Network Working Group P. Calhoun, Editor
Internet-Draft Cisco Systems, Inc.
Expires: September 5, 2007 M. Montemurro, Editor
Research In Motion
D. Stanley, Editor
Aruba Networks
March 4, 2007
CAPWAP Protocol Specification
draft-ietf-capwap-protocol-specification-05
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Abstract
This specification defines the Control And Provisioning of Wireless
Access Points (CAPWAP) Protocol. The CAPWAP protocol meets the IETF
CAPWAP working group protocol requirements. 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. The CAPWAP protocol binding which defines extensions for
use with the IEEE 802.11 wireless LAN protocol is available in [13].
Extensions are expected to be defined to enable use of the CAPWAP
protocol with additional wireless technologies.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2. Conventions used in this document . . . . . . . . . . . . 8
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 . . . . . . . . . . . . . 14
2.3.1. CAPWAP Protocol State Transitions . . . . . . . . . . 16
2.3.2. CAPWAP/DTLS Interface . . . . . . . . . . . . . . . . 27
2.4. Use of DTLS in the CAPWAP Protocol . . . . . . . . . . . 29
2.4.1. DTLS Handshake Processing . . . . . . . . . . . . . . 29
2.4.2. DTLS Session Establishment . . . . . . . . . . . . . 30
2.4.3. DTLS Error Handling . . . . . . . . . . . . . . . . . 31
2.4.4. DTLS EndPoint Authentication and Authorization . . . 32
3. CAPWAP Transport . . . . . . . . . . . . . . . . . . . . . . 36
3.1. UDP Transport . . . . . . . . . . . . . . . . . . . . . . 36
3.2. AC Discovery . . . . . . . . . . . . . . . . . . . . . . 36
3.3. Fragmentation/Reassembly . . . . . . . . . . . . . . . . 37
4. CAPWAP Packet Formats . . . . . . . . . . . . . . . . . . . . 38
4.1. CAPWAP Preamble . . . . . . . . . . . . . . . . . . . . . 40
4.2. CAPWAP Header . . . . . . . . . . . . . . . . . . . . . . 40
4.3. CAPWAP Data Messages . . . . . . . . . . . . . . . . . . 44
4.3.1. CAPWAP Data Keepalive . . . . . . . . . . . . . . . . 44
4.3.2. Data Payload . . . . . . . . . . . . . . . . . . . . 45
4.3.3. Establishment of a DTLS Data Channel . . . . . . . . 46
4.4. CAPWAP Control Messages . . . . . . . . . . . . . . . . . 46
4.4.1. Control Message Format . . . . . . . . . . . . . . . 47
4.4.2. Control Message Quality of Service . . . . . . . . . 49
4.4.3. Retransmissions . . . . . . . . . . . . . . . . . . . 50
4.5. CAPWAP Protocol Message Elements . . . . . . . . . . . . 50
4.5.1. AC Descriptor . . . . . . . . . . . . . . . . . . . . 53
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4.5.2. AC IPv4 List . . . . . . . . . . . . . . . . . . . . 54
4.5.3. AC IPv6 List . . . . . . . . . . . . . . . . . . . . 55
4.5.4. AC Name . . . . . . . . . . . . . . . . . . . . . . . 55
4.5.5. AC Name with Index . . . . . . . . . . . . . . . . . 56
4.5.6. AC Timestamp . . . . . . . . . . . . . . . . . . . . 56
4.5.7. Add MAC ACL Entry . . . . . . . . . . . . . . . . . . 57
4.5.8. Add Station . . . . . . . . . . . . . . . . . . . . . 57
4.5.9. Add Static MAC ACL Entry . . . . . . . . . . . . . . 58
4.5.10. CAPWAP Control IPv4 Address . . . . . . . . . . . . . 58
4.5.11. CAPWAP Control IPv6 Address . . . . . . . . . . . . . 59
4.5.12. CAPWAP Timers . . . . . . . . . . . . . . . . . . . . 60
4.5.13. Data Transfer Data . . . . . . . . . . . . . . . . . 60
4.5.14. Data Transfer Mode . . . . . . . . . . . . . . . . . 61
4.5.15. Decryption Error Report . . . . . . . . . . . . . . . 61
4.5.16. Decryption Error Report Period . . . . . . . . . . . 62
4.5.17. Delete MAC ACL Entry . . . . . . . . . . . . . . . . 62
4.5.18. Delete Station . . . . . . . . . . . . . . . . . . . 63
4.5.19. Delete Static MAC ACL Entry . . . . . . . . . . . . . 63
4.5.20. Discovery Type . . . . . . . . . . . . . . . . . . . 64
4.5.21. Duplicate IPv4 Address . . . . . . . . . . . . . . . 65
4.5.22. Duplicate IPv6 Address . . . . . . . . . . . . . . . 65
4.5.23. Idle Timeout . . . . . . . . . . . . . . . . . . . . 66
4.5.24. Image Data . . . . . . . . . . . . . . . . . . . . . 67
4.5.25. Image Identifier . . . . . . . . . . . . . . . . . . 67
4.5.26. Image Information . . . . . . . . . . . . . . . . . . 68
4.5.27. Initiate Download . . . . . . . . . . . . . . . . . . 69
4.5.28. Location Data . . . . . . . . . . . . . . . . . . . . 69
4.5.29. Maximum Message Length . . . . . . . . . . . . . . . 69
4.5.30. MTU Discovery Padding . . . . . . . . . . . . . . . . 70
4.5.31. Radio Administrative State . . . . . . . . . . . . . 70
4.5.32. Radio Operational State . . . . . . . . . . . . . . . 71
4.5.33. Result Code . . . . . . . . . . . . . . . . . . . . . 72
4.5.34. Returned Message Element . . . . . . . . . . . . . . 73
4.5.35. Session ID . . . . . . . . . . . . . . . . . . . . . 74
4.5.36. Statistics Timer . . . . . . . . . . . . . . . . . . 74
4.5.37. Vendor Specific Payload . . . . . . . . . . . . . . . 75
4.5.38. WTP Board Data . . . . . . . . . . . . . . . . . . . 75
4.5.39. WTP Descriptor . . . . . . . . . . . . . . . . . . . 76
4.5.40. WTP Fallback . . . . . . . . . . . . . . . . . . . . 78
4.5.41. WTP Frame Tunnel Mode . . . . . . . . . . . . . . . . 79
4.5.42. WTP IPv4 IP Address . . . . . . . . . . . . . . . . . 80
4.5.43. WTP MAC Type . . . . . . . . . . . . . . . . . . . . 80
4.5.44. WTP Name . . . . . . . . . . . . . . . . . . . . . . 81
4.5.45. WTP Operational Statistics . . . . . . . . . . . . . 81
4.5.46. WTP Radio Statistics . . . . . . . . . . . . . . . . 82
4.5.47. WTP Reboot Statistics . . . . . . . . . . . . . . . . 83
4.5.48. WTP Static IP Address Information . . . . . . . . . . 84
4.6. CAPWAP Protocol Timers . . . . . . . . . . . . . . . . . 85
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4.6.1. ChangeStatePendingTimer . . . . . . . . . . . . . . . 85
4.6.2. DataChannelKeepAlive . . . . . . . . . . . . . . . . 85
4.6.3. DataChannelDeadInterval . . . . . . . . . . . . . . . 86
4.6.4. DiscoveryInterval . . . . . . . . . . . . . . . . . . 86
4.6.5. DTLSRehandshake . . . . . . . . . . . . . . . . . . . 86
4.6.6. DTLSSessionDelete . . . . . . . . . . . . . . . . . . 86
4.6.7. EchoInterval . . . . . . . . . . . . . . . . . . . . 86
4.6.8. KeyLifetime . . . . . . . . . . . . . . . . . . . . . 86
4.6.9. MaxDiscoveryInterval . . . . . . . . . . . . . . . . 87
4.6.10. MaxFailedDTLSSessionRetry . . . . . . . . . . . . . . 87
4.6.11. NeighborDeadInterval . . . . . . . . . . . . . . . . 87
4.6.12. ResponseTimeout . . . . . . . . . . . . . . . . . . . 87
4.6.13. RetransmitInterval . . . . . . . . . . . . . . . . . 87
4.6.14. SilentInterval . . . . . . . . . . . . . . . . . . . 87
4.6.15. StatisticsTimer . . . . . . . . . . . . . . . . . . . 88
4.6.16. WaitDTLS . . . . . . . . . . . . . . . . . . . . . . 88
4.6.17. WaitJoin . . . . . . . . . . . . . . . . . . . . . . 88
4.7. CAPWAP Protocol Variables . . . . . . . . . . . . . . . . 88
4.7.1. AdminState . . . . . . . . . . . . . . . . . . . . . 88
4.7.2. DiscoveryCount . . . . . . . . . . . . . . . . . . . 88
4.7.3. FailedDTLSAuthFailCount . . . . . . . . . . . . . . . 88
4.7.4. FailedDTLSSessionCount . . . . . . . . . . . . . . . 88
4.7.5. IdleTimeout . . . . . . . . . . . . . . . . . . . . . 89
4.7.6. MaxDiscoveries . . . . . . . . . . . . . . . . . . . 89
4.7.7. MaxRetransmit . . . . . . . . . . . . . . . . . . . . 89
4.7.8. ReportInterval . . . . . . . . . . . . . . . . . . . 89
4.7.9. RetransmitCount . . . . . . . . . . . . . . . . . . . 89
4.7.10. WTPFallBack . . . . . . . . . . . . . . . . . . . . . 89
4.8. WTP Saved Variables . . . . . . . . . . . . . . . . . . . 89
4.8.1. AdminRebootCount . . . . . . . . . . . . . . . . . . 89
4.8.2. FrameEncapType . . . . . . . . . . . . . . . . . . . 89
4.8.3. LastRebootReason . . . . . . . . . . . . . . . . . . 90
4.8.4. MacType . . . . . . . . . . . . . . . . . . . . . . . 90
4.8.5. PreferredACs . . . . . . . . . . . . . . . . . . . . 90
4.8.6. RebootCount . . . . . . . . . . . . . . . . . . . . . 90
4.8.7. Static ACL Table . . . . . . . . . . . . . . . . . . 90
4.8.8. Static IP Address . . . . . . . . . . . . . . . . . . 90
4.8.9. WTPLinkFailureCount . . . . . . . . . . . . . . . . . 90
4.8.10. WTPLocation . . . . . . . . . . . . . . . . . . . . . 90
4.8.11. WTPName . . . . . . . . . . . . . . . . . . . . . . . 90
5. CAPWAP Discovery Operations . . . . . . . . . . . . . . . . . 91
5.1. Discovery Request Message . . . . . . . . . . . . . . . . 91
5.2. Discovery Response Message . . . . . . . . . . . . . . . 92
5.3. Primary Discovery Request Message . . . . . . . . . . . . 93
5.4. Primary Discovery Response . . . . . . . . . . . . . . . 94
6. CAPWAP Join Operations . . . . . . . . . . . . . . . . . . . 95
6.1. Join Request . . . . . . . . . . . . . . . . . . . . . . 95
6.2. Join Response . . . . . . . . . . . . . . . . . . . . . . 96
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7. Control Channel Management . . . . . . . . . . . . . . . . . 98
7.1. Echo Request . . . . . . . . . . . . . . . . . . . . . . 98
7.2. Echo Response . . . . . . . . . . . . . . . . . . . . . . 98
8. WTP Configuration Management . . . . . . . . . . . . . . . . 100
8.1. Configuration Consistency . . . . . . . . . . . . . . . . 100
8.1.1. Configuration Flexibility . . . . . . . . . . . . . . 101
8.2. Configuration Status . . . . . . . . . . . . . . . . . . 101
8.3. Configuration Status Response . . . . . . . . . . . . . . 102
8.4. Configuration Update Request . . . . . . . . . . . . . . 103
8.5. Configuration Update Response . . . . . . . . . . . . . . 104
8.6. Change State Event Request . . . . . . . . . . . . . . . 104
8.7. Change State Event Response . . . . . . . . . . . . . . . 105
8.8. Clear Configuration Request . . . . . . . . . . . . . . . 106
8.9. Clear Configuration Response . . . . . . . . . . . . . . 106
9. Device Management Operations . . . . . . . . . . . . . . . . 107
9.1. Firmware Management . . . . . . . . . . . . . . . . . . . 107
9.1.1. Image Data Request . . . . . . . . . . . . . . . . . 110
9.1.2. Image Data Response . . . . . . . . . . . . . . . . . 111
9.2. Reset Request . . . . . . . . . . . . . . . . . . . . . . 112
9.3. Reset Response . . . . . . . . . . . . . . . . . . . . . 112
9.4. WTP Event Request . . . . . . . . . . . . . . . . . . . . 113
9.5. WTP Event Response . . . . . . . . . . . . . . . . . . . 114
9.6. Data Transfer Request . . . . . . . . . . . . . . . . . . 114
9.7. Data Transfer Response . . . . . . . . . . . . . . . . . 114
10. Station Session Management . . . . . . . . . . . . . . . . . 116
10.1. Station Configuration Request . . . . . . . . . . . . . . 116
10.2. Station Configuration Response . . . . . . . . . . . . . 116
11. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 117
12. Security Considerations . . . . . . . . . . . . . . . . . . . 119
12.1. CAPWAP Security . . . . . . . . . . . . . . . . . . . . . 119
12.1.1. Converting Protected Data into Unprotected Data . . . 120
12.1.2. Converting Unprotected Data into Protected Data
(Insertion) . . . . . . . . . . . . . . . . . . . . . 120
12.1.3. Deletion of Protected Records . . . . . . . . . . . . 120
12.1.4. Insertion of Unprotected Records . . . . . . . . . . 120
12.2. Session ID Security . . . . . . . . . . . . . . . . . . . 120
12.3. Discovery Attacks . . . . . . . . . . . . . . . . . . . . 121
12.4. Interference with a DTLS Session . . . . . . . . . . . . 121
12.5. Use of Preshared Keys in CAPWAP . . . . . . . . . . . . . 121
12.6. Use of Certificates in CAPWAP . . . . . . . . . . . . . . 122
12.7. AAA Security . . . . . . . . . . . . . . . . . . . . . . 123
13. Management Considerations . . . . . . . . . . . . . . . . . . 124
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 125
14.1. CAPWAP Message Types . . . . . . . . . . . . . . . . . . 125
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 126
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 127
16.1. Normative References . . . . . . . . . . . . . . . . . . 127
16.2. Informational References . . . . . . . . . . . . . . . . 127
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16.3. Informational References . . . . . . . . . . . . . . . . 128
Editors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 129
Intellectual Property and Copyright Statements . . . . . . . . . 130
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1. Introduction
This document describes the CAPWAP Protocol, a standard,
interoperable protocol which 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 technology.
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 [13] 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 RF interfaces controlled by the AC. The CAPWAP
protocol supports two modes of operation: Split and Local MAC. 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 bridging function. In either
case the L2 wireless management frames are processed locally by the
WTP, and then forwarded to the AC. Figure 2 shows the Local MAC
mode, in which a station transmits a wireless frame which is
encapsulated in an 802.3 frame and forwarded to the AC.
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+-+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 the subject to severe
cost pressure.
3. To provide 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.
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 [1].
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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) [add reference, when available] to be used as the
basis of the CAPWAP protocol specification. The following people are
authors of the LWAPP document:
Bob O'Hara, Cisco Systems, Inc.,170 West Tasman Drive, San Jose, CA 95134
Phone: +1 408-853-5513, Email: bob.ohara@cisco.com
Pat Calhoun, Cisco Systems, Inc., 170 West Tasman Drive, San Jose, CA 95134
Phone: +1 408-853-5269, 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, Nexthop Technologies, Inc., 825 Victors Way, Suite 100, Ann Arbor, MI 48108
Phone: +1 734 222 1610, Email: shares@nexthop.com
DTLS is used as the security solution for the CAPWAP protocol. The
following people are authors of significant DTLS-related text
included in this document:
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 [add
reference when available]. The following people are authors of the
SLAPP proposal:
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Partha Narasimhan, Aruba Networks, 1322 Crossman Ave, Sunnyvale, CA 94089
Phone: +1 408-480-4716, Email: partha@arubanetworks.com
Dan Harkins, Tropos Networks, 555 Del Rey Avenue, Sunnyvale, CA, 95085
Phone: +1 408 470 7372, Email: dharkins@tropos.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 draft:
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: skelly@arubanetworks.com
1.4. Terminology
Access Controller (AC): The network entity that provides WTPs access
to the network infrastructure in the data plane, control plane,
management plane, or a combination therein.
Station (STA): A device that contains an IEEE 802.11 conformant
medium access control (MAC) and physical layer (PHY) interface to the
wireless medium (WM).
Wireless Termination Point (WTP): The physical or network entity that
contains an RF antenna and wireless PHY to transmit and receive
station traffic for wireless access networks.
This document uses additional terminology defined in [8].
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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)
[7]. 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. CAPWAP protocol
messages will be fragmented to the maximum length discovered to be
supported by the network.
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.
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
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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. 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
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 [13], begins with "IEEE 802.11".
The CAPWAP binding concept is also 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.
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2.2. CAPWAP Session Establishment Overview
This section describes the session establishment process message
exchanges in the ideal case. 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.
============ ============
WTP AC
============ ============
[----------- begin optional discovery ------------]
Discover Request ------>
<------ Discover Response
[----------- end optional discovery ------------]
(--- begin DTLS handshake ---)
ClientHello ------>
<------ HelloVerifyRequest
(with cookie)
ClientHello ------>
(with cookie)
<------ ServerHello
<------ Certificate
<------ ServerHelloDone
(WTP callout for AC authorization)
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ------>
(AC callout for WTP
authorization)
[ChangeCipherSpec]
<------ Finished
(--- DTLS session is established now ---)
Join Request ------>
<------ Join Response
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( ---assume image is up to date ---)
Configuration Status Request ------->
<------ Configuration Status Response
(--- enter RUN state ---)
:
:
Echo Request ------->
<------ Echo Response
:
:
Event Request ------->
<------ Event Response
:
:
At the end of the illustrated CAPWAP message exchange, the AC and WTP
are securely exchanging CAPWAP control messages. This is an
idealized illustration, provided to clarify protocol operation.
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.
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/-------------------------\
w| |
5+----------+ x +------------+ |
| Run |-->| Reset |-\|
+----------+ +------------+ ||
u ^ ^ ^ y||
+------------+--------/ | | ||
| Data Check | /-------/ | ||
+------------+<-------\ | | ||
| | ||
/------------------+--------\ | ||
r| t| s| 4 v o| ||
+--------+ +-----------+ +--------------+||
| Join |---->| Configure | | Image Data |||
+--------+ q +-----------+ +--------------+||
^ p| V| x| ||
| | \-------------------\ | ||
| \--------------------------------------\| | ||
\------------------------\ || | ||
/--------------<----------------+--------------\ || | ||
| /------------<-------------\ | | || | ||
| | m| |n z| vv v vv
| | +----------------+ +--------------+ +-----------+
| | | DTLS Setup | | DTLS Connect | | DTLS TD |
| | +----------------+ +--------------+ +-----------+
| | g| ^ ^ |h ^ ^
v v | | | | | |
| | | | | \-------\ | /-----------/
| | | | | | | |
| | v |e f| 2 v |j |k
| \->+------+ +------+ +-----------+
| | Idle |-->| Disc | | Authorize |
\--->+------+ a +------+ +-----------+
b| ^ |c
| | /----/
v d| |
+---------+ |
| Sulking |<-/
3 +---------+
Figure 3: 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
called out in the transition descriptions below. For every state
defined, only certain messages are permitted to be sent and received.
The CAPWAP control messages definitions specify the state(s) in which
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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 AC has a separate
instance of the CAPWAP state machine per WTP it is communicating
with.
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.
Idle to Discovery (a): This transition occurs once device
initialization is complete.
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 4.6). The WTP resets the DiscoveryCount counter
to zero (0) (see Section 4.7). The WTP also clears all
information from ACs it may have received during a previous
Discovery phase.
AC: The AC does not maintain state information for the WTP upon
reception of the Discovery Request message, but it SHOULD
respond with a Discovery Response message (see Section 5.2).
This transition is a no-op for the AC.
Idle to Sulking (b): This transition occurs to force the WTP and AC
to enter a quiet period to avoid repeatedly attempting to
establish a connection.
WTP: The WTP enters this state when the FailedDTLSSessionCount or
the FailedDTLSAuthFailCount counter reaches
MaxFailedDTLSSessionRetry variable (see Section 4.7). 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: The AC enters this state with the specific WTP when the
FailedDTLSSessionCount or the FailedDTLSAuthFailCount counter
reaches MaxFailedDTLSSessionRetry variable (see Section 4.7).
Upon entering this state, the AC MUST start the SilentInterval
timer. While in the Sulking state, all received CAPWAP and
DTLS protocol messages received from the WTP MUST be ignored.
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Discovery to Discovery (2): In the Discovery state, the WTP
determines which AC to connect to.
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 a no-op.
Discovery to Sulking (c): This transition occurs on a WTP when
Discovery or connectivity to the AC fails.
WTP: The WTP enters this state when the DiscoveryInterval timer
expires or the DiscoveryCount variable is equal to the
MaxDiscoveries variable (see Section 4.7). Upon entering this
state, the WTP MUST start the SilentInterval timer. While in
the Sulking state, all received CAPWAP protocol messages
received MUST be ignored.
AC: This is a no-op.
Sulking to Idle (d): 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 4.6) expires. The FailedDTLSSessionCount,
DiscoveryCount and FailedDTLSAuthFailCount counters are reset
to zero.
AC: The AC enters this state when the SilentInterval timer (see
Section 4.6) expires. The FailedDTLSSessionCount,
DiscoveryCount and FailedDTLSAuthFailCount counters are reset
to zero.
Sulking to Sulking (3): 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.
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AC: All packets receive from the WTP while in the sulking state
are ignored.
Idle to DTLS Setup (e): This transition occurs to establish a secure
DTLS session with the peer.
WTP: The WTP initiates this transition by invoking the DTLSStart
command, which starts the DTLS session establishment with the
chosen AC. When the discovery phase is bypassed, it is assumed
the WTP has a locally configured AC.
AC: The AC initiates this transition by invoking the DTLSListen
command, which informs the DTLS stack that it is willing to
listen for an incoming session. The AC MAY provide optional
qualifiers in the DTLSListen command to only accept session
requests from specific WTPs.
Discovery to DTLS Setup (f): 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 which AC to
connect to is the result of the discovery phase, which is
described in Section 3.2.
AC: The AC initiates this transition by invoking the DTLSListen
command (see Section 2.3.2.1), which informs the DTLS stack
that it is willing to listen for an incoming session. The AC
MAY have maintained state information when it received the
Discovery Request message to provide optional qualifiers in the
DTLSListen command to only accept session requests from a
specific WTP. Note that maintaining state information based on
an unsecured Discovery Request message MAY lead to a Denial of
Service attack. Therefore the AC SHOULD ensure that the state
information is freed after a period, which is implementation
specific.
DTLS Setup to Idle (g): This transition occurs when the DTLS Session
failed to be established.
WTP: The WTP 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.
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AC: The WTP 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.
DTLS Setup to Authorize (h): 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 occurs when the AC receives the
DTLSPeerAuthorize notification (see Section 2.3.2.2). Upon
entering this state, the AC performs an authorization check
against the WTP credentials. See Section 2.4.4 for more
information on WTP authorization.
Authorize to DTLS Connect (j): This transition occurs to notify the
DTLS stack that the session should be established.
WTP: This state transition occurs when the WTP has either opted
to forgo the authorization check of the AC's credentials, or
the credentials were successfully authorized. This is done by
invoking the DTLSAccept DTLS command (see Section 2.3.2.1).
AC: This state transition occurs when the AC has either opted to
forgo the authorization check of the WTP's credentials, or the
credentials were successfully authorized. This is done by
invoking the DTLSAccept DTLS command (see Section 2.3.2.1).
Authorize to DTLS Teardown (k): This transition occurs to notify the
DTLS stack that the session should be aborted.
WTP: This state transition occurs when the WTP was 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).
AC: This state transition occurs when the AC was 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).
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DTLS Connect to Idle (m): 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.
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. When this transition occurs due to
the DTLSAuthenticateFail notification, the
FailedDTLSAuthFailCount is incremented, otherwise the
FailedDTLSSessionCount counter is incremented.
DTLS Connect to Join (n): 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 FailedDTLSSessionCount counter is
set to zero.
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, and the WaitJoin timer is started (see
Section 4.6).
Join to DTLS Teardown (p): This transition occurs when the join
process failed.
WTP: This state transition occurs when the WTP receives a Join
Response message with a Result Code message element containing
an error, 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, or if the WaitDTLS timer expires. 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.
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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.
Join to Image Data (r): 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 and the
included 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 transmits the Image
Data Request message (see Section 9.1.1) requesting the start
of the firwware download.
AC: This state transition occurs when the AC receives the Image
Data Request message from the WTP. 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 (q): 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, and determines that the included
Image Identifier message element is the same as its currently
running image. The WTP transmits the Configuration Status
message (see Section 8.2) to the AC with message elements
describing its current configuration. The WTP also starts the
ResponseTimeout timer (see Section 4.6).
AC: This state transition occurs immediately after the AC
transmits the Join Response message to the WTP. If the AC
receives the Configuration Status message from the WTP, the AC
must transmit a Configuration Status Response message (see
Section 8.3) to the WTP, and may include specific message
elements to override the WTP's configuration. The WTP also
starts the ChangeStatePendingTimer timer (see Section 4.6).
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Configure to Reset (s): 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.
WTP: The WTP enters the Reset state when it receives a
Configuration Status Response 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.
Configure to DTLS Teardown (V): 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.
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 WTP MAY tear down the DTLS session if it
receives frequent DTLSDecapFailure notifications.
Image Data to Image Data (4): 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.
AC: This state transition occurs when the AC receives the Image
Data Request message from the WTP while already in the Image
Data state, and it detects that the firmware download has not
completed.
Image Data to Reset (o): This state transition is used to reset the
DTLS connection prior to restarting the WTP after an image
download.
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WTP: When an image download completes, 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 when the image download is
complete, or if an error occurs during the image download
process.
Image Data to DTLS Teardown (x): 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.
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 WTP MAY tear down the DTLS session if it
receives frequent DTLSDecapFailure notifications.
Configure to Data Check (t): 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
initializes the HeartBeat timer (see Section 4.6), and
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 NeighborDeadInterval timer
(see Section 4.6).
Data Check to Run (u): This state transition occurs when the linkage
between the control and data channels has occured, 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 4.6) and transmits a Data Channel Keep Alive packet
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(see Section 4.3.1). The WTP then starts the
DataChannelDeadInterval timer (see Section 4.6).
AC: This state transition occurs when the AC receives the Data
Channel Keep Alive packet (see Section 4.3.1), with a Session
ID message element matching that included by the WTP in the
Join Request message. 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 (u): This state transition occurs when an error
has occured in the DTLS stack, causing the DTLS session to be
torndown.
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 4.6).
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 WTP 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 4.6).
Run to Run (5): This is the normal state of operation.
WTP: This is the WTP's normal state of operation. There are many
events that result 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.
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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.
Clear Config Request: The WTP receives a Clear Configuration
Request message (see Section 8.8). 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).
Data Transfer: The WTP sends a Data Transfer Request message
to the AC (see Section 9.6). The WTP receives a Data
Transfer Response message from the AC (see Section 9.7).
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:
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 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).
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Data Transfer: The AC receives a Data Transfer Request message
from the WTP (see Section 9.6) and MUST generate a
corresponding Data Transfer Response message (see
Section 9.7).
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 (x): This state transition is used when either the AC
or WTP tear 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 (y): This transition occurs when the CAPWAP
reset is complete, to terminate the DTLS session.
WTP: This state transition occurs when the WTP receives a Reset
Response message. This causes the WTP to initiate the
DTLSShutdown command (see Section 2.3.2.1).
AC: This state transition occurs when the AC transmits a Reset
Response message. The AC does not invoke the DTLSShutdown
command (see Section 2.3.2.1).
DTLS Teardown to Idle (z): This transition occurs when the DTLS
session has been shutdown.
WTP: This state transition occurs when the WTP has successfully
cleaned up all resources associated with the control plane DTLS
session. The data plane DTLS session is also shutdown, and all
resources freed, 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 state transition occurs when the AC has successfully
cleaned up all resources associated with the control plane DTLS
session. The data plane DTLS session is also shutdown, and all
resources freed, if a DTLS session was established for the data
plane. Any timers set for the current instance of the state
machine are also cleared.
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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 API:
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.
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. 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
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below occurs, the target CAPWAP state shown in Figure 3 becomes the
current state.
Below is a list of the API notifications:
o DTLSIncomingSession 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
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.
o DTLSAuthenticateFail is sent when DTLS session establishment
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.
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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 entitites; 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 [9]. 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 follows (NOTE: this example uses
certificates, but preshared keys are also supported):
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============ ============
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
DTLS, as specified, provides its own retransmit timers with an
exponential back-off. However, DTLS will never terminate the
handshake due to non-responsiveness; instead, DTLS will continue to
increase its back-off timer period. Hence, timing out incomplete
DTLS handshakes is entirely the responsiblity of the CAPWAP module.
The DTLS implementation used by CAPWAP MUST support TLS Session
Resumption. Session resumption is used to establish the DTLS session
used for the data channel. The DTLS implementation on the WTP MUST
return some unique identifier to the CAPWAP module to enable
subsequent establishment of a DTLS-encrypted data channel, if
necessary.
2.4.2. DTLS Session Establishment
The WTP, either through the Discovery process, or through pre-
configuration, determines the AC to connect to. 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
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WTP sets the WaitDTLS timer. Upon receiving the DTLSIncomingSession
DTLS notification, the AC sets 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 does not respond to any DTLS messages sent by the WTP, the
DTLS specification calls for the WTP to retransmit these messages.
If the WaitDTLS timer expires, CAPWAP will issue the DTLSAbortSession
command, causing DTLS to terminate the handshake and remove any
allocated session context. Note that DTLS MAY send a single TLS
Alert message to the AC to indicate session termination.
If the WTP does not respond to any DTLS messages sent by the AC, the
CAPWAP protocol allows for three possiblities, listed below. Note
that DTLS MAY send a single TLS Alert message to the AC to indicate
session termination.
o The message was lost in transit; in this case, the WTP will re-
transmit its last outstanding message, since it did not receive a
reply.
o The WTP sent a DTLS Alert, which was lost in transit; in this
case, the AC's WaitDTLS timer will expire, and the session will be
terminated.
o Communication with the WTP has completely failed; in this case,
the AC's WaitDTLS timer will expire, and the session will be
terminated.
The DTLS specification provides for retransmission of unacknowledged
requests. If retransmissions remain unacknowledged, the WaitDTLS
timer will eventually expire, at which time the CAPWAP component will
terminate the session.
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, but SHOULD NOT attempt to reply to the WTP.
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
errors, including overlapping and/or lost fragments. The DTLS
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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 DTLS component returns this notification to the CAPWAP component
whenever a transmission request will result in a packet which exceeds
the MTU.
2.4.4. DTLS EndPoint Authentication and Authorization
DTLS supports endpoint authentication with certificates or preshared
keys. The TLS algorithm suites for each endpoint authentication
method are described below.
2.4.4.1. Authenticating with Certificates
Note that only block ciphers are currently recommended for use with
DTLS. To understand the reasoning behind this, see [16]. At
present, the following algorithms MUST be supported when using
certificates for CAPWAP authentication:
o TLS_RSA_WITH_AES_128_CBC_SHA
The following algorithms SHOULD be supported when using certificates:
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o TLS_DH_RSA_WITH_AES_128_CBC_SHA
The following algorithms MAY be supported when using certificates:
o TLS_RSA_WITH_AES_256_CBC_SHA
o TLS_DH_RSA_WITH_AES_256_CBC_SHA
2.4.4.2. Authenticating with Preshared 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 preshared keys are defined
[6], and we focus on the following two:
o 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 alorithm 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
preshared keys:
o TLS_PSK_WITH_AES_128_CBC_SHA
o TLS_DHE_PSK_WITH_AES_128_CBC_SHA
The following algorithms MAY be supported when using preshared keys:
o TLS_PSK_WITH_AES_256_CBC_SHA
o TLS_DHE_PSK_WITH_AES_256_CBC_SHA
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
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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 [4].
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 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 }
For an AC, the id-kp-capwapAC EKU MUST be present in the certificate.
For a WTP, the id-kp-capwapWTP EKU MUST be present in the
certificate.
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.
The certificate common name (CN) for both the WTP and AC MUST be the
MAC address of that device. The MAC address MUST be formatted as
ASCII HEX, e.g. 01:23:45:67:89:ab.
ACs and WTPs SHOULD authorize (e.g. through access control lists)
certificates of devices to which they are connecting, based on the
MAC address and organizational information specified in the O and OU
fields. The identities specified in the certificates bind a
particular DTLS session to a specific pair of mutually-authenticated
and authorized MAC addresses.
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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.
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3. CAPWAP Transport
The CAPWAP protocol uses UDP as a transport protocol, and can be used
with IPv4 or IPv6. This section details the specifics of how the
CAPWAP protocol works with IP.
3.1. UDP Transport
Communication between a WTP and an AC is established according to the
standard UDP client/server model. One of the CAPWAP protocol
requirements is to allow a WTP to reside behind a firewall and/or
Network Address Translation (NAT) device. Since the connection is
initiated by the WTP (client) to the well-known UDP port of the AC
(server), the use of UDP is a logical choice.
CAPWAP protocol control packets sent between the WTP and the AC use
well known UDP port [to be IANA assigned]. CAPWAP protocol data
packets sent between the WTP and the AC use UDP port [to be IANA
assigned].
3.2. AC Discovery
The AC discovery phase allows the WTP to determine which ACs are
available, and chose 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),
a well known multicast address or to the unicast IP address of the
AC. For IPv6 networks, since broadcast does not exist, the use of
"All ACs multicast address" 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.
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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:
DHCP: A comma delimited ASCII encoded list of AC IP addresses is
embedded in DHCP code number TBD. An example of the actual format
of the vendor specific payload for IPv4 is of the form "10.1.1.1,
10.1.1.2".
DNS: The DNS name "CAPWAP-AC-Address" MAY be resolvable to one or
more AC addresses.
An AC MAY also communicate alternative ACs to the WTP within the
Discovery Response message through the AC IPv4 List (see
Section 4.5.2) and AC IPv6 List (see Section 4.5.2). 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 Index message element (see Section 4.5.5), 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 4.5.4) 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.
3.3. 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 "middle box"
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 configuration.
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4. CAPWAP Packet Formats
This section contains the CAPWAP protocol packet formats. A CAPWAP
protocol packet consists of a CAPWAP Transport Layer packet header
followed by a CAPWAP message. The CAPWAP message can be either of
type Control or Data, where Control packets carry 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 | CAPWAP | Control | Message |
| Hdr | Hdr | Preamble| 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. The format of these packets is as follows:
CAPWAP Control Packet (DTLS Security Required):
+-----------------------------------------------------------------+
| IP | UDP | CAPWAP | DTLS | CAPWAP | Control| Message | DTLS |
| Hdr | Hdr | Preamble| 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. The
format of CAPWAP data packets is shown below:
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CAPWAP Plain Text Data Packet :
+-----------------------------------------+
| IP | UDP | CAPWAP | CAPWAP | Wireless |
| Hdr | Hdr | Preamble| Header | Payload |
+-----------------------------------------+
DTLS Secured CAPWAP Data Packet:
+-------------------------------------------------------+
| IP | UDP | CAPWAP | DTLS | CAPWAP | Wireless | DTLS |
| Hdr | Hdr | Preamble| Hdr | Hdr | Payload | Trlr |
+-------------------------------------------------------+
\------ authenticated -----/
\------- encrypted --------/
UDP Header: All CAPWAP packets are encapsulated within UDP. Section
Section 3.1 defines the specific UDP usage.
CAPWAP Preamble: All CAPWAP protocol packets are prefixed with the
CAPWAP Preamble header, used to identify the frame type that
follows. The CAPWAP Preamble header is defined in Section 4.1.
DTLS Header: The DTLS header provides authentication and encrytion
services to the CAPWAP payload it encapsulates. This protocol is
defined in RFC 4347 [9].
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 4.2.
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 4.3.
Control Header: The CAPWAP protocol includes a signalling component,
known as the CAPWAP control protocol. All CAPWAP control packets
include a Control Header, which is defined in Section 4.4.1.
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 4.5.
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
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including the Maximum Message Length message element, see
Section 4.5.29 in the Join Request message or the Join Response
message.
4.1. CAPWAP Preamble
The CAPWAP Preamble header is used to identify the payload type that
immediately follows, to avoid needing to perform byte comparisons to
determine if the packet is DTLS encrypted or not. The format of the
CAPWAP Preamble is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: A 4 bit field which contains the version of CAPWAP used in
this packet. The value for this specification is zero (0).
Payload Type: A 4 bit field which specifies the payload type that
follows the preamble header. The following values are supported:
0 - Clear text. If the packet is received on the data UDP port,
the CAPWAP stack MUST treat the packet as a clear text CAPWAP
data packet. If received on the control UDP port, the CAPWAP
stack MUST treat the packet as a clear text CAPWAP control
packet. If the control packet is not a Discovery Request or
Response packet, the packet MUST be dropped.
1 - DTLS Payload. The packet is a DTLS packet and MAY be a data
or control packet, based on the UDP port it was received on
(see Section 3.1).
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.
4.2. 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.
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The Version field in the CAPWAP header MUST NOT be modified in any
future CAPWAP specifications unless the Version field in the CAPWAP
Preamble Header is modified. The Version field value is used to
parse the CAPWAP headers.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| RID | HLEN | WBID |T|F|L|W|M|K| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment ID | Frag Offset |Rsvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (optional) Radio MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (optional) Wireless Specific Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: A 4 bit field which contains the version of the CAPWAP
protocol used in this packet. The value of this field MUST match
the version field set in the CAPWAP preamble header (see
Section 4.1). The reason for this duplicate field is to avoid any
possible tampering of the version field in the preamble header
which is not encrypted or authenticated.
RID: A 5 bit field which contains the Radio ID number for this
packet. Given that MAC Addresses are not necessarily unique
across physical radios in a WTP, the Radio Identifier (RID) field
is used to indiciate which physical radio the message is
associated with.
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.
WBID: A 5 bit field which is the wireless binding identifier. The
identifier will indicate the type of wireless packet type
associated with the radio. The following values are defined:
1 - IEEE 802.11
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2 - IEEE 802.16
3 - 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 1, the packet is
the last fragment. When this bit is 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 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. This field MUST NOT be set to one in packets
sent by the AC to the WTP.
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 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.
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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 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. This is useful in packets sent
from the WTP to the AC, when the native wireless frame format is
converted to 802.3 by the WTP. 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.
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 number of bytes in the MAC Address field. The length
field is present since some technologies (e.g., IEEE 802.16)
use 64 bit MAC addresses.
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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Wireless ID | Length | Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Wireless ID: The wireless binding identifier. The following
values are defined:
1 - IEEE 802.11
2 - IEEE 802.16
3 - EPCGlobal
Length: The length of the data field
Data: Wireless specific information, defined by the wireless
specific binding.
Payload: This field contains the header for a CAPWAP Data Message or
CAPWAP Control Message, followed by the data contained in the
message.
4.3. 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.
Both CAPWAP data messages are transmitted on the data channel UDP
port.
4.3.1. CAPWAP Data Keepalive
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. 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
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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 4.2)
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 Length field indicates the number of
bytes following the CAPWAP Header.
Message Element[0..N]: The message element(s) carry the information
pertinent to each of the CAPWAP Data Keepalive message. The
following message elements MUST be present in this CAPWAP message:
Session ID, see Section 4.5.35
4.3.2. 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 the bridging 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.
If the encapsulated frame would exceed the transport layer's MTU, the
sender is responsible for fragmentation of the frame, as specified in
Section 3.3.
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4.3.3. 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
MUST be initiated using the TLS session resumption feature [11].
When establishing the DTLS-encrypted data channel, the WTP MUST
provide the identifier returned during the initialization of the
control channel to the DTLS component so it can perform the
resumption using the proper session information.
The AC DTLS implementation MUST NOT accept a session resumption
request for a DTLS session in which the control channel for the
session has been torn down.
4.4. 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.
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 which 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
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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.
4.4.1. Control Message Format
All CAPWAP control messages are sent encapsulated within the CAPWAP
header (see Section 4.2). 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] ...
+-+-+-+-+-+-+-+-+-+-+-+-+
4.4.1.1. Message Type
The Message Type field identifies the function of the CAPWAP control
message. The Message Type field is comprised of an IANA Enterprise
Number and an enterprise specific message type number. The first
three octets contain the enterprise number in network byte order,
with zero used for CAPWAP protocol defined message types and the IEEE
802.11 IANA assigned enterprise number 13277 is used for IEEE 802.11
technology specific 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
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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.
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 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
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4.4.1.2. 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.
4.4.1.3. Message Element Length
The Length field indicates the number of bytes following the Sequence
Number field.
4.4.1.4. Flags
The Flags field MUST be set to zero.
4.4.1.5. 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).
4.4.2. Control Message Quality of Service
It is recommended that CAPWAP control messages be sent by both the AC
and the WTP with an appropriate Quality of Service precedence value,
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ensuring that congestion in the network minimizes occurrences of
CAPWAP control channel disconnects. Therefore, a Quality of Service
enabled CAPWAP device SHOULD use the following values:
802.1P: The precedence value of 7 SHOULD be used.
DSCP: The DSCP tag value of 46 SHOULD be used.
4.4.3. 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 4.4.1).
Response messages are not explicitly acknowledged, therefore if a
Response message is not received, the original Request message is
retransmitted. Implementations MAY cache Response messages to
respond to a retransmitted Request messages with minimal local
processing. Retransmitted Request messages MUST NOT be altered by
the sender. The sender MUST assume that the original Request message
was processed, but that the Response message was lost. Any
alterations to the original Request message MUST have a new Sequence
Number, and be treated as a new Request message by the receiver.
After transmitting a Request message, the RetransmitInterval (see
Section 4.6) timer and MaxRetransmit (see Section 4.7) variable are
used to determine if the original Request message needs to be
retransmitted. Response messages are not subject to these timers.
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.
4.5. CAPWAP Protocol Message Elements
This section defines the CAPWAP Protocol message elements which are
included in CAPWAP protocol control messages.
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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 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.
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.
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. Type field values are allocated as follows:
Usage Type Values
CAPWAP Protocol Message Elements 1-1023
IEEE 802.11 Message Elements 1024-2047
IEEE 802.16 Message Elements 2048 - 3071
EPCGlobal Message Elements 3072 - 4095
Reserved for Future Use 4096 - 65024
The table below lists the CAPWAP protocol Message Elements and their
Type values.
CAPWAP Message Element Type Value
AC Descriptor 1
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AC IPv4 List 2
AC IPv6 List 3
AC Name 4
AC Name with Index 5
AC Timestamp 6
Add MAC ACL Entry 7
Add Station 8
Add Static MAC ACL Entry 9
CAPWAP Control IPV4 Address 10
CAPWAP Control IPV6 Address 11
CAPWAP Timers 12
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
Delete Static MAC ACL Entry 19
Discovery Type 20
Duplicate IPv4 Address 21
Duplicate IPv6 Address 22
Idle Timeout 23
Image Data 24
Image Identifier 25
Image Info 26
Initiate Download 27
Location Data 28
Maximum Message Length 29
MTU Discovery Padding 30
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
WTP IPv4 IP Address 42
WTP MAC Type 43
WTP Name 44
WTP Operational Statistics 45
WTP Radio Statistics 46
WTP Reboot Statistics 47
WTP Static IP Address Information 48
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4.5.1. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=4 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=5 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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: A 8 bit bit mask specifying the authentication credential
type supported by the AC. The following values are supported (see
Section 2.4.4):
1 - X.509 Certificate Based
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2 - Pre-Shared Secret
R-MAC Field: The AC supports the optional Radio MAC Address field
in the CAPWAP transport Header (see Section 4.2).
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
following bit field values are supported:
1 - Clear Text Data Channel Supported
2 - DTLS Enabled Data Channel Supported
Vendor Identifier: A 32-bit value containing the IANA assigned "SMI
Network Management Private Enterprise Codes"
Type: Vendor specific encoding of AC information. The following
values are supported. The Hardware and Software Version values
MUST be included.
4 - Hardware Version: The AC's hardware version number.
5 - Software Version: The AC's Software (firmware) version
number.
Length: Length of vendor specific encoding of AC information.
Value: Vendor specific encoding of AC information.
4.5.2. 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[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type: 2 for AC IPv4 List
Length: >= 4
The AC IP Address: An array of 32-bit integers containing AC IPv4
Addresses.
4.5.3. 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[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 3 for AC IPV6 List
Length: >= 16
The AC IP Address: An array of 128-bit integers containing AC IPv6
Addresses.
4.5.4. AC Name
The AC Name message element contains an UTF-8 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 ...
+-+-+-+-+-+-+-+-+
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Type: 4 for AC Name
Length: > 0
Name: A variable length UTF-8 encoded string containing the AC's
name
4.5.5. AC Name with Index
The AC Name with Index 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.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index | AC Name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 5 for AC Name with Index
Length: > 2
Index: The index of the preferred server (1=primary, 2=secondary).
AC Name: A variable length UTF-8 encoded string containing the AC
name.
4.5.6. 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 [3].
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4.5.7. 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.
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| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 7 for Add MAC ACL Entry
Length: >= 7
Num of Entries: The number of MAC Addresses in the array.
MAC Address: An array of MAC Addresses to add to the ACL.
4.5.8. 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 | MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address | VLAN Name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type: 8 for Add Station
Length: >= 7
Radio ID: An 8-bit value representing the radio
MAC Address: The station's MAC Address
VLAN Name: An optional variable length UTF-8 encoded string
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.
4.5.9. Add Static MAC ACL Entry
The Add Static MAC ACL Entry message element is used by an AC to add
a permanent ACL entry on a WTP, ensuring that the WTP no longer
provides any service to the MAC addresses provided in the message.
The MAC Addresses provided in this message element are expected to be
saved in non-volative memory 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num of Entries| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 9 for Add Static MAC ACL Entry
Length: >= 7
Num of Entries: The number of MAC Addresses in the array.
MAC Address: An array of MAC Addresses to add to the permanent ACL.
4.5.10. 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.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
4.5.11. 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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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4.5.12. 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.
Echo Request: The number of seconds between WTP Echo Request CAPWAP
messages. The default value for this message element is specified
in Section 4.6.7.
4.5.13. 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
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Type | Data Length | Data ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 13 for Data Transfer Data
Length: >= 3
Data Type: An 8-bit value the type of information being sent. The
following values are supported:
1 - WTP Crash Data
2 - WTP Memory Dump
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Data Length: Length of data field.
Data: Debug information.
4.5.14. 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 Type |
+-+-+-+-+-+-+-+-+
Type: 14 for Data Transfer Mode
Length: 1
Data Type: An 8-bit value the type of information being requested.
The following values are supported:
1 - WTP Crash Data
2 - WTP Memory Dump
4.5.15. 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 | Station MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Station MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 15 for Decryption Error Report
Length: >= 8
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Radio ID: The Radio Identifier refers to an interface index on the
WTP.
Num Of Entries: An 8-bit unsigned integer indicating the number of
station MAC addresses.
Station MAC Address: An array of station MAC addresses that have
caused decryption errors.
4.5.16. 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.
Report Interval: A 16-bit unsigned integer indicating the time, in
seconds. The default value for this message element can be found
in Section 4.7.8.
4.5.17. 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| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type: 17 for Delete MAC ACL Entry
Length: >= 7
Num of Entries: The number of MAC Addresses in the array.
MAC Address: An array of MAC Addresses to delete from the ACL.
4.5.18. 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 | MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 18 for Delete Station
Length: 7
Radio ID: An 8-bit value representing the radio
MAC Address: The station's MAC Address
4.5.19. Delete Static MAC ACL Entry
The Delete Static MAC ACL Entry message element is used by an AC to
delete a previously added static MAC ACL entry on a WTP, ensuring
that the WTP provides service to the MAC addresses provided in the
message.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num of Entries| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 19 for Delete Static MAC ACL Entry
Length: >= 7
Num of Entries: The number of MAC Addresses in the array.
MAC Address: An array of MAC Addresses to delete from the static
MAC ACL entry.
4.5.20. 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.
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 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)
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4.5.21. 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 MUSY 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 21 for Duplicate IPv4 Address
Length: 11
IP Address: The IP Address currently used by the WTP.
MAC Address: The MAC Address of the offending device.
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.
4.5.22. 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.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 23 for Duplicate IPv6 Address
Length: 22
IP Address: The IP Address currently used by the WTP.
MAC Address: The MAC Address of the offending device.
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.
4.5.23. 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 to be enforced by the WTP. The
default value for this message element is specified in
Section 4.7.5.
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4.5.24. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 24 for Image Data
Length: >= 1
Opcode: An 8-bit value representing the transfer opcode. The
following values are supported:
1 - Image data is included
2 - Last Image Data Block is included (EOF)
5 - An error occurred. Transfer is aborted
Value: The Image Data field contains up to 1024 characters. If the
block being sent is the last one, the Opcode is set to 2. The AC
MAY opt to abort the data transfer by setting the Opcode to 5.
When the Opcode is 5, the Value field has a zero length.
4.5.25. Image Identifier
The Image Identifier message element is sent by the AC to the WTP and
is used to indicate the expected active software version that is to
be run on the WTP. The value is a variable length UTF-8 encoded
string, 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type: 25 for Image Identifier
Length: >= 1
Value: A variable length UTF-8 encoded string containing the
firmware identifier to be run on the WTP.
4.5.26. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 26 for Image Information
Length: 18
File Size: A 16-bit value containing the size of the file that will
be transfered by the AC to the WTP.
Hash: A 16 octet hash of the image. The hash is computed using
MD5, using the following pseudo-code:
#include <md5.h>
CapwapCreateHash(char *hash, char *image, int image_len)
{
MD_CTX context;
MDInit (&context);
MDUpdate (&context, buffer, len);
MDFinal (hash, &context);
}
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4.5.27. Initiate Download
The Initiate Download message element is used by the AC to inform the
WTP that the WTP should initiate a firmware upgrade. The WTP
subsequently transmits an Image Data Request message which includes
the Image Download message element. This message element does not
contain any data.
Type: 27 for Initiate Download
Length: 0
4.5.28. Location Data
The Location Data message element is a variable length byte UTF-8
encoded string 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: > 0
Location: A non-zero terminated UTF-8 encoded string containing the
WTP location.
4.5.29. 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 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Maximum Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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Type: 29 for Maximim Message Length
Length: 2
Maximum Message Length An 16-bit unsigned integer indicating the
maximum message length.
4.5.30. 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: 30 for MTU Discovery Padding
Length: variable
Pad: A variable length pad.
4.5.31. 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 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
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Length: 2
Radio ID: An 8-bit value representing the radio to configure. 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 4.7.1. The following values are supported:
1 - Enabled
2 - Disabled
4.5.32. 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. 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.
A value of one disables the radio, while a value of two enables
it.
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Cause: When a radio is inoperable, the cause field contains the
reason the radio is out of service. The following values are
supported:
0 - Normal
1 - Radio Failure
2 - Software Failure
3 - Administratively Set
4.5.33. 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 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)
6 Join Failure (Incorrect Data)
7 Join Failure (Session ID already in use)
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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
4.5.34. 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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason | Message Element...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type: 34 for Returned Message Element
Length: >= 1
Reason: The reason why the configuration in the offending message
element could not be applied by the WTP.
1 - Unknown Message Element
2 - Unsupported Message Element
3 - Unknown Message Element Value
4 - Unsupported Message Element Value
Message Element: The Message Element field encapsulates the message
element sent by the AC in the Configuration Status Response
message that caused the error.
4.5.35. Session ID
The Session ID message element value contains a randomly generated
unsigned 32-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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 35 for Session ID
Length: 16
Session ID: A 32-bit unsigned integer used as a random session
identifier
4.5.36. 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 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 4.6.15.
4.5.37. Vendor Specific Payload
The Vendor Specific Payload message element is used to communicate
vendor specific information between the WTP and the AC. 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 | Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 37 for Vendor Specific
Length: >= 7
Vendor Identifier: A 32-bit value containing the IANA assigned "SMI
Network Management Private Enterprise Codes" [15]
Element ID: A 16-bit Element Identifier which is managed by the
vendor.
Value: The value associated with the vendor specific element.
4.5.38. WTP Board Data
The WTP Board Data message element is sent by the WTP to the AC and
contains information about the hardware present.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional additional vendor specific WTP board data TLVs.....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 38 for WTP Board Data
Length: >=14
Vendor Identifier: A 32-bit value containing the IANA assigned "SMI
Network Management Private Enterprise Codes"
Type: The following values are supported:
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 mesage element.
3 - Board Revision A revision number of the board, which MAY be
included in the WTP Board Data message element.
4 - Base MAC Addres The WTP's Base MAC Address, which MAY be
assigned to the primary Ethernet interface.
4.5.39. 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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Radios | Radios in use | Encryption Capabilities |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 39 for WTP Descriptor
Length: >= 31
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.
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.
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Vendor Identifier: A 32-bit value containing the IANA assigned "SMI
Network Management Private Enterprise Codes".
Type: The following values are supported. The Hardware Version,
Active Software Version, and Boot Version values MUST be included.
Zero or more Other Software Version values MAY be included.
0 - Hardware Version: The WTP hardware version number.
1 - Active Software Version: The WTP running software version
number.
2 - Boot Version: The WTP boot loader version number.
3 - Other Software Version: The WTP non-running software
(firmware) version number.
Length: Length of vendor specific encoding of WTP information.
Value: Vendor specific data of WTP information encoded in the UTF-8
format.
4.5.40. 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, and is not currently connected to it.
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 4.7.10. The following values
are supported:
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1 - Enabled
2 - Disabled
4.5.41. WTP Frame Tunnel Mode
The WTP Frame Tunnel Mode message element allows the WTP to
communicate the tunneling modes of operation which 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
+-+-+-+-+-+-+-+-+
| Tunnel Mode |
+-+-+-+-+-+-+-+-+
Type: 41 for WTP Frame Tunnel Mode
Length: 1
Frame Tunnel Mode: The Frame Tunnel mode specifies the tunneling
modes for station data that are supported by the WTP. The
following values are supported:
1 - Local Bridging: 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.
2 - 802.3 Frame Tunnel Mode: The 802.3 Frame Tunnel Mode
requires the WTP and AC to encapsulate all user payload as
native IEEE 802.3 frames (see Section 4.3). All user traffic
is tunneled to the AC. This value MUST NOT be used when the
WTP MAC Type is set to Split-MAC.
4 - Native Frame Tunnel Mode: 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 4.3).
7 - All: The WTP is capable of supporting all frame tunnel
modes.
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4.5.42. WTP IPv4 IP Address
The WTP IPv4 address is used to perform NAT detection.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| WTP IPv4 IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 42 for WTP IPv4 IP Address
Length: 4
WTP IPv4 IP Address: The IPv4 address from which the WTP is sending
packets. This field is used for NAT detection.
4.5.43. 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: 43 for WTP MAC Type
Length: 1
MAC Type: The MAC mode of operation supported by the WTP. The
following values are supported
0 - Local-MAC: Local-MAC is the default mode that MUST be
supported by all WTPs.
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.
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4.5.44. WTP Name
The WTP Name message element is a variable length byte UTF-8 encoded
string. The string is not zero terminated.
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-
| WTP Name ...
+-+-+-+-+-+-+-+-+-
Type: 44 for WTP Name
Length: variable
WTP Name: A non-zero terminated UTF-8 encoded string containing the
WTP name.
4.5.45. WTP Operational Statistics
The WTP Operational Statistics message element is sent by the WTP to
the AC to provide statistics related to the operation of 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Radio ID | Tx Queue Level | Wireless Link Frames per Sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 45 for WTP Operational Statistics
Length: 4
Radio ID: The radio ID of the radio to which the statistics apply.
Wireless Transmit Queue Level: The percentage of Wireless Transmit
queue utilization, calculated as the sum of utilized transmit
queue lengths divided by the sum of maximum transmit queue
lengths, multiplied by 100. The Wireless Transmit Queue Level is
representative of congestion conditions over wireless interfaces
between the WTP and stations.
Wireless Link Frames per Sec: The number of frames transmitted or
received per second by the WTP over the air interface.
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4.5.46. 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.
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: 46 for WTP Radio Statistics
Length: 20
Radio ID: The radio ID of the radio to which the statistics apply.
Last Failure Type: The last WTP failure. The following 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 that the radio has been
reset.
SW Failure Count: The number of times that the radio has failed due
to software related reasons.
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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.
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 which indicates the noise
floor of the radio receiver in units of dBm.
4.5.47. 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.
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: 47 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.
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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.
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 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)
4.5.48. 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.
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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: 48 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.
Netmask: The IP Netmask. This field is only valid if the static
field is set to one.
Static: An 8-bit boolean stating whether the WTP should use a
static IP address or not. A value of zero disables the static IP
address, while a value of one enables it.
4.6. CAPWAP Protocol Timers
This section contains the CAPWAP timers.
4.6.1. 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. The default value is 25
seconds.
4.6.2. DataChannelKeepAlive
The minimum time, in seconds, between sending Data Channel Keep Alive
packets to the AC that the WTP has joined. The default value is 30
seconds.
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4.6.3. 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: 5
4.6.4. DiscoveryInterval
The minimum time, in seconds, that a WTP MUST wait after receiving a
Discovery Response message, before initiating a DTLS handshake.
Default: 5
4.6.5. DTLSRehandshake
The minimum time, in seconds, a WTP MUST wait for DTLS rehandshake to
complete.
Default: 10
4.6.6. DTLSSessionDelete
The minimum time, in seconds, a WTP MUST wait for DTLS session
deletion.
Default: 5
4.6.7. EchoInterval
The minimum time, in seconds, between sending Echo Request messages
to the AC with which the WTP has joined.
Default: 30
4.6.8. KeyLifetime
The maximum time, in seconds, which a CAPWAP DTLS session key is
valid.
Default: 28800
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4.6.9. 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.
4.6.10. MaxFailedDTLSSessionRetry
The maximum number of failed DTLS session establishment attempts
before the CAPWAP device enters a silent period.
Default: 3.
4.6.11. NeighborDeadInterval
The minimum time, in seconds, a WTP MUST wait without having received
an Echo Response message to its Echo Request message, before the
destination for the Echo Request may be considered dead. This value
MUST be no less than 2*EchoInterval seconds and no greater than 240
seconds.
Default: 60
4.6.12. ResponseTimeout
The minimum time, in seconds, in which the WTP or AC must respond to
a CAPWAP Request message.
Default: 1
4.6.13. RetransmitInterval
The minimum time, in seconds, in which a non-acknowledged CAPWAP
packet will be retransmitted.
Default: 3
4.6.14. 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 a
establish 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
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4.6.15. StatisticsTimer
The default Statistics Interval is 120 seconds.
4.6.16. 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
4.6.17. WaitJoin
The maximum time, in seconds, after which the DTLS session has been
established that the AC will wait before receiving a Join Request
message. This timer must be greater than 30 seconds.
Default: 60
4.7. CAPWAP Protocol Variables
A WTP or AC that implements the CAPWAP Discovery phase MUST allow for
the following variables to be configured by system management;
default values are specified, making explicit configuration
unnecessary in many cases. If the default values are explicitly
overriden by the AC, the WTP MUST save the values sent by the AC.
4.7.1. AdminState
The default Administrative State value is enabled (1).
4.7.2. DiscoveryCount
The number of Discovery Request messages transmitted by a WTP to a
single AC. This is a monotonically increasing counter.
4.7.3. FailedDTLSAuthFailCount
The number of failed DTLS session establishment attempts due to
authentication failures.
4.7.4. FailedDTLSSessionCount
The number of failed DTLS session establishment attempts.
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4.7.5. IdleTimeout
The default Idle Timeout is 300 seconds.
4.7.6. MaxDiscoveries
The maximum number of Discovery Request messages that will be sent
after a WTP boots.
Default: 10
4.7.7. MaxRetransmit
The maximum number of retransmissions for a given CAPWAP packet
before the link layer considers the peer dead.
Default: 5
4.7.8. ReportInterval
The default Report Interval is 120 seconds.
4.7.9. RetransmitCount
The number of retransmissions for a given CAPWAP packet. This is a
monotonically increasing counter.
4.7.10. WTPFallBack
The default WTP Fallback value is enabled (1).
4.8. WTP Saved Variables
In addition to the values defined in Section 4.7, 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.
4.8.1. AdminRebootCount
The number of times the WTP has rebooted administratively, defined in
Section 4.5.47.
4.8.2. 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 4.5.41.
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4.8.3. LastRebootReason
The reason why the WTP last rebooted, defined in Section 4.5.47.
4.8.4. MacType
For WTPs that support multiple MAC Types, it is useful to save the
value configured by the AC. The MACType is defined in
Section 4.5.43.
4.8.5. PreferredACs
The preferred ACs, with the index, defined in Section 4.5.5.
4.8.6. RebootCount
The number of times the WTP has rebooted, defined in Section 4.5.47.
4.8.7. Static ACL Table
The static ACL table saved on the WTP, as configured by the Add
Static MAC ACL Entry message element, see Section 4.5.9.
4.8.8. Static IP Address
The static IP Address assigned to the WTP, as configured by the WTP
Static IP Address Information message element (see Section 4.5.48).
4.8.9. WTPLinkFailureCount
The number of times the link to the AC has failed, see
Section 4.5.47.
4.8.10. WTPLocation
The WTP Location, defined in Section 4.5.28.
4.8.11. WTPName
The WTP Name, defined in Section 4.5.44.
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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
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.
Discovery Request messages MUST be sent by a WTP when no Echo
Response messages are received for NeighborDeadInterval and the WTP
returns to the Idle state. Discovery Request messages are sent after
NeighborDeadInterval. They MUST be sent after waiting for a random
delay less than MaxDiscoveryInterval. A WTP MAY send up to a maximum
of MaxDiscoveries Discovery Request messages, waiting for a random
delay less than MaxDiscoveryInterval between each successive message.
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.
It is possible for the AC to receive a cleartext 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,
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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 4.5.20
o WTP Board Data, see Section 4.5.38
o WTP Descriptor, see Section 4.5.39
o WTP Frame Tunnel Mode, see Section 4.5.41
o WTP MAC Type, see Section 4.5.43
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).
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
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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:
o AC Descriptor, see Section 4.5.1
o AC Name, see Section 4.5.4
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 4.5.10
* CAPWAP Control IPv6 Address, see Section 4.5.11
5.3. Primary Discovery Request Message
The Primary Discovery Request message is sent by the WTP to determine
whether its preferred (or primary) AC is available.
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.
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o Discovery Type, see Section 4.5.20
o WTP Board Data, see Section 4.5.38
o WTP Descriptor, see Section 4.5.39
o WTP Frame Tunnel Mode, see Section 4.5.41
o WTP MAC Type, see Section 4.5.43
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).
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 4.5.1
o AC Name, see Section 4.5.4
o CAPWAP Control IPv4 Address, see Section 4.5.10
o CAPWAP Control IPv6 Address, see Section 4.5.11
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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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 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. A Join Request message is sent by a WTP after (optionally)
receiving one or more Discovery Response messages, and completion of
DTLS session establishment. When an AC receives a Join Request
message it responds with a Join Response message.
Upon completion of the DTLS handshake, and receiving 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.
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 4.5.28
o WTP Board Data, see Section 4.5.38
o WTP Descriptor, see Section 4.5.39
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o WTP IPv4 IP Address, see Section 4.5.42
o WTP Name, see Section 4.5.44
o Session ID, see Section 4.5.35
o WTP Frame Tunnel Mode, see Section 4.5.41
o WTP MAC Type, see Section 4.5.43
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 element MAY be included in the Join Request
message.
o Maximum Message Length, see Section 4.5.29
o WTP Reboot Statistics, see Section 4.5.47
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) attempts 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 which 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 either change its currently running
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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 MAY be included in the Join Response
message.
o AC IPv4 List, see Section 4.5.2
o AC IPv6 List, see Section 4.5.3
o Image Identifier, see Section 4.5.25
o Maximum Message Length, see Section 4.5.29
The following message elements MUST be included in the Join Response
message.
o Result Code, see Section 4.5.33
o AC Descriptor, see Section 4.5.1
o AC Name, see Section 4.5.4
o CAPWAP Control IPv4 Address, see Section 4.5.10
o CAPWAP Control IPv6 Address, see Section 4.5.11
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).
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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.
Echo Request messages are sent periodically by a WTP in the 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 Heartbeat timer expires. The WTP MUST start its
NeighborDeadInterval timer when the Heartbeat timer expires.
The Echo Request message is sent by the WTP when in the Run State.
The AC does not transmit this message.
The Echo Request message carries no message elements.
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 Heartbeat timer to expire in the value configured
for EchoInterval. 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 Echo Response message carries no message elements.
When a WTP receives an Echo Response message it stops the
NeighborDeadInterval timer, and initializes the Heartbeat timer to
the EchoInterval.
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If the NeighborDeadInterval timer expires prior to receiving an Echo
Response message, or other control message, the WTP enters the Idle
state.
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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 has two options:
1. The WTP retains no configuration and accepts the configuration
provided by the AC.
2. The WTP retains the configuration of parameters provided by the AC
that are non-default values.
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 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.
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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
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
The Configuration Status message is sent by a WTP to deliver its
current configuration to the AC.
The Configuration Status message carries binding specific message
elements. Refer to the appropriate binding for the definition of
this structure.
When an AC receives a Configuration Status message it acts upon the
content of the message and responds to the WTP with a Configuration
Status Response message.
The Configuration Status message includes multiple Radio
Administrative State message elements, one for the WTP, and one for
each radio in the WTP.
The Configuration Status 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 message.
o AC Name, see Section 4.5.4
o AC Name with Index, see Section 4.5.5
o Radio Administrative State, see Section 4.5.31
o Statistics Timer, see Section 4.5.36
o WTP Reboot Statistics, see Section 4.5.47
The following message elements MAY be included in the Configuration
Status message.
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o WTP Static IP Address Information, see Section 4.5.48
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 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 AC IPv4 List, see Section 4.5.2
o AC IPv6 List, see Section 4.5.3
o CAPWAP Timers, see Section 4.5.12
o Decryption Error Report Period, see Section 4.5.16
o Idle Timeout, see Section 4.5.23
o WTP Fallback, see Section 4.5.40
The following message element MAY be included in the Configuration
Status Response message.
o WTP Static IP Address Information, see Section 4.5.48
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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 and Initiate Download message
elements 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 (see Section 9.1.1).
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 Index, see Section 4.5.5
o AC Timestamp, see Section 4.5.6
o Add MAC ACL Entry, see Section 4.5.7
o Add Static MAC ACL Entry, see Section 4.5.9
o CAPWAP Timers, see Section 4.5.12
o Decryption Error Report Period, see Section 4.5.16
o Delete MAC ACL Entry, see Section 4.5.17
o Delete Static MAC ACL Entry, see Section 4.5.19
o Idle Timeout, see Section 4.5.23
o Location Data, see Section 4.5.28
o Radio Administrative State, see Section 4.5.31
o Statistics Timer, see Section 4.5.36
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o WTP Fallback, see Section 4.5.40
o WTP Name, see Section 4.5.44
o WTP Static IP Address Information, see Section 4.5.48
o Image Identifier, see Section 4.5.25
o Initiate Download, see Section 4.5.27
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 4.5.33
The following message elements MAY be present in the Configuration
Update Response message.
o Radio Operational State, see Section 4.5.32
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
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the WTP's radio operational state.
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 specfic configuration
request, it indicates the failure by including one or more Returned
Message Element message elements (see Section 4.5.34).
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 4.5.32
o Result Code, see Section 4.5.33
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 4.5.34
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.
The Change State Event Response message carries no message elements.
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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 Clear Configuration Request message carries no message elements.
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 Request message MUST include the following
message element.
o Result Code, see Section 4.5.33
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9. Device Management Operations
This section defines CAPWAP operations responsible for debugging,
gathering statistics, logging, and firmware management.
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.
Figure 4 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.
WTP AC
Join Request
-------------------------------------------------------->
Join Response (Image Identifier = x)
<------------------------------------------------------
Image Data Request (Image Identifier = x)
-------------------------------------------------------->
Image Data Response (Result Code = Success,
Image Information = {size,hash},
Initiate Download)
<------------------------------------------------------
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)
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Figure 4: WTP Firmware Download Case 1
Figure 5 provides an example in which the WTP has the image specified
by the AC in its non-volative storage. The WTP opts to NOT download
the firmware and immediately reset.
WTP AC
Join Request
-------------------------------------------------------->
Join Response (Image Identifier = x)
<------------------------------------------------------
(WTP enters the Reset State)
Figure 5: WTP Firmware Download Case 2
Figure 6 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.
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WTP AC
Configuration Update Request (Image Identifier = x)
<------------------------------------------------------
Configuration Update Response (Result Code = Success)
-------------------------------------------------------->
Image Data Request (Image Identifier = x)
-------------------------------------------------------->
Image Data Response (Result Code = Success,
Image Information = {size,hash},
Initiate Download)
<------------------------------------------------------
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 6: WTP Firmware Download Case 3
Figure 7 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-volative storage. The WTP opts to NOT
download the firmware. The WTP resets upon receipt of a Reset
Request message from the AC.
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WTP AC
Configuration Update Request (Image Identifier = x,
Image Information = {size,hash},
Initiate Download)
<------------------------------------------------------
Configuration Update Response (Result Code = Already Have Image)
-------------------------------------------------------->
.....
(administratively requested reboot request)
Reset Request (Image Identifier = x)
<------------------------------------------------------
Reset Response (Result Code = Success)
-------------------------------------------------------->
Figure 7: 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, it simply resets.
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 Initiate Download
and and Image Identifier message elements. The WTP then transmits
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the Image Data Request message, which includes the Image
Identifier message element to start the download process. 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 transfered.
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 Image Data, see Section 4.5.24
o Image Identifier, see Section 4.5.25
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.
The following message element MUST be included in the Image Data
Response message.
o Result Code, see Section 4.5.33
The following message elements MAY be included in the Image Data
Response message.
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o Image Information, see Section 4.5.26
o Initiate Download, see Section 4.5.27
Upon receiving an Image Data Response message indicating an error,
the WTP MAY retransmit a previous Image Data Reqest 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.
The Reset Request is sent by the AC when in the Run State. The WTP
does not transmit this message.
The following message elements MUST be included in the Reset Request
message.
o Image Identifier, see Section 4.5.25
When a WTP receives a Reset Request message, it responds with a Reset
Response message indicating success and then reinitialize 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,
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 element MAY be included in the Image Data
Request message.
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o Result Code, see Section 4.5.33
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 4.5.23), due to 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.
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 messsage element
listed may be included in the WTP Event Request message.
o Decryption Error Report, see Section 4.5.15
o Duplicate IPv4 Address, see Section 4.5.21
o Duplicate IPv6 Address, see Section 4.5.22
o WTP Operational Statistics, see Section 4.5.45
o WTP Radio Statistics, see Section 4.5.46
o WTP Reboot Statistics, see Section 4.5.47
o Delete Station, see Section 4.5.18
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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 WTP Event Response message carries no message elements.
9.6. Data Transfer Request
The Data Transfer Request message is used to deliver debug
information from the WTP to the AC.
Data Transfer Request messages are sent by the WTP to the AC when the
WTP determines that it has important information to send to the AC.
For instance, if the WTP detects that its previous reboot was caused
by a system crash, it can send the crash file to the AC. The remote
debugger function in the WTP also uses the Data Transfer Request
message to send console output to the AC for debugging purposes.
When the 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.
The Data Transfer Request message is sent by the WTP when in the Run
State. The AC does not transmit this message.
The Data Transfer Request message MUST contain one of the message
elements listed below.
o Data Transfer Data, see Section 4.5.13
o Data Transfer Mode, see Section 4.5.14
9.7. 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.
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The Data Transfer Response message is sent by the AC when in the Run
State. The WTP does not transmit this message.
The Data Transfer Response message carries no message elements.
Upon receipt of a Data Transfer Response message, the WTP transmits
more information, if more information is available.
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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.
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 4.5.8
o Delete Station, see Section 4.5.18
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 4.5.33
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.
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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 cannot differentiate the originating WTP of
the connection requests. The CAPWAP Data Check state, which
establishes the data plane connection and communicates the Data
Keepalive, 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).
In the third configuration, the AC is deployed behind a NAT. Two
issues exist in this situation. First, an AC communicates its
interfaces and corresponding WTP load using the CAPWAP Control
IP(v4/v6) Address message element. This message element is currently
mandatory, and if NAT compliance becomes an issue, it is possible to
either:
1. Make the CAPWAP Control IP (v4/v6) Address optional, allowing the
WTP to use the known IP Address. Note that this approach
eliminates the ability to perform load balancing of WTP across
ACs, and therefore is not the recommended approach.
2. Allow an AC to configure a NAT'ed address for every AC that would
otherwise be communicated in the CAPWAP Control IP (v4/v6) Address
message element.
3. Require that if a WTP determines that the AC List message element
contains a set of IP Addresses that are different from the AC IP
Address the WTP is currently using, then assume that NAT is
present, and require that the WTP communicate with the AC IP
Address (and ignore the CAPWAP Control IP (v4/v6) Address message
element(s)).
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 disabled when the AC is behind a NAT and the IP
Address that is embedded is invalid.
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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.
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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 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)
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 un-
protected data and attempting to convert un-protected data into
protected data. These concerns are addressed below.
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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 MiM can 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.
12.2. 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
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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 64-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.
12.3. Discovery Attacks
Since the Discovery Request messages are sent in the clear, it is
important that AC implementations NOT assume that receiving such a
request from a WTP implies that it 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.
12.4. Interference with a DTLS Session
If a WTP or AC repeatedly receives packets which 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.
In the state machine (section 2.3), transitions to the DTLS tear down
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. Use of Preshared Keys in CAPWAP
While use of preshared keys may provide deployment and provisioning
advantages not found in public key based deployments, it also
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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 preshared 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 preshared-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 1750 [2]
into account.
o Preshared 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
unqiue PSK. This prevents the domino effect (see Guidance for AAA
Key Management [14]). 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.
12.6. 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.
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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
the peer certificate itself. At a minimum, devices SHOULD use SSH-
style certificate caching to guarantee consistency. If devices have
access to a certificate authority, they SHOULD properly validate the
trust chain. 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 OCSP server or otherwise obtain
an up to date 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.
12.7. AAA Security
The AAA protocol is used to distribute EAP keys to the ACs, and
consequently its security is important to the overall system
security. When used with TLS or IPsec, security guidelines specified
in RFC 3539 [5] 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.
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13. Management 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 CLI or SNMP, could lead to the AC state being out of sync
with the WTP.
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14. IANA Considerations
A separate UDP port for data channel communications is (currently)
the selected demultiplexing mechanism, and a port must be assigned
for this purpose.
IANA needs to assign a DHCP code point, currently identified as TBD
in the Section 3.2. DHCP options are defined in RFC 1533 [10], and
are listed by IANA at
http://www.iana.org/assignments/bootp-dhcp-parameters.
IANA needs to assign an organization local multicast address called
the "All ACs multicast address" from the IPv6 multicast address
registry in Section 3.2
14.1. CAPWAP Message Types
The Message Type field in the CAPWAP header (Section 4.4.1.1) is used
to identify the operation performed by the message. There are
multiple namespaces, which is identified via the first three octets
of the field containing the IANA Enterprise Number [12]. When the
Enterprise Number is set to zero, the message types are reserved for
use by the base CAPWAP specification which are controlled and
maintained by IANA.
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15. Acknowledgements
The following individuals are acknowledged for their contributions to
this protocol specification: Puneet Agarwal, Saravanan Govindan,
Peter Nilsson, and David Perkins.
Michael Vakulenko contributed text to describe how CAPWAP can be used
over layer 3 (IP/UDP) networks.
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16. References
16.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[3] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305, March 1992.
[4] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[5] Aboba, B. and J. Wood, "Authentication, Authorization and
Accounting (AAA) Transport Profile", RFC 3539, June 2003.
[6] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for
Transport Layer Security (TLS)", RFC 4279, December 2005.
[7] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.1", RFC 4346, April 2006.
[8] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[9] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[10] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 1533, October 1993.
[11] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[12] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
16.2. Informational References
[13] "draft-ietf-capwap-protocol-binding-specification-ieee802dot11-
02".
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16.3. Informational References
[14] "draft-housley-aaa-key-mgmt-06".
[15] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-
line Database", RFC 3232, January 2002.
[16] Modadugu et al, N., "The Design and Implementation of Datagram
TLS", Feb 2004.
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Editors' Addresses
Pat R. Calhoun
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Phone: +1 408-853-5269
Email: pcalhoun@cisco.com
Michael P. Montemurro
Research In Motion
5090 Commerce Blvd
Mississauga, ON L4W 5M4
Canada
Phone: +1 905-629-4746 x4999
Email: mmontemurro@rim.com
Dorothy Stanley
Aruba Networks
1322 Crossman Ave
Sunnyvale, CA 94089
Phone: +1 630-363-1389
Email: dstanley@arubanetworks.com
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