Creating a L2CAP Client Connection to a Remote Device
An L2CAP client profile driver is a profile driver that requests an Asynchronous Connectionless Link (ACL) connection to a remote device. If the device accepts the connection, the L2CAP client profile driver is notified of any changes to the connection. For example, a L2CAP client profile driver can request a connection to a remote printer, and after the printer accepts the request, the Bluetooth driver stack can notify the profile driver when the printer is turned off or removed.
The L2CAP client profile driver must have information about the remote device, such as the protocol/service multiplexer (PSM) that the device uses, in order to request a connection to the device. The client profile driver can obtain this information through the Service Discovery Protocol (SDP) DDIs, or through a service's fixed PSM. For more information about how to obtain this information, see Accessing SDP Service Information.
To initiate a L2CAP connection to a remote device, after the client profile driver has the required information about the device, it should build and send a BRB_L2CA_OPEN_CHANNEL request.
When the client profile driver builds the request, it supplies a pointer to a _BRB_L2CA_OPEN_CHANNEL structure in the Parameters.Others.Argument1 member of the IRP associated with the request. This structure contains the Bluetooth address for the remote device, the PSM registered for the device, and additional configuration parameters.
If the remote device accepts the open channel request, the OutResults and InResults members of the _BRB_L2CA_OPEN_CHANNEL structure contain information about the newly created connection. The OutResults member specifies the parameters for the outbound half of the channel and the InResults member specifies the parameters for the inbound half of the channel.
Several of the configuration values passed in the _BRB_L2CA_OPEN_CHANNEL structure, such as the Mtu member, are used to negotiate the connection with the remote device. Client profile drivers should provide as wide a range as possible to increase the chances of successful channel negotiation. Specifying a minimum message transfer units (MTU) size greater than the basic Bluetooth minimum MTU size should only be done when absolutely necessary. If negotiation fails, the connection will fail.
The IncomingQueueDepth member of the _BRB_L2CA_OPEN_CHANNEL structure specifies the maximum number of MTUs that the Bluetooth driver stack will receive and queue on the connection before the Bluetooth driver stack begins to discard them. Setting this value to a very small number increases the chances of data loss, while setting it to a very large number increases memory requirements. Microsoft recommends setting this member to 10.
When the profile driver no longer requires the L2CAP connection to the remote device, it should build and send a BRB_L2CA_CLOSE_CHANNEL request.