Network Working Group S. Govindan, Ed.
Request for Comments: 4564 H. Cheng
Category: Informational Panasonic
ZH. Yao
Huawei
WH. Zhou
China Mobile
L. Yang
Intel
July 2006
Objectives for
Control and Provisioning of Wireless Access Points (CAPWAP)
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 (2006).
Abstract
This document presents objectives for an interoperable protocol for
the Control and Provisioning of Wireless Access Points (CAPWAP). The
document aims to establish a set of focused requirements for the
development and evaluation of a CAPWAP protocol. The objectives
address architecture, operation, security, and network operator
requirements that are necessary to enable interoperability among
Wireless Local Area Network (WLAN) devices of alternative designs.
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Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Requirements Notation ...........................................4
4. Objectives Overview .............................................4
5. Objectives ......................................................5
5.1. Mandatory and Accepted Objectives ..........................5
5.1.1. Logical Groups ......................................5
5.1.2. Support for Traffic Separation ......................6
5.1.3. Wireless Terminal Transparency ......................8
5.1.4. Configuration Consistency ...........................8
5.1.5. Firmware Trigger ....................................9
5.1.6. Monitoring and Exchange of System-wide
Resource State .....................................10
5.1.7. Resource Control Objective .........................11
5.1.8. CAPWAP Protocol Security ...........................12
5.1.9. System-wide Security ...............................14
5.1.10. IEEE 802.11i Considerations .......................15
5.1.11. Interoperability Objective .......................17
5.1.12. Protocol Specifications ..........................18
5.1.13. Vendor Independence ..............................19
5.1.14. Vendor Flexibility ...............................19
5.1.15. NAT Traversal ....................................20
5.2. Desirable Objectives ......................................21
5.2.1. Multiple Authentication Mechanisms .................21
5.2.2. Support for Future Wireless Technologies ...........21
5.2.3. Support for New IEEE Requirements ..................22
5.2.4. Interconnection Objective ..........................23
5.2.5. Access Control ....................................24
5.3. Non-Objectives ............................................25
5.3.1. Support for Non-CAPWAP WTPs ........................25
5.3.2. Technical Specifications ...........................26
5.4. Operator Requirements .....................................27
5.4.1. AP Fast Handoff ....................................27
6. Summary and Conclusion .........................................27
7. Security Considerations ........................................28
8. Acknowledgements ...............................................29
9. Normative References ...........................................29
10. Informative References ........................................29
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1. Introduction
The growth in large-scale Wireless Local Area Network (WLAN)
deployments has brought into focus a number of technical challenges.
Among them is the complexity of managing large numbers of Wireless
Termination Points (WTPs), which is further exacerbated by variations
in their design. Another challenge is the maintenance of consistent
configurations among the numerous WTPs of a system. The dynamic
nature of the wireless medium is also a concern together with WLAN
security. The challenges affecting large-scale WLAN deployments have
been highlighted in [RFC3990].
Many vendors have addressed these challenges by developing new
architectures and solutions. A survey of the various developments
was conducted to better understand the context of these challenges.
This survey is a first step towards designing interoperability among
the solutions. The Architecture Taxonomy [RFC4118] is a result of
this survey in which major WLAN architecture families are classified.
Broadly, these are the autonomous, centralized WLAN, and distributed
mesh architectures.
The Architecture Taxonomy identified the centralized WLAN
architecture as one in which portions of the wireless medium access
control (MAC) operations are centralized in a WLAN controller. This
centralized WLAN architecture is further classified into remote-MAC,
split-MAC, and local-MAC designs. Each differs in the degree of
separation of wireless MAC layer capabilities between WTPs and WLAN
controller.
This document puts forward critical objectives for achieving
interoperability in the CAPWAP framework. It presents requirements
that address the challenges of controlling and provisioning large-
scale WLAN deployments. The realization of these objectives in a
CAPWAP protocol will ensure that WLAN equipment of major design types
may be integrally deployed and managed.
2. Terminology
This document uses terminology defined in [RFC4118], [802.11],
[802.11i], and [802.11e]. Additionally, the following terms are
defined.
Centralized WLAN: A WLAN based on the centralized WLAN Architecture
[RFC4118].
Switching Segment: Those aspects of a centralized WLAN that primarily
deal with switching or routing of control and data information
between Wireless Termination Points (WTPs) and the WLAN controller.
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Wireless Medium Segment: Those aspects of a centralized WLAN that
primarily deal with the wireless interface between WTPs and wireless
terminals. The Wireless Medium Segment is specific to layer 2
wireless technology, such as IEEE 802.11.
CAPWAP Framework: A term that covers the local-MAC and split-MAC
designs of the Centralized WLAN Architecture. Standardization
efforts are focused on these designs.
CAPWAP Protocol: The protocol between WLAN controller and WTPs in the
CAPWAP framework. It facilitates control, management, and
provisioning of WTPs in an interoperable manner.
Logical Group: A logical separation of a physical WTP is termed
logical group. So a single physical WTP will operate a number of
logical groups. Virtual access points (APs) are examples of logical
groups. Here, each Basic Service Set Identifier (BSSID) and
constituent wireless terminals' radios are denoted as distinct
logical groups of a physical WTP. Logical groups are maintained
without conflicting with the CAPWAP objectives, particularly the
'Wireless Terminal Transparency' objective.
3. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
4. Objectives Overview
The objectives for CAPWAP have been broadly classified to address
architecture, operation, and security requirements of managing
large-scale WLAN deployments.
Architecture objectives deal with system-level aspects of the CAPWAP
protocol. They address issues of protocol extensibility, diversity
in network deployments and architecture designs, and differences in
transport technologies.
Operational objectives address the control and management features of
the CAPWAP protocol. They deal with operations relating to WLAN
monitoring, resource management, Quality of Service (QoS), and access
control.
Security objectives address potential threats to WLANs and their
containment. In the CAPWAP context, security requirements cover the
protocol between the WLAN controller and WTPs and also the WLAN
system as a whole.
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Additionally, a general classification is used for objectives
relating to the overall impact of the CAPWAP protocol specifications.
5. Objectives
The objectives described in this document have been prioritized based
on their immediate significance in the development and evaluation of
a control and provisioning protocol for large-scale WLAN deployments.
The priorities are:
i. Mandatory and Accepted Objectives
ii. Desirable Objectives
iii. Non-Objectives
The priorities have been assigned to individual objectives in
accordance with working group discussions.
Furthermore, a distinct category of objectives is provided based on
requirements gathered from network service operators. These are
specific needs that arise from operators' experiences in deploying
and managing large-scale WLANs.
a. Operator Requirements
5.1. Mandatory and Accepted Objectives
Objectives prioritized as mandatory and accepted have been deemed
crucial for the control and provisioning of WTPs. They directly
address the challenges of large-scale WLAN deployments and MUST be
realized by a CAPWAP protocol.
5.1.1. Logical Groups
Classification: Architecture
Description:
Large WLAN deployments are complex and expensive. Furthermore,
enterprises deploying such networks are under pressure to improve the
efficiency of their expenditures.
Shared WLAN deployments, where a single physical WLAN infrastructure
supports a number of logical networks, are increasingly used to
address these two issues of large-scale WLANs. These are popular as
they allow deployment and management costs to be spread across
businesses.
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In traditional WLANs, each physical WTP represents one complete
subset of a larger WLAN system. Shared WLANs differ in that each
physical WTP represents a number of logical subsets of possibly a
number of larger WLAN systems. Each logical division of a physical
WTP is referred to as a logical group (see definition in Section 2).
So WLANs are managed in terms of logical groups instead of physical
WTPs. Logical groups are based on BSSIDs and other types of virtual
APs.
Protocol Requirement:
The CAPWAP protocol MUST be capable of controlling and managing
physical WTPs in terms of logical groups including BSSID-based
groups.
For all operating modes, including those in which the WTP performs
local bridging and those in which the Access Controller (AC) performs
centralized bridging, the protocol MUST provide provisions for
configuring logical groups at the WTP.
Motivation and Protocol Benefits:
Commercial realities necessitate that WLANs be manageable in terms of
their logical groups. This allows separation of logical services and
underlying infrastructure management. A protocol that realizes this
need ensures simpler and cost-effective WLANs, which directly address
the requirements of network service operators.
Relation to Problem Statement:
This objective addresses the problem of management complexity in
terms of costs. Cost complexity is reduced by sharing WLAN
deployments. Consequently, deployment and management cost-
efficiencies are realized.
5.1.2. Support for Traffic Separation
Classification: Operations
Description:
The centralized WLAN architecture simplifies complexity associated
with large-scale deployments by consolidating portions of wireless
MAC functionality at a central WLAN controller and distributing the
remaining across WTPs. As a result, WTPs and WLAN controller
exchange control and data information between them. This objective
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states that control and data aspects of the exchanges be mutually
separated for further simplicity. This will allow solutions for each
type of exchange to be independently optimized.
Furthermore, in the context of shared WLAN deployments, the mutual
separation of control and data also addresses security concerns. In
particular, given the likelihood of different logical groups, such as
those established by different virtual APs, being managed by
different administrators, separation of control and data is a first
step towards individually containing and securing the logical groups.
It is also important to ensure that traffic from each logical group
is mutually separated to maintain the integrity and independence of
the logical groups.
Protocol Requirement:
The CAPWAP protocol MUST define transport control messages such that
the transport of control messages is separate from the transport of
data messages.
Motivation and Protocol Benefits:
The aim of separating data and control aspects of the protocol is to
simplify the protocol. It also allows for the flexibility of
addressing each type of traffic in the most appropriate manner.
Furthermore, this requirement will help remotely located WTPs to
handle data traffic in alternative ways without the need for
forwarding them across a wide network to the WLAN controller.
Separation of WTP control and data also aids in the secure
realization of shared WLAN deployments.
Relation to Problem Statement:
Broadly, this objective relates to the challenge of managing
complexity in large-scale WLANs. The requirement for traffic
separation simplifies control as this is separated from the task of
data transport.
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5.1.3. Wireless Terminal Transparency
Classification: Operations
Description:
The CAPWAP protocol is applicable between a centralized WLAN
controller and a number of WTPs; i.e., it affects only the switching
segment of the centralized WLAN architecture. Its operations should
therefore be independent of the wireless terminal. Wireless
terminals should not be required to be aware of the existence of the
CAPWAP protocol.
Protocol Requirement:
Wireless terminals MUST NOT be required to recognize or be aware of
the CAPWAP protocol.
Motivation and Protocol Benefits:
IEEE 802.11-based wireless terminals are mature and widely available.
It would be beneficial for CAPWAP not to impose new requirements on
these wireless terminals. In effect, this requirement ensures that
the setup cost of the protocol is reduced as the numerous existing
wireless terminals need not be altered.
Relation to Problem Statement:
The Problem Statement highlights the challenges faced by large WLANs
consisting of many WTPs. It does not refer to the operations of
wireless terminals and this objective emphasizes the independence.
5.1.4. Configuration Consistency
Classification: Operations
Description:
WLANs in the CAPWAP framework contain numerous WTPs, each of them
needing to be configured and managed in a consistent manner. The
main concern in ensuring consistency is availability of appropriate
information corresponding to WTP configuration states. So
configuration consistency can be achieved by providing the
centralized WLAN controller with regular updates on the state of WTP
operations. The centralized WLAN controller can in turn apply
information from the regular updates to ensure consistently among the
WTPs.
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Protocol Requirement:
The CAPWAP protocol MUST include support for regular exchanges of
state information between WTPs and the WLAN controller. Examples of
state information include WTP processing load and memory utilization.
Motivation and Protocol Benefits:
A protocol that provides access to regular state information can in
turn be used to enhance WLAN configuration and performance. The
CAPWAP protocol will be better equipped to address configuration-
related problems with the regularly available state information. So
with greater state information, control and management operations can
be improved.
Relation to Problem Statement:
One of the major challenges described in the Problem Statement is
that of maintaining consistent configuration across the numerous WTPs
of a WLAN. This objective addresses the fundamental issue behind
this -- availability of timely state information.
5.1.5. Firmware Trigger
Classification: Operations
Description:
One specific aspect of configuration consistency is the firmware used
by various WTPs. The scale of large WLANs introduces possibilities
for variations in the firmware used among WTPs. This objective
highlights the need for the CAPWAP protocol to trigger the delivery
of appropriate versions of firmware to WTPs. The actual delivery of
firmware need not be inclusive to the protocol.
Protocol Requirement:
The CAPWAP protocol MUST support a trigger for delivery of firmware
updates.
Motivation and Protocol Benefits:
The CAPWAP protocol interfaces many WTPs to a centralized WLAN
controller. Firmware distribution allows these interfaces to be
compatible. This in turn results in consistent configuration and
simplified management. So the protocol benefits by including
triggers for the distribution of firmware updates.
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Relation to Problem Statement:
Inconsistencies in the configuration of WTPs have been identified as
a major challenge for large-scale WTPs. This objective helps
overcome the challenge by providing a way for the CAPWAP protocol to
initiate delivery of firmware updates that are compatible among all
WTPs.
5.1.6. Monitoring and Exchange of System-wide Resource State
Classification: Operations
Description:
The centralized WLAN architecture is made up of a switching segment
and wireless medium segment. In the switching segment, network
congestion, WTP status, and firmware information have to be
monitored. In the wireless medium segment, the dynamic nature of the
medium itself has to be monitored. Overall, there are also various
statistics that need to be considered for efficient WLAN operation.
The CAPWAP protocol should be capable of monitoring the various
information sources and deliver the resulting information to the
relevant WLAN devices -- either WTPs or the WLAN controller.
Moreover, given the relationship among information sources, the
CAPWAP protocol should combine state information from them. For
example, statistics information and status signals from WTPs may be
merged before being exchanged.
Examples of statistics information that the CAPWAP protocol should
monitor and exchange include congestion state, interference levels,
loss rates, and various delay factors.
Protocol Requirement:
The CAPWAP protocol MUST allow for the exchange of statistics,
congestion, and other WLAN state information.
Motivation and Protocol Benefits:
The effectiveness of a protocol is based on the relevance of
information on which it operates. This requirement for resource
monitoring and exchange can provide the appropriate information to
the CAPWAP protocol.
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Relation to Problem Statement:
The Problem Statement highlights the challenge of dealing with large
numbers of WTPs and the dynamic nature of the wireless medium.
Information on the state of WTPs and the medium is important to deal
with them effectively. So this objective relates to the problem of
managing consistency in large WLANs.
5.1.7. Resource Control Objective
Classification: Operations
Description:
Integral to the success of any wireless network system is the
performance and quality it can offer its subscribers. Since CAPWAP-
based WLANs combine a switching segment and a wireless medium
segment, performance and quality need to be coordinated across both
of these segments. So QoS performance must be enforced system-wide.
This objective highlights QoS over the entire WLAN system, which
includes the switching segment and the wireless medium segment.
Given the fundamental differences between the two, it is likely that
there are alternate QoS mechanisms between WTPs and wireless service
subscribers and between WTPs and WLAN controllers. For instance, the
former will be based on IEEE 802.11e, whereas the latter will be an
alternative. So resources need to be adjusted in a coordinated
fashion over both segments. The CAPWAP protocol should ensure that
these adjustments are appropriately exchanged between WLAN
controllers and WTPs.
In addition to IEEE 802.11e, there are a number of other IEEE 802.11
task groups that may affect network resources. These include IEEE
802.11 TGk, TGu, and TGv, which are currently in progress. CAPWAP
should therefore not be restricted to IEEE 802.11e-based mapping.
Protocol Requirement:
The CAPWAP protocol MUST map the IEEE 802.11e QoS priorities to
equivalent QoS priorities across the switching and wireless medium
segments.
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Motivation and Protocol Benefits:
A protocol that addresses QoS aspects of WLAN systems will deliver
high performance thereby being beneficial for subscribers and for
resource utilization efficiency. Since CAPWAP deals with WTPs
directly and with the wireless medium indirectly, both of these must
be considered for performance.
For the wireless medium segment, QoS aspects in the protocol enable
high-quality communications within the domain of a WLAN controller.
Since each domain generally covers an enterprise or a group of
service providers, such protocol performance has wide-ranging
effects.
Within the switching segment of CAPWAP, a QoS-enabled protocol
minimizes the adverse effects of dynamic traffic characteristics so
as to ensure system-wide performance.
Relation to Problem Statement:
QoS control is critical to large WLANs and relates to a number of
aspects. In particular, this objective can help address the problem
of managing dynamic conditions of the wireless medium.
Furthermore, traffic characteristics in large-scale WLANs are
constantly varying. So network utilization becomes inefficient, and
user experience is unpredictable.
The interaction and coordination between the two aspects of system-
wide QoS are therefore critical for performance.
5.1.8. CAPWAP Protocol Security
Classification: Security
Description:
This objective addresses the security of the CAPWAP protocol.
The CAPWAP protocol MUST first provide for the participating entities
-- the WLAN controller and WTPs -- to be explicitly mutually
authenticated. This is to ensure that rogue elements do not gain
access to the WLAN system. Rogue WTPs should not be allowed to
breach legitimate WLANs, and at the same time rogue WLAN controllers
should not be allowed to gain control of legitimate WTPs. For
example, WTPs may need to regularly renew their authentication state
with the WLAN controller and similarly for WLAN controllers.
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If authentication is performed via an authenticated key exchange,
future knowledge of derived keys is not sufficient for
authentication.
Any session keys used between the WLAN controller and WTPs MUST be
mutually derived using entropy contributed by both parties. This
ensures that no one party has control over the resulting session
keys.
Once WTPs and the WLAN controller have been mutually authenticated,
information exchanges between them must be secured against various
security threats. So the CAPWAP protocol MUST provide integrity
protection and replay protection. The protocol SHOULD provide
confidentiality through encryption. This should cover illegitimate
modifications to protocol exchanges, eavesdropping, and Denial of
Service (DoS) attacks, among other potential compromises. So the
protocol must provide confidentiality, integrity, and authenticity
for those exchanges.
As a result of realizing this objective, it should not be possible
for individual WTP breaches to affect the security of the WLAN as a
whole. So WTP misuse will be protected against.
Additionally, the key establishment protocol for authentication and
securing CAPWAP exchanges must be designed to minimize the
possibility of future compromises after the keys are established.
CAPWAP MUST NOT prevent the use of asymmetric authentication. The
security considerations of such asymmetric authentication are
described in the Security Considerations section.
If the CAPWAP protocol meets the criteria to require automated key
management per BCP 107 [RFC4107], then mutual authentication MUST be
accomplished via an authenticated key exchange.
Protocol Requirement:
The CAPWAP protocol MUST support mutual authentication of WTPs and
the centralized controller. It also MUST ensure that information
exchanges are integrity protected and SHOULD ensure confidentiality
through encryption.
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Motivation and Protocol Benefits:
WLANs are increasingly deployed in critical aspects of enterprise and
consumer networks. In these contexts, protocol security is crucial
to ensure the privacy and integrity expected from network
administrators and end-users. So securing the CAPWAP protocol has
direct benefits in addressing these concerns.
In many cases, the network path between a WTP and WLAN controller
contains untrusted links. Such links could be leveraged by rogue
WTPs to gain access to the WLAN system. They could also be used by
rogue WLAN controllers to gain control of legitimate WTPs and their
associated terminals to either redirect or compromise terminal
traffic. These security concerns can be mitigated with this
objective.
Relation to Problem Statement:
Security problems in large-scale WLANs are detailed in the Problem
Statement. These include complications arising from rogue WTPs and
compromised interfaces between WTPs and the WLAN controller. The
requirement for protocol security addresses these problems and
highlights the importance of protecting against them.
5.1.9. System-wide Security
Classification: Security
Description:
The emphasis of this objective is on the security threats external to
the centralized CAPWAP segment of a WLAN system. The focus is
therefore on rogue wireless clients and other illegitimate wireless
interferences. There are a number of specific external threats that
need to be addressed within the CAPWAP framework.
i. PMK Sharing
One aspect of this objective relates to recent discussions on
Pairwise Master Key (PMK) sharing in the CAPWAP framework. This
objective highlights the need to prevent exploitation of this
ambiguity by rogue wireless clients. It is to ensure that any
ambiguities arising from the CAPWAP framework are not cause for
security breaches.
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Protocol Requirement:
The design of the CAPWAP protocol MUST NOT allow for any compromises
to the WLAN system by external entities.
Motivation and Protocol Benefits:
The external threats to the centralized WLAN architecture become
increasingly crucial given the low cost of wireless clients. Since
it is relatively inexpensive for rogue individuals to mount attacks,
it is important that WLAN systems are protected against them.
Adequate mechanisms to thwart such external threats will be of
tremendous benefit to the WLAN systems controlled and managed with
the CAPWAP protocol.
Relation to Problem Statement:
This objective is based on the security needs highlighted in the
Problem Statement. Specifically, the Problem Statement discusses the
effects of the shared wireless medium. This represents the external
aspects of the CAPWAP framework from which certain threats can arise.
The system-wide security objective addresses such threats in relation
to the Problem Statement.
5.1.10. IEEE 802.11i Considerations
Classification: Operations
Description:
The CAPWAP protocol must support authentication in the centralized
WLAN architecture in which the authenticator and encryption points
can be located on distinct entities, i.e., WLAN controller or WTP.
The Architecture Taxonomy illustrates a number of variants, in both
local-MAC and split-MAC designs, in which the authenticator is
located at the WLAN controller and the encryption points are at the
WTPs. The CAPWAP protocol must be applicable to these variants and
allow authentication mechanisms and their constituent processes to be
operable in these cases.
An important issue to consider in this case is the exchange of key
information when authenticator and encryption points are located on
distinct entities. For example, consider the case where IEEE 802.11i
is used in a WLAN in which the WLAN controller realizes the
authenticator, some WTPs realize encryption (possibly local-MAC
WTPs), and other WTPs rely on the WLAN controller for encryption
(possibly split-MAC WTPs).
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Here, CAPWAP will first need to identify the location of the
authenticator and encryption points between each WLAN controller-WTP
pair. This will likely be part of the initial WTP configuration.
Subsequently, the WTPs that realize encryption will need CAPWAP to
exchange key information with the authenticator at the WLAN
controller. For the WTPs that do not realize encryption, CAPWAP
needs to adapt its control to bypass the key exchange phase.
Clearly, the centralized WLAN architecture presents a different
platform for authentication mechanisms compared to legacy WLANs in
which a WTP realized both authenticator and encryption roles. So
this objective highlights the need for CAPWAP to support
authentication and key management in the centralized WLAN
architecture.
Protocol Requirement:
The CAPWAP protocol MUST determine the exact structure of the
centralized WLAN architecture in which authentication needs to be
supported, i.e., the location of major authentication components.
This may be achieved during WTP initialization where major
capabilities are distinguished.
The protocol MUST allow for the exchange of key information when
authenticator and encryption roles are located in distinct entities.
Motivation and Protocol Benefits:
The immediate focus of CAPWAP is on supporting IEEE 802.11-based
WLANs. As such, it is necessary for the protocol to recognize the
major distinction in WLAN design with respect to IEEE 802.11i
authenticator and encryption points. This represents a significant
variation that has been highlighted in the Architecture Taxonomy.
The CAPWAP protocol benefits by accommodating such a major
consideration from IEEE 802.11i.
These requirements will be common for all authentication mechanisms
over the centralized WLAN architecture. So they are applicable to
IEEE 802.11i, Universal Access Method (UAM), and other mechanisms.
Relation to Problem Statement:
The Problem Statement highlights the availability of different WTP
designs and the need to ensure interoperability among them. In this
regard, operational changes occurring due to the separation of the
IEEE 802.11i authenticator and encryption points need to be
accommodated within the CAPWAP protocol.
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5.1.11. Interoperability Objective
Classification: Architecture
Description:
Two major designs of the centralized WLAN architecture are local-MAC
and split-MAC. With the focusing of standardization efforts on these
two designs, it is crucial to ensure mutual interoperation among
them.
This objective for the CAPWAP protocol is to ensure that WTPs of both
local-MAC and split-MAC architecture designs are capable of
interoperation within a single WLAN. Consequently, a single WLAN
controller will be capable of controlling both types of WTPs using a
single CAPWAP protocol. Integral support for these designs comprises
a number of protocol aspects.
i. Capability negotiations between WLAN controller and WTPs
WTP designs differ in the degree of IEEE 802.11 MAC functionalities
that each type of WTP realizes. The major distinctions, split-MAC
and local-MAC, differ in the processing of IEEE 802.11 MAC frames.
In this regard, the CAPWAP protocol should include functionality that
allows for negotiations of significant capabilities between WTPs and
the WLAN controller.
As a first step, such negotiations could cover the type of WTP,
split-MAC or local-MAC, as this provides substantial information on
their respective capabilities.
ii. Establishment of alternative interfaces
The capability differences among different WTPs essentially equate to
alternative interfaces with a WLAN controller. So the CAPWAP
protocol should be capable of adapting its operations to the major
different interfaces. In a first case, this would include
accommodating capability differences between local-MAC and split-MAC
WTPs.
The definition of these interfaces in terms of finer granularity of
functionalities will be based on AP functionality documents produced
by the IEEE 802.11 AP Functionality (APF) Ad-Hoc Committee.
Protocol Requirement:
The CAPWAP protocol MUST include sufficient capabilities negotiations
to distinguish between major types of WTPs.
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Motivation and Protocol Benefits:
The benefits of realizing this architecture objective are both
technical and practical. First, there are substantial overlaps in
the control operations of local-MAC and split-MAC architecture
designs. The Architecture Taxonomy tabulates major common features
of the two designs. As a result, it is technically practical to
devise a single protocol that manages both types of devices.
Next, the ability to operate a CAPWAP protocol for both types of
architectural designs enhances its practical prospects as it will
have wider appeal.
Furthermore, the additional complexity resulting from such
alternative interfaces is marginal. Consequently, the benefits of
this objective will far outweigh any cost of realizing it.
Relation to Problem Statement:
The objective for supporting both local-MAC and split-MAC WTPs is
fundamental to addressing the Problem Statement. It forms the basis
for those problems to be uniformly addressed across the major WLAN
architectures. This is the ultimate aim of standardization efforts.
The realization of this objective will ensure the development of a
comprehensive set of mechanisms that address the challenges of
large-scale WLAN deployments.
5.1.12. Protocol Specifications
Classification: General
Description:
WLAN equipment vendors require sufficient details from protocol
specifications so that implementing them will allow for compatibility
with other equipment that runs the same protocol. In this light, it
is important for the CAPWAP protocol specifications to be reasonably
complete for realization.
Protocol Requirement:
Any WTP or WLAN controller vendor or any person MUST be able to
implement the CAPWAP protocol from the specification itself and by
that it is required that all such implementations do interoperate.
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Motivation and Protocol Benefits:
It is beneficial for WLAN equipment vendors to refer to a single set
of specifications while implementing the CAPWAP protocol. This helps
to ease and quicken the development process.
Relation to Problem Statement:
This requirement is based on WG discussions that have been determined
to be important for CAPWAP.
5.1.13. Vendor Independence
Classification: General
Description:
Rapid developments in WLAN technologies result in equipment vendors
constantly modifying their devices. In many cases, developments are
independently made for WLAN controllers and WTPs. The CAPWAP
protocol should not affect the independence of device modifications.
Protocol Requirement:
A WTP vendor SHOULD be able to make modifications to hardware without
any WLAN controller vendor involvement.
Motivation and Protocol Benefits:
Independence in the type of hardware for WLAN equipment ensures that
new developments do not hamper protocol operation.
Relation to Problem Statement:
This requirement is based on WG discussions that have been determined
to be important for CAPWAP.
5.1.14. Vendor Flexibility
Classification: General
Description:
The CAPWAP protocol must not be specified for a particular type of
wireless MAC design. It should be compatible with both local-MAC and
split-MAC WTPs.
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Protocol Requirement:
The CAPWAP protocol MUST NOT limit WTP vendors in their choice of
local-MAC or split-MAC WTPs. It MUST be compatible with both types
of WTPs.
Motivation and Protocol Benefits:
This requirement is to ensure that WTP vendors have sufficient
flexibility in selecting the type of wireless MAC design that they
consider best for deployments.
Relation to Problem Statement:
This requirement is based on WG discussions that have been determined
to be important for CAPWAP.
5.1.15. NAT Traversal
Classification: General
Description:
WLAN deployments may involve WTPs and the WLAN controller
communicating across Network Address Translators (NATs). The CAPWAP
protocol must be capable of operating across topologies that contain
known NAT configurations. It requires appropriate discovery and
identification mechanisms for NAT traversal.
Protocol Requirement:
The CAPWAP protocol MUST NOT prevent the operation of established
methods of NAT traversal.
Motivation and Protocol Benefits:
The widespread adoption of WLANs raises the possibility for WLAN
topologies containing NATs. It is important for the CAPWAP protocol
to be applicable within such topologies. This requirement aims to
make the CAPWAP protocol relevant for NAT traversal.
Relation to Problem Statement:
This requirement is based on WG discussions that have been determined
to be important for CAPWAP.
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5.2. Desirable Objectives
These objectives have been determined to be desirable for a CAPWAP
protocol but not mandatory. Realizing these objectives may help
improve control of WLANs but need not necessarily be required for all
networks or scenarios.
5.2.1. Multiple Authentication Mechanisms
Classification: Architecture
Description:
Shared WLAN infrastructure raises the issue of multiple
authentication mechanisms. This is because each logical group is
likely to be associated with different service providers or WLAN
domains. As a result, the authentication needs within them will be
different. Although CAPWAP is required to support IEEE 802.11i, it
is also necessary for it to support other authentication mechanisms.
For example, one logical group may use IEEE 802.11i, whereas another
may use web authentication. CAPWAP must be able to operate in such
shared WLANs.
Protocol Requirement:
The CAPWAP protocol MUST support different authentication mechanisms
in addition to IEEE 802.11i.
Motivation and Protocol Benefits:
The benefit of supporting various authentication mechanisms is that
the protocol then becomes flexible for use in various deployments.
The protocol will therefore not mandate the use of any particular
mechanisms that may not be appropriate for a particular deployment.
Relation to Problem Statement:
This objective relates to the problem of management complexity.
Shared WLAN deployments simplify management of large networks.
5.2.2. Support for Future Wireless Technologies
Classification: Architecture
Description:
The rapid pace of technology developments means that new advances
need to be catered to in current analyses. Among these is the
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support for new wireless technologies within the CAPWAP protocol,
such as IEEE 802.16. The protocol should therefore not rely on
specifics of IEEE 802.11 technology.
In all cases where the CAPWAP protocol messages contain specific
layer 2 information elements, the definition of the protocol needs to
provide for extensibility so that these elements can be defined for
specific layer 2 wireless protocols. This may entail assigning a
layer 2 wireless protocol type and version field to the message PDU.
Examples of other wireless protocols that might be supported include
but are not limited to 802.16e, 802.15.x, etc.
Protocol Requirement:
CAPWAP protocol messages MUST be designed to be extensible for
specific layer 2 wireless technologies. It should not be limited to
the transport of elements relating to IEEE 802.11.
Motivation and Protocol Benefits:
There are many benefits to an extensible protocol. It allows for
application in different networks and provides greater scope.
Furthermore, service providers require WLAN solutions that will be
able to meet current and future market requirements.
Relation to Problem Statement:
The Problem Statement describes some of the advances taking place in
other standards bodies like the IEEE. It is important for the CAPWAP
protocol to reflect the advances and provide a framework in which
they can be supported.
5.2.3. Support for New IEEE Requirements
Classification: Architecture
Description:
The IEEE 802.11 APF Ad-Hoc Committee has reviewed IEEE 802.11
functionality and has made more thorough definitions for the new
requirements. The CAPWAP protocol must be able to incorporate these
definitions with minimal change. Furthermore, a number of extensions
for IEEE 802.11 are currently being standardized. The CAPWAP
protocol must also be able to incorporate these new extensions with
minimal change.
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Protocol Requirement:
The CAPWAP protocol MUST be openly designed to support new IEEE
802.11 definitions and extensions.
Motivation and Protocol Benefits:
There are a number of advances being made within the IEEE regarding
the functionality of IEEE 802.11 technology. Since this represents
one of the major wireless technologies in use today, it will be
beneficial for CAPWAP to incorporate the relevant new extensions.
Relation to Problem Statement:
The Problem Statement presents an overview of the task of the IEEE
802.11 working group. This group is focused on defining the
functional architecture of WTPs and new extensions for it. It is
necessary for the CAPWAP protocol to reflect these definitions and
extensions.
5.2.4. Interconnection Objective
Classification: Architecture
Description:
Large-scale WLAN deployments are likely to use a variety of
interconnection technologies between different devices of the
network. It should therefore be possible for the CAPWAP protocol to
operate over various interconnection technologies.
As a result of realizing this objective, the protocol will be capable
of operation over both IPv4 and IPv6. It will also be designed such
that it can operate within tightly administered networks, such as
enterprise networks, or on open, public access networks. For
example, VLAN tunnels can be used across different types of networks
over which CAPWAP will operate.
Protocol Requirement:
The CAPWAP protocol MUST NOT be constrained to specific underlying
transport mechanisms.
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Motivation and Protocol Benefits:
The main aim of the CAPWAP protocol is to achieve interoperability
among various WTPs and WLAN controllers. As such, the motivation for
this requirement is for the protocol to be operable independent of
underlying interconnection technologies.
Relation to Problem Statement:
The Problem Statement discusses the complexity of configuring large
WLANs. The selection of available interconnection technologies for
large-scale deployments further intensifies this complexity. This
requirement avoids part of the complexity by advocating independence
of the operational aspects of the protocol from underlying transport.
5.2.5. Access Control
Classification: Operations
Description:
This objective focuses on the informational needs of WLAN access
control and specifically the role of the CAPWAP protocol in
transporting this information between WTPs and their WLAN controller.
The following are some specific information aspects that need to be
transported by the CAPWAP protocol:
i. IEEE 802.11 association and authentication
The association of wireless clients is distinct for initial and
roaming cases. As a result, access control mechanisms require
specific contextual information regarding each case. Additionally,
load balancing, QoS, security, and congestion information in both
wireless medium segments and switching segments need to be
considered.
ii. WTP Access Control
In addition to controlling access for wireless clients, it is also
necessary to control admission of new WTPs. Given the threat of
rogue WTPs, it is important for CAPWAP to relay appropriate
authentication information between new WTPs and the WLAN controller.
Protocol Requirement:
The CAPWAP protocol MUST be capable of exchanging information
required for access control of WTPs and wireless terminals.
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Motivation and Protocol Benefits:
Due to the scale of deployments in which CAPWAP will be employed,
comprehensive access control is crucial. The effectiveness of access
control in turn is affected by the information on which such control
is based. As a result, this objective has critical relevance to a
CAPWAP protocol.
Relation to Problem Statement:
This objective addresses the issue of access control in large WLANs.
Broadly, it relates the problem of managing the complexity scale of
such networks. With collective information of both switching and
wireless medium segments, realizing this objective will help control
and manage complexity.
5.3. Non-Objectives
The following objectives have been prioritized as non-objectives
during the course of working group consultations. They have been
prioritized so in the context of CAPWAP and its considerations. They
may, however, be applicable in alternative contexts.
5.3.1. Support for Non-CAPWAP WTPs
Classification: Architecture
Description:
The CAPWAP protocol should provide an engine-mechanism to spring WTP
auto-configuration and/or software version updates and should support
integration with existing network management system. WLAN controller
as a management agent is optional.
If entities other than WLAN controllers manage some aspects of WTPs,
such as software downloads, the CAPWAP protocol may be used for WTPs
to notify WLAN controllers of any changes made by the other entities.
Protocol Requirement:
The CAPWAP protocol SHOULD be capable of recognizing legacy WTPs and
existing network management systems.
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Motivation and Protocol Benefits:
It is expected that in many cases, the centralized WLAN architecture
will be deployed incrementally with legacy systems. In this regard,
it is necessary for the protocol to be used in scenarios with mixed
WLAN devices.
Relation to Problem Statement:
The Problem Statement highlights management complexity as a major
issue with large WLANs. One part of this complexity can be related
to the incremental deployment of centralized WLAN devices for which
this objective is applicable.
5.3.2. Technical Specifications
Classification: General
Description:
The CAPWAP protocol must not require AC and WTP vendors to share
technical specifications to establish compatibility. The protocol
specifications alone must be sufficient for compatibility.
Protocol Requirement:
WTP vendors SHOULD NOT have to share technical specifications for
hardware and software to AC vendors in order for interoperability to
be achieved.
Motivation and Protocol Benefits:
It is beneficial for WLAN equipment vendors to refer to a single set
of specifications while implementing the CAPWAP protocol. This helps
to ease and quicken the development process.
Relation to Problem Statement:
This requirement is based on WG discussions that have been determined
to be important for CAPWAP.
This objective has been prioritized as a non-objective as it is a
duplicate of the Protocol Specifications objective (Section 5.1.12).
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5.4. Operator Requirements
The following objectives have been provided by network service
operators. They represent the requirements from those ultimately
deploying the CAPWAP protocol in their WLANs.
5.4.1. AP Fast Handoff
Classification: Operations
Description:
Network service operators consider handoffs crucial because of the
mobile nature of their customers. In this regard, the CAPWAP
protocol should not adversely affect AP fast-handoff procedures. The
protocol may support optimizations for fast handoff procedures so as
to allow better support for real-time services during handoffs.
Protocol Requirement:
CAPWAP protocol operations MUST NOT impede or obstruct the efficacy
of AP fast-handoff procedures.
6. Summary and Conclusion
The objectives presented in this document address three main aspects
of the CAPWAP protocol, namely:
i. Architecture
ii. Operations
iii. Security
These requirements are aimed at focusing standardization efforts on a
simple, interoperable protocol for managing large-scale WLANs. The
architecture requirements specify the structural features of the
protocol such as those relating to WTP types (local-MAC and split-
MAC) and WTP structures (logical groups). The operations
requirements address the functional aspects dealing with WTP
configuration and management. Finally, the security requirements
cover authentication and integrity aspects of protocol exchanges.
The objectives have additionally been prioritized to reflect their
immediate significance to the development and evaluation of an
interoperable CAPWAP protocol. The priorities are Mandatory and
Accepted, Desirable, and Non-Objectives. They reflect working group
consensus on the effectiveness of the requirements in the context of
protocol design.
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Additionally, this document includes requirements from network
service operators that have been derived based on their experience in
operating large-scale WLANs.
The resulting requirements from this document will be used in
conjunction with the CAPWAP Problem Statement [RFC3990] and CAPWAP
Architecture Taxonomy [RFC4118] to develop and evaluate an
interoperable protocol for the control and provisioning of WTPs in
large-scale WLANs.
7. Security Considerations
The CAPWAP framework highlights support for both local-MAC and
split-MAC WTPs. In deployments where both types of WTPs are used, it
is crucial to ensure that each be secured in consideration of its
capabilities. The Architecture Taxonomy illustrates how different
WTPs incorporate varying levels of functionalities. Development of
the CAPWAP protocol should ensure that the deployment of both local-
MAC and split-MAC WTPs within a single WLAN do not present loopholes
for security compromises.
In shared WLAN deployments made of a number of logical groups,
traffic from each group needs to be mutually separated. So in
addition to protocol-related exchanges, data traffic from wireless
terminals should also be segregated with respect to the logical
groups to which they belong. It should not be possible for data or
control traffic from one logical group to stray to or influence
another logical group.
The use of IEEE 802.11i over the centralized WLAN architecture allows
for implementations in which the PMK is shared across WTPs. This
raises the ambiguity between legitimate sharing and illegitimate
copies. Wireless terminals may unknowingly fall prey to or exploit
this ambiguity. The resolution of this issue is currently being
evaluated by the IEEE 802 and IETF liaisons.
The low cost of launching attacks on WLANs makes the CAPWAP protocol
a target. A first step in securing against any form of attacks is to
continuously monitor the WLAN for conditions of potential threats
from rogue WTPs or wireless terminals. For example, profiles for DoS
and replay attacks need to be considered for the CAPWAP protocol to
effectively monitor security conditions.
The open environment of many WLAN deployments makes physical security
breaches highly probable. Compromises resulting from theft and
physical damage must be considered during protocol development. For
instance, it should not be possible for a single compromised WTP to
affect the WLAN as a whole.
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Considering asymmetric, non-mutual authentication between WTPs and
the WLAN controller, there is a risk of a rogue participant
exploiting such an arrangement. It is preferable to avoid non-mutual
authentication. In some cases, the legitimacy of the protocol
exchange participants may be verified externally, for example, by
means of physical containment within a close environment. Asymmetric
authentication may be appropriate here without risk of security
compromises.
8. Acknowledgements
The authors would like to thank the working group chairs, Dorothy
Gellert and Mahalingam Mani, for their support and patience with this
document. We would also like to thank participants of the working
group who have helped shape the objectives. In particular, the
authors thank James Kempf, Pat Calhoun, Inderpreet Singh, Dan
Harkins, T. Sridhar, Charles Clancy, and Emek Sadot for their
invaluable inputs. We also extend our gratitude to the IEEE 802.11
Ad-Hoc Committee for its evaluation of the document. The authors
also acknowledge the contributions from Meimei Dang, Satoshi Iino,
Mikihito Sugiura, and Dong Wang.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3990] O'Hara, B., Calhoun, P., and J. Kempf, "Configuration and
Provisioning for Wireless Access Points (CAPWAP) Problem
Statement", RFC 3990, February 2005.
[RFC4118] Yang, L., Zerfos, P., and E. Sadot, "Architecture Taxonomy
for Control and Provisioning of Wireless Access Points
(CAPWAP)", RFC 4118, June 2005.
10. Informative References
[802.11] IEEE Standard 802.11, "Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications", June 2003.
[802.11i] IEEE Standard 802.11i, "Medium Access Control (MAC)
Security Enhancements", July 2004.
[802.11e] IEEE Standard 802.11e, "Medium Access Control (MAC)
Quality of Service Enhancements", November 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
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Authors' Addresses
Saravanan Govindan
Panasonic Singapore Laboratories
Block 1022, Tai Seng Industrial Estate
#06-3530, Tai Seng Avenue
Singapore 534 415
Singapore
Phone: +65 6550 5441
EMail:
[email protected]
Zhonghui Yao
Huawei Longgang Production Base
Shenzhen 518 129
P. R. China
Phone: +86 755 2878 0808
EMail:
[email protected]
Wenhui Zhou
China Mobile
53A, Xibianmen Ave, Xuanwu District
Beijing 100 053
P. R. China
Phone: +86 10 6600 6688 ext.3061
EMail:
[email protected]
L. Lily Yang
Intel Corp.
JF3-206, 2111 NE 25th Ave.
Hilsboro, OR 97124
USA
Phone: +1 503 264 8813
EMail:
[email protected]
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Hong Cheng
Panasonic Singapore Laboratories
Block 1022, Tai Seng Industrial Estate
#06-3530, Tai Seng Avenue
Singapore 534 415
Singapore
Phone: +65 6550 5447
EMail:
[email protected]
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Full Copyright Statement
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