TCP Congestion Control
๐
The key questions of TCP congestion control ๐
We want to control congestion.
- When congestion exists, reduce the sending rateto avoid, alleviate, or remove it.
- When there is no congestion, increase the sending rateto use bandwidth more effectively.
Note: We will consider only the main features of
TCP Reno
, one version of TCP.- TCP contains much more than we will discuss here.
TCP congestion-control terminology ๐
Main states and mechanisms
- Slow start: Exponential growth in the sending rate.
- Congestion avoidance:AIMD, Additive Increase/Multiplicative Decrease.
- Fast recovery.
Main parameters
- Congestion window: cwnd.
- Slow-start threshold: ssthresh.
- MSS: Maximum Segment Size.
- RTT: Round-Trip Time.
Main events
- ACK received: Things are proceeding well.
- Timeout: A very bad situation.
- Three duplicate ACKs received: A bad situation, but not as bad as a timeout.
The TCP congestion window: cwnd ๐
An important parameter adjusted according to network congestion.
It determines the maximum amount of data the sender can have in flightโthat is, sent but not yet acknowledged.
cwnd
is dynamic and responds to detected congestion.Last byte sent - last byte acknowledged < = cwnd.
TCP sending rate
:- Roughly, send cwndbytes, wait an RTT for acknowledgments, then send more.Sending rateโ cwnd / RTT (bytes/sec).
TCP slow start ๐
At the start of a connection, increase the sending rate exponentially until the first loss event.
- Initial congestion window (cwnd) = 1 MSS.
- Double the congestion window each RTT.
- Increase cwndfor every received ACK.
- Increase
The initial rate is slow but grows rapidly.
Return to
slow start
after every timeout
.
Move to congestion avoidance
after every triple-duplicate ACK
event.TCP: Detecting congestion and reacting to loss ๐
Detect congestion from inferred packet loss, usingtimeoutsorthree duplicate ACKs.
Loss inferred from a
timeout
:- Similar to returning to slow start.
- Set cwndto1 MSS.
- Grow the window exponentially until the ssthresh threshold, as in slow start.
- Set
- Then increase it linearly, entering congestion avoidance.
Loss inferred from
three duplicate ACKs
:- Duplicate ACKs show that the network can still deliver some segments.
- Halve cwnd.
- Then grow it linearly, entering congestion avoidance.
TCP congestion avoidance: AIMD ๐
The sender explores available bandwidth by increasing its rate, or window size, until loss occurs.
Additive increase
:- Increase cwndby 1 MSS each RTT until loss is detected.
Multiplicative decrease
:- Halve cwndwhenever loss is detected.
AIMD's sawtooth behavior reflects this search for bandwidth.
Transitioning from slow start to congestion avoidance ๐
Transition from exponential to linear increase:
- Once the ssthreshthreshold, half the pre-timeout value, is exceeded, increasecwndlinearly.
