Privacy key management for BSs and CISs in 802.16 LE Systems (for IEEE 802.16 Session #38,2005-07)

Privacy key management for BSs and CISs in 802.16 LE Systems (for IEEE 802.16 Session #38,2005-07)


Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Privacy key management for BSs and CISs in 802.16 LE Systems
Date Submitted
2005-07-11
Source(s)
Hung-Lin Chou,
Keng-Ming Huang, 
Chi-Chen Lee, Fang-Ching Ren
Industrial Technology Research Institute, Computer and Communications Research Labs, Taiwan
Bldg. 11, 195 Sec. 4, 
Chung Hsing Rd.Chutung, HsinChuTaiwan 310, R.O.C.
Voice: +886-3-5912042
Fax: +886-3-5829733
mailto: hunglinchou@itri.org.tw
Re:
Call for Contributions, IEEE 802.16h Task Group on License-Exempt Coexistence, IEEE 802.16h-05/014, 2005/06/09
Abstract
Propose the PKM protocol for intercommunications in 802.16 LE.
Purpose
Information.
Notice
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Privacy Key Management for BSs and CISs in 802.16 LE Systems
Hung-Lin Chou, Keng-Ming Huang , Chi-Chen Lee, Fang-Ching Ren
Computer & Communications Research Labs, ITRI, Taiwan

1. Introduction

This document proposes an enhanced network architecture which is distributed and more flexible. Besides, the related Privacy Key Management protocol for 802.16 LE systems is also introduced.
2. Background
In session#37, the architecture proposed in [1] was accepted. However, the accepted architecture requires a regional centralized RADIUS server and CIS which may lack for flexibility and scalability. Moreover, it is more reasonable that each operator has its own RADIUS server for authentication. This proposal intends to reduce the key management complexity of the RADIUS server and the maintenance overhead of CIS. In the enhanced architecture, only the global RADIUS server (here may be more than one global server) called “root” RADIUS server remains and the CIS will be ditributed. All RADIUS servers and CISs of the 802.16 LE operators shall register IP addresses of RADIUS servers and CISs as well as the country code of the operator to the root RADIUS server(s). An 802.16 LE system learns other existing 802.16 LE systems by querying the root RADIUS server(s) using its own country code and neighboring country code. 
A new re-key mechanism is proposed, as the previous re-key procedures rely on Radius-Server to generate security blocks and Security Parameters Index (SPIs) (a field of ESP header which identifies the security parameters in combination with IP address) and Keys for the BSs/CISs. The loading of SPIs/Keys update of Radius-Server will be an issue as the number of BSs increases. For multiple Radius-Servers environment, the new PKM protocol provides an easier way to regenerate the session-key that secures the communications between BSs/CISs based on the Master-Key, which is for generating session-key. The original IAPP-based solution relies on Radius Server to keep security information parameters and BSs mapping (ex: SPISecurity Association and Supporting Transform/Authentication Algorithm…etc). While BSs need to re-key or to create a new SPI/SA, Radius Server must involve and handle message exchange between BSs/CIS. The proposed mechanism resolves the issue of SPIs/Keys mapping in multiple Radius-Servers environment by avoid the SPI/SA mapping, i.e. the Radius Server will not involve the re-key procedures.
 
Acronyms
CIS - Coexistence Identification Server
PKM  Private Key Management
IPsec Internet Protocol Security
ESP  IP Encapsulating Security Payload
AH Authentication Header
 

3. Suggested remedy

(1) Proposed enhancement of general architecture for inter-network communication

[insert the following section into 2.1.2.1 Architecture]
Figure 1 shows the original network architecture. Figure 2 demonstrates the IEEE 802.16 LE inter-network communication architecture under multi-Operators with multi-Radius Servers.
If BS-1 want to communicate with BS-2, it must get BS-2’s Country’s Code, Operator ID and BSID from local CIS first. And then work as the following steps
(2) BS-1 send Radius-Access-Request frame with BS-2’s Country’s Code, Operator ID and BSID to Radius-Server
(3) Radius-Server will act as Radius-Proxy and transfer this Radius-Access-Request to the target Radius-Server
(4) Target Radius-Server will response Radius-Access-Accept with BS-2 Master-Key and MK-index
(5) BS-1 will receive Radius-Access-Accept from its local Radius-Server and get the BS-2 Master-Key and MK-index
(6) BS-1 will act as a PKM-initiator to send Session-Key-Start to BS-2
(7) BS-2 will calculate the ESP-Key-Stuffs with Master Key and response Session-Key-Request to BS-1
(8) BS-1 will also calculate the ESP-Key-Stuffs with Master Key to verify Key-Signature and response Session-Key-Response to BS-2
(9) BS-2 will verify Key-Signature and response Session-Key-Accept to BS-1
(10) After the above procedures, BS-1 and BS-2 could communicate in IPSec with the ESP-Key-Stuffs generated dynamically
The following figure shows the IEEE 802.16 LE Network Hierarchy for Radius-Servers to find another Radius Servers via the Root-Radius-Server by the Country Code.


Figure 1 Original Network Architecture

Figure 2 Network Architecture under multi-Operators with multi-Radius Servers

Figure 3 Network Hierarchy for Radius-Servers

For the IEEE 802.16 LE, we want it to work with multi-Radius-Servers, but how to find each Radius-Server near local Radius-Server would be a critical issue. The Root-Radius-Server will place a Key-Role for each Radius-Server to find the Radius-Servers in the near countries by Country-Codes. Based on this mechanism, each Radius-Server need to keep shared-key with the Root-Radius-Server and use the Radius-Access-Request frame to request the target Country-Code’s Radius-Servers information. The Root-Radius-Server will response those information in Radius-Access-Accept frame. The response information will include Operators’ Radius-Servers and CISs information of the target Country-Code.
The following figure shows the each connection of BSs/CISs will be encrypted in individual Session-Key.
Figure4 Individual Session-Key
For the BSs/CISs, each connection with different BSs/CISs will use individual Session-Key. Those Session Keys would be generated from PKM-Handshaking with Master-Keys of target BSs/CISs. The re-key procedures also don’t need Radius-Servers and just use Master-Keys of target BSs/CISs.
(2) Proposed enhancement of RADIUS protocol usage
[insert the following section into 3.2.4.3 RADIUS Protocol Usage]
For future interoperability consideration, similar mechanisms in [2] are maintained. Secure exchange of 802.16 LE signaling information can be achieved after successful procedures of the RADIUS protocol. To include RADIUS support, the RADIUS server and the BS/CIS RADIUS client must be configured with the shared secret and with each other’s IP address. Each BS/CIS acts as a RADIUS client and has its own shared-secret with the RADIUS server. The shared secret may be different from that of any other BS/CIS.
Figure 5 RADIUS protocol example
Figure 5 shows the RADIUS protocol message exchange sequence. At starting up, each BS or CIS must send a Radius-BS/CIS-Registration-Access-Request (shown in table 1) to the RADIUS server for authentication purpose and leave the address mapping (BSID to IP) information in the server. At this time, the RADIUS server will retain the following information of registered BS or CIS:
(a) Wireless medium address of BS (BSID) or medium address of CIS,
(b) RADIUS BSID Secret at least 160 bits in length,
(c) IP address or DNS name,
(d) Cipher suites supported by the BS or CIS for the protection of CRN protocol communications, and
(e) The Master-Key for BS or CIS to establish Session-Key-Handshaking procedures
Same as [2], Microsoft Point-to-Point Encryption (MPPE) (RFC 2548:1999) key is introduced. The MS-MPPE-Send-Key, which could be got in the Radius-BS/CIS-Registration-Access-Accept message (shown in table 2), is used for encrypting the security parameters in the accept message. A registration access reject message may be issued due to a BS not supporting the ESP Transform or ESP Authentication algorithm selected for use in securing the following intercommunication, or for other RADIUS configuration reasons not discussed here.
Once a BS wants to get the knowledge of neighbor topology, it must first send Radius-BS/CIS-Access-Request message (shown in table 3) to the RADIUS server in order to acquire the regional CIS’s IP address, and also to deliver the ESP-Transforms-and-Authentication-Algorithms-initiator-send-Codes/ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Codes necessary for establishing a secure connection with the CIS. The wireless medium addresses of regional CIS, similar to BSID, well known by all BSs supporting LE operation, is sent in the Radius-BS/CIS-Access-Request message to the RADIUS server for looking up IP address of the CIS. Upon receiving the request message, the RADIUS server will respond with a Radius-BS/CIS-Access-Accept message (shown in table 4) if the BS is a valid member which is allowed to perform inter-communication.
After succeeded query process between the BS and the regional CIS (detailed please refer to [6]). The CIS will respond to the BS with possible neighbor BSs candidates and their BSIDs. The BS, then, tries to establish secure connections with the neighbor BSs after evaluating the coexistence relationships with these candidates. The BS sends Radius-BS/CIS-Access-Request message to the RADIUS server to query the IP address and ESP-Transforms-and-Authentication-Algorithms-initiator-send-Codes/ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Codes for each evaluated neighbor BS.
An access reject message may be issued due to a BS or the regional CIS not supporting the ESP Transform or ESP Authentication algorithm selected for use in securing the following intercommunication, or for other RADIUS configuration reasons not discussed here.
The attribute 26 field of Radius-Frame(shown in table 2 and 4) would be encrypted in 32-bytes MPPE-Send-Key with the following manner (‘+’ indicates concatenation):
b(1) = MD5(MPPE-Send-Key+BSID) c(1) = p(1) xor b(1) C = c(1)
b(2) = MD5(MPPE-Send-Key+BSID + c(1)) c(2) = p(2) xor b(2) C = C + c(2)
. .
. .
. .
b(i) = MD5(MPPE-Send-Key+BSID + c(i-1)) c(i) = p(i) xor b(i) C = C + c(i)
Break plain text into 16 octet chunks p(1), p(2)…p(i), where i = len(P)/16. Call the ciphertext blocks c(1), c(2)…c(i) and the final ciphertext C. Intermediate values b(1), b(2)…c(i) are required. The resulting encrypted String field will contain c(1)+c(2)+…+c(i).
The RADIUS-BS/CIS-Registration-Access-Accept and RADIUS-BS/CIS- Access-Accept would use each time MPPE-Send-Key separately. The BSs/CISs/Radius-Servers don’t need to retain the MPPE-Send-Key. The MPPE-Send-Key is generated dynamically for the RADIUS-BS/CIS-Registration-Access-Accept and RADIUS-BS/CIS- Access-Accept messages by Radius-Servers.

RADIUS protocol messages

[insert the following section into 6.2 RADIUS Protocol Messages]
The following messages are listed to support RADIUS protocol:
Note that TBD means To Be Defined.
Radius-BS/CIS-Registration-Request (BS/CIS → RADIUS server): A startup BS/CIS sends this message for authentication purpose.
Table 1 RADIUS-BS/CIS-Registration-Access-Request
Attribute number
Attribute name
Value
1
User-Name
BSID. The BSID should be represented in ASCII format, with octet values separated by a “-“. Example: “00-10-A4-23-19-C0”.
4
NAS-IP-Address
BS’s IP Address
6
Service-Type
CRN-Register (value = TBD, ex. IAPP-Register, value = 15)
26
26-TBD
26-TBD
Vendor-Specific-Attribute (VSA)
Supported-ESP-Authentication-Algorithms

Supported-ESP-Transforms
The list of ESP Authentication IDs corresponding to the ESP Authentication algorithms supported by this BS (See Table 6)
The list of ESP Transform IDs corresponding to the ESP transforms supported by this BS (See Table 5)
32
NAS-Identifier
BS’s NAS Identifier
80
Message-Authenticator
The RADIUS message’s authenticator
According to RFC 2865:2000, other RADIUS attributes may be included in the RADIUS-BS/CIS-Registration-Access-Request packet in addition to the ones listed in Table 1.
Radius-BS/CIS-Registration-Accept (RADIUS server → BS/CIS): After RADIUS server verifies the valid membership, it will respond with this accept message.
Table 2 RADIUS-BS/CIS-Registration-Access-Accept
Attribute number
Attribute name
Value
1
User-Name
BSID.
6
Service-Type
CRN-Register (value = TBD, ex. IAPP-Register, value = 15)
26
26-TBD
26-TBD
Vendor-Specific-Attribute (VSA)
MK Index

Supported-ESP Transforms and Authentication-Algorithms Code
The index number of Master Key (0-255)

The list of ESP Transforms and
Authentication-Algorithms Codes approved by Radius Server
27
Session-Timeout
Number of seconds until the BS should re-issue the registration Access-Request to the RADIUS server to obtain new key information.
80
Message-Authenticator
The RADIUS message’s authenticator
The RADIUS-ESP-Transform-ID, RADIUS-ESP-Authentication-ID and RADIUS-ESP-SPI attributes are encrypted as described for the MS-MPPE-Send-Key attribute in RFC 2548:1999
Radius-BS/CIS-Access-Request (BS/CIS → RADIUS server): The BS sends this message to request for inter-communication with another neighbor BS or a regional CIS.
Table 3 RADIUS-BS/CIS- Access-Request
Attribute number
Attribute name
Value
1
User-Name
Regional CIS’s WM address or neighbor BS’s BSID.
2
User-Password
NULL.
4
NAS-IP-Address
Original BS’s IP Address (the BS sending this request message)
6
Service-Type
CS/CIS-Check (value = TBD, ex. IAPP-AP-Check, value = 16)
61
NAS-Port-Type
Wireless – Other (value = 18)
80
Message-Authenticator
The RADIUS message’s authenticator
Radius-BS/CIS-Access-Accept (RADIUS server → BS/CIS): After verifying that the neighbor BS is valid member, RADIUS server will respond with the security parameters necessary for establishing a secure connection between the neighbor BS and requesting BS or between CIS and requesting BS.
Table 4 RADIUS-BS/CIS- Access-Accept
Attribute number
Attribute name
Value
1
User-Name
Regional CIS’s WM address or neighbor BS’s BSID.
8
Framed-IP-Address
IP Address of Regional CIS or neighbor BS.
26
26-TBD

26-TBD

26-TBD
26-TBD
Vendor-Specific-Attribute (VSA)
Terminated-BS/CIS-MK Index
Terminated-BS/CIS-Master-KEY
ESP-Transforms-and-Authentication-Algorithms-initiator-send-Codes
ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Codes
The index number of Master Key for Terminated-BS/CIS (0-255)
Terminated-BS/CIS-MPPE-RECV-Key (MK) encrypted using current BS’s MPPE-SEND-Key
Codes used to identify ESP algorithm codes to the regional CIS or neighbor BS
Codes used to identify ESP algorithm codes from the regional CIS or neighbor BS
80
Message-Authenticator
The RADIUS message’s authenticator
Table 5 ESP Transform identifiers
Transform identifier
Value
Reference
RESERVED
0
[RFC2407]
ESP_DES_IV64
1
[RFC2407]
ESP_DES
2
[RFC2407]
ESP_3DES
3
[RFC2407]
ESP_RC5
4
[RFC2407]
ESP_IDEA
5
[RFC2407]
ESP_CAST
6
[RFC2407]
ESP_BLOWFISH
7
[RFC2407]
ESP_3IDEA
8
[RFC2407]
ESP_DES_IV32
9
[RFC2407]
ESP_RC4
10
[RFC2407]
ESP_NULL
11
[RFC2407]
ESP_AES
12
[Leech]
Reserved for privacy use
249-255
[RFC2407]
Table 6 ESP Authentication algorithm identifiers
Transform identifier
Value
Reference
RESERVED
0
[RFC2407]
HMAC-MD5
1
[RFC2407]
HMAC-SHA
2
[RFC2407]
DES-MAC
3
[RFC2407]
KPDK
4
[RFC2407]
HMAC-SHA2-256
5
[Leech]
HMAC-SHA2-384
6
[Leech]
HMAC-SHA2-512
7
[Leech]
HMAC-RIPEMD
8
[RFC2857]
RESERVED
9-61439
Reserved for privacy use
61440-65535
(3) Proposed enhancement of Privacy Key Management protocol usage
[insert the following section into 3.2.4.4 Privacy Key Management protocol usage]
The PKM protocol would provide a flexible and easy-to-maintain key exchange mechanism. The PKM is based on the Master-Key (MPPE-Recv-Key from RADIUS-BS/CIS-Registration-Access-Accept message) to provide a symmetric key for the PKM-Initiator and PKM-Target side.
The following figure shows the PKM Session-Key-Handshaking procedures
Figure 6 PKM Session-Key-Handshaking procedures
The PKM-Initiator will need to get the PKM-Target’s Master Key from Radius-Server. And the perform the following steps
(1) PKM-Initiator would get Masket-Key-IndexMaster-KeyESP-Transforms-and-Authentication-Algorithms-initiator-send-Codes and ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Codes of PKM-Target in RADIUS-BS/CIS- Access-Accept message from Radius-Server and then generate a random 32-bytes ANonce.
(2) PKM-Initiator would will send Session-Key-Start message to PKM-Target with “ANonce””Master-Key-Index” ”ESP-Transforms-and-Authentication-Algorithms-initiator-send-Codes” and “ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Codes”.
(3) After receiving Session-Key-Start message, PKM-Target would generate a random 32-bytes BNonce. And perform the PRF640 algorithm to generate the 640-bits Key. Keep the first 512-bits ESP-Transform/Authentication Keys and use the last 128-bits M-Key as the HMAC-MD5 key to generate 16-bytes Key-Signature.
(4) PKM-Target would will send Session-Key-Request message to PKM-Initiator with “BNonce””Master-Key-Index” ”ESP-Transforms-and-Authentication-Algorithms-initiator-send-Code”(PKM-Target chosen) and “ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Code” (PKM-Target chosen)
(5) After receiving Session-Key-Request message, PKM-Initiator would perform the PRF640 algorithm to generate the 640-bits Key. Keep the first 512-bits ESP-Transform/Authentication Keys and use the last 128-bits M-Key as the HMAC-MD5 key to generate 16-bytes Key-Signature to verify the Key-Signature field on the Session-Key-Request message. If it is wrong, PKM-Initiator would perform silent-drop and doesn’t response any message. If it is correct, PKM-Initiator would prepare the Session-Key-Response message and use HMAC-MD5 generate Key-Signature filed.
(6) PKM-Initiator would will send Session-Key-Response message to PKM-Target with “ANonce””Master-Key-Index” ”ESP-Transforms-and-Authentication-Algorithms-initiator-send-Code”(PKM-Initiator chosen) and “ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Code” (PKM-Initiator chosen)
(7) After receiving Session-Key- Response message, PKM-Target would check the ANonce value if equal to the previous ANonce value in Session-Key-Start message and use HMAC-MD5 generate Key-Signature filed to verify the Key-Signature field. Compare the values of ”ESP-Transforms-and-Authentication-Algorithms-initiator-send-Code” and “ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Code” to make sure the security parameters.
(8) After the above, PKM-Target will send Session-Key-Accept with Key-Signature filed to PKM-Initiator to verify.
(9) The following IP connection will use the first 512-bits ESP-Transform/Authentication Keys from PRF640 as keys and perform the ESP-Transform/Authentication algorithms from ”ESP-Transforms-and-Authentication-Algorithms-initiator-send-Code” and “ESP-Transforms-and-Authentication-Algorithms-initiator-receive-Code”.
The following figure shows the PKM Session-Key Re-Key procedures
Figure 7 PKM Session-Key Re-Key procedures
Each Session-Key would set a Key-Lifetime, and PKM-Initiator could set a Session-Key grace time to perform Session-Key-Handshaking for the next new Session-Key#2 to be generated until the end of the key lifetime. The Session-Key#1 could use up its lifetime and then activate the Session-Key#2. If each side use the Session-Key#2 first in IPsec connection, it could also activate the Session-Key#2. If the lifetime of Session-Key#1 use up, the PKM-Initiator doesn’t perform the Session-Key Re-Key procedures. PKM-Target would disconnect the IP connection until the Session-Key#2 generated.
The following figure shows the PKM Session-Key Re-Key procedures with the MK update of PKM-Target

Figure 8 PKM Session-Key Re-Key procedures with the MK update of PKM-Target
The PKM-Initiator will check the current MK-Index if it is equal to the MK-index of PKM-Target in Session-Key-Request message. If the PKM-Initiator detects the MK-Index different of the MK-Index of PKM-Target, it would perform RADIUS-BS/CIS- Access-Request/Accept procedures to get the latest Master-Key of PKM-Target from Radius-Server.
The following figure shows the Master-Key Re-Key procedures
Figure 9 Master-Key Re-Key procedures
Each Master-Key would set a Key-Lifetime, and BSs/CISs could set a Master-Key grace time to perform Registration-Access-Request/Accept procedures for the new Master-Key (MPPE-Recv-Key) until the end of the key lifetime. If the lifetime of Master-Key use up, the BSs/CISs don’t perform the Registration-Access-Request/Accept procedures. Radius-Server would discard the Master-Key of the BS and reject the Radius-Access-Request from the other BSs/CISs for this BS.
The following figure shows the 640-bits Key generated by PRF640
Figure 10 the 640-bits Key generated by PRF640
The BSs/CISs get Master-Key from Radius-Servers and generate 32-bytes Nonce value to derive 640-bits key as follows
PRF-640(MK, “BS-CIS key expansion”, Min(BS1ID,BS2ID) || Max (BS1ID,BS2ID)|| Min(ANonce,BNonce) || Max(ANonce,BNonce))
Where
PRF-640 (K,A,B) =
for i=0 to 4 do
R=R|HMAC-SHA-1(K, A|0|B|I)
return LeastSignificant-640-bits( R )
and “|” denotes bitstring concatenation
Privacy Key Management protocol messages
[insert the following section into 6.3 Privacy Key Management protocol messages]
The following figure shows the 32-bits ESP-Transforms-and-Authentication-Algorithms-Code format
Figure 11 32-bits ESP-Transforms-and-Authentication-Algorithms-Code format
The PKM protocol procedures contain 4 message actions, and each-side could check the code value of the begin of PKM message to recognize which action need to perform this moment. The meaning of codes for PKM message as follows
– 0 = Session Key Start
– 1 = Session Key Request
– 2 = Session Key Response
– 3 = Session Key Accept
The Length field contains a 16-bits value to record the whole frames size starting from Code field, with the ESP-Transforms-and-Authentication-Algorithms-Codes field filled in if present.
The MK-Index field contains a 8-bits value to record the current Master-Key-Index each PKM-side used. If the PKM-Initiator detects the MK-Index different of PKM-Target, it must perform Radius procedures with Radius-Server to retrieve latest Master-Key.
The Replay-Counter field contains a 64-bits random number (such as 64-bit NTP timestamp) and does not repeat within the life of the Master-Key material.
The Key-Lifetime field contains a 64-bits value to record the Session-Key lifetime in seconds.
The Key-Signature field contains an HMAC-MD5 message integrity check computed over the Session-Key-Frame starting from Code field, with the ESP-Transforms-and-Authentication-Algorithms-Codes field filled in if present, but with the Key Signature field set to zero. The M-Key is used as the HMAC-MD5 key.
The Security-Parameters-Index field contains a 32-bits value to assign to the IPsec Security Association (including the encryption and authentication keys, the authentication algorithm for AH and ESP, the encryption algorithm for ESP, the lifetime of encryption keys…etc in this session). PKM-Initiator/Target could check the SPI value in ESP-Header or AH-Header to detect to use which SA for this IPsec connection.
The ESP-Transforms-and-Authentication-Algorithms-Codes field contains 8-bits Codes-Number value to record the number of ESP-Transforms-and-Authentication-Algorithms-initiator-send/receive-Codes, each length of ESP-Transforms-and-Authentication-Algorithms-Codes is 4-bytes. The BSs/CISs would compare each ESP-Transforms-and-Authentication-Algorithms-initiator-send/receive-Code to find out which code is supported.
The following figure shows the Session-Key-Start message format
Figure 12 Session-Key-Start message format
The following figure shows the Session-Key-Request message format
Figure 13 Session-Key-Request message format
The following figure shows the Session-Key-Response message format
Figure 14 Session-Key-Response message format
The following figure shows the Session-Key-Accept message format

Figure 15 Session-Key-Accept message format

References

[1] IEEE C802.16h – 05/012r1 –General Architecture for Inter-network Communication Across 802.16 LE Systems, 2005-04-29
[2] IEEE Std 802.11F-2003, IEEE Trial-Use Recommended Practice for Multi-Vendor Access Point Interoperability via an Inter-Access Point Protocol Across Distribution Systems Supporting IEEE 802.11? Operation.






















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