragments and a minimum reassembled size of 3,600
bytes. This fits nicely with the 1,280 byte minimum MTU over IPv6, allowing
for a total of 66 bytes of extra overhead (e.g., 22 bytes of per-fragment
UDP options in each fragment). If the MTU is larger, as is typically the
case, one of the three fragments would not be full size, which is not a
problem.

I would appreciate hearing from the WG whether this change would be
acceptable.

Feel free to comment on the list or on github
<https://github.com/tsvwg/draft-ietf-tsvwg-udp-options/issues/52>.

Thanks and regards,

Mike Heard