Re: MAP client: Add support for receiving notifications

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

Hi Marcel,

On 03/20/2013 06:10 PM, Marcel Holtmann wrote:
> Hi Christian,
> 
>> 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.
> 
> are you sure about this? In theory you could just use another OBEX connection id for the second connection. Has this actually been tested?
> 
>> 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?
> 
> This proposal is inherintly racy. I also do not understand it. When you have a different ACL link, then you can have the same RFCOMM channel established twice. So why are we bothering with this.
> 
> If you are expecting one client connection from each MSE, then there is no need to play any of these tricks. And if a MSE wants to establish more than one client connection, then it is better able to use different connection ids.
> 
> Regards
> 
> Marcel

I agree. As written in response to Luiz's Mail I have tested connecting
two devices to one MNS instance a couple of weeks ago and I couldn't
get it to work. Because of some misleading old mails, saying that it is
possible only with L2CAP, I didn't pursue the simple approach.

However, after your helpful hints, I have retested it and confirm that
dual connections to the same MNS server instance works fine. This makes
both, the proposal and a dedicated API call for setting up the
notification connection obsolete.

I will split up my patch set as requested into smaller parts and send
it as different sets to simplify the review.

Br,
Christian