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Why the IPv4 need Don't fragment Flag?

We know the IPv4 Flags options are: Reserved bit, Don't fragment, More fragments, fragment offset options.

Why there need the Don't fragment Flag? if it is small than MTU of transmission nodes what will happen?

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  • Many routers are configured to drop fragments because they can be used in an attack. Modern networking doesn't use fragmentation, and it was eliminated for intermediate devices in IPv6 because the IETF improved IP in IPv6 by eliminating things that seemed like a good idea in IPv4, but have proved problematic or are misused. Things like broadcast and fragmentation.
    – Ron Maupin ♦
    Aug 22, 2019 at 13:25
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Dec 15, 2019 at 18:39

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If frame is bigger than MTU and have don't fragment bit set then it will drop the packet. Discarding router will send back to sender ICMP message Fragmentation Needed (Type 3, Code 4) which contains MTU size and then sender should send this packet again adjusted to MTU size which he received in ICMP message.

IPv4 don't need fragment bit but it can be set in IP stack. Fragmentation causes extra CPU overhead which can affect total throughput so most of the times this DF bit is set.

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  • 4
    Since most modern IP stacks perform PMTUd (Path MTU Discovery) by default, the DF bit ist most often set, not only "sometimes".
    – Marc 'netztier' Luethi
    Aug 22, 2019 at 11:19
  • Ok sorry fixed :)
    – kubn2
    Aug 22, 2019 at 11:27

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