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Source port was what I had forgotten.

Thanks again for the enlightenment.

Have a great Thanksgiving.

John McKee

-----Original message-----
From: Scott Klement midrange-l@xxxxxxxxxxxxxxxx
Date: Wed, 25 Nov 2009 17:55:32 -0600
To: Midrange Systems Technical Discussion midrange-l@xxxxxxxxxxxx
Subject: Re: LPR over SSH

John,

You understand that there are two IP addresses and two ports on every
connection, right? Not just one as you seem to be implying? Every
packet has a source address & source port, as well as a destination
address and destination port. You rarely hear people talk about the
"source port", as it's usually managed by the OS and will vary with each
connection.

But, basically, in TCP it creates a "tuple".
SrcAddr/SrcPort/DestAddr/DestPort. I believe there's also some sort of
TCP sequence number involved as well. These things are used to identify
a specific TCP session in the TCP/IP stack of your operating system.

I don't know if SSH uses a separate TCP session for everything, or if it
multiplexes within one. If it multiplexes, then it doubtless includes
it's own fields for this as well.

At any rate, I know that it works... I don't need to fully understand
the mechanism! :)


jmmckee wrote:
Hi Scott,

Thank you for the clarification. My confusion comes from thinking
that the packets would need to be separated, either by adifferent IP
address, port, or both. If an sftp session is initiated on the i to
the remote location, on the default port, how is an additional
session created, on the default port, when the IP addresses on both
ends are the same, and yet the packets get routed to the appropriate
connection? I thought the connection was the port. If I understand
the process correctly, in addition to ip address and port, there is
also a connection number?

John McKee

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