Re: Yet more on RTTs


Van Jacobson (van@lbl-csam.arpa)
Sat, 15 Nov 86 11:51:38 PST


RTTs are not a red herring. As the specs now stand, RTT and the
associated TCP retransmit algorithm are the *only* congestion
control for 99%+ of the Internet traffic. The Nagle algorithm is
not for congestion control, it increases the line efficiency so
you are less likely to need congestion control. This only
postpones the day of reckoning. We have on the order of 30,000
networked computers sitting behind the Internet backbone and the
number is increasing exponentially. Long-haul services like
NSFNet supercomputer access are going to increase the traffic
those computers send across the backbone. There is a factor of
100 difference between number of customers and number of backbone
circuits. There is a factor of 200 impedance mismatch between
the local nets and the backbone. With these numbers, congestion
is guaranteed, even with everyone running every algorithm that
John devises.

The problem could be avoided if there was some way to solve it in
the gateways. RFC970 proposes one such algorithm. I started to
implement it but took some data that convinced me it wouldn't
help. In fact, I couldn't see anything that would help short of
improving the congestion algorithms in the endnodes. I saw a
useful endnode change, implemented part of it and it worked. But
to work well it requires more topology information going between
the gateways and the endnodes. This is undesirable, as is the
thought of changing the tcp in all 1000 of our local computers.
Thus it seemed worthwhile to continue the discussion in this
forum.

If congestion problems can be solved in the gateways, we have
quite a bit of time and only need to do trial implementations and
measurements to verify that the proposed algorithms work in real
world traffic. If problems have to be solved in the endnodes, we
have to implement and verify solutions now, then start leaning
hard on vendors to adopt those solutions. If we went to the
vendors today, it would be at least a year until we could buy the
fruits of our labor.

With luck, John Nagle's algorithms, and DCA/NSF infusions of
money into the backbone, we have a year to solve the next set of
problems. But we still have to solve them. Telling vendors to
hurry up and market things they should have had yesterday is very
important. So is figuring out what to tell them tomorrow.

  - Van



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