AQM & TCP models

Courtesy of

Sally Floyd with ICIR

Raj Jain with OSU

• Queue management- Passive- Active

• AQM: RED Variants• ECN• TCP models

Agenda

the majority in router

• Passive queue management (PQM)– No preventive packet drop– Buffer level > threshold, drop packets– Two dropping schemes

• Tail-drop• Drop-from-front

Drop-tail

Drop-from-front

Which is better?

Problems with PQM

• A trade-off between the buffer size and QoS• Larger buffer results in higher throughput, but

longer delay• Lock out: A single connection monopolises

the buffer space– Give rise to fairness problem

• Full queue: Queue is full for a long period of time– Long queuing delay

Global synchronization

Global Synchronization

• When queue overflows, several connections decrease congestion windows simultaneously

Bias Against Bursty Traffic• Bursty traffic more likely to be dropped

average queue length

V.S.

Objective: Congestion Avoidance

• Maintains low delay and high throughput– Average queue size kept low– Actual queue size grows enough to

handle:• Bursty Traffic• Transient Congestion

Active Queue Management (AQM)

• Provide preventive measures to manage a buffer to eliminate problems associated with PQM

• Characteristics:– Preventive random packet drop is performed

before the buffer is full– The probability of preventive packet drop

increases with the increasing level of congestion

• Goals:– Reduce dropped packets– Support low-delay interactive services– Avoid lock-out

Random Early Detection (RED)

• A router maintains two thresholds:• Min_th:

– Accept all packets until the queue reaches Min_th– Drop packets with a linear drop probability when

the queue is greater than Min_th

• Max_th: All packets are dropped with probability of 1 when the queue exceeds this threshold

RED Algorithm

drop probability

Qminth maxth

Max_drop

1

Selection of Maximum Drop Probability for RED

• Selection of Max_drop significantly affects the performance of RED– Too small: Active packet drops not enough to

prevent global synchronisation– Too large: Decreases the throughput– Optimal value depends on number of connections,

round trip time, etc.

• Selection of an optimal value for Max_drop remains an open issue

RED: Calculating Average Queue Size

• Use low-pass filter (exponential weighted moving average)

• wq should be small enough to filter out transient congestion, and large enough for the average to be responsive

qwavgwavg qq )1(

RED solves the problems

• Global synchronization

• Transient congestion (short queue)

• Bias against bursty Traffic

• Drop packets when congestion eminent

• Select packets at random

• Use average queue length as indicator of congestion

RED Variants

• RED variants can be classified into two categories:– Aggregate control

• Modifying the calculation of the control variable and/or drop function

• Determines packet drop probability

– Per-flow control• Configuring and setting RED’s parameters• Addresses fairness problem

BLUE (aggregate)

• RED: depends only on Q length– For optimal operating point, long Q is

necessary

• Uses packet loss and link utilisation to measure network congestion directly

• Fewer configuration parameters• Advantages:

– Reduces packet loss rate– Keeps the gateway queue stable

• Increases marking/dropping prob. when detects packet loss due to buffer overflow• Decreases marking/dropping prob. when detects that the marking prob. is too aggressive

BLUE

RED Variants Using Per-Flow Accounting

• Flow RED (FRED)

• Fair Buffering RED (FB-RED)

• XRED

• Class-Based Threshold RED (CBT-RED)

• Balanced RED (BRED)

• Stochastic Fair Blue (SFB)

Two variants

• FRED (Fair RED)– fairness among TCP connection– uses per-active-flow accounting (flow’s use of buffer

space)– Scalability problem

• FBRED (Fair Buffering RED)– use of individual bandwidth delay product for each

link to modify the packet drop probability• inverse of the bandwidth delay product to calculate

Max_drop• inverse of the square root of the bandwidth delay product to

calculate Max_drop

Explicit congestion notification (ECN)

RFC 3168

Packet dropped or packet marked

• Instead of dropping packets, packets could be marked. Such marking is called ECN (explicit congestion notification)

• The benefits of ECN– A packet does not have to be retransmitted. (Not that

big of a deal when drop probabilities are small, e.g., 1%)

– Has a dramatic effect when congestion window is small. • Because timeout is avoided.• But why is the congestion window small

– If it small because the link is heavily congested, ECN might not be possible because the queue might truly be full.

ECN in IP header

ECT: ECN-capable transport

TCP should change for ECN

• TCP connection setup– Find out whether endpoints are ECN-

capable

• To inform sender of congestion– ECN-echo (ECE) flag in TCP header

• To inform receiver of window reduction – Congestion Window Reduction (CWR) flag

TCP throughput modeling

Motivation for TCP Modeling

• TCP operating scale is very large– Models are required to gain deeper

understanding of TCP dynamics

• Uncertainties can be modeled as stochastic processes

• Drive the design of TCP-friendly algorithms for multimedia applications

• Optimize TCP performance

TCP Modeling Essentials

• Mainly Reno flavors are modeled• Two main features are modeled

– Window dynamics– Packet loss process

Packet Loss Process

• Packet loss triggers window decrease

• Packet loss is uncertain• This uncertainty is typically

modeled as a stochastic process– E.g. probability p of losing a packet

Window Dynamics

• Linear increase and multiplicative decrease is modeled

• The standard assumption– X(t) = W(t)/RTT

Gallery of TCP Models

• Periodic model• Detailed packet loss model• Finite state machine• Fluid flow model• And others…

Periodic model

TCP Congestion Control: window algorithm

Window: can send W packets

• increase window by one per RTT if no loss, W <- W+1 each RTT

• decrease window by half on detection of loss W <- W/2

sender

receiver

W

1 RTT

Window: can send W packets• increase window by one per RTT if no

loss, W <- W+1 each RTT • decrease window by half on detection of

loss W W/2, when receiving 3 DUPACKs

sender

receiver

W

TCP Congestion Control: window algorithm

TCP throughput/loss relationshipIdealized model:• W is maximum supportable

window size (then loss occurs)

• TCP window starts at w/2 grows to W, then halves, then grows to W, then halves…

• one window worth of packets each rtt

• to find: throughput as function of loss, RTT

TCPwindow

size

time (rtt)

W/2

W

loss occurs

period

# packets sent per “period”

period

222

8

3

8

1

4

1WWW

1 packet lost per period implies 2

8

31 Wp

where

Detailed packet loss model

(TDP)

b = 2 (delayed ACK)Xi = total number of rounds in TDP i

RTT

p1

2][WE

RTT

][

][

AE

YE

MSS is not shown

TCP as an FSM