[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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