WEEK 5 CS2005 Transport Layer Concepts in Networks

CS2005 Networks and Operating Systems Transport Layer I

• J.F Kurose and K.W. Ross 1996-2016

Session Objectives

• To understand:

  • Transport layer services and protocols

  • Multiplexing and demultiplexing

  • Connectionless transport

  • User Datagram Protocol (UDP)

Transport Layer Services and Protocols

• Provides logical communication between application processes across different hosts.

• Implemented in end systems, not in network routers.

Sending Side Process

• Converts application layer messages into transport layer packets (segments).

• May break messages into smaller chunks and add a transport layer header.

• Segments passed to network layer, encapsulating them into datagrams.

Receiving Side Process

• Network layer extracts transport layer segment from datagram.

• Transport layer processes the segment to make data available to the application.

• Network routers only act on network layer fields and do not examine transport-layer segments.

Relationship Between Transport Layer and Network Layer

• Transport layer: logical communication between processes.

• Network layer: logical communication between hosts.

• Transport layer enhances network layer services.

• IP service model is a best-effort delivery service.

Available Transport Protocols

• Two main protocols available on the Internet:

  • Transmission Control Protocol (TCP)

    • Reliable, in-order delivery.

    • Includes connection setup, flow control, and congestion control.

  • User Datagram Protocol (UDP)

    • Unreliable, unordered delivery (no extension of IP's best effort).

Transport Layer Services to Applications

• Process-Level Addressing

  • Differentiates between application processes.

  • Network layer addressing identifies hosts.

• Multiplexing and Demultiplexing

  • Sender side: multiplexing data from various processes.

  • Receiver side: demultiplexing received data to appropriate processes.

• Integrity Checking

  • Error detection mechanisms.

• Connection Management

  • Establishment and termination processes (TCP specific).

• Acknowledgments and Retransmissions

  • Important for reliable service (TCP specific).

• Flow Control

  • Ensures in-order delivery service (TCP specific).

Multiplexing and Demultiplexing

• Extends host-to-host delivery to process-to-process delivery for applications.

Sending Host Functions

• Multiplexing: Gathering data from different sockets, creating segments, and passing them to the network layer.

Destination Host Functions

• Demultiplexing: Delivering received segments to the correct socket.

• Each socket has a unique identifier.

Connectionless Demultiplexing (UDP)

• Identified by a 2-tuple: destination IP address and destination port number.

• Different source IPs or ports to the same destination will connect to the same process.

Connection-Oriented Demultiplexing (TCP)

• Identified by a 4-tuple: source IP address, source port number, destination IP address, and destination port number.

• Uses all four values for proper demultiplexing to the correct socket, differing from UDP in that different source tuples will connect to different sockets.

Web Server Usage of Port Numbers

• Web server (e.g., Apache) running on port 80 handles segments directed to it from clients.

• Distinguishes segments from clients using source IP addresses and port numbers.

• Each connection can spawn a new process or thread with a unique connection socket.

Persistent vs Non-Persistent HTTP

• Persistent HTTP: Client and server exchange messages via the same server socket throughout connection.

• Non-Persistent HTTP: New TCP connection and socket created for each request/response cycle, potentially impacting performance negatively.

Connectionless Transport: User Datagram Protocol (UDP)

• Provides mux/demux service to direct data to correct application-level processes.

• Only adds light error checking to IP functionality.

UDP Segment Structure

• Length Field: Indicates the total number of bytes in the UDP segment (header plus data).

• Checksum: Used for error checking to verify data integrity during transmission.

  • Sender computes checksum and receiver verifies against received checksum value.

Reading Material Overview

• Introduction to transport-layer services focusing on:

  • Relationship between transport and network layers

  • Multiplexing and demultiplexing

  • Connectionless transport, UDP, UDP segment structure, and UDP checksum.