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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)

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

The transport layer in a protocol suite is responsible for a specific set of functions. For this reason, one might expect that the TCP/IP suite would have a single main transport protocol to perform those functions, just as it has IP as its core protocol at the network layer. It is a curiosity, then, that there are two different widely-used TCP/IP transport layer protocols. This arrangement is probably one of the best examples of the power of protocol layering—and hence, an illustration that it was worth all the time you spent learning to understand that pesky OSI Reference Model. J

Let's start with a look back at layer three. In my overview of the key operating characteristics of the Internet Protocol, I described several important limitations of how IP works. The most important of these are that IP is connectionless, unreliable and unacknowledged. Data is sent over an IP internetwork without first establishing a connection, using a “best effort” paradigm. Messages usually get where they need to go, but there are no guarantees, and the sender usually doesn't even know if the data got to its destination.

These characteristics present serious problems to software. Many, if not most, applications need to be able to count on the fact that the data they send will get to its destination without loss or error. They also want the connection between two devices to be automatically managed, with problems such as congestion and flow control taken care of as needed. Unless some mechanism is provided for this at lower layers, every application would need to perform these jobs, which would be a massive duplication of effort.

In fact, one might argue that establishing connections, providing reliability, and handling retransmissions, buffering and data flow is sufficiently important that it would have been best to simply build these abilities directly into the Internet Protocol. Interestingly, that was exactly the case in the early days of TCP/IP. “In the beginning” there was just a single protocol called “TCP” that combined the tasks of the Internet Protocol with the reliability and session management features just mentioned.

There's a big problem with this, however. Establishing connections, providing a mechanism for reliability, managing flow control and acknowledgments and retransmissions: these all come at a cost: time and bandwidth. Building all of these capabilities into a single protocol that spans layers three and four would mean all applications got the benefits of reliability, but also the costs. While this would be fine for many applications, there are others that both don't need the reliability, and “can't afford” the overhead required to provide it.

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