Some QoS services may require traffic policing at some or all of (1) the edge of the network, (2) a merging point for data from multiple senders, and/or (3) a branch point where traffic flow from upstream may be greater than the downstream reservation being requested. RSVP knows where such points occur and must so indicate to the traffic control mechanism. On the other hand, RSVP does not interpret the service embodied in the flowspec and therefore does not know whether policing will actually be applied in any particular case.
The RSVP process passes to traffic control a separate policing flag for each of these three situations.
This flag is set in the first-hop RSVP node that implements traffic control (and is therefore capable of policing).
For example, sender hosts must implement RSVP but currently many of them do not implement traffic control. In this case, the E_Police_Flag should be off in the sender host, and it should only be set on when the first node capable of traffic control is reached. This is controlled by the E_Police flag in SESSION objects.
This flag should be set on for a reservation using a shared style (WF or SE) when flows from more than one sender are being merged.
This flag should be set on when the flowspec being installed is smaller than, or incomparable to, a FLOWSPEC in place on any other interface, for the same FILTER_SPEC and SESSION.
RSVP must also test for the presence of non-RSVP hops in the path and pass this information to traffic control. From this flag bit that the RSVP process supplies and from its own local knowledge, traffic control can detect the presence of a hop in the path that is not capable of QoS control, and it passes this information to the receivers in Adspecs [RFC 2210].
With normal IP forwarding, RSVP can detect a non-RSVP hop by comparing the IP TTL with which a Path message is sent to the TTL with which it is received; for this purpose, the transmission TTL is placed in the common header. However, the TTL is not always a reliable indicator of non-RSVP hops, and other means must sometimes be used. For example, if the routing protocol uses IP encapsulating tunnels, then the routing protocol must inform RSVP when non-RSVP hops are included. If no automatic mechanism will work, manual configuration will be required.