[e2e] Rationale for EWMA filters in RTTM

[Detlef Bosau]

On 08/06/2011 01:20 AM, Anoop Ghanwani wrote:
> Have you looked at RFC 6298?  Based on your last email
> it looks like you were reading an obsoleted RFC.

O.k., now we've learned that RFC 6298 obsoletes RFC 2988.

Of course, it is always useful to know the most recent RFC numbers. 
However, I don't see a solution for my problem.

>
> I don't think this timer needs to be super accurate since
> it kicks in only when duplicate ACKs don't already solve
> the problem, e.g. under severe forward or reverse congestion
> because of which ACKs aren't making it back.

Hang on.

First, refer to Manus post some few days ago and the paper

Sharad Jaiswal, Gianluca Iannaccone, Christophe Diot, James F. Kurose,
Donald F. Towsley,
Measurement and classification of out-of-sequence packets in a tier-1
IP backbone.
IEEE/ACM Trans. Netw. (TON) 15(1):54-66 (2007)

Manu points out, that according to this paper, 40% of the  observed 
links exhibit more or less sever packet reordering.

In addition, we know the state variable DUPACKTHRESH for tcp senders for 
years now - which was particularly intended to address packet reordering.
 From what I've read in recent literature, not even the least effort is 
spent, to address this problem in practical implementations.

Consequence: Triple Dupacks may or may not happen - according to the 
phases of the moon or the water level, however, when they are related to 
congestion or packet loss, this is pure luck.

In the same way,  spurious timeout may occur on the same "basis", caused 
by RTO values being unreasonable small.

>
> Having it be a moving average just allows us to pick an
> initial value that could be terribly wrong for the environment
> (data center at one end, satellite links at the other end)

I'm with you to up to now, but:
> and we still find a reasonable value after a few RTT.
>

from what I've seen with some playing around in Octave, I would like to 
herewith suggest THE one and only reasonable RTO for TCP:

10 milliseconds times Bill Gates' birthday.

At the moment, I'm quite convinced that this is neither worse nor better 
than those values we're using today.

I have to apologize for my frustration. However, I'm still to overcome 
this huge difference between a marvelous, splendid theory and a very 
ugly practice.

Please correct me, when I'm completely wrong here.



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