Summary: TCP congestion control

TCP Reno congestion control

Important events:

Receive a new ACK
Action:

Receive 3 duplicate ACKs

Action:

Retransmit the last packet unacknowledgedpacket (Fast retransmit)
Set SSThresh = 0.5 × CWND
Set CWND = SSThresh + 3 × MSS (Fasy recovery)

Sender times out
Action:

Congestion window increase algorithms:

When CWND ≤ SSThresh (Slow Start)

CWND = CWND + MSS (for every new ACK)
When CWND > SSThresh (Congestion Avoidance)

CWND = CWND + MSS × MSS/CWND (for every new ACK)

TCP Vegas

Differences:

Reno

Vegas

Reno timer expires once every 500 msec
When the timer expires, TCP Reno will compute the RTT for the acknowledged packets and examine which packets need to be retransmitted.

Vegas time stamps every transmitted packet
When an ACK is received, TCP Vegas computes the RTT

Reno detects packet loss with 3 duplicate ACKs

Vegas detects packet loss with 1 duplicate ACK using:

Now - (Packet Transmission Time) > Timeout for packet Situation: Packet Transmission Recv Dup. ACK time (Now) ---+---------------------+---------- <---------------> Packet Timeout

This check is done for every unackownledged packet

Reno has no equivalent mechanism

Vegas use the first 2 new non-duplicate ACKs as a re-transmission trigger

Now - (Packet Transmission Time) > Timeout for packet Situation: Packet Transmission Recv Dup. ACK time (Now) ---+---------------------+---------- <---------------> Packet Timeout

This check is done for every unackownledged packet

(Reason: click here )

Increase/decrease decision:

Increase/decrease decision:

Increase: when (Expected (best) Throughput - Actual Throughput) < &alpha
CWND unchanged: when &alpha ≤ (Expected (best) Throughput - Actual Throughput) < &beta
Decrease: when (Expected (best) Throughput - Actual Throughput) > &beta

Increase/decrease algorithm:

Increase/decrease algorithm:

When (Expected Throughput - Actual Throughput) ≤ δ:
When (Expected Throughput - Actual Throughput) > δ: