When deploying a single application that uses XML parser, you typically understand the application to be receiving an infoset through some type of XML parser. That infoset should by definition be a tree of elements, character data, attachments and attributes.
On the other hand, IETF talks about applications in the sense that any single endpoint is an application -- this could be an IIS server or a FTP client. The OSI model assumes more layers are required in the stack because of the definition of the single applications. The stack for an IIS server would contain the HTTP server, the session management, the authentication, etc. It will actually do all the WS layers inside the upper OSI layers.
In the Windows world, we have built a lot of applications to handle low-level protocols such as HTTP. In the mainframe world, we build applications to handle business-level protocols, where services such as transport, queuing and sessions are handled by the operating system.
However, we need to recognize that the term "transport" has different meaning in different circumstances. When talking about the OSI 7-layer model, "transport" refers to level 4. In the domain of the application, "transport" could mean the network protocols that I use to transfer my message -- which is equivalent to the entire OSI 7 layer stack.
One of the transports for Web services is SOAP over HTTP, which in turn is over TCP/IP -- which is not an application layer protocol. In the OSI world, it sits at levels 5 (TCP) and 4(IP).
Why the difference in definitions? An application can't just use TCP/IP. It also needs something like HTTP, SMTP, IIOP, etc. So from the network's perspective, SOAP itself is an application, and as with any other application, it accesses the network using an application layer protocol. Unlike other middleware systems, SOAP has been designed so that it can use any application layer protocol to access the network and can use HTTP, SMTP, IIOP, etc. as its "transport".
So, the answer is yes -- a Web services application could map to the OSI 7 layers, and no -- a Web services architecture exists across multiple OSI 7 layers and extends beyond the scope of a "single application".
This was first published in December 2003