[e2e] Some questions about TCP.

Bob Braden braden at ISI.EDU
Wed Nov 25 12:03:22 PST 2009

John Kristoff wrote:
> On Tue, Nov 24, 2009 at 07:08:04PM -0500, Noel Chiappa wrote:
>> If you are seeing multiple ack-only TCP packets for the same segment, and
>> those aren't due to a lost packet causing a hole, then either i) you
>> retransmitted that segment several times, and either the original data
>> segments, or the acks, all got wildly delayed (so long that you timed out and
>> retransmitted it several times _before_ either a) the first segment got to
>> the far end, or b) the first ack got back to you), or ii) it got duplicated
>> somehow in the network.
> There is also the TCP keep-alive mechanism, which I believe may result
> in this type of behavior too, though I'm not familiar with implementation
> details or the scale of its deployment.
> John
Note that there ISs no keepalive mechanism in the TCP spec (see the 
Discussion in Section of RFC 1122). Jon Postel, the Arbiter of 
Good Taste in Protocols, was vehement against the idea.  Berkeley Unix 
created one, and it caused a lot of mischief in the early days. 
Community concensus in the IETF forced RFC 1122 to allow ("MAY") 
implementation of a TCP keepalive, but it tried to severely limit its 
use (MUST default to "off").

Bob Braden

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