3 – Socket Programming

Socket is the interface between the application and transport layer.

Port number is a 16-bit integer (0 to 65535) identifier for a socket with IP address. (Note: 0 to 1023 are reserved for standard use. (HTTP service, DNS service etc.)

IP address is an identifier, not for a host but for a network interface. (TBC in IP layer)

2 socket types:

• UDP: Unreliable datagram, connectionless
• TCP: reliable byte stream, connection-oriented

UDP

No connection:

• No handshaking, but every packet needs to include address information
• Sender (client) explicitly attaches IP destination (server) address and port number on every packet sent
• Receiver (server) extracts sender IP and port number from received packet
• Data may be lost, or received out of order.
  • Unreliable groups of bytes (datagrams)

Note: The term datagrams is used to refer to data at the IP layer, in transport layer it’s called segments, in application layer, messages. Different terminologies at different layers.

High level program

Client code

from socket import *
serverName = 'hostname'
serverPort = 12000

# automatically creates random feasible port number for you here.
clientSocket = socket(AF_INET, SOCK_DGRAM)
message = input("input message: ")
# attach server name, port to header and send bytes to server
clientSocket.sendTo(message.encode(), (serverName, serverPort))
# 2048 is the upper limit on the size of the response/buffer size.
response, serverAddress = clientSocket.recvfrom(2048) 

print(response.decode())
clientSocket.close()

AF_INET is the address family for IPv4, SOCK_DGRAM to specify that we are creating a datagram socket for UDP, not TCP.

Does server not require client info (port number, IP)? No it does! The port number is generated in socket(AF_INET, SOCK_DGRAM)

Server code

from socket import *
serverPort = 12000

serverSocket = socket(AF_INET, SOCK_DGRAM)
# bind socket to localhost "", local port number 12000
serverSocket.bind(("", serverPort))

while True:
    # clientAddress = (IP address, port number)
    msg, clientAddress = serverSocket.recvfrom(2048)
    modifiedMessage = message.decode().upper()
    serverSocket.sendto(modifiedMessage.encode(), clientAddress)

Note: Binding the port number to the socket is “assigning” anyone who sends a packet to 12000 to this socket. This ensures a fixed port number for senders to request from.

UDP server enters a while loop to receive and process packets. This omits a lot of the burdens that are on the developer to handle:

• order of the data to be read then, as there are a lot of processes reading from the same socket
• concurrency
• preventing starvation

DNS servers use UDP to send/received requests for IP addresses.

TCP

For each client:

• after it sends the initial request through server’s welcoming socket
• server must create a unique connection socket to welcome that client’s contact
  • server uses the source port numbers (each unique socket’s) to distinguish client.
• the client creates TCP socket, specifies IP address, port number of server: creating the socket establishes the connection to TCP server
• data in reliable, in-order byte-stream transfer

The welcoming socket is not the same as the socket for subsequent communicating with the client. Hence there is a three-way handshake.

Client code

from socket import *
serverName = 'hostname'
serverPort = 12000

# SOCK_STREAM for stream connection instead of datagrams
clientSocket = socket(AF_INET, SOCK_STREAM)
# connect to establish TCP connection
clientSocket.connect((serverName, serverPort))
message = input("input message: ")
# send won't need to attach server name/port
clientSocket.send(message.encode())
response, serverAddress = clientSocket.recvfrom(2048) 

print(response.decode())
clientSocket.close()

Server code

from socket import *
serverPort = 12000

serverSocket = socket(AF_INET, SOCK_STREAM)
serverSocket.bind(("", serverPort))
# the welcoming/serverSocket listens for TCP requests
serverSocket.listen(1)

while True:
    # server blocks on accept until incoming request is received
    # a connectionSocket will be created.
    connectionSocket, addr = serverSocket.accept()
    message = connectionSocket.recvfrom(2048).decode()
    connectionSocket.send(message.upper().encode())
	connectionSocket.close()