Re-cap: Socket programming

Re-cap: Socket programming

• Goal: learn how to build client/server applications that communicate using sockets

• Socket: door between application process and end-end-transport protocol

Re-cap: Socket programming

Two socket types for two transport services:

1. UDP: unreliable datagram
2. TCP: reliable, byte stream-oriented

Re-cap: UDP

• UDP: no “connection” between client & server
  • No handshaking before sending data
  • Sender explicitly attaches IP destination address and port # to each packet
  • Receiver extracts sender IP address and port# from received packet
• UDP: transmitted data may be lost or received out-of-order
• Application viewpoint:
  • UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server

Example app: UDP client

Application Example:

1. Client reads a line of characters (data) from its keyboard and sends data to server
2. Server receives the data and converts characters to uppercase
3. Server sends modified data to client
4. Client receives modified data and displays line on the screen

Example app: UDP client

import socket

def start_client():
    serverName = ""
    serverPort = 8800
    clientSocket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

    message = input("Input lowercase sentence: ")

    clientSocket.sendto(message.encode(), (serverName, serverPort))
    modifiedMessage, serverAddress = clientSocket.recvfrom(1024)
    print(modifiedMessage.decode())
    clientSocket.close()

if __name__ == "__main__":
    start_client()

Example app: UDP server

import socket

def start_server():
    serverPort = 8800
    serverSocket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) 
    serverSocket.bind(("", serverPort))
    print("The server is ready to receive")
    while True: 
        message, clientAddress = serverSocket.recvfrom(1024)
        modifiedMessage = message.decode().upper() 
        serverSocket.sendto(modifiedMessage.encode(), clientAddress)

if __name__=="__main__":
    start_server()

Re-cap: TCP

Application viewpoint:

• TCP provides reliable, in-order byte-stream transfer (“pipe”) between client and serve

Re-cap: TCP

• Server process must first be running and must have created socket that accepts client’s connection
• Client contacts server by:
  • Creating TCP socket, specifying IP address, port number of server process
  • When client creates socket: client TCP establishes connection to server TCP
• When contacted by client, server TCP creates new socket for server process to communicate with that particular client
  • Allows server to talk with multiple clients
  • Source port numbers used to distinguish clients

Outline

• Transport-layer services
• Multiplexing and demultiplexing
• Connectionless transport: UDP
• Principles of reliable data transfer

Re-cap: Layer Model

Transport services and protocols

• Provide logical communication between app processes running on different hosts
• Transport protocols run in end systems
  • Send side: breaks app messages into segments, passes to network layer
  • Recieve side: reassembles segments into messages, passes to app layer
• More than one transport protocol available to apps
  • Internet: TCP and UDP

Transport services and protocols

</img>

Transport vs. network layer

Network layer:

• Logical communication between hosts

Transport layer:

• Logical communication between processes
  • Relies on & enhances network layer services

Internet transport-layer protocols

TCP (Transmission Control Protocol)

• Reliable, in-order delivery
• Congestion control, flow control, ack., timer
  </br>

  UDP (User Datagram Protocol)

• Unreliable, unordered delivery
• No-frills extension of IP
  </br>

Multiplexing/demultiplexing

Multiplexing/demultiplexing

Multiplexing at sender

• Handle data from multiple sockets, add transport header (later used for demultiplexing)
  </br>

Demultiplexing at receiver

• Use header info to deliver received segments to correct socket

Multiplexing/demultiplexing

</img>

Multiplexing/demultiplexing

Example

- Host C initiates 2 HTTP</br> sessions to B - Host A initiates 1 HTTP</br> session to B - Server B demultiplexes
correctly as the two</br> identical port numbers</br> come from different IP addresses </img>

TCP/UDP Segment format

• Host receives IP datagrams
• Each datagram has source IP address, destination IP address
• Each datagram carries one transport-layer segment
• Each segment has source, destination port number

TCP/UDP Segment format

- Host uses IP addresses</br> & port numbers to direct </br> segment to appropriate</br> socket - Each socket has a unique identifier </img>

Connectionless multiplexing/demultiplexing

• All sockets have host-local port #
• Assigned automatically, or via bind()
• serverSocket.bind((ip,port))
  </br>

Connectionless multiplexing/demultiplexing

• When host receives UDP segment:
  • Checks destination port # in segment
  • Directs UDP segment to socket with that port #
    </br>

Connectionless multiplexing/demultiplexing

- serverSocket.bind((’’, 6428)) - clientSocket = socket(...)</br> #assigned port 9157 - clientSocket = socket(...)</br> #assigned port 5775 </img>

Reliable data transfer

Reliable data transfer

• Important in application, transport, link layers
  • Top-10 list of important networking topics
• Characteristics of unreliable channel will determine the complexity of a reliable data transfer protocol (rdt)

Reliable data transfer

</img>

Reliable data transfer

• A reliable protocol (TCP) may be implemented on top of an unreliable end-to-end network layer (IP)
• Reliable transfer over UDP:
  • Add reliability at application layer
  • Application-specific error recovery

Summary

• Principles behind transport layer services
• Multiplexing, demultiplexing
• Connectionless transport
• UDP: datagram protocol
• UDP segment format and checksum
• Connection-oriented transport
• Basic structure of reliable data transfer