Subject: Re: Pushing more than 936kb/sec via satellite

From: [email-masked] (Anil Agarwal)
Date: Thu, 21 Jan 99 11:05:19 EST

Dear Hank,
	A few weeks ago, you wrote -

> Today, is there any way without modifying the TCP stacks in NT, W95, AIX,
> Solaris, SG, HP, etc. to push more than 936kb/sec via satellite?  Can one
> put a black box on each end and fake things out (window size, ACKs, NAKs,
> etc.) for all TCP streams so as to get 20-30Mb/sec per TCP stream via GEO
> satellites?  Does anyone know anyone doing this?  Does anyone know of
> equipment that can handle T3/OC-3 speeds?

> What if the GEO satellite link was simplex with a fiber uplink?  This would
> alter the numbers - has anyone had any experience with hybrid systems being
> able to pump 20-40Mb/sec per single TCP stream?
	
We (COMSAT) have just released version 2.1 of the COMSAT Link Accelerator
(CLA-2000/Internet) product. This release includes support for
"TCP Spoofing" (we call it TCP Proxy). It enables non-RFC-1323 TCP 
implementations to achieve throughputs of up to 3 Mbps per single TCP stream
over geo-synchronous satellite links.

The CLA-2000/Internet is a small, stand-alone box, that sits between
a network and the satellite modem. Two such units are needed, one on each
side of the satellite link. Besides TCP Proxy, it performs a number of
other functions to improve performance of TCP over satellite links,
such as (1) adaptive Reed-Solomon coding to improve link quality and
(2) loss-less data compression. 

The CLA-2000 can also be used effectively over asymmetric rate/delay links
(e.g., asymmetric rate satellite links or hybrid satellite/fiber links).

The CLA-2000/Internet can be used in a "transparent" mode - the CLA-2000
is inserted between the serial port of a router and the satellite 
modem or it can be used in a "routing" mode, in which case it "sits"
on an ethernet and acts as a IP router between ethernet and the serial
port connected to the modem. In either mode, TCP Proxy can be enabled.

The product does not run at DS3/OC-3 rates quite yet - it can do a maximum of
3 Mbit/s, due to hardware limitations.  The next model, the CLA-2100,
will support rates up to 45 Mbit/s, and will be released around Aug. 1999.

Our "TCP Proxy" implementation is a fairly robust implementation - it
does a number of things to correctly perform TCP Proxy, without breaking
the end-to-end semantics of TCP. Acknowledgements for data packets
are generated locally, but TCP connect (SYN) and disconnect (FIN) packets
and their acknowledgements retain their end-to-end significance.
Over the satellite link, our implementation uses standard TCP with large
windows and a slightly more aggressive window control scheme. It performs
flow/congestion control, by appropriately setting TCP windows that are 
advertised to the sender.  We have run 1 to 64 TCP sessions, using 
8K windows at the end-user workstations, and have obtained aggregate 
throughputs around 3 Mbps in each case.
	
Anyone interested in trying out this technology can contact me or
preferably Mr. Nalin Kapila at 301-428-4363, [email-masked].

Anil K. Agarwal
COMSAT Labs
22300 COMSAT Drive
Clarksburg, MD 20871
(301)428-4655
[email-masked]