Layer 4: The Transport Layer

The Transport layer of the OSI model is associated with reliable delivery. With reliable delivery, the sender and receiver establish a connection, and the receiver acknowledges receipt of the data by sending an acknowledgment (ACK) packet to the sender.

Reliable delivery is a concept that can be applied to the telephone system. You place a call to a friend hoping that they answer. When they answer, you begin a conversation. If everything is going well, you will each take turns talking. But if the connection is bad, you may not hear a question that was asked. In that case, your friend, having not received an answer, would ask you again.

Most protocols have two mechanisms for sending data through the Transport layer: connection-oriented transmission and connectionless transmission.

Connection-oriented transmission:

• Is reliable

• Involves more overhead, so is slower and less efficient

• Can provide guaranteed file transfer

  

Connectionless transmission:

• Is less reliable

• Involves less overhead, so is faster and more efficient

• Does not resend packets

  

Other Transport Layer Functions

Data moves down the layers of the OSI model in large chunks. After data reaches the Transport layer, it is segmented into smaller units that do not exceed the maximum packet length specified by the network in use. The Transport layer is responsible for dividing and then reassembling the data back into the original order at the receiver's end.

The Transport layer has the capability to perform some error control. If a particular unit of data does not arrive at the receiver's end, the Transport layer can request that the missing data be retransmitted.

This layer of the OSI model also performs some flow control. The sender and receiver determine how much data can be sent before an acknowledgment is required.

Note 

Data at the Transport layer is referred to as a datagram, segment, or packet.

Network and Transport Layer Technologies

The Network and Transport layers of the OSI model work closely together. Many protocols function at these layers. Two are presented below:

TCP/IP  The leading industry protocol for the Internet. Transmission Control Protocol (TCP) functions at the Transport layer and Internet Protocol (IP) functions at the Network layer.

IPX/SPX  A proprietary protocol developed by Novell for their NetWare software. With Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX), IPX functions primarily at the Network layer, and SPX functions at the Transport layer.