[Edu-sig] Re: socket-module

[Jeremy Hylton]

>>>>> "A" ==   <A> writes:

  A> A stupid question maybe. I want to find the parts where is
  A> described HOW a TCP-packet is created. Something like:

  A> -IP from sender 
  A> -IP from receiver 
  A> -data

  A> Is it possible?

The socket interface hides the all the details of TCP/IP from you.  If
you call send on a socket, the socket library implementation handles
all the details.  It handles buffering of the data, putting into
appropriately-sized TCP packets, retransmission of packets, etc.

If you're interested in TCP at that level of detail, you're going to
have to look at a TCP stack implementation in the OS kernel.  I
*strongly* recommend W. Richard Steven's TCP/IP Illustrated series if
you go this route.

http://www.kohala.com/start/tcpipiv1.html -- protocols
http://www.kohala.com/start/tcpipiv2.html -- BSD implementation

One thing you can play with is sending raw IP packets, using the a
socket create with the SOCK_RAW protocol.  I believe this just works
on Windows, but requires a setuid root script on Unix. An example:

s = socket.socket(socket.AF_INET, socket.SOCK_RAW,
                  socket.IPPROTO_ICMP)

The OS kernel still does some work for you in this case.  The only
example I can think of is that it computes the checksum for the
packet.

My ping and traceroute code in Python, while a little buggy and out of
date, might get you started.  http://www-tech.mit.edu/~jeremy/python/

Jeremy