[e2e] Once again: TCP RTT and RTO

Detlef Bosau detlef.bosau at web.de
Mon Aug 8 15:50:53 PDT 2011

Hi to all.

Let me quote a sentence from the congavoid paper:

> A good round trip time estimator, the core of the
> retransmit timer, is the single most important feature of any
> protocol implementation that expects to survive heavy load.

To my understanding, a goot RTT estimator and a good RTO as well are not 
only important from a simple viewpoint of network utilization, too small 
an RTO would be to aggressive, too large an RTO would result in 
underutilization, but important for network stability at all.

J&K outline that a congested network needs an exponential damping in 
order to return to stability. To my understanding, this is achieved by 
the AIMD scheme in CA/Slow Start on the one hand and by the exponential 
RTO back off on the other: Particularly in flows which did not yet reach 
equilibrium, the RTO and the RTO backoff impose a, NB exponentially 
damped!, "rate" on otherwise self clocking flows.

For this reason, I'm particularly interested in whether we have, or at 
least expect to achieve, a reliable means of getting RTT measurements 
_AND_, which is presumably anything but an orthogonal question, we 
expect the RTTM process to be weakly stationary. These two issues are 
obviously intertwined as a "stationary" process means a process which 
has settled after some time. (Although "some time" may be infinite from 
a mathematical point of view ;-))

Hence, for the process to become weakly stationary and for the flow to 
reach equilibrium is basically the same story.

I would appreciate a discussion about what is already achieved in this 
field and what we expect. Perhaps even with respect to different 
situations in wired networks and mobile wide area networks.


Detlef Bosau
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