IntServ, DiffServ, and TCP - What Does It All Mean?

Glynn Rogers

Research Leader - Advanced Networks Technology

CSIRO Telecommunications and Industrial Physicsglynn.rogers@tip.csiro.au

Quality of Service (QoS)

A major driving force in Internet evolution Not simply defined - means many things to

many people Has sense of predictable network behaviour Central idea is provision of network

resources that an application requires to perform adequately

QoS is Generating a Confusing Array of Acronyms

Diffserv

QoS

CoS

Intserv RSVP

MPLS

But Its All Beginning to Fit Together

Primary aim is to convey my emerging picture of how Secondary aim is to argue that something new and

important is happening here– a whole new area of networking is developing– merging of traditional ‘routing and addressing’ IP world

with telecommunications engineering– the technical consequence of ‘convergence’– complex - won’t happen overnight

Firstly, a Caveat or Two

What follows is based squarely on the documentation of the relevant industry ‘standards’ organisations:– the ATM Forum – the Internet Engineering Task Force (IETF)

These are my interpretations - any confusions are mine. Its the ‘big picture’ that counts - don’t worry about the detail.

– not a tutorial - convey general impression by example -no attempt at completeness

Why Do We Need Such a Revolutionary Change?

Current ‘best effort’ technology is essentially a quarter of a century old

Two factors driving the development of a new generation of multimedia applications– commercialisation of the Internet– Increasing availability and decreasing cost of bandwidth

No evidence of ‘free bandwidth’ scenario emerging– rejected in RFC1633 (1994) - still true– demand always rises to meet supply

QoS is Not New

Telephone network has QoS– economics and technology based on a single application – highly developed engineering– but one size fits all

BISDN an attempt by telephony world to generalise network to encompass diverse applications

ATM technology - first full exploration of QoS on demand concepts

A Quick Look at ATM

ATM is connection oriented – end to end virtual connection established with negotiated

QoS characteristics» Service category - CBR, VBR etc » traffic characteristics - peak rate, sustained rate, burst size etc» QoS parameters - loss rate, delay, delay jitter

SVC establishment requires both – ‘QoS routing’ (PNNI) and – resource allocation in traversed switches (signaling)

Quality of Service and Resource Management

Fundamental resource is output link rate Access managed via scheduling discipline Bursty input traffic held in buffers

– adds delay and jitter– overflow causes packet loss

These factors determine QoS at network level Optimise via buffer management and scheduler

parameter setting

QoS in the Internet

Internet Engineering Task Force (IETF) is evolving QoS support mechanisms for the Internet - two approaches– The Integrated Services Internet

» QoS for individual microflows

» perhaps too complex for large networks - won’t scale easily

– Differentiated Services - more scaleable» lose sight of individual microflows - Behaviour Aggregates

Why not Just Stick with ATM?

Original ATM concept was QoS overkill– end-to-end defined channel– assumed long lived flows with specific requirements– connection setup overheads relatively small

OK for telephony, high quality VoD etc But Internet traffic is dominated by TCP

– significant proportion of short lived flows (eg Web downloads of text and image pages

– even streaming video applications are using TCP

IETF IntServ Introduces Another Traffic Class

Newer ‘real time’ applications (Web based in particular) are elastic or adaptable to modest fluctuations in network performance

An example is streaming video over TCP– TCP provides rate adaptation to network load

– application can respond to blocking at socket calls» change frame rate (but careful with audio)

» hierarchical coding provides graceful degradation

» MPEG 4 supplies a formal framework

IntServ Controlled Load Service

Based on observation that for this class of traffic the existing Internet works fine if it is not heavily loaded

Use resource allocation to provide performance equivalent to a lightly loaded network

Can base definition on qualitative specifications as distinct from quantitative specifications of ATM

IntServ Also Provides for Established Traffic Classes

A growing number of ‘demanding’ applications– VoIP has stringent requirements on packet loss and delay– Guaranteed Service designed for such applications

Traditional ‘best effort’ service class is still required for non real time applications

IntServ provides a framework for defining new service types

The Integrated Services Concept

Internal network resources are committed to individual end-to-end microflows to provide the QoS the service requires - connection setup

Applications must specify the traffic characteristics of the microflow – token bucket model - rate and burst size specs.– flows are policed to ensure conformance

Network performs Connection Admission Control Method of resource allocation up to implementor

Why Not Just Extend ATM?

ATM is based on Layer 2 switching IntServ retains Layer 3 forwarding mechanism

– essentially a connectionless environment– flows are more abstract than a VC - akin to ‘traffic

trunk’ concept in MPLS

IntServ’s signalling protocol - RSVP - is receiver driven and ‘soft state’ based– much greater compatibility with multicast

So What Went Wrong?

‘RSVP is dead’ reports are exaggerated– QoS is complex – requires systems rather than individual protocol approach– more time required for development and acceptance

Nevertheless there is a problem– IntServ inconsistent with Internet philosophy of keeping

complexity to the network edge– requires interior nodes to retain state for each microflow– ‘state explosion’ problem in interior of big networks

Enter Differentiated Services DiffServ distinguishes between end-to-end services and the

behavior of the individual network components required to support them

DiffServ is based on a set of defined Per Hop Behaviours (PHB’s) specified via an IP header byte, the DS byte

3 types of PHB so far defined in RFC’s– ‘Class Selectors’ - priority based - cf IP priority

– Expedited Forwarding (EF)

– Assured Forwarding (AF)

Diffserv Emphasis is on Individual Interfaces

‘State explosion’ problem is avoided by aggregating traffic requiring the same QoS at each interface

Each Behaviour Aggregate experiences the node performance specified in the required Per Hop Behaviour

The behaviour aggregate and PHB are determined by the DiffServ Code Point (DSCP) carried in the DS Byte

Expedited Forwarding and Assured Forwarding PHB’s

‘… about bandwidth allocation’ - via schedulers such as weighted fair queuing as well as buffer management

Expedited Forwarding - reserved resources (aggregated) - signalling (RSVP?) - VoIP

Assured Forwarding - 4 classes – intended for controllable sources such as TCP – controlled packet drops - 3 levels of drop precedence with

a separate DSCP for each level

Example of an Assured Forwarding Mechanism

drop probability

buffer with 3 levelRED mechanism

AF class 1

AF class 2

AF class 3

AF class 4

WeightedFairQueuingScheduler

Seeing the Woods for the Trees- Diffserv Domains

Domain - collection of nodes under one administration with common policies for routing, QoS, etc

Domains interact via Service Level Agreements– traffic policy written as Service Level Specifications – traffic managed using Traffic Conditioning Specifications

Domains interconnnect via boundary nodes which contain Traffic Conditioning Elements– packet filters, meters, shapers, policers etc– note these all act on aggregates specified by the DSCP

Management Issues - Provisioning Diffserv Domains

Both EF and AF PHB’s require explicit resource allocation - bandwidth, buffer space etc

Mechanisms for allocating resources over domain a research issue– static allocation - management systems– dynamic allocation - bandwidth broker - active networks

Routing implications - traffic engineering – constrained routing– MPLS

Of Microflows and Macroflows - IntServ over DiffServ

Policing at domain boundaries on aggregates Without individual CAC all flows in an aggregate can

suffer from over commitment IETF Integrated Services over Specific Lower Layers

(ISSLL) working group proposes using DiffServ network as akin to, say, ATM link– Aggregation of RSVP requests into single RSVP action– mapping of IntServ services onto Diffserv Per Domain

Behaviours - determined by node PHB’s

Example Scenario - TCP based Streaming Video

Assume a properly resourced Diffserv domain Assume a Bandwidth Broker which can interact with RSVP

to provide IntServ admission control Combine Controlled Load with Assured Forwarding

– both in spirit of elastic flows on lightly loaded network

Require policing to control average TCP flow rate– nonconforming packets ‘marked down’ to a DSCP giving higher

drop probability in AF class– we have experimentally demonstrated that this works!

IntServ Domain

Traffic Generator #1

Traffic Generator #3

Traffic Sink

CISCO 7505 Router

Linux Router

IntServ Domain DiffServ Domain

Traffic Generator #2

TCP Rate Control Using Source Policing and Assured Forwarding

Video On Demand Server

Traffic Generator

Video On Demand Client

Accelar Switch Router

Linux Router

IntServ Domain DiffServ Domain IntServ Domain