Download sock - Kansas State University

Introduction to Network
Programming with Sockets
Network Programming
Kansas State University at Salina
Protocols
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Rules that determine how communication is
achieved.
Human protocols: vocal cords to ears, sign
language to eyes, English, French,
Spanish…
Layers of protocols – may replace any
protocol with another functionally equivalent.
Network protocols – Ethernet, IP, TCP, UDP,
HTTP, SMTP, etc…
TCP/IP
OSI Model

TCP/IP
Transmission
Control
Protocol/Internet
Protocol (TCP/IP)
 Suite of small,
specialized
protocols called
subprotocols
TCP/IP compared to the OSI Model
Internet Protocol (IP)

IP datagram
 IP portion of
TCP/IP
frame that
acts as an
envelope for
data
 Contains
information
necessary
for routers to
transfer data
between
subnets
Components of an IP datagram
Transport Control Protocol (TCP)
TCP provides reliable end to end communication.
A TCP
segment
User Datagram Protocol (UDP)

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Connectionless transport service
Only four fields in UDP header (10 in TCP
header)
Unreliable
Used for broadcast applications and
voice/video data.
Getting Data from Source to
Destination

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IP address – IP header
Protocol (TCP or UDP) – IP header
Port – TCP or UDP protocol
A port is a rendezvous point between TCP
and the application.
A socket allows the application to read from
and write to the port.
Postal Mail Analogy
• IP gets the data to the
right place.
• TCP puts the data in
the right port (mail
box).
• The socket goes and
gets the data from the
port (mail box) and
delivers it to the
application.
Sockets
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API for applications to read and write data
from TCP/IP or UDP/IP
File abstraction (open, read, write, close)
Abstract operating system resource.
First introduced with BSD UNIX
De-facto standard API for TCP/IP
Ports

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Well Known Ports are in the range of 0 to
1023 only the operating system or an
Administrator of the system can access
Registered Ports are in the range of 1024 to
49151.
Dynamic and/or Private Ports are those from
49152 through 65535 and are open for use
without restriction
The Socket API Commands
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See example session
Initialize
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sock = socket.socket(socket.AF_INET,
socket.SOCK_STREAM)
Will always use above options.
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First option says it is IP version 4 (another value for IP
version 6), in UNIX it can also be set to a named pipe.
Second option says that it will connect to a TCP port.
These names are the original ones used.
Socket API (cont.) Server Side

sock.bind(( hostname, port ))
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Hostname can be one of: socket.gethostname(),
‘localhost’ or the machine host name as a string.

The port should usually be between 1024 to
49151.
Can not bind to a port with another socket bound
to it.
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Note the double parenthesis, bind() takes a single
tuple argument. The extra set of parenthesis
makes the two values a tuple.
Socket API (cont.) Server Side

sock.listen(n)
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Tells the socket to listen for incoming connections
n here is an integer for number of new
connections to hold in queue. For now, we can
set n to 1.
(newsock, address) = ss.accept()
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Wait for a connection – blocking
Returns a tuple of a new allocated socket and the
IP address and port number of the client. The
address is a tuple.
A new socket is allocated because a server will
usually want to continue listening to the old
socket.
Socket API (cont.) Client Side

sock.connect(( hostname, portNumber ))
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Connect to a server that is listening for
connections
For most exercises in this class, just set hostname
to ‘localhost’.
portNumber should match the port number that
the sever was bound to.
A blocking statement, until the connection is
established.
Socket API (cont.)
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message = sock.recv(N)
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sock.send( msg )
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Read a message from the socket and return a string.
N is the maximum message size in characters.
Blocking
Send a string messages (msg) over an established socket
connection.
sock.close()

Always a good idea to close sockets when finished with
them.