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Light Weight Access Point Protocol(LWAPP)
Pat R. Calhoun
Bob OHara
Rohit SuriNancy Cam Winget
Scott Kelly
Michael Williams
Sue Hares
draft-ohara-capwap-lwapp-03.txt
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Introduction
LWAPP is a candidate protocol for CAPWAPthat supports both Split
and Local MACapproaches
The protocol specification is mature andcomplete Products have
been shipping for well over 2 years
LWAPP specs have been available through individualcontributions
for well over 18 months
Many comments have been received (both technicaland editorial),
which have been included in thespecification.
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Introduction (cont.)
LWAPP Version 03 was submitted to the IETF
This document comprises of many changes: Addresses all comments
and issues identified in
Charles Clancys security review:
(http://www.cs.umd.edu/~clancy/docs/lwapp-review.pdf) Addresses
all non-conforming objectives listed in
LWAPP self evaluation version 00
Complete text for Local MAC support was added Although initially
supported, normative text was missing
Support for IPv6 Added significant amount of behavioral text to
aid in
interoperability e.g., BSSID/SSID Mapping recommendation
http://www.cs.umd.edu/~clancy/docs/lwapp-review.pdfhttp://www.cs.umd.edu/~clancy/docs/lwapp-review.pdfhttp://www.cs.umd.edu/~clancy/docs/lwapp-review.pdfhttp://www.cs.umd.edu/~clancy/docs/lwapp-review.pdf
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Why use 802.11 Frames?
An AC can perform its task better if it hascomplete
information
e.g., BSSID enforcement at the AC
Signal strength allows AC to make accesspolicy decisions based
on RF information
Also useful for Local MAC
Proxy MAC allows WTP to make accesscontrol decisions, while
providing visibility tothe AC
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Addressing Security Review
Comments We worked directly with Charles in addressing
identified
issues, ensuring the solution was technically
(andcryptographically) sound, including: Simplified the state
machine to provide key confirmation for all
security mechanisms supported
Mutual Derivation of LWAPP Session Keys and
InitializationVector
Unified Key Exchange protocol for both X.509 (asymmetric)
andpre-shared key (symmetric) security modes
Included an X.509 certificate profile to ease interoperability
(andeliminate man-in-the-middle attacks)
Text describing the use of 802.11i, and how to handle handoffsin
conjunction with 802.11i to avoid vulnerabilities
Makes use of NIST approved cryptographic algorithmsonly
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Basic LWAPP Architecture
AC
WTP
STA
802.11
AssocReq
802.11
Data Frame
802.11AssocReq
LWAPP(C=0)
802.11Data Frame
LWAPP(C=0)
802.11
AssocResp
802.11AssocResp
LWAPP(C=0)
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Advantages of using 802.11 frames
The design goal behind LWAPP was to allow for
802.11 extensions to be added with minimal (if
any) protocol changes.
Minimize lag time between IEEE 802.11extension publication and
ability to deliver
CAPWAP based solutions
LWAPP is also efficient on AP processor as it
only requires tunneling
Local MAC requires additional processing on AP to
provide Proxy MAC
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LWAPP Configuration Mgmt
AC
WTP
Config Request
(Override Configuration)(SSID=foobar, RSN, WMM)
Config Update
(Configuration)(e.g., External Antenna)
Override
Configuration
By default, WTP uses AC configuration, but can have its own
override configuration
for APs that require different configuration from the norm (e.g,
corner of building AP
requires only left antenna to be enabled).
Global
Configuration
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Advantages of Configuration Mgmt
Allows for centralized (global AC) configuration
policies to be enforced
Allows for localized configuration override for
specific WTPs Allows for WTP to provide localized
configuration to one of many ACs, without a
need for a global WTP configuration database
No complex configuration versioning problem
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Modes of Operation
Split MAC Encryption at WTP Mandatory to implement for Split
MAC
Split MAC Encryption in AC Optional
Local MAC Encryption at WTP Mandatory to implement
Small number of modes of operation
Provides sufficient flexibility
Mandatory to implement modes guarantee interoperability
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Quality of Service
The LWAPP Spec contains complete QoS
handling, including:
Marking of tunneled packets between AC and
WTP
Configuration of 802.11e EDCA Parameter in
the WTP
Enforcement of 802.11e at the WTP Configuration of
802.11e/802.1P/DSCP table
mapping
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Objectives Comparison
Feature ComplianceRating
Logical Groups S
Support for Traffic Separation S
Wireless Terminal Transparency S
Configuration Consistency S
Firmware Trigger S
Monitoring and Exchange of System-wide Resource State SResource
Control Objective S
CAPWAP Protocol Security S
System-wide Security S
IEEE 802.11i Considerations S
Interoperability Objective S
Protocol Specifications S
Vendor Independence S
Vendor Flexibility S
Multiple Authentication Mechanisms SSupport for Future Wireless
Technologies S
Support for New IEEE Requirements S
Interconnection Objective S
Access Control S
Support for Non-CAPWAP WTPs S
Technical Specifications S
AP Fast Handoff S
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Questions?
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Backup
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LWAPP Packet FormatsLWAPP Header:
0 1 2 30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
9 0
1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|VER|
RID |C|F|L| Frag ID | Length
|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
Status/WLANs | Payload...
|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Control Packets (C=1):
0 1 2 30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
9 0
1+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
Message Type | Seq Num | Msg Element Length
|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
Session ID
|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
Msg Element [0..N] |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Packets (C=0):
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
1
+---------------------------------------------------------------+
| RSSI | SNR | 802.11 Frame...
+---------------------------------------------------------------+:
Status Field Payload
Payload
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Revised LWAPP State Machine
/------------\| v| +------------+| C| Idle |+-------+| |
b+--------------+ +-------------+ | Reset || | |d f| ^ | Configure
|------->+-------+
| | | | | +-------------+p ^| |e v | | ^ || +---------+ v |i 2|
|| C| Sulking | +------------+ +--------------+ || +---------+ C|
Join |--->| Join-Confirm | || g+------------+z +--------------+
|| |h m| 3| |4 || | | | v |o|\ | | |
+------------+\\-----------------/ \--------+---->| Image Data
|C\------------------------------------/ +------------+n
Key Confirmation
Phase
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Unified Key ExchangeJoin-Req(SID, XNonce, WTP-Cert)
Join-Resp(SID, RSA-E(wtp-Kpub, XNonce XOR ANonce), AC-Cert)
Join-Ack(AES(RK0E, WNonce), AES-CMAC(SK1M, Join-Ack))
Join-Confirm(AES-CMAC(SK1M, Join-Confirm))
*SK1=KDF(WNonce || ANonce, string || SID || WTP-MAC ||
AC-MAC)SK1E (Encryption Key), SK1M (MICing Key), SK1R (Rekey Key),
IV
RK0=KDF(psk, string || SID || WTP-MAC || AC-MAC)RK0E (Encryption
Key), RK0M(MICing Key)
First frame uses IV from AC, SK1E plumbed into crypto engine
Join-Resp(SID, AES(RK0E, XNonce XOR ANonce), AES-CMAC(RK0M,
Join-Resp))
PSK:
CERT:
Join-Ack(RSA-E(ac-Kpub, WNonce), AES-CMAC(SK1M, Join-Ack))
*WTP generates K1
*WTP generates K1
*AC generates K1
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Proposed ReKey Exchange
Rekey-Req(new-SID, XNonce)
Rekey-Ack(AES(RK0E, WNonce), AES-CMAC(SK2M, Join-Ack))
Rekey-Confirm(AES-CMAC(SK2M, Join-Confirm))
RK0=KDF(SK1R, string || SID || WTP-MAC || AC-MAC)RK0E
(Encryption Key), RK0M(MICing Key)
SK2E & new IV plumbed into crypto engineSK1R replaced with
SK2R
Rekey-Resp(new-SID, AES(RK0E, XNonce XOR ANonce),
AES-CMAC(RK0M, Join-Resp))*WTP generates K2
*AC generates K2
*SK2=KDF(WNonce || ANonce, string || SID || WTP-MAC ||
AC-MAC)
SK2E (Encryption Key), SK2M (MICing Key), SK2R (Rekey Key),
IV
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X.509 Certificate Profile
Latest LWAPP specification includes an
X.509 certificate profile to facilitate
interoperability
The X.509 profile defines a field that
indicates a devices CAPWAP role (AC or
WTP)
Embedding the role eliminates the
possibility for man-in-the-middle attacks
