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Network Working Group D. Loher
Request for Comments: 4565 Envysion, Inc.
Category: Informational D. Nelson
Enterasys Networks, Inc.
O. Volinsky
Colubris Networks, Inc.
B. Sarikaya
Huawei USA
July 2006
Evaluation of Candidate Control and Provisioning
of Wireless Access Points (CAPWAP) Protocols
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document is a record of the process and findings of the Control
and Provisioning of Wireless Access Points Working Group (CAPWAP WG)
evaluation team. The evaluation team reviewed the 4 candidate
protocols as they were submitted to the working group on June 26,
2005.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
1.2. Terminology ................................................3
2. Process Description .............................................3
2.1. Ratings ....................................................3
3. Member Statements ...............................................4
4. Protocol Proposals and Highlights ...............................5
4.1. LWAPP ......................................................5
4.2. SLAPP ......................................................6
4.3. CTP ........................................................6
4.4. WiCoP ......................................................7
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RFC 4565 Evaluation of Candidate CAPWAP Protocols July 2006
5. Security Considerations .........................................7
6. Mandatory Objective Compliance Evaluation .......................8
6.1. Logical Groups .............................................8
6.2. Traffic Separation .........................................8
6.3. STA Transparency ...........................................9
6.4. Configuration Consistency .................................10
6.5. Firmware Trigger ..........................................11
6.6. Monitor and Exchange of System-wide Resource State ........12
6.7. Resource Control ..........................................13
6.8. Protocol Security .........................................15
6.9. System-Wide Security ......................................16
6.10. 802.11i Considerations ...................................17
6.11. Interoperability .........................................17
6.12. Protocol Specifications ..................................18
6.13. Vendor Independence ......................................19
6.14. Vendor Flexibility .......................................19
6.15. NAT Traversal ............................................20
7. Desirable Objective Compliance Evaluation ......................20
7.1. Multiple Authentication ...................................20
7.2. Future Wireless Technologies ..............................21
7.3. New IEEE Requirements .....................................21
7.4. Interconnection (IPv6) ....................................22
7.5. Access Control ............................................23
8. Evaluation Summary and Conclusions .............................24
9. Protocol Recommendation ........................................24
9.1. High-Priority Recommendations Relevant to
Mandatory Objectives ......................................25
9.1.1. Information Elements ...............................25
9.1.2. Control Channel Security ...........................25
9.1.3. Data Tunneling Modes ...............................26
9.2. Additional Recommendations Relevant to Desirable
Objectives ................................................27
9.2.1. Access Control .....................................27
9.2.2. Removal of Layer 2 Encapsulation for Data
Tunneling ..........................................28
9.2.3. Data Encapsulation Standard ........................28
10. Normative References ..........................................29
11. Informative References ........................................29
1. Introduction
This document is a record of the process and findings of the Control
and Provisioning of Wireless Access Points Working Group (CAPWAP WG)
evaluation team. The evaluation team reviewed the 4 candidate
protocols as they were submitted to the working group on June 26,
2005.
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RFC 4565 Evaluation of Candidate CAPWAP Protocols July 2006
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Terminology
This document uses terminology defined in RFC 4118 [ARCH], RFC 4564
[OBJ], and IEEE 802.11i [802.11i].
2. Process Description
The process to be described here has been adopted from a previous
evaluation in IETF [RFC3127]. The CAPWAP objectives in RFC 4564
[OBJ] were used to set the scope and direction for the evaluators and
was the primary source of requirements. However, the evaluation team
also used their expert knowledge and professional experience to
consider how well a candidate protocol met the working group
objectives.
For each of the 4 candidate protocols, the evaluation document editor
assigned 2 team members to write evaluation briefs. One member was
assigned to write a "Pro" brief and could take a generous
interpretation of the proposal; this evaluator could grant benefit of
doubt. A second evaluator was assigned to write a "Con" brief and
was required to use strict criteria when performing the evaluation.
2.1. Ratings
The "Pro" and "Con" members independently evaluated how well the
candidate protocol met each objective. Each objective was scored as
an 'F' for failure, 'P' for partial, or 'C' for completely meeting
the objective.
F - Failure to Comply
The evaluation team believes the proposal does not meet the
objective. This could be due to the proposal completely missing any
functionality towards the objective. A proposal could also receive
an 'F' for improperly implementing the objective.
P - Partial Compliance
The proposal has some functionality that addresses the objective, but
it is incomplete or ambiguous.
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C - Compliant
The proposal fully specifies functionality meeting the objective.
The specification must be detailed enough that interoperable
implementations are likely from reading the proposal alone. If the
method is ambiguous or particularly complex, an explanation, use
cases, or even diagrams may need to be supplied in order to receive a
compliant rating.
The 4-person evaluation team held a teleconference for each candidate
to discuss the briefs. One of the working group chairs was also
present at the meeting in an advisory capacity. Each evaluator
presented a brief with supporting details. The team discussed the
issues and delivered a team rating for each objective. These
discussions are documented in the meeting minutes. The team ratings
are used for the compliance evaluation.
The candidate protocols were scored only on the information written
in their draft. This means that a particular protocol might actually
meet the specifics of a requirement, but if the proposal did not
state, describe, or reference how that requirement was met, it might
be scored lower.
3. Member Statements
Darren Loher, Roving Planet
I am employed as the senior architect at Roving Planet, which writes
network and security management software for wireless networks. I
have over 11 years of commercial experience designing and operating
networks. I have implemented and operated networks and network
management systems for a university, large enterprises, and a major
Internet service provider for over 4 years. I also have software
development experience and have written web-based network and systems
management tools including a system for managing a very large
distributed DNS system. I have witnessed the IETF standards process
for several years, my first event being IETF 28. I have rarely
directly participated in any working group activities before this
point. To my knowledge, my company has no direct relationship with
any companies that have authored the CAPWAP protocol submissions.
David Nelson, Enterasys
I am currently cochair of the RADEXT WG, AAA Doctor in O&M Area, and
employed in the core router engineering group of my company. I have
previously served on a protocol evaluation team in the AAA WG, and am
a coauthor of RFC 3127 [RFC3127]. I was an active contributor in the
IEEE 802.11i task group, and previously employed in the WLAN
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engineering group of my company. I have had no participation in any
of the submitted protocols. My company does have an OEM relationship
with at least one company whose employees have coauthored one of the
submissions, but I have no direct involvement with our WLAN product
at this time.
Oleg Volinsky, Colubris Networks
I am a member of the Enterprise group of Colubris Networks, a WLAN
vendor. I have over 10 years of experience in design and development
of network products from core routers to home networking equipment.
Over years I have participated in various IETF groups. I have been a
member of CAPWAP WG for over a year. In my current position I have
been monitoring the developments of CAPWAP standards and potential
integration of the resulting protocol into the company's products. I
have not participated in any of the candidate protocol drafts. I
have not worked for any of the companies whose staff authored any of
the candidate protocols.
Behcet Sarikaya, University of Northern British Columbia
I am currently Professor of Computer Science at UNBC. I have so far
5 years of experience in IETF as a member of mobile networking-
related working groups. I have made numerous I-D contributions and
am a coauthor of one RFC. I have submitted an evaluation draft (with
Andy Lee) that evaluated LWAPP, CTP, and WiCoP. Also I submitted
another draft (on CAPWAPHP) that used LWAPP, CTP, WiCoP, and SLAPP as
transport. I also have research interests on next-generation access
point/controller architectures. I have no involvement in any of the
candidate protocol drafts, have not contributed any of the drafts. I
have not worked in any of the companies whose staff has produced any
of the candidate protocols.
4. Protocol Proposals and Highlights
The following proposals were submitted as proposals to the CAPWAP
working group.
4.1. LWAPP
The "Light Weight Access Point Protocol" [LWAPP] was the first CAPWAP
protocol originally submitted to Seamoby Working Group. LWAPP
proposes original solutions for authentication and user data
encapsulation as well as management and configuration information
elements. LWAPP originated as a "split MAC" protocol, but recent
changes have added local MAC support as well. LWAPP has received a
security review from Charles Clancy of the University of Maryland
Information Systems Security Lab.
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LWAPP is the most detailed CAPWAP proposal. It provides a thorough
specification of the discovery, security, and system management
methods. LWAPP focuses on the 802.11 WLAN-specific monitoring and
configuration. A key feature of LWAPP is its use of raw 802.11
frames that are tunneled back to the Access Controller (AC) for
processing. In both local- and split-MAC modes, raw 802.11 frames
are forwarded to the AC for management and control. In addition, in
split-MAC mode, user data is tunneled in raw 802.11 form to the AC.
While in concept, LWAPP could be used for other wireless
technologies, LWAPP defines very few primitives that are independent
of the 802.11 layer.
4.2. SLAPP
"Secure Light Access Point Protocol" [SLAPP] distinguishes itself
with the use of well-known, established technologies such as Generic
Routing Encapsulation (GRE) for user data tunneling between the AC
and Wireless Termination Point (WTP) and the proposed standard
Datagram Transport Layer Security [DTLS] for the control channel
transport.
4 modes of operation are supported, 2 local-MAC modes and 2 split-MAC
modes. STA control may be performed by the AC using native 802.11
frames that are encapsulated in SLAPP control packets across all
modes. (STA refers to a wireless station, typically a laptop.)
In SLAPP local-MAC modes, user data frames may be bridged or tunneled
back using GRE to the AC as 802.3 frames. In the split-MAC modes,
user data is always tunneled back to the AC as native 802.11 frames.
Encryption of user data may be performed at either the AC or the WTP
in split-MAC mode.
4.3. CTP
"CAPWAP Tunneling Protocol" [CTP] distinguishes itself with its use
of Simple Network Management Protocol (SNMP) to define configuration
and management data that it then encapsulates in an encrypted control
channel. CTP was originally designed as a local-MAC protocol but the
new version has split-MAC support as well. In addition, CTP is
clearly designed from the beginning to be compatible with multiple
wireless technologies.
CTP defines information elements for management and control between
the AC and WTP. CTP control messages are specified for STA session
state, configuration, and statistics.
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In local-MAC mode, CTP does not forward any native wireless frames to
the AC. CTP specifies control messages for STA session activity,
mobility, and radio frequency (RF) resource management between the AC
and WTP. CTP local-MAC mode specifies that the integration function
from the wireless network to 802.3 Ethernet is performed at the WTP
for all user data. User data may either be bridged at the WTP or
encapsulated as 802.3 frames in CTP packets at the WTP and tunneled
to the AC.
CTP's split-MAC mode is defined as an extension to local-MAC mode.
In CTP's version of split-MAC operation, wireless management frames
are forwarded in their raw format to the AC. User data frames may be
bridged locally at the WTP, or they may be encapsulated in CTP
packets and tunneled in their native wireless form to the AC.
CTP supplies STA control abstraction, methods for extending the
forwarding of multiple types of native wireless management frames,
and many options for user data tunneling. Configuration management
is an extension of SNMP. This makes CTP one of the most flexible of
the proposed CAPWAP protocols. However, it does define new security
and data tunneling mechanisms instead of leveraging existing
standards.
4.4. WiCoP
"Wireless LAN Control Protocol" [WICOP] introduces new discovery,
configuration, and management of Wireless LAN (WLAN) systems. The
protocol defines a distinct discovery mechanism that integrates WTP-
AC capabilities negotiation.
WiCoP defines 802.11 Quality of Service (QoS) parameters. In
addition, the protocol proposes to use standard security and
authentication methods such as IPsec and Extensible Authentication
Protocol (EAP). The protocol needs to go into detail with regards to
explicit use of the above-mentioned methods. To ensure interoperable
protocol implementations, it is critical to provide users with
detailed unambiguous specification.
5. Security Considerations
Each of the candidate protocols has a Security Considerations
section, as well as security properties. The CAPWAP objectives
document [OBJ] contains security-related requirements. The
evaluation team has considered if and how the candidate protocols
implement the security features required by the CAPWAP objectives.
However, this evaluation team is not a security team and has not
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performed a thorough security evaluation or tests. Any protocol
coming out of the CAPWAP working group must undergo an IETF security
review in order to fully meet the objectives.
6. Mandatory Objective Compliance Evaluation
6.1. Logical Groups
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP provides a control message called "Add WLAN". This message is
used by the AC to create a WLAN with a unique ID, i.e., its Service
Set Identifier (SSID). The WTPs in this WLAN have their own Basic
Service Set Identifiers (BSSIDs). LWAPP meets this objective.
SLAPP
SLAPP explicitly supports 0-255 BSSIDs.
CTP
CTP implements a NETWORK_ID attribute that allows a wireless-
technology-independent way of creating logical groups. CTP meets
this objective.
WiCoP
WiCoP provides control tunnels to manage logical groups. There is
one control tunnel for each logical group. WiCoP meets this
objective.
6.2. Traffic Separation
LWAPP:C, SLAPP:C, CTP:P, WiCoP:P
If a protocol distinguishes a data message from a control message,
then it meets this objective.
LWAPP
LWAPP separates control messages from data messages using "C-bit".
"C-bit" is defined in the LWAPP transport header. When C-bit is
equal to zero, the message is a data message. When C-bit is equal to
one, the message is a control message. So, LWAPP meets this
objective.
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SLAPP
The SLAPP protocol encapsulates control using DTLS and optionally,
user data with GRE. Of particular note, SLAPP defines 4
"architecture modes" that define how user data is handled in relation
to the AC. SLAPP is compliant with this objective.
CTP
CTP defines separate packet frame types for control and data.
However, the evaluation team could not find a way to configure the
tunneling of user data, so it opted to rate CTP as only partially
compliant. It appears that CTP would rely on SNMP MIB Object
Identifiers (OIDs) for this function, but none were defined in the
specification. Defining the necessary OIDs would make CTP fully
compliant.
WiCoP
WiCoP provides for separation between control and data channels.
However, tunneling methods are not explicitly described. Because of
this, WiCoP partially meets this objective.
6.3. STA Transparency
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
If a protocol does not indicate that STA needs to know about the
protocol, then this objective is met.
The protocol must not define any message formats between STA and
WTP/AC.
LWAPP
LWAPP does not require a STA to be aware of LWAPP. No messages or
protocol primitives are defined that the STA must interact with
beyond the 802.11 standard. LWAPP is fully compliant.
SLAPP
SLAPP places no requirements on STA network elements. No messages or
protocol primitives are defined that the STA must interact with
beyond the 802.11 standard.
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CTP
CTP does not require a terminal to know CTP. So, CTP meets this
objective.
WiCoP
WiCoP does not require a terminal to know WiCoP. So, WiCoP meets
this objective.
6.4. Configuration Consistency
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
Given the objective of maintaining configurations for a large number
of network elements involved in 802.11 wireless networks, the
evaluation team would like to recommend that a token, key, or serial
number for configuration be implemented for configuration
verification.
LWAPP
It is possible to obtain and verify all configurable values through
LWAPP. Notably, LWAPP takes an approach that only "non-default"
settings (defaults are specified by LWAPP) are necessary for
transmission when performing configuration consistency checks. This
behavior is explicitly specified in LWAPP. LWAPP is compliant with
this objective.
SLAPP
Numerous events and statistics are available to report configuration
changes and WTP state. SLAPP does not have any built-in abilities to
minimize or optimize configuration consistency verification, but it
is compliant with the objective.
CTP
CTP's use of SNMP makes configuration consistency checking
straightforward. Where specified in a MIB, one could take advantage
of default values.
WICOP
The WiCoP configuration starts with exchange of capability messages
between the WTP and AC. Next, configuration control data is sent to
the WTP.
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WiCoP defines configuration values in groups of configuration data
messages. In addition, the protocol supports configuration using MIB
objects. To maintain data consistency, each configuration message
from the AC is acknowledged by the WTP.
6.5. Firmware Trigger
LWAPP:P, SLAPP:P, CTP:P, WiCoP:C
The evaluation team considered the objective and determined that for
full compliance, the protocol state machine must support the ability
to initiate the process for checking and performing a firmware update
independently of other functions.
Many protocols perform a firmware check and update procedure only on
system startup time. This method received a partial compliance. The
team believed that performing the firmware check only at startup time
was unnecessarily limiting and that allowing it to occur at any time
in the state machine did not increase complexity of the protocol.
Allowing the firmware update process to be initiated during the
running state allows more possibilities for minimizing downtime of
the WTP during the firmware update process.
For example, the firmware check and download of the image over the
network could potentially occur while the WTP was in a running state.
After the file transfer was complete, the WTP could be rebooted just
once and begin running the new firmware image. This could pose a
meaningful reduction in downtime when the firmware image is large,
the link for loading the file is very slow, or the WTP reboot time is
long.
A protocol would only fail compliance if no method was specified for
updating of firmware.
LWAPP
Firmware download is initiated by the WTP only at the Join phase
(when a WTP is first associating with an AC) and not at any other
time. The firmware check and update could be "triggered" indirectly
by the AC by sending a reset message to the WTP. The resulting
reboot would cause a firmware check and update to be performed.
LWAPP is partially compliant because its firmware trigger can only be
used in the startup phases of the state machine.
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SLAPP
SLAP includes a firmware check and update procedure that is performed
when a WTP is first connecting to an AC. The firmware check and
update can only be "triggered" indirectly by the AC by sending a
reset message to the WTP. SLAPP is partially compliant because its
firmware trigger can only be used in the startup phases of the state
machine.
CTP
The CTP state machine specifies that the firmware upgrade procedure
must be performed immediately after the authentication exchange as
defined in section 6.2 of [CTP]. However, section 5.2.5 of [CTP]
states that the SW-Update-Req message MAY be sent by the AC. This
indirectly implies that CTP could support an AC-triggered software
update during the regular running state of the WTP. So it seems that
CTP might be fully compliant, but the proposal should be clarified
for full compliance.
WiCoP
In WiCoP, firmware update may be triggered any time in the active
state, so WiCoP is fully compliant.
6.6. Monitor and Exchange of System-wide Resource State
LWAPP:C, SLAPP:C, CTP:P, WiCoP:C
The evaluation team focused on the protocols supplying 3 methods
relevant to statistics from WTPs: The ability to transport
statistics, a minimum set of standard data, and the ability to extend
what data could be reported or collected.
LWAPP
Statistics are sent by the WTP using an "Event Request" message.
LWAPP defines an 802.11 statistics message that covers 802.11 MAC
layer properties. LWAPP is compliant.
SLAPP
WLAN statistics transport is supplied via the control channel and
encoded in SLAPP-defined TLVs called information elements. 802.11
configuration and statistics information elements are supplied in
[SLAPP] 6.1.3.1. These are extendable and include vendor-specific
extensions.
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CTP
CTP defines a control message called "CTP Stats-Notify". This
control message contains statistics in the form of SNMP OIDs and is
sent from the WTP to AC. This approach is novel because it leverages
the use of standard SNMP.
Section 5.3.10 of [CTP] recommends the use of 802.11 MIBs where
applicable. However, the proposal acknowledges that additional
configuration and statistics information is required, but does not
specify these MIB extensions. CTP needs to add these extensions to
the proposal. Also, this minimum set of statistics and configuration
OIDs must become requirements in order to fully meet the objective.
WiCoP
The feedback control message sent by the WTP contains many
statistics. WiCoP specifies 15 statistics that the WTP needs to send
to the AC. New versions of WiCoP can address any new statistics that
the AC needs to monitor the WTP. WiCoP meets this objective.
6.7. Resource Control
LWAPP:C, SLAPP:P, CTP:P, WiCoP:P
The evaluation team interpreted the resource control objective to
mean that the CAPWAP protocol must map 802.11e QoS markings to the
wired network. This mapping must include any encapsulation or
tunneling of user data defined by the CAPWAP protocol. Of particular
note, the evaluation team agreed that the CAPWAP protocol should
supply an explicit capability to configure this mapping. Since most
of the protocols relied only on the 802.11e statically defined
mapping, most received a partial compliance.
LWAPP
LWAPP defines its own custom TLV structure, which consists of an
8-bit type or class of information value and an additional 8-bit
value that indexes to a specific variable.
LWAPP allows the mobile station-based QoS configuration in each Add
Mobile Request sent by AC to WTP for each new mobile station that is
attached. Packet prioritization is left to individual WTPs. 4
different QoS policies for each station to enforce can be configured.
Update Mobile QoS message element can be used to change QoS policy at
the WTP for a given mobile station. LWAPP should support 8 QoS
policies as this matches 802.11e 802.1p and IP TOS, but for this
objective, 4 classes is compliant.
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Overall, LWAPP conforms to the resource control objective. It
enables QoS configuration and mapping. The control can be applied on
a logical group basis and also enables the wireless traffic to be
flexibly mapped to the wired segment.
SLAPP
Although 802.11e specifies 802.1p and Differentiated Service Code
Point (DSCP) mappings, there is no explicit support for 802.11e in
SLAPP. SLAPP must be updated to add 802.11e as one of the standard
capabilities that a WTP could support and specify a mechanism that
would allow configuration of mapping the QoS classes.
CTP
CTP requires that the WTP and AC copy the QoS marking of user data to
the data message encapsulation. This mapping is accomplished by the
CTP Header's 1-byte policy field. However, no configuration of QoS
mapping other than copying the user data's already existing markings
is defined in CTP. It seems clear that SNMP could be used to
configure the mapping to occur differently, but no OIDs are defined
that would enable this. Partial compliance is assigned to CTP for
this objective.
WiCoP
Note: WiCoP rating for resource control objectives has been upgraded
from Failed to Partial. After an additional review of the WiCoP
protocol proposal, it was determined that the protocol partially
meets resource control objectives.
WiCoP protocol starts its QoS configuration with 802.11e capability
exchange between the WTP and AC. The QoS capabilities primitives are
included in the capability messages.
WiCoP defines the QoS-Value message that contains 802.11e
configuration parameters. This is sent for each group supported by
the WTP. WiCoP does not provide an explicit method for configuration
of DSCP tags and 802.1P precedence values. It is possible to
configure these parameters through SNMP OID configuration method, but
WiCoP does not explicitly identify any specific MIBs. Overall, WiCoP
partially meets resource control CAPWAP objectives. In order to be
fully compliant with the given objective, the protocol needs to
identify a clear method to configure 802.1p and DSCP mappings.
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6.8. Protocol Security
LWAPP:C, SLAPP:C, CTP:F, WiCoP:F
For the purposes of the protocol security objective, the evaluation
team primarily considered whether or not the candidate protocols
implement the security features required by the CAPWAP objectives.
Please refer to the Security Considerations section of this document.
LWAPP
It appears that the security mechanisms, including the key management
portions in LWAPP, are correct. One third-party security review has
been performed. However, further security review is warranted since
a CAPWAP-specific key exchange mechanism is defined. LWAPP is
compliant with the objective.
SLAPP
The SLAPP protocol implements authentication of the WTP by the AC
using the DTLS protocol. This behavior is defined in both the
discovery process and the 802.11 control process. SLAPP allows
mutual and asymmetric authentication. SLAPP also gives informative
examples of how to properly use the authentication. SLAPP should add
another informative example for authentication of the AC by the WTP.
SLAPP is compliant with the objective.
CTP
The original presentation at IETF63 of the preliminary findings of
the evaluation team reported that CTP failed this objective. This
was on the basis of asymmetric authentication not being supported by
CTP. This was due to a misunderstanding of what was meant by
asymmetric authentication by the evaluation team. The definitions of
the terminology used in [OBJ] were clarified on the CAPWAP mailing
list. CTP in fact does implement a form of asymmetric authentication
through the use of public keys.
However, CTP still fails to comply with the objective for two
reasons:
First, CTP does not mutually derive session keys. Second, CTP does
not perform explicit mutual authentication because the 2 parties
authenticating do not confirm the keys.
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WiCoP
There is not enough specific information to implement WiCoP protocol
security features. Although in concept EAP and IPsec make sense,
there is no explicit description on how these methods would be used.
6.9. System-Wide Security
LWAPP:C, SLAPP:C, CTP:F, WiCoP:F
LWAPP
LWAPP wraps all control and management communication in its
authenticated and encrypted control channel. LWAPP does not seem
particularly vulnerable to Denial of Service (DoS). LWAPP should
make a recommendation that the Join method be throttled to reduce the
impact of DoS attacks against it. Use of an established security
mechanism such as IPsec would be preferred. However, LWAPP's
independent security review lent enough confidence to declare LWAPP
compliant with the objective.
SLAPP
SLAPP is compliant due to wrapping all control and management
communication in DTLS. SLAPP also recommends measures to protect
against discovery request DoS attacks. DTLS has undergone security
review and has at least one known implementation outside of SLAPP.
At the time of this writing, DTLS is pending proposed standard status
in the IETF.
CTP
CTP introduces a new, unestablished mechanism for AC-to-WTP
authentication. For complete compliance, use of an established
security mechanism with detailed specifications for its use in CTP is
preferred. Alternatively, a detailed security review could be
performed. CTP does not point out or recommend or specify any DoS
attack mitigation requirements against Reg-Req and Auth-Req floods,
such as a rate limiter. Because CTP received an 'F' on its protocol
security objective, it follows that system-wide security must also be
rated 'F'.
WiCoP
WiCop does not address DoS attack threats. Also, as with the
protocol security objective, the protocol needs to explicitly
describe its tunnel and authentication methods.
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6.10. 802.11i Considerations
LWAPP:C, SLAPP:C, CTP:F, WiCoP:P
LWAPP
LWAPP explicitly defines mechanisms for handling 802.11i in its modes
with encryption terminated at the WTP. In order to accomplish this,
the AC sends the Pairwise Transient Key (PTK) using the encrypted
control channel to the WTP using the Add Mobile message. When
encryption is terminated at the AC, there are no special
requirements. LWAPP is compliant.
SLAPP
SLAPP defines a control message to send the PTK and Group Temporal
Key (GTK) to the WTP when the WTP is the encryption endpoint. This
control message is carried on the DTLS protected control channel.
SLAPP is compliant.
CTP
CTP lacks a specification for a control message to send 802.11i PTK
and GTK keys to a WTP when the WTP is an encryption endpoint. Based
on this, CTP fails compliance for this objective. This requirement
could be addressed either by defining new control channel information
elements or by simply defining SNMP OIDs. The transport of these
OIDs would be contained in the secure control channel and therefore
protected.
WiCoP
WiCoP lacks documentation on how to handle 4-way handshake. The case
for encryption at the AC needs clarification.
6.11. Interoperability
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP supports both split- and local-MAC architectures and is
therefore compliant to the letter of the objectives. LWAPP is
particularly rich in its support of the split-MAC architecture.
However, LWAPP's support of local-MAC is somewhat limited and could
be expanded. LWAPP is lacking a mode that allows local-MAC data
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frames to be tunneled back to the AC. A discussion of possible
extensions and issues is discussed in the recommendations section of
this evaluation.
SLAPP
SLAPP is compliant.
CTP
CTP is compliant.
WiCoP
WiCoP is compliant.
6.12. Protocol Specifications
LWAPP:C, SLAPP:P, CTP:P, WiCoP:P
LWAPP
LWAPP is nearly fully documented. Only a few sections are noted as
incomplete. Detailed descriptions are often given to explain the
purpose of the protocol primitives defined that should encourage
interoperable implementations.
SLAPP
SLAPP is largely implementable from its specification. It contains
enough information to perform an interoperable implementation for its
basic elements; however, additional informative references or
examples should be provided covering use of information elements,
configuring multiple logical groups, and so on.
CTP
As noted earlier, there are a few areas where CTP lacks a complete
specification, primarily due to the lack of specific MIB definitions.
WiCoP
Due to the lack of specific tunnel specifications and authentication
specifications, WiCoP is only partially compliant.
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6.13. Vendor Independence
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP is compliant.
SLAPP
SLAPP is compliant.
CTP
CTP is compliant.
WiCoP
WiCoP is compliant.
6.14. Vendor Flexibility
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP is compliant.
SLAPP
SLAPP is compliant.
CTP
CTP is compliant.
WiCoP
WiCoP is compliant.
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6.15. NAT Traversal
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP may require special considerations due to it carrying the IP
address of the AC and data termination points in the payload of
encrypted control messages. To overcome Network Address Translation
(NAT), static NAT mappings may need to be created at the NAT'ing
device if the AC or data termination points addresses are translated
from the point of view of the WTP. A WTP should be able to function
in the hidden address space of a NAT'd network.
SLAPP
SLAPP places no out-of-the-ordinary constraints regarding NAT. A WTP
could function in the hidden address space of a NAT'd network without
any special configuration.
CTP
CTP places no out-of-the-ordinary constraints regarding NAT. A WTP
could function in the hidden address space of a NAT'd network without
any special configuration.
WiCoP
WiCoP places no out-of-the-ordinary constraints regarding NAT. A WTP
could function in the hidden address space of a NAT'd network without
any special configuration.
7. Desirable Objective Compliance Evaluation
7.1. Multiple Authentication
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP allows for multiple STA authentication mechanisms.
SLAPP
SLAPP does not constrain other authentication techniques from being
deployed.
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CTP
CTP supports multiple STA authentication mechanisms.
WiCoP
WiCoP allows for multiple STA authentication mechanisms.
7.2. Future Wireless Technologies
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP could be used for other wireless technologies. However, LWAPP
defines very few primitives that are independent of the 802.11 layer.
SLAPP
SLAPP could be used for other wireless technologies. However, SLAPP
defines very few primitives that are independent of the 802.11 layer.
CTP
CTP supplies STA control abstraction, methods for extending the
forwarding of multiple types of native wireless management frames,
and many options for user data tunneling. Configuration management
is an extension of SNMP, to which new MIBs could, in concept, be
easily plugged in. This helps makes CTP a particularly flexible
proposal for supporting future wireless technologies. In addition,
CTP has already defined multiple wireless protocol types in addition
to 802.11.
WiCoP
WiCoP could be used for other wireless technologies.
7.3. New IEEE Requirements
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP's extensive use of native 802.11 frame forwarding allows it to
be transparent to many 802.11 changes. It, however, shifts the
burden of adapting MAC layer changes to the packet processing
capabilities of the AC.
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SLAPP
SLAPP's use of native 802.11 frames for control and management allows
SLAPP a measure of transparency to changes in 802.11. Because SLAPP
also supports a mode that tunnels user data as 802.3 frames, it has
additional architectural options for adapting to changes on the
wireless infrastructure.
CTP
CTP has perhaps the greatest ability to adapt to changes in IEEE
requirements. Architecturally speaking, CTP has several options
available for adapting to change. SNMP OIDs are easily extended for
additional control and management functions. Native wireless frames
can be forwarded directly to the AC if necessary. Wireless frames
can be bridged to 802.3 frames and tunneled back to the AC to protect
the AC from changes at the wireless MAC layer. These options allow
many possible ways to adapt to change of the wireless MAC layer.
WiCoP
Because WiCoP uses 802.11 frames for the data transport, it is
transparent to most IEEE changes. Any new IEEE requirements may need
new configuration and new capability messages between the WTP and AC.
The AC would need to be modified to handle new 802.11 control and
management frames.
7.4. Interconnection (IPv6)
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP explicitly defines measures for accommodating IPv6. LWAPP is
more sensitive to this in part because it carries IP addresses in two
control messages.
SLAPP
SLAPP is transparent to the interconnection layer. DTLS and GRE will
both operate over IPv6.
CTP
CTP is transparent to the interconnection layer. CTP should be able
to operate over IPv6 without any changes.
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WiCoP
WiCoP is transparent to the interconnection layer and should be able
to operate over IPv6 without changes.
7.5. Access Control
LWAPP:C, SLAPP:C, CTP:C, WiCoP:C
LWAPP
LWAPP uses native 802.11 management frames forwarded to the AC for
the purpose of performing STA access control. WTPs are authenticated
in LWAPP's control protocol Join phase.
SLAPP
SLAPP has support for multiple authentication methods for WTPs. In
addition, SLAPP can control STA access via 802.11 management frames
forwarded to the AC or via SLAPP's information element primitives.
CTP
CTP specifies STA access control primitives.
WiCoP
WiCoP specifies access control in [WICOP] section 5.2.2.
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8. Evaluation Summary and Conclusions
See Figure 1 (section numbers correspond to RFC 4564 [OBJ]).
---------------------------------------------------------------
| CAPWAP Evaluation | LWAPP | SLAPP | CTP | WiCoP |
|---------------------------------------------------------------|
| 5.1.1 Logical Groups | C | C | C | C |
| 5.1.2 Traffic Separation | C | C | P | P |
| 5.1.3 STA Transparency | C | C | C | C |
| 5.1.4 Config Consistency | C | C | C | C |
| 5.1.5 Firmware Trigger | P | P | P | C |
| 5.1.6 Monitor System | C | C | P | C |
| 5.1.7 Resource Control | C | P | P | P |
| 5.1.8 Protocol Security | C | C | F | F |
| 5.1.9 System Security | C | C | F | F |
| 5.1.10 802.11i Consideration | C | C | F | P |
|---------------------------------------------------------------|
| 5.1.11 Interoperability | C | C | C | C |
| 5.1.12 Protocol Specifications | C | P | P | P |
| 5.1.13 Vendor Independence | C | C | C | C |
| 5.1.14 Vendor Flexibility | C | C | C | C |
| 5.1.15 NAT Traversal | C | C | C | C |
|---------------------------------------------------------------|
| Desirable |
|---------------------------------------------------------------|
| 5.2.1 Multiple Authentication | C | C | C | C |
| 5.2.2 Future Wireless | C | C | C | C |
| 5.2.3 New IEEE Requirements | C | C | C | C |
| 5.2.4 Interconnection (IPv6) | C | C | C | C |
| 5.2.5 Access Control | C | C | C | C |
---------------------------------------------------------------
Figure 1: Summary Results
9. Protocol Recommendation
The proposals presented offer a variety of novel features that
together would deliver a full-featured, flexible, and extensible
CAPWAP protocol. The most novel of these features leverage existing
standards where feasible. It is this evaluation team's opinion that
a mix of the capabilities of the proposals will produce the best
CAPWAP protocol.
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The recommended features are described below. Many of these novel
capabilities come from CTP and SLAPP and WiCoP. However, LWAPP has
the most complete base protocol and is flexible enough to be extended
or modified by the working group. We therefore recommend that LWAPP
be used as the basis for the CAPWAP protocol.
The evaluation team recommends that the working group carefully
consider the following issues and recommended changes. The
evaluation team believes that a more complete CAPWAP protocol will be
delivered by addressing these issues and changes.
9.1. High-Priority Recommendations Relevant to Mandatory Objectives
9.1.1. Information Elements
LWAPP's attribute value pair system meets the objectives as defined
by the working group. However, it has only 8 bits assigned for
attribute types, with an additional 8 bits for a specific element
within an attribute type. The evaluation team strongly recommends
that a larger number of bits be assigned for attribute types and
information elements.
9.1.2. Control Channel Security
LWAPP's security mechanisms appear satisfactory and could serve
CAPWAP going forward. However, the evaluation team recommends
adoption of a standard security protocol for the control channel.
There are several motivations for a standards-based security
protocol, but the primary disadvantage of a new security protocol is
that it will take longer and be more difficult to standardize than
reusing an existing IETF standard. First, a new security protocol
will face a longer, slower approval processes from the Security Area
Directorate and the IESG. The new CAPWAP security protocol will need
to pass several tests including the following:
What is uniquely required by CAPWAP that is not available from an
existing standard protocol? How will CAPWAP's security protocol meet
security area requirements for extensibility, such as the ability to
support future cipher suites and new key exchange methods? How does
this ability compare to established security protocols that have
these capabilities?
Points such as these are continually receiving more attention in the
industry and in the IETF. Extensibility of key exchange methods and
cipher suites are becoming industry standard best practices. These
issues are important topics in the IETF Security Area Advisory Group
(SAAG) and the SecMech BOF, held during the 63rd IETF meeting.
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These issues could be nullified by adopting an appropriate existing
standard security protocol. IPsec or DTLS could be a standards
alternative to LWAPP's specification. DTLS presents a UDP variant of
Transport Layer Security (TLS). Although DTLS is relatively new, TLS
is a heavily used, tried-and-tested security protocol.
The evaluation team recommends that whatever security protocol is
specified for CAPWAP, its use cases must be described in detail.
LWAPP does a good job of this with its proposed, proprietary method.
If an updated specification is developed, it should contain at least
one mandatory authentication and cipher method. For example, pre-
shared key and x.509 certificates could be specified as mandatory
authentication methods, and Advanced Encryption Standard (AES)
Counter Mode with CBC-MAC Protocol (CCMP) could be selected as a
mandatory cipher.
Given the possibilities for code reuse, industry reliance on TLS, and
the future for TLS, DTLS may be a wise alternative to a security
method specific to CAPWAP. In addition, use of DTLS would likely
expedite the approval of CAPWAP as a proposed standard over the use
of CAPWAP-specific security mechanisms.
9.1.3. Data Tunneling Modes
9.1.3.1. Support for Local MAC User Data Tunneling
The issue of data encapsulation is closely related to the split- and
local-MAC architectures. The split-MAC architecture requires some
form of data tunneling. All the proposals except LWAPP offer a
method of tunneling in local-MAC mode as well. By local-MAC data
tunneling, we mean the tunneling of user data as 802.3 Ethernet
frames back to the AC from a WTP that is otherwise in local-MAC mode.
Tunneling data in local-MAC mode offers the ability for implementers
to innovate in several ways even while using a local-MAC
architecture. For example, functions such as mobility, flexible user
data encryption options, and fast handoffs can be enabled through
tunneling of user data back to an AC, or as LWAPP defines, a data
termination endpoint, which could be different from the AC. In
addition, there are special QoS or application-aware treatments of
user data packets such as voice or video. Improved transparency and
compatibility with future wireless technologies are also possible
when encapsulating user data in a common format, such as 802.3,
between the access point and the AC or other termination point in the
network.
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Another possibility is when a native wireless MAC changes in the
future, if a new WTP that supports this MAC change can also support a
wireless MAC -> 802.3 integration function, then the wireless MAC
layer change may remain transparent to an AC and still maintain many
of the benefits that data tunneling can bring.
LWAPP does support a header for tunneled user data that contains
layer 1 wireless information (Received Signal Strength Indication
(RSSI) and Signal-to-Noise Ratio (SNR)) that is independent of the
wireless layer 2 MAC. Innovations related to the use of RSSI and SNR
at the AC may be retained even when tunneling 802.3 user data across
different wireless MACs.
It is likely that many other features could be created by innovative
implementers using this method. However, LWAPP narrowly defines the
local-MAC architecture to exclude an option of tunneling data frames
back to the AC. Given the broad support for tunneling 802.3 data
frames between the WTP and AC across all the proposals and existing
proprietary industry implementations, the evaluation team strongly
recommends that the working group consider a data tunneling mode for
local-MAC be added to the LWAPP proposal and become part of the
standard CAPWAP protocol.
9.1.3.2. Mandatory and Optional Tunneling Modes
If more than one tunneling mode is part of the CAPWAP protocol, the
evaluation team recommends that the working group choose one method
as mandatory and other methods as optional. In addition, the CAPWAP
protocol must implement the ability to negotiate which tunneling
methods are supported through a capabilities exchange. This allows
ACs and WTPs freedom to implement a variety of modes but always have
the option of falling back to a common mode.
The choice of which mode(s) should be mandatory is an important
decision and may impact many decisions implementers have to make with
their hardware and software choices for both WTPs and ACs. The
evaluation team believes that the working group should address this
issue of local-MAC data tunneling and carefully choose which mode(s)
should be mandatory.
9.2. Additional Recommendations Relevant to Desirable Objectives
9.2.1. Access Control
Abstraction of STA access control, such as that implemented in CTP
and WiCoP, stands out as a valuable feature as it is fundamental to
the operational capabilities of many types of wireless networks, not
just 802.11. LWAPP implements station access control as an 802.11-
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specific function via forwarding of 802.11 control frames to the
access controller. LWAPP has abstracted the STA Delete function out
of the 802.11 binding. However, the Add STA function is part of the
802.11 binding. It would be useful to implement the wireless MAC
independent functions for adding a STA outside of the 802.11 binding.
9.2.2. Removal of Layer 2 Encapsulation for Data Tunneling
LWAPP currently specifies layer 2 and layer 3 methods for data
tunneling. The evaluation team believes that the layer 2 method is
redundant to the layer 3 method. The team recommends that the layer
2 method encapsulation be removed from the LWAPP protocol.
9.2.3. Data Encapsulation Standard
LWAPP's layer 3 data encapsulation meets the working group
objectives. However, the evaluation team recommends the use of a
standards-based protocol for encapsulation of user data between the
WTP and AC. GRE or Layer 2 Tunneling Protocol (L2TP) could make good
candidates as standards-based encapsulation protocols for data
tunneling.
Using a standard gives the opportunity for code reuse, whether it is
off-the-shelf microcode for processors, code modules that can be
purchased for real-time operating systems, or open-source
implementations for Unix-based systems. In addition, L2TP and GRE
are designed to encapsulate multiple data types, increasing
flexibility for supporting future wireless technologies.
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10. Normative References
[802.11i] IEEE Standard 802.11i, "Medium Access Control (MAC)
Security Enhancements", July 2004.
[ARCH] Yang, L., Zerfos, P., and E. Sadot, "Architecture Taxonomy
for Control and Provisioning of Wireless Access Points
(CAPWAP)", RFC 4118, June 2005.
[OBJ] Govindan, S., Ed., Cheng, H., Yao, ZH., Zhou, WH., and L.
Yang, "Objectives for Control and Provisioning of Wireless
Access Points (CAPWAP)", RFC 4564, July 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
11. Informative References
[CTP] Singh , I., Francisco, P., Pakulski , K., and F. Backes,
"CAPWAP Tunneling Protocol (CTP)", Work in Progress, April
2005.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[LWAPP] Calhoun, P., O'Hara, B., Kelly, S., Suri, R., Williams,
M., Hares, S., and N. Cam Winget, "Light Weight Access
Point Protocol (LWAPP)", Work in Progress, March 2005.
[RFC3127] Mitton, D., St.Johns, M., Barkley, S., Nelson, D., Patil,
B., Stevens, M., and B. Wolff, "Authentication,
Authorization, and Accounting: Protocol Evaluation", RFC
3127, June 2001.
[SLAPP] Narasimhan, P., Harkins, D., and S. Ponnuswamy, "SLAPP :
Secure Light Access Point Protocol", Work in Progress, May
2005.
[WICOP] Iino, S., Govindan, S., Sugiura, M., and H. Cheng,
"Wireless LAN Control Protocol (WiCoP)", Work in Progress,
March 2005.
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Authors' Addresses
Darren P. Loher
Envysion, Inc.
2010 S. 8th Street
Boulder, CO 80302
USA
Phone: +1.303.667.8761
EMail: dplore@gmail.com
David B. Nelson
Enterasys Networks, Inc.
50 Minuteman Road
Anover, MA 01810-1008
USA
Phone: +1.978.684.1330
EMail: dnelson@enterasys.com
Oleg Volinsky
Colubris Networks, Inc.
200 West Street
Waltham, MA 02451
USA
Phone: +1.781.547.0329
EMail: ovolinsky@colubris.com
Behcet Sarikaya
Huawei USA
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Phone: +1.972.509.5599
EMail: sarikaya@ieee.org
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Full Copyright Statement
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