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RFC 6864, "Updated Specification of the IPv4 ID field," in section 4 defines "atomic datagrams" as "datagrams not yet fragmented and for which further fragmentation has been inhibited." That is, further fragmentation has been inhibited by setting the Don't Fragment bit. So, they haven't been fragmented and they won't be fragmented. In section 4.1 it states "In atomic datagrams, the IPv4 ID field has no meaning; thus, it can be set to an arbitrary value...."

So, there are some IPv4 hosts that continue to increment the IP ID field in atomic datagrams, regardless of the fact that the field is meaningless in those packets, and, as you see, there are other hosts that just put zero in the IP ID field. They can put any value, but if they aren't going to increment to the next IP ID value, they generally put zero.

All IPv4 hosts are required to handle IP datagrams of at least 576 bytes, so IP datagrams that are 576 bytes or less also will not be fragmented, regardless of whether the Don't Fragment bit is set or not, so some hosts will also put zero in the IP ID field if the packet is 576 bytes or less, although that case is not discussed in RFC 6864.

Technically, an atomic datagram is defined as: The Don't Fragment bit is set, the More Fragments bit is not set, and the Fragment Offset is 0.

RFC 6864, "Updated Specification of the IPv4 ID field," in section 4 defines "atomic datagrams" as "datagrams not yet fragmented and for which further fragmentation has been inhibited." That is, further fragmentation has been inhibited by setting the Don't Fragment bit. So, they haven't been fragmented and they won't be fragmented. In section 4.1 it states "In atomic datagrams, the IPv4 ID field has no meaning; thus, it can be set to an arbitrary value...."

So, there are some IPv4 hosts that continue to increment the IP ID field in atomic datagrams, regardless of the fact that the field is meaningless in those packets, and, as you see, there are other hosts that just put zero in the IP ID field. They can put any value, but if they aren't going to increment to the next IP ID value, they generally put zero.

All IPv4 hosts are required to handle IP datagrams of at least 576 bytes, so IP datagrams that are 576 bytes or less also will not be fragmented, regardless of whether the Don't Fragment bit is set or not, so some hosts will also put zero in the IP ID field if the packet is 576 bytes or less, although that case is not discussed in RFC 6864.

Technically, an atomic datagram is defined in RFC 6864 as: The Don't Fragment bit is set, the More Fragments bit is not set, and the Fragment Offset is 0.