1 Multimedia Networking: Beyond Best-Effort Internet.

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Multimedia Networking: Beyond Best-Effort Internet

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Outline

Providing QoS in the Internet Scheduling and policing mechanisms

Integrated services Differentiated services

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QoS in IP Networks

Thus far: “making the best of best effort”Future: next-generation Internet with QoS

guarantees Differentiated Services: differential guarantees Integrated Services: firm guarantees

Example: guarantees an audio application 1Mbps; the remaining to 0.5Mbps to Web transfer

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Principles for QoS Guarantees

packet classification: router can distinguish between different classes

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prevents applications from misbehaving (e.g., multimedia app. sends higher than declared rate)

scheduling and policing: provide protection (isolation)

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Allocating fixed (non-sharable) bandwidth to flow: inefficient use of bandwidth if flows doesn’t use its allocation

high utilization: while providing isolation, it is desirable to use resources as efficiently as possible

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Basic fact of life: cannot support traffic demands beyond link capacity

call admission: flow declares its needs, network may block call (e.g., busy signal) if it cannot meet needs

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Summary of QoS Principles

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Outline

Providing QOS in the Internet Scheduling and policing mechanisms


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Traffic Specification

Three common-used criteria: (Long term) Average Rate: how many pkts can be

sent per unit time (in the long run) crucial question: what is the interval length: 100 packets

per sec or 6000 packets per min have same average!

Peak Rate: e.g., 6000 pkts per min. (ppm) avg.; 1500 ppm peak rate

(Max.) Burst Size: max. number of pkts sent consecutively (with no intervening idle)

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Traffic SpecificationToken Bucket: limit input to specified Burst Size

and Average Rate.

bucket can hold b tokens tokens generated at rate r token/sec unless

bucket full over interval of length t: number of packets

admitted less than or equal to (r t + b).

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A More General Version of Token Bucket

A flow is characterized by three parameters (r, b, p) b – token depth r – average rate p bps (peak rate, e.g., link capacity)

r bps

b bits

≤ p bps

regulator time

b

slope p

slope r

arrival curve

The traffic of a flow at the source is regulated by a token bucket: A bit is transmitted only when there is an available token

Arrival curve – maximum amount of bits transmitted during time period of length t

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How is Traffic Specification Used

Part of service agreement, for admission control

For policing, scheduling then can be easier

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Scheduling Mechanisms scheduling: choose next packet to send on link FIFO (first in first out) scheduling: send in order of

arrival to queue discard policy: if packet arrives to full queue: who to

discard?• tail drop: drop arriving packet• priority: drop/remove on priority basis• random: drop/remove randomly

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Scheduling Policies: morePriority scheduling: transmit highest

priority queued packet multiple classes, with different priorities

class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc..

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Scheduling Policies: still moreround robin scheduling: multiple classes cyclically scan class queues, serving one from each class (if available)

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Scheduling Policies: still moreWeighted Fair Queuing: generalized Round Robin each class gets weighted amount of

service in each cycle

For implementations, see backup slides on Virtual Clock.

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Delay Guarantees

token bucket, WFQ combine to guarantee upper bound on delay, i.e., QoS guarantee!

WFQ

token rate, r

bucket size, b

assigned flow rate, R

D = b/Rmax

arrivingtraffic

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Service Curve: Arrival and Departure Processes

Network ElementRin Rout

Rin(t) = arrival process = amount of data arriving up to time t

Rout(t) = departure process = amount of data departing up to time t

bits

t

delay

buffer

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Traffic Envelope (Arrival Curve) Maximum amount of traffic that a flow can

send during an interval of time t

slope = max average rate

b(t) = envelope

slope = peak rate

t

“burstness constraint”

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Service Curve

Assume a flow that is idle at time s and it is backlogged during the interval (s, t)

Service curve: the minimum service received by the flow during the interval (s, t)

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Big Picture

t t

slope = C

t

Rin(t)

service curvebits bits

bits

Rout(t)

arrival curve

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Delay and Buffer Bounds

t

S (t) = service curve

Arrival curve

Maximum delay

Maximum buffer

bits

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Service Curve-based Earliest Deadline (SCED)

Packet deadline – time at which the packet would be served assuming that the flow receives no more than its service curve

Serve packets in the increasing order of their deadlines

Properties If sum of all service curves ≤ C*t All packets will meet their deadlines modulo the transmission

time of the packet of maximum length, i.e., Lmax/C

bits

deadline of 4-th packet

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t

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Linear Service Curves: Example

t

bits

t

bits

t

arrival curves

t

bits

t t

bitsbits

bits

service curves

arrival process

deadlinecomputation

VideoFTP

t

Audio packets have to wait after ftp packets

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Non-Linear Service Curves: Example

t

bits

t

bits

t

Arrival curves

t

bits

t t

bitsbits

bits

Service curves

Arrival process

Deadlinecomputation

t

Video FTP

Audio packets transmittedas soon as they arrive

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Outline



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IETF IntServ Services Architecture for providing QoS guarantees in IP

networks for individual application sessions

Assumptions use a common infrastructure for both real-time

and non-real-time communications

resource must be explicitly managed in order to meet the requirements of real-time applications

• resource reservation: routers maintain state info (a la VC) of allocated resources, QoS req’s

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Intserv QoS: Service Models [rfc2211, rfc 2212]

Guaranteed service: worst case traffic arrival:

leaky-bucket-policed source simple (mathematically

provable) bound on delay [Parekh 1992, Cruz 1988]

Controlled load service: "a quality of service

closely approximating the QoS that same flow would receive from an unloaded network element."

WFQ

token rate, r

bucket size, b

per-flowrate, R

D = b/Rmax

arrivingtraffic Controlled link sharing:

Sharing link among different classes

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Reference Architecture

Admission Control

Data InData Out

Con

trol P

lan

eD

ata

Pla

ne

Scheduler

Routing Routing

MessagesRSVP

messages

Classifier

RSVP

Route Lookup

Forwarding Table Per Flow QoS Table

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Classifier

Buffermanagement

Scheduler

flow 1

flow 2

flow n

…

Per-flow State

A Closer Look at the Data Path

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Intserv: QoS Guarantee Scenario Resource reservation

call setup, signaling (RSVP) traffic, QoS declaration per-element admission control

QoS-sensitive scheduling (e.g., WFQ)

request/reply

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RSVP Protocol

A flow needs performance guarantee must :

declare its QoS requirementR-spec: defines the QoS being requested

characterize traffic it will send into network T-spec: defines traffic characteristics

signaling protocol: needed to carry R-spec and T-spec to routers (where reservation is required)RSVP

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RSVP: Soft-state Receiver-initiatedEnd-to-End Reservation

Sender periodically sends PATH messages to receiver R, each router updates the PATH message by increasing hop count and adding its propagation delay

When receiver R gets the PATH message, it knows Traffic characteristics (tspec): (r,b,R) Number of hops, propagation delay introduced by the routers

Receiver R sends back this information + required worst-case delay in RESV

Each router along path provides a per-hop delay guarantee and forwards RESV with updated info

In the simplest case, the routers can just split the delay State timed out if not refreshed

R1R1

R2R2

R3R3

SS

RR(b,r,R,0,0)

(b,r,R,2,D-D3 )

(b,r,R,1,D-D 3

-D 2)

(b,r,R,0,0) (b,r,R,3,D

)

delay budget

PATH

RESV

(b,r,R,1,d 1

) (b,r,R,2,d1 +d

2 ) (b,r,

R,3,d 1+d 2

+d 3)

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Implementing IntServ: Big Picture

Use WFQ to implement controlled link sharing among different organizations

WFQ provides guaranteed service

Controlled-load and best-effort flows are separated by priority

WFQ

guaranteedflow n

priority

10%30%

controlledflows

best effortflows

WFQ

guaranteedflow 1

CS department gets 50%

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Outline



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IETF Differentiated ServicesConcerns with Intserv: Scalability: signaling, maintaining per-flow router state difficult with large

number of flows Flexible Service Models: Intserv has only two classes. Also want “qualitative”

service classes “behaves like a wire” relative service distinction: Platinum, Gold, Silver

Diffserv approach: simple functions in network core, relatively complex functions at edge routers

(or hosts) Don’t define service classes, provide functional components to build service

classes

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DiffServ Architecture

Edge router:- per-flow traffic management

- marks packets as in-profile and out-profile

Core router:

- per class traffic management

- buffering and scheduling

based on marking at edge

- preference given to in-profile packets- Assured Forwarding

scheduling

...

r

b

marking

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Edge-router Packet Marking

class-based marking: packets of different classes marked differently

intra-class marking: conforming portion of flow marked differently than non-conforming one

profile: pre-negotiated rate A, bucket size B packet marking at edge based on per-flow profile

Possible usage of marking:

User packets

Rate A

B

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Classification and Conditioning

Packet is marked in the Type of Service (TOS) in IPv4, and Traffic Class in IPv6

6 bits used for Differentiated Service Code Point (DSCP) and determine PHB that the packet will receive

2 bits are currently unused

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Classification and Conditioning

may be desirable to limit traffic injection rate of some class:

user declares traffic profile (eg, rate, burst size)

traffic metered, shaped if non-conforming

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Forwarding (PHB)

PHB result in a different observable (measurable) forwarding performance behavior

PHB does not specify what mechanisms to use to ensure required PHB performance behavior

Examples: Class A gets x% of outgoing link bandwidth

over time intervals of a specified length Class A packets leave first before packets from

class B

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Forwarding (PHB)

PHBs being developed: Expedited Forwarding: pkt departure rate

of a class equals or exceeds specified rate logical link with a minimum guaranteed rate

Assured Forwarding: 4 classes of traffic each guaranteed minimum amount of

bandwidth each with three drop preference partitions

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Backup Slides

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Hard Real Time: Guaranteed Services

Service contract Network to client: guarantee a deterministic

upper bound on delay for each packet in a session

Client to network: the session does not send more than it specifies

Algorithm support Admission control based on worst-case

analysis Per flow classification/scheduling at routers

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Soft Real Time: Controlled Load (Predicative) Service

Service contract: Network to client: similar performance as an

unloaded best-effort network Client to network: the session does not send

more than it specifies Algorithm Support

Admission control based on measurement of aggregates

Scheduling for aggregate possible

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Generalized Processor Share: Assume we can arbitrarily divide a bit

0 152 104 6 8

5 1 1 11 1

Red session has packets backlogged between time 0 and 10

Other sessions have packets continuously backlogged

flows

link

Two interpretations: (1) simultaneous transmissions; 2) still round-robin, but we are allowed to transmit wi bits per round for flow I, here wi is extremely small.

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From GPS to Packet Scheduling

General Processor Share (GPS) is defined in an idealized fluid flow model multiple queues can be serviced

simultaneously no non-preemption unit

Real systems are packet systems only one queue is served at a given time packet transmission will not be preempted

Goal define packet algorithms that approximate

the fluid system maintain most of the important properties

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Implementation: Virtual Clock

V(t): virtual clock (or virtual round) If we serve wi bits per round for a

backlogged flow with weight wi, how fast will V(t) progress?

)(tNj jac

w

CV

t

whereNac(t) – number of flows backlogged

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System Virtual Time in GPS

0 4 128 16

1/2

1/81/81/81/8

)(tV

2*C

C2*C

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Virtual Start and Finish Times

Sik : the virtual time (round) the packet k of flow i

would start in a fluid system Fi

k: the virtual finish time

0 4 128 16

kiFkiS

kk-1 k+1

)(tV

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Virtual Time Implementation of Weighted Fair Queueing

wi

kik

ik

i

LSF

111

0)0( V

))(,max(1 ki

ki

ki aVFS

If flow i backlogged

if session j un-backlogged

aik – arrival time of packet k of flow i

Sik – virtual starting time of packet k of flow i

Fik – virtual finishing time of packet k of flow i

Lik – length of packet k of flow i

ki

ki FS 1

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RSVP: Per-hop Reservation

Question: Given (b,r,R) and target per-hop delay d Determine:

• Rate allocation ra, and buffer size Ba

bits

b

slope rarrival curve

d

Ba

slope ra

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Diffserv Architecture Ingress routers

Keep per flow state, police/shape traffic Set Differentiated Service Code Point (DSCP) in Diffserv (DS)

field(think of it as a general priority scheme)

Core routers Implement Per Hop Behavior (PHB) for each DSCP Process packets based on DSCP

Ingress

EgressEgressIngress

EgressEgress

DS-1 DS-2

Edge router Core router

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Control Path Each domain is assigned a Bandwidth

Broker (BB) usually, used to perform ingress-egress

bandwidth allocation BB is responsible to perform admission

control in the entire domain BB not easy to implement

require complete knowledge about domain single point of failure, may be performance

bottleneck designing BB still a research problem

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Example

Achieve end-to-end bandwidth guarantee

BBBB BBBB BBBB1

2 3

579

sender

receiver8 profile 6

profile4 profile

Can be setup ahead of time manually

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Data Path: Edge Router

classifier

traffic conditioner

traffic conditioner

scheduler

class 1

class 2

Best-effort

marked traffic

Ingress

per aggregateclassification (e.g., user)

data traffic

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Edge Router: Example Traffic Conditioning: Metering/Marking

In-profile traffic is marked: A-bit is set in every packet

Out-of-profile (excess) traffic is unmarked A-bit is cleared (if it was previously set) in every

packet; this traffic treated as best-effort

r bps

b bits

metering in-profile traffic

out-of-profile traffic

assured traffic

user profile (token bucket)

set A-bit

clear A-bit

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Edge Router: Example Traffic Conditioning: Metering/Marking/Shaping In-profile traffic marked:

Set P-bit in each packet Out-of-profile traffic is delayed, and when buffer

overflows it is dropped

r bps

b bits

Metering/Shaper/Set P-bit

in-profile traffic

out-of-profile traffic(delayed and dropped)

premium traffic

user profile(token bucket)

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Differentiated Service (DS) Field

Version HLen TOS Length

Identification Fragment offsetFlags

Source address

Destination address

TTL Protocol Header checksum

0 4 8 16 19 31

Data

IPheader

DS field reuses the first 6 bits from the former Type of Service (TOS) byte

The other two bits are proposed to be used by Explicit Congestion Notification (ECN)

DS Filed0 5 6 7

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Data Path: Core Router: Example Scheduler

Packets marked with the P-bit (Premium traffic) sent at high priority

Packets marked with the A-bit (Assured traffic) and best-effort traffic sent at low priority

P-bit set?

A-bit set? RIO

yes

noyes

no

high priority or WFQ

low priority

1 Multimedia Networking: Beyond Best-Effort Internet.

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