Internet Protocol Connectionless Datagram Delivery

1
Internet Protocol--- Connectionless Datagram
Delivery

• Linda Wu
• (CMPT 471 2003-3)

2
Content

• TCP/IP internet services
• Internet datagram format
• Fragmentation Reassembly
• Datagram fields
• Reference chapter 7

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TCP/IP Internet Services

• Networking services are arranged in a hierarchy
• Internet software is designed around three
  conceptual networking services
• Replacing one service will not disturb others

4
TCP/IP Internet Services (cont.)

• Connectionless delivery system
• Data are split into several packets
• Unreliable delivery is not guaranteed packet
  may be lost, duplicated, delayed
• Connectionless each packet is treated
  independently from all others
• Best-effort internet software makes an earnest
  attempt to deliver packets unreliability arises
  only when resources are exhausted or networks fail

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TCP/IP Internet Services (cont.)

• Internet protocol (IP)
• IP protocol defines the unreliable,
  connectionless delivery mechanism, and provides 3
  important definitions
• Format of data passed across the internet
• internet transfer unit is called internet
  datagram
• Routing function
• Rules of unreliable packet delivery packet
  processing, error handling, etc.

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Internet Datagram Format

• Internet datagram (IP datagram, datagram)
• Datagram vs. frame

7
Internet Datagram Format (cont.)

• Datagram format

8
Fragmentation Reassembly

• IPv4 datagram length
• ? 216 1 65,535
• Includes the length of header and data
• Network MTU
• Maximum transfer unit (MTU) each hardware
  technology places a fixed upper bound on the
  amount of data transferred in one frame
• Ethernet MTU 1500 (bytes/frame)
• FDDI MTU 4470

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Fragmentation Reassembly (cont.)

• Datagram encapsulation
• Carrying one datagram in one frame
• Hardware does not recognize datagram format
• Efficient transportation

? MTU
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Fragmentation Reassembly (cont.)

• MTU vs. datagram length
• Limiting datagram to fit the smallest possible
  MTU makes transfer inefficient
• Allowing datagrams to be larger than the minimum
  MTU means a datagram cannot always fit into a
  single frame
• Solution in IP protocol datagram fragmentation

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Fragmentation Reassembly (cont.)

• Datagram fragmentation
• The source chooses a convenient datagram size
• Intermediate routers divide large datagram into
  small fragments when needed
• Fragments may not be of equal size their sizes
  must be multiple of 8
• Each fragment can be shipped in a single frame
• Router must accept datagram up to the maximal
  MTUs of the networks they attach to
• Fragments travel to the destination as separate
  datagrams

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Fragmentation Reassembly (cont.)
Original datagram A ? B (1400 bytes of data)
Fragmentation at R1
Fragment header duplicates most of the fields in
the datagram header
Fragment 1 (offset 0/8)
Fragment 2 (offset 600/8)
Fragment 3 (offset 1200/8)
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Fragmentation Reassembly (cont.)

• Reassembly at ultimate destination
• Reassembly timer
• Started when an initial fragment is received
• If the timer expires before all fragments arrive,
  all the received fragments will be discarded
• Advantages
• Fragments are routed independently
• Intermediate routers are not required to store or
  reassemble fragments
• Disadvantages
• Small fragments may traverse on networks with
  large MTU capability
• If any fragment is lost, datagram cannot be
  reassembled

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Datagram Fields

• VERS
• 4 bits
• Specifies the version of IP protocol used to
  create the datagram
• To verify that sender, receiver and any router in
  between them agree on the datagram format
• Machines reject datagram with protocol version
  that differs from theirs
• Current version 4 (IPv4)

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Datagram Fields (cont.)

• HLEN
• 4 bits
• Specifies datagram header length (unit 32-bit
  word / 4 bytes)
• TOTAL LENGTH
• 16 bits
• Specifies of bytes in header and data (unit
  byte)
• TOTAL LENGTH ? 216 1 65,535
• Size of data TOTAL LENGTH - HLEN

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Datagram Fields (cont.)

• SERVICE TYPE
• 8 bits
• Original TOS interpretation

Precedence importance of datagram D low
delay T high throughput R high reliability
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Datagram Fields (cont.)

• Revised differentiated services interpretation
• A codepoint value maps to an underlying service
  definition
• 3 right-most bits of codepoint
• All 0s xxx000 (backward compatible with
  original interpretation)
• Not all 0s 3 administrative groups

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Datagram Fields (cont.)

• Fragmentation control fields
• IDENTIFICATION
• Datagram sender generates a unique IDENTIFICATION
  for each datagram
• It is copied to fragment header for reassembly at
  a later time
• Source address IDENTIFICATION to identify
  datagram
• FRAGMENT OFFSET
• The offset in the original datagram of the data
  carried in the fragment (unit 8 bytes)

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Datagram Fields (cont.)

• FLAGS 3 bits
• D do not fragment
• When a router needs to fragment a datagram with D
  bit set, it will discard the datagram and send an
  error message to the source
• M more fragments
• To indicate whether a fragment is the tail of the
  datagram
• Destination gets the TOTAL LENGTH of the fragment
  instead of the original datagram
• Upon receiving a fragment F with M bit off,
  calculate total length of the datagram based on
  Fs total length and Fs fragment offset

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Datagram Fields (cont.)

• TIME TO LIVE (TTL)
• Defines hop limit of a datagram
• Each router along the path from source to
  destination decrements TTL by 1
• If datagrams TTL filed is 0, router discards
  datagram and sends back an error message to the
  source
• Objective to guarantees that datagrams cannot
  travel around internet forever

21
Datagram Fields (cont.)

• PROTOCOL
• 8 bits
• Defines the higher-level protocol to which
  datagram should be delivered

22
Datagram Fields (cont.)

• HEADER CHECKSUM
• To ensure integrity of header
• Calculated at the sender and the value obtained
  is sent with datagram
• The receiver repeats the same calculation
• If the result is satisfactory, accept the
  datagram
• Otherwise, reject it
• Only applies to values in header, not to data

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Datagram Fields (cont.)

• SOURCE / DESTINATION IP ADDRESS
• Specifies sender / intended recipient of datagram
• Never changes when datagram is routed through
  intermediate routers
• DATA
• Specifies the data sent in datagram
• Variable length
• PADDING
• Composed of all 0s
• Its length depends on IP OPTIONS selected
• To ensure the datagram header extends to an exact
  multiple of 32 bits

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Datagram Fields (cont.)

• IP OPTIONS
• Optional fields with variable length
• Appear contiguously without separators between
  them
• Format

• COPY
• 0 Copy only in the first fragment
• 1 Copy in all fragments
• CLASS
• 00 Datagram control
• 01 Reserved
• Debugging and management
• 11 Reserved
• NUMBER
• See examples on P109

LENGTH total length of the option, including
code field and length field DATA data that
specific options require
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Datagram Fields (cont.)

• Record route option
• To record the routers that handle the datagram
• POINTER the offset within option of the next
  available slot
• When a machine handles a datagram with record
  route option
• If POINTER gt LENGTH, forward datagram without
  inserting its entry
• Otherwise, insert its entry, increase POINTER by 4

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Datagram Fields (cont.)

• Source route option
• Used by the source to predetermine a route for
  the datagram
• At the router side
• If POINTER gt LENGTH, do routing as usual
• Otherwise, follow POINTER to pickup IP address,
  replace it with its outgoing IP address, increase
  POINTER by 4, and forward the datagram

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Datagram Fields (cont.)

• Strict source route (CODE 137)
• All the routers in the option must be visited
• A router not listed in the option must not be
  visited
• If datagram visits a router not in the list, or,
  if it arrives at destination with some listed
  routers not visited, the datagram is discarded
  and an error message is issued
• Loose source route (CODE 131)
• Each router in the list must be visited
• The datagram can visit other routers as well

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Datagram Fields (cont.)

• Timestamp option
• To record the time at which router processes
  datagram
• To keep track of router behavior in the internet
• OFLOW of routers that cannot supply a
  timestamp because the option is full
• FLAGS specifies the visited router
  responsibilities
• Timestamp time and date at which router
  processes datagram expressed as milliseconds
  since midnight, Universal Time

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Datagram Fields (cont.)

• Subfield FLAGS

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Datagram Fields (cont.)

• Processing IP OPTIONs during fragmentation
• The function of COPY bit in fragmentation
• Some IP OPTIONs are replicated in all fragments
• Some IP OPTIONs are placed in one fragment
• Record route option should only be copied into
  one of the fragments
• Source route option should be copied into all
  fragments