(a) Which field shows number of hop count?
Hop count: This is an 8-bit field defining the maximum number of hops the packet can travel.
(b) If HLEN value is 5 and length of data is 24 bytes, Calculate option:
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 24 bytes, which means the packet is carrying 4 bytes of data (24 − 20).
When there are no options, the header length is 20 bytes, and the value of HLEN field is 5 (5 × 4 = 20).
(c) What are differentiating services?
Service type: In the original design of IP header, this field was referred to as type of service (TOS), which defined how the datagram should be handled. Part of the field was used to define the precedence of the datagram; the rest defined the type of service (low delay, high throughput, and so on). IETF has changed the interpretation of this 8-bit field. This field now defines a set of differentiated services. The new interpretation is shown in Figure1.
In this interpretation, the first 6 bits make up the codepoint subfield and the last 2 bits are not used. The codepoint subfield can be used in two different ways.
a. 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. In other words, it is compatible with the old interpretation. The precedence defines the eight-level priority of the datagram (0 to 7) in issues such as congestion. If a router is congested and needs to discard some datagrams, those datagrams with lowest precedence are discarded first. Some datagrams in the Internet are more important than the others.
b. When the 3 right-most bits are not all 0s, the 6 bits define 56 (64 − 8) services based on the priority assignment by the Internet or local authorities according to Table 1. The first category contains 24 service types; the second and the third each contain 16.
The first category is assigned by the Internet authorities (IETF). The second category can be used by local authorities (organizations). The third category is temporary and can be used for experimental purposes. Note that these assignments have not yet been finalized.
(d) Packet version of.010 is discarded. Justify.
This 4-bit field defines the version of the IP protocol. There is an error in this packet. The 4 left-most bits (0010) show the version, which is wrong. The packet has been corrupted in transmission. Hence packet version of.010 is discarded
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