Answer to Question #314551 in Computer Networks for Sunny

Question

In the IPv4 Header format following information is given in decimal number system, with

respect to the given information calculate the header checksum value. (10)

VER=4

HLEN =6

Service type=9

Total length=26

Identification=2

Flag=3

Fragmentation offset=0

Time to live=5

Protocol=12

Source IP Address=11.13.12.6

Destination IP Address=10.6.8.11

Answer 1

• If header length field contains decimal value 5 (represented as 0101), then-

Header length = 5 x 4 = 20 bytes

• If header length field contains decimal value 10 (represented as 1010), then-

Header length = 10 x 4 = 40 bytes

• If header length field contains decimal value 15 (represented as 1111), then-

Header length = 15 x 4 = 60 bytes

1.Version

The first header field is a 4-bit version indicator. In the case of IPv4, the value of its four bits is set to 0100, which indicates 4 in binary.

2.Internet Header Length

 IHL is the 2^nd field of an IPv4 header, and it is of 4 bits in size. This header component is used to show how many 32-bit words are present in the header. As we know, IPv4 headers have a variable size, so this is used to specify the size of the header to avoid any errors. This size can be between 20 bytes to 60 bytes.

3.Type of Service

ToS is also called Differentiated Services Code Point or DSCP. This field is used to provide features related to service quality, such as for data streaming or Voice over IP (VoIP) calls. It is used to specific how a datagram will be handled.

4.Explicit Congestion Notification

 ECN is used to send notifications to the sender or receive in situations where network congestion happens. This is an optional feature of IPv4 can; if one of the endpoints don’t support it, it is not used.

5.Total Length

 This field’s size is 16 bit, and it is used to denote the size of the entire datagram. The minimum size of an IP datagram is 20 bytes, and at the maximum, it can be 65,535 bytes. Practically, all hosts are required to be able to read 576-byte datagrams. If a datagram is too large for the hosts in the network, fragmentation is used, which is handled in the host or packet switch.

6.Identification

 The identification or ID field in a packet can identify an IP datagram’s fragments uniquely. Some have suggested using this field for other things such as adding information for packet tracing etc.

7.Flags

Flag in an IPv4 header is a three-bit field that is used to control and identify fragments. The following can be their possible configuration:

• Bit 0: this is reserved and has to be set to zero
• Bit 1: DF or do not fragment
• Bit 2: MF or more fragments.

8.Fragment Offset

This field is 13 bit long in length, and it is measured by blocks that units of 8-byte blocks. These are used to specify the offset of a fragment relative to the start of the IP datagram, which when it was not fragmented. As you can expect, the first offset of a fragment is always set to zero. The maximum possible offset is ( 2¹³-1 ) * 8 = 65528, but it is more than the maximum possible IP Packet length, which is 65,535 bytes long with the length of a header added in.

9.Time to live

Time to live (or TTL in short) is an 8-bit field to indicate the maximum time the datagram will be live in the internet system. The time here is measured in seconds, and in case the value of TTL is zero, the datagram is erased. Every time a datagram is processed, it’s Time to live is decreased by one second. These are used so that datagrams that are not delivered are discarded automatically. TTL can be between 0 – 255.

10.Protocol

This is a filed in the IPv4 header reserved to denote which protocol is used in the later (data) portion of the datagram. For Example, number 6 is used to denote TCP and 17 is used to denote UDP protocol.

11.The header’s checksum

The checksum field is of 16-bit length, and it is used to check the header for any errors. The header is compared to the value of its checksum at each hop, and in case the header checksum is not matching, the packet is discarded. Keep in mind that this is only for the header, and its protocol handles the data field. UDP and TCP, for example, have their own checksum fields.

12.Source Address

It is a 32-bit address of the source of the IPv4 packet.

13.Destination Address

The destination address is also 32 bit in size, and it contains the receiver’s address.

14.Options

This is an optional field of the IPv4 header. It is used only when the value of IHL is set to more than 5. These options contain values and settings for things related to security. Record route and time stamp etc. You will find that the list of options component ends with an End of Options or EOL in many cases.