Module 5.2: Internet Protocol

1
Module 5.2 Internet Protocol

• CO vs. CL protocols
• IP Features
• Fragmentation
• Routing
• IP Datagram Format
• IPv6

2
CO vs. CL

• CO Connection Oriented
• Modeled after the telephone system
• When PDU are sequenced, I.e. logical connection
• CL Connectionless
• Modeled after the postal system
• When PDUs are not sequenced. Each PDU is treated
  independently from each other.
• IP is a CL protocol!
• Advantages
• Flexibility
• Robust
• Smaller Buffers Needed
• No unnecessary overhead
• Unreliable
• Not guaranteed delivery
• packets can be lost, duplicated, damaged.
• Not guaranteed order of delivery
• Packets can take different routes
• Reliability is responsibility of next layer up
  (e.g. TCP)

3
IP Features

• IP has two primary responsibilities
• Routing
• Providing CL, best-effort delivery of datagrams
  through an internetwork and
• Fragmentation
• Providing fragmentation and reassembly of
  datagrams to support data links with different
  maximum transmission unit (MTU) sizes.

4
Routing

• IP relies on two tools to help it route
  datagrams
• Subnet mask
• IP routing table
• If source and destination network and subnet
  parts are the same, then the destination host is
  in the same network and the routing is direct.
• The datagram is wrapped in a frame and
  transmitted directly to its destination on the
  local LAN.
• The destination address that is placed in the
  frame header must be the physical address of the
  destination.
• ARP (Address Resolution Protocol) will be used to
  find the physical address of the destination.
• If destination is not on the local subnet, IP
  must consult its local routing table.
• In such a case, the datagram is sent to the
  router specified in the routing table.
• If no router (or default gateway) is found in the
  routing table, report error.

5
Fragmentation

• Each LAN and WAN technology imposes a different
  size limit on its frames.
• For example, the maximum frame size of the
  ethernet (MTU) is 1500 bytes, which is far below
  the maximum size of an IP datagram.
• Maximum IP packet size is (65537) or 216 bytes.
• IP solves the size problem by chopping the
  datagram into several smaller datagrams called
  fragments. Fragmentation is performed by routers
  and hosts.
• It is up to IP in the destination host to gather
  up the incoming fragments and rebuild the
  original datagram, before passing it to the upper
  layer.
• Fragmentation most often is performed in a
  router.
• Fragmentation is a performance killer.

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Fragmentation (Cont.)

• When to re-assemble
• At destination
• Results in packets getting smaller as data
  traverses internet
• Intermediate re-assembly
• Need large buffers at routers
• Buffers may fill with fragments
• All fragments must go through same router
• Inhibits dynamic routing
• IP re-assembles at destination only

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Fragmentation (Cont.)

• Uses fields in header
• Data Unit Identifier (ID)
• Identifies end system originated datagram
• Source and destination address
• Protocol layer generating data (e.g. TCP)
• Identification supplied by IP layer
• Data length
• Length of user data in octets
• Offset
• Position of fragment of user data in original
  datagram
• In multiples of 64 bits (8 octets)
• More flag
• Indicates that this is not the last fragment

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Fragmentation Example
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Dealing with Failure

• Re-assembly may fail if some fragments get lost
• Need to detect failure
• Re-assembly time out
• Assigned to first fragment to arrive
• If timeout expires before all fragments arrive,
  discard partial data
• Use packet lifetime (remaining time to live in
  IP)
• If time to live runs out, kill partial data

10
IP Datagram format
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Header Fields (1)

• Version
• Currently 4
• IP v6 - see later
• Internet header length (HLEN)
• In 32 bit words
• Including options
• Type of service
• Total length
• Of datagram (headerdata) in octets
• Identification
• unique integer
• Used with addresses and user protocol to identify
  datagram uniquely
• This parameter is needed for reassembly and error
  reporting.

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Header Fields (2)

• Flags (only 2 bits used)
• More bit
• Dont fragment
• If a node does not know how to reassemble
• Useful in bootstrapping. The node initially has a
  lightweight IP stack
• Fragmentation offset
• Time to live
• Protocol
• Next higher layer to receive data field at
  destination

13
Header Fields (3)

• Header checksum
• Reverified and recomputed at each router
• 16 bit ones complement sum of all 16 bit words in
  header
• Set to zero during calculation
• Source address
• Destination address
• Options (variable)
• Padding (variable)
• Used to ensure that the IP header is a multiple
  of 32 bits in length.
• Data (variable)
• Must be an integer multiple of 8 bits in legth
• The maximum length of datagram (dataheader) is
  65,535 bytes

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Type of Service

• Precedence
• Measurement of packets relative importance.
• 8 levels
• Reliability
• Try not to drop the packet.
• Delay
• Try to minimize the delay for this packet.
• Throughput
• Choose a network with high bandwidth.
• Cost
• Choose a network with least cost

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Options

• Security
• Attach classified information level to packet.
  For DOD military application. RFC 1108.
• Source routing
• List of all routers.
• Route recording
• List of routers visited.
• Stream identification
• For special handling of voice and data
• Timestamping
• Add a timestamp at each router

16
IPv6

• IP v 1-3 defined and replaced
• IP v4 - current version
• IP v5 - streams protocol
• IP v6 - replacement for IP v4
• During development it was called IPng
• Next Generation
• Why Change IP?
• Address space exhaustion
• 232 different addresses gives over 4 billion
  addresses is not enough!
• Due to growth of wireless, PDA, and Internet.
• Other enhancements

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IPv6 vs. IPv4

• The changes from IPv4 to IPv6 are primarily in
• expanded addressing capabilities
• header format simplification
• flow labeling capability
• Support for resource allocation
• improved support for extensions, options, and
  QoS
• Support for more authentication and security.

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IPv6 Format
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Comparison

• The header length field is eliminated.
• The service type field is eliminated in IPv6.
• The total length field is eliminated.
• The identification, flag, and offset fields are
  eliminated.
• The TTL field is called hop limit.
• The protocol field is replaced by the next
  header field.
• The header checksum is eliminated.
• The option fields in IPv4 changed to extension
  headers.

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Extension Headers
21
Extension Headers
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Status of IPv6

• Smooth transition is key factor in success of
  IPv6
• Dual stack
• IPv6 Tunneling for IPv4 packets.
• Header translation
• In reality, we have a slow adoption of IPv6. This
  is due to the invention of NAT.
• NAT may work only with certain styles of
  applications, but not adequate for say IP
  telephony. Also, it does not scale very well.
• The urge is not there yet, but surely growing!