Chapter 17 - Service Configuration on the Internet

Many services require a mechanism for allowing users to manage their service configuration. For example, a presence server requires presentities (users) to authorize which watchers can see their presence information. A Push-to-talk over Cellular (PoC) service requires users to create and manage groups. Likewise a conference may require users to configure a dial-out or dial-in list of participants, their privileges (who can speak or who can send or receive which media type), and so on.
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Chapter 17 - Service Configuration on the Internet Chapter 17Service Configuration on theInternetMany services require a mechanism for allowing users to manage their service configuration.For example, a presence server requires presentities (users) to authorize which watchers cansee their presence information. A Push-to-talk over Cellular (PoC) service requires users tocreate and manage groups. Likewise a conference may require users to configure a dial-outor dial-in list of participants, their privileges (who can speak or who can send or receivewhich media type), and so on. All of these use cases share many commonalities: a user hasto perform non-real-time operations on a server to manipulate one or more documents thatconfigure or personalize their instance of the service. Usually the user creates a configuration document locally in their terminal and thenuploads it to a server. Sometimes, the user just needs to make a small change to an existingdocument, so it is not worth uploading the complete document. Instead, it is desireable tohave the capability to update part of the document. In some other cases the user changes theirusual terminal and uses a different one, so they may first need to download a fresh copy ofthe current configuration document, make some changes, and upload it (either complete or apart of it) to the server. Typically, configuration documents are highly structured. Owing to this, the trendnowadays is to use the Extensible Markup Language (XML) (specified by the World WideWeb Consortium in the W3C recommendation XML 1.1 [93]), for formatting documents thatpersonalize the instance of the service. So, we know that configuration documents are effectively XML documents, but how arethey sent to and received from the server that stores them? The XML Configuration AccessProtocol (XCAP) solves this vacuum.17.1 The XML Configuration Access Protocol (XCAP)The problem of document management can be depicted in its most simplified way as inFigure 17.1. A user creates an XML document in his computer and wants to upload it to aserver. The server will use the document at a later time to produce a personalized instance ofthe service. The problem to be solved requires designing a protocol that allows such an uploadprocedure. HTTP (specified in RFC 2616 [144]) provides a good baseline, since it providesThe 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third EditionGonzalo Camarillo and Miguel A . Garc ıa-Mart´n ´ ı© 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51662-1 CHAPTER 17. SERVICE CONFIGURATION ON THE INTERNET372 Terminal server Figure 17.1: Overview of Document Managementthe POST and PUT methods for transferring files from the client to the server. It also containsa GET method for downloading a document from the server. However, there are additionalrequirements that prevent the usage of HTTP. For example, on many occasions users justneed to modify a small piece of an existing XML document. For instance, when a user wantsto add a “buddy” to his list of friends in the presence service: the user just wants to edit a fewbytes of a potentially large document. It is not worth uploading the whole XML documentagain, since the document is mostly unchanged, just the modified content. HTTP alone doesnot offer that functionality. The IETF decided then to develop a set of conventions and rules for using HTTP to uploadand download complete or portions of XML documents to and from a server. This resulted inthe creation of XCAP (specified in RFC 4825 [277]). XCAP is, therefore, not a new protocol,but a set of conventions for using HTTP for managing remotely stored XML documents.Figure 17.2 shows a schematic representation of the protocol stack used by XCAP. XCAP HTTP TCP IP Figure 17.2: The XCAP protocol stack XCAP provides a client with the means to read, write, and modify XML applicationconfiguration data remotely stored on a server. This includes, for example, modifying the listof users in a presence list, authorization policies (e.g., a list of authorized watchers) or the listof participants in a conference. XCAP does not control the user interface (e.g., the graphicalrepresentation of the list), rather, it focuses on the data structure. XCAP defines conventions that map XML documents and their components (e.g.,elements, attributes) to HTTP URIs. Therefore, XCAP provides a unique way to represent anXML document or a component with an HTTP URI. It also defines the rules that govern how17.1. THE X ...