GATE CSE 2024 | Set 2 | Question: 22

Question

​​​​Which of the following fields of an $\text{IP}$ header is/are always modified by any router before it forwards the $\text{IP}$ packet?

• A. Source $\text{IP}$ Address
• B. Protocol
• C. Time to Live $\text{(TTL)}$
• D. Header Checksum

Comments

On each Hop we wil decrement TTL(ALWAYS) because of TTL change in TTL checksum got updated as well

Rest will remain the same

 

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Tanenbaum

“A router does not modify the source and destination IP addresses; it only forwards the packet toward the next hop.”

Kurose & Ross

“Routers do not alter IP addresses; they only decrement TTL and recompute the checksum.”

Only NAT devices change IP addresses, and that is a special case, not a generic router.
 

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==> At input ports verify the IP header checksum... Output ports perform fragmentation update header fields including TTL and checksum, and transmit bits onto the link

==> a) Router can change  --> Flag ,total length , DF ,MF , Fragment offset , TTL , Header Checksum .. ..b) Router can't happen --> Identification no , Source ip , Destination ip , version , IHL ..

PS : I am not sure about "ToS" ,is it changing or not ...if anyone know plz share with some references ..

Answer 1

A) Source IP address remains unchanged as it identifies the original sender and is used for sending acknowledgement by destination or an ICMP message by router/destination.

B) Protocol indicates the type of data carried (e.g., TCP, UDP) and isn't altered by routers.

C) Time to Live(TTL) value is decremented by 1 by each router that forwards the packet.

D) As TTL value of IP header is changed by every router, corresponding Header Checksum is recalculated based on the new value of TTL.

So, correct options are $C,D.$

Comments

ALWAYS is written dear. @Manish Kumar 24

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i think in this question they have mentioned "always" keyword, but if i forward that packet in the same network then sip will remain unchanged

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yeah you are correct but they have mentioned the term ALWAYS, our private IP
let us assume is given by our home WIFI for all the other routers in the internet they will forward to the packet to our home WIFI, because the public IP of it will be fixed.

I have also realized GATE is the game of words also.

Answer 2

 

• Source Address: This remains unchanged, as it identifies the original sender of the packet. However, if Network Address Translation (NAT) is used, the source address may be modified, but that is not something that occurs with every packet.

• Protocol: This field specifies the type of data being carried (e.g., TCP, UDP). Routers do not change this value.

• Time to Live (TTL): Every router that forwards the packet decrements the TTL value by 1. This helps prevent packets from looping indefinitely in a network.

• Header Checksum: Since TTL changes at each hop, the router recalculates the header checksum accordingly.

Thus, the fields Time to Live (TTL) and Header Checksum are always modified by any router before forwarding an IP packet. Hope this helps!

C & D Answer

Answer 3

TTL and Header Checksum definitely changed

Protocol and SIP Doesn't changed

Option C, D answer

Answer 4

Answer 5

Option C,D are correct

A:Source IP Address: it ensure source to destination delivery so it can not be changed by router.

B:Protocol:  it is also not altered .

C:Time to Live (TTL): TTL is decremented at every router so it is changing at every router.

D:Header Checksum : Router after decrement recompute the checksum so it get also altered.