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Let's talk XMPP - episode 1 - the basics

This article was initially published in French, and it got some interest, so we started to translate the whole series in English. If you can read French, you can follow the whole series there: http://www.goffi.org/tag/parlons_xmpp

The translation to English was done thanks to: Poulet, Éfrit, Paco and mbarbarosa. The whole series is under CC By-SA, and any help would be appreciated to help with the translation of next articles.

Well, as it’s a shame that XMPP is not well known or understood, I decided to start a series of articles to explain what it is.
These articles are for a technical audience, but not necessarily for developers, and I hope it will help you to understand the advantages of this protocol and to use your software in a better way.
Being a developer of the “Salut à Toi” project, I’ll probably give examples with it quite often.
So let’s start with basis.
What’s XMPP? It’s a standard message and presence protocol, and extensible (XMPP means « eXtensible Message and Presence Protocol », that is it’s documented and used as a reference (validated by standards organization: the IETF). It allows all software which use it to speak the same language, and so to be interoperable. It’s a libre protocol, that is you can get the documentation and use it for free, without any legal or technical restrictions, and you can improve or modify it (but if you divert and don’t suggest your modifications, you risk to lose the compatibility with other software).
XMPP is decentralized and federated, that is you can have servers anywhere, they could (if they don’t forbid it explicitly in their configuration) communicate with each other.
It’s a popular protocol, many software allow to use it: for servers, there are Prosody, Ejabberd, Openfire, Tigase, Mongooselm, Metronome, etc. For clients: Gajim, Poezio, Pidgin, Psi, Swift, Jappix, Movim and of course Libervia/Salut à Toi (I let you find the official websites by yourself). A complete list (servers, clients and libraries) is available here: https://xmpp.org/xmpp-software/.
If you cannot install your server at home, many public ones are available: in France one can cite those of APINC ((jabber.fr, im.apinc.org, etc), of the Quadrature du Net, etc. A small list (it’s not up-to-date, don’t hesitate to contribute) is available here in French,, if not you can check on https://xmpp.net/directory.php or http://www.jabberes.org/servers (yes, there are a lot!). But I strongly suggest you to install your own server or to go meet a local organization that you can contact easily: on the one hand if you install it by yourself you’ll have a better control on your own data and on the other hand if you want a specific feature you’ll be able to ask admins for an update.
Knowing all that, let’s try to install an account.
Once the server installed or a public server chosen, you can create an account. You’ll have then an address, a “JID” (Jabber ID, “Jabber” is the former name of the protocol, this name is now owned by a private company and is kept here for historical reasons).
This address is on the form of local_name@domain.tld/resource, or in canonical form (or what is called “bare JID”) local_name@domain.tld. For example, mine is goffi@jabber.fr. Does it sound familiar? Yes, it really looks like email addresses!
So what is this resource? The resource is linked with the client software that you use to connect: XMPP has been designed from the beginning to allow several clients to connect at once (10 years ago, a few messaging protocols were able to do that, and connecting again from a different place was often resulting in the disconnection of the first client) and this resource is used to identify them. In other words, you have a resource only once connected, and it will be a different one for each client that you are using: if I connect with Libervia, Gajim and Movim, I’ll have 3 different resources.
It used to be several strategies to name the resource, sometimes it was used to show the connection location (“home”, “work”), clients often used their names as default value (“gajim”, “psi”). Today, it’s well accepted that it’s better to have a resource that we cannot guess, as somebody can know if you are online or not (even if you don’t want this information to be public) by doing a request to this resource. The best way is to let the server choose the resource for you.
Finally, the resource is linked with a priority: it allows to indicate, if several resources are available, which one will get the message. But we’ll see that later.
In the next episode, I'll talk about extensions an features discovery
That’s it. Let me know if you are interested in this series, if it seems too technical for you, or if you have any comment or fix to suggest. Everything is under the license CC By-SA, so don't hesitate to re-use, share or modify !
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