1. Chapter 8: Internet Protocol (IP)

2. Contents 8.1 DATAGRAM 8.2 FRAGMENTATION 8.3 OPTIONS 8.4 CHECKSUM
8.5 IP PACKAGE
8.6 KEY TERMS
SUMMARY

3. 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

4. Position of IP in TCP/IP protocol suite

5. Internet Protocol (IP)
Packet delivery mechanism in TCP/IP
Unreliable and 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

6. 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

7. 8.1 DATAGRAM Version(VER) Header Length(HLEN) 4 BIT field
Currently, the version is 4
Header Length(HLEN)
The total length of the datagram header in 4 byte words
No options : 5(5*4=20), with maximum option size : 15(15*4=60)

8. 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 (in case of 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
Normal (default)
Minimize cost
Maximize reliability
Maximize throughput
Minimize delay
The precedence subfield was designed, but never used in version 4.

9. Table 8.2 Default types of service
Protocol TOS Bits Description
ICMP Normal
BOOTP Normal
NNTP Minimize cost
IGP Maximize reliability
SNMP Maximize reliability
TELNET Minimize delay
FTP (data) Maximize throughput
FTP (control) Minimize delay
TFTP Minimize delay
SMTP (command) Minimize delay
SMTP (data) Maximize throughput
DNS (UDP query) Minimize delay
DNS (TCP query) Normal
DNS (zone) Maximize throughput

10. Temporary or experimental
8.1 DATAGRAM
2. Differentiated Services (Type)
First 6 bits : code point
Last 2 bits : not used
The codepoint subfield can be used in two different ways
When the 3 right-most bits are 0s, the 3 left-most bits are interpreted the same as the precedence bits in the Service Type Interpretation
When the 3 right-most bits are not all 0s, the 6 bits define 64 services based on the priority assignment
Temporary or experimental
XXXX01 3
Local
XXXX11 2
Internet
XXXXX0 1
Assignment Authority
Codepoint
Category

11. 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 : bytes (216 – 1)
Encapsulation is needed to transfer datagram in some case
(some padding added)

12. 8.1 DATAGRAM (cont.) fragmentation Identification
Used in fragmentation
Flags.
Fragmentation offset
fragmentation

13. 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 25 25 24 24 22 23 23

14. 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
Value Protocol
1 ICMP
2 IGMP
6 TCP
8 EGP
17 UDP
41 IPv6
89 OSPF

15. 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

16. 8.1 DATAGRAM (cont.)
Example 1 : An IP packet has arrived with the first 8 bits as shown: 
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.

17. 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 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: 
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.

18. Ethernet formatted frame Token ring formatted frame
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
router
Ethernet formatted frame
Token ring formatted frame
Ethernet network
Token ring network

19. IP datagram 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

20. - MTUs for different network -
8.2 FRAGMENTATION
Table 8.5 MTUs for some networks
- MTUs for different network -

21. 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

22. 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

23. 8.2 FRAGMENTATION 1 : must not fragment the datagram
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

24. 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.

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

26. 8.2 FRAGMENTATION (cont.)

27. 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 don’t 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.

28. 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.

29. 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

30. 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

31. 8.3 OPTIONS Code (8bits) : Type Length Data : Value 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

32. 8.3 OPTIONS Option types(6 types) Two types Four types 1 byte
Do not require the length or the the data fields
Four types
Multiple bytes
Require the length and the data fields

33. 8.3 OPTIONS (cont.) No Operation (00001) 1 byte option
Used as a filler between options

34. 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

35. 8.3 OPTIONS (cont.) 경로 기록 - Record Route (00111)
Used to record the internet routers that handle the datagram
Nine router IP can be contained(4byte×9 = 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

36. 8.3 OPTIONS (cont.)

37. 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

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

39. 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

40. 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

41. Use of flag in timestamp
8.3 OPTIONS (cont.)
Use of flag in timestamp
0 : add only the timestamp in the provided field
1 : add each router’s 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

42. 8.3 OPTIONS (cont.)
Timestamp concept

43. 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 ; no copy.
End of option: Code is ; no copy.
Record route: Code is ; no copy.
Strict source route: Code is ; copied.
Loose source route: Code is ; copied.
Timestamp: Code is ; 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 ; control.
End of option: Code is ; control.
Record route: Code is ; control.
Strict source route: Code is ; control.
Loose source route: Code is ; control.
Timestamp: Code is ; debugging

44. 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
$ ping fhda.edu PING fhda.edu ( ) 56(84) bytes of data. 64 bytes from tiptoe.fhda.edu ( ): ....
The result shows the IP address of the host and the number of bytes used.

45. Example 13
We can also use the ping utility with the -R option to implement the record route option.
$ ping -R fhda.edu PING fhda.edu ( ) 56(124) bytes of data. 64 bytes from tiptoe.fhda.edu ( ): icmp_seq=0 ttl=62 time=2.70 ms RR: voyager.deanza.fhda.edu ( ) Dcore_G0_3-69.fhda.edu ( ) Dbackup_V13.fhda.edu ( ) tiptoe.fhda.edu ( ) Dbackup_V62.fhda.edu ( ) Dcore_G0_1-6.fhda.edu ( ) voyager.deanza.fhda.edu ( )
The result shows the interfaces and IP addresses.

46. Example 14
The traceroute utility can also be used to keep track of the route of a packet.
$ traceroute fhda.edu traceroute to fhda.edu ( ), 30 hops max, 38 byte packets 1 Dcore_G0_1-6.fhda.edu ( ) ms ms ms 2 Dbackup_V69.fhda.edu ( ) ms ms ms 3 tiptoe.fhda.edu ( ) ms ms ms
The result shows the three routers visited.

47. 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
$ traceroute -g fhda.edu.
traceroute to fhda.edu ( ), 30 hops max, 46 byte packets
1 Dcore_G0_1-6.fhda.edu ( ) ms ms ms
2 Dbackup_V69.fhda.edu ( ) ms ms ms

48. 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 , not any other one.
$ traceroute -G fhda.edu.
traceroute to fhda.edu ( ), 30 hops max, 46 byte packets
1 Dbackup_V69.fhda.edu ( ) ms ms ms

49. 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

50. 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 one’s complement arithmetic.
3. The final result is complemented to make the checksum.
Checksum in one’s complement arithmetic

51. Checksum concept

52. An example of IP header checksum calculation in binary and hexadecimal

53. Note:
Check Appendix C for a detailed description of checksum calculation and the handling of carries.

54. 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

55. 8.5 IP PACKAGE

56. 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

57. 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

58. 8.5 IP PACKAGE MTU table Fragmentation module
Receive : an IP datagram from the routing module
1. Extract the size of the datagram
2. If(size > 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

59. 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)

60. 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