see http://xmpp.org/extensions/
XEP-0047: In-Band Bytestreams
XEP-0066: Out of Band Data
XEP-0030: Service Discovery
XEP-0060: Publish-Subscribe
XEP-0045: Multi-User Chat
XEP-0149: Time Periods
XEP-0166: Jingle
XEP-0174: Serverless Messaging
XEP-0199: XMPP Ping
XEP-0224: Attention
XEP-0077: In-Band Registration