SOAP extensibility and processing model

SOAP feature: abstract piece of functionality
Extensions expressed formally as a SOAP module
- Headers can be added to have extra processing done to the message; e.g.:
  - Integrity checks
  - Encryption
- One can ensure that a header will be processed as intended:
  - Optional extensions
  - Mandatory extensions, marked with mustUnderstand='true'; e.g.:
    - Process header block in a particular order, or don't do anything