Subject: Re: Retransmissions without explanation

From: [email-masked]
Date: Fri, 21 May 1999 14:33:14 -0700

As I recall, Van's basic argument for making as many RTT measurements
as possible was to defeat possible "aliasing" effects.  Aliasing, as I
understand it, refers to sampling a periodic signal using a sampling
rate that is approximately a multiple of the period of the signal.
This can clearly give completely bogus results.  One sample per RTT
could fall into that trap, since traffic effects might be periodic of
the same order.

Please correct me if I have misunderstood this.

Bob Braden

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From: Francesco Potorti` <[email-masked]>
Date: Sat, 22 May 1999 11:44:59 +0200 (CEST)

More  precisely, a  signal which  is  reconstructed by  sampling has  bogus
features  not found  in  the  original signal  (aliasing)  if the  sampling
frequency is less  than twice the highest frequency  present in the sampled
signal.

-- 
Francesco Potortì (researcher)         Voice:    +39-050-593 203 (op. 211)
Computer Networks Group                Fax:      +39-050-904052
CNUCE-CNR, Via Santa Maria 36          Email:    [email-masked]
56126 Pisa - Italy                     Web:	 http://fly.cnuce.cnr.it/

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From: Joo Change-hee <[email-masked]>
Date: Sat, 22 May 1999 18:57:42 +0900

> could fall into that trap, since traffic effects might be periodic of

I don't think that a RTT measurement for one RTT is a simple periodic
sampling but a filtered sampling.

For a example, it does not contain RTT measurements after a packet
drop until fast recovery ends. Because the RTT measurements has peak value

during one RTT before a drop dectection, a measurement for one RTT has
smaller value than every measurement for packets like timestamp option.
As a result, TCP usually has a smaller timeout value with a measurement
for one RTT.

If TCP with timestamp option is used in a dedicated line, the sender's
averaged RTT has a overshoot before its stable value at the end of
slow-start
period.TCP(slow-start)'s excessive probing packets makes the network
queue overflow. But the averaged RTT of TCP without timestamp option
usually
doesnot has this overshoot. (If it has, it is smaller than that of
TCP with timestamp option.)
I show this measurement difference only in a simualtion, so it can be
different in a real network.