Second, to maximize the reusability of exiting tools, XP does not introduce into the data streams any content which would not be recognized by an XML processor. Specification XP specifies the data exchanged on a session-layer connection between two endpoints. The session-layer connection will often be a TCP connection. The data on the session-layer connection must be an uninterrupted stream of complete XML documents in each direction. Well-formedness may and should be ensured before data is sent.
All error reporting which involves additional document transmission is left to derived protocols.
Accordingly, we introduce mechanisms for handling both the sequential and interrelated aspects of the document streams. XP introduces three constructs into the document streams. All of these constructs are XML processing instructions. All are optional. Requests Requests, defined as documents not solicited by a request from the opposite endpoint or documents with which responses may be associated, may be sent at any time, by either endpoint.