Fall 2006 CS 561 1

Congestion Control

OutlineQueuing Discipline

Reacting to Congestion

Avoiding Congestion

Quality of Service

Fall 2006 CS 561 2

Issues• Two sides of the same coin

– pre-allocate resources so at to avoid congestion– control congestion if (and when) is occurs

• Two points of implementation– hosts at the edges of the network (transport protocol)– routers inside the network (queuing discipline)

• Underlying service model– best-effort– multiple qualities of service (QoS)

Destination1.5-Mbps T1 link

Router

Source2

Source1

100-Mbps FDDI

10-Mbps Ethernet

Fall 2006 CS 561 3

Framework• Connectionless flows

– sequence of packets sent between source/destination pair– maintain soft state at the routers

• Taxonomy– router-centric versus host-centric– reservation-based versus feedback-based– window-based versus rate-based

Router

Source2

Source1

Source3

Router

Router

Destination2

Destination1

Fall 2006 CS 561 4

Evaluation

• Fairness• Power (ratio of throughput to delay)

Optimalload Load

Th

rou

ghp

ut/d

elay

Fall 2006 CS 561 5

Queuing Discipline• First-In-First-Out (FIFO)

– does not discriminate between traffic sources– drop policy (tail-drop, random early drop)

• Fair Queuing (FQ)– explicitly segregates traffic based on flows– ensures no flow captures more than its share of capacity– variation: weighted fair queuing (WFQ)

• Problem?Flow 1

Flow 2

Flow 3

Flow 4

Round-robinservice

Fall 2006 CS 561 6

FQ Algorithm

• Suppose clock ticks each time a bit is transmitted• Let Pi denote the length of packet i• Let Si denote the time when start to transmit packet i• Let Fi denote the time when finish transmitting packet i• Fi = Si + Pi

• When does router start transmitting packet i?– if before router finished packet i - 1 from this flow, then

immediately after last bit of i - 1 (Fi-1)– if no current packets for this flow, then start transmitting

when arrives (call this Ai)• Thus: Fi = MAX (Fi - 1, Ai) + Pi

Fall 2006 CS 561 7

FQ Algorithm (cont)

• For multiple flows– calculate Fi for each packet that arrives on each flow– treat all Fi’s as timestamps– next packet to transmit is one with lowest timestamp

• Not perfect: can’t preempt current packet• Example

Flow 1 Flow 2

(a) (b)

Output Output

F = 8 F = 10F = 5

F = 10

F = 2

Flow 1(arriving)

Flow 2(transmitting)

Fall 2006 CS 561 8

TCP Congestion Control

• Idea– assumes best-effort network (FIFO or FQ routers) each

source determines network capacity for itself

– uses implicit feedback

– ACKs pace transmission (self-clocking)

• Challenge– determining the available capacity in the first place

– adjusting to changes in the available capacity

Fall 2006 CS 561 9

Additive Increase/Multiplicative Decrease

• Objective: adjust to changes in the available capacity• New state variable per connection: CongestionWindow

– limits how much data source has in transit

MaxWin = MIN(CongestionWindow, AdvertisedWindow)

EffWin = MaxWin - (LastByteSent - LastByteAcked)

• Idea:– increase CongestionWindow when congestion goes down– decrease CongestionWindow when congestion goes up

Fall 2006 CS 561 10

AIMD (cont)

• Question: how does the source determine whether or not the network is congested?

• Answer: a timeout occurs– timeout signals that a packet was lost– packets are seldom lost due to transmission error– lost packet implies congestion

Fall 2006 CS 561 11

AIMD (cont)

• In practice: increment a little for each ACKIncrement = (MSS * MSS)/CongestionWindow

CongestionWindow += Increment

Source Destination

…

• Algorithm– increment CongestionWindow by

one packet per RTT (linear increase)

– divide CongestionWindow by two whenever a timeout occurs (multiplicative decrease)

Fall 2006 CS 561 12

AIMD (cont)

• Trace: sawtooth behavior

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

KB

Time (seconds)

70

304050

10

10.0

Fall 2006 CS 561 13

Slow Start

• Objective: determine the available capacity in the first place

• Idea:– begin with CongestionWindow = 1

packet– double CongestionWindow each RTT

(increment by 1 packet for each ACK)

Source Destination

…

Fall 2006 CS 561 14

Slow Start (cont)• Exponential growth, but slower than all at once• Used…

– when first starting connection– when connection goes dead waiting for timeout

• Trace

• Problem: lose up to half a CongestionWindow’s worth of data

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

KB

70

304050

10

Fall 2006 CS 561 15

Fast Retransmit and Fast Recovery

• Problem: coarse-grain TCP timeouts lead to idle periods

• Fast retransmit: use duplicate ACKs to trigger retransmission

Packet 1

Packet 2

Packet 3

Packet 4

Packet 5

Packet 6

Retransmitpacket 3

ACK 1

ACK 2

ACK 2

ACK 2

ACK 6

ACK 2

Sender Receiver

Fall 2006 CS 561 16

Results

• Fast recovery– skip the slow start phase– go directly to half the last successful CongestionWindow (ssthresh)

60

20

1.0 2.0 3.0 4.0 5.0 6.0 7.0

KB

70

304050

10

Fall 2006 CS 561 17

Congestion Avoidance• TCP’s strategy

– control congestion once it happens

– repeatedly increase load in an effort to find the point at which congestion occurs, and then back off

• Alternative strategy– predict when congestion is about to happen

– reduce rate before packets start being discarded

– call this congestion avoidance, instead of congestion control

• Two possibilities – router-centric: DECbit and RED Gateways

– host-centric: TCP Vegas

Fall 2006 CS 561 18

DECbit• Add binary congestion bit to each packet header• Router

– monitors average queue length over last busy+idle cycle

– set congestion bit if average queue length > 1– attempts to balance throughout against delay

Queue length

Currenttime

TimeCurrent

cyclePrevious

cycleAveraginginterval

Fall 2006 CS 561 19

DECbit (cont)

• Destination echoes bit back to source• Source records how many packets resulted in set bit• If less than 50% of last window’s worth had bit set

– increase CongestionWindow by 1 packet

• If 50% or more of last window’s worth had bit set – decrease CongestionWindow by 0.875 times

Fall 2006 CS 561 20

Random Early Detection (RED)

• Notification is implicit – just drop the packet (TCP will timeout)– could make explicit by marking the packet

• Early random drop– rather than wait for queue to become full, drop each

arriving packet with some drop probability whenever the queue length exceeds some drop level

Fall 2006 CS 561 21

RED Details• Compute average queue length

AvgLen = (1 - Weight) * AvgLen + Weight * SampleLen

0 < Weight < 1 (usually 0.002)SampleLen is queue length each time a packet arrives

MaxThreshold MinThreshold

AvgLen

Fall 2006 CS 561 22

RED Details (cont)

• Two queue length thresholds

if AvgLen <= MinThreshold then

enqueue the packet

if MinThreshold < AvgLen < MaxThreshold then

calculate probability P

drop arriving packet with probability P

if MaxThreshold <= AvgLen then

drop arriving packet

Fall 2006 CS 561 23

RED Details (cont)• Computing probability P

TempP = MaxP * (AvgLen - MinThreshold)/ (MaxThreshold - MinThreshold)

P = TempP/(1 - count * TempP)

• Drop Probability CurveP(drop)

1.0

MaxP

MinThresh MaxThresh

AvgLen

Fall 2006 CS 561 24

Tuning RED• Probability of dropping a particular flow’s packet(s) is roughly

proportional to the share of the bandwidth that flow is currently getting• MaxP is typically set to 0.02, meaning that when the average queue size

is halfway between the two thresholds, the gateway drops roughly one out of 50 packets.

• If traffic id bursty, then MinThreshold should be sufficiently large to allow link utilization to be maintained at an acceptably high level

• Difference between two thresholds should be larger than the typical increase in the calculated average queue length in one RTT; setting MaxThreshold to twice MinThreshold is reasonable for traffic on today’s Internet

• Penalty Box for Offenders

Fall 2006 CS 561 25

TCP Vegas• Idea: source watches for some sign that router’s queue is

building up and congestion will happen too; e.g.,– RTT grows

– sending rate flattens 60

20

0.5 1.0 1.5 4.0 4.5 6.5 8.0

KB

Time (seconds)

Time (seconds)

70

304050

10

2.0 2.5 3.0 3.5 5.0 5.5 6.0 7.0 7.5 8.5

900

300100

0.5 1.0 1.5 4.0 4.5 6.5 8.0

Sen

din

g K

Bp

s 1100

500700

2.0 2.5 3.0 3.5 5.0 5.5 6.0 7.0 7.5 8.5

Time (seconds)0.5 1.0 1.5 4.0 4.5 6.5 8.0Q

ueu

e s

ize

in r

oute

r

5

10

2.0 2.5 3.0 3.5 5.0 5.5 6.0 7.0 7.5 8.5

Fall 2006 CS 561 26

Algorithm • Let BaseRTT be the minimum of all measured RTTs (commonly the

RTT of the first packet)• If not overflowing the connection, then

ExpectRate = CongestionWindow/BaseRTT• Source calculates sending rate (ActualRate) once per RTT• Source compares ActualRate with ExpectRate

Diff = ExpectedRate - ActualRateif Diff <

increase CongestionWindow linearlyelse if Diff >

decrease CongestionWindow linearlyelse

leave CongestionWindow unchanged

Fall 2006 CS 561 27

Algorithm (cont)

• Parameters = 1 packet = 3 packets

• Even faster retransmit– keep fine-grained timestamps for each packet – check for timeout on first duplicate ACK

70605040302010

KB

Time (seconds)

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0

CA

M K

Bps

2402001601208040

Time (seconds)

Fall 2006 CS 561 28

Realtime Applications• Require “deliver on time” assurances

– must come from inside the network

• Example application (audio)– sample voice once every 125us– each sample has a playback time– packets experience variable delay in network– add constant factor to playback time: playback point

Microphone

Speaker

Sampler,A D

converter

Buffer,D A

Fall 2006 CS 561 29

Playback BufferS

eque

nce

num

ber

Packetgeneration

Networkdelay

Buffer

Playback

Time

Packetarrival

Fall 2006 CS 561 30

Example Distribution of Delays

1

2

3

Pa

cke

ts (

%)

90% 97% 98% 99%

150 20010050

Delay (milliseconds)

Fall 2006 CS 561 31

Integrated Services

• Service Classes– guaranteed

– controlled-load

• Mechanisms– signalling protocol

– admission control

– policing

– packet scheduling

Fall 2006 CS 561 32

Flowspec

• Rspec: describes service requested from network– controlled-load: none– guaranteed: delay target

• Tspec: describes flow’s traffic characteristics– average bandwidth + burstiness: token bucket filter– token rate r– bucket depth B– must have a token to send a byte– must have n tokens to send n bytes– start with no tokens– accumulate tokens at rate of r per second– can accumulate no more than B tokens

Fall 2006 CS 561 33

Differentiated Services• Problem with IntServ: scalability• Idea: segregate packets into a small number of classes

– e.g., premium vs best-effort

• Packets marked according to class at edge of network• Core routers implement some per-hop-behavior (PHB)• Example: Expedited Forwarding (EF)

– rate-limit EF packets at the edges– PHB implemented with class-based priority queues or WFQ

Fall 2006 CS 561 34

DiffServ (cont)

• Assured Forwarding (AF)– customers sign service

agreements with ISPs

– edge routers mark packets as being “in” or “out” of profile

– core routers run RIO: RED with in/out

P(drop)

1.0

MaxP

Min in MaxinMaxoutMinout

AvgLen