What is Ssthresh in TCP?

The slow start threshold (ssthresh) determines the (de)activation of slow start. When a new connection is made, cwnd is initialized to one TCP data or acknowledgment packet, and waits for an acknowledgement, or ACK. When that ACK is received, the congestion window is incremented until the cwnd is less than ssthresh.

What does Ssthresh stand for?

slow start threshold value
the slow start threshold value (ssthresh) with an initial value equal to the receiver window.

What is the value of Ssthresh at the 25th transmission round?

7
At transmission round 25, ssthresh is 7.

What is congestion window size?

TCP uses what is called the “congestion window”, or CWND, to determine how many packets can be sent at one time. The larger the congestion window size, the higher the throughput. The TCP “slow start” and “congestion avoidance” algorithms determine the size of the congestion window.

When does ssthresh initialize to 32 MSS?

We can see in the graph that this happens when cwnd is 32. So, we immediately know that ssthresh is initialized to 32 MSS. The book shows a very similar graph on page 276 (Figure 3.53), where the authors draw a similar conclusion:

When to set ssthresh to half the congestion window size?

On timeout: ssthresh is set to half the congestion window size before the timeout. slow start is initiated. A system administrator may adjust the maximum window size limit, or adjust the constant added during additive increase, as part of TCP tuning .

How to calculate ssthresh in a graph stack?

Note that the time unit in the graph is a round. So, if at time T=X we transmit N segments, then it is assumed that at time T=X+1 these N segments have been ACKed (assuming they weren’t lost, of course). Also note how we can tell the value of ssthresh just by looking at the graph.