So I'm reading about IPv4 and one of the questions are about how IPv4 knows that a fragment has arrived.

Is it a certain header that is set?

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There are three fields in the IPv4 header that are relavent to fragment detection and reassembly.

  • The "fragment offset", a number where a fragment fits into the complete packet.
  • The "more fragments" flag, a single bit flag that specifies whether there are more fragments after this one.
  • The "ID" field, this identifies which packet a fragment belongs to.

If "fragment offset" and "more fragments" are both zero then it is a complete un-fragmented packet. Otherwise we are dealing with a fragment.

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