Designing Grid services for multimedia streaming in an e-learning environment

CONCURRENCY AND COMPUTATION: PRACTICE AND EXPERIENCEConcurrency Computat.: Pract. Exper. 2006; 18:911–923Published online 17 November 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/cpe.985

Designing Grid services formultimedia streaming in ane-learning environment

M. Amoretti, R. Bertolazzi, M. Reggiani, F. Zanichelli∗,†

and G. Conte

Dip. di Ingegneria dell’Informazione, Universita degli Studi di Parma, Parco Area delle Scienze, 181A,43100 Parma, Italy

SUMMARY

Next generation e-learning platforms will support cooperative use of geographically distributed educationalresources as an aggregated environment, thus enabling a more effective knowledge exchange. Sharing thisgoal, our research activity addresses the definition and implementation of a service-based infrastructurefor content distribution and multimedia streaming. The proposed architecture provides enough flexibility,extensibility, and scalability to cope with the ever-increasing heterogeneity and mobility of edge devices.In this paper, we initially illustrate the general objectives and requirements in the development of ane-learning oriented application for multimedia content discovery, access, and distribution in a virtualcommunity. Architectural considerations have motivated the use of Grid technologies as they provideaccess to resources and services of different administrative domains in a transparent, seamless, and secureway. The Grid-based architecture has been endowed with a number of services including discovery andstreaming of multimedia objects, multimedia content update, quality of service (QoS) management, andsupport for user authentication and authorization. A multimedia application based on the architecture isdescribed and evaluated on a multi-domain network. Copyright c© 2005 John Wiley & Sons, Ltd.

KEY WORDS: e-learning; Grid Services; distributed multimedia

1. INTRODUCTION

In a global knowledge-based society, communities play a pivotal role and re-shape the process oflearning and sharing knowledge in and among organizations. Knowledge sharing is the process whereindividuals mutually exchange their (implicit or explicit) knowledge and jointly create new knowledge.

∗Correspondence to: F. Zanichelli, Dip. di Ingegneria dell’Informazione, Universita degli Studi di Parma, Parco Area delleScienze, 181A, 43100 Parma, Italy.†E-mail: [email protected]

Contract/grant sponsor: IBM Faculty AwardContract/grant sponsor: National Research Ministry ‘WEB-MINDS’ FIRB Project

Copyright c© 2005 John Wiley & Sons, Ltd.Received 20 November 2004

Revised 30 January 2005Accepted 20 February 2005

912 M. AMORETTI ET AL.

The role of technology, especially of information and communication technology (ICT), is crucial asit allows bridging gaps of space and time between members of communities, often termed virtualorganizations.

Providing a rich communication medium for work or interest groups, community support systemsare gaining more and more attention in application areas ranging from leisure and customer supportto knowledge management. Important application areas are also related to teaching and researchactivities in universities. ICT, in particular, is becoming of paramount importance for the so-calledinformal learning, which includes all of the activities taking place outside classes and courses, thataim at understanding, creation of knowledge, and skills acquisition [1]. In the context of universitycampuses, the ever-increasing joint availability of portable devices and wireless technologies and theirforthcoming functional integration will eventually change both the way in which students attend theireducation in the university and the way the institution and the professors communicate with the studentsas well as the way in which students communicate with each other. In this time devoted to consolidationof notions and self-confidence development, students will obtain great advantage by a 24/7 access toinstructional material, lecture notes, and multimedia contents, such as video recordings of previousclasses, made available by teachers and instructors or even by other students.

In this paper we present our results in the design and on-going development of a campus-wide servicefor multimedia content access and distribution. The goal of our work is an extensible service-orientedarchitecture where Grid technologies enable uniform search and access capabilities to multimediacontents available at several heterogeneous video servers. Our service-oriented design attempts toexploit both basic, (e.g. Grid Security Infrastructure) and advanced Grid capabilities. Recent activitiesin Grid computing are defining a service-oriented design methodology [2] to cope with dynamic andflexible applications which need integration and interoperability among heterogeneous off-the-shelfsoftware components (e.g. databases, service proxies, collaborative platforms, streaming servers, etc.).The key concept is the Grid Service [3], essentially a stateful (and potentially transient) Web Servicewith an associated lifetime and whose behaviors must conform to a set of interfaces defining the servicesemantics. Moreover, Grid Services should be compliant with standard mechanisms to support statemanagement, referenceable handles and event notification.

The main contribution of this paper is an effective approach to the complex design problem ofdistributed multimedia systems for virtual organizations (e.g. university campuses). We show how thedesign can be tackled by leveraging upon the capabilities of recent service-oriented Grid technologies.By encapsulating architectural components, from the infrastructure level (e.g. quality of service (QoS)management) to higher application levels (e.g. content search), into Grid Services the distributedmultimedia system improves in terms of flexibility, scalability, extensibility, and reuse. The paper isorganized as follows. Section 2 introduces the general issues and requirements for a campus-wide mul-timedia service and establishes our long-term goals. The overall service-oriented architecture and theservices developed so far in the current prototype are illustrated in Section 3 along with the role playedby relevant Grid technologies. Finally, some remarks and an outline of further work conclude the paper.

2. GENERAL REQUIREMENTS FOR A CAMPUS-WIDE MULTIMEDIA SERVICE

As multimedia content distribution is rapidly becoming one of the most important network applications,several universities around the world have already deployed or are experimenting in solutions

Copyright c© 2005 John Wiley & Sons, Ltd. Concurrency Computat.: Pract. Exper. 2006; 18:911–923

DESIGNING GRID SERVICES FOR MULTIMEDIA STREAMING IN E-LEARNING 913

for multimedia content distribution over their TCP/IP wired/wireless networks and the Internet.Recordings of class lectures, seminars, and lab sessions are made available in digital form to bestreamed and viewed either off- or on-campus, to the benefit of special e-learning programs, of blendedlearning services aiming at improving the experience of attending students who can also greatly benefitfrom a wireless infrastructure.

In the following, we introduce a set of objectives and requirements to be taken into account in thedesign of a campus-wide service for multimedia content access and distribution. The effectiveness ofsuch a complex service is influenced by several interacting issues, namely:

• the functional integration of search and delivery capabilities;• the representation level at which search is performed (i.e. metadata or knowledge based);• the ability to integrate pre-existing multimedia services hiding their heterogeneity;• security management (authentication, authorization, accounting, etc.);• the ability to interact with resource managers to request resource allocations;• the conformance to standards to allow interoperability and easier integration into other systems.

The effective exploitation of a potentially huge amount of instructional multimedia material callsfor powerful search capabilities. Within most video services in use today a rather limited set ofmetadata is exploited to describe the available multimedia contents. Although a set of standards isrecently emerging regarding audio/video metadata [4,5], widely used descriptions often only includea minimal set of common attributes (e.g. title, author, date, description, etc.) together with key-words-based indexes. Higher levels of search capabilities can be obtained by means of ontology-basedsemantic descriptions [6,7] to automate search or to assembly course materials on account of userpreferences.

Additional goals for designing a campus-wide video service can be related to system integration.On the one hand, it should be designed so as to be able to integrate pre-existing video servicesset up by different university groups. This integration should be obtained with the goal of hidingthe heterogeneity of such systems to users, by providing some unifying layer to search and accessconsistently over all available services of the same kind. On the other hand, a campus-wide videoservice should also be easily integrated into other larger e-learning or collaborative frameworkswhich can take advantage of distributed multimedia capabilities. From our viewpoint, both forms ofintegration of applications can be pursued by means of computing technologies relying on high level,standardized, self-describing interfaces.

One more system-level goal is represented by security management. The usual authentication,authorization and accounting capabilities should be available to the campus-wide video service.However, we deem very important the ability to finely tune access control lists to multimedia materialas well as making this service part of a single sign-on solution which does not require users toauthenticate multiple times at different services. Finally, we envision that such a system could interactwith resource managers on behalf of its users, possibly hiding the complexity of a feature-richnetwork infrastructure. For example, on best-effort IP networks, Content Delivery Networks (CDNs)exploit caching proxies to move Web and multimedia content towards network edges and closer toclients. These network intermediaries can also offer additional services such as content adaptation,personalization, or location-aware data manipulation. The campus-wide video service should bedesigned taking advantage of lower-level infrastructure components and capabilities whenever



available (e.g. proxies and resource allocation services). This could be carried out transparently, that isdirectly determining all of the information required to acquire and reserve the needed resources, giventhe requested content, the network status and the characteristics of user terminal and connectivity.However, QoS parameters could also be explicitly considered in the phase of content search, to thepurpose of constraining the search only to material which can be adequately experienced by the enduser. Furthermore, coallocation should also be supported, i.e. the simultaneous allocation of multipleresources enabling concurrent streaming tasks (distribution of some live or prerecorded content toseveral classrooms). Unfortunately, coallocation poses significant additional challenges with respect tothe reservation of resources for a single streaming task.

2.1. Related work

A few related projects concerning service-oriented architectures (mainly based on Web Services)for multimedia applications are described in the literature. However, it is quite difficult to comparethem with our approach as prototypes are generally incomplete and not supported by experimentalresults.

A formal problem domain definition for multimedia services can be found in [8]. A three-tierframework aimed at achieving transparent dynamic QoS-enabled Web Services is also introduced.The lack of implementation details and experimental results prevents to infer any conclusions aboutthe scalability of the proposed solution.

Multimedia applications based on Web Services are also presented in [9,10]. A Web-Service-based framework for conference control is presented in [9]. The framework implements user andaudio/video (A/V) application session management and floor control function in a scalable structureover heterogeneous collaboration systems. The same approach, i.e. achieving interoperability throughthe use of Web Services, has also been exploited in our architecture to support heterogeneous videoservers. In [10], a QoS-enabled Web Service architecture deploying a QoS Broker between clients andproviders was proposed. The service quality was evaluated in term of latency, availability, timeliness,and realiability. The concept of service instances introduced in the Open Grid Service Architecture(OGSA), partially relieves the problem of service availability, as shown in Section 3.

Grid technologies supporting multimedia applications are presented in [11,12]. mmGrid [11] is anarchitecture supporting multimedia applications in a Grid computing environment. The implementationis based on a non-standard solution for the development of required functionalities (authentication,registration, resource discovery, scheduling) as the system has been developed before OGSAstandardization. The different meaning of the service concept could introduce some difficulties inexploiting research results in future work. An OGSA-based approach is used in [12] to implement aservice reservation framework (G-QoSm) that supports QoS in service-oriented Grids. While our goalis to exploit Grid technologies to enable uniform search and access capabilities to multimedia contentsavailable at heterogeneous video servers, G-QoSm uses Grid capabilities with the goal of advertisingand trading available resources.

Finally, Grid technologies have been also used for e-learning applications as proposed in [13,14].These projects mainly deal with the problem of handling, storing, and exchanging learning objectswhile our efforts are towards the development of a campus-wide service for multimedia content accessand distribution.



3. GRID-BASED PROTOTYPE OF A MULTIMEDIA SERVICE

In this section we describe the on-going development of a Multimedia Service prototype based onOGSA [2]. Service-orientation has been adopted primarily on account of the need of our system tooperate on a wide range of resources such as data, storage, network, software applications, throughstandard high-level descriptions. In order to cope with the general requirements described in theprevious section we have developed several services (Figure 1) together making up the core of ourprototype.

The user interested in a video content (Figure 2) accesses the Multimedia Discovery Service (MDS)and submit his/her query. The MDS activates several Search Services to process user queries on theremote video servers listed by the Video Server Index. The search results (e.g. title, format, length,location) are collected by the MDS and returned to the user, which can then select the desiredmultimedia object. In addition, the user can request the reservation of network resources through theBandwidth Broker Service or obtain information about the current quality of the relevant network pathusing the Monitoring Service.

The implementation of each service has been extensively based on the factory approach, animprovement provided by Grid Services to overcome stateless and non-transient weaknesses of WebServices. When a Grid service is deployed in the container, it generates a factory that will be used tocreate individual instances of the service for each client application. As shown in Figure 2, each servicegets created as an instance since it must be dedicated to a single user application and needs to be ableto store intermediate query results. The possibility to have a single instance serving multiple differentuser applications is still available and it is used with the Video Server Index, which provides the sameinformation to all of its clients.

In the following we describe in more detail the aforementioned services and components of theimplemented streaming system together with other ancillary services, such as those devoted to userauthentication.

MDS

After being authenticated, the user can submit a request to a MDS. Currently supported queries formultimedia objects are: browse, which returns the complete list of available multimedia objects;searchForProfessor, searchForKey and searchForCourse which return only matching contents.Additional user requests about network reservation and monitoring are also handled by the multimediadiscovery service that, exploiting Web Service protocols, interacts with the other developed servicesto answer user needs. The MDS plays a key role inside our prototype as it hides to the userthe heterogeneity and the actual location of streaming servers and provides a simplified accessto other commodity services, such as those devoted to resource management (Figure 1). For anyuser application, the MDS Factory spawns a dedicated instance allowing different queries to beprocessed concurrently, while subsequent refinements of a query are evaluated within the same instance(Figure 3).

Search Service

To hide to the user the heterogeneity and the actual location of the content repositories, the MDS shouldbe consistent and interoperable with all the remote multimedia servers. This could be unfeasible as



SearchService

BandwidthBrokerService

Client

MultimediaDiscoveryService

MonitoringService

VideoServerIndex

usesuses

uses usesuses

Figure 1. Component diagram.

:Client

:Multimedia DiscoveryService

:Bandwidth BrokerService

:Monitoring Service

create

query

results

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MonitoringQuery

destroy

Monitoring Query

create

destroy

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results

query

:Search Service

QoSresultQoSresult

MonitoringResult

MonitoringResult

destroy

BrowseSearchFor....

create aninstance

Server:VideoIndex Service

Active Serverslist of

an instancedestroy

Figure 2. Sequence diagram.



Figure 3. Interaction between user and Multimedia Discovery Service Factory.

the multimedia servers could greatly differ in terms of metadata, search capabilities and managementpolicies. A possible solution, adopted in our framework, is to deploy a Search Service on eachmultimedia server, empowered with mechanisms to access the specific information data structure ofthe local server and to return the list of matching multimedia objects to the MDS. Search Servicesmust present a common interface which is enforced by the GWSDL [3] portType shown in Figure 4.

Video Server Index Service

Monitoring the state of multimedia servers is required to ensure that the search service will deal onlywith fully operational resources. Indeed, while a university campus might host a large number ofmultimedia servers, many of these could be temporarily off-line or mismanaged. A simple idea wouldbe to include a polling functionality in the MDS, periodically asking each resource about its liveness.This solution can be inefficient as it increases the workload of the service and reduces the efficiencyof the whole system. Therefore, we chose instead a push model: each resource periodically notifies itsaliveness to a Video Index Service, which updates a data structure that contains information about thestate (active or inactive) of each multimedia server. If the update from a resource is not received withina time deadline, the Video Index Service attempts at querying the resource and accordingly updates thelist of multimedia servers.

Bandwidth Broker Service

Through the use of a bandwidth broker, the MDS is able to ensure adequate QoS on the streaming ofthe multimedia content chosen by the user.

Our prototype exploits a Diffserv network architecture, performing traffic classification andprioritization, and a bandwidth broker component, responsible for admission control and edge-routerconfiguration (marking and shaping). Among the available bandwidth brokers [15], we have chosenthe Kansas University Bandwidth Broker (KUBB) [16], an intra-domain network manager allocatingnetwork resource to users as per their requests and the existing service policy. KUBB stores in apolicy database the necessary information about existing user service level agreements and bandwidthallocation requests.



...

<gwsdl:portType name="SearchServicePortType" extends="ogsi:GridService">

<operation name="searchForProfessor"><input message="tns:SearchForProfessorInputMessage"/><output message="tns:SearchForProfessorOutputMessage"/><fault name="Fault" message="ogsi:FaultMessage"/>

</operation>

<operation name="searchForCourse"><input message="tns:SearchForCourseInputMessage"/><output message="tns:SearchForCourseOutputMessage"/><fault name="Fault" message="ogsi:FaultMessage"/>

</operation>

<operation name="searchForKey"><input message="tns:SearchForKeyinputMessage"/><output message="tns:SearchForKeyOutputMessage"/><fault name="Fault" message="ogsi:FaultMessage"/>

</operation>

<operation name="browse"><input message="tns:BrowseInputMessage"/><output message="tns:BrowseOutputMessage"/><fault name="Fault" message="ogsi:FaultMessage"/>

</operation>

</gwsdl:portType>

...

Figure 4. GWSDL portType for the Search Service.

The KUBB has been wrapped inside a Grid Service (Bandwidth Broker Service) to simplify theinteraction between the bandwidth broker and the MDS and to support multi-domain reservation andrelated security issues. When an authenticated user requests QoS for his/her video streaming session,the MDS interacts on behalf of the user with the Bandwidth Broker Service to allocate resources withthe KUBB.

If the streaming server is in a different domain, it is necessary to propagate the request through thewhole set of domains included in the routing table. As KUBB has not been developed for inter-domainapplications, in our Bandwidth Broker Service we have introduced the capabilities to interact withBandwidth Broker Services of the neighbor domains, using a chained-based approach.

Network Monitoring Service

When the Bandwidth Broker Service is not able to allocate network resources, the platform can stillprovide a best effort content delivery. To collect the information required to choose the most suitable



streaming server, a Network Monitoring Service has been introduced into the prototype architecture.At regular intervals, the Service probes the network path to a set of streaming servers and gathersperformance characteristics about available bandwidth and roundtrip time. These measurements canbe treated as time series and used for the purpose of forecasting as proposed in [17]. The Service canalso perform a rapid and intrusive probing on the network (using the netest utility) to support thechoice among a set of potential servers and monitor the effective resource consumption during thestreaming session.

Authentication and authorization

The MDS can only operate if the user is identified by the system and his/her requests are compatiblewith resource access rights. In practice, network resources can be reserved only by a specific subset ofusers, e.g. those who have negotiated a service level agreement with the domain network administrator.One of our strongest motivations in using Grid technologies to build our multimedia streaming system,is that security functionalities can be naturally embedded in Grid Services which can be located andused as needed by applications. According to the security model proposed in the OGSA standard (GridSecurity Infrastructure (GSI)), security policies are published so that users can dynamically discoverwhich credentials and mechanisms are needed to establish trust with the service.

To support the Bandwidth Broker Service, we have merged the OGSA security model with theauthorization policy provided by the KUBB. While authentication is provided by GSI, to performdelegation of credentials in multi-domain applications, the authorization is implemented using thepolicy database provided by the KUBB. We decided to preserve the KUBB authorization mechanismbecause it is capable of performing fine grain authorization for user access not yet fully availablein GSI. This implementation shows the flexibility of OGSA, able to integrate available securitymechanisms with native solutions.

The prototype at work

We have validated our prototype through the deployment of a campus-wide multimedia application.Figure 5 shows an overview of the testbed used in the experiments. Several RTSP streaming servers,based on either Darwin or Helix packages, are distributed across the university LANs.

The distributed nature is completely hidden to the user, interacting with the application througha simple Java-based graphical interface. Figure 6 shows a common use case scenario, involving astudent looking for multimedia resources related to a course. The submission of a query (a) results ina list of video files matching with client requests (b). The student can browse the list and receivefurther information (c), then she/he can request a streaming session. Depending upon the state ofnetwork resources, she/he will transparently obtain a QoS-enabled session or will be offered a best-effort delivery (d).

The experiments performed so far have exhibited satisfactory results in terms of correctness,robustness, and performance of the distributed system. For example, we set up a MDS connected withfive video servers, each one managing a database of more than 200 content descriptors, to evaluate itsperformance and to assess factors affecting service response time. The average execution time is aslow as 300 ms when the MDS replies to a single query. The minimum answer time is influenced bythe number of matching entries returned by the video servers. While the time spent in the execution



Figure 5. Overview of current testbed.

Figure 6. Screenshot of the user interface in the current prototype.

of Search Service is approximatively constant, the delivery and handling of returned data increase theaverage response time to a couple of seconds for queries returning more than 100 items. A largernumber of concurrent queries to the MDS, as could be foreseen, increases the service response time.Nevertheless, the relation between the number of queries and the response time remains satisfactory,as processing 30 concurrent queries still requires less than three seconds.

As an excerpt of experimental results obtained for streaming sessions, Figure 7 reports some networkperformance data for (a) a best-effort delivery and (b) a QoS-enabled session. Both refer to the same



0 10 20 30 40 50 600

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ate

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best delivery

0 10 20 30 40 50 60Time

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Figure 7. Quicktime network statistics for (a) best delivery and (b) QoS-enabled streaming sessions for a 802.11bwireless client. (c) Measured bitrates for the same sessions.

300 Kbps MPEG-4 stream served by one of the Darwin RTSP Servers to a Quicktime player of a802.11b wireless client (Figure 5).

In order to assess the effectiveness of QoS support in presence of network resource scarcity, these teststreaming sessions have been performed while injecting a synthetic UDP stream along the server–clientpath. This competing stream has a high bitrate of 8 Mbps so as to exhaust completely the minimumcapacity link along the path, namely the 802.11b wireless access network.

In the case of best-effort delivery (Figure 7(a)), the streaming session is sensibly perturbed by thecompeting traffic as made clear particularly by the large packet loss and by the insufficient averagebandwidth in the player statistics. Differently, when the streaming session has obtained end-to-endQoS guarantees from the multimedia service and the Bandwidth Broker enables the prioritization of itstraffic, the user experiences smooth playback of his/her multimedia content, as testified by the muchimproved streaming statistics (Figure 7(b)).

Additional information on the two streaming sessions is provided by the graphs in Figure 7(c)showing the measured throughput over time to the wireless streaming client. In order to extend theDiffserv domain to the wireless portion of our network we have exploited the traffic prioritizationcapabilities (WMM/802.11e) of Cisco Access Points along with a novel technique not described heredue to the lack of space.



In the case of best-effort delivery (top of Figure 7(c)), as soon as the competing background trafficis activated (Time = 10 s) the bitrate of the streaming session is almost annihilated. During the QoS-enabled session (bottom of Figure 7(c)), instead, streaming throughput is unaffected by the injectedtraffic and its variations are only due to VBR encoding and buffering schemes.

4. CONCLUSIONS AND FUTURE WORK

In this paper we presented our on-going research concerning a service-oriented architecture formultimedia content access and distribution in an e-learning environment. The Grid Service approachwe are following is essentially motivated by the strong need for interoperability and scalability, reuse ofcommon infrastructure services, consistent navigational model for users, and support for heterogeneousdevices. Initially, we described a number of general requirements that need to be taken into accountin the design of campus-wide services for multimedia content access and distribution. Throughoutthe presentation of the overall architecture and the services developed so far, we discussed how GridServices technologies have provided an effective support to build a system prototype coping with aninitial subset of the general requirements. The prototype has been experimented successfully with atestbed including several distributed multimedia resources available on our campus network.

Further work will investigate several extensions to the service-oriented architecture for multimediaaccess and distribution. Methodologically, we will attempt at gaining a deeper understanding of theissues related to the dynamic integration of high-level Grid services (e.g. an e-learning service) withlower-level infrastructural services (e.g. caching proxies, databases, bandwidth brokers). Moreover,we are already introducing semantic and QoS-constrained search capabilities as well as profile-basedcontent adaptation services.

ACKNOWLEDGEMENTS

This work has been partially supported by a IBM Faculty Award and by the ‘WEB-MINDS’ FIRB project of theNational Research Ministry.

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Designing Grid services for multimedia streaming in an e-learning environment