Network Working Group P. Calhoun, Editor
Internet-Draft Cisco Systems, Inc.
Expires: July 27, 2007 M. Montemurro, Editor
Research In Motion
D. Stanley, Editor
Aruba Networks
January 23, 2007
CAPWAP Protocol Specification
draft-ietf-capwap-protocol-specification-04
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Internet-Draft CAPWAP Protocol Specification January 2007
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 [12].
Extensions are expected to be defined to enable use of the CAPWAP
protocol with additional wireless technologies.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2. Conventions used in this document . . . . . . . . . . . . 7
1.3. Contributing Authors . . . . . . . . . . . . . . . . . . 8
1.4. Acknowledgements . . . . . . . . . . . . . . . . . . . . 9
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 10
2.1. Wireless Binding Definition . . . . . . . . . . . . . . . 11
2.2. CAPWAP Session Establishment Overview . . . . . . . . . . 11
2.3. CAPWAP State Machine Definition . . . . . . . . . . . . . 13
2.3.1. CAPWAP Protocol State Transitions . . . . . . . . . . 15
2.3.2. CAPWAP/DTLS Interface . . . . . . . . . . . . . . . . 24
2.4. Use of DTLS in the CAPWAP Protocol . . . . . . . . . . . 26
2.4.1. DTLS Handshake Processing . . . . . . . . . . . . . . 26
2.4.2. DTLS Session Establishment . . . . . . . . . . . . . 28
2.4.3. DTLS Error Handling . . . . . . . . . . . . . . . . . 28
2.4.4. DTLS EndPoint Authentication . . . . . . . . . . . . 29
3. CAPWAP Transport . . . . . . . . . . . . . . . . . . . . . . 33
3.1. UDP Transport . . . . . . . . . . . . . . . . . . . . . . 33
3.2. AC Discovery . . . . . . . . . . . . . . . . . . . . . . 33
3.3. Fragmentation/Reassembly . . . . . . . . . . . . . . . . 34
4. CAPWAP Packet Formats . . . . . . . . . . . . . . . . . . . . 35
4.1. CAPWAP preamble . . . . . . . . . . . . . . . . . . . . . 37
4.2. CAPWAP Header . . . . . . . . . . . . . . . . . . . . . . 37
4.3. CAPWAP Data Messages . . . . . . . . . . . . . . . . . . 41
4.3.1. CAPWAP Data Keepalive . . . . . . . . . . . . . . . . 41
4.3.2. Station Data Payloads . . . . . . . . . . . . . . . . 42
4.4. CAPWAP Control Messages . . . . . . . . . . . . . . . . . 43
4.4.1. Control Message Format . . . . . . . . . . . . . . . 43
4.4.2. Control Message Quality of Service . . . . . . . . . 46
4.5. CAPWAP Protocol Message Elements . . . . . . . . . . . . 46
4.5.1. AC Descriptor . . . . . . . . . . . . . . . . . . . . 49
4.5.2. AC IPv4 List . . . . . . . . . . . . . . . . . . . . 50
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4.5.3. AC IPv6 List . . . . . . . . . . . . . . . . . . . . 51
4.5.4. AC Name . . . . . . . . . . . . . . . . . . . . . . . 51
4.5.5. AC Name with Index . . . . . . . . . . . . . . . . . 52
4.5.6. AC Timestamp . . . . . . . . . . . . . . . . . . . . 52
4.5.7. Add MAC ACL Entry . . . . . . . . . . . . . . . . . . 53
4.5.8. Add Station . . . . . . . . . . . . . . . . . . . . . 53
4.5.9. Add Static MAC ACL Entry . . . . . . . . . . . . . . 54
4.5.10. CAPWAP Control IPv4 Address . . . . . . . . . . . . . 55
4.5.11. CAPWAP Control IPv6 Address . . . . . . . . . . . . . 55
4.5.12. CAPWAP Timers . . . . . . . . . . . . . . . . . . . . 56
4.5.13. Data Transfer Data . . . . . . . . . . . . . . . . . 56
4.5.14. Data Transfer Mode . . . . . . . . . . . . . . . . . 57
4.5.15. Decryption Error Report . . . . . . . . . . . . . . . 57
4.5.16. Decryption Error Report Period . . . . . . . . . . . 58
4.5.17. Delete MAC ACL Entry . . . . . . . . . . . . . . . . 58
4.5.18. Delete Station . . . . . . . . . . . . . . . . . . . 59
4.5.19. Delete Static MAC ACL Entry . . . . . . . . . . . . . 59
4.5.20. Discovery Type . . . . . . . . . . . . . . . . . . . 60
4.5.21. Duplicate IPv4 Address . . . . . . . . . . . . . . . 61
4.5.22. Duplicate IPv6 Address . . . . . . . . . . . . . . . 61
4.5.23. Idle Timeout . . . . . . . . . . . . . . . . . . . . 62
4.5.24. Image Data . . . . . . . . . . . . . . . . . . . . . 63
4.5.25. Image Filename . . . . . . . . . . . . . . . . . . . 63
4.5.26. Initiate Download . . . . . . . . . . . . . . . . . . 64
4.5.27. Location Data . . . . . . . . . . . . . . . . . . . . 64
4.5.28. MTU Discovery Padding . . . . . . . . . . . . . . . . 65
4.5.29. Radio Administrative State . . . . . . . . . . . . . 65
4.5.30. Radio Operational State . . . . . . . . . . . . . . . 66
4.5.31. Result Code . . . . . . . . . . . . . . . . . . . . . 67
4.5.32. Returned Message Element . . . . . . . . . . . . . . 68
4.5.33. Session ID . . . . . . . . . . . . . . . . . . . . . 69
4.5.34. Statistics Timer . . . . . . . . . . . . . . . . . . 69
4.5.35. Vendor Specific Payload . . . . . . . . . . . . . . . 70
4.5.36. WTP Board Data . . . . . . . . . . . . . . . . . . . 70
4.5.37. WTP Descriptor . . . . . . . . . . . . . . . . . . . 71
4.5.38. WTP Fallback . . . . . . . . . . . . . . . . . . . . 73
4.5.39. WTP Frame Tunnel Mode . . . . . . . . . . . . . . . . 73
4.5.40. WTP IPv4 IP Address . . . . . . . . . . . . . . . . . 74
4.5.41. WTP MAC Type . . . . . . . . . . . . . . . . . . . . 75
4.5.42. WTP Name . . . . . . . . . . . . . . . . . . . . . . 75
4.5.43. WTP Operational Statistics . . . . . . . . . . . . . 76
4.5.44. WTP Radio Statistics . . . . . . . . . . . . . . . . 76
4.5.45. WTP Reboot Statistics . . . . . . . . . . . . . . . . 78
4.5.46. WTP Static IP Address Information . . . . . . . . . . 79
4.6. CAPWAP Protocol Timers . . . . . . . . . . . . . . . . . 80
4.6.1. DataChannelKeepAlive . . . . . . . . . . . . . . . . 80
4.6.2. DataChannelDeadInterval . . . . . . . . . . . . . . . 80
4.6.3. DiscoveryInterval . . . . . . . . . . . . . . . . . . 81
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4.6.4. DTLSRehandshake . . . . . . . . . . . . . . . . . . . 81
4.6.5. DTLSSessionDelete . . . . . . . . . . . . . . . . . . 81
4.6.6. EchoInterval . . . . . . . . . . . . . . . . . . . . 81
4.6.7. KeyLifetime . . . . . . . . . . . . . . . . . . . . . 81
4.6.8. MaxDiscoveryInterval . . . . . . . . . . . . . . . . 81
4.6.9. MaxFailedDTLSSessionRetry . . . . . . . . . . . . . . 82
4.6.10. NeighborDeadInterval . . . . . . . . . . . . . . . . 82
4.6.11. ResponseTimeout . . . . . . . . . . . . . . . . . . . 82
4.6.12. RetransmitInterval . . . . . . . . . . . . . . . . . 82
4.6.13. SilentInterval . . . . . . . . . . . . . . . . . . . 82
4.6.14. StatisticsTimer . . . . . . . . . . . . . . . . . . . 82
4.6.15. WaitDTLS . . . . . . . . . . . . . . . . . . . . . . 82
4.7. CAPWAP Protocol Variables . . . . . . . . . . . . . . . . 83
4.7.1. AdminState . . . . . . . . . . . . . . . . . . . . . 83
4.7.2. DiscoveryCount . . . . . . . . . . . . . . . . . . . 83
4.7.3. FailedDTLSSessionCount . . . . . . . . . . . . . . . 83
4.7.4. IdleTimeout . . . . . . . . . . . . . . . . . . . . . 83
4.7.5. MaxDiscoveries . . . . . . . . . . . . . . . . . . . 83
4.7.6. MaxRetransmit . . . . . . . . . . . . . . . . . . . . 83
4.7.7. ReportInterval . . . . . . . . . . . . . . . . . . . 83
4.7.8. RetransmitCount . . . . . . . . . . . . . . . . . . . 84
4.7.9. WTPFallBack . . . . . . . . . . . . . . . . . . . . . 84
4.8. WTP Saved Variables . . . . . . . . . . . . . . . . . . . 84
4.8.1. AdminRebootCount . . . . . . . . . . . . . . . . . . 84
4.8.2. FrameEncapType . . . . . . . . . . . . . . . . . . . 84
4.8.3. LastRebootReason . . . . . . . . . . . . . . . . . . 84
4.8.4. MacType . . . . . . . . . . . . . . . . . . . . . . . 84
4.8.5. PreferredACs . . . . . . . . . . . . . . . . . . . . 84
4.8.6. RebootCount . . . . . . . . . . . . . . . . . . . . . 84
4.8.7. Static ACL Table . . . . . . . . . . . . . . . . . . 85
4.8.8. Static IP Address . . . . . . . . . . . . . . . . . . 85
4.8.9. WTPLinkFailureCount . . . . . . . . . . . . . . . . . 85
4.8.10. WTPLocation . . . . . . . . . . . . . . . . . . . . . 85
4.8.11. WTPName . . . . . . . . . . . . . . . . . . . . . . . 85
5. CAPWAP Discovery Operations . . . . . . . . . . . . . . . . . 86
5.1. Discovery Request Message . . . . . . . . . . . . . . . . 86
5.2. Discovery Response Message . . . . . . . . . . . . . . . 87
5.3. Primary Discovery Request Message . . . . . . . . . . . . 87
5.4. Primary Discovery Response . . . . . . . . . . . . . . . 88
6. CAPWAP Join Operations . . . . . . . . . . . . . . . . . . . 90
6.1. Join Request . . . . . . . . . . . . . . . . . . . . . . 90
6.2. Join Response . . . . . . . . . . . . . . . . . . . . . . 91
7. Control Channel Management . . . . . . . . . . . . . . . . . 92
7.1. Echo Request . . . . . . . . . . . . . . . . . . . . . . 92
7.2. Echo Response . . . . . . . . . . . . . . . . . . . . . . 92
8. WTP Configuration Management . . . . . . . . . . . . . . . . 93
8.1. Configuration Consistency . . . . . . . . . . . . . . . . 93
8.1.1. Configuration Flexibility . . . . . . . . . . . . . . 94
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8.2. Configuration Status . . . . . . . . . . . . . . . . . . 94
8.3. Configuration Status Response . . . . . . . . . . . . . . 95
8.4. Configuration Status Acknowledge . . . . . . . . . . . . 96
8.5. Configuration Update Request . . . . . . . . . . . . . . 96
8.6. Configuration Update Response . . . . . . . . . . . . . . 97
8.7. Change State Event Request . . . . . . . . . . . . . . . 97
8.8. Change State Event Response . . . . . . . . . . . . . . . 98
8.9. Clear Configuration Request . . . . . . . . . . . . . . . 99
8.10. Clear Configuration Response . . . . . . . . . . . . . . 99
9. Device Management Operations . . . . . . . . . . . . . . . . 100
9.1. Image Data Request . . . . . . . . . . . . . . . . . . . 100
9.2. Image Data Response . . . . . . . . . . . . . . . . . . . 101
9.3. Reset Request . . . . . . . . . . . . . . . . . . . . . . 101
9.4. Reset Response . . . . . . . . . . . . . . . . . . . . . 101
9.5. WTP Event Request . . . . . . . . . . . . . . . . . . . . 102
9.6. WTP Event Response . . . . . . . . . . . . . . . . . . . 102
9.7. Data Transfer Request . . . . . . . . . . . . . . . . . . 103
9.8. Data Transfer Response . . . . . . . . . . . . . . . . . 103
10. Station Session Management . . . . . . . . . . . . . . . . . 104
10.1. Station Configuration Request . . . . . . . . . . . . . . 104
10.2. Station Configuration Response . . . . . . . . . . . . . 104
11. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 105
12. Security Considerations . . . . . . . . . . . . . . . . . . . 107
12.1. CAPWAP Security . . . . . . . . . . . . . . . . . . . . . 107
12.1.1. Converting Protected Data into Unprotected Data . . . 108
12.1.2. Converting Unprotected Data into Protected Data
(Insertion) . . . . . . . . . . . . . . . . . . . . . 108
12.1.3. Deletion of Protected Records . . . . . . . . . . . . 108
12.1.4. Insertion of Unprotected Records . . . . . . . . . . 108
12.2. Use of Preshared Keys in CAPWAP . . . . . . . . . . . . . 108
12.3. Use of Certificates in CAPWAP . . . . . . . . . . . . . . 109
12.4. AAA Security . . . . . . . . . . . . . . . . . . . . . . 110
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 111
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 112
14.1. Normative References . . . . . . . . . . . . . . . . . . 112
14.2. Informational References . . . . . . . . . . . . . . . . 112
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 114
Intellectual Property and Copyright Statements . . . . . . . . . 115
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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 [12] 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 (or STA for short), 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, or station to AC communication is
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. [Note: This section
needs work to accurately reflect the contribution of each author and
this work will be done a future revision of this document.]
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
1.4. Acknowledgements
The authors thank Michael Vakulenko for contributing text that
describes how CAPWAP can be used over a layer 3 (IP/UDP) network.
The authors thank Russ Housley and Charles Clancy for their
assistance in provide a security review of the LWAPP specification.
Charles' review can be found at [11].
1.5. 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 frames can exceed the PMTU,
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 will select 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 to 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
statistical information collected by the WTP.
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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 the definition for a binding-
specific Statistics message element, carried in the WTP Event Request
message, a message element carried in the Station Configure Request
to configure STA information on the WTP, and a WTP Radio Information
message element carried in the Discovery Request, Primary Discovery
Request and and Join Request messages, indicating the binding
specific radio types supported at the WTP. 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 [12], begins with "IEEE 802.11"."
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 ------------]
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(--- 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
( ---assume image is up to date ---)
Configure Request ------->
<------ Configure Response
(--- enter RUN state ---)
:
:
Echo Request ------->
<------ Echo Response
:
:
EventRequest ------->
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<------ 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 (from CAPWAP to DTLS) and notifications
(from DTLS to CAPWAP). 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 | /-------/ | ||
+------------+<-------\ | | ||
t| s| 4 o| ||
+--------+ +-----------+ +------------+ ||
| Join |---->| Configure |---->| Image Data | ||
+--------+ q +-----------+ r +------------+ ||
^ p| ||
| \------------------------------------\ ||
\---------------------\ | ||
/--------------<----------------+---------------\ | ||
| /------------<-------------\ | | | ||
| | m| |n z| 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
each message is valid.
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2.3.1. CAPWAP Protocol State Transitions
The following text discusses the various state transitions, and the
events that cause them. This section does not discuss interactions
between DTLS- and CAPWAP-specific states. Those interactions, as
well as 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
counter reaches MaxFailedDTLSSessionRetry variable (see
Section 4.7). Upon entering this state, the WTP shall start
the SilentInterval timer. While in the Sulking state, all
received CAPWAP and DTLS protocol messages received shall be
ignored.
AC: The AC enters this state with the specific WTP when the
FailedDTLSSessionCount counter reaches
MaxFailedDTLSSessionRetry variable (see Section 4.7). Upon
entering this state, the AC shall start the SilentInterval
timer. While in the Sulking state, all received CAPWAP and
DTLS protocol messages received from the WTP shall be ignored.
Discovery to Discovery (2): In the Discovery state, the WTP
determines which AC to connect to.
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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 shall start the SilentInterval timer. While in
the Sulking state, all received CAPWAP protocol messages
received shall 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 and
DiscoveryCount counters are reset to zero.
AC: The AC enters this state when the SilentInterval timer (see
Section 4.6) expires. The FailedDTLSSessionCount and
DiscoveryCount counters are reset to zero.
Sulking to Sulking (3): The Sulking state provides the silent
period, minimizing the possibility for Denial of service attacks.
WTP: All packets received from the AC while in the sulking state
are ignored.
AC: All packets receive from the WTP while in the sulking state
are ignored.
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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. This decision is performed via local configuration
of the 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 to only accept session requests
from specific WTP.
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 in order to provide optional qualifiers in
the DTLSListen command to only accept session requests from
specific WTP. Note that maintaining state information based on
an unsecured discovery request 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 transition occurs, the
FailedDTLSSessionCount counter is incremented.
AC: The WTP initiates this state transition when it receives a
DTLSEstablishFail notification from DTLS (see Section 2.3.2.2).
This error notification means a DTLS session was attempted with
a WTP, but failed. The notification should include information
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such as the offending WTP, and the reason for the failure.
When this transition occurs, the FailedDTLSSessionCount counter
is incremented.
DTLS Setup to Authorize (h): This transition occurs 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 MAY perform an authorization check
against the AC's credentials. The method by which this
authorization is performed is outside the scope of the CAPWAP
specification.
AC: This state transition occurs when the AC receives the
DTLSPeerAuthorize notification (see Section 2.3.2.2). Upon
entering this state, the AC MAY perform an authorization check
against the WTP's credentials. The method by which this
authorization is performed is outside the scope of the CAPWAP
specification.
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, via its credentials. The WTP then aborts the
DTLS session, which is done by invoking DTLSAbortSession (see
Section 2.3.2.1).
AC: This state transition occurs when the AC was unable to
authorize the WTP, via its credentials. The AC then aborts the
DTLS session, which is done by invoking DTLSAbortSession (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 the
DTLSAborted notification (see Section 2.3.2.2), indicating that
the DTLS session was not successfully established. When this
notification is received, the FailedDTLSSessionCount counter is
incremented.
AC: This state transition occurs when the AC receives the
DTLSAborted notification (see Section 2.3.2.2), indicating that
the DTLS session was not successfully established. When this
notification is received, 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.
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 with a Result Code message element containing an
error. 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 Join
Response with a Result Code message element containing an
error. This causes the WTP to initiate the DTLSShutdown
command (see Section 2.3.2.1).
Join to Configure (g): This state transition is used by the WTP and
the AC to exchange configuration information.
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WTP: The WTP enters the Configure state when it successfully
completes the Join operation. If it determines that its
version number and the version number advertised by the AC are
compatible, the WTP transmits the Configuration Status message
(see Section 8.2) to the AC with a snapshot of its current
configuration. The WTP also starts the ResponseTimeout timer
(see Section 4.6). If the version numbers are not compatible,
the WTP will immediately transition to Image Data state (see
transition (g)). If the AC determines that a new firmware
image should be installed on the WTP, the AC initiates a
firmware download by sending an Image Data Request Message with
an Initiate Download message element to the WTP
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. If the AC
instead receives the Image Data Request from the WTP, it
immediately transitions to the Image Data state (see transition
(g)).
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 the AC's policy does not permit the WTP providing
service.
Configure 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 successfully
comletes DTLS session establishment, and determines that its
version number and the version number advertised by the AC are
different. The WTP transmits the Image Data Request (see
Section 9.1) message requesting that a download of the AC's
latest firmware be initiated.
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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 9.2) to the WTP, which
includes a portion of the firmware.
Image Data to Image Data (4): The Image Data state is used by 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.
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 from the AC (see Section 9.2)
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.
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 8.7).
AC: This state transition occurs when the AC receives the Change
State Event Request message (see Section 8.7) from the WTP.
The AC responds with a Change State Event Response (see
Section 8.8) message. The AC must start the
NeighborDeadInterval timer (see Section 4.6).
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Data Check to Run (u): This state transition occurs once the linkage
between the control and data channels has occured, which causes
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 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 (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 (see Section 4.3.1), whose Session ID
message element matches the one included by the WTP in the Join
Request. Note that if the AC's policy is to require the data
channel to be encrypted, this process would also require the
establishment of the data channel's DTLS session. Upon
receiving the Data Channel Keep Alive, the AC transmits its own
Data Channel Keep Alive.
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 a one of the
following DTLS notifications: DTLSAborted,
DTLSReassemblyFailure, DTLSDecapFailure or DTLSPeerDisconnect
(see Section 2.3.2.2).
AC: The AC enters this state when it receives a one of the
following DTLS notifications: DTLSAborted,
DTLSReassemblyFailure, DTLSDecapFailure or DTLSPeerDisconnect
(see Section 2.3.2.2).
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 8.5). The WTP MUST respond with
a Configuration Update Response message (see Section 8.6).
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 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 Request: The WTP receives a Clear Configuration
Request message (see Section 8.9). The WTP MUST reset its
configuration back to manufacturer defaults.
WTP Event: The WTP generates a WTP Event Request message to
send information to the AC (see Section 9.5). The WTP
receives a WTP Event Response message from the AC (see
Section 9.6).
Data Transfer: The WTP generates a Data Transfer Request
message to the AC (see Section 9.7). The WTP receives a
Data Transfer Response message from the AC (see
Section 9.8).
Station Configuration Request: The WTP receives a Station
Config Request message (see Section 10.1), to which it MUST
respond with a Station Config 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 8.5) to the WTP to update its
configuration. The AC receives a Configuration Update
Response message (see Section 8.6) from the WTP.
Change State Event: The AC receives a Change State Event
Request message (see Section 8.7), to which it MUST respond
with the Change State Event Response message (see
Section 8.8).
Echo: The AC sends an Echo Request message Section 7.1 or
receives the corresponding Echo Response message, see
Section 7.2 from the WTP.
Clear Config Response: The AC receives a Clear Configuration
Response message (see Section 8.10).
Station Config: 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.
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Data Transfer: The AC receives a Data Transfer Request message
from the AC (see Section 9.7) and MUST generate a
corresponding Data Transfer Response message (see
Section 9.8).
WTP Event: The AC receives a WTP Event Request message from
the AC (see Section 9.5) and MUST generate a corresponding
WTP Event Response message (see Section 9.6).
Run to Reset (x): This state transition is used when the AC or WTP
wish to 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 from the AC.
AC: The AC enters the reset state when it transmits a Reset
Request 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. 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. This causes the WTP to initiate 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 receives a
DTLSPeerDisconnect notification (see Section 2.3.2.2).
AC: This state transition occurs when the AC receives a
DTLSPeerDisconnect notification (see Section 2.3.2.2).
2.3.2. CAPWAP/DTLS Interface
This section describes the DTLS Commands used by CAPWAP, as well as
the notifications received from DTLS to the CAPWAP protocol stack.
2.3.2.1. CAPWAP to DTLS Commands
Four commands are defined for the CAPWAP to DTLS API. These
"commands" are conceptual, and may be implemented as one or more
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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 module to cause a DTLS session to be
established. Upon invoking the DTLSStart command, the WaitDTLS
timer is started. The WTP is the only CAPWAP device that
initiates this DTLS command, as the AC does not initiate DTLS
sessions.
o DTLSListen is sent to the DTLS module to allow the DTLS to listen
for incoming DTLS session requests.
o DTLSAccept is sent to the DTLS module to allow the DTLS session
establishment to continue successfully.
o DTLSAbortSession is sent to the DTLS module 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 module to cause session teardown.
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 Section 2.3.1. When a notification
listed 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 protocol stack during
the DTLS session establishment once the peer's identity has been
received. This notification MAY be used by the CAPWAP protocol
stack in order to authorize the session, based on the peer's
identity. The authorization process will lead to the CAPWAP
protocol stack initiating either the DTLSAccept or
DTLSAbortSession commands.
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o DTLSEstablished is sent to the CAPWAP module 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 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 reset to zero. When this
notification is received, the WaitDTLS is stopped.
o DTLSAborted is sent to the CAPWAP module 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 is stopped.
o DTLSReassemblyFailure may be sent to the CAPWAP module to indicate
DTLS fragment reassembly failure.
o DTLSDecapFailure may be sent to CAPWAP to indicate an
decapsulation failure. DTLSDecapFailure may be sent to CAPWAP to
indicate an encryption/authentication failure.
o DTLSPeerDisconnect is sent to the CAPWAP module to indicate the
DTLS session has been torn down. Note that this notification is
only received if the DTLS session has been established.
2.4. Use of DTLS in the CAPWAP Protocol
DTLS is used as a tightly-integrated, secure wrapper for the CAPWAP
protocol. In this document DTLS and CAPWAP are discussed as
nominally distinct 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
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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):
============ ============
WTP AC
============ ============
<DTLS Idle> <DTLS Idle>
ClientHello ------>
<------ HelloVerifyRequest
(with cookie)
<DTLS Setup>
ClientHello ------>
(with cookie)
<DTLS Setup>
<------ ServerHello
<------ Certificate
<------ ServerHelloDone
(WTP callout for AC authorization
occurs in CAPWAP Auth state)
<DTLS Run>
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ------>
(AC callout for WTP authorization
occurs in CAPWAP Auth state)
<DTLS Run>
[ChangeCipherSpec]
<------ Finished
DTLS, as specified, provides its own retransmit timers with an
exponential back-off. However, it will never terminate the handshake
due to non-responsiveness; rather, it will continue to increase its
back-off timer period. Hence, timing out incomplete DTLS handshakes
is entirely the responsiblity of the CAPWAP protocol.
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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
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
state to transition back 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
the 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 module 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
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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 module
MUST send a DTLSReassemblyFailure notification to CAPWAP. 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 protocol implementation
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,
and 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, these are reported as authentication failures.
The DTLS module MUST provide a DTLSDecapFailure notification to
CAPWAP 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
exceeeded. As part of DTLS session establishment, CAPWAP informs
DTLS of the MTU size. This may be dynamically modified at any time
when CAPWAP sends the DTLSMtuUpdate command to DTLS. DTLS returns
this notification to CAPWAP whenever a transmission request will
result in a packet which exceeds the MTU.
2.4.4. DTLS EndPoint Authentication
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 [14]. However,
support for AES counter mode encryption is currently progressing in
the TLS working group, and once protocol identifiers are available,
they will be added below. At present, the following algorithms MUST
be supported when using certificates for CAPWAP authentication:
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o TLS_RSA_WITH_AES_128_CBC_SHA
o TLS_RSA_WITH_3DES_EDE_CBC_SHA
The following algorithms SHOULD be supported when using certificates:
o TLS_DH_RSA_WITH_AES_128_CBC_SHA
o TLS_DH_RSA_WITH_3DES_EDE_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.
However, [6] defines several different methods for authenticating
with preshared keys, 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_PSK_WITH_3DES_EDE_CBC_SHA
o TLS_DHE_PSK_WITH_AES_128_CBC_SHA
o TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA
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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
When using certificates, both authentication and authorization must
be considered. Section 12.3 provides recommendations on how to
authenticate a certificate and bind that to a CAPWAP entity. This
section deals with certificate authorization.
Certificate authorization by the AC and WTP is required so that only
an AC may perform the functions of an AC and that only a WTP may
perform the functions of a WTP. This restriction of functions to the
AC or WTP requires that the certificates used by the AC MUST be
distinguishable from the certificate used by the WTP. To accomplish
this differentiation, the x.509 certificates MUST include the
Extended Key Usage (EKU) certificate extension [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 those 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-capwapWTP OBJECT IDENTIFIER ::= { id-kp 19 }
id-kp-capwapAC OBJECT IDENTIFIER ::= { id-kp 18 }
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
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that the proper EKU is included and only allowing the CAPWAP session
to be established if the extension properly represents the device.
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3. CAPWAP Transport
The CAPWAP protocol uses UDP as a transport, and can be used with
IPv4 or IPv6. This section details the specifics of how the CAPWAP
protocol works in conjunction 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 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
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. This section describes how AC discovery is
performed by WTPs.
As the WTP attempts to establish communication with an AC, it sends
the Discovery Request message and receives the corresponding 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. Upon receipt of the Discovery Request message, the AC issues 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.
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:
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DHCP: A comma delimited ASCII encoded list of AC IP addresses is
embedded in the 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.
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, Network Address Translation
(NAT) and "middle box" devices, which tend to drop IP fragments in
order to minimize possible Denial of Service 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 is not impacted by any network
configurations.
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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. See section Section 3.1 for more information on the use of
UDP.
The CAPWAP Control protocol includes two messages that are never
protected by DTLS. These messages, called the Discovery Request and
Discovery Response, need to be in the clear in order for 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 | p-amble|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 are as follows:
CAPWAP Control Packet (DTLS Security Required):
+------------------------------------------------------------------+
| IP | UDP | CAPWAP | DTLS | CAPWAP | Control | Message | DTLS |
| Hdr | Hdr | p-amble| Hdr | Header | Header | Element(s) | Trlr |
+------------------------------------------------------------------+
\----------- authenticated ------------/
\------------- encrypted -------------/
The CAPWAP protocol allows optional encryption of the data frames,
once again using the DTLS protocol. Whether or not the data frames
are encrypted is a matter of policy, which is described in a later
section of this specification. The format of these packets is as
follows:
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CAPWAP Plain Text Data Packet :
+-----------------------------------------+
| IP | UDP | CAPWAP | CAPWAP | Wireless |
| Hdr | Hdr | p-amble| Header | Payload |
+-----------------------------------------+
DTLS Secured CAPWAP Data Packet:
+------------------------------------------------------+
| IP | UDP | CAPWAP | DTLS | CAPWAP | Wireless | DTLS |
| Hdr | Hdr | p-amble| Hdr | Hdr | Payload | Trlr |
+------------------------------------------------------+
\----- authenticated -----/
\------- encrypted --------/
UDP: 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
preable header, which is used to identify the frame type that
follows. This 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 UDP header. This header, is defined in
Section 4.2.
Wireless Payload: A CAPWAP protocol packet that contains a wireless
payload is known as a data frame. The CAPWAP protocol does not
dictate 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.
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.
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4.1. CAPWAP preamble
The CAPWAP preamble header is used to help identify the payload type
that immediately follows. The reason for this header to is avoid
needing the perform byte comparisons in order to guess whether the
frame is DTLS encrypted or not. The format of the frame 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 draft 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 this as a clear text CAPWAP data
packet. If received on the control UDP port, the CAPWAP stack
MUST treat this as a clear text CAPWAP control packet. If the
control packet is not a Discovery Request or Response packet,
it is illegal and MUST be dropped.
1 - DTLS Payload. The packet is either a DTLS packet and MAY be
a data or control packet, based on the UDP port it was received
on (see section 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.
Note that the optional fields defined in this section MUST be present
in the precise order shown below.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 CAPWAP 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. WTPs with multiple radios but a single MAC Address range
use this field to indicate which radio is associated with the
packet.
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
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.
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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 when the native wireless packet. 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: An 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.
Fragment Offset: A 13 bit field that indicates where in the payload
will this fragment belong 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.
For instance, fragment 0 would include offset 0 with a payload
length of 1000, while fragment 1 include offset 900 with a payload
length of 600.
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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. Given the HLEN field assumes 4 byte alignment,
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)
are now using 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. Given the HLEN field assumes 4 byte alignment,
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Wireless ID | Length | Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Wireless ID: The wireless binding identifier. The following
values are defined:
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1 - : IEEE 802.11
IEEE 802.16
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 associated with that
message.
4.3. CAPWAP Data Messages
There are two different types of CAPWAP data messages; keepalive and
user payload. The first is used by the WTP to synchronize the
control and data channels, as well as to maintain freshness of the
data channel. The second is used to transmit user payloads between
the AC and WTP. This section will detail both types of CAPWAP data
messages.
Both CAPWAP data messages are transmitted on the data channel UDP
port.
4.3.1. CAPWAP Data Keepalive
The CAPWAP data keepalive is used to bind the CAPWAP control channel
with the data channel. The keep alive is also used to maintain
freshness of the data channel, meaning ensuring the channel is still
in functioning. The CAPWAP Data Keepalive is transmitted by the WTP
when the DataChannelKeepAlive timer expires. When the CAPWAP Data
Keepalive is transmitted, the WTP sets the DataChannelDeadInterval
timer.
All of the fields in the CAPWAP header, other than the HLEN and K
bit, are set to zero upon transmission. Upon receiving a CAPWAP Data
Keepalive, the AC transmits a CAPWAP Data Keepalive message back to
the WTP. The contents of the CAPWAP message is assumed to be
identical to the one received.
Upon receiving a CAPWAP Data Keepalive, the WTP cancels the
DataChannelDeadInterval timer and resets the DataChannelKeepAlive
timer. The CAPWAP Data Keepalive is retranmitted by the WTP in the
same manner as the CAPWAP control messages. If the
DataChannelDeadInterval timer expires the WTP tears down the control
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DTLS session, as well as the data DTLS session if one existed.
The CAPWAP Data Keepalive 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Msg Element Length | Msg 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 4.5.33
4.3.2. Station Data Payloads
A CAPWAP protocol data message 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 actual format of the encapsulated CAPWAP data frame is
subject to the rules defined under the specific wireless technology
binding. As a consequence, each wireless technology binding MUST
define a section entitled "Payload encapsulation", which defines the
format of the wireless payload that is encapsulated within the CAPWAP
Data messages.
In the event that the encapsulated frame would exceed the transport
layer's MTU, the sender is responsible for the fragmentation of the
frame, as specified in Section 3.3.
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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
distinct message types:
Discovery: CAPWAP Discovery messages are used to identify potential
ACs, their load and capabilities.
Join: CAPWAP Join messages are used to for 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 push a specific configuration to the WTP.
Messages which provide retrieval of statistics from the WTP also
fall in this category.
Station Session Management: Station session management messages are
used by the AC to push 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 Frames: 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 Message Management, WTP Configuration
Management and Station Session Management CAPWAP control messages
MUST be implemented. Device Operations Management 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:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 is the enterprise number in network byte order, with
zero being used for CAPWAP generic message types and the IEEE 802.11
IANA assigned enterprise number 13277 being 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 can be expressed as:
Message Type = IANA Enterprise Number * 256 + enterprise specific message type number
The valid values for base CAPWAP Message Types are given in the table
below:
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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 Status Acknowledge ???
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 Status Acknowledge ???
4.4.1.2. Sequence Number
The Sequence Number Field is an identifier value to match request and
response packet exchanges. When a CAPWAP packet with a request
message type is received, the value of the sequence number field is
copied into the corresponding response packet.
When a CAPWAP control message is sent, its internal sequence number
counter is monotonically incremented, ensuring that no two requests
pending have the same sequence number. This field will wrap 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.
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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,
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.
4.5. CAPWAP Protocol Message Elements
This section defines the CAPWAP Protocol message elements which are
included in CAPWAP protocol control messages.
Message elements are used to carry information needed in control
messages. Every message element is identified by the Type field,
whose numbering space is 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 in
order 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.
Note that 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 order
the message elements as convenient. Furthermore, unless specifically
noted, any individual message element may exist one or more times
within 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 ... |
+-+-+-+-+-+-+-+-+
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Where Type (16 bit) identifies the character of 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.
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CAPWAP Message Element Type Value
AC Descriptor 1
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 Filename 25
Initiate Download 26
Location Data 27
MTU Discovery Padding 28
Radio Administrative State 29
Radio Operational State 30
Result Code 31
Returned Message Element 46
Session ID 32
Statistics Timer 33
Vendor Specific Payload 34
WTP Board Data 35
WTP Descriptor 36
WTP Fallback 37
WTP Frame Tunnel Mode 38
WTP IPv4 IP Address 39
WTP MAC Type 40
WTP Name 41
WTP Operational Statistics 42
WTP Radio Statistics 43
WTP Reboot Statistics 44
WTP Static IP Address Information 45
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4.5.1. AC Descriptor
The AC payload message element is used by the AC to communicate it's
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 |Wireless Field | 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 associated with 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).
Wireless Field: The AC supports the optional Wireless Specific
Information field in the CAPWAP Header (see Section 4.2).
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 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 List
Length: 4
The AC IP Address: An array of 32-bit integers containing an AC's
IPv4 Address.
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 32-bit integers containing an AC's
IPv6 Address.
4.5.4. AC Name
The AC name message element contains an UTF-8 representation of the
AC's 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 where this
message element would be present 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 (e.g., 1=primary,
2=secondary).
AC Name: A variable length UTF-8 encoded string containing the AC's
name.
4.5.6. AC Timestamp
The AC Timestamp message element is sent by the AC to synchronize the
WTP's 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
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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].
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 any 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 particular station. The Add
Station message element will be accompanied by technology specific
binding information element which may include security parameters.
Consequently, the security parameters must be applied by the WTP for
the particular station.
Once a station's policy has been pushed to the WTP through this
message element, an AC may change any policies by simply 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 it may have for the station in question. The latest
Add Station message element data overrides any previously received
messages.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Radio ID | MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address | VLAN Name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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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. In the event that multiple CAPWAP Control IPV4 Address
message elements are returned, the WTP is expected to perform load
balancing across the 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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.
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 packets,
when the WTP is in the discovery mode.
Echo Request: The number of seconds between WTP Echo Request CAPWAP
messages. The default value for this message element can be found
in Section 4.6.6.
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
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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
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 via 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[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type: 15 for Decryption Error Report
Length: >= 8
Radio ID: The Radio Identifier, which typically 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.
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, typically refers to some 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 4.7.7.
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 an WTP
that it should no longer provide service to a particular station.
The WTP must terminate service immediately upon receiving this
message element.
The transmission of a Delete Station message element could occur for
various reasons, including for administrative reasons, as a result of
the fact that the station has roamed to another WTP, etc.
The Delete Station message element MAY be sent by the WTP, through
the WTP Event Request, 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
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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[] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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4 - AC Referral
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 it is currently using.
The WTP shall transmit this message element after it has detected a
duplicate IP address. The WTP will consider the condition cleared
once it has successfully received a frame from the AC.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 21 for Duplicate IPv4 Address
Length: 10
IP Address: The IP Address currently used by the WTP.
MAC Address: The MAC Address of the offending device.
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 it
is currently using.
The WTP shall transmit this message element after it has detected a
duplicate IP address. The WTP will consider the condition cleared
once it has successfully received a frame from the AC.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 22 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.
4.5.23. Idle Timeout
The Idle Timeout message element is sent by the AC to the WTP to
provide it with the idle timeout that it should enforce on its active
station entries. The value applies for 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 can be found in
Section 4.7.4.
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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 | Checksum | Image Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Image Data ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 24 for Image Data
Length: >= 4 (allows 0 length element if last data unit is 1024
bytes)
Opcode: An 8-bit value representing the transfer opcode. The
following values are supported:
3 - Image data is included
5 - An error occurred. Transfer is aborted
Checksum: A 16-bit value containing a checksum of the image data
that follows. The checksum field is the 16 bit one's complement
of the one's complement sum of all 16 bit words in the header.
For purposes of computing the checksum, the value of the checksum
field is zero.
Image Data: The Image Data field contains 1024 characters, unless
the payload being sent is the last one (end of file). If the last
block was 1024 in length, an Image Data with a zero length payload
is sent.
4.5.25. Image Filename
The image filename message element is sent by the WTP to the AC and
is used to initiate the firmware download process. This message
element contains the image filename, which the AC subsequently
transfers to the WTP via the Image Data message element. The value
is a variable length UTF-8 encoded string, which is NOT zero
terminated.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Filename ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 25 for Image Filename
Length: >= 1
Filename: A variable length UTF-8 encoded string containing the
filename to download.
4.5.26. Initiate Download
The Initiate Download message element is used by the AC to inform the
WTP that it should initiate a firmware upgrade. This is performed by
having the WTP initiate its own Image Data Request, with the Image
Download message element. This message element does not contain any
data.
Type: 24 for Initiate Download
Length: 0
4.5.27. 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 for the WTP location to be determined
through this field. The string is not zero terminated.
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-
| Location ...
+-+-+-+-+-+-+-+-+-
Type: 27 for Location Data
Length: > 0
Location: A non-zero terminated UTF-8 encoded string containing the
WTP location.
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4.5.28. 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: 28 for MTU Discovery Padding
Length: variable
Pad: A variable length pad.
4.5.29. Radio Administrative State
The radio administrative state message element is used to communicate
the state of a particular radio. The configuration of the Radio
Administrative State is sent by the AC to change the state of the
WTP, which saves the value to ensure its effect remains across WTP
resets. The WTP communicates this message element during the
configuration phase to ensure that AC has the WTP radio's current
administrative state settings. The value contains the following
fields.
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 | Admin State | Cause |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 29 for Administrative State
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 itself. Therefore, if an AC wishes to change
the administrative state of a WTP, it would include 0xff in the
Radio ID field.
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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 Section 4.7.1. The following values are
supported:
1 - Enabled
2 - Disabled
Cause: In the event of a radio being inoperable, the cause field
would contain the reason the radio is out of service. The
following values are supported:
0 - Normal
1 - Radio Failure
2 - Software Failure
3 - Radar Detection
4.5.30. Radio Operational State
The Radio Operational State message element is sent by the WTP to the
AC to communicate a change in the operational state of a radio. For
instance, if the WTP were to detect that a hardware failure existed
with a radio, which caused the radio to be taken offline, the WTP
would indicate this event to the AC via the message element. The AC
MAY also send this message element to change the operational state of
a specific radio. Note that the operational state setting is not
saved on the WTP, and therefore does not remain across WTP resets.
The value contains two fields, as shown.
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: 30 for Radio Operational State
Length: 3
Radio ID: The Radio Identifier, typically refers to some interface
index on the WTP. A value of 0xFF is invalid, as it is not
possible to change the WTP's operational state.
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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.
Cause: In the event of a radio being inoperable, the cause field
would contain the reason the radio is out of service. The
following values are supported:
0 - Normal
1 - Radio Failure
2 - Software Failure
3 - Administratively Set
4.5.31. Result Code
The Result Code message element value is a 32-bit integer value,
indicating the result of the request operation corresponding to the
sequence number 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 31 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 Failure (unspecified)
4 Failure (Join Failure, Resource Depletion)
5 Failure (Join Failure, Unknown Source)
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6 Failure (Join Failure, Incorrect Data)
7 Failure (Join Failure, Session ID already in use)
8 Failure (Join Failure, WTP Hardware not supported)
9 Failure (Unable to Reset)
10 Failure (Unable to Apply Requested Configuration - Service
Provided Anyhow)
11 Failure (Unable to Apply Requested Configuration - Service Not
Provided)
12 Image Data Error (Invalid Checksum)
13 Image Data Error (Invalid Data Length)
14 Image Data Error (Other Error)
4.5.32. Returned Message Element
The Returned Message Element is sent by the WTP within the Change
State Event Request in order to communicate to the AC which message
elements in the Configuration Status Response it was unable to apply
locally. The Returned Message Element contains a result code that is
used to indicate the reason why the configuration could not be
applied, and encapsulates the offending 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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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Message Element: The Message Element field encapsulates the message
element sent by the AC in the Configuration Status Response
message that caused the error.
4.5.33. 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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 32 for Session ID
Length: 16
Session ID: A 16 octet random session identifier
4.5.34. Statistics Timer
The statistics timer message element value is used by the AC to
inform the WTP of the frequency which it expects to receive updated
statistics.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Statistics Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 33 for Statistics Timer
Length: 2
Statistics Timer: A 16-bit unsigned integer indicating the time, in
seconds. The default value for this timer can be found in section
Section 4.6.14.
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4.5.35. Vendor Specific Payload
The Vendor Specific Payload is used to communicate vendor specific
information between the WTP and the AC. The value contains 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: 34 for Vendor Specific
Length: >= 7
Vendor Identifier: A 32-bit value containing the IANA assigned "SMI
Network Management Private Enterprise Codes" [13]
Element ID: A 16-bit Element Identifier which is managed by the
vendor.
Value: The value associated with the vendor specific element.
4.5.36. WTP Board Data
The WTP Board Data message element is sent by the WTP to the AC and
contains information about the hardware present.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional additional vendor specific WTP board data TLVs
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Type: 35 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.
4.5.37. WTP Descriptor
The WTP descriptor message element is used by a WTP to communicate
it's current hardware/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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 36 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, or RID, field)
supported by the WTP
Radios in use: An 8-bit value representing the number of radios
present in the WTP
Encryption Capabilities: This 16-bit field is used by the WTP to
communicate it's 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.
Vendor Identifier: A 32-bit value containing the IANA assigned "SMI
Network Management Private Enterprise Codes"
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Type: The following values are supported. The Hardware Version,
Software Version, and Boot Version values MUST be included.
0 - Hardware Version: The WTP's hardware version number.
1 - Software Version: The WTP's Firmware version number.
2 - Boot Version: The WTP's boot loader's version number.
Length: Length of vendor specific encoding of WTP information.
Value: Vendor specific data of WTP information encoded in the UTF-8
format.
4.5.38. 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: 37 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 it is not connected to it, it SHOULD
automatically disconnect from its current AC and reconnect to its
primary. If disabled, the WTP will only reconnect to its primary
through manual intervention (e.g., through the Reset Request
command). The default value for this field can be found in
section Section 4.7.9. The following values are supported:
1 - Enabled
2 - Disabled
4.5.39. 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
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select which type will be used, based on its local policy.
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Tunnel Mode |
+-+-+-+-+-+-+-+-+
Type: 38 for WTP Frame Tunnel Mode
Length: 1
Frame Tunnel Mode: The Frame Tunnel mode specifies the tunneling
modes for station data which 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.
4.5.40. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type: 39 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.
4.5.41. 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: 40 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.
4.5.42. 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 ...
+-+-+-+-+-+-+-+-+-
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Type: 41 for WTP Name
Length: variable
WTP Name: A non-zero terminated UTF-8 encoded string containing the
WTP name.
4.5.43. 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: 42 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, calaculated 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 wireless terminals.
Wireless Link Frames per Sec: The number of frames transmitted or
received per second by the WTP over the air interface.
4.5.44. 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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Radio ID | Last Fail Type| Reset Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SW Failure Count | HW Failure Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Other Failure Count | Unknown Failure Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Config Update Count | Channel Change Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Band Change Count | Current Noise Floor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 43 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.
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.
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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.
4.5.45. 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: 44 for WTP Reboot Statistics
Length: 15
Reboot Count: The number of reboots that have occurred due to a WTP
crash. A value of 65535 implies that this information is not
available on the WTP.
AC Initiated Count: The number of reboots that have occurred at the
request of a CAPWAP protocol message, such as a change in
configuration that required a reboot or an explicit CAPWAP
protocol reset request. A value of 65535 implies that this
information is not available on the WTP.
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Link Failure Count: The number of times that a CAPWAP protocol
connection with an AC has failed due to link failure.
SW Failure Count: The number of times that a CAPWAP protocol
connection with an AC has failed due to software related reasons.
HW Failure Count: The number of times that a CAPWAP protocol
connection with an AC has failed due to hardware related reasons.
Other Failure Count: The number of times that a CAPWAP protocol
connection with an AC has failed due to known reasons, other than
AC initiated, link, SW or HW failure.
Unknown Failure Count: The number of times that a CAPWAP protocol
connection with an AC has failed for unknown reasons.
Last Failure Type: The failure type of the most recent WTP failure.
The following values are supported:
0 - Not Supported
1 - AC Initiated (see Section 9.3)
2 - Link Failure
3 - Software Failure
4 - Hardware Failure
5 - Other Failure
255 - Unknown (e.g., WTP doesn't keep track of info)
4.5.46. 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: 45 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
A WTP or AC that implements CAPWAP discovery MUST implement the
following timers.
4.6.1. DataChannelKeepAlive
The minimum time, in seconds, between sending data channel keep-alive
packets to the AC with which the WTP has joined. The default value
is 30 seconds.
4.6.2. DataChannelDeadInterval
The minimum time, in seconds, a WTP MUST wait without having received
data channel keep-alive packets before the destination for the data
channel keep-alive packets may be considered dead. Must be no less
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than 2*DataChannelKeepAlive seconds and no greater that 240 seconds.
Default: 5
4.6.3. DiscoveryInterval
The minimum time, in seconds, that a WTP MUST wait after receiving a
Discovery Response, before initiating a DTLS handshake.
Default: 5
4.6.4. DTLSRehandshake
The minimum time, in seconds, a WTP MUST wait for DTLS rehandshake to
complete.
Default: 10
4.6.5. DTLSSessionDelete
The minimum time, in seconds, a WTP MUST wait for DTLS session
deletion.
Default: 5
4.6.6. EchoInterval
The minimum time, in seconds, between sending echo requests to the AC
with which the WTP has joined.
Default: 30
4.6.7. KeyLifetime
The maximum time, in seconds, which a CAPWAP DTLS session key is
valid.
Default: 28800
4.6.8. MaxDiscoveryInterval
The maximum time allowed between sending discovery requests from the
interface, in seconds. Must be no less than 2 seconds and no greater
than 180 seconds.
Default: 20 seconds.
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4.6.9. MaxFailedDTLSSessionRetry
The maximum number of failed DTLS session establishment attempts
before the CAPWAP device enters a silent period.
Default: 3.
4.6.10. NeighborDeadInterval
The minimum time, in seconds, a WTP MUST wait without having received
Echo Responses to its Echo Requests, before the destination for the
Echo Request may be considered dead. Must be no less than
2*EchoInterval seconds and no greater than 240 seconds.
Default: 60
4.6.11. ResponseTimeout
The minimum time, in seconds, which the WTP or AC must respond to a
CAPWAP Request message.
Default: 1
4.6.12. RetransmitInterval
The minimum time, in seconds, which a non-acknowledged CAPWAP packet
will be retransmitted.
Default: 3
4.6.13. SilentInterval
For a WTP, this is the minimum time, in seconds, a WTP MUST wait
before it MAY again send discovery requests or attempt to a establish
DTLS session. For an AC, this is the minimum time, in seconds, which
the AC should ignore all CAPWAP and DTLS packets received from the
WTP that is in the sulking state.
Default: 30
4.6.14. StatisticsTimer
The default Statistics Interval is 120 seconds.
4.6.15. WaitDTLS
The maximum time, in seconds, a WTP MUST wait without having received
a DTLS Handshake message from an AC. This timer must be greater than
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30 seconds.
Default: 60
4.7. CAPWAP Protocol Variables
A WTP or AC that implements CAPWAP discovery 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, t he 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 discoveries transmitted by a WTP to a single AC. This
is a monotonically increasing counter.
4.7.3. FailedDTLSSessionCount
The number of failed DTLS session establishment attempts.
4.7.4. IdleTimeout
The default Idle Timeout is 300 seconds.
4.7.5. MaxDiscoveries
The maximum number of discovery requests that will be sent after a
WTP boots.
Default: 10
4.7.6. MaxRetransmit
The maximum number of retransmissions for a given CAPWAP packet
before the link layer considers the peer dead.
Default: 5
4.7.7. ReportInterval
The default Report Interval is 120 seconds..
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4.7.8. RetransmitCount
The number of retransmissions for a given CAPWAP packet. This is a
monotonically increasing counter.
4.7.9. 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 4.5.45.
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 4.5.39.
4.8.3. LastRebootReason
The reason why the WTP last rebooted, defined in Section 4.5.45.
4.8.4. MacType
For WTPs that support multiple MAC Types, it is usefule to save the
value configured by the AC. The MAC Type is defined in
Section 4.5.41.
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 4.5.45.
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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
message element, see Section 4.5.46.
4.8.9. WTPLinkFailureCount
The number of times the link to the AC has failed, see
Section 4.5.45.
4.8.10. WTPLocation
The WTP Location, defined in Section 4.5.27.
4.8.11. WTPName
The WTP Name, defined in Section 4.5.42.
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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. A WTP must transmit this command even if it has a
statically configured AC.
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.
The following message elements MUST be included in the Discovery
Request message:
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o Discovery Type, see Section 4.5.20
o WTP Board Data, see Section 4.5.36
o WTP Descriptor, see Section 4.5.37
o WTP Frame Tunnel Mode, see Section 4.5.39
o WTP MAC Type, see Section 4.5.41
5.2. Discovery Response Message
The Discovery Response message provides a mechanism for an AC to
advertise its services to requesting WTPs.
The Discovery Response message is sent by an AC after receiving a
Discovery Request message from a WTP. As with the Discovery Request,
the Session ID field in the CAPWAP header MUST be set to zero.
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.
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 CAPWAP Control IPv4 Address, see Section 4.5.10
o 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. As a
consequence, this message is only sent by a WTP when it is in the Run
state.
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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 Discovery Request message, the AC responds with a
Primary Discovery Response message sent to the address in the source
address of the received Primary Discovery Request message.
The following message elements MUST be included in the Primary
Discovery Request message.
o Discovery Type, see Section 4.5.20
o WTP Board Data, see Section 4.5.36
o WTP Descriptor, see Section 4.5.37
o WTP Frame Tunnel Mode, see Section 4.5.39
o WTP MAC Type, see Section 4.5.41
o WTP Radio Information Element(s)that the AC supports; These are
defined by the individual link layer CAPWAP Binding Protocols.
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 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
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o WTP Radio Information Element(s)that the AC supports; These are
defined by the individual link layer CAPWAP Binding Protocols.
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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 inform an AC that it
wishes to provide services through the AC. A Join Request message is
sent by a WTP after (optionally) receiving one or more Discovery
Responses, 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, which the WTP is notified via
the DTLSEstablished notification, 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 generates a Join Response message
and sends it to the WTP, indicating success or failure.
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 following message elements MUST be included in the Join Request
message.
o Location Data, see Section 4.5.27
o WTP Board Data, see Section 4.5.36
o WTP Descriptor, see Section 4.5.37
o WTP IPv4 IP Address, see Section 4.5.40
o WTP Name, see Section 4.5.42
o Session ID, see Section 4.5.33
The following message element MAY be included in the Join Request
message.
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o WTP Reboot Statistics, see Section 4.5.45
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 it.
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 associated
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. In this
way, the WaitDTLS timer will eventually expire, in which case the WTP
may (if it is so configured) attempt to join with an alternative AC.
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 Result Code, see Section 4.5.31
o WTP Radio Information Element(s)that the AC supports; These are
defined by the individual link layer CAPWAP Binding Protocols.
The following message element MUST be included in the Join Response
message.
o AC Descriptor, see Section 4.5.1
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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 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 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, and
is only processed while in the Run state (see Section 2.3).
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 be reachable.
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.
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
Wireless Termination Point 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 parameter that is non-
default. One example is that 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 own configuration. This allows the WTP to inherit the
configuration and policies from the AC.
An AC maintains a copy of each active WTP's configuration. There is
no need for versioning or other means to identify configuration
changes. If a WTP becomes inactive, the AC MAY delete the
configuration associated with it. If a WTP fails, and connects to a
new AC, it provides its overridden configuration parameters, allowing
the new AC to be aware of the WTP's configuration.
This model allows for resiliency in case of an AC failure, that
another AC can provide service to the WTP. In this scenario, the new
AC would be automatically updated with WTP configuration changes,
eliminating the need for inter-AC communication or 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 quite 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 operations to deal with the WTP
according to its functionalities.
8.2. Configuration Status
The Configuration Status message is sent by a WTP to deliver its
current configuration to its AC.
Configuration Status messages are sent by a WTP while in the
Configure state.
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 will act upon
the content of the packet and respond to the WTP with a Configuration
Status Response message.
The Configuration Status message includes multiple Radio
Administrative State message Elements. There is one such message
element for the WTP, and one message element per radio in the WTP.
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 4.5.29
o Statistics Timer, see Section 4.5.34
o WTP Reboot Statistics, see Section 4.5.45
The following message elements MAY be included in the Configuration
Status message.
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o WTP Static IP Address Information, see Section 4.5.46
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.
Configuration Status Response messages are sent by an AC after
receiving a Configure 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 Radio, the WTP will transmit a Change State Event to
the AC, as an acknowledgement of the change in state.
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 Radio Operational Event, see Section 4.5.30
o Decryption Error Report Period, see Section 4.5.16
o Idle Timeout, see Section 4.5.23
o WTP Fallback, see Section 4.5.38
The following message element MAY be included in the Configuration
Status Response message.
o WTP Static IP Address Information, see Section 4.5.46
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8.4. Configuration Status Acknowledge
The Configuration Status Acknowledge message is sent by a WTP and
provides a mechanism for the WTP to acknowledge or report an error
condition to the AC for a requested configuration.
Configuration Status Acknowledge messages are sent by a WTP after
receiving a Configure Response message.
The Configuration Status Acknowledge message carries a status code
and may contain any specific binding message elements that could not
be set as requested by the AC. If the WTP successfully applies the
configuration, it shall set the return code value to "Success". If
the WTP is unable to apply any part of the configuration, it shall
set the return code value to "Unable to Apply Requested
Configuration" Refer to the appropriate binding for the definition of
this structure.
When a AC receives a Configuration Status Acknowledge 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 Radio, the WTP will transmit a Change State Event to
the AC, as an acknowledgement of the change in state.
The following message elements MUST be included in the Configuration
Status Acknowledge message.
o Result Code, see Section 4.5.31
8.5. 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 an AC receives a Configuration Update Request message it will
respond with a Configuration Update Response message, with the
appropriate Result Code.
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
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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 4.5.27
o Radio Operational State, see Section 4.5.30
o Statistics Timer, see Section 4.5.34
o WTP Fallback, see Section 4.5.38
o WTP Name, see Section 4.5.42
o WTP Static IP Address Information, see Section 4.5.46
8.6. 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 following message element MUST be present in the Configuration
Update message.
Result Code, see Section 4.5.31
8.7. Change State Event Request
The Change State Event Request message is used by the WTP for two
main purposes:
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o When sent by the WTP following the reception Configuration Status
Response from the AC, the WTP uses the Change State Event to
provide an update on the WTP radio's operational state as well as
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 to notify the AC of an unexpected change in the WTP's radio
operational state.
When an AC receives a Change State Event Request message it will
respond with a Change State Event Response message and make any
necessary modifications to internal WTP data structures. The AC MAY
decide not to provide service to the WTP if it receives an error,
based on local policy, which is done by transitioning to the CAPWAP
Reset state.
The Change State Event Request is sent by a WTP to acknowledge or
report an error condition to the AC for a requested configuration
through the Configuration Status Response. The Change State Event
Request 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 4.5.32).
The following message elements MUST be present in the Change State
Event Request message.
o Radio Operational State, see Section 4.5.30
o Result Code, see Section 4.5.31
One or more of the following message elements MAY be present in the
Change State Event Request message.
o Returned Message Element, see Section 4.5.32
8.8. 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 carries no message elements.
Its purpose is to acknowledge the receipt of the Change State Event
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Request message.
The WTP does not need to perform any special processing of the Change
State Event Response message.
8.9. Clear Configuration Request
The Clear Configuration Request message is used to reset a WTP's
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 CAPWAP state.
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.
8.10. 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 back to the manufacturing default values.
The Clear Configuration Request message carries the message elements.
o Result Code, see Section 4.5.31
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9. Device Management Operations
This section defines CAPWAP operations responsible for debugging,
gathering statistics, logging, and firmware management.
9.1. 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 will respond 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 methods:
When the WTP joins the AC, and each exchange their software
revision, the WTP may opt to initiate a firmware download by
sending an Image Data Request, which contains an Image Filename
message element.
Once the WTP is in the Configure state, it is possible for the AC
to cause the WTP to initiate a firmware download by sending an
Image Data Request message, with the Initiate Download and and
Image Filename message elements. The WTP then transmits the Image
Data Request message,which includes the Image Filename message
element to start the download process.
Regardless of how the download was initiated, once the AC receives an
Image Data Request with the Image Filename message element, it begins
the transfer process by transmitting its own request with the Image
Data message element. This continues until the firmware image has
been transfered.
The following message elements MAY be included in the Image Data
Request message.
o Image Data, see Section 4.5.24
o Image Filename, see Section 4.5.25
o Initiate Download, see Section 4.5.26
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9.2. 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 following message elements MUST be included in the Image Data
Response message.
o Result Code, see Section 4.5.31
Upon receiving an error, the WTP MAY decide to retransmit a previous
Image Data Reqest, or abandon the firmware download to the WTP by
transitioning to the Reset state machine.
9.3. 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 carries no message elements.
When a WTP receives a Reset Request it will respond with a Reset
Response indicating success and then reinitialize itself. In the
event the WTP is unable to reset, including a hardware reset, it can
respond with a Reset Response whose Result-Code message element
indicates failure.
9.4. 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 following message elements MAY be included in the Image Data
Request Message.
o Result Code, see Section 4.5.31
When an AC receives a successful Reset Response message, it is
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notified that the WTP will reinitialize its operation. An AC that
receives a Reset Response indicating failure may opt to no longer
provide service to the WTP in question.
9.5. WTP Event Request
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 MUST contain one of the message
elements listed below, or a message element that is defined for a
specific wireless technology.
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 4.5.43
o WTP Radio Statistics, see Section 4.5.44
o WTP Reboot Statistics, see Section 4.5.45
o Delete Station, see Section 4.5.18
9.6. WTP Event Response
The WTP Event Response message acknowledges receipt of the WTP Event
Request message.
A WTP Event Response message issent by an AC after receiving a WTP
Event Request message.
The WTP Event Response message carries no message elements.
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9.7. 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 ith a Data Transfer Response message. The AC MAY log the
information received.
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
9.8. 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.
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 section or document for the definitions of the
messages elements that may be used in this control message.
The following CAPWAP Control message elements 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
following message element MUST be present in the Station
Configuration Response message.
o Result Code, see Section 4.5.31
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 two specific situations in which a NAT system may be used
in conjunction with a CAPWAP-enabled system. The first consists of a
configuration where the WTP is behind a NAT system. Given that 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.
It is, however, possible for two or more WTPs to reside behind the
same NAT system. In this instance, the AC would receive multiple
connection requests from the same IP address, and could end up
thinking all of the connection requests come from the same WTP. It
is important that the AC not disconnect another WTP's session as a
result of this situation occuring. Therefore, the AC should consider
the WTP's identity, which is communicated within the DTLS exchange
used to secure the CAPWAP control channel. The CAPWAP header
includes a Session Identifier field, which is used to match the
control and data plane. This allows the AC to match the control and
data plane flows from multiple WTPs behind the same NAT system
(therefore all sharing the same IP address).
The second configuration is one where the AC sits behind a NAT. Two
issues exist in this situation. First, an AC communicates its
interfaces, and associated WTP load on these interfaces, through the
WTP Manager Control IP Address. This message element is currently
mandatory, and if NAT compliance became an issue, it would be
possible to either:
1. Make the WTP Manager Control IP Address optional, allowing the WTP
to simply use the known IP Address. However, note that this
approach would eliminate the ability to perform load balancing of
WTP across ACs, and therefore is not the recommended approach.
2. Allow an AC to be able to configure a NAT'ed address for every
associated AC that would generally be communicated in the WTP
Manager Control IP Address message element.
3. Require that if a WTP determines that the AC List message element
consists of a set of IP Addresses that are different from the AC's
IP Address it is currently communicating with, then assume that
NAT is being enforced, and require that the WTP communicate with
the original AC's IP Address (and ignore the WTP Manager Control
IP Address message element(s)).
Another issue related to having an AC behind a NAT system is CAPWAP's
support for the CAPWAP Objective to allow the control and data plane
to be separated. In order to support this requirement, the CAPWAP
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protocol defines the WTP Manager Data IP Address message element,
which allows the AC to inform the WTP that the CAPWAP data frames are
to be forwarded to a separate IP Address. This feature MUST be
disabled when an AC is behind a NAT. However, there is no easy way
to provide some default mechanism that satisfies both the data/
control separation and NAT objectives, as they directly conflict with
each other. As a consequence, user intervention will be required to
support such networks.
The CAPWAP protocol allows for all of the ACs 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 would be invalid.
The CAPWAP protocol has a feature that allows an AC to configure a
static IP address on a WTP. The WTP Static IP Address Information
message element provides such a function, however this feature 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. 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
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, we make the following
recommendations:
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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.
12.3. Use of Certificates in CAPWAP
For public-key-based DTLS deployments, each device SHOULD have unique
credentials, with an extended key usage authorizing them to act as
either a WTP or AC. If devices do not have unique credentials, it is
possible that by compromising one, any other one using the same
credential may also be considered to be compromised.
Certificate validation involves checking a large variety of things.
Since the necessary things to validate are often environment-
specific, many are beyond the scope of this document. In this
section, we provide some basic guidance on certificate validation.
Each device is responsible for authenticating and authorizing devices
with which they communicate. Authentication entails validation of
the chain of trust leading to the peer certificate, followed by the
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),
there is a chicken and egg problem, in that 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.
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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.
Another important part of certificate authentication is binding a
certificate to a particular device. There are many ways to
accomplish this. Specification of the certificate common name (CN)
as the WTP or AC MAC address formatted as ASCII HEX, for example, 01:
23:45:67:89:ab is REQUIRED for use with the CAPWAP protocol. During
authentication, devices SHOULD ensure that the MAC address matches
the MAC address specified in the CAPWAP header. If this mechanism is
used, the ACs SHOULD maintain list of all authorized WTP MAC
addresses and the WTP SHOULD save the AC credential or credential
identifier.
12.4. 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. 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 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.
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14. References
14.1. 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] Clancy, C., "Security Review of the Light Weight Access Point
Protocol", May 2005,
<http://www.cs.umd.edu/~clancy/docs/lwapp-review.pdf>.
14.2. Informational References
[12] "draft-ietf-capwap-protocol-binding-specification-ieee802dot11-
00".
[13] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by an On-
line Database", RFC 3232, January 2002.
[14] Modadugu et al, N., "The Design and Implementation of Datagram
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TLS", Feb 2004.
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Authors' 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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