[e2e] Any Data on Fast Retransmit vs RTO Expiry Numbers?

Paddy Ganti pganti at gmail.com
Tue Dec 18 11:11:09 PST 2007

Hello e2e,

Does anyone have some quantitative experimental data on what percentage of
reliable packet delivery in TCP is done through Fast Retransmit versus that
of a RTO expiry? Specifically I am looking at such data being available for
HTTP class of traffic.

Some raw issues that lead for such data to be interesting:

(a) When the initial cwnd is less than 4 then there is a chance that initial
SYN/SYNACK  oss cannot be recovered using FastRetransmit (this is also worse
than RTO expiry because of additional 3 seconds).Magic value of 4 seems to
be from the paper Morris'  *Scalable TCP Congestion Control*

(b) The last packet isnt eligible for fast retransmit as well by the same
logic albeit this time the recovert via RTO

(c) In between (a) and (b) lets say we have a train of packets (dictated by
the cwnd size or the application's PSH). If you imagine this flight of
packets as a train, the last packet of such a burst cannot also be recovered
using fast retransmit

(d) Some other cases that I am not thinking of here.

Given that HTTP traffic seems to be like small bursts of packet trains,
there will be many last packets in a train and hence  response time suffers
on lossy/congested networks.

-Paddy Ganti
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