The Transport Layer

The Transport Layer
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

A Review

Application Layer:
- Involves user requests
Transport Layer:
- Responsible for between processes/applications on different hosts
Network/Internetwork Layer:
- Responsible for routing and delivering frames/datagrams

The Abstraction

Processes/applications use the services provided by the transport layer without worrying about the details of the lower levels.

Implementation of Transport Layer Protocols

Where is the code?
- It resides on the host computers, not on routers
The Sender:
- Process/application sends messages to the transport layer
- Transport-layer software creates segments (each with a header)
- Transport-layer gives the segments to the network-layer software which transmits them
The Receiver:
- The network-layer software receives the datagrams
- The network-layer software gives the segments contained in the datagrams to the transport-layer software
- The transport-layer software gives the segments to the appropriate process/application

An Analogy

The Situation:
- You want to send a letter to a friend at another university
- The address includes a postal code that is specific to the University and your friend's box number
The Network Layer:
- The U.S. postal service delivers the letter from your university to your friend's university (through a variety of intermediate points)
The Transport Layer:
- Your university's mail system gave the letter to the U.S. postal service
- Your friend's university's mail system got the letter from the U.S. postal service and delivered it to your friend