2014-03-23 15:53:55 +00:00
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Observations on implementing XMPP
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After spending the last two and a half month basically writing my own XMPP
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library from scratch I decided to share some of the observations I made in the
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process.. In part this article can be seen as a response to a blog post made by
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Dr. Ing. Georg Lukas. The blog post introduces a couple of XEP (XMPP Extensions)
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which make the life on mobile devices a lot easier but states that they are
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currently very few implementations of those XEPs. So I went ahead and
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implemented all of them in my Android XMPP client.
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2014-03-23 15:57:47 +00:00
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###General observations
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2014-03-23 15:53:55 +00:00
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The first thing I noticed is that XMPP is actually okish designed. If you were
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to design a new chat protocol today you probably wouldn’t choose XML again
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however the protocol basically consists of only three different packages which
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are quickly hidden under some sort of abstraction layer within your library.
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Getting from zero to sending messages to other users actually was very simple
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and straight forward. But then came the XEPs.
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2014-03-23 15:57:47 +00:00
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###Multi-User Chat
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2014-03-23 15:53:55 +00:00
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The first one was XEP-0045 Multi-User Chat. This is the one XEP of the XEPs I’m
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going to mention in my article which is actually wildly adopted. Most clients
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and servers I know of support MUC. However the level of completeness varies.
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MUC actually introduces access and permission roles which are far more complex
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than what some of us are used to from IRC but a lot of clients just don’t
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implement them. I’m not implementing them myself (at least for now) because I
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somewhat doubt that someone would actually use them. (How ever this might be
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some sort of chicken or egg problem.) I did find some strange bugs though which
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might be interesting for other library developers. In theory a MUC server
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implementation can allow a single user (same jid) to join a conference room
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multiple times with the same nick from different clients. This means if someone
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wants to participate in a conference from two different devices (mobile and
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desktop for example) one wouldn’t have to name oneself userDesktop and
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userMobile but just user. Both ejabberd and prosody support this but with
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strange side effects. prosody for example doesn’t allow a user to change its
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name once two clients are “merged” by having the same nick.
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2014-03-23 15:57:47 +00:00
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###Carbons and Stream Management
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2014-03-23 15:53:55 +00:00
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Two of the other XEPs Lukas’ mentions - Carbons (XEP-0280) and Stream Management
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(XEP-0198) - were actually fairly easy to implement. The only challenges were to
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find a server to support them (I ended up running my own prosody server) and a
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desktop client to test them with. For carbons there is a patched mcabber version
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and gajim. After implementing stream management I had very good results on my
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mobile device. I had sessions running for up to 24 hours with a walking outside,
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loosing mobile coverage for a few minutes and so on. The only limitation was
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that I had to keep on developing and reinstalling my app.
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2014-03-23 15:57:47 +00:00
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###Off the record
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2014-03-23 15:53:55 +00:00
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And then came OTR... This is were I spend the most time debugging stuff and
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trying to get things right and compatible with other clients. This is the part
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were I want to help other developers not to make the same mistakes and maybe
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come to some sort of consent among XMPP developers to ultimately increase the
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interoperability. OTR has some down sides which make it difficult or at times
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even dangerous to implement within XMPP. First of all it is a synchronous
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protocol which is tunneled through a different protocol (XMPP). Synchronous
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means - among other things - auto replies. (An OTR session begins with “hi I’m
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speaking otr give me your key” “ok cool here is my key”) And auto replies - we
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know that since the first time an out of office auto responder went postal - are
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dangerous. Things really start to get messy when you use one of the best
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features of XMPP - multiple clients. The way XMPP works is that clients are
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encouraged to send their messages to the raw jid and let the server decide what
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full jid the messages are routed to. If in doubt even all of them. So what
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happens when Alice sends a start-otr-message to Bobs raw jid? Bob receives the
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message on his notebook as well as his cell phone. Both of them answer. Alice
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gets two different replies. Shit explodes. Even if Alice sends the message to
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bob/notebook chances are that Bob has carbon messages enabled and still receives
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the messages on both devices. Now assuming that Bobs client is clever enough not
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to auto reply to carbonated messages Bob/cellphone will still end up with a lot
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of garbage messages. (Essentially the entire conversation between Alice and
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Bob/notebook but unreadable of course) Therefor it should be good practice to
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tag OTR messages as both private and no-copy. (private is part of the carbons
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XEP, no-copy is a general hint. I found that prosody for some reasons doesn’t
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honor the private tag on outgoing messages. While this is easily fixed I presume
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that having both the private and the no-copy tag will make it more compatible
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with servers or clients I don’t know about yet)
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2014-03-23 15:57:47 +00:00
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####Rules to follow when implementing OTR
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2014-03-23 15:53:55 +00:00
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To summarize my observations on implementing OTR in XMPP let me make the
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following three statements.
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1. While it is good practice for unencrypted messages to be send to the raw jid
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and have the receiving server or user decide how they should be routed OTR
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messages must be send to a specific resource. To make this work the user should
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be given the option to select the presence (which can be assisted with some
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educated guessing by the client based on previous messages).
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Furthermore a client should encourage a user to choose meaningful presences
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instead of the clients name or even random ones. Something like /mobile,
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/notebook, /desktop is a greater assist to any one who wants to start an otr
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session then /Gajim, /mcabber or /pidgin
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2. Messages should be tagged private and no-copy to avoid unnecessary traffic or
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otr error loops with faulty clients. This tagging should be done even if your
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own client doesn’t support carbons.
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3. When dealing with “legacy clients” - meaning clients which don’t follow my
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advise a client should be extra careful not to create message loops. This means
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to not respond with otr errors if a client is not 100% sure it is the only
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client which received the message
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