Network Working Group D. Loher
Internet-Draft Roving Planet, Inc.
Expires: February 2, 2006 D. Nelson
Enterasys Networks, Inc.
O. Volinsky
Colubris Networks, Inc.
B. Sarikaya
UNBC
August 2005
Evaluation of Candidate CAPWAP Protocols
draft-ietf-capwap-eval-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document is a record of the process and findings of the
Configuration and Provisioning of Wireless Access Points Working
Group (CAPWAP WG) evaluation team. The evaluation team reviewed the
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four candidate protocols as they were submitted to the working group
on the 26th of June, 2005.
Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions used in this document . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Process Description . . . . . . . . . . . . . . . . . . . . . 5
2.1. Ratings . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Member Statements . . . . . . . . . . . . . . . . . . . . . . 7
4. Protocol Proposals and Highlights . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Mandatory Objective Compliance Evaluation . . . . . . . . . . 12
6.1. Logical Groups . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Traffic Separation . . . . . . . . . . . . . . . . . . . . 12
6.3. STA Transparency . . . . . . . . . . . . . . . . . . . . . 13
6.4. Configuration Consistency . . . . . . . . . . . . . . . . 14
6.5. Firmware Trigger . . . . . . . . . . . . . . . . . . . . . 14
6.6. Monitor and Exchange of System-wide Resource State . . . . 16
6.7. Resource Control . . . . . . . . . . . . . . . . . . . . . 17
6.8. Protocol Security . . . . . . . . . . . . . . . . . . . . 18
6.9. System-Wide Security . . . . . . . . . . . . . . . . . . 19
6.10. 802.11i Considerations . . . . . . . . . . . . . . . . . . 20
6.11. Interoperability . . . . . . . . . . . . . . . . . . . . . 21
6.12. Protocol Specifications . . . . . . . . . . . . . . . . . 21
6.13. Vendor Independence . . . . . . . . . . . . . . . . . . . 22
6.14. Vendor Flexibility . . . . . . . . . . . . . . . . . . . . 22
6.15. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 23
7. Desirable Objective Compliance Evaluation . . . . . . . . . . 24
7.1. Multiple Authentication . . . . . . . . . . . . . . . . . 24
7.2. Future Wireless Technologies . . . . . . . . . . . . . . . 24
7.3. New IEEE Requirements . . . . . . . . . . . . . . . . . . 25
7.4. Interconnection (IPv6) . . . . . . . . . . . . . . . . . . 25
7.5. Access Control . . . . . . . . . . . . . . . . . . . . . . 26
8. Evaluation Summary and Conclusions . . . . . . . . . . . . . . 27
9. Protocol Recommendation . . . . . . . . . . . . . . . . . . . 28
9.1. High priority recommendations relevant to mandatory
objectives . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1.1. Information Elements . . . . . . . . . . . . . . . . . 28
9.1.2. Control Channel Security . . . . . . . . . . . . . . . 28
9.1.3. Data Tunneling Modes . . . . . . . . . . . . . . . . . 29
9.2. Additional Recommendations Relevant to Desirable
Objectives . . . . . . . . . . . . . . . . . . . . . . . . 31
9.2.1. Access Control . . . . . . . . . . . . . . . . . . . . 31
9.2.2. Removal of Layer 2 Encapsulation for Data Tunneling . 31
9.2.3. Data Encapsulation Standard . . . . . . . . . . . . . 31
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10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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1. Definitions
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 follows the terminology defined in [ARCH] (a work in
progress), and [OBJ] (also a work in progress).
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2. Process Description
The process to be described here has been adopted from a previous
evaluation in IETF [RFC3127]. The CAPWAP objectives draft [OBJ] was
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 four candidate protocols, the evaluation draft editor
assigned two 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.
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 four 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 their brief with supporting details. The team discussed
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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.
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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 co-chair 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 co-author of RFC 3127 [RFC3127]. I was an active contributor in
the IEEE 802.11i task group, and previously employed in the WLAN
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 co-authored 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
five years of experience in IETF as a member of mobile networking
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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.
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4. Protocol Proposals and Highlights
The following proposals were submitted as proposals to the CAPWAP
working group.
[LWAPP] "Light Weight Access Point Protocol", Work in progress.
LWAPP was the first CAPWAP protocol orginally submitted to Seamoby
Working Group. LWAPP proposes original solutions for authentication,
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.
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 it's use of raw 802.11
frames which are tunneled back to the AC for processing. In both
Local and Split MAC mode, 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. Although in concept,
LWAPP could be used for other wireless technologies, LWAPP defines
very few primitives that are independant of the 802.11 layer.
[SLAPP]"Secure Light Access Point Protocol", Work in progress.
SLAPP distinguishes itself with the use of well known, established
technologies such as GRE for user data tunneling between AC and WTP
and the proposed standard DTLS for the control channel transport.
Four 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 which are encapsulated in SLAPP control packets across
all modes.
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.
[CTP]"CAPWAP Tunneling Protocol", Work in progress.
CTP distinguishes itself with its use of SNMP to define configuration
and management data which it then encapsulates in an encrypted
control channel. CTP was originally designed as a local MAC protocol
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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.
In local MAC mode, CTP does not forward any native wireless frames to
the AC. CTP specifies control messages for STA session activity,
mobilitiy and 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.
[WICOP]"Wireless LAN Control Protocol", Work in progress.
The WiCoP protocol introduces new discovery, configuration and
management of WLAN systems. The protocol defines a distinct
discovery mechanism that integrates WTP-AC capabilities negotiation.
WiCoP defines 802.11 QoS parameters. In addition, the protocol
proposes to use standard security and authentication methods such as
IPSec and EAP. The protocol needs to go into detail with regards to
explicit use of the above mentioned methods. To assure interoperable
protocol implementations, it is critical to provide user with
detailed unambiguous specification.
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5. Security Considerations
Each of the candidate protocols has a Security Considerations
section, as well as security properties. The CAPWAP Objectives
document contains security related requirements. The evaluation team
has considered if and how the candidate candidate protocols implement
the security features required by the CAPWAP objectives. However,
this evaluation team is not a security team and has not performed a
thorough security evaluation or tests. Any protocol coming out of
the CAPWAP working group must undergo a IETF security review in order
to fully meet the objectives.
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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 AC to create a WLAN with a unique ID, i.e. its SSID. The
WTPs in this WLAN have their own BSSIDs. LWAPP meets this objective.
SLAPP
SLAPP explicitly supports 0-255 BSSID's.
CTP
CTP implements a NETWORK_ID attribute which allows a wireless
technology independant 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, message is a control message. So, LWAPP meets this objective
SLAPP
The SLAPP protocol encapsulates control using DTLS and optionally,
user data with GRE. Of particular note, SLAPP defines 4
"architecture modes" which define how user data is handled in
relation to the AC. SLAPP is compliant with this objective.
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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 opted to rate CTP as only partially
compliant. It appears that CTP would rely on SNMP MIB OID's for this
function, but none were defined in the specification. Defining the
necessary OID's 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.
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.
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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 event and statistics are available to report config 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
straightfoward. Where specified in an a MIB, one could take
advantage of default values.
WICOP
The WiCoP configuration starts with exchange of capability messages
between WTP and AC. Next, configuration control data is sent to WTP.
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 AC is acknowledged by 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
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to initiate the process for checking and performing a firmware update
independantly 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 occur 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 being 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 it 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.
SLAPP
SLAP{ includes a firmware check and update procedure which 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
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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 three methods
relevant to statistics from WTP's; 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 WTP using "Event Request" message. LWAPP
defines an 802.11 statistics message which covers 802.11 MAC layer
properties. LWAPP is compliant
SLAPP
WLAN statisitics transport is supplied via the control channel and
encoded in SLAPP defined TLV's 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.
CTP
CTP defines a control message called "CTP Stats-Notify". This
control message contains statistics in the form of SNMP OID's and is
sent from the WTP to AC. This approach is novel because it leverages
the use of standard SNMP.
[CTP] 5.3.10 reccommends 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 OID's must
become requirements in order to fully meet the objective.
WiCoP
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The feedback control message sent by WTP contains many statistics.
WiCoP specifies fifteen statistics WTP needs to send to AC. New
versions of WiCoP can address any new statistics AC needs to monitor
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 it's own custom TLV structure which consists of an 8
bit type or class of information value and an additional 8 bit value
which 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.
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 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 which would allow configuration of mapping
the QoS classes.
CTP
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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 OID's are defined
which would enable this. Partial compliance is assigned to CTP for
this objective.
WiCoP
Note: WiCoP rating for Resource Control objective has been upgraded
from FAILED to PARTIAL. After an additional review of the WiCoP
protocol proposal, it was determined that the protocol partially
meats Resource control objectives.
WiCoP protocol starts its QoS configuration with 802.11e capability
exchange between WTP and AC. The QoS capabilities primitives are
included in the capability messages.
WiCoP defines the QoS-Value message which contains 802.11e
configuration parameters. This is sent for each group supported by
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 objects. 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.
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
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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 two parties
authenticating do not confirm the keys.
WiCoP
There is not enough specific information to implement WiCOP protocol
security features. While 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 it's
authenticated and encrypted control channel. LWAPP does not seem
particularly vulnerable to 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 independant security review
lended enough confidence to declare LWAPP compliant with the
objective.
SLAPP
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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 it's 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 it's 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.
6.10. 802.11i Considerations
LWAPP:C, SLAPP:C, CTP:F, WiCoP:P
LWAPP
LWAPP explicitly defines mechanisms for handling 802.11i in it's
modes with encryption terminated at the WTP. In order to accomplish
this, the AC sends the 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 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
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could be addressed either by defining new control channel information
elements or by simply defining SNMP OID's. The transport of these
OID's would be contained in the secure control channel and therefore
protected.
WiCoP
WiCoP lacks documentation in the draft 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 which allows Local MAC data
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 which should encourage
interoperable implementations.
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SLAPP
SLAPP is largely implementable from it's specification. It contains
enough information to perform an interoperable implementation for
it's basic elements, however additional informative references or
examples should be provided covering use of information elements,
configuring multiple logical groups, etc.
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.
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
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SLAPP is compliant.
CTP
CTP is compliant.
WiCoP
WiCoP is compliant.
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 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 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.
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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 for being
deployed.
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 independant of the 802.11 layer.
SLAPP
SLAPP could be used for other wireless technologies. However, SLAPP
defines very few primitives that are independant 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, for which new MIB's 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.
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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.
SLAPP
SLAPP's use of native 802.11 frames for control and management allow
SLAPP a measure of transparency to changes in 802.11. Because SLAPP
also supports a mode which tunnels user data as 802.3 frames, it has
additonal 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 OID's 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 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
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LWAPP explicitly defines measures for accomodating 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.
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. WTP's are
authenticated in LWAPP's control protocol Join phase.
SLAPP
SLAPP has support for multiple authentication methods for WTP's. 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.
---------------------------------------------------------------
| 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
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9. Protocol Recommendation
The proposals presented offer a variety of novel features which
together would deliver a full featured, flexible and extensible
CAPWAP protocol. The most novel of these features leverage existing
standards where feasable. It is this evaluation team's opinion that
a mix of the capabilities of the proposals will produce the best
CAPWAP protocol.
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 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 a 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:
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
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support future cipher suites and new key exchange methods? How does
this ability compare to established security protocols which 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
ciper suites are becoming industry standard best practices. These
issues are important topics in the IETF Security Area Advisory Group
(SAAG) and the recent SecMech BOF.
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
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, that it's use cases must be described in
detail. LWAPP does a good job of this with it's 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 AES 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
expidite 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 which is otherwise in Local MAC
mode.
Tunneling data in Local MAC mode offers the ability for implementors
to innovate in several ways even while using a Local MAC
architecture. For example, functions such as mobility, flexible user
data encryption options, fast handoffs can be enabled through
tunneling of user data back to an AC, or as LWAPP defines, a data
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termination endpoint, which could be different than the AC. In
addition, 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.
Another possibility is when a native wireless MAC changes in the
future, if a new WTP which 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 which contains
layer 1 wireless information (RSSI and SNR) that is independant 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 MAC's.
It is likely many other features could be created by innovative
implementors 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 WTP and AC across all the proposals and existing
proprietary industry implementations, the evaluation team strongly
recommends 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
AC's and WTP's 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 an implementor has to make
with their hardware and software choices for both WTP's and AC's.
The evaluation team believes the working group should address this
issue of Local MAC data tunneling and carefully choose which mode(s)
should be mandatory.
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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 a 802.11
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
independant 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 the layer 2 method is
redundant to the layer 3 method. The team recommends the layer 2
method encapsulation be removed from the LWAPP protocol.
9.2.3. Data Encapsulation Standard
LWAPP's layer 3 data encapsulation meet's 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 L2TP could make good candidates as standards
based encapsulation protocols for data tunneling.
Using a standard gives the opprotunity 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.
10. References
[ARCH] Yang, L., Zerfos, P., and E. Sadot, "Architecture Taxonomy
for Control and Provisioning of Wireless Access
Points(CAPWAP)", November 2004.
[CTP] Singh , I., Francisco, P., Pakulski , K., and F. Backes ,
"CAPWAP Tunneling Protocol (CTP)", April 2005.
[LWAPP] Calhoun, P., O'Hara, B., Kelly, S., Suri, R., Williams,
M., Hares, S., and N. Cam Winget, "Light Weight Access
Point Protocol (LWAPP)", March 2005.
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[OBJ] Govindan, S., Yao, ZH., Zhou, WH., Yang, L., and H. Cheng,
"Objectives for Control and Provisioning of Wireless
Access Points (CAPWAP)", June 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[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", May 2005.
[WICOP] Iino, S., Govindan, S., Sugiura, M., and H. Cheng,
"Wireless LAN Control Protocol (WiCoP)", March 2005.
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Authors' Addresses
Darren P. Loher
Roving Planet, Inc.
7237 Church Ranch Blvd.
Westminster, CO 80021
USA
Phone: +1.303.996.7560
Email: dloher@rovingplanet.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
UNBC
Computer Science Dept.
3333 University Way
Prince George, BC V2N 4Z9
Canada
Phone: +1.250.960.5551
Email: sarikaya@ieee.org
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