1. 18 Nov 2009 TCP VEGAS Mohammad AlKurbi CMPT – 771: Internet Architecture and Protocols

2. Introduction  Another congestion control algorithm.  Developed by L. Brakmo, S. O'Malley, and L. Peterson on 1994.  Objective  Anticipate congestion at early stage.  Increase throughput and decrease losses. 18 Nov 2009 2 TCP Vegas

3. Concept 18 Nov 2009 TCP Vegas 3  Enhances congestion control in TCP Reno:  Sending part only.  It does not change TCP specification.  Avoids/minimizes congestion by 3 mechanisms:  New Retransmission mechanism.  Congestion Avoidance mechanism.  Modified Slow-Start mechanism.

4. 1.New Retransmission mechanism 18 Nov 2009 TCP Vegas 4  Fast retransmission is not used most of the time [2]:  Losses are so great, that three duplicate ACKs are not received.  TCP Vegas calculate an exact RTT time from a previous segment(Send, Ack received)  Timeout = one RTT.  A retransmission happen if:  [ time(ACK (Packet-n)) – time(Packet-n+1) ] > timeout(Packet-n+1). When  A duplicate Ack is received.  OR, a (1’st/2’nd) non-duplicate Ack is received after a retransmission.

5. 1.New Retransmission mechanism (2) 18 Nov 2009 TCP Vegas 5 Host A Pkt-13 Rtt TCP Vegas Retransmission mechanism Host B time Packet 11,12 X ACK 11 Packet 13 Packet 14 ACK 12 Packet 15 D. ACK 12 Pkt-15 Rtt Re. Packet 13 TCP Reno’d wait for 3 rd D. Ack ACK 13 ACK 14 Re. Packet 15

6. 1.New Retransmission mechanism (3). 18 Nov 2009 TCP Vegas 6  Reducing the number of duplicate ACKs unwisely from 3 to 2 or 1, can result in many unnecessary retransmissions.  TCP Vegas reduces congestion window for losses that happened at the current sending rate, and not at before higher rate.

7. 2.Congestion Avoidance Mechanism 18 Nov 2009 TCP Vegas 7  Mitigate congestion to avoid losses (TCP Reno react to loss).  Expected = WindowSize / BaseRTT  BaseRtt: minimum RTT so far.  For every RTT, calculate the following:  Diff = Expected Throughput – Actual Throughput  If ( Diff < α) then increase CongWin linearly.  If ( Diff > β) then derease CongWin linearly.  Otherwise (i.e. α < Diff < β), no changes.  the algorithm is not in effect during slow-start.

8. TCP Vegas sending rate adaptation [2] 18 Nov 2009 TCP Vegas 8 Background traffic Shared Link utilization

9. TCP Reno Behavior 18 Nov 2009 TCP Vegas 9 TCP Reno Background traffic Shared Link utilization

10. 3.Modified Slow-Start Mechanism 18 Nov 2009 TCP Vegas 10  Incorporated CAM mechanism into slow-start.  Increase CongWin exponentially every other RTT.  In between,  If ( Diff ≥ β : one router buffer size ), then switch from Slow-start mode to linear increase mode.  During Initial Slow-Start, doubling CongWin size on the next Rtt, may exceed available bandwidth, so:  Set CongWin Threshold based on the estimated available bandwidth.  Such technique is not included: It does not improve throughput. It does not help if two connections decided to send at once.

11. Performance Evaluation [2] (Under Internet Real Traffic) 18 Nov 2009 TCP Vegas 11  Under Real traffic (Internet):  (37% ~ 71%) higher throughput.  (49% ~ 83%) less loss/retransmission.

12. Performance Evaluation [2] (Under Experimental traffic) 18 Nov 2009 TCP Vegas 12  Fairness (Another metric factor: [0~1]):  By running 2, 4 and 16 connections sharing a bottleneck link, It turns that: Vegas f airness ≈ Reno.  Does Vegas impose on Reno ?

13. Summary 18 Nov 2009 TCP Vegas 13  TCP Vegas has better throughput/fairness [2,3] and decreases losses by:  Reducing time required to detect lost and retransmit.  Slowdown sending rate before losses.  Switch from slow-start to linear mode on the right time.  TCP Vegas does not gain improvement by imposing Reno.  TCP Vegas is as fair as TCP Reno.

14. References 18 Nov 2009 TCP Vegas 14 1. L. Brakmo et al. TCP Vegas: New techniques for congestion detection and avoidance. In Proceedings of ACM SIGCOMM ’94, pages 24–35, May, 1994. 2. L. Brakmo and L. Peterson. TCP Vegas: End to End Congestion Avoidance on a global Internet. IEEE Journal on Selected Areas in Communications, 13(8):1465–1480,Oct 1995. 3. J. Mo, R.J. La, V. Anantharam, and J.Walrand. Analysis and Comparison of TCP Reno and Vegas. In Proceedings of INFOCOM ’99, pages 1556–1563,March 1999.

15. Thank you 18 Nov 2009 TCP Vegas