Question

IP Security and IPSec

Suppose you have an IPv4 packet with 825964 bytes length. Assume that this packet is to be sent through a network having 1500 bytes MTU.

Question:

(a) How many minimum fragments must be created from the IP packet?

(b) What would be the value of “flag” and “offset” fields of the first fragment? Justify your answer.

(c) What would be the value of “flag”, “offset”, and “length” fields of the last fragment? Justify your answer.

(d) What is a security issue with IPv4 fragmentation? Explain in one paragraph.

Answer 1

(a) the minimum fragment should be 559 as each of the fragment will send a packet of size of 1480Bytes. and total we have 825964Bytes to send. Also MTU 1500Bytes will have 20 Bytes as the header size.

(b) See when we are sending the first fragment its offset will be 0 as before this we havent send any packet. And the value of MF Flag will be 1 because this is not the last fragment we have to send more fragment. so flag will be 1.

(c) In the last fragment As the fragment is last that means there will not be any fragment after this one so. the MF flag will be 0 means there will not be any more fragment. The offset will be the no. of bytes till now sent before the last fragment. i.e 825940

Also the length of the data will be 24Bytes + 20 Bytes of header size AS only this much data left after second last data movement. so total length of last fragment will be 44 bytes.

(d) We will have issue with the Security of IPV4 fragmentation. the sender normally fragment the large packet into multiple packet and at the receiver end those fragment packets got reassembled. But there is danger in that when the fragment packet is send in that case there might be fragmentation attacks. means the receiver resources will be used by the attacker and it will be bombarded with large no. of fragments by attackers. due to which i may cause to creash the server and resources of the receiver system.