RSVP was originally a protocol for resource reservation. In the context of resource reservation, different reservation styles were developed to determine the degree to which resources are shared; the FF and SE reservation styles.
The FF reservation style dedicates a particular reservation to an individual sender (ingress router). This reservation style is useful for concurrent and independent traffic from different senders. When used with Multiprotocol Label Switching (MPLS), the FF reservation style allows the establishment of multiple parallel unicast point-to-point LSPs to support load balancing. It can also be used with primary and secondary paths to achieve minimal disruption to traffic. Examples of applications that use FF-style reservations are video applications and unicast applications, which both require flows that have a separate reservation for each sender.
The SE reservation style allows an explicit list of senders to share the largest bandwidth request across shared links. In an MPLS environment, this style is important for rerouting LSPs with no disruption to the flow of subscriber traffic. An example application for shared explicit reservations is an audio application in which each sender transmits a distinct data stream. Typically, only a few senders are transmitting at any one time. Such a flow does not require a separate reservation for each sender; a single reservation is sufficient.
In RSVP with traffic engineering, the ingress router can request the SE style by setting the appropriate bit in the Session Attribute object. If the Session Attribute object is present but the particular bit is not present, the egress router can use either style (FF or SE). All values in the Session Attribute object are advisory, so an egress router can ignore the bits when it selects a style; however, to date, this behavior has not been implemented. Selection of a style can be determined by non-support of a particular style, an explicit policy, or available resources.
In the context of traffic engineering, FF is the default reservation style. The SE style allows an LSP to share reservations which is useful when the ingress router is trying to set up an alternate path before tearing down the existing path. Clearly traffic is sent on the active path only, but from the point of view of reservations, sharing resources avoids double counting the resources.