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Transport Layer (Layer 4)
(Page 3 of 3)
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.)
 Key Concept: The fourth and middle OSI Reference Model layer is the transport layer. This is another very important conceptual layer in the model; it represents the transition point between the lower layers that deal with data delivery issues, and the higher layers that work with application software. The transport layer is responsible for enabling end-to-end communication between application processes, which it accomplishes in part through the use of process-level addressing and multiplexing/demultiplexing. Transport layer protocols are responsible for dividing application data into blocks for transmission, and may be either connection-oriented or connectionless. Protocols at this layer also often provide data delivery management services such as reliability and flow control.
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