[e2e] TCP improved closing strategies?

Craig Partridge craig at aland.bbn.com
Wed Aug 12 16:01:18 PDT 2009

Hi Bill:

Couple of questions and idea.

Question first -- why is port cycling an issue?  In TCP, one has to keep
the tuple <src addr, dst addr, src port, dst port> unique.  To run out
of ports you'd have to use so many ports with ONE peer that you'd have
problems..  Seems unlikely (even if a peer is, actually, a NATed network).

The second question is why having a hashed PCB in TIME WAIT is such an
issue for 2 MSL...  (Is it purely the size of the hash database -- if so,
there are ways to compress the hash table...).

That said, the problem is fun.

As I recall Andy Tanenbaum used to point out that TP4 had an abrupt close
and it worked.  It does require somewhat more application coordination but
perhaps we can fake that by, say, retransmitting the last segment and the FIN
a few times to seek to ensure that all data is received by the client???



> With the advent of more widespread DNSsec deployment, more UDP sessions
> are likely to fallover into TCP sessions.
> I've been informed that even today, with a more limited TCP activity,
> busy servers cannot wait 2MSL to finish closing.
> Also, busy caching servers run out of port numbers, and cycle quickly.
> So there's ample opportunity for seemingly duplicate transmissions.
> I've been searching my personal copy of the e2e-interest archives back to
> '98 (the previous years are only on backup somewhere), and haven't found
> anything on improved closing strategies.  Ideas?
> Of course, there's T/TCP, but wasn't closing one of its Achilles heels?
Craig Partridge
Chief Scientist, BBN Technologies
E-mail: craig at aland.bbn.com or craig at bbn.com
Phone: +1 517 324 3425

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