Quality of Service Support

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Quality of Service Support
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QOS in IP Networks

• IETF groups are working on proposals to provide
  QOS control in IP networks, i.e., going beyond
  best effort to provide some assurance for QOS
• Work in Progress includes RSVP, Differentiated
  Services, and Integrated Services
• Simple model for sharing and congestion
  studies

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Principles for QOS Guarantees

• Consider a phone application at 1Mbps and an FTP
  application sharing a 1.5 Mbps link.
• bursts of FTP can congest the router and cause
  audio packets to be dropped.
• want to give priority to audio over FTP
• PRINCIPLE 1 Marking of packets is needed for
  router to distinguish between different classes
  and new router policy to treat packets accordingly

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Principles for QOS Guarantees (more)

• Applications misbehave (audio sends packets at a
  rate higher than 1Mbps assumed above)
• PRINCIPLE 2 provide protection (isolation) for
  one class from other classes
• Require Policing Mechanisms to ensure sources
  adhere to bandwidth requirements Marking and
  Policing need to be done at the edges

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Principles for QOS Guarantees (more)

• Alternative to Marking and Policing allocate a
  set portion of bandwidth to each application
  flow can lead to inefficient use of bandwidth if
  one of the flows does not use its allocation
• PRINCIPLE 3 While providing isolation, it is
  desirable to use resources as efficiently as
  possible

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Principles for QOS Guarantees (more)

• Cannot support traffic beyond link capacity
• Two phone calls each requests 1 Mbps
• PRINCIPLE 4 Need a Call Admission Process
  application flow declares its needs, network may
  block call if it cannot satisfy the needs

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Summary
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Scheduling And Policing Mechanisms

• Scheduling choosing the next packet for
  transmission
• FIFO
• Priority Queue
• Round Robin
• Weighted Fair Queuing
• We had a lecture on that!

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(No Transcript)
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Discussion of RED

• Advantages
• Early drop
• TCP congestion
• Fairness in drops
• Bursty versus non-Bursy
• Disadvantages
• Many additional parameters
• Increasing the loss

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Policing Mechanisms

• (Long term) Average Rate
• 100 packets per sec or 6000 packets per min??
• crucial aspect is the interval length
• Peak Rate
• e.g., 6000 p p minute Avg and 1500 p p sec Peak
• (Max.) Burst Size
• Max. number of packets sent consecutively, ie
  over a short period of time
• Units of measurement
• Packets versus bits

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Policing Mechanisms

• 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 number of
  packets that are admitted is less than or equal
  to (r t b).

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Token bucket example
arrival queue bucket sent
p1 (5) - 0 -
p2 (2) p1 3 -
p3 (1) p2 1 p1
1 p3,p2
4
5
parameters b5 r3
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Integrated Services

• An architecture for providing QOS guarantees in
  IP networks for individual application sessions
• relies on resource reservation, and routers need
  to maintain state info (Virtual Circuit??),
  maintaining records of allocated resources and
  responding to new Call setup requests on that
  basis

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Call Admission

• Session must first declare its QOS requirement
  and characterize the traffic it will send through
  the network
• R-spec defines the QOS being requested
• T-spec defines the traffic characteristics
• A signaling protocol is needed to carry the
  R-spec and T-spec to the routers where
  reservation is required
• RSVP is a leading candidate for such signaling
  protocol

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RSVP request (T-Spec)

• A token bucket specification
• bucket size, b
• token rate, r
• the packet is transmitted onward only if the
  number of tokens in the bucket is at least as
  large as the packet
• peak rate, p
• p gt r
• maximum packet size, M
• minimum policed unit, m
• All packets less than m bytes are considered to
  be m bytes
• Reduces the overhead to process each packet
• Bound the bandwidth overhead of link-level
  headers

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Call Admission

• Call Admission routers will admit calls based on
  their R-spec and T-spec and base on the current
  resource allocated at the routers to other calls.

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Integrated Services Classes

• Guaranteed QOS this class is provided with firm
  bounds on queuing delay at a router envisioned
  for hard real-time applications that are highly
  sensitive to end-to-end delay expectation and
  variance
• Controlled Load this class is provided a QOS
  closely approximating that provided by an
  unloaded router envisioned for todays IP
  network real-time applications which perform well
  in an unloaded network

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R-spec

• An indication of the QoS control service
  requested
• Controlled-load service and Guaranteed service
• For Controlled-load service
• Simply a Tspec
• For Guaranteed service
• A Rate (R) term, the bandwidth required
• R ? r, extra bandwidth will reduce queuing delays
• A Slack (S) term
• The difference between the desired delay and the
  delay that would be achieved if rate R were used
• With a zero slack term, each router along the
  path must reserve R bandwidth
• A nonzero slack term offers the individual
  routers greater flexibility in making their local
  reservation
• Number decreased by routers on the path.

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QoS Routing Multiple constraints

• A request specifies the desired QoS requirements
• e.g., BW, Delay, Jitter, packet loss, path
  reliability etc
• Two type of constraints
• Additive e.g., delay
• Maximum (or Minimum) e.g., Bandwidth
• Task
• Find a (min cost) path which satisfies the
  constraints
• if no feasible path found, reject the connection

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Example of QoS Routing
D 24, BW 55
D 30, BW 20
A
B
D 5, BW 90
D 14, BW 90
D 5, BW 90
D 5, BW 90
D 7, BW 90
D 10, BW 90
D 5, BW 90
D 3, BW 105
Constraints Delay (D) lt 25, Available Bandwidth
(BW) gt 30
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Differentiated Services

• Intended to address the following difficulties
  with Intserv and RSVP
• Scalability maintaining states by routers in
  high speed networks is difficult sue to the very
  large number of flows
• Flexible Service Models Intserv has only two
  classes, want to provide more qualitative service
  classes want to provide relative service
  distinction (Platinum, Gold, Silver, )
• Simpler signaling (than RSVP) many applications
  and users may only want to specify a more
  qualitative notion of service

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Differentiated Services

• Approach
• Only simple functions in the core, and relatively
  complex functions at edge routers (or hosts)
• Do not define service classes, instead provides
  functional components with which service classes
  can be built

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Edge Functions at DiffServ (DS)

• At DS-capable host or first DS-capable router
• Classification edge node marks packets according
  to classification rules to be specified (manually
  by admin, or by some TBD protocol)
• Traffic Conditioning edge node may delay and
  then forward or may discard

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Core Functions

• Forwarding according to Per-Hop-Behavior or
  PHB specified for the particular packet class
  such PHB is strictly based on class marking (no
  other header fields can be used to influence PHB)
• BIG ADVANTAGE
• No state info to be maintained by routers!

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Classification and Conditioning

• Packet is marked in the Type of Service (TOS) in
  IPv4, and Traffic Class in IPv6
• 6 bits used for Differentiated Service Code Point
  (DSCP) and determine PHB that the packet will
  receive
• 2 bits are currently unused

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Classification and Conditioning

• It may be desirable to limit traffic injection
  rate of some class user declares traffic profile
  (eg, rate and burst size) traffic is metered and
  shaped if non-conforming

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Forwarding (PHB)

• PHB result in a different observable (measurable)
  forwarding performance behavior
• PHB does not specify what mechanisms to use to
  ensure required PHB performance behavior
• Examples
• Class A gets x of outgoing link bandwidth over
  time intervals of a specified length
• Class A packets leave first before packets from
  class B

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Forwarding (PHB)

• PHBs under consideration
• Expedited Forwarding departure rate of packets
  from a class equals or exceeds a specified rate
  (logical link with a minimum guaranteed rate)
• Assured Forwarding 4 classes, each guaranteed a
  minimum amount of bandwidth and buffering each
  with three drop preference partitions

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Differentiated Services Issues

• AF and EF are not even in a standard track yet
  research ongoing
• Virtual Leased lines and Olympic services are
  being discussed
• Impact of crossing multiple ASs and routers that
  are not DS-capable

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DiffServ Routers

DiffServ Edge Router
Classifier
Meter
Policer
Marker

DiffServ Core Router
PHB
PHB
Select PHB
Local conditions
PHB
PHB
Extract DSCP
Packet treatment
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IntServ vs. DiffServ
IP
IntServ network
DiffServ network
"Call blocking" approach
"Prioritization" approach
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Comparison of Intserv Diffserv Architectures
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Comparison of Intserv Diffserv Architectures
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Diffserv Theoretical Model
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Basic Theoretical Model

• Single FIFO queue.
• Bounded capacity holds up to B packets
• All packets have same size
• Packet Arrival arbitrary
• Packet Send 1 packet/time unit
• Actions
• Non-Preemptive model accept or reject
• Preemptive model also preempt

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Packet Values

• Goal
• Each packet has an intrinsic value
• maximize the total value of packet sent!
• Cheap and expensive packets (two values)
• low value of 1 and high value of ?
• Continuous packet values
• any value in 1,?

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Competitive Analysis

• Analysis for online algorithms
• For a given sequence S VA(S) / Vopt(S)
• Competitive Ratio MINS VA(S) / Vopt(S)
• Worse case guarantee

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Non-Preemptive Policies

• Fixed Partition(x)
• At most xB low value and (1-x)B high value.
• Flexible Partition (x)
• At most xB low value and any high value.
• Round Robin(x)
• Like fixed partition.
• send x low and (1-x) high fractional!
• Simulate it using FIFO queue.

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Implementing Round Robin

• Implementation
• Maintain two variables
• high
• low
• If low packet arrives tests low 1 lt xB
• IF YES ACCEPT
• IF NO REJECT
• High packets the same
• Sending
• low low x
• high high (1-x)
• Main observation
• once a packet is accepted it will be sent
  eventually.
• Sending order not important!

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Analysis of Round Robin

• Consider the case that all packet values are 1.
• Claim
• For any input sequence
• The number of packet a buffer of size B/2 accepts
• is at least half of a buffer of size B
• Let x ½
• Consider Low and High packets separately
• RR(½)
• Accepts at least half High and half Low
• Benefit at least half

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Preemptive Policies

• Greedy
• Always accept if the buffer is not full
• Preempt a low value packet to accept a high one
• COMPETITIVE RATIO 2
• ?-Preemptive
• Drop from the head packets with total value ?/?
• Active queue management (AQM)

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Preemptive Model ?1/2 -Preemptive

• We consider ?1/2-Preemptive Policy
• There are two packet values 1 and ?
• For ?9 each high value packet preempts 3 low
  value packets (pro-active preemptions)

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?1/2-Preemptive Theorem

• Claim 1 VA(Slow) ?VOPT(Slow) 1/?1/2
  VOPT(Shigh)
• Claim 2 VA(Shigh) ? VOPT(Shigh) 1/?1/2
  VOPT(Shigh)
• Theorem VA(S) ? VOPT(S) 2/?1/2
  VOPT(S)

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Optimal Offline

• Process the packet in decreasing order of value.
• Accept a packet if possible.
• otherwise reject
• Two values
• Maximizes the number of high value packets
• Given a buffer of size B
• Maximizes the total number of packets
• Using the remaining buffer space.

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Proof Outline Claim 2

• We partition the schedule to intervals
• Intervals ends when the buffer is empty.
• Overloaded intervals some high value packet is
  lost and only high value packets are scheduled.
• Underloaded intervals no high value packet is
  lost

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Proof (Claim 1)

• We show VA(Slow) ?VOPT(Slow) 1/?1/2 VA(Shigh)
• Low packet loss overflow Preemption
• Low packet lost in overflow
• Opt also lost a packet.
• Low packet preempted by a high packet
• Value of high ?
• Preempted ?1/2
• Value is 1/?1/2 V(high)
• Recall VA(Shigh) ? VOPT(Shigh)

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Proof Outline (Claim2)

• We divide the HIGH packet loss into two subsets
• The packets lost by OPT (easy case)
• The packets scheduled by OPT

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Proof Outline (Claim 2)

• Observation 1
• When some high value packet is lost the buffer is
  full of high value packets

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Proof Outline (Claim 2)
Observation 2 If there are at least B/?1/2 high
value packets in the buffer then the next packet
to be scheduled is a high value packet.
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Proof Outline (Claim 2)

• Observation 1 ? The length of an overloaded
  interval is at least B
• Observation 2 ? An optimal offline policy could
  have scheduled at most B/?1/2 additional high
  value packets
• The ratio between the additional loss and the
  benefit of the overloaded interval is bounded by
  1/?1/2
• VA(Shigh) ? VOPT(Shigh) 1/?1/2 VOPT(Shigh)

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Lower bound (Non-Preemptive)

• Scenario
• B low value packets
• maybe B high value packets
• Online accepts xB low value
• Case I only low values
• Online xB Offline B
• Case II Both low and high value
• online xB (1-x) aB offline aB
• Competitive ratio ? a/(2a-1)
• For large values of a we have a/(2a-1) ? ½

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Lower bound Preemptive model

• Scenario
• B low value packets
• For zB time units
• one high value packet arrives each time unit
• Maybe B high value packets
• Let zB be the time the Online sends the last low
• (1) No more packets arrive
• (2) B high value packets arrive
• Online Benefit (1) zB z?B (2) zB ?B
• Offline Benefit (1) B z?B (2) z?B ?B
• Solving for best z gives a lower bound (about 0.8)

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Fixed vs. Flexible Partition
Fixed Flexible time Arrival event
B/2 high B/2 low B high 1 B high B/2 low
B/2 high B/2 low B/2 B/2 low B/2 high
B/2 low B B/2 low
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Summary of Results Non-preemptive
Two values
Multiple Values
Competitive ratio 1/(2 ln a) 1/(1 ln a)
Policies cont RR Impossibility
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Summary of Results Preemptive
Multiple Values
Policies Greedy Better Than G Impossibility
Competitive ratio ½ 1/(1.98..) 0.8
2 Values
Policies ?1/2-Preemptive Impossibility
Competitive ratio 1-2/?1/2 1-1/(2?1/2)