Internet Engineering Task Force               Raj Yavatkar, Intel
  INTERNET-DRAFT                                Dimitrios Pendarakis, IBM
                                                Roch Guerin, U. Of Pennsylvania

                                                April 1999
                                                Expires: December 1999

                A Framework for Policy-based Admission Control

                             Status of this Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  This document is an Internet Draft. Internet Drafts are working
  documents of the Internet Engineering Task Force (IETF), its areas,
  and its working groups. Note that other groups may also distribute
  working documents as Internet-Drafts.

  Internet Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time. It is inappropriate to use Internet Drafts as reference
  material or to cite them other than as ``work in progress.''

  The list of current Internet-Drafts can be accessed at

  The list of Internet-Draft Shadow Directories can be accessed at

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                A Framework for Policy-based Admission Control    March 1999

  1. Abstract

  The IETF working groups such as Integrated Services (called "int-serv")
  and RSVP [1] have developed extensions to the IP architecture and the
  best-effort service model so that applications or end users can request
  specific quality (or levels) of service from an internetwork in addition
  to the current IP best-effort service. Recent efforts in the
  Differentiated Services Working Group are also directed at definition of
  mechanisms that support aggregate QoS services. The int-serv model for
  these new services requires explicit signaling of the QoS (Quality of
  Service) requirements from the end points and provision of admission and
  traffic control at Integrated Services routers. The proposed standards for
  RSVP [RFC 2205] and Integrated Services [RFC 2211, RFC 2212] are examples
  of a new reservation setup protocol and new service definitions
  respectively.  Under the int-serv model, certain data flows receive
  preferential treatment over other flows; the admission control component
  only takes into account the requester's  resource reservation request and
  available capacity to determine whether or not to accept a QoS request.
  However, the int-serv mechanisms do not include  an important aspect of
  admission control: network managers and service providers must be able to
  monitor, control, and enforce use of network resources and services based
  on policies derived from criteria such as the identity of users and
  applications, traffic/bandwidth requirements, security considerations, and
  time-of-day/week. Similarly, diff-serv mechanisms also need to take into
  account policies that take into account various criteria such as customer
  identity, ingress points, and so on.

  This document is concerned with specifying a framework for providing
  policy-based control over admission control decisions. In particular, it
  focuses on policy-based control over admission control using RSVP as an
  example of the QoS signaling mechanism. Even though the focus of the work
  is on RSVP-based admission control, the document outlines a framework that
  can provide policy-based admission control in other QoS contexts. We argue
  that policy-based control must be applicable to different kinds and
  qualities of services offered in the same network and our goal is to
  consider such extensions whenever possible.

  We begin with a list of definitions in Section 2. Section 3 lists the
  requirements and goals of the mechanisms capable of controlling and
  enforcing access to better QoS.  We then outline the architectural
  elements of the framework in Section 4 and describe the functionality
  assumed for each component.  Section 5 discusses example policies,
  possible scenarios, and policy support needed for those scenarios. Section
  6 specifies the requirements for a client-server protocol for
  communication between a policy server (PDP) and its client (PEP) and

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                A Framework for Policy-based Admission Control    March 1999

  evaluates suitability of some of the existing protocols for this purpose.

  2. Terminology

  The following is a list of terms used in this document.

    -    Administrative Domain: A collection of networks under the same
         administrative control and grouped together for administrative

    -    Network Element or Node: Routers, switches, hubs are examples of
         network nodes. They are the entities where resource allocation
         decisions have to be made and the decisions have to be enforced. A
         RSVP router which allocates part of a link capacity (or buffers) to
         a particular flow and ensures that only the admitted flows have
         access to their reserved resources is an example of a network
         element of interest in our context.

         In this document, sometimes we use the terms router,  network
         element, and network node interchangeably, but should be
         interpreted as reference to a network element.

    -    QoS Signaling Protocol: A signaling protocol that carries an
         admission control request for a bandwidth resource, e.g., RSVP.

    -    Policy: The combination of rules and services where rules define
         the criteria for resource access and usage.

    -    Policy control: The application of rules to determine whether or
         not access to a particular resource should be granted.

    -    Policy Object:  Contains policy-related info such as policy
         elements and is carried in a request or response related to
         resource allocation decision.

    -    Policy Element: Subdivision of policy objects; contains single
         units of information necessary for the evaluation of policy rules.
         A single policy element carries an user or application
         identification whereas another policy element may carry user
         credentials or credit card information.  Examples of policy

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                A Framework for Policy-based Admission Control    March 1999

         elements include identity of the requesting user or application,
         user/app credentials, etc. The policy elements themselves are
         expected to be independent of which QoS signaling protocol is used.

    -    Policy Decision Point (PDP): The point where policy decisions are

    -    Policy Enforcement Point (PEP): The point where the policy
         decisions are actually enforced.

    -    Policy Ignorant Node (PIN): A network element that does not
         explicitly support policy control using the mechanisms defined in
         this document.

    -    Resource: Something of value in a network infrastructure to which
         rules or policy criteria are first applied before access is
         granted. Examples of resources include the buffers in a router and
         bandwidth on an interface.

    -    Service Provider: Controls the network infrastructure  and may be
         responsible for the charging and accounting of services.

    -    Soft State Model - Soft state is a form of the stateful model that
         times out installed state at a PEP or PDP. It is an automatic way
         to erase state in the presence of communication or network element
         failures. For example, RSVP uses the soft state model for
         installing reservation state at network elements along the path of
         a data flow.

    -    Installed State: A new and unique request made from a PEP to a PDP
         that must be explicitly deleted.

    -    Trusted Node: A node that is within the boundaries of an
         administrative domain (AD) and is trusted in the sense that the
         admission control requests from such a node do not necessarily need
         a PDP decision.

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  3. Policy-based Admission Control: Goals and Requirements

  In this section, we describe the goals and requirements of mechanisms and
  protocols designed to provide policy-based control over admission control

    -    Policies vs Mechanisms: An important point to note is that the
         framework does not include any discussion of any  specific policy
         behavior or does not require use of specific policies. Instead, the
         framework only outlines the architectural elements and mechanisms
         needed to allow a wide variety of possible policies to be carried

    -    RSVP-specific: The mechanisms must be designed to meet the policy-
         based control requirements specific to the problem of bandwidth
         reservation using RSVP as the signaling protocol. However, our goal
         is to allow for the application of this framework for admission
         control involving other types of resources and QoS services (e.g.,
         Diff-Serv) as long as we do not diverge from our central goal.

    -    Support for preemption: The mechanisms designed must include
         support for preemption. By preemption, we mean an ability to remove
         a previously installed state in favor of accepting a new admission
         control request.  For example, in the case of RSVP, preemption
         involves the ability to remove one or more currently installed
         reservations to make room for a new resource reservation request.

    -    Support for many styles of policies: The mechanisms designed must
         include support for many policies and policy configurations
         including bi-lateral and multi-lateral service agreements and
         policies based on the notion of relative priority.  In general, the
         determination and configuration of viable policies are the
         responsibility of the service provider.

    -    Provision for Monitoring and Accounting Information:  The
         mechanisms must include support for monitoring policy state,
         resource usage, and provide access information. In particular,
         mechanisms must be included to provide usage and access information
         that may be used for accounting and billing purposes.

    -     Fault tolerance and recovery: The mechanisms designed on the basis

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         of this framework must include provisions for fault tolerance and
         recovery from failure cases such as failure of PDPs, disruption in
         communication including network partitions (and subsequent merging)
         that separate a PDP from its peer PEPs.

    -    Support for Policy-Ignorant Nodes (PINs):  Support for the
         mechanisms described in this document should not be mandatory for
         every node in a network. Policy based admission control could be
         enforced at a subset of nodes, for example the boundary nodes
         within an administrative domain. These policy capable nodes would
         function as trusted nodes from the point of view of the policy-
         ignorant nodes in that administrative domain.

    -    Scalability:  One of the important requirements for the mechanisms
         designed for policy control is scalability. The mechanisms must
         scale at least to the same extent that RSVP scales in terms of
         accommodating multiple flows and network nodes in the path of a
         flow. In particular, scalability must be considered when specifying
         default behavior for merging policy data objects and merging should
         not result in duplicate policy elements or objects. There are
         several sensitive areas in terms of scalability for policy control
         over RSVP. First, not every policy aware node in an infrastructure
         should be expected to contact a remote PDP. This would cause
         potentially long delays in verifying requests that must travel up
         hop by hop. Secondly, RSVP is capable of setting up resource
         reservations for multicast flows. This implies that the policy
         control model must be capable of servicing the special requirements
         of large multicast flows. Thus, the policy control architecture
         must scale at least as well as RSVP based on factors such as the
         size of RSVP messages, the time required for the network to service
         an RSVP request, local processing time required per node, and local
         memory consumed per node.

    -    Security and denial of service considerations: The policy control
         architecture must be secure as far as the following aspects are
         concerned. First, the mechanisms proposed under the framework must
         minimize theft and denial of service threats. Second, it must be
         ensured that the entities (such as PEPs and PDPs) involved in
         policy control can verify each other's identity and establish
         necessary trust before communicating.

  4. Architectural Elements

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  The two main architectural elements for policy control are the PEP (Policy
  Enforcement Point) and the PDP (Policy Decision Point). Figure 1 shows a
  simple configuration involving these two elements; PEP is a component at a
  network node and PDP is a remote entity that may reside at a policy
  server.  The PEP represents the component that always runs on the policy
  aware node. It is the point at which policy decisions are actually
  enforced. Policy decisions are made primarily at the PDP. The PDP itself
  may make use of additional mechanisms and protocols to achieve additional
  functionality such as user authentication, accounting, policy information
  storage, etc. For example, the PDP is likely to use an LDAP-based
  directory service for storage and retrieval of policy information[6]. This
  document does not include discussion of these additional mechanisms and
  protocols and how they are used.

  The basic interaction between the components begins with the PEP. The PEP
  will receive a notification or a message that requires a policy decision.
  Given such an event, the PEP then formulates a request for a policy
  decision and sends it to the PDP.  The request for policy control from a
  PEP to the PDP may contain one or more policy elements (encapsulated into
  one or more policy objects) in addition to the admission control
  information (such as a flowspec or amount of bandwidth requested) in the
  original message or event that triggered the policy decision request.  The
  PDP returns the policy decision and the PEP then enforces the policy
  decision by appropriately accepting or denying the request.  The PDP may
  also return additional information to the PEP which includes one or more
  policy elements. This information need not be associated with an admission
  control decision. Rather, it can be used to formulate an error message or
  outgoing/forwarded message.

        ________________         Policy server
       |                |        ______
       |  Network Node  |        |     |------------->
       |    _____       |        |     |   May use LDAP,SNMP,.. for accessing
       |   |     |      |        |     |  policy database, authentication,etc.
       |   | PEP |<-----|------->| PDP |------------->
       |   |_____|      |        |_____|
       |                |

  Figure 1: A simple configuration with the primary policy control
  architecture components. PDP may use additional mechanisms and protocols
  for the purpose of accounting, authentication, policy storage, etc.

  The PDP might optionally contact other external servers, e.g., for
  accessing configuration, user authentication, accounting and billing
  databases. Protocols defined for network management (SNMP) or directory
  access (LDAP) might be used for this communication. While the specific

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                A Framework for Policy-based Admission Control    March 1999

  type of access and the protocols used may vary among different
  implementations, some of these interactions will have network-wide
  implications and could impact the interoperability of different devices.

  Of particular importance is the "language" used to specify the policies
  implemented by the PDP. The number of policies applicable at a network
  node might potentially be quite large. At the same time, these policies
  will exhibit high complexity, in terms of number of fields used to arrive
  at a decision, and the wide range of decisions. Furthermore, it is likely
  that several policies could be applicable to the same request profile. For
  example, a policy may prescribe the treatment of requests from a general
  user group (e.g., employees of a company) as well as the treatment of
  requests from specific members of that group (e.g., managers of the
  company). In this example, the user profile "managers" falls within the
  specification of two policies, one general and one more specific.

  In order to handle the complexity of policy decisions and to ensure a
  coherent and consistent application of policies network-wide, the policy
  specification language should ensure unambiguous mapping of a request
  profile to a policy action. It should also permit the specification of the
  sequence in which different policy rules should be applied and/or the
  priority associated with each one. Some of these issues are addressed in

  In some cases, the simple configuration shown in Figure 1 may not be
  sufficient as it might be necessary to apply local policies (e.g.,
  policies specified in access control lists) in addition to the policies
  applied at the remote PDP. In addition, it is possible for the PDP to be
  co-located with the PEP at the same network node. Figure 2 shows the
  possible configurations.

  The configurations shown in Figures 1 and 2 illustrate the flexibility in
  division of labor. On one hand, a centralized policy server, which could
  be responsible for policy decisions on behalf of multiple network nodes in
  an administrative domain, might be implementing policies of a wide scope,
  common across the AD. On the other hand, policies which depend on
  information and conditions local to a particular router and which are more
  dynamic, might be better implemented locally, at the router.

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     ________________                        ____________________
    |                |                      |                    |
    |  Network Node  |  Policy Server       |    Network Node    |
    |    _____       |      _____           |  _____      _____  |
    |   |     |      |     |     |          | |     |    |     | |
    |   | PEP |<-----|---->| PDP |          | | PEP |<-->| PDP | |
    |   |_____|      |     |_____|          | |_____|    |_____| |
    |    ^           |                      |                    |
    |    |    _____  |                      |____________________|
    |    \-->|     | |
    |        | LPDP| |
    |        |_____| |
    |                |

  Figure 2: Two other possible configurations of policy control
  architecture components. The configuration on left shows a local decision
  point at a network node and the configuration on the left shows PEP and
  PDP co-located at the same node.

  If it is available, the PEP will first use the LPDP to reach a local
  decision. This partial decision and the original policy request are next
  sent to the PDP which  renders a final decision (possibly, overriding the
  LPDP). It must be noted that the PDP acts as the final authority for the
  decision returned to the PEP and the PEP must enforce the decision
  rendered by the PDP. Finally, if a shared state has been established for
  the request and response between the PEP and PDP, it is the responsibility
  of the PEP to notify the PDP that the original request is no longer in

  Unless otherwise specified, we will assume the configuration shown on the
  left in Figure 2 in the rest of this document.

  Under this policy control model, the PEP module at a network node must use
  the following steps to reach a policy decision:

    1.   When a local event or message invokes PEP for a policy decision,
         the PEP creates a request that includes information from the
         message (or local state) that describes the admission control
         request. In addition, the request includes appropriate policy
         elements as described below.

    2.   The PEP may consult a local configuration database to identify a
         set of policy elements (called set A) that are to be evaluated
         locally. The local configuration specifies the types of policy

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         elements that are evaluated locally. The PEP passes the request
         with the set A to the Local Decision point (LPDP) and collects the
         result of the LPDP (called "partial result" and referred to as D(A)

    3.   The PEP then passes the request with ALL the policy elements and
         D(A) to the PDP. The PDP applies policies based on all the policy
         elements and the request and reaches a decision (let us call it
         D(Q)). It then combines its result with the partial result D(A)
         using a combination operation to reach a final decision.

    4.   The PDP returns the final policy decision (one after the
         combination operation) to the PEP.

  Note that in the above model, the PEP  *must* contact the PDP even if no
  (or NULL) policy objects are received in the admission control request.
  This requirement would help ensure that a request cannot bypass policy
  control by omitting policy elements in a reservation request. However,
  ``short circuit'' processing is permitted, i.e., if the result of D(A),
  above, is ``no'', then there is no need to proceed with further policy
  processing at the policy server. Still, the PDP must be informed of the
  failure of local policy processing. The same applies to the case when
  policy processing is successful but admission control (at the resource
  management level due to unavailable capacity) fails; again the policy
  server has to be informed of the failure.

  It must also be noted that the PDP may, at any time, send an asynchronous
  notification to the PEP to change its earlier decision or to generate a
  policy error/warning message.

  4.1. Example of a RSVP Router

  In the case of a RSVP router, Figure 3 shows the interaction between a PEP
  and other int-serv components within the router.  For the purpose of this
  discussion, we represent all the components of RSVP-related processing by
  a single RSVP module, but more detailed discussion of the exact
  interaction and interfaces between RSVP and PEP will be described in a
  separate document [3].

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    |                              |
    |           Router             |
    |  ________           _____    |            _____
    | |        |         |     |   |           |     |
    | |  RSVP  |<------->| PEP |<--|---------->| PDP |
    | |________|         |_____|   |           |_____|
    |      ^                       |
    |      |      Traffic control  |
    |      |      _____________    |
    |      \---->|  _________  |   |
    |            | |capacity | |   |
    |            | | ADM CTL | |   |
    |            | |_________| |   |
  --|----------->|  ____ ____  |   |
    |   Data     | | PC | PS | |   |
    |            | |____|____| |   |
    |            |_____________|   |
    |                              |

  Figure 3: Relationship between PEP and other int-serv components
  within an RSVP router. PC -- Packet Classifier, PS -- Packet Scheduler

  When a RSVP message arrives at the router (or an RSVP related event
  requires a policy decision), the RSVP module is expected to hand off the
  request (corresponding to the event or message) to its PEP module. The PEP
  will use the PDP (and LPDP) to obtain the policy decision and communicate
  it back to the RSVP module.

  4.2. Additional functionality at the PDP

  Typically, PDP returns the final policy decision based on an admission
  control request and the associated policy elements. However, it should be
  possible for the PDP to sometimes ask the PEP (or the admission control
  module at the network element where PEP resides) to generate policy-
  related error messages. For example, in the case of RSVP, the PDP may
  accept a request and allow installation and forwarding of a reservation to
  a previous hop, but, at the same time, may wish to generate a
  warning/error message to a downstream node (NHOP) to warn about conditions
  such as "your request may have to be torn down in 10 mins, etc."
  Basically, an ability to create policy-related errors and/or warnings and
  to propagate them using the native QoS signaling protocol (such as RSVP)
  is needed. Such a policy error returned by the PDP must be able to also
  specify whether the reservation request should still be accepted,
  installed,  and forwarded to allow continued normal RSVP processing. In
  particular, when a PDP  sends back an error, it specifies that:

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    1. the message that generated the admission control request should be
    processed further as usual, but an error message (or warning) be sent in
    the other direction and include the policy objects supplied in that
    error message

    2. or, specifies that an error be returned, but the RSVP message should
    not be forwarded  as usual.

  4.3. Interactions between PEP, LPDP, and PDP at a RSVP router

  All the details of RSVP message processing and associated interactions
  between different elements at an RSVP router (PEP, LPDP) and PDP are
  included in separate documents [3,8]. In the following, a few, salient
  points related to the framework are listed:

    *    LPDP is optional and may be used for making decisions based on
         policy elements handled locally. The LPDP, in turn, may have to go
         to external entities (such as a directory server or an
         authentication server, etc.) for making its decisions.

    *    PDP is stateful and  may make decisions even if no policy objects
         are received (e.g., make decisions based on information such as
         flowspecs and session object in the RSVP messages). The PDP may
         consult other PDPs, but discussion of inter-PDP communication and
         coordination is outside the scope of this document.

    *    PDP sends asynchronous notifications to PEP whenever necessary to
         change earlier decisions, generate errors etc.

    *    PDP exports the information useful for usage monitoring  and
         accounting purposes. An example of a useful mechanism for this
         purpose is a MIB or a relational database. However, this document
         does not specify any particular mechanism for this purpose and
         discussion of such mechanisms is out of the scope of this document.

  4.4. Placement of Policy Elements in a Network

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  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-3.
  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
          +-------+   +-----+    +-------+    +-------+    +-------+
          | RSVP  |   | RSVP|    | RSVP  |    | RSVP  |    | RSVP  |
  +----+  |-------|   |-----|    |-------|    |-------|    |-------|
  | S1 |--| P | L |---|     |----| P | L |----| P | P |----|   P   |    +----+
  +----+  | E | D |   +-----+    | E | D |    | E | D |    |   E   |----| R1 |
          | P | P |              | P | P |    | P | P |    |   P   |    +----+
          +-------+              +-------+    +------+     +-------+
             ^                         ^                           ^
             |                         |                           |
             |                         |                           |
             |                         |                       +-------+
             |                         |                       | PDP   |
             |         +------+        |                       |-------|
             +-------->| PDP  |<-------+                       |       |
                       |------|                                +-------+
                       |      |                                   PS-2

           Figure 4: Placement of Policy Elements in an internet

  5. Example Policies, Scenarios, and  Policy Support

  In the following, we present examples of desired policies and scenarios
  requiring policy control that should possibly be addressed by the policy

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  control framework. In some cases,  possible approach(es) for achieving the
  desired goals are also outlined with a list of open issues to be resolved.

  5.1. Admission control policies based on factors such as Time-of-Day, User
  Identity, or credentials

  Policy control must be able to express and enforce rules with temporal
  dependencies. For example, a group of users might be allowed to make
  reservations at certain levels only during off-peak hours.  In addition,
  the policy control must also support policies that take into account
  identity or credentials of users requesting a particular service or
  resource. For example, an RSVP reservation request may be denied or
  accepted based on the credentials or identity supplied in the request.

  5.2. Bilateral agreements between service providers

  Until recently, usage agreements between service providers for traffic
  crossing their boundaries have been quite simple. For example, two ISPs
  might agree to accept all traffic from each other, often without
  performing any accounting or billing for the ``foreign'' traffic carried.
  However, with the availability of QoS mechanisms based on Integrated and
  Differentiated Services, traffic differentiation and quality of service
  guarantees are being phased into the Internet. As ISPs start to sell their
  customers different grades of service and can differentiate among
  different sources of traffic, they will also seek mechanisms for charging
  each other for traffic (and reservations) transiting their networks. One
  additional incentive in establishing such mechanisms is the potential
  asymmetry in terms of the customer base that different providers will
  exhibit: ISPs focused on servicing corporate traffic are likely to
  experience much higher demand for reserved services than those that
  service the consumer market. Lack of sophisticated accounting schemes for
  inter-ISP traffic could lead to inefficient allocation of costs among
  different service providers.

  Bilateral agreements could fall into two broad categories; local or
  global. Due to the complexity of the problem, it is expected that
  initially only the former will be deployed. In these, providers which
  manage a network cloud or administrative domain contract with their
  closest point of contact (neighbor) to establish ground rules and
  arrangements for access control and accounting. These contracts are mostly
  local and do not rely on global agreements; consequently, a policy node
  maintains information about its neighboring nodes only. Referring to
  Figure 4, this model implies that provider AD-1 has established
  arrangements with AD-2, but not with AD-3, for usage of each other's
  network. Provider AD-2, in turn, has in place agreements with AD-3 and so
  on. Thus, when forwarding a reservation request to AD-2, provider AD-2
  will charge AD-1 for use of all resources beyond AD-1's network.  This

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  information is obtained by recursively applying the bilateral agreements
  at every boundary between (neighboring) providers, until the recipient of
  the reservation request is reached. To implement this scheme under the
  policy control architecture, boundary nodes have to add an appropriate
  policy object to the RSVP message before forwarding it to a neighboring
  provider's network. This policy object will contain information such as
  the identity of the provider that generated them and the equivalent of an
  account number where charges can be accumulated. Since agreements only
  hold among neighboring nodes, policy objects have to be rewritten as RSVP
  messages cross the boundaries of administrative domains or provider's

  5.3. Priority based admission control policies

  In many settings, it is useful to distinguish between reservations on the
  basis of some level of "importance".  For example, this can be useful to
  avoid that the first reservation being granted the use of some resources,
  be able to hog those resources for some indefinite period of time.
  Similarly, this may be useful to allow emergency calls to go through even
  during periods of congestion.  Such functionality can be supported by
  associating priorities with reservation requests, and conveying this
  priority information together with other policy information.

  In its basic form, the priority associated with a reservation directly
  determines a reservation's rights to the resources it requests.  For
  example, assuming that priorities are expressed through integers in the
  range 0 to 32 with 32 being the highest priority, a reservation of
  priority, say, 10, will always be accepted, if the amount of resources
  held by lower priority reservations is sufficient to satisfy its
  requirements.  In other words, in case there are not enough free resources
  (bandwidth, buffers, etc.) at a node to accommodate the priority 10
  request, the node will attempt to free up the necessary resources by
  preempting existing lower priority reservations.

  There are a number of requirements associated with the support of priority
  and their proper operation.  First, traffic control in the router needs to
  be aware of priorities, i.e., classify existing reservations according to
  their priority, so that it is capable of determining how many and which
  ones to preempt, when required to accommodate a higher priority
  reservation request.  Second, it is important that preemption be made
  consistently at different nodes, in order to avoid transient
  instabilities.  Third and possibly most important, merging of priorities
  needs to be carefully architected and its impact clearly understood as
  part of the associated policy definition.

  Of the three above requirements, merging of priority information is the
  more complex and deserves additional discussions.  The complexity of
  merging priority information arises from the fact that this merging is to

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  be performed in addition to the merging of reservation information.  When
  reservation (FLOWSPEC) information is identical, i.e., homogeneous
  reservations, merging only needs to consider priority information, and the
  simple rule of keeping the highest priority provides an adequate answer.
  However, in the case of heterogeneous reservations, the * two-dimensional
  nature} of the (FLOWSPEC, priority) pair makes their ordering, and
  therefore merging, difficult. A description of the handling of different
  cases of RSVP priority objects is presented in [7].

  5.4. Pre-paid calling card or Tokens

  A model of increasing popularity in the telephone network is that of the
  pre-paid calling card. This concept could also be applied to the Internet;
  users purchase ``tokens'' which can be redeemed at a later time for access
  to network services. When a user makes a reservation request through, say,
  an RSVP RESV message, the user supplies a unique identification number of
  the ``token'', embedded in a policy object. Processing of this object at
  policy capable routers results in decrementing the value, or number of
  remaining units of service, of this token.

  Referring to Figure 4, suppose receiver R1 in the administrative domain
  AD3 wants to request a reservation for a service originating in AD1. R1
  generates a policy data object of type PD(prc, CID), where ``prc'' denotes
  pre-paid card and CID is the card identification number. Along with other
  policy objects carried in the RESV message, this object is received by
  node E, which forwards it to its PEP, PEP_E, which, in turn, contacts PDP
  PS-3. PS-3 either maintains locally, or has remote access to, a database
  of pre-paid card numbers. If the amount of remaining credit in CID is
  sufficient, the PDP accepts the reservation and the policy object is
  returned to PEP_E. Two issues have to be resolved here:

  *    What is the scope of these charges?

  *    When are charges (in the form of decrementing the remaining credit)
       first applied?

  The answer to the first question is related to the bilateral agreement
  model in place. If, on the one hand, provider AD-3 has established
  agreements with both AD-2 and AD-1, it could charge for the cost of the
  complete reservation up to sender S1. In this case PS-2 removes the
  PD(prc,CID) object from the outgoing RESV message.

  On the other hand, if AD-3 has no bilateral agreements in place, it will
  simply charge CID for the cost of the reservation within AD-3 and then
  forward PD(prc,CID) in the outgoing RESV message. Subsequent PDPs in other

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  administrative domains will charge CID for their respective reservations.
  Since multiple entities are both reading (remaining credit) and writing
  (decrementing credit) to the same database, some coordination and
  concurrency control might be needed.  The issues related to location,
  management, coordination of credit card (or similar) databases is outside
  the scope of this document.

  Another problem in this scenario is determining when the credit is
  exhausted. The PDPs should contact the database periodically to submit a
  charge against the CID; if the remaining credit reaches zero, there must
  be a mechanism to detect that and to cause revocation or termination of
  privileges granted based on the credit.

  Regarding the issue of when to initiate charging, ideally that should
  happen only after the reservation request has succeeded. In the case of
  local charges, that could be communicated by the router to the PDP.

  5.5. Sender Specified Restrictions on Receiver Reservations

  The ability of senders to specify restrictions on reservations, based on
  receiver identity, number of receivers or reservation cost might be useful
  in future network applications. An example could be any application in
  which the sender pays for service delivered to receivers. In such a case,
  the sender might be willing to assume the cost of a reservation, as long
  as it satisfies certain criteria, for example, it originates from a
  receiver who belongs to an access control list (ACL) and satisfies a limit
  on cost. (Notice that this could allow formation of "closed" multicast

  In the policy based admission control framework such a scheme could be
  achieved by having the sender generate appropriate policy objects, carried
  in a PATH message, which install state in routers on the path to
  receivers. In accepting reservations, the routers would have to compare
  the RESV requests to the installed state.

  A number of different solutions can be built to address this scenario;
  precise description of a solution is beyond the scope of this document.

  6. Interaction Between the Policy Enforcement Point (PEP) and the Policy
  Decision Point (PDP)

  In the case of an external PDP, the need for a communication protocol
  between the PEP and PDP arises. In order to allow for interoperability
  between different vendors networking elements and (external) policy
  servers, this protocol should be standardized.

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                A Framework for Policy-based Admission Control    March 1999

  6.1. PEP to PDP Protocol Requirements

  This section describes a set of general requirements for the communication
  protocol between the PEP and an external PDP.

  *    Reliability:  The sensitivity of policy control information
       necessitates reliable operation. Undetected loss of policy queries or
       responses may lead to inconsistent network control operation and are
       clearly unacceptable for actions such as billing and accounting. One
       option for providing reliability is the re-use of the TCP as the
       transport protocol.

  *    Small delays: The timing requirements of policy decisions related to
       QoS signaling protocols are expected to be quite strict. The PEP to
       PDP protocol should add small amount of delay to the response delay
       experienced by queries placed by the PEP to the PDP.

  *    Ability to carry opaque objects: The protocol should allow for
       delivery of self-identifying, opaque objects, of variable length,
       such as RSVP messages, RSVP policy objects and other objects that
       might be defined as new policies are introduced. The protocol should
       not have to be changed every time a new object has to be exchanged.

  *    Support for PEP-initiated, two-way Transactions:  The protocol must
       allow for two-way transactions (request-response exchanges) between a
       PEP and a PDP. In particular, PEPs must be able to initiate requests
       for policy decision, re-negotiation of previously made policy
       decision, and exchange of policy information. To some extent, this
       requirement is closely tied to the goal of meeting the requirements
       of RSVP-specific, policy-based admission control. RSVP signaling
       events such as arrival of RESV refresh messages, state timeout, and
       merging of reservations require that a PEP (such as an RSVP router)
       request a policy decision from PDP at any time. Similarly, PEP must
       be able to report monitoring information and policy state changes to
       PDP at any time.

  *    Support for asynchronous notification: This is required in order to
       allow both the policy server and client to notify each other in the
       case of an asynchronous change in state, i.e., a change that is not
       triggered by a signaling message. For example, the server would need
       to notify the client if a particular reservation has to be terminated
       due to expiration of a user's credentials or account balance.
       Likewise, the client has to inform the server of a reservation

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       rejection which is due to admission control failure.

  *    Handling of multicast groups: The protocol should provision for
       handling of policy decisions related to multicast groups.

  *    QoS Specification: The protocol should allow for precise
       specification of level of service requirements in the PEP requests
       forwarded to the PDP.

  7. Security Considerations

  The communication tunnel between policy clients and policy servers should
  be secured by the use of an IPSEC [4] channel. It is advisable that this
  tunnel makes use of both the AH (Authentication Header) and ESP
  (Encapsulating Security Payload) protocols, in order to provide
  confidentiality, data origin authentication, integrity and replay

  In the case of the RSVP signaling mechanism, RSVP MD5 [2] message
  authentication can be used to secure communications between network

  8. References

  [1] R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin, "Resource
  ReSerVation Protocol (RSVP) -- Version 1 Functional Specification ", RFC
  2205, September 1997.

  [2] F. Baker., "RSVP Cryptographic Authentication", draft-ietf-rsvp-md5-
  05.txt, August 1997.

  [3] S. Herzog., "RSVP Extensions for Policy Control",  Internet Draft},
  draft-ietf-rsvp-policy-ext-03.[ps,txt], August 1998.

  [4] R. Atkinson. Security Architecture for the Internet Protocol. RFC1825,
  Aug. 1995.

  [5] C. Rigney, A Rubens, W. Simpson and S. Willens. Remote Authentication
  Dial In User Service (RADIUS). RFC 2138.

  [6] R. Rajan et al. Schema for Differentiated Services and Integrated
  Services in Networks, draft-rajan-policy-qosschema-00.txt, October 1998.

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                A Framework for Policy-based Admission Control    March 1999

  [7] S. Herzog, "RSVP Preemption Priority Policy", Internet Draft, draft-
  ietf-rap-priority-00.txt, Nov. 1998.

  [8] S. Herzog, "COPS Usage for RSVP", Internet Draft, draft-ietf-rap-
  cops-rsvp-00.txt, August 1998.

  8. Acknowledgements

  This is a result of an ongoing discussion among many members of the RAP
  group including Jim Boyle, Ron Cohen, Laura Cunningham, Dave Durham, Shai
  Herzog, Tim O'Malley, Raju Rajan, and Arun Sastry.

  9.  Authors` Addresses

          Raj Yavatkar
          Intel Corporation
          2111 N.E. 25th Avenue,
          Hillsboro, OR 97124
          phone: +1 503-264-9077

          Dimitrios Pendarakis
          IBM T.J. Watson Research Center
          P.O. Box 704
          Yorktown Heights
          NY 10598
          phone: +1 914-784-7536

          Roch Guerin
          University of Pennsylvania
          Dept. of Electrical Engineering
          200 South 33rd Street
          Philadelphia, PA  19104

          phone: +1 215 898-9351

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