Subject: Re: Doubt regarding the MANAGING THE RTO TIMER

From: [email-masked]
Date: Tue, 11 Dec 2001 09:57:03 -0600

Dear All,

I think one of our papers may cast some new insight on this RTO timer 
issue. In this paper, the dependent properties of measured delay samples 
are investigated and a new delay estimation method is proposed. The 
understanding gained in this study may provide some clue on how to set the 
constant gains of the TCP-style SRTT estimator under different conditions.

The paper is:

 Qiong Li and Dave L. Mills, "Jitter Based Delay Boundary Prediction of 
Wide-Area Networks", appears in ACM/IEEE Transactions on Networking, Vol. 
9, No. 5, Oct., 2001. Available at: 
http://www.ee.udel.edu/~qli/paper/acm_delay_pred.ps

Abstract

The delay boundary prediction algorithms currently implemented by 
transport protocols are lowpass filters based on autoregressive and moving 
average (ARMA) models. However, recent studies have revealed a 
fractal-like structure of delay sequences, which may not be well suited to 
ARMA models. In this paper we propose a novel delay boundary prediction 
algorithm based on a deviation-lag function (DLF) to characterize 
end-to-end delay variations. Compared to conventional algorithms derived 
from ARMA models, the new algorithm can adapt to delay variations more 
rapidly and share the delay's robust high-order statistical information 
(jitter deviation) among competing connections along a common network 
path. Preliminary experiments show it outperforms Jacobson's algorithm, 
which is based on an ARMA model, by significantly reducing the prediction 
error rate. To show the practical feasibility of the DLF algorithm, we 
also propose a skeleton implementation model.

Thanks,

-qiong

_____________________________________
Qiong Li (Stan)
Senior Member Research Staff
Wireless Communications and Networking (WiCAN)
Philips Research USA
345 Scarborough Road, Briarcliff Manor, NY 10510-2099

Tel:(914)945-6408/Fax:(914)945-6580
email: [email-masked]
** My opinions do not reflect those of my employer **

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From: Mark Allman <[email-masked]>
Date: Wed, 12 Dec 2001 13:19:12 -0500

> >The minimum of 1 second is a traditional aspect of the RTO timer.
> 
> That's the one thing about RFC2988 that I never bought, and I
> raised that issue when RFC2988 was last called. What you codified
> was pretty much what is found in the BSD4.4-Lite implementation of
> TCP, and that one sets RTO to 2x granularity, and not 1 second.

The 2x the granularity is not the fundemental point, though.  That
is simply an implementation detail.  The ultimate effect is we end
up with a 1 second minimum.  The analysis in our SIGCOMM paper
showed that 2x the granularity was not the key to the performance,
but the minimum (in seconds) was.

allman

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Mark Allman -- NASA GRC/BBN -- http://roland.grc.nasa.gov/~mallman/