Networking - The OSI Seven-Layer Model - The Transport Layer

The purpose of the Transport layer is to provide end to end communication services between applications running on different hosts. Where the Network layer is concerned with individual packets, the Transport layer is concerned with complete conversations.

The Transport layer is responsible for:

• Segmenting and reassembling application data into smaller units for transmission,
• Providing reliable or unreliable delivery services as required,
• Controlling the rate of data flow so that fast senders do not overwhelm slow receivers, and
• Multiplexing many logical conversations over a single Network layer connection.

Unlike the lower layers, the Transport layer is usually implemented entirely in software within the end systems. Dedicated devices are rare, but some sophisticated firewalls and multilayer switches inspect and filter Transport layer information.

The Transport layer is responsible for several processes, each of which is responsible for a series of methods:

1. Segmentation and Reassembly
   • Breaking application messages into segments that fit within the Network layer maximum transmission unit, and
   • Reassembling those segments at the receiving host in the proper order.
2. Connection Services
   • Connection oriented transport
     • Three way handshakes to establish sessions,
     • Orderly connection teardown, and
     • Use of sequence numbers and acknowledgments to keep track of data.
   • Connectionless transport
     • Best effort delivery without prior setup,
     • Useful when applications provide their own recovery, and
     • Often used for real time or simple query response protocols.
3. Reliability and Error Recovery
   • Positive acknowledgments and retransmissions,
   • Checksums to detect corrupted segments, and
   • Timers to detect lost data.
4. Flow Control
   • Windowing techniques that allow multiple segments to be in transit at once,
   • Dynamic adjustment of window sizes in response to congestion, and
   • Graceful reduction of sending rate when necessary.
5. Multiplexing
   • Use of port numbers to distinguish among multiple applications on the same host, and
   • Simultaneous support for many logical connections between the same pair of hosts.

Let us move on to layer 5, the Introduction

Copyright 1999, Marc Elliot Hall, DBA Sensation! Services