XMPP Protocol Explained: Server, Client, and IoT Applications

This page explores the XMPP protocol, its relevance in the Internet of Things (IoT), and the communication between XMPP servers and clients.

XMPP stands for Extensible Messaging and Presence Protocol. As the name suggests, here are its key features:

• Extensible: It can be customized to meet individual user needs.
• Messaging: Employs short messages for communication between clients (users) and servers.
• Presence: Monitors user presence and status, reacting accordingly.
• Protocol: An open platform, constantly evolving, based on XML and asynchronous communication.

The base protocol is specified in RFC 2778 and RFC 2779, which provide a general model for presence and instant messaging. Furthermore, RFC 3920 details the XMPP core, and RFC 3921 covers XMPP IM (Instant Messaging).

XMPP Core Overview

• XMPP is an open protocol for streaming XML elements, facilitating near real-time exchange of messages and presence information.
• The protocol operates within a typical client-server architecture. An XMPP client interacts with an XMPP server using a TCP socket.

XMPP Addressing Scheme (JID)

The Jabber Identifier (JID) format is: [node@]domain[/resource]

• Domain Identifier:
  • The only required identifier of a JID.
  • Functions as the networking gateway or primary server to which all entities connect.
  • Can also be a service addressed as a subdomain of a server, not necessarily a server itself.
• Node Identifier:
  • An optional secondary identifier.
  • Represents a client or chat room.
  • A “bare JID” has the form <node@domain>.
• Resource Identifier:
  • An optional tertiary identifier.
  • Represents a specific session, connection, or object.
  • Typically defined by a client implementation.

Communication is broken down into stanzas. Stanzas are short, self-contained XML messages exchanged between XMPP clients and servers. There are three types of stanzas:

• Presence
• IQ (Info/Query)
• Message

The basic structure of a stanza is as follows:

<[Stanza type][from=""][to=""][type=""][id="]>
  <child element>
    <sub-child element>
    </child element>
</Stanza type>

XMPP Protocol: Server and Client Interaction

Figure 2 depicts the XML message exchange between a client “Mike” and the server “Ollie.org.”

Instant messaging provides a means for immediate message transmission and reception to online users.

The following are key characteristics of the XMPP protocol used for communication between XMPP clients and servers:

• XMPP uses port 5222 for client-to-server (C2S) communication.
• XMPP uses port 5269 for server-to-server (S2S) communication.
• Discovery and XML streams are utilized for both S2S and C2S communications.
• It employs security mechanisms like TLS and SASL.
• Unlike email, there are no intermediate servers for federation.

XMPP’s features, including addressing, scalability, federation, and security, make it well-suited for Internet of Things (IoT) applications. Federation allows users from different business domains to communicate with each other. For instance, connecting a thermostat to a web server allows for easy access via a mobile phone.