3.6. Rate Control [RFC6446] extends the SIP events framework by defining the following three Event header field parameters that allow a subscriber to set a minimum, a maximum, and an average rate adaptive minimum of event notifications generated by the notifier. This allows a subscriber to have overall control over the stream of notifications, for example to avoid being flooded. Two of the parameters, namely "min-interval" "min-rate" (which specifies a minimum notification time period between two notifications, in seconds) rate per second) and "max-interval" "max-rate" (which specifies a maximum notification time period between two notifications, in seconds) rate per second) are used by this document. Only the implementation of these two attributes is required from the attributes defined in [RFC6446]. Whenever the time since the most recent notification exceeds the value in corresponding to the "max-interval" "max-rate" parameter, the current state would be sent in its entirety, just like after a subscription refresh. A notifier is required to send a NOTIFY request immediately after creation of a subscription. If state is not available at that time, then the NOTIFY request may be sent with no content. A separate NOTIFY containing location is subsequently generated some time between the time included in "min-interval" "min-rate" and the time in "max- interval". "max-rate". An important use case for location-based applications focuses on the behavior of the initial NOTIFY message(s) and the information it returns, for example in case of emergency call routing. When an initial NOTIFY is transmitted, it might not include complete state. Subscriber Notifier |---SUBSCRIBE(1)--->| Create subscription (w/small (w/large value |<-------200--------| for min-interval min-rate and max-interval) max-rate) |<-----NOTIFY(2)----| Return initial notify with no state |--------200------->| |<-----NOTIFY(3)----| Return full state (between min-interval min-rate |--------200------->| and max-interval) max-rate) |---SUBSCRIBE(4)--->| Update subscription (to update |<-------200--------| min-interval min-rate and max-interval) max-rate) Figure 9: SUBSCRIBE/NOTIFY with Rate Control Figure 9 shows a SUBSCRIBE/NOTIFY exchange. The initial SUBSCRIBE message (1) has filters attached and contains a "max-interval" "max-rate" rate control parameter. In certain situations, it is important to obtain some amount of location information within a relatively short and pre-defined period of time, even if the obtained location information contains a high amount of uncertainty and location information with less uncertainty at a later point in time. An example is emergency call routing where an emergency services routing proxy may need to obtain location information suitable for routing rather quickly and subsequently a Public Safety Answering Point requests location information for dispatch. To obtain location information in a timely fashion using the SUBSCRIBE/NOTIFY mechanism, it is RECOMMENDED that the initial SUBSCRIBE contain a "max-interval" "max-rate" rate control parameter (with a small large value) that is in a later message updated to a more sensible value. This provides equivalent functionality to the 'responseTime' attribute in Section 6.1 of [RFC5985]. The "max-interval" "max-rate" for this first request is therefore much lower larger than thereafter. Updating the "max-interval" "max- rate" for the subscription can be performed in the 200 response (see message 3) to the NOTIFY that contains state. Depending on the value in the "max-interval" "max-rate" parameter, the Notifier may create a NOTIFY message (see message 2) immediately in response to the SUBSCRIBE that might be empty in case no location information is available at this point in time. The desired location information may then arrive in the subsequent NOTIFY message (see message 4).
