ECE544: Communication Networks-II, Spring 2012

D. RaychaudhuriLecture 8 (QoS)

Includes teaching materials from D. Raychaudhuri, L. Peterson

Today’s Lecture

• Congestion control in best effort networks– Basic principles & mechanisms– FQ, WFQ, congestion feedback, TCP,

RED• Quality-of-service (QoS)

– Mechanisms (traffic shaping, admission control, reservation, priority queuing)

– RSVP Intserv and Diffserv, RIO– Comparison to ATM (CBR, VBR; ABR)

Congestion Control & QoS in Packet Networks

• Congestion control – reactive methods used in best effort networks– Packet scheduling at network nodes– Feedback congestion control

• End-to-end• Hop-by-hop

• QoS control – proactive methods used for premium or guaranteed services:– Source traffic shaping & policing at entry

points– Priority queuing and packet drop at routers– End-to-end reservation and admission

control

Network Congestion• All networks have saturating throughput

– Reduction in performance beyond max capacity

– Need to keep input load below G0– Also must avoid unstable equilibrium point in overload

region

Overloadregion

Normal operatingPoint (G0)

Capacity LimitSmax

Offered Traffic (G)

ThruTrafficmargin

Congestion control policies

Unstable network load

Stable network load lineswith congestion control

Queue Scheduling

• A queue scheduler employs 2 strategies:– Scheduling discipline: Which packet

to serve (transmit) next– Drop policy: Which packet to drop

next (when required)

FIFO Queuing

• FIFO:first-in-first-out (or FCFS: first-come-first-serve)

• Arriving packets get dropped when queue is full regardless of flow or importance - implies drop-tail

• Important distinction:– FIFO: scheduling discipline– Drop-tail: drop policy

Fair Queuing

• Main idea:– maintain a separate queue for each

flow currently flowing through router– router services queues in Round-

Robin fashion

FQ illustrationFlow 1

Flow 2

Flow n

I/P O/P

Variation: Weighted Fair Queuing (WFQ)

Some Complications

• Packets are of different length• We really need bit-by-bit round-

robin (RR)• FQ simulates bit-by-bit RR

– Not feasible to interleave bits!

Bit-by-bit RR• Single flow: suppose clock ticks when a bit is

transmitted. For packet i:– Pi: length, Ai = arrival time, Si: begin transmit time, Fi:

finish transmit time. Fi = Si+Pi

– Fi = max (Fi-1, Ai) + Pi

• Multiple flows: clock ticks when a bit from all active flows is transmitted, that is, the clock advances by one tick when n bits are transmitted (assuming n flows)– calculate Fi for each packet– transmit packet with earliest Fi

• RR is only simulated, packet in transmission is not interrupted.

• If n flows have data to transmit, each gets 1/nth bandwidth.

Bit-by-bit RR

Flow 1 Flow 2

Pkt 2-1=3 units

Pkt 1-1=2 units

Pkt 2-2=2 units

Pkt 1-2=1 unit

Pkt 1-3=1 unit

Channel clock - 1-1

P(1,1) = 2P(1,2) = 1P(1,3) = 1

P(2,1) = 3P(2,2) = 2

Start with A(*,*)=0 (all pkts arrive at T=0)

F(1,1) = 1F(1,2) = 1.5F(1,3) = 2

F(2,1) = 1.5F(2,2) = 2.5

Fi = max (Fi-1, Ai) + Pi

2-1 1-2 1-3 2-2

Weighted Fair Queuing (WFQ)

• Weighted Fair Queuing (WFQ): assign a weight to each flow– Assume transmitting wq bits each time the

router serves queue q (simulate in packet scheduling)

– Control the percentage of the link’s bandwidth that a flow will get• The bandwidth that flow q gets (n active queues

sending data):

– FQ is a special case of WFQ with a weight of 1 for each queue

n

qq

qq

w

wB

1

Congestion Control and Congestion Avoidance

• TCP’s “blind” approach:– Detect congestion (loss) after it happens and back off on

offered rate– Increase load trying to maximize utilization until loss

occurs

SourceRate(bps)

Congestion detected(via packet loss)

Time-out

• Alternatively:– We can try to predict congestion and reduce rate before

packets start being discarded– This is called congestion avoidance

Congestion Control via Router Feedback

• Router has unified view of queuing behavior

• Routers can distinguish between propagation and persistent queuing delays

• Routers can decide on transient congestion, based on workload

Solving the Full Queues Problem

• Router monitors the load• Drop (or mark) packets before

queue becomes full (early drop)• Intuition: notify senders of incipient

congestion– Simple example:

• If qlen > drop level, drop (or mark) each new packet with a fixed probability p

• Does not control misbehaving users

Random Early Detection (RED)

• Motivation:– High bw-delay flows have large

queues to accommodate transient congestion

– TCP detects congestion from loss - after queues have built up and increase delay

• Aim:– Keep throughput high and delay low– Accommodate bursts

Random Early Detection (RED)

• Detect incipient congestion, allow bursts

• Keep power (throughput/delay) high– keep average queue size low– assume hosts respond to lost packets

• Avoid window synchronization– randomly mark packets, instead of

dropping• Avoid bias against bursty traffic• Some protection against ill-behaved

users

RED Algorithm

• Maintain running average of queue length

• If avg < minth do nothing– Low queuing, send packets through

• If avg > maxth, drop all packets– Protection from misbehaving sources

• Else randomly drop (or mark) some packets in a manner proportional to queue length– Notify sources of incipient congestion

RED Operation

Min threshold

Max threshold

Average queuelength

minthresh maxthresh

MaxP

1.0

Avg length

P(drop)

• If AvgLen <= MinThreshold– Queue the packet

• If MinThreshold < Avglen < MaxThreshold– Calculate probability P – Drop the arriving packets with probability P

• If Avglen >= MaxThreshold– Drop the arriving packet

• AveLen = (1-W) x AveLen + W x SampleLen

)1/(

)(

)(

TempPcountTemPP

ldMinThresholdMaxThresho

ldMinThreshoAvgLenMaxPTempP

• Count:# of newly arriving packets that have been queued (not dropped) while AvgLen has been between the two thresholds– Count increases, P increases– Make drop more evenly distributed (Avoid

bias against bursty traffic)

Explicit Congestion Control

• Router can signal the congestion by marking packets instead of dropping using RED– Set the ECN bit (bit 7 of

the IP TOS field) in IP header

• The destination copies the ECN bit into the ACK sent back to the source

• The source TCP responds to the ECN bit set in the same way as a packet drop

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

• ToS field => now used for DiffServ and ECN– Bits 0-5: Differentiated

Services Code Point (DSCP)– Bit 6: ECN-capable– Bit 7: ECN

Quality of Service

OutlineRealtime ApplicationsIntegrated ServicesDifferentiated Services

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– Use initial buffering delay to compensate jitter, but

result in longer end-to-end delay

Microphone

Speaker

Sampler,A D

converter

Buffer,D A

Playback BufferS

eque

nce

num

ber

Packetgeneration

Networkdelay

Buffer

Playback

Time

Packetarrival

Example Distribution of Delays

1

2

3

Pa

cke

ts (

%)

90% 97% 98% 99%

150 20010050

Delay (milliseconds)

Application requirements & Services Classes

• Different application requirements – Elastic: no restrict delay requirements, traditional data – Real-time: delay bound, jitter, loss

• Loss: intolerant or tolerant to some loss• Delay: adaptive (e.g. lengthening/shortening the silence between

words, playing back video a little slower, etc) or not adaptive • Data rate: adaptive (e.g. reduce video quality by compressing video

more) or not adaptive• Different application requirements=>different service

classes (not only best effort anymore)• A network that can provide these different levels of service is

said to support QoS– Integrated service: fine-grained approach, provide QoS to

individual applications or flows• Allow individual application flows to specify their needs to the

routers using an explicit signaling mechanism (RSVP)• Scalability is an issue

– Differentiated Service: coarse-grained approach, provide QoS to several classes of data or aggregated traffic

• Assign packets into a small number of classes that receive differentiated treatment in the routers

Taxonomy of applications

Applications

ElasticReal-Time

Loss, delay tolerant

IntolerantInteractive

Non-adaptiveadaptive Non-adaptive

Delayadaptive

Rateadaptive

Rateadaptive

Asynchronous

Interactive-bulk

Components of Integrated Services architecture

• Flowspec: information of the flow traffic characteristics and its service

• Reservations (includes reservation signaling protocol)

• Admission control based on flow description and current load

• Scheduling to meet the reservation• Traffic shaping at edges to fit reservation• Traffic policing to mark or drop non-conforming

traffic• Some application adaptation

Types of guarantees

• Absolute bound on delay and jitter• Absolute bound on delay only• Statistical bound on delay• No quantitative delay bound but

admission control and preferential treatment

• None

Internet service classes proposed by IETF

• Guaranteed service– firm bounds on e2e delays and bandwidth

• Controlled load– “a QoS closely approximating the QoS that

same flow would receive from an unloaded network element, but uses capacity (admission) control to assure that this service is received even when the network element is overloaded”• Use a queuing mechanism such as WFQ to isolate

the controlled load traffic from other traffic• Admission control to limit the total amount of

controlled load traffic

• Best effort

Overview of mechanisms

• Flow specification (flowspec)– type of service we require

• Admission control– can the network provide the

requested service?• Resource reservation protocol

– RSVP• Packet scheduling

Flowspecs

• Tspec: describes the flow’s traffic characteristics

• Rspec: describes the service requested from the network

Token bucket filter• Described by 2 parameters:

– token rate r: rate of tokens placed in the bucket

– bucket depth B: capacity of the bucket

• Operation:– tokens are placed in bucket at rate r– if bucket fills, tokens are discarded– sending a packet of size P uses P tokens– if bucket has P tokens, packet sent at max

rate, else must wait for tokens to accumulate

Token bucket operation

tokens

Packet

overflow

tokens

tokens

Packet

Enough tokenspacket goes through,tokens removed

Not enoughtokens - wait fortokens toaccumulate

TB characteristics

• On the long run, rate is limited to r• On the short run, a burst of size B

can be sent at peak data rate• Amount of traffic entering at

interval T is bounded by:– traffic = B + r*T

• Information useful to admission algorithm

Token bucket specs

BW

Time

1

2

1 2 3

Flow A

Flow BFlow A: r = 1 Mbps, B=1 byte

Flow B: r = 1 Mbps, B=1MB

Admission control

• When new flow request arrives, look at Rspec and Tspec and decide whether to admit or reject– Can it provide the desired service

requested by the flow, given the currently available resources without causing any previously admitted flow to receive worse service that agreed?

• Not policing

Reservation protocol: RSVP

Upper layer protocols and applications

IP

Link layer modules

ICMP IGMP RSVP

IP service interface

Link layer service interface

RSVP• Used on connectionless networks• Relies on soft state: reservations

must be refreshed and do not have to be explicitly deleted

• Aims to support multicast as effectively as unicast flows - mcast apps good candidates for real-time, and are heterogeneous

• Receiver-oriented approach

Basic message types

• PATH message• RESV message• CONFIRMATION message

– generated only upon request– unicast to receiver when RESV

reaches node with established state• TEARDOWN message• ERROR message (if path or RESV

fails)

Making a reservation

• Receivers make reservation• Before making a reservation,

receiver must know:– type of traffic sender will send (Tspec)– path the sender’s packets will follow

• Both can be accomplished by sending PATH messages by the sender

PATH messages• PATH messages carry sender’s Tspec

and sent from the sender to the receiver• Record the path from the sender to the

receiver in the PATH message• Receivers send RESV messages that

follow reverse path and setup reservations

• If reservation cannot be made, user gets an error

PATH and RESV messages

R

Sender 1

Sender 2

receiver 1

receiver 2

R R

R

PATH

PATH RESV

RESV

RESV (merged)

Soft State• Allow increasing or decreasing the level of

resource reservation• Adapt to link or router failure and topology

changes– Routing protocol makes routing changes, RSVP

adjusts reservation state– In absence of route or membership changes,

periodic PATH and RESV msgs refresh established reservation state

– When change, new PATH msgs follow new path, new RESV msgs set reservation

– Non-refreshed state times out automatically

Router handling of RESV messages

• If new request rejected, send error message

• If admitted:– install packet filter into forwarding

dbase– pass flow parameters to scheduler– activate packet policing if needed– forward RESV msg upstream

Packet classifying and scheduling

• Each arriving packet must be:– classified: associated with the

application reservation• Examining up to five fields in the packet:

source + destination address, protocol number, source + destination port

– scheduled: managed in the queue so that it receives the requested service• implementation not specified in the

service model

RSVP and multicast

• Reservations from multiple receivers for a single sender are merged together at branching points

• Reservations for multiple senders may not be added up:– audio conference, not many talk at same

time– Different reservation styles:

• Reserve resources for all speakers• Reserve resources for any n speakers• Reserve resources for speakers A and B only

RSVP versus ATM (Q.2931)• RSVP

– receiver generates reservation– soft state (refresh/timeout)– separate from route establishment– QoS can change dynamically– receiver heterogeneity

• ATM– sender generates connection request– hard state (explicit delete)– concurrent with route establishment– QoS is static for life of connection– uniform QoS to all receivers

ATM Service CategoriesATM Service Categories• CBR

– Constant Bit Rate– Continuous flow of data with tight bounds on delay and delay

variation

• rt-VBR– Real-Time Variable Bit Rate– Variable bandwidth with tight bounds on delay and delay

variation

• nrt-VBR– Non-Real-Time Variable Bit Rate– Variable bandwidth with tight bound on cell loss

• UBR– Unspecified Bit Rate– No guarantees (i.e., best effort delivery)

• ABR– Available Bit Rate– Flow control on source with tight bound on cell loss

Traffic Shaping• Traffic shaping: control traffic in order to

conform to the traffic contract by delaying packets to meet certain criteria.– To optimize or guarantee performance (lower

latency, higher usable bandwidth)– commonly applied at the network edges to control

traffic entering the network, but can also be applied by the traffic source or in the network

• Token bucket– tokens are placed in bucket at rate r– if bucket fills, tokens are discarded– sending a packet of size P uses P tokens– if bucket has P tokens, packet sent at max rate, – else must wait for tokens to accumulate

Traffic Policing• Traffic policing: router monitors the flow traffic

for conformity with a traffic contract – Drop or tag the packets not conforming to the TSpec

that used to make the reservation– to enforce compliance with that contract

• Token bucket– tokens are placed in bucket at rate r– if bucket fills, tokens are discarded– When a packet of p bytes arrives, p tokens are

removed from the bucket, and the packet is sent to the network.

– If fewer than p tokens are available, no tokens are removed from the bucket, and the packet is considered to be non-conformant.

– Drop or mark the non-conformant packets

Differentiated Services (DiffServ)

• Analogy:– airline service, first class, coach,

various restrictions on coach as a function of payment

• Best-effort expected to make up bulk of traffic, but revenue from “premium” service important to economic base (will pay for more plentiful bandwidth overall)– Not motivated by real-time,

motivated by economics and assurances

Differentiated Services (cont)• Divide traffic into a small number of

classes and allocates resource on a per-class basis, instead of individual flows– Scalable– No need for reservations: just mark

packets• E.g. packet marking can be done at

administrative boundaries before injecting packets into network

– Significant savings in signaling, much simpler overall

IP DiffServ

• IP packets carry 6-bit service code points (DSCP) – Potentially support

64-different classes of services

– In implementation, the number of DSCPs used in a network is much smaller, e.g. simple two-class network

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

• ToS field => now used for DiffServ and ECN– Bits 0-5: Differentiated

Services Code Point (DSCP)– Bit 6: ECN-capable– Bit 7: ECN

DiffServ• Where and how to set the DSCP value

– Host set DSCP based on knowledge of application requirements

– Router at an administrative boundary set DSCP according to a certain policy

• What does a router do when it receives a packet marked with a certain DSCP– Routers map DSCP to per-hop-behavior (PHB)– PHBs can be standard or local– Standard PHBs include

• Default: No special treatment or best effort• Expedited forwarding (EF): should be forwarded with

low delay and loss• Assured forwarding (AF): Multiple classes, each class

with multiple drop preference

Expedited Forwarding (EF)

• A router should forward the EF packets with minimal delay and low loss

• To implement, put all EF packets in a dedicated EF queue and ensure that the arrival rate of packets to the queue is less than the service rate– Configure the router at the edge of an

administrative domain to allow a certain max rate of EF packets arrivals into the domain• Limit the EF packet rate < the bandwidth of the

slowest link in the domain– Give EF packet queue high priority, e.g.

WFQ with a high weight to EF queue

Assured Forwarding (AF)• A set of AF PHBs is defined• AFxy:

– x: class, typically determine a queue for the packet

– y:drop preference– For example: AF11, AF12 and AF13 packets

are put into the same queue, but AF13 would have a greater chance to be dropped due to congestion. AF11 and AF2y would go to different queues

• 12 AF PHBs are recommended by IETF, 4 AF classes with three drop preference levels each

An Example DiffServ Network Implementation

• Support Two-type DiffServ –Premium (expedited) service: (type P)

• User sends within profile, network commits to delivery with the agreed profile

– Classify packet in one of two queues, use priority– Shaping at trust boundaries only, using token bucket

• Allocate resource based on the agreed peak rate– conservative, virtual wire services– unused premium goes to best effort (subsidy!)

–Assured service: (type A)• based on expected capacity usage profiles• traffic unlikely to be dropped if user maintains

profile. Out-of-profile traffic marked

Premium traffic flow

first hoprouter

internalrouter

borderrouter

host

borderrouter

ISP

Company A

Unmarkedpacket flow

Packets in premiumflows have bit set

Premium packet flowrestricted to R bytes/sec

2-bit differentiated service

• Use precedence field to encode P & A type packets

• P packets are queued at higher priority than ordinary best effort

• A packets treated preferentially wrt dropping probability in the normal queue

• Leaf and border routers have input and output tasks - other routers just output

Leaf router input functionality

ClearA & P

bits

Packetclassifier

Marker 1

Marker N

Forwardingengine

Arrivingpacket Best effort

Flow

1Fl

ow N

• Marker: Leaf routers have traffic profiles - they classify packets based on packet header (destination addr & port, source addr & port, protocol ID, etc)– If no profile present, pass as best effort– If profile is for A:

• mark in-profile packets with A, forward others unmarked– If profile is for P:

• delay out-of -profile packets to shape into profile

Markers to implement two different services

Wait fortoken

Set P bitPacketinput

Packetoutput

Drop on overflow

P service

Test iftoken

Set A bittoken

No token

Packetinput

Packetoutput

A service

Router output interface for two-bit architecture

• 2 queues: P packets on higher priority queue• Lower priority queue implements RED “In or

Out” scheme (RIO)

P-bit set?

If A-bit setincr A_cnt

High-priority Q

Low-priority Q

If A-bit setdecr A_cnt

RIO queuemanagement

Packets out

yes

no

Border router input interface Profile Meters

Arrivingpacket

Is packetmarked?

Tokenavailable?

Tokenavailable?

Clear A-bit

Drop packet

Forwardingengine

A set

P set

token

token

Not marked

no

no

• At border routers, profile meters test marked flows:– drop P packets out of profile– unmark A packets

Red with In or Out (RIO)• Similar to RED, but with two separate probability curves• Has two classes, “In” and “Out” (of profile)• “Out” class has lower Minthresh, so packets are dropped from

this class first• As avg queue length increases, “in” packets are dropped

MaxP

1.0

Minout Minin MaxinMaxout

P(drop)

AvgLen

65

Today’s Homework• Chapter 6

-6.13 4th ed-6.32 -6.43 -6.44

Due 3/30/12