[tcpm] 793bis IESG comment on NAT64 and MSS

[Wesley Eddy <[e-mail]>]

Here is another interesting point in the IESG ballots from Erik Kline 
that it would be good to get list feedback on:

    [S3.7.1, comment]

    * In networks where NAT64 is employed, the default MSS assumed by a sender
       will differ from the default assumed by a receiver, since the address
       families sent and received will be different.

       This may bolster the case for MAY-3 being a SHOULD (or even a MUST ;-) but,
       more to the point, may be a caveat to note w.r.t. SHLD-5.

       Alas, I could find no discussion of MSS option handling in RFC 6146,
       so I wonder if that's something that we missed...

For reference, MAY-3 and SHLD-5 come from:

    TCP implementations SHOULD send an MSS option in every SYN segment
    when its receive MSS differs from the default 536 for IPv4 or 1220
    for IPv6 (SHLD-5), and MAY send it always (MAY-3).

It's not entirely clear to me whether there is something within the 
793bis scope to do about this, or if it should be saved as a possible 
bit of "future work" for maintenance regarding NAT64 and transport 
notions of MSS (which should impact more than just TCP).