IETF RFC 3181 PDF

Scope and Applicability

The Framework document for policy-based admission control [RAP] describes the various components that participate in policy decision making (i.e., PDP, PEP and LDP). The emphasis of PREEMPTION_PRI elements is to be simple, stateless, and light-weight such that they could be implemented internally within a node's LDP (Local Decision Point).

Certain base assumptions are made in the usage model for PREEMPTION_PRI elements:

- They are created by PDPs

In a model where PDPs control PEPs at the periphery of the policy domain (e.g., in border routers), PDPs reduce sets of relevant policy rules into a single priority criterion. This priority as expressed in the PREEMPTION_PRI element can then be communicated to downstream PEPs of the same policy domain, which have LDPs but no controlling PDP.

- They can be processed by LDPs

PREEMPTION_PRI elements are processed by LDPs of nodes that do not have a controlling PDP. LDPs may interpret these objects, forward them as is, or perform local merging to forward an equivalent merged PREEMPTION_PRI policy element. LDPs must follow the merging strategy that was encoded by PDPs in the PREEMPTION_PRI objects. (Clearly, a PDP, being a superset of LDP, may act as an LDP as well).

- They are enforced by PEPs

PREEMPTION_PRI elements interact with a node's traffic control module (and capacity admission control) to enforce priorities, and preempt previously admitted flows when the need arises.