3.6.2.1 SIP Proxy Operation

If the SIP proxy uses a Hash-based Message Authentication Code (HMAC) algorithm, as described in [FIPS198a], to protect the integrity of the Record-Route, Contact, or Via headers and it periodically changes the key used in the HMAC computation, as recommended by [FIPS198a], or if it uses a similar algorithm that depends on periodically updated keys, the proxy MUST start a timer per key when the key is last used to compute the HMAC before it gets changed and it MUST retain the key until the timer fires. The timer SHOULD fire no earlier than 1 hour after it is started for keys used to protect information in Via and Record-Route header fields that are copied from the request to the response. The timer SHOULD fire no earlier than 8 hours for keys used to protect information in Contact and Record-Route header field URIs that is preserved in the dialog route set and used to populate Route header fields in mid-dialog requests.