The TCP send window, in the context of z/OS Communications Server, is the amount of data that a sending TCP endpoint is permitted to transmit before receiving an acknowledgment (ACK) from the receiving TCP endpoint. It's essentially the sender's view of the receiver's advertised receive window.
Here's a breakdown:
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Relationship to Receive Window: The send window is constrained by the receiver's advertised receive window. The receiver tells the sender how much buffer space it has available to receive data. The sender cannot send more data than the receiver has advertised it can handle.
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Flow Control: The send window is a core mechanism for TCP flow control. It prevents the sender from overwhelming the receiver with data, ensuring reliable data transmission.
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Congestion Control (Indirectly): While primarily related to flow control, the send window interacts with TCP's congestion control mechanisms. The size of the send window, coupled with the congestion window, determines how much data is sent.
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z/OS Implementation: The z/OS Communications Server implementation specifically sets the send window to the advertised receive window of the partner (the receiving endpoint). This means the sender adheres directly to the receiver's buffer limitations.
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Send Window vs. Congestion Window:
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Send Window (Flow Control): Determined by the receiver's advertised receive window. It prevents the sender from overwhelming that specific receiver.
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Congestion Window (Congestion Control): Determined by the network conditions and prevents the sender from overwhelming the network itself. TCP uses algorithms like slow start and congestion avoidance to adjust the congestion window.
The actual amount of data sent is the minimum of the send window and the congestion window.
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Example: If the receiver advertises a receive window of 64KB, the sender's send window will initially be set to 64KB. However, if network congestion is detected, the congestion window might be reduced to 32KB. In this case, the sender will only send 32KB of data before receiving an ACK, even though the receiver could theoretically handle 64KB.
In short, the TCP send window helps manage the rate at which data is transmitted, based on the receiver's buffer capacity, preventing data loss and ensuring reliable communication. It works in conjunction with the congestion window to optimize data flow across the network.
