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The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  OSI Reference Model Layers

Network Layer (Layer 3) 1 2 3 Session Layer (Layer 5)

Relationship Between the Transport Layer and Network Layer

In theory, the transport layer and network layer are distinct, but in practice, they are often very closely related to each other. You can see this easily just by looking at the names of common protocol stacks—they are often named after the layer three and four protocols in the suite, implying their close relationship. For example, the name “TCP/IP” comes from the suite’s most commonly used transport layer protocol (TCP) and network layer protocol (IP). Similarly, the Novell NetWare suite is often called “IPX/SPX” for its layer three (IPX) and layer four (SPX) protocols. Typically, specific transport layer protocols use the network layers in the same family. You won't often find a network using the transport layer protocol from one suite and the network layer protocol from another.

The most commonly used transport layer protocols are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) in the TCP/IP suite, the Sequenced Packet Exchange (SPX) protocol in the NetWare protocol suite, and the NetBEUI protocol in the NetBIOS/NetBEUI/NBF suite (though NetBEUI is more difficult to categorize.)

Network Layer (Layer 3) 1 2 3 Session Layer (Layer 5)