[e2e] Some questions about TCP.
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
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.
there is no rest for the wicked, yet the virtuous have no pillows
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