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INFORMATIONAL
Network Working Group L. Yang
Request for Comments: 4118 Intel Corp.
Category: Informational P. Zerfos
UCLA
E. Sadot
Avaya
June 2005
Architecture Taxonomy 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 (2005).
Abstract
This document provides a taxonomy of the architectures employed in
the existing IEEE 802.11 products in the market, by analyzing
Wireless LAN (WLAN) functions and services and describing the
different variants in distributing these functions and services among
the architectural entities.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. IEEE 802.11 WLAN Functions . . . . . . . . . . . . . . 3
1.2. CAPWAP Functions . . . . . . . . . . . . . . . . . . . 5
1.3. WLAN Architecture Proliferation . . . . . . . . . . . 6
1.4. Taxonomy Methodology and Document Organization . . . . 8
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. IEEE 802.11 Definitions . . . . . . . . . . . . . . . 9
3.2. Terminology Used in This Document . . . . . . . . . . 11
3.3. Terminology Used Historically but Not Recommended . . 13
4. Autonomous Architecture . . . . . . . . . . . . . . . . . . 13
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . 13
4.2. Security . . . . . . . . . . . . . . . . . . . . . . . 14
5. Centralized WLAN Architecture . . . . . . . . . . . . . . . 15
5.1. Interconnection between WTPs and ACs . . . . . . . . . 16
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5.2. Overview of Three Centralized WLAN Architecture
Variants . . . . . . . . . . . . . . . . . . . . . . . 17
5.3. Local MAC . . . . . . . . . . . . . . . . . . . . . . 19
5.4. Split MAC . . . . . . . . . . . . . . . . . . . . . . 22
5.5. Remote MAC . . . . . . . . . . . . . . . . . . . . . . 27
5.6. Comparisons of Local MAC, Split MAC, and Remote MAC. . 27
5.7. Communication Interface between WTPs and ACs . . . . . 29
5.8. Security . . . . . . . . . . . . . . . . . . . . . . . 29
5.8.1. Client Data Security . . . . . . . . . . . . . 30
5.8.2. Security of Control Channel between
the WTP and AC . . . . . . . . . . . . . . . . 30
5.8.3. Physical Security of WTPs and ACs . . . . . . 31
6. Distributed Mesh Architecture . . . . . . . . . . . . . . . 32
6.1. Common Characteristics . . . . . . . . . . . . . . . . 32
6.2. Security . . . . . . . . . . . . . . . . . . . . . . . 33
7. Summary and Conclusions . . . . . . . . . . . . . . . . . . 33
8. Security Considerations . . . . . . . . . . . . . . . . . . 36
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
10. Normative References . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
As IEEE 802.11 Wireless LAN (WLAN) technology matures, large scale
deployment of WLAN networks is highlighting certain technical
challenges. As outlined in [2], management, monitoring, and control
of large number of Access Points (APs) in the network may prove to be
a significant burden for network administration. Distributing and
maintaining a consistent configuration throughout the entire set of
APs in the WLAN is a difficult task. The shared and dynamic nature
of the wireless medium also demands effective coordination among the
APs to minimize radio interference and maximize network performance.
Network security issues, which have always been a concern in WLANs,
present even more challenges in large deployments and new
architectures.
Recently many vendors have begun offering partially proprietary
solutions to address some or all of the above mentioned problems.
Since interoperable systems allow for a broader choice of solutions,
a standardized interoperable solution addressing the aforementioned
problems is desirable. As the first step toward establishing
interoperability in the market place, this document provides a
taxonomy of the architectures employed in existing WLAN products. We
hope to provide a cohesive understanding of the market practices for
the standard bodies involved (including the IETF and IEEE 802.11).
This document may be reviewed and utilized by the IEEE 802.11 Working
Group as input in defining the functional architecture of an AP.
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1.1. IEEE 802.11 WLAN Functions
The IEEE 802.11 specifications are wireless standards that specify an
"over-the-air" interface between a wireless client Station (STA) and
an Access Point (AP), and also among wireless clients. 802.11 also
describes how mobile devices can associate into a basic service set
(BSS). A BSS is identified by a basic service set identifier (BSSID)
or name. The WLAN architecture can be considered as a type of 'cell'
architecture, in which each cell is the Basic Service Set (BSS), and
each BSS is controlled by the AP. When two or more APs are connected
via a broadcast layer 2 network and all are using the same SSID, an
extended service set (ESS) is created.
The architectural component used to interconnect BSSs is the
distribution system (DS). An AP is an STA that provides access to
the DS by providing DS services, as well as acting as an STA.
Another logical architectural component, portal, is introduced to
integrate the IEEE 802.11 architecture with a traditional wired LAN.
It is possible for one device to offer both the functions of an AP
and a portal.
IEEE 802.11 does not specify the details of DS implementations
explicitly. Instead, the 802.11 standard defines services that
provide functions that the LLC layer requires for sending MAC Service
Data Units (MSDUs) between two entities on the network. These
services can be classified into two categories: the station service
(SS) and the distribution system service (DSS). Both categories of
service are used by the IEEE 802.11 MAC sublayer. Station services
consist of the following four services:
o Authentication: Establishes the identity of one station as a
member of the set of stations that are authorized to associate
with one another.
o De-authentication: Voids an existing authentication relationship.
o Confidentiality: Prevents the content of messages from being read
by others than the intended recipients.
o MSDU Delivery: Delivers the MAC service data unit (MSDU) for the
stations.
Distribution system services consist of the following five
services:
o Association: Establishes Access Point/Station (AP/STA) mapping and
enables STA invocation of the distribution system services.
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o Disassociation: Removes an existing association.
o Reassociation: Enables an established association (between AP and
STA) to be transferred from one AP to another or the same AP.
o Distribution: Provides MSDU forwarding by APs for the STAs
associated with them. MSDUs can be either forwarded to the
wireless destination or to the wired (Ethernet) destination (or
both) using the "Distribution System" concept of 802.11.
o Integration: Translates the MSDU received from the Distribution
System to a non-802.11 format and vice versa. Any MSDU that is
received from the DS invokes the 'Integration' services of the DSS
before the 'Distribution' services are invoked. The point of
connection of the DS to the wired LAN is termed as 'portal'.
Apart from these services, the IEEE 802.11 also defines additional
MAC services that must be implemented by the APs in the WLAN. For
example:
o Beacon Generation
o Probe Response/Transmission
o Processing of Control Frames: RTS/CTS/ACK/PS-Poll/CF-End/CF-ACK
o Synchronization
o Retransmissions
o Transmission Rate Adaptation
o Privacy: 802.11 Encryption/Decryption
In addition to the services offered by the 802.11, the IEEE 802.11 WG
is also developing technologies to support Quality of Service
(802.11e), Security Algorithms (802.11i), Inter-AP Protocol (IAPP, or
802.11F -- recommended practice) to update APs when a STA roams from
one BSS to another, Radio Resource Measurement Enhancements
(802.11k), etc.
IEEE 802.11 does not specify exactly how these functions are
implemented, nor does it specify that they be implemented in one
physical device. It only requires that the APs and the rest of the
DS together implement all these services. Typically, vendors
implement not only the services defined in the IEEE 802.11 standard,
but also a variety of value-added services or functions, such as load
balancing support, QoS, station mobility support, and rogue AP
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detection. What becomes clear from this document is that vendors
take advantage of the flexibility in the 802.11 architecture, and
have come up with many different flavors of architectures and
implementations of the WLAN services.
Because many vendors choose to implement these WLAN services across
multiple network elements, we want to make a clear distinction
between the logical WLAN access network functions and the individual
physical devices by adopting different terminology. We use "AP" to
refer to the logical entity that provides access to the distribution
services, and "WTP" (Wireless Termination Point) to the physical
device that allows the RF antenna and 802.11 PHY to transmit and
receive station traffic in the BSS network. In the Centralized
Architecture (see section 5), the combination of WTPs with Access
Controller (AC) implements all the logical functions. Each of these
physical devices (WTP or AC) may implement only part of the logical
functions. But the DS, including all the physical devices as a
whole, implements all or most of the functions.
1.2. CAPWAP Functions
To address the four problems identified in [2] (management,
consistent configuration, RF control, security) additional functions,
especially in the control and management plane, are typically offered
by vendors to assist in better coordination and control across the
entire ESS network. Such functions are especially important when the
IEEE 802.11 WLAN functions are implemented over multiple entities in
a large scale network, instead of within a single entity. Such
functions include:
o RF monitoring, such as Radar detection, noise and interference
detection, and measurement.
o RF configuration, e.g., for retransmission, channel selection,
transmission power adjustment.
o WTP configuration, e.g., for SSID.
o WTP firmware loading, e.g., automatic loading and upgrading of WTP
firmware for network wide consistency.
o Network-wide STA state information database, including the
information needed to support value-added services, such as
mobility and load balancing.
o Mutual authentication between network entities, e.g., for AC and
WTP authentication in a Centralized WLAN Architecture.
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The services listed are concerned with the configuration and control
of the radio resource ('RF Monitoring' and 'RF Configuration'),
management and configuration of the WTP device ('WTP Configuration',
'WTP Firmware upgrade'), and also security regarding the registration
of the WTP to an AC ('AC/WTP mutual authentication'). Moreover, the
device from which other services, such as mobility management across
subnets and load balancing, can obtain state information regarding
the STA(s) associated with the wireless network, is also reported as
a service ('STA state info database').
The above list of CAPWAP functions is not an exhaustive enumeration
of all additional services offered by vendors. We included only
those functions that are commonly represented in the survey data, and
are pertinent to understanding the central problem of
interoperability.
Most of these functions are not explicitly specified by IEEE 802.11,
but some of the functions are. For example, the control and
management of the radio-related functions of an AP are described
implicitly in the MIB, such as:
o Channel Assignment
o Transmit Power Control
o Radio Resource Measurement (work is currently under way in IEEE
802.11k)
The 802.11h [5] amendment to the base 802.11 standard specifies the
operation of a MAC management protocol to accomplish the requirements
of some regulatory bodies (principally in Europe, but expanding to
others) in the following areas:
o RADAR detection
o Transmit Power Control
o Dynamic Channel Selection
1.3. WLAN Architecture Proliferation
This document provides a taxonomy of the WLAN network architectures
developed by the vendor community in an attempt to address some or
all of the problems outlined in [2]. As the IEEE 802.11 standard
purposely avoids specifying the details of DS implementations,
different architectures have proliferated in the market. While all
these different architectures conform to the IEEE 802.11 standard as
a whole, their individual functional components are not standardized.
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Interfaces between the network architecture components are mostly
proprietary, and there is no guarantee of cross-vendor
interoperability of products, even within the same family of
architectures.
To achieve interoperability in the market place, the IETF CAPWAP
working group is first documenting both the functions and the network
architectures currently offered by the existing WLAN vendors. The
end result is this taxonomy document.
After analyzing more than a dozen different vendors' architectures,
we believe that the existing 802.11 WLAN access network architectures
can be broadly categorized into three distinct families, based on the
characteristics of the Distribution Systems that are employed to
provide the 802.11 functions.
o Autonomous WLAN Architecture: The first architecture family is the
traditional autonomous WLAN architecture, in which each WTP is a
single physical device that implements all the 802.11 services,
including both the distribution and integration services, and the
portal function. Such an AP architecture is called Autonomous
WLAN Architecture because each WTP is autonomous in its
functionality, and no explicit 802.11 support is needed from
devices other than the WTP. In such architecture, the WTP is
typically configured and controlled individually, and can be
monitored and managed via typical network management protocols
like SNMP. The WTPs are the traditional APs with which most
people are familiar. Such WTPs are sometimes referred to as "Fat
APs" or "Standalone APs".
o Centralized WLAN Architecture: The second WLAN architecture family
is an emerging hierarchical architecture utilizing one or more
centralized controllers for managing a large number of WTP
devices. The centralized controller is commonly referred to as an
Access Controller (AC), whose main function is to manage, control,
and configure the WTP devices that are present in the network. In
addition to being a centralized entity for the control and
management plane, it may also become a natural aggregation point
for the data plane since it is typically situated in a centralized
location in the wireless access network. The AC is often co-
located with an L2 bridge, a switch, or an L3 router, and may be
referred to as Access Bridge or Access Router in those particular
cases. Therefore, an Access Controller could be either an L3 or
L2 device, and is the generic term we use throughout this
document. It is also possible that multiple ACs are present in a
network for purposes of redundancy, load balancing, etc. This
architecture family has several distinct characteristics that are
worth noting. First, the hierarchical architecture and the
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centralized AC affords much better manageability for large scale
networks. Second, since the IEEE 802.11 functions and the CAPWAP
control functions are provided by the WTP devices and the AC
together, the WTP devices themselves may no longer fully implement
the 802.11 functions as defined in the standards. Therefore, it
can be said that the full 802.11 functions are implemented across
multiple physical network devices, namely, the WTPs and ACs.
Since the WTP devices only implement a portion of the functions
that standalone APs implement, WTP devices in this architecture
are sometimes referred to as light weight or thin APs.
o Distributed WLAN Architecture: The third emerging WLAN
architecture family is the distributed architecture in which the
participating wireless nodes are capable of forming a distributed
network among themselves, via wired or wireless media. A wireless
mesh network is one example within the distributed architecture
family, where the nodes themselves form a mesh network and connect
with neighboring mesh nodes via 802.11 wireless links. Some of
these nodes also have wired Ethernet connections acting as
gateways to the external network.
1.4. Taxonomy Methodology and Document Organization
Before the IETF CAPWAP working group started documenting the various
WLAN architectures, we conducted an open survey soliciting WLAN
architecture descriptions via the IETF CAPWAP mailing list. We
provided the interested parties with a common template that included
a number of questions about their WLAN architectures. We received 16
contributions in the form of short text descriptions answering those
questions. 15 of them are from WLAN vendors (AireSpace, Aruba,
Avaya, Chantry Networks, Cisco, Cranite Systems, Extreme Networks,
Intoto, Janusys Networks, Nortel, Panasonic, Trapeze, Instant802,
Strix Systems, Symbol) and one from the academic research community
(UCLA). Out of the 16 contributions, one describes an Autonomous
WLAN Architecture, three are Distributed Mesh Architectures, and the
remaining twelve entries represent architectures in the family of the
Centralized WLAN Architecture.
The main objective of this survey was to identify the general
categories and trends in WLAN architecture evolution, discover their
common characteristics, and determine what is performed differently
among them and why. In order to represent the survey data in a
compact format, a "Functional Distribution Matrix" is used in this
document, (mostly in the Centralized WLAN architecture section), to
tabulate the various services and functions in the vendors'
offerings. These services and functions are classified into three
main categories:
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o Architecture Considerations: The choice of the connectivity
between the AC and the WTP. The design choices regarding the
physical device on which processing of management, control, and
data frames of the 802.11 takes place.
o 802.11 Functions: As described in Section 1.1.
o CAPWAP Functions: As described in Section 1.2.
For each one of these categories, the mapping of each individual
function to network entities implemented by each vendor is shown in
tabular form. The rows in the Functional Distribution Matrix
represent individual functions that are organized into the above
mentioned three categories. Each column of the Matrix represents one
vendor's architecture offering in the survey data. See Figure 7 as
an example of the Matrix.
This Functional Distribution Matrix is intended for the sole purpose
of organizing the architecture taxonomy data, and represents the
contributors' views of their architectures from an engineering
perspective. It does not necessarily imply that a product exists or
will be shipped, nor an intent by the vendor to build such a product.
The next section provides a list of definitions used in this
document. The rest of this document is organized around the three
broad WLAN architecture families that were introduced in Section 1.3.
Each architecture family is discussed in a separate section. The
section on Centralized Architecture contains more in-depth details
than the other two families, largely due to the large number of the
survey data (twelve out of sixteen) collected that fall into the
Centralized Architecture category. Summary and conclusions are
provided at the end to highlight the basic findings from this
taxonomy exercise.
2. Conventions
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 RFC 2119 [3].
3. Definitions
3.1. IEEE 802.11 Definitions
Station (STA): A device that contains an IEEE 802.11 conformant
medium access control (MAC) and physical layer (PHY) interface to the
wireless medium (WM).
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Access Point (AP): An entity that has station functionality and
provides access to distribution services via the wireless medium (WM)
for associated stations.
Basic Service Set (BSS): A set of stations controlled by a single
coordination function.
Station Service (SS): The set of services that support transport of
medium access control (MAC) service data units (MSDUs) between
stations within a basic service set (BSS).
Distribution System (DS): A system used to interconnect a set of
basic service sets (BSSs) and integrated local area networks (LANs)
to create an extended service set (ESS).
Extended Service Set (ESS): A set of one or more interconnected basic
service sets (BSSs) with the same SSID and integrated local area
networks (LANs), which appears as a single BSS to the logical link
control layer at any station associated with one of those BSSs.
Portal: The logical point at which medium access control (MAC)
service data units (MSDUs) from a non-IEEE 802.11 local area network
(LAN) enter the distribution system (DS) of an extended service set
(ESS).
Distribution System Service (DSS): The set of services provided by
the distribution system (DS) that enable the medium access control
(MAC) layer to transport MAC service data units (MSDUs) between
stations that are not in direct communication with each other over a
single instance of the wireless medium (WM). These services include
the transport of MSDUs between the access points (APs) of basic
service sets (BSSs) within an extended service set (ESS), transport
of MSDUs between portals and BSSs within an ESS, and transport of
MSDUs between stations in the same BSS in cases where the MSDU has a
multicast or broadcast destination address, or where the destination
is an individual address, but the station sending the MSDU chooses to
involve DSS. DSSs are provided between pairs of IEEE 802.11 MACs.
Integration: The service that enables delivery of medium access
control (MAC) service data units (MSDUs) between the distribution
system (DS) and an existing, non-IEEE 802.11 local area network (via
a portal).
Distribution: The service that, by using association information,
delivers medium access control (MAC) service data units (MSDUs)
within the distribution system (DS).
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3.2. Terminology Used in This Document
One of the motivations in defining new terminology is to clarify
ambiguity and confusion surrounding some conventional terms. One
such term is "Access Point (AP)". Typically, when people talk about
"AP", they refer to the physical entity (box) that has an antenna,
implements 802.11 PHY, and receives/transmits the station (STA)
traffic over the air. However, the 802.11 Standard [1] describes the
AP mostly as a logical entity that implements a set of logical
services so that station traffic can be received and transmitted
effectively over the air. When people refer to "AP functions", they
usually mean the logical functions the whole WLAN access network
supports, and not just the subset of functions supported by the
physical entity (box) that the STAs communicate with directly. Such
confusion can be especially acute when logical functions are
implemented across a network instead of within a single physical
entity. To avoid further confusion, we define the following
terminology:
CAPWAP: Control and Provisioning of Wireless Access Points
IEEE 802.11 WLAN Functions: A set of logical functions defined by the
IEEE 802.11 Working Group, including all the MAC services, Station
Services, and Distribution Services. These logical functions are
required to be implemented in the IEEE 802.11 Wireless LAN (WLAN)
access networks by the IEEE 802.11 Standard [1].
CAPWAP Functions: A set of WLAN control functions that are not
directly defined by IEEE 802.11 Standards, but deemed essential for
effective control, configuration, and management of 802.11 WLAN
access networks.
Wireless Termination Point (WTP): The physical or network entity that
contains an RF antenna and 802.11 PHY to transmit and receive station
traffic for the IEEE 802.11 WLAN access networks. Such physical
entities were often called "Access Points" (AP), but "AP" can also
refer to the logical entity that implements 802.11 services. We
recommend "WTP" as the generic term that explicitly refers to the
physical entity with the above property (e.g., featuring an RF
antenna and 802.11 PHY), applicable to network entities of both
Autonomous and Centralized WLAN Architecture (see below).
Autonomous WLAN Architecture: The WLAN access network architecture
family in which all the logical functions, including both IEEE 802.11
and CAPWAP functions (wherever applicable), are implemented within
each Wireless Termination Point (WTP) in the network. The WTPs in
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such networks are also called standalone APs, or fat APs, because
these devices implement the full set of functions that enable the
devices to operate without any other support from the network.
Centralized WLAN Architecture: The WLAN access network architecture
family in which the logical functions, including both IEEE 802.11 and
CAPWAP functions (wherever applicable), are implemented across a
hierarchy of network entities. At the lower level are the WTPs,
while at the higher level are the Access Controllers (ACs), which are
responsible for controlling, configuring, and managing the entire
WLAN access network.
Distributed WLAN Architecture: The WLAN access network architecture
family in which some of the control functions (e.g., CAPWAP
functions) are implemented across a distributed network consisting of
peer entities. A wireless mesh network can be considered an example
of such an architecture.
Access Controller (AC): The network entity in the Centralized WLAN
Architecture that provides WTPs access to the centralized
hierarchical network infrastructure in the data plane, control plane,
management plane, or a combination therein.
Standalone WTP: Refers to the WTP in Autonomous WLAN Architecture.
Controlled WTP: Refers to the WTP in Centralized WLAN Architecture.
Split MAC Architecture: A subgroup of the Centralized WLAN
Architecture whereby WTPs in such WLAN access networks only implement
the delay sensitive MAC services (including all control frames and
some management frames) for IEEE 802.11, while all the remaining
management and data frames are tunnelled to the AC for centralized
processing. The IEEE 802.11 MAC, as defined by IEEE 802.11 Standards
in [1], is effectively split between the WTP and AC.
Remote MAC Architecture: A subgroup of the Centralized WLAN
Architecture, where the entire set of 802.11 MAC functions (including
delay-sensitive functions) is implemented at the AC. The WTP
terminates the 802.11 PHY functions.
Local MAC Architecture: A subgroup of the Centralized WLAN
Architecture, where the majority or entire set of 802.11 MAC
functions (including most of the 802.11 management frame processing)
are implemented at the WTP. Therefore, the 802.11 MAC stays intact
and local in the WTP, along with PHY.
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3.3. Terminology Used Historically but Not Recommended
While some terminology has been used by vendors historically to
describe "Access Points", we recommend deferring its use, in order to
avoid further confusion. A list of such terms and the recommended
new terminology is provided below:
Split WLAN Architecture: Use Centralized WLAN Architecture.
Hierarchical WLAN Architecture: Use Centralized WLAN Architecture.
Standalone Access Point: Use Standalone WTP.
Fat Access Point: Use Standalone WTP.
Thin Access Point: Use Controlled WTP.
Light weight Access Point: Use Controlled WTP.
Split AP Architecture: Use Local MAC Architecture.
Antenna AP Architecture: Use Remote MAC Architecture.
4. Autonomous Architecture
4.1. Overview
Figure 1 shows an example network of the Autonomous WLAN
Architecture. This architecture implements all the 802.11
functionality in a single physical device, the Wireless Termination
Point (WTP). An embodiment of this architecture is a WTP that
translates between 802.11 frames to/from its radio interface and
802.3 frames to/from an Ethernet interface. An 802.3 infrastructure
that interconnects the Ethernet interfaces of different WTPs provides
the distribution system. It can also provide portals for integrated
802.3 LAN segments.
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+---------------+ +---------------+ +---------------+
| 802.11 BSS 1 | | 802.11 BSS 2 | | 802.11 BSS 3 |
| ... | | ... | | ... |
| +-----+ | | +-----+ | | +-----+ |
+----| WTP |----+ +----| WTP |----+ +----| WTP |----+
+--+--+ +--+--+ +--+--+
|Ethernet | |
+------------------+ | +------------------+
| | |
+---+--+--+---+
| Ethernet |
802.3 LAN --------------+ Switch +-------------- 802.3 LAN
segment 1 | | segment 2
+------+------+
Figure 1: Example of Autonomous WLAN Architecture
A single physical WTP can optionally be provisioned as multiple
virtual WTPs by supporting multiple SSIDs to which 802.11 clients may
associate. In some cases, this will involve putting a corresponding
802.1Q VLAN tag on each packet forwarded to the Ethernet
infrastructure and removing 802.1Q tags prior to forwarding the
packets to the wireless medium.
The scope of the ESS(s) created by interconnecting the WTPs will be
confined by the constraints imposed by the Ethernet infrastructure.
Authentication of 802.11 clients may be performed locally by the WTP
or by using a centralized authentication server.
4.2. Security
Since both the 802.11 and CAPWAP functions are tightly integrated
into a single physical device, security issues with this architecture
are confined to the WTP. There are no extra implications from the
client authentication and encryption/decryption perspective, as the
AAA interface and the key generation mechanisms required for 802.11i
encryption/decryption are integrated into the WTP.
One of the security needs in this architecture is for mutual
authentication between the WTP and the Ethernet infrastructure. This
can be ensured by existing mechanisms such as 802.1X between the WTP
and the Ethernet switch to which it connects. Another critical
security issue is the fact that the WTP is most likely not under lock
and key, but contains secret information to communicate with back-end
systems, such as AAA and SNMP. Because IT personnel uses the common
management method of pushing a "template" to all devices, theft of
such a device would potentially compromise the wired network.
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5. Centralized WLAN Architecture
Centralized WLAN Architecture is an emerging architecture family in
the WLAN market. Contrary to the Autonomous WLAN Architecture, where
the 802.11 functions and network control functions are all
implemented within each Wireless Termination Point (WTP), the
Centralized WLAN Architecture employs one or more centralized
controllers, called Access Controller(s), to enable network-wide
monitoring, improve management scalability, and facilitate dynamic
configurability.
The following figure schematically shows the Centralized WLAN
Architecture network diagram, where the Access Controller (AC)
connects to multiple Wireless Termination Points (WTPs) via an
interconnection medium. This can be a direct connection, an L2-
switched, or an L3-routed network as described in Section 5.1. The
AC exchanges configuration and control information with the WTP
devices, allowing the management of the network from a centralized
point. Designs of the Centralized WLAN Architecture family do not
presume (as the diagram might suggest) that the AC necessarily
intercedes in the data plane to/from the WTP(s). More details are
provided later in this section.
+---------------+ +---------------+ +---------------+
| 802.11 BSS 1 | | 802.11 BSS 2 | | 802.11 BSS 3 |
| ... | | ... | | ... |
| +-------+ | | +-------+ | | +-------+ |
+----| WTP |--+ +----| WTP |--+ +----| WTP |--+
+---+---+ +---+---+ +---+---+
| | |
+------------------+ | +-----------------+
| |...|
+----+--+---+--------+
| Interconnection |
+-------+------------+
|
|
+-----+----+
| AC |
+----------+
Figure 2: Centralized WLAN Architecture Diagram
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In the diagram above, the AC is shown as a single physical entity
that provides all of the CAPWAP functions listed in Section 1.2.
However, this may not always be the case. Closer examination of the
functions reveals that their different resource requirements (e.g.,
CPU, memory, storage) may be distributed across different devices.
For instance, complex radio control algorithms can be CPU intensive.
Storing and downloading images and configurations can be storage
intensive. Therefore, different CAPWAP functions might be
implemented on different physical devices due to the different nature
of their resource requirements. The network entity marked 'AC' in
the diagram above should be thought of as a multiplicity of logical
functions, and not necessarily as a single physical device. The ACs
may also choose to implement some control functions locally, and
provide interfaces to access other global network management
functions, which are typically implemented on separate boxes, such as
a SNMP Network Management Station and an AAA back-end server (e.g.,
Radius Authentication Server).
5.1. Interconnection between WTPs and ACs
There are several connectivity options to consider between the AC(s)
and the WTPs, including direct connection, L2 switched connection,
and L3 routed connection, as shown in Figures 3, 4, and 5.
-------+------ LAN
|
+-------+-------+
| AC |
+----+-----+----+
| |
+---+ +---+
| |
+--+--+ +--+--+
| WTP | | WTP |
+--+--+ +--+--+
Figure 3: Directly Connected
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-------+------ LAN
|
+-------+-------+
| AC |
+----+-----+----+
| |
+---+ +---+
| |
+--+--+ +-----+-----+
| WTP | | Switch |
+--+--+ +---+-----+-+
| |
+-----+ +-----+
| WTP | | WTP |
+-----+ +-----+
Figure 4: Switched Connections
+-------+-------+
| AC |
+-------+-------+
|
--------+------ LAN
|
+-------+-------+
| Router |
+-------+-------+
|
-----+--+--+--- LAN
| |
+---+ +---+
| |
+--+--+ +--+--+
| WTP | | WTP|
+--+--+ +--+--+
Figure 5: Routed Connections
5.2. Overview of Three Centralized WLAN Architecture Variants
Dynamic and consistent network management is one of the primary
motivations for the Centralized Architecture. The survey data from
vendors also shows that different varieties of this architecture
family have emerged to meet a complex set of different requirements
for various possible deployment scenarios. This is also a direct
result of the inherent flexibility in the 802.11 standard [1]
regarding the implementation of the logical functions that are
Yang, et al. Informational [Page 17]
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broadly described under the term "Access Point (AP)". Because there
is no standard mapping of these AP functions to physical network
entities, several design choices have been made by vendors that offer
related products. Moreover, the increased demand for monitoring and
consistent configuration of large wireless networks has resulted in a
set of 'value-added' services provided by the various vendors, most
of which share common design properties and service goals.
In the following, we describe the three main variants observed from
the survey data within the family of Centralized WLAN Architecture,
namely the Local MAC, Split MAC, and Remote MAC approaches. For each
approach, we provide the mapping characteristics of the various
functions into the network entities from each vendor. The naming of
Local MAC, Split MAC, and Remote MAC reflects how the functions, and
especially the 802.11 MAC functions, are mapped onto the network
entities. Local MAC indicates that the MAC functions stay intact and
local to WTPs, while Remote MAC denotes that the MAC has moved away
from the WTP to a remote AC in the network. Split MAC shows the MAC
being split between the WTPs and ACs, largely along the line of
realtime sensitivity. Typically, Split MAC vendors choose to put
realtime functions on the WTPs while leaving non-realtime functions
to the ACs. 802.11 does not clearly specify what constitutes
realtime functions versus non-realtime functions, and so a clear and
definitive line does not exist. As shown in Section 5.4, each vendor
has its own interpretation on this, and there are some discrepancies
about where to draw the line between realtime and non-realtime
functions. However, vendors agree on the characterization of the
majority of MAC functions. For example, every vendor classifies the
DCF as a realtime function.
The differences among Local MAC, Split MAC and Remote MAC
architectures are shown graphically in the following figure:
+--------------+--- +---------------+--- +--------------+---
| CAPWAP | | CAPWAP | | CAPWAP |
| functions |AC | functions |AC | functions |
|==============|=== |---------------| |--------------|
| | | non RT MAC | | |AC
| 802.11 MAC | |===============|=== | 802.11 MAC |
| |WTP | Realtime MAC | | |
|--------------| |---------------|WTP |==============|===
| 802.11 PHY | | 802.11 PHY | | 802.11 PHY |WTP
+--------------+--- +---------------+--- +--------------+---
(a) "Local MAC" (b) "Split MAC" (c) "Remote MAC"
Figure 6: Three Architectural Variants within the Centralized
WLAN Architecture Family
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5.3. Local MAC
The main motivation of the Local MAC architecture model, as shown in
Figure 6 (a), is to offload network access policies and management
functions (CAPWAP functions described in Section 1.2) to the AC
without splitting the 802.11 MAC functionality between WTPs and AC.
The whole 802.11 MAC resides on the WTPs locally, including all the
802.11 management and control frame processing for the STAs. On the
other hand, information related to management and configuration of
the WTP devices is communicated with a centralized AC to facilitate
management of the network and maintain a consistent network-wide
configuration for the WTP devices.
Figure 7 shows a tabular representation of the design choices made by
the six vendors in the survey that follow the Local MAC approach,
with respect to the above mentioned architecture considerations.
"WTP-AC connectivity" shows the type connectivity between the WTPs
and AC that every vendor's architecture can support. Clearly, all
the vendors can support L3 routed network connectivity between WTPs
and the AC, which implies that direct connections and L2 switched
networks are also supported by all vendors. By '802.11 mgmt
termination', and '802.11 control termination', we denote the
physical network device on which processing of the 802.11 management
and control frames is done respectively. All the vendors here choose
to terminate 802.11 management and control frames at the WTPs. The
last row of the table, '802.11 data aggregation', refers to the
device on which aggregation and delivery of 802.11 data frames from
one STA to another (possibly through a DS) is performed. As shown by
the table, vendors make different choices as to whether all the
802.11 data traffic is aggregated and routed through the AC. The
survey data shows that some vendors choose to tunnel or encapsulate
all the station traffic to or from the ACs, implying that the AC also
acts as the access router for this WLAN access network. Other
vendors choose to separate the control and data plane by letting the
station traffic be bridged or routed locally, while keeping the
centralized control at the AC.
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Arch7 Arch8 Arch9 Arch10 Arch11
----- ----- ----- ------ ------
WTP-AC
connectivity L3 L3 L3 L3 L3
802.11 mgmt
termination WTP WTP WTP WTP WTP
802.11 control
termination WTP WTP WTP WTP WTP
802.11 data
aggregation AC AC WTP AC WTP
Figure 7: Architecture Considerations for Local MAC Architecture
Figure 8 reveals that most of the CAPWAP functions, as described in
Section 1.2, are implemented at the AC with help from WTPs to monitor
RF channels, and collect statistics and state information from the
STAs, as the AC offers the advantages of network-wide visibility,
which is essential for many of the control, configuration, and
value-added services.
Arch7 Arch8 Arch9 Arch10 Arch11
----- ----- ----- ------ ------
RF
Monitoring WTP WTP AC/WTP WTP WTP
RF
Config. AC AC AC AC AC
WTP config. AC AC AC AC AC
WTP
Firmware AC AC AC AC AC
STA state
info
database AC AC/WTP AC/WTP AC/WTP AC
AC/WTP
mutual
authent. AC/WTP AC/WTP AC/WTP AC/WTP AC/WTP
Figure 8: Mapping of CAPWAP Functions for Local MAC Architecture
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The matrix in Figure 9 shows that most of the 802.11 functions are
implemented at the WTPs for Local MAC Architecture, with some minor
differences among the vendors regarding distribution service, 802.11e
scheduling, and 802.1X/EAP authentication. The difference in
distribution service is consistent with that described earlier
regarding "802.11 data aggregation" in Figure 7.
Arch7 Arch8 Arch9 Arch10 Arch11
----- ----- ----- ------ ------
Distribution
Service AC AC WTP AC WTP
Integration
Service WTP WTP WTP WTP WTP
Beacon
Generation WTP WTP WTP WTP WTP
Probe
Response WTP WTP WTP WTP WTP
Power mgmt
Packet
Buffering WTP WTP WTP WTP WTP
Fragmentation/
Defragment. WTP WTP WTP WTP WTP
Association
Disassoc.
Reassociation AC WTP WTP WTP WTP
WME/11e
--------------
classifying AC WTP
scheduling WTP AC/WTP WTP WTP WTP
queuing WTP WTP WTP WTP
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Authentication
and Privacy
--------------
802.1X/EAP AC AC AC/WTP AC AC/WTP
Keys
Management AC AC WTP AC AC
802.11
Encryption/
Decryption WTP WTP WTP WTP WTP
Figure 9: Mapping of 802.11 Functions for Local MAC Architecture
From Figures 7, 8, and 9, it is clear that differences among vendors
in the Local MAC Architecture are relatively minor, and most of the
functional mapping appears to be common across vendors.
5.4. Split MAC
As depicted in Figure 6 (b), the main idea behind the Split MAC
architecture is to implement part of the 802.11 MAC functionality on
a centralized AC instead of the WTPs, in addition to providing the
required services for managing and monitoring the WTP devices.
Usually, the decision of which functions of the 802.11 MAC need to be
provided by the AC is based on the time-criticality of the services
considered.
In the Split MAC architecture, the WTP terminates the infrastructure
side of the wireless physical link, provides radio-related
management, and also implements time-critical functionality of the
802.11 MAC. In addition, the non-realtime management functions are
handled by a centralized AC, along with higher level services, such
as configuration, QoS, policies for load balancing, and access
control lists. The key distinction between Local MAC and Split MAC
relates to non-realtime functions: in Split MAC architecture, the AC
terminates 802.11 non realtime functions, whereas in Local MAC
architecture, the WTP terminates the 802.11 non-realtime functions
and consequently sends appropriate messages to the AC.
There are several motivations for taking the Split MAC approach. The
first is to offload functionality that is specific and relevant only
to the locality of each BSS to the WTP, in order to allow the AC to
scale to a large number of 'light weight' WTP devices. Moreover,
realtime functionality is subject to latency constraints and cannot
tolerate delays due to transmission of 802.11 control frames (or
other realtime information) over multiple-hops. The latter would
limit the available choices for connectivity between the AC and the
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RFC 4118 CAPWAP Architecture Taxonomy June 2005
WTP. Therefore, the realtime criterion is usually employed to
separate MAC services between the devices. Another consideration is
cost reduction of the WTP to make it as cheap and simple as possible.
Finally, moving functions like encryption and decryption to the AC
reduces vulnerabilities from a compromised WTP, since user encryption
keys no longer reside on the WTP. As a result, any advancements in
security protocol and algorithm designs do not necessarily obsolete
the WTPs; the ACs implement the new security schemes instead, which
simplifies the management and update task. Additionally, the network
is protected against LAN-side eavesdropping.
Since there is no clear definition in the 802.11 specification as to
which 802.11 MAC functions are considered "realtime", each vendor
interprets this in their own way. Most vendors agree that the
following services of 802.11 MAC are examples of realtime services,
and are chosen to be implemented on the WTPs.
o Beacon Generation
o Probe Response/Transmission
o Processing of Control Frames: RTS/CTS/ACK/PS-Poll/CF-End/CF-ACK
o Synchronization
o Retransmissions
o Transmission Rate Adaptation
The following list includes examples of non-realtime MAC functions as
interpreted by most vendors:
o Authentication/De-authentication
o Association/Disassociation/Reassociation/Distribution
o Integration Services: Bridging between 802.11 and 802.3
o Privacy: 802.11 Encryption/Decryption
o Fragmentation/Defragmentation
However, some vendors may choose to classify some of the above "non-
realtime" functions as realtime functions in order to support
specific applications with strict QoS requirements. For example,
Reassociation is sometimes implemented as a "realtime" function to
support VoIP applications.
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The non-realtime aspects of the 802.11 MAC are handled by the AC
through the processing of raw 802.11 management frames (Split MAC).
The following matrix in Figure 10 offers a tabular representation of
the design choices made by the six vendors that follow the Split MAC
design regarding the architecture considerations. While most vendors
support L3 connectivity between WTPs and ACs, some can only support
L2 switched connections due to the tighter delay constraint resulting
from splitting MAC between two physical entities across a network.
In Figure 7, it is clear that the WTP processes the 802.11 control
frames in both the Split MAC and Local MAC. The difference between
the two lies in the termination point for 802.11 management frames.
Local MAC terminates 802.11 management frames at WTP, while at least
some of the 802.11 management frames are terminated at the AC for the
Split MAC Architecture. Since in most cases WTP devices are IP-
addressable, any of the direct connection, L2-switched, or L3-routed
connections of Section 1.2 can be used. If only Ethernet-
encapsulation is performed (e.g., as in Architecture 4), then only
direct connection and L2-switched connections are supported.
Arch1 Arch2 Arch3 Arch4 Arch5 Arch6
----- ----- ----- ----- ----- -----
WTP-AC
connectivity L3 L3 L3 L2 L3 L3
802.11 mgmt
termination AC AC AC AC AC/WTP AC
802.11 control
termination WTP WTP WTP WTP WTP WTP
802.11 data
aggregation AC AC AC AC AC AC
Figure 10: Architecture Considerations for Split MAC Architecture
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Similar to the Local MAC Architecture, the matrix in Figure 11 shows
that most of the CAPWAP control functions are implemented at the AC.
The exception is RF monitoring, and in some cases RF configuration,
which are performed locally at the WTPs.
Arch1 Arch2 Arch3 Arch4 Arch5 Arch6
----- ----- ----- ----- ----- -----
RF
Monitoring WTP WTP WTP WTP WTP WTP
RF
Config. AC/WTP AC/WTP AC AC AC
WTP config. AC AC AC AC AC
WTP
Firmware AC AC AC AC AC
STA state
info
database AC AC AC AC AC
AC/WTP
mutual
authent. AC/WTP AC/WTP AC/WTP AC/WTP
Figure 11: Mapping of CAPWAP Functions for Split MAC Architecture
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The most interesting matrix for Split MAC Architecture is the
Functional Distribution Matrix for 802.11 functions, as shown below
in Figure 12. Vendors map the functions onto the WTPs and AC with a
certain regularity. For example, all vendors choose to implement
Distribution, Integration Service at the AC, along with 802.1X/EAP
authentication and keys management. All vendors also choose to
implement beacon generation at WTPs. On the other hand, vendors
sometimes choose to map many of the other functions differently.
Therefore, Split MAC Architectures are not consistent regarding the
exact way the MAC is split.
Arch1 Arch2 Arch3 Arch4 Arch5 Arch6
----- ----- ----- ------ ----- -----
Distribution
Service AC AC AC AC AC AC
Integration
Service AC AC AC AC AC AC
Beacon
Generation WTP WTP WTP WTP WTP WTP
Probe
Response WTP AC/WTP WTP WTP WTP WTP
Power mgmt
Packet
Buffering WTP WTP WTP AC AC/WTP WTP
Fragmentation
Defragment. WTP WTP AC AC AC
Association
Disassoc.
Reassociation AC AC AC AC WTP AC
WME/11e
--------------
classifying AC AC AC AC
scheduling WTP/AC AC WTP AC AC WTP/AC
queuing WTP/AC WTP WTP AC WTP WTP
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Authentication
and Privacy
--------------
802.1X/EAP AC AC AC AC AC AC
Keys
Management AC AC AC AC AC AC
802.11
Encryption/
Decryption WTP AC WTP AC AC AC
Figure 12: Mapping of 802.11 Functions for Split MAC Architecture
5.5. Remote MAC
One of the main motivations for the Remote MAC Architecture is to
keep the WTPs as light weight as possible, by having only the radio
interfaces on the WTPs and offloading the entire set of 802.11 MAC
functions (including delay-sensitive ones) to the Access Controller.
This leaves all the complexities of the MAC and other CAPWAP control
functions to the centralized controller.
The WTP acts only as a pass-through between the Wireless LAN clients
(STA) and the AC, though they may have an additional feature to
convert the frames from one format (802.11) to the other (i.e.,
Ethernet, TR, Fiber). The centralized controller provides network
monitoring, management and control, an entire set of 802.11 AP
services, security features, resource management, channel selection
features, and guarantees Quality of Service to the users. Because
the MAC is separated from the PHY, we call this the "Remote MAC
Architecture". Typically, such architecture is deployed with special
attention to the connectivity between the WTPs and AC so that the
delay is minimized. The Radio over Fiber (RoF) from Architecture 5
is an example of Remote MAC Architecture.
5.6. Comparisons of Local MAC, Split MAC, and Remote MAC
Two commonalities across all three Centralized Architectures (Local
MAC, Split MAC, and Remote MAC) are:
o Most of the CAPWAP functions related to network control and
configuration reside on the AC.
o IEEE 802.11 PHY resides on the WTP.
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There is a clear difference between Remote MAC and the other two
Centralized Architectures (namely, Local MAC and Split MAC), as the
802.11 MAC is completely separated from the PHY in the former, while
the other two keep some portion of the MAC functions together with
PHY at the WTPs. The implication of PHY and MAC separation is that
it severely limits the kind of interconnection between WTPs and ACs,
so that the 802.11 timing constraints are satisfied. As pointed out
earlier, this usually results in tighter constraint over the
interconnection between WTP and AC for the Remote MAC Architecture.
The advantage of Remote MAC Architecture is that it offers the
lightest possible WTPs for certain deployment scenarios.
The commonalities and differences between Local MAC and Split MAC are
most clearly seen by comparing Figure 7 to Figure 10. The
commonality is that 802.11 control frames are terminated at WTPs in
both cases. The main difference between Local MAC and Split MAC is
that the WTP terminates only the 802.11 control frames in the Split
MAC, while the WTP may terminate all 802.11 frames in the Local MAC.
An interesting consequence of this difference is that the Integration
Service, which essentially refers to bridging between 802.11 and
802.3 frames, is implemented by the AC in the Split MAC and by the
WTP in the Local MAC, as shown in Figures 9 and 12, respectively.
As a second note, the Distribution Service, although usually provided
by the AC, can also be implemented at the WTP in some Local MAC
architectures. This approach is meant to increase performance in
delivering STAs data traffic by avoiding tunneling it to the AC, and
relaxing the dependency of the WTP from the AC. Therefore, it is
possible for the data and control planes to be separated in the Local
MAC Architecture.
Even though all the 802.11 traffic is aggregated at ACs in the case
of Split MAC Architecture, the data and control planes can still be
separated by employing multiple ACs. For example, one AC can
implement most of the CAPWAP functions (control plane), while other
ACs can be used for 802.11 frames bridging (data plane).
Each of the three architectural variants may be advantageous for
certain deployment scenarios. While the Local MAC retains most of
the STA's state information at the local WTPs, Remote MAC centralizes
most of the state into the back-end AC. Split MAC sits somewhat in
the middle of this spectrum, keeping some state information locally
at the WTPs, and the rest centrally at the AC. Many factors should
be taken into account to determine the exact balance desired between
the centralized and decentralized state. The impact of such balance
on network manageability is currently a matter of dispute within the
technical community.
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5.7. Communication Interface between WTPs and ACs
Before any messages can be exchanged between an AC and WTP, the WTP
needs to discover, authenticate, and register with the AC first, then
download the firmware and establish a control channel with the AC.
Message exchanges between the WTP and AC for control and
configuration can happen after that. The following list outlines the
basic operations that are typically performed between the WTP and the
AC in their typical order:
1. Discovery: The WTPs discover the AC with which they will be bound
to and controlled by. The discovery procedure can employ either
static or dynamic configuration. In the latter case, a protocol
is used in order for the WTP to discover candidate AC(s).
2. Authentication: After discovery, the WTP device authenticates
itself with the AC. However, mutual authentication, in which the
WTP also authenticates the AC, is not always supported since some
vendors strive for zero-configuration on the WTP side. This is
not necessarily secure as it leaves the possible vulnerability of
the WTP being attached to a rogue AC.
3. WTP Association: After successful authentication, a WTP registers
with the AC in order to start receiving management and
configuration messages.
4. Firmware Download: After successful association, the WTP may
pull, or the AC may push, the WTPs firmware, which may be
protected in some manner, such as digital signatures.
5. Control Channel Establishment: The WTP establishes either an IP-
tunnel or performs Ethernet encapsulation with the AC in order to
transfer data traffic and management frames.
6. Configuration Download: Following the control channel
establishment process, the AC may push configuration parameters
to the WTPs.
5.8. Security
Given the varied distribution of functionalities for the Centralized
Architecture, as surveyed in Section 4.3, it is obvious that an extra
network binding is created between the WTP and the AC. This brings
new and unique security issues and subsequent requirements.
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5.8.1. Client Data Security
The survey shows clearly that the termination point for "over the
air" 802.11 encryption [4] can be implemented either in the WTP or in
the AC. Furthermore, the 802.1X/EAP [6] functionality is distributed
between the WTP and the AC where, in most cases, the AC performs the
necessary functions as the authenticator in the 802.1X exchange.
If the STA and AC are the parties in the 4-way handshake (defined in
[4]), and 802.11i traffic encryption terminates at the WTP, then the
Pairwise Transient Key (PTK) has to be transferred from the AC to the
WTP. Since the keying material is part of the control and
provisioning of the WTPs, a secure encrypted tunnel for control
frames is employed to transport the keying material.
The centralized model encourages AC implementations to use one PMK
for many different WTPs. This practice facilitates speedy transition
by an STA from one WTP to another that is connected to the same AC
without establishing a separate PMK. However, this leaves the STA in
a difficult position, as the STA cannot distinguish between a
compromised PMK and one that is intentionally being shared. This
issue must be resolved, but the resolution is beyond the scope of the
CAPWAP working group. The venue for this resolution is to be
determined by the IEEE 802 and IETF liaisons.
When the 802.11i encryption/decryption is performed in the AC, the
key exchange and state transitions occur between the AC and the STA.
Therefore, there is no need to transfer any crypto material between
the AC and the WTP.
Regardless of where the 802.11i termination point occurs, the
Centralized WLAN Architecture records two practices for "over the
wire" client data security. In some cases there is an encrypted
tunnel (IPsec or SSL) between the WTP and AC, which assumes that the
security boundary is in the AC. In other cases, an end-to-end
mutually authenticated secure VPN tunnel is assumed between the
client and AC, other security gateway, or end host entity.
5.8.2. Security of Control Channel between the WTP and AC
In order for the CAPWAP functions to be implemented in the
Centralized WLAN Architecture, a control channel is necessary between
the WTP and AC.
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To address potential security threats against the control channel,
existing implementations feature one or more of the following
security mechanisms:
1. Secure discovery of WTP and AC.
2. Authentication of the WTPs to the ACs (and possibly mutual
authentication).
3. Confidentiality, integrity, and replay protection of control
channel frames.
4. Secure management of WTPs and ACs, including mechanisms for
securely setting and resetting secrets and state.
Discovery and authentication of WTPs are addressed in the submissions
by implementing authentication mechanisms that range from X.509
certificates, AAA authentication to pre-shared credential
authentication. In all cases, confidentiality, integrity, and
protection against man-in-the-middle attacks of the control frames
are addressed by a secure encrypted tunnel between the WTP and AC(s),
utilizing keys derived from the authentication methods mentioned
previously. Finally, one of the motivations for the Centralized WLAN
Architecture is to minimize the storage of cryptographic and security
sensitive information, in addition to operational configuration
parameters within the WTPs. It is for that reason that the majority
of the submissions under the Centralized Architecture category have
employed a post WTP authenticated discovery phase of configuration
provisioning, which in turn protects against the theft of WTPs.
5.8.3. Physical Security of WTPs and ACs
To provide comprehensive radio coverage, WTPs are often installed in
locations that are difficult to secure physically; it is relatively
easier to secure the AC physically. If high-value secrets, such as a
RADIUS shared secret, are stored in the AC instead of WTPs, then the
physical loss of an WTP does not compromise these secrets. Hence,
the Centralized Architecture may reduce the security consequences of
a stolen WTP. On the other hand, concentrating all the high-value
secrets in one place makes the AC an attractive target that requires
strict physical, procedural, and technical controls to protect the
secrets.
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6. Distributed Mesh Architecture
Out of the sixteen architecture survey submissions, three belong to
the Distributed Mesh Architecture family. An example of the
Distributed Mesh Architecture is shown in Figure 13, and reflects
some of the common characteristics found in these three submissions.
+-----------------+ +-----------------+
| 802.11 BSS 1 | | 802.11 BSS 2 |
| ... | | ... |
| +---------+ | | +---------+ |
+----|mesh node|--+ +----|mesh node|--+
+-+---+---+ +-+-+-----+
| | | |
| | | | +----------+
| +-----------------------+ | Ethernet | Ethernet |
| 802.11 wireless links | +--------+ Switch |
| +-----------------------+ | | | |
| | | | | +----------+
+-+---+---+ +-+--+----+
+----|mesh node|--+ +----|mesh node|--+
| +---------+ | | +---------+ |
| ... | | ... |
| 802.11 BSS 4 | | 802.11 BSS 3 |
+-----------------+ +-----------------+
Figure 13: Example of Distributed Mesh Architecture
6.1. Common Characteristics
To provide wider wireless coverage, mesh nodes in the network may act
as APs to client stations in their respective BSS, as well as traffic
relays to neighboring mesh nodes via 802.11 wireless links. It is
also possible that some mesh nodes in the network may serve only as
wireless traffic relays for other mesh nodes, but not as APs for any
client stations. Instead of pulling Ethernet cable connections to
every AP, wireless mesh networks provide an attractive alternative to
relaying backhaul traffic.
Mesh nodes can also keep track of the state of their neighboring
nodes, or even nodes beyond their immediate neighborhood by
exchanging information periodically amongst them; this way, mesh
nodes can be fully aware of the dynamic network topology and RF
conditions around them. Such peer-to-peer communication model allows
mesh nodes to actively coordinate among themselves to achieve self-
configuration and self-healing. This is the major distinction
between this Distributed Architecture family and the Centralized
Architecture -- much of the CAPWAP functions can be implemented
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across the mesh nodes in a distributed fashion, without a centralized
entity making all the control decisions.
It is worthwhile to point out that mesh networks do not necessarily
preclude the use of centralized control. It is possible that a
combination of centralized and distributed control co-exists in mesh
networks. Some global configuration or policy change may be better
served in a coordinated fashion if some form of Access Controller
(AC) exists in the mesh network (even if not the full blown version
of the AC, as defined in the Centralized WLAN Architecture). For
example, a centralized management entity can be used to update every
mesh node's default configuration. It may also be more desirable to
leave certain functions, such as user authentication to a single
centralized end point (such as a RADIUS server), but mesh networks
allow each mesh AP to directly talk to the RADIUS server. This
eliminates the single point of failure and takes advantage of the
client distribution in the network.
The backhaul transport network of the mesh network can be either an
L2 or L3 networking technology. Currently, vendors are using
proprietary mesh technologies on top of standard 802.11 wireless
links to enable peer-to-peer communication between the mesh nodes.
Hence, there is no interoperability among mesh nodes from different
vendors. The IEEE 802.11 WG has recently started a new Task Group
(TGs) to define the mesh standard for 802.11.
6.2. Security
Similar security concerns for client data security, as described in
Section 5.8.1, also apply to the Distributed Mesh Architecture.
Additionally, one important security consideration for the mesh
networks is that the mesh nodes must authenticate each other within
the same administrative domain. To protect user and management data
that may not be secured at layer 3, data transmission among
neighboring nodes should be secured by a layer 2 mechanism of
confidentiality, integrity, and replay protection.
7. Summary and Conclusions
We requested existing WLAN vendors and other interested parties to
submit a short description of existing or desired WLAN access network
architectures to define a taxonomy of possible WLAN access network
architectures. The information from the 16 submissions was condensed
and summarized in this document.
New terminology has been defined wherever existing terminology was
found to be either insufficient or ambiguous in describing the WLAN
architectures and supporting functions listed in the document. For
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example, the broad set of Access Point functions has been divided
into two categories: 802.11 functions, which include those that are
required by the IEEE 802.11 standards, and CAPWAP functions, which
include those that are not required by the IEEE 802.11, but are
deemed essential for control, configuration, and management of 802.11
WLAN access networks. Another term that has caused considerable
ambiguity is "Access Point", which usually reflected a physical box
that has the antennas, but did not have a uniform set of externally
consistent behavior across submissions. To remove this ambiguity, we
have redefined the AP as the set of 802.11 and CAPWAP functions,
while the physical box that terminates the 802.11 PHY is called the
Wireless Termination Point.
Based on the submissions during the architecture survey phase, we
have classified the existing WLAN architectures into three broad
classes:
1. Autonomous WLAN Architecture: Indicates a family of architectures
in which all the 802.11 functions and, where applicable, CAPWAP
functions are implemented in the WTPs.
2. Centralized WLAN Architecture: Indicates a family of architectures
in which the AP functions are split between the WTPs and the AC,
with the AC acting as a centralized control point for multiple
WTPs.
3. Distributed WLAN Architecture: Indicates a family of architectures
in which part of the control functions is implemented across a
distributed network of peer entities.
Within the Centralized WLAN Architecture, there are a few visible
sub-categories that depend on how one maps the MAC functions (at a
high-level), between the WTP and the AC. Three prominent sub-
categories emerged from the information in the submissions:
1. Split MAC Architecture: The 802.11 MAC functions are split between
the WTP and the AC. This subgroup includes all architectures that
split the 802.11 MAC functions even though individual submissions
differed on the specifics of the split.
2. Local MAC Architecture: The entire set of 802.11 MAC functions is
implemented on the WTP.
3. Remote MAC Architecture: The entire set of 802.11 MAC functions is
implemented on the AC.
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The following tree diagram summarizes the architectures documented in
this taxonomy.
+----------------+
|Autonomous |
+---------->|Architecture |
| |Family |
| +----------------+
| +--------------+
| |Local |
| +---->|MAC |
| | |Architecture |
| | +--------------+
| |
| +----------------+ | +--------------+
| |Centralized | | |Split |
+---------->|Architecture |--+---->|MAC |
| |Family | | |Architecture |
| +----------------+ | +--------------+
| |
| | +--------------+
| | |Remote |
| +---->|MAC |
| |Architecture |
| +--------------+
| +----------------+
| |Distributed Mesh|
+---------->|Architecture |
|Family |
+----------------+
A majority of the submitted WLAN access network architectures (twelve
out of sixteen) followed the Centralized WLAN Architecture. All but
one of the Centralized WLAN Architecture submissions were grouped
into either a Split MAC Architecture or a Local MAC Architecture.
One submission followed the Autonomous WLAN Architecture, and three
followed the Distributed WLAN Architecture.
The WLAN access network architectures in the submissions indicated
that the connectivity assumptions were:
o Direct connection between the WTP and the AC.
o L2 switched connection between the WTP and the AC.
o L3 routed connection between the WTP and the AC.
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o Wireless connection between the mesh nodes in the distributed mesh
architecture.
Interoperability between equipment from different vendors is one of
the fundamental problems in the WLAN market today. To achieve
interoperability via open standard development, the following steps
are suggested for IETF and IEEE 802.11.
Using this taxonomy, a functional model of an Access Point should be
defined by the new study group recently formed within the IEEE
802.11. The functional model will consist of defining functional
elements of an 802.11 Access Point that are considered atomic, i.e.,
not subject to further splitting across multiple network elements.
Such a functional model should serve as a common foundation to
support the existing WLAN architectures as outlined in this taxonomy,
and any further architecture development within or outside the IEEE
802.11 group. It is possible, and even recommended, that work on the
functional model definition may also include impact analysis of
implementing each functional element on either the WTP or the AC.
As part of the functional model definition, interfaces must be
defined as primitives between these functional elements. If a pair
of functional elements that have an interface defined between them is
being implemented on two different network entities, then a protocol
specification definition between such a pair of network elements is
required, and should be developed by the IETF.
8. Security Considerations
This document does not intend to provide a comprehensive threat
analysis of all of the security issues with the different WLAN
architectures. Nevertheless, in addition to documenting the
architectures employed in the existing IEEE 802.11 products in the
market, this taxonomy document also catalogues the security issues
that arise and the manner in which vendors address these security
threats. The WLAN architectures are broadly categorized into three
families: Autonomous Architecture, Centralized Architecture, and
Distributed Architecture. While Sections 4, 5, and 6 are devoted to
each of these three architecture families, respectively, each section
also contains a subsection to address the security issues within each
architecture family.
In summary, the main security concern in the Autonomous Architecture
is the mutual authentication between the WTP and the wired (Ethernet)
infrastructure equipment. Physical security of the WTPs is also a
network security concern because the WTPs contain secret information
and theft of these devices could potentially compromise even the
wired network.
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In the Centralized Architecture there are a few new security concerns
due to the new network binding between the WTP and AC. The following
security concerns are raised for this architecture family: keying
material for mobile client traffic may need to be securely
transported from the AC to WTP; secure discovery of the WTP and AC is
required, as well as mutual authentication between the WTPs and AC;
man-in-the-middle attacks to the control channel between WTP and AC,
confidentiality, integrity and replay protection of control channel
frames, and theft of WTPs for extraction of embedded secrets within.
Each of the survey results for this broad architecture category has
presented mechanisms to address these security issues.
The new security issue in the Distributed Mesh Architecture is the
need for mesh nodes to authenticate each other before forming a
secure mesh network. Encrypted communication between mesh nodes is
recommended to protect both control and user data.
9. Acknowledgements
This taxonomy is truly a collaborative effort with contributions from
a large group of people. First, we want to thank all the CAPWAP
Architecture Design Team members who have spent many hours in the
teleconference calls, over e-mails, and in writing and reviewing the
document. The full Design Team is listed here:
o Peyush Agarwal
STMicroelectronics
Plot# 18, Sector 16A
Noida, U.P 201301
India
Phone: +91-120-2512021
EMail: peyush.agarwal@st.com
o Dave Hetherington
Roving Planet
4750 Walnut St., Suite 106
Boulder, CO 80027
United States
Phone: +1-303-996-7560
EMail: Dave.Hetherington@RovingPlanet.com
o Matt Holdrege
Strix Systems
26610 Agoura Road
Calabasas, CA 91302
Phone: +1 818-251-1058
EMail: matt@strixsystems.com
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o Victor Lin
Extreme Networks
3585 Monroe Street
Santa Clara, CA 95051
Phone: +1 408-579-3383
EMail: vlin@extremenetworks.com
o James M. Murphy
Trapeze Networks
5753 W. Las Positas Blvd.
Pleasanton, CA 94588
Phone: +1 925-474-2233
EMail: jmurphy@trapezenetworks.com
o Partha Narasimhan
Aruba Wireless Networks
180 Great Oaks Blvd
San Jose, CA 95119
Phone: +1 408-754-3018
EMail: partha@arubanetworks.com
o Bob O'Hara
Airespace
110 Nortech Parkway
San Jose, CA 95134
Phone: +1 408-635-2025
EMail: bob@airespace.com
o Emek Sadot (see Authors' Addresses)
o Ajit Sanzgiri
Cisco Systems
170 W Tasman Drive
San Jose, CA 95134
Phone: +1 408-527-4252
EMail: sanzgiri@cisco.com
o Singh
Chantry Networks
1900 Minnesota Court
Mississauga, Ontario L5N 3C9
Canada
Phone: +1 905-567-6900
EMail: isingh@chantrynetworks.com
o L. Lily Yang (Editor, see Authors' Addresses)
o Petros Zerfos (see Authors' Addresses)
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In addition, we would also like to acknowledge contributions from the
following individuals who participated in the architecture survey and
provided detailed input data in preparation of the taxonomy: Parviz
Yegani, Cheng Hong, Saravanan Govindan, Bob Beach, Dennis Volpano,
Shankar Narayanaswamy, Simon Barber, Srinivasa Rao Addepalli,
Subhashini A. Venkataramanan, Kue Wong, Kevin Dick, Ted Kuo, and
Tyan-shu Jou. It is simply impossible to write this taxonomy without
the large set of representative data points that they provided to us.
We would also like to thank our CAPWAP WG co-chairs, Mahalingam Mani
and Dorothy Gellert, and our Area Director, Bert Wijnen, for their
unfailing support.
10. Normative References
[1] "IEEE WLAN MAC and PHY Layer Specifications", August 1999, <IEEE
802.11-99>.
[2] O'Hara, B., Calhoun, P., and J. Kempf, "Configuration and
Provisioning for Wireless Access Points (CAPWAP) Problem
Statement", RFC 3990, February 2005.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] "IEEE Std 802.11i: Medium Access Control (MAC) Security
Enhancements", April 2004.
[5] "IEEE Std 802.11h: Spectrum and Transmit Power Management
Extensions in the 5 GHz Band in Europe", October 2003.
[6] "IEEE Std 802.1X: Port-based Network Access Control", June 2001.
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Authors' Addresses
L. Lily Yang
Intel Corp.
MS JF3 206, 2111 NE 25th Avenue
Hillsboro, OR 97124
Phone: +1 503-264-8813
EMail: lily.l.yang@intel.com
Petros Zerfos
UCLA - Computer Science Department
4403 Boelter Hall
Los Angeles, CA 90095
Phone: +1 310-206-3091
EMail: pzerfos@cs.ucla.edu
Emek Sadot
Avaya
Atidim Technology Park, Building #3
Tel-Aviv 61131
Israel
Phone: +972-3-645-7591
EMail: esadot@avaya.com
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Full Copyright Statement
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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Yang, et al. Informational [Page 41]
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