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diff --git a/docs/obeservations.md b/docs/obeservations.md new file mode 100644 index 00000000..f4e4bf17 --- /dev/null +++ b/docs/obeservations.md @@ -0,0 +1,97 @@ +Observations on implementing XMPP +================================= +After spending the last two and a half month basically writing my own XMPP +library from scratch I decided to share some of the observations I made in the +process.. In part this article can be seen as a response to a blog post made by +Dr. Ing. Georg Lukas. The blog post introduces a couple of XEP (XMPP Extensions) +which make the life on mobile devices a lot easier but states that they are +currently very few implementations of those XEPs. So I went ahead and +implemented all of them in my Android XMPP client. + +###General observations +The first thing I noticed is that XMPP is actually okish designed. If you were +to design a new chat protocol today you probably wouldn’t choose XML again +however the protocol basically consists of only three different packages which +are quickly hidden under some sort of abstraction layer within your library. +Getting from zero to sending messages to other users actually was very simple +and straight forward. But then came the XEPs. + +###Multi-User Chat +The first one was XEP-0045 Multi-User Chat. This is the one XEP of the XEPs I’m +going to mention in my article which is actually wildly adopted. Most clients +and servers I know of support MUC. However the level of completeness varies. +MUC actually introduces access and permission roles which are far more complex +than what some of us are used to from IRC but a lot of clients just don’t +implement them. I’m not implementing them myself (at least for now) because I +somewhat doubt that someone would actually use them. (How ever this might be +some sort of chicken or egg problem.) I did find some strange bugs though which +might be interesting for other library developers. In theory a MUC server +implementation can allow a single user (same jid) to join a conference room +multiple times with the same nick from different clients. This means if someone +wants to participate in a conference from two different devices (mobile and +desktop for example) one wouldn’t have to name oneself userDesktop and +userMobile but just user. Both ejabberd and prosody support this but with +strange side effects. prosody for example doesn’t allow a user to change its +name once two clients are “merged” by having the same nick. + +###Carbons and Stream Management +Two of the other XEPs Lukas’ mentions - Carbons (XEP-0280) and Stream Management +(XEP-0198) - were actually fairly easy to implement. The only challenges were to +find a server to support them (I ended up running my own prosody server) and a +desktop client to test them with. For carbons there is a patched mcabber version +and gajim. After implementing stream management I had very good results on my +mobile device. I had sessions running for up to 24 hours with a walking outside, +loosing mobile coverage for a few minutes and so on. The only limitation was +that I had to keep on developing and reinstalling my app. + +###Off the record +And then came OTR... This is were I spend the most time debugging stuff and +trying to get things right and compatible with other clients. This is the part +were I want to help other developers not to make the same mistakes and maybe +come to some sort of consent among XMPP developers to ultimately increase the +interoperability. OTR has some down sides which make it difficult or at times +even dangerous to implement within XMPP. First of all it is a synchronous +protocol which is tunneled through a different protocol (XMPP). Synchronous +means - among other things - auto replies. (An OTR session begins with “hi I’m +speaking otr give me your key” “ok cool here is my key”) And auto replies - we +know that since the first time an out of office auto responder went postal - are +dangerous. Things really start to get messy when you use one of the best +features of XMPP - multiple clients. The way XMPP works is that clients are +encouraged to send their messages to the raw jid and let the server decide what +full jid the messages are routed to. If in doubt even all of them. So what +happens when Alice sends a start-otr-message to Bobs raw jid? Bob receives the +message on his notebook as well as his cell phone. Both of them answer. Alice +gets two different replies. Shit explodes. Even if Alice sends the message to +bob/notebook chances are that Bob has carbon messages enabled and still receives +the messages on both devices. Now assuming that Bobs client is clever enough not +to auto reply to carbonated messages Bob/cellphone will still end up with a lot +of garbage messages. (Essentially the entire conversation between Alice and +Bob/notebook but unreadable of course) Therefor it should be good practice to +tag OTR messages as both private and no-copy. (private is part of the carbons +XEP, no-copy is a general hint. I found that prosody for some reasons doesn’t +honor the private tag on outgoing messages. While this is easily fixed I presume +that having both the private and the no-copy tag will make it more compatible +with servers or clients I don’t know about yet) + +####Rules to follow when implementing OTR +To summarize my observations on implementing OTR in XMPP let me make the +following three statements. + +1. While it is good practice for unencrypted messages to be send to the raw jid +and have the receiving server or user decide how they should be routed OTR +messages must be send to a specific resource. To make this work the user should +be given the option to select the presence (which can be assisted with some +educated guessing by the client based on previous messages). +Furthermore a client should encourage a user to choose meaningful presences +instead of the clients name or even random ones. Something like /mobile, +/notebook, /desktop is a greater assist to any one who wants to start an otr +session then /Gajim, /mcabber or /pidgin + +2. Messages should be tagged private and no-copy to avoid unnecessary traffic or +otr error loops with faulty clients. This tagging should be done even if your +own client doesn’t support carbons. + +3. When dealing with “legacy clients” - meaning clients which don’t follow my +advise a client should be extra careful not to create message loops. This means +to not respond with otr errors if a client is not 100% sure it is the only +client which received the message |