# [e2e] Analytic Model of Download Time as a Function of TCP ConnectTime

Cottrell, Les cottrell at slac.stanford.edu
Thu Jul 26 16:28:08 PDT 2007

```The sounds a bit like an extnesion of "ITU-T Rec.G1040 "Network contribution to transaction time" <http://www.itu.int/rec/T-REC-G/recommendation.asp?lang=en&parent=T-REC-G.1040>  which calculates the network contribution to transaction time. The contribution depends on the RTT, loss probability (p), the Retransmission Time Out (RTO) and the number of round trips involved (n) in a transaction. The Network Contribution to Transation Time (NCTT) is given as:

Average(NCTT) = (n * RTT) + (p * n * RTO)

In our case (PingER) typical values for n are 8, for RTO we take 2.5 seconds, we take the RTT and loss probability (p) from the PingER measurements.  The main difference is that you seem to ignore the losses.

________________________________

From: end2end-interest-bounces at postel.org [mailto:end2end-interest-bounces at postel.org] On Behalf Of Paddy Ganti
Sent: Thursday, July 26, 2007 2:44 PM
To: end2end-interest at postel.org
Subject: [e2e] Analytic Model of Download Time as a Function of TCP ConnectTime

I am thinking of an approach to analytically determine the download time as a function of RTT given a few initial real world samples. Say, I measured a web page from 4 locations around the globe. Knowing this sample, what can I infer anything about the population of download times as a function of RTT.

dt = n* RTT + c

where n is the number of round trips (RTT ping pongs, includes one burst of data which can be multiple packets) with c being the server stall time between sending the data or server processing time plus some random noise all factored into once constant.

The above equation is of the form y=mx +c and I can equate the slope with that of number of round trips (makes sense as the lesser the number of round trips the lower the response time) while x is RTT.

So if I take enough sampls, say 10, and perform a regression analysis on those to generate the equation wouldnt that classify the population. If I have such an equation then I would plug in various RTT(s) and asuming the R-squared value is high wouldnt that be representative of real performance. A few initial measurements showed encouraging results but a few measurements didnt converge and a few had negative valus,etc.

Before I go further and present this to an internal audience I want to poll this group for any feedback/remarks/comments on using this method and its pitfalls.