Re: BBC TV 4 - Hawkwind: Do not panic

On 2007-04-01 18:11:33 +0100, real-address-in-sig@xxxxxxxxxxxxxxx (Rowland McDonnell) said:

Chris Ridd <chrisridd@xxxxxxx> wrote:

(Rowland McDonnell) said:

Jaimie Vandenbergh <jaimie@xxxxxxxxxxxxxxxxxxxxx> wrote:

Close enough, because that's all they are: an abstract addressing
system designed to get an incoming request to the correct application
to respond to it.

Well, yes, but how *exactly* is it all managed? You've explained some
of the missing info above - I'm getting it now.

Very simply - there's a list of "well known" ports that everyone
understands run particular services.

Umm. But that's how people look up which port to type in to their
program code. It doesn't explain how the OS and applications manage the
data transfer in practice, and that's what I'm trying to get my head
round.

Oh, that's straightforward too.

Server processes actively have to tell the OS they want to listen to the network on port such-and-such (also whether it is UDP or TCP etc). The OS therefore has a list of what addresses (remember Macs support multiple network interfaces with multiple addresses) and ports being listened to by particular processes, so when a patching packet comes in, it knows where to send it to.

The same procedure applies to client processes, except they say connect me to port such-and-such at address a.b.c.d, and the OS obliges.

How so? If you've got a computer with no Web server running on it
(i.e., not the normal server listening on socket 80), and you set up the
Telnet server to listen on socket 80, surely it should all `just work'
if you then tell your Telnet client to connect on socket 80?

Yes, that would certainly work.

Cheers,

Chris

.

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