to the same IPsec processing. That is to say, that all fragments are ESP,
or AH, or AH+ESP, or none.
The reassembly mechanism can be used both on the wire and inside an IPsec
tunnel, so we need to make sure all fragments of a packet were received
on only one side.
Even though I haven't tried, I believe there are configurations where it
would be possible for an attacker to inject an unencrypted fragment into a
legitimate stream of already-decrypted-and-authenticated fragments.
Typically on IPsec gateways with ESP tunnels, where we can encapsulate
fragments (as opposed to the general case, where we fragment encapsulated
data).
Note, for the record: a funnier thing, under IPv4, would be to send a
zero-sized !MFF fragment at the head of the packet, and manage to trigger
an ICMP error; M_DECRYPTED gets lost by the reassembly, and ICMP will reply
with the packet in clear (not encrypted).
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