@Gaduc, where did this terminology come from? The LoRaWAN 1.0 specification pretty much leaves the internet-side of things unspecified, referring to a "network server" only ("application server" is mentioned once in the spec, without really defining it).
In the current prototype TTN design, there aren't really any MAC level commands set, so you can just keep the session key to yourself and do the decryption yourself too (but if you use the Semtech default key, I think some decryption happens automatically).
In the proposed architecture, this separation is made more clearly, though the terminology is different (the architecture is probably also different from the one you describe, which I think fits a typical centralized telco usecase). See this topic for more info on the proposed architecture, you should probably direct your questions (if any are left after reading) there: http://forum.thethingsnetwork.org/t/request-for-comments-network-architecture/287/16