Elsevier

Information Sciences

Volume 179, Issue 11, 13 May 2009, Pages 1678-1693
Information Sciences

Algorithms for validating E-tickets in mobile computing environment

https://doi.org/10.1016/j.ins.2009.01.018Get rights and content

Abstract

E-tickets issued online can be used by different vendors to provide services such as discount coupons for E-shopping, or an entrance for a football game. One major issue associated with E-tickets is the problem of validation, which becomes more severe in a mobile environment due to mobility of a mobile host (MH) and frequent failure/disconnection of validation servers known as mobile support stations (MSSs). Some additional problems include the identification of duplicate submissions of an E-ticket by a MH. Thus, this paper proposes two protocols using the Flat and Hierarchical schemes for E-ticket processing and validation in a mobile environment that guarantee at least and at most once property (each E-ticket is validated at least and at most once). The protocols have been validated and compared through complexity analysis and experiments.

Introduction

The mobile wireless communication revolution has introduced some wireless miniature devices so that a user can get connected irrespective of its geographical location. This form of remote access is termed as mobile computing [11], [16], [10]. There have been studies for adaptability of existing distributed algorithms in this new environment [2], [3], [18], [19], [20], [9], [7]. It is very common over the Internet where producers and consumers participate in business transactions using wireless communication network. A user on the move may avail oneself to services like listening music or playing games over the Internet [1] by submitting E-tickets (it is digital form of a ticket). An E-ticket is an online service that gives permission to its owner to use it for diverse purposes like for admission to a movie or in other forms of entertainment like downloading songs. Therefore, it is natural to extend E-commerce support for mobile users referred to as Mobile commerce (M-commerce). However, M-commerce has its own limitations, and to derive the benefits of M-commerce these hindrances must be addressed. The limitations are mostly due to constraints such as small portable devices with limited power and storage, low bandwidth, intermittent disconnections, high latency and frequent location changes.

Researchers have provided distributed system architectures [22], [23], [31], [26], [8] suitable for Internet and smart-card technology-based E-ticket management system to deal with validation, authorization, issuance, transfer and usage of E-tickets. Note that we assume here that distributed hash table (DHT) like structures cannot be used among MSSs due confidential nature of E-commerce business to protect the details of terms and conditions associated with E-tickets. It is only when validation is required MSSs that collaborate themselves. In other words, they only provide service to each other’s customers in validating E-tickets. There is no replication of data and their indexes are not shared among MSSs. In addition, it is difficult to maintain DHT in a mobile environment.

Despite their convenience and ease of use, E-tickets can lead to problems such as duplicate submission and maintaining atomicity (E-ticket should be accepted only once). Therefore, E-tickets have to be validated or authorized by a vendor before they can be used. The validation of an E-ticket prevents duplication of the provider’s services and ensures integrity of the E-tickets. Duplication of E-tickets can be intentional or unintentional, where a user purposefully or unknowingly submits an E-ticket more than once for validation. This could happen either due to untrusted users who misusing E-tickets or due to a wireless network disconnection and a user may resubmit an E-ticket twice with the assumption that the last request may not have reached the server. While a service provider must assure a service to genuine E-ticket holders against all odds, and at the same time, it must also prevent multiple uses of the same E-ticket.

Validation of E-tickets becomes much more difficult in a mobile environment [21], [33] because of constraints such as constantly changing locations of the mobile host (MH), disconnection between the MH and the mobile service station (MSS), possible submission of multiple copies of the E-tickets and failure of the server (MSS).

In this paper, we propose protocols for online validation of E-tickets in a mobile computing environment. In Phase I, if an E-ticket is not used, a MSS verifies it. The protocol enters Phase II only when a MSS gets multiple copies of the same E-ticket or it receives a negative acknowledgment from some other MSSs in response. A negative acknowledgment signifies that this E-ticket has been seen by at least one other MSS before and therefore, may have been validated earlier, or some other MSS is in the process of validating this E-ticket. Our algorithm guarantees that one of the MSSs provides service to a user while all other MSSs reject the validation request. That is, our protocol ensures that despite disconnections/failures, a mobile user eventually makes use of an E-ticket “at least once” and “at most once”. The “at least once” scenario is when a verifying server may get crashed or malfunction whereas in the “at most once”, a user who is either impatient to wait or unaware of the status of the submitted E-ticket, re-submits the E-ticket. In either case, the system assures that the E-ticket is validated only once and therefore, our protocol prevents multiple uses of the same E-ticket as well as provides service to the genuine ticket holder. There are two algorithms proposed to validate an E-ticket in a mobile environment; two-phase E-ticket protocol (TPEP), called a Flat scheme and tree-based protocol (TBP) called a Hierarchical scheme.

The simulations are performed using NS-2 [32] to observe the behavior of the two algorithms to validate E-tickets. The experiments are performed comparing the TPEP and TBP protocols to evaluate the efficiency of the two algorithms in terms of throughput, average response and waiting times under changing node density (to check latency), and location changes which may impact the optimal path length (in case of a congestion), route changes, link changes and unreachable destinations (when a MH is switched off or is in a disconnected mode). As far our knowledge goes, we are the first who addressed the shortcomings of earlier work [22], [23] and discussed E-tickets validation process in a mobile environment considering two different models (flat and hierarchical) while guaranteeing at least once and at most once property and provided extensive simulations to study insight into the latency of the two algorithms due to a MH’s location change or due to the submission of the duplicate E-tickets. We do not compare with protocols in [22], [23] experimentally as these algorithms are not proposed for mobile computing environment. However, we did compare their algorithmic complexity with ours.

The paper is organized into the following sections. The review of related work is presented in Section 2. The infrastructure support for mobile E-ticket services is discussed in Section 3. Section 4 presents the two-phase E-ticket validation problem and discusses the algorithm. The details of the Tree-based validation strategy are presented in Section 5. The analysis is given in Section 6. The performance study is presented in Section 7. The paper concludes with Section 8.

Section snippets

Related work

A ticket holder is entitled for a service from vendors. Ticket based service process includes steps such as ticket submission, validation and service collection. Using the Internet, a customer can put a request electronically to buy a ticket and can even pay through electronic transactions. Due to advances in the wireless communication network, the service provider accepts electronic identity more liberally to increase the customer base. This change in attitude is manifested in terms of

E-ticket and mobile system architecture

An E-ticket is defined by (I, S, O, Y); the ticket has digital signature of the issuer I with a service promise of S to the ticket owner O with the ticket status Y. During the E-ticket issuance, an E-ticket issuer provides the information (name, signature), service category, validity period, and conditions of transferability and service accesses. If the E-ticket is issued to a particular customer then the identification of the customer can also be included in a ticket. A customer availing an

E-tickets validation protocols in mobile environment

Each E-ticket has a particular user and is to be validated for a particular service. The E-ticket validation process is intended to prevent duplication, to avoid multiple uses of an E-ticket by the same or different users. In a mobile environment, a MSS works as a verification server to validate the E-ticket submitted by an MH within its cell. The validation process, called E-ticket validation problem, results in either the acceptance or rejection of the E-ticket. The process works as follows:

Tree-based protocol

In the previous algorithm, a network wide search has been reduced by restricting it to the servers at locations visited earlier by a mobile user. However, the difficulty in this strategy is the communication overhead associated with the increasing length of the message as it has to follow backward pointers [25], [24] to find the list of the locations visited by a mobile user or MH itself carries the list. In order to overcome this problem, we propose a scheme that uses a hierarchical scheme

Comparison of the protocols

Our comparison assumes executions without failures and the most common case where the same E-ticket is used once by the users. We compare the protocols based on (a) their resilience, and (b) the latency and (c) the number of messages exchanged to validate an E-ticket. In the two-phase E-ticket protocol for Internet users [22], E-tickets are accepted in Phase I after two communication steps: the initial acknowledgement request sent to all the servers by the server that receives the E-ticket and

Experimental evaluations and observations

We assume that the links between wired nodes and between wired and base station (BS) nodes are bi-directional links. NS-2 stores routing information based on the topology [5] (e.g. how nodes are connected to each other through links). A BS node plays the role of a gateway between the wired and wireless domains. A BS node is responsible for delivering packets into and out of the wireless domain. Each wireless domain assigns a unique domain address to a BS node. All packets destined to a wireless

Conclusions and future work

In this paper, we proposed two algorithms that ensure atomic use of an E-ticket. That is, our proposed algorithm guarantees validation of an E-ticket “at least once and at most once”. In addition to proposing algorithms for E-ticket validation, a strategy for restructuring a distributed algorithm for mobile computing environment is also demonstrated. Our tree-based algorithms provides better response time, throughput and waiting time than the flat version of the algorithm. The algorithms are

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