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rfc:rfc5415

Network Working Group P. Calhoun, Ed. Request for Comments: 5415 Cisco Systems, Inc. Category: Standards Track M. Montemurro, Ed.

                                                    Research In Motion
                                                       D. Stanley, Ed.
                                                        Aruba Networks
                                                            March 2009
    Control And Provisioning of Wireless Access Points (CAPWAP)
                       Protocol Specification

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Calhoun, et al. Standards Track [Page 1] RFC 5415 CAPWAP Protocol Specification March 2009

Abstract

 This specification defines the Control And Provisioning of Wireless
 Access Points (CAPWAP) Protocol, meeting the objectives defined by
 the CAPWAP Working Group in RFC 4564.  The CAPWAP protocol is
 designed to be flexible, allowing it to be used for a variety of
 wireless technologies.  This document describes the base CAPWAP
 protocol, while separate binding extensions will enable its use with
 additional wireless technologies.

Table of Contents

 1. Introduction ....................................................7
    1.1. Goals ......................................................8
    1.2. Conventions Used in This Document ..........................9
    1.3. Contributing Authors .......................................9
    1.4. Terminology ...............................................10
 2. Protocol Overview ..............................................11
    2.1. Wireless Binding Definition ...............................12
    2.2. CAPWAP Session Establishment Overview .....................13
    2.3. CAPWAP State Machine Definition ...........................15
         2.3.1. CAPWAP Protocol State Transitions ..................17
         2.3.2. CAPWAP/DTLS Interface ..............................31
    2.4. Use of DTLS in the CAPWAP Protocol ........................33
         2.4.1. DTLS Handshake Processing ..........................33
         2.4.2. DTLS Session Establishment .........................35
         2.4.3. DTLS Error Handling ................................35
         2.4.4. DTLS Endpoint Authentication and Authorization .....36
 3. CAPWAP Transport ...............................................40
    3.1. UDP Transport .............................................40
    3.2. UDP-Lite Transport ........................................41
    3.3. AC Discovery ..............................................41
    3.4. Fragmentation/Reassembly ..................................42
    3.5. MTU Discovery .............................................43
 4. CAPWAP Packet Formats ..........................................43
    4.1. CAPWAP Preamble ...........................................46
    4.2. CAPWAP DTLS Header ........................................46
    4.3. CAPWAP Header .............................................47
    4.4. CAPWAP Data Messages ......................................50
         4.4.1. CAPWAP Data Channel Keep-Alive .....................51
         4.4.2. Data Payload .......................................52
         4.4.3. Establishment of a DTLS Data Channel ...............52
    4.5. CAPWAP Control Messages ...................................52
         4.5.1. Control Message Format .............................53
         4.5.2. Quality of Service .................................56
         4.5.3. Retransmissions ....................................57
    4.6. CAPWAP Protocol Message Elements ..........................58
         4.6.1. AC Descriptor ......................................61

Calhoun, et al. Standards Track [Page 2] RFC 5415 CAPWAP Protocol Specification March 2009

         4.6.2. AC IPv4 List .......................................64
         4.6.3. AC IPv6 List .......................................64
         4.6.4. AC Name ............................................65
         4.6.5. AC Name with Priority ..............................65
         4.6.6. AC Timestamp .......................................66
         4.6.7. Add MAC ACL Entry ..................................66
         4.6.8. Add Station ........................................67
         4.6.9. CAPWAP Control IPv4 Address ........................68
         4.6.10. CAPWAP Control IPv6 Address .......................68
         4.6.11. CAPWAP Local IPv4 Address .........................69
         4.6.12. CAPWAP Local IPv6 Address .........................69
         4.6.13. CAPWAP Timers .....................................70
         4.6.14. CAPWAP Transport Protocol .........................71
         4.6.15. Data Transfer Data ................................72
         4.6.16. Data Transfer Mode ................................73
         4.6.17. Decryption Error Report ...........................73
         4.6.18. Decryption Error Report Period ....................74
         4.6.19. Delete MAC ACL Entry ..............................74
         4.6.20. Delete Station ....................................75
         4.6.21. Discovery Type ....................................75
         4.6.22. Duplicate IPv4 Address ............................76
         4.6.23. Duplicate IPv6 Address ............................77
         4.6.24. Idle Timeout ......................................78
         4.6.25. ECN Support .......................................78
         4.6.26. Image Data ........................................79
         4.6.27. Image Identifier ..................................79
         4.6.28. Image Information .................................80
         4.6.29. Initiate Download .................................81
         4.6.30. Location Data .....................................81
         4.6.31. Maximum Message Length ............................81
         4.6.32. MTU Discovery Padding .............................82
         4.6.33. Radio Administrative State ........................82
         4.6.34. Radio Operational State ...........................83
         4.6.35. Result Code .......................................84
         4.6.36. Returned Message Element ..........................85
         4.6.37. Session ID ........................................86
         4.6.38. Statistics Timer ..................................87
         4.6.39. Vendor Specific Payload ...........................87
         4.6.40. WTP Board Data ....................................88
         4.6.41. WTP Descriptor ....................................89
         4.6.42. WTP Fallback ......................................92
         4.6.43. WTP Frame Tunnel Mode .............................92
         4.6.44. WTP MAC Type ......................................93
         4.6.45. WTP Name ..........................................94
         4.6.46. WTP Radio Statistics ..............................94
         4.6.47. WTP Reboot Statistics .............................96
         4.6.48. WTP Static IP Address Information .................97
    4.7. CAPWAP Protocol Timers ....................................98

Calhoun, et al. Standards Track [Page 3] RFC 5415 CAPWAP Protocol Specification March 2009

         4.7.1. ChangeStatePendingTimer ............................98
         4.7.2. DataChannelKeepAlive ...............................98
         4.7.3. DataChannelDeadInterval ............................99
         4.7.4. DataCheckTimer .....................................99
         4.7.5. DiscoveryInterval ..................................99
         4.7.6. DTLSSessionDelete ..................................99
         4.7.7. EchoInterval .......................................99
         4.7.8. IdleTimeout ........................................99
         4.7.9. ImageDataStartTimer ...............................100
         4.7.10. MaxDiscoveryInterval .............................100
         4.7.11. ReportInterval ...................................100
         4.7.12. RetransmitInterval ...............................100
         4.7.13. SilentInterval ...................................100
         4.7.14. StatisticsTimer ..................................100
         4.7.15. WaitDTLS .........................................101
         4.7.16. WaitJoin .........................................101
    4.8. CAPWAP Protocol Variables ................................101
         4.8.1. AdminState ........................................101
         4.8.2. DiscoveryCount ....................................101
         4.8.3. FailedDTLSAuthFailCount ...........................101
         4.8.4. FailedDTLSSessionCount ............................101
         4.8.5. MaxDiscoveries ....................................102
         4.8.6. MaxFailedDTLSSessionRetry .........................102
         4.8.7. MaxRetransmit .....................................102
         4.8.8. RetransmitCount ...................................102
         4.8.9. WTPFallBack .......................................102
    4.9. WTP Saved Variables ......................................102
         4.9.1. AdminRebootCount ..................................102
         4.9.2. FrameEncapType ....................................102
         4.9.3. LastRebootReason ..................................103
         4.9.4. MacType ...........................................103
         4.9.5. PreferredACs ......................................103
         4.9.6. RebootCount .......................................103
         4.9.7. Static IP Address .................................103
         4.9.8. WTPLinkFailureCount ...............................103
         4.9.9. WTPLocation .......................................103
         4.9.10. WTPName ..........................................103
 5. CAPWAP Discovery Operations ...................................103
    5.1. Discovery Request Message ................................103
    5.2. Discovery Response Message ...............................105
    5.3. Primary Discovery Request Message ........................106
    5.4. Primary Discovery Response ...............................107
 6. CAPWAP Join Operations ........................................108
    6.1. Join Request .............................................108
    6.2. Join Response ............................................110
 7. Control Channel Management ....................................111
    7.1. Echo Request .............................................111
    7.2. Echo Response ............................................112

Calhoun, et al. Standards Track [Page 4] RFC 5415 CAPWAP Protocol Specification March 2009

 8. WTP Configuration Management ..................................112
    8.1. Configuration Consistency ................................112
         8.1.1. Configuration Flexibility .........................113
    8.2. Configuration Status Request .............................114
    8.3. Configuration Status Response ............................115
    8.4. Configuration Update Request .............................116
    8.5. Configuration Update Response ............................117
    8.6. Change State Event Request ...............................117
    8.7. Change State Event Response ..............................118
    8.8. Clear Configuration Request ..............................119
    8.9. Clear Configuration Response .............................119
 9. Device Management Operations ..................................120
    9.1. Firmware Management ......................................120
         9.1.1. Image Data Request ................................124
         9.1.2. Image Data Response ...............................125
    9.2. Reset Request ............................................126
    9.3. Reset Response ...........................................127
    9.4. WTP Event Request ........................................127
    9.5. WTP Event Response .......................................128
    9.6. Data Transfer ............................................128
         9.6.1. Data Transfer Request .............................130
         9.6.2. Data Transfer Response ............................131
 10. Station Session Management ...................................131
    10.1. Station Configuration Request ...........................131
    10.2. Station Configuration Response ..........................132
 11. NAT Considerations ...........................................132
 12. Security Considerations ......................................134
    12.1. CAPWAP Security .........................................134
         12.1.1. Converting Protected Data into Unprotected Data ..135
         12.1.2. Converting Unprotected Data into
                 Protected Data (Insertion) .......................135
         12.1.3. Deletion of Protected Records ....................135
         12.1.4. Insertion of Unprotected Records .................135
         12.1.5. Use of MD5 .......................................136
         12.1.6. CAPWAP Fragmentation .............................136
    12.2. Session ID Security .....................................136
    12.3. Discovery or DTLS Setup Attacks .........................137
    12.4. Interference with a DTLS Session ........................137
    12.5. CAPWAP Pre-Provisioning .................................138
    12.6. Use of Pre-Shared Keys in CAPWAP ........................139
    12.7. Use of Certificates in CAPWAP ...........................140
    12.8. Use of MAC Address in CN Field ..........................140
    12.9. AAA Security ............................................141
    12.10. WTP Firmware ...........................................141
 13. Operational Considerations ...................................141
 14. Transport Considerations .....................................142
 15. IANA Considerations ..........................................143
    15.1. IPv4 Multicast Address ..................................143

Calhoun, et al. Standards Track [Page 5] RFC 5415 CAPWAP Protocol Specification March 2009

    15.2. IPv6 Multicast Address ..................................144
    15.3. UDP Port ................................................144
    15.4. CAPWAP Message Types ....................................144
    15.5. CAPWAP Header Flags .....................................144
    15.6. CAPWAP Control Message Flags ............................145
    15.7. CAPWAP Message Element Type .............................145
    15.8. CAPWAP Wireless Binding Identifiers .....................145
    15.9. AC Security Types .......................................146
    15.10. AC DTLS Policy .........................................146
    15.11. AC Information Type ....................................146
    15.12. CAPWAP Transport Protocol Types ........................146
    15.13. Data Transfer Type .....................................147
    15.14. Data Transfer Mode .....................................147
    15.15. Discovery Types ........................................147
    15.16. ECN Support ............................................148
    15.17. Radio Admin State ......................................148
    15.18. Radio Operational State ................................148
    15.19. Radio Failure Causes ...................................148
    15.20. Result Code ............................................149
    15.21. Returned Message Element Reason ........................149
    15.22. WTP Board Data Type ....................................149
    15.23. WTP Descriptor Type ....................................149
    15.24. WTP Fallback Mode ......................................150
    15.25. WTP Frame Tunnel Mode ..................................150
    15.26. WTP MAC Type ...........................................150
    15.27. WTP Radio Stats Failure Type ...........................151
    15.28. WTP Reboot Stats Failure Type ..........................151
 16. Acknowledgments ..............................................151
 17. References ...................................................151
    17.1. Normative References ....................................151
    17.2. Informative References ..................................153

Calhoun, et al. Standards Track [Page 6] RFC 5415 CAPWAP Protocol Specification March 2009

1. Introduction

 This document describes the CAPWAP protocol, a standard,
 interoperable protocol that enables an Access Controller (AC) to
 manage a collection of Wireless Termination Points (WTPs).  The
 CAPWAP protocol is defined to be independent of Layer 2 (L2)
 technology, and meets the objectives in "Objectives for Control and
 Provisioning of Wireless Access Points (CAPWAP)" [RFC4564].
 The emergence of centralized IEEE 802.11 Wireless Local Area Network
 (WLAN) architectures, in which simple IEEE 802.11 WTPs are managed by
 an Access Controller (AC), suggested that a standards-based,
 interoperable protocol could radically simplify the deployment and
 management of wireless networks.  WTPs require a set of dynamic
 management and control functions related to their primary task of
 connecting the wireless and wired mediums.  Traditional protocols for
 managing WTPs are either manual static configuration via HTTP,
 proprietary Layer 2-specific or non-existent (if the WTPs are self-
 contained).  An IEEE 802.11 binding is defined in [RFC5416] to
 support use of the CAPWAP protocol with IEEE 802.11 WLAN networks.
 CAPWAP assumes a network configuration consisting of multiple WTPs
 communicating via the Internet Protocol (IP) to an AC.  WTPs are
 viewed as remote radio frequency (RF) interfaces controlled by the
 AC.  The CAPWAP protocol supports two modes of operation: Split and
 Local MAC (medium access control).  In Split MAC mode, all L2
 wireless data and management frames are encapsulated via the CAPWAP
 protocol and exchanged between the AC and the WTP.  As shown in
 Figure 1, the wireless frames received from a mobile device, which is
 referred to in this specification as a Station (STA), are directly
 encapsulated by the WTP and forwarded to the AC.
            +-+         wireless frames        +-+
            | |--------------------------------| |
            | |              +-+               | |
            | |--------------| |---------------| |
            | |wireless PHY/ | |     CAPWAP    | |
            | | MAC sublayer | |               | |
            +-+              +-+               +-+
            STA              WTP                AC
      Figure 1: Representative CAPWAP Architecture for Split MAC
 The Local MAC mode of operation allows for the data frames to be
 either locally bridged or tunneled as 802.3 frames.  The latter
 implies that the WTP performs the 802.11 Integration function.  In
 either case, the L2 wireless management frames are processed locally

Calhoun, et al. Standards Track [Page 7] RFC 5415 CAPWAP Protocol Specification March 2009

 by the WTP and then forwarded to the AC.  Figure 2 shows the Local
 MAC mode, in which a station transmits a wireless frame that is
 encapsulated in an 802.3 frame and forwarded to the AC.
            +-+wireless frames +-+ 802.3 frames +-+
            | |----------------| |--------------| |
            | |                | |              | |
            | |----------------| |--------------| |
            | |wireless PHY/   | |     CAPWAP   | |
            | | MAC sublayer   | |              | |
            +-+                +-+              +-+
            STA                WTP               AC
      Figure 2: Representative CAPWAP Architecture for Local MAC
 Provisioning WTPs with security credentials and managing which WTPs
 are authorized to provide service are traditionally handled by
 proprietary solutions.  Allowing these functions to be performed from
 a centralized AC in an interoperable fashion increases manageability
 and allows network operators to more tightly control their wireless
 network infrastructure.

1.1. Goals

 The goals for the CAPWAP protocol are listed below:
 1. To centralize the authentication and policy enforcement functions
    for a wireless network.  The AC may also provide centralized
    bridging, forwarding, and encryption of user traffic.
    Centralization of these functions will enable reduced cost and
    higher efficiency by applying the capabilities of network
    processing silicon to the wireless network, as in wired LANs.
 2. To enable shifting of the higher-level protocol processing from
    the WTP.  This leaves the time-critical applications of wireless
    control and access in the WTP, making efficient use of the
    computing power available in WTPs, which are subject to severe
    cost pressure.
 3. To provide an extensible protocol that is not bound to a specific
    wireless technology.  Extensibility is provided via a generic
    encapsulation and transport mechanism, enabling the CAPWAP
    protocol to be applied to many access point types in the future,
    via a specific wireless binding.
 The CAPWAP protocol concerns itself solely with the interface between
 the WTP and the AC.  Inter-AC and station-to-AC communication are
 strictly outside the scope of this document.

Calhoun, et al. Standards Track [Page 8] RFC 5415 CAPWAP Protocol Specification March 2009

1.2. Conventions Used in This Document

 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 [RFC2119].

1.3. Contributing Authors

 This section lists and acknowledges the authors of significant text
 and concepts included in this specification.
 The CAPWAP Working Group selected the Lightweight Access Point
 Protocol (LWAPP) [LWAPP] to be used as the basis of the CAPWAP
 protocol specification.  The following people are authors of the
 LWAPP document:
    Bob O'Hara
    Email: bob.ohara@computer.org
    Pat Calhoun, Cisco Systems, Inc.
    170 West Tasman Drive, San Jose, CA  95134
    Phone: +1 408-902-3240, Email: pcalhoun@cisco.com
    Rohit Suri, Cisco Systems, Inc.
    170 West Tasman Drive, San Jose, CA  95134
    Phone: +1 408-853-5548, Email: rsuri@cisco.com
    Nancy Cam Winget, Cisco Systems, Inc.
    170 West Tasman Drive, San Jose, CA  95134
    Phone: +1 408-853-0532, Email: ncamwing@cisco.com
    Scott Kelly, Aruba Networks
    1322 Crossman Ave, Sunnyvale, CA 94089
    Phone: +1  408-754-8408, Email: skelly@arubanetworks.com
    Michael Glenn Williams, Nokia, Inc.
    313 Fairchild Drive, Mountain View, CA  94043
    Phone: +1 650-714-7758, Email: Michael.G.Williams@Nokia.com
    Sue Hares, Green Hills Software
    825 Victors Way, Suite 100, Ann Arbor, MI  48108
    Phone: +1 734 222 1610, Email: shares@ndzh.com
 Datagram Transport Layer Security (DTLS) [RFC4347] is used as the
 security solution for the CAPWAP protocol.  The following people are
 authors of significant DTLS-related text included in this document:

Calhoun, et al. Standards Track [Page 9] RFC 5415 CAPWAP Protocol Specification March 2009

    Scott Kelly, Aruba Networks
    1322 Crossman Ave, Sunnyvale, CA 94089
    Phone: +1  408-754-8408
    Email: skelly@arubanetworks.com
    Eric Rescorla, Network Resonance
    2483 El Camino Real, #212,Palo Alto CA, 94303
    Email: ekr@networkresonance.com
 The concept of using DTLS to secure the CAPWAP protocol was part of
 the Secure Light Access Point Protocol (SLAPP) proposal [SLAPP].  The
 following people are authors of the SLAPP proposal:
    Partha Narasimhan, Aruba Networks
    1322 Crossman Ave, Sunnyvale, CA  94089
    Phone: +1 408-480-4716
    Email: partha@arubanetworks.com
    Dan Harkins
    Trapeze Networks
    5753 W. Las Positas Blvd, Pleasanton, CA  94588
    Phone: +1-925-474-2212
    EMail: dharkins@trpz.com
    Subbu Ponnuswamy, Aruba Networks
    1322 Crossman Ave, Sunnyvale, CA  94089
    Phone: +1 408-754-1213
    Email: subbu@arubanetworks.com
 The following individuals contributed significant security-related
 text to the document [RFC5418]:
    T. Charles Clancy, Laboratory for Telecommunications Sciences,
    8080 Greenmead Drive, College Park, MD 20740
    Phone: +1 240-373-5069, Email: clancy@ltsnet.net
    Scott Kelly, Aruba Networks
    1322 Crossman Ave, Sunnyvale, CA 94089
    Phone: +1  408-754-8408, Email: scott@hyperthought.com

1.4. Terminology

 Access Controller (AC): The network entity that provides WTP access
 to the network infrastructure in the data plane, control plane,
 management plane, or a combination therein.

Calhoun, et al. Standards Track [Page 10] RFC 5415 CAPWAP Protocol Specification March 2009

 CAPWAP Control Channel: A bi-directional flow defined by the AC IP
 Address, WTP IP Address, AC control port, WTP control port, and the
 transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Control
 packets are sent and received.
 CAPWAP Data Channel: A bi-directional flow defined by the AC IP
 Address, WTP IP Address, AC data port, WTP data port, and the
 transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Data
 packets are sent and received.
 Station (STA): A device that contains an interface to a wireless
 medium (WM).
 Wireless Termination Point (WTP): The physical or network entity that
 contains an RF antenna and wireless Physical Layer (PHY) to transmit
 and receive station traffic for wireless access networks.
 This document uses additional terminology defined in [RFC3753].

2. Protocol Overview

 The CAPWAP protocol is a generic protocol defining AC and WTP control
 and data plane communication via a CAPWAP protocol transport
 mechanism.  CAPWAP Control messages, and optionally CAPWAP Data
 messages, are secured using Datagram Transport Layer Security (DTLS)
 [RFC4347].  DTLS is a standards-track IETF protocol based upon TLS.
 The underlying security-related protocol mechanisms of TLS have been
 successfully deployed for many years.
 The CAPWAP protocol transport layer carries two types of payload,
 CAPWAP Data messages and CAPWAP Control messages.  CAPWAP Data
 messages encapsulate forwarded wireless frames.  CAPWAP protocol
 Control messages are management messages exchanged between a WTP and
 an AC.  The CAPWAP Data and Control packets are sent over separate
 UDP ports.  Since both data and control packets can exceed the
 Maximum Transmission Unit (MTU) length, the payload of a CAPWAP Data
 or Control message can be fragmented.  The fragmentation behavior is
 defined in Section 3.
 The CAPWAP Protocol begins with a Discovery phase.  The WTPs send a
 Discovery Request message, causing any Access Controller (AC)
 receiving the message to respond with a Discovery Response message.
 From the Discovery Response messages received, a WTP selects an AC
 with which to establish a secure DTLS session.  In order to establish
 the secure DTLS connection, the WTP will need some amount of pre-
 provisioning, which is specified in Section 12.5.  CAPWAP protocol
 messages will be fragmented to the maximum length discovered to be
 supported by the network.

Calhoun, et al. Standards Track [Page 11] RFC 5415 CAPWAP Protocol Specification March 2009

 Once the WTP and the AC have completed DTLS session establishment, a
 configuration exchange occurs in which both devices agree on version
 information.  During this exchange, the WTP may receive provisioning
 settings.  The WTP is then enabled for operation.
 When the WTP and AC have completed the version and provision exchange
 and the WTP is enabled, the CAPWAP protocol is used to encapsulate
 the wireless data frames sent between the WTP and AC.  The CAPWAP
 protocol will fragment the L2 frames if the size of the encapsulated
 wireless user data (Data) or protocol control (Management) frames
 causes the resulting CAPWAP protocol packet to exceed the MTU
 supported between the WTP and AC.  Fragmented CAPWAP packets are
 reassembled to reconstitute the original encapsulated payload.  MTU
 Discovery and Fragmentation are described in Section 3.
 The CAPWAP protocol provides for the delivery of commands from the AC
 to the WTP for the management of stations that are communicating with
 the WTP.  This may include the creation of local data structures in
 the WTP for the stations and the collection of statistical
 information about the communication between the WTP and the stations.
 The CAPWAP protocol provides a mechanism for the AC to obtain
 statistical information collected by the WTP.
 The CAPWAP protocol provides for a keep-alive feature that preserves
 the communication channel between the WTP and AC.  If the AC fails to
 appear alive, the WTP will try to discover a new AC.

2.1. Wireless Binding Definition

 The CAPWAP protocol is independent of a specific WTP radio
 technology, as well its associated wireless link layer protocol.
 Elements of the CAPWAP protocol are designed to accommodate the
 specific needs of each wireless technology in a standard way.
 Implementation of the CAPWAP protocol for a particular wireless
 technology MUST follow the binding requirements defined for that
 technology.
 When defining a binding for wireless technologies, the authors MUST
 include any necessary definitions for technology-specific messages
 and all technology-specific message elements for those messages.  At
 a minimum, a binding MUST provide:
 1. The definition for a binding-specific Statistics message element,
    carried in the WTP Event Request message.
 2. A message element carried in the Station Configuration Request
    message to configure station information on the WTP.

Calhoun, et al. Standards Track [Page 12] RFC 5415 CAPWAP Protocol Specification March 2009

 3. A WTP Radio Information message element carried in the Discovery,
    Primary Discovery, and Join Request and Response messages,
    indicating the binding-specific radio types supported at the WTP
    and AC.
 If technology-specific message elements are required for any of the
 existing CAPWAP messages defined in this specification, they MUST
 also be defined in the technology binding document.
 The naming of binding-specific message elements MUST begin with the
 name of the technology type, e.g., the binding for IEEE 802.11,
 provided in [RFC5416], begins with "IEEE 802.11".
 The CAPWAP binding concept MUST also be used in any future
 specifications that add functionality to either the base CAPWAP
 protocol specification, or any published CAPWAP binding
 specification.  A separate WTP Radio Information message element MUST
 be created to properly advertise support for the specification.  This
 mechanism allows for future protocol extensibility, while providing
 the necessary capabilities advertisement, through the WTP Radio
 Information message element, to ensure WTP/AC interoperability.

2.2. CAPWAP Session Establishment Overview

 This section describes the session establishment process message
 exchanges between a CAPWAP WTP and AC.  The annotated ladder diagram
 shows the AC on the right, the WTP on the left, and assumes the use
 of certificates for DTLS authentication.  The CAPWAP protocol state
 machine is described in detail in Section 2.3.  Note that DTLS allows
 certain messages to be aggregated into a single frame, which is
 denoted via an asterisk in Figure 3.
         ============                         ============
             WTP                                   AC
         ============                         ============
          [----------- begin optional discovery ------------]
                         Discover Request
               ------------------------------------>
                         Discover Response
               <------------------------------------
          [----------- end optional discovery ------------]
                    (-- begin DTLS handshake --)
                           ClientHello
               ------------------------------------>

Calhoun, et al. Standards Track [Page 13] RFC 5415 CAPWAP Protocol Specification March 2009

                    HelloVerifyRequest (with cookie)
               <------------------------------------
                      ClientHello (with cookie)
               ------------------------------------>
                              ServerHello,
                              Certificate,
                              ServerHelloDone*
               <------------------------------------
              (-- WTP callout for AC authorization --)
                      Certificate (optional),
                       ClientKeyExchange,
                   CertificateVerify (optional),
                       ChangeCipherSpec,
                           Finished*
               ------------------------------------>
              (-- AC callout for WTP authorization --)
                       ChangeCipherSpec,
                           Finished*
               <------------------------------------
              (-- DTLS session is established now --)
                            Join Request
               ------------------------------------>
                            Join Response
               <------------------------------------
                    [-- Join State Complete --]
                 (-- assume image is up to date --)
                    Configuration Status Request
               ------------------------------------>
                    Configuration Status Response
               <------------------------------------
                  [-- Configure State Complete --]
                     Change State Event Request
               ------------------------------------>
                     Change State Event Response
               <------------------------------------
                 [-- Data Check State Complete --]

Calhoun, et al. Standards Track [Page 14] RFC 5415 CAPWAP Protocol Specification March 2009

                      (-- enter RUN state --)
                                 :
                                 :
                            Echo Request
               ------------------------------------>
                           Echo Response
               <------------------------------------
                                 :
                                 :
                            Event Request
               ------------------------------------>
                           Event Response
               <------------------------------------
                                 :
                                 :
              Figure 3: CAPWAP Control Protocol Exchange
 At the end of the illustrated CAPWAP message exchange, the AC and WTP
 are securely exchanging CAPWAP Control messages.  This illustration
 is provided to clarify protocol operation, and does not include any
 possible error conditions.  Section 2.3 provides a detailed
 description of the corresponding state machine.

2.3. CAPWAP State Machine Definition

 The following state diagram represents the lifecycle of a WTP-AC
 session.  Use of DTLS by the CAPWAP protocol results in the
 juxtaposition of two nominally separate yet tightly bound state
 machines.  The DTLS and CAPWAP state machines are coupled through an
 API consisting of commands (see Section 2.3.2.1) and notifications
 (see Section 2.3.2.2).  Certain transitions in the DTLS state machine
 are triggered by commands from the CAPWAP state machine, while
 certain transitions in the CAPWAP state machine are triggered by
 notifications from the DTLS state machine.

Calhoun, et al. Standards Track [Page 15] RFC 5415 CAPWAP Protocol Specification March 2009

                          /-------------------------------------\
                          |          /-------------------------\|
                          |         p|                         ||
                          |    q+----------+ r +------------+  ||
                          |     |   Run    |-->|   Reset    |-\||
                          |     +----------+   +------------+ |||
                         n|  o      ^           ^     ^      s|||
              +------------+--------/           |     |       |||
              | Data Check |             /-------/    |       |||
              +------------+<-------\   |             |       |||
                                    |   |             |       |||
                     /------------------+--------\    |       |||
                    f|             m|  h|    j   v   k|       |||
             +--------+     +-----------+     +--------------+|||
             |  Join  |---->| Configure |     |  Image Data  ||||
             +--------+  n  +-----------+     +--------------+|||
              ^   |g                 i|                    l| |||
              |   |                   \-------------------\ | |||
              |   \--------------------------------------\| | |||
              \------------------------\                 || | |||
       /--------------<----------------+---------------\ || | |||
       | /------------<----------------+-------------\ | || | |||
       | |  4                          |d           t| | vv v vvv
       | |   +----------------+   +--------------+   +-----------+
       | |   |   DTLS Setup   |   | DTLS Connect |-->|  DTLS TD  |
     /-|-|---+----------------+   +--------------+ e +-----------+
     | | |    |$  ^  ^   |5  ^6         ^              ^  |w
     v v v    |   |  |   |   \-------\  |              |  |
     | | |    |   |  |   \---------\ |  |  /-----------/  |
     | | |    |   |  \--\          | |  |  |              |
     | | |    |   |     |          | |  |  |              |
     | | |    v  3|  1  |%     #   v |  |a |b             v
     | | \->+------+-->+------+   +-----------+    +--------+
     | |    | Idle |   | Disc |   | Authorize |    |  Dead  |
     | |    +------+<--+------+   +-----------+    +--------+
     | |     ^   0^  2      |!
     | |     |    |         |   +-------+
    *| |u    |    \---------+---| Start |
     | |     |@             |   +-------+
     | \->+---------+<------/
     \--->| Sulking |
          +---------+&
               Figure 4: CAPWAP Integrated State Machine
 The CAPWAP protocol state machine, depicted above, is used by both
 the AC and the WTP.  In cases where states are not shared (i.e., not
 implemented in one or the other of the AC or WTP), this is explicitly

Calhoun, et al. Standards Track [Page 16] RFC 5415 CAPWAP Protocol Specification March 2009

 called out in the transition descriptions below.  For every state
 defined, only certain messages are permitted to be sent and received.
 The CAPWAP Control message definitions specify the state(s) in which
 each message is valid.
 Since the WTP only communicates with a single AC, it only has a
 single instance of the CAPWAP state machine.  The state machine works
 differently on the AC since it communicates with many WTPs.  The AC
 uses the concept of three threads.  Note that the term thread used
 here does not necessarily imply that implementers must use threads,
 but it is one possible way of implementing the AC's state machine.
 Listener Thread:   The AC's Listener thread handles inbound DTLS
    session establishment requests, through the DTLSListen command.
    Upon creation, the Listener thread starts in the DTLS Setup state.
    Once a DTLS session has been validated, which occurs when the
    state machine enters the "Authorize" state, the Listener thread
    creates a WTP session-specific Service thread and state context.
    The state machine transitions in Figure 4 are represented by
    numerals.  It is necessary for the AC to protect itself against
    various attacks that exist with non-authenticated frames.  See
    Section 12 for more information.
 Discovery Thread:   The AC's Discovery thread is responsible for
    receiving, and responding to, Discovery Request messages.  The
    state machine transitions in Figure 4 are represented by numerals.
    Note that the Discovery thread does not maintain any per-WTP-
    specific context information, and a single state context exists.
    It is necessary for the AC to protect itself against various
    attacks that exist with non-authenticated frames.  See Section 12
    for more information.
 Service Thread:   The AC's Service thread handles the per-WTP states,
    and one such thread exists per-WTP connection.  This thread is
    created by the Listener thread when the Authorize state is
    reached.  When created, the Service thread inherits a copy of the
    state machine context from the Listener thread.  When
    communication with the WTP is complete, the Service thread is
    terminated and all associated resources are released.  The state
    machine transitions in Figure 4 are represented by alphabetic and
    punctuation characters.

2.3.1. CAPWAP Protocol State Transitions

 This section describes the various state transitions, and the events
 that cause them.  This section does not discuss interactions between
 DTLS- and CAPWAP-specific states.  Those interactions, and DTLS-
 specific states and transitions, are discussed in Section 2.3.2.

Calhoun, et al. Standards Track [Page 17] RFC 5415 CAPWAP Protocol Specification March 2009

 Start to Idle (0):  This transition occurs once device initialization
    is complete.
    WTP:  This state transition is used to start the WTP's CAPWAP
          state machine.
    AC:   The AC creates the Discovery and Listener threads and starts
          the CAPWAP state machine.
 Idle to Discovery (1):  This transition occurs to support the CAPWAP
    discovery process.
    WTP:  The WTP enters the Discovery state prior to transmitting the
          first Discovery Request message (see Section 5.1).  Upon
          entering this state, the WTP sets the DiscoveryInterval
          timer (see Section 4.7).  The WTP resets the DiscoveryCount
          counter to zero (0) (see Section 4.8).  The WTP also clears
          all information from ACs it may have received during a
          previous Discovery phase.
    AC:   This state transition is executed by the AC's Discovery
          thread, and occurs when a Discovery Request message is
          received.  The AC SHOULD respond with a Discovery Response
          message (see Section 5.2).
 Discovery to Discovery (#):  In the Discovery state, the WTP
    determines to which AC to connect.
    WTP:  This transition occurs when the DiscoveryInterval timer
          expires.  If the WTP is configured with a list of ACs, it
          transmits a Discovery Request message to every AC from which
          it has not received a Discovery Response message.  For every
          transition to this event, the WTP increments the
          DiscoveryCount counter.  See Section 5.1 for more
          information on how the WTP knows the ACs to which it should
          transmit the Discovery Request messages.  The WTP restarts
          the DiscoveryInterval timer whenever it transmits Discovery
          Request messages.
    AC:   This is an invalid state transition for the AC.
 Discovery to Idle (2):  This transition occurs on the AC's Discovery
    thread when the Discovery processing is complete.
    WTP:  This is an invalid state transition for the WTP.

Calhoun, et al. Standards Track [Page 18] RFC 5415 CAPWAP Protocol Specification March 2009

    AC:   This state transition is executed by the AC's Discovery
          thread when it has transmitted the Discovery Response, in
          response to a Discovery Request.
 Discovery to Sulking (!):  This transition occurs on a WTP when AC
    Discovery fails.
    WTP:  The WTP enters this state when the DiscoveryInterval timer
          expires and the DiscoveryCount variable is equal to the
          MaxDiscoveries variable (see Section 4.8).  Upon entering
          this state, the WTP MUST start the SilentInterval timer.
          While in the Sulking state, all received CAPWAP protocol
          messages MUST be ignored.
    AC:   This is an invalid state transition for the AC.
 Sulking to Idle (@):  This transition occurs on a WTP when it must
    restart the Discovery phase.
    WTP:  The WTP enters this state when the SilentInterval timer (see
          Section 4.7) expires.  The FailedDTLSSessionCount,
          DiscoveryCount, and FailedDTLSAuthFailCount counters are
          reset to zero.
    AC:   This is an invalid state transition for the AC.
 Sulking to Sulking (&):  The Sulking state provides the silent
    period, minimizing the possibility for Denial-of-Service (DoS)
    attacks.
    WTP:  All packets received from the AC while in the sulking state
          are ignored.
    AC:   This is an invalid state transition for the AC.
 Idle to DTLS Setup (3):  This transition occurs to establish a secure
    DTLS session with the peer.
    WTP:  The WTP initiates this transition by invoking the DTLSStart
          command (see Section 2.3.2.1), which starts the DTLS session
          establishment with the chosen AC and the WaitDTLS timer is
          started (see Section 4.7).  When the Discovery phase is
          bypassed, it is assumed the WTP has locally configured ACs.

Calhoun, et al. Standards Track [Page 19] RFC 5415 CAPWAP Protocol Specification March 2009

    AC:   Upon entering the Idle state from the Start state, the newly
          created Listener thread automatically transitions to the
          DTLS Setup and invokes the DTLSListen command (see
          Section 2.3.2.1), and the WaitDTLS timer is started (see
          Section 4.7).
 Discovery to DTLS Setup (%):  This transition occurs to establish a
    secure DTLS session with the peer.
    WTP:  The WTP initiates this transition by invoking the DTLSStart
          command (see Section 2.3.2.1), which starts the DTLS session
          establishment with the chosen AC.  The decision of to which
          AC to connect is the result of the Discovery phase, which is
          described in Section 3.3.
    AC:   This is an invalid state transition for the AC.
 DTLS Setup to Idle ($):  This transition occurs when the DTLS
    connection setup fails.
    WTP:  The WTP initiates this state transition when it receives a
          DTLSEstablishFail notification from DTLS (see
          Section 2.3.2.2), and the FailedDTLSSessionCount or the
          FailedDTLSAuthFailCount counter have not reached the value
          of the MaxFailedDTLSSessionRetry variable (see Section 4.8).
          This error notification aborts the secure DTLS session
          establishment.  When this notification is received, the
          FailedDTLSSessionCount counter is incremented.  This state
          transition also occurs if the WaitDTLS timer has expired.
    AC:   This is an invalid state transition for the AC.
 DTLS Setup to Sulking (*):  This transition occurs when repeated
    attempts to set up the DTLS connection have failed.
    WTP:  The WTP enters this state when the FailedDTLSSessionCount or
          the FailedDTLSAuthFailCount counter reaches the value of the
          MaxFailedDTLSSessionRetry variable (see Section 4.8).  Upon
          entering this state, the WTP MUST start the SilentInterval
          timer.  While in the Sulking state, all received CAPWAP and
          DTLS protocol messages received MUST be ignored.
    AC:   This is an invalid state transition for the AC.
 DTLS Setup to DTLS Setup (4):  This transition occurs when the DTLS
    Session failed to be established.
    WTP:  This is an invalid state transition for the WTP.

Calhoun, et al. Standards Track [Page 20] RFC 5415 CAPWAP Protocol Specification March 2009

    AC:   The AC's Listener initiates this state transition when it
          receives a DTLSEstablishFail notification from DTLS (see
          Section 2.3.2.2).  This error notification aborts the secure
          DTLS session establishment.  When this notification is
          received, the FailedDTLSSessionCount counter is incremented.
          The Listener thread then invokes the DTLSListen command (see
          Section 2.3.2.1).
 DTLS Setup to Authorize (5):  This transition occurs when an incoming
    DTLS session is being established, and the DTLS stack needs
    authorization to proceed with the session establishment.
    WTP:  This state transition occurs when the WTP receives the
          DTLSPeerAuthorize notification (see Section 2.3.2.2).  Upon
          entering this state, the WTP performs an authorization check
          against the AC credentials.  See Section 2.4.4 for more
          information on AC authorization.
    AC:   This state transition is handled by the AC's Listener thread
          when the DTLS module initiates the DTLSPeerAuthorize
          notification (see Section 2.3.2.2).  The Listener thread
          forks an instance of the Service thread, along with a copy
          of the state context.  Once created, the Service thread
          performs an authorization check against the WTP credentials.
          See Section 2.4.4 for more information on WTP authorization.
 Authorize to DTLS Setup (6):  This transition is executed by the
    Listener thread to enable it to listen for new incoming sessions.
    WTP:  This is an invalid state transition for the WTP.
    AC:   This state transition occurs when the AC's Listener thread
          has created the WTP context and the Service thread.  The
          Listener thread then invokes the DTLSListen command (see
          Section 2.3.2.1).
 Authorize to DTLS Connect (a):  This transition occurs to notify the
    DTLS stack that the session should be established.
    WTP:  This state transition occurs when the WTP has successfully
          authorized the AC's credentials (see Section 2.4.4).  This
          is done by invoking the DTLSAccept DTLS command (see
          Section 2.3.2.1).
    AC:   This state transition occurs when the AC has successfully
          authorized the WTP's credentials (see Section 2.4.4).  This
          is done by invoking the DTLSAccept DTLS command (see
          Section 2.3.2.1).

Calhoun, et al. Standards Track [Page 21] RFC 5415 CAPWAP Protocol Specification March 2009

 Authorize to DTLS Teardown (b):  This transition occurs to notify the
    DTLS stack that the session should be aborted.
    WTP:  This state transition occurs when the WTP has been unable to
          authorize the AC, using the AC credentials.  The WTP then
          aborts the DTLS session by invoking the DTLSAbortSession
          command (see Section 2.3.2.1).  This state transition also
          occurs if the WaitDTLS timer has expired.  The WTP starts
          the DTLSSessionDelete timer (see Section 4.7.6).
    AC:   This state transition occurs when the AC has been unable to
          authorize the WTP, using the WTP credentials.  The AC then
          aborts the DTLS session by invoking the DTLSAbortSession
          command (see Section 2.3.2.1).  This state transition also
          occurs if the WaitDTLS timer has expired.  The AC starts the
          DTLSSessionDelete timer (see Section 4.7.6).
 DTLS Connect to DTLS Teardown (c):  This transition occurs when the
    DTLS Session failed to be established.
    WTP:  This state transition occurs when the WTP receives either a
          DTLSAborted or DTLSAuthenticateFail notification (see
          Section 2.3.2.2), indicating that the DTLS session was not
          successfully established.  When this transition occurs due
          to the DTLSAuthenticateFail notification, the
          FailedDTLSAuthFailCount is incremented; otherwise, the
          FailedDTLSSessionCount counter is incremented.  This state
          transition also occurs if the WaitDTLS timer has expired.
          The WTP starts the DTLSSessionDelete timer (see
          Section 4.7.6).
    AC:   This state transition occurs when the AC receives either a
          DTLSAborted or DTLSAuthenticateFail notification (see
          Section 2.3.2.2), indicating that the DTLS session was not
          successfully established, and both of the
          FailedDTLSAuthFailCount and FailedDTLSSessionCount counters
          have not reached the value of the MaxFailedDTLSSessionRetry
          variable (see Section 4.8).  This state transition also
          occurs if the WaitDTLS timer has expired.  The AC starts the
          DTLSSessionDelete timer (see Section 4.7.6).
 DTLS Connect to Join (d):  This transition occurs when the DTLS
    Session is successfully established.
    WTP:  This state transition occurs when the WTP receives the
          DTLSEstablished notification (see Section 2.3.2.2),
          indicating that the DTLS session was successfully
          established.  When this notification is received, the

Calhoun, et al. Standards Track [Page 22] RFC 5415 CAPWAP Protocol Specification March 2009

          FailedDTLSSessionCount counter is set to zero.  The WTP
          enters the Join state by transmitting the Join Request to
          the AC.  The WTP stops the WaitDTLS timer.
    AC:   This state transition occurs when the AC receives the
          DTLSEstablished notification (see Section 2.3.2.2),
          indicating that the DTLS session was successfully
          established.  When this notification is received, the
          FailedDTLSSessionCount counter is set to zero.  The AC stops
          the WaitDTLS timer, and starts the WaitJoin timer.
 Join to DTLS Teardown (e):  This transition occurs when the join
    process has failed.
    WTP:  This state transition occurs when the WTP receives a Join
          Response message with a Result Code message element
          containing an error, or if the Image Identifier provided by
          the AC in the Join Response message differs from the WTP's
          currently running firmware version and the WTP has the
          requested image in its non-volatile memory.  This causes the
          WTP to initiate the DTLSShutdown command (see
          Section 2.3.2.1).  This transition also occurs if the WTP
          receives one of the following DTLS notifications:
          DTLSAborted, DTLSReassemblyFailure, or DTLSPeerDisconnect.
          The WTP starts the DTLSSessionDelete timer (see
          Section 4.7.6).
    AC:   This state transition occurs either if the WaitJoin timer
          expires or if the AC transmits a Join Response message with
          a Result Code message element containing an error.  This
          causes the AC to initiate the DTLSShutdown command (see
          Section 2.3.2.1).  This transition also occurs if the AC
          receives one of the following DTLS notifications:
          DTLSAborted, DTLSReassemblyFailure, or DTLSPeerDisconnect.
          The AC starts the DTLSSessionDelete timer (see
          Section 4.7.6).
 Join to Image Data (f):  This state transition is used by the WTP and
    the AC to download executable firmware.
    WTP:  The WTP enters the Image Data state when it receives a
          successful Join Response message and determines that the
          software version in the Image Identifier message element is
          not the same as its currently running image.  The WTP also
          detects that the requested image version is not currently
          available in the WTP's non-volatile storage (see Section 9.1
          for a full description of the firmware download process).
          The WTP initializes the EchoInterval timer (see

Calhoun, et al. Standards Track [Page 23] RFC 5415 CAPWAP Protocol Specification March 2009

          Section 4.7), and transmits the Image Data Request message
          (see Section 9.1.1) requesting the start of the firmware
          download.
    AC:   This state transition occurs when the AC receives the Image
          Data Request message from the WTP, after having sent its
          Join Response to the WTP.  The AC stops the WaitJoin timer.
          The AC MUST transmit an Image Data Response message (see
          Section 9.1.2) to the WTP, which includes a portion of the
          firmware.
 Join to Configure (g):  This state transition is used by the WTP and
    the AC to exchange configuration information.
    WTP:  The WTP enters the Configure state when it receives a
          successful Join Response message, and determines that the
          included Image Identifier message element is the same as its
          currently running image.  The WTP transmits the
          Configuration Status Request message (see Section 8.2) to
          the AC with message elements describing its current
          configuration.
    AC:   This state transition occurs when it receives the
          Configuration Status Request message from the WTP (see
          Section 8.2), which MAY include specific message elements to
          override the WTP's configuration.  The AC stops the WaitJoin
          timer.  The AC transmits the Configuration Status Response
          message (see Section 8.3) and starts the
          ChangeStatePendingTimer timer (see Section 4.7).
 Configure to Reset (h):  This state transition is used to reset the
    connection either due to an error during the configuration phase,
    or when the WTP determines it needs to reset in order for the new
    configuration to take effect.  The CAPWAP Reset command is used to
    indicate to the peer that it will initiate a DTLS teardown.
    WTP:  The WTP enters the Reset state when it receives a
          Configuration Status Response message indicating an error or
          when it determines that a reset of the WTP is required, due
          to the characteristics of a new configuration.
    AC:   The AC transitions to the Reset state when it receives a
          Change State Event message from the WTP that contains an
          error for which AC policy does not permit the WTP to provide
          service.  This state transition also occurs when the AC
          ChangeStatePendingTimer timer expires.

Calhoun, et al. Standards Track [Page 24] RFC 5415 CAPWAP Protocol Specification March 2009

 Configure to DTLS Teardown (i):  This transition occurs when the
    configuration process aborts due to a DTLS error.
    WTP:  The WTP enters this state when it receives one of the
          following DTLS notifications: DTLSAborted,
          DTLSReassemblyFailure, or DTLSPeerDisconnect (see
          Section 2.3.2.2).  The WTP MAY tear down the DTLS session if
          it receives frequent DTLSDecapFailure notifications.  The
          WTP starts the DTLSSessionDelete timer (see Section 4.7.6).
    AC:   The AC enters this state when it receives one of the
          following DTLS notifications: DTLSAborted,
          DTLSReassemblyFailure, or DTLSPeerDisconnect (see
          Section 2.3.2.2).  The AC MAY tear down the DTLS session if
          it receives frequent DTLSDecapFailure notifications.  The AC
          starts the DTLSSessionDelete timer (see Section 4.7.6).
 Image Data to Image Data (j):  The Image Data state is used by the
    WTP and the AC during the firmware download phase.
    WTP:  The WTP enters the Image Data state when it receives an
          Image Data Response message indicating that the AC has more
          data to send.  This state transition also occurs when the
          WTP receives the subsequent Image Data Requests, at which
          time it resets the ImageDataStartTimer time to ensure it
          receives the next expected Image Data Request from the AC.
          This state transition can also occur when the WTP's
          EchoInterval timer (see Section 4.7.7) expires, in which
          case the WTP transmits an Echo Request message (see
          Section 7.1), and resets its EchoInterval timer.  The state
          transition also occurs when the WTP receives an Echo
          Response from the AC (see Section 7.2).
    AC:   This state transition occurs when the AC receives the Image
          Data Response message from the WTP while already in the
          Image Data state.  This state transition also occurs when
          the AC receives an Echo Request (see Section 7.1) from the
          WTP, in which case it responds with an Echo Response (see
          Section 7.2), and resets its EchoInterval timer (see
          Section 4.7.7).

Calhoun, et al. Standards Track [Page 25] RFC 5415 CAPWAP Protocol Specification March 2009

 Image Data to Reset (k):  This state transition is used to reset the
    DTLS connection prior to restarting the WTP after an image
    download.
    WTP:  When an image download completes, or if the
          ImageDataStartTimer timer expires, the WTP enters the Reset
          state.  The WTP MAY also transition to this state upon
          receiving an Image Data Response message from the AC (see
          Section 9.1.2) indicating a failure.
    AC:   The AC enters the Reset state either when the image transfer
          has successfully completed or an error occurs during the
          image download process.
 Image Data to DTLS Teardown (l):  This transition occurs when the
    firmware download process aborts due to a DTLS error.
    WTP:  The WTP enters this state when it receives one of the
          following DTLS notifications: DTLSAborted,
          DTLSReassemblyFailure, or DTLSPeerDisconnect (see
          Section 2.3.2.2).  The WTP MAY tear down the DTLS session if
          it receives frequent DTLSDecapFailure notifications.  The
          WTP starts the DTLSSessionDelete timer (see Section 4.7.6).
    AC:   The AC enters this state when it receives one of the
          following DTLS notifications: DTLSAborted,
          DTLSReassemblyFailure, or DTLSPeerDisconnect (see
          Section 2.3.2.2).  The AC MAY tear down the DTLS session if
          it receives frequent DTLSDecapFailure notifications.  The AC
          starts the DTLSSessionDelete timer (see Section 4.7.6).
 Configure to Data Check (m):  This state transition occurs when the
    WTP and AC confirm the configuration.
    WTP:  The WTP enters this state when it receives a successful
          Configuration Status Response message from the AC.  The WTP
          transmits the Change State Event Request message (see
          Section 8.6).
    AC:   This state transition occurs when the AC receives the Change
          State Event Request message (see Section 8.6) from the WTP.
          The AC responds with a Change State Event Response message
          (see Section 8.7).  The AC MUST start the DataCheckTimer
          timer and stops the ChangeStatePendingTimer timer (see
          Section 4.7).
 Data Check to DTLS Teardown (n):  This transition occurs when the WTP
    does not complete the Data Check exchange.

Calhoun, et al. Standards Track [Page 26] RFC 5415 CAPWAP Protocol Specification March 2009

    WTP:  This state transition occurs if the WTP does not receive the
          Change State Event Response message before a CAPWAP
          retransmission timeout occurs.  The WTP also transitions to
          this state if the underlying reliable transport's
          RetransmitCount counter has reached the MaxRetransmit
          variable (see Section 4.7).  The WTP starts the
          DTLSSessionDelete timer (see Section 4.7.6).
    AC:   The AC enters this state when the DataCheckTimer timer
          expires (see Section 4.7).  The AC starts the
          DTLSSessionDelete timer (see Section 4.7.6).
 Data Check to Run (o):  This state transition occurs when the linkage
    between the control and data channels is established, causing the
    WTP and AC to enter their normal state of operation.
    WTP:  The WTP enters this state when it receives a successful
          Change State Event Response message from the AC.  The WTP
          initiates the data channel, which MAY require the
          establishment of a DTLS session, starts the
          DataChannelKeepAlive timer (see Section 4.7.2) and transmits
          a Data Channel Keep-Alive packet (see Section 4.4.1).  The
          WTP then starts the EchoInterval timer and
          DataChannelDeadInterval timer (see Section 4.7).
    AC:   This state transition occurs when the AC receives the Data
          Channel Keep-Alive packet (see Section 4.4.1), with a
          Session ID message element matching that included by the WTP
          in the Join Request message.  The AC disables the
          DataCheckTimer timer.  Note that if AC policy is to require
          the data channel to be encrypted, this process would also
          require the establishment of a data channel DTLS session.
          Upon receiving the Data Channel Keep-Alive packet, the AC
          transmits its own Data Channel Keep Alive packet.
 Run to DTLS Teardown (p):  This state transition occurs when an error
    has occurred in the DTLS stack, causing the DTLS session to be
    torn down.
    WTP:  The WTP enters this state when it receives one of the
          following DTLS notifications: DTLSAborted,
          DTLSReassemblyFailure, or DTLSPeerDisconnect (see
          Section 2.3.2.2).  The WTP MAY tear down the DTLS session if
          it receives frequent DTLSDecapFailure notifications.  The
          WTP also transitions to this state if the underlying
          reliable transport's RetransmitCount counter has reached the
          MaxRetransmit variable (see Section 4.7).  The WTP starts
          the DTLSSessionDelete timer (see Section 4.7.6).

Calhoun, et al. Standards Track [Page 27] RFC 5415 CAPWAP Protocol Specification March 2009

    AC:   The AC enters this state when it receives one of the
          following DTLS notifications: DTLSAborted,
          DTLSReassemblyFailure, or DTLSPeerDisconnect (see
          Section 2.3.2.2).  The AC MAY tear down the DTLS session if
          it receives frequent DTLSDecapFailure notifications.  The AC
          transitions to this state if the underlying reliable
          transport's RetransmitCount counter has reached the
          MaxRetransmit variable (see Section 4.7).  This state
          transition also occurs when the AC's EchoInterval timer (see
          Section 4.7.7) expires.  The AC starts the DTLSSessionDelete
          timer (see Section 4.7.6).
 Run to Run (q):  This is the normal state of operation.
    WTP:  This is the WTP's normal state of operation.  The WTP resets
          its EchoInterval timer whenever it transmits a request to
          the AC.  There are many events that result in this state
          transition:
          Configuration Update:  The WTP receives a Configuration
                Update Request message (see Section 8.4).  The WTP
                MUST respond with a Configuration Update Response
                message (see Section 8.5).
          Change State Event:  The WTP receives a Change State Event
                Response message, or determines that it must initiate
                a Change State Event Request message, as a result of a
                failure or change in the state of a radio.
          Echo Request:  The WTP sends an Echo Request message
                (Section 7.1) or receives the corresponding Echo
                Response message, (see Section 7.2) from the AC.  When
                the WTP receives the Echo Response, it resets its
                EchoInterval timer (see Section 4.7.7).
          Clear Config Request:  The WTP receives a Clear
                Configuration Request message (see Section 8.8) and
                MUST generate a corresponding Clear Configuration
                Response message (see Section 8.9).  The WTP MUST
                reset its configuration back to manufacturer defaults.
          WTP Event:  The WTP sends a WTP Event Request message,
                delivering information to the AC (see Section 9.4).
                The WTP receives a WTP Event Response message from the
                AC (see Section 9.5).

Calhoun, et al. Standards Track [Page 28] RFC 5415 CAPWAP Protocol Specification March 2009

          Data Transfer:  The WTP sends a Data Transfer Request or
                Data Transfer Response message to the AC (see
                Section 9.6).  The WTP receives a Data Transfer
                Request or Data Transfer Response message from the AC
                (see Section 9.6).  Upon receipt of a Data Transfer
                Request, the WTP transmits a Data Transfer Response to
                the AC.
          Station Configuration Request:  The WTP receives a Station
                Configuration Request message (see Section 10.1), to
                which it MUST respond with a Station Configuration
                Response message (see Section 10.2).
    AC:   This is the AC's normal state of operation.  Note that the
          receipt of any Request from the WTP causes the AC to reset
          its EchoInterval timer (see Section 4.7.7).
          Configuration Update:  The AC sends a Configuration Update
                Request message (see Section 8.4) to the WTP to update
                its configuration.  The AC receives a Configuration
                Update Response message (see Section 8.5) from the
                WTP.
          Change State Event:  The AC receives a Change State Event
                Request message (see Section 8.6), to which it MUST
                respond with the Change State Event Response message
                (see Section 8.7).
          Echo Request:  The AC receives an Echo Request message (see
                Section 7.1), to which it MUST respond with an Echo
                Response message (see Section 7.2).
          Clear Config Response:  The AC sends a Clear Configuration
                Request message (see Section 8.8) to the WTP to clear
                its configuration.  The AC receives a Clear
                Configuration Response message from the WTP (see
                Section 8.9).
          WTP Event:  The AC receives a WTP Event Request message from
                the WTP (see Section 9.4) and MUST generate a
                corresponding WTP Event Response message (see
                Section 9.5).
          Data Transfer:  The AC sends a Data Transfer Request or Data
                Transfer Response message to the WTP (see
                Section 9.6).  The AC receives a Data Transfer Request

Calhoun, et al. Standards Track [Page 29] RFC 5415 CAPWAP Protocol Specification March 2009

                or Data Transfer Response message from the WTP (see
                Section 9.6).  Upon receipt of a Data Transfer
                Request, the AC transmits a Data Transfer Response to
                the WTP.
          Station Configuration Request:  The AC sends a Station
                Configuration Request message (see Section 10.1) or
                receives the corresponding Station Configuration
                Response message (see Section 10.2) from the WTP.
 Run to Reset (r):  This state transition is used when either the AC
    or WTP tears down the connection.  This may occur as part of
    normal operation, or due to error conditions.
    WTP:  The WTP enters the Reset state when it receives a Reset
          Request message from the AC.
    AC:   The AC enters the Reset state when it transmits a Reset
          Request message to the WTP.
 Reset to DTLS Teardown (s):  This transition occurs when the CAPWAP
    reset is complete to terminate the DTLS session.
    WTP:  This state transition occurs when the WTP transmits a Reset
          Response message.  The WTP does not invoke the DTLSShutdown
          command (see Section 2.3.2.1).  The WTP starts the
          DTLSSessionDelete timer (see Section 4.7.6).
    AC:   This state transition occurs when the AC receives a Reset
          Response message.  This causes the AC to initiate the
          DTLSShutdown command (see Section 2.3.2.1).  The AC starts
          the DTLSSessionDelete timer (see Section 4.7.6).
 DTLS Teardown to Idle (t):  This transition occurs when the DTLS
    session has been shut down.
    WTP:  This state transition occurs when the WTP has successfully
          cleaned up all resources associated with the control plane
          DTLS session, or if the DTLSSessionDelete timer (see
          Section 4.7.6) expires.  The data plane DTLS session is also
          shut down, and all resources released, if a DTLS session was
          established for the data plane.  Any timers set for the
          current instance of the state machine are also cleared.
    AC:   This is an invalid state transition for the AC.

Calhoun, et al. Standards Track [Page 30] RFC 5415 CAPWAP Protocol Specification March 2009

 DTLS Teardown to Sulking (u):  This transition occurs when repeated
    attempts to setup the DTLS connection have failed.
    WTP:  The WTP enters this state when the FailedDTLSSessionCount or
          the FailedDTLSAuthFailCount counter reaches the value of the
          MaxFailedDTLSSessionRetry variable (see Section 4.8).  Upon
          entering this state, the WTP MUST start the SilentInterval
          timer.  While in the Sulking state, all received CAPWAP and
          DTLS protocol messages received MUST be ignored.
    AC:   This is an invalid state transition for the AC.
 DTLS Teardown to Dead (w):  This transition occurs when the DTLS
    session has been shut down.
    WTP:  This is an invalid state transition for the WTP.
    AC:   This state transition occurs when the AC has successfully
          cleaned up all resources associated with the control plane
          DTLS session , or if the DTLSSessionDelete timer (see
          Section 4.7.6) expires.  The data plane DTLS session is also
          shut down, and all resources released, if a DTLS session was
          established for the data plane.  Any timers set for the
          current instance of the state machine are also cleared.  The
          AC's Service thread is terminated.

2.3.2. CAPWAP/DTLS Interface

 This section describes the DTLS Commands used by CAPWAP, and the
 notifications received from DTLS to the CAPWAP protocol stack.

2.3.2.1. CAPWAP to DTLS Commands

 Six commands are defined for the CAPWAP to DTLS API.  These
 "commands" are conceptual, and may be implemented as one or more
 function calls.  This API definition is provided to clarify
 interactions between the DTLS and CAPWAP components of the integrated
 CAPWAP state machine.
 Below is a list of the minimal command APIs:
 o  DTLSStart is sent to the DTLS component to cause a DTLS session to
    be established.  Upon invoking the DTLSStart command, the WaitDTLS
    timer is started.  The WTP initiates this DTLS command, as the AC
    does not initiate DTLS sessions.
 o  DTLSListen is sent to the DTLS component to allow the DTLS
    component to listen for incoming DTLS session requests.

Calhoun, et al. Standards Track [Page 31] RFC 5415 CAPWAP Protocol Specification March 2009

 o  DTLSAccept is sent to the DTLS component to allow the DTLS session
    establishment to continue successfully.
 o  DTLSAbortSession is sent to the DTLS component to cause the
    session that is in the process of being established to be aborted.
    This command is also sent when the WaitDTLS timer expires.  When
    this command is executed, the FailedDTLSSessionCount counter is
    incremented.
 o  DTLSShutdown is sent to the DTLS component to cause session
    teardown.
 o  DTLSMtuUpdate is sent by the CAPWAP component to modify the MTU
    size used by the DTLS component.  See Section 3.5 for more
    information on MTU Discovery.  The default size is 1468 bytes.

2.3.2.2. DTLS to CAPWAP Notifications

 DTLS notifications are defined for the DTLS to CAPWAP API.  These
 "notifications" are conceptual and may be implemented in numerous
 ways (e.g., as function return values).  This API definition is
 provided to clarify interactions between the DTLS and CAPWAP
 components of the integrated CAPWAP state machine.  It is important
 to note that the notifications listed below MAY cause the CAPWAP
 state machine to jump from one state to another using a state
 transition not listed in Section 2.3.1.  When a notification listed
 below occurs, the target CAPWAP state shown in Figure 4 becomes the
 current state.
 Below is a list of the API notifications:
 o  DTLSPeerAuthorize is sent to the CAPWAP component during DTLS
    session establishment once the peer's identity has been received.
    This notification MAY be used by the CAPWAP component to authorize
    the session, based on the peer's identity.  The authorization
    process will lead to the CAPWAP component initiating either the
    DTLSAccept or DTLSAbortSession commands.
 o  DTLSEstablished is sent to the CAPWAP component to indicate that a
    secure channel now exists, using the parameters provided during
    the DTLS initialization process.  When this notification is
    received, the FailedDTLSSessionCount counter is reset to zero.
    When this notification is received, the WaitDTLS timer is stopped.
 o  DTLSEstablishFail is sent when the DTLS session establishment has
    failed, either due to a local error or due to the peer rejecting
    the session establishment.  When this notification is received,
    the FailedDTLSSessionCount counter is incremented.

Calhoun, et al. Standards Track [Page 32] RFC 5415 CAPWAP Protocol Specification March 2009

 o  DTLSAuthenticateFail is sent when DTLS session establishment has
    failed due to an authentication error.  When this notification is
    received, the FailedDTLSAuthFailCount counter is incremented.
 o  DTLSAborted is sent to the CAPWAP component to indicate that
    session abort (as requested by CAPWAP) is complete; this occurs to
    confirm a DTLS session abort or when the WaitDTLS timer expires.
    When this notification is received, the WaitDTLS timer is stopped.
 o  DTLSReassemblyFailure MAY be sent to the CAPWAP component to
    indicate DTLS fragment reassembly failure.
 o  DTLSDecapFailure MAY be sent to the CAPWAP module to indicate a
    decapsulation failure.  DTLSDecapFailure MAY be sent to the CAPWAP
    module to indicate an encryption/authentication failure.  This
    notification is intended for informative purposes only, and is not
    intended to cause a change in the CAPWAP state machine (see
    Section 12.4).
 o  DTLSPeerDisconnect is sent to the CAPWAP component to indicate the
    DTLS session has been torn down.  Note that this notification is
    only received if the DTLS session has been established.

2.4. Use of DTLS in the CAPWAP Protocol

 DTLS is used as a tightly integrated, secure wrapper for the CAPWAP
 protocol.  In this document, DTLS and CAPWAP are discussed as
 nominally distinct entities; however, they are very closely coupled,
 and may even be implemented inseparably.  Since there are DTLS
 library implementations currently available, and since security
 protocols (e.g., IPsec, TLS) are often implemented in widely
 available acceleration hardware, it is both convenient and forward-
 looking to maintain a modular distinction in this document.
 This section describes a detailed walk-through of the interactions
 between the DTLS module and the CAPWAP module, via 'commands' (CAPWAP
 to DTLS) and 'notifications' (DTLS to CAPWAP) as they would be
 encountered during the normal course of operation.

2.4.1. DTLS Handshake Processing

 Details of the DTLS handshake process are specified in [RFC4347].
 This section describes the interactions between the DTLS session
 establishment process and the CAPWAP protocol.  Note that the
 conceptual DTLS state is shown below to help understand the point at
 which the DTLS states transition.  In the normal case, the DTLS
 handshake will proceed as shown in Figure 5.  (NOTE: this example
 uses certificates, but pre-shared keys are also supported.)

Calhoun, et al. Standards Track [Page 33] RFC 5415 CAPWAP Protocol Specification March 2009

         ============                         ============
             WTP                                   AC
         ============                         ============
         ClientHello           ------>
                               <------       HelloVerifyRequest
                                                 (with cookie)
         ClientHello           ------>
         (with cookie)
                               <------       ServerHello
                               <------       Certificate
                               <------       ServerHelloDone
         (WTP callout for AC authorization
                  occurs in CAPWAP Auth state)
         Certificate*
         ClientKeyExchange
         CertificateVerify*
         ChangeCipherSpec
         Finished              ------>
                              (AC callout for WTP authorization
                               occurs in CAPWAP Auth state)
                                             ChangeCipherSpec
                               <------       Finished
                       Figure 5: DTLS Handshake
 DTLS, as specified, provides its own retransmit timers with an
 exponential back-off.  [RFC4347] does not specify how long
 retransmissions should continue.  Consequently, timing out incomplete
 DTLS handshakes is entirely the responsibility of the CAPWAP module.
 The DTLS implementation used by CAPWAP MUST support TLS Session
 Resumption.  Session resumption is typically used to establish the
 DTLS session used for the data channel.  Since the data channel uses
 different port numbers than the control channel, the DTLS
 implementation on the WTP MUST provide an interface that allows the
 CAPWAP module to request session resumption despite the use of the
 different port numbers (TLS implementations usually attempt session
 resumption only when connecting to the same IP address and port
 number).  Note that session resumption is not guaranteed to occur,
 and a full DTLS handshake may occur instead.

Calhoun, et al. Standards Track [Page 34] RFC 5415 CAPWAP Protocol Specification March 2009

 The DTLS implementation used by CAPWAP MUST use replay detection, per
 Section 3.3 of [RFC4347].  Since the CAPWAP protocol handles
 retransmissions by re-encrypting lost frames, any duplicate DTLS
 frames are either unintentional or malicious and should be silently
 discarded.

2.4.2. DTLS Session Establishment

 The WTP, either through the Discovery process or through pre-
 configuration, determines to which AC to connect.  The WTP uses the
 DTLSStart command to request that a secure connection be established
 to the selected AC.  Prior to initiation of the DTLS handshake, the
 WTP sets the WaitDTLS timer.  Upon invoking the DTLSStart or
 DTLSListen commands, the WTP and AC, respectively, set the WaitDTLS
 timer.  If the DTLSEstablished notification is not received prior to
 timer expiration, the DTLS session is aborted by issuing the
 DTLSAbortSession DTLS command.  This notification causes the CAPWAP
 module to transition to the Idle state.  Upon receiving a
 DTLSEstablished notification, the WaitDTLS timer is deactivated.

2.4.3. DTLS Error Handling

 If the AC or WTP does not respond to any DTLS handshake messages sent
 by its peer, the DTLS specification calls for the message to be
 retransmitted.  Note that during the handshake, when both the AC and
 the WTP are expecting additional handshake messages, they both
 retransmit if an expected message has not been received (note that
 retransmissions for CAPWAP Control messages work differently: all
 CAPWAP Control messages are either requests or responses, and the
 peer who sent the request is responsible for retransmissions).
 If the WTP or the AC does not receive an expected DTLS handshake
 message despite of retransmissions, the WaitDTLS timer will
 eventually expire, and the session will be terminated.  This can
 happen if communication between the peers has completely failed, or
 if one of the peers sent a DTLS Alert message that was lost in
 transit (DTLS does not retransmit Alert messages).
 If a cookie fails to validate, this could represent a WTP error, or
 it could represent a DoS attack.  Hence, AC resource utilization
 SHOULD be minimized.  The AC MAY log a message indicating the
 failure, and SHOULD treat the message as though no cookie were
 present.
 Since DTLS Handshake messages are potentially larger than the maximum
 record size, DTLS supports fragmenting of Handshake messages across
 multiple records.  There are several potential causes of re-assembly

Calhoun, et al. Standards Track [Page 35] RFC 5415 CAPWAP Protocol Specification March 2009

 errors, including overlapping and/or lost fragments.  The DTLS
 component MUST send a DTLSReassemblyFailure notification to the
 CAPWAP component.  Whether precise information is given along with
 notification is an implementation issue, and hence is beyond the
 scope of this document.  Upon receipt of such an error, the CAPWAP
 component SHOULD log an appropriate error message.  Whether
 processing continues or the DTLS session is terminated is
 implementation dependent.
 DTLS decapsulation errors consist of three types: decryption errors,
 authentication errors, and malformed DTLS record headers.  Since DTLS
 authenticates the data prior to encapsulation, if decryption fails,
 it is difficult to detect this without first attempting to
 authenticate the packet.  If authentication fails, a decryption error
 is also likely, but not guaranteed.  Rather than attempt to derive
 (and require the implementation of) algorithms for detecting
 decryption failures, decryption failures are reported as
 authentication failures.  The DTLS component MUST provide a
 DTLSDecapFailure notification to the CAPWAP component when such
 errors occur.  If a malformed DTLS record header is detected, the
 packets SHOULD be silently discarded, and the receiver MAY log an
 error message.
 There is currently only one encapsulation error defined: MTU
 exceeded.  As part of DTLS session establishment, the CAPWAP
 component informs the DTLS component of the MTU size.  This may be
 dynamically modified at any time when the CAPWAP component sends the
 DTLSMtuUpdate command to the DTLS component (see Section 2.3.2.1).
 The value provided to the DTLS stack is the result of the MTU
 Discovery process, which is described in Section 3.5.  The DTLS
 component returns this notification to the CAPWAP component whenever
 a transmission request will result in a packet that exceeds the MTU.

2.4.4. DTLS Endpoint Authentication and Authorization

 DTLS supports endpoint authentication with certificates or pre-shared
 keys.  The TLS algorithm suites for each endpoint authentication
 method are described below.

2.4.4.1. Authenticating with Certificates

 CAPWAP implementations only use cipher suites that are recommended
 for use with DTLS, see [DTLS-DESIGN].  At present, the following
 algorithms MUST be supported when using certificates for CAPWAP
 authentication:
 o  TLS_RSA_WITH_AES_128_CBC_SHA [RFC5246]

Calhoun, et al. Standards Track [Page 36] RFC 5415 CAPWAP Protocol Specification March 2009

 The following algorithms SHOULD be supported when using certificates:
 o  TLS_DHE_RSA_WITH_AES_128_CBC_SHA [RFC5246]
 The following algorithms MAY be supported when using certificates:
 o  TLS_RSA_WITH_AES_256_CBC_SHA [RFC5246]
 o  TLS_DHE_RSA_WITH_AES_256_CBC_SHA [RFC5246]
 Additional ciphers MAY be defined in subsequent CAPWAP
 specifications.

2.4.4.2. Authenticating with Pre-Shared Keys

 Pre-shared keys present significant challenges from a security
 perspective, and for that reason, their use is strongly discouraged.
 Several methods for authenticating with pre-shared keys are defined
 [RFC4279], and we focus on the following two:
 o  Pre-Shared Key (PSK) key exchange algorithm - simplest method,
    ciphersuites use only symmetric key algorithms.
 o  DHE_PSK key exchange algorithm - use a PSK to authenticate a
    Diffie-Hellman exchange.  These ciphersuites give some additional
    protection against dictionary attacks and also provide Perfect
    Forward Secrecy (PFS).
 The first approach (plain PSK) is susceptible to passive dictionary
 attacks; hence, while this algorithm MUST be supported, special care
 should be taken when choosing that method.  In particular, user-
 readable passphrases SHOULD NOT be used, and use of short PSKs SHOULD
 be strongly discouraged.
 The following cryptographic algorithms MUST be supported when using
 pre-shared keys:
 o  TLS_PSK_WITH_AES_128_CBC_SHA [RFC5246]
 o  TLS_DHE_PSK_WITH_AES_128_CBC_SHA [RFC5246]
 The following algorithms MAY be supported when using pre-shared keys:
 o  TLS_PSK_WITH_AES_256_CBC_SHA [RFC5246]
 o  TLS_DHE_PSK_WITH_AES_256_CBC_SHA [RFC5246]
 Additional ciphers MAY be defined in following CAPWAP specifications.

Calhoun, et al. Standards Track [Page 37] RFC 5415 CAPWAP Protocol Specification March 2009

2.4.4.3. Certificate Usage

 Certificate authorization by the AC and WTP is required so that only
 an AC may perform the functions of an AC and that only a WTP may
 perform the functions of a WTP.  This restriction of functions to the
 AC or WTP requires that the certificates used by the AC MUST be
 distinguishable from the certificate used by the WTP.  To accomplish
 this differentiation, the x.509 certificates MUST include the
 Extended Key Usage (EKU) certificate extension [RFC5280].
 The EKU field indicates one or more purposes for which a certificate
 may be used.  It is an essential part in authorization.  Its syntax
 is described in [RFC5280] and [ISO.9834-1.1993] and is as follows:
       ExtKeyUsageSyntax  ::=  SEQUENCE SIZE (1..MAX) OF KeyPurposeId
       KeyPurposeId  ::=  OBJECT IDENTIFIER
 Here we define two KeyPurposeId values, one for the WTP and one for
 the AC.  Inclusion of one of these two values indicates a certificate
 is authorized for use by a WTP or AC, respectively.  These values are
 formatted as id-kp fields.
           id-kp  OBJECT IDENTIFIER  ::=
               { iso(1) identified-organization(3) dod(6) internet(1)
                 security(5) mechanisms(5) pkix(7) 3 }
            id-kp-capwapAC   OBJECT IDENTIFIER  ::=  { id-kp 18 }
            id-kp-capwapWTP  OBJECT IDENTIFIER  ::=  { id-kp 19 }
 All capwap devices MUST support the ExtendedKeyUsage certificate
 extension if it is present in a certificate.  If the extension is
 present, then the certificate MUST have either the id-kp-capwapAC or
 the id-kp-anyExtendedKeyUsage keyPurposeID to act as an AC.
 Similarly, if the extension is present, a device MUST have the id-kp-
 capwapWTP or id-kp-anyExtendedKeyUsage keyPurposeID to act as a WTP.
 Part of the CAPWAP certificate validation process includes ensuring
 that the proper EKU is included and allowing the CAPWAP session to be
 established only if the extension properly represents the device.
 For instance, an AC SHOULD NOT accept a connection request from
 another AC, and therefore MUST verify that the id-kp-capwapWTP EKU is
 present in the certificate.
 CAPWAP implementations MUST support certificates where the common
 name (CN) for both the WTP and AC is the MAC address of that device.

Calhoun, et al. Standards Track [Page 38] RFC 5415 CAPWAP Protocol Specification March 2009

 The MAC address MUST be encoded in the PrintableString format, using
 the well-recognized MAC address format of 01:23:45:67:89:ab.  The CN
 field MAY contain either of the EUI-48 [EUI-48] or EUI-64 [EUI-64]
 MAC Address formats.  This seemingly unconventional use of the CN
 field is consistent with other standards that rely on device
 certificates that are provisioned during the manufacturing process,
 such as Packet Cable [PacketCable], Cable Labs [CableLabs], and WiMAX
 [WiMAX].  See Section 12.8 for more information on the use of the MAC
 address in the CN field.
 ACs and WTPs MUST authorize (e.g., through access control lists)
 certificates of devices to which they are connecting, e.g., based on
 the issuer, MAC address, or organizational information specified in
 the certificate.  The identities specified in the certificates bind a
 particular DTLS session to a specific pair of mutually authenticated
 and authorized MAC addresses.  The particulars of authorization
 filter construction are implementation details which are, for the
 most part, not within the scope of this specification.  However, at
 minimum, all devices MUST verify that the appropriate EKU bit is set
 according to the role of the peer device (AC versus WTP), and that
 the issuer of the certificate is appropriate for the domain in
 question.

2.4.4.4. PSK Usage

 When DTLS uses PSK Ciphersuites, the ServerKeyExchange message MUST
 contain the "PSK identity hint" field and the ClientKeyExchange
 message MUST contain the "PSK identity" field.  These fields are used
 to help the WTP select the appropriate PSK for use with the AC, and
 then indicate to the AC which key is being used.  When PSKs are
 provisioned to WTPs and ACs, both the PSK Hint and PSK Identity for
 the key MUST be specified.
 The PSK Hint SHOULD uniquely identify the AC and the PSK Identity
 SHOULD uniquely identify the WTP.  It is RECOMMENDED that these hints
 and identities be the ASCII HEX-formatted MAC addresses of the
 respective devices, since each pairwise combination of WTP and AC
 SHOULD have a unique PSK.  The PSK Hint and Identity SHOULD be
 sufficient to perform authorization, as simply having knowledge of a
 PSK does not necessarily imply authorization.
 If a single PSK is being used for multiple devices on a CAPWAP
 network, which is NOT RECOMMENDED, the PSK Hint and Identity can no
 longer be a MAC address, so appropriate hints and identities SHOULD
 be selected to identify the group of devices to which the PSK is
 provisioned.

Calhoun, et al. Standards Track [Page 39] RFC 5415 CAPWAP Protocol Specification March 2009

3. CAPWAP Transport

 Communication between a WTP and an AC is established using the
 standard UDP client/server model.  The CAPWAP protocol supports both
 UDP and UDP-Lite [RFC3828] transport protocols.  When run over IPv4,
 UDP is used for the CAPWAP Control and Data channels.
 When run over IPv6, the CAPWAP Control channel always uses UDP, while
 the CAPWAP Data channel may use either UDP or UDP-Lite.  UDP-Lite is
 the default transport protocol for the CAPWAP Data channel.  However,
 if a middlebox or IPv4 to IPv6 gateway has been discovered, UDP is
 used for the CAPWAP Data channel.
 This section describes how the CAPWAP protocol is carried over IP and
 UDP/UDP-Lite transport protocols.  The CAPWAP Transport Protocol
 message element, Section 4.6.14, describes the rules to use in
 determining which transport protocol is to be used.
 In order for CAPWAP to be compatible with potential middleboxes in
 the network, CAPWAP implementations MUST send return traffic from the
 same port on which they received traffic from a given peer.  Further,
 any unsolicited requests generated by a CAPWAP node MUST be sent on
 the same port.

3.1. UDP Transport

 One of the CAPWAP protocol requirements is to allow a WTP to reside
 behind a middlebox, firewall, and/or Network Address Translation
 (NAT) device.  Since a CAPWAP session is initiated by the WTP
 (client) to the well-known UDP port of the AC (server), the use of
 UDP is a logical choice.  When CAPWAP is run over IPv4, the UDP
 checksum field in CAPWAP packets MUST be set to zero.
 CAPWAP protocol control packets sent from the WTP to the AC use the
 CAPWAP Control channel, as defined in Section 1.4.  The CAPWAP
 control port at the AC is the well-known UDP port 5246.  The CAPWAP
 control port at the WTP can be any port selected by the WTP.
 CAPWAP protocol data packets sent from the WTP to the AC use the
 CAPWAP Data channel, as defined in Section 1.4.  The CAPWAP data port
 at the AC is the well-known UDP port 5247.  If an AC permits the
 administrator to change the CAPWAP control port, the CAPWAP data port
 MUST be the next consecutive port number.  The CAPWAP data port at
 the WTP can be any port selected by the WTP.

Calhoun, et al. Standards Track [Page 40] RFC 5415 CAPWAP Protocol Specification March 2009

3.2. UDP-Lite Transport

 When CAPWAP is run over IPv6, UDP-Lite is the default transport
 protocol, which reduces the checksum processing required for each
 packet (compared to the use of UDP over IPv6 [RFC2460]).  When UDP-
 Lite is used, the checksum field MUST have a coverage of 8 [RFC3828].
 UDP-Lite uses the same port assignments as UDP.

3.3. AC Discovery

 The AC Discovery phase allows the WTP to determine which ACs are
 available and choose the best AC with which to establish a CAPWAP
 session.  The Discovery phase occurs when the WTP enters the optional
 Discovery state.  A WTP does not need to complete the AC Discovery
 phase if it uses a pre-configured AC.  This section details the
 mechanism used by a WTP to dynamically discover candidate ACs.
 A WTP and an AC will frequently not reside in the same IP subnet
 (broadcast domain).  When this occurs, the WTP must be capable of
 discovering the AC, without requiring that multicast services are
 enabled in the network.
 When the WTP attempts to establish communication with an AC, it sends
 the Discovery Request message and receives the Discovery Response
 message from the AC(s).  The WTP MUST send the Discovery Request
 message to either the limited broadcast IP address (255.255.255.255),
 the well-known CAPWAP multicast address (224.0.1.140), or to the
 unicast IP address of the AC.  For IPv6 networks, since broadcast
 does not exist, the use of "All ACs multicast address" (FF0X:0:0:0:0:
 0:0:18C) is used instead.  Upon receipt of the Discovery Request
 message, the AC sends a Discovery Response message to the unicast IP
 address of the WTP, regardless of whether the Discovery Request
 message was sent as a broadcast, multicast, or unicast message.
 WTP use of a limited IP broadcast, multicast, or unicast IP address
 is implementation dependent.  ACs, on the other hand, MUST support
 broadcast, multicast, and unicast discovery.
 When a WTP transmits a Discovery Request message to a unicast
 address, the WTP must first obtain the IP address of the AC.  Any
 static configuration of an AC's IP address on the WTP non-volatile
 storage is implementation dependent.  However, additional dynamic
 schemes are possible, for example:

Calhoun, et al. Standards Track [Page 41] RFC 5415 CAPWAP Protocol Specification March 2009

 DHCP:  See [RFC5417] for more information on the use of DHCP to
    discover AC IP addresses.
 DNS:  The WTP MAY support use of DNS Service Records (SRVs) [RFC2782]
    to discover the AC address(es).  In this case, the WTP first
    obtains (e.g., from local configuration) the correct domain name
    suffix (e.g., "example.com") and performs an SRV lookup with
    Service name "capwap-control" and Proto "udp".  Thus, the name
    resolved in DNS would be, e.g., "_capwap-
    control._udp.example.com".  Note that the SRV record MAY specify a
    non-default port number for the control channel; the port number
    for the data channel is the next port number (control channel port
    + 1).
 An AC MAY also communicate alternative ACs to the WTP within the
 Discovery Response message through the AC IPv4 List (see
 Section 4.6.2) and AC IPv6 List (see Section 4.6.2).  The addresses
 provided in these two message elements are intended to help the WTP
 discover additional ACs through means other than those listed above.
 The AC Name with Priority message element (see Section 4.6.5) is used
 to communicate a list of preferred ACs to the WTP.  The WTP SHOULD
 attempt to utilize the ACs listed in the order provided by the AC.
 The Name-to-IP Address mapping is handled via the Discovery message
 exchange, in which the ACs provide their identity in the AC Name (see
 Section 4.6.4) message element in the Discovery Response message.
 Once the WTP has received Discovery Response messages from the
 candidate ACs, it MAY use other factors to determine the preferred
 AC.  For instance, each binding defines a WTP Radio Information
 message element (see Section 2.1), which the AC includes in Discovery
 Response messages.  The presence of one or more of these message
 elements is used to identify the CAPWAP bindings supported by the AC.
 A WTP MAY connect to an AC based on the supported bindings
 advertised.

3.4. Fragmentation/Reassembly

 While fragmentation and reassembly services are provided by IP, the
 CAPWAP protocol also provides such services.  Environments where the
 CAPWAP protocol is used involve firewall, NAT, and "middlebox"
 devices, which tend to drop IP fragments to minimize possible DoS
 attacks.  By providing fragmentation and reassembly at the
 application layer, any fragmentation required due to the tunneling
 component of the CAPWAP protocol becomes transparent to these
 intermediate devices.  Consequently, the CAPWAP protocol can be used
 in any network topology including firewall, NAT, and middlebox
 devices.

Calhoun, et al. Standards Track [Page 42] RFC 5415 CAPWAP Protocol Specification March 2009

 It is important to note that the fragmentation mechanism employed by
 CAPWAP has known limitations and deficiencies, which are similar to
 those described in [RFC4963].  The limited size of the Fragment ID
 field (see Section 4.3) can cause wrapping of the field, and hence
 cause fragments from different datagrams to be incorrectly spliced
 together (known as "mis-associated").  For example, a 100Mpbs link
 with an MTU of 1500 (causing fragmentation at 1450 bytes) would cause
 the Fragment ID field wrap in 8 seconds.  Consequently, CAPWAP
 implementers are warned to properly size their buffers for reassembly
 purposes based on the expected wireless technology throughput.
 CAPWAP implementations SHOULD perform MTU Discovery (see
 Section 3.5), which can avoid the need for fragmentation.  At the
 time of writing of this specification, most enterprise switching and
 routing infrastructure were capable of supporting "mini-jumbo" frames
 (1800 bytes), which eliminates the need for fragmentation (assuming
 the station's MTU is 1500 bytes).  The need for fragmentation
 typically continues to exist when the WTP communicates with the AC
 over a Wide Area Network (WAN).  Therefore, future versions of the
 CAPWAP protocol SHOULD consider either increasing the size of the
 Fragment ID field or providing alternative extensions.

3.5. MTU Discovery

 Once a WTP has discovered the AC with which it wishes to establish a
 CAPWAP session, it SHOULD perform a Path MTU (PMTU) discovery.  One
 recommendation for performing PMTU discovery is to have the WTP
 transmit Discovery Request (see Section 5.1) messages, and include
 the MTU Discovery Padding message element (see Section 4.6.32).  The
 actual procedures used for PMTU discovery are described in [RFC1191]
 for IPv4; for IPv6, [RFC1981] SHOULD be used.  Alternatively,
 implementers MAY use the procedures defined in [RFC4821].  The WTP
 SHOULD also periodically re-evaluate the PMTU using the guidelines
 provided in these two RFCs, using the Primary Discovery Request (see
 Section 5.3) along with the MTU Discovery Padding message element
 (see Section 4.6.32).  When the MTU is initially known, or updated in
 the case where an existing session already exists, the discovered
 PMTU is used to configure the DTLS component (see Section 2.3.2.1),
 while non-DTLS frames need to be fragmented to fit the MTU, defined
 in Section 3.4.

4. CAPWAP Packet Formats

 This section contains the CAPWAP protocol packet formats.  A CAPWAP
 protocol packet consists of one or more CAPWAP Transport Layer packet
 headers followed by a CAPWAP message.  The CAPWAP message can be
 either of type Control or Data, where Control packets carry

Calhoun, et al. Standards Track [Page 43] RFC 5415 CAPWAP Protocol Specification March 2009

 signaling, and Data packets carry user payloads.  The CAPWAP frame
 formats for CAPWAP Data packets, and for DTLS encapsulated CAPWAP
 Data and Control packets are defined below.
 The CAPWAP Control protocol includes two messages that are never
 protected by DTLS: the Discovery Request message and the Discovery
 Response message.  These messages need to be in the clear to allow
 the CAPWAP protocol to properly identify and process them.  The
 format of these packets are as follows:
     CAPWAP Control Packet (Discovery Request/Response):
     +-------------------------------------------+
     | IP  | UDP | CAPWAP | Control | Message    |
     | Hdr | Hdr | Header | Header  | Element(s) |
     +-------------------------------------------+
 All other CAPWAP Control protocol messages MUST be protected via the
 DTLS protocol, which ensures that the packets are both authenticated
 and encrypted.  These packets include the CAPWAP DTLS Header, which
 is described in Section 4.2.  The format of these packets is as
 follows:
  CAPWAP Control Packet (DTLS Security Required):
  +------------------------------------------------------------------+
  | IP  | UDP | CAPWAP   | DTLS | CAPWAP | Control| Message   | DTLS |
  | Hdr | Hdr | DTLS Hdr | Hdr  | Header | Header | Element(s)| Trlr |
  +------------------------------------------------------------------+
                         \---------- authenticated -----------/
                                \------------- encrypted ------------/
 The CAPWAP protocol allows optional protection of data packets, using
 DTLS.  Use of data packet protection is determined by AC policy.
 When DTLS is utilized, the optional CAPWAP DTLS Header is present,
 which is described in Section 4.2.  The format of CAPWAP Data packets
 is shown below:

Calhoun, et al. Standards Track [Page 44] RFC 5415 CAPWAP Protocol Specification March 2009

     CAPWAP Plain Text Data Packet :
     +-------------------------------+
     | IP  | UDP | CAPWAP | Wireless |
     | Hdr | Hdr | Header | Payload  |
     +-------------------------------+
     DTLS Secured CAPWAP Data Packet:
     +--------------------------------------------------------+
     | IP  | UDP |  CAPWAP  | DTLS | CAPWAP | Wireless | DTLS |
     | Hdr | Hdr | DTLS Hdr | Hdr  |  Hdr   | Payload  | Trlr |
     +--------------------------------------------------------+
                            \------ authenticated -----/
                                   \------- encrypted --------/
 UDP Header:  All CAPWAP packets are encapsulated within either UDP,
    or UDP-Lite when used over IPv6.  Section 3 defines the specific
    UDP or UDP-Lite usage.
 CAPWAP DTLS Header:  All DTLS encrypted CAPWAP protocol packets are
    prefixed with the CAPWAP DTLS Header (see Section 4.2).
 DTLS Header:  The DTLS Header provides authentication and encryption
    services to the CAPWAP payload it encapsulates.  This protocol is
    defined in [RFC4347].
 CAPWAP Header:  All CAPWAP protocol packets use a common header that
    immediately follows the CAPWAP preamble or DTLS Header.  The
    CAPWAP Header is defined in Section 4.3.
 Wireless Payload:  A CAPWAP protocol packet that contains a wireless
    payload is a CAPWAP Data packet.  The CAPWAP protocol does not
    specify the format of the wireless payload, which is defined by
    the appropriate wireless standard.  Additional information is in
    Section 4.4.
 Control Header:  The CAPWAP protocol includes a signaling component,
    known as the CAPWAP Control protocol.  All CAPWAP Control packets
    include a Control Header, which is defined in Section 4.5.1.
    CAPWAP Data packets do not contain a Control Header field.
 Message Elements:  A CAPWAP Control packet includes one or more
    message elements, which are found immediately following the
    Control Header.  These message elements are in a Type/Length/Value
    style header, defined in Section 4.6.
 A CAPWAP implementation MUST be capable of receiving a reassembled
 CAPWAP message of length 4096 bytes.  A CAPWAP implementation MAY
 indicate that it supports a higher maximum message length, by

Calhoun, et al. Standards Track [Page 45] RFC 5415 CAPWAP Protocol Specification March 2009

 including the Maximum Message Length message element, see
 Section 4.6.31, in the Join Request message or the Join Response
 message.

4.1. CAPWAP Preamble

 The CAPWAP preamble is common to all CAPWAP transport headers and is
 used to identify the header type that immediately follows.  The
 reason for this preamble is to avoid needing to perform byte
 comparisons in order to guess whether or not the frame is DTLS
 encrypted.  It also provides an extensibility framework that can be
 used to support additional transport types.  The format of the
 preamble is as follows:
       0
       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |Version| Type  |
      +-+-+-+-+-+-+-+-+
 Version:  A 4-bit field that contains the version of CAPWAP used in
    this packet.  The value for this specification is zero (0).
 Type:  A 4-bit field that specifies the payload type that follows the
    UDP header.  The following values are supported:
    0 -   CAPWAP Header.  The CAPWAP Header (see Section 4.3)
          immediately follows the UDP header.  If the packet is
          received on the CAPWAP Data channel, the CAPWAP stack MUST
          treat the packet as a clear text CAPWAP Data packet.  If
          received on the CAPWAP Control channel, the CAPWAP stack
          MUST treat the packet as a clear text CAPWAP Control packet.
          If the control packet is not a Discovery Request or
          Discovery Response packet, the packet MUST be dropped.
    1 -   CAPWAP DTLS Header.  The CAPWAP DTLS Header (and DTLS
          packet) immediately follows the UDP header (see
          Section 4.2).

4.2. CAPWAP DTLS Header

 The CAPWAP DTLS Header is used to identify the packet as a DTLS
 encrypted packet.  The first eight bits include the common CAPWAP
 Preamble.  The remaining 24 bits are padding to ensure 4-byte
 alignment, and MAY be used in a future version of the protocol.  The
 DTLS packet [RFC4347] always immediately follows this header.  The
 format of the CAPWAP DTLS Header is as follows:

Calhoun, et al. Standards Track [Page 46] RFC 5415 CAPWAP Protocol Specification March 2009

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CAPWAP Preamble|                    Reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 CAPWAP Preamble:  The CAPWAP Preamble is defined in Section 4.1.  The
    CAPWAP Preamble's Payload Type field MUST be set to one (1).
 Reserved:  The 24-bit field is reserved for future use.  All
    implementations complying with this protocol MUST set to zero any
    bits that are reserved in the version of the protocol supported by
    that implementation.  Receivers MUST ignore all bits not defined
    for the version of the protocol they support.

4.3. CAPWAP Header

 All CAPWAP protocol messages are encapsulated using a common header
 format, regardless of the CAPWAP Control or CAPWAP Data transport
 used to carry the messages.  However, certain flags are not
 applicable for a given transport.  Refer to the specific transport
 section in order to determine which flags are valid.
 Note that the optional fields defined in this section MUST be present
 in the precise order shown below.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CAPWAP Preamble|  HLEN   |   RID   | WBID    |T|F|L|W|M|K|Flags|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Fragment ID          |     Frag Offset         |Rsvd |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 (optional) Radio MAC Address                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            (optional) Wireless Specific Information           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Payload ....                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 CAPWAP Preamble:  The CAPWAP Preamble is defined in Section 4.1.  The
    CAPWAP Preamble's Payload Type field MUST be set to zero (0).  If
    the CAPWAP DTLS Header is present, the version number in both
    CAPWAP Preambles MUST match.  The reason for this duplicate field
    is to avoid any possible tampering of the version field in the
    preamble that is not encrypted or authenticated.

Calhoun, et al. Standards Track [Page 47] RFC 5415 CAPWAP Protocol Specification March 2009

 HLEN:  A 5-bit field containing the length of the CAPWAP transport
    header in 4-byte words (similar to IP header length).  This length
    includes the optional headers.
 RID:  A 5-bit field that contains the Radio ID number for this
    packet, whose value is between one (1) and 31.  Given that MAC
    Addresses are not necessarily unique across physical radios in a
    WTP, the Radio Identifier (RID) field is used to indicate with
    which physical radio the message is associated.
 WBID:  A 5-bit field that is the wireless binding identifier.  The
    identifier will indicate the type of wireless packet associated
    with the radio.  The following values are defined:
    0 -  Reserved
    1 -  IEEE 802.11
    2 -  Reserved
    3 -  EPCGlobal [EPCGlobal]
 T: The Type 'T' bit indicates the format of the frame being
    transported in the payload.  When this bit is set to one (1), the
    payload has the native frame format indicated by the WBID field.
    When this bit is zero (0), the payload is an IEEE 802.3 frame.
 F: The Fragment 'F' bit indicates whether this packet is a fragment.
    When this bit is one (1), the packet is a fragment and MUST be
    combined with the other corresponding fragments to reassemble the
    complete information exchanged between the WTP and AC.
 L: The Last 'L' bit is valid only if the 'F' bit is set and indicates
    whether the packet contains the last fragment of a fragmented
    exchange between WTP and AC.  When this bit is one (1), the packet
    is the last fragment.  When this bit is (zero) 0, the packet is
    not the last fragment.
 W: The Wireless 'W' bit is used to specify whether the optional
    Wireless Specific Information field is present in the header.  A
    value of one (1) is used to represent the fact that the optional
    header is present.
 M: The Radio MAC 'M' bit is used to indicate that the Radio MAC
    Address optional header is present.  This is used to communicate
    the MAC address of the receiving radio.

Calhoun, et al. Standards Track [Page 48] RFC 5415 CAPWAP Protocol Specification March 2009

 K: The Keep-Alive 'K' bit indicates the packet is a Data Channel
    Keep-Alive packet.  This packet is used to map the data channel to
    the control channel for the specified Session ID and to maintain
    freshness of the data channel.  The 'K' bit MUST NOT be set for
    data packets containing user data.
 Flags:  A set of reserved bits for future flags in the CAPWAP Header.
    All implementations complying with this protocol MUST set to zero
    any bits that are reserved in the version of the protocol
    supported by that implementation.  Receivers MUST ignore all bits
    not defined for the version of the protocol they support.
 Fragment ID:  A 16-bit field whose value is assigned to each group of
    fragments making up a complete set.  The Fragment ID space is
    managed individually for each direction for every WTP/AC pair.
    The value of Fragment ID is incremented with each new set of
    fragments.  The Fragment ID wraps to zero after the maximum value
    has been used to identify a set of fragments.
 Fragment Offset:  A 13-bit field that indicates where in the payload
    this fragment belongs during re-assembly.  This field is valid
    when the 'F' bit is set to 1.  The fragment offset is measured in
    units of 8 octets (64 bits).  The first fragment has offset zero.
    Note that the CAPWAP protocol does not allow for overlapping
    fragments.
 Reserved:  The 3-bit field is reserved for future use.  All
    implementations complying with this protocol MUST set to zero any
    bits that are reserved in the version of the protocol supported by
    that implementation.  Receivers MUST ignore all bits not defined
    for the version of the protocol they support.
 Radio MAC Address:  This optional field contains the MAC address of
    the radio receiving the packet.  Because the native wireless frame
    format to IEEE 802.3 format causes the MAC address of the WTP's
    radio to be lost, this field allows the address to be communicated
    to the AC.  This field is only present if the 'M' bit is set.  The
    HLEN field assumes 4-byte alignment, and this field MUST be padded
    with zeroes (0x00) if it is not 4-byte aligned.

Calhoun, et al. Standards Track [Page 49] RFC 5415 CAPWAP Protocol Specification March 2009

    The field contains the basic format:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Length    |                  MAC Address
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Length:  The length of the MAC address field.  The formats and
       lengths specified in [EUI-48] and [EUI-64] are supported.
    MAC Address:  The MAC address of the receiving radio.
 Wireless Specific Information:  This optional field contains
    technology-specific information that may be used to carry per-
    packet wireless information.  This field is only present if the
    'W' bit is set.  The WBID field in the CAPWAP Header is used to
    identify the format of the Wireless-Specific Information optional
    field.  The HLEN field assumes 4-byte alignment, and this field
    MUST be padded with zeroes (0x00) if it is not 4-byte aligned.
    The Wireless-Specific Information field uses the following format:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Length     |                Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Length:  The 8-bit field contains the length of the data field,
       with a maximum size of 255.
    Data:  Wireless-specific information, defined by the wireless-
       specific binding specified in the CAPWAP Header's WBID field.
 Payload:  This field contains the header for a CAPWAP Data Message or
    CAPWAP Control Message, followed by the data contained in the
    message.

4.4. CAPWAP Data Messages

 There are two different types of CAPWAP Data packets: CAPWAP Data
 Channel Keep-Alive packets and Data Payload packets.  The first is
 used by the WTP to synchronize the control and data channels and to
 maintain freshness of the data channel.  The second is used to
 transmit user payloads between the AC and WTP.  This section
 describes both types of CAPWAP Data packet formats.

Calhoun, et al. Standards Track [Page 50] RFC 5415 CAPWAP Protocol Specification March 2009

 Both CAPWAP Data messages are transmitted on the CAPWAP Data channel.

4.4.1. CAPWAP Data Channel Keep-Alive

 The CAPWAP Data Channel Keep-Alive packet is used to bind the CAPWAP
 control channel with the data channel, and to maintain freshness of
 the data channel, ensuring that the channel is still functioning.
 The CAPWAP Data Channel Keep-Alive packet is transmitted by the WTP
 when the DataChannelKeepAlive timer expires (see Section 4.7.2).
 When the CAPWAP Data Channel Keep-Alive packet is transmitted, the
 WTP sets the DataChannelDeadInterval timer.
 In the CAPWAP Data Channel Keep-Alive packet, all of the fields in
 the CAPWAP Header, except the HLEN field and the 'K' bit, are set to
 zero upon transmission.  Upon receiving a CAPWAP Data Channel Keep-
 Alive packet, the AC transmits a CAPWAP Data Channel Keep-Alive
 packet back to the WTP.  The contents of the transmitted packet are
 identical to the contents of the received packet.
 Upon receiving a CAPWAP Data Channel Keep-Alive packet, the WTP
 cancels the DataChannelDeadInterval timer and resets the
 DataChannelKeepAlive timer.  The CAPWAP Data Channel Keep-Alive
 packet is retransmitted by the WTP in the same manner as the CAPWAP
 Control messages.  If the DataChannelDeadInterval timer expires, the
 WTP tears down the control DTLS session, and the data DTLS session if
 one existed.
 The CAPWAP Data Channel Keep-Alive packet contains the following
 payload immediately following the CAPWAP Header (see Section 4.3).
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Message Element Length     |  Message Element [0..N] ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Message Element Length:   The 16-bit Length field indicates the
    number of bytes following the CAPWAP Header, with a maximum size
    of 65535.
 Message Element[0..N]:   The message element(s) carry the information
    pertinent to each of the CAPWAP Data Channel Keep-Alive message.
    The following message elements MUST be present in this CAPWAP
    message:
       Session ID, see Section 4.6.37.

Calhoun, et al. Standards Track [Page 51] RFC 5415 CAPWAP Protocol Specification March 2009

4.4.2. Data Payload

 A CAPWAP protocol Data Payload packet encapsulates a forwarded
 wireless frame.  The CAPWAP protocol defines two different modes of
 encapsulation: IEEE 802.3 and native wireless.  IEEE 802.3
 encapsulation requires that for 802.11 frames, the 802.11
 *Integration* function be performed in the WTP.  An IEEE 802.3-
 encapsulated user payload frame has the following format:
     +------------------------------------------------------+
     | IP Header | UDP Header | CAPWAP Header | 802.3 Frame |
     +------------------------------------------------------+
 The CAPWAP protocol also defines the native wireless encapsulation
 mode.  The format of the encapsulated CAPWAP Data frame is subject to
 the rules defined by the specific wireless technology binding.  Each
 wireless technology binding MUST contain a section entitled "Payload
 Encapsulation", which defines the format of the wireless payload that
 is encapsulated within CAPWAP Data packets.
 For 802.3 payload frames, the 802.3 frame is encapsulated (excluding
 the IEEE 802.3 Preamble, Start Frame Delimiter (SFD), and Frame Check
 Sequence (FCS) fields).  If the encapsulated frame would exceed the
 transport layer's MTU, the sender is responsible for the
 fragmentation of the frame, as specified in Section 3.4.  The CAPWAP
 protocol can support IEEE 802.3 frames whose length is defined in the
 IEEE 802.3as specification [FRAME-EXT].

4.4.3. Establishment of a DTLS Data Channel

 If the AC and WTP are configured to tunnel the data channel over
 DTLS, the proper DTLS session must be initiated.  To avoid having to
 reauthenticate and reauthorize an AC and WTP, the DTLS data channel
 SHOULD be initiated using the TLS session resumption feature
 [RFC5246].
 The AC DTLS implementation MUST NOT initiate a data channel session
 for a DTLS session for which there is no active control channel
 session.

4.5. CAPWAP Control Messages

 The CAPWAP Control protocol provides a control channel between the
 WTP and the AC.  Control messages are divided into the following
 message types:
 Discovery:  CAPWAP Discovery messages are used to identify potential
    ACs, their load and capabilities.

Calhoun, et al. Standards Track [Page 52] RFC 5415 CAPWAP Protocol Specification March 2009

 Join:  CAPWAP Join messages are used by a WTP to request service from
    an AC, and for the AC to respond to the WTP.
 Control Channel Management:  CAPWAP Control channel management
    messages are used to maintain the control channel.
 WTP Configuration Management:  The WTP Configuration messages are
    used by the AC to deliver a specific configuration to the WTP.
    Messages that retrieve statistics from a WTP are also included in
    WTP Configuration Management.
 Station Session Management:  Station Session Management messages are
    used by the AC to deliver specific station policies to the WTP.
 Device Management Operations:  Device management operations are used
    to request and deliver a firmware image to the WTP.
 Binding-Specific CAPWAP Management Messages:  Messages in this
    category are used by the AC and the WTP to exchange protocol-
    specific CAPWAP management messages.  These messages may or may
    not be used to change the link state of a station.
 Discovery, Join, Control Channel Management, WTP Configuration
 Management, and Station Session Management CAPWAP Control messages
 MUST be implemented.  Device Management Operations messages MAY be
 implemented.
 CAPWAP Control messages sent from the WTP to the AC indicate that the
 WTP is operational, providing an implicit keep-alive mechanism for
 the WTP.  The Control Channel Management Echo Request and Echo
 Response messages provide an explicit keep-alive mechanism when other
 CAPWAP Control messages are not exchanged.

4.5.1. Control Message Format

 All CAPWAP Control messages are sent encapsulated within the CAPWAP
 Header (see Section 4.3).  Immediately following the CAPWAP Header is
 the control header, which has the following format:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Message Type                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Seq Num    |        Msg Element Length     |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Msg Element [0..N] ...
   +-+-+-+-+-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 53] RFC 5415 CAPWAP Protocol Specification March 2009

4.5.1.1. Message Type

 The Message Type field identifies the function of the CAPWAP Control
 message.  To provide extensibility, the Message Type field is
 comprised of an IANA Enterprise Number [RFC3232] and an enterprise-
 specific message type number.  The first three octets contain the
 IANA Enterprise Number in network byte order, with zero used for
 CAPWAP base protocol (this specification) defined message types.  The
 last octet is the enterprise-specific message type number, which has
 a range from 0 to 255.
 The Message Type field is defined as:
       Message Type =
               IANA Enterprise Number * 256 +
                   Enterprise Specific Message Type Number
 The CAPWAP protocol reliability mechanism requires that messages be
 defined in pairs, consisting of both a Request and a Response
 message.  The Response message MUST acknowledge the Request message.
 The assignment of CAPWAP Control Message Type Values always occurs in
 pairs.  All Request messages have odd numbered Message Type Values,
 and all Response messages have even numbered Message Type Values.
 The Request value MUST be assigned first.  As an example, assigning a
 Message Type Value of 3 for a Request message and 4 for a Response
 message is valid, while assigning a Message Type Value of 4 for a
 Response message and 5 for the corresponding Request message is
 invalid.
 When a WTP or AC receives a message with a Message Type Value field
 that is not recognized and is an odd number, the number in the
 Message Type Value Field is incremented by one, and a Response
 message with a Message Type Value field containing the incremented
 value and containing the Result Code message element with the value
 (Unrecognized Request) is returned to the sender of the received
 message.  If the unknown message type is even, the message is
 ignored.

Calhoun, et al. Standards Track [Page 54] RFC 5415 CAPWAP Protocol Specification March 2009

 The valid values for CAPWAP Control Message Types are specified in
 the table below:
         CAPWAP Control Message           Message Type
                                            Value
         Discovery Request                    1
         Discovery Response                   2
         Join Request                         3
         Join Response                        4
         Configuration Status Request         5
         Configuration Status Response        6
         Configuration Update Request         7
         Configuration Update Response        8
         WTP Event Request                    9
         WTP Event Response                  10
         Change State Event Request          11
         Change State Event Response         12
         Echo Request                        13
         Echo Response                       14
         Image Data Request                  15
         Image Data Response                 16
         Reset Request                       17
         Reset Response                      18
         Primary Discovery Request           19
         Primary Discovery Response          20
         Data Transfer Request               21
         Data Transfer Response              22
         Clear Configuration Request         23
         Clear Configuration Response        24
         Station Configuration Request       25
         Station Configuration Response      26

4.5.1.2. Sequence Number

 The Sequence Number field is an identifier value used to match
 Request and Response packets.  When a CAPWAP packet with a Request
 Message Type Value is received, the value of the Sequence Number
 field is copied into the corresponding Response message.
 When a CAPWAP Control message is sent, the sender's internal sequence
 number counter is monotonically incremented, ensuring that no two
 pending Request messages have the same sequence number.  The Sequence
 Number field wraps back to zero.

4.5.1.3. Message Element Length

 The Length field indicates the number of bytes following the Sequence
 Number field.

Calhoun, et al. Standards Track [Page 55] RFC 5415 CAPWAP Protocol Specification March 2009

4.5.1.4. Flags

 The Flags field MUST be set to zero.

4.5.1.5. Message Element [0..N]

 The message element(s) carry the information pertinent to each of the
 control message types.  Every control message in this specification
 specifies which message elements are permitted.
 When a WTP or AC receives a CAPWAP message without a message element
 that is specified as mandatory for the CAPWAP message, then the
 CAPWAP message is discarded.  If the received message was a Request
 message for which the corresponding Response message carries message
 elements, then a corresponding Response message with a Result Code
 message element indicating "Failure - Missing Mandatory Message
 Element" is returned to the sender.
 When a WTP or AC receives a CAPWAP message with a message element
 that the WTP or AC does not recognize, the CAPWAP message is
 discarded.  If the received message was a Request message for which
 the corresponding Response message carries message elements, then a
 corresponding Response message with a Result Code message element
 indicating "Failure - Unrecognized Message Element" and one or more
 Returned Message Element message elements is included, containing the
 unrecognized message element(s).

4.5.2. Quality of Service

 The CAPWAP base protocol does not provide any Quality of Service
 (QoS) recommendations for use with the CAPWAP Data messages.  Any
 wireless-specific CAPWAP binding specification that has QoS
 requirements MUST define the application of QoS to the CAPWAP Data
 messages.
 The IP header also includes the Explicit Congestion Notification
 (ECN) bits [RFC3168].  Section 9.1.1 of [RFC3168] describes two
 levels of ECN functionality: full functionality and limited
 functionality.  CAPWAP ACs and WTPs SHALL implement the limited
 functionality and are RECOMMENDED to implement the full functionality
 described in [RFC3168].

Calhoun, et al. Standards Track [Page 56] RFC 5415 CAPWAP Protocol Specification March 2009

4.5.2.1. Applying QoS to CAPWAP Control Message

 It is recommended that CAPWAP Control messages be sent by both the AC
 and the WTP with an appropriate Quality-of-Service precedence value,
 ensuring that congestion in the network minimizes occurrences of
 CAPWAP Control channel disconnects.  Therefore, a QoS-enabled CAPWAP
 device SHOULD use the following values:
 802.1Q:   The priority tag of 7 SHOULD be used.
 DSCP:   The CS6 per-hop behavior Service Class SHOULD be used, which
    is described in [RFC2474]).

4.5.3. Retransmissions

 The CAPWAP Control protocol operates as a reliable transport.  For
 each Request message, a Response message is defined, which is used to
 acknowledge receipt of the Request message.  In addition, the control
 header Sequence Number field is used to pair the Request and Response
 messages (see Section 4.5.1).
 Response messages are not explicitly acknowledged; therefore, if a
 Response message is not received, the original Request message is
 retransmitted.
 Implementations MUST keep track of the sequence number of the last
 received Request message, and MUST cache the corresponding Response
 message.  If a retransmission with the same sequence number is
 received, the cached Response message MUST be retransmitted without
 re-processing the Request.  If an older Request message is received,
 meaning one where the sequence number is smaller, it MUST be ignored.
 A newer Request message, meaning one whose sequence number is larger,
 is processed as usual.
 Note: A sequence number is considered "smaller" when s1 is smaller
 than s2 modulo 256 if and only if (s1<s2 and (s2-s1)<128) or
 (s1>s2 and (s1-s2)>128).
 Both the WTP and the AC can only have a single request outstanding at
 any given time.  Retransmitted Request messages MUST NOT be altered
 by the sender.
 After transmitting a Request message, the RetransmitInterval (see
 Section 4.7) timer and MaxRetransmit (see Section 4.8) variable are
 used to determine if the original Request message needs to be
 retransmitted.  The RetransmitInterval timer is used the first time
 the Request is retransmitted.  The timer is then doubled every

Calhoun, et al. Standards Track [Page 57] RFC 5415 CAPWAP Protocol Specification March 2009

 subsequent time the same Request message is retransmitted, up to
 MaxRetransmit but no more than half the EchoInterval timer (see
 Section 4.7.7).  Response messages are not subject to these timers.
 If the sender stops retransmitting a Request message before reaching
 MaxRetransmit retransmissions (which leads to transition to DTLS
 Teardown, as described in Section 2.3.1), it cannot know whether the
 recipient received and processed the Request or not.  In most
 situations, the sender SHOULD NOT do this, and instead continue
 retransmitting until a Response message is received, or transition to
 DTLS Teardown occurs.  However, if the sender does decide to continue
 the connection with a new or modified Request message, the new
 message MUST have a new sequence number, and be treated as a new
 Request message by the receiver.  Note that there is a high chance
 that both the WTP and the AC's sequence numbers will become out of
 sync.
 When a Request message is retransmitted, it MUST be re-encrypted via
 the DTLS stack.  If the peer had received the Request message, and
 the corresponding Response message was lost, it is necessary to
 ensure that retransmitted Request messages are not identified as
 replays by the DTLS stack.  Similarly, any cached Response messages
 that are retransmitted as a result of receiving a retransmitted
 Request message MUST be re-encrypted via DTLS.
 Duplicate Response messages, identified by the Sequence Number field
 in the CAPWAP Control message header, SHOULD be discarded upon
 receipt.

4.6. CAPWAP Protocol Message Elements

 This section defines the CAPWAP Protocol message elements that are
 included in CAPWAP protocol control messages.
 Message elements are used to carry information needed in control
 messages.  Every message element is identified by the Type Value
 field, defined below.  The total length of the message elements is
 indicated in the message element's length field.
 All of the message element definitions in this document use a diagram
 similar to the one below in order to depict its format.  Note that to
 simplify this specification, these diagrams do not include the header
 fields (Type and Length).  The header field values are defined in the
 message element descriptions.

Calhoun, et al. Standards Track [Page 58] RFC 5415 CAPWAP Protocol Specification March 2009

 Unless otherwise specified, a control message that lists a set of
 supported (or expected) message elements MUST NOT expect the message
 elements to be in any specific order.  The sender MAY include the
 message elements in any order.  Unless otherwise noted, one message
 element of each type is present in a given control message.
 Unless otherwise specified, any configuration information sent by the
 AC to the WTP MAY be saved to non-volatile storage (see Section 8.1)
 for more information).
 Additional message elements may be defined in separate IETF
 documents.
 The format of a message element uses the TLV format shown here:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Value ...   |
   +-+-+-+-+-+-+-+-+
 The 16-bit Type field identifies the information carried in the Value
 field and Length (16 bits) indicates the number of bytes in the Value
 field.  The value of zero (0) is reserved and MUST NOT be used.  The
 rest of the Type field values are allocated as follows:
            Usage                              Type Values
 CAPWAP Protocol Message Elements                   1 - 1023
 IEEE 802.11 Message Elements                    1024 - 2047
 Reserved for Future Use                         2048 - 3071
 EPCGlobal Message Elements                      3072 - 4095
 Reserved for Future Use                         4096 - 65535
 The table below lists the CAPWAP protocol Message Elements and their
 Type values.

Calhoun, et al. Standards Track [Page 59] RFC 5415 CAPWAP Protocol Specification March 2009

 CAPWAP Message Element                            Type Value
 AC Descriptor                                         1
 AC IPv4 List                                          2
 AC IPv6 List                                          3
 AC Name                                               4
 AC Name with Priority                                 5
 AC Timestamp                                          6
 Add MAC ACL Entry                                     7
 Add Station                                           8
 Reserved                                              9
 CAPWAP Control IPV4 Address                          10
 CAPWAP Control IPV6 Address                          11
 CAPWAP Local IPV4 Address                            30
 CAPWAP Local IPV6 Address                            50
 CAPWAP Timers                                        12
 CAPWAP Transport Protocol                            51
 Data Transfer Data                                   13
 Data Transfer Mode                                   14
 Decryption Error Report                              15
 Decryption Error Report Period                       16
 Delete MAC ACL Entry                                 17
 Delete Station                                       18
 Reserved                                             19
 Discovery Type                                       20
 Duplicate IPv4 Address                               21
 Duplicate IPv6 Address                               22
 ECN Support                                          53
 Idle Timeout                                         23
 Image Data                                           24
 Image Identifier                                     25
 Image Information                                    26
 Initiate Download                                    27
 Location Data                                        28
 Maximum Message Length                               29
 MTU Discovery Padding                                52
 Radio Administrative State                           31
 Radio Operational State                              32
 Result Code                                          33
 Returned Message Element                             34
 Session ID                                           35
 Statistics Timer                                     36
 Vendor Specific Payload                              37
 WTP Board Data                                       38
 WTP Descriptor                                       39
 WTP Fallback                                         40
 WTP Frame Tunnel Mode                                41
 Reserved                                             42

Calhoun, et al. Standards Track [Page 60] RFC 5415 CAPWAP Protocol Specification March 2009

 Reserved                                             43
 WTP MAC Type                                         44
 WTP Name                                             45
 Unused/Reserved                                      46
 WTP Radio Statistics                                 47
 WTP Reboot Statistics                                48
 WTP Static IP Address Information                    49

4.6.1. AC Descriptor

 The AC Descriptor message element is used by the AC to communicate
 its current state.  The value contains the following fields.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Stations           |             Limit             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Active WTPs          |            Max WTPs           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Security   |  R-MAC Field  |   Reserved1   |  DTLS Policy  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  AC Information Sub-Element...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   1 for AC Descriptor
 Length:   >= 12
 Stations:   The number of stations currently served by the AC
 Limit:   The maximum number of stations supported by the AC
 Active WTPs:   The number of WTPs currently attached to the AC
 Max WTPs:   The maximum number of WTPs supported by the AC
 Security:   An 8-bit mask specifying the authentication credential
    type supported by the AC (see Section 2.4.4).  The field has the
    following format:
       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |Reserved |S|X|R|
      +-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 61] RFC 5415 CAPWAP Protocol Specification March 2009

    Reserved:  A set of reserved bits for future use.  All
       implementations complying with this protocol MUST set to zero
       any bits that are reserved in the version of the protocol
       supported by that implementation.  Receivers MUST ignore all
       bits not defined for the version of the protocol they support.
    S:    The AC supports the pre-shared secret authentication, as
          described in Section 12.6.
    X:    The AC supports X.509 Certificate authentication, as
          described in Section 12.7.
    R:    A reserved bit for future use.  All implementations
          complying with this protocol MUST set to zero any bits that
          are reserved in the version of the protocol supported by
          that implementation.  Receivers MUST ignore all bits not
          defined for the version of the protocol they support.
 R-MAC Field:   The AC supports the optional Radio MAC Address field
    in the CAPWAP transport header (see Section 4.3).  The following
    enumerated values are supported:
    0 -  Reserved
    1 -  Supported
    2 -  Not Supported
 Reserved:  A set of reserved bits for future use.  All
    implementations complying with this protocol MUST set to zero any
    bits that are reserved in the version of the protocol supported by
    that implementation.  Receivers MUST ignore all bits not defined
    for the version of the protocol they support.
 DTLS Policy:   The AC communicates its policy on the use of DTLS for
    the CAPWAP data channel.  The AC MAY communicate more than one
    supported option, represented by the bit field below.  The WTP
    MUST abide by one of the options communicated by AC.  The field
    has the following format:
       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |Reserved |D|C|R|
      +-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 62] RFC 5415 CAPWAP Protocol Specification March 2009

    Reserved:  A set of reserved bits for future use.  All
       implementations complying with this protocol MUST set to zero
       any bits that are reserved in the version of the protocol
       supported by that implementation.  Receivers MUST ignore all
       bits not defined for the version of the protocol they support.
    D:    DTLS-Enabled Data Channel Supported
    C:    Clear Text Data Channel Supported
    R:    A reserved bit for future use.  All implementations
          complying with this protocol MUST set to zero any bits that
          are reserved in the version of the protocol supported by
          that implementation.  Receivers MUST ignore all bits not
          defined for the version of the protocol they support.
 AC Information Sub-Element:   The AC Descriptor message element
    contains multiple AC Information sub-elements, and defines two
    sub-types, each of which MUST be present.  The AC Information sub-
    element has the following format:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                AC Information Vendor Identifier               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      AC Information Type      |     AC Information Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     AC Information Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    AC Information Vendor Identifier:   A 32-bit value containing the
       IANA-assigned "Structure of Management Information (SMI)
       Network Management Private Enterprise Codes".
    AC Information Type:   Vendor-specific encoding of AC information
       in the UTF-8 format [RFC3629].  The following enumerated values
       are supported.  Both the Hardware and Software Version sub-
       elements MUST be included in the AC Descriptor message element.
       The values listed below are used in conjunction with the AC
       Information Vendor Identifier field, whose value MUST be set to
       zero (0).  This field, combined with the AC Information Vendor
       Identifier set to a non-zero (0) value, allows vendors to use a
       private namespace.

Calhoun, et al. Standards Track [Page 63] RFC 5415 CAPWAP Protocol Specification March 2009

       4 -   Hardware Version: The AC's hardware version number.
       5 -   Software Version: The AC's Software (firmware) version
             number.
    AC Information Length:   Length of vendor-specific encoding of AC
       information, with a maximum size of 1024.
    AC Information Data:   Vendor-specific encoding of AC information.

4.6.2. AC IPv4 List

 The AC IPv4 List message element is used to configure a WTP with the
 latest list of ACs available for the WTP to join.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       AC IP Address[]                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   2 for AC IPv4 List
 Length:   >= 4
 AC IP Address:   An array of 32-bit integers containing AC IPv4
    Addresses, containing no more than 1024 addresses.

4.6.3. AC IPv6 List

 The AC IPv6 List message element is used to configure a WTP with the
 latest list of ACs available for the WTP to join.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       AC IP Address[]                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       AC IP Address[]                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       AC IP Address[]                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       AC IP Address[]                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 64] RFC 5415 CAPWAP Protocol Specification March 2009

 Type:   3 for AC IPV6 List
 Length:   >= 16
 AC IP Address:   An array of 128-bit integers containing AC IPv6
    Addresses, containing no more than 1024 addresses.

4.6.4. AC Name

 The AC Name message element contains an UTF-8 [RFC3629]
 representation of the AC identity.  The value is a variable-length
 byte string.  The string is NOT zero terminated.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |   Name ...
   +-+-+-+-+-+-+-+-+
 Type:   4 for AC Name
 Length:   >= 1
 Name:   A variable-length UTF-8 encoded string [RFC3629] containing
    the AC's name, whose maximum size MUST NOT exceed 512 bytes.

4.6.5. AC Name with Priority

 The AC Name with Priority message element is sent by the AC to the
 WTP to configure preferred ACs.  The number of instances of this
 message element is equal to the number of ACs configured on the WTP.
 The WTP also uses this message element to send its configuration to
 the AC.
    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Priority  |   AC Name...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   5 for AC Name with Priority
 Length:   >= 2
 Priority:   A value between 1 and 255 specifying the priority order
    of the preferred AC.  For instance, the value of one (1) is used
    to set the primary AC, the value of two (2) is used to set the
    secondary, etc.

Calhoun, et al. Standards Track [Page 65] RFC 5415 CAPWAP Protocol Specification March 2009

 AC Name:   A variable-length UTF-8 encoded string [RFC3629]
    containing the AC name, whose maximum size MUST NOT exceed 512
    bytes.

4.6.6. AC Timestamp

 The AC Timestamp message element is sent by the AC to synchronize the
 WTP clock.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   6 for AC Timestamp
 Length:   4
 Timestamp:   The AC's current time, allowing all of the WTPs to be
    time synchronized in the format defined by Network Time Protocol
    (NTP) in RFC 1305 [RFC1305].  Only the most significant 32 bits of
    the NTP time are included in this field.

4.6.7. Add MAC ACL Entry

 The Add MAC Access Control List (ACL) Entry message element is used
 by an AC to add a MAC ACL list entry on a WTP, ensuring that the WTP
 no longer provides service to the MAC addresses provided in the
 message.  The MAC addresses provided in this message element are not
 expected to be saved in non-volatile memory on the WTP.  The MAC ACL
 table on the WTP is cleared every time the WTP establishes a new
 session with an AC.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Num of Entries|    Length     |         MAC Address ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   7 for Add MAC ACL Entry
 Length:   >= 8
 Num of Entries:   The number of instances of the Length/MAC Address
    fields in the array.  This value MUST NOT exceed 255.

Calhoun, et al. Standards Track [Page 66] RFC 5415 CAPWAP Protocol Specification March 2009

 Length:  The length of the MAC Address field.  The formats and
    lengths specified in [EUI-48] and [EUI-64] are supported.
 MAC Address:   MAC addresses to add to the ACL.

4.6.8. Add Station

 The Add Station message element is used by the AC to inform a WTP
 that it should forward traffic for a station.  The Add Station
 message element is accompanied by technology-specific binding
 information element(s), which may include security parameters.
 Consequently, the security parameters MUST be applied by the WTP for
 the station.
 After station policy has been delivered to the WTP through the Add
 Station message element, an AC MAY change any policies by sending a
 modified Add Station message element.  When a WTP receives an Add
 Station message element for an existing station, it MUST override any
 existing state for the station.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Radio ID   |     Length    |          MAC Address ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  VLAN Name...
   +-+-+-+-+-+-+-+-+
 Type:   8 for Add Station
 Length:   >= 8
 Radio ID:   An 8-bit value representing the radio, whose value is
    between one (1) and 31.
 Length:  The length of the MAC Address field.  The formats and
    lengths specified in [EUI-48] and [EUI-64] are supported.
 MAC Address:   The station's MAC address.
 VLAN Name:   An optional variable-length UTF-8 encoded string
    [RFC3629], with a maximum length of 512 octets, containing the
    VLAN Name on which the WTP is to locally bridge user data.  Note
    this field is only valid with WTPs configured in Local MAC mode.

Calhoun, et al. Standards Track [Page 67] RFC 5415 CAPWAP Protocol Specification March 2009

4.6.9. CAPWAP Control IPv4 Address

 The CAPWAP Control IPv4 Address message element is sent by the AC to
 the WTP during the Discovery process and is used by the AC to provide
 the interfaces available on the AC, and the current number of WTPs
 connected.  When multiple CAPWAP Control IPV4 Address message
 elements are returned, the WTP SHOULD perform load balancing across
 the multiple interfaces (see Section 6.1).
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           WTP Count           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   10 for CAPWAP Control IPv4 Address
 Length:   6
 IP Address:   The IP address of an interface.
 WTP Count:   The number of WTPs currently connected to the interface,
    with a maximum value of 65535.

4.6.10. CAPWAP Control IPv6 Address

 The CAPWAP Control IPv6 Address message element is sent by the AC to
 the WTP during the Discovery process and is used by the AC to provide
 the interfaces available on the AC, and the current number of WTPs
 connected.  This message element is useful for the WTP to perform
 load balancing across multiple interfaces (see Section 6.1).
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           WTP Count           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 68] RFC 5415 CAPWAP Protocol Specification March 2009

 Type:   11 for CAPWAP Control IPv6 Address
 Length:   18
 IP Address:   The IP address of an interface.
 WTP Count:   The number of WTPs currently connected to the interface,
    with a maximum value of 65535.

4.6.11. CAPWAP Local IPv4 Address

 The CAPWAP Local IPv4 Address message element is sent by either the
 WTP, in the Join Request, or by the AC, in the Join Response.  The
 CAPWAP Local IPv4 Address message element is used to communicate the
 IP Address of the transmitter.  The receiver uses this to determine
 whether a middlebox exists between the two peers, by comparing the
 source IP address of the packet against the value of the message
 element.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   30 for CAPWAP Local IPv4 Address
 Length:   4
 IP Address:   The IP address of the sender.

4.6.12. CAPWAP Local IPv6 Address

 The CAPWAP Local IPv6 Address message element is sent by either the
 WTP, in the Join Request, or by the AC, in the Join Response.  The
 CAPWAP Local IPv6 Address message element is used to communicate the
 IP Address of the transmitter.  The receiver uses this to determine
 whether a middlebox exists between the two peers, by comparing the
 source IP address of the packet against the value of the message
 element.

Calhoun, et al. Standards Track [Page 69] RFC 5415 CAPWAP Protocol Specification March 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   50 for CAPWAP Local IPv6 Address
 Length:   16
 IP Address:   The IP address of the sender.

4.6.13. CAPWAP Timers

 The CAPWAP Timers message element is used by an AC to configure
 CAPWAP timers on a WTP.
    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Discovery   | Echo Request  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   12 for CAPWAP Timers
 Length:   2
 Discovery:   The number of seconds between CAPWAP Discovery messages,
    when the WTP is in the Discovery phase.  This value is used to
    configure the MaxDiscoveryInterval timer (see Section 4.7.10).
 Echo Request:   The number of seconds between WTP Echo Request CAPWAP
    messages.  This value is used to configure the EchoInterval timer
    (see Section 4.7.7).  The AC sets its EchoInterval timer to this
    value, plus the maximum retransmission time as described in
    Section 4.5.3.

Calhoun, et al. Standards Track [Page 70] RFC 5415 CAPWAP Protocol Specification March 2009

4.6.14. CAPWAP Transport Protocol

 When CAPWAP is run over IPv6, the UDP-Lite or UDP transports MAY be
 used (see Section 3).  The CAPWAP IPv6 Transport Protocol message
 element is used by either the WTP or the AC to signal which transport
 protocol is to be used for the CAPWAP data channel.
 Upon receiving the Join Request, the AC MAY set the CAPWAP Transport
 Protocol to UDP-Lite in the Join Response message if the CAPWAP
 message was received over IPv6, and the CAPWAP Local IPv6 Address
 message element (see Section 4.6.12) is present and no middlebox was
 detected (see Section 11).
 Upon receiving the Join Response, the WTP MAY set the CAPWAP
 Transport Protocol to UDP-Lite in the Configuration Status Request or
 Image Data Request message if the AC advertised support for UDP-Lite,
 the message was received over IPv6, the CAPWAP Local IPv6 Address
 message element (see Section 4.6.12) and no middlebox was detected
 (see Section 11).  Upon receiving either the Configuration Status
 Request or the Image Data Request, the AC MUST observe the preference
 indicated by the WTP in the CAPWAP Transport Protocol, as long as it
 is consistent with what the AC advertised in the Join Response.
 For any other condition, the CAPWAP Transport Protocol MUST be set to
 UDP.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |   Transport   |
   +-+-+-+-+-+-+-+-+
 Type:   51 for CAPWAP Transport Protocol
 Length:   1
 Transport:   The transport to use for the CAPWAP Data channel.  The
    following enumerated values are supported:
    1 -   UDP-Lite: The UDP-Lite transport protocol is to be used for
          the CAPWAP Data channel.  Note that this option MUST NOT be
          used if the CAPWAP Control channel is being used over IPv4.
    2 -   UDP: The UDP transport protocol is to be used for the CAPWAP
          Data channel.

Calhoun, et al. Standards Track [Page 71] RFC 5415 CAPWAP Protocol Specification March 2009

4.6.15. Data Transfer Data

 The Data Transfer Data message element is used by the WTP to provide
 information to the AC for debugging purposes.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Data Type   |   Data Mode   |         Data Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Data ....
   +-+-+-+-+-+-+-+-+
 Type:   13 for Data Transfer Data
 Length:   >= 5
 Data Type:   An 8-bit value representing the transfer Data Type.  The
    following enumerated values are supported:
    1 -  Transfer data is included.
    2 -  Last Transfer Data Block is included (End of File (EOF)).
    5 -  An error occurred.  Transfer is aborted.
 Data Mode:   An 8-bit value describing the type of information being
    transmitted.  The following enumerated values are supported:
    0 -  Reserved
    1 -  WTP Crash Data
    2 -  WTP Memory Dump
 Data Length:   Length of data field, with a maximum size of 65535.
 Data:   Data being transferred from the WTP to the AC, whose type is
    identified via the Data Mode field.

Calhoun, et al. Standards Track [Page 72] RFC 5415 CAPWAP Protocol Specification March 2009

4.6.16. Data Transfer Mode

 The Data Transfer Mode message element is used by the WTP to indicate
 the type of data transfer information it is sending to the AC for
 debugging purposes.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |   Data Mode   |
   +-+-+-+-+-+-+-+-+
 Type:   14 for Data Transfer Mode
 Length:   1
 Data Mode:   An 8-bit value describing the type of information being
    requested.  The following enumerated values are supported:
    0 -  Reserved
    1 -  WTP Crash Data
    2 -  WTP Memory Dump

4.6.17. Decryption Error Report

 The Decryption Error Report message element value is used by the WTP
 to inform the AC of decryption errors that have occurred since the
 last report.  Note that this error reporting mechanism is not used if
 encryption and decryption services are provided in the AC.
    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Radio ID    |Num Of Entries |     Length    | MAC Address...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   15 for Decryption Error Report
 Length:   >= 9
 Radio ID:   The Radio Identifier refers to an interface index on the
    WTP, whose value is between one (1) and 31.
 Num of Entries:   The number of instances of the Length/MAC Address
    fields in the array.  This field MUST NOT exceed the value of 255.

Calhoun, et al. Standards Track [Page 73] RFC 5415 CAPWAP Protocol Specification March 2009

 Length:  The length of the MAC Address field.  The formats and
    lengths specified in [EUI-48] and [EUI-64] are supported.
 MAC Address:   MAC address of the station that has caused decryption
    errors.

4.6.18. Decryption Error Report Period

 The Decryption Error Report Period message element value is used by
 the AC to inform the WTP how frequently it should send decryption
 error report messages.  Note that this error reporting mechanism is
 not used if encryption and decryption services are provided in the
 AC.
    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Radio ID    |        Report Interval        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   16 for Decryption Error Report Period
 Length:   3
 Radio ID:   The Radio Identifier refers to an interface index on the
    WTP, whose value is between one (1) and 31.
 Report Interval:   A 16-bit unsigned integer indicating the time, in
    seconds.  The default value for this message element can be found
    in Section 4.7.11.

4.6.19. Delete MAC ACL Entry

 The Delete MAC ACL Entry message element is used by an AC to delete a
 MAC ACL entry on a WTP, ensuring that the WTP provides service to the
 MAC addresses provided in the message.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Num of Entries|     Length    |          MAC Address ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   17 for Delete MAC ACL Entry
 Length:   >= 8

Calhoun, et al. Standards Track [Page 74] RFC 5415 CAPWAP Protocol Specification March 2009

 Num of Entries:   The number of instances of the Length/MAC Address
    fields in the array.  This field MUST NOT exceed the value of 255.
 Length:  The length of the MAC Address field.  The formats and
    lengths specified in [EUI-48] and [EUI-64] are supported.
 MAC Address:   An array of MAC addresses to delete from the ACL.

4.6.20. Delete Station

 The Delete Station message element is used by the AC to inform a WTP
 that it should no longer provide service to a particular station.
 The WTP MUST terminate service to the station immediately upon
 receiving this message element.
 The transmission of a Delete Station message element could occur for
 various reasons, including for administrative reasons, or if the
 station has roamed to another WTP.
 The Delete Station message element MAY be sent by the WTP, in the WTP
 Event Request message, to inform the AC that a particular station is
 no longer being provided service.  This could occur as a result of an
 Idle Timeout (see section 4.4.43), due to internal resource shortages
 or for some other reason.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Radio ID   |     Length    |        MAC Address...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   18 for Delete Station
 Length:   >= 8
 Radio ID:   An 8-bit value representing the radio, whose value is
    between one (1) and 31.
 Length:  The length of the MAC Address field.  The formats and
    lengths specified in [EUI-48] and [EUI-64] are supported.
 MAC Address:   The station's MAC address.

4.6.21. Discovery Type

 The Discovery Type message element is used by the WTP to indicate how
 it has come to know about the existence of the AC to which it is
 sending the Discovery Request message.

Calhoun, et al. Standards Track [Page 75] RFC 5415 CAPWAP Protocol Specification March 2009

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   | Discovery Type|
   +-+-+-+-+-+-+-+-+
 Type:   20 for Discovery Type
 Length:   1
 Discovery Type:   An 8-bit value indicating how the WTP discovered
    the AC.  The following enumerated values are supported:
    0 -   Unknown
    1 -   Static Configuration
    2 -   DHCP
    3 -   DNS
    4 -   AC Referral (used when the AC was configured either through
          the AC IPv4 List or AC IPv6 List message element)

4.6.22. Duplicate IPv4 Address

 The Duplicate IPv4 Address message element is used by a WTP to inform
 an AC that it has detected another IP device using the same IP
 address that the WTP is currently using.
 The WTP MUST transmit this message element with the status set to 1
 after it has detected a duplicate IP address.  When the WTP detects
 that the duplicate IP address has been cleared, it MUST send this
 message element with the status set to 0.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IP Address                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Status    |     Length    |          MAC Address ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   21 for Duplicate IPv4 Address
 Length:   >= 12
 IP Address:   The IP address currently used by the WTP.

Calhoun, et al. Standards Track [Page 76] RFC 5415 CAPWAP Protocol Specification March 2009

 Status:   The status of the duplicate IP address.  The value MUST be
    set to 1 when a duplicate address is detected, and 0 when the
    duplicate address has been cleared.
 Length:  The length of the MAC Address field.  The formats and
    lengths specified in [EUI-48] and [EUI-64] are supported.
 MAC Address:   The MAC address of the offending device.

4.6.23. Duplicate IPv6 Address

 The Duplicate IPv6 Address message element is used by a WTP to inform
 an AC that it has detected another host using the same IP address
 that the WTP is currently using.
 The WTP MUST transmit this message element with the status set to 1
 after it has detected a duplicate IP address.  When the WTP detects
 that the duplicate IP address has been cleared, it MUST send this
 message element with the status set to 0.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IP Address                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IP Address                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IP Address                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IP Address                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Status    |     Length    |         MAC Address ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   22 for Duplicate IPv6 Address
 Length:   >= 24
 IP Address:   The IP address currently used by the WTP.
 Status:   The status of the duplicate IP address.  The value MUST be
    set to 1 when a duplicate address is detected, and 0 when the
    duplicate address has been cleared.
 Length:  The length of the MAC Address field.  The formats and
    lengths specified in [EUI-48] and [EUI-64] are supported.
 MAC Address:   The MAC address of the offending device.

Calhoun, et al. Standards Track [Page 77] RFC 5415 CAPWAP Protocol Specification March 2009

4.6.24. Idle Timeout

 The Idle Timeout message element is sent by the AC to the WTP to
 provide the Idle Timeout value that the WTP SHOULD enforce for its
 active stations.  The value applies to all radios on the WTP.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Timeout                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   23 for Idle Timeout
 Length:   4
 Timeout:   The current Idle Timeout, in seconds, to be enforced by
    the WTP.  The default value for this message element is specified
    in Section 4.7.8.

4.6.25. ECN Support

 The ECN Support message element is sent by both the WTP and the AC to
 indicate their support for the Explicit Congestion Notification (ECN)
 bits, as defined in [RFC3168].
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  ECN Support  |
   +-+-+-+-+-+-+-+-+
 Type:   53 for ECN Support
 Length:   1
 ECN Support:   An 8-bit value representing the sender's support for
    ECN, as defined in [RFC3168].  All CAPWAP Implementations MUST
    support the Limited ECN Support mode.  Full ECN Support is used if
    both the WTP and AC advertise the capability for "Full and Limited
    ECN" Support; otherwise, Limited ECN Support is used.
    0 -  Limited ECN Support
    1 -  Full and Limited ECN Support

Calhoun, et al. Standards Track [Page 78] RFC 5415 CAPWAP Protocol Specification March 2009

4.6.26. Image Data

 The Image Data message element is present in the Image Data Request
 message sent by the AC and contains the following fields.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Data Type   |                    Data ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   24 for Image Data
 Length:   >= 1
 Data Type:   An 8-bit value representing the image Data Type.  The
    following enumerated values are supported:
    1 -  Image data is included.
    2 -  Last Image Data Block is included (EOF).
    5 -  An error occurred.  Transfer is aborted.
 Data:   The Image Data field contains up to 1024 characters, and its
    length is inferred from this message element's length field.  If
    the block being sent is the last one, the Data Type field is set
    to 2.  The AC MAY opt to abort the data transfer by setting the
    Data Type field to 5.  When the Data Type field is 5, the Value
    field has a zero length.

4.6.27. Image Identifier

 The Image Identifier message element is sent by the AC to the WTP to
 indicate the expected active software version that is to be run on
 the WTP.  The WTP sends the Image Identifier message element in order
 to request a specific software version from the AC.  The actual
 download process is defined in Section 9.1.  The value is a variable-
 length UTF-8 encoded string [RFC3629], which is NOT zero terminated.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Vendor Identifier                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 79] RFC 5415 CAPWAP Protocol Specification March 2009

 Type:   25 for Image Identifier
 Length:   >= 5
 Vendor Identifier:   A 32-bit value containing the IANA-assigned "SMI
    Network Management Private Enterprise Codes".
 Data:   A variable-length UTF-8 encoded string [RFC3629] containing
    the firmware identifier to be run on the WTP, whose length MUST
    NOT exceed 1024 octets.  The length of this field is inferred from
    this message element's length field.

4.6.28. Image Information

 The Image Information message element is present in the Image Data
 Response message sent by the AC to the WTP and contains the following
 fields.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           File Size                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Hash                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Hash                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Hash                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Hash                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   26 for Image Information
 Length:   20
 File Size:   A 32-bit value containing the size of the file, in
    bytes, that will be transferred by the AC to the WTP.
 Hash:   A 16-octet MD5 hash of the image using the procedures defined
    in [RFC1321].

Calhoun, et al. Standards Track [Page 80] RFC 5415 CAPWAP Protocol Specification March 2009

4.6.29. Initiate Download

 The Initiate Download message element is used by the WTP to inform
 the AC that the AC SHOULD initiate a firmware upgrade.  The AC
 subsequently transmits an Image Data Request message, which includes
 the Image Data message element.  This message element does not
 contain any data.
 Type:   27 for Initiate Download
 Length:   0

4.6.30. Location Data

 The Location Data message element is a variable-length byte UTF-8
 encoded string [RFC3629] containing user-defined location information
 (e.g., "Next to Fridge").  This information is configurable by the
 network administrator, and allows the WTP location to be determined.
 The string is not zero terminated.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-
   | Location ...
   +-+-+-+-+-+-+-+-+-
 Type:   28 for Location Data
 Length:   >= 1
 Location:   A non-zero-terminated UTF-8 encoded string [RFC3629]
    containing the WTP location, whose maximum size MUST NOT exceed
    1024.

4.6.31. Maximum Message Length

 The Maximum Message Length message element is included in the Join
 Request message by the WTP to indicate the maximum CAPWAP message
 length that it supports to the AC.  The Maximum Message Length
 message element is optionally included in Join Response message by
 the AC to indicate the maximum CAPWAP message length that it supports
 to the WTP.
       0              1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Maximum Message Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 81] RFC 5415 CAPWAP Protocol Specification March 2009

 Type:   29 for Maximum Message Length
 Length:   2
 Maximum Message Length  A 16-bit unsigned integer indicating the
    maximum message length.

4.6.32. MTU Discovery Padding

 The MTU Discovery Padding message element is used as padding to
 perform MTU discovery, and MUST contain octets of value 0xFF, of any
 length.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Padding...
   +-+-+-+-+-+-+-+-
 Type:   52 for MTU Discovery Padding
 Length:   Variable
 Pad:   A variable-length pad, filled with the value 0xFF.

4.6.33. Radio Administrative State

 The Radio Administrative State message element is used to communicate
 the state of a particular radio.  The Radio Administrative State
 message element is sent by the AC to change the state of the WTP.
 The WTP saves the value, to ensure that it remains across WTP resets.
 The WTP communicates this message element during the configuration
 phase, in the Configuration Status Request message, to ensure that
 the AC has the WTP radio current administrative state settings.  The
 message element contains the following fields:
       0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Radio ID    |  Admin State  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   31 for Radio Administrative State
 Length:   2

Calhoun, et al. Standards Track [Page 82] RFC 5415 CAPWAP Protocol Specification March 2009

 Radio ID:   An 8-bit value representing the radio to configure, whose
    value is between one (1) and 31.  The Radio ID field MAY also
    include the value of 0xff, which is used to identify the WTP.  If
    an AC wishes to change the administrative state of a WTP, it
    includes 0xff in the Radio ID field.
 Admin State:   An 8-bit value representing the administrative state
    of the radio.  The default value for the Admin State field is
    listed in Section 4.8.1.  The following enumerated values are
    supported:
    0 -  Reserved
    1 -  Enabled
    2 -  Disabled

4.6.34. Radio Operational State

 The Radio Operational State message element is sent by the WTP to the
 AC to communicate a radio's operational state.  This message element
 is included in the Configuration Update Response message by the WTP
 if it was requested to change the state of its radio, via the Radio
 Administrative State message element, but was unable to comply to the
 request.  This message element is included in the Change State Event
 message when a WTP radio state was changed unexpectedly.  This could
 occur due to a hardware failure.  Note that the operational state
 setting is not saved on the WTP, and therefore does not remain across
 WTP resets.  The value contains three fields, as shown below.
    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Radio ID    |     State     |     Cause     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   32 for Radio Operational State
 Length:   3
 Radio ID:   The Radio Identifier refers to an interface index on the
    WTP, whose value is between one (1) and 31.  A value of 0xFF is
    invalid, as it is not possible to change the WTP's operational
    state.
 State:   An 8-bit Boolean value representing the state of the radio.
    The following enumerated values are supported:

Calhoun, et al. Standards Track [Page 83] RFC 5415 CAPWAP Protocol Specification March 2009

    0 -  Reserved
    1 -  Enabled
    2 -  Disabled
 Cause:   When a radio is inoperable, the cause field contains the
    reason the radio is out of service.  The following enumerated
    values are supported:
    0 -  Normal
    1 -  Radio Failure
    2 -  Software Failure
    3 -  Administratively Set

4.6.35. Result Code

 The Result Code message element value is a 32-bit integer value,
 indicating the result of the Request message corresponding to the
 sequence number included in the Response message.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Result Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   33 for Result Code
 Length:   4
 Result Code:   The following enumerated values are defined:
    0  Success
    1  Failure (AC List Message Element MUST Be Present)
    2  Success (NAT Detected)
    3  Join Failure (Unspecified)
    4  Join Failure (Resource Depletion)
    5  Join Failure (Unknown Source)

Calhoun, et al. Standards Track [Page 84] RFC 5415 CAPWAP Protocol Specification March 2009

    6  Join Failure (Incorrect Data)
    7  Join Failure (Session ID Already in Use)
    8  Join Failure (WTP Hardware Not Supported)
    9  Join Failure (Binding Not Supported)
    10 Reset Failure (Unable to Reset)
    11 Reset Failure (Firmware Write Error)
    12 Configuration Failure (Unable to Apply Requested Configuration
       - Service Provided Anyhow)
    13 Configuration Failure (Unable to Apply Requested Configuration
       - Service Not Provided)
    14 Image Data Error (Invalid Checksum)
    15 Image Data Error (Invalid Data Length)
    16 Image Data Error (Other Error)
    17 Image Data Error (Image Already Present)
    18 Message Unexpected (Invalid in Current State)
    19 Message Unexpected (Unrecognized Request)
    20 Failure - Missing Mandatory Message Element
    21 Failure - Unrecognized Message Element
    22 Data Transfer Error (No Information to Transfer)

4.6.36. Returned Message Element

 The Returned Message Element is sent by the WTP in the Change State
 Event Request message to communicate to the AC which message elements
 in the Configuration Status Response it was unable to apply locally.
 The Returned Message Element message element contains a result code
 indicating the reason that the configuration could not be applied,
 and encapsulates the failed message element.

Calhoun, et al. Standards Track [Page 85] RFC 5415 CAPWAP Protocol Specification March 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Reason     |    Length     |       Message Element...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   34 for Returned Message Element
 Length:   >= 6
 Reason:   The reason the configuration in the offending message
    element could not be applied by the WTP.  The following enumerated
    values are supported:
    0 -  Reserved
    1 -  Unknown Message Element
    2 -  Unsupported Message Element
    3 -  Unknown Message Element Value
    4 -  Unsupported Message Element Value
 Length:   The length of the Message Element field, which MUST NOT
    exceed 255 octets.
 Message Element:   The Message Element field encapsulates the message
    element sent by the AC in the Configuration Status Response
    message that caused the error.

4.6.37. Session ID

 The Session ID message element value contains a randomly generated
 unsigned 128-bit integer.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 86] RFC 5415 CAPWAP Protocol Specification March 2009

 Type:   35 for Session ID
 Length:   16
 Session ID:   A 128-bit unsigned integer used as a random session
    identifier

4.6.38. Statistics Timer

 The Statistics Timer message element value is used by the AC to
 inform the WTP of the frequency with which it expects to receive
 updated statistics.
    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Statistics Timer       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   36 for Statistics Timer
 Length:   2
 Statistics Timer:   A 16-bit unsigned integer indicating the time, in
    seconds.  The default value for this timer is specified in
    Section 4.7.14.

4.6.39. Vendor Specific Payload

 The Vendor Specific Payload message element is used to communicate
 vendor-specific information between the WTP and the AC.  The Vendor
 Specific Payload message element MAY be present in any CAPWAP
 message.  The exchange of vendor-specific data between the MUST NOT
 modify the behavior of the base CAPWAP protocol and state machine.
 The message element uses the following format:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Vendor Identifier                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Element ID           |    Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   37 for Vendor Specific Payload
 Length:   >= 7

Calhoun, et al. Standards Track [Page 87] RFC 5415 CAPWAP Protocol Specification March 2009

 Vendor Identifier:   A 32-bit value containing the IANA-assigned "SMI
    Network Management Private Enterprise Codes" [RFC3232].
 Element ID:   A 16-bit Element Identifier that is managed by the
    vendor.
 Data:   Variable-length vendor-specific information, whose contents
    and format are proprietary and understood based on the Element ID
    field.  This field MUST NOT exceed 2048 octets.

4.6.40. WTP Board Data

 The WTP Board Data message element is sent by the WTP to the AC and
 contains information about the hardware present.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Vendor Identifier                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Board Data Sub-Element...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   38 for WTP Board Data
 Length:   >=14
 Vendor Identifier:   A 32-bit value containing the IANA-assigned "SMI
    Network Management Private Enterprise Codes", identifying the WTP
    hardware manufacturer.  The Vendor Identifier field MUST NOT be
    set to zero.
 Board Data Sub-Element:   The WTP Board Data message element contains
    multiple Board Data sub-elements, some of which are mandatory and
    some are optional, as described below.  The Board Data Type values
    are not extensible by vendors, and are therefore not coupled along
    with the Vendor Identifier field.  The Board Data sub-element has
    the following format:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Board Data Type        |       Board Data Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Board Data Value...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Calhoun, et al. Standards Track [Page 88] RFC 5415 CAPWAP Protocol Specification March 2009

    Board Data Type:   The Board Data Type field identifies the data
       being encoded.  The CAPWAP protocol defines the following
       values, and each of these types identify whether their presence
       is mandatory or optional:
    0 -   WTP Model Number: The WTP Model Number MUST be included in
          the WTP Board Data message element.
    1 -   WTP Serial Number: The WTP Serial Number MUST be included in
          the WTP Board Data message element.
    2 -   Board ID: A hardware identifier, which MAY be included in
          the WTP Board Data message element.
    3 -   Board Revision: A revision number of the board, which MAY be
          included in the WTP Board Data message element.
    4 -   Base MAC Address: The WTP's Base MAC address, which MAY be
          assigned to the primary Ethernet interface.
 Board Data Length:   The length of the data in the Board Data Value
    field, whose length MUST NOT exceed 1024 octets.
 Board Data Value:   The data associated with the Board Data Type
    field for this Board Data sub-element.

4.6.41. WTP Descriptor

 The WTP Descriptor message element is used by a WTP to communicate
 its current hardware and software (firmware) configuration.  The
 value contains the following fields:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Max Radios  | Radios in use |  Num Encrypt  |Encryp Sub-Elmt|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Encryption Sub-Element    |    Descriptor Sub-Element...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   39 for WTP Descriptor
 Length:   >= 33

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 Max Radios:   An 8-bit value representing the number of radios (where
    each radio is identified via the Radio ID field) supported by the
    WTP.
 Radios in use:   An 8-bit value representing the number of radios in
    use in the WTP.
 Num Encrypt:   The number of 3-byte Encryption sub-elements that
    follow this field.  The value of the Num Encrypt field MUST be
    between one (1) and 255.
 Encryption Sub-Element:   The WTP Descriptor message element MUST
    contain at least one Encryption sub-element.  One sub-element is
    present for each binding supported by the WTP.  The Encryption
    sub-element has the following format:
    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Resvd|  WBID   |  Encryption Capabilities      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Resvd:  The 3-bit field is reserved for future use.  All
       implementations complying with this protocol MUST set to zero
       any bits that are reserved in the version of the protocol
       supported by that implementation.  Receivers MUST ignore all
       bits not defined for the version of the protocol they support.
    WBID:   A 5-bit field that is the wireless binding identifier.
       The identifier will indicate the type of wireless packet
       associated with the radio.  The WBIDs defined in this
       specification can be found in Section 4.3.
    Encryption Capabilities:   This 16-bit field is used by the WTP to
       communicate its capabilities to the AC.  A WTP that does not
       have any encryption capabilities sets this field to zero (0).
       Refer to the specific wireless binding for further
       specification of the Encryption Capabilities field.
 Descriptor Sub-Element:   The WTP Descriptor message element contains
    multiple Descriptor sub-elements, some of which are mandatory and
    some are optional, as described below.  The Descriptor sub-element
    has the following format:

Calhoun, et al. Standards Track [Page 90] RFC 5415 CAPWAP Protocol Specification March 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Descriptor Vendor Identifier                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Descriptor Type        |       Descriptor Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Descriptor Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Descriptor Vendor Identifier:   A 32-bit value containing the
       IANA-assigned "SMI Network Management Private Enterprise
       Codes".
    Descriptor Type:   The Descriptor Type field identifies the data
       being encoded.  The format of the data is vendor-specific
       encoded in the UTF-8 format [RFC3629].  The CAPWAP protocol
       defines the following values, and each of these types identify
       whether their presence is mandatory or optional.  The values
       listed below are used in conjunction with the Descriptor Vendor
       Identifier field, whose value MUST be set to zero (0).  This
       field, combined with the Descriptor Vendor Identifier set to a
       non-zero (0) value, allows vendors to use a private namespace.
       0 -   Hardware Version: The WTP hardware version number MUST be
             present.
       1 -   Active Software Version: The WTP running software version
             number MUST be present.
       2 -   Boot Version: The WTP boot loader version number MUST be
             present.
       3 -   Other Software Version: The WTP non-running software
             (firmware) version number MAY be present.  This type is
             used to communicate alternate software versions that are
             available on the WTP's non-volatile storage.
    Descriptor Length:   Length of the vendor-specific encoding of the
       Descriptor Data field, whose length MUST NOT exceed 1024
       octets.
    Descriptor Data:   Vendor-specific data of WTP information encoded
       in the UTF-8 format [RFC3629].

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4.6.42. WTP Fallback

 The WTP Fallback message element is sent by the AC to the WTP to
 enable or disable automatic CAPWAP fallback in the event that a WTP
 detects its preferred AC to which it is not currently connected.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |     Mode      |
   +-+-+-+-+-+-+-+-+
 Type:   40 for WTP Fallback
 Length:   1
 Mode:   The 8-bit value indicates the status of automatic CAPWAP
    fallback on the WTP.  When enabled, if the WTP detects that its
    primary AC is available, and that the WTP is not connected to the
    primary AC, the WTP SHOULD automatically disconnect from its
    current AC and reconnect to its primary AC.  If disabled, the WTP
    will only reconnect to its primary AC through manual intervention
    (e.g., through the Reset Request message).  The default value for
    this field is specified in Section 4.8.9.  The following
    enumerated values are supported:
    0 -  Reserved
    1 -  Enabled
    2 -  Disabled

4.6.43. WTP Frame Tunnel Mode

 The WTP Frame Tunnel Mode message element allows the WTP to
 communicate the tunneling modes of operation that it supports to the
 AC.  A WTP that advertises support for all types allows the AC to
 select which type will be used, based on its local policy.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |Reservd|N|E|L|U|
   +-+-+-+-+-+-+-+-+

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 Type:   41 for WTP Frame Tunnel Mode
 Length:   1
 Reservd:   A set of reserved bits for future use.  All
    implementations complying with this protocol MUST set to zero any
    bits that are reserved in the version of the protocol supported by
    that implementation.  Receivers MUST ignore all bits not defined
    for the version of the protocol they support.
 N:    Native Frame Tunnel mode requires the WTP and AC to encapsulate
       all user payloads as native wireless frames, as defined by the
       wireless binding (see for example Section 4.4)
 E:    The 802.3 Frame Tunnel Mode requires the WTP and AC to
       encapsulate all user payload as native IEEE 802.3 frames (see
       Section 4.4).  All user traffic is tunneled to the AC.  This
       value MUST NOT be used when the WTP MAC Type is set to Split
       MAC.
 L:    When Local Bridging is used, the WTP does not tunnel user
       traffic to the AC; all user traffic is locally bridged.  This
       value MUST NOT be used when the WTP MAC Type is set to Split
       MAC.
 R:    A reserved bit for future use.  All implementations complying
       with this protocol MUST set to zero any bits that are reserved
       in the version of the protocol supported by that
       implementation.  Receivers MUST ignore all bits not defined for
       the version of the protocol they support.

4.6.44. WTP MAC Type

 The WTP MAC-Type message element allows the WTP to communicate its
 mode of operation to the AC.  A WTP that advertises support for both
 modes allows the AC to select the mode to use, based on local policy.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |   MAC Type    |
   +-+-+-+-+-+-+-+-+
 Type:   44 for WTP MAC Type

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 Length:   1
 MAC Type:   The MAC mode of operation supported by the WTP.  The
    following enumerated values are supported:
    0 -   Local MAC: Local MAC is the default mode that MUST be
          supported by all WTPs.  When tunneling is enabled (see
          Section 4.6.43), the encapsulated frames MUST be in the
          802.3 format (see Section 4.4.2), unless a wireless
          management or control frame which MAY be in its native
          format.  Any CAPWAP binding needs to specify the format of
          management and control wireless frames.
    1 -   Split MAC: Split MAC support is optional, and allows the AC
          to receive and process native wireless frames.
    2 -   Both: WTP is capable of supporting both Local MAC and Split
          MAC.

4.6.45. WTP Name

 The WTP Name message element is a variable-length byte UTF-8 encoded
 string [RFC3629].  The string is not zero terminated.
    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-
   |  WTP Name ...
   +-+-+-+-+-+-+-+-+-
 Type:   45 for WTP Name
 Length:   >= 1
 WTP Name:   A non-zero-terminated UTF-8 encoded string [RFC3629]
    containing the WTP name, whose maximum size MUST NOT exceed 512
    bytes.

4.6.46. WTP Radio Statistics

 The WTP Radio Statistics message element is sent by the WTP to the AC
 to communicate statistics on radio behavior and reasons why the WTP
 radio has been reset.  These counters are never reset on the WTP, and
 will therefore roll over to zero when the maximum size has been
 reached.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Radio ID    | Last Fail Type|          Reset Count          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       SW Failure Count        |        HW Failure Count       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Other  Failure Count      |     Unknown Failure Count     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Config Update Count      |     Channel Change Count      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Band Change Count       |      Current Noise Floor      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   47 for WTP Radio Statistics
 Length:   20
 Radio ID:   The radio ID of the radio to which the statistics apply,
    whose value is between one (1) and 31.
 Last Failure Type:   The last WTP failure.  The following enumerated
    values are supported:
    0 -  Statistic Not Supported
    1 -  Software Failure
    2 -  Hardware Failure
    3 -  Other Failure
    255 -  Unknown (e.g., WTP doesn't keep track of info)
 Reset Count:   The number of times that the radio has been reset.
 SW Failure Count:   The number of times that the radio has failed due
    to software-related reasons.
 HW Failure Count:   The number of times that the radio has failed due
    to hardware-related reasons.
 Other Failure Count:   The number of times that the radio has failed
    due to known reasons, other than software or hardware failure.

Calhoun, et al. Standards Track [Page 95] RFC 5415 CAPWAP Protocol Specification March 2009

 Unknown Failure Count:   The number of times that the radio has
    failed for unknown reasons.
 Config Update Count:   The number of times that the radio
    configuration has been updated.
 Channel Change Count:   The number of times that the radio channel
    has been changed.
 Band Change Count:   The number of times that the radio has changed
    frequency bands.
 Current Noise Floor:   A signed integer that indicates the noise
    floor of the radio receiver in units of dBm.

4.6.47. WTP Reboot Statistics

 The WTP Reboot Statistics message element is sent by the WTP to the
 AC to communicate reasons why WTP reboots have occurred.  These
 counters are never reset on the WTP, and will therefore roll over to
 zero when the maximum size has been reached.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Reboot Count          |      AC Initiated Count       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Link Failure Count       |       SW Failure Count        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       HW Failure Count        |      Other Failure Count      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Unknown Failure Count     |Last Failure Type|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type:   48 for WTP Reboot Statistics
 Length:   15
 Reboot Count:   The number of reboots that have occurred due to a WTP
    crash.  A value of 65535 implies that this information is not
    available on the WTP.
 AC Initiated Count:   The number of reboots that have occurred at the
    request of a CAPWAP protocol message, such as a change in
    configuration that required a reboot or an explicit CAPWAP
    protocol reset request.  A value of 65535 implies that this
    information is not available on the WTP.

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 Link Failure Count:   The number of times that a CAPWAP protocol
    connection with an AC has failed due to link failure.
 SW Failure Count:   The number of times that a CAPWAP protocol
    connection with an AC has failed due to software-related reasons.
 HW Failure Count:   The number of times that a CAPWAP protocol
    connection with an AC has failed due to hardware-related reasons.
 Other Failure Count:   The number of times that a CAPWAP protocol
    connection with an AC has failed due to known reasons, other than
    AC initiated, link, SW or HW failure.
 Unknown Failure Count:   The number of times that a CAPWAP protocol
    connection with an AC has failed for unknown reasons.
 Last Failure Type:   The failure type of the most recent WTP failure.
    The following enumerated values are supported:
    0 -  Not Supported
    1 -  AC Initiated (see Section 9.2)
    2 -  Link Failure
    3 -  Software Failure
    4 -  Hardware Failure
    5 -  Other Failure
    255 -  Unknown (e.g., WTP doesn't keep track of info)

4.6.48. WTP Static IP Address Information

 The WTP Static IP Address Information message element is used by an
 AC to configure or clear a previously configured static IP address on
 a WTP.  IPv6 WTPs are expected to use dynamic addresses.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IP Address                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Netmask                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Gateway                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Static     |
   +-+-+-+-+-+-+-+-+
 Type:   49 for WTP Static IP Address Information
 Length:   13
 IP Address:   The IP address to assign to the WTP.  This field is
    only valid if the static field is set to one.
 Netmask:   The IP Netmask.  This field is only valid if the static
    field is set to one.
 Gateway:   The IP address of the gateway.  This field is only valid
    if the static field is set to one.
 Static:   An 8-bit Boolean stating whether or not the WTP should use
    a static IP address.  A value of zero disables the static IP
    address, while a value of one enables it.

4.7. CAPWAP Protocol Timers

 This section contains the definition of the CAPWAP timers.

4.7.1. ChangeStatePendingTimer

 The maximum time, in seconds, the AC will wait for the Change State
 Event Request from the WTP after having transmitted a successful
 Configuration Status Response message.
 Default: 25 seconds

4.7.2. DataChannelKeepAlive

 The DataChannelKeepAlive timer is used by the WTP to determine the
 next opportunity when it must transmit the Data Channel Keep-Alive,
 in seconds.
 Default: 30 seconds

Calhoun, et al. Standards Track [Page 98] RFC 5415 CAPWAP Protocol Specification March 2009

4.7.3. DataChannelDeadInterval

 The minimum time, in seconds, a WTP MUST wait without having received
 a Data Channel Keep-Alive packet before the destination for the Data
 Channel Keep-Alive packets may be considered dead.  The value of this
 timer MUST be no less than 2*DataChannelKeepAlive seconds and no
 greater that 240 seconds.
 Default: 60

4.7.4. DataCheckTimer

 The number of seconds the AC will wait for the Data Channel Keep
 Alive, which is required by the CAPWAP state machine's Data Check
 state.  The AC resets the state machine if this timer expires prior
 to transitioning to the next state.
 Default: 30

4.7.5. DiscoveryInterval

 The minimum time, in seconds, that a WTP MUST wait after receiving a
 Discovery Response message, before initiating a DTLS handshake.
 Default: 5

4.7.6. DTLSSessionDelete

 The minimum time, in seconds, a WTP MUST wait for DTLS session
 deletion.
 Default: 5

4.7.7. EchoInterval

 The minimum time, in seconds, between sending Echo Request messages
 to the AC with which the WTP has joined.
 Default: 30

4.7.8. IdleTimeout

 The default Idle Timeout is 300 seconds.

Calhoun, et al. Standards Track [Page 99] RFC 5415 CAPWAP Protocol Specification March 2009

4.7.9. ImageDataStartTimer

 The number of seconds the WTP will wait for its peer to transmit the
 Image Data Request.
 Default: 30

4.7.10. MaxDiscoveryInterval

 The maximum time allowed between sending Discovery Request messages,
 in seconds.  This value MUST be no less than 2 seconds and no greater
 than 180 seconds.
 Default: 20 seconds.

4.7.11. ReportInterval

 The ReportInterval is used by the WTP to determine the interval the
 WTP uses between sending the Decryption Error message elements to
 inform the AC of decryption errors, in seconds.
 The default Report Interval is 120 seconds.

4.7.12. RetransmitInterval

 The minimum time, in seconds, in which a non-acknowledged CAPWAP
 packet will be retransmitted.
 Default: 3

4.7.13. SilentInterval

 For a WTP, this is the minimum time, in seconds, a WTP MUST wait
 before it MAY again send Discovery Request messages or attempt to
 establish a DTLS session.  For an AC, this is the minimum time, in
 seconds, during which the AC SHOULD ignore all CAPWAP and DTLS
 packets received from the WTP that is in the Sulking state.
 Default: 30 seconds

4.7.14. StatisticsTimer

 The StatisticsTimer is used by the WTP to determine the interval the
 WTP uses between the WTP Events Requests it transmits to the AC to
 communicate its statistics, in seconds.
 Default: 120 seconds

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4.7.15. WaitDTLS

 The maximum time, in seconds, a WTP MUST wait without having received
 a DTLS Handshake message from an AC.  This timer MUST be greater than
 30 seconds.
 Default: 60

4.7.16. WaitJoin

 The maximum time, in seconds, an AC will wait after the DTLS session
 has been established until it receives the Join Request from the WTP.
 This timer MUST be greater than 20 seconds.
 Default: 60

4.8. CAPWAP Protocol Variables

 This section defines the CAPWAP protocol variables, which are used
 for various protocol functions.  Some of these variables are
 configurable, while others are counters or have a fixed value.  For
 non-counter-related variables, default values are specified.
 However, when a WTP's variable configuration is explicitly overridden
 by an AC, the WTP MUST save the new value.

4.8.1. AdminState

 The default Administrative State value is enabled (1).

4.8.2. DiscoveryCount

 The number of Discovery Request messages transmitted by a WTP to a
 single AC.  This is a monotonically increasing counter.

4.8.3. FailedDTLSAuthFailCount

 The number of failed DTLS session establishment attempts due to
 authentication failures.

4.8.4. FailedDTLSSessionCount

 The number of failed DTLS session establishment attempts.

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4.8.5. MaxDiscoveries

 The maximum number of Discovery Request messages that will be sent
 after a WTP boots.
 Default: 10

4.8.6. MaxFailedDTLSSessionRetry

 The maximum number of failed DTLS session establishment attempts
 before the CAPWAP device enters a silent period.
 Default: 3

4.8.7. MaxRetransmit

 The maximum number of retransmissions for a given CAPWAP packet
 before the link layer considers the peer dead.
 Default: 5

4.8.8. RetransmitCount

 The number of retransmissions for a given CAPWAP packet.  This is a
 monotonically increasing counter.

4.8.9. WTPFallBack

 The default WTP Fallback value is enabled (1).

4.9. WTP Saved Variables

 In addition to the values defined in Section 4.8, the following
 values SHOULD be saved on the WTP in non-volatile memory.  CAPWAP
 wireless bindings MAY define additional values that SHOULD be stored
 on the WTP.

4.9.1. AdminRebootCount

 The number of times the WTP has rebooted administratively, defined in
 Section 4.6.47.

4.9.2. FrameEncapType

 For WTPs that support multiple Frame Encapsulation Types, it is
 useful to save the value configured by the AC.  The Frame
 Encapsulation Type is defined in Section 4.6.43.

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4.9.3. LastRebootReason

 The reason why the WTP last rebooted, defined in Section 4.6.47.

4.9.4. MacType

 For WTPs that support multiple MAC-Types, it is useful to save the
 value configured by the AC.  The MAC-Type is defined in
 Section 4.6.44.

4.9.5. PreferredACs

 The preferred ACs, with the index, defined in Section 4.6.5.

4.9.6. RebootCount

 The number of times the WTP has rebooted, defined in Section 4.6.47.

4.9.7. Static IP Address

 The static IP address assigned to the WTP, as configured by the WTP
 Static IP address Information message element (see Section 4.6.48).

4.9.8. WTPLinkFailureCount

 The number of times the link to the AC has failed, see
 Section 4.6.47.

4.9.9. WTPLocation

 The WTP Location, defined in Section 4.6.30.

4.9.10. WTPName

 The WTP Name, defined in Section 4.6.45.

5. CAPWAP Discovery Operations

 The Discovery messages are used by a WTP to determine which ACs are
 available to provide service, and the capabilities and load of the
 ACs.

5.1. Discovery Request Message

 The Discovery Request message is used by the WTP to automatically
 discover potential ACs available in the network.  The Discovery
 Request message provides ACs with the primary capabilities of the

Calhoun, et al. Standards Track [Page 103] RFC 5415 CAPWAP Protocol Specification March 2009

 WTP.  A WTP must exchange this information to ensure subsequent
 exchanges with the ACs are consistent with the WTP's functional
 characteristics.
 Discovery Request messages MUST be sent by a WTP in the Discover
 state after waiting for a random delay less than
 MaxDiscoveryInterval, after a WTP first comes up or is
 (re)initialized.  A WTP MUST send no more than the maximum of
 MaxDiscoveries Discovery Request messages, waiting for a random delay
 less than MaxDiscoveryInterval between each successive message.
 This is to prevent an explosion of WTP Discovery Request messages.
 An example of this occurring is when many WTPs are powered on at the
 same time.
 If a Discovery Response message is not received after sending the
 maximum number of Discovery Request messages, the WTP enters the
 Sulking state and MUST wait for an interval equal to SilentInterval
 before sending further Discovery Request messages.
 Upon receiving a Discovery Request message, the AC will respond with
 a Discovery Response message sent to the address in the source
 address of the received Discovery Request message.  Once a Discovery
 Response has been received, if the WTP decides to establish a session
 with the responding AC, it SHOULD perform an MTU discovery, using the
 process described in Section 3.5.
 It is possible for the AC to receive a clear text Discovery Request
 message while a DTLS session is already active with the WTP.  This is
 most likely the case if the WTP has rebooted, perhaps due to a
 software or power failure, but could also be caused by a DoS attack.
 In such cases, any WTP state, including the state machine instance,
 MUST NOT be cleared until another DTLS session has been successfully
 established, communicated via the DTLSSessionEstablished DTLS
 notification (see Section 2.3.2.2).
 The binding specific WTP Radio Information message element (see
 Section 2.1) is included in the Discovery Request message to
 advertise WTP support for one or more CAPWAP bindings.
 The Discovery Request message is sent by the WTP when in the
 Discovery state.  The AC does not transmit this message.
 The following message elements MUST be included in the Discovery
 Request message:
 o  Discovery Type, see Section 4.6.21

Calhoun, et al. Standards Track [Page 104] RFC 5415 CAPWAP Protocol Specification March 2009

 o  WTP Board Data, see Section 4.6.40
 o  WTP Descriptor, see Section 4.6.41
 o  WTP Frame Tunnel Mode, see Section 4.6.43
 o  WTP MAC Type, see Section 4.6.44
 o  WTP Radio Information message element(s) that the WTP supports;
    These are defined by the individual link layer CAPWAP Binding
    Protocols (see Section 2.1).
 The following message elements MAY be included in the Discovery
 Request message:
 o  MTU Discovery Padding, see Section 4.6.32
 o  Vendor Specific Payload, see Section 4.6.39

5.2. Discovery Response Message

 The Discovery Response message provides a mechanism for an AC to
 advertise its services to requesting WTPs.
 When a WTP receives a Discovery Response message, it MUST wait for an
 interval not less than DiscoveryInterval for receipt of additional
 Discovery Response messages.  After the DiscoveryInterval elapses,
 the WTP enters the DTLS-Init state and selects one of the ACs that
 sent a Discovery Response message and send a DTLS Handshake to that
 AC.
 One or more binding-specific WTP Radio Information message elements
 (see Section 2.1) are included in the Discovery Request message to
 advertise AC support for the CAPWAP bindings.  The AC MAY include
 only the bindings it shares in common with the WTP, known through the
 WTP Radio Information message elements received in the Discovery
 Request message, or it MAY include all of the bindings supported.
 The WTP MAY use the supported bindings in its AC decision process.
 Note that if the WTP joins an AC that does not support a specific
 CAPWAP binding, service for that binding MUST NOT be provided by the
 WTP.
 The Discovery Response message is sent by the AC when in the Idle
 state.  The WTP does not transmit this message.
 The following message elements MUST be included in the Discovery
 Response Message:

Calhoun, et al. Standards Track [Page 105] RFC 5415 CAPWAP Protocol Specification March 2009

 o  AC Descriptor, see Section 4.6.1
 o  AC Name, see Section 4.6.4
 o  WTP Radio Information message element(s) that the AC supports;
    these are defined by the individual link layer CAPWAP Binding
    Protocols (see Section 2.1 for more information).
 o  One of the following message elements MUST be included in the
    Discovery Response Message:
  • CAPWAP Control IPv4 Address, see Section 4.6.9
  • CAPWAP Control IPv6 Address, see Section 4.6.10
 The following message elements MAY be included in the Discovery
 Response message:
 o  Vendor Specific Payload, see Section 4.6.39

5.3. Primary Discovery Request Message

 The Primary Discovery Request message is sent by the WTP to:
 o  determine whether its preferred (or primary) AC is available, or
 o  perform a Path MTU Discovery (see Section 3.5).
 A Primary Discovery Request message is sent by a WTP when it has a
 primary AC configured, and is connected to another AC.  This
 generally occurs as a result of a failover, and is used by the WTP as
 a means to discover when its primary AC becomes available.  Since the
 WTP only has a single instance of the CAPWAP state machine, the
 Primary Discovery Request is sent by the WTP when in the Run state.
 The AC does not transmit this message.
 The frequency of the Primary Discovery Request messages should be no
 more often than the sending of the Echo Request message.
 Upon receipt of a Primary Discovery Request message, the AC responds
 with a Primary Discovery Response message sent to the address in the
 source address of the received Primary Discovery Request message.
 The following message elements MUST be included in the Primary
 Discovery Request message.
 o  Discovery Type, see Section 4.6.21

Calhoun, et al. Standards Track [Page 106] RFC 5415 CAPWAP Protocol Specification March 2009

 o  WTP Board Data, see Section 4.6.40
 o  WTP Descriptor, see Section 4.6.41
 o  WTP Frame Tunnel Mode, see Section 4.6.43
 o  WTP MAC Type, see Section 4.6.44
 o  WTP Radio Information message element(s) that the WTP supports;
    these are defined by the individual link layer CAPWAP Binding
    Protocols (see Section 2.1 for more information).
 The following message elements MAY be included in the Primary
 Discovery Request message:
 o  MTU Discovery Padding, see Section 4.6.32
 o  Vendor Specific Payload, see Section 4.6.39

5.4. Primary Discovery Response

 The Primary Discovery Response message enables an AC to advertise its
 availability and services to requesting WTPs that are configured to
 have the AC as its primary AC.
 The Primary Discovery Response message is sent by an AC after
 receiving a Primary Discovery Request message.
 When a WTP receives a Primary Discovery Response message, it may
 establish a CAPWAP protocol connection to its primary AC, based on
 the configuration of the WTP Fallback Status message element on the
 WTP.
 The Primary Discovery Response message is sent by the AC when in the
 Idle state.  The WTP does not transmit this message.
 The following message elements MUST be included in the Primary
 Discovery Response message.
 o  AC Descriptor, see Section 4.6.1
 o  AC Name, see Section 4.6.4
 o  WTP Radio Information message element(s) that the AC supports;
    These are defined by the individual link layer CAPWAP Binding
    Protocols (see Section 2.1 for more information).

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 One of the following message elements MUST be included in the
 Discovery Response Message:
 o  CAPWAP Control IPv4 Address, see Section 4.6.9
 o  CAPWAP Control IPv6 Address, see Section 4.6.10
 The following message elements MAY be included in the Primary
 Discovery Response message:
 o  Vendor Specific Payload, see Section 4.6.39

6. CAPWAP Join Operations

 The Join Request message is used by a WTP to request service from an
 AC after a DTLS connection is established to that AC.  The Join
 Response message is used by the AC to indicate that it will or will
 not provide service.

6.1. Join Request

 The Join Request message is used by a WTP to request service through
 the AC.  If the WTP is performing the optional AC Discovery process
 (see Section 3.3), the join process occurs after the WTP has received
 one or more Discovery Response messages.  During the Discovery
 process, an AC MAY return more than one CAPWAP Control IPv4 Address
 or CAPWAP Control IPv6 Address message elements.  When more than one
 such message element is returned, the WTP SHOULD perform "load
 balancing" by choosing the interface that is servicing the least
 number of WTPs (known through the WTP Count field of the message
 element).  Note, however, that other load balancing algorithms are
 also permitted.  Once the WTP has determined its preferred AC, and
 its associated interface, to which to connect, it establishes the
 DTLS session, and transmits the Join Request over the secured control
 channel.  When an AC receives a Join Request message it responds with
 a Join Response message.
 Upon completion of the DTLS handshake and receipt of the
 DTLSEstablished notification, the WTP sends the Join Request message
 to the AC.  When the AC is notified of the DTLS session
 establishment, it does not clear the WaitDTLS timer until it has
 received the Join Request message, at which time it sends a Join
 Response message to the WTP, indicating success or failure.
 One or more WTP Radio Information message elements (see Section 2.1)
 are included in the Join Request to request service for the CAPWAP
 bindings by the AC.  Including a binding that is unsupported by the
 AC will result in a failed Join Response.

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 If the AC rejects the Join Request, it sends a Join Response message
 with a failure indication and initiates an abort of the DTLS session
 via the DTLSAbort command.
 If an invalid (i.e., malformed) Join Request message is received, the
 message MUST be silently discarded by the AC.  No response is sent to
 the WTP.  The AC SHOULD log this event.
 The Join Request is sent by the WTP when in the Join State.  The AC
 does not transmit this message.
 The following message elements MUST be included in the Join Request
 message.
 o  Location Data, see Section 4.6.30
 o  WTP Board Data, see Section 4.6.40
 o  WTP Descriptor, see Section 4.6.41
 o  WTP Name, see Section 4.6.45
 o  Session ID, see Section 4.6.37
 o  WTP Frame Tunnel Mode, see Section 4.6.43
 o  WTP MAC Type, see Section 4.6.44
 o  WTP Radio Information message element(s) that the WTP supports;
    these are defined by the individual link layer CAPWAP Binding
    Protocols (see Section 2.1 for more information).
 o  ECN Support, see Section 4.6.25
 At least one of the following message element MUST be included in the
 Join Request message.
 o  CAPWAP Local IPv4 Address, see Section 4.6.11
 o  CAPWAP Local IPv6 Address, see Section 4.6.12
 The following message element MAY be included in the Join Request
 message.
 o  CAPWAP Transport Protocol, see Section 4.6.14
 o  Maximum Message Length, see Section 4.6.31

Calhoun, et al. Standards Track [Page 109] RFC 5415 CAPWAP Protocol Specification March 2009

 o  WTP Reboot Statistics, see Section 4.6.47
 o  Vendor Specific Payload, see Section 4.6.39

6.2. Join Response

 The Join Response message is sent by the AC to indicate to a WTP that
 it is capable and willing to provide service to the WTP.
 The WTP, receiving a Join Response message, checks for success or
 failure.  If the message indicates success, the WTP clears the
 WaitDTLS timer for the session and proceeds to the Configure state.
 If the WaitDTLS Timer expires prior to reception of the Join Response
 message, the WTP MUST terminate the handshake, deallocate session
 state and initiate the DTLSAbort command.
 If an invalid (malformed) Join Response message is received, the WTP
 SHOULD log an informative message detailing the error.  This error
 MUST be treated in the same manner as AC non-responsiveness.  The
 WaitDTLS timer will eventually expire, and the WTP MAY (if it is so
 configured) attempt to join a new AC.
 If one of the WTP Radio Information message elements (see
 Section 2.1) in the Join Request message requested support for a
 CAPWAP binding that the AC does not support, the AC sets the Result
 Code message element to "Binding Not Supported".
 The AC includes the Image Identifier message element to indicate the
 software version it expects the WTP to run.  This information is used
 to determine whether the WTP MUST change its currently running
 firmware image or download a new version (see Section 9.1.1).
 The Join Response message is sent by the AC when in the Join State.
 The WTP does not transmit this message.
 The following message elements MUST be included in the Join Response
 message.
 o  Result Code, see Section 4.6.35
 o  AC Descriptor, see Section 4.6.1
 o  AC Name, see Section 4.6.4
 o  WTP Radio Information message element(s) that the AC supports;
    these are defined by the individual link layer CAPWAP Binding
    Protocols (see Section 2.1).

Calhoun, et al. Standards Track [Page 110] RFC 5415 CAPWAP Protocol Specification March 2009

 o  ECN Support, see Section 4.6.25
 One of the following message elements MUST be included in the Join
 Response Message:
 o  CAPWAP Control IPv4 Address, see Section 4.6.9
 o  CAPWAP Control IPv6 Address, see Section 4.6.10
 One of the following message elements MUST be included in the Join
 Response Message:
 o  CAPWAP Local IPv4 Address, see Section 4.6.11
 o  CAPWAP Local IPv6 Address, see Section 4.6.12
 The following message elements MAY be included in the Join Response
 message.
 o  AC IPv4 List, see Section 4.6.2
 o  AC IPv6 List, see Section 4.6.3
 o  CAPWAP Transport Protocol, see Section 4.6.14
 o  Image Identifier, see Section 4.6.27
 o  Maximum Message Length, see Section 4.6.31
 o  Vendor Specific Payload, see Section 4.6.39

7. Control Channel Management

 The Control Channel Management messages are used by the WTP and AC to
 maintain a control communication channel.  CAPWAP Control messages,
 such as the WTP Event Request message sent from the WTP to the AC
 indicate to the AC that the WTP is operational.  When such control
 messages are not being sent, the Echo Request and Echo Response
 messages are used to maintain the control communication channel.

7.1. Echo Request

 The Echo Request message is a keep-alive mechanism for CAPWAP control
 messages.

Calhoun, et al. Standards Track [Page 111] RFC 5415 CAPWAP Protocol Specification March 2009

 Echo Request messages are sent periodically by a WTP in the Image
 Data or Run state (see Section 2.3) to determine the state of the
 control connection between the WTP and the AC.  The Echo Request
 message is sent by the WTP when the EchoInterval timer expires.
 The Echo Request message is sent by the WTP when in the Run state.
 The AC does not transmit this message.
 The following message elements MAY be included in the Echo Request
 message:
 o  Vendor Specific Payload, see Section 4.6.39
 When an AC receives an Echo Request message it responds with an Echo
 Response message.

7.2. Echo Response

 The Echo Response message acknowledges the Echo Request message.
 An Echo Response message is sent by an AC after receiving an Echo
 Request message.  After transmitting the Echo Response message, the
 AC SHOULD reset its EchoInterval timer (see Section 4.7.7).  If
 another Echo Request message or other control message is not received
 by the AC when the timer expires, the AC SHOULD consider the WTP to
 be no longer reachable.
 The Echo Response message is sent by the AC when in the Run state.
 The WTP does not transmit this message.
 The following message elements MAY be included in the Echo Response
 message:
 o  Vendor Specific Payload, see Section 4.6.39
 When a WTP receives an Echo Response message it initializes the
 EchoInterval to the configured value.

8. WTP Configuration Management

 WTP Configuration messages are used to exchange configuration
 information between the AC and the WTP.

8.1. Configuration Consistency

 The CAPWAP protocol provides flexibility in how WTP configuration is
 managed.  A WTP can behave in one of two ways, which is
 implementation specific:

Calhoun, et al. Standards Track [Page 112] RFC 5415 CAPWAP Protocol Specification March 2009

 1. The WTP retains no configuration and accepts the configuration
    provided by the AC.
 2. The WTP saves the configuration of parameters provided by the AC
    that are non-default values into local non-volatile memory, and
    are enforced during the WTP's power up initialization phase.
 If the WTP opts to save configuration locally, the CAPWAP protocol
 state machine defines the Configure state, which allows for
 configuration exchange.  In the Configure state, the WTP sends its
 current configuration overrides to the AC via the Configuration
 Status Request message.  A configuration override is a non-default
 parameter.  As an example, in the CAPWAP protocol, the default
 antenna configuration is internal omni antenna.  A WTP that either
 has no internal antennas, or has been explicitly configured by the AC
 to use external antennas, sends its antenna configuration during the
 configure phase, allowing the AC to become aware of the WTP's current
 configuration.
 Once the WTP has provided its configuration to the AC, the AC sends
 its configuration to the WTP.  This allows the WTP to receive
 configuration and policies from the AC.
 The AC maintains a copy of each active WTP configuration.  There is
 no need for versioning or other means to identify configuration
 changes.  If a WTP becomes inactive, the AC MAY delete the inactive
 WTP configuration.  If a WTP fails, and connects to a new AC, the WTP
 provides its overridden configuration parameters, allowing the new AC
 to be aware of the WTP configuration.
 This model allows for resiliency in case of an AC failure, ensuring
 another AC can provide service to the WTP.  A new AC would be
 automatically updated with WTP configuration changes, eliminating the
 need for inter-AC communication and the need for all ACs to be aware
 of the configuration of all WTPs in the network.
 Once the CAPWAP protocol enters the Run state, the WTPs begin to
 provide service.  It is common for administrators to require that
 configuration changes be made while the network is operational.
 Therefore, the Configuration Update Request is sent by the AC to the
 WTP to make these changes at run-time.

8.1.1. Configuration Flexibility

 The CAPWAP protocol provides the flexibility to configure and manage
 WTPs of varying design and functional characteristics.  When a WTP
 first discovers an AC, it provides primary functional information

Calhoun, et al. Standards Track [Page 113] RFC 5415 CAPWAP Protocol Specification March 2009

 relating to its type of MAC and to the nature of frames to be
 exchanged.  The AC configures the WTP appropriately.  The AC also
 establishes corresponding internal state for the WTP.

8.2. Configuration Status Request

 The Configuration Status Request message is sent by a WTP to deliver
 its current configuration to the AC.
 The Configuration Status Request message carries binding-specific
 message elements.  Refer to the appropriate binding for the
 definition of this structure.
 When an AC receives a Configuration Status Request message, it acts
 upon the content of the message and responds to the WTP with a
 Configuration Status Response message.
 The Configuration Status Request message includes multiple Radio
 Administrative State message elements, one for the WTP, and one for
 each radio in the WTP.
 The Configuration Status Request message is sent by the WTP when in
 the Configure State.  The AC does not transmit this message.
 The following message elements MUST be included in the Configuration
 Status Request message.
 o  AC Name, see Section 4.6.4
 o  Radio Administrative State, see Section 4.6.33
 o  Statistics Timer, see Section 4.6.38
 o  WTP Reboot Statistics, see Section 4.6.47
 The following message elements MAY be included in the Configuration
 Status Request message.
 o  AC Name with Priority, see Section 4.6.5
 o  CAPWAP Transport Protocol, see Section 4.6.14
 o  WTP Static IP Address Information, see Section 4.6.48
 o  Vendor Specific Payload, see Section 4.6.39

Calhoun, et al. Standards Track [Page 114] RFC 5415 CAPWAP Protocol Specification March 2009

8.3. Configuration Status Response

 The Configuration Status Response message is sent by an AC and
 provides a mechanism for the AC to override a WTP's requested
 configuration.
 A Configuration Status Response message is sent by an AC after
 receiving a Configuration Status Request message.
 The Configuration Status Response message carries binding-specific
 message elements.  Refer to the appropriate binding for the
 definition of this structure.
 When a WTP receives a Configuration Status Response message, it acts
 upon the content of the message, as appropriate.  If the
 Configuration Status Response message includes a Radio Operational
 State message element that causes a change in the operational state
 of one of the radios, the WTP transmits a Change State Event to the
 AC, as an acknowledgement of the change in state.
 The Configuration Status Response message is sent by the AC when in
 the Configure state.  The WTP does not transmit this message.
 The following message elements MUST be included in the Configuration
 Status Response message.
 o  CAPWAP Timers, see Section 4.6.13
 o  Decryption Error Report Period, see Section 4.6.18
 o  Idle Timeout, see Section 4.6.24
 o  WTP Fallback, see Section 4.6.42
 One or both of the following message elements MUST be included in the
 Configuration Status Response message:
 o  AC IPv4 List, see Section 4.6.2
 o  AC IPv6 List, see Section 4.6.3
 The following message element MAY be included in the Configuration
 Status Response message.
 o  WTP Static IP Address Information, see Section 4.6.48
 o  Vendor Specific Payload, see Section 4.6.39

Calhoun, et al. Standards Track [Page 115] RFC 5415 CAPWAP Protocol Specification March 2009

8.4. Configuration Update Request

 Configuration Update Request messages are sent by the AC to provision
 the WTP while in the Run state.  This is used to modify the
 configuration of the WTP while it is operational.
 When a WTP receives a Configuration Update Request message, it
 responds with a Configuration Update Response message, with a Result
 Code message element indicating the result of the configuration
 request.
 The AC includes the Image Identifier message element (see
 Section 4.6.27) to force the WTP to update its firmware while in the
 Run state.  The WTP MAY proceed to download the requested firmware if
 it determines the version specified in the Image Identifier message
 element is not in its non-volatile storage by transmitting an Image
 Data Request (see Section 9.1.1) that includes the Initiate Download
 message element (see Section 4.6.29).
 The Configuration Update Request is sent by the AC when in the Run
 state.  The WTP does not transmit this message.
 One or more of the following message elements MAY be included in the
 Configuration Update message:
 o  AC Name with Priority, see Section 4.6.5
 o  AC Timestamp, see Section 4.6.6
 o  Add MAC ACL Entry, see Section 4.6.7
 o  CAPWAP Timers, see Section 4.6.13
 o  Decryption Error Report Period, see Section 4.6.18
 o  Delete MAC ACL Entry, see Section 4.6.19
 o  Idle Timeout, see Section 4.6.24
 o  Location Data, see Section 4.6.30
 o  Radio Administrative State, see Section 4.6.33
 o  Statistics Timer, see Section 4.6.38
 o  WTP Fallback, see Section 4.6.42
 o  WTP Name, see Section 4.6.45

Calhoun, et al. Standards Track [Page 116] RFC 5415 CAPWAP Protocol Specification March 2009

 o  WTP Static IP Address Information, see Section 4.6.48
 o  Image Identifier, see Section 4.6.27
 o  Vendor Specific Payload, see Section 4.6.39

8.5. Configuration Update Response

 The Configuration Update Response message is the acknowledgement
 message for the Configuration Update Request message.
 The Configuration Update Response message is sent by a WTP after
 receiving a Configuration Update Request message.
 When an AC receives a Configuration Update Response message, the
 result code indicates if the WTP successfully accepted the
 configuration.
 The Configuration Update Response message is sent by the WTP when in
 the Run state.  The AC does not transmit this message.
 The following message element MUST be present in the Configuration
 Update message.
 Result Code, see Section 4.6.35
 The following message elements MAY be present in the Configuration
 Update Response message.
 o  Radio Operational State, see Section 4.6.34
 o  Vendor Specific Payload, see Section 4.6.39

8.6. Change State Event Request

 The Change State Event Request message is used by the WTP for two
 main purposes:
 o  When sent by the WTP following the reception of a Configuration
    Status Response message from the AC, the WTP uses the Change State
    Event Request message to provide an update on the WTP radio's
    operational state and to confirm that the configuration provided
    by the AC was successfully applied.
 o  When sent during the Run state, the WTP uses the Change State
    Event Request message to notify the AC of an unexpected change in
    the WTP's radio operational state.

Calhoun, et al. Standards Track [Page 117] RFC 5415 CAPWAP Protocol Specification March 2009

 When an AC receives a Change State Event Request message it responds
 with a Change State Event Response message and modifies its data
 structures for the WTP as needed.  The AC MAY decide not to provide
 service to the WTP if it receives an error, based on local policy,
 and to transition to the Reset state.
 The Change State Event Request message is sent by a WTP to
 acknowledge or report an error condition to the AC for a requested
 configuration in the Configuration Status Response message.  The
 Change State Event Request message includes the Result Code message
 element, which indicates whether the configuration was successfully
 applied.  If the WTP is unable to apply a specific configuration
 request, it indicates the failure by including one or more Returned
 Message Element message elements (see Section 4.6.36).
 The Change State Event Request message is sent by the WTP in the
 Configure or Run state.  The AC does not transmit this message.
 The WTP MAY save its configuration to persistent storage prior to
 transmitting the response.  However, this is implementation specific
 and is not required.
 The following message elements MUST be present in the Change State
 Event Request message.
 o  Radio Operational State, see Section 4.6.34
 o  Result Code, see Section 4.6.35
 One or more of the following message elements MAY be present in the
 Change State Event Request message:
 o  Returned Message Element(s), see Section 4.6.36
 o  Vendor Specific Payload, see Section 4.6.39

8.7. Change State Event Response

 The Change State Event Response message acknowledges the Change State
 Event Request message.
 A Change State Event Response message is sent by an AC in response to
 a Change State Event Request message.
 The Change State Event Response message is sent by the AC when in the
 Configure or Run state.  The WTP does not transmit this message.

Calhoun, et al. Standards Track [Page 118] RFC 5415 CAPWAP Protocol Specification March 2009

 The following message element MAY be included in the Change State
 Event Response message:
 o  Vendor Specific Payload, see Section 4.6.39
 The WTP does not take any action upon receipt of the Change State
 Event Response message.

8.8. Clear Configuration Request

 The Clear Configuration Request message is used to reset the WTP
 configuration.
 The Clear Configuration Request message is sent by an AC to request
 that a WTP reset its configuration to the manufacturing default
 configuration.  The Clear Config Request message is sent while in the
 Run state.
 The Clear Configuration Request is sent by the AC when in the Run
 state.  The WTP does not transmit this message.
 The following message element MAY be included in the Clear
 Configuration Request message:
 o  Vendor Specific Payload, see Section 4.6.39
 When a WTP receives a Clear Configuration Request message, it resets
 its configuration to the manufacturing default configuration.

8.9. Clear Configuration Response

 The Clear Configuration Response message is sent by the WTP after
 receiving a Clear Configuration Request message and resetting its
 configuration parameters to the manufacturing default values.
 The Clear Configuration Response is sent by the WTP when in the Run
 state.  The AC does not transmit this message.
 The Clear Configuration Response message MUST include the following
 message element:
 o  Result Code, see Section 4.6.35
 The following message element MAY be included in the Clear
 Configuration Request message:
 o  Vendor Specific Payload, see Section 4.6.39

Calhoun, et al. Standards Track [Page 119] RFC 5415 CAPWAP Protocol Specification March 2009

9. Device Management Operations

 This section defines CAPWAP operations responsible for debugging,
 gathering statistics, logging, and firmware management.  The
 management operations defined in this section are used by the AC to
 either push/pull information to/from the WTP, or request that the WTP
 reboot.  This section does not deal with the management of the AC per
 se, and assumes that the AC is operational and configured.

9.1. Firmware Management

 This section describes the firmware download procedures used by the
 CAPWAP protocol.  Firmware download can occur during the Image Data
 or Run state.  The former allows the download to occur at boot time,
 while the latter is used to trigger the download while an active
 CAPWAP session exists.  It is important to note that the CAPWAP
 protocol does not provide the ability for the AC to identify whether
 the firmware information provided by the WTP is correct or whether
 the WTP is properly storing the firmware (see Section 12.10 for more
 information).
 Figure 6 provides an example of a WTP that performs a firmware
 upgrade while in the Image Data state.  In this example, the WTP does
 not already have the requested firmware (Image Identifier = x), and
 downloads the image from the AC.

Calhoun, et al. Standards Track [Page 120] RFC 5415 CAPWAP Protocol Specification March 2009

           WTP                                               AC
                              Join Request
       -------------------------------------------------------->
                   Join Response (Image Identifier = x)
       <------------------------------------------------------
            Image Data Request (Image Identifier = x,
                                Initiate Download)
       -------------------------------------------------------->
         Image Data Response (Result Code = Success,
                              Image Information = {size,hash})
       <------------------------------------------------------
              Image Data Request (Image Data = Data)
       <------------------------------------------------------
              Image Data Response (Result Code = Success)
       -------------------------------------------------------->
                                .....
              Image Data Request (Image Data = EOF)
       <------------------------------------------------------
              Image Data Response (Result Code = Success)
       -------------------------------------------------------->
                   (WTP enters the Reset State)
                Figure 6: WTP Firmware Download Case 1
 Figure 7 provides an example in which the WTP has the image specified
 by the AC in its non-volatile storage, but is not its current running
 image.  In this case, the WTP opts to NOT download the firmware and
 immediately reset to the requested image.

Calhoun, et al. Standards Track [Page 121] RFC 5415 CAPWAP Protocol Specification March 2009

           WTP                                               AC
                              Join Request
       -------------------------------------------------------->
                   Join Response (Image Identifier = x)
       <------------------------------------------------------
                   (WTP enters the Reset State)
                Figure 7: WTP Firmware Download Case 2
 Figure 8 provides an example of a WTP that performs a firmware
 upgrade while in the Run state.  This mode of firmware upgrade allows
 the WTP to download its image while continuing to provide service.
 The WTP will not automatically reset until it is notified by the AC,
 with a Reset Request message.

Calhoun, et al. Standards Track [Page 122] RFC 5415 CAPWAP Protocol Specification March 2009

           WTP                                               AC
              Configuration Update Request (Image Identifier = x)
       <------------------------------------------------------
          Configuration Update Response (Result Code = Success)
       -------------------------------------------------------->
            Image Data Request (Image Identifier = x,
                                Initiate Download)
       -------------------------------------------------------->
            Image Data Response (Result Code = Success,
                                 Image Information = {size,hash})
       <------------------------------------------------------
              Image Data Request (Image Data = Data)
       <------------------------------------------------------
              Image Data Response (Result Code = Success)
       -------------------------------------------------------->
                                .....
              Image Data Request (Image Data = EOF)
       <------------------------------------------------------
              Image Data Response (Result Code = Success)
       -------------------------------------------------------->
                                .....
              (administratively requested reboot request)
                 Reset Request (Image Identifier = x)
       <------------------------------------------------------
                Reset Response (Result Code = Success)
       -------------------------------------------------------->
                Figure 8: WTP Firmware Download Case 3
 Figure 9 provides another example of the firmware download while in
 the Run state.  In this example, the WTP already has the image
 specified by the AC in its non-volatile storage.  The WTP opts to NOT
 download the firmware.  The WTP resets upon receipt of a Reset
 Request message from the AC.

Calhoun, et al. Standards Track [Page 123] RFC 5415 CAPWAP Protocol Specification March 2009

           WTP                                               AC
           Configuration Update Request (Image Identifier = x)
       <------------------------------------------------------
    Configuration Update Response (Result Code = Already Have Image)
       -------------------------------------------------------->
                                .....
              (administratively requested reboot request)
                 Reset Request (Image Identifier = x)
       <------------------------------------------------------
                Reset Response (Result Code = Success)
       -------------------------------------------------------->
                Figure 9: WTP Firmware Download Case 4

9.1.1. Image Data Request

 The Image Data Request message is used to update firmware on the WTP.
 This message and its companion Response message are used by the AC to
 ensure that the image being run on each WTP is appropriate.
 Image Data Request messages are exchanged between the WTP and the AC
 to download a new firmware image to the WTP.  When a WTP or AC
 receives an Image Data Request message, it responds with an Image
 Data Response message.  The message elements contained within the
 Image Data Request message are required to determine the intent of
 the request.
 The decision that new firmware is to be downloaded to the WTP can
 occur in one of two ways:
    When the WTP joins the AC, the Join Response message includes the
    Image Identifier message element, which informs the WTP of the
    firmware it is expected to run.  If the WTP does not currently
    have the requested firmware version, it transmits an Image Data
    Request message, with the appropriate Image Identifier message
    element.  If the WTP already has the requested firmware in its
    non-volatile flash, but is not its currently running image, it
    simply resets to run the proper firmware.
    Once the WTP is in the Run state, it is possible for the AC to
    cause the WTP to initiate a firmware download by sending a
    Configuration Update Request message with the Image Identifier
    message elements.  This will cause the WTP to transmit an Image

Calhoun, et al. Standards Track [Page 124] RFC 5415 CAPWAP Protocol Specification March 2009

    Data Request with the Image Identifier and the Initiate Download
    message elements.  Note that when the firmware is downloaded in
    this way, the WTP does not automatically reset after the download
    is complete.  The WTP will only reset when it receives a Reset
    Request message from the AC.  If the WTP already had the requested
    firmware version in its non-volatile storage, the WTP does not
    transmit the Image Data Request message and responds with a
    Configuration Update Response message with the Result Code set to
    Image Already Present.
 Regardless of how the download was initiated, once the AC receives an
 Image Data Request message with the Image Identifier message element,
 it begins the transfer process by transmitting an Image Data Request
 message that includes the Image Data message element.  This continues
 until the firmware image has been transferred.
 The Image Data Request message is sent by the WTP or the AC when in
 the Image Data or Run state.
 The following message elements MAY be included in the Image Data
 Request message:
 o  CAPWAP Transport Protocol, see Section 4.6.14
 o  Image Data, see Section 4.6.26
 o  Vendor Specific Payload, see Section 4.6.39
 The following message elements MAY be included in the Image Data
 Request message when sent by the WTP:
 o  Image Identifier, see Section 4.6.27
 o  Initiate Download, see Section 4.6.29

9.1.2. Image Data Response

 The Image Data Response message acknowledges the Image Data Request
 message.
 An Image Data Response message is sent in response to a received
 Image Data Request message.  Its purpose is to acknowledge the
 receipt of the Image Data Request message.  The Result Code is
 included to indicate whether a previously sent Image Data Request
 message was invalid.
 The Image Data Response message is sent by the WTP or the AC when in
 the Image Data or Run state.

Calhoun, et al. Standards Track [Page 125] RFC 5415 CAPWAP Protocol Specification March 2009

 The following message element MUST be included in the Image Data
 Response message:
 o  Result Code, see Section 4.6.35
 The following message element MAY be included in the Image Data
 Response message:
 o  Vendor Specific Payload, see Section 4.6.39
 The following message element MAY be included in the Image Data
 Response message when sent by the AC:
 o  Image Information, see Section 4.6.28
 Upon receiving an Image Data Response message indicating an error,
 the WTP MAY retransmit a previous Image Data Request message, or
 abandon the firmware download to the WTP by transitioning to the
 Reset state.

9.2. Reset Request

 The Reset Request message is used to cause a WTP to reboot.
 A Reset Request message is sent by an AC to cause a WTP to
 reinitialize its operation.  If the AC includes the Image Identifier
 message element (see Section 4.6.27), it indicates to the WTP that it
 SHOULD use that version of software upon reboot.
 The Reset Request is sent by the AC when in the Run state.  The WTP
 does not transmit this message.
 The following message element MUST be included in the Reset Request
 message:
 o  Image Identifier, see Section 4.6.27
 The following message element MAY be included in the Reset Request
 message:
 o  Vendor Specific Payload, see Section 4.6.39
 When a WTP receives a Reset Request message, it responds with a Reset
 Response message indicating success and then reinitializes itself.
 If the WTP is unable to write to its non-volatile storage, to ensure
 that it runs the requested software version indicated in the Image
 Identifier message element, it MAY send the appropriate Result Code
 message element, but MUST reboot.  If the WTP is unable to reset,

Calhoun, et al. Standards Track [Page 126] RFC 5415 CAPWAP Protocol Specification March 2009

 including a hardware reset, it sends a Reset Response message to the
 AC with a Result Code message element indicating failure.  The AC no
 longer provides service to the WTP.

9.3. Reset Response

 The Reset Response message acknowledges the Reset Request message.
 A Reset Response message is sent by the WTP after receiving a Reset
 Request message.
 The Reset Response is sent by the WTP when in the Run state.  The AC
 does not transmit this message.
 The following message elements MAY be included in the Reset Response
 message.
 o  Result Code, see Section 4.6.35
 o  Vendor Specific Payload, see Section 4.6.39
 When an AC receives a successful Reset Response message, it is
 notified that the WTP will reinitialize its operation.  An AC that
 receives a Reset Response message indicating failure may opt to no
 longer provide service to the WTP.

9.4. WTP Event Request

 The WTP Event Request message is used by a WTP to send information to
 its AC.  The WTP Event Request message MAY be sent periodically, or
 sent in response to an asynchronous event on the WTP.  For example, a
 WTP MAY collect statistics and use the WTP Event Request message to
 transmit the statistics to the AC.
 When an AC receives a WTP Event Request message it will respond with
 a WTP Event Response message.
 The presence of the Delete Station message element is used by the WTP
 to inform the AC that it is no longer providing service to the
 station.  This could be the result of an Idle Timeout (see
 Section 4.6.24), due to resource shortages, or some other reason.
 The WTP Event Request message is sent by the WTP when in the Run
 state.  The AC does not transmit this message.

Calhoun, et al. Standards Track [Page 127] RFC 5415 CAPWAP Protocol Specification March 2009

 The WTP Event Request message MUST contain one of the message
 elements listed below, or a message element that is defined for a
 specific wireless technology.  More than one of each message element
 listed MAY be included in the WTP Event Request message.
 o  Decryption Error Report, see Section 4.6.17
 o  Duplicate IPv4 Address, see Section 4.6.22
 o  Duplicate IPv6 Address, see Section 4.6.23
 o  WTP Radio Statistics, see Section 4.6.46
 o  WTP Reboot Statistics, see Section 4.6.47
 o  Delete Station, see Section 4.6.20
 o  Vendor Specific Payload, see Section 4.6.39

9.5. WTP Event Response

 The WTP Event Response message acknowledges receipt of the WTP Event
 Request message.
 A WTP Event Response message is sent by an AC after receiving a WTP
 Event Request message.
 The WTP Event Response message is sent by the AC when in the Run
 state.  The WTP does not transmit this message.
 The following message element MAY be included in the WTP Event
 Response message:
 o  Vendor Specific Payload, see Section 4.6.39

9.6. Data Transfer

 This section describes the data transfer procedures used by the
 CAPWAP protocol.  The data transfer mechanism is used to upload
 information available at the WTP to the AC, such as crash or debug
 information.  The data transfer messages can only be exchanged while
 in the Run state.
 Figure 10 provides an example of an AC that requests that the WTP
 transfer its latest crash file.  Once the WTP acknowledges that it
 has information to send, via the Data Transfer Response, it transmits
 its own Data Transfer Request.  Upon receipt, the AC responds with a

Calhoun, et al. Standards Track [Page 128] RFC 5415 CAPWAP Protocol Specification March 2009

 Data Transfer Response, and the exchange continues until the WTP
 transmits a Data Transfer Data message element that indicates an End
 of File (EOF).
           WTP                                               AC
         Data Transfer Request (Data Transfer Mode = Crash Data)
       <------------------------------------------------------
            Data Transfer Response (Result Code = Success)
       -------------------------------------------------------->
            Data Transfer Request (Data Transfer Data = Data)
       -------------------------------------------------------->
            Data Transfer Response (Result Code = Success)
       <------------------------------------------------------
                                .....
              Data Transfer Request (Data Transfer Data = EOF)
       -------------------------------------------------------->
            Data Transfer Response (Result Code = Success)
       <------------------------------------------------------
                  Figure 10: WTP Data Transfer Case 1
 Figure 11 provides an example of an AC that requests that the WTP
 transfer its latest crash file.  However, in this example, the WTP
 does not have any crash information to send, and therefore sends a
 Data Transfer Response with a Result Code indicating the error.
          WTP                                               AC
        Data Transfer Request (Data Transfer Mode = Crash Data)
      <------------------------------------------------------
           Data Transfer Response (Result Code = Data Transfer
                                   Error (No Information to Transfer))
      -------------------------------------------------------->
                  Figure 11: WTP Data Transfer Case 2

Calhoun, et al. Standards Track [Page 129] RFC 5415 CAPWAP Protocol Specification March 2009

9.6.1. Data Transfer Request

 The Data Transfer Request message is used to deliver debug
 information from the WTP to the AC.
 The Data Transfer Request messages can be sent either by the AC or
 the WTP.  When sent by the AC, it is used to request that data be
 transmitted from the WTP to the AC, and includes the Data Transfer
 Mode message element, which specifies the information desired by the
 AC.  The Data Transfer Request is sent by the WTP in order to
 transfer actual data to the AC, through the Data Transfer Data
 message element.
 Given that the CAPWAP protocol minimizes the need for WTPs to be
 directly managed, the Data Transfer Request is an important
 troubleshooting tool used by the AC to retrieve information that may
 be available on the WTP.  For instance, some WTP implementations may
 store crash information to help manufacturers identify software
 faults.  The Data Transfer Request message can be used to send such
 information from the WTP to the AC.  Another possible use would be to
 allow a remote debugger function in the WTP to use the Data Transfer
 Request message to send console output to the AC for debugging
 purposes.
 When the WTP or AC receives a Data Transfer Request message, it
 responds to the WTP with a Data Transfer Response message.  The AC
 MAY log the information received through the Data Transfer Data
 message element.
 The Data Transfer Request message is sent by the WTP or AC when in
 the Run state.
 When sent by the AC, the Data Transfer Request message MUST contain
 the following message element:
 o  Data Transfer Mode, see Section 4.6.16
 When sent by the WTP, the Data Transfer Request message MUST contain
 the following message element:
 o  Data Transfer Data, see Section 4.6.15
 Regardless of whether the Data Transfer Request is sent by the AC or
 WTP, the following message element MAY be included in the Data
 Transfer Request message:
 o  Vendor Specific Payload, see Section 4.6.39

Calhoun, et al. Standards Track [Page 130] RFC 5415 CAPWAP Protocol Specification March 2009

9.6.2. Data Transfer Response

 The Data Transfer Response message acknowledges the Data Transfer
 Request message.
 A Data Transfer Response message is sent in response to a received
 Data Transfer Request message.  Its purpose is to acknowledge receipt
 of the Data Transfer Request message.  When sent by the WTP, the
 Result Code message element is used to indicate whether the data
 transfer requested by the AC can be completed.  When sent by the AC,
 the Result Code message element is used to indicate receipt of the
 data transferred in the Data Transfer Request message.
 The Data Transfer Response message is sent by the WTP or AC when in
 the Run state.
 The following message element MUST be included in the Data Transfer
 Response message:
 o  Result Code, see Section 4.6.35
 The following message element MAY be included in the Data Transfer
 Response message:
 o  Vendor Specific Payload, see Section 4.6.39
 Upon receipt of a Data Transfer Response message, the WTP transmits
 more information, if more information is available.

10. Station Session Management

 Messages in this section are used by the AC to create, modify, or
 delete station session state on the WTPs.

10.1. Station Configuration Request

 The Station Configuration Request message is used to create, modify,
 or delete station session state on a WTP.  The message is sent by the
 AC to the WTP, and MAY contain one or more message elements.  The
 message elements for this CAPWAP Control message include information
 that is generally highly technology specific.  Refer to the
 appropriate binding document for definitions of the messages elements
 that are included in this control message.
 The Station Configuration Request message is sent by the AC when in
 the Run state.  The WTP does not transmit this message.

Calhoun, et al. Standards Track [Page 131] RFC 5415 CAPWAP Protocol Specification March 2009

 The following CAPWAP Control message elements MAY be included in the
 Station Configuration Request message.  More than one of each message
 element listed MAY be included in the Station Configuration Request
 message:
 o  Add Station, see Section 4.6.8
 o  Delete Station, see Section 4.6.20
 o  Vendor Specific Payload, see Section 4.6.39

10.2. Station Configuration Response

 The Station Configuration Response message is used to acknowledge a
 previously received Station Configuration Request message.
 The Station Configuration Response message is sent by the WTP when in
 the Run state.  The AC does not transmit this message.
 The following message element MUST be present in the Station
 Configuration Response message:
 o  Result Code, see Section 4.6.35
 The following message element MAY be included in the Station
 Configuration Response message:
 o  Vendor Specific Payload, see Section 4.6.39
 The Result Code message element indicates that the requested
 configuration was successfully applied, or that an error related to
 processing of the Station Configuration Request message occurred on
 the WTP.

11. NAT Considerations

 There are three specific situations in which a NAT deployment may be
 used in conjunction with a CAPWAP-enabled deployment.  The first
 consists of a configuration in which a single WTP is behind a NAT
 system.  Since all communication is initiated by the WTP, and all
 communication is performed over IP using two UDP ports, the protocol
 easily traverses NAT systems in this configuration.
 In the second case, two or more WTPs are deployed behind the same NAT
 system.  Here, the AC would receive multiple connection requests from
 the same IP address, and therefore cannot use the WTP's IP address
 alone to bind the CAPWAP Control and Data channel.  The CAPWAP Data
 Check state, which establishes the data plane connection and

Calhoun, et al. Standards Track [Page 132] RFC 5415 CAPWAP Protocol Specification March 2009

 communicates the CAPWAP Data Channel Keep-Alive, includes the Session
 Identifier message element, which is used to bind the control and
 data plane.  Use of the Session Identifier message element enables
 the AC to match the control and data plane flows from multiple WTPs
 behind the same NAT system (multiple WTPs sharing the same IP
 address).  CAPWAP implementations MUST also use DTLS session
 information on any encrypted CAPWAP channel to validate the source of
 both the control and data plane, as described in Section 12.2.
 In the third configuration, the AC is deployed behind a NAT.  In this
 case, the AC is not reachable by the WTP unless a specific rule has
 been configured on the NAT to translate the address and redirect
 CAPWAP messages to the AC.  This deployment presents two issues.
 First, an AC communicates its interfaces and corresponding WTP load
 using the CAPWAP Control IPv4 Address and CAPWAP Control IPv6 Address
 message elements.  This message element is mandatory, but contains IP
 addresses that are only valid in the private address space used by
 the AC, which is not reachable by the WTP.  The WTP MUST NOT utilize
 the information in these message elements if it detects a NAT (as
 described in the CAPWAP Transport Protocol message element in
 Section 4.6.14).  Second, since the addresses cannot be used by the
 WTP, this effectively disables the load-balancing capabilities (see
 Section 6.1) of the CAPWAP protocol.  Alternatively, the AC could
 have a configured NAT'ed address, which it would include in either of
 the two control address message elements, and the NAT would need to
 be configured accordingly.
 In order for a CAPWAP WTP or AC to detect whether a middlebox is
 present, both the Join Request (see Section 6.1) and the Join
 Response (see Section 6.2) include either the CAPWAP Local IPv4
 Address (see Section 4.6.11) or the CAPWAP Local IPv6 Address (see
 Section 4.6.12) message element.  Upon receiving one of these
 messages, if the packet's source IP address differs from the address
 found in either one of these message elements, it indicates that a
 middlebox is present.
 In order for CAPWAP to be compatible with potential middleboxes in
 the network, CAPWAP implementations MUST send return traffic from the
 same port on which it received traffic from a given peer.  Further,
 any unsolicited requests generated by a CAPWAP node MUST be sent on
 the same port.
 Note that this middlebox detection technique is not foolproof.  If
 the public IP address assigned to the NAT is identical to the private
 IP address used by the AC, detection by the WTP would fail.  This
 failure can lead to various protocol errors, so it is therefore
 necessary for deployments to ensure that the NAT's IP address is not
 the same as the ACs.

Calhoun, et al. Standards Track [Page 133] RFC 5415 CAPWAP Protocol Specification March 2009

 The CAPWAP protocol allows for all of the AC identities supporting a
 group of WTPs to be communicated through the AC List message element.
 This feature MUST be ignored by the WTP when it detects the AC is
 behind a middlebox.
 The CAPWAP protocol allows an AC to configure a static IP address on
 a WTP using the WTP Static IP Address Information message element.
 This message element SHOULD NOT be used in NAT'ed environments,
 unless the administrator is familiar with the internal IP addressing
 scheme within the WTP's private network, and does not rely on the
 public address seen by the AC.
 When a WTP detects the duplicate address condition, it generates a
 message to the AC, which includes the Duplicate IP Address message
 element.  The IP address embedded within this message element is
 different from the public IP address seen by the AC.

12. Security Considerations

 This section describes security considerations for the CAPWAP
 protocol.  It also provides security recommendations for protocols
 used in conjunction with CAPWAP.

12.1. CAPWAP Security

 As it is currently specified, the CAPWAP protocol sits between the
 security mechanisms specified by the wireless link layer protocol
 (e.g., IEEE 802.11i) and Authentication, Authorization, and
 Accounting (AAA).  One goal of CAPWAP is to bootstrap trust between
 the STA and WTP using a series of preestablished trust relationships:
       STA            WTP           AC            AAA
       ==============================================
                          DTLS Cred     AAA Cred
                       <------------><------------->
                       EAP Credential
        <------------------------------------------>
         wireless link layer
         (e.g., 802.11 PTK)
        <--------------> or
        <--------------------------->
            (derived)
                     Figure 12: STA Session Setup

Calhoun, et al. Standards Track [Page 134] RFC 5415 CAPWAP Protocol Specification March 2009

 Within CAPWAP, DTLS is used to secure the link between the WTP and
 AC.  In addition to securing control messages, it's also a link in
 this chain of trust for establishing link layer keys.  Consequently,
 much rests on the security of DTLS.
 In some CAPWAP deployment scenarios, there are two channels between
 the WTP and AC: the control channel, carrying CAPWAP Control
 messages, and the data channel, over which client data packets are
 tunneled between the AC and WTP.  Typically, the control channel is
 secured by DTLS, while the data channel is not.
 The use of parallel protected and unprotected channels deserves
 special consideration, but does not create a threat.  There are two
 potential concerns: attempting to convert protected data into
 unprotected data and attempting to convert un-protected data into
 protected data.  These concerns are addressed below.

12.1.1. Converting Protected Data into Unprotected Data

 Since CAPWAP does not support authentication-only ciphers (i.e., all
 supported ciphersuites include encryption and authentication), it is
 not possible to convert protected data into unprotected data.  Since
 encrypted data is (ideally) indistinguishable from random data, the
 probability of an encrypted packet passing for a well-formed packet
 is effectively zero.

12.1.2. Converting Unprotected Data into Protected Data (Insertion)

 The use of message authentication makes it impossible for the
 attacker to forge protected records.  This makes conversion of
 unprotected records to protected records impossible.

12.1.3. Deletion of Protected Records

 An attacker could remove protected records from the stream, though
 not undetectably so, due the built-in reliability of the underlying
 CAPWAP protocol.  In the worst case, the attacker would remove the
 same record repeatedly, resulting in a CAPWAP session timeout and
 restart.  This is effectively a DoS attack, and could be accomplished
 by a man in the middle regardless of the CAPWAP protocol security
 mechanisms chosen.

12.1.4. Insertion of Unprotected Records

 An attacker could inject packets into the unprotected channel, but
 this may become evident if sequence number desynchronization occurs
 as a result.  Only if the attacker is a man in the middle (MITM) can

Calhoun, et al. Standards Track [Page 135] RFC 5415 CAPWAP Protocol Specification March 2009

 packets be inserted undetectably.  This is a consequence of that
 channel's lack of protection, and not a new threat resulting from the
 CAPWAP security mechanism.

12.1.5. Use of MD5

 The Image Information message element (Section 4.6.28) makes use of
 MD5 to compute the hash field.  The authenticity and integrity of the
 image file is protected by DTLS, and in this context, MD5 is not used
 as a cryptographically secure hash, but just as a basic checksum.
 Therefore, the use of MD5 is not considered a security vulnerability,
 and no mechanisms for algorithm agility are provided.

12.1.6. CAPWAP Fragmentation

 RFC 4963 [RFC4963] describes a possible security vulnerability where
 a malicious entity can "corrupt" a flow by injecting fragments.  By
 sending "high" fragments (those with offset greater than zero) with a
 forged source address, the attacker can deliberately cause
 corruption.  The use of DTLS on the CAPWAP Data channel can be used
 to avoid this possible vulnerability.

12.2. Session ID Security

 Since DTLS does not export a unique session identifier, there can be
 no explicit protocol binding between the DTLS layer and CAPWAP layer.
 As a result, implementations MUST provide a mechanism for performing
 this binding.  For example, an AC MUST NOT associate decrypted DTLS
 control packets with a particular WTP session based solely on the
 Session ID in the packet header.  Instead, identification should be
 done based on which DTLS session decrypted the packet.  Otherwise,
 one authenticated WTP could spoof another authenticated WTP by
 altering the Session ID in the encrypted CAPWAP Header.
 It should be noted that when the CAPWAP Data channel is unencrypted,
 the WTP Session ID is exposed and possibly known to adversaries and
 other WTPs.  This would allow the forgery of the source of data-
 channel traffic.  This, however, should not be a surprise for
 unencrypted data channels.  When the data channel is encrypted, the
 Session ID is not exposed, and therefore can safely be used to
 associate a data and control channel.  The 128-bit length of the
 Session ID mitigates online guessing attacks where an adversarial,
 authenticated WTP tries to correlate his own data channel with
 another WTP's control channel.  Note that for encrypted data
 channels, the Session ID should only be used for correlation for the
 first packet immediately after the initial DTLS handshake.  Future
 correlation should instead be done via identification of a packet's
 DTLS session.

Calhoun, et al. Standards Track [Page 136] RFC 5415 CAPWAP Protocol Specification March 2009

12.3. Discovery or DTLS Setup Attacks

 Since the Discovery Request messages are sent in the clear, it is
 important that AC implementations NOT assume that receiving a
 Discovery Request message from a WTP implies that the WTP has
 rebooted, and consequently tear down any active DTLS sessions.
 Discovery Request messages can easily be spoofed by malicious
 devices, so it is important that the AC maintain two separate sets of
 states for the WTP until the DTLSSessionEstablished notification is
 received, indicating that the WTP was authenticated.  Once a new DTLS
 session is successfully established, any state referring to the old
 session can be cleared.
 Similarly, when the AC is entering the DTLS Setup phase, it SHOULD
 NOT assume that the WTP has reset, and therefore should not discard
 active state until the DTLS session has been successfully
 established.  While the HelloVerifyRequest provides some protection
 against denial-of-service (DoS) attacks on the AC, an adversary
 capable of receiving packets at a valid address (or a malfunctioning
 or misconfigured WTP) may repeatedly attempt DTLS handshakes with the
 AC, potentially creating a resource shortage.  If either the
 FailedDTLSSessionCount or the FailedDTLSAuthFailCount counter reaches
 the value of MaxFailedDTLSSessionRetry variable (see Section 4.8),
 implementations MAY choose to rate-limit new DTLS handshakes for some
 period of time.  It is RECOMMENDED that implementations choosing to
 implement rate-limiting use a random discard technique, rather than
 mimicking the WTP's sulking behavior.  This will ensure that messages
 from valid WTPs will have some probability of eliciting a response,
 even in the face of a significant DoS attack.
 Some CAPWAP implementations may wish to restrict the DTLS setup
 process to only those peers that have been configured in the access
 control list, authorizing only those clients to initiate a DTLS
 handshake.  Note that the impact of this on mitigating denial-of-
 service attacks against the DTLS layer is minimal, because DTLS
 already uses client-side cookies to minimize processor consumption
 attacks.

12.4. Interference with a DTLS Session

 If a WTP or AC repeatedly receives packets that fail DTLS
 authentication or decryption, this could indicate a DTLS
 desynchronization between the AC and WTP, a link prone to
 undetectable bit errors, or an attacker trying to disrupt a DTLS
 session.

Calhoun, et al. Standards Track [Page 137] RFC 5415 CAPWAP Protocol Specification March 2009

 In the state machine (section 2.3), transitions to the DTLS Tear Down
 (TD) state can be triggered by frequently receiving DTLS packets with
 authentication or decryption errors.  The threshold or technique for
 deciding when to move to the tear down state should be chosen
 carefully.  Being able to easily transition to DTLS TD allows easy
 detection of malfunctioning devices, but allows for denial-of-service
 attacks.  Making it difficult to transition to DTLS TD prevents
 denial-of-service attacks, but makes it more difficult to detect and
 reset a malfunctioning session.  Implementers should set this policy
 with care.

12.5. CAPWAP Pre-Provisioning

 In order for CAPWAP to establish a secure communication with a peer,
 some level of pre-provisioning on both the WTP and AC is necessary.
 This section will detail the minimal number of configuration
 parameters.
 When using pre-shared keys, it is necessary to configure the pre-
 shared key for each possible peer with which a DTLS session may be
 established.  To support this mode of operation, one or more entries
 of the following table may be configured on either the AC or WTP:
 o  Identity: The identity of the peering AC or WTP.  This format MAY
    be in the form of either an IP address or host name (the latter of
    which needs to be resolved to an IP address using DNS).
 o  Key: The pre-shared key for use with the peer when establishing
    the DTLS session (see Section 12.6 for more information).
 o  PSK Identity: Identity hint associated with the provisioned key
    (see Section 2.4.4.4 for more information).
 When using certificates, the following items need to be pre-
 provisioned:
 o  Device Certificate: The local device's certificate (see
    Section 12.7 for more information).
 o  Trust Anchor: Trusted root certificate chain used to validate any
    certificate received from CAPWAP peers.  Note that one or more
    root certificates MAY be configured on a given device.
 Regardless of the authentication method, the following item needs to
 be pre-provisioned:

Calhoun, et al. Standards Track [Page 138] RFC 5415 CAPWAP Protocol Specification March 2009

 o  Access Control List: The access control list table contains the
    identities of one or more CAPWAP peers, along with a rule.  The
    rule is used to determine whether communication with the peer is
    permitted (see Section 2.4.4.3 for more information).

12.6. Use of Pre-Shared Keys in CAPWAP

 While use of pre-shared keys may provide deployment and provisioning
 advantages not found in public-key-based deployments, it also
 introduces a number of operational and security concerns.  In
 particular, because the keys must typically be entered manually, it
 is common for people to base them on memorable words or phrases.
 These are referred to as "low entropy passwords/passphrases".
 Use of low-entropy pre-shared keys, coupled with the fact that the
 keys are often not frequently updated, tends to significantly
 increase exposure.  For these reasons, the following recommendations
 are made:
 o  When DTLS is used with a pre-shared key (PSK) ciphersuite, each
    WTP SHOULD have a unique PSK.  Since WTPs will likely be widely
    deployed, their physical security is not guaranteed.  If PSKs are
    not unique for each WTP, key reuse would allow the compromise of
    one WTP to result in the compromise of others.
 o  Generating PSKs from low entropy passwords is NOT RECOMMENDED.
 o  It is RECOMMENDED that implementations that allow the
    administrator to manually configure the PSK also provide a
    capability for generation of new random PSKs, taking RFC 4086
    [RFC4086] into account.
 o  Pre-shared keys SHOULD be periodically updated.  Implementations
    MAY facilitate this by providing an administrative interface for
    automatic key generation and periodic update, or it MAY be
    accomplished manually instead.
 Every pairwise combination of WTP and AC on the network SHOULD have a
 unique PSK.  This prevents the domino effect (see "Guidance for
 Authentication, Authorization, and Accounting (AAA) Key Management"
 [RFC4962]).  If PSKs are tied to specific WTPs, then knowledge of the
 PSK implies a binding to a specified identity that can be authorized.
 If PSKs are shared, this binding between device and identity is no
 longer possible.  Compromise of one WTP can yield compromise of
 another WTP, violating the CAPWAP security hierarchy.  Consequently,
 sharing keys between WTPs is NOT RECOMMENDED.

Calhoun, et al. Standards Track [Page 139] RFC 5415 CAPWAP Protocol Specification March 2009

12.7. Use of Certificates in CAPWAP

 For public-key-based DTLS deployments, each device SHOULD have unique
 credentials, with an extended key usage authorizing the device to act
 as either a WTP or AC.  If devices do not have unique credentials, it
 is possible that by compromising one device, any other device using
 the same credential may also be considered to be compromised.
 Certificate validation involves checking a large variety of things.
 Since the necessary things to validate are often environment-
 specific, many are beyond the scope of this document.  In this
 section, we provide some basic guidance on certificate validation.
 Each device is responsible for authenticating and authorizing devices
 with which they communicate.  Authentication entails validation of
 the chain of trust leading to the peer certificate, followed by the
 peer certificate itself.  Implementations SHOULD also provide a
 secure method for verifying that the credential in question has not
 been revoked.
 Note that if the WTP relies on the AC for network connectivity (e.g.,
 the AC is a Layer 2 switch to which the WTP is directly connected),
 the WTP may not be able to contact an Online Certificate Status
 Protocol (OCSP) server or otherwise obtain an up-to-date Certificate
 Revocation List (CRL) if a compromised AC doesn't explicitly permit
 this.  This cannot be avoided, except through effective physical
 security and monitoring measures at the AC.
 Proper validation of certificates typically requires checking to
 ensure the certificate has not yet expired.  If devices have a real-
 time clock, they SHOULD verify the certificate validity dates.  If no
 real-time clock is available, the device SHOULD make a best-effort
 attempt to validate the certificate validity dates through other
 means.  Failure to check a certificate's temporal validity can make a
 device vulnerable to man-in-the-middle attacks launched using
 compromised, expired certificates, and therefore devices should make
 every effort to perform this validation.

12.8. Use of MAC Address in CN Field

 The CAPWAP protocol is an evolution of an existing protocol [LWAPP],
 which is implemented on a large number of already deployed ACs and
 WTPs.  Every one of these devices has an existing X.509 certificate,
 which is provisioned at the time of manufacturing.  These X.509
 certificates use the device's MAC address in the Common Name (CN)
 field.  It is well understood that encoding the MAC address in the CN
 field is less than optimal, and using the SubjectAltName field would
 be preferable.  However, at the time of publication, there is no URN

Calhoun, et al. Standards Track [Page 140] RFC 5415 CAPWAP Protocol Specification March 2009

 specification that allows for the MAC address to be used in the
 SubjectAltName field.  As such a specification is published by the
 IETF, future versions of the CAPWAP protocol MAY require support for
 the new URN scheme.

12.9. AAA Security

 The AAA protocol is used to distribute Extensible Authentication
 Protocol (EAP) keys to the ACs, and consequently its security is
 important to the overall system security.  When used with Transport
 Layer Security (TLS) or IPsec, security guidelines specified in RFC
 3539 [RFC3539] SHOULD be followed.
 In general, the link between the AC and AAA server SHOULD be secured
 using a strong ciphersuite keyed with mutually authenticated session
 keys.  Implementations SHOULD NOT rely solely on Basic RADIUS shared
 secret authentication as it is often vulnerable to dictionary
 attacks, but rather SHOULD use stronger underlying security
 mechanisms.

12.10. WTP Firmware

 The CAPWAP protocol defines a mechanism by which the AC downloads new
 firmware to the WTP.  During the session establishment process, the
 WTP provides information about its current firmware to the AC.  The
 AC then decides whether the WTP's firmware needs to be updated.  It
 is important to note that the CAPWAP specification makes the explicit
 assumption that the WTP is providing the correct firmware version to
 the AC, and is therefore not lying.  Further, during the firmware
 download process, the CAPWAP protocol does not provide any mechanisms
 to recognize whether the WTP is actually storing the firmware for
 future use.

13. Operational Considerations

 The CAPWAP protocol assumes that it is the only configuration
 interface to the WTP to configure parameters that are specified in
 the CAPWAP specifications.  While the use of a separate management
 protocol MAY be used for the purposes of monitoring the WTP directly,
 configuring the WTP through a separate management interface is not
 recommended.  Configuring the WTP through a separate protocol, such
 as via a command line interface (CLI) or Simple Network Management
 Protocol (SNMP), could lead to the AC state being out of sync with
 the WTP.

Calhoun, et al. Standards Track [Page 141] RFC 5415 CAPWAP Protocol Specification March 2009

 The CAPWAP protocol does not deal with the management of the ACs.
 The AC is assumed to be configured through some separate management
 interface, which could be via a proprietary CLI, SNMP, Network
 Configuration Protocol (NETCONF), or some other management protocol.
 The CAPWAP protocol's control channel is fairly lightweight from a
 traffic perspective.  Once the WTP has been configured, the WTP sends
 periodic statistics.  Further, the specification calls for a keep-
 alive packet to be sent on the protocol's data channel to make sure
 that any possible middleboxes (e.g., NAT) maintain their UDP state.
 The overhead associated with the control and data channel is not
 expected to impact network traffic.  That said, the CAPWAP protocol
 does allow for the frequency of these packets to be modified through
 the DataChannelKeepAlive and StatisticsTimer (see Section 4.7.2 and
 Section 4.7.14, respectively).

14. Transport Considerations

 The CAPWAP WG carefully considered the congestion control
 requirements of the CAPWAP protocol, both for the CAPWAP Control and
 Data channels.
 CAPWAP specifies a single-threaded command/response protocol to be
 used on the control channel, and we have specified that an
 exponential back-off algorithm should be used when commands are
 retransmitted.  When CAPWAP runs in its default mode (Local MAC), the
 control channel is the only CAPWAP channel.
 However, CAPWAP can also be run in Split MAC mode, in which case
 there will be a DTLS-encrypted data channel between each WTP and the
 AC.  The WG discussed various options for providing congestion
 control on this channel.  However, due to performance problems with
 TCP when it is run over another congestion control mechanism and the
 fact that the vast majority of traffic run over the CAPWAP Data
 channel is likely to be congestion-controlled IP traffic, the CAPWAP
 WG felt that specifying a congestion control mechanism for the CAPWAP
 Data channel would be more likely to cause problems than to resolve
 any.
 Because there is no congestion control mechanism specified for the
 CAPWAP Data channel, it is RECOMMENDED that non-congestion-controlled
 traffic not be tunneled over CAPWAP.  When a significant amount of
 non-congestion-controlled traffic is expected to be present on a
 WLAN, the CAPWAP connection between the AC and the WTP for that LAN
 should be configured to remain in Local MAC mode with Distribution
 function at the WTP.

Calhoun, et al. Standards Track [Page 142] RFC 5415 CAPWAP Protocol Specification March 2009

 The lock step nature of the CAPWAP protocol's control channel can
 cause the firmware download process to take some time, depending upon
 the round-trip time (RTT).  This is not expected to be a problem
 since the CAPWAP protocol allows firmware to be downloaded while the
 WTP provides service to wireless clients/devices.
 It is necessary for the WTP and AC to configure their MTU based on
 the capabilities of the path.  See Section 3.5 for more information.
 The CAPWAP protocol mandates support of the Explicit Congestion
 Notification (ECN) through a mode of operation named "limited
 functionality option", detailed in section 9.1.1 of [RFC3168].
 Future versions of the CAPWAP protocol should consider mandating
 support for the "full functionality option".

15. IANA Considerations

 This section details the actions that IANA has taken in preparation
 for publication of the specification.  Numerous registries have been
 created, and the contents, document action (see [RFC5226], and
 registry format are all included below.  Note that in cases where bit
 fields are referred to, the bit numbering is left to right, where the
 leftmost bit is labeled as bit zero (0).
 For future registration requests where an Expert Review is required,
 a Designated Expert should be consulted, which is appointed by the
 responsible IESG Area Director.  The intention is that any allocation
 will be accompanied by a published RFC, but given that other SDOs may
 want to create standards built on top of CAPWAP, a document the
 Designated Expert can review is also acceptable.  IANA should allow
 for allocation of values prior to documents being approved for
 publication, so the Designated Expert can approve allocations once it
 seems clear that publication will occur.  The Designated Expert will
 post a request to the CAPWAP WG mailing list (or a successor
 designated by the Area Director) for comment and review.  Before a
 period of 30 days has passed, the Designated Expert will either
 approve or deny the registration request and publish a notice of the
 decision to the CAPWAP WG mailing list or its successor, as well as
 informing IANA.  A denial notice must be justified by an explanation,
 and in the cases where it is possible, concrete suggestions on how
 the request can be modified so as to become acceptable should be
 provided.

15.1. IPv4 Multicast Address

 IANA has registered a new IPv4 multicast address called "capwap-ac"
 from the Internetwork Control Block IPv4 multicast address registry;
 see Section 3.3.

Calhoun, et al. Standards Track [Page 143] RFC 5415 CAPWAP Protocol Specification March 2009

15.2. IPv6 Multicast Address

 IANA has registered a new organization local multicast address called
 the "All ACs multicast address" in the Variable Scope IPv6 multicast
 address registry; see Section 3.3.

15.3. UDP Port

 IANA registered two new UDP Ports, which are organization-local
 multicast addresses, in the registered port numbers registry; see
 Section 3.1.  The following values have been registered:
 Keyword         Decimal    Description                  References
 -------         -------    -----------                  ----------
 capwap-control  5246/udp   CAPWAP Control Protocol      This Document
 capwap-data     5247/udp   CAPWAP Data Protocol         This Document

15.4. CAPWAP Message Types

 The Message Type field in the CAPWAP Header (see Section 4.5.1.1) is
 used to identify the operation performed by the message.  There are
 multiple namespaces, which are identified via the first three octets
 of the field containing the IANA Enterprise Number [RFC5226].
 IANA maintains the CAPWAP Message Types registry for all message
 types whose Enterprise Number is set to zero (0).  The namespace is 8
 bits (0-255), where the value of zero (0) is reserved and must not be
 assigned.  The values one (1) through 26 are allocated in this
 specification, and can be found in Section 4.5.1.1.  Any new
 assignments of a CAPWAP Message Type whose Enterprise Number is set
 to zero (0) requires an Expert Review.  The registry maintained by
 IANA has the following format:
         CAPWAP Control Message           Message Type     Reference
                                            Value

15.5. CAPWAP Header Flags

 The Flags field in the CAPWAP Header (see Section 4.3) is 9 bits in
 length and is used to identify any special treatment related to the
 message.  This specification defines bits zero (0) through five (5),
 while bits six (6) through eight (8) are reserved.  There are
 currently three unused, reserved bits that are managed by IANA and
 whose assignment require an Expert Review.  IANA created the CAPWAP
 Header Flags registry, whose format is:
         Flag Field Name                   Bit Position    Reference

Calhoun, et al. Standards Track [Page 144] RFC 5415 CAPWAP Protocol Specification March 2009

15.6. CAPWAP Control Message Flags

 The Flags field in the CAPWAP Control Message header (see
 Section 4.5.1.4) is used to identify any special treatment related to
 the control message.  There are currently eight (8) unused, reserved
 bits.  The assignment of these bits is managed by IANA and requires
 an Expert Review.  IANA created the CAPWAP Control Message Flags
 registry, whose format is:
         Flag Field Name                   Bit Position    Reference

15.7. CAPWAP Message Element Type

 The Type field in the CAPWAP Message Element header (see Section 4.6)
 is used to identify the data being transported.  The namespace is 16
 bits (0-65535), where the value of zero (0) is reserved and must not
 be assigned.  The values one (1) through 53 are allocated in this
 specification, and can be found in Section 4.5.1.1.
 The 16-bit namespace is further divided into blocks of addresses that
 are reserved for specific CAPWAP wireless bindings.  The following
 blocks are reserved:
       CAPWAP Protocol Message Elements                   1 - 1023
       IEEE 802.11 Message Elements                    1024 - 2047
       EPCGlobal Message Elements                      3072 - 4095
 This namespace is managed by IANA and assignments require an Expert
 Review.  IANA created the CAPWAP Message Element Type registry, whose
 format is:
         CAPWAP Message Element           Type Value       Reference

15.8. CAPWAP Wireless Binding Identifiers

 The Wireless Binding Identifier (WBID) field in the CAPWAP Header
 (see Section 4.3) is used to identify the wireless technology
 associated with the packet.  This specification allocates the values
 one (1) and three (3).  Due to the limited address space available, a
 new WBID request requires Expert Review.  IANA created the CAPWAP
 Wireless Binding Identifier registry, whose format is:
         CAPWAP Wireless Binding Identifier  Type Value      Reference

Calhoun, et al. Standards Track [Page 145] RFC 5415 CAPWAP Protocol Specification March 2009

15.9. AC Security Types

 The Security field in the AC Descriptor message element (see
 Section 4.6.1) is 8 bits in length and is used to identify the
 authentication methods available on the AC.  This specification
 defines bits five (5) and six (6), while bits zero (0) through four
 (4) as well as bit seven (7) are reserved and unused.  These reserved
 bits are managed by IANA and assignment requires Standards Action.
 IANA created the AC Security Types registry, whose format is:
         AC Security Type                  Bit Position    Reference

15.10. AC DTLS Policy

 The DTLS Policy field in the AC Descriptor message element (see
 Section 4.6.1) is 8 bits in length and is used to identify whether
 the CAPWAP Data Channel is to be secured.  This specification defines
 bits five (5) and six (6), while bits zero (0) through four (4) as
 well as bit seven (7) are reserved and unused.  These reserved bits
 are managed by IANA and assignment requires Standards Action.  IANA
 created the AC DTLS Policy registry, whose format is:
         AC DTLS Policy                    Bit Position    Reference

15.11. AC Information Type

 The Information Type field in the AC Descriptor message element (see
 Section 4.6.1) is used to represent information about the AC.  The
 namespace is 16 bits (0-65535), where the value of zero (0) is
 reserved and must not be assigned.  This field, combined with the AC
 Information Vendor ID, allows vendors to use a private namespace.
 This specification defines the AC Information Type namespace when the
 AC Information Vendor ID is set to zero (0), for which the values
 four (4) and five (5) are allocated in this specification, and can be
 found in Section 4.6.1.  This namespace is managed by IANA and
 assignments require an Expert Review.  IANA created the AC
 Information Type registry, whose format is:
         AC Information Type              Type Value       Reference

15.12. CAPWAP Transport Protocol Types

 The Transport field in the CAPWAP Transport Protocol message element
 (see Section 4.6.14) is used to identify the transport to use for the
 CAPWAP Data Channel.  The namespace is 8 bits (0-255), where the
 value of zero (0) is reserved and must not be assigned.  The values
 one (1) and two (2) are allocated in this specification, and can be

Calhoun, et al. Standards Track [Page 146] RFC 5415 CAPWAP Protocol Specification March 2009

 found in Section 4.6.14.  This namespace is managed by IANA and
 assignments require an Expert Review.  IANA created the CAPWAP
 Transport Protocol Types registry, whose format is:
         CAPWAP Transport Protocol Type   Type Value       Reference

15.13. Data Transfer Type

 The Data Type field in the Data Transfer Data message element (see
 Section 4.6.15) and Image Data message element (see Section 4.6.26)
 is used to provide information about the data being carried.  The
 namespace is 8 bits (0-255), where the value of zero (0) is reserved
 and must not be assigned.  The values one (1), two (2), and five (5)
 are allocated in this specification, and can be found in
 Section 4.6.15.  This namespace is managed by IANA and assignments
 require an Expert Review.  IANA created the Data Transfer Type
 registry, whose format is:
         Data Transfer Type               Type Value       Reference

15.14. Data Transfer Mode

 The Data Mode field in the Data Transfer Data message element (see
 Section 4.6.15) and Data Transfer Mode message element (see
 Section 15.14) is used to provide information about the data being
 carried.  The namespace is 8 bits (0-255), where the value of zero
 (0) is reserved and must not be assigned.  The values one (1) and two
 (2) are allocated in this specification, and can be found in
 Section 15.14.  This namespace is managed by IANA and assignments
 require an Expert Review.  IANA created the Data Transfer Mode
 registry, whose format is:
         Data Transfer Mode               Type Value       Reference

15.15. Discovery Types

 The Discovery Type field in the Discovery Type message element (see
 Section 4.6.21) is used by the WTP to indicate to the AC how it was
 discovered.  The namespace is 8 bits (0-255).  The values zero (0)
 through four (4) are allocated in this specification and can be found
 in Section 4.6.21.  This namespace is managed by IANA and assignments
 require an Expert Review.  IANA created the Discovery Types registry,
 whose format is:
         Discovery Types                  Type Value       Reference

Calhoun, et al. Standards Track [Page 147] RFC 5415 CAPWAP Protocol Specification March 2009

15.16. ECN Support

 The ECN Support field in the ECN Support message element (see
 Section 4.6.25) is used by the WTP to represent its ECN Support.  The
 namespace is 8 bits (0-255).  The values zero (0) and one (1) are
 allocated in this specification, and can be found in Section 4.6.25.
 This namespace is managed by IANA and assignments require an Expert
 Review.  IANA created the ECN Support registry, whose format is:
         ECN Support                      Type Value       Reference

15.17. Radio Admin State

 The Radio Admin field in the Radio Administrative State message
 element (see Section 4.6.33) is used by the WTP to represent the
 state of its radios.  The namespace is 8 bits (0-255), where the
 value of zero (0) is reserved and must not be assigned.  The values
 one (1) and two (2) are allocated in this specification, and can be
 found in Section 4.6.33.  This namespace is managed by IANA and
 assignments require an Expert Review.  IANA created the Radio Admin
 State registry, whose format is:
         Radio Admin State                Type Value       Reference

15.18. Radio Operational State

 The State field in the Radio Operational State message element (see
 Section 4.6.34) is used by the WTP to represent the operational state
 of its radios.  The namespace is 8 bits (0-255), where the value of
 zero (0) is reserved and must not be assigned.  The values one (1)
 and two (2) are allocated in this specification, and can be found in
 Section 4.6.34.  This namespace is managed by IANA and assignments
 require an Expert Review.  IANA created the Radio Operational State
 registry, whose format is:
         Radio Operational State          Type Value       Reference

15.19. Radio Failure Causes

 The Cause field in the Radio Operational State message element (see
 Section 4.6.34) is used by the WTP to represent the reason a radio
 may have failed.  The namespace is 8 bits (0-255), where the value of
 zero (0) through three (3) are allocated in this specification, and
 can be found in Section 4.6.34.  This namespace is managed by IANA
 and assignments require an Expert Review.  IANA created the Radio
 Failure Causes registry, whose format is:
         Radio Failure Causes             Type Value       Reference

Calhoun, et al. Standards Track [Page 148] RFC 5415 CAPWAP Protocol Specification March 2009

15.20. Result Code

 The Result Code field in the Result Code message element (see
 Section 4.6.35) is used to indicate the success or failure of a
 CAPWAP Control message.  The namespace is 32 bits (0-4294967295),
 where the value of zero (0) through 22 are allocated in this
 specification, and can be found in Section 4.6.35.  This namespace is
 managed by IANA and assignments require an Expert Review.  IANA
 created the Result Code registry, whose format is:
         Result Code                      Type Value       Reference

15.21. Returned Message Element Reason

 The Reason field in the Returned Message Element message element (see
 Section 4.6.36) is used to indicate the reason why a message element
 was not processed successfully.  The namespace is 8 bits (0-255),
 where the value of zero (0) is reserved and must not be assigned.
 The values one (1) through four (4) are allocated in this
 specification, and can be found in Section 4.6.36.  This namespace is
 managed by IANA and assignments require an Expert Review.  IANA
 created the Returned Message Element Reason registry, whose format
 is:
         Returned Message Element Reason  Type Value       Reference

15.22. WTP Board Data Type

 The Board Data Type field in the WTP Board Data message element (see
 Section 4.6.40) is used to represent information about the WTP
 hardware.  The namespace is 16 bits (0-65535).  The WTP Board Data
 Type values zero (0) through four (4) are allocated in this
 specification, and can be found in Section 4.6.40.  This namespace is
 managed by IANA and assignments require an Expert Review.  IANA
 created the WTP Board Data Type registry, whose format is:
         WTP Board Data Type              Type Value       Reference

15.23. WTP Descriptor Type

 The Descriptor Type field in the WTP Descriptor message element (see
 Section 4.6.41) is used to represent information about the WTP
 software.  The namespace is 16 bits (0-65535).  This field, combined
 with the Descriptor Vendor ID, allows vendors to use a private
 namespace.  This specification defines the WTP Descriptor Type
 namespace when the Descriptor Vendor ID is set to zero (0), for which
 the values zero (0) through three (3) are allocated in this

Calhoun, et al. Standards Track [Page 149] RFC 5415 CAPWAP Protocol Specification March 2009

 specification, and can be found in Section 4.6.41.  This namespace is
 managed by IANA and assignments require an Expert Review.  IANA
 created the WTP Board Data Type registry, whose format is:
         WTP Descriptor Type              Type Value       Reference

15.24. WTP Fallback Mode

 The Mode field in the WTP Fallback message element (see
 Section 4.6.42) is used to indicate the type of AC fallback mechanism
 the WTP should employ.  The namespace is 8 bits (0-255), where the
 value of zero (0) is reserved and must not be assigned.  The values
 one (1) and two (2) are allocated in this specification, and can be
 found in Section 4.6.42.  This namespace is managed by IANA and
 assignments require an Expert Review.  IANA created the WTP Fallback
 Mode registry, whose format is:
         WTP Fallback Mode                Type Value       Reference

15.25. WTP Frame Tunnel Mode

 The Tunnel Type field in the WTP Frame Tunnel Mode message element
 (see Section 4.6.43) is 8 bits and is used to indicate the type of
 tunneling to use between the WTP and the AC.  This specification
 defines bits four (4) through six (6), while bits zero (0) through
 three (3) as well as bit seven (7) are reserved and unused.  These
 reserved bits are managed by IANA and assignment requires an Expert
 Review.  IANA created the WTP Frame Tunnel Mode registry, whose
 format is:
         WTP Frame Tunnel Mode             Bit Position    Reference

15.26. WTP MAC Type

 The MAC Type field in the WTP MAC Type message element (see
 Section 4.6.44) is used to indicate the type of MAC to use in
 tunneled frames between the WTP and the AC.  The namespace is 8 bits
 (0-255), where the value of zero (0) through two (2) are allocated in
 this specification, and can be found in Section 4.6.44.  This
 namespace is managed by IANA and assignments require an Expert
 Review.  IANA created the WTP MAC Type registry, whose format is:
         WTP MAC Type                     Type Value       Reference

Calhoun, et al. Standards Track [Page 150] RFC 5415 CAPWAP Protocol Specification March 2009

15.27. WTP Radio Stats Failure Type

 The Last Failure Type field in the WTP Radio Statistics message
 element (see Section 4.6.46) is used to indicate the last WTP
 failure.  The namespace is 8 bits (0-255), where the value of zero
 (0) through three (3) as well as the value 255 are allocated in this
 specification, and can be found in Section 4.6.46.  This namespace is
 managed by IANA and assignments require an Expert Review.  IANA
 created the WTP Radio Stats Failure Type registry, whose format is:
         WTP Radio Stats Failure Type     Type Value       Reference

15.28. WTP Reboot Stats Failure Type

 The Last Failure Type field in the WTP Reboot Statistics message
 element (see Section 4.6.47) is used to indicate the last reboot
 reason.  The namespace is 8 bits (0-255), where the value of zero (0)
 through five (5) as well as the value 255 are allocated in this
 specification, and can be found in Section 4.6.47.  This namespace is
 managed by IANA and assignments require an Expert Review.  IANA
 created the WTP Reboot Stats Failure Type registry, whose format is:
         WTP Reboot Stats Failure Type    Type Value       Reference

16. Acknowledgments

 The following individuals are acknowledged for their contributions to
 this protocol specification: Puneet Agarwal, Abhijit Choudhury, Pasi
 Eronen, Saravanan Govindan, Peter Nilsson, David Perkins, and Yong
 Zhang.
 Michael Vakulenko contributed text to describe how CAPWAP can be used
 over Layer 3 (IP/UDP) networks.

17. References

17.1. Normative References

 [RFC1191]          Mogul, J. and S. Deering, "Path MTU discovery",
                    RFC 1191, November 1990.
 [RFC1321]          Rivest, R., "The MD5 Message-Digest Algorithm",
                    RFC 1321, April 1992.
 [RFC1305]          Mills, D., "Network Time Protocol (Version 3)
                    Specification, Implementation", RFC 1305,
                    March 1992.

Calhoun, et al. Standards Track [Page 151] RFC 5415 CAPWAP Protocol Specification March 2009

 [RFC1981]          McCann, J., Deering, S., and J. Mogul, "Path MTU
                    Discovery for IP version 6", RFC 1981,
                    August 1996.
 [RFC2119]          Bradner, S., "Key words for use in RFCs to
                    Indicate Requirement Levels", BCP 14, RFC 2119,
                    March 1997.
 [RFC2460]          Deering, S. and R. Hinden, "Internet Protocol,
                    Version 6 (IPv6) Specification", RFC 2460,
                    December 1998.
 [RFC2474]          Nichols, K., Blake, S., Baker, F., and D. Black,
                    "Definition of the Differentiated Services Field
                    (DS Field) in the IPv4 and IPv6 Headers",
                    RFC 2474, December 1998.
 [RFC2782]          Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS
                    RR for specifying the location of services (DNS
                    SRV)", RFC 2782, February 2000.
 [RFC3168]          Ramakrishnan, K., Floyd, S., and D. Black, "The
                    Addition of Explicit Congestion Notification (ECN)
                    to IP", RFC 3168, September 2001.
 [RFC3539]          Aboba, B. and J. Wood, "Authentication,
                    Authorization and Accounting (AAA) Transport
                    Profile", RFC 3539, June 2003.
 [RFC3629]          Yergeau, F., "UTF-8, a transformation format of
                    ISO 10646", STD 63, RFC 3629, November 2003.
 [RFC3828]          Larzon, L-A., Degermark, M., Pink, S., Jonsson,
                    L-E., and G. Fairhurst, "The Lightweight User
                    Datagram Protocol (UDP-Lite)", RFC 3828,
                    July 2004.
 [RFC4086]          Eastlake, D., Schiller, J., and S. Crocker,
                    "Randomness Requirements for Security", BCP 106,
                    RFC 4086, June 2005.
 [RFC4279]          Eronen, P. and H. Tschofenig, "Pre-Shared Key
                    Ciphersuites for Transport Layer Security (TLS)",
                    RFC 4279, December 2005.
 [RFC5246]          Dierks, T. and E. Rescorla, "The Transport Layer
                    Security (TLS) Protocol Version 1.2", RFC 5246,
                    August 2008.

Calhoun, et al. Standards Track [Page 152] RFC 5415 CAPWAP Protocol Specification March 2009

 [RFC4347]          Rescorla, E. and N. Modadugu, "Datagram Transport
                    Layer Security", RFC 4347, April 2006.
 [RFC4821]          Mathis, M. and J. Heffner, "Packetization Layer
                    Path MTU Discovery", RFC 4821, March 2007.
 [RFC4963]          Heffner, J., Mathis, M., and B. Chandler, "IPv4
                    Reassembly Errors at High Data Rates", RFC 4963,
                    July 2007.
 [RFC5226]          Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 5226, May 2008.
 [RFC5280]          Cooper, D., Santesson, S., Farrell, S., Boeyen,
                    S., Housley, R., and W. Polk, "Internet X.509
                    Public Key Infrastructure Certificate and
                    Certificate Revocation List (CRL) Profile",
                    RFC 5280, May 2008.
 [ISO.9834-1.1993]  International Organization for Standardization,
                    "Procedures for the operation of OSI registration
                    authorities - part 1: general procedures",
                    ISO Standard 9834-1, 1993.
 [RFC5416]          Calhoun, P., Ed., Montemurro, M., Ed., and D.
                    Stanley, Ed., "Control And Provisioning of
                    Wireless Access Points (CAPWAP) Protocol Binding
                    for IEEE 802.11", RFC 5416, March 2009.
 [RFC5417]          Calhoun, P., "Control And Provisioning of Wireless
                    Access Points (CAPWAP) Access Controller DHCP
                    Option", RFC 5417, March 2009.
 [FRAME-EXT]        IEEE, "IEEE Standard 802.3as-2006", 2005.

17.2. Informative References

 [RFC3232]          Reynolds, J., "Assigned Numbers: RFC 1700 is
                    Replaced by an On-line Database", RFC 3232,
                    January 2002.
 [RFC3753]          Manner, J. and M. Kojo, "Mobility Related
                    Terminology", RFC 3753, June 2004.

Calhoun, et al. Standards Track [Page 153] RFC 5415 CAPWAP Protocol Specification March 2009

 [RFC4564]          Govindan, S., Cheng, H., Yao, ZH., Zhou, WH., and
                    L. Yang, "Objectives for Control and Provisioning
                    of Wireless Access Points (CAPWAP)", RFC 4564,
                    July 2006.
 [RFC4962]          Housley, R. and B. Aboba, "Guidance for
                    Authentication, Authorization, and Accounting
                    (AAA) Key Management", BCP 132, RFC 4962,
                    July 2007.
 [LWAPP]            Calhoun, P., O'Hara, B., Suri, R., Cam Winget, N.,
                    Kelly, S., Williams, M., and S. Hares,
                    "Lightweight Access Point Protocol", Work in
                    Progress, March 2007.
 [SLAPP]            Narasimhan, P., Harkins, D., and S. Ponnuswamy,
                    "SLAPP: Secure Light Access Point Protocol", Work
                    in Progress, May 2005.
 [DTLS-DESIGN]      Modadugu, et al., N., "The Design and
                    Implementation of Datagram TLS", Feb 2004.
 [EUI-48]           IEEE, "Guidelines for use of a 48-bit Extended
                    Unique Identifier", Dec 2005.
 [EUI-64]           IEEE, "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER
                    (EUI-64) REGISTRATION AUTHORITY".
 [EPCGlobal]        "See http://www.epcglobalinc.org/home".
 [PacketCable]      "PacketCable Security Specification PKT-SP-SEC-
                    I12-050812", August 2005, <PacketCable>.
 [CableLabs]        "OpenCable System Security Specification OC-SP-
                    SEC-I07-061031", October 2006, <CableLabs>.
 [WiMAX]            "WiMAX Forum X.509 Device Certificate Profile
                    Approved Specification V1.0.1", April 2008,
                    <WiMAX>.
 [RFC5418]          Kelly, S. and C. Clancy, "Control And Provisioning
                    for Wireless Access Points (CAPWAP) Threat
                    Analysis for IEEE 802.11 Deployments", RFC 5418,
                    March 2009.

Calhoun, et al. Standards Track [Page 154] RFC 5415 CAPWAP Protocol Specification March 2009

Editors' Addresses

 Pat R. Calhoun (editor)
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA  95134
 Phone: +1 408-902-3240
 EMail: pcalhoun@cisco.com
 Michael P. Montemurro (editor)
 Research In Motion
 5090 Commerce Blvd
 Mississauga, ON  L4W 5M4
 Canada
 Phone: +1 905-629-4746 x4999
 EMail: mmontemurro@rim.com
 Dorothy Stanley (editor)
 Aruba Networks
 1322 Crossman Ave
 Sunnyvale, CA  94089
 Phone: +1 630-363-1389
 EMail: dstanley@arubanetworks.com

Calhoun, et al. Standards Track [Page 155]

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