12. traffic management ininternet - netlab.tkk.fi filelect12.ppt s-38.145 - introduction to...

1lect12.ppt S-38.145 - Introduction to Teletraffic Theory - Fall 2001

12. Traffic management in Internet

2


Contents

• Introduction

• The Integrated Services architecture• The Differentiated Services architecture

3


Why not ATM?

• ATM offers a lot ...– traffic management

– voice/data integration: CBR, VBR, ABR, UBR– signaling, QoS routing

• ... but to reach the desktop requires new interfaces (HW+SW) at theend-systems!

– ATM signaling (UNI) considered complex, thus HW + SW for it has beenconsidered expensive for end-systems

– Difficult to use the QoS capabilities of ATM: standards do not consider, howapplications should choose all the traffic contract parameters

– Design favors voice (short cells)– Too much, too soon ...

4


Why not ATM?

• More importantly, while the HW+SW for ATM devices were beingdeveloped, the Internet exploded (WWW)!

– the web browser created an easy access to the Internet

– exponential growth in the number of users/traffic amounts

• Internet won the race to the desktop!– Internet is now ubiquitous (everywhere)

• With new requirements from the applications demanding also QoSsupport, the approach is to try to enhance Internet’s capabilities, ratherthan build a new network (based e.g. on ATM)

– Cheaper to fix the old house than to build a new one!

5


Why did the web explode?

• The user side– The web browser created an easy access to the Internet (accessible even

to your grand mother!)

– Allows the integration of new types of applications through the browserinterface (streaming media, interactive chats, games etc.)

– Number of users and the amount of traffic (still) increases exponentially

• Internet technology is very scalable– The networking paradigm is based on best-effort service (no guarantees are

made about the service quality) and the network is connectionless

– The nodes of the network do not store any state information of theusers/connections

– New nodes and users can be added to the network (almost) without anycomplexity increases

– Only the routing is affected by the increase in the number of nodes (routecomputation complexity grows with the number of nodes)

6


Current trends driving the evolution [2]

• Decreasing HW costs (CPU, memory)

• Increasing computing power– Moore’s law:nof transistors per integrated circuit doubles every 18 months

– More powerful machines � more bandwidth hogging applications

• The link speeds increase dramatically– 1993: 100 Mbit/s (FDDI), 2000: 1 Tbit/s (dense WDM technologies)

• Traffic > capacity vs. capacity > traffic– If traffic is much greater than capacity, engineering and traffic control

mechanisms are required. If the opposite holds, just add more capacity ...– Capacity likely to be a problem in WANs.

7


Current trends driving the evolution [2]

• Data traffic > voice traffic– 1998-99 amount of data traffic exceeded voice traffic

• ATM used in the core network

• Everything over IP– Data over IP (requires traffic engineering)– Voice (real time) over IP (requires QoS support)

– IETF is the center of all standardization work.

• From integrated services (IntServ) to differentiated services (DiffServ)– more on this in the next slides ...

• Traffic engineering– Traffic engineering methods needed to manage the complex new Internet

8


Overview of QoS architectures in Internet

• Integrated services (IntServ)– defined in IETF in 1995-97

– fine grained QoS approach: provides QoS on a per connection (flow) basis– similar to ATM: resource reservation through signaling

– scalability problems ...

• Differentiated services (DiffServ)– Tries to solve the scalability problems of IntServ

– coarse grained approach: QoS provided for large aggregated traffic streams(nothing is guaranteed for individual flows)

• ATM can be thought of as both fine or coarse grained– QoS per each VC fine grained

– QoS per VPs (aggregation of VCs) coarse grained

9


Contents

• Introduction


10


Integrated Services

• Specifications of service classes and mechanisms to implement them

• Service classes (= Service Category in ATM)– Guaranteed service: for intolerant applications that need a bound on the

maximum delay that any packet will experiance

– Controlled service: emulates a lightly loaded network– Controlled service traffic is isolated from guaranteed service traffic

• Mechanisms– Flow specification (flowspec): information characterizing the flow (traffic

contract in ATM)

– Admission control

– Flow reservation protocol RSVP (signaling protocol, e.g. Q.2931, in ATM)– Packet scheduling for implementing the given QoS guarantees

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Flow specification

• flowspec consists of 2 parts: RSpec and TSpec

• RSpec (Request Specification): requested service type– controlled service: no parameters– guaranteed service: delay target

• TSpec (traffic cpecification): bandwidth requirements– used as a basis for admission control– source behavior described by a token bucket

• chracterizes source’s mean rate + burstiness

• token rate (r), bucket depth (B) (+ some other parameters)

– Token bucket description:• To send n bytes, must have n tokens

• Tokens accumulate at rate r

• At most B tokens can be stored– Token bucket determines if a packet is conformant or not

12


Router functionality

• Connection admission control– Input: the flow’s RSpec and TSpec

– Decision: The router determines if the connection can be admitted withoutdegrading the QoS of other already admitted connections, or not.

• Policing may be used to enforce each flow’s flowspec (cf. UPC in ATM)– The answer depends heavily on the used scheduling mechanisms implemented

by the router.

• Packet classification and scheduling– Packet classification: associate each packet with the appropriate reservation

• Based on source/destination address, protocol number, source/destinationport

– Different combinations of scheduling mechanisms (processor sharing, priorities,round-robin, ...) can be used to realize the guaranteed and conrolled loadservices

• Vendors can try to create as efficient implementations as possible.

13


RSVP

• Reservation protocol RSVP (corresponds to signaling in ATM)

• Features– Connectionless networks are robust (user state is not stored by the

network)

– RSVP tries to achieve robustness by using soft state information• Connections live as long as they are periodically refreshed (every 30 s)

– RSVP tries to support multicast as effectively as unicast

– Receiver oriented approach:

• Receiver decides how much resources he needs (in ATM andconnection oriented protocols in general, the sender decides theresource requirements)

• Receiver can change resource requirements dynamically with therefresh messages

14


RSVP exampleSource: White paper - QoS protocols & architectures, http://www.qosforum.com/tech_resources.htm

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Problems with RSVP

• Router complexity– Admission control

– Classification and scheduling

• Not scalable with the number of flows– Soft state & admission control for every flow

– Route and allocation refreshment messages every 30s for each flow!

• Not suitable for core network (# of flows is too large in the core)– A method for dealing with traffic aggregates is needed � DiffServ

16


Contents

• Introduction


17


Differentiated Services - Basic philosophy

• No signaling (instead service agreements)

• Network nodes implement defined per hop behaviors (PHBs) (notservices)

– Expedited Forwarding (EF), Assured Forwarding (AF):

• QoS is differential– No guaranteed QoS

– “Higher priority traffic gets better QoS than low priority traffic”

• Complex (per flow) traffic management functions only at the edge of thenetwork

– Classification and conditioning ...

– For each flow the traffic is classified as belonging to some traffic class

• Core functionality as simple as possible– Traffic management per traffic class (= aggregated traffic)

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Per Hop Behavior (PHB)

• Traffic class = PHB

• Different PHBs provide different QoS– However, no quantifiable guarantees are given!

• Operators configure PHB such that it meets desired (offered) servicequality in the DiffServ domain

• Traffic class of a packet marked in the packet header (Type of Servicefield in IPv4, Traffic Class in IPv6)

– 6 bits reserved for this

– Each bit combination is called a Differentiated Service Code Point (DSCP)– DSCP identifies the PHB of a packet

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Per Hop Behavior (PHB)

• So far, 2 PHBs have been defined

• Expedited Forwarding (EF) PHB– the packet service rate equals or exceeds a specified rate– “premium service”

– not affected by other PHBs (has presedence)

• Assured Forwarding (AF) PHB Group– 4 traffic classes

• Each class has a guaranteed minimum rate

• Within a class, 3 drop precedence priorities

• Total of 12 DSCP values

• PHB definitions do not specify how the PHB should be realized

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SLA (Service Level Agreement)

• DiffServ assumes that a Service Level Agreement (SLA) existsbetween networks (or users) sharing a border with a DiffServ network

• SLA defines the traffic profile of the “customer”– SLA is done off-line, i.e., no signaling to set set up dynamic SLAs!

SLA

SLASLA

SLA

DiffServ network 1

DiffServ network 2

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DiffServ - Functional Overview

SLA

SLASLA

SLA

DiffServ network 1

DiffServ network 2

Edge router

• performs per flow traffic management• marks packets as in-profile or out-profile

• assigns DSCP to the packets

Core router

• performs per class traffic management

• scheduling based on DSCP of packets• “preference given to in-profile packets”

• implements PHBs (EF, AF)

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DiffServ - Edge Router Functions

• Packet classification– Packet classifiers select packets in a traffic stream based on the content of

some portion of the packet header.

– BA (Behavior Aggregate) classifier classifies packets based on DSCP only

– MF (Multi-Field) classifier selects packets based on the value of acombination of several header fields (addresses, DS field, protocol id, ...)

Classifier Shaper/DropperMarkerpackets

discard

enque

Metermeasurements

affects affects

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DiffServ - Edge Router Functions

• Packet conditioning: based on the traffic profile obtained from classification– Meter: used to measure the traffic stream against a traffic profile

– Marker: based on information from the meter, packets are marked as in-profileor out-profile, out-profile packets may be demoted to a lower priority traffic class

– Shaper/Dropper: may delay out-profile packets (shaping) or drop them

Classifier Shaper/DropperMarkerpackets

discard

enque

Metermeasurements

affects affects

24


DiffServ - Core Router Functions

• Performs only packet forwarding– Inspects the DSCP of each packet

– Forwards the packet to the appropriate queue

• Efficient implementation of the queing disciplines required by differentPHBs not trivial!

ClassifierPS

processor sharing or weightedround robin scheduler

EF queue

AF group

2

1

1>2

packets

class 1 has priorityover class 2

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Problems with DiffServ

• End-to-end QoS difficult to realize just based on per-hop QoS

• SLAs are static– Network conditions and traffic needs change highly dynamically

• Network dimensioning inside the DiffServ network is difficult

• QoS provided to traffic aggregates, not individual flows– Long lasting and high bandwidth (e.g. video) flows need per flow

guarantees

• Currently some proposals consider having IntServ (RSVP) on theedges and DiffServ in the core.

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Literature

1 L. L. Peterson and B. S. Davie (2nd ed., 2000)– “Computer Networks: A Systems Approach”

– Morgan Kaufmann Publishers, San Fransisco, California

2 Professor Raj Jain’s homepage– http://www.cis.ohio-state.edu/~jain/

– e.g. course material from “Recent Advances in Networking (1999)”

3 Quality of Service Forum Homepage– http://www.qosforum.com/tech_resources.htm

– e.g. white papers on QoS and links to related sites

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