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

On Feb 25, 10:56 am, Thomas Bellman <bell...@xxxxxxxxxxxxxx> wrote:
7stud <bbxx789_0...@xxxxxxxxx> wrote:
The question I'm really trying to answer is: if a client connects to a
host at a specific port, but the server changes the port when it
creates a new socket with accept(), how does data sent by the client
arrive at the correct port?  Won't the client be sending data to the
original port e.g. port 5052 in the client code above?

The answer is that the server *doesn't* change its port.  As you
could see in the output of your server, the socket that accept()
returned also had local port 5052.  Each *client* will however
get a unique local port at *its* end.

A TCP connection is identified by a four-tuple:

    ( localaddr, localport, remoteaddr, remoteport )

Note that what is local and what is remote is relative to which
process you are looking from.  If the four-tuple for a specific
TCP connection is ( 127.0.0.1, 5052, 127.0.0.1, 50816 ) in your
server, it will be ( 127.0.0.1, 50816, 127.0.0.1, 5052 ) in the
client for the very same TCP connection.

Since your client hasn't bound its socket to a specific port, the
kernel will chose a local port for you when you do a connect().
The chosen port will be more or less random, but it will make
sure that the four-tuple identifying the TCP connection will be
unique.


You seem to be describing what I see:

----server output-----
original socket: ('0.0.0.0', 5053)
new socket, self: ('127.0.0.1', 5053)
new socket, peer: ('127.0.0.1', 49302)

original socket: ('0.0.0.0', 5053)
new socket, self: ('127.0.0.1', 5053)
new socket, peer: ('127.0.0.1', 49303)

---client1 output-----
('0.0.0.0', 0)
('127.0.0.1', 49302)

---client2 output-----
('0.0.0.0', 0)
('127.0.0.1', 49303)


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. Here are two sources for that claim:

Socket Programming How To:
http://www.amk.ca/python/howto/sockets/

Tutorial on Network Programming with Python:
http://heather.cs.ucdavis.edu/~matloff/Python/PyNet.pdf

In either case, there are still some things about the output that
don't make sense to me. Why does the server initially report that its
ip address is 0.0.0.0:

original socket: ('0.0.0.0', 5053)

I would expect the reported ip address to be '127.0.0.1'. Also, since
a socket is uniquely identified by an ip address and port number, then
the ('0.0.0.0', 5053) socket is not the same as this socket:

new socket, self: ('127.0.0.1', 5053)
.

Relevant Pages

  • Re: Socket switch delay
    ... both at the client and at the server (and why ... would you set the send buffer size to zero on a non-overlapped ... One glaring error is your client does ... So when you use a single socket, ...
    (microsoft.public.win32.programmer.networks)
  • Re: Final Confusions on Concurrent Server
    ... the client is using ... Final Confusions on Concurrent Server ... listener socket). ... would think already knows what listener sockets are mapped to what port. ...
    (bit.listserv.ibm-main)
  • Re: Locking on async calls
    ... you must synchronize the entire SendMessage routine as an atomic ... operation to prevent mixed messages from being transmitted to the server. ... You are correct that read and write on the socket do not interfere with each ... If you want to handle multiple client connections from one server object ...
    (microsoft.public.dotnet.general)
  • Re: socket communication: socket doesnt connect
    ... Microsoft MVP, MCSD ... As far as your server code goes, ... accept the listening socket. ... Client client = new Client; ...
    (microsoft.public.vc.language)
  • Re: TCP server stop receiving new connections
    ... reset the event mask of your listening socket each time you ... I have a strange problem in my class library used by all our client ... server applications. ... incomming connections, but keeps current connections. ...
    (microsoft.public.win32.programmer.networks)