[e2e] High Packet Loss and TCP

[Jonathan Stone]

My experience was that TCP collapses at around 25%-30%, in a very
special sense of "collapse", namely that "HTTP pages never finish
downloading, faster to kill the conneciton and start from scratch".
I'd thought this was fairly well-known.

Someone at Stanford once asked me why this phenomenon happened.  I
took packet traces from some sites in Euroe traversing a badly
overloaded link.  A tcptrace-like tool showed packet loss rates were
so high that fast recovery/fast retransmit never got 3 dupacks (due to
drop, the receiver got at most two segments, so the receiver got at
most 2 dupacks.)

At that point not only has throughput gone to hell, but the
time-constant on whatever goodput can be gotten, has shifted from (a
constant factor of) network RTT, to (some factor of) the slow-retransmit
timeout.  That's quite a different regime from the 3% to 5% 5 loss
over, (for example) overloaded CDMA nets, where (with careful tuning)
even soft-real-time flows can be quite doable.