IPv4 (The Internet Protocol version 4)

IPv4:

The Internet Protocol version 4 (IPv4) is the delivery mechanism used by the TCP/IP protocols.

IPv4 is an unreliable and connectionless datagram protocol-a best-effort delivery service. The term best-effort means that IPv4 provides no error control or flow control (except for error detection on the header). IPv4 assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees.

1. Datagram

Packets in the IPv4 layer are called datagrams. A datagram is 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. It is customary in TCP/IP to show the header in 4-byte sections.

· Version (VER): This 4-bit field defines the version of the IPv4 protocol. Currently theversion is 4. However, version 6 (or IPv6) may totally replace version 4 in the future. This field tells the IPv4 software running in the processing machine that the datagram has the format of version 4. All fields must be interpreted as specified in the fourth version of the protocol. If the machine is using some other version of IPv4, the datagram is discarded rather than interpreted incorrectly

· Header length (HLEN): This 4-bit field defines the total length of the datagram headerin 4-byte words. This field is needed because the length of the header is variable (between 20 and 60 bytes). When there are no options, the header length is 20 bytes, and the value of this field is 5 (5 x 4 = 20). When the option field is at its maximum size, the value of this field is 15 (15 x 4 = 60)

· Services: IETF has changed the interpretation and name of this 8-bit field. This field,previously called service type, is now called differentiated services.

1. Service Type

In this interpretation, the first 3 bits are called precedence bits. The next 4 bits are called type of service (TOS) bits, and the last bit is not used.

·           Precedence is a 3-bit subfield ranging from 0 (000 in binary) to 7 (111 in binary).The precedence defines the priority of the datagram in issues such as congestion.

·           TOS bits are a 4-bit subfield with each bit having a special meaning. Although abit can be either 0 or 1, one and only one of the bits can have the value of 1 in each datagram. The bit patterns and their interpretations are given in Table 3.4. With only 1 bit set at a time, we can have five different types of services.

2. Differentiated Services

In this interpretation, the first 6 bits make up the code point subfield, and the last 2 bits are not used. The code point subfield can be used in two different ways.

·           When the 3 rightmost bits are 0s, the 3 leftmost bits are interpreted the same as the precedence bits in the service type interpretation. In other words, it is compatible with the old interpretation.

·           When the 3 rightmost bits are not all 0s, the 6 bits define 64 services based on the priority assignment by the Internet or local authorities. The first category contains 32 service types; the second and the third each contain 16. The first category (numbers 0, 2, 4, ... ,62) is assigned by the Internet authorities (IETF). The second category (3, 7, 11, 15, , 63) can be used by local authorities (organizations). The third category (1, 5, 9, .., 61) is temporary and can be used for experimental purposes

· Total length. This is a In-bit field that defines the total length (header plus data) of theIPv4 datagram in bytes. To find the length of the data coming from the upper layer, subtract the header length from the total length. The header length can be found by multiplying the value in the HLEN field by 4.

If the size of an IPv4 datagram is less than 46 bytes, some padding will be added to meet this requirement. In this case, when a machine decapsulates the datagram, it needs to check the total length field to determine how much is really data and how much is padding.

·           Identification. This field is used in fragmentation.

·           Flags. This field is used in fragmentation.

·           Fragmentation offset. This field is used in fragmentation.

·           Time to live: A datagram has a limited lifetime in its travel through an internet.This field was originally designed to hold a timestamp, which was decremented by each visited router. The datagram was discarded when the value became zero.

·           Protocol: This 8-bit field defines the higher-level protocol that uses the servicesof the IPv4 layer. An IPv4 datagram can encapsulate data from several higher-level protocols such as TCP, UDP, ICMP, and IGMP. This field specifies the final destination protocol to which the IPv4 datagram is delivered. In other words, since the IPv4 protocol carries data from different other protocols.

·           Checksum: The checksum concept and its calculation.

·           Source address: This 32-bit field defines the IPv4 address of the source. Thisfield must remain unchanged during the time the IPv4 datagram travels from the source host to the destination host.

·           Destination address: This 32-bit field defines the IPv4 address of the destination

Example 3.12

An IPv4 packet has arrived with the first 8 bits as shown: 01000010

The receiver discards the packet. Why?

Solution

There is an error in this packet. The 4 leftmost bits (0100) show the version, which is correct. The next 4 bits (0010) show an invalid header length (2 x 4 =8). The minimum number of bytes in the header must be 20. The packet has been corrupted in transmission.

Example 3.13

In an IPv4 packet, the value of HLEN is 1000 in binary. How many bytes of options are being carried by this packet?

Solution

The HLEN value is 8, which means the total number of bytes in the header is 8 x 4, or 32 bytes. The first 20 bytes are the base header; the next 12 bytes are the options.

Example 3.14

In an IPv4 packet, the value of HLEN is 5, and the value of the total length field is 0x0028. How many bytes of data are being carried by this packet?

Solution

The HLEN value is 5, which means the total number of bytes in the header is 5 x 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 3.15

An IPv4 packet has arrived with the first few hexadecimal digits as shown. 0x45000028000100000102 ...

How many hops can this packet travel before being dropped? The data belong to what upper-layer protocol?

Solution

To find the time-to-live field, we 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..

Fragmentation

A datagram can travel through different networks. Each router decapsulates the IPv4 datagram from the frame it receives, processes it, and then encapsulates it in another frame. The format and size of the received frame depend on the protocol used by the physical network through which the frame has just traveled. The format and size of the sent frame depend on the protocol used by the physical network through which the frame is going to travel. For example, if a router connects a LAN to a WAN, it receives a frame in the LAN format and sends a frame in the WAN format.

Checksum

The implementation of the checksum in the IPv4 packet follows the same principles. First, the value of the checksum field is set to O. Then the entire header is divided into 16-bit sections and added together. The result (sum) is complemented and inserted into the checksum field.

Example 3.16

Figure 3.23 shows an example of a checksum calculation for an IPv4 header without options. The header is divided into 16-bit sections. All the sections are added and the sum is complemented.

The result is inserted in the checksum field.

Options

The header of the IPv4 datagram is made of two parts: a fixed part and a variable part. The fixed part is 20 bytes long and was discussed in the previous section. The variable part comprises the options that can be a maximum of 40 bytes. Options, as the name implies, are not required

for a datagram. They can be used for network testing and debugging. Although options are not a required part of the IPv4 header, option processing is required of the IPv4 software. This means that all implementations must be able to handle options if they are present in the header.

No Operation

A no-operation option is a 1-byte option used as filler between options.

End of Option

An end-of-option option is a 1-byte option used for padding at the end of the option field. It, however, can only be used as the last option.

Record Route

A record route option is used to record the Internet routers that handle the datagram. It can list up to nine router addresses. It can be used for debugging and management purposes.

Strict Source Route

A strict source route option is used by the source to predetermine a route for the datagram as it travels through the Internet. Dictation of a route by the source can be useful for several purposes. The sender can choose a route with a specific type of service, such as minimum delay or maximum throughput. Alternatively, it may choose a route that is safer or more reliable for the sender's purpose.

Loose Source Route

A loose source route option is similar to the strict source route, but it is less rigid. Each router in the list must be visited, but the datagram can visit other routers as well.

Timestamp

A timestamp option is used to record the time of datagram processing by a router. The time is expressed in milliseconds from midnight, Universal time or Greenwich mean time. Knowing the time a datagram is processed can help users and managers track the behavior of the routers in the Internet.