TCP Flow Control Mechanisms
Explore the operation of sliding window flow control in TCP, the relationship between window size and buffer space, and the management of TCP windows for efficient data transmission and acknowledgement handling.
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Presentation Transcript
TCP Part 4: Flow Control ECE 461 Professor Jorg Liebeherr 1
Takeaways Takeaways Operation of sliding window flow control in TCP Size of window relates to available buffer space at receiver 2
TCP flow control TCP flow control TCP uses a version of the sliding window flow control, where Sending acknowledgements is separated from setting the window size at sender Acknowledgements do not automatically increase the window size During connection establishment, both ends of a TCP connection set the initial size of the sliding window 3
Window management in TCP Window management in TCP The receiver is returning two parameters to the sender Window size (Win) AckNo 32 bits 16 bits The interpretation is: I am ready to receive new data with SeqNo= AckNo, AckNo+1, ., AckNo+Win-1 Receiver can acknowledge data without opening the window Receiver can change the window size without acknowledging data 4
Sliding window flow control Sliding window flow control Sliding window flow control is performed at the byte level: Advertised window 1 2 3 4 5 6 7 8 9 10 11 sent and acknowledged sent but not acknowledged can be sent Usable window can't sent Here: Sender can transmit sequence numbers 6,7,8 5
Sliding window: Sliding window: window closes window closes Transmission of a single byte (with SeqNo = 6) and acknowledgement is received (AckNo = 5, Win=4): 1 2 3 4 5 6 7 8 9 10 11 Transmit Byte 6 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 4 is received 1 2 3 4 5 6 7 8 9 10 11 6
Sliding window: Sliding window: window closes window closes Transmission of a single byte (with SeqNo = 6) and acknowledgement is received (AckNo = 5, Win=4): 1 2 3 4 5 6 7 8 9 10 11 Transmit Byte 6 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 4 is received 1 2 3 4 5 6 7 8 9 10 11 7
Sliding window: Sliding window: window opens window opens Acknowledgement is received that enlarges the window to the right (AckNo = 5, Win=6): 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 6 is received 1 2 3 4 5 6 7 8 9 10 11 A receiver opens a window when its buffer empties (meaning that data is delivered to the application). 8
Sliding window: Sliding window: window opens window opens Acknowledgement is received that enlarges the window to the right (AckNo = 5, Win=6): 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 6 is received 1 2 3 4 5 6 7 8 9 10 11 A receiver opens a window when TCP buffer empties (meaning that data is delivered to the application). 9
Sliding window: Sliding window: window shrinks window shrinks Acknowledgement is received that reduces the window from the right (AckNo = 5, Win=3): 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 3 is received 1 2 3 4 5 6 7 8 9 10 11 Windows should not never shrink. Why? 10
Sliding window: Sliding window: window shrinks window shrinks Acknowledgement is received that reduces the window from the right (AckNo = 5, Win=3): 1 2 3 4 5 6 7 8 9 10 11 AckNo = 5, Win = 3 is received 1 2 3 4 5 6 7 8 9 10 11 Windows should not never shrink. Why? 11
Sliding Window: Example Sliding Window: Example Receiver Buffer Sender sends 2K of data 0 4K 2K SeqNo=0 2K AckNo=2048 Win=2048 Sender sends 2K of data 2K SeqNo=2048 Sender blocked 4K AckNo=4096 Win=0 3K AckNo=4096 Win=1024 12
