In tcp congestion control, what is slow start?

In TCP congestion control, slow start means that when the host just starts sending a message segment, the congestion window cwnd can be set to a value of the maximum message segment MSS; after each pair is received After the new message segment is confirmed, the congestion window is increased to a value of at most one MSS; in this way, the congestion window cwnd of the sender is gradually increased.

The operating environment of this article: Windows 7 system, Dell G3 computer.

Slow start:

When the host just starts sending message segments, the congestion window cwnd can be set to a value of the maximum message segment MSS. After receiving an acknowledgment of a new segment, the congestion window is increased by up to one MSS value. By gradually increasing the congestion window cwnd of the sender in this way, the rate of packet injection into the network can be more reasonable.

Congestion avoidance:

When the congestion window value is greater than the slow start threshold, stop using the slow start algorithm and switch to the congestion avoidance algorithm. The congestion avoidance algorithm increases the size of the sending congestion window by one MSS every time a round-trip delay RTT passes.

Fast retransmission algorithm stipulates:

As long as the sender receives three repeated ACKs in a row, it can conclude that a packet has been lost, and it should retransmit it immediately. segment without having to wait for the retransmission timer set for the segment to expire.

Fast recovery algorithm:

When the sender receives three consecutive repeated ACKs, it resets the slow start threshold ssthresh

and slow The first difference is that the congestion window cwnd is not set to 1, but to ssthresh

If the number of duplicate AVKs received is n (n>3), set cwnd to ssthresh

If the sending window value still allows the sending of message segments, continue to send message segments according to the congestion avoidance algorithm.

If an ACK confirming the new segment is received, the cwnd will be reduced to ssthresh

Multiply reduction:

means regardless of Whether in the slow start phase or the congestion avoidance phase, as long as a timeout occurs (that is, a network congestion occurs), the slow start threshold value ssthresh is set to the current congestion window value multiplied by 0.5.

When the network is frequently congested, the ssthresh value drops quickly to greatly reduce the number of packets injected into the network.

Additional increase:

means that after executing the congestion avoidance algorithm, after receiving confirmation of all message segments (that is, after a round trip time), Increase the congestion window cwnd by an MSS size to slowly increase the congestion window to prevent premature network congestion.

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