[e2e] Agility of RTO Estimates

[Craig Partridge]

In message <42D7EB1D.8050003 at web.de>, Detlef Bosau writes:

>My question is, with repect to mobile wireless networks as UMTS or GPRS:
>How "quickly" does RTO adapt? I expect, this is restricted by the ES-ES
>latency, the packet rate (i.e. "samplin rate"), the burstiness of
>traffic etc.
>Can this "RTO model" follow e.g. the latency variations met on the
>mobile network in "real time"?
>Or are there basic limiations. (At least, I expect so.)

I'll take a stab at this and be delighted to be corrected by others who
know better.

I believe the immediate issue is not the "RTO model" but rather the
question of what RTO estimator you use.  In the late 1980s there was
a crisis of confidence in RTO estimators -- a problem we dealt with by
developing Karn's algorithm (to deal with retransmission ambiguity) and
improving the RSRE estimation algorithm with Van Jacobson's replacement.

Van did a bunch of testing of his estimator on real Internet traffic and
looked to see how often the estimator failed.  (Note that spurious
timeouts are only one failure -- delaying a retransmission overly long
after the loss is also a failure.)  He picked an estimator that was
easy to compute and gave good results in the real world.

If there's reason to believe the estimator today is working less well, we
could obviously replace it.  That doesn't mean the RTO model needs fixing.

Second point is that the RTO model now works in concert with other
mechanisms.  I.e. it used to be that we relied only on RTO to determine
if we should retransmit.  Now we have Fast Retransmit to catch certain
types of loss.

Craig