ISRO CSE 2014 | Question: 55

Question

An IP packet has arrived with the first $8$ bits as $0100 0010.$ Which of the following is correct?

• A. The number of hops this packet can travel is $2$
• B. The total number of bytes in header is $16$ bytes
• C. The upper layer protocol is ICMP
• D. The receiver rejects the packet

Answer 1

There must be an error in it because See this figure

It is given in Question that arrived packet has 8 bits and a/c to the given figure  4 bits is for Header Length .That must be invalid because the min header length in Bytes must be 20. So

Answer : It must be Option D

The Receiver rejects the packet

Comments

yes...only first 8 bit...r enough 2 check whether 2 accept it or reject it..

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If instead of 0100 0010 we have 0100 1001 then in this case first 4 bit indicates version i.e 4 and last 4 bit means 9 ie 4*9 =36. which is between 20 & 60. so should we accept it?

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yes,,

Answer 2

It is clear from below diagram that max bits in header is=4bits;So how it can be 8bits.Therefore Option -D will be correct answer.

Answer 3

As the HLEN is of 4 bit but (0000 to 0100) is treated as don't care condition and receiver accepts when HLEN (0101 to 1111) The HLEN indicates the rows and each row has 4 bytes (IPV4) so header ranges(5*4 to 15*4) i.e (20 to 60)

Answer 4

There 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 wrong header length (2 × 4 = 8). The minimum number of bytes in the header must be 20. The packet has been corrupted in transmission.and receiver will reject the packet 

Comments

y are we multiplying header length by 4 ??

Answer 5

IP Packet header length = 20B to 60B. Given:

0100	0010
VER	HLEN

Version = 4. So IPv4. This is all right.

Header length = 2. HLEN is actually scaled down by the factor of 4.

So, actual HLEN = $2*4=8$. This specifies that the header is of length 8B, which is invalid.

So, Option D