A Framework for Policy-based Admission Control
draft-ietf-rap-framework-03
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 2753.
|
|
|---|---|---|---|
| Authors | Dimitrios Pendarakis , Dr. Raj Yavatkar , Dr. Roch Guerin | ||
| Last updated | 2013-03-02 (Latest revision 1999-05-17) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 2753 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-rap-framework-03
Internet Engineering Task Force Raj Yavatkar, Intel
INTERNET-DRAFT Dimitrios Pendarakis, IBM
Roch Guerin, U. Of Pennsylvania
April 1999
Expires: December 1999
draft-ietf-rap-framework-03.txt
A Framework for Policy-based Admission Control
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
[Page 1]
A Framework for Policy-based Admission Control March 1999
1. Abstract
The IETF working groups such as Integrated Services (called "int-serv")
and RSVP [1] have developed extensions to the IP architecture and the
best-effort service model so that applications or end users can request
specific quality (or levels) of service from an internetwork in addition
to the current IP best-effort service. Recent efforts in the
Differentiated Services Working Group are also directed at definition of
mechanisms that support aggregate QoS services. The int-serv model for
these new services requires explicit signaling of the QoS (Quality of
Service) requirements from the end points and provision of admission and
traffic control at Integrated Services routers. The proposed standards for
RSVP [RFC 2205] and Integrated Services [RFC 2211, RFC 2212] are examples
of a new reservation setup protocol and new service definitions
respectively. Under the int-serv model, certain data flows receive
preferential treatment over other flows; the admission control component
only takes into account the requester's resource reservation request and
available capacity to determine whether or not to accept a QoS request.
However, the int-serv mechanisms do not include an important aspect of
admission control: network managers and service providers must be able to
monitor, control, and enforce use of network resources and services based
on policies derived from criteria such as the identity of users and
applications, traffic/bandwidth requirements, security considerations, and
time-of-day/week. Similarly, diff-serv mechanisms also need to take into
account policies that take into account various criteria such as customer
identity, ingress points, and so on.
This document is concerned with specifying a framework for providing
policy-based control over admission control decisions. In particular, it
focuses on policy-based control over admission control using RSVP as an
example of the QoS signaling mechanism. Even though the focus of the work
is on RSVP-based admission control, the document outlines a framework that
can provide policy-based admission control in other QoS contexts. We argue
that policy-based control must be applicable to different kinds and
qualities of services offered in the same network and our goal is to
consider such extensions whenever possible.
We begin with a list of definitions in Section 2. Section 3 lists the
requirements and goals of the mechanisms capable of controlling and
enforcing access to better QoS. We then outline the architectural
elements of the framework in Section 4 and describe the functionality
assumed for each component. Section 5 discusses example policies,
possible scenarios, and policy support needed for those scenarios. Section
6 specifies the requirements for a client-server protocol for
communication between a policy server (PDP) and its client (PEP) and
[Page 2]
A Framework for Policy-based Admission Control March 1999
evaluates suitability of some of the existing protocols for this purpose.
2. Terminology
The following is a list of terms used in this document.
- Administrative Domain: A collection of networks under the same
administrative control and grouped together for administrative
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, sometimes we use the terms router, network
element, and network node interchangeably, but should be
interpreted as reference to a network element.
- QoS Signaling Protocol: A signaling protocol that carries an
admission control request for a bandwidth resource, e.g., RSVP.
- Policy: The combination of rules and services where rules define
the criteria for resource access and usage.
- Policy control: The application of rules to determine whether or
not access to a particular resource should be granted.
- Policy Object: Contains policy-related info such as policy
elements and is carried in a request or response related to
resource allocation decision.
- Policy Element: Subdivision of policy objects; contains single
units of information necessary for the evaluation of policy rules.
A single policy element carries an user or application
identification whereas another policy element may carry user
credentials or credit card information. Examples of policy
[Page 3]
A Framework for Policy-based Admission Control March 1999
elements include identity of the requesting user or application,
user/app credentials, etc. The policy elements themselves are
expected to be independent of which QoS signaling protocol is used.
- Policy Decision Point (PDP): The point where policy decisions are
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.
- 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.
[Page 4]
A Framework for Policy-based Admission Control March 1999
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.
- Fault tolerance and recovery: The mechanisms designed on the basis
[Page 5]
A Framework for Policy-based Admission Control March 1999
of this framework must include provisions for fault tolerance and
recovery from failure cases such as failure of PDPs, disruption in
communication including network partitions (and subsequent merging)
that separate a PDP from its peer PEPs.
- Support for Policy-Ignorant Nodes (PINs): Support for the
mechanisms described in this document should not be mandatory for
every node in a network. Policy based admission control could be
enforced at a subset of nodes, for example the boundary nodes
within an administrative domain. These policy capable nodes would
function as trusted nodes from the point of view of the policy-
ignorant nodes in that administrative domain.
- Scalability: One of the important requirements for the mechanisms
designed for policy control is scalability. The mechanisms must
scale at least to the same extent that RSVP scales in terms of
accommodating multiple flows and network nodes in the path of a
flow. In particular, scalability must be considered when specifying
default behavior for merging policy data objects and merging should
not result in duplicate policy elements or objects. There are
several sensitive areas in terms of scalability for policy control
over RSVP. First, not every policy aware node in an infrastructure
should be expected to contact a remote PDP. This would cause
potentially long delays in verifying requests that must travel up
hop by hop. Secondly, RSVP is capable of setting up resource
reservations for multicast flows. This implies that the policy
control model must be capable of servicing the special requirements
of large multicast flows. Thus, the policy control architecture
must scale at least as well as RSVP based on factors such as the
size of RSVP messages, the time required for the network to service
an RSVP request, local processing time required per node, and local
memory consumed per node.
- Security and denial of service considerations: The policy control
architecture must be secure as far as the following aspects are
concerned. First, the mechanisms proposed under the framework must
minimize theft and denial of service threats. Second, it must be
ensured that the entities (such as PEPs and PDPs) involved in
policy control can verify each other's identity and establish
necessary trust before communicating.
4. Architectural Elements
[Page 6]
A Framework for Policy-based Admission Control March 1999
The two main architectural elements for policy control are the PEP (Policy
Enforcement Point) and the PDP (Policy Decision Point). Figure 1 shows a
simple configuration involving these two elements; PEP is a component at a
network node and PDP is a remote entity that may reside at a policy
server. The PEP represents the component that always runs on the policy
aware node. It is the point at which policy decisions are actually
enforced. Policy decisions are made primarily at the PDP. The PDP itself
may make use of additional mechanisms and protocols to achieve additional
functionality such as user authentication, accounting, policy information
storage, etc. For example, the PDP is likely to use an LDAP-based
directory service for storage and retrieval of policy information[6]. This
document does not include discussion of these additional mechanisms and
protocols and how they are used.
The basic interaction between the components begins with the PEP. The PEP
will receive a notification or a message that requires a policy decision.
Given such an event, the PEP then formulates a request for a policy
decision and sends it to the PDP. The request for policy control from a
PEP to the PDP may contain one or more policy elements (encapsulated into
one or more policy objects) in addition to the admission control
information (such as a flowspec or amount of bandwidth requested) in the
original message or event that triggered the policy decision request. The
PDP returns the policy decision and the PEP then enforces the policy
decision by appropriately accepting or denying the request. The PDP may
also return additional information to the PEP which includes one or more
policy elements. This information need not be associated with an admission
control decision. Rather, it can be used to formulate an error message or
outgoing/forwarded message.
________________ Policy server
| | ______
| Network Node | | |------------->
| _____ | | | May use LDAP,SNMP,.. for accessing
| | | | | | policy database, authentication,etc.
| | PEP |<-----|------->| PDP |------------->
| |_____| | |_____|
| |
|________________|
Figure 1: A simple configuration with the primary policy control
architecture components. PDP may use additional mechanisms and protocols
for the purpose of accounting, authentication, policy storage, etc.
The PDP might optionally contact other external servers, e.g., for
accessing configuration, user authentication, accounting and billing
databases. Protocols defined for network management (SNMP) or directory
access (LDAP) might be used for this communication. While the specific
[Page 7]
A Framework for Policy-based Admission Control March 1999
type of access and the protocols used may vary among different
implementations, some of these interactions will have network-wide
implications and could impact the interoperability of different devices.
Of particular importance is the "language" used to specify the policies
implemented by the PDP. The number of policies applicable at a network
node might potentially be quite large. At the same time, these policies
will exhibit high complexity, in terms of number of fields used to arrive
at a decision, and the wide range of decisions. Furthermore, it is likely
that several policies could be applicable to the same request profile. For
example, a policy may prescribe the treatment of requests from a general
user group (e.g., employees of a company) as well as the treatment of
requests from specific members of that group (e.g., managers of the
company). In this example, the user profile "managers" falls within the
specification of two policies, one general and one more specific.
In order to handle the complexity of policy decisions and to ensure a
coherent and consistent application of policies network-wide, the policy
specification language should ensure unambiguous mapping of a request
profile to a policy action. It should also permit the specification of the
sequence in which different policy rules should be applied and/or the
priority associated with each one. Some of these issues are addressed in
[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.
[Page 8]
A Framework for Policy-based Admission Control March 1999
________________ ____________________
| | | |
| Network Node | Policy Server | Network Node |
| _____ | _____ | _____ _____ |
| | | | | | | | | | | |
| | PEP |<-----|---->| PDP | | | PEP |<-->| PDP | |
| |_____| | |_____| | |_____| |_____| |
| ^ | | |
| | _____ | |____________________|
| \-->| | |
| | LPDP| |
| |_____| |
| |
|________________|
Figure 2: Two other possible configurations of policy control
architecture components. The configuration on left shows a local decision
point at a network node and the configuration on the left shows PEP and
PDP co-located at the same node.
If it is available, the PEP will first use the LPDP to reach a local
decision. This partial decision and the original policy request are next
sent to the PDP which renders a final decision (possibly, overriding the
LPDP). It must be noted that the PDP acts as the final authority for the
decision returned to the PEP and the PEP must enforce the decision
rendered by the PDP. Finally, if a shared state has been established for
the request and response between the PEP and PDP, it is the responsibility
of the PEP to notify the PDP that the original request is no longer in
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
[Page 9]
A Framework for Policy-based Admission Control March 1999
elements that are evaluated locally. The PEP passes the request
with the set A to the Local Decision point (LPDP) and collects the
result of the LPDP (called "partial result" and referred to as D(A)
).
3. The PEP then passes the request with ALL the policy elements and
D(A) to the PDP. The PDP applies policies based on all the policy
elements and the request and reaches a decision (let us call it
D(Q)). It then combines its result with the partial result D(A)
using a combination operation to reach a final decision.
4. The PDP returns the final policy decision (one after the
combination operation) to the PEP.
Note that in the above model, the PEP *must* contact the PDP even if no
(or NULL) policy objects are received in the admission control request.
This requirement would help ensure that a request cannot bypass policy
control by omitting policy elements in a reservation request. However,
``short circuit'' processing is permitted, i.e., if the result of D(A),
above, is ``no'', then there is no need to proceed with further policy
processing at the policy server. Still, the PDP must be informed of the
failure of local policy processing. The same applies to the case when
policy processing is successful but admission control (at the resource
management level due to unavailable capacity) fails; again the policy
server has to be informed of the failure.
It must also be noted that the PDP may, at any time, send an asynchronous
notification to the PEP to change its earlier decision or to generate a
policy error/warning message.
4.1. Example of a RSVP Router
In the case of a RSVP router, Figure 3 shows the interaction between a PEP
and other int-serv components within the router. For the purpose of this
discussion, we represent all the components of RSVP-related processing by
a single RSVP module, but more detailed discussion of the exact
interaction and interfaces between RSVP and PEP will be described in a
separate document [3].
[Page 10]
A Framework for Policy-based Admission Control March 1999
______________________________
| |
| Router |
| ________ _____ | _____
| | | | | | | |
| | RSVP |<------->| PEP |<--|---------->| PDP |
| |________| |_____| | |_____|
| ^ |
| | Traffic control |
| | _____________ |
| \---->| _________ | |
| | |capacity | | |
| | | ADM CTL | | |
| | |_________| | |
--|----------->| ____ ____ | |
| Data | | PC | PS | | |
| | |____|____| | |
| |_____________| |
| |
|______________________________|
Figure 3: Relationship between PEP and other int-serv components
within an RSVP router. PC -- Packet Classifier, PS -- Packet Scheduler
When a RSVP message arrives at the router (or an RSVP related event
requires a policy decision), the RSVP module is expected to hand off the
request (corresponding to the event or message) to its PEP module. The PEP
will use the PDP (and LPDP) to obtain the policy decision and communicate
it back to the RSVP module.
4.2. Additional functionality at the PDP
Typically, PDP returns the final policy decision based on an admission
control request and the associated policy elements. However, it should be
possible for the PDP to sometimes ask the PEP (or the admission control
module at the network element where PEP resides) to generate policy-
related error messages. For example, in the case of RSVP, the PDP may
accept a request and allow installation and forwarding of a reservation to
a previous hop, but, at the same time, may wish to generate a
warning/error message to a downstream node (NHOP) to warn about conditions
such as "your request may have to be torn down in 10 mins, etc."
Basically, an ability to create policy-related errors and/or warnings and
to propagate them using the native QoS signaling protocol (such as RSVP)
is needed. Such a policy error returned by the PDP must be able to also
specify whether the reservation request should still be accepted,
installed, and forwarded to allow continued normal RSVP processing. In
particular, when a PDP sends back an error, it specifies that:
[Page 11]
A Framework for Policy-based Admission Control March 1999
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
[Page 12]
A Framework for Policy-based Admission Control March 1999
By allowing division of labor between an LPDP and a PDP, the policy
control architecture allows staged deployment by enabling routers of
varying degrees of sophistication, as far as policy control is concerned,
to communicate with policy servers. Figure 4 depicts an example set of
nodes belonging to three different administrative domains (AD) (Each AD
could correspond to a different service provider in this case). Nodes A,
B and C belong to administrative domain AD-1, advised by PDP PS-1, while D
and E belong to AD-2 and AD-3, respectively. E communicates with PDP PS-3.
In general, it is expected that there will be at least one PDP per
administrative domain.
Policy capable network nodes could range from very unsophisticated, such
as E, which have no LPDP, and thus have to rely on an external PDP for
every policy processing operation, to self-sufficient, such as D, which
essentially encompasses both an LPDP and a PDP locally, at the router.
AD-1 AD-2 AD-3
________________/\_______________ __/\___ __/\___
{ } { } { }
A B C D E
+-------+ +-----+ +-------+ +-------+ +-------+
| RSVP | | RSVP| | RSVP | | RSVP | | RSVP |
+----+ |-------| |-----| |-------| |-------| |-------|
| S1 |--| P | L |---| |----| P | L |----| P | P |----| P | +----+
+----+ | E | D | +-----+ | E | D | | E | D | | E |----| R1 |
| P | P | | P | P | | P | P | | P | +----+
+-------+ +-------+ +------+ +-------+
^ ^ ^
| | |
| | |
| | +-------+
| | | PDP |
| +------+ | |-------|
+-------->| PDP |<-------+ | |
|------| +-------+
| | PS-2
+------+
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 should possibly be addressed by the policy
[Page 13]
A Framework for Policy-based Admission Control March 1999
control framework. In some cases, possible approach(es) for achieving the
desired goals are also outlined with a list of open issues to be resolved.
5.1. Admission control policies based on factors such as Time-of-Day, User
Identity, or credentials
Policy control must be able to express and enforce rules with temporal
dependencies. For example, a group of users might be allowed to make
reservations at certain levels only during off-peak hours. In addition,
the policy control must also support policies that take into account
identity or credentials of users requesting a particular service or
resource. For example, an RSVP reservation request may be denied or
accepted based on the credentials or identity supplied in the request.
5.2. Bilateral agreements between service providers
Until recently, usage agreements between service providers for traffic
crossing their boundaries have been quite simple. For example, two ISPs
might agree to accept all traffic from each other, often without
performing any accounting or billing for the ``foreign'' traffic carried.
However, with the availability of QoS mechanisms based on Integrated and
Differentiated Services, traffic differentiation and quality of service
guarantees are being phased into the Internet. As ISPs start to sell their
customers different grades of service and can differentiate among
different sources of traffic, they will also seek mechanisms for charging
each other for traffic (and reservations) transiting their networks. One
additional incentive in establishing such mechanisms is the potential
asymmetry in terms of the customer base that different providers will
exhibit: ISPs focused on servicing corporate traffic are likely to
experience much higher demand for reserved services than those that
service the consumer market. Lack of sophisticated accounting schemes for
inter-ISP traffic could lead to inefficient allocation of costs among
different service providers.
Bilateral agreements could fall into two broad categories; local or
global. Due to the complexity of the problem, it is expected that
initially only the former will be deployed. In these, providers which
manage a network cloud or administrative domain contract with their
closest point of contact (neighbor) to establish ground rules and
arrangements for access control and accounting. These contracts are mostly
local and do not rely on global agreements; consequently, a policy node
maintains information about its neighboring nodes only. Referring to
Figure 4, this model implies that provider AD-1 has established
arrangements with AD-2, but not with AD-3, for usage of each other's
network. Provider AD-2, in turn, has in place agreements with AD-3 and so
on. Thus, when forwarding a reservation request to AD-2, provider AD-2
will charge AD-1 for use of all resources beyond AD-1's network. This
[Page 14]
A Framework for Policy-based Admission Control March 1999
information is obtained by recursively applying the bilateral agreements
at every boundary between (neighboring) providers, until the recipient of
the reservation request is reached. To implement this scheme under the
policy control architecture, boundary nodes have to add an appropriate
policy object to the RSVP message before forwarding it to a neighboring
provider's network. This policy object will contain information such as
the identity of the provider that generated them and the equivalent of an
account number where charges can be accumulated. Since agreements only
hold among neighboring nodes, policy objects have to be rewritten as RSVP
messages cross the boundaries of administrative domains or provider's
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
[Page 15]
A Framework for Policy-based Admission Control March 1999
be performed in addition to the merging of reservation information. When
reservation (FLOWSPEC) information is identical, i.e., homogeneous
reservations, merging only needs to consider priority information, and the
simple rule of keeping the highest priority provides an adequate answer.
However, in the case of heterogeneous reservations, the * two-dimensional
nature} of the (FLOWSPEC, priority) pair makes their ordering, and
therefore merging, difficult. A description of the handling of different
cases of RSVP priority objects is presented in [7].
5.4. Pre-paid calling card or Tokens
A model of increasing popularity in the telephone network is that of the
pre-paid calling card. This concept could also be applied to the Internet;
users purchase ``tokens'' which can be redeemed at a later time for access
to network services. When a user makes a reservation request through, say,
an RSVP RESV message, the user supplies a unique identification number of
the ``token'', embedded in a policy object. Processing of this object at
policy capable routers results in decrementing the value, or number of
remaining units of service, of this token.
Referring to Figure 4, suppose receiver R1 in the administrative domain
AD3 wants to request a reservation for a service originating in AD1. R1
generates a policy data object of type PD(prc, CID), where ``prc'' denotes
pre-paid card and CID is the card identification number. Along with other
policy objects carried in the RESV message, this object is received by
node E, which forwards it to its PEP, PEP_E, which, in turn, contacts PDP
PS-3. PS-3 either maintains locally, or has remote access to, a database
of pre-paid card numbers. If the amount of remaining credit in CID is
sufficient, the PDP accepts the reservation and the policy object is
returned to PEP_E. Two issues have to be resolved here:
* What is the scope of these charges?
* When are charges (in the form of decrementing the remaining credit)
first applied?
The answer to the first question is related to the bilateral agreement
model in place. If, on the one hand, provider AD-3 has established
agreements with both AD-2 and AD-1, it could charge for the cost of the
complete reservation up to sender S1. In this case PS-2 removes the
PD(prc,CID) object from the outgoing RESV message.
On the other hand, if AD-3 has no bilateral agreements in place, it will
simply charge CID for the cost of the reservation within AD-3 and then
forward PD(prc,CID) in the outgoing RESV message. Subsequent PDPs in other
[Page 16]
A Framework for Policy-based Admission Control March 1999
administrative domains will charge CID for their respective reservations.
Since multiple entities are both reading (remaining credit) and writing
(decrementing credit) to the same database, some coordination and
concurrency control might be needed. The issues related to location,
management, coordination of credit card (or similar) databases is outside
the scope of this document.
Another problem in this scenario is determining when the credit is
exhausted. The PDPs should contact the database periodically to submit a
charge against the CID; if the remaining credit reaches zero, there must
be a mechanism to detect that and to cause revocation or termination of
privileges granted based on the credit.
Regarding the issue of when to initiate charging, ideally that should
happen only after the reservation request has succeeded. In the case of
local charges, that could be communicated by the router to the PDP.
5.5. Sender Specified Restrictions on Receiver Reservations
The ability of senders to specify restrictions on reservations, based on
receiver identity, number of receivers or reservation cost might be useful
in future network applications. An example could be any application in
which the sender pays for service delivered to receivers. In such a case,
the sender might be willing to assume the cost of a reservation, as long
as it satisfies certain criteria, for example, it originates from a
receiver who belongs to an access control list (ACL) and satisfies a limit
on cost. (Notice that this could allow formation of "closed" multicast
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.
[Page 17]
A Framework for Policy-based Admission Control March 1999
6.1. PEP to PDP Protocol Requirements
This section describes a set of general requirements for the communication
protocol between the PEP and an external PDP.
* Reliability: The sensitivity of policy control information
necessitates reliable operation. Undetected loss of policy queries or
responses may lead to inconsistent network control operation and are
clearly unacceptable for actions such as billing and accounting. One
option for providing reliability is the re-use of the TCP as the
transport protocol.
* Small delays: The timing requirements of policy decisions related to
QoS signaling protocols are expected to be quite strict. The PEP to
PDP protocol should add small amount of delay to the response delay
experienced by queries placed by the PEP to the PDP.
* Ability to carry opaque objects: The protocol should allow for
delivery of self-identifying, opaque objects, of variable length,
such as RSVP messages, RSVP policy objects and other objects that
might be defined as new policies are introduced. The protocol should
not have to be changed every time a new object has to be exchanged.
* Support for PEP-initiated, two-way Transactions: The protocol must
allow for two-way transactions (request-response exchanges) between a
PEP and a PDP. In particular, PEPs must be able to initiate requests
for policy decision, re-negotiation of previously made policy
decision, and exchange of policy information. To some extent, this
requirement is closely tied to the goal of meeting the requirements
of RSVP-specific, policy-based admission control. RSVP signaling
events such as arrival of RESV refresh messages, state timeout, and
merging of reservations require that a PEP (such as an RSVP router)
request a policy decision from PDP at any time. Similarly, PEP must
be able to report monitoring information and policy state changes to
PDP at any time.
* Support for asynchronous notification: This is required in order to
allow both the policy server and client to notify each other in the
case of an asynchronous change in state, i.e., a change that is not
triggered by a signaling message. For example, the server would need
to notify the client if a particular reservation has to be terminated
due to expiration of a user's credentials or account balance.
Likewise, the client has to inform the server of a reservation
[Page 18]
A Framework for Policy-based Admission Control March 1999
rejection which is due to admission control failure.
* Handling of multicast groups: The protocol should provision for
handling of policy decisions related to multicast groups.
* QoS Specification: The protocol should allow for precise
specification of level of service requirements in the PEP requests
forwarded to the PDP.
7. Security Considerations
The communication tunnel between policy clients and policy servers should
be secured by the use of an IPSEC [4] channel. It is advisable that this
tunnel makes use of both the AH (Authentication Header) and ESP
(Encapsulating Security Payload) protocols, in order to provide
confidentiality, data origin authentication, integrity and replay
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] R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin, "Resource
ReSerVation Protocol (RSVP) -- Version 1 Functional Specification ", RFC
2205, September 1997.
[2] F. Baker., "RSVP Cryptographic Authentication", draft-ietf-rsvp-md5-
05.txt, August 1997.
[3] S. Herzog., "RSVP Extensions for Policy Control", Internet Draft},
draft-ietf-rsvp-policy-ext-03.[ps,txt], August 1998.
[4] R. Atkinson. Security Architecture for the Internet Protocol. RFC1825,
Aug. 1995.
[5] C. Rigney, A Rubens, W. Simpson and S. Willens. Remote Authentication
Dial In User Service (RADIUS). RFC 2138.
[6] R. Rajan et al. Schema for Differentiated Services and Integrated
Services in Networks, draft-rajan-policy-qosschema-00.txt, October 1998.
[Page 19]
A Framework for Policy-based Admission Control March 1999
[7] S. Herzog, "RSVP Preemption Priority Policy", Internet Draft, draft-
ietf-rap-priority-00.txt, Nov. 1998.
[8] S. Herzog, "COPS Usage for RSVP", Internet Draft, draft-ietf-rap-
cops-rsvp-00.txt, August 1998.
8. Acknowledgements
This is a result of an ongoing discussion among many members of the RAP
group including Jim Boyle, Ron Cohen, Laura Cunningham, Dave Durham, Shai
Herzog, Tim O'Malley, Raju Rajan, and Arun Sastry.
9. Authors` Addresses
Raj Yavatkar
Intel Corporation
2111 N.E. 25th Avenue,
Hillsboro, OR 97124
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
[Page 20]