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 Transmission Control Protocol (TCP)                      9  TCP Fundamentals and General Operation

TCP Data Handling and Processing: Streams, Segments and Sequence Numbers 1 2 3 4 5 6 7 8 9 TCP Ports, Connections and Connection Identification

What differentiates the Transmission Control Protocol from simpler transport protocols like UDP is the quality of the manner in which it sends data between devices. Rather than just sticking data in a message and saying “off you go”, TCP carefully keeps track of the data it sends and what happens to it. This management of data is required to facilitate two key requirements of the protocol:

• Reliability: Ensuring that data that is sent actually arrives at its destination, and if not, detecting this and re-sending the data.
  
  
• Data Flow Control: Managing the rate at which data is sent so that it does not overwhelm the device that is receiving it.

To accomplish these tasks, the entire operation of the protocol is oriented around something called the sliding window acknowledgment system. It is no exaggeration to say that comprehending how sliding windows works is critical to understanding just about everything else in TCP. It is also, unfortunately, a bit hard to follow if you try to grasp it all at once, which means many people's eyes glaze over trying to make sense of it.

Since you can’t really get TCP without understanding sliding windows, I wanted to make sure that I explained the mechanism thoroughly—and without assuming you already understand a great deal, as most references do. For this reason I am going to start with the concepts behind sliding windows and eventually explain how the technique works in general terms and why it is so powerful. Doing this properly required a considerable amount of explanation (which took a long time to get right, I might add!) so buckle your seat belt. J

TCP Data Handling and Processing: Streams, Segments and Sequence Numbers 1 2 3 4 5 6 7 8 9 TCP Ports, Connections and Connection Identification