TCP Congestion Control

TCP Congestion Control

Overview of TCP Congestion Control

• TCP (Transmission Control Protocol) manages congestion in order to maintain efficient and effective transmission rates across networks.
• The process involves both the sender and potentially the network (routers, etc.) to adapt to changing conditions.

1. Approaches to Congestion Control

• Loss-Based Approach:
  • Mechanism: The sender increases the sending rate until packet loss is detected, then decreases the rate accordingly.
  • Timeouts: Response involves a timeout when a packet is lost.
  • RTT Measurement: Delay-based methods increase the rate until the round-trip time (RTT) reaches a congested state.
• Network-Assisted Approach:
  • Involves cooperation between sender, network routers, and receiver.

2. TCP’s AIMD (Additive Increase-Multiplicative Decrease)

• How It Works:
  • Increases the sending rate by increasing the congestion window (cwnd) until a loss occurs.
  • Upon detecting loss, it either cuts the sending rate in half (Multiplicative Decrease).
  • Sawtooth Behavior: Sending rate follows a sawtooth pattern, probing for bandwidth.

Details of AIMD:

• Additive Increase (AI): Increase cwnd by 1 MSS (Maximum Segment Size) for each RTT until loss is detected.
• Multiplicative Decrease (MD): On loss event, cut the cwnd in half.
• Versions:
  • TCP Reno: Cuts the cwnd roughly in half upon detection via triple duplicate ACKs.
  • TCP Tahoe: Cuts the cwnd to 1 MSS following a timeout or loss event.

3. TCP Congestion Control States

• Three States:
  • Slow Start:
  • Initiates with cwnd = 1 MSS, increases exponentially until the first loss.
  • Congestion Avoidance:
  • Linear increase strategy kicks in when the cwnd reaches half of the previous maximum value.
  • Fast Recovery:
  • Reacts to duplicate acknowledgments, adjusts cwnd accordingly.
• Transition Rules:
  • Slow Start Threshold (ssthresh): When a loss occurs, ssthresh is set to half the cwnd before loss.

4. TCP CUBIC

• Comparison with Reno:
  • More aggressive initially, then becomes conservative as it approaches the maximum window size (Wmax).
  • Adjusts the window size with respect to the cube of the distance from K (the point where the window reaches Wmax).

5. Delay-Based Congestion Control

• Objective: Monitor throughput to keep the network pipe fully utilized but not overloaded.
  • Uses measured RTT to evaluate whether network conditions are congested.
• Throughput Calculation:
  • If throughput is close to cwnd/RTTmin, increase cwnd; if it’s low, decrease cwnd linearly.

6. Network-Assisted Congestion Control

• Explicit Congestion Notification (ECN):
  • Uses 2 bits in the IP header to signal congestion without dropping packets.
  • Enables communication of congestion indications from routers to the sender.

7. TCP Fairness

• Goal: Fair distribution of bandwidth among multiple TCP connections.
  • Ideal Case: Each session should have an equal share (R/K, where R is bandwidth and K is the number of TCP sessions).
• Influencing Factors:
  • Same RTT, controlled sessions contribute to fairness; differing conditions can impact this balance.

Conclusion

• TCP utilizes a combination of strategies for congestion control to optimize the flow of data.
• Understanding the details of AIMD, TCP CUBIC, and network-assisted controls is essential for enhancing network reliability and performance.