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

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

• Glynn Rogers
• Research Leader - Advanced Networks Technology
• CSIRO Telecommunications and Industrial Physics
• glynn.rogers_at_tip.csiro.au

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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

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QoS is Generating a Confusing Array of Acronyms
Diffserv
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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 - wont happen overnight

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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 - dont worry
  about the detail.
• not a tutorial - convey general impression by
  example -no attempt at completeness

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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

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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

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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)

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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

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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 - wont
  scale easily
• Differentiated Services - more scaleable
• lose sight of individual microflows - Behaviour
  Aggregates

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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

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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

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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

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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

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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

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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
• IntServs signalling protocol - RSVP - is
  receiver driven and soft state based
• much greater compatibility with multicast

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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

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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 (PHBs) specified via an IP header
  byte, the DS byte
• 3 types of PHB so far defined in RFCs
• Class Selectors - priority based - cf IP
  priority
• Expedited Forwarding (EF)
• Assured Forwarding (AF)

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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

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Expedited Forwarding and Assured Forwarding PHBs

• 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

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Example of an Assured Forwarding Mechanism
AF class 1
AF class 2
Weighted Fair Queuing Scheduler
drop probability
buffer with 3 level RED mechanism
AF class 3
AF class 4
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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

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Management Issues - Provisioning Diffserv Domains

• Both EF and AF PHBs 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

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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 PHBs

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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!

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Traffic Generator 1
Traffic Sink
CISCO 7505 Router
Linux Router
Traffic Generator 2
Traffic Generator 3
IntServ Domain
IntServ Domain
DiffServ Domain
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TCP Rate Control Using Source Policing and
Assured Forwarding
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Video On Demand Server
Video On Demand Client
Accelar Switch Router
Linux Router
Traffic Generator
IntServ Domain
DiffServ Domain
IntServ Domain