[ee122] Fast recovery homework question
vern at cs.berkeley.edu
vern at cs.berkeley.edu
Thu Dec 6 23:23:13 PST 2007
> Just to clarify, what is fast recovery? Online definitions mention inflating
> the CWND size, but this is somewhat confusing. I am interpreting this
> to mean that on the fourth DUP and beyond, the sender would continue
> sending packets despite not having received any ACKs
> for new data.
That's the flavor of it.
In particular, additional dups eventually result in transmission of
additional packets. The actual mechanism used is a bit tricky - see
RFC 2581 if you want details. For our purposes, the key things to
know about it are:
(1) It starts off the same as Fast Retransmit
(2) It can send additional packets if enough dups arrive
(3) It implements true AIMD - when the lost packet has
been successfully retransmitted, the sender continues
with CWND = half the value it had prior to loss (whereas
Fast Retransmit enters Slow Start at this point)
(4) Like Fast Retransmit, if more than one packet is lost
in a single flight, it will in general wind up recovering
via a Timeout followed by Slow Start (unless the TCP also
uses SACK)
- Vern
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