Overview
Selected Notions in Quality of Service
(QoS)
• General QoS concepts
• Enforcing QoS in IP networks
• Allocation Optimisation
Calin Curescu
• Conclusions
Real-Time Systems Laboratory
Department of Computer and Information Science
Linköping University, Sweden
Selected Notions in Quality of Service (QoS)
Calin Curescu
22 pages
TDDB47, 2006
Selected Notions in Quality of Service (QoS)
Calin Curescu
Why care about QoS?
• Nowadays communication networks (Internet, Wireless)
support several types of applications:
• Voice Service, Audio Phone
• Videoconferences
• Video on Demand, Interactive Multimedia
• Email, SMS
• Remote login, e-commerce, online banking
• File Transfer Services (FTP, HTTP download)
A QoS definition
• Performance assurance
– Applications have different requirements
• FTP – no strict requirements
• Videoconference – strict bandwidth and delay
requirements
– System should provide guarantees
• Service differentiation
– Network has to differentiate between applications and
offer the right service
• E.g: Videoconference more important than FTP
• Applications can work at different QoS levels
• User requirements (payment) can also vary
• Only one type of service: best-effort
• All packets are treated similarly – FCFS basis
• What if the network gets overloaded?
• Can we prevent?
• Can we control?
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Providing QoS
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QoS parameters
• By Allocating Resources
– CPU (application scheduling on the hosts)
– Network link bandwidth (packet scheduling)
– Memory (packet buffers, RAM pages)
• Enforcement level (low)
– bandwidth, delay, jitter, packet loss rate, energy
consumption, CPU time
• Frameworks and Mechanisms to enforce resource allocation
– Intserv, Diffserv, etc.
• Application level (high)
– Picture quality, compression levels, window size
– Security, redundancy
• Performance Optimisation (policy level)
– How to allocate/utilise resources in the best way
– Traffic engineering
• QoS – based routing
• User satisfaction
– Perceived benefit (utility)
– Willingness to pay a certain amount for a certain service
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Classic Real-time Systems
• Resource allocation -- Scheduling
• Resource -- CPU time
• QoS parameters
– CPU utilisation
– Deadlines
– Jitter
• Control plane
– QoS routing (path establishment)
– Admission control
– Resource reservation
– Corrective control of congestions (overloads)
• Data plane (packet level)
– Classifier
– Meter
– Shaper
– Queue
– Scheduler (FCFS, WFQ, PQ, EDF)
• Optimisation criterion
– Construct a feasible schedule (if possible)
• RT Scheduling is subset of QoS allocation
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QoS enforcement in IP networks
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Internet QoS frameworks: Intserv
• Session must first declare its QOS requirement and
characterize the traffic it will send through the network
– T-spec: defines the traffic characteristics
• Token bucket – average rate, burst size, burst rate
– R-spec: defines the QOS being requested
• Rate, delay
• A signalling (reservation) protocol
– Needed to carry the R-spec and T-spec to the routers where
reservation is required
– Resource Reservation Protocol (RSVP)
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Token Bucket Shaping
• Token Bucket mechanism, provides a means for limiting
input to specified Burst Size and Average Rate.
• Bucket can hold b tokens;
• tokens are generated at a rate of r token/sec
– unless bucket is full of tokens.
• Over an interval of length t, the size of all admitted packets
is less than or equal to (r t + b).
• Admission control
– At every router
– Depends on: T-spec & R-spec & already allocated resources
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T-spec and R-spec
• T-spec (traffic specification)
– A token bucket specification
• token rate - r
• bucket size - b
• peak rate - p
• maximum packet size - M
• minimum policed unit - m
Intserv Classes
• Guaranteed Service
– Deterministic guarantees
• On bandwidth and delay
• Admission control based on worst case
– For hard real-time applications
• Controlled Load
– Statistic guarantee
• Admission control based on average
• Only T-spec used
– Simulates a “lightly loaded best effort traffic”.
• R-spec (reservation specification)
– Service Rate – R
• The bandwidth requirement
– Slack Term – S
• The delay requirement
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QoS routing
D
D=
5, B
W
= 90
A
BW
5
=5
=5
,B
W
D=
3, B
W
= 20
• Scalability
– Maintaining “flow states” by routers in high speed
networks is difficult due to the very large number of flows
B
=9
0
D
W
,B
=5
=9
0
D = 14, BW = 90
D
W
,B
=5
D = 10, BW = 90
=9
0
D = 3,
10
BW =
• Complicated signalling in RSVP
– Admissions, reservations, timeouts
= 90
20,
D = 7, BW
D=
Intserv Shortcomings
• Rigid service models
– Intserv has only two classes
5
• A “relative” notion of service might be desired
• Instead of T-spec and R-spec
• Constraints
– total delay < 25; available bandwidth > 30
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Diffserv
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Per-Hop-Behaviour (PHB)
• Complex functionality in edge routers
– Classification:
• marks packets according to some specified rules
– Traffic Conditioning:
• may delay and then forward or may discard
• Diffserv does not specify service classes
• instead provides functional components for creating them
• Proposed PHBs
– Expedited Forwarding
• Simple functionality in core routers
– Class-based forwarding
• Per class instead of per flow
– No state info must be maintained in the routers
• Big advantage over Intserv
Selected Notions in Quality of Service (QoS)
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• Absolute assurance
• Always prioritised
• Admission control
– Assured Forwarding
• Relative assurance
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Performance Optimisation
• Intserv & Diffserv provide the framework & mechanisms
– but not the intelligence.
• Optimise system-wide performance by
Utility functions
• Which is the more important connection ?
• Using resource-utility functions for optimisation of allocation
• Single resource case:
– Allocating resources to the “best” connections first
– Proposing the “best” route to a new connection
• Shortest path routing usually creates uneven load
a-2
bandwidth
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b-
2
bandwidth
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1
b-
a-2
conn. b utility
b1
conn. a utility
a-1
b-3
– Only knowing the status of the whole network one can hope
for a best allocation
• unrealistic in a large, open, dynamic network
optimal system utility
a-1
• Global vs. local allocation decisions
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b-3
2
b-
bandwidth
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Then again, is QoS really needed?
• How can QoS management be avoided?
• Always resource constrained compared to wireline
– Limited spectrum, energy, processing power
– Just add another wire…
• But also more complicated
– Interference
• A node’s bandwidth is influenced by neighbour nodes
– Mobility
• Old routes break, other are formed, resource availability
can quickly change
– Hard to offer guarantees
• Reasons to not use QoS
–
–
–
–
–
Wireless Networks
Too complicated to set up
Too big overhead
More cost-effective to increase bandwidth
Too many technologies
Different Internet providers
• Where QoS is really needed?
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Concluding remarks
ATM & QoS
• Active research is going on concerning performance
optimisation
– Both in wireline and wireless networks
• Asynchronous Transfer Mode – network technology
– Small, fixed size packets (cells)
– Connection oriented
– Adm. ctrl. + res. alloc. at connection setup
• New hot area: Sensor Networks
– Tradeoffs between performance and energy consumption
• Service Types
– Constant bit rate (CBR)
– Constant Rate, delay guarantees
• Pricing schemes from economics
– To calculate the costs and benefits of using a certain
resource, used for optimisation
– Variable bit rate (VBR)
– Rt-VBR – avg. rate, burst rate, delay guarantees
– Nonrt-VBR – avg. rate, burst rate guarantees
• The more restrictive and complex environment, the higher the
need for QoS management
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– Available bit rate
– Minimum rate guarantees, uses network feedback for higher rates
– Unspecified bit rate (UBR)
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Intserv vs. Diffserv
Intserv
Granularity of service
differentiation
State in routers(e.g.
scheduling, buffer
management)
Traffic Classification
Basis
Type of service
differentiation
Individual Flow
Admission Control
Required
Signaling Protocol
Required(RSVP)
Selected Notions in Quality of Service (QoS)
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Diffserv
Per Flow
Aggregate of
flows
Per Aggregate
Several header fields
DS Field
Deterministic or
statistical guarantees
Absolute or
relative
assurance
Required for
absolute
differentiation
Not required for
relative schemes
Intserv vs. Diffserv
Coordination for
service differentiation
Scope of Service
Differentiation
Scalabilty
Network Accounting
Network Management
Interdomain
deployment
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Intserv
Diffserv
End-to-End
Local (Per-Hop)
A Unicast or Multicast
path
Anywhere in a
Network or in
specific paths
Limited by the number Limited by the
of flows
number of classes
of service
Based on flow
Based on class
characteristics and QoS usage
requirement
Similar to Circuit
Similar to existing
Switching networks
IP networks
Multilateral
Bilateral
Agreements
Agreements
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