Chapter 2 Internet Protocol IP Recommended reading: Comer, Vol 1, Chapters 4, 7

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Chapter 2Internet Protocol (IP) (Recommended
reading Comer, Vol 1, Chapters 4, 7)

• Internet Protocol (IP)
• a protocol used in the internet layer.
• IP makes use of the existing networks to deliver
  information, where these networks may use a
  variety of protocols.
  
• IP Addresses
• Hardware Address and Protocol Address
• Each network adopts its own addressing scheme.
• Different networks may adopt different hardware
  addresses with different sizes and different
  formats.
  
• E.g, Ethernet address 48 bits
• When these networks are interconnected, there
  must be a uniform addressing scheme for all the
  computers and routers.
  
• Each computer / router is assigned a unique
  protocol address.

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• Remarks
• Each computer has two addresses
• hardware address used by the underlying network
  protocol
  
• protocol address used by the internetworking
  protocols.
  
• Hardware address is also known as physical
  address.
  
• IP Addressing Scheme
• Each computer / router is assigned a unique IP
  address having 32 bits.
  
• Each IP address has two parts
• The prefix specifies the network to which the
  computer is attached. (network id or netid)
  
• The suffix specifies a particular computer on a
  network. (hostid)
  
• Problem
• Given only 32 bits, how many bits should be
  allocated to the prefix and the suffix?
  
• Considerations
• If the prefix has many bits, the internet can
  accommodate many networks but each network can
  only have a few computers.
  
• If the prefix has a few bits, the internet can
  only accommodate a few networks although each
  network can have many computers.

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• The IP addressing scheme defines three primary
  classes, where each class has a distinct
  prefix/suffix size. (The Classful Addressing
  Scheme)
• The internet can accommodate large networks,
  medium networks, and small networks.
  
• Classes A, B, C are the primary classes. The IP
  addresses of computers and routers belong to
  these classes.
  
• Class D is used for multicasting. When a packet
  is sent to an IP multicast address, all the
  computers sharing this address will receive this
  packet.

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• In each primary class, the number of networks and
  the number of computers per network are as
  follows
  
• Each packet sent across the internet contains
• the IP address of the source, and
• the IP address of the destination.
• Dotted Decimal Notation
• Commonly we use the dotted decimal notation to
  represent the 32-bit IP address.
  
• more convenient for human to manipulate
• Each octet (8-bit) is expressed as a decimal
  value, and adjacent decimal values are separated
  by a dot.
  
• Example

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• The first decimal value defines the class of the
  IP address as follows
  
• Loopback address
• 127.x.x.x
• intended for use in testing TCP/IP and for
  inter-process communication on the local
  computer
  

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• Assigning IP Addresses
• Assigning Prefix Address
• Each network must have a unique prefix address
  throughout an internet.
  
• The prefix must be coordinated globally.
• To connect a network to the global internet, an
  organization obtains a unique prefix address from
  the Internet Service Provider (ISP).
  
• In turn, the ISP coordinates with a central
  organization (the Internet Assigned Number
  Authority (IANA, on or before 1998) the Internet
  Corporation for Assigned Names and Numbers
  (ICANN, after 1998)) to ensure the uniqueness of
  the prefix.
• To connect a network to a private internet
  (Intranet), the organization can determine the
  prefix while ensuring its uniqueness.
  

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• Assigning Suffix Address
• Each computer must have a unique suffix address
  in the same network while two computers in two
  different networks can have identical suffix
  address.
• Assign any unique suffix to each computer /
  router within a network without considering the
  addresses in the other networks.
  
• If the suffix is 000 or 111, the corresponding
  IP addresses have special meaning. Do not assign
  these suffixes.
  
• An IP address with suffix equal to 000 is used
  to refer to the network itself.
  
• An IP address with suffix equal to 111 is a
  directed broadcast address, i.e., it refers to
  all hosts on the network.
  
• Example
• An organization wants to form a private TCP/IP
  internet with four networks, where one network is
  large (with many computers), two are medium, and
  one is small.

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• Firstly, assign a unique prefix to each network
• Assign a class A prefix for the large network
  (say, 10).
  
• Assign a class B prefix for each of the two
  medium networks (say, 128.10 and 128.11).
  
• Assign a class C prefix for the small network
  (say, 192.5.48).
  
• Secondly, assign a unique suffix to each computer
  within each network

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• IP Addresses for Routers
• A router is connected to multiple networks.
• It belongs to multiple networks.
• It is assigned multiple IP addresses where every
  IP address corresponds to one network.
  
• Example
• Correction of misconception
• An internet address does not really identifies a
  host, but only a network connection at which a
  host attaches to a network.
  
• Can all the IP addresses of a router have the
  same suffix?

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• Subnet addressing
• also called subnet routing, or subnetting
• one IP network address, but with two or more
  physical networks
  
• Only local routers know that there are multiple
  physical networks and how to route traffic among
  them.
  
• Example

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• Hierarchical addressing concept
• Subdivide the 32-bit IP address into network
  number, subnet number, and host number.
  
• Analogous to telephone system
• (852) 3411-XXXX area code, exchange number,
  connection number
  

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• Common scenario
• The number of subnets may grow in the future.
  (See figure above.)
  
• The number of machines in each subnet may be
  uneven.
  

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• Flexibility in subnet address assignment
• Fixed-length subnetting
• Number of bits for subnets in the IP address is
  the same for all physical networks in the same
  organization.
  
• (Recall that all 11s and 00s addresses are
  reserved.)
  
• Disadvantage there is a maximum number of
  machines in each subnet.
  
• Advantage simplicity of management and routing.

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• Flexibility in subnet address assignment (cont.)
• Variable-length subnetting
• Number of bits for subnets varies. Hence number
  of machines in each subnet may vary.
  
• This scheme is uncommon because it causes
  confusion easily.
  

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• Advantage of classful addressing scheme
• small routing table for routers
• one routing entry per network
• Weaknesses in classful addressing scheme
• Addresses refer to network connections
• If a host computer (notebook) moves from one
  network to another, its IP address must change
  because the network id has changed.
  
• When any class C network grows to more than 255
  hosts, it must have its address changed to a
  class B address.
  
• The change must be done abruptly for all
  machines.
  
• difficult to debug individual machines
• insufficient number of network addresses
• Remedies
• Interim solution assign a few Class C blocks
  instead of a Class B network address to fulfill a
  network address application. (Classless
  Addressing)
• Internet Protocol Version 6 (IPV6) 128-bit IP
  addresses
  

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• The Internet Layer
• The most fundamental internet service consists of
  a packet delivery system.
  
• unreliable
• The packet may be lost, duplicated, delayed, or
  delivered out of order, but the service will not
  detect such conditions, nor will it inform the
  sender or receiver.
• best-effort
• The internet tries its best to deliver the
  packet, but delivery is not guaranteed.
  
• connectionless
• The path used by each packet may be different.
• Purpose of the Internet Protocol (IP)
• The IP protocol defines the basic unit of data
  transfer used throughout a TCP/IP internet.
  
• exact format of all data specified
• IP software performs the routing function.
• IP includes a set of rules that embody the idea
  of unreliable packet delivery.
  
• How hosts and routers should process packets.
• How and when error messages should be generated.
• The conditions under which packets can be
  discarded.
  

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• IP Datagrams
• An internet connects many different networks
  having different packet formats.
  
• IP datagram is defined as a network-independent
  packet format
  
• The header contains the IP addresses of the
  source and the destination.
  
• The data area can contain a variable amount of
  data.
  

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• The details of an IP datagram
• Header
• The header is composed of two parts
• 20-byte fixed part
• an optional part that has a variable length and
  can have up to 40 bytes
  
• Maximum header size is 60 bytes.
• Data
• The data can have a variable size, subject to the
  constraint that the maximum IP datagram size is
  216 - 1 65535 bytes.

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• VERS IP protocol version number
• HLEN Header length measured in 32-bit words
• minimum value 5
• TOTAL LENGTH (16-bit field) total length of
  datagram in octets, including header and data
  
• Max possible size of datagram 216 1 65535
  octets
  
• SERVICE TYPE hints to intermediate routers about
  special treatment of datagram, such as
  priority/precedence, low delay, etc.
  
• IDENTIFICATION, FLAGS, AND FRAGMENT OFFSET
  fields used by intermediate routers to break down
  a large datagram into smaller fragments
  
• The smaller fragments will only be reassembled at
  the final destination.
  
• TYPE which high-level protocol was used to
  create the data.
  
• IP OPTIONS mainly for network testing or
  debugging.
  

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• (MTU Maximum Transfer Unit)

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• Delivery of IP Datagrams
• A datagram may go through several intermediate
  networks before reaching the destination.
  
• Problem
• Each intermediate network uses its own address
  and frame format, and it does not understand IP
  address and datagram format.
  
• Solution Encapsulation
• The frame header contains the field frame type,
  which indicates that it carries an IP datagram.
  
• The frame header contains the hardware address of
  the next hop, where the hardware address can be
  obtained by address resolution.

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• Example
• A datagram is encapsulated and unencapsulated as
  it travels from a source to a destination through
  three networks and two routers
  

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• Network byte order
• used for representing integers (4 bytes) on the
  internet
  
• All intermediate routers must be able to read,
  say, the TOTAL LENGTH field, in an IP datagram
  correctly.
  
• Little Endian
• Lowest memory address contains the low-order byte
  of the integer.
  
• Intel 80x86, Pentium, Dec VAX, Dec PDP-11
• Big Endian
• opposite of Little Endian
• IBM 370, Motorola 68000 series, Pyramid
• The network standard byte order uses Big Endian.

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• Tutorial Problems
• Each IP address consists of a prefix and a
  suffix. State the advantages and disadvantages
  of this addressing scheme.
  
• Are the following valid IP addresses
• 131.108.99.5
• 128.256.1.2
• 78.0.0.17
• An organization has two networks network A has
  100 computers and network B has 500 computers.
  These networks will be connected to a router, and
  this router will be connected to the Internet.
  The organization is going to get two prefix
  addresses. Select suitable prefix addresses and
  assign IP addresses to the computers and
  routers.
• Computer A sends several IP datagrams to computer
  B through the Internet. Explain briefly why
  computer B
  
• may not receive some of the IP datagrams
• may receive the IP datagrams in a wrong order.
• A student argued that IP is useless because IP is
  unreliable but we need reliable information
  retrieval/delivery through the Internet. Do you
  agree?