window advertised by the receiver. A sender can send data less than its own congestion window and the receive window. Slow start Slow start, defined by . is part of the congestion control strategy used by TCP in conjunction with other algorithms to avoid sending more data than the network is capable of forwarding, that is, to avoid causing network congestion. Slow start begins initially with a congestion window size (CWND) of 1, 2, 4 or 10 MSS. The value for the congestion window size can be increased by 1 MSS with each acknowledgement (ACK) received, effectively doubling the window size each RTT. The transmission rate will be increased by the slow-start algorithm until either a packet loss is detected, or the receiver's advertised window (rwnd) is the limiting factor. Or slow start threshold (ssthresh) is reached,which is used to determine whether the slow start or congestion avoidance algorithm is used, which is a value set to limit slow start If the CWND reaches ssthresh, TCP changes to congestion avoidance algorithm. It should be increased by up to 1 MSS for each RTT. A common formula is that each new ACK increases the CWND by MSS * It increases almost linearly and provides an acceptable approximation. If a loss event occurs, TCP assumes that it is due to network congestion and takes steps to reduce the offered load on the network. These measurements depend on the exact TCP congestion avoidance algorithm used. When a TCP sender detects segment loss using the retransmission timer and the given segment has not yet been resent by way of the retransmission timer, the value of ssthresh must be set to no more than half of the amount of data that has been sent but not yet cumulatively acknowledged. Or 2 * MSS TCP Tahoe When a loss occurs, retransmit is sent, half of the current CWND is saved as ssthresh and slow start begins again from its initial CWND. TCP Reno A fast retransmit is sent, half of the current CWND is saved as ssthresh and as new CWND, thus skipping slow start and going directly to the congestion avoidance algorithm. The overall algorithm here is called . Slow start assumes that unacknowledged segments are due to network congestion. While this is an acceptable assumption for many networks, segments may be lost for other reasons, such as poor data link layer transmission quality. Thus,