Consider sending a 5000byte IP datagram into a link that has

Consider sending a 5000-byte IP datagram into a link that has an MTU of 900 bytes. Suppose the original datagram is stamped with the identification number 325. Assume an IP header size of 20 byte, how many fragments are generated? What are the values in the various fields in the IP datagram(s) generated related to fragmentation, i.e., ID, fragmentation flag, and offset?

Solution

Given that original IP datagram size is 5000 bytes with the IP header size of 20 bytes.

Maximum transfer (MTU) given as 900 bytes

packet identification number(ID)-325

The original datagram was 5000 bytes, subtracting 20 bytes for header, gives 4980 bytes of data.
With an MTU of 900 bytes, 900 - 20 = 880 bytes of data may be transmitted in each packet
Therefore, ceiling(4980 / 880) = 6 packets are needed to carry the data.

fragement offset calculated as (880/8)=110

MF=1 measn we have to fragement the data or we have more fragments .
Packet 1: ID=325, Total_len=900, MF=1, Frag_offset=0
Packet 2: ID=325, Total_len=900, MF=1, Frag_offset=110
Packet 3: ID=325, Total_len=900, MF=1, Frag_offset=220
Packet 4: ID=325, Total_len=900, MF=1, Frag_offset=330
Packet 5: ID=325, Total_len=900, MF=1, Frag_offset=440
Packet 6: ID=325, Total_len=500, MF=0, Frag_offset=550