What is meant by propagation delay, latency, RTT?
Propagation delay is the time it takes for a signal to propagate over (traverse) a given distance. For example, if an electromagnetic wave propagates through a given medium at 2/3 the speed of light and travels for a distance of 100 km, the time to traverse this distance (propagation delay) would be tp = (100 km) (1000 m/km) / 2 x 108 m/s = 0.5 ms. The RTT is the round trip time, which in this example is 1 ms (2 tp). The latency is defined to be latency = RTT + transmit time + queuing delay + processing time In our examples we have ignored queuing delay (assumed lightly loaded network) and neglected processing time (small compared to other times). The latency is the (minimum) time it takes to send a packet and get an acknowledgement back (assuming time to transmit the ACK is also negligible). If we divide the latency by the time to transmit (and neglect the last two terms) we get latency / ttrans = RTT/ ttrans + 1 = 2 tp/ ttrans + 1 or 1 + 2a where a = tp/ ttrans The value 1 + 2a is us
