Multiplexing and Demultiplexing Applications, Computer Networking

Multiplexing and Demultiplexing Applications

In this topic we discuss the multiplexing/demultiplexing of messages by the transport layer from/to the application layer. In order to keep the discussion concrete, we'll discuss this basic service in the context of the Internet's transport layer. We emphasize, however, that multiplexing and demultiplexing services are provided in almost every protocol architecture ever designed. Moreover, multiplexing/demultiplexing are generic services, often found in several layers within a given protocol stack. Although the multiplexing/demultiplexing service is not among the most exciting services that can be provided by a transport layer protocol, it is an absolutely critical one. To understand why it so critical, consider the fact that IP delivers data between two end systems, with each end system identified with a unique IP address. IP does not deliver data between the application processes that run on these end systems. Extending host-to-host delivery to a process-to-process delivery is the job of the transport layer's application multiplexing and demultiplexing service.

At the destination host, the transport layer receives segments (i.e., transport-layer PDUs) from the network layer just below. The transport layer has the responsibility of delivering the data in these segments to the appropriate application process running in the host. Let's take a look at an example. Suppose you are sitting in front of your computer, and you are downloading Web pages while running one FTP session and two Telnet sessions. You therefore have four network application processes running -- two Telnet processes, one FTP process, and one HTTP process. When the transport layer in your computer receives data from the network layer below, it needs to

direct the received data to one of these four processes. Let's now examine how this is done.

Each transport-layer segment has a field that contains information that is used to determine the process to which the segment's data is to be delivered. At the receiving end, the transport layer can then examine this field to determine the receiving process, and then direct the segment to that process. This job of delivering the data in a transport-layer segment to the correct application process is called demultiplexing. The job of gathering data at the source host from different application processes, enveloping the data with header information (which will later be used in demultiplexing) to create segments, and passing the segments to the network layer is called multiplexing.

To illustrate the demultiplexing job, let us return to the household saga in the previous section. Each of the kids is distinguished by his or her name. When Bob receives a batch of mail from the mail person, he performs a demultiplexing operation by observing to whom the letters are addressed and then hand delivering the mail to his brothers and sisters. Alice performs a multiplexing operation when she collects letters from her brothers and sisters and gives the collected mail to the mail person.

UDP and TCP perform the demultiplexing and multiplexing jobs by including two special fields in the segment headers: the source port number field and the destination port number field.