Congestion Control

Packet Conservation and Additive Increase/Multiplicative Decrease

packet conservation

for each acknowledgement a sender receives, it knows one packet has left the network and it is therefore safe to send another

window calculation

window = min (congestionWindow, advertisedWindow)

how is congestion window set?

by the sender based on its perception of congestion in the network

additive increase (when sender is close to network capacity)

for each RTT, the congestion window will increase by 1 TCP segment

multiplicative decrease

in the event of congestion, the congestion window will be halved

why is the congestion window reduced much quicker than it is increased?

sending at a rate that is too aggressive has compounding effects - congestion leads to loss which leads to retransmissions which leads to more congestion

what two signals does TCP use to indicate congestion?

a timeout or 3 duplicate acknowledgements

slow start (when the connection is just beginning)

congestion window initially set to 1 and is doubled each RTT

slow start is used when the sender doesn’t know what the network capacity is. what 2 scenarios would this occur in?

beginning of a connection or when a timeout occurs

ecn

explicit congestion notification, allows routers to notify endpoints of congestion before packets are dropped so TCP can reduce its sending rate earlier

why can’t we just add more buffering 1

harmful to performance - TCP uses packet loss as an indicator of performance and without that indicator the large buffers will just fill up

why can’t we just add more buffering 2

large buffers increase latency and jitter