[e2e] Some questions about TCP.

Ted Faber faber at ISI.EDU
Mon Nov 23 11:49:08 PST 2009

On Mon, Nov 23, 2009 at 07:00:12PM +0100, Dominik Kaspar wrote:
> Hi Detlef,
> Maybe the answer to your questions is that a DupACK never contains a
> payload. 

While a specific implementation may never generate a DUPACK with data
(and I'm skeptical about that), the specification of the protocol
certainly allows DUPACKs to exist on segments with payloads. 

TCP connections allow both endpoints to be sending data at once.  You
may have experienced this when you accidentally catted a large file
through an ssh session.  The server is pumping out streams of characters
while you're trying to control-c the process.  If one of your packets
gets lost, the server need not stop to send 2 empty ACK segments
back; it will simply duplicate the ACK on three data packets full of
stuff you're trying to avoid seeing. :-)

Ted Faber
http://www.isi.edu/~faber           PGP: http://www.isi.edu/~faber/pubkeys.asc
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