Internet Engineering Task Force Raj Yavatkar, Intel
INTERNET-DRAFT Dimitrios Pendarakis, IBM
Roch Guerin, IBM
November 21, 1997
Expires: May 20, 1998
A Framework for Policy-based Admission Control
Status of this Memo
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
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Coast).
This document is a product of the RSVP Admission Policy (RAP) working
group in the Transport Area of the Internet Engineering Task Force.
Comments are
solicited and should be addressed to the working group's mailing list at
ipc@iphighway.com, and/or the author(s).
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INTERNET-DRAFTA Framework for Policy-based Admission Control Nov. 1997
Abstract
TO BE FILLED IN.
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INTERNET-DRAFTA Framework for Policy-based Admission Control Nov. 1997
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. 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 treat-
ment over other flows; the admission control component only takes into
account the requester's resource reservation request and available capa-
city to determine whether or not to accept a QoS request. However, the
int-serv mechanisms do not include an important aspect of admission con-
trol: network managers and service providers must be able to monitor, con-
trol, 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.
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. For
instance, it may be argued 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 ele-
ments 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 evalu-
ates existing solutions such as RADIUS and LDAP and discusses their appli-
cability to the problem of policy control.
2. Terminology
The following is a list of terms used in this document.
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- Administrative Domain: A collection of networks under the same
administrative control and grouped together for administrative pur-
poses.
- 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 ele-
ment of interest in our context.
In this document, sometimes we use the terms router, network ele-
ment, and network node interchangeably, but should be interpreted
as reference to a network element.
- QoS Signaling Protocol: A signaling protocol that carries an admis-
sion 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 ele-
ments and is carried by the QoS signaling protocol.
- 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 identifica-
tion whereas another policy element may carry user credentials or
credit card information. Examples of policy elements include iden-
tity 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.
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- Policy Enforcement Point (PEP): The point where the policy deci-
sions are actually enforced.
- Policy Ignorant Node (PIN): A network element that does not expli-
citly 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 instal-
ling 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 adminis-
trative 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
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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 as long as we do not
diverge from our central goal.
- Support for preemption: The mechanisms designed must include sup-
port 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 includ-
ing bi-lateral and multi-lateral service agreements and policies
based on the notion of relative priority. In general, the determi-
nation 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
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 mechan-
isms described in this document should not be mandatory for every
node in a network. Policy based admission control could be enforced
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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 poten-
tially 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 mul-
ticast 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 pol-
icy control can verify each other's identity and establish neces-
sary 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 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
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storage, etc. This document does not include discussion of these addi-
tional 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 deci-
sion 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 mes-
sage 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 ele-
ments. 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.
In some cases, the simple configuration shown in Figure 1 may not be suf-
ficient 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 confi-
gurations.
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 informa-
tion 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 | |
| |_____| | |_____| | |_____| |_____| |
| ^ | | |
| | _____ | |____________________|
| \-->| | |
| | LDP | |
| |_____| |
| |
|________________|
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 LDP to reach a local deci-
sion. This partial decision and the original policy request are next sent
to the PDP which renders a final decision (possibly, overriding the LDP).
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 mes-
sage (or local state) that describes the admission control request.
In addition, the request include appropriate policy elements to the
request 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 ele-
ments that are evaluated locally. The PEP passes the request with
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INTERNET-DRAFTA Framework for Policy-based Admission Control Nov. 1997
the set A to the Local Decision point (LDP) and collects the result
of the LDP (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 combina-
tion 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 interac-
tion and interfaces between RSVP and PEP will be described in a separate
document [3].
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______________________________
| |
| Router |
| ________ _____ | _____
| | | | | | | |
| | RSVP |<------->| PEP |<--|---------->| PDP |
| |________| |_____| | |_____|
| ^ |
| | Traffic control |
| | _____________ |
| \---->| _________ | |
| | |capacity | | |
| | | ADM CTL | | |
| | |_________| | |
--|----------->| ____ ____ | |
| Data | | PC | PS | | |
| | |____|____| | |
| |_____________| |
| |
|______________________________|
Figure 3: Relationship between PEP and other int-serv components
within an RSVP router. PC -- Packet Classifier, PS -- Packet Scheduler
When a RSVP message arrives at the router (or an RSVP related event
requires a policy decision), the RSVP module is expected to hand off the
request (corresponding to the event or message) to its PEP module. The PEP
will use the PDP (and LDP) 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." Basi-
cally, an ability to create policy-related errors and/or warnings and to
propagate them using the native QoS signaling protocol (such as RSVP) is
needed. Such a policy error returned by the PDP must be able to also
specify whether the reservation request should still be accepted,
installed, and forwarded to allow continued normal RSVP processing. In
particular, when a PDP sends back an error, it specifies that:
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INTERNET-DRAFTA Framework for Policy-based Admission Control Nov. 1997
1. the message that generated the adm control request should be pro-
cessed 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.
OPEN ISSUE -- What happens in case of the blockade state in RSVP?
4.3. Interactions between PEP, LDP, and PDP at a RSVP router
[NOTE: This section only reflects the ongoing discussion].
At the interim meeting held in October, the working group members present
discussed the issue of defining the interaction (and interfaces) among
different policy control components at a RSVP router. The discussion was
inconclusive and the following lists the items covered and the issues
raised for information purpose:
* PEP is invoked whenever RSVP events happen. Examples of events are
timeout, refresh events, a message arrives on an incoming interface
or is to be sent on an outgoing interface or reservations have to
be merged and installed or forwarded on an interface.
* PEP is responsible for notifying RSVP whenever asynch notifications
come from the PDP.
* PEP may cache the results returned by the PDP for later use to
avoid checking with the PDP every time. However, this raises an
interesting issue as the PEP must be able to detect changes to the
previously received policy elements/objects so that the PEP can
contact the PDP whenever changes occur.
* LDP is optional and may be used for making decisions based on pol-
icy elements handled locally. The LDP, in turn, may have to go to
external entities (such as a directory server or an authentication
server, etc.) for making its decisions.
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* 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: Examples of such information may include a
MIB.
* How or when to merge policy elements and default rules on merging?
In case of RSVP node without a PEP, what and if default behavior
for merging should be specified?
4.4. Placement of Policy Elements in a Network
By allowing division of labor between an LDP 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 pro-
vider 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 LDP, 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 LDP and a PDP locally,
at the router.
AD-1 AD-2 AD-3
________________/\_______________ __/\___ __/\___
{ } { } { }
A B C D E
+-------+ +-----+ +-------+ +-------+ +-------+
| RSVP | | RSVP| | RSVP | | RSVP | | RSVP |
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INTERNET-DRAFTA Framework for Policy-based Admission Control Nov. 1997
+----+ |-------| |-----| |-------| |-------| |-------|
| 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 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 tem-
poral 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 iden-
tity suppled in the request.
5.2. Bilateral agreements between service providers
Today, usage agreements between service providers for traffic
crossing their boundaries tend to be quite simple, for example, two
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ISPs might agree to accept all traffic from each other, often
without performing any accounting or billing for the ``foreign''
traffic carried. There are strong reasons to expect that such a
model cannot survive once traffic differentiation and quality of
service guarantees begin to be phased in the Internet. Once ISPs
start charging end users based on usage and quality of service, it
is only natural that they will also seek mechanisms for charging
each other for reservations transiting their networks. One addi-
tional 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. (ISSUE:
ISn't this an accounting issue among ISPs? We can only provide
mechanism to facilitate resolution of such issues, but not suggest
solutions.)
Bilateral agreements could fall into two broad categories. In the
first, 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 informa-
tion 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. Pro-
vider 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 message before
forwarding it to a neighboring provider's network. This policy
object will contain information such as the identity of the pro-
vider 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.
Bilateral agreements could also be established on a *global* scale.
In this second category, providers contract with an arbitrary
number of other providers, not necessarily neighboring. The objec-
tive of global agreements would be to provide a global network
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picture that allows a provider to satisfy (most) reservation
requests without reliance on third party agreements. This category
of agreements can also be supported in the context of the policy
control architecture. As above, the originating node includes a
policy object containing identity and account information in the
RSVP message; however, as long as global agreements are in place,
this object is not rewritten as the message crosses administrative
boundaries. Instead, each provider ``charges'' to the account the
incremental cost incurred in carrying the reservation request
through its network.
5.3. 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 identifica-
tion 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 suffi-
cient, 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
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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 respective reser-
vations. Since multiple entities are both reading (remaining credit)
and writing (decrementing credit) to the same database, some coordina-
tion 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.
5.4. 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 use-
ful 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 to reservation requests, and convey-
ing 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 prior-
ity 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
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priority and their proper operation. First, traffic control in the
router needs to be aware of priorities, i.e., classify existing reserva-
tions 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 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 order-
ing, and therefore merging, difficult. Two possible cases can be iden-
tified:
1. (FLOWSPEC1, priority1)= (high, high);
(FLOWSPEC2, priority2)= (low, low)
2. (FLOWSPEC1, priority1) = (high, low);
(FLOWSPEC2, priority2) = (low, high)
In the first case, the ordering is immediate, i.e.,
(FLOWSPEC1, priority1) >= (FLOWSPEC2, priority2),
so that the merged value can readily be chosen as (FLOWSPEC1, prior-
ity1). However, in the second case, no obvious ordering is available,
and each possible merged combination introduces a problem of its own
which we briefly review.
2.a (FLOWSPEC_OUT, priority_OUT) = (high, high) = {FLOWSPEC1, prior-
ity2}
The main problem with this selection is the so-called "free-rider"
problem, in that the high bandwidth requirement of a low priority
request is now entitled to the higher priority that was initially
granted to a low bandwidth request. Such "upgrades" can all but dis-
able the ability of priorities to give preference to certain flows,
and are therefore not acceptable.
2.b (FLOWSPEC_OUT, priority_OUT) = (high, low) = (FLOWSPEC1,
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priority1)
This selection amounts to (initially) giving preference to high
bandwidth requests, irrespective of their priority. The main benefit
is that larger reservations are allowed to try and succeed in case the
resources are available. The main disadvantage is that if a high
FLOWSPEC and low priority reservation is preempted, the entire reser-
vation is taken down. As a result, a high priority and low FLOWSPEC
reservation is left without resources until a new reservation is ulti-
mately reestablished at the higher priority level.
2.c (FLOWSPEC_OUT, priority_OUT) = (low, high) = (FLOWSPEC2, prior-
ity2)
This is the opposite selection to 2.b, and as a result, it has the
opposite disadvantages and benefits. Specifically, a low FLOWSPEC but
high priority request can preclude higher FLOWSPEC but lower priority
requests, *irrespective* of how lightly loaded the network is. On the
other hand, the high priority request is *guaranteed* unperturbed ser-
vice unless it itself needs to be preempted.
2.d (FLOWSPEC_OUT, priority_OUT) = (low, low)
This last option is mentioned for completeness, but is not particu-
larly meaningful as it offers no benefits and only disadvantages.
As can be seen from the above discussion, support for priorities and the
associated merging operations raises a number issues. There are possi-
ble solutions to most of the above problems, e.g.,
- Disable or limit the amount of merging that can take place, - Select
one of the above rules 2.b or 2.c based on the relative difference
between the FLOWSPEC being merged,
but it is likely that different environments (set of criteria) may man-
date different selections. In addition, it may be desirable to extend
the concept of priorities to let applications specify both holding and
preemption priorities, with the former being always greater than the
latter. The preemption priority would be a function of the urgency of
satisfying the reservation request being made, while the holding prior-
ity would reflect the application's tolerance to being interrupted.
In general, support for priority provides a powerful tool to manage
access to network resources, and should represent one of the key bene-
fits of the introduction of policy support. However, rules determining
how priorities are to be handled may differ, and likely to be specified
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in the context of each policy supporting priorities. (OPEN ISSUE:
Should we specify default merging rules?)
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 use-
ful 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 satis-
fies 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, car-
ried 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. Evaluation of Existing Frameworks or Solutions
6.1. RADIUS
The Remote Authentication Dial In User Service (RADIUS) [REF] protocol,
specifies how authentication, authorization and configuration informa-
tion is exchanged between a network access server wishing to authenti-
cate its users and a (shared) authentication server. RADIUS has been
designed to operate at the level of a * session}, which is defined as
the interval between the time that service is first provided and the
time service is ended. The latter definition is geared towards login-
type service, for example network access for dial up users through PPP.
RADIUS is an important protocol for authenticating login users. However,
some of its characteristics make it unsuitable for use, at least in its
current form, in the context of policy control for QoS reservations:
* Use of the UDP protocol: The sensitivity of policy control
information requires reliable operation. Use of UDP would mandate
specifying retry and fallback algorithms, which can be quite
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complicated. In addition, while timing requirements for a user
logging onto a network might permit a delay of several seconds
([REF]), this will not be the case for reservations proceeding in
the network. Moreover, acknowledgments are essential for actions
like billing and accounting.
* Difficulty in carrying opaque objects: Use of RADIUS for QoS pol-
icy control would require defining additional RADIUS * attri-
butes. To carry opaque objects of variable length, the format of
RADIUS attributes would have to be extended; it currently sup-
ports four data types of pre-specified length; string, address,
integer and time.
* No support for asynchronous notification: Asynchronous notifica-
tion 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 expira-
tion 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.
* Restricted Message Types: The restriction to messages of type
``access-request'' and ``access-accept'' (or reject) does not
facilitate information gathering for monitoring and accounting
purposes. New message types would have to be defined.
* Multicast Groups: It is not clear how requests from a group of
multicast receivers could be handled in the context of RADIUS.
* Identifying Requests: Several changes will be needed in the way
requests are identified. Currently this is done using a user
name/IP address attribute. This might be too narrow for reserva-
tion protocols, i.e. one might need to specify a source port, a
destination address/port and provide some user credentials as
well.
* QoS Specification: Changes will be needed to incorporate the new
service (resource reservation) and the ability to specify the
desired level of service (QoS, Tspec)
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In short, we believe that if the RADIUS protocol were to be used for the
purpose of QoS policy control, the number of changes required would make
the task as difficult or more than defining a new protocol designed with
QoS reservations in mind.
6.2. LDAP
LDAP is very effective as a directory service protocol. Nevertheless, it
restricts the flexibility of an implementation by requiring a very
specific and well understood format of policy information and policy
rules, common to both a policy server and a client (e.g., a standardized
schema). The client must be able to interpret the format of policy
information and rules directly. In addition, LDAP is more suitable for
accessing directory information, information that is predefined and
changes infrequently.
Given the rather experimental state of policy control for QoS reserva-
tion protocols, the dynamic nature of policy decisions and rules and the
limitations of clients (routers) in interpreting policy information, an
LDAP based solution for client-server communication would be inadequate
at this point. Also, a wideer variety of policy element types and
objects need to be represented using a policy control protocol than sup-
ported by LDAP.
However, LDAP could conceivably be used in downloading policy rules from
an LDAP server to policy servers. Furthermore, future versions of LDAP
promise to make it more suitable for the exchange of dynamic informa-
tion. As these modifications are introduced the role of LDAP in QoS pol-
icy control should be reevaluated.
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 con-
fidentiality, data origin authentication, integrity and replay preven-
tion.
In the case of the RSVP signaling mechanism, RSVP MD5 [2] message
authentication can be used to secure communications between network ele-
ments.
8. References
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[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-02.[ps,txt], Apr. 1997.
[4] R. Atkinson. Security Architecture for the Internet Protocol.
RFC1825, Aug. 1995.
[5] C. Rigney, A Rubens, W. Simpson and S. Willens. Remote Authentica-
tion Dial In User Service (RADIUS). RFC 2138.
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: yavatkar@ibeam.intel.com
Dimitrios Pendarakis
IBM T.J. Watson Research Center
P.O. Box 704
Yorktown Heights
NY 10598
phone: +1 914-784-7536
email: dimitri@watson.ibm.com
Roch Guerin
IBM T.J. Watson Research Center
P.O. Box 704
Yorktown Heights
NY 10598
phone: +1 914-784-7038
email: guerin@watson.ibm.com
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