Transcript An Introduction to Computer Networks

An Introduction
to
Computer Networks
Lecture 12: Quality of Service
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
Univ. of Tehran
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Outline
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Realtime Applications
Integrated Services
Differentiated Services
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Realtime Applications
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Require “deliver on time” assurances
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must come from inside the network
Microphone
,
Sampler
,
Buffer
A D
converter
D
A
Speaker
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Example application (audio)
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sample voice once every 125us
each sample has a playback time
packets experience variable delay in network
add constant factor to playback time: playback point
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Sequence number
Playback Buffer
•Real-Time Audio
 rate one per
125 micro-sec
• Limit on delay,
at most 300 ms.
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Packet
arrival
Packet
generation
Playback
Network
delay
Buffer
Time
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Distribution of Delays
90% 97% 98%
Packets (%)
3
99%
2
1
50
• Blue is the delay distribution
•Others are cumulative percentages
•Variability of delay is consistent for
almost all paths
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100
150
200
Delay (milliseconds)
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Taxonomy
Applications
Real time
Tolerant
Elastic
Intolerant
Interactive
Interactive
Asynchronous
bulk
Adaptive
Nonadaptive
Delay-
Rate-
adaptive
adaptive
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Rate-adaptive
Delay adaptive
• Tolerant: Audio
•Intolerant: a robot control.
•Adaptive: can adjust playback time
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Approaches to QoS
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Fine –grained- Provide QoS to individual
flows. Integrated service or RSVP and
usually ATM
Coarse-grained- For aggregated traffics.
Differentiated Services.
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Integrated Services (RSVP)
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Produced by IETF around 95-97
Service Classes
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Guaranteed, never arrive late. For tolerant,
adaptive applications.
controlled-load, not heavily loaded (i.e vat)
Mechanisms
signaling protocol, flowspec, resource resevation.
 admission control
 policing
 Univ.
packet
of Tehran scheduling
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Flowspec
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Rspec: describes service requested from network
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controlled-load: none
guaranteed: delay target
Tspec: describes flow’s traffic characteristics
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average bandwidth + burstiness: token bucket filter
token rate r
bucket depth B
must have a token to send a byte
must have n tokens to send n bytes
start with no tokens
accumulate tokens at rate of r per second
can accumulate no more than B tokens
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Per-Router Mechanisms
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Admission Control
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decide if a new flow can be supported
answer depends on service class and policy
not the same as policing
Packet Processing
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classification: associate each packet with the
appropriate reservation
scheduling: manage queues so each packet
receives the requested service
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Reservation Protocol
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Called signaling in ATM
Proposed Internet standard: RSVP
Consistent with robustness of today’s connectionless
model
Uses soft state (refresh periodically)
Designed to support multicast
Receiver-oriented
Two messages: PATH and RESV
Source transmits PATH messages every 30 seconds
Destination responds with RESV message
Merge requirements in case of multicast
Can specify number of speakers
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RSVP Example
Sender 1
PA TH
R
Sender 2
R
PA TH
RESV
(merged)
R
RESV
R
R
Receiver A
RESV
Receiver B
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RSVP versus ATM (Q.2931)
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RSVP
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receiver generates reservation
soft state (refresh/timeout)
separate from route establishment
QoS can change dynamically
receiver heterogeneity
ATM
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sender generates connection request
hard state (explicit delete)
concurrent with route establishment
QoS is static for life of connection
uniform QoS to all receivers
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Differentiated Services
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Problem with IntServ: scalability
Idea: support two classes of packets
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premium
best-effort
Can be done by a bit.
Who set the bit?
What the router does with this bit?
Per-hop behavior, means not end-to-end.
Use TOS field in IP packet, DiffServ code points
(DSCP)
EF class: Expedite Forwarding, with min delay.
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Differentiated Services
Guarantee Delay: Max entry to router
Strict priority
Assured Forwarding P(drop)
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RED with In and Out
Mechanisms
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1.0
packets: ‘in’ and ‘out’
bit
edge routers: tag
MaxP
packets
core routers: RIO
(RED with In and Out)
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A vgLen
Min out
Min in Max out
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Max in
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