Subject: State of TCP Fairness Question

From: Chris Metz <[email-masked]>
Date: Mon, 12 Mar 2001 14:01:12 -0500

Hi-
Seeking to understand that status of the TCP fairness issue. Here are my 
data points:

- TCP thoughput is a function of packet size, RTT, loss probability and RTO 
as shown in the "Modeling TCP Throughput ..." paper by Padhye, et al from 
Sigcomm '98.

- TCP may be biased against connections with long RTTs because the additive 
increase time is roughly one segment per RTT. Connections with longer RTTs 
take longer to ramp up and while this is happening those with smaller RTTs 
can acquire available bandwidth. TCP constant rate algorithm as suggested 
by Floyd and later Henderson, et al paper could overcome this limitation by 
"equalizing" the rate by which senders with different RTTs increase their 
sending rates during congestion avoidance. Not implemented.

- New techniques (larger initial windows, limited transmit) can help the 
performance of small flows.

Anything else to look at? What about TCP and Diffserv? It was suggested by 
recent Yeom and Reddy paper that it may be difficult to achieve 
"contracted" TCP throughputs for higher rates and that those with smaller 
rates would fare better.

My apologies if this is not relevant for the mailing list but I was 
thinking that sat links can encompass the entire spectrum of 
characteristics that impact TCP throughput.

Thanks ...

Chris Metz
Lead IP Architect
Solutions Integration
Service Provider Line of Business
Cisco Systems
email: [email-masked]
offic phone: 408-525-3275
home office: 914-241-0423
pager: 800-365-4578
Internal URL: http://wwwin-people.cisco.com/chmetz/chmetz.htm

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From: Alhussein Abouzeid <[email-masked]>
Date: Mon, 12 Mar 2001 14:48:23 -0800 (PST)

An additional point: In the case of congestion, TCP fairness is improved
by a factor of sqrt if you use RED instead of Tail Drop. Thus, if the
ratio of the lowest to the highest flow (in steady state) through a
congested Tail Drop gateway is 0.2, this ratio is improved to 0.45 if you
use an equivalent RED.

For details/derivation, please see the paper
Alhussein A. Abouzeid, Sumit Roy, "Analytic Understanding of RED Gateways
with Multiple Competing TCP Flows", in Proceedings of  GLOBECOM'00, also
available electronically at 
http://students.washington.edu/abouzeid/globecom.pdf

-Hussein.