RFC2753 - A Framework for Policy-based Admission Control

4.4. Placement of Policy Elements in a Network

   By allowing division of labor between an LPDP and a PDP, the policy
   control architecture allows staged deployment by enabling routers of
   varying degrees of sophistication, as far as policy control is
   concerned, to communicate with policy servers. Figure 4 depicts an
   example set of nodes belonging to three different administrative
   domains (AD) (Each AD could correspond to a different service

   provider in this case).  Nodes A, B and C belong to administrative
   domain AD-1, advised by PDP PS-1, while D and E belong to AD-2 and
   AD-3, respectively. E communicates with PDP PS-2.  In general, it is
   expected that there will be at least one PDP per administrative
   domain.

   Policy capable network nodes could range from very unsophisticated,
   such as E, which have no LPDP, and thus have to rely on an external
   PDP for every policy processing operation, to self-sufficient, such
   as D, which essentially encompasses both an LPDP and a PDP locally,
   at the router.

                        AD-1                    AD-2         AD-3
      ________________/\_______________     __/\___      __/\___
     {                                 }   {       }    {       }
             A           B            C            D            E
        +-------+  +-----+    +-------+    +-------+    +-------+

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