Re: BBC TV 4 - Hawkwind: Do not panic

Woody <usenet@xxxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:

[snip]

Here's a question: let's say I'm Web browsing. My Web browser sends a
request for a Web page over port 80 to a server. Got that - but what'
the mechanism for ensuring that the data sent back from the Web server
to the originally requesting Web browser goes to the right place?

Obviously there's an IP address to send it to the right device on the
network (with NAT getting involved and so on if needful) - but how does
the OS at the browser end sort things out?

In exactly the same way as it went out.

Umm. But the way it went out was sending out a request for data via a
port on a server. `The same way it went out' implies to me that the
server then sends a similar request to the client, on a `A.N. Other'
socket, and that's just bollocks and gibberish, innit?

A TCP header consists of a source port and a destination port (and some
other flags and counters to do with ensuring that data hasn't been
lost). The answer goes to the port where the question came from.

Aha! Cunning! Righto. The source port - is that, as Jim suggested,
`just some arbitrary bignum', or what?

The operating system on the browsers machine keeps a routing table which
makes an entry that the web browser sent a request out on a certain
port, to a destination, so an answer from that destination goes back to
the web browser.

Gotcha.

Like everything though, this is a simplistic overview,

Well, yeah, but it's the basic principles I'm trying to get the hang of.
Seems to do the job - at least, I think I'm understanding it well enough
to be getting on with, so that'll do.

and there is a
lot going on behind the scenes because it is on a TCP stream connection
which has to be set up (which involves more communication between the
source and destination),

I'll leave all that to the machinery ;-)

but the effect is the same either way.

Righto - ta.

Cheers,
Rowland.

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