Documentation/Maemo 5 Developer Guide/Architecture/RTCOM

=Real Time Communications domain= The RTCom domain provides both services and applications for real time Internet communications, such as Voice Over IP / Video calls and Instant Messaging, and also cellular/CS communications on Maemo platform. It also provides and distributes presence information within the system. The architecture consists of a few major components; connection managers which abstract implementation details of different protocols to a common API, mission control to handle transient events and enforce system policies, and user interfaces to present information to the user. There is also a streaming server to handle actual media streaming communications. It is not conceptually a separate architectural element of the framework, but more like one UI implementation. Farsight interface is used to manage media pipelines.

System decomposition view
The diagram below shows the entities of the RTCOM subsystem, dependencies between them, provided interfaces and required external interfaces. The client applications are shown as packages without the fill-color. Different UI components (Messaging, Call, Log Viewer, Desktop Elements, Control Panel Applets) provide user interface layer to the system.

On intermediate layer, there’s a Mission Control package, which handle transient events and enforce system policies, providing application logic services, and libtelepathy-glib component providing GObject-style access to the Telepathy DBUS interfaces.

On lowest level, there are Streaming Engine to handle actual media streaming communications, and Connection managers that abstract implementation details of different protocols to a common API. The interface is modular and platform includes Jabber (instant messaging) and SIP (voice and video calls).

The Telepathy framework is used to handle real time communication related signaling and abstracting all protocol dependencies to a single D-BUS interface.

Centralized addressbook is provided by the Contacts subsystem and used by the RTcom subsystem for all contact information storage and lookup purposes and it also stores transient contacts/states.

RTCOM framework
In Fremantle, RTCom framework provides the following functionality:
 * Support for comprehensive logging
 * SSO related components
 * Suport for heartbeat
 * Device presence profiles
 * Support for BT HFP
 * Location support for presence

Connection Managers
Connection managers can be thought of as a kind of protocol plugins of the system. They conform to Telepathy DBUS interface specification and provide DBUS interface abstraction of some protocol. Each connection manager usually provides support for one protocol. The provided interfaces depend on the capabilities of the connection manager and protocol.

Connection managers only signaling protocols. In case of a media streaming session, SDP or similar negotiation signaling is done in Connection Manager using the signaling protocol, but actual payload streaming is handled by Streaming Engine (Eg: telepathy-stream-engine).

Functionalities provided

 * Support for Skype integrated into Telepathy framework

Streaming Engines
Media engines handle media streaming needs for multimedia communication methods like voice and video conversation. Generic telepathy-stream-engine provides voice and video streaming capabilities over RTP protocol. If there is a need for other kind of media streaming for some connection manager, it might be reasonable to provide separate media engine for that purpose. Independent of possible other uses, libjingle is used to provide NAT traversal using STUN, GTalk relay and ICE technologies for the GTalk service. libnice provides fully standards compliant ICE implementation.

Functionalities provided

 * Support for QoS analysis and actions, partially based on the (refreshed) implementation of RTCP support.
 * Support for fully standards compliant ICE through libnice.
 * Support for H.264
 * Support for BT HFP.
 * Support for Skype in Telepathy framework.

telepathy-glib
Telepathy Library’s role is to provide common GObject-based C-language API for applications. It wraps Telepathy DBUS interfaces to GObjects and also provides some utility functions for accessing and enumerating DBUS interfaces. It’s code is mostly autogenerated from DBUS interface spec.

Interfaces used by libtelepathy-glib

Mission Control
Communication system policies are usually dependent on application and environment, thus some method external to framework has to exist for handling those. Mission Control thus enforces communication system policies. It also handles configuration information storage, network connectivity from policy point of view and is thus responsible for starting and stopping connection managers and engines. While starting up connection managers and engines, it provides the configuration information. In case of network side event like presence subscription or start of communication session, Mission Control handles these transient events based on system policy and possibly starts associated UI components. Misson Control employs plugin methods to provide possibility to add special event handlers called “mission control filters”. These plugins can provide special device or system dependent function handling.

Messaging UI
Messaging UI is a user interface component for instant messaging sessions. It handles all kinds of non-media streaming communications like text chat.

Event logger
Logger is a database engine built into a library for logging all kinds of events. It’s particularly powerful for logging messages (optionally with attachments), calls and network events.

Notification UI
This is a Hildon Status menu plugin for RTCom related notifications.

Presence UI
This is the presence user interface.

Single-Sign-On (SSO)
SSO subsystem is a separate subsystem providing credential storage and a client library for applications. This way, user is not requested to login or enter credentials as he’s navigating between different service types within single service provider. In case of wide ticket-based system like OpenID (reference), user can even use the same credential between different service providers as long as service provider accepts tickets from external providers.

SSO-subsystem consists of server, here called “signond” and application libraries libsignon-glib and libsignonui. Server stores credentials in optionally encrypted database. It is running under different user id than applications. Application library accesses the storage over IPC mechanism.

Protocol Stack
Realtime communication framework connects to services using protocol plugins. Currently, XMPP/Jabber and SIP are the officially supported protocols. There’s also support for media extension of Jabber called Jingle. As described earlier, each of the protocol plugins create abstraction of some protocol to a common DBUS protocol.

GoogleTalk uses Jingle protocol to handle voice streaming.

Standard Jabber communication always goes through persistent TCP connection to the server. SIP protocol uses either UDP or TCP protocol for signaling. UDP is older and established method, while TCP is more recommended nowadays, mainly due to some particular message size and NAT traversal problems associated with UDP use. Together with SIP, standard RTP is always used for the media.