E-tickets validation protocols in mobile environment MSS ： verification server maintains a list of E-tickets validated by it. MH ： submit the E-ticket E-ticket validation problem ： validation process  The same sequence of an E-ticket should not get validated more than once.  An E-ticket should get validated at least once. That is, at least one MSS should accept this sequence of an E- ticket.

E-tickets validation protocols in mobile environment E-ticket problem can be defined in short by the following two properties:  (P.1) If a MSS accepts the E-ticket T and it does not crash, then no other MSS accepts T and a MSS does not accept the same E-ticket more than once.  (P.2) Let δ(T) be the set of MSSs that validate the same E-ticket T. If not all MSSs in δ(T) crashes then there is at least one MSS in δ(T) that accepts T.

E-tickets validation protocols in mobile environment Two Phase E-ticket Validation Protocol (TPEP)  Flat scheme algorithm Tree-Based Protocol (TBP)  Hierarchical scheme algorithm

Two Phase E-ticket Validation Protocol When MH send the E-ticket to the MSS Phase I  The MSS sends an inquiry to all other MSSs in the system visited by the MH, and it validates an E-ticket if none of the responses received from all other MSSs for the same E-ticket is negative. Phase II  It only starts if the MSS gets at least one negative acknowledgment. When more than one MSS tries to validate an E-ticket in Phase II, the MSS that is in the same cell as the MH will validate the E-ticket and all others will reject it.

Two Phase E-ticket Validation Protocol At the end of the protocol, the MSS S i informs the decision on the validation of the E-ticket T to the mobile host, H i, holding T. If H i is disconnected, S i waits for the reconnection of H i and delivers the message.

Message Type NEWTKT REQUESTACK ACK  POSITIVE  NEGATIVE  REJECT ACCEPT MH 將 E-ticket 傳送給 MSS 做認證 MSS 詢問其他 MSSs 1. 驗證過此 E-ticket ? 2. 其他 MSS 發送相同的 REQUESTACK ? MSS 回覆 REQUESTACK 此 E-ticket 沒有被認證過且沒有其他 MSS 發送過此 E-ticket 的 REQUESTACK 給我 同時有其他 MSS 正為此 E-ticket 作認證 此 E-ticket 我已認證過了 MSS 通知所有 MSSs 此 E-ticket 已被我認 證了

MSS S i All Positive REJECT ACK-NEGATIVEACK-POSITIVE Has Reject Has Negative Other MSSs Already received Otherwise ACCEPT ACK-REJECT Phase I MH H i NEWTKT REQUESTACK Already validated ACKs Phase II Wait replies

MSS S i Phase II More than one MSS tries to validate the same E- ticket T in case the same ticket is submitted for validation by H i from several places. MSS S j MSS S k MH H i E-Ticket MH H i E-Ticket MH H i E-Ticket cell Accept Reject Phase II

Two Phase E-ticket Validation Protocol S i ： the mobile support station (MSS) H i ： the mobile host (MH) λ S ： the set of MSSs λ H ： the set of MHs δ H i ： the set of MSSs where H i visited visited_ list ( H i ) → 回傳 H i 之前拜訪過的 MSS List Search ( H i ) → 回傳目前正在服務 H i 的 MSS Nearst ( CurrentMSS, List ) → 回傳 List 中離 CurrentMSS 最近的 MSS

E-ticket’s attribute  status: accept or reject  ackStatus: positive, negative or reject  currentValidator: the set of validators which are validating this E-ticket

Algorithm to be executed at each MSS S i Format: (Message Type, ticket, sender, receiver) Two Phase E-ticket Validation Protocol 存放 REQUESTACK 紀錄 (MSS,T)

傳回結果給 Hi H i 之前拜訪過的 MSS 清單 紀錄其他 MSSs 回覆訊息的數量 紀錄 Positive ACK 的數量 紀錄其他同時認證此 ticket 之 MSSs PHASE I

此 ticket 是否有其他 MSS 同時在認證 ? 將該 MSS 加入 Otherwise Already received Already validated

Has Reject 從 other MSSs 接收的 ACK 回應 Positive ACK Negative ACK 記錄目前正在驗證 的 MSS

Otherwise All Positive 當 replies >= (RequestACK / 2) 告知所有的 MSS ，此 ticket 已被我驗證過

更新 vTkts, aTkts 資訊

PHASE II S i 是否為 H i 目前所在的 MSS 中 ? Nearest Accept Other Reject 查處 H i 目前所在的 MSS

H i 在 S i 範圍且 H i 未斷線 則傳遞 H i 在 S i 範圍且 H i 斷線 則等 △ t 時間後再傳 H i 不在 S i 範圍 則搜尋 H i 所在的 S i

MSS S 1 MSS S 2 MSS S 3 MSS S 4 Correctness of protocol NewTkt Req_Ack +ACK H1 submits T T accepted H1 submits T NewTkt Accept Req_Ack -ACKReject T rejected

MSS S 1 MSS S 2 MSS S 3 MSS S 4 Correctness of protocol NewTkt Req_Ack H1 submits T NewTkt Req_Ack -ACK T rejected Req_Ack +ACK -ACK T accepted Phase II

Analysis of protocol Three types of messages exchanged in the Phase I of the protocol  REQUESTACK  ACK  ACCEPT Best Case : MH is residing in only one cell. In this case the number of messages exchanged will be zero. Worst Case : MH is highly mobile and may cover the entire network. In this case the number of messages exchanged will be 3(m-1) where m is the number of MSSs. Avg. Case : The mobile host is limited to a few cells. The number of messages exchanged will be 3(δ MH -1) where δ MH < m is the number of cells that MH visited.

Tree-based protocol Hierarchy of geographical region  Each node is a MSS  Each region is a collection of some MSS’s information and is represented by a internal node.  The root node represents the total area spanning over the entire region  Each MSS in a higher level of hierarchy knows the information about E-tickets validated by the lower level nodes Use the timestamp to choose nearest MSS

Tree-Based Protocol S1S1 S7S7 S4S4 S8S8 S6S6 S3S3 S5S5 S2S2 Has information of validated ticket Has information of validated ticket Has information of validated ticket

Tree-Based Protocol S1S1 S7S7 S4S4 S8S8 S6S6 S3S3 S5S5 S2S2 NewTkt Req_Ack NewTkt Req_Ack Already validated Reject

Tree-Based Protocol Algorithm to be executed at each MSS S i Format: (Message Type, ticket, sender, receiver)

將 Request ACK 往 Parent 送

表此 ticket 已被驗證過 回傳 Negative ACK 若此 MSS 為 root ，且此 ticket 未被驗證過 回傳 Positive ACK Otherwise 將 Request ACK 往上傳

表此 MSS 為驗證的源起者 表此 MSS 非驗證的源起者 將 ACK 訊息往下傳 判斷路由 Stack 是否為空

H i 在 S i 範圍且 H i 未斷線 則傳遞 H i 在 S i 範圍且 H i 斷線 則等 △ t 時間後再傳 H i 不在 S i 範圍 則搜尋 H i 所在的 S i

S1S1 S7S7 S4S4 S8S8 S6S6 S3S3 S5S5 S2S2 -ACK+ACK Correctness of protocol NewTkt Req_Ack NewTkt Req_Ack If S 4 faster than S 5 If S 4, S 5 At the same time S5’s timestamp > S4’s timestamp +ACK-ACK

Analysis of protocol Two types of messages exchanged in the protocol  REQUESTACK  ACK Best Case : The server that gets the ticket and the root being the same, the number of messages is zero. Worst Case : The server that gets the ticket is at the highest level, the number of messages will be 2(h - 1) where h is the height of the tree. Avg. Case : The server that gets the ticket is at a level i, the number of messages will be 2(h – i - 1), where h is the height of the tree.

Comparsion of the protocol Assumption  Without failure  The same E-ticket is used once by the users Comparsion  Resilience  Latency  The number of messages exchanged to validate an E-ticket.

Comparsion of the protocol E-ticket protocol 2PE Internet users 2PE mobile users Tree protocol Resilience < n/2 < h Latency 2θ< 2θ<< 2θ Message 2( n - 1)2(δ s - 1) + ( n - 1)2( h - 1) Θ : maximum message delay δs : the number of servers visited by the client h : the height of tree Request & ACK & Accept Request & ACK