Conference lassie::ucx

5330.0. "TCP MSS and Slow Start setup ?" by AUBER::LEZY (Il n'y a qu'MI qui m'aille) Tue Mar 11 1997 11:44

Abstract : How can you specify the MSS TCP option within a VMS application ?
	   How to bypass the TCP slow start mechanism for some special cases ?


 	Hello,

One of our customer (Satellite TV provider) will have the following
configuration :




					Satellite
					/	\
				       /	 \
				      /		  \      
				     /		   \
	       system X ->- transmitter		  receiver->-TV
		 |					      |
		 |					   special-box
		 |					      |
		 |--------<-------  Telephone -------<--------|



Notes :
-------

- system X is not really chosen : Digital (Unix or VMS/UCX), SUN...
depending on the functionnalities. I suppose here that UCX will be the 
winner.

- The satellite datalink is unidirectionnal (you just have a receiver 
at home)

- The good guy in front of his TV will reclaim some program (big MPEG files)
using his special-box (created by the same TV provider) that will do
something very similar to the web concept : a TCP connect, a little request
(by the telphone), a BIG reply (by the satellite) and a TCP close.

- The principal need for system X is to be able to use the satellite correctly,
that is sending VERY large amounts of data IMMEDIATELY after the request.
Why ? because the satellite line has a high latency but accepts large data.

- The special-box will contain customized TCP/IP code so will be able to 
specify whatever wanted for TCP MSS, Window size...

First approach :
----------------

- To be able to transmit a large amount of data without waiting an ACK, just
use the large window size specified by the special-box.

BUT....the two sides will not be in the same subnet and will fall into the
slow start mechanism, transmitting one or two segments and waiting an ACK.
And with 1460 bytes segments, (or even worst 576) it is not useable for 
the satellite.

So if somebody has a little time to answer this :

 if UCX receives a TCP connect request with a Window Scale option and a segment
specifying a large window, is it possible to make it answer using IMMEDIATELY
this big window and not the slow start mechanism ?

Second approach :
-----------------

- If we suppose that the UCX system will just send one segment before an ACK
at the beginning (slow start), the only solution is to use a BIG segment size.
The special-box will be able to connect with a TCP option 
MSS=<as-big-as-needed>

But what for UCX ? The customer found that with Digital Unix, one can specify
the MSS for one connection using the setsockopt TCP_MAXSEG option.
This option does not work with UCX, but can you specify the MSS anyway in
an application ?

This is at this moment the main question of the customer.

Third (last ?) approach :
-------------------------

Have the tramistter directly on the VMS system, a special driver and be
able to special a very very big MTU for the corresponding interface, but
that is another story.


I would really appreciate (and the customer, former Digital VMS support too)
any light on these questions.


Stephane.

You should *never* be able to bypass slow start.