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Part III. Appendix 2: Data Transportgous to the pointer-dereference operator of ANSI C. That is, if the variable my-url is defined as aURL data type, then my-url indicates the string spelling out the URL, and *my-url indicates theactual data indicated by the URL.FUNCTIONALITIES OF OPENDAP/NVODSThe motivation behind communication protocols tends to be the desire to provide the most functionalityand convenience to the greatest number of users for the lowest overhead imposed uponthe parties individually and in total. Certainly, OPeNDAP’s design and implementation wereguided by this desire. The design goal of bringing data transfer functions to all OPeNDAP/NVODSclients and yet not requiring them or all the system’s servers to become complex (“thick”) is beingachieved by giving the responsibility for fulfilling certain kinds of client requests to just a few servers.The organization of this section reflects this dichotomy:• Section 1 discusses services which contribute to the “thickness” of every OPeNDAP/NVODS clientand server;• Section 2 considers services which are presently implemented at selected servers.The next three sections list platforms, formats, and applications currently in use and supported byOPeNDAP/NVODS:• Section 3: Platforms and operating systems• Section 4: Server-side data storage formats• Section 5: Clients (applications)The remaining section examines one aspect of OPeNDAP/NVODS which is in flux:• Section 6: XMLUniversal minimal requests and facilitiesBy “universal minimal requests and facilities” we mean those requests that every OPeNDAP-compliantclient can make in every appropriate request to any OPeNDAP-compliant server with the fullexpectation that the request will be satisfied by that server without participation of an intermediateOPeNDAP server.Since all communication from client to server takes the form of a qualified URL, these universalservices depend on the expressiveness of that URL. As noted above, the client, by using variousfilename extensions, is able to ask for the DDS, the DAS, the DataDDS, an information page aboutthe server, and help documentation from the server. A query (constraint expression) added to theDataDDS request can trigger projection and selection by the server. Since the constraint expressioncan invoke functions implemented on the server and made available to clients, the constraint expressionis actually a powerful tool.189