Subject: [tcpm] Comments on draft-kang-tcpm-subtype-capability-exchange-00

From: Martin Duke <[email-masked]>
Date: Thu, 11 Mar 2021 14:22:45 -0800

I can't speak to the problems subtype mismatches are causing, if any, but I
do like the idea of negotiation extension of new subtypes without having to
roll whole new MPTCP versions.

I wonder if we could save some bits by assuming that 0x0-0x7 will be
supported by all MPTCP implementations and omitting them from this field.

The location of the OptionSupported field probably has to change along with
other details of Figure 2. If I understand correctly, OptionSupported has
to begin with the 5th byte, as this first goes out with the SYN where the
entire (original) option is 4 bytes. Which means, we're relying on the
length field to indicate this is not like other MP-CAPABLE options. I'm not
sure this interoperability model is fully thought out.

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From: Martin Duke <[email-masked]>
Date: Fri, 12 Mar 2021 05:40:36 -0800

After hearing the talk, I'm unclear on the use case this is addressing. I
see two possibilities:

1) Current v0 and v1 implementations don't support all the specified
subtypes, and there ought to be a way of negotiating this explicitly. I
haven't surveyed MPTCP implementations so I don't know if this is the case.

2) There should be a way for endpoints to negotiate use of new subtypes
without having to roll out a new version.

I don't think adoption is dependent on the answer here, but I would like to
understand the problem it is trying to solve.

On Thu, Mar 11, 2021 at 2:22 PM Martin Duke <[email-masked]> wrote:

> I can't speak to the problems subtype mismatches are causing, if any, but
> I do like the idea of negotiation extension of new subtypes without having
> to roll whole new MPTCP versions.
>
> The location of the OptionSupported field probably has to change along
> with other details of Figure 2. If I understand correctly, OptionSupported
> has to begin with the 5th byte, as this first goes out with the SYN where
> the entire (original) option is 4 bytes. Which means, we're relying on the