Advanced Computer Networks: Lecture 38 - Dr. Amir Qayyum
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CS716
Advanced Computer Networks
By Dr. Amir Qayyum
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Lecture No. 38
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RealTimeApplications
ã Requiredeliverontimeassurances
Mustcomefrominsidethenetwork
Microphone
Sampler
A D
converter
Buffer
D A
Speaker
ã Exampleapplication(audio)
Samplevoiceonceevery125às
Eachsamplehasaplaybacktime
Packetsexperiencevariabledelayinnetwork
Addconstantfactortoplaybacktime:
playbackpoint
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Real Time Application Example:
LiveSource Audio
• One byte per 125 microseconds
• Delay between speaker & listener
should be fixed
• Each packet has “playback time”
based on delay
• But network delay is variable
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Real Time Application Example:
LiveSource Audio
• Solution
– Fix delay at 9599% mark on distribution
– i.e. delay by which 9599% of packets
arrive
• Some packets (15%) still lost, but tolerable
• Early packets are buffered until playback
time
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Playback Buffer
Sequence number
Packet
arrival
Packet
generation
Playback
Network
delay
Buffer
Time
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Example Distribution of Delays
90% 97% 98%
Packets (%)
3
99%
2
1
50
100
150
200
Delay (milliseconds)
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Taxonomy of Applications
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Taxonomy
Applications
Real time
Tolerant
Adaptive
Delayadaptive
Nonadaptive
Elastic
Intolerant
Rate-adaptive
Interactive
Interactive
bulk
Nonadaptive
Rateadaptive
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Asynchronous
Elastic Applications
• Old data are useful, just not as good
– Interactive applications (e.g. ssh,
rlogin, telnet)
– Bulk transfers (e.g. http, ftp)
– Asynchronous interaction (e.g. email)
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Realtime Applications
• Intolerant to loss
• Tolerant to loss
– Nonadaptive: quality fixed (sample delay and
throughput, then assume worstcase variation)
– Adaptive: adjust quality to current parameters
• Delay adaptive: adjust to achieved delay
• Rate adaptive: adjust to achieved throughput
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Integrated Services :
Resource Reservation Protocol
(RSVP)
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Integrated Services
• Service Classes
– Guaranteed
– Controlledload
• Mechanisms
– Signaling protocol
– Admission control
– Policing
– Packet scheduling
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Three Classes of Service
• Guaranteed: specified maximum delay
• Controlled load
– For losstolerant, adaptive applications
– Network appears lightly loaded (via
weighted fair queuing)
• Best effort
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Mechanisms to Support Integrated
Services
• Flow specification: delay requirements for flow
• Admission control: network decision to support
flow
• Resource reservation: protocol for exchanging
flowspecs, performing admission control, etc.
• Packet classification: mapping packets to flows
• Packet scheduling: forwarding policy
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Integrated Services Example
• Flowspec: 100 ms guaranteed to www.nsf.gov
• Reservation: spec travels down path for approval
• Delay guarantee approved by all routers, so
admitted
• My packets marked as guaranteed
• EXAMPLE policy: guaranteed packets sent first
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Flow Specification Components
• RSpec: describes service requested from network
– Controlleddelay: level of delay required
– Guaranteed/predictive: delay target
• TSpec: describe flow traffic characterization
– Characterized by token bucket filter
– Average throughput r
– Maximum buffering requirement B
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Flowspec
• Rspec: describes service requested from network
– Controlledload: none
– Guaranteed: delay target
• Tspec: describes flow’s traffic characteristics
–
–
–
–
–
–
–
–
Average bandwidth + burstness: 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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Flows with Equal Average Rates but
Different Token Bucket Descriptions
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Unrealistic Expectations …
• Be explicit about requirements
• “In general, it is good to be as explicit
about the bandwidth needs of an
application as possible...”
• How much bandwidth does your
browser need? What about buffering?
Please be explicit!
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Token Bucket Filters
r tokens/sec
Token Bucket,
Capacity B
Each byte needs a
token in order to
pass
Data
• Dropping filter: drops packets if token is not available
• Buffered filter: buffers data until tokens become
available
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Token Bucket
• Given a rate r and a finite data trace, what
is minimum bucket capacity B such that the
filter has no effect?
• Simply observe the maximum buffer size
• Does this approach work? Why?
• For buffer of size b, the number of empty
buffer positions is equal to the number of
tokens in an (r,b) token bucket filter
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Token Bucket
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PerRouter (RouterCentric)
Mechanisms
• Admission Control
– Decide if a new flow can be supported
– Answer depends on service class
– Not the same as policing
• Packet Processing
– Classification: associate each packet with
the appropriate reservation
– Scheduling: manage queues so each packet
receives the requested service
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Reservation Protocol
• 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 (can specify
number of speakers)
– Receiveroriented
• RSVP uses two messages
– PATH transmitted by source every 30 sec
– Destination responds with RESV message
• Requirements must be merged for multicast
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