[e2e] Some questions about TCP.

David P. Reed dpreed at reed.com
Tue Nov 24 10:01:10 PST 2009

On 11/24/2009 11:41 AM, rick jones wrote:
> Since reordering is presumed rare, when three of these duplicate ACKs 
> have been received, we can presume that three segments beyond the 
> possibly lost segment were received and be statistically certain that 
> segment was indeed lost.
I may have gotten confused by the way this conversation has wandered, 
but there is no statistical certainty  of loss here.   There are lots of 
ways that three duplicate ACKs can be sent, and then received.  Every 
time the receiver receives a duplicate of a previously accepted segment 
it will transmit another ACK with the same content - there need be no 
"hole" and no segments to the "right" of the hole.

TCP doesn't estimate certainties of loss - at best it tries to minimize 
the frequency that an IP datagram is unnecessarily retransmitted, while 
at the same time trying to estimate the short-term non-congesting rate 
that can be sustained by using the statistics of the queue-drop erasure 
channel and other inputs such as ECN bits as input to a channel model 
that is used to control the probability of congestion by controlling the 

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