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OK. Let's try this again. Is this acceptable?
2.21. Error Handling
There are many kinds of errors that can occur during IKE processing. If a request is received that is badly formatted, or unacceptable for
reasons of policy (e.g., no matching cryptographic algorithms), the response MUST contain a Notify payload indicating the error. If an error occurs in the processing of a response, then the initiator SHOULD initiate an INFORMATIONAL exchange with a Notify payload describing the problem. If an error occurs outside the context of an
IKE request (e.g., the node is getting ESP messages on a nonexistent SPI), the node SHOULD initiate an INFORMATIONAL exchange with a Notify payload describing the problem.
Errors that occur before a cryptographically protected IKE SA is established must be handled very carefully. There is a trade-off between wanting to be helpful in diagnosing a problem and responding to it and wanting to avoid being a dupe in a denial of service attack
based on forged messages.
If a node receives a message on UDP port 500 or 4500 outside the context of an IKE SA known to it (and not a request to start one), it
may be the result of a recent crash of the node. If the message is marked as a response, the node MAY audit the suspicious event but MUST NOT respond. If the message is marked as a request, the node MAY audit the suspicious event and MAY send a response. If a response is sent, the response MUST be sent to the IP address and port from whence it came with the same IKE SPIs and the Message ID copied. The response MUST NOT be cryptographically protected and MUST contain a Notify payload indicating INVALID_IKE_SPI. The INVALID_IKE_SPI notification indicates an IKE message was received with an unrecognized destination SPI; this usually indicates that the
recipient has rebooted and forgotten the existence of an IKE SA.
A node receiving such an unprotected Notify payload MUST NOT respond and MUST NOT change the state of any existing SAs. The message might
be a forgery or might be a response, the genuine correspondent was tricked into sending. A node should treat such a message (and also a
network message like ICMP destination unreachable) as a hint that there might be problems with SAs to that IP address and should initiate a liveness check for any such IKE SA. An implementation SHOULD limit the frequency of such tests to avoid being tricked into participating in a denial of service attack.
A node receiving a suspicious message from an IP address (and port, if NAT traversal is used) with which it has an IKE SA SHOULD send an IKE Notify payload in an IKE INFORMATIONAL exchange over that SA. The recipient MUST NOT change the state of any SAs as a result, but may wish to audit the event to aid in diagnosing malfunctions. A node MUST limit the rate at which it will send messages in response to unprotected messages.
All errors that occur in an IKE_AUTH exchange, causing the authentication to fail for whatever reason (invalid shared secret, invalid ID, untrusted certificate issuer, revoked or expired certificate, etc.) SHOULD result in an AUTHENTICATION_FAILED notification. If the error occurred on the responder, the notification is returned in the protected response, and should be the
only payload in that response. If the error occurs on the initiator,
the notification MAY be returned in a separate INFORMATIONAL exchange, usually with no other payloads. Note, however, that messages that contain an unsupported critical payload, or where the whole message is malformed (rather than just bad payload contents), MUST be rejected in their entirety, and only lead to an UNSUPPORTED_CRITICAL_PAYLOAD or INVALID_SYNTAX Notification. The receiver should not verify the payloads related to authentication in this case.
If authentication has succeeded in the IKE_AUTH exchange, the IKE SA is established, but establishing the child SA, or requesting configuration information may still fail. This failure does not automatically cause the IKE SA to be deleted. Specifically, a responder may include all the payloads associated with authentication
(IDr, Cert and AUTH) while sending error notifications for the piggybacked exchanges (FAILED_CP_REQUIRED, INVALID_SELECTORS, NO_PROPOSAL_CHOSEN, etc.), and the initiator MUST NOT fail the authentication because of this. The initiator MAY, of course, for reasons of policy later delete such an IKE SA.
Only authentication failures and malformed messages lead to a deletion of the IKE SA without requiring an explicit INFORMATIONAL exchange carrying a DELETE payload. Other error conditions require such an exchange, if policy dictates that this is needed.
In an IKE_SA_INIT exchange, any error notification causes the exchange to fail, although some, like COOKIE, INVALID_KE_PAYLOAD or INVALID_MAJOR_VERSION may lead to a subsequent successful exchange. In an IKE_AUTH exchange, or in the INFORMATIONAL exchnage immediately
following it, only the following notifications cause the IKE SA to be
deleted or not created, without a DELETE payload:
Extension documents may define new error notifications with these semantics, but MUST NOT use them unless the peer is known to understand them.