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A PRACTICAL GUIDE TO TCP/IP PROTOCOLS

by Tracy Thomas

Start ı Data Delivery ı Regulating Data ı Pros and Cons ı Sources and PDF

REGULATING DATA

TCP regulates the data rate, or throughput, with a sliding window. With TCP, a client cannot establish a connection and blast the remote host with data at full speed. The data flow is regulated by the requirement for acknowledgement of data and by the TCP window. In each TCP segment, the host advertises the amount of data itıs ready to accept. This amount is the TCP window (see Figure 1). The sliding window refers to the progression of data sequence numbers as data is sent throughout a connection. The window size advertised specifies the maximum amount of data the sender can have outstanding (not acknowledged).

Figure 1ıThe TCP data window slides to the right as data is sent and acknowledged. TCPıs sliding window acts as a throttle on a connection to limit the data rate to what the data receiver can actually handle.

Data to the left of the current window is in the past and canıt be resent. Data to the right of the current window canıt be sent until the window moves. So, only data within the sliding window may be sent to the remote host. The sliding window moves to the right only when a data segment is acknowledged. Thus, the acknowledgement of data along with the window keeps TCP from sending data too fast.

For example, as shown in Figure 1, the client can send segments 7, 8, and 9, while segments 4, 5, and 6 are waiting for acknowledgement. When you receive an acknowledgement for segment 4 and the window slides one place to the right, you can send segment 10. This concept of the sliding window is also referred to as flow control and limits the amount of traffic one host can place on the network to the amount that the remote host is actually processing. The remote host can close its window (advertise zero bytes can be sent). If the remote window is closed, the client can no longer send data to the remote host, but instead must probe the remote host to discover when the window opens.

One important feature of TCP to understand is that the data is delivered as a stream. If the client application performs ten 128-byte writes, there is no way to know if the underlying TCP will deliver ten 128-byte segments, or one 1280-byte segment. Application developers note that an additional protocol often needs to be used to mark the beginning, end, and type of data. FTP, Telnet, and e-mail (SMTP) are common application examples that use protocol headers in addition to TCP. When you design your own application with TCP, youıll need a way to break up the stream of data into recognizable parts.

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