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From: minshall@OPAL.BERKELEY.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Separation of Layers
Message-ID: <8711041745.AA20491@opal.berkeley.edu>
Date: Wed, 4-Nov-87 12:45:49 EST
Article-I.D.: opal.8711041745.AA20491
Posted: Wed Nov  4 12:45:49 1987
Date-Received: Tue, 10-Nov-87 06:05:42 EST
References: <8710312107.AA14385@PARIS.MIT.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 23

Well, I've never been great on understanding which level is which.

However, a TCP address is the two-ple <IP address, TCP port number>.
This is the TCP address.  That is the way it is.  The nice thing here
is that the TCP address contains sufficient information to determine
the IP address (and, the bad thing here is that...).

Next, a TCP connection is defined by the two-ple <local TCP address,
remote TCP address>.  A TCP connection *cannot* be defined by the
two-ple <local TCP port, remote TCP port> - port numbers only have
significance when concatenated with an IP address (imagine telnet
clients on two machines trying to connect to the telnet server, port
23 or whatever, on one server machine).

If I understand the world of ISO correctly, *everything* has an
address (or is it a name?) which is (maybe) globally unique.
In that case what you are saying would make sense - ISO TCP
shouldn't care about the ISO IP source address some packet of
data came from.  However, DARPA TCP/IP is not built that way.
(It would, it is true, be nice if every host had exactly one "name";
then a TCP address could be <host "name", port number>.)

Greg Minshall