EE 122: Lecture 15 (Quality of Service)

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EE 122 Lecture 15(Quality of Service)

• Ion Stoica
• October 25, 2001

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Limitations of IP Architecture in Supporting
Resource Management

• IP provides only best effort service
• IP does not participate in resource management
• Cannot provide service guarantees on a per flow
  basis
• Cannot provide service differentiation among
  traffic aggregates
• Early efforts
• Tenet group at Berkeley (Ferrari and Verma)
• Asynchronous Transfer Mode (ATM)
• IETF (Internet Engineering Task Force) efforts
• Integrated services initiative
• Differentiated services initiative

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Service Classes

• Multiple service classes
• Service can be viewed as a contract between
  network and communication client
• End-to-end service (multicast and anycast)
• Other service scopes possible, e.g.,
• Aggregates all packets between to points (not
  necessary end-hosts) in the Internet
• Three common services
• Best-effort (elastic applications)
• Hard real-time (real-time applications)
• Soft real-time (tolerant applications)

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Example Integrated Services

• Enhance IPs service model
• Old model single best-effort service class
• New model multiple service classes, including
  best-effort and QoS classes
• Create protocols and algorithms to support new
  service models
• Old model no resource management at IP level
• New model explicit resource management at IP
  level
• Key architecture difference
• Old model stateless
• New model per flow state maintained at routers
• Used for admission control and scheduling
• Set up by signaling protocol

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QoS Network

• Flow or session as QoS abstractions
• Each flow has a fixed or stable path
• Routers along the path maintain the state of the
  flow

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QoS Network Operations

• Control plane admission control
• Reserve resources (i.e., link capacity and buffer
  space) at every router along the path
• Data plane perform per flow
• Classification classify each packet to the flow
  it belongs to
• Buffer management decide when and which packet
  to drop
• Packet scheduling decide when and which packet
  to send

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Control Plane Admission Control

• Example achieve per-flow bandwidth and delay
  guarantees
• Example guarantee 1MBps and lt 100 ms delay to a
  flow

Receiver
Sender







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Control Plane Admission Control

• Allocate resources - perform per-flow admission
  control

Receiver
Sender







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Control Plane Admission Control

• Install per-flow state

Receiver
Sender







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Control Plane Admission Control

• Install per flow state

Receiver
Sender







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Data Plane

• Per-flow classification

Receiver
Sender











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Data Plane

• Per-flow buffer management

Receiver
Sender











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Data Plane

• Per-flow scheduling

Receiver
Sender











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Service Classes

• Multiple service classes
• Service can be viewed as a contract between
  network and communication client
• End-to-end service
• Other service scopes possible
• Three common services
• Best-effort (elastic applications)
• Hard real-time (real-time applications)
• Soft real-time (tolerant applications)

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Service Specification

• Loss probability that a flows packet is lost
• Delay time it takes a packets flow to get from
  source to destination
• Delay jitter maximum difference between the
  delays experienced by two packets of the flow
• Bandwidth maximum rate at which the soource can
  send traffic

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Hard Real Time Guaranteed Services

• Service contract
• Network to client guarantee a deterministic
  upper bound on delay for each packet in a session
  
• Client to network the session does not send more
  than it specifies
• Algorithm support
• Admission control based on worst-case analysis
• Per flow classification/scheduling at routers

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Soft Real Time Controlled Load Service

• Service contract
• Network to client similar performance as an
  unloaded best-effort network
• Client to network the session does not send more
  than it specifies
• Algorithm Support
• Admission control based on measurement of
  aggregates
• Scheduling for aggregate possible

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Traffic and Service Characterization

• To quantify a service one has two know
• Flows traffic arrival
• Service provided by the router, i.e., resources
  reserved at each router
• Examples
• Traffic characterization token bucket
• Service provided by router fix rate and fix
  buffer space

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Token Bucket

• Characterized by three parameters (b, r, R)
• b token depth
• r average arrival rate
• R maximum arrival rate (e.g., R link capacity)
• A bit is transmitted only when there is an
  available token
• When a bit is transmitted exactly one token is
  consumed

r tokens per second
bits
slope r
bR/(R-r)
b tokens
slope R
lt R bps
time
regulator
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Characterizing a Source by Token Bucket

• Arrival curve maximum amount of bits
  transmitted by time t
• Use token bucket to bound the arrival curve

bits
bps
Arrival curve
time
time
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Example

• Arrival curve maximum amount of bits
  transmitted by time t
• Use token bucket to bound the arrival curve

Arrival curve
bits
4
bps
3
2
2
1
1
0
1
2
3
4
5
1
2
3
4
5
size of time interval
time
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Per-hop Reservation

• Given b,r,R and per-hop delay d
• Allocate bandwidth ra and buffer space Ba such
  that to guarantee d

slope ra
slope r
bits
Arrival curve
b
Ba
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End-to-End Reservation

• Source S sends a message containing traffic
  characteristics
• r,b,R
• This message is used to computes the number of
  hops
• Receiver R sends back this information
  worst-case delay (D)
• Each router along path provide a per-hop delay
  guarantee and forwards the message
• In simplest case routers split the delay D

S2
R
(b,r,R)
S
(b,r,R,2,D-d1)
S1
S3
(b,r,R,1,D-d1-d2)
(b,r,R,0,0)
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Summary

• Service a contract between end-hosts and network
• QoS goal provide better than best-effort
  services to support new applications with more
  stringent delay and bandwidth requirements, e.g.,
  IP telephony, videoconferencing
• QoS requires to manage flows/aggregates both on
  data and control plane
• Two major proposals
• Integrated Services (this is mostly what we did
  this lecture well do more next lecture)
• Differentiated Services (see next lectures)

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Administrative Stuff

• 2nd midterm exam next Tuesday, November 6
• Similar to the 1st midterm exam
• Conceptual questions
• Problems similar to the ones in homeworks
• Close books
• All material up to and including IP Multicast
  (i.e., lecture on October 16)
• 4th homework also handed out next Tuesday
• We are trying to have a review session for the
  2nd project one week or so before the deadline
  details to be announced