This question is from Computer Network Security, (Applied Cryptography) In IPsec

Question

This question is from Computer Network Security, (Applied Cryptography)

In IPsec authentication, suppose that the current replay window spans from 120 to 530. a. If the next incoming authenticated packet has sequence number 105, what will the receiver do with the packet, and what will be the parameters of the window after that? If instead the next incoming authenticated packet has sequence number 440, what will the receiver do with the packet, and what will be the parameters of the window after that? If instead the next incoming authenticated packet has sequence number 540, what will the receiver do with the packet, and what will be the parameters of the window after that?

Explanation / Answer

Answer:

a)

If the tunnel mode is being used, then the new router IP header is constructed. In both the versions of the IP headers, the relationship between the each outer IP header and extended header at the outer packet to the corresponding field are defined.

Thus, when the authenticated packet is received with sequence number 105, the receiver will discard the packet that is the received packed will be left out of window.

So, no changes are made at window parameters.

b)

When the authenticated packet is received with sequence number 440 at receiver side, the packet is accepted. Since, the sequence number falls within the window range.

If is for the first time, it checks the MAC. If it is authenticated, the corresponding slot of the window is marked.

If it not a new, the packet is discarded.

There will be no change in the window parameters.

c)

When the authenticated packet is received with sequence number 540 at receiver side, it is checked. The sequence number is to the right of the window.

Since it is new, the MAC of the authenticated packet is checked.

If it is authenticated, the window will be advanced so the sequence number is at right edge of the window.

The respective slot of the window is marked.

So, now the window span is changed from 120 to 540. Thus, the window parameters are changed.

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