EE%20122:%20Network%20Applications

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EE 122 Network Applications

• Kevin Lai
• Aug 28, 2002

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Network Functionality

• Network functionality is only useful if it
  benefits users
• also applies to any computer system, but easier
  to forget in networking
• e.g., Is it worth it to upgrade a 56Kb/s modem
  link in your network to 100Mb/s Ethernet? Only if
  it significantly improves user experience
• Generalization of Amdahls Law from computer
  architecture
• Also applies to metrics other than time

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Network Applications

• how networks affect users
• buy stock in networking companies, work for
  networking companies
• use network applications
• need to understand application requirements
• to build useful networks
• to understand why networks are built the way they
  are
• to understand why some ideas failed

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Different Network Applications

• telephony (making a phone call)
• sending email
• web browsing
• buying something
• reading news
• sending instant messages
• file sharing
• playing a distributed game
• watching/listening to streaming media

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Requirements of the Network

• throughput
• latency
• cost
• compatibility with existing equipment usually
  lowers cost
• qualifications
• distribution mean, worst-case
• in small bursts / all the time
• ease of programmability
• routing
• unicast, multicast, broadcast

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Web Browsing Network Requirements

• Transfer 1KB-100KB files
• e.g., HTML file, JPEG image
• Depends on size of content at sites visited
• News oriented sizes (e.g., www.nytimes.com)
• Average throughput 32Kb/s 1Mb/s
• Average latency 100 250ms
• Average loss lt 10
• Flat rate 20-50/month

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Telephony Network Requirements

• Stream data at 9.6Kb/s 128Kb/s
• an encoded audio signal
• Wired
• Worst case throughput 64Kb/s 128Kb/s
• Worst case latency 100ms 125ms
• Loss lt 1
• .01-.10/minute
• Mobile
• Worst case throughput 9.6Kb/s 128Kb/s
• Worst case latency 100ms 250ms
• Loss lt 10
• .10-1/minute

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File Sharing Requirements

• Transfer 3MB 600MB file
• MP3 audio file, MPEG video file
• Inbound throughput 128Kb/s 10Mb/s
• Outbound throughput 128Kb/s ? Mb/s
• Flat rate 20-50/month

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Distributed Game Requirements

• e.g., WarCraft III, Quake III, EverQuest
• unlimited number of players
• stream data about players state
• e.g., location in the world, appearance, items
  carried, units controlled, etc.
• Throughput 128Kb/s ? Mb/s
• Latency 0ms 250ms
• Flat rate 20-50/month

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Comparing Applications

• Research community consistently failed to predict
  next popular application
• predicted telnet actually email
• predicted ftp actually WWW
• predicted IP telephony actually instant
  messaging
• predicted streaming actually file sharing
• predicted streaming maybe distributed
  gaming
• Otherwise, little uniformity in application
  requirements

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Designing for Requirements

• Given different application requirements, how to
  design network that can run different
  applications well?
• define network service model
• implement service model on physical technology
• satisfy service model when multiple applications
  and users are sharing network
• replace obsolete service model

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Network Service Model

• Specifies what the network does for an
  application
• leaves the rest to the application
• Examples
• Establishes a circuit that guarantees 1Mb/s
  from one node to another for .10/s
• regardless of other traffic
• Deliver a packet from one node to another with a
  delay of at most 100ms for .10/Mb
• Deliver a packet with unknown delay for .10/Mb,
  but less delay than people paying .05/Mb
• Deliver a packet with unknown delay, maybe
• Which model satisfies which application?
• What model do existing networks provide?

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Service Model Philosophies
Less Functionality
More Functionality

• Provide functionality for existing popular
  application
• e.g., telephone network
• less work for application developers
• some functionality can only be provided by the
  network
• more expensive network
• per flow state
• not all applications may use functionality
• target applications may be replaced with new
  applications
• Put only the most basic functionality in
• e.g., internet
• invert above properties

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Service Model Philosophies
Less Functionality
More Functionality
-Integrated Services -Asynchronous Transfer Mode
(ATM)
-Best effort delivery -Ethernet -Unicast
-Differentiated Services -Multicast -Explicit
Congestion Notification (ECN)

• Simpler has predominated (so far)
• Reduced Instruction Set Computing (RISC) analogy

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Implementing service model

• How to implement service model on physical
  medium?
• e.g., Category 5 cabling, optical fiber, radio
  waves
• analog ? digital, digital ? analog
• encoding
• how to separate packets
• framing
• physical medium is subject to errors
• e.g., fading, multipath, microwave ovens, sun
  spots, etc.
• service model may specify higher reliability than
  physical medium provides
• reliability, error detection

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Sharing

• Many users of same application using network
• Users of different applications using network
• How to do resource allocation?
• resources bandwidth, memory, CPU cycles
• Not all packets are created equal
• IP telephony packet must be delivered with low
  delay
• File sharing packet can be delayed
• Not all users created equal
• some users pay more
• some users follow rules
• some do not for personal gain (selfish users)
• some do not just to be mean (malicious users)

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Replacing Network Model

• Every service model developed so far has
  eventually become obsolete
• service model optimizes for a particular
  application mix
• application mix changes
• Network is much harder to upgrade than other
  systems
• bad news 100s M nodes ? full upgrade takes
  decades
• good news use old network to bootstrap new
  network
• e.g., first Internet nodes used phone lines
• How to implement new network service model on top
  of existing network
• solution overlay networks