<- RFC Index (2701..2800)
RFC 2753
Network Working Group R. Yavatkar
Request for Comments: 2753 Intel
Category: Informational D. Pendarakis
IBM
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
possible.
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
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.
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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
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.
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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
Scheduler
When a RSVP message arrives at the router (or an RSVP related event
requires a policy decision), the RSVP module is expected to hand off
the request (corresponding to the event or message) to its PEP
module. The PEP will use the PDP (and LPDP) to obtain the policy
decision and communicate it back to the RSVP module.
4.2. Additional functionality at the PDP
Typically, PDP returns the final policy decision based on an
admission control request and the associated policy elements.
However, it should be possible for the PDP to sometimes ask the PEP
(or the admission control module at the network element where PEP
resides) to generate policy-related error messages. For example, in
the case of RSVP, the PDP may accept a request and allow installation
and forwarding of a reservation to a previous hop, but, at the same
time, may wish to generate a warning/error message to a downstream
node (NHOP) to warn about conditions such as "your request may have
to be torn down in 10 mins, etc." Basically, an ability to create
policy-related errors and/or warnings and to propagate them using the
native QoS signaling protocol (such as RSVP) is needed. Such a policy
error returned by the PDP must be able to also specify whether the
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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
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 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
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 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
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.
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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: 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.edu
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11. Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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