Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The XML Configuration Access Protocol (XCAP), is an application layer protocol that allows a client to read, write, and modify application configuration data stored in XML format on a server. XCAP maps XML document sub-trees and element attributes to HTTP URIs, so that these components can be directly accessed by clients using HTTP protocol. An XCAP server is used by XCAP clients to store data like buddy lists and presence policy in combination with a SIP Presence server that supports PUBLISH, SUBSCRIBE and NOTIFY methods to provide a complete SIP SIMPLE server solution.