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RFC 2753

A Framework for Policy-based Admission Control

Pages: 20

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Network Working Group                                         R. Yavatkar
Request for Comments: 2753                                          Intel
Category: Informational                                     D. Pendarakis
                                                                R. Guerin
                                                       U. Of Pennsylvania
                                                             January 2000

             A Framework for Policy-based Admission Control

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

1. Introduction

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 the 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-
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   of-day/week. Similarly, diff-serv mechanisms also need to take into
   account policies that involve 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

   We begin with a list of definitions in Section 2. Section 3 lists the
   requirements and goals of the mechanisms used to control and enforce
   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
   evaluates the suitability of some existing protocols for this

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 purposes. - 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, we use the terms router, network element, and network node interchangeably, but the should all be interpreted as references to a network element. - QoS Signaling Protocol: A signaling protocol that carries an admission control request for a resource, e.g., RSVP.
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   -  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 information such as policy
      elements and is carried in a request or response related to a
      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 may carry an user or application
      identification whereas another policy element may carry user
      credentials or credit card information.  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.
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   -  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.

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 decisions. - 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 out. - 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.
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   -  Fault tolerance and recovery: The mechanisms designed on the basis
      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 associated 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

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
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   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 type of access and the protocols used may vary among
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   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 [6].

   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 the left shows a local
   decision point at a network node and the configuration on the right
   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 use.

   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
      elements that are evaluated locally. The PEP passes the request
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      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 (obtained from 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 helps 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 PDP. 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 PDP 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 an 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 a more detailed discussion of the exact interaction and interfaces between RSVP and the PEP is provided 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

   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
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   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:

      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

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
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   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

   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 the policy control framework should be able to support. 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.

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   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 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
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   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 be performed in addition to the merging of reservation
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   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 administrative domains will charge CID for their
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   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

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 groups). 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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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 rejection which is due to admission control failure.
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   *  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 prevention. In the case of the RSVP signaling mechanism, RSVP MD5 [2] message authentication can be used to secure communications between network elements.

8. References

[1] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, September 1997. [2] Baker, F., Lindell, B. and M. Talwar, "RSVP Cryptographic Authentication", RFC 2747, January 2000. [3] Herzog, S., "RSVP Extensions for Policy Control", RFC 2750, January 2000. [4] Atkinson, R., "Security Architecture for the Internet Protocol", RFC 1825, August 1995. [5] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote Authentication Dial In User Service (RADIUS)", RFC 2138, April 1997. [6] Rajan, R., et al., "Schema for Differentiated Services and Integrated Services in Networks", Work in Progress. [7] Herzog, S., "RSVP Preemption Priority Policy", Work in Progress. [8] Herzog, S., "COPS Usage for RSVP", RFC 2749, January 2000.
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9. 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.

10. Authors' Addresses

Raj Yavatkar Intel Corporation 2111 N.E. 25th Avenue, Hillsboro, OR 97124 USA Phone: +1 503-264-9077 EMail: Dimitrios Pendarakis IBM T.J. Watson Research Center P.O. Box 704 Yorktown Heights NY 10598 Phone: +1 914-784-7536 EMail: Roch Guerin University of Pennsylvania Dept. of Electrical Engineering 200 South 33rd Street Philadelphia, PA 19104 Phone: +1 215 898-9351 EMail:
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11. Full Copyright Statement

Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.