Path: utzoo!attcan!uunet!munnari.oz.au!mtiame!ubeaut!mwp
From: mwp@ubeaut.oz.au (Michael Paddon)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: TCP segment size -- user defined?
Message-ID: <256@ubeaut.oz.au>
Date: 3 Oct 90 05:27:08 GMT
References: <266@aldetec.oz.au>
Organization: Digital Equipment Corp., Melbourne, Australia
Lines: 32

From article <266@aldetec.oz.au>, by mawson@aldetec.oz.au (Graeme Mawson):
> 
> We are presently experimenting with the TCP/IP protcol suite over an Ethernet
> LAN.  A useful guide to TCP/IP by Comer seems to suggest that TCP segment
> size is user-definable.  Is this true?  Does anyone know how to define it?

It surely is. On BSD based systems, the macro TCP_MSS is defined in
/usr/include/netinet/tcp.h. This may be changed with impunity since the
negotiation undergone at connection setup time chooses the smaller of
the segment sizes supported by each implementation.

There is not much point setting TCP_MSS to be greater than
	(maximum IP packet size - IP header size - TCP header size)
[536 octets] since IP fragmentation will take place. Receipt of a
fragmented packet is an all or nothing proposition; a good thing to
avoid for throughput reasons.

In general, the BSD TCP parameters in tcp.h and tcp_timers.h are
close to optimal for ethernet (default RTT is a little pessimistic,
but that is so the latter stages of backoff work properly). I found
I had to do a fair bit of tuning to get everything right for a
SLIP environment with flakey link hardware.

					Michael

-------------------------------------------------------------------
|                     |     Internet: paddon@meo78b.enet.dec.com  |
|                     |     ACSnet:   mwp@ubeaut.oz.au            |
|  Michael Paddon     |     ACSnet:   mwp@munnari.oz.au           |
|                     |     EasyNet:  meo78b::paddon              |
|                     |     Voice:    +61 3 895 9392              |
-------------------------------------------------------------------