client-server architecture

6.3 IP header fileds updated by a router-fragmentation/non-fragmentation

Suppose a router receives an IPv4 datagram on one interface, and forwards it on another interface.

Which IPv4 header fields will the router modify in the forwarded datagram, if fragmentation is not required? Select all that apply

• TTL
• Header Checksum

ID(identification): Each value of the ID header field is 17663 (0x44ff). The ID header field is the same for all fragments of a datagram sent by sender to allow for reassembly. X modified

Total Length: Total length is modified to the size of the fragment.

Flags: "more fragments" bit will be modified , meaning there is no need for further fragmentation. “Don’t Frag” bit will be modified.

Fragment Offset: Each offset will be mofidied as 0, 1480, 2960 bytes. ( 0, 185, 370 in 8-byte units). The offset means how much bytes of data in the datagram we have received from previous packet at the position of current fragment. So the second fragment we receive 1480 bytes of data and the third we receive 2960 bytes of data from previous packet.

TTL: (both frag/non-frag) & Header Checksum: (both frag/non-frag) will be modified Since the router changes the IPv4 header (it decrements the TTL), it needs to calculate a new value for the checksum, otherwise subsequent devices receiving the packet will think it is damaged.