1. Introduction

1.1. Overview

   The Extensible Messaging and Presence Protocol (XMPP) is a protocol
   for streaming XML [XML] elements in order to exchange messages and
   presence information in close to real time.  The core features of
   XMPP are defined in Extensible Messaging and Presence Protocol
   (XMPP): Core [XMPP-CORE].  These features -- mainly XML streams, use
   of TLS and SASL, and the <message/>, <presence/>, and <iq/> children
   of the stream root -- provide the building blocks for many types of
   near-real-time applications, which may be layered on top of the core
   by sending application-specific data qualified by particular XML
   namespaces [XML-NAMES].  This memo describes extensions to and
   applications of the core features of XMPP that provide the basic
   functionality expected of an instant messaging (IM) and presence
   application as defined in RFC 2779 [IMP-REQS].

1.2. Requirements

   For the purposes of this memo, the requirements of a basic instant
   messaging and presence application are defined by [IMP-REQS], which
   at a high level stipulates that a user must be able to complete the
   following use cases:

   o  Exchange messages with other users
   o  Exchange presence information with other users
   o  Manage subscriptions to and from other users
   o  Manage items in a contact list (in XMPP this is called a "roster")
   o  Block communications to or from specific other users

   Detailed definitions of these functionality areas are contained in
   [IMP-REQS], and the interested reader is directed to that document
   regarding the requirements addressed herein.

   [IMP-REQS] also stipulates that presence services must be separable
   from instant messaging services; i.e., it must be possible to use the
   protocol to provide a presence service, an instant messaging service,
   or both.  Although the text of this memo assumes that implementations
   and deployments will want to offer a unified instant messaging and
   presence service, there is no requirement that a service must offer
   both a presence service and an instant messaging service, and the
   protocol makes it possible to offer separate and distinct services
   for presence and for instant messaging.

   Note: While XMPP-based instant messaging and presence meets the
   requirements of [IMP-REQS], it was not designed explicitly with that
   specification in mind, since the base protocol evolved through an
   open development process within the Jabber open-source community
   before RFC 2779 was written.  Note also that although protocols
   addressing many other functionality areas have been defined in the
   Jabber community, such protocols are not included in this memo
   because they are not required by [IMP-REQS].

1.3. Terminology

   This memo inherits the terminology defined in [XMPP-CORE].

   The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, RFC 2119 [TERMS].