John;
I just had occasion to look again at RFC 2428, "FTP
Extensions for IPv6 and NATs",
Please consider this a fairly narrow question.
I'm afraid that your question is still too broad.
Are you asking the question for IPv6 or for NATs?
I am asking the question about FTP, about a piece of syntax in
the protocol, and about the options that syntax permits.
For what purpose?
I am asking only about why we haven't structured that syntax
Seemingly because there was no reason to have one.
If you think you now have a reason, you should state it.
If you don't, we can debate forever on infinite variations of
reasons and associated extensions.
For example, how about extending FTP to be able to treat
realtime stream? It is a question about FTP, about a piece
of syntax (MODE/STRU commands) in the protocol, and about
the options that syntax permits.
Masataka Ohta