Part 3: Internetworking

1
Part 3 Internetworking

• Internet architecture, addressing, encapsulation,
  reliable transport and the TCP/IP protocol suite

2
Internetworking concepts, architecture and
protocols

• Motivation, architecture, routers, TCP/IP
  protocols, internet reference model

3
Motivation

• A large organisation will use several networking
  technologies
• Inter-organisational communication is significant
• Universal service - any two computers should be
  able to communicate
• However, different network technologies cannot
  just be wired together

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Internetworking

• Interconnect heterogeneous networks and provide
  universal service
• Hardware routers connect different networks
• Internet protocols provide universal service by
  creating a single virtual network

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Internet architecture

• Although a single router can connect many
  networks, most organisations use multiple routers

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Virtual network

• The illusion that there is a single universal
  network

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Internetworking protocols

• The TCP/IP Internet Protocols
• begun in the 1970s
• The Internet has emerged into the public domain
  in the 1990s
• Controlled by the Internet Engineering Task Force
  (IETF)

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Internet Reference Model
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Host computers

• TCP/IP used the term host computer to refer to
  any system that connects to an Internet and that
  runs applications
• Both hosts and routers use TCP/IP protocol
  software

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IP Internet protocol addresses

• Uniform addressing, the IP address hierarchy,
  address classes, dotted decimal notation, special
  addresses, routers and addresses, address
  resolution

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Uniform addressing

• Internet protocols deal in packets and provide
  uniform addressing
• The addressing scheme is defined in software and
  is used transparently by applications
• Internet addressing is specified in the IP
  protocol
• Each host is assigned a unique 32 bit address

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The IP address hierarchy

• Each 32 bit address is divided into two parts
• prefix physical network to which the host is
  attached - the network number
• suffix a host attached to a given physical
  network
• Prefixes are coordinated globally and suffixes
  locally

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Classes of IP address

• Size of prefix and suffix determines maximum
  number of networks and maximum number of hosts
  per network
• IP defines different classes of address with
  different sized prefixes and suffixes
• The first four bits of the address specify its
  class

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The five classes of IP address
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Dotted decimal notation

• Makes it easier to for humans to use addresses
  (names are also possible)

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Classes and dotted decimal
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Division of the address space

• Public Internet network numbers are assigned by
  Internet Service providers (ISPs) and these are
  coordinated by the Internet Assigned Number
  Authority

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An addressing example
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Classless Addressing

• The Internet is running out of addresses
• Allow division between prefix and suffix to
  appear at an arbitrary boundary
• Consider network with only 9 hosts
• Only need four bits for host suffix
• Class C (smallest) address uses 8 bits for host
  suffix
• Can subdivide a class C address into 16
  addresses with a 28 bit prefix and 4 bit suffix
• Extend dotted decimal notation
• 193.68.138.0/28, 193.68.138.16/28,
  ,193.68.138.240/28

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Special IP addresses
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Routers and IP addressing

• Routers are assigned two or more IP addresses
• So are multi-homed computers

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Binding protocol addresses

• An Internet packet passes through a series of
  routers
• each hop takes it over a particular network,
  either to a specific computer on that network or
  to the next router
• in either case, the sending router has to map
  between the protocol (IP) address and a hardware
  address
• this is called address resolution

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Address resolution techniques

• Table lookup
• Closed-form computation
• Message exchange
• send message to specific server computers
• broadcast message, only the required computer
  responds

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Pros and cons of techniques
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Address resolution protocol

• TCP/IP defines the Address Resolution Protocol
  (ARP) which defines the format of resolution
  requests and responses
• This technique is usually combined with local
  caching of hardware addresses

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Dynamic Host Configuration Protocol (DHCP)

• Special DHCP server that assigns IP addresses to
  hosts
• Newly booted machine broadcasts a DHCP discover
  packet
• DHCP server sends back an IP address
• Permanent IP addresses
• Manually assigned by administrator
• Automatic IP address from a pool of addresses to
  be allocated on demand
• Leased for a finite period of time

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DHCP Operation

• DHCP server does not need to be on the same
  network as the host

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Summary

• Uniform addressing
• Address classes
• Dotted decimal notation
• Classless addressing
• Special IP addresses
• Address resolution (ARP) and ssignment (DHCP)