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

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Presentation on theme: "1 Computer Networks Lecture 18 TCP Cubic, TCP in 4G LTE 11/5/2013 Lecturer: Namratha Vedire."— Presentation transcript:

1 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 Recap : Congestion Control Congestion is too many sources sending too much data too fast. Manifestation 1.Lost packets 2. High Delay 3. Wasted Bandwidth Load Delay Throughput kneecliff congestion collapse packet loss 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 User 2 User n  d =  x i > X goal ? x1x1 x2x2 xnxn

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 Time cwnd SS CA TD TO ssthresh CA TD CA SS CA TD SS – Slow Start CA – Congestion Avoidance TD – Three Duplicate ACKs TO - Timeout

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

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

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 W m segments, round trip time RTT & pack size S -Throughput ≈ W m * S RTT bytes/sec

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

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

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 infocom-04.pdf TCP BIC

21 Growth functions – Consider TCP/Reno growth function Time cwnd CA TD ssthresh TD WmWm CA Grows linearly throughout

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 -W max = cwnd size before the reduction -W min = β*W max – just after reduction -midpoint = (Wmax + Wmin)/2 BIC performs binary search between W max and W min looking for the midpoint.

24 BIC Algorithm Packet loss event W max W min = β*W max midpoint = (W min + W max )/2 W min – midpoint > S max W min + S max W min midpoint = (W min + W max )/2 W min + S max W min Additive Increase midpoint = (W min + W max )/2 W min + S min W min (W min – midpoint) < S min Binary Search W max + S min W max +S max Slow Start W max + 2S max W max + 3S max Additive Inc. Max Probing

25 BIC Algorithm while (cwnd != W max ){ If ((W min – midpoint) > S max ) cwnd = cwnd + S max else If ((W min – midpoint) < S min ) cwnd = W max else cwnd = midpoint If (no packet loss) W min = cwnd else W min = β*cwnd W max = cwnd midpoint = (W max + W min )/2 } Additive Increase Binary Search

26 BIC Algorithm while (cwnd >= W max ){ If (cwnd < W max + S max ) cwnd = cwnd + S min else cwnd = cwnd + S max If (packet loss) W min = β*cwnd W max = cwnd } Slow Start Additive Increase Max Probing

27 TCP BIC - Summary Packet loss event Binary Increase Additive Increase Slow Start Max Probing + S max Time Additive Increase W max jump to midpoint + S min + S max

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 W max 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

32  cwnd = C( t – K) 3 + W max - W max = cwnd before last reduction - βmultiplicative decrease factor - C scaling factor - - t is the time elapsed since last window reduction

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

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 W max (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 pdf TCP in 4G LTE

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 - Ts a – Timestamp of SYN - TS b – Timestamp of ACK - G – inverse of clock frequency - Promo Delay = G(TS b – TS a )

40 4G LTE - Latency – 3G Networks - 2 s from idle to high power state s from low to high power state  4G Networks 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 in-flight bytes growing 4G LTE – Undesired Slow Start

44 Packet loss 4G LTE – Undesired Slow Start

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

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

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

48  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 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

51 Netflix App Case Study


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