OptionalblockOptionalcontentOptionalcontentOptionalfromExplicit sender JID — required when the session is a component.
OptionalnameFile name announced in transfer metadata. Default "body".
OptionalonFailures after the descriptor was already sent land here.
OptionalttlOffers expire unclaimed after this.
A body to hand to a handshake-driven transport (sipub, jingle). The transport builds its descriptor synchronously and registers pending state;
open()is called (at most once) when the peer completes the handshake and bytes actually start flowing.