MAP client: Add support for receiving notifications

[Christian Fetzer <christian.fetzer@oss.bmw-carit.de>]

Hi,

This is a follow-up on the discussion started in my patch set
"MAP client: notification support". The patches were implementing
notification support only for one remote device and needed an additional
API function for choosing that device.

It's better to implement this implicitly and support multiple devices
from the beginning.

In this E-Mail I'd like to suggest how we could implement the
notification connection setup in BlueZ. I'll try to explain the relevant
part of the spec to have a base for discussion.

MAP consists of two different OBEX applications, Message Access Service
(MAS) for browsing messages and Message Notification Service (MNS) for
receiving event reports about changes on the server (like new messages,
moved messages, delivery reports, ...).

The spec describes the single phone use case where the MAP client (MCE)
consists of several MAS client instances for the different accounts and
one common MNS server instance:

  MCE (Car Kit)       MSE (Phone)
+---------------+   +-------------------------+
| MAS client 1  |<->| MAS server 1 (SMS)      |
| MAS client 2  |<->| MAS server 2 (E-Mail A) |
|     ...       |   |     ...                 |
| MNS server    |<->| MNS client              |
+---------------+   +-------------------------+

Every MAS client instance, that wants to use notifications, needs to
use the SetNotificationRegistration function to enable it.
If the MNS connection isn't established already, the server (MSE) will
start the SDP lookup for the MNS instance on the client and connect to
it. The event reports that are sent on the MNS channel contain the MAS
instance ID, that can be used to map the event to the MAS client.

Since an OBEX instance, like the MNS server, cannot be connected to
different remote devices at the same time, it will not work to have one
static MNS server instance. Instead we need one MNS server instance for
every connected remote device (that we want to use with MAP). Since the
MNS connection establishment is based on SDP, we have to make sure that
there's only one MNS server SDP record visible. Otherwise, the MSE
cannot know which one to connect to.

If we want to hide this all from the application level, we could do the
connection establishment for two devices (MSE 1 and MSE 2) as described
here.

  MCE (Car Kit)       MSE 1 (Business phone)
+---------------+   +-------------------------+
| MAS client 1  |<->| MAS server 1 (SMS)      |
| MAS client 2  |<->| MAS server 2 (E-Mail A) |
|     ...       |   |     ...                 |
| MNS server 1  |<->| MNS client              |
|               |   +-------------------------+
|               |
|               |     MSE 2 (private phone)
|               |   +-------------------------+
| MAS client 3  |<->| MAS server 1 (SMS)      |
|     ...       |   |     ...                 |
| MNS server 2  |<->| MNS client              |
+---------------+   +-------------------------+

- MCE connects MAS client 1 to MSE 1
- MCE dynamically instantiates MNS server 1 and registers its SDP record
- MAS client 1 sends SetNotificationRegistration(on)
- MSE does SDP lookup and connects to the found MNS server 1
- MCE removes SDP record for MNS server 1
- MCE connects MAS client 2 to MSE 1
- MAS client 2 sends SetNotificationRegistration(on)
     (MNS channel is already up, MSE just responds OK)
- MCE connects MAS client 3 to MSE 2
- MCE dynamically instantiates MNS server 2 and registers its SDP record
- MAS client 3 sends SetNotificationRegistration(on)
- MSE does SDP lookup and connects to the found MNS server 2
- MCE removes SDP record for MNS server 2
...

Does that sound reasonable for you?

Currently it's not possible to dynamically add OBEX servers, as they are
all instantiated at startup on predefined RFCOMM channels.
Could you comment on how this should be implemented best?

Thanks and Br,
Christian