Re: network programming: how does s.accept() work?

7stud wrote:
On Feb 25, 10:00 pm, Roy Smith <r...@xxxxxxxxx> wrote:
In article
<e4138617-bb4b-4f59-ab78-a49104854...@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,

7stud <bbxx789_0...@xxxxxxxxx> wrote:
But your claim that the server doesn't change its port flies in the
face of every description I've read about TCP connections and
accept(). The articles and books I've read all claim that the server
port 5053 is a 'listening' port only. Thereafter, when a client sends
a request for a connection to the listening port, the accept() call on
the server creates a new socket for communication between the client
and server, and then the server goes back to listening on the original
socket.
You're confusing "port" and "socket".

A port is an external thing. It exists in the minds and hearts of packets
on the network, and in the RFCs which define the TCP protocol (UDP too, but
let's keep this simple).

A socket is an internal thing. It is a programming abstraction. Sockets
can exist that aren't bound to ports, and several different sockets can be
bound to the same port. Just like there can be multiple file descriptors
which are connected to a given file on disk.

The TCP protocol defines a state machine which determines what packets
should be sent in response when certain kinds of packets get received. The
protocol doesn't say how this state machine should be implemented (or even
demands that it be implemented at all). It only requires that a TCP host
behave in a way which the state machine defines.

In reality, whatever operating system you're running on almost certainly
implements in the kernel a state machine as described by TCP. That state
machine has two sides. On the outside is the network interface, which
receives and transmits packets. On the inside is the socket interface to
user-mode applications. The socket is just the API by which a user program
interacts with the kernel to get it to do the desired things on the network
interface(s).

Now, what the articles and books say is that there is a listening SOCKET. And when you accept a connection on that socket (i.e. a TCP three-way
handshake is consummated on the network), the way the socket API deals with
that is to generate a NEW socket (via the accept system call). There
really isn't any physical object that either socket represents. They're
both just programming abstractions.

Does that help?

If two sockets are bound to the same host and port on the server, how
does data sent by the client get routed? Can both sockets recv() the
data?

"Routing" traditionally means passing hop-by-hop from one IP address to another, but I'll assume that you are actually concerned about delivery of packets from two separate clients - lets call them (addr1, p1) and (addrs, p2) - to two server processes both using the same endpoint address which we will call (addrS, pS). In all cases the first memebr of the tuple is an IP address and the second is a port number.

Note that the condition I mentioned earlier (with the caveat added by Roy) ensures that while addr1 and addr2 might be the same, or p1 and p2 might be the same, they can *never* be the same together: if the TCP layer at addr1 allocates port p1 to one client process, when another client process asks for an ephemeral port TCP guarantees that it wonn't be given p1, because that is already logged as in use by another process.

So, in Python terms that represents a guarantee that

(addr1, p1) != (addr2, p2)

and consequently (addr1, p1, addrS, pS) != (addr2, p2, addrS, pS)

Now, when a packet arrives at the server system addressed to the server endpoint, the TCP layer (whcih maintains a record of *both* endpoints for each connection) simply looks at the incoming address and port number to determine which process, of the potentially many using (addrS, pS), it needs to be delivered to.

If this isn't enough then you should really take this problem to a TCP/IP group. It's pretty basic, and until you understand it you will never make sense of TCP/IP client/server communications.

http://holdenweb.com/linuxworld/NetProg.pdf

might help, but I don't guarantee it.

regards
Steve

--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/

.

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