PEP Extensions

Examples

• AccessCounter
• EscapeBody
• EncoderDispatcher

Types

For network security reasons, it may be usefull to categorize extensions according to their purpose and allowed actions.

• Stream - extensions that operate on a data stream without providing their own message body.
  • Encoder - a stream that renders its data for downstream extensions, presenters, or agents. Most encoders will be occlusive or ordered. The output from one will affect downstream encoders. Non-occlusive encoders depend on the medium being transmitted, for example in HTML, one could have a tag encoder and a text language translater that could be run in either order as they would opperate on seperate parts of the body.
  • Extender - a stream that enhances the data stream given read access to system resources. An example would be an extension that performed server-side includes or executed some embedded code.
• Resource - extensions that provide a message body or perform a terminal action on the transmitted body. The client should allow only one of these.
• Presenter - display data for browser.
• Moderator - Orders other extensions. May be an encoders, like EscapeBody/SumBody or a more complicated extension that produces a cohesive product from a combination resource extensions, for example, a mix and match from generator. These may be coded eiather by
  • assuming the roles of the moderated extensions
  • hiding the headers from the moderated extensions and generating the calls to the extensions on its own, see EncoderDispatcher.

Reasons

There are different reasons an extension may be called to map itself into a message or supply meta information about itself. The most common of these are likely to be:

• Always - an extension that is always called. These must determine for themselves whether to ap themselves.
• URLTree - an extension that is associated with accessing a specific resource or set of resources. These may lie on the local file system, as on a server, or in URL space, as with a client.

Other protocols may be invoked for many reasons. For instance, the server may elect to map a resource because of the state of some system resources. It is unlikely that the standard interface will able to support dynamic loading of these, so they would only be using the mapping collision advantages of PEP.

Downloading

PEPExtensions may be downloaded:

• Directly from the site listed in the URL.
• From an independent clearinghouse.
• Directly from agent who requests its use. This creates the most complex security scenario, but is also the most efficient use of a persistent connection.

Locating the extension

PEP extensions are uniquely identified by the URI. PEPsample extensions may be located by inserting "PEPsample" after the last segment of the URI. For instance, a description of the compressions simulation http://www.w3.org/PEP/extensions/EscapeBody could be located by requesting that URL. The PEPsample implementations of that extension are located in the directory http://www.w3.org/PEP/extensions/EscapeBody/PEPsample/. The PEPAgent downloads the classes for its role, for example, the PEPClient will download the class http://www.w3.org/PEP/extensions/EscapeBody/Client.class.

Associated classes

Most extensions will have at least an instance class in addition to seperate classes for the client, proxy, and server. during downloading, the URLClassLoader calls java.lang.ClassLoader.resolveClass. This insures that any other classes associated with the extension will be downloaded too. These classes may be located anywhere underneath the extensions directory. For instance, http://www.w3.org/PEP/extensions/EscapeBody/Client.class may make a reference to http://www.w3.org/PEP/extensions/Escape/EscapeStream.class.