CN | ACE Practice Book | Q47

Question

Host A (on TCP/ IPv4 network A) sends an IP datagram to host B (also on TCP/IPV4 network B). Assume that, no error occurred during the transmission of D. When D reaches B, which of the following IP header field(s) may be different from that of the original datagram D?

• A. TTL
• B. Checksum
• C. Fragment offset
• D. HLEN (Header length)

Comments

@anupamsworld a,b,c,d?

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Given answer is A,B,C.

However,
as per your answer, I understand A,B,C but could you please describe how HLEN may change?
I understand HLEN is depending on OPTION field.
So, may OPTION field of an ip packet be changed in transit? If yes, how?  @Sunnidhya Roy

Answer 1

@anupamsworld Options field can hold Record Route right?? So as and when the datagram passes through intermediate nodes it adds the IP addresses in the Record Route in the options field which in turn affects the HLEN so that is why I am saying that option D may change on transit.

If the question asked was “will always change on transit” then A and B would have been the answer because TTL will always change which in turn will change Checksum value but Fragmentation and Record Route might not always happen in the channel. 

Comments

@Sunnidhya Roy

Got it , Thanks :)

“will always change on transit” then A and B would have been the answer because TTL will always change which in turn will change Checksum value but Fragmentation and Record Route might not always happen in the channel. 

Here , if MTU is bigger than the data to be transmitted, then fragment offset will not change right?

and what about record route? when there is only one route in that case it will not change? 

is this correct reasoning for both of these cases?

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@Pranavpurkar Yes the reasoning for Fragmentation is absolutely correct, for the Record Route Part there can be another case when the configuration only does not support/ implement Record Route as it is an optional requirement.

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got it ,  Thanks @Sunnidhya Roy.