Networking - The TCP/IP Five-Layer Model - The Transport Layer

The Transport layer of the TCP/IP model sits above the Network layer and provides end to end communication services between processes running on different hosts. It is concerned with conversations rather than individual packets.

In particular, the Transport layer is responsible for:

• Segmenting application data into units suitable for transmission and reassembling them at the destination,
• Offering reliable or unreliable delivery as required by applications, and
• Multiplexing many logical conversations over a single Network layer path using port numbers.

The two best known Transport layer protocols in TCP/IP are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). TCP provides connection oriented, reliable service, while UDP provides connectionless, best effort delivery.

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

1. Connection Establishment and Termination (for connection oriented transport)
   • Setting up a session with three way handshakes,
   • Managing state information for the duration of the connection, and
   • Closing the connection in an orderly fashion.
2. Reliability and Flow Control
   • Numbering and acknowledging segments,
   • Retransmitting lost or damaged data, and
   • Adjusting the rate of transmission so that fast senders do not overwhelm slow receivers.
3. Multiplexing with Port Numbers
   • Using source and destination port numbers to distinguish among many simultaneous conversations, and
   • Allowing multiple applications on the same host to share the same Network layer connection.

Let us move on to layer 5, the Application Layer.

Nah, I want to skip around:

• Introduction
• Physical (Layer 1)
• Data Link (Layer 2)
• Network (Layer 3)
• Transport (Layer 4)
• Application (Layer 5)
• Conclusion

Copyright 1999, Marc Elliot Hall, DBA Sensation! Services