Flow control in TCP • End-to-end control across a path possibly passing through multiple networks (internet transport protocol) • Flow Control: How to prevent receiver buffer overflows? • Flow Control in TCP • Window-based flow control • Called sliding window flow control
TCP header fields related to flow control and error control • Three fields of interest to error control and flow control in the TCP header • The interpretation of the AckNo and Window size fields: • The sender of the TCP segment with the Ack Number field set to “AckNo” and Window size field set to “Win” is communicating to the far end that it is 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
TCP header fields - Sequence Number • Sequence Number (SeqNo): • Sequence number is 32 bits long. • So the range of SeqNo is 0 <= SeqNo <= 232 -1 4.3 Gbyte • The sequence number identifies the byte in the stream of data from the sending TCP to the receiving TCP that the first byte of data in this segment represents. • Initial Sequence Number (ISN) of a connection is set during connection establishment
Sliding Window Flow Control • Sliding Window Protocol is performed at the byte level: • Here: Sender can transmit sequence numbers 6,7,8.
Sliding Window: “Window Closes” • Transmission of a single byte (with SeqNo = 6) and acknowledgement is received (AckNo = 5, Win=4):
Sliding Window: “Window Opens” • Acknowledgement is received that enlarges the window to the right (AckNo = 5, Win=6): • A receiver opens a window when TCP buffer empties (meaning that data is delivered to the application).
Sliding Window: “Window Shrinks” • Acknowledgement is received that reduces the window from the right (AckNo = 5, Win=3): • Shrinking a window should not be used - Host requirements RFC strongly discourages this
TCP error control • Cumulative ACKs are used • In popular implementations, an ACK-every-other-segment strategy is used • Sender keeps an associated timeout value for segments sent. If an ACK is not received before the timer times out, it resends the segment • Retransmission scheme is Go-Back-N ARQ • But Selective repeat along with Selective ACKs have been added as extensions • Piggybacking of ACKs on data segments is indeed used. • If there is no data to send before a delayed-ACK timer expires, a zero-payload segment is sent as an ACK