1. Chapter 8 Internet Protocol (IP)

2. Position of IP in TCP/IP protocol suite

3. IP is the transmission mechanism used by the TCP/IP protocol
Introduction
IP is the transmission mechanism used by the TCP/IP protocol
It is unreliable and connectionless datagram protocol
Providing Best-effort delivery service (best-effort : no error checking and tracking)

4. 8.1 Datagram Packets in the IP layer : called datagrams
IP datagram format
Variable-length packet consisting of header and data
Header
20 ~ 60 bytes
Containing information that is essential for routing and delivery
IP header
Version (VER) : Version 4 or 6 (IPng)
Header length (HLEN) : represented by in 4 byte words
Ex) if HLEN = 5, the real header length is 20 bytes

5. Datagram (cont’d)

6. Datagram (cont’d) Service Type
Defining how the datagram should be handled by the routers
Precedence : 3 bits
Defining the priority of the datagram in issues such as congestion
Ex) a datagram for network management vs. optional information to a group of people
At present, not used in version 4
service type : 4 bits (TOS bits)
With only one bit set at a time
Remaining bit : not used

7. Datagram (cont’d)
Service type or Differentiated Services

8. Datagram (cont’d) Types of service TOS bits Description 0000 Normal
0001
Minimize cost
0010
Maximize reliability
0100
Maximize throughput
1000
Minimize delay

9. Datagram (cont’d) Default types for some applications in use of TOS
Protocol
TOS bits
Description
ICMP
0000
Normal
BOOTP
NNTP
0001
Minimize cost
IGP
0010
Maximize reliability
SNMP
TELNET
1000
Minimize delay
FTP (data)
0100
Maximize throughput
FTP (control)
TFTP
SMTP (command)
SMTP (data)
DNS (UDP query)
DNS (TCP query)
DNS (zone)

10. Datagram (cont’d) Differentiated Services Category Codepoint
The first 6 bits : codepoint subfield
Values for codepoints
Category
Codepoint
Assigning Authority 1
XXXXX0
Internet 2
XXXX11
Local 3
XXXX01
Temporary or experiment

11. Datagram (cont’d) Total Length : head + data
Defining the total length of the datagram including the header
Length of data = total length – header length
Limited to 65,535 (216 – 1) bytes
Encapsulation of a small datagram in an Ethernet Frame
Ethernet Frame size : 46 ~ 1500 bytes

12. Datagram (cont’d) Flags : used in fragmentation
Fragmentation offset : used in fragmentation
Time to live
Used to control the maximum number of hops (routers) visited by the datagram
If the value is Zero, the routers discarded
If the source wants to confine the packet to the local network, it can store 1 in this field

13. Datagram (cont’d) Protocol
Defining the higher level protocol that uses the services of the IP layer
TCP, UDP, ICMP, and IGMP
Multiplexing data from different higher level protocols
Value
Protocol 1
ICMP 2
IGMP 6
TCP 8
EGP 17
UDP 89
OSPF

14. Datagram (cont’d)
Example 1
An arriving IP packet : 
The receiver discards the packet, Why ?
- 2 x 4 bytes = 8bytes : Minimum number of bytes in the header must be 20

15. Datagram (cont’d)
Example 2
The value of HLEN is 1000 in binary
How many bytes of options are being carried by this packet ? 
8 x 4 bytes = 32 bytes : 20 bytes + 12 bytes (option)

16. Datagram (cont’d)
Example 3
- In an IP packet, the value of HLEN is 5 16 and the value of the total length field is How many bytes of data are being carried by this packet?
Answer
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 ( ).

17. Datagram (cont’d)
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?

18. Datagram (cont’d)
Answer
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.

19. Datagram (cont’d) Checksum : header checksum- 16 bits
Source IP address : 32 bit-field
Destination IP address : 32 bit-field

20. 8.2 Fragmentation
The format and size of the received frame depend on the protocol used by the physical network
Ex) A router connecting Ethernet to token ring

21. Fragmentation (cont’d)
MTU (Maximum Transfer Unit)
When a datagram is encapsulated in a frame, the total size of the datagram must be less than this maximum size

22. Fragmentation (cont’d)
MTUs for different networks
Protocol
MTU
Hyperchannel
65,535
Token ring (16Mbps)
17,914
Token ring (4Mbps)
4,464
FDDI
4,352
Ethernet
1,500
X.25
576
PPP
296
Hyperchannel : Network Systems Corporation, 1988 (RFC 1044)

23. Fragmentation (cont’d)
The maximum length of the IP datagram equals to the largest MTU defined so far (65,535 bytes)
Therefore, for the other physical networks we must divide the datagram : fragmentation
datagram that can be fragmented by the source host or any router in the path, but the reassembly of datagram is done by the destination
When a datagram is fragmented, required parts of the header must be copied by all fragments.
Changing the values of the three fields : flags, fragmentation offset, and total length
The rest of fields must be copied
Checksum must be recalculated

24. Fragmentation (cont’d)
Fields related to fragmentation
Identification : 16 bit-field
Datagram id that is originated by the source host
Therefore, Source IP address + datagram id (identification)
All fragments having same identification number
Identification No. to be used for the destination in reassembling the datagram
Flags : 3 bit-field
D : Do not fragment (1)
If it can not pass the datagram through any available physical network, it discards the datagram and send ICMP error message to the source host
M : More fragment (0)
0 : last fragment or only fragment

25. Fragmentation (cont’d)
Fragmentation offset : 13-bit field
Showing relative position of this fragment with respect to the whole datagram
Measured in units of 8 bytes : forcing hosts or routers that fragment datagrams to choose the size of each fragment so that the first byte number is divisible by eight

26. Fragmentation (cont’d)

27. 8.3 Options Variable part of the IP datagram : the maximum of 40 bytes
Format : Code, Length, and Data

28. Options (cont’d) Code field
8 bits length and containing 3 subfields : copy, class, and number
Copy
Controlling the presence of the option in fragmentation
0 : meaning that option must be copied only to the first fragment
1 : meaning the option must be copied to all fragments
Class
Defining the general purpose of the option
00 : datagram control, 01 : reserved, 10 : Debugging and management, 11: reserved

29. Options (cont’d) Length Number
Defining the type of the option : only 6 options that are currently being used
Length
defining the total length of the option including the code field and length field itself
Data
containing the data that specific options require

30. Options (cont’d)
Option Types

31. Options (cont’d) No Operation
one byte option used as a filler between options

32. Options (cont’d) End of Option
one-byte option used for padding at the end of the option field
used as the last option

33. Options (cont’d) Record Route
used to record the internet routers that handle the datagram
list up to 9 router IP addresses since the max. size of the header is 60 bytes (Base header : 20 bytes)
pointer field
An offset integer field containing the byte number of the first empty entry (available entry)
When leaving the source, the pointer field has a value of four, pointing to the first empty field

34. Options (cont’d)
Record route option

35. Options (cont’d)

36. Options (cont’d) Strict Source Route
used by the source to predetermine a route for the datagram as it travels through the Internet
can choose a route with specific type of service : minimum delay or maximum throughput

37. Options (cont’d)
Strict source route concept

38. Options (cont’d) Loose Source Route
similar to the strict source route, but it is more relaxed
each router in the list must be visited, but the datagram can visit other routers as well

39. Options (cont’d) Time Stamp
used to record the time of datagram processing by a router
expressed in millisecond from the midnight, Universal Time

40. Options (cont’d)
overflow field : recording the number of routers that could not add their timestamp because no more fields were available
Use of flag in timestamp

41. Options (cont’d)
Timestamp concept (when flag =1)

42. Error detection method used by most TCP/IP protocols
8.4 Checksum
Error detection method used by most TCP/IP protocols
Checksum calculation at the sender
The packet is divided into k sections, each of n bits ( n is usually 16)
All sections are added together using one’s complement arithmetic
The final result is complemented to make the checksum

43. Checksum calculation at the receiver
Checksum (cont’d)
Checksum calculation at the receiver
The packet is divided into k sections, each of n bits.
All sections are added together using one’s complement arithmetic
The result is complemented
If the final result is 0, the packet is accepted; otherwise it is rejected

44. Checksum (cont’d)
Checksum concept

45. Checksum (cont’d)
Checksum in one’s complement arithmetic

46. Checksum in the IP Packet
Checksum (cont’d)
Checksum in the IP Packet
covering only the header, not the data
all higher level protocols that encapsulate data in the IP datagram have a checksum field that covers the whole packet
the header changes with each visited router, but data does not. So the checksum includes only the part which has changed
if each router must recalculates the checksum, it is needed to have the more processing time for each router

47. Checksum (cont’d)
Example

48. IP package : 8 components
Header-adding module
Processing module
Routing module
fragmentation module
reassembly module
routing table
MTU table
reassembly table

49. IP Package (cont’d)
IP components

50. The operation of IP package
IP Package(cont’d)
The operation of IP package
receiving an IP packet, either from the data link layer or a higher level protocol
if the packet comes from a upper layer protocol, it should be delivered to the data link layer
if the packet comes from the data link layer, forwarding to data link or a upper layer ( the destination is same as the station address)

51. IP Package (cont’d) Header-adding Module
Receive : data, destination address
Encapsulate the data in an IP datagram
Calculate the checksum and insert it in the checksum field
Send the data to the corresponding input queue
Return

52. IP Package (cont’d) Processing Module
Remove one datagram from one of the input queues
if (destination address is 127.X.Y.Z or matches one of the local addresses)
Send datagram to the reassembly module.
Return
if (machine is a router)
Decrement TTL
if (TTL less than or equal to zero)
Discard the datagram
Send an ICMP error message
Send the datagram to the routing module

53. IP Package (cont’d) Queues Routing table Routing module
Input queues and output queues
Routing table
used by the routing module to determine the next-hop address of the packet
Routing module
receiving an IP packet from the processing module
sending the packet with the information to the fragmentation module

54. IP Package (cont’d) MTU Table
to find the maximum transfer unit of a particular interface.

55. IP Package (cont’d) Fragmentation Module
Receive : an IP packet from routing module
Extract the size of the datagram
if (size > MTU of the corresponding network)
If (D (do not fragment) bit is set
Discard the datagram
Send an ICMP error message
Return
Else
Calculate the maximum size
Divide the datagram into fragments
Add header to each fragment
Add required options to each fragment
Send the datagram

56. IP Package (cont’d) Reassembly Table State field : FREE or IN-USE
Source IP address of datagram
Datagram ID
Time-out : a predetermined amount of time in which all fragments must arrive
Fragment field : a pointer to a linked list of fragments

57. IP Package (cont’d)

58. IP Package (cont’d) Reassembly Module
Receive : an IP packet from the processing module
If (offset value is zero and the M bit is 0)
Send the datagram to the appropriate queue
Return
Search the reassembly table for the corresponding entry
If (not found)
Create a new entry

59. IP Package (cont’d) 5. Return
4. Insert the fragment at the appropriate place in the linked list
if (all fragments have arrived)
Reassemble the fragments
Deliver the datagram to the corresponding upper layer protocol
Return
Else
Check the time-out
if (time-out expired)
Discard all fragment
Send an ICMP error message
5. Return

60. Summary (1)
IP is an unreliable connectionless protocol responsible for source-to-destination delivery.
Packets in the IP layer are called datagrams
A datagram consists of a header (20 to 60 bytes) and data.
The IP header contains the following information: version number, header length, differentiated services, datagram length, identification number, fragmentation flags, fragmentation offset, time to live, protocol, checksum, source address, destination address, and options.
The maximum length of a datagram is 65,535 bytes.
The MTU is the maximum number of bytes that a data link protocol can encapsulate. MTUs vary from protocol to protocol.

61. Summary (2)
Fragmentation is the division of a datagram into smaller units to accommodate the MTU of a data link protocol.
The fields in the IP header that relate to fragmentation are the identification number, the fragmentation flags, and the fragmentation offset.
The IP datagram header consists of a fixed, 20-byte section and a variable options section with a maximum of 40 bytes.
The options section of the IP header is used for network testing and debugging.
The options header contains the following information: a code field that identifies the option, option length, and the specific data.

62. Summary (3)
The six IP options each have a specific function. They are as follows: filler between options for alignment purposes, padding, recording the route the datagram takes, selection of a mandatory route by the sender, selection of certain routers that must be visited, and recording of processing times at routers.
The ping and traceroute utilities in UNIX can be used to implement some of the IP options.
The error detection method used by IP is the checksum.
The checksum uses one's complement arithmetic to add equal-size sections of the IP header. The complemented result is stored in the checksum field. The receiver also uses one's complement arithmetic to check the header.
An IP package can consist of the following: a header-adding module, a processing module, a forwarding module, a fragmentation module, a reassembly module, a routing table, an MTU table, and a reassembly table.