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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 Transmission Control Protocol (TCP)

TCP Priority Data Transfer: "Urgent" Function 1 TCP Segment Retransmission Timers and the Retransmission Queue

The main task of the Transmission Control Protocol is simple: packaging and sending data. Of course, almost every protocol packages and sends data. What distinguishes TCP from these protocols is the sliding window mechanism that controls the flow of data between devices. This system not only manages the basic data transfer process, it is also used to ensure that data is sent reliably, and also to manage the flow of data between devices to ensure that data is transferred efficiently without either device sending data faster than the other can receive it.

To enable TCP to provide the features and quality of data transfer that applications require, the protocol had to be enhanced beyond the simplified data transfer mechanism we saw in preceding sections. Extra “smarts” needed to be given to the protocol to handle potential problems, and changes to the basic way that devices send data were implemented to avoid inefficiencies that might otherwise have resulted.

In this section I describe how TCP ensures that devices on a TCP connection communicate in a reliable and efficient manner. I begin with an explanation of the basic method by which TCP detects lost segments and retransmits them. I discuss some of the issues associated with TCP's acknowledgment scheme and an optional feature for improving its efficiency. I then describe the system by which TCP adjusts how long it will wait before deciding that a segment is lost. I discuss how the window size can be adjusted to implement flow control, and some of the issues involved in window size management. This includes a look at the infamous “Silly Window Syndrome” problem, and special heuristics for addressing issues related to small window size that modify the basic sliding windows scheme. I conclude with a discussion of TCP's mechanisms for handling and avoiding congestion.

TCP Priority Data Transfer: "Urgent" Function 1 TCP Segment Retransmission Timers and the Retransmission Queue