Basics of Networking

Defining Networking Protocols

• Python supports two interenet protocols:

  • TCP
  • UDP

• TCP protocol is a reliable connection-oriented protocol

  • Used to establish a two-way stream of communication between machines
  • Uses three-way handshake
  • More secure
  • Heavyweight
  • Slower communication

• UDP protocol is a connectionless protocol

  • Machines send a receive packets without establishing a connection
  • Used for streaming
  • Less secure
  • Lightweight
  • Faster communication
• Consequently, the TCP protocol is more popular

Defining Sockets

• Both network protocols are handled through a programming abstraction known as a socket

• A socket allows a program to do the following:

  • Receive incoming connections
  • Make outgoing connections
  • Send and receive data
• A socket is represented as an unnamed file used by the system
• In other words, a socket is an endpoint of a connection between a client and a server

• An endpoint is defined as:

  • An address (IP address + port)
  • A number of connections that will be backlogged until connections are refused
  • Other properties
• Before two machines communicate, both must create a socket object
• Essentially, sockets allow us to exchange information between processes across a network

Defining Ports

• The machine receiving a connection must bind its socket object to a known port number
• Typically, this machine is the server
• A port is a 16-bit number in the range of $0-65535$
• A port is managed by the operating system
• A port is used by clients to uniquely identify servers
• Ports $0-1023$ are reserved by the system
• FOR TCP, only one application can listen on the same port at a time

Summarizing the Network and Transport Layers

• An IP address belongs to the network layer
• A port number belong to the transport layer
• An IP address is used to identify the destination machine within a network
• A port number is used to identify the destination service within the system
• First, an IP Packet is received at the destination IP address
• Then, a port number is used by the OS to deliver the packets to the correct process
• A port number will be specified in the transport header within the IP packet
• Without the port number, the OS will have no way of knowing which process the packets should be delivered to

Why use Ports instead of Process IDs?

• You may be thinking we could use process IDs instead of ports

• You may be asking why did we create an entirely new concept when:

  • Process ids are unique
  • They identify which process is running on a machine

• However, we needed to create ports for the following reasons:

  • Process ids aren't stable

    • They can change at any time (e.g. reboot, crashes, etc.)

  • Running applications couldn't listen on multiple ports

    • Applications listen on ports ($80$, $443$, etc.)
    • If the system was bound to a process id, it couldn't listen to different ports for different protocols]

  • Process ids are inconsistent

    • If our application is running on different machines (i.e. different IP addresses), then the OS for those machines would almost always choose different process ids for the applications to run on

Establishing a TCP Connection

• Typically, a server is considered a TCP server if it is responsible for handling TCP connections

• There are two different sockets created for the following:

  • Receiving connections
  • Performing subsequent communication with the client
• Specifically, the accept() system call returns a new socket object
• This socket object is actually used for the connection
• This allows a server to manage connections from many clients simultaneously
• This involves the following steps using the socket module:

Example Server Program using socket

>>> import socket, time

>>> s = socket(AF_INET, SOCK_STREAM) # Create a TCP socket
>>> s.bind(('', 8888))               # Listen on port 8888
>>> s.listen(5)                      # Allow 5 connections

>>> while True:                      # Starts web service
...     client, addr = s.accept()    # Get a connection
...     c = 'Connection received'
...     client.send(c.encode('ascii'))
...     client.close()

Example Client Program using socket

>>> import socket
>>> s = socket(AF_INET, SOCK_STREAM) # Create a TCP socket
>>> s.connect(('localhost', 8888))   # Connect to server
>>> tm = s.recv(1024)                # Receive 1024 bytes
>>> s.close()

• Notice, text sent across the network needs to be encoded to ascii characters
• In Python 3, all strings are unicode by default
• Therefore, we need to call encode('ascii')

References

• Python Essential Reference
• Details about How Sockets are Represented
• Can Two Applications Listen to the Same Port
• What do Sockets look like?
• Why are Ports Necessary?
• Why use Ports instead of Process IDs?