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Chapter 8

Chapter 8. Internet Protocol. 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

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Chapter 8

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  1. Chapter 8 Internet Protocol 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 TCP/IP Protocol Suite

  2. Position of IP in TCP/IP protocol suite TCP/IP Protocol Suite

  3. INTERNET PROTOCOL (IP) • It is the transmission mechanism used by TCP/IP protocols. • It is an unreliable & connectionless datagram protocol-a best delivery service. • Best effort means that IP provides no error checking or tracking. • IP is also a connectionless protocol for a packet switching network that uses the datagram approach. • This means that each datagram is handled independently & each datagram can follow a different route to the destination. TCP/IP Protocol Suite

  4. 8.1 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. TCP/IP Protocol Suite

  5. IP datagram TCP/IP Protocol Suite

  6. Service type or differentiated services (DS) Precedence: (0 to 7 bits). The precedence defines the priority of datagram in issues such as congestion. If a router is congested & needs to discard some datagram, those datagram with lowest precedence are discarded first. The precedence subfield was part of version 4, but never used. TCP/IP Protocol Suite

  7. 4-bit subfield with each bit having special meaning. • The bit pattern & their interpretation are given below: Types of service TCP/IP Protocol Suite

  8. Default types of service TCP/IP Protocol Suite

  9. The first 6 bits make up codepoints subfield & last 2 bits are not used. • The first category (0, 2, 4, ……..62) is assigned by the Internet authority IETF. • The second category (3, 7, 11, 15, ….63)can be used by local authorities. • The third category (1, 5, 9….61) is temporary & can be used for experimental purposes. Values for code points TCP/IP Protocol Suite

  10. Note: Total Length=Header + Data Length of Data=Total Length – Header Length The total length field defines the total length of the datagram including the header. TCP/IP Protocol Suite

  11. Encapsulation of a small datagram in an Ethernet frame TCP/IP Protocol Suite

  12. Figure 8.5Multiplexing TCP/IP Protocol Suite

  13. Protocols TCP/IP Protocol Suite

  14. Example 1 An IP packet has arrived with the first 8 bits as shown: 01000010 The receiver discards the packet. Why? TCP/IP Protocol Suite

  15. Example 1 An IP packet has arrived with the first 8 bits as shown: 01000010 The receiver discards the packet. Why? SolutionThere 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. TCP/IP Protocol Suite

  16. 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? TCP/IP Protocol Suite

  17. 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? SolutionThe 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 base header, the next 12 bytes are the options. TCP/IP Protocol Suite

  18. Example 3 In an IP packet, the value of HLEN is 516 and the value of the total length field is 002816 . Howmany bytes of data are being carried by this packet? TCP/IP Protocol Suite

  19. Example 3 In an IP packet, the value of HLEN is 516 and the value of the total length field is 002816 . Howmany bytes of data are being carried by this packet? SolutionThe 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). TCP/IP Protocol Suite

  20. 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 TCP/IP Protocol Suite

  21. MTU Each data link layer protocol has its own protocols. One of the field in the format is maximum size of the data field. The value of the MTU differs from one physical network protocol to another. TCP/IP Protocol Suite

  22. MTUs for some networks When a datagram is fragmented, each fragment has its own header with most of the field repeated, but some changed. That means, datagram can be fragmented many times before it reaches the final destination. TCP/IP Protocol Suite

  23. FIELDS RELATED TO FRAGMENTATION: • Identification: • This is a 16-bit field identifies a datagram & uniquely defined • IP protocol uses a counter to label the datagram. • When the IP protocol sends a datagram , it copies the current value of the counter to the identification field & increments the counter by one. • All fragments have the same identification number. • Flags: • This is a 3-bit field. • The first bit is reserved. • If D=1, the machine must not fragment the datagram. • If D=0, the datagram can be fragmented. • If M=1, there are more fragments after this fragments. • If M=0, this is the last or only fragment. TCP/IP Protocol Suite

  24. FIELDS RELATED TO FRAGMENTATION: • Fragmentation Offset: • This is a 13-bit field shows the relative position of this fragment with respect to the whole datagram. • It is the offset of data in the original datagram measured in units of 8 bytes. TCP/IP Protocol Suite

  25. Fragmentation example The following ex. Shows a datagram with a data size of 4000 bytes in units of 8 bytes. TCP/IP Protocol Suite

  26. Detailed fragmentation example TCP/IP Protocol Suite

  27. 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? TCP/IP Protocol Suite

  28. 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? SolutionIf 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. TCP/IP Protocol Suite

  29. 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? TCP/IP Protocol Suite

  30. 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? SolutionIf 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). See also the next example. TCP/IP Protocol Suite

  31. 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?. TCP/IP Protocol Suite

  32. 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?. SolutionBecause 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. TCP/IP Protocol Suite

  33. 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? TCP/IP Protocol Suite

  34. 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? SolutionTo 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. TCP/IP Protocol Suite

  35. OPTIONS The header of the IP datagram is made of two parts: a fixed part and a variable part. The fixed part is 20 bytes & the variable part comprises the options that can be a maximum of 40 bytes. They can be used for network testing & debugging. TCP/IP Protocol Suite

  36. Option format Length: It define the total length of the option including the code field & the length field itself. This field is not present in all of the option types. Data: It contains the data that specific options require. Like the length field , this field also not present in all option types. TCP/IP Protocol Suite

  37. Categories of options TCP/IP Protocol Suite

  38. No operation option It is a 1-byte option used as a filler between options. For ex., it can be used to align the next option on a 16-bit or 32-bit boundary. TCP/IP Protocol Suite

  39. End of option It is a 1-byte option used for padding at the end of the option field. Only one end of option can be used. After this option, the receiver looks for the payload data. TCP/IP Protocol Suite

  40. Record route option • It is used to record the internet routers that handle the datagram. • Pointer field is an offset integer field containing the byte number of the first empty entry. TCP/IP Protocol Suite

  41. Record route concept TCP/IP Protocol Suite

  42. Strict source route option • It is used by the source to predetermine a route for the datagram as it travels through the Internet. • The sender can choose a route with a specific type of services, such as minimum delay or maximum throughput. • If a datagram specifies a strict route , all of the routers defined in the option must be visited by the datagram. TCP/IP Protocol Suite

  43. Strict source route concept TCP/IP Protocol Suite

  44. Loose source route option • It is similar to strict source route, but it is more relaxed. • In this each router in the list must be visited, but the datagram can visit other routers as well. TCP/IP Protocol Suite

  45. Timestamp option • It is used to record the time of datagram processing by a router. • We can estimate the time it takes for a datagram to go from one router to another. • Although all routers may use Universal Time, their local clocks may not be synchronized. • Overflow field records the number of routers that could not add their timestamp. • Flag=0,each router adds only the timestamp in the provided field • Flag=1, each router must add its outgoing IP address & the timestamp. • Flag=3, IP addresses are given & each router must check given IP address with its own incoming IP address. TCP/IP Protocol Suite

  46. Use of flag in timestamp TCP/IP Protocol Suite

  47. Timestamp concept TCP/IP Protocol Suite

  48. Example 10 Which of the six options must be copied to each fragment? SolutionWe look at the first (left-most) bit of the code for each option. a. No operation: Code is 00000001; not copied.b. End of option: Code is 00000000; not copied.c. Record route: Code is 00000111; not copied.d. Strict source route: Code is 10001001; copied.e. Loose source route: Code is 10000011; copied.f. Timestamp: Code is 01000100; not copied. TCP/IP Protocol Suite

  49. Example 11 Which of the six options are used for datagram control and which are used for debugging and management? SolutionWe look at the second and third (left-most) bits of the code. a. No operation: Code is 00000001; datagram control.b. End of option: Code is 00000000; datagram control.c. Record route: Code is 00000111; datagram control.d. Strict source route: Code is 10001001; datagram control.e. Loose source route: Code is 10000011; datagram control.f. Time stamp: Code is 01000100; debugging and management control. TCP/IP Protocol Suite

  50. 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 checksum is calculated at the sender & the value obtained is sent with the packet. • The receiver repeats the same calculation on the whole packet including the checksum. • If the result is satisfactory, the packet is accepted; otherwise it is rejected TCP/IP Protocol Suite

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