Re: 4.2/4.3 TCP and long RTTs


Bob Braden (braden@isi.edu)
Mon, 8 Dec 86 08:57:21 PST


Craig:

The amount of your hair pulling must be small compared to the time integral
of hair pulled by our UCL friends over the years. Quite simply, their
conclusion was that most TCP implementations have design problems
that make them behave poorly over paths which have very long delay and
moderate to high loss. SATNET sometimes (often?) exhibits that behaviour.
Recently, the ARPANET+core_gateway system has also exhibited that
behaviour, and many TCP's have not been up to it... lots of broken
connections, etc.

I suggest that the cause of this situation is a performance/ robustness
tradeoff inherent to TCP implementations. Most of the
currently-available TCPs have been implemented and tested in an LAN
environment, to provide optimal performance in a low-delay, low-error
situation. On the other hand, when we wrote the original experimental
implementations of TCP, we found the little beasties to be amazingly
robust; they would tenaciously hold on for minutes (or hours!)
retransmitting until a path came back, and would get the data through in
spite of terrible bugs. But we were writing and testing them for
equally-experimental gateway implementations and frequently testing to
UCL, and did not demand high throughput or low delay.

It would certainly be interesting to understand exactly how these TCP;s
have failed. I suspect it is a combination of a Zhang-catastrophe (RTT
measurement diverging towards infinity due to high loss rate) with an
implementation-imposed upper bound on retransmission time before the
connection breaks. On the other hand, the answer may be that selective
retransmission is really absolutely essential to deal with the
long-delay, lossy situation. I would like to get someone interesting in
running some experiments on this (maybe you just did??) Would it be
possible for you to disable just the selective retransmisssion feature of
RDP and try again?

Bob Braden



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