The following exerts from the proposal provide a nice short and concise description of the XPL/Argot concept:
“XPL was created to combine (i) the extensibility provided by XML and XML Schema, (ii) the ability to describe and encode binary information like IDL+IOP and ASN.1, (iii) the ability to create remote procedure call services like CORBA, and (iv) to be inherently 'version-aware', to ensure that a change to one aspect of the system did not create cost across the whole network.
The XPL solution is to have each application in the network contain metadata about all the information it can communicate (send and receive). This allows each XPL application to negotiate directly with other peers the information they are able to exchange. It is this fundamental change of moving the metadata knowledge into the application or file, rather than keeping it externally, which is the most important aspect of XPL.
The XPL type system embedded (or notionally embedded) in each device is combined with methods for performing type comparisons with external systems. The type comparisons are designed to find the common definition type set shared by two XPL type systems.”
The proposal breaks the technology into two parts. The following descriptions are also from the proposal:
- XPL, a powerful Extensible Presentation Language which is used both at protocol design time and by applications at runtime via very simple and small code libraries to communicate both primitives and compound types. XPL creates an internally consistent directed graph, which may be cyclic, to define complete versioned type systems to be stored with (or notionally with) an application or device.
- XPL/TRP, a compact Type Resolution Protocol, in turn provides (i) dynamic negotiation of protocols and their constituent types between disparate devices; (ii) protocol versioning all the way down to the type level; and (iii) dynamic discovery of device capabilities and versions. An XPL device is able to describe and communicate its own application protocol. All these features are built-in to the protocol and intrinsic to its operation, rather than being extra-cost additions to it.
The XPL system is a departure from how most application protocols are designed. Protocols are normally designed and then the implementation is created from the design. XPL binds the design and implementation together so that they are interrelated. This has interesting consequences for versioning and detailed discovery. A device can be interrogated to discovery the structure of all the data that can be sent/received to it. Combined with scripting languages and other methods it would allow clients to be built automatically with zero code. XPL is the first time that you can implant a formal protocol description in any device or application down to the smallest of devices.
There's a lot of other interesting proposals that have been delivered to the IETF. In the application protocols area includes, XPL, Binary HTTP (chopan), Binary XML (exi), ZigBee Alliance requirements, and others have been submitted. The area of M2M (Machine 2 Machine) is going to continue to hot up over the next few years. There's still plenty more work to do, and it will take a while before a consensus is reached over the various protocols. Some interesting times are ahead!