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TCP Window Size Adjustment and Flow Control
(Page 4 of 4)
Closing the Send Window
The process of window adjustment
can continue, and of course, can be done by both deviceswe are
just only considering the client-sends-to-server side of the equation
here. If the server continues to receive data from the client faster
than it can pump it out to the application, it will continue to reduce
the size of its receive window. To continue our example above, suppose
that after the send window is reduced to 80, the client sends a third
request, this one 80 bytes in length, but the server is still busy.
The server then reduces its window all the way down to 0, which is called
closing the window. This tells the client the server is very
overloaded, and it should stop routine sending of data entirely, as
shown in the bottom third of Figure 226.
Later on, when the server is less loaded down, it can increase the window
size for this connection back up again, permitting more data to be transferred.
While conceptually simple, flow control
using window size adjustment can be very tricky. If we aren't careful
about how we make changes to window size, we can introduce serious problems
in the operation of TCP. There are also special situations that can
occur, especially in cases where the window size is made small in response
to a device becoming busy. The next two topics explore window
management issues, as well as changes
that need to be made to the basic sliding windows system
to address them.
Key Concept: The TCP sliding window system is used not just for ensuring reliability through acknowledgments and retransmissionsit is also the basis for TCPs flow control mechanism. By increasing or reducing the size of its receive window, a device can raise or lower the rate at which its connection partner sends it data. In the case where a device becomes extremely busy, it can even reduce the receive window to zero, closing it; this will halt any further transmissions of data until the window is reopened.
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Version 3.0 - Version Date: September 20, 2005
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