RSVP-mediated QoS requests allow particular user(s) to obtain preferential access to network resources. To prevent abuse, some form of back pressure will generally be required on users who make reservations. For example, such back pressure may be accomplished by administrative access policies, or it may depend upon some form of user feedback such as real or virtual billing for the "cost" of a reservation. In any case, reliable user identification and selective admission will generally be needed when a reservation is requested.
The term "policy control" is used for the mechanisms required to support access policies and back pressure for RSVP reservations. When a new reservation is requested, each node must answer two questions: "Are enough resources available to meet this request?" and "Is this user allowed to make this reservation?" These two decisions are termed the "admission control" decision and the "policy control" decision, respectively, and both must be favorable in order for RSVP to make a reservation. Different administrative domains in the Internet may have different reservation policies.
The input to policy control is referred to as "policy data", which RSVP carries in POLICY_DATA objects. Policy data may include credentials identifying users or user classes, account numbers, limits, quotas, etc. Like flowspecs, policy data is opaque to RSVP, which simply passes it to policy control when required. Similarly, merging of policy data must be done by the policy control mechanism rather than by RSVP itself. Note that the merge points for policy data are likely to be at the boundaries of administrative domains. It may therefore be necessary to carry accumulated and unmerged policy data upstream through multiple nodes before reaching one of these merge points.
Carrying user-provided policy data in Resv messages presents a potential scaling problem. When a multicast group has a large number of receivers, it will be impossible or undesirable to carry all receivers' policy data upstream. The policy data will have to be administratively merged at places near the receivers, to avoid excessive policy data. Further discussion of these issues and an example of a policy control scheme will be found in [PolArch96]. Specifications for the format of policy data objects and RSVP processing rules for them are under development.