1 computer networks lecture 18 tcp cubic, tcp in 4g lte 11/5/2013 lecturer: namratha vedire

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Computer Networks

Lecture 18TCP Cubic, TCP in 4G LTE

11/5/2013

Lecturer: Namratha Vedire

Admin

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

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

Recap : Congestion Control

• Congestion istoo many sources sending too much data too fast.

• Manifestation1. Lost packets 2. High Delay

3. Wasted Bandwidth

LoadD

elay

Thro

ughp

ut

knee cliff

congestioncollapse

packetloss

Load

Recap : Congestion Control

• Efficiency- Close to full utilization but low delay.- Fast convergence after disturbance.

• Fairness- Resource Sharing

• Distributed- No central knowledge necessary- Scalability

Recap : Simple Model

Flows observe congestion signal d, and locally take actions to adjust rates.

User 1

User 2

User n

d = xi > Xgoal?

x1

x2

xn

Recap : A(M)I - MD Protocol

•Apply the A(M)I – MD algorithm to a sliding window protocol

Recap : TCP/ Reno

• Two cases- 3 duplicate ACKs (network capable of delivering some packets)-Timeout (more alarming)

• Two phases1. 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

Recap : TCP/ Reno

Time

cwnd

SS CA

TD

TOssthresh

ssthresh ssthreshssthresh

CA

TD

CA SS CA

TD

SS – Slow StartCA – Congestion AvoidanceTD – Three Duplicate ACKsTO - Timeout

Recap : TCP/ Reno

When cwnd is cut to half, why does sending rate not get cut?

Recap : TCP/ Reno

There is a filling and draining of buffers for each TCP flow.

Time

cwnd

CA

TD

ssthresh

bottleneckbandwidth

filling buffer

draining buffer

çç

TCP/ Reno Analysis

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

RTT bytes/sec

Deterministic Analysis

• Consider congestion avoidance- Assume one packet is lost per cycle- Total packets sent per cycle - Packet loss (p)

• Throughput =

83

243 2WWW

Time

cwnd

CA

TD

ssthresh

availablebandwidth

W/2W

S*Wm

RTT

= ½*(W + W/2) * W/2 = 3W2/8= 1/(3W2/8) = 8/(3W2)

TCP/ Reno Drawbacks

• Multiple packets lost simultaneously cannot be accounted for

ACK for segment 7

segment 1

segment 2segment 3segment 4segment 5

cwnd = 6

3 duplicate ACK’sRe-transmit segment 1cwnd = 3 segment 1

segment 6segment 7

3 duplicate ACK’sRe-transmit segment 2cwnd = 1 segment 2

cwnd might reduce twice for packets lost in same window

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!!

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

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

TCP BIC

Growth functions

– Consider TCP/Reno growth function

Time

cwnd

CA

TD

ssthresh

TD TD TD

Wm

CA CA CA

Grows linearly throughout

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

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.

BIC Algorithm

Packet loss event

Wmax

Wmin = β*Wmax

midpoint = (Wmin + Wmax)/2

Wmin – midpoint > Smax

Wmin + SmaxWmin

midpoint = (Wmin + Wmax)/2

Wmin + Smax

Wmin

Additive Increase

midpoint = (Wmin + Wmax)/2Wmin + Smin

Wmin(Wmin – midpoint) < Smin

Binary Search

Wmax + SminWmax + Smin

Wmax +Smax

Slow Start

Wmax + 2Smax

Wmax + 3Smax

Additive Inc.

Max Probing

BIC Algorithm

while (cwnd != Wmax){

If ((Wmin – midpoint) > Smax)

cwnd = cwnd + Smax

elseIf ((Wmin – midpoint) < Smin)

cwnd = Wmax

else cwnd = midpoint

If (no packet loss)Wmin = cwnd

elseWmin = β*cwnd

Wmax = cwnd

midpoint = (Wmax + Wmin)/2

}

Additive Increase

Binary Search

BIC Algorithm

while (cwnd >= Wmax){

If (cwnd < Wmax + Smax) cwnd = cwnd + Smin

elsecwnd = cwnd + Smax

If (packet loss)Wmin = β*cwnd

Wmax = cwnd

}

Slow Start

Additive IncreaseMax Probing

TCP BIC - Summary

Packet loss event

Binary Increase

Additive Increase

SlowStart

Max Probing

+ Smax

Time

Additive Increase

Wmax

jump to midpoint

+ Smin + Smax

TCP BIC in Action

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.

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

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

TCP Cubic

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

TCP CUBIC

Packet loss event

Time

Steady State Behavior

Max Probing

Wmax

Around Wmax, window growth almost becomes zero

Fast growth upon reduction

Cubic starts probing for more Bandwidth

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

TCP Cubic Drawbacks

Slow Convergence- Flows with higher cwnd are more aggressive initially- Prolonged unfairness between flows

Bandwidth Delay Products- Linear increase artefacts

http://conferences.sigcomm.org/sigcomm/2013/papers/sigcomm/p363.pdf

TCP in 4G LTE

4G LTE

Bandwidths match (often exceed) home broadband speeds.

Higher Energy Efficiency- New resource management policy

Higher Throughputs

Lower Latency

4G LTE - Architecture

UE – User EquipmentRAN – Radio Access NetworkCN – Core NetworkSGW – Switching GatewayPGW – Packet Data Network Gateway

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)

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

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.

4G LTE – Undesired Slow Start

in-flight bytes growing


Packet loss


Fast retransmission

Fast retransmission allows TCP to directly send the lost segment to the receiver possibly preventing retransmission timeout


RTT: 262msRTO: 290ms

TCP uses RTT estimate to update retransmission timeout (RTO)However, TCP does not update RTO based on duplicate ACKs

Duplicate ACKs


RTT: 356msRTO: 290msRTT > RTO, timeout!

Retransmission timeout causes slow start

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

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

Backup

Netflix App Case Study

1 computer networks lecture 18 tcp cubic, tcp in 4g lte 11/5/2013 lecturer: namratha vedire

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