4th Edition: Chapter 1

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CSCD 443/547Advanced Networks
Lecture 3 Network Design and Performance Read
Chapter 1 and Other Reading Course Notes page
(links)
People Network
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Topics

• Review Network layers
• Overview of services each layer
• What exactly is a protocol?
• Network Architecture
• How does everyone connect to the Internet?
• Packet Switched vs. Circuit Switched Networks

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Designing a Network

• If you were designing a network
• Want to accommodate many users
• Want to handle different types of applications
• Be as flexible as possible
• Handle multiple types of network physical medium
• Be able to accommodate change with least amount
  of cost
• Be able to scale with growth since demand
  continues to increase
• How would you do it?

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General Requirements for Networks

• Multiple hosts share common links
• Diverse applications use the network
• Text based such as file transfer, Web page
• Multi-media too, as in video, images and voice
• Cant always anticipate the next Killer
  application
• Electronic commerce needs privacy and security
• Performance is critical, so network efficiency is
  important
• Reliability is also critical but varies by
  application
• Accommodate network failure
• What else is a requirement?

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Multiple Users

• Must share common network links
• What works best in an environment where the
  number of users is unknown and their network use
  varies?
• Statistical Multiplexing
• Has been shown to work best rather than
  statically allocating resources like time or
  frequency division multiplexing
• Fairness is done by limiting the size of packets
  so that packets from multiple users each have a
  turn

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Diverse Applications Common Services

• While applications are variable, can define
  common set of communication patterns
• Helps in planning network resources
• File transfer pattern
• User requests file, Server provides it
• Small amount of data in request
• Larger amount of data in reply
• What applications are like this?
• What are characteristics of a channel that
  supports this pattern?

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Diverse Applications Common Services

• Common Services
• Video-on-demand
• Video is streamed when requested
• Also, can go both ways as in video conferencing
• Think of it as a stream of bytes instead of a
  distinct set of packets
• Channel Characteristics
• Must be seen in-order, no out-of-order frames
• Can tolerate some loss,
• Should support multi-cast for conferencing

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Diverse Applications Common Services

• Common Services
• Audio on-demand
• Variation of video on-demand
• Has some real-time aspects to it, must be given
  priority over other traffic, known as Quality of
  Service (QOS)
• Has real limitations on delay it can tolerate
• Audio signal must be played in order, can
  tolerate some frame loss

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Location of Services

• A fundamental design decision, affects
  performance
• Where do I put my network services?
• Services are needed to coordinate the traffic
  between the hosts
• Must deal with errors, packet loss,
  retransmission and congestion control
• Should we make the routers and switches more
  intelligent to handle these things or let the
  applications take care of them in the hosts?
• What would you do?

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Telephone Network Dumb Edge, Smart Core

• Dumb phones
• Let you dial a number
• Speak and listen thats it
• Smart switches
• Set up and tear down a circuit
• Forward audio along the path
• Limited services
• Audio
• Later, caller-id, call forwarding etc.
• A monopoly for a long time

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Internet Smart Edge, Dumb Core
End-to-End Principle Whenever possible,
communications protocol operations should be
defined to occur at the end-points of a
communications system.
Programmability With programmable end hosts, new
network services can be added at any time, by
anyone.
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End Host Can Take Care of Details

• No error detection or correction
• Higher-level protocol can provide error checking
• Successive packets may not follow same path
• No problem as long as packets reach destination
• Packets can be delivered out-of-order
• Receiver can put packets back in order (if
  needed)
• Packets may be lost or arbitrarily delayed
• Sender can send the packets again (if desired)
• No reaction to congestion, beyond drop
• Sender can slow down in response to loss or delay

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Internet Architecture

• Three features of the architecture worth
  highlighting in terms of design
• 1. Layering is not strict
• Application can bypass the Transport layer and
  utilize IP layer directly or lower layer
• 2. Hourglass shape
• IP is focal point of architecture
• Allows common method of exchanging packets
• But, also allows anything to run over IP and
  IP runs over a multitude of mediums

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Internet Architecture

• 3. To include new protocol, must have a protocol
  specification and one implementation
• Assures protocol can be implemented efficiently

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Layering A Modular Approach

• Sub-divide the problem
• Each layer relies on services from layer below
• Each layer exports services to layer above
• Interface between layers defines interaction
• Hides implementation details
• Layers can change without disturbing other layers

Application
Application-to-application channels
Host-to-host connectivity
Link hardware
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The Narrow Waist of IP
Applications
FTP
HTTP
TFTP
NV
TCP
UDP
Waist
IP
Data Link
NET1
NET2
NETn

Physical
The Hourglass Model
The waist facilitates interoperability
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Above and Below the Waist

• IP over anything
• Internetworking protocol that runs on anything
• Accommodate innovation in link technology
• and heterogeneity throughout the network
• Anything over IP
• Variety of transport protocols can be built
• Though, in practice, mainly just TCP and UDP
• TCP ordered, reliable stream of bytes
• UDP simple (unreliable) message delivery
• And any applications on top of that

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Why is the Internet So Successful?

• Authors believe one factor is that so many
  network applications are just software running on
  commodity machines
• Means do not need hardware upgrades to provide
  new functionality
• Computers became faster and could support
  multimedia applications
• More and more innovative applications using
  multimedia
• Examples Web browser and server, video over the
  Web YouTube, Google everything, chat programs
  and VOIP applications

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Decentralized Control

• How you choose to implement network organization
  is critical to the success of and adaptability of
  a network
• Could have one entity deciding everything and
  imposing updates on everything
• Simpler and more efficient, but not feasible in
  the long run
• Turns out decentralization is better for large
  scale, networks

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Benefits of Decentralization Scalability

• Scalability
• Limit amount of state, and frequency of updates
• Addressing
• Internet routers only need to know how to reach
  blocks of addresses (e.g., 12.0.0.0/8)
• Routing
• Link failure in one network is typically not
  visible in another
• Naming
• Look-up of www.cnn.com doesnt go to same server
  as look-up of www.princeton.edu

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Benefits of Decentralization Autonomy

• Autonomy
• Allow different parties to manage different parts
  of the system, and apply their own policies
• Addressing
• ARIN delegates address space to ATT, who
  delegates smaller blocks to its customers
• Routing
• ATT controls flow of traffic through its
  backbone
• Naming
• CNN controls addresses for www.cnn.com

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Problems Related to Design Decisions

• Turns out that not all the design decisions
  resulted in a perfect network
• There are continuing problems related to some of
  the original design decisions
• A few of these are outlines as follows

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Challenges Tied to Early Design Decisions

• Power of Programmable end hosts
• Easy to spoof IP addresses, e-mail addresses,
• Incentives for users to violate congestion
  control
• Malicious users can launch Denial-of-Service
  attacks
• Best-effort packet-delivery service
• Inefficient in high-loss environments (wireless)
• Poor performance for interactive applications
• Expensive per-packet handling on high-speed links

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Challenges Tied to Early Design Decisions

• Layering and the IP narrow waist
• Low efficiency due to many layers of headers
• Poor visibility into underlying shared risks
• Complex network management due to multiple
  interconnected protocols and systems
• Decentralized control
• Hierarchical addressing makes mobility difficult,
  and requires careful configuration
• Autonomy makes measurement (and troubleshooting
  and accountability) hard
• Autonomy makes protocol changes difficult

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Recurring Challenges

• Security
• Weak notions of identity that are easy to spoof
• Protocols that rely on good behavior
• Incomplete or non-existent registries, keys,
• Mobility and disconnected operation
• Hierarchical addressing closely tied with routing
• Presumption that hosts are connected
• Network management
• Many coupled, decentralized control loops
• Limited visibility into across layers and
  networks
• Application performance requirements
• Real-time, interactive applications
• Throughput sensitive vs. delay-sensitive

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Summary

• Designing a successful network is challenging
• Many requirements, not all of them well-defined
• Design decisions can allow a network to adapt
  successfully to change over time
• Some of the attributes of large-scale networks
  are similar to any complex system
• Will study this more later

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• No class on Monday, Tuesday 1/12, 1/13 I will
  be at a conference
• See CourseNotes page. Substitute lecture
  Assigned papers and written summary due Friday,
  1/15
• There is class on Wed. 8 am on Network
  Programming.
• There is also a lab you can do at home using
  Wireshark.
• This is another tool you should have on your
  home computer.
• Download this if you dont already have it, Lab
  is on Labs page.
• You will use this information later in a program.
• Reading
• Papers on Course Notes page, summary to be
  turned in on Friday.
• Book Chapter 1 Continued