Internetworking

1
Internetworking

• An internetwork is typically comprised of many
  physical networks over which data travels
• There are many different types of physical
  networks
• Ethernet
• FDDI
• ATM
• Goals of internetworking
• Make all the diverse network technologies
  function as a coordinated whole
• Hide the details of the underlying network
  hardware
• Provide universal communication services

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Review Ethernet

• A popular packet-switched LAN technology invented
  at Xerox PARC in the 1970s

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Properties of an Ethernet

• 10 100 Mbps
• Broadcast bus
• Best-effort delivery
• Distributed access control (CSMA/CD)

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Distributed Access Control

• Carrier Sense Multiple Access (CSMA)
• If two transceivers both sense that the network
  is idle and begin transmission simultaneously a
  collision occurs
• Collision Detection (CD)
• Recovery

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Ethernet Addressing

• Each machine connected to the network has a
  unique 48-bit number (its address)
• To send a packet a computer puts the destination
  address in the destination field
• That address can be
• A unicast address
• A broadcast address
• A multicast address

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Ethernet Frame Format

• Variable length (no shorter than 64 octets and no
  larger than 1518 octets)
• Fields
• Preamble
• Destination address
• Source address
• Frame type
• Data
• Cyclic Redundancy Check (CRC)

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An Ethernet Frame

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Extending an Ethernet

• Repeaters - hardware devices used to relay
  electrical signals from one cable to another
• Advantages
• Disadvantages

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Extending an Ethernet (cont)

• Bridges - receives valid frames and retransmits
  it on another another cable
• Advantages
• Disadvantages

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Access Control Alternatives

• Ethernet - distributed
• Advantages
• Disadvantages
• Can you think of other alternatives?

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Fiber Distributed Data Interconnect (FDDI)

• Popular LAN technology
• Higher bandwidth (100 Mbps) than Ethernet
• Uses optical fiber
• Implements a token ring

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FDDI (cont)

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FDDIs Self-Healing Capability

• Two rings
• Counter rotating
• Uses the backup ring to bypass a failed computer
  or link

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FDDI Self-Healing (cont)

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Asynchronous Transfer Mode (ATM)

• A high-speed connection-oriented network
  technology
• Operates from 100 Mbps to gt1 Gbps
• Expensive

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Recall How Internetworking Works

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Goals of Internetworking

• Make all the diverse network technologies
  function as a coordinated whole
• Hide the details of the underlying network
  hardware
• Provide universal communication services

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Application-Level Interconnection

• One application-level program per machine
  cooperating/interoperating with other programs
• Example?
• Advantages
• Disadvantages

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Network-Level Interconnection

• Have a set of communication conventions
• The O.S. for each computer provides communication
  services for all application programs
• Example?
• Advantages
• Disadvantages

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Internetworking

• Takes the network-level interconnection approach
• Functions as a virtual packet-switched network
• Attempts to build a unified, cooperative
  interconnection of networks that supports a
  universal communication service

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Properties of the Internet

• Hides underlying internet architecture from the
  user
• Does not mandate a network interconnection
  topology
• Establishes network independence in the user
  interface

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Network Interconnection

• Internet router
• A small computer that physically attaches to two
  networks
• Passes packets from one network to the other
• As the Internets topology becomes more complex,
  routers need to know about that topology
• More on routers later

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Routing Decisions

• Can be complex
• Want to keep routing hardware simple and cheap
• So routers only use the destination network (not
  the destination machine) to make routing
  decisions
• Routing table is proportional to the of
  networks in the internet (not the of hosts)

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All Networks Are Equal

• Any communication system capable of transferring
  packets counts as a single network independent
  of
• its delay
• throughput characteristics
• packet size
• scale

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Big Advantages of Internetworking

• Users perceive a single, virtual network through
  which all machines can be reached
• Users do not need to know the underlying details
  of the network
• Application-level programs do not need to know
  the underlying details of the network

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Big Responsibilities of Internetworking

• Cooperation
• Communication conventions
• Each participating network agrees to handle
  traffic to/from other hosts in return for their
  handling of its traffic

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Unanswered Questions

• What is the addressing scheme in an internet?
• What do packets look like?
• How is routing done?
• What happens when routers/hosts malfunction?
• What happens when routers/hosts get overloaded?