[e2e] High Packet Loss and TCP

[Craig Partridge]

In message <4.3.2.20030501153756.02d4fbe0 at zircon.juniper.net>, Ross Callon writ
es:

>My understanding is that there is some level of packet loss 
>which causes TCP to back off to the point of stopping. My 
>impression is that this is a sufficiently high loss rate that it 
>shouldn't happen in a network which is behaving properly, 
>and if it happens this should be considered a network failure 
>rather than a TCP problem. (I am pretty sure that I saw this 
>sort of behavior a few years ago when trying to access large 
>files over a very bad link). 

Hi Ross:

Well, in the extreme, TCP just fails.  And that's a fairly simple
computation, namely, does:

    the maximum number of retransmissions * the loss rate * total segments
      to be sent in the connection

give you a number that starts getting close to 1.0?  If so, you've got
non-working TCP.

If you want to do a little better, you can evaluate the forward and
reverse loss rates (the equation above assumes all acks get through, which
usually isn't true if the loss rate in the forward direction is high).

If you want to make your head hurt, you can try to puzzle out just how
poorly TCP works with lower loss rates -- that's a function of retransmission
policies, cumulative acks, and such like.  Most studies I've seen suggest
that single digit loss rates are enough to seriously slow things down
(though exact behavior varies widely according to things like use of SACK).

Craig