Identical Principles, Higher Layers: Modeling Web Services as Protocol Stack

Mario Jeckle, Erik Wilde


Web Services and their potential applications are currently under heavy discussion in industry, research, and standardization. As a result of evaluation and experience by early adopters, the technology is expected to mature through the advent of new standards and solutions leveraging Web Service's power. In essence, the efforts undertaken to create and complete a stack of Web Service protocols lead to a new communication architecture and extends the stack of classical network protocols. This evolving architecture could serve as a future-proof infrastructure for businesses to rely on. However the growth of the Web Service stack with respect to the addition of new layers and expansion of the resulting infrastructure has not been studied in comparison with well-established protocol suites like the ISO/OSI stack or the set of protocols constituting the Internet. Strictly speaking, industry's demand for functionality and services enhancing the basic Web Service protocols such as XML-RPC or SOAP, leads to the creation of a full-fledged layered protocol suite on-top of the existing ones. Nevertheless, the various standards, specifications, and ideas have neither been consolidated on a common terminological basis, nor been integrated in a single framework of reference. This observation also applies to the established trio of Web Service standards composing of SOAP, WSDL, and UDDI. According to the specific usage patterns of these specifications, they are not operating on one layer as the well-known triangular relationship graph suggests, but instead they are connected by means of unidirectional usage dependencies. From this point of view, the message patterns (MP) defined by WSDL 2.0 offer services to layers organized on top of WSDL which rely on the service interfaces exposed by SOAP. More precisely, not the interface definition with WSDL but the accompanying MPs act as the transport layer of the service stack. Based on this and other criteria, SOAP can be categorized as the basic low-level layer of the Web Service infrastructure corresponding to the network-dependent layers of the classical protocol suites. Based on these facts, all of the various efforts relying on the seminal Web Service protocols can be categorized at the various levels layered above the transport layer. This is especially true for specifications dealing with the management of sessions and transactions which are layered directly above the MPs. Also, security standards like XML digital signatures and XML encryption fit well into this by classifying them as part of the presentation layer. Furthermore, within the Web Service environment quite analogous application layer mechanisms (e.g. firewalls for content filtering) emerge are commonly known for classical network operation. Taking this congruency of established protocol stacks and the Web Service's one step further the analogy may serve as a valuable framework for the comparison of different architectural styles in Web Service deployment. Taking the continuing debate weighing services based on representational state transfer (REST) against those based on RPC-style SOAP as an example, both approaches reveal themselves as heterogeneous protocols. Both ideas are not mutually exclusive nor conflicting at all. Both protocols can be made interoperable by the use of bridges or gateways arbitrating between the two parties. Our analysis shows that Web Services are a true but yet incomplete protocol suite deploying classical Internet protocols as basic services by the continued addition of supplemental specifications and standards.


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