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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)                      9  TCP Reliability and Flow Control Features and Protocol Modifications

TCP Adaptive Retransmission and Retransmission Timer Calculations 1 2 3 4 TCP Window Management Issues

Reducing Send Window Size To Stop The Sending of New Data

What if the server is so bogged down that it can't process any of the bytes received? Let’s suppose that the next transmission from the client is 180 bytes in size, but the server is so busy it can’t remove any of them. It could buffer the 180 bytes and in the acknowledgment it sends for those bytes, reduce the window size by the same amount: from 260 to 80. When the client received the acknowledgment for 180 bytes it would see the window size had reduced by 180 bytes as well. It would “slide” its window by the same amount as the window size was reduced! This is effectively like the server saying “I acknowledge receipt of 180 bytes, but I am not allowing you to send any new bytes to replace them”.

Another way of looking at this is that the left edge of the window slides 180 bytes while the right edge remained fixed. And as long as the right edge of the window doesn't move, the client can't send any more data than it could before receipt of the acknowledgment. This is the middle exchange (Steps #4 to #6) in Figure 226.

TCP Adaptive Retransmission and Retransmission Timer Calculations 1 2 3 4 TCP Window Management Issues