Question

What should be the m flag value to indicate the last fragment?

Answer 1

Suppose I have two hosts, host A and host B, and host A sends a 1500 byte packet to B and there are two routers between them with a MTU of 800 bytes for the first link, and MTU of 500 bytes for the second link between them.

As I understand it the packet will have to be fragmented both times. The packet will have to be fragmented into 3 packets (2 equally sized packets and one smaller one).

Then when we hit this second router we will have each of the first two fragmented packets be fragmented once more. The first two original fragments will produce 1 larger fragment (close to 500 bytes), and one smaller one.

This is where I am confused.

I know that the 'More Fragment' bit should be set to 1 on all of the first 3 fragments except the last indicating that the packets are part of a fragment up to and including the next packet with 'More Fragment' set to 0. However, for the second set of fragments I am not so sure. If this was the first time the packet had been fragmented, the last fragment would have 'More Fragment' of 0, but since this is actually just a fragment somewhere in the middle of the original message, I have a feeling it should be 1.

I am hoping someone with more experience can shed some light on this for me. Is the 'More Fragment' bit used in reassembly? If so I would imagine that when the fragments are fragmented then we do not set the last fragment to 0.

So the algorithm would go:

If fragmenting a packet with 'More Fragment' set to 0, then set 'More Fragment' to 1 in all fragmented packets except the last. Else If fragmenting a packet with 'More Fragment' set to 1, then set 'More Fragment' to 1 in all of the fragmented packets.
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