Congestion control
This presentation is the property of its rightful owner.
Sponsored Links
1 / 34

Congestion Control PowerPoint PPT Presentation


  • 68 Views
  • Uploaded on
  • Presentation posted in: General

Congestion Control. Outline Queuing Discipline Reacting to Congestion Avoiding Congestion Quality of Service. Source. 1. 10-Mbps Ethernet. Router. Destination. 1.5-Mbps T1 link. 100-Mbps FDDI. Source. 2. Issues. Two sides of the same coin

Download Presentation

Congestion Control

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Congestion control

Congestion Control

Outline

Queuing Discipline

Reacting to Congestion

Avoiding Congestion

Quality of Service

CS 561


Issues

Source

1

10-Mbps Ethernet

Router

Destination

1.5-Mbps T1 link

100-Mbps FDDI

Source

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)

CS 561


Framework

Source

1

Router

Destination

1

Router

Source

2

Router

Destination

2

Source

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

CS 561


Evaluation

Throughput/delay

Optimal

Load

load

Evaluation

  • Fairness

  • Power (ratio of throughput to delay)

CS 561


Queuing discipline

Flow 1

Flow 2

Round-robin

service

Flow 3

Flow 4

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?

CS 561


Fq algorithm

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

CS 561


Fq algorithm cont

Flow 1

Flow 2

Flow 1

Flow 2

Output

(arriving)

(transmitting)

Output

F = 10

F = 10

F = 8

F = 5

F = 2

(a)

(b)

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

CS 561


Tcp congestion control

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

CS 561


Additive increase multiplicative decrease

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

CS 561


Aimd cont

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

CS 561


Aimd cont1

Source

Destination

AIMD (cont)

  • Algorithm

    • increment CongestionWindow by one packet per RTT (linear increase)

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

  • In practice: increment a little for each ACK

    Increment = (MSS * MSS)/CongestionWindow

    CongestionWindow += Increment

CS 561


Aimd cont2

70

60

50

40

KB

30

20

10

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

T

ime (seconds)

AIMD (cont)

  • Trace: sawtooth behavior

CS 561


Slow start

Source

Destination

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)

CS 561


Slow start cont

70

60

50

KB

40

30

20

10

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

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

CS 561


Fast retransmit and fast recovery

Sender

Receiver

Packet 1

Packet 2

ACK 1

Packet 3

ACK 2

Packet 4

ACK 2

Packet 5

Packet 6

ACK 2

ACK 2

Retransmit

packet 3

ACK 6

Fast Retransmit and Fast Recovery

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

  • Fast retransmit: use duplicate ACKs to trigger retransmission

CS 561


Results

70

60

50

40

KB

30

20

10

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Results

  • Fast recovery

    • skip the slow start phase

    • go directly to half the last successful CongestionWindow (ssthresh)

CS 561


Congestion avoidance

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

CS 561


Decbit

Queue length

Current

time

T

ime

Previous

Current

cycle

cycle

A

veraging

interval

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

CS 561


Decbit cont

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

CS 561


Random early detection red

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

CS 561


Red details

MaxThreshold

MinThreshold

A

vgLen

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

CS 561


Red details cont

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

CS 561


Red details cont1

P(drop)

1.0

MaxP

A

vgLen

MinThresh

MaxThresh

RED Details (cont)

  • Computing probability P

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

    P = TempP/(1 - count * TempP)

  • Drop Probability Curve

CS 561


Tuning red

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

CS 561


Tcp vegas

70

60

50

40

KB

30

20

10

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

8.5

Time (seconds)

1100

900

700

Sending KBps

500

300

100

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

8.5

Time (seconds)

10

5

Queue size in router

0.5

1.0

1.5

4.0

4.5

6.5

8.0

Time (seconds)

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

2.0

2.5

3.0

3.5

5.0

5.5

6.0

7.0

7.5

8.5

CS 561


Algorithm

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 - ActualRate

    if Diff < a

    increase CongestionWindow linearly

    else if Diff > b

    decrease CongestionWindow linearly

    else

    leave CongestionWindow unchanged

CS 561


Algorithm cont

70

60

50

40

KB

30

20

10

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

T

ime (seconds)

240

200

160

CAM KBps

120

80

40

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

T

ime (seconds)

Algorithm (cont)

  • Parameters

    • a = 1 packet

    • b = 3 packets

  • Even faster retransmit

    • keep fine-grained timestamps for each packet

    • check for timeout on first duplicate ACK

CS 561


Realtime applications

Sampler

,

Microphone

Buffer

,

A D

D A

converter

Speaker

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

CS 561


Playback buffer

Playback Buffer

Packet

arrival

Packet

generation

Playback

Sequence number

Buffer

Network

delay

T

ime

CS 561


Example distribution of delays

Example Distribution of Delays

90%

97%

98%

99%

3

2

Packets (%)

1

50

100

150

200

Delay (milliseconds)

CS 561


Integrated services

Integrated Services

  • Service Classes

    • guaranteed

    • controlled-load

  • Mechanisms

    • signalling protocol

    • admission control

    • policing

    • packet scheduling

CS 561


Flowspec

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

CS 561


Differentiated services

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

CS 561


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

A

vgLen

Min

Min

Max

Max

out

in

out

in

DiffServ (cont)

CS 561


  • Login