TCP experts - improving dial-up performance?

[Kelsay, Brian - Kansas City, MO]

Do you have call waiting on your phone line??   There is a code to send when you initiate your data call.  I think it is *70 to disable call waiting when you start the call to your ISP.

You may also want to look into changing your MTU settings.   I recall that I had to change this with the OLD dial modems to optimize the packet size.   This could be causing the buffer overflow, start and restart of the session.

Really those two items are what immediately comes to mind.


Brian Kelsay

-----Original Message-----
From: kclug-bounces at kclug.org [mailto:kclug-bounces at kclug.org] On Behalf Of Shawn C. Powell
Sent: Saturday, June 25, 2011 10:09 AM
To: kclug at kclug.org
Subject: TCP experts - improving dial-up performance?

Hi All,

Yeah, so I have dial-up.  Stop laughing. :) 

I would like some advice on making the best of my slow dial-up connection.  

The behavour I observe when downloading files (http get and ftp both) is:
	a) Download holds steady for some seconds, say about 30
	b) Rate stumbles, falling to zero for a few seconds
	c) Rate shoots up to 4 or five times nominal
	d) Rate declines back to nominal over a few seconds
	e) repeat from a)

A wireshark capture, if I am interpreting correctly, reveals that the window ( 
this is the TCP congestion window cgwnd?) increases during (a), whereupon at 
(b) a segment or two gets dropped, dup ACKS and retransmissions ensue (some 
which seem to me to be unnecessary), the window is shortened, progress resumes 
as normal for awhile, then the whole cycle repeats.

So I imagine that packets from the high-speed sender are piling up somewhere 
in the path until eventually a buffer gets exceeded and a packet gets dropped.

Am I reading the situation correctly?  Is there any way I encourage the other 
end of the link to throttle back?

Thanks for any comments and education.  And really, stop laughing!

-Shawn