Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!thunder.mcrcim.mcgill.edu!snorkelwacker.mit.edu!apple!usc!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!lll-winken!prang!ejmmips.NOC.Vitalink.COM!ejm
From: ejm@ejmmips.NOC.Vitalink.COM (Erik J. Murrey)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: When is a link saturated?
Message-ID: <32@prang.TEST.Vitalink.COM>
Date: 21 Jan 91 23:55:42 GMT
References: <1991Jan21.141530.7031@ccs.carleton.ca> <9101150900.AA08526@jerry.inria.fr> <9101150724.AA12358@mcsun.EU.net> <24@prang.TEST.Vitalink.COM> <1991Jan20.040130.18339@quick.com>
Sender: usenet@prang.TEST.Vitalink.COM
Reply-To: ejm@ejmmips.NOC.Vitalink.COM (Erik J. Murrey)
Organization: Vitalink Communications
Lines: 47
Nntp-Posting-Host: ejmmips.noc.vitalink.com

In article <1991Jan21.141530.7031@ccs.carleton.ca>,
jstewart@ccs.carleton.ca (John Stewart) writes:
> In article <1991Jan20.040130.18339@quick.com> srg@quick.com (Spencer
Garrett) writes:
> >-> 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.
> >
> >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.)
> 
> 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.
> -- 

Wait a minute.  On most BSD implementations, "inetd" spawns a separate
rlogind or telnetd process for each incoming telnet or rlogin session
requested.  The processes share the same local port number (23 or 513)
since TCP/IP allows them to do so.  (The connection is still unique
based on (source ip address, source TCP port, dest ip address, dest TCP port)

I will quote from RFC 854 (telnet)
"  The TELNET TCP connection is established between the user's port U
   and the server's port L.  The server listens on its well known port L
   for such connections.  Since a TCP connection is full duplex and
   identified by the pair of ports, the server can engage in many
   simultaneous connections involving its port L and different user
   ports U.
"

A netstat -n on all of the machines I can access show port 23 or 513
as the host's local port for incoming or foreign port for outgoing
telnet/rlogin sessions.

This allows a router to look at the source/dest TCP port to determine
whether this is a rlogin or telnet session.

---
Erik J. Murrey
Vitalink Communications NOC
ejm@NOC.Vitalink.COM	...!uunet!NOC.Vitalink.COM!ejm