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The fourth and “middle” layer of the OSI Reference Model protocol stack is the transport layer. I consider the transport layer in some ways to be part of both the lower and upper “groups” of layers in the OSI model.

Recall that layers 1, 2 and 3 are concerned with the actual packaging, addressing, routing and delivery of data; the physical layer handles the bits; the data link layer deals with local networks and the network layer handles routing between networks. The transport layer, in contrast, is sufficiently conceptual that it no longer concerns itself with these “nuts and bolts” matters. It relies on the lower layers to handle the process of moving data between devices.

The transport layer really acts as a “liaison” of sorts between the abstract world of applications at the higher layers, and the concrete functions of layers one to three. Due to this role, the transport layer’s overall job is to provide the necessary functions to enable communication between software application processes on different computers. This encompasses a number of different but related duties

Modern computers are multitasking, and at any given time may have many different software applications all trying to send and receive data. The transport layer is charged with providing a means by which these applications can all send and receive data using the same lower-layer protocol implementation. Thus, the transport layer is sometimes said to be responsible for end-to-end or host-to-host transport (in fact, the equivalent layer in the TCP/IP model is called the “host-to-host transport layer”).

Accomplishing this communication between processes requires that the transport layer perform several different, but related jobs. For transmission, the transport layer protocol must keep track of what data comes from each application, then combine this data into a single flow of data to send to the lower layers. The device receiving information must reverse these operations, splitting data and funneling it to the appropriate recipient processes. The transport layer is also responsible for defining the means by which potentially large amounts of application data are divided into smaller blocks for transmission.

Another key function of the transport layer is to provide connection services for the protocols and applications that run at the levels above it. These can be categorized as either connection-oriented services or connectionless services. Neither is better or worse than the other; they each have their uses. While connection-oriented services can be handled at the network layer as well, they are more often seen in the transport layer in the “real world”. Some protocol suites, such as TCP/IP, provide both a connection-oriented and a connectionless transport layer protocol, to suit the needs of different applications.

The transport layer is also the place in the layer stack where functions are normally included to add features to end-to-end data transport. Where network layer protocols are normally concerned with just “best effort” communications, where delivery is not guaranteed. Transport layer protocols are given intelligence in the form of algorithms that ensure that reliable and efficient communication between devices takes place. This encompasses several related jobs, including lost transmission detection and handling, and managing the rate at which data is sent to ensure that the receiving device is not overwhelmed.

Transmission quality, meaning ensuring that transmissions are received as sent, is so important that some networking references define the transport layer on the basis of reliability and flow-control functions. However, not all transport layer protocols provide these services. Just as a protocol suite may have a connection-oriented and a connectionless transport layer protocol, it may also have one that provides reliability and data management services, and one that does not. Again, this is the case with TCP/IP: there is one main transport layer protocol, TCP, that includes reliability and flow control features, and a second, UDP, that doesn't.

Transport Layer Functions

Let’s look at the specific functions often performed at the transport layer in more detail:

• Process-Level Addressing: Addressing at layer two deals with hardware devices on a local network, and layer three addressing identifies devices on a logical internetwork. Addressing is also performed at the transport layer, where it is used to differentiate between software programs. This is part of what enables many different software programs to use a network layer protocol simultaneously, as mentioned above. The best example of transport-layer process-level addressing is the TCP and UDP port mechanism used in TCP/IP, which allows applications to be individually referenced on any TCP/IP device. 
  
  
• Multiplexing and Demultiplexing: Using the addresses I just mentioned, transport layer protocols on a sending device multiplexthe data received from many application programs for transport, combining them into a single stream of data to be sent. The same protocols receive data and then demultiplex it from the incoming stream of datagrams, and direct each package of data to the appropriate recipient application processes. 
  
  
• Segmentation, Packaging and Reassembly: The transport layer segments the large amounts of data it sends over the network into smaller pieces on the source machine, and then reassemble them on the destination machine. This function is similar conceptually to the fragmentation function of the network layer; just as the network layer fragments messages to fit the limits of the data link layer, the transport layer segments messages to suit the requirements of the underlying network layer. 
  
  
• Connection Establishment, Management and Termination: Transport layer connection-oriented protocols are responsible for the series of communications required to establish a connection, maintain it as data is sent over it, and then terminate the connection when it is no longer required. 
  
  
• Acknowledgments and Retransmissions: As mentioned above, the transport layer is where many protocols are implemented that guarantee reliable delivery of data. This is done using a variety of techniques, most commonly the combination ofacknowledgments and retransmission timers. Each time data is sent a timer is started; if it is received, the recipient sends back an acknowledgment to the transmitter to indicate successful transmission. If no acknowledgment comes back before the timer expires, the data is retransmitted. Other algorithms and techniques are usually required to support this basic process. 
  
  
• Flow Control: Transport layer protocols that offer reliable delivery also often implement flow control features. These features allow one device in a communication to specify to another that it must "throttle back" the rate at which it is sending data, to avoid bogging down the receiver with data. These allow mismatches in speed between sender and receiver to be detected and dealt with.

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.)