The 3rd International Conference on Grid and Pervasive Computing - Workshops

The 3rd International Conference on Grid and Pervasive Computing - Workshops Layered Peer-to-Peer Architecture for Mobile Web Services via Converged Cellular and Ad Hoc Networks Zhonghong Ou, Meina Song, Hui Chen, Junde Song PCN&CAD Center, Department of Electronics Engineering Beijing University of Posts and Telecommunications Beijing, China zhhouyang@gmail.com, mnsong@bupt.edu.cn, xtchenhui@gmail.com, jdsong@bupt.edu.cn Abstract Mobile communication technology is rapidly developed with the enhanced networking capacities as well as the expanding population of mobile subscribers. Providing Web services via converged mobile cellular and Ad Hoc networks has attracted growing attentions. In this paper, a layered peer-to-peer (P2P) architecture is designed for providing Web services via converged mobile cellular and Ad Hoc networks. To speed up the service discovery, a vertical tunneling model is developed as well as the service being classified into hot, warm and cold. A plug-and-play middleware architecture is developed enabling mobile Web services communication and adapting the heterogeneity of mobile terminals. The diagram of the message sequences implementing the plug-and-play middleware architecture is designed with the case of weather service provision. Keywords- layered p2p architecture; vertical tunneling model; Web services; Cellular network; Ad Hoc network 1. Introduction With the rapid development of mobile communication technology and the expanding population of mobile subscribers, providing Web services via converged cellular networks and Mobile Ad Hoc Networks (MANET) attracts growing attentions by researchers and practitioners in terms of Web services, cellular and Ad Hoc networks and P2P networks. Web services are being proliferated and enriched with several standards, e.g. Simple Object Access Protocol (SOAP) [1], Web Service Description Language (WSDL) [2], Universal Description Discovery and Integration (UDDI) [3], Web Ontology Language for Web Service (OWL-S) [4], etc., which enable applications to communicate across heterogeneous platforms and programming languages. The new generation of mobile access technologies, e.g. Wideband Code-Division Multiple Access (WCDMA) [5] etc., are being developed towards enhancing capacities for voice and data communications as well as the delivery of Web services. The MANET challenges data and service communication in a dynamical way because each node in MANET is selfconfigured and willing to forward data to other nodes [6]. P2P networks have the potentialities of realizing highly scalable, extensible, and efficiently distributed applications. Wellknown applications are Napster, Skype, Gnutella and so on. The consistently enhanced capability of mobile terminals together with extended and converged functionalities provided by network operators envisions a paradigm of providing mobile subscribers with Web services via converged cellular and Ad Hoc networks at any given time and anywhere. This paper aims at tackling the vision of Mobile Web Service (MWS) delivery by developing a layered P2P network architecture which enables mobile Web services provision via converged cellular and Ad Hoc networks as well as taking advantages of P2P connections. The remainder of the paper is organized as follows. In Section 2, a converged MWS network architecture is developed, which enables the provision of Web services via both cellular networks and Ad Hoc networks. Section 3, a plug-and-play middleware architecture is designed, which enables MWS communication across heterogeneous platforms. Section 4, the diagram of message sequences implementing the plug-and-play middleware architecture is presented with the case of weather service application. Section 5 concludes the paper. 2. Converged MWS Network Architecture 2.1. Converged Network Architecture A converged MWS network architecture is developed as Figure 1. The network architecture is composed of three components [7]: (1) Cellular and Ad Hoc network; (2) UMTS Terrestrial Radio Access Network (UTRAN); (3) Core network. The cellular and Ad Hoc network consists of mobile terminals which have both the cellular network interfaces and short-range wireless communication interfaces, such as Wi-Fi and Bluetooth. The UTRAN is composed of Radio Network Subsystems (RNS) which consist of Node B and Radio Network Controller (RNC) [8]. The core network is mainly made up of Serving GPRS Support Node (SGSN) and GPRS Gateway Support Node (GGSN), wherein GGSN is in charge of the routing and switching to the outer networks, such as the Internet and other cellular networks. In order to support Web services provision via converged networks, an extra network element of Mobile Service Broker (MSB) is added in each of the above three network components. The cellular network interfaces of the mobile terminals have the following functionalities: 978-0-7695-3177-9/08 $25.00 2008 IEEE DOI 10.1109/GPC.WORKSHOPS.2008.42 195

T-MSB Cellular and Ad Hoc network G-MSB R-MSB Iur T-MSB MSR MSP G-MSB R-MSB Figure 1. Converged network architecture of MWS RNS Cellular and Ad Hoc network Mobile Service Brokers (MSBs) are mainly located in RNC and GGSN of each Public Land Mobile Network (PLMN), still some are located in powerful terminals. Mobile terminals (denoted as T1, T2, etc., see Figure 2) connect with the MSB located in RNC (denoted as R1, R2, etc., see Figure 2), which in turn connect with the MSB located in GGSN of each PLMN (denoted as G1, G2, etc., see Figure 2). Meanwhile, all the MSBs included in one RNC construct a terminal-layer P2P network, briefly, T-layer P2P network, while the MSBs located in one GGSN construct a RNC-layer P2P network, briefly, R-layer P2P network. All the MSBs located in all GGSN collectively form a GGSN-layer P2P network, briefly, G-layer P2P network. The T- layer, R-layer and G-layer P2P network utilize Distributed Hash Table (DHT) [9] to look up services. The nodes which act as Mobile Service Providers (MSPs) register their service to specific MSB, create service descriptions, publish the service descriptions through one or more means of discovery, and make the specific MWSs ready to receive messages from Mobile Service Requestors (MSRs) [10]. The nodes which act as MSRs look up, through corresponding MSB, service descriptions of interest and use them to bind with MWS provided by MSPs. The short-range wireless communication interfaces of the mobile terminals have the following functions: Searching for other peers within their communication range which have the same interfaces to construct pure Ad Hoc networks. Communicating with each other through wireless multihop networking. As no infrastructure exists in MANET, no centralized MSB is available, some node in the MANET should act as the MSB. The heterogeneous environment of MANET is characterized by the diversity of the mobile terminals; some nodes have a powerful computing capacity, while others have relatively low processing power, so the roles a node can act in MANET are different from each other. MSR, which can only request services from the MSB; MSP, which can only provide services for the MSB; MSR and MSP, which can both request services from and provide services for MSBs; MSB, MSR and MSP, at least one node in MANET has to act as MSB when no centralized MSB exists. Meanwhile, for the convenience of providing and discovering services, the MSB node should also act as MSR and MSP. 2.2. Layered Peer-to-Peer Mobile Service Broker Abstracting all the MSBs from the converged network architecture of MWS in Figure 1, we can get a layered P2P overlay network, which consists of three layers, from up to down, G-layer, R-layer and T-layer, standing for GGSNlocated, RNC-located and Terminal-located P2P network respectively, as shown in Figure 2. Infrastructure-based cellular network makes it easy to sense the location of mobile terminals. Thus we can partition terminals into different P2P networks according to their respective RNC and in turn form different P2P networks according to respective GGSN. All the MSBs in the charge of each RNC form one P2P network, T-layer P2P network. T2 and T4 in Figure 2 compose one T-layer P2P network while T6 and T8 construct another. All the Service Identifiers (SIs) in the T-layer are included in the associated R-layer nodes, illustrated in Figure 2, SIs of T2 and T4 are duplicated in R3, while SIs of T6 and T8 are done in R4. Meanwhile, all the MSBs located in each GGSN form another layer P2P network, R-layer P2P network. R3 and R4 in the range of G1 constitute one P2P network. Similar to T-layer P2P network, all the SIs in the R-layer are associated with corresponding G-layer nodes. For example, SIs of R3 and R4, with duplicated SIs of T2, T4, T6 and T8, are specified in G1 as shown in Figure 2. All the GGSN-located MSBs collectively construct a third layer P2P network, the G-layer P2P network, also the top layer in the layered P2P network. Each G-layer node includes part of the network SIs, all the G-layer nodes collectively possesses SIs of the whole network. The communication process among MSBs, MSRs, MSPs applies the following steps: 196

T7 G3 R5 T8 T6 R4 G1 T1 T5 R3 G2 R1 T2 T4 R2 Figure 2. Layered P2P overlay network of MSB MSRs and MSPs receive IP addresses through activating the Packet Data Protocol (PDP) context; MSRs and MSPs register themselves to the responsible T-layer MSB which is further duplicated in the related R-layer MSBs and further in the G-layer MSB; The MSP publishes services to the corresponding T- layer MSB following the similar process as the above; The MSR sends a request to the T-layer MSB to search for a given service. If appropriate results are found then the service discovery procedure is terminated and the service binding is started. If no proper result is found in the T-layer MSB, the request will be forwarded to the upper layer MSB, namely corresponding R-layer MSB. If still no right service is matched, the request will be forwarded to the top layer, the G-layer, where either successful or failure response will be finally given according to the specific service. In the structured P2P network, the key and value pair is stored in DHT, wherein the key stands for the SI which is used for discovering services, value stands for the index information of the peers which provide the service [11]. There are two operations between the MSBs of the P2P overlay network, Lookup (key) and Result (key, value). When the requesting services cannot be found in the local MSB, the requests can be forwarded through sending Lookup (key) to other MSBs. After receiving the Lookup(key) requests, the MSB checks up its service repository, if the proper service identifier is just in it then a Result(key, value) is given back to the original MSB, or the request will be forwarded to other MSBs, until the exact service is found or a failure response is given. When the requesting MSR and the targeting MSP are in the same cellular Ad Hoc network, the communication can be initiated directly through the short-range wireless communication interface provided by the terminals, which will not only save the precious wireless radio resource, but also mitigate the communication overload of Node B in UTRAN. T3 2.3. Service Layered and Vertical Tunneling Model To speed up the service discovery, a vertical tunneling model (VTM) is developed as shown in Figure 3. According to the popularity, a given service can be categorized into three classes: hot service has the most popularity; warm service has the medium popularity; cold service has the least popularity. The service popularity can be evaluated by the frequency of service discovery. Appropriate thresholds can be set to classify services. When the system is just started, all the service should be treated as cold. MSBs will calculate the requesting times for every service in a given time span. If the times exceed the warm threshold, the service is labeled as warm ; while exceeding the hot threshold is labeled as hot. VTM breaks down the normal layered P2P network to some extent. To hot services, no change takes place to the service discovery in the layered P2P model. To warm services and cold services, additional specific vertical tunnels are specified from lower layer to upper layer directly. Vertical Tunnels (VTs) between the T-layer and the R-layer are matched with warm services, while VTs between the T-layer and the G-layer are matched with cold services. As shown in Figure 3, a vertical tunnel is built up between T6 and R4 to match with the warm service. If a warm service request is received by the MSB in the T-layer, instead of looking for the given service in the T-layer, the request will be forwarded to R4 directly, and handled in the R-layer. If no proper result is found in the R-layer, the request will be forwarded to the G- layer. A tunnel connecting T8 and G1 is built up for the cold service discovery. If a cold service request is sent to the T- layer MSB T8, the request would not be handled in the T-layer. Instead it will be forwarded to the G-layer directly. In this way, the VTM can reduce the workload of service discovery effectively. T7 G3 R5 T8 T6 R4 G1 T1 T5 R3 G2 R1 T2 T4 R2 Figure 3. Vertical tunnelling model (VTM) T3 197

3. Plug-and-Play Middleware Architecture Enabling Mobile Web Services This section describes a plug-and-play [12] middleware architecture (PPMA) for MWS, as shown in Figure 4. PPMA consists of eight modules: Transport Adaptation Layer (TAL), Simple Object Access Protocol (SOAP), Service, Graphical User Interface (GUI), Statistical, Intelligent Policy, Migration and P2P. To provide adaptation for the heterogeneity of various mobile terminals, the modules are partitioned into mandatory and optional [13]: the frames in the shadow are mandatory, while other frames are optional. In this way, less powerful terminals can only implement the mandatory modules, while more powerful terminals can provide full functionalities by implementing all the modules, which provides PPMA with the necessary flexibility. 3.1. Transport Adaptation Layer (TAL) TAL is the first mandatory module of PPMA, with the objective of enabling PPMA to interact with different types of transport mechanism within a uniform interface. TAL is the lowest layer within PPMA and provides transport-like functionality to all other communication modules. SOAP Service Application Services GUI Intelligent Statistic Migration Policy Transport Adaptation Layer (TAL) TCP/IP P2P Figure 4. Plug-and-play middleware architecture (PPMA) for MWS 3.2. SOAP The SOAP is the core component in PPMA, which is responsible for generating and parsing SOAP messages for the communication between a provider and a client. Meanwhile, SOAP also provides an XML parser, transport protocols, security algorithms which is given in [10] [14-16], and the client/server ports. This module is the interface to the distributed environment and is responsible for message exchanging, data encryption, and data confidentiality. There are a number of SOAP toolkits available for mobile devices [17]. gsoap for C++ is a platform independent toolkit for Web services, which includes a WSDL parser and a stub/skeleton compiler. ksoap is an open source SOAP API for J2ME devices and provides a lightweight way to access SOAP based Web services. 3.3 Service Service is deployed within the framework and publishes the service description in WSDL to specific MSB. When some other node requests the service provided by this node, it will retrieve the WSDL firstly, and then invoke the remote methods provided by this service. Incoming requests from other nodes will be parsed by SOAP, the SOAP content of the request will be analyzed by Service, and the corresponding service will be invoked. 3.4. GUI GUI consists of GUI adaptation and GUI [18]. These two parts are mainly supported by devices which support GUI. GUI adaptation module reconfigures the GUI which needs a user input. This module can be used to access on runtime remote services with a default user interface. 3.5. Statistical Statistical is mainly a MSB-side module, which collects and accumulates the service frequency information from each service request. It provides Intelligent Policy with service popularity information to aid its service categorizing policy. Meanwhile, it receives frequency feedback and obeys the policy from Intelligent Policy about the different classified service. 3.6. Intelligent Policy Intelligent Policy is also primarily a MSB-side module, which works cooperatively with Statistical to decide different layered services. It utilizes the statistical information provided by Statistical to classify the whole service set into three sub-categories: hot service, warm service and cold service. Since mobile devices are limited in terms of processor speed, memory space, and battery lifetime, Intelligent Policy can also be used collaboratively with Service to provide MSP with additional information to publish only hot service in order to efficiently use the limited resources. 3.7. Migration Migration [17] is used to migrate MWSs from one node to another. The MWS migration is carried out by the request of a service provider. When the battery level of an original server is lower than a given value, or the MWS cannot be provided due to the location change of a device, the service provider can request a service migration. At this time, the migration module transmits the codes and instances of the MWS, and its WSDL document to a relevant device. 198

3.8. P2P P2P is mainly a MSB-side module, which builds up communication connections between MSB P2P nodes of different layers and constructs P2P networks in the respective layers, such as T-layer, R-layer or G-layer. Together with the Intelligent Policy, P2P can decide which layer the request should be forwarded to and which response should be replied back to the user. 4. Message Sequences Design In this section, the message sequences implementing the layered P2P network architecture and the PPMA middleware is designed as Figure 5, with the analysis of a typical application of weather inquiry service provision. Node A acts just as a MSR and wants to request the weather service. Node T1, R1 and G1 are MSBs which stands for T- layer MSB, R-layer MSB and G-layer MSB respectively. Node R1 is in charge of the domain in which node T1 is included, while Node G1 is responsible for the domain in which R1 is involved. Thus Node R1 is the parent node of Node T1 and owns all the SIs of Node T1. In turn, Node G1 is the parent node of Node R1, the grandparent node of Node T1. Node G1 also has all the SIs of Node T1. Meanwhile, Node G1 is the grandparent node of Node B as well, a node which can act as MSP but not MSB. To make the message sequences more readable, we make an assumption that Node B can detect the existence of Node G1 and connect with it directly. In practice, Node B can only connect with some T-layer MSBs whose grandparent node is G1. But from the functional viewpoint, Node G1 also has the whole SIs of Node B, can replace T-layer MSBs and act as the role of service discovery. At the same time, we omit the service discovery in the P2P network in T-layer, R-layer and G-layer. 4.1. Service Publishing The service publishing applies the following steps: Node B acts as a MSP and registers itself as a business entity to Node G1. Node G1, acting as a MSB, responds Node B with either a successful or failure registry. Node B publishes the service description of the weather information on a given region in WSDL to Node G1 after a successful registry, makes the service description visible to consuming applications and the service accessible to the external. MSR A T-MSB T1 R-MSB R1 G-MSB G1 MSP B Connected Inquiry a Success Response Service Publishing Service Discovering b Failure Inquiry forward a Success Inquiry response 4.2. Service Discovering Service Binding Service binding Inquiry response Binding response b Failure Inquiry forward Figure 5. Messages sequences diagram Connected Register Response Service publish a Success or b Failure Invoke The service discovering applies the following steps: Node A sends a request to its directly connected MSB, Node T1 (see Figure 5), to look up the nodes which provide weather service of a given region. Node T1 matches the service based on its own service repository. If the right service is found in its local repository, it responds Node A with a successful message which includes the service description of the weather service (see the first a Success in Figure 5). If the match fails, Node T1 forwards the request to R- MSB R1. Node R1 looks up the service in its local service repository. If the right service is found in the local repository of Node R1, it responds Node A with a successful message (see the second a Success message in Figure 5). If the match still fails, the request will be further forwarded to the G-layer MSB G1. Node G1 looks up the service in its local service repository and responds to Node A with a successful or failure message by routing through Node R1 and T1. 4.3. Service Binding 199

After successful service discovering, service binding initiates. Node A initiates a request of service binding directly to Node B. After that, the weather service can be used in Node A locally. 5. Conclusion and Future Work In this paper, a layered P2P network architecture for MWS is developed, which converges cellular and Ad Hoc networks by taking advantages of P2P communication. All the MSBs are regarded as layered P2P nodes which are located in three layers, i.e., Terminal-located, RNC-located and GGSN-located MSB. To improve the efficiency of service discovery, MWSs are categorized into three sub-categories, hot service, warm service and cold service based on the popularity of a given service. A vertical tunneling model (VTM) is developed for allowing the direct communication between the lower layer and the upper layer which will reduce the workload of service discovery. Moreover, a Plug-and-Play Middleware Architecture (PPMA) is designed to mask the heterogeneity of mobile terminals. Eight key modules in PPMA are defined, i.e. Transport Adaptation Layer (TAL), SOAP, Service, GUI, Statistical, Intelligent Policy, Migration and P2P. Finally, a diagram of message sequences with the case of mobile weather service provision is designed for implementing the layered P2P network and PPMA architecture. Future work includes implementing and evaluating the scalability and performance of the layered P2P architecture, VTM and PPMA architectures. 6. Acknowledgment This work was financially supported by the National High- Tech Research and Development Plan (863) of China under Grant No. 2006AA01Z206 and National Key Project of Scientific and Technical Supporting Programs Funded by Ministry of Science & Technology of China NO. 2006BAH02A03. 7. References [1] W3C-SOAP-Part0, SOAP Version 1.2 Part 0: Primer, 2003. [2] W3C-WSDL, WSDL: Web Services Description Language (WSDL) 1.1, Http://www.w3.org/TR/wsdl, 2005. [3] UDDI, UDDI Version 3.0.2, Http://www.Oasis- Open.org/committees/uddi-spec/doc/spec/v3/uddi-v3.0.2-20041019.Htm, 2004. [4] DAML, OWL-S: OWL-based Web service ontology, v.1.1, 2004. [5] S. G. Glisic, Advanced Wireless Communications 4G Cognitive and Cooperative Broadband Technology. Chichester : Wiley, 2007. [6] B. Stefano, C. Marco, G. Silvia and S. Ivan, Mobile Ad Hoc Networking. Hoboken (NJ) : Wiley, 2004. [7] Zhonghong OU,Meina Song,Hui Chen,Junde Song. "Harnessing Peer-to-Peer Paradigm to Implement Mobile Web Services in Cellular Ad Hoc Networks". Future Telecommunications Conference,11-12, October, 2007, Beijing.Page(s):546-550. [8] CHENG Jiujun, LI Yuhong, CHENG Shiduan, MA Jian. The Architecture on the Mobile P2P System and the Study for the Key Technology. Journal of Beijing University of Posts and Telecommunications. 2006(04), 86-89. [9] Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.R., Kaashoek, M.F.,Dabek, F., Balakrishnan, H.. Chord: a scalable peer-to-peer lookup protocol for Internet applications. Networking, IEEE/ACM Transactions on Volume 11, Issue 1, Feb. 2003 Page(s):17-32. [10] Open Mobile Alliance. OMA Web Services Enabler (OWSER): Overview, OMA-AD-OWSER_Overview-V1_1-20060328-A. Available at URL:http://www.openmobilealliance.org/. [11] Lifeng Le, Geng-Sheng (G.S.) Kuo. Hierarchical and Breathing Peer-to-Peer SIP System.. ICC '07. IEEE International Conference on Communications, 2007. 24-28 June 2007 Page(s):1887 1892. [12] Harjula, Erkki; Ala-Kurikka, Jussi; Howie, Douglas; Ylianttila, Mika. Analysis of Peer-to-Peer SIP in a Distributed Mobile Middleware System. Global Telecommunications Conference, 2006. GLOBECOM '06. IEEE. Nov. 2006 Page(s):1 6. [13] Sumino, H.; Ishikawa, N.; Kato, T.. Design and implementation of P2P protocol for mobile phones. Pervasive Computing and Communications Workshops, 2006. PerCom Workshops 2006. Fourth Annual IEEE International Conference on 13-17 March 2006 Page(s):6 pp. [14]Open Mobile Alliance. OMA Web Services Enabler (OWSER) Best Practices:WSDL Style Guide, OMA-TS-OWSER- Best_Practice_WSDL_Style_Guide-V1_1-0060328-A. Available at URL:http://www.openmobilealliance.org/. [15] Open Mobile Alliance. OMA Web Services Enabler (OWSER):Core Specifications, OMA-TS- OWSER_Core_Specification-V1_1-20060328-A. Available at URL:http://www.openmobilealliance.org/. [16]Open Mobile Alliance. Mobile Web Services Requirements, OMA-RD-OWSER-V1_1-20060328-A. Available at URL:http://www.openmobilealliance.org/. [17]Kim, Yeon-Seok; Lee, Kyong-Ho. A Light-weight Framework for Hosting Web Services on Mobile Devices. ECOWS '07. Fifth European Conference on Web Services. 26-28 Nov. 2007 Page(s):255 263. [18]Gehlen, G.; Pham, L.. Mobile Web Services for Peer-to-Peer Applicatios, Second IEEE Consumer Communications and Networking Conference, 2005. CCNC. 3-6 Jan. 2005.Page(s):427 433. 200