[e2e] High Packet Loss and TCP

[Christian Huitema]

There are two issues, the behavior of TCP and then the possible measures
to fix it.

At high loss rate, TCP will tend to operate with a window of 1 or 2, in
a cycle of send one or two packets, wait for a timer, and repeat until
an ack is received -- possibly a delayed ack. Each failed transmission
will result in an increase of the timer value, until the timer
eventually reaches some pre-configured maximum. Eventually, the TCP
connections will settle to something like sending one packet every 4
seconds, and receiving only a few of those.

One can argue that this can lead to congestion collapse: if too many
connections try to use a too small link, then they will all pack-off to
this one packet every 4 second behavior, which may well exceed the link
capacity. Also, the slow progress will lead to many job aborted midway,
which is a loss of resource. It has been shown that, in these
circumstances, admission control helps: preferably drop the SYN packets,
so that the remaining connections have some bandwidth and can actually
complete.

-- Christian Huitema