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Network Working Group                                         R. YavatkarRequest for Comments: 2753                                          IntelCategory: Informational                                     D. Pendarakis                                                                      IBM                                                                R. Guerin                                                       U. Of Pennsylvania                                                             January 2000A Framework for Policy-based Admission ControlStatus 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-Yavatkar, et al.             Informational                      [Page 1]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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   possible.   We begin with a list of definitions inSection 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 inSection 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   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      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.Yavatkar, et al.             Informational                      [Page 2]

RFC 2753      Framework for Policy-based Admission Control  January 2000   -  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      made.   -  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.Yavatkar, et al.             Informational                      [Page 3]

RFC 2753      Framework for Policy-based Admission Control  January 2000   -  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.Yavatkar, et al.             Informational                      [Page 4]

RFC 2753      Framework for Policy-based Admission Control  January 2000   -  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 thatYavatkar, et al.             Informational                      [Page 5]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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 amongYavatkar, et al.             Informational                      [Page 6]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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.Yavatkar, et al.             Informational                      [Page 7]

RFC 2753      Framework for Policy-based Admission Control  January 2000    ________________                        ____________________   |                |                      |                    |   |  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 requestYavatkar, et al.             Informational                      [Page 8]

RFC 2753      Framework for Policy-based Admission Control  January 2000      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].Yavatkar, et al.             Informational                      [Page 9]

RFC 2753      Framework for Policy-based Admission Control  January 2000        ______________________________       |                              |       |           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 theYavatkar, et al.             Informational                     [Page 10]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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 serviceYavatkar, et al.             Informational                     [Page 11]

RFC 2753      Framework for Policy-based Admission Control  January 2000   provider in this case).  Nodes A, B and C belong to administrative   domain AD-1, advised by PDP PS-1, while D and E belong to AD-2 and   AD-3, respectively. E communicates with PDP PS-2.  In general, it is   expected that there will be at least one PDP per administrative   domain.   Policy capable network nodes could range from very unsophisticated,   such as E, which have no LPDP, and thus have to rely on an external   PDP for every policy processing operation, to self-sufficient, such   as D, which essentially encompasses both an LPDP and a PDP locally,   at the router.                        AD-1                    AD-2         AD-3      ________________/\_______________     __/\___      __/\___     {                                 }   {       }    {       }             A           B            C            D            E        +-------+  +-----+    +-------+    +-------+    +-------+        | 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                     +------+                       PS-1         Figure 4: Placement of Policy Elements in an internet5. Example Policies, Scenarios, andPolicy 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.Yavatkar, et al.             Informational                     [Page 12]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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 RSVPYavatkar, et al.             Informational                     [Page 13]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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   networks.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 reservationYavatkar, et al.             Informational                     [Page 14]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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 theirYavatkar, et al.             Informational                     [Page 15]

RFC 2753      Framework for Policy-based Admission Control  January 2000   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 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.Yavatkar, et al.             Informational                     [Page 16]

RFC 2753      Framework for Policy-based Admission Control  January 20006.1. 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.Yavatkar, et al.             Informational                     [Page 17]

RFC 2753      Framework for Policy-based Admission Control  January 2000   *  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.Yavatkar, et al.             Informational                     [Page 18]

RFC 2753      Framework for Policy-based Admission Control  January 20009. 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: raj.yavatkar@intel.com   Dimitrios Pendarakis   IBM T.J. Watson Research Center   P.O. Box 704   Yorktown Heights   NY 10598   Phone: +1 914-784-7536   EMail: dimitris@watson.ibm.com   Roch Guerin   University of Pennsylvania   Dept. of Electrical Engineering   200 South 33rd Street   Philadelphia, PA  19104   Phone: +1 215 898-9351   EMail: guerin@ee.upenn.eduYavatkar, et al.             Informational                     [Page 19]

RFC 2753      Framework for Policy-based Admission Control  January 200011.  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.Yavatkar, et al.             Informational                     [Page 20]