Re: Server Ports

From: Karl Levinson [x y] mvp (levinson_k_at_despammed.com)
Date: 12/01/03


Date: 1 Dec 2003 14:51:53 -0800

I would question what that manual is and what they are talking about
[or whether they know what they are talking about].

This is not the case with normal HTTP web servers or SMTP servers.
[The quote would seem to me to be poorly written, inaccurate and made
unnecessarily complicated.]

I don't understand the sentence stating "That is how a web server can
listen for (and handle) thousands of connections from users." TCP
connections are uniquely identified not just by source and destination
port but also by source and destination IP address. A connection from
you:1024 --> www.microsoft.com:80 need not be confused with
someone_else:1024 --> www.microsoft.com:80

I'm not sure I see how switching connections to a different port
number improves either performance or number of simultaneous
connections possible. [Some FTP servers do work in a manner similar
to the one described below.]

You can use a sniffer such as Ethereal and/or launch Telnet such as
TELNET YOURMAILSERVER 25 to see the ports being used and see that
the port number does not change.

"ClareOldie" <ClareOldie@nowhere.ie> wrote in message news:<O0MyIJBuDHA.3536@tk2msftngp13.phx.gbl>...

> the last part of the below quote from the LanSuite
> manual is what had me confused. Any comments??
>
> Quote from LanSuite manual:
>
> You might think that the application sending e-mail uses port 25, but that
> is not the case. The usual procedure involves an application requesting and
> being given a socket by the operating system; that is, it asks for and
> receives a port. Any port will do (the application doesn't even need to know
> what the exact port number is), but the operating system will issue a port
> from somewhere above 1023. This port is used briefly, and then returned to
> the pool for another application to use later. The application sending the
> e- mail, using a port above 1023 sends a connection request to the standard
> port. When the connection is established, part of the information in each
> packet is the source IP address and port as well as the destination IP
> address and port. The port above 1023 is the source port; the standard port
> is the destination port. The destination machine will return packets using
> the original port above 1023 as its destination port. Although this sounds
> complicated, the underlying principle is easy to grasp: when a program uses
> a port above 1023, replies arrive back at that same port. Here's one last
> bit of complexity.
> ******** Since standard listening ports are for everybody, the
> destination machine does not actually use it for data transfer. It only
> listens on that port. As soon as a connection is established it hands that
> connection to a local port above 1023 and immediately resumes listening for
> a new incoming connection request on the standard port. That is how' a web
> server can listen for (and handle) thousands of connections from users.
>
> End Quote



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