The Information Query (IQ) stanza is where the work is done in XMPP. Rather than being used in a fire-and-forget fashion like message stanzas, IQ requests represent a request for information or a request to set some piece of information. Therefore unlike message stanzas, they always return a response, even to inform the requester of an error.

There are two types of IQ requests: The GET request, which is comparable with GET in HTTP; and the SET request, which, as you may have guessed, equates to a POST, PUT, or DELETE in HTTP. The response stanzas come in the form of RESULT or ERROR.

Unlike message or presence stanzas, IQ stanzas also have the requirement of an id attribute, which is a unique identifier that allows the mapping of a response to the original request, since XMPP is asynchronous, and so responses may be returned out of order.

A typical IQ request might be from a client to a server to see what features it supports (like our entity capabilities from before, it works on...