[e2e] Some questions about TCP.

rick jones perfgeek at mac.com
Tue Nov 24 08:41:30 PST 2009

On Nov 24, 2009, at 3:36 AM, Detlef Bosau wrote:

> rick jones wrote:
>> IIRC the duplicate ACK test includes the advertised window holding  
>> fixed.  If the arriving segment updates the window,
> On the sender side? Or on the receiver side?
> I can well imagine that a receiver may hand over data to the  
> application between to acknowledging packets.
> So, why should particularly a receiver keep its advertised window  
> fixed?

The receiver cannot hand to the application any data to the "right" of  
the first hole in the received sequence number space, there for it  
cannot advance the window past that hole.  I believe it is this, and  
the "immediate" nature of ACKing segments arriving "out of order"  
coupled with the presumption that packet reordering is "rare" that  
allows the three "duplicate" ACKs from the receiver to signal that the  
segment starting at that sequence number was most likely lost.

If the ACK advances the window, it cannot, by definition (?) say  
anything about reordering or loss.  All the sender can infer from its  
receipt is that all is happiness and joy.  Once there is a hole in the  
sequence space, there will be no advancing the window beyond it.  So,  
ACKs arriving at the sender, for the same sequence number, and which  
do not advance the window can be presumed to have been triggered by  
segments "beyond" a hole starting at that point in the sequence  
space.  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.

rick jones
there is no rest for the wicked, yet the virtuous have no pillows

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