RFC 2751 (rfc2751) - Page 3 of 12

Signaled Preemption Priority Policy Element

RFC 2751      Signaled Preemption Priority Policy Element   January 2000


2  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.

3  Stateless Policy

   Signaled Preemption Priority is stateless (does not require past
   history or external information to be interpreted). Therefore, when
   carried in COPS messages for the outsourcing of policy decisions,
   these objects are included as COPS Stateless Policy Data Decision
   objects (see [COSP, COPS-RSVP]).







Herzog                      Standards Track