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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

I believe the most sensible place to start learning about how the TCP/IP protocol suite works is by examining the Internet Protocol (IP) itself, and the support protocols that function in tandem with it at the network layer. IP is the foundation upon which most of the rest of TCP/IP is built. It is the mechanism by which data is packaged and routed throughout a TCP/IP internetwork.

It makes sense, then, that when we examine the operation of TCP/IP from the perspective of the Internet Protocol, we talk very generically about sending and receiving datagrams. To the IP layer software that sends and received IP datagrams, the higher-level application they come from and go to is really unimportant: to IP, “a datagram is a datagram”, pretty much. All datagrams are packaged and routed in the same way, and IP is mainly concerned with lower-level aspects of moving them between devices in an efficient manner.

It's important to remember, however, that this is really an abstraction, for the convenience of describing layer three operation. It doesn't consider how datagrams are really generated and used above layer three. Layer four represents a transition point between the OSI model hardware-related layers (one, two and three) and the software-related layers (five to seven). This means the TCP/IP transport layer protocols, TCP and UDP, do need to pay attention to the way that software uses TCP/IP, even if IP really does not.

Ultimately, the entire point of having networks, internetworks and protocols suites like TCP/IP is to enable networking applications. Most Internet users employ these applications on a daily basis. In fact, most of us will be running many different applications simultaneously. For example, you might be using a World Wide Web browser to check the news, an FTP client to upload some pictures to share with family, and an Internet Relay Chat program to discuss something with a friend or colleague. In fact, it is common to have multiple instances of a single application. The most common example is having multiple Web browser windows open (I sometimes find myself with as many as 30 going at one time!)

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