+To further reduce the message overhead and allow more fine-grained control over
+information filtering, \term{Temporary Subscription for Presence} is introduced.
+This technique builds on top of presence stanzas as defined in core XMPP, which
+are sent by default without a \code{to} or \code{from} attribute, and therefore
+fit into a single TCP/IP packet over IEEE~802.15.4. However, a drawback of the
+presence mechanism defined by core XMPP is the fact that a client must manually
+subscribe to presence information of another client in order to receive it,
+which requires further communication between the clients. Since the network can
+change rapidly, and clients can frequently join and leave the network,
+subscriptions would often be outdated and must be renewed, leading to overhead
+of subscriptions and unsubscription messages, which would inhibit the flow of
+the actual information.
+
+To solve this problem, a dynamic, topic-based roster is implemented on top of
+Multi-User Chats (XEP-0045). Every topic corresponds with a chat room, and nodes
+join the chat rooms which they are interested in. This allows nodes to inform
+only interested nodes about updates. This has the advantage that existing
+clients supporting Multi-User Chats can be used by a user, but Chatty Things and
+XMPP servers need to be adapted to the new subscription model. Also, this
+mechanism does not work with Serverless Messaging.