Outline
¾ Fundamental issues
Lecture 8
Quality of Service
(QoS)
¾ Basic functions and prerequisites
¾ Differentiated services
¾ Integrated services
Paper: ”Fundamental Design Issues for the
Future Internet”, Shenker, Scott
Kurose-Ross: 7.1, 7.3, 7.5-7.6
2007-05-04
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David Gundlegård, ITN
QoS Definition
¾ ”Determining, setting and acting on priority
levels for traffic flows” [McCabe, 2003]
Fundamental Issues
¾ ”A defined measure of performance in a data
communications system” [TechEncyclopedia]
¾ ”Service quality” vs ”Guaranteed service
quality”
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David Gundlegård, ITN
Internet Characteristics
Application Requirements
¾ Packet oriented datagram approach
¾ Best effort
¾ Packets traverse multiple (unknown) networks with
different characteristics
¾ FIFO is typically used in routers
¾ Time varying capacity in core network
¾ Elastic applications
¾ E-mail
¾ FTP
¾ Real-time applications
¾ IP telephony
¾ E.g. http://www.internettrafficreport.com
¾ Video conferences
¾ Semi elastic
¾ HTTP
¾ Streaming of stored/live audio and video
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David Gundlegård, ITN
Quality of Service
Real-time Applications
Real-time Applications
¾ Constant playout delay (buffering)
¾ Adaptive playout delay
¾ Estimate delay at every burst of data
¾ When is this relevant?
¾ Silent time will vary…
packets
t i = timestamp of the i : th packet
ri = the time packet i is received by receiver
Trade-off for buffer size?
loss
packets
generated
packets
received
playout schedule
p' - r
playout schedule
p-r
time
p i = the time packet i is played at receiver
ri − t i = network delay for i : th packet
Sid 7
p
David Gundlegård, ITN
vi = (1 − u )vi −1 + u | ri − ti − d i |
d i = estimate of average network delay
after receiving i : th packet
v i = average delay variation
Tid för uppspelning: pi = ti + d i + Kvi
r
2007-05-04
d i = (1 − u )d i −1 + u (ri − ti )
p'
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David Gundlegård, ITN
Application Adaption
Internet Fundamental Issue - QoS
¾ Adjust application parameters according to
network state
¾ Changed service model for Internet?
¾ Priority/Call admission/QoS gurantees
¾ As mentioned: playout delay
¾ Stay with best effort/complexity in end-systems?
¾ Coding
¾ Still no answer
¾ Compression (audio/video quality)
¾ Related: telephone/Internet/cellular convergence:
¾ Consencus in using IP
¾ Example: GSM full speech and half speech
¾ IP Multimedia Subsystem (IMS)
¾ Application to large-scale Internet?
¾ Today: ”Islands” of QoS
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David Gundlegård, ITN
David Gundlegård, ITN
Goal of Network Design?
Efficacy Example
¾ si = service delivered to i:th user
¾ Network with a single
link with exponential
server (capacity=1)
¾ Depending on delay, throughput, packet loss etc.
¾ U i ( si ) = application performance as function of
service delivered
¾ Flows with Poisson
arrivals
¾ Network design goal:
¾ Maximize the efficacy V =
∑U (s )
i
i
i
¾ Not always most important (fairness, ”platinum
subscriptions”, etc.)
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¾ FIFO:
d1 = d 2 =
1
(1 − 0.5)
=2
¾ V=2
¾ Priority queuing:
1
4
3
d1 =
(1 − 0.25)
¾ U1 = 4 − 2d1
d2 =
(1 − 0.25)(1 − 0.5)
¾ U 2 = 4 − d2
¾ Where d = average
queuing delay
¾ V=8/3
¾ R=0.25
¾ Observe: maximizes collective benefit
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David Gundlegård, ITN
1
=
=
8
3
¾ Priority efficient!
¾ Dependent on utility
functions, priority
mechanism cost...
Application Performance
Overprovisioning
¾ Networks with real-time applications get
overloaded
⎛B⎞
⎝n⎠
¾ i.e. V (n) = nU ⎜ ⎟ is maximized at n < infinity
¾ Maximum benefit is decreased when a new flow is
added
¾ Overprovisioning difficult due to large
demand variations
¾ Who should pay for this?
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Who Decides Flow Priority?
¾ Implicitly
¾ Network chooses level of priority (based on what?)
¾ Same service interface
¾ Network layer needs application layer information
¾ Violates the layer concept
¾ Fixed set of classes, new applications?
¾ Explicitly
¾ Who chooses a lower priority?
¾ Pricing?
¾ Needs to be stable but also extensible
¾ New interface and end-to-end communication
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David Gundlegård, ITN
Basic Functions and
Prerequisites
Quality of Service
Quality of Service
QoS-principles
Scheduling
¾ Decrease queuing delay for a certain type of
traffic
¾ Split capacity
¾ First-in-first-out (FIFO)
¾ Priority queuing
¾ Classification based on e.g.
ToS, IP address,
port number…
¾ Long queuing delay for non-prioritized traffic
¾ How can we achieve this…?
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Quality of Service
Quality of Service
Scheduling
Policing (part I)
¾ Wighted fair queuing (WFQ)
¾ To be able to offer QoS, users traffic input
rate needs to be restricted somehow
¾ Take turn between different queues
¾ Weight decides how many packets to transmit from every
queue
¾ What should be restricted?
¾ Guaranteed part of capacity for a certain class
¾ Average rate (during how long time?)
¾ Equal weights = Round Robin
¾ Peak rate
¾ Burst size
¾ One common way to do it is to use a ”Leaky
bucket”
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David Gundlegård, ITN
Quality of Service
Leaky Bucket + WFQ = Max Delay
Leaky Bucket
¾ A token is removed for every packet sent
WFQ : Ri ≥
¾ What is restricted by r and b?
Rtot ⋅ wi
∑ wj
(where Ri is throughput for class i)
j =1..n
How to restrict
peak rate?
Suppose ri < Ri ⇒ d max =
Rtot
bi
wi
∑w
j =1..n
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David Gundlegård, ITN
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David Gundlegård, ITN
Random Early Detection (RED)
RED
¾ ”Policing in the core network”
¾ Drop packets as a function of queue length
¾ Standard router behaviour:
¾ Objectives
¾ Tail drop when buffers are full
¾ Reduce number of dropped packets and hence
increase throughput
¾ What happens with the aggregated throughput of
multiple TCP-connections when a router buffer fills
up?
¾ Decrease delay (due to less packets in the queue)
¾ Increase fairness among different flows
¾ ”Global ”synchronization”
¾ Use ”early” packet drops to increase
throughput and decrease total number of
packet drops
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j
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RED Standard version
RED Performance
¾ Average queue length avg (t ) = (1 − w)avg (t − 1) + wq (t )
¾ RED performance very dependent on
parameters
¾ w = parameter
¾ q(t) = current queue
length
¾ Drop probability
p = Maxdrop
¾ Might degrade performance in both throughput
and jitter
avg − Minth
Maxth − Minth
¾ Fairness
¾ Per flow management require heavy processing
¾ Between TCP flows
¾ Between TCP/UDP flows
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Quality of Service
Examples
Internet QoS
¾ 1 Mbps Video telephone (UDP) + FTPtransfer (TCP)
¾ So far
¾ What happens?
¾ General principles and methods to offer QoS in packet
oriented networks
Classification,
scheduling, policing,
call admission,
resource utilisation,
layer 2 flow control…
¾ Quite easy to implement in ”self controlled” networks
¾ Difficult in the Internet
¾ IETF has two suggestions in order to offer QoS in
the Internet
¾ Integrated services (Intserv)
¾ Differentiated services (Diffserv)
¾ 1.5 Mbps Video telephone + FTP-transfer?
¾ Still open question which one that will be mostly
used
¾ 2 x 1 Mbps Video telephones?
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David Gundlegård, ITN
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David Gundlegård, ITN
Quality of Service
Quality of Service
Diffserv
Diffserv
¾ Straightforward and scalable way to introduce priority
in the Internet
¾ IP-traffic is divided into different classes (flows) using
the DS-field in the IP-header (previous ToS)
¾ Simple functionality in the core network
¾ How to assign different
classes?
¾ Classification in hosts or
edge routers
¾ Probably in edge routers
(why?)
¾ Class priority
¾ Not decided exactly how (scheduling, policing…)
¾ Combine with policing
¾ Complexity in network edges
¾ The meter function
compares user traffic
with an agreed traffic
profile
¾ Basic Internet idea
¾ Relative priority between service classes
¾ More information can be found in RFC 2475:
http://www.ietf.org/rfc/rfc2475.txt
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Quality of Service
Diffserv
Intserv
¾ Per-hop behaviour
¾ Much more extensive way of introducing
(guaranteed) QoS in the Internet
¾ Expedited forwarding (EF)
¾ Departure rate for a specific (aggregated) class must reach a
configured rate
¾ With Intserv every application will be able to
negotiate and receive guranteed QoS parameters
¾ Isolation
¾ To achieve this, we need
¾ Assured forwarding (AF)
¾ Four (aggregated) classes, each with minimum resource
requirements and three drop categories
¾ Reserved resources
¾ Call setup
¾ Not specified how this should be met (WFQ, policing…)
¾ Classification does not depend only on traffic type
(Diffserv), but to which application the flow belongs
¾ Diffserv will not give any guarantees, since flows are
aggregated into classes of traffic in the core network
¾ However, Intserv does…
2007-05-04
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David Gundlegård, ITN
2007-05-04
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David Gundlegård, ITN
Quality of Service
Quality of Service
Intserv
RSVP
¾ Call setup
¾ Resource reservation protocol (RSVP)
¾ Traffic characterization and
specification of desired QoS
¾ A protocol to reserve capacity in a IP-network
¾ Rspec/Tspec
¾ Signalling protocol
¾ Call setup signalling
¾ Nothing to do with how the capacity actually
is reserved
¾ Rspec/Tspec forwarding
¾ RSVP
¾ Per-element call admission
¾ Reservations is made for multicast trees
¾ Service class Guranteed Quality of Service
¾ Unicast a degenerate case of unicast
¾ Proven bounds on delays and throughput (WFQ + leaky
bucket + packetization effects)
¾ Receiver oriented
¾ Controlled-load network service
¾ The receiver of a flow initiates the reservation
¾ ”Very high percentage of packets will pass without being
dropped and have a queuing delay close to zero”
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David Gundlegård, ITN
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David Gundlegård, ITN
RSVP
Next Lecture
¾ Heterogeneous receivers
¾ TCP performance in Wireless Networks
¾ Streams divided into different layers
¾ TCP performance in Assymmetric Networks
¾ Wireless QoS
Important RSVP question: Scalable??
2007-05-04
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David Gundlegård, ITN
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