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When Device A sends an IP datagram toward Device B, the IP Layer of Device A computes if it must or not fragment the IP datagram to fit the link MTU. Sent to the next hop, the process will be repeated.

My question is, must every intermediary device on network wait for all the IP datagram fragments to arrive and reassemble them in the correct order and re-fragment if necessary?

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As the diagram shows, fragmentation happens along the path, as needed. It is up to the end-device to reassemble any fragments.

Notice in the drawing how the colored boxes (representing IP packets) are fragmented by the first hop router, and they remain fragmented upon leaving the second hop router. Device B is responsible to reassemble the fragment into the IP packets before passing the reassembled packets to the upper-layer protocols.

Note:

This is only for IPv4. IPv6 requires the sending host to pre-fragment any IPv6 packets before sending them since the intermediate devices will not fragment the IPv6 packets.

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  • Thanks Ron, I thought that intermediary devices would reassemble the fragments , but maybe it would result in a big processing and delay
    – Shinomoto Asakura
    Sep 28, 2015 at 2:16
  • The key is that the packet fragments need to be reassembled before passing to upper-layer protocols. A router doesn't need to do that unless it is the end-device because it is operating at layer-3, the IP layer.
    – Ron Maupin ♦
    Sep 28, 2015 at 2:21
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    You can find detailed information about fragmentation below. The best practice is not to exceed the known reassembly limit of the remote host for exactly the reason that Ron has explained. [1]: tools.ietf.org/html/rfc791 [2]: tools.ietf.org/html/rfc815
    – Daniel
    Sep 28, 2015 at 2:54
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My question is, must every intermediary device on network wait for all the IP datagram fragments to arrive and reassemble them in the correct order and re-fragment if necessary?

The original intent was that reassembly would be performed only on the final end host.

However some intermediate devices will reassemble fragmented packets because it makes firewalling and NAT much simpler.

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