3.6.5.1 SIP Proxy Operation

If the SIP proxy uses an HMAC algorithm, as specified in [FIPS198a], to protect the integrity of the Record-Route or Contact header fields, and it periodically changes the key used in the HMAC computation, as recommended by the [FIPS198a], or if it uses a similar algorithm that depends on periodically updated keys, and it receives a SIP request that contains the HMAC that the SIP proxy previously inserted, and the SIP proxy no longer has the key to compute the HMAC, the SIP proxy SHOULD reject the request with a 481 Call Leg Does Not Exist response.<23> However, if the SIP proxy implements the extensions for dialog state recovery, as described in section 3.7, it SHOULD follow the procedure defined there to send a 430 Flow Failed or a 481 Call Leg Does Not Exist response.<24>