[e2e] An alternative algorithm of calculating RTO

[Jason Gao]

>jaglee at peoplemail.com.cn wrote:
>> The algorithm in a nutshell:
>> (1) Calculate mean measured round-trip time and measured round-trip time variation: Suppose mrtt[0], mrtt[1], ...,  mrtt[N-1] are the measured round trip time of the N most recently acknowledged packets without re-sending,
>> 
>> let mmrtt <- (mrtt[0] + mrtt[1] + ... + mrtt[N-1]) / N;
>> and 
>> mrttvar <-  (abs(mrtt[0] - mmrtt) + abs(mrtt[1] - mmrtt) + ... + abs(mrtt[N-1) - mmrtt) / N;

>This is known as a "rolling average" and "rolling variance", and have 
>been tried in a number of contexts before, in particular, for a ECG 
>scanning system:

>"An Experimental ECG Scanning System," (extended abstract)
>K.P. Birman and J.D. Touch, Proc. Of the Ninth Annual Conf. Of the IEEE 
>Engineering in Medicine and Biology, Boston, Nov. 13-16, 1987. The note 
>on which this extended abstract is based is available at 
>http://www.isi.edu/touch/pubs/tri.pdf

Thanks!

>The use of 'calibrating factors' was used in that work to describe the 
>difference between signal and noise - literally, signal was when the 
>data exceeded the average by more than some multiple of the variance.

>It has been raised as an alternative to TCP's capacitive decay model 
>several times, notably at several IETF TCPIMPL meetings. Whether it is 
>strictly more stable or useful than the current 'capacitative decay' 
>model has not yet been proven.

Well, I think the usefulness of the rolling average may be apparent in adaptive rate control, where we may make the floor (rate_sstart) of the sending rate to, say, HDRSIZE / (rartt + 4 * rvrtt), and the ceil (rate_sthresh) to MSS / rartt. (rartt: rolling average of rtt; rvrtt: rolling variance of rtt; rate_sstart: rate on slow start; rate_sthresh: threshold rate of slow-start. Well, it's true that this adaptive rate control algorithm is not TCP-friendly).

Stableness is really a problem. Thanks again!

Jason Gao.
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