XMPP

Extensible Messaging and Presence Protocol (XMPP, originally named Jabber) is an open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance. Based on XML (Extensible Markup Language), it enables the near-real-time exchange of structured data between two or more network entities. Designed to be extensible, the protocol offers a multitude of applications beyond traditional IM in the broader realm of message-oriented middleware, including signalling for VoIP, video, file transfer, gaming and other uses.

Unlike most commercial instant messaging protocols, XMPP is defined in an open standard in the application layer. The architecture of the XMPP network is similar to email; anyone can run their own XMPP server and there is no central master server. This federated open system approach allows users to interoperate with others on any server using a 'JID' user account, similar to an email address. XMPP implementations can be developed using any software license and many server, client, and library implementations are distributed as free and open-source software. Numerous freeware and commercial software implementations also exist.

Originally developed by the open-source community, the protocols were formalized as an approved instant messaging standard in 2004 and has been continuously developed with new extensions and features. Various XMPP client software are available on both desktop and mobile platforms and devices - by 2003 the protocol was used by over ten million people worldwide on the network, according to the XMPP Standards Foundation.

Protocol characteristics

Decentralization

The XMPP network architecture is reminiscent of the Simple Mail Transfer Protocol (SMTP), a client–server model; clients do not talk directly to one another as it is decentralized - anyone can run a server. By design, there is no central authoritative server as there is with messaging services such as AIM, WLM, WhatsApp or Telegram. Some confusion often arises on this point as there is a public XMPP server being run at jabber.org, to which many users subscribe. However, anyone may run their own XMPP server on their own domain.

Addressing

Every user on the network has a unique XMPP address, called ''JID'' (for historical reasons, XMPP addresses are often called ''Jabber IDs''). The JID is structured like an email address with a username and a domain name (or IP address) for the server where that user resides, separated by an at sign (@) - for example, “[e-mail]“: here alice is the username and example.com the server with which the user is registered.

Since a user may wish to log in from multiple locations, they may specify a resource. A resource identifies a particular client belonging to the user (for example home, work, or mobile). This may be included in the JID by appending a slash followed by the name of the resource. For example, the full JID of a user's mobile account could be [e-mail]/mobile.

Each resource may have specified a numerical value called priority. Messages simply sent to [e-mail] will go to the client with highest priority, but those sent to [e-mail]/mobile will go ''only'' to the mobile client. The highest priority is the one with largest numerical value.

JIDs without a username part are also valid, and may be used for system messages and control of special features on the server. A resource remains optional for these JIDs as well.

The means to route messages based on a logical endpoint identifier - the JID, instead of by an explicit IP Address present opportunities to use XMPP as an Overlay network implementation on top of different underlay networks.

XMPP via HTTP

The original and "native" transport protocol for XMPP is Transmission Control Protocol (TCP), using open-ended XML streams over long-lived TCP connections. As an alternative to the TCP transport, the XMPP community has also developed an HTTP transport for web clients as well as users behind restricted firewalls. In the original specification, XMPP could use HTTP in two ways: ''polling'' and ''binding''. The polling method, now deprecated, essentially implies messages stored on a server-side database are being fetched (and posted) regularly by an XMPP client by way of HTTP 'GET' and 'POST' requests. The binding method, implemented using ''Bidirectional-streams Over Synchronous HTTP'' ( BOSH), allows servers to push messages to clients as soon as they are sent. This push model of notification is more efficient than polling, where many of the polls return no new data.

Because the client uses HTTP, most firewalls allow clients to fetch and post messages without any hindrances. Thus, in scenarios where the TCP port used by XMPP is blocked, a server can listen on the normal HTTP port and the traffic should pass without problems. Various websites let people sign into XMPP via a browser. Furthermore, there are open public servers that listen on standard http (port 80) and https (port 443) ports, and hence allow connections from behind most firewalls. However, the IANA-registered port for BOSH is actually 5280, not 80.

Extensibility

The XMPP Standards Foundation or XSF (formerly the Jabber Software Foundation) is active in developing open XMPP extensions, so called ''XEP''. However, extensions can also be defined by any individual, software project, or organization. To maintain interoperability, common extensions are managed by the XSF. XMPP applications beyond IM include: chat rooms, network management, content syndication, collaboration tools, file sharing, gaming, remote systems control and monitoring, geolocation, middleware and cloud computing, VoIP, and identity services.

Building on its capability to support discovery across local network domains, XMPP is well-suited for cloud computing where virtual machines, networks, and firewalls would otherwise present obstacles to alternative service discovery and presence-based solutions. Cloud computing and storage systems rely on various forms of communication over multiple levels, including not only messaging between systems to relay state but also the migration or distribution of larger objects, such as storage or virtual machines. Along with authentication and in-transit data protection, XMPP can be applied at a variety of levels and may prove ideal as an extensible middleware or Message-oriented middleware (MOM) protocol.

Current limitations

At the moment, XMPP does not support Quality of Service (QoS); assured delivery of messages has to be built on top of the XMPP layer. There are two XEPs proposed to deal with this issue, XEP-0184 Message delivery receipts which is currently a draft standard, and XEP-0333 Chat Markers which is considered experimental.

Since XML is text based, normal XMPP has a higher network overhead compared to purely binary solutions. This issue was being addressed by the experimental XEP-0322: Efficient XML Interchange (EXI) Format, where XML is serialized in a very efficient binary manner, especially in schema-informed mode. This XEP is currently deferred.

In-band binary data transfer is limited. Binary data must be first base64 encoded before it can be transmitted in-band. Therefore, any significant amount of binary data (e.g., file transfers) is best transmitted out-of-band, using in-band messages to coordinate. The best example of this is the Jingle XMPP Extension Protocol, XEP-0166.

Features

Peer-to-peer sessions

Using the extension called Jingle, XMPP can provide an open means to support machine-to-machine or peer-to-peer communications across a diverse set of networks. This feature is mainly used for IP telephony (VoIP).

Multi-user chat

XMPP supports conferences with multiple users, using the specification Multi-User Chat (MUC) (XEP-0045). From the point of view of a normal user, it is comparable to Internet Relay Chat (IRC).

Security and encryption

XMPP servers can be isolated (e.g., on a company intranet), and secure authentication ( SASL) and point-to-point encryption (TLS) have been built into the core XMPP specifications, as well as

Off-the-Record Messaging (OTR) is an extension of XMPP enabling encryption of messages and data. It has since been replaced by a better extension, multi-end-to-multi-end encryption (OMEMO, XEP-0384) end-to-end encryption between users. This gives a higher level of security, by encrypting all data from the source client and decrypting again at the target client; the server operator cannot decrypt the data they are forwarding.

Messages can also be encrypted with OpenPGP, for example with the software Gajim.

Service discovery

While several service discovery protocols exist today (such as zeroconf or the Service Location Protocol), XMPP provides a solid base for the discovery of services residing locally or across a network, and the availability of these services (via presence information), as specified by XEP-0030 DISCO.

Connecting to other protocols

One of the original design goals of the early Jabber open-source community was enabling users to connect to multiple instant messaging systems (especially non-XMPP systems) through a single client application. This was done through entities called ''transports'' or ''gateways'' to other instant messaging protocols like ICQ, AIM or Yahoo Messenger, but also to protocols such as SMS, IRC or email. Unlike multi-protocol clients, XMPP provides this access at the server level by communicating via special gateway services running alongside an XMPP server. Any user can "register" with one of these gateways by providing the information needed to log on to that network, and can then communicate with users of that network as though they were XMPP users. Thus, such gateways function as client proxies (the gateway authenticates on the user's behalf on the non-XMPP service). As a result, any client that fully supports XMPP can access any network with a gateway without extra code in the client, and without the need for the client to have direct access to the Internet. However, the client proxy model may violate terms of service on the protocol used (although such terms of service are not legally enforceable in several countries) and also requires the user to send their IM username and password to the third-party site that operates the transport (which may raise privacy and security concerns).

Another type of gateway is a server-to-server gateway, which enables a non-XMPP server deployment to connect to native XMPP servers using the built in interdomain federation features of XMPP. Such server-to-server gateways are offered by several enterprise IM software products, including:

* IBM Lotus Sametime
* Skype for Business Server (formerly named Microsoft Lync Server and Microsoft Office Communications Server – OCS)

Software

XMPP is implemented by many clients, servers, and code libraries. These implementations are provided under a variety of software licenses.

Servers

Numerous XMPP server software exist, some well known ones include ejabberd and Prosody.

Clients

A large number of XMPP client software exist on various modern and legacy platforms, including both graphical and command line based clients. According to the XMPP website, some of the most popular software include Conversations (Android), Converse.js (web browser, Linux, Windows, macOS), Gajim (Windows, Linux), Monal (macOS, iOS), and Swift.IM (macOS, Windows, Linux).

Other clients include: Bombus, ChatSecure, Coccinella, JWChat.org, MCabber, Miranda NG, Pidgin, Psi, Tkabber, Trillian, and Xabber.

Deployment and distribution

There are thousands of XMPP servers worldwide, many public ones as well as private individuals or organizations running their own servers without commercial intent. Numerous websites show a list of public XMPP servers where users may register at (for example on the XMPP.net website).

Several large public IM services natively use or used XMPP, including LiveJournal's "LJ Talk", Nimbuzz, and HipChat. Various hosting services, such as DreamHost, enable hosting customers to choose XMPP services alongside more traditional web and email services. Specialized XMPP hosting services also exist in form of cloud so that domain owners need not directly run their own XMPP servers, including Cisco Webex Connect, Chrome.pl, Flosoft.biz, i-pobox.net, and hosted.im.

XMPP is also used in deployments of non-IM services, including smart grid systems such as demand response applications, message-oriented middleware, and as a replacement for SMS to provide text messaging on many smartphone clients.

Non-native deployments

Some of the largest messaging providers use, or have been using, various forms of XMPP based protocols in their backend systems without necessarily exposing this fact to their end users. One example is Google, which in August 2005 introduced Google Talk, a combination VoIP and IM system that uses XMPP for instant messaging and as a base for a voice and file transfer signaling protocol called Jingle. The initial launch did not include server-to-server communications; Google enabled that feature on January 17, 2006. Google later added video functionality to Google Talk, also using the Jingle protocol for signaling. In May 2013, Google announced XMPP compatibility would be dropped from Google Talk for server-to-server federation, although it would retain client-to-server support. Google talk has since been dropped from Google's line of products. In January 2008, AOL introduced experimental XMPP support for its AOL Instant Messenger (AIM) service, allowing AIM users to communicate using XMPP. However, in March 2008, this service was discontinued. As of May 2011, AOL offers limited XMPP support. In February 2010, the social-networking site Facebook opened up its chat feature to third-party applications via XMPP. Some functionality was unavailable through XMPP, and support was dropped in April 2014. Similarly, in December 2011, Microsoft released an XMPP interface to its Microsoft Messenger service. Skype, its de facto successor, also provided limited XMPP support. Apache Wave is another example.

XMPP is the de facto standard for private chat in gaming related platforms such as Origin, and PlayStation, as well as the now discontinued Xfire and Raptr. Two notable exceptions are Steam and Xbox LIVE; both use their own proprietary