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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets

TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets 1 2 3 TCP/IP Ports: Transport Layer (TCP/UDP) Addressing

Multiplexing and Demultiplexing

Most communication in TCP/IP takes the form of exchanges of information between a program running on one device, and a matching program on another device. Each instance of an application represents a copy of that application software that needs to send and receive information. These application instances are commonly called processes. A TCP/IP application process is any piece of networking software that sends and receives information using the TCP/IP protocol suite. This includes both “classic” end-user applications such as the ones described above, as well as support protocols that behave as applications when they send messages. Examples of the latter would include a network management protocol like SNMP, or even the routing protocol BGP (which sends messages using TCP like an application does).

So, a typical TCP/IP host has multiple processes each needing to send and receive datagrams. All of them, however, must be sent using the same interface to the internetwork, using the IP layer. This means that the data from all applications (with some possible exceptions) is “funneled down”, initially to the transport layer, where it is handled by either TCP or UDP. From there, messages pass to the device's IP layer, where they are packaged in IP datagrams and sent out over the internetwork to different destinations. The technical term for this is multiplexing. This term simply means combining, and its use here is a software analog to the way it is done with signals.

A complementary mechanism is responsible for receipt of datagrams. At the same time that the IP layer multiplexes datagrams from many application processes to be sent out, it receives many datagrams that are intended for different processes. The IP layer must take this stream of unrelated datagrams, and eventually pass them to the correct process (through the transport layer protocol above it). This is the reverse of multiplexing: demultiplexing. You can see an illustration of the basic concept behind TCP/IP process multiplexing and demultiplexing in Figure 197.

TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets 1 2 3 TCP/IP Ports: Transport Layer (TCP/UDP) Addressing