1. 1 TCP-BFA: Buffer Fill Avoidance {aaa,crai}@stanford.edu http://klamath.stanford.edu/~aaa/tcp-bfa September 1998 Amr A. Awadallah Chetan Rai Computer Systems Laboratory Stanford University

2. 2 TCP Congestion Avoidance Offered Load Delay Throughput (Goodput) Disadvantages: 1. High Buffer Occupancies 2. High Delay 3. Periodic Packet Losses 4. Bursty Traffic Pattern Knee Cliff

3. 3 Operate Closer to the Knee Offered Load Delay Throughput Advantages: 1. Smaller Buffers 2. Lower Delay 3. Fewer Packet Losses 4. Faster Loss Detection 5. Smoother Traffic Patterns Knee Cliff

4. 4 How To Estimate ? Signed RTT variance (srv): where = current RTT - smoothed RTT and = filter gain constant Time srv Disable Threshold Enable Threshold Enable BFA Mode Disable BFA Mode

5. 5 Buffer Fill Avoidance A) Incrementing the Congestion Window: if( BFA Flag is set ) hold cwnd else allow normal cwnd increase. B) Setting the Buffer Fill Avoidance Flag: Rule 1: if( BFA Flag is set ) if(Signed_RTT_Variance ) set BFA Flag Rule 2: Clear the BFA Flag whenever the congestion window drops due to loss.

6. 6 Simulation Simulations using the ns network simulator. 64 Kbps, 100 ms 10 Mbps, 10 ms Source 1 Source 2 Sink 1 Sink 2 Node 1 Node 2

7. Simulation Single Sources Competing Sources Buffer Occupancy (packets) Buffer Size (packets) TCP-BFA Reno 0200100300 0 5 10 15 20 Network Power (Throughput/Delay) 0 50 100150200 0 10 5 15 20 25 TCP-BFA Reno Time (sec) 0200 100 300 0 5 10 15 20 Buffer Occupancy (packets) Reno vs. Reno TCP-BFA vs. TCP-BFA Buffer Size (packets) 0 1020 30 40 50 0 10 5 15 20 Network Power (Throughput/Delay) Reno1 TCP-BFA 1&2 Reno2 TCP-BFA Reno Time (sec)

8. 8 Simulation - Fairness Buffer Size (packets) 010 2030 40 50 0 0.4 0.6 0.8 1.0 0.2 Jain’s Fairness Index ( 1 = fair, 0 = unfair) Reno (starting together) Reno (starting at different times) TCP-BFA (starting together) TCP-BFA (starting at different times)

9. 9 Measurements Modified the FreeBSD TCP kernel to perform buffer fill avoidance. Internet (20 hops) SourceSink Stanford, USA 64Kbps 320 ms Cairo, Egypt

10. Low Congestion TCP-BFATCP-Reno 0 200 400600 0 5 10 15 20 25 30 Time (s) Congestion Window (KB) 0 200 400600 Time (s) 10 15 20 25 30 0 5 Advertised Window Congestion Window (KB) cwnd (KB) 0510 1520 2530 RTT (sec) 0 1.0 2.0 3.0 4.0 5.0 cwnd (KB) 510 1520 2530 0 RTT (sec) 0 1.0 2.0 3.0 4.0 5.0

11. Medium Congestion TCP-BFATCP-Reno 200 400 600 0 5 10 15 20 25 30 Time (s) cwnd (KB) 0 5 10 15 202530 RTT (sec) 0 1.0 2.0 3.0 4.0 5.0 Congestion Window (KB) 0 800 1000 200 400600 0 5 10 15 20 25 30 Time (s) Congestion Window (KB) 0 800 1000 cwnd (KB) 0 5 10 15 20 25 30 RTT (sec) 0 1.0 2.0 3.0 4.0 5.0

12. 12 Summary Advantages: –Lower buffer occupancies  Fewer Losses –Lower delays  Faster loss detection –Smoother traffic patterns –Easy to integrate into current implementations Disadvantages: –Slightly lower throughput –Unfairness in ideal situations TCP-BFA: Operate closer to knee than cliff