Computer Networks

1. Computer Networks Lecture 18 TCP Cubic, TCP in 4G LTE 11/5/2013 Lecturer: Namratha Vedire

2. Admin • Assignment 4 Check Point 1: Nov 15, 11:55 pm To Do: Discuss design with instructor or a TF Nov 11 Code and Report: Nov 19, 11:55 pm To Do: Discuss design with instructor or a TF Nov 14

3. Demo

4. Recap

5. Recap : RTT & Timeout • RTT Sample RTT EstimatedRTT = (1-α)*EstimatedRTT + α*SampleRTT (α=0.125) DevRTT = (1-β)*DevRTT + β*|SampleRTT – EstimatedRTT| (β=0. • Timeout = EstimatedRTT + 4*DevRTT SEG ACK

6. Delay Load Recap :Congestion Control • Congestion is too many sources sending too much data too fast. • Manifestation • Lost packets 2. High Delay 3. Wasted Bandwidth packet loss knee cliff congestion collapse Throughput Load

7. Recap : Congestion Control • Efficiency - Close to full utilization but low delay. - Fast convergence after disturbance. • Fairness - Resource Sharing • Distributed - No central knowledge necessary - Scalability

8. Recap : Simple Model Flows observe congestion signal d, and locally take actions to adjust rates. User 1 x1 x2  User 2 d = xi > Xgoal? xn User n

9. Recap : A(M)I - MD Protocol • Apply the A(M)I – MD algorithm to a sliding window protocol

10. Recap : TCP/ Reno • Two cases - 3 duplicate ACKs (network capable of delivering some packets) • Timeout (more alarming) • Two phases 1. Slow start (SS) - MI 2*cwnd per RTT till congestion 2. Congestion avoidance(CA) – AIMD cwnd increase by 1 per RTT - 3 duplicate ACKs  cwnd = cwnd/2 - Timeout  cwnd =1 - In timeout  timeout = 2*timeout

11. Recap : TCP/ Reno TD cwnd TD TD TO ssthresh ssthresh ssthresh ssthresh Time CA SS CA CA SS CA SS – Slow Start CA – Congestion Avoidance TD – Three Duplicate ACKs TO - Timeout

12. Recap : TCP/ Reno When cwndis cut to half, why does sending rate not get cut?

13. filling buffer draining buffer Recap : TCP/ Reno ç There is a filling and draining of buffers for each TCP flow. cwnd TD bottleneck bandwidth ssthresh Time CA

14. TCP/ Reno Analysis

15. TCP/ Reno Throughput Analysis • Understand throughput in terms of • RTT • Packet loss rate (p) • Packet size (S) • Throughput calculations • Assume congestion avoidance and no timeouts occur • Mean window size Wm segments, round trip time RTT & pack size S • Throughput ≈ Wm * S bytes/sec RTT

16. Deterministic Analysis • Consider congestion avoidance • Assume one packet is lost per cycle • Total packets sent per cycle • Packet loss (p) • Throughput = • = ½*(W + W/2) * W/2 = 3W2/8 • = 1/(3W2/8) = 8/(3W2)  cwnd TD available bandwidth ssthresh W W/2 S*Wm Time RTT CA

17. segment 1 ACK for segment 7 segment 2 segment 3 segment 4 segment 1 segment 7 segment 2 segment 5 segment 6 TCP/ Reno Drawbacks • Multiple packets lost simultaneously cannot be accounted for cwnd = 6 3 duplicate ACK’s Re-transmit segment 1 cwnd = 3 cwnd might reduce twice for packets lost in same window 3 duplicate ACK’s Re-transmit segment 2 cwnd = 1

18. TCP/ Reno Drawbacks • RTT unfairness • Flows with different RTT’s grow their congestion windows differently • Users with shorter RTT ramp up faster! • On long distance links, RTT is high and cwnd takes longer to increase leading to underutilization of link. • Synchronized losses • Simultaneous packet loss events for multiple competing flows. New Protocol Necessary!!

19. Desired Characteristics in TCP • Adaptive schemes that grow the congestion window depending on network conditions • Scalable • RTT Fairness • Faster convergence to better utilize full bandwidth

20. TCP BIC http://www.land.ufrj.br/~classes/coppe-redes-2007/projeto/BIC-TCP-infocom-04.pdf

21. Growth functions • Consider TCP/Reno growth function cwnd TD TD TD TD ssthresh Wm Grows linearly throughout Time CA CA CA CA

22. TCP BIC Binary Increase Congestion Control (BIC) algorithm • PHASE 1 • cwnd < low_wind, follows TCP • ACK received : cwnd = cwnd + 1 • Loss event: cwnd = cwnd/2 • PHASE 2 • cwnd > low_wind, follows BIC

23. BIC Algorithm • Some preliminaries • βmultiplicative decrease factor • Wmax = cwnd size before the reduction • Wmin = β*Wmax – just after reduction • midpoint = (Wmax + Wmin)/2 BIC performs binary search between Wmax and Wmin looking for the midpoint.

24. BIC Algorithm Max Probing Packet loss event Wmax + Smin Wmax + Smin Wmax (Wmin – midpoint) < Smin Wmax + 3Smax Wmax + 2Smax Wmax +Smax Wmin midpoint = (Wmin + Wmax)/2 Wmin Wmin + Smin midpoint = (Wmin + Wmax)/2 midpoint = (Wmin + Wmax)/2 Wmin + Smax Wmin Wmin + Smax Wmin – midpoint > Smax Wmin = β*Wmax Additive Inc. Slow Start Additive Increase Binary Search

25. BIC Algorithm while (cwnd != Wmax){ If ((Wmin – midpoint) > Smax) cwnd = cwnd + Smax else If ((Wmin – midpoint) < Smin) cwnd = Wmax else cwnd = midpoint If (no packet loss) Wmin = cwnd else Wmin = β*cwnd Wmax =cwnd midpoint = (Wmax + Wmin)/2 } Additive Increase Binary Search

26. BIC Algorithm while (cwnd >= Wmax){ If (cwnd < Wmax + Smax) cwnd = cwnd + Smin else cwnd = cwnd + Smax If (packet loss) Wmin = β*cwnd Wmax =cwnd } Slow Start Max Probing Additive Increase

27. TCP BIC - Summary Max Probing + Smax + Smin Packet loss event Wmax Time + Smax jump to midpoint Slow Start Binary Increase Additive Increase Additive Increase

28. TCP BIC in Action

29. TCP BIC Advantages • Scalability: quickly scales to fair BW share • Fairness and convergence: Achieves better fairness and faster convergence • Slow Growth around Wmax ensures that unnecessary timeouts do not occur.

30. TCP BIC Drawbacks • cwnd growth is aggressive for TCP with short RTT or low speed • Short RTT makes cwnd ramp up soon • Still dependent on RTT • Proportional to inverse square of the RTT like TCP/ Reno • Complex window growth function • Difficult for analysis and actual implementation

31. TCP Cubic http://www4.ncsu.edu/~rhee/export/bitcp/cubic-paper.pdf

32. TCP Cubic • cwnd = C( t – K)3 + Wmax • Wmax = cwnd before last reduction • βmultiplicative decrease factor • C scaling factor • t is the time elapsed since last window reduction

33. TCP CUBIC Max Probing Cubic starts probing for more Bandwidth Packet loss event Wmax Time Around Wmax, window growth almost becomes zero Fast growth upon reduction Steady State Behavior

34. TCP Cubic Advantages • Good RTT fairness • Growth dominated by t, competing flows have same t after synchronized packet loss • Real-time dependent • Similar to BIC but linear increases are time dependent • Does not depend on ACK’s like TCP/ Reno • Scalability • Cubic increases window to Wmax (or its vicinity) quickly and keeps it there longer

35. TCP Cubic Drawbacks • Slow Convergence • Flows with higher cwnd are more aggressive initially • Prolonged unfairness between flows • Bandwidth Delay Products • Linear increase artefacts

36. TCP in 4G LTE http://conferences.sigcomm.org/sigcomm/2013/papers/sigcomm/p363.pdf

37. 4G LTE • Bandwidths match (often exceed) home broadband speeds. • Higher Energy Efficiency • New resource management policy • Higher Throughputs • Lower Latency

38. 4G LTE - Architecture UE – User Equipment RAN – Radio Access Network CN – Core Network SGW – Switching Gateway PGW – Packet Data Network Gateway

39. 4G LTE - Latency • End-to-end latency of a packet that requires a UE’s radio interface is long - RRC promotion delay • Promotion delay is not included in either uplink or downlink as the delay has already finished when it reaches the server • Estimating the Promo Delay • Tsa – Timestamp of SYN • TSb – Timestamp of ACK • G – inverse of clock frequency • Promo Delay = G(TSb – TSa)

40. 4G LTE - Latency • 3G Networks • 2 s from idle to high power state • 1.5 s from low to high power state • 4G Networks • 600 ms promotion delays

41. 4G LTE - Queuing Delays • During data transfer phase, a TCP sender will increase its congestion window, allowing number of unacknowledged packets to grow. • “in-flight” packets buffered by routers in network path • buffers extensively accommodate cellular network conditions and conceal packet loss • In-flight bytes of more than 200KB leads to longer queuing delays.

42. 4G LTE – Undesired Slow Start

43. 4G LTE – Undesired Slow Start in-flight bytes growing

44. 4G LTE – Undesired Slow Start Packet loss

45. 4G LTE – Undesired Slow Start Fast retransmission allows TCP to directly send the lost segment to the receiver possibly preventing retransmission timeout Fast retransmission

46. 4G LTE – Undesired Slow Start TCP uses RTT estimate to update retransmission timeout (RTO) However, TCP does not update RTO based on duplicate ACKs RTT: 262ms RTO: 290ms Duplicate ACKs

47. 4G LTE – Undesired Slow Start Retransmission timeout causes slow start RTT: 356ms RTO: 290ms RTT > RTO, timeout! SLOW START

48. 4G LTE – Undesired Slow Start • If large number of packets are in flight and one packet is lost • large number of duplicate ACKs trigger fast re-transmission • avoid timeout • Large in-network queues hold many packets and delay the retransmitted packet • If specified ACK does not arrive within timeout, this triggers timeout and cwnd = 1 • Undesired Slow Start SOLUTION:Update the estimated RTT with duplicate ACKs

49. 4G LTE – TCP Receive Window • In 4G LTE networks, receive windows have become the bottleneck • Initial receive window is not large (mostly 131.8 KB) • Application is not reading data fast enough from the receive buffer • TCP rate is jointly controlled by congestion window and receive window • a full receive window prevents the server from sending more data • This leads to bandwidth underutilization SOLUTION • Move data from transport layer buffers to application layer buffers to empty receive window • Increase receive window at network level – deployment is challenging

50. Backup