Question 35

1. (i) Time To Live (TTL):
   Every router that forwards an IP datagram decrements its TTL field by at least 1 (to prevent routing loops). Because datagram D travels from network A to network B across one or more routers, its TTL will be reduced. Thus, TTL changes.

2. (ii) Header Checksum:
   The IPv4 header checksum covers only the IPv4 header fields. Because the TTL field changes at every intermediate router (and potentially options/length fields if fragmented), the router must recompute and update the header checksum. Thus, Checksum changes.

3. (iii) Fragment Offset:
   If any intermediate network between host A and host B has a Maximum Transmission Unit (MTU) smaller than the datagram size, the router will fragment the datagram into multiple smaller fragments. In fragmented packets, the Fragment Offset field is set to indicate the relative offset of the fragment payload. Thus, when D reaches B (as one of the fragments), the Fragment Offset field may differ from that of the original datagram D.

Hence, all three fields (i), (ii), and (iii) may be different from the original datagram (option D).