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.
On Mon, 12 Mar 2001, Chris Metz wrote:
> 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 protected]
> 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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