Path: utzoo!utgpu!watserv1!watmath!att!news.cs.indiana.edu!know!sdd.hp.com!spool2.mu.edu!uunet!nwnexus!amc-gw!quick!srg
From: srg@quick.com (Spencer Garrett)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: When is a link saturated?
Message-ID: <1991Jan22.191059.5523@quick.com>
Date: 22 Jan 91 19:10:59 GMT
References: <9101150900.AA08526@jerry.inria.fr> <9101150724.AA12358@mcsun.EU.net> <1991Jan21.141530.7031@ccs.carleton.ca>
Organization: Quicksilver Engineering, Seattle USA
Lines: 30

Someone else wrote:
-> >-> I don't understand why the "remember the first exchange" is necessary. 
-> >-> Both telnet and rlogin use a reserved port number that appears in either
-> >-> the source or destination TCP port fields on *every* packet that is
-> >-> routed for the entire session.
-> >
-> In article <1991Jan20.040130.18339@quick.com> I wrote:
-> >Alas, no.  A server is free to answer the connection request
-> >with a different port number, and they commonly do.  (The reason
-> >for this eludes me.  It is permitted by the RFC's, but not
-> >required or particularly encouraged.)
-> 
In article <1991Jan21.141530.7031@ccs.carleton.ca>,
	jstewart@ccs.carleton.ca (John Stewart) writes:
-> The main reason for doing so is to facilitate multiple sessions.  For example
-> if 10 people telnet to a machine, each user will get their own telnetd 
-> process communicating to them via a unique set of ports.  Now imagine how
-> difficult this would be to do if you could only have one process running 
-> connected to the well known telnet port.

Not so.  A "connection" is identified by both source and destination
addresses *and port numbers*, so as long as the originator of each
session grabs a unique port number all is well.  My own networking
code does not shift away from the original well-known-port, and
multiple sessions work just fine.  I think the reason BSD does shift
port numbers may have something to do with the notion of "priviledged
port numbers" being those less than some small fixed number (1024
as I recall).  They may have thought it would be easier to implement
some security features if neither end of a regular session used
a small port number.