Chapter%208%20Internet%20Protocol%20(IP)

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Chapter 8 Internet Protocol (IP)
Mi-Jung Choi Dept. of Computer Science and
Engineering mjchoi_at_postech.ac.kr
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Contents

• 8.1 DATAGRAM
• 8.2 FRAGMENTATION
• 8.3 OPTIONS
• 8.4 CHECKSUM
• 8.5 IP PACKAGE
• 8.6 KEY TERMS
• 8.7 SUMMARY

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Objectives

• Upon completion you will be able to
• Understand the format and fields of a datagram
• Understand the need for fragmentation and the
  fields involved
• Understand the options available in an IP
  datagram
• Be able to perform a checksum calculation
• Understand the components and interactions of an
  IP package

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Position of IP in TCP/IP protocol suite
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Internet Protocol (IP)

• Packet delivery mechanism in TCP/IP
• Unreliable connectionless datagram protocol
• Each datagram handled independently
• Each datagram can follow a different route to
  destination.
• datagrams sent by same source and destination
  could arrive out of order
• Best effort delivery
• No error checking or tracking

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8.1 IP Datagram
A packet in the IP layer is called a datagram, a
variable-length packet consisting of two parts
header and data. The header is 20 to 60 bytes in
length and contains information essential to
routing and delivery
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8.1 DATAGRAM

• Version(VER)
• 4BIT field
• Currently, the version is 4
• Header Length(HLEN)
• 4BIT field
• The total length of the datagram header in 4byte
  words
• No options 5(5420), with maximum option size
  15(15460)

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8.1 DATAGRAM

• Differentiated Services (formerly Service Type)
  8bits
• 1. Service type
• The first 3bit precedence bit
• Precedence
• 1(000) to 7(111)
• The priority of the datagram (congestion)
• Not used in version 4
• 000 routine
• 001 Priority
• 010 Immediate
• 011 Flash
• 100 Flash override
• 101 Critical
• 110 Internetwork control
• 111 Network control

• TOS Bits Description
• 0000 Normal (default)
• 0001 Minimize cost
• 0010 Maximize reliability
• 0100 Maximize throughput
• 1000 Minimize delay
• The precedence subfield was designed, but never
  used in version 4.

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Table 8.2 Default types of service
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8.1 DATAGRAM

• 2. Differentiated Services
• First 6bit code point
• Last 2bit not used
• The codepoint subfield can be used in two
  different ways
• When the 3bit right-most bits are 0s, the 3bit
  left-most bits are interpreted the same as the
  precedence bits in the Service Type
  Interpretation
• When the 3bit right-most bits are not all 0s, the
  6bits define 64 services based on the priority
  assignment

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8.1 DATAGRAM (cont.)

• Total length
• The total length field defines the total length
  of the datagram including the header.
• Total length of the IP datagram in byte(header
  data)
• Length of data total length HLEN
• Maximum size 65535 bytes (216 1)
• Encapsulation is needed to transfer datagram in
  some case
• (some padding added)

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8.1 DATAGRAM (cont.)

• Identification
• Used in fragmentation
• Flags.
• Used in fragmentation
• Fragmentation offset
• Used in fragmentation

fragmentation
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8.1 DATAGRAM (cont.)

• Time to live
• Datagram should have a limited lifetime
• Decremented by each visited router
• Discarded when zero
• All the machine must have synchronized clocks and
  how long it takes for a datagram to go from one
  machine to another
• Cases
• Corrupted router
• Intentionally limit the journey of the packet

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8.1 DATAGRAM(cont.)

• Protocol
• Define the higher level protocol that uses the
  services of the IP layer.
• Encapsulate data from several higher level
  protocol(TCP,UDP,ICMP,IGMP)
• Specify the final destination protocol to which
  datagram should be delivered

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8.1 DATAGRAM (cont.)

• Checksum
• Later present
• Source IP address
• Define the IP address of the source
• Destination IP address
• Define the IP address of the destination

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8.1 DATAGRAM (cont.)

• Example 1 An IP packet has arrived with the
  first 8 bits as shown ? 01000010
• The receiver discards the packet. Why?
• There is an error in this packet. The 4 left-most
  bits (0100) show the version, which is correct.
  The next 4 bits (0010) show the header length,
  which means (2 ? 4 8), which is wrong. The
  minimum number of bytes in the header must be 20.
  The packet has been corrupted in transmission.
• Example 2 In an IP packet, the value of HLEN is
  1000 in binary. How many bytes of options are
  being carried by this packet?
• The HLEN value is 8, which means the total number
  of bytes in the header is 8 ? 4 or 32 bytes. The
  first 20 bytes are the main header, the next 12
  bytes are the options.

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8.1 DATAGRAM (cont.)

• Example 3 In an IP packet, the value of HLEN
  is 516 and the value of the total length field is
  002816. How many bytes of data are being carried
  by this packet?
• The HLEN value is 5, which means the total
  number of bytes in the header is 5 ? 4 or 20
  bytes (no options). The total length is 40 bytes,
  which means the packet is carrying 20 bytes of
  data (40-20).
• Example 4 An IP packet has arrived with the
  first few hexadecimal digits as shown below ?
  45000028000100000102...................
• How many hops can this packet travel before being
  dropped? The data belong to what upper layer
  protocol?
• To find the time-to-live field, we should skip
  8 bytes (16 hexadecimal digits). The time-to-live
  field is the ninth byte, which is 01. This means
  the packet can travel only one hop. The protocol
  field is the next byte (02), which means that the
  upper layer protocol is IGMP.

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8.2 FRAGMENTATION

• The format and size of a frame depend on the
  protocol used by the physical network. A datagram
  may have to be fragmented to fit the protocol
  regulations.
• The topics discussed in this section include
• Maximum Transfer Unit (MTU)
• Fields Related to Fragmentation

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8.2 FRAGMENTATION

• Maximum Transfer Unit (MTU)
• Maximum size of the data field
• The total size of encapsulated datagram in a
  frame must be less than maximum size
• Differ from one physical network protocol to
  another

IP datagram
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8.2 FRAGMENTATION
Table 8.5 MTUs for some networks
- MTUs for different network -
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8.2 FRAGMENTATION

• Fragmentation
• Divide the datagram to make it possible to pass
  some networks
• Each fragment has its own header
• With most of the fields repeated, but some
  changed
• A datagram can be fragmented several times
• Only final destination can reassembly of the
  datagram
• Required parts of the header must be copied by
  all fragments
• Three fields(flags, fragmentation offsets, total
  length) changed

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8.2 FRAGMENTATION

• Fields related to fragmentation
• Identification(16bits)
• Identify a datagram originating from the source
  host
• Combination of the identification and source IP
  address must be unique
• IP protocol uses positive number
  counter(guarantee uniqueness)
• Kept in the Main Memory
• All fragment of a datagram have the same
  identification number

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8.2 FRAGMENTATION

• Flags (3bits)
• First bit reserved
• Second bit do not fragment
• 1 must not fragment the datagram
• If cannot pass the datagram through any physical
  network, discards the datagram and return ICMP
  error message to source host
• 0 the datagram can be fragmented if necessary
• Third bit more fragment
• 1 more fragments after this one
• 0 last or only fragment

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8.2 FRAGMENTATION

• Fragmentation offset (13bits)
• Relative position of fragment with respect to the
  whole datagram
• Offset of the data in the original datagram
  measured in units of eight bytes (The size of
  first fragment is divisible by eight)
• Reassemble step in final destination host
• First fragment offset value is 0.
• Divide the length of 1st fragment by 8. This
  value is the offset value of 2nd fragment.
• Divide the length of 2st fragment by 8. This
  value is the offset value of 3rd fragment.
• Continue the process. The last fragment has more
  bit value of 0.

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8.2 FRAGMENTATION (cont.)
Fragmentation example
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8.2 FRAGMENTATION (cont.)
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8.2 FRAGMENTATION (cont.)

• Example 5 A packet has arrived with an M bit
  value of 0. Is this the first fragment, the last
  fragment, or a middle fragment? Do we know if the
  packet was fragmented?
• If the M bit is 0, it means that there are no
  more fragments the fragment is the last one.
  However, we cannot say if the original packet was
  fragmented or not. A nonfragmented packet is
  considered the last fragment.
• Example 6 A packet has arrived with an M bit
  value of 1. Is this the first fragment, the last
  fragment, or a middle fragment? Do we know if the
  packet was fragmented?
• If the M bit is 1, it means that there is at
  least one more fragment. This fragment can be the
  first one or a middle one, but not the last one.
  We dont know if it is the first one or a middle
  one we need more information (the value of the
  fragmentation offset). However, we can definitely
  say the original packet has been fragmented
  because the M bit value is 1.

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8.2 FRAGMENTATION (cont.)

• Example 7 A packet has arrived with an M bit
  value of 1 and a fragmentation offset value of
  zero. Is this the first fragment, the last
  fragment, or a middle fragment?
• Because the M bit is 1, it is either the first
  fragment or a middle one. Because the offset
  value is 0, it is the first fragment.
• Example 8 A packet has arrived in which the
  offset value is 100. What is the number of the
  first byte? Do we know the number of the last
  byte?
• To find the number of the first byte, we multiply
  the offset value by 8. This means that the first
  byte number is 800. We cannot determine the
  number of the last byte unless we know the length
  of the data.
• Example 9 A packet has arrived in which the
  offset value is 100, the value of HLEN is 5 and
  the value of the total length field is 100. What
  is the number of the first byte and the last
  byte?
• The first byte number is 100 ? 8 800. The total
  length is 100 bytes and the header length is 20
  bytes (5 ? 4), which means that there are 80
  bytes in this datagram. If the first byte number
  is 800, the last byte number must 879.

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8.3 Options

• The header of the IP datagram is made of two
  parts a fixed part and a variable part. The
  variable part comprises the options that can be a
  maximum of 40 bytes.
• The topics discussed in this section include
• Format
• Option Types

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8.3 OPTIONS

• Security, Source routing, Record route, Timestamp
• Used for network testing and debugging
• Not required for every datagram
• TLV (Type, Length, Value) Format

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8.3 OPTIONS

• Code (8bits) Type
• Copy(1bit)
• Control the presence of the option in
  fragmentation
• 0 only copied to the first fragment
• 1 coped to all fragments
• Class(2bits)
• General purpose of the option
• 00 datagram control
• 10 debugging and management(01 11 not defined)
• Number(5bits)
• Type of the option(only six types are in use)
• Length
• Total length of the option(including code and
  length fields)
• Not present in all of the option types
• Data Value
• The data that specific options require
• Not present in all of the option types

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8.3 OPTIONS

• Option types(6 types)
• Two types
• 1 byte
• Do not require the length or the the data fields
• Four types
• Multiple bytes
• Require the length and the data fields

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8.3 OPTIONS (cont.)

• No Operation (00001)
• 1 byte option
• Used as a filler between options

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8.3 OPTIONS (cont.)

• End of Option (00000)
• 1 byte option
• Used for padding at the end of the option field
• Only one end of option can be used
• Search payload(data) after option

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8.3 OPTIONS (cont.)

• ?? ?? - Record Route (00111)
• Used to record the internet routers that handle
  the datagram
• Nine router IP can be contained(4byte9 36
  bytes 40bytes)
• Uses a pointer field containing the byte number
  of the first empty entry
• Initialized by 4 and increased by 4 until over
  the length value

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8.3 OPTIONS (cont.)
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8.3 OPTIONS (cont.)

• Strict Source Route (01001)
• Used by the source to predetermine a rout for the
  datagram
• All of the routers defined in the option must be
  visited by datagram
• If the datagram visits a router that is not on
  the list, the datagram is discarded and an error
  message is issued
• If the datagram arrives at the destination and
  some of the entries were not visited, it is also
  discarded and an error message issued

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8.3 OPTIONS (cont.)
Strict source route concept
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8.3 OPTIONS (cont.)

• Loose Source Route (00011)
• Similar to the strict source route
• Each router in the list must be visited, but the
  datagram can visit other routers

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8.3 OPTIONS(cont.)

• Timestamp (00101)
• Used to record the time of datagram processing by
  a router
• Milliseconds from midnight, Universal Time
• Overflow field
• Records the number of routers that could not add
  their timestamp because of no more fields
  available
• Flags field
• Specify the visited router responsibilities

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8.3 OPTIONS (cont.)
Use of flag in timestamp

• 0 add only the timestamp in the provided field
• 1 add each routers outgoing IP address and the
  timestamp
• 3 each router must check the given IP address
  with its own incoming IP address
• If matched, the router overwrites the IP
  address with its outgoing IP address and
• adds the timestamp

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8.3 OPTIONS (cont.)
Timestamp concept
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8.3 OPTIONS (cont.)

• Example 10 Which of the six options must be
  copied to each fragment?
• We look at the first (left-most) bit of the code
  for each option.
• No operation Code is 00000001 no copy.
• End of option Code is 00000000 no copy.
• Record route Code is 00000111 no copy.
• Strict source route Code is 10001001 copied.
• Loose source route Code is 10000011 copied.
• Timestamp Code is 01000100 no copy.
• Example 11 Which of the six options are used
  for datagram control and which are used for
  debugging and management?
• We look at the second and third (left-most) bits
  of the code.
• No operation Code is 00000001 control.
• End of option Code is 00000000 control.
• Record route Code is 00000111 control.
• Strict source route Code is 10001001 control.
• Loose source route Code is 10000011 control.
• Timestamp Code is 01000100 debugging

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Example 12
One of the utilities available in UNIX to check
the travelling of the IP packets is ping. In the
next chapter, we talk about the ping program in
more detail. In this example, we want to show how
to use the program to see if a host is available.
We ping a server at De Anza College named
fhda.edu. The result shows that the IP address of
the host is 153.18.8.1.
ping fhda.eduPING fhda.edu (153.18.8.1) 56(84)
bytes of data.64 bytes from tiptoe.fhda.edu
(153.18.8.1) ....
The result shows the IP address of the host and
the number of bytes used.
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Example 13
We can also use the ping utility with the -R
option to implement the record route option.
ping -R fhda.eduPING fhda.edu (153.18.8.1)
56(124) bytes of data.64 bytes from
tiptoe.fhda.edu (153.18.8.1) icmp_seq0 ttl62
time2.70 msRR voyager.deanza.fhda.edu
(153.18.17.11) Dcore_G0_3-69.fhda.edu
(153.18.251.3) Dbackup_V13.fhda.edu
(153.18.191.249) tiptoe.fhda.edu (153.18.8.1)
Dbackup_V62.fhda.edu (153.18.251.34)
Dcore_G0_1-6.fhda.edu (153.18.31.254)
voyager.deanza.fhda.edu (153.18.17.11)
The result shows the interfaces and IP addresses.
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Example 14
The traceroute utility can also be used to keep
track of the route of a packet.
traceroute fhda.edutraceroute to fhda.edu
(153.18.8.1), 30 hops max, 38 byte packets 1
Dcore_G0_1-6.fhda.edu (153.18.31.254) 0.972 ms
0.902 ms 0.881 ms 2 Dbackup_V69.fhda.edu
(153.18.251.4) 2.113 ms 1.996 ms 2.059 ms 3
tiptoe.fhda.edu (153.18.8.1) 1.791 ms 1.741 ms
1.751 ms
The result shows the three routers visited.
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Example 15
The traceroute program can be used to implement
loose source routing. The -g option allows us to
define the routers to be visited, from the source
to destination. The following shows how we can
send a packet to the fhda.edu server with the
requirement that the packet visit the router
153.18.251.4.
traceroute -g 153.18.251.4 fhda.edu. traceroute
to fhda.edu (153.18.8.1), 30 hops max, 46 byte
packets 1 Dcore_G0_1-6.fhda.edu (153.18.31.254)
0.976 ms 0.906 ms 0.889 ms 2 Dbackup_V69.fhda.ed
u (153.18.251.4) 2.168 ms 2.148 ms 2.037 ms
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Example 16
The traceroute program can also be used to
implement strict source routing. The -G option
forces the packet to visit the routers defined in
the command line. The following shows how we can
send a packet to the fhda.edu server and force
the packet to visit only the router 153.18.251.4,
not any other one.
traceroute -G 153.18.251.4 fhda.edu. traceroute
to fhda.edu (153.18.8.1), 30 hops max, 46 byte
packets 1 Dbackup_V69.fhda.edu (153.18.251.4)
2.168 ms 2.148 ms 2.037 ms
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8.4 CheckSum

• The error detection method used by most TCP/IP
  protocols is called the checksum. The checksum
  protects against the corruption that may occur
  during the transmission of a packet. It is
  redundant information added to the packet.
• The topics discussed in this section include
• Checksum Calculation at the Sender
• Checksum Calculation at the Receiver
• Checksum in the IP Packet

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8.4 CheckSum in IP

• To create the checksum, the sender does the
  following
• 1. The packet is divided into k sections,
  each of n bits.
• 2. All sections are added together using
  ones complement arithmetic.
• 3. The final result is complemented to make
  the checksum.
• Checksum in ones complement arithmetic

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Checksum concept
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An example of IP header checksum calculation in
binary and hexadecimal
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Note
Check Appendix C for a detailed description of
checksum calculation and the handling of carries.
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8.5 IP PACKAGE

• We give an example of a simplified IP software
  package to show its components and the
  relationships between the components. This IP
  package involves eight modules.
• The topics discussed in this section include
• Header-adding module
• Processing module
• Routing module
• Fragmentation module
• Reassembly module
• Routing table
• MTU table
• Reassembly table

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8.5 IP PACKAGE
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8.5 IP PACKAGE

• Header-adding module
• Receive data, destination address
• 1. Encapsulate the data in an IP datagram
• 2.Calculate the checksum and insert it in the
  checksum field
• 3. Send the data to the corresponding input queue
• 4. Return

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8.5 IP PACKAGE

• Processing module
• 1. Remove one datagram from one of the input
  queues
• 2. If(destination address is 127.X.Y.Z or matches
  one of the local addresses)
• 1. Send the datagram to the reassembly module
• 2. Return
• 3. If(machine is a router)
• 1. Decrement TTL
• 4. If(TTL less than or equal to zero)
• 1. Discard the datagram
• 2. Send an ICMP error message
• 3. Return
• 5. Send the datagram to the routing module
• 6. Return

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8.5 IP PACKAGE

• Fragmentation module
• Receive an IP datagram from the routing module
• 1. Extract the size of the datagram
• 2. If(size gt MTU of the corresponding network)
• 1. If(D(do not fragment) bit is set)
• 1. Discard the datagram
• 2. Send an ICMP error message
• 3. Return
• 2. Else
• 1. Calculate the maximum size
• 2. Divide the datagram into fragments
• 3. Add header to each fragment
• 4. Add required options to each fragment
• 5. Send the datagrams
• 6. Return
• 3. Else
• 1. Send the datagram
• 4. Return

MTU table
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8.5 IP PACKAGE

• Reassembly table
• Used by the reassembly module
• Five field
• State(FREE or IN-USE)
• Source IP address(source IP of the datagram)
• Datagram ID(uniquely define a datagram)
• Time-out(predetermined amount of time, in which
  all fragments must arrive)
• Fragments(a pointer to a linked list of
  fragments)

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8.5 IP PACKAGE (cont.)

• Reassembly module
• Receive an IP datagram from the processing
  module
• 1. If(offset value is zero and the M bit is 0)
• 1. Send the datagram to the appropriate queue
• 2. Return.
• 2. Search the reassembly table for the
  corresponding entry
• 3. If(not found)
• 1. Create a new entry
• 4. Insert the fragment at the appropriate place
  in the link list
• 1. If(all fragments have arrived)
• 1. Reassemble the fragments
• 2. Deliver the datagram to the corresponding
  upper layer protocol
• 3.Return
• 2. Else
• 1. Check the time-out
• 2. If(time-out expired)
• 1. Discard all fragments
• 2. Send an ICMP error message
• 5. Return