XEP-0198: Stream Management
"The basic concept behind stream management is that the initiating entity (either a client or a server) and the receiving entity (a server) can exchange "commands" for active management of the stream. The following stream management features are of particular interest because they are expected to improve network reliability and the end-user experience:
Stanza Acknowledgements – the ability to know if a stanza or series of stanzas has been received by one's peer.
Stream Resumption – the ability to quickly resume a stream that has been terminated.
Stream management implements these features using short XML elements at the root stream level. These elements are not "stanzas" in the XMPP sense (i.e., not <iq/>, <message/>, or <presence/> stanzas as defined in RFC 6120) and are not counted or acked in stream management, since they exist for the purpose of managing stanzas themselves."