Background Bluetooth Services on Windows Phone 8.1

Added in Windows Phone 8.1 was a new RfcommConnectionTrigger which allows you to host a Bluetooth service on the device and have your background task triggered when an incoming connection is established. This makes a lot of sense as having to have your app in the foreground to receive connections limits the usage somewhat.

In its simplest form you just need to specify an RfcommServiceId for your service, register your background task and the system publishes the required SDP record which allows other devices to discover your service. Your background task is executed when an incoming connection is received and you can cast the TriggerDetails of the received IBackgroundTaskInstance to an RfcommConnectionTriggerDetails. This gives you two useful things – RemoteDevice – a BluetoothDevice which describes the device which initiated the connection and Socket a StreamSocket which allows you to talk to the remote device.

This works well for most devices but what if you have more specific requirements for the published SDP record. In a foreground app you’d use RfcommServiceProvider and this allows you to set individual SDP attributes which are appended to the default record. The RfcommConnectionTrigger mechanism provides something similar but there is literally no documentation on how it is expected to work. The difference from the foreground approach is that rather than setting attributes with id and raw body it has a property called SdpRecord which accepts an iBuffer. It’s very easy to get from a byte array to an iBuffer but we still have to create the SDP record…

At first I thought the best approach was to look at what the default record contained – but unless you assign a record the property returns null (even once the background task is registered and the SDP record is published). The only way to see the exposed record is to read it remotely from another Bluetooth device (one which is a desktop app since device and service discovery is not supported from WinRT). I used Alan’s SdpBrowserDesktop app from 32feet.This showed a fairly standard set of attributes:-

• Record:
AttrId: 0x0000 -- ServiceRecordHandle
UInt32: 0x10009
AttrId: 0x0001 -- ServiceClassIdList
ElementSequence
    Uuid16: 0x1101 -- SerialPort
AttrId: 0x0004 -- ProtocolDescriptorList
ElementSequence
    ElementSequence
        Uuid16: 0x100 -- L2CapProtocol
        UInt16: 0x3
    ElementSequence
        Uuid16: 0x3 -- RFCommProtocol
        UInt8: 0x6
( ( L2Cap, PSM=Rfcomm ), ( Rfcomm, ChannelNumber=6 ) )
AttrId: 0x0005 -- BrowseGroupList
ElementSequence
    Uuid16: 0x1002 -- PublicBrowseGroup

The ServiceRecordHandle is allocated by the host device as is the Rfcomm channel (here 6). If I want to add a service name to this I realised I would have a problem – how can I build a record when I don’t yet know the port that will be assigned (and there is never a way to find this out from WinRT). I built the record anyway hoping that the system would simply read it and replace the value at registration. You get an ArgumentException at the point you call BackgroundTaskBuilder.Register() and it doesn’t contain any useful information. I tried a few variations of this approach and then happened upon another approach – what if I build a valid record but just for the attributes I want to add. Luckily this approach works – your custom attributes are appended to the end of the record and registration succeeds.

I’m documenting this here because there isn’t a way to add to the MSDN documentation and hopefully someone searching will find the answer. I’m working on porting code across so that there is a friendly API available for all current Windows flavours which helps in reading and writing SDP records and attribute values. I can only assume the approaches used by the foreground and background APIs differ because they were written at different times by different teams.

My custom record consists of an ElementSequence, with a 16bit UUID for the attribute (0x0100 – ServiceName) followed by a UTF-8 encoded string e.g.

Element Sequence
   Uuid16: 0x0100
   String (len 11): "Hello World"

Windows 10 supports this same approach and it is no longer for Phones only – it should be universal across phones, desktop and IOT so I anticipate people doing more advanced things with Bluetooth…

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