4.3 測試結果
4.3.3 Loads of Anchor Authenticator
本論文所提出的方法，是採用分散式方法(distributed)，由 serving BS 傳送 AK Context 給 candidate target BSs，來取代中央控管(centralized)的方式，由 anchor authenticator 傳送 AK 給 candidate target BSs。因此每當 MS 在 handover 時採用分散 式的方式取得 AK 時，anchor authenticator 節省 AK Assignment 所需的封包(Context Request、Context Report、AK Request、AK Transfer，Context Request)。亦即多台 MS 同時 handover 的數量越多，anchor authenticator 節省的負載量越多。因此在 NAP 的 範圍內：
anchor authenticator 可以節省計算 AK 的計算量為：HNAP(l) * NMS * 6 * c HNAP(l)：為 MS 在 NAP 內平均換手的次數；
NMS：為單一時間內，anchor authenticator 同時收到 MS 提出計算 AK 的需求；
6：表示 MS 在執行換手動作時，candidate target BS 也跟著提出計算 AK 的需求；
c：表示 anchor authenticator 在計算 AK 時所花費的時間。
T T
NAP_O(l)
T T
NAP_P(l)
0 5000 10000 15000
1 2 3 4 5 6 7 8
# of Trffic Loads
Level l+1
第 5 章
結論與未來工作
綜合考量 AK 重新取得的次數與網路傳輸的負載度，本論文所提出的方法在 效能上明顯比原先的流程較好。此外不僅是在 ASN 或是在 NAP 的環境下，隨著 l 的階層數越高，降低網路負載的效果較為明顯，減少 anchor authenticator 計算 AK 所花費的時間也越明顯。
另外，本論文在提出方論點時，是按照 WiMAX 的架構及原先設計的精神，
在兼顧安全性的考量下所設計。所以在討論結論與未來工作之前，首先了解本論 文所以提出的論述與考慮方向，經由哪些考量點，再來評估往後的發展。因此本 論文先針對：1) 修改 AK 公式的各種可行性分析。2) AK Context 在 handover 過程 時的傳送路徑，在此提出分析過程。
針對修改AK的產生公式，根據[3]所提出關於AK的公式：
AK=Dot16KDF(PMK, SS MAC Address | BSID | “AK＂, 160)
在欲修改BSID為之前，首先分析目前WiMAX網路上有哪些ID可以選擇？參照[5]
所提供的ID (表格 A-6)，除了有原先的BS ID之外，還包含Operator ID (NAP ID)、
NSP ID、Authenticator ID。我們針對各個ID做評估分析：
1) NSP ID：
優點：[5]中所描述，NSP ID為全域變數，因此NSP ID為整個WiMAX中均可以 獲得。所以MS在initial network entry時可以透過SBC-REQ向BS提出，BS即可透過 SBC-RSP給予回應。MS不需為獲得NSP ID而增加額外的負擔。
缺點：若將 NSP ID 帶入公式，則 AK 可使用的涵蓋範圍擴大到 NSP 所對應到 的 NAP(1 或多個 NAP)，除非跨越不同的 NSP，否則 AK 在這 NSP 底下的多個 NAP 範圍內不會改變。這幾乎否決 key 存在的意義。在安全性的考量下，理應 key 的範
圍越小越好，知道的人也越少越好。
2) Authenticator ID：
優點：Authenticator ID 的涵蓋範圍與 NSP ID 比較下相對較小。
缺點：如(表格 A-6)所示，Authenticator ID為在Intra-ASN範圍內傳送，所以 MS並無法獲得Authenticator ID。此外為了讓MS獲得此ID則必須增加額外的流程。
另外，當MS在做handover時，若將AK產生公式的參數由原先的target BS ID改為target Authenticator ID，對於先前描述AK的產生是由anchor authenticator所產生，anchor authenticator 裡 應 不 知 道 target Authenticator ID (anchor authenticator 不 曉 得 MS handover的target BS是屬於哪一個authentication domain)，若是選擇anchor Authenticator ID，則又與原先 802.16e訂定MS再換手時須使用target BS ID來產生AK的精神相違 背。
3) NAP ID：
優點：
a) MS 透過 DL-MAP 可以獲得 NAP ID(BS ID 的前 24bits)，不需增加額 外流程。
b) NAP 的涵蓋範圍比 NSP 小。
c) MS handover過程中，在mobility domain的範圍內(包含一個或多個 authentication domain)，一般為NAP，AK統一由anchor authenticator所 產生。因此認為使用NAP ID比較符合[4]的設計架構。
d) NAP ID 帶入公式後，MS 在 handover 過程中，只要是在 NAP(mobility domain)的涵蓋範圍下，在生命週期內(lifetime)不需重新產生 AK，可 減少後端網路的負擔。
缺點：
AK 產生的涵蓋範圍由原先的 MS 與 BS 之間擴大為 MS 與 NAP 之間。在 NAP 涵蓋範圍下 MS 與任何 BS 相連接均使用同一把 AK。為了安全性的考量，必須儘 量讓越少的 BS 知道 MS 的 AK Context。
綜合前面三種 ID 的評估後，本論文因此選擇使用 NAP ID 為最後決定，至於 NAP 所產生的缺點，則藉由減少 AK Context 的分布，補強使用 NAP ID 所帶來的缺 點。
既然選擇NAP ID為本論文修改公式的參數後，在Mobility domain (通常為NAP) 底下的AK為同一把，接下來的討論為AK Context應該由誰傳送以及誰可收到AK Context。[4]描述在Mobility domain底下由anchor authenticator依據target BS提出AK Request 和 Context Request傳送AK Context給target BS；為了節省 AK Request與 Context Request的傳輸訊息所花費的時間與縮短anchor authenticator與target BS之間 的距離，本論文利用AK在NAP底下為同一把的特性，選擇serving BS在通知candidate target BSs有開始啟動handover的同時，挾帶著AK Context資料給candidate target BSs。
藉此只讓serving BS週遭的candidate target BSs擁有MS的AK Context，以減少AK的分 布，來強化其安全性。另外，也因為serving BS取代anchor authenticator來傳送AK Context，原先anchor authenticator與candidate target BSs之間的距離縮短為serving BS與 candidate target BSs之間的距離，減少因距離而產生的延遲(參照2.3.2)。
既然已選擇由serving BS取代anchor authenticator傳送AK給target BS，該透過什麼 時間
過程中，後端(backhaul) 網路
析與設計，本論文在經過實驗與分析後，實際加速了 MS 在 hand
點與什麼訊息傳送，本論文考慮2.3.5所描述的三個時間點，以及目前後端訊息 傳送的各種封包訊息，發現透過HO Request封包夾帶AK Context，若選擇已知的HO Request通知candidate target BSs並不會增加額外的機制，而且可以讓target BS事先獲 的MS的AK Context。因此決定在serving BS欲發出HO Request的訊息給candidate target BSs的時間點同時夾帶AK Context。
在原先 WiMAX Forum 的設計架構下，MS 在 handover 的
須負責為 MS 處理相當多的流程(包含 authentication、path registration、MIP 等 等)，也因此大大增加了 handover 的延長時間與網路負擔，因此簡化後端流程為相 當重要的課題。
透過以上的分
over 的流程，也降低了網路後端的負擔，更減少 anchor authenticator 的運算負
載。
關於未來工作的部分，WiMAX Forum持續在針對[4][5]做修正與改進，[2]也持 續在為認證程序構思簡化方法，以求達到seamless的效果。
Reference
[1] B. Aboba, et al., “Extensible Authentication Protocol (EAP)”, RFC 3748, June 2004.
[2] IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1, IEEE Std 802.16e-2005 and IEEE Std 802.16-2004 / Cor 1-2005, 2006.
[3] IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems, IEEE Std 802.16-2004, 2004.
[4] WiMAX Forum NWG Stage 2, release 1 V&V draft, August 8, 2006.
[5] WiMAX Forum NWG Stage 3, release 1 V&V draft, August 8, 2006.
[6] Simon Blake-Wilson “TLS Inner Application Extension(TLS/IA)”, TLS Working Group, October 2004
[7] I. F. Akyildiz,Y.-B. Lin,W.-R. Lai, and R.-J. Chen, "A new random walk model for PCS networks," IEEE J. Select. Areas Commun., vol. 18, pp. 1254–1260, 2000.
[8] G. Xue, “An Improved Random Walk Model for PCS Networks,” IEEE Transactions on Communications, vol. 50, no. 8, pp. 1224-1226, August 2002.
[9] David Johnston and Hassan Yaghoobi, “Peering Into the WiMAX Spec,”
CommsDesign, www.commsdesign.com, 21 Jan, 2004.
附錄A
表格 A-1 HO Request transmitted within ASN
IE Description M/O
HO Type Describes type of the HO (FBSS, MDHO, HHO) M
MS Info Contains HO-related MS context in the nested IEs M
>MS ID 6 octet MS ID (MAC Address) M
>Anchor GW ID Identifies the Anchor ASN GW M
>Authenticator GW ID Identifies the Authenticator GW M
>R3MM Context R3MM related Context Info O
>SF Info (one or more) Each IE of the list contains context of a particular SF. M
>>SFID SFID associated with the Service Flow M
>>CID CID associated with the Service Flow in the Serving BS M
>>SAID SAID associated with the Service Flow M
>>TEK Context TEK context might be included if there is a desire to share TEKs between the Serving and Target BS upon HO.
O
>>Packet Classification Rule
(one or more)
Each IE in the list contains IEEE 802.16e Packet Classification Rule
O
>>>Classifier Rule Priority IEEE 802.16e Classifier Rule Priority O
>>>Classifiers Set of IEEE 802.16e Classifiers associated with the Classifier Rule
O
>>QoS Info QoS Parameters associated with the Service Flow O
>>>QoS Parameters IEEE 802.16 QoS Parameters O
Serving BS Info Contains Serving BS context in the nested IEs. M
>BS ID Serving BS ID M
>Round Trip Delay Round Trip Delay (RTD) between the MS and the Serving BS O
>DL PHY Quality Info Downlink PHY Quality between the MS and the Serving BS O
>UL PHY Quality Info Uplink PHY Quality between the MS and the Serving BS O Target BS Info (one or
more)
Each IE in the list contains Target BS context in the nested IEs. M
>BS ID Target BS ID M
>Data Path Establishment Option
A flag indicating whether or not Data Path should be established before responding to the HO Request.
O
>Relative Delay Indicates the delay of neighbor DL signals relative to the serving BS,
as measured by the MS for the particular BS.
O
>DL PHY Quality Info Downlink PHY Quality between the MS and the Serving BS O
>UL PHY Quality Info Uplink PHY Quality between the MS and the Serving BS O
表格 A-2 Context Request from Target BS
IE Description M/O
HO Type Describes type of the HO (FBSS, MDHO, HHO) M
MS Info Contains HO-related MS context in the nested IEs. M
>MS ID 6 octet MS ID (MAC Address) M
Serving BS Info Contains relevant Serving BS context in the nested IEs. O
>BS ID Serving BS ID O
Target BS Info Contains relevant Target BS context in the nested IEs. M
>BS ID Target BS ID M
表格 A-3 Context Report to Target BS
IE Description M/O
MS Info Contains HO-related MS context in the nested IEs. M
>MS ID 6 octet MS ID (MAC Address) M
>Service Authorization Indicates whether or not the service is authorized, if not specifies reason.
O
>Authenticator AK Context Contains AK Context in the nested IEs M
>>AK 160-bit AK M
>>AK ID 64-bit AK ID M
>>AK Lifetime 16-bit AK Lifetime (in seconds) M
>>AK SN 4-bit AK SN M
>>PMK SN 4-bit PMK SN M
表格 A-4 HO Response
IE Description M/O
HO Type Describes type of the HO (FBSS, MDHO, HHO) M
Result Code The result of the Request M
MS Info Contains HO-related MS context in the nested IEs. M
>MS ID 6 octet MS ID (MAC Address) M
Serving BS Info Contains relevant Serving BS context in the nested IEs. M
>BS ID Serving BS ID M
Target BS Info (one or more)
Contains relevant Target BS context in the nested IEs. M
>BS ID Target BS ID M
>Temporary BS ID Temporary ID assigned to the target BS O
>HO ID ID assigned for use in initial ranging to the target BS once this BS is selected as the target BS
O
>Service Level Prediction Service Level Prediction code. O
>Preamble Index / Subchannel Index
Preamble Index / Sub-channel Index code O
>HO Process Optimization
HO Process Optimization code O
>SF Info (one or more) Each IE of the list contains context of a particular SF. M
>>SFID SFID associated with the Service Flow M
>>CID CID replacement M
>>SAID SAID replacement M
>>SDU Info First Buffered/Multicast SDU context for Data Integrity. Relevant only for DL U-Cast Service Flows.
O
>>>SDU SN Sequence Number of the First Buffered/Multicast SDU context for Data Integrity. Relevant only for DL U-Cast SF.
O
表格 A-5 AK Context
Parameter Size(bits) Usage
AK 160 The authorization key
AKID 64 AKID = Dot16KDF(AK, AK SN|SS MAC
Address|BSID|"AK", 64). The AK_SN in the Dot16KDF function is an 8-bit number which consists of leading 4 zero bits and appending 4-bit AK_SN in MSB first order.
AK Sequence Number 4 Sequence number of AK. If AK = f (PMK and PMK2), then AK SN = PMK SN + PMK2 SN
If AK = f (PMK), then AK SN = PMK SN
AK Lifetime This is the time this key is valid; it is calculated AK lifetime
=MIN(PMK lifetime, PMK2 lifetime) - when this expires, re -authentication is needed.
PMK Sequence Number 4 The sequence number of the PMK that this AK is derived from
PMK2 Sequence umber 4 The sequence number of the PMK2 that this AK is derived from. In Single-EAP, it shall always set to zero
CMAC_KEY_U 160/128 The key which is used for signing UL management messages
CMAC_PN_U 32 Used to avoid UL replay attack on the management
connection–before this will expire (for example, when
CMAC_PN_D value bigger than 232 – 10, 000) re-authentication is needed. The initial value of CMAC_PN_U is zero and the value of CMAC_PN_U is reset to zero whenever CMAC_KEY_COUNT is increased.
CMAC_KEY_D 160/128 The key which is used for signing DL management messages
CMAC_PN_D 32 Used to avoid DL reply attack on the management
connection –before this will expire (for example, when CMAC_PN_D value bigger than 232 – 10, 000) re-authentication is needed. The initial value of CMAC_PN_D is zero and the value of CMAC_PN_D is reset to zero whenever CMAC_KEY_COUNT is increased.
KEK 160 Used to encrypt transport keys from the BS to the SS EIK 160 EAP Integrity Key for authenticating Authenticated EAP
message.
CMAC_KEY_COUNT 16 Value of the Entry Counter that is used to guarantee
freshness of computed CMAC_KEY_* with every entry and provide replay protection. Upon initial network entry, count is reset to 0 in the MS and Serving BS, and to 1 in the Authenticator.
表格 A-6 List of Identifier
Identifier Type Size Scope(area of validity) BS ID binary 48 bits Global
Operator ID binary 24 bits Global NSP ID binary 24 bits/32 char string Global
Authenticator ID binary 4 octets/16 octets NAP/NSP
表格 A-7 Proposed HO Request transmitted within ASN
IE Description M/O
HO Type Describes type of the HO (FBSS, MDHO, HHO) M
MS Info Contains HO-related MS context in the nested IEs M
>MS ID 6 octet MS ID (MAC Address) M
>Anchor GW ID Identifies the Anchor ASN GW M
>Authenticator GW ID Identifies the Authenticator GW M
>R3MM Context R3MM related Context Info O
>SF Info (one or more) Each IE of the list contains context of a particular SF. M
>>SFID SFID associated with the Service Flow M
>>CID CID associated with the Service Flow in the Serving BS M
>>SAID SAID associated with the Service Flow M
>>TEK Context TEK context might be included if there is a desire to share TEKs
>>TEK Context TEK context might be included if there is a desire to share TEKs