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First, let me say this is a real question, and not just a joke.
NULL authentication algorithm and NULL encryption algorithm are mandatory-to-implement in ESP. RFC 2406 and revised version (RFC 4303) states in section 3.2:
Note that although both confidentiality and integrity are optional, at least one of these services MUST be selected, hence both algorithms MUST NOT be simultaneously NULL.
I obviously understand that this configuration makes ESP useless in term of protection and just consumes bandwidth. Is there any other reason behind the "MUST NOT" statement? I tried to dig but did not find any.
Now, please consider the following architecture (it's not just hypothetical). In a domain with dynamic keying mandatory for IPsec SA negotiation between all nodes and filtering dropping everything except 500/udp and esp (*), i have some specific needs (security at upper layer, performance, ...) to _explicitly_ allow few unprotected flows between some nodes or sets of nodes, based on a policy and credentials (X.509 certs).
For that purpose, _negotiated_ NULL/NULL is the right solution, as the tiny mtu loss (say 12 bytes in worst case) spares me the burden of static configuration in the core and on the nodes.
At the moment, even if some implementations allow it, the standard prevents it. I'll be interested in comments on that.