Bluetooth and Socket Options

Bluetooth for Windows supports the following socket options. Socket options are set and queried using the setsockopt and getsockopt functions, respectively. All of the following options can be used with the setsockopt function, but only the SO_BTH_MTU option is available for use with the getsockopt function.

The following settings are required for working with Bluetooth socket options:

  • The s parameter must be a Bluetooth socket.
  • The level parameter must be SOL_RFCOMM.

SO_BTH_AUTHENTICATE

For disconnected sockets, the SO_BTH_AUTHENTICATE specifies that authentication is required for a connect or accept operation to complete successfully. Setting this socket option actively initiates authentication during connection establishment, if the two Bluetooth devices were not previously authenticated. The user interface for passkey exchange, if necessary, is provided by the operating system outside the application context.

For outgoing connections that require authentication, the connect operation fails with WSAEACCES if authentication is not successful. In response, the application may prompt the user to authenticate the two Bluetooth devices before connection.

For incoming connections, the connection is rejected if authentication cannot be established and returns a WSAEHOSTDOWN error. For more information about authenticating Bluetooth devices, see BluetoothAuthenticateDevice.

For the SO_BTH_AUTHENTICATE socket option, optval is a pointer to ULONG bAuthenticate and must be TRUE; optlen is equivalent to "sizeof(ULONG)".

Windows XP with SP2:  SO_BTH_AUTHENTICATE starts authentication for connected sockets, and forces authentication upon connection for unconnected sockets. For incoming connections, the connection is rejected if authentication cannot be performed.

SO_BTH_ENCRYPT

On unconnected sockets, the SO_BTH_ENCRYPT socket option enforces encryption to establish a connection. Encryption is only available for authenticated connections. For incoming connections, a connection for which encryption cannot be established is automatically rejected and returns WSAEHOSTDOWN as the error. For outgoing connections, the connect function fails with WSAEACCES if encryption cannot be established. In response, the application may prompt the user to authenticate the two Bluetooth devices before connection. For more information about authenticating Bluetooth devices, see BluetoothAuthenticateDevice.

For the SO_BTH_ENCRYPT socket option, optval is a pointer to ULONG bEncrypt and must be TRUE; optlen is equivalent to sizeof(ULONG).

Windows XP with SP2:  For a socket that is connected and authenticated, SO_BTH_ENCRYPT starts encryption.

SO_BTH_MTU

The SO_BTH_MTU socket option is an advanced option used primarily for validation. The SO_BTH_MTU option obtains or sets default RFCOMM MTU (maximum transmission unit) for connection negotiation to a value different than the RFCOMM protocol-default value.

Because RFCOMM MTU is affected by the underlying L2CAP MTU, and protocol and application minimums and maximums, the default value for SO_BTH_MTU is only a starting point for negotiation with the remote peer, and the final negotiated MTU is likely to vary from the default. Setting the SO_BTH_MTU value may negatively affect throughput, and as such, any modification should be performed with knowledge of the underlying Bluetooth protocol.

The SO_BTH_MTU socket option can be performed on connected sockets, but has no effect if the negotiation has already completed. Setting it on the listening (server) socket has no effect.

The amount of data that an application can send or receive in a single socket call is not affected by the MTU; MTU only affects how the underlying Windows Sockets service provider segments packets for transport. Both the proposed MTU and the MTU ultimately negotiated must be between RFCOMM_MIN_MTU and RFCOMM_MAX_MTU, as defined in the Ws2bth.h header file.

For the SO_BTH_MTU socket option, optval is a pointer to ULONG mtu; optlen is equivalent to "sizeof(ULONG)".

SO_BTH_MTU_MAX

The SO_BTH_MTU_MAX socket option is an advanced option used primarily for validation. The SO_BTH_MTU_MAX socket option sets the maximum RFCOMM MTU (maximum transmission unit) for connection negotiation. Connections with an RFCOMM MTU equal to or greater than this value fail during the connect/accept process. While setting this socket option is allowed for a connected socket, it has no effect if the negotiation has completed. Setting this socket option on a listening socket propagates the value for all incoming connections. The MAX MTU value must be between RFCOMM_MIN_MTU and RFCOMM_MAX_MTU, as defined in the Ws2bth.h header file.

For the SO_BTH_MTU_MAX socket option, optval is a pointer to ULONG max_mtu; optlen is equivalent to "sizeof(ULONG)".

SO_BTH_MTU_MIN

The SO_BTH_MTU_MIN socket option is an advanced option used primarily for validation. The SO_BTH_MTU_MIN socket option sets the minimum RFCOMM MTU (maximum transmission unit) for connection negotiation. Connections with an RFCOMM MTU smaller than this value fail during the connect/accept process. While setting this socket option is allowed for a connected socket, it has no effect if the negotiation has completed. Setting this socket option on a listening socket propagates the value for all incoming connections.

Only a listening socket can revise the MTU downward, therefore if the value proposed by the connecting socket is less than the value set for SO_BTH_MTU_MIN on the listening socket, the connection is refused. The minimum MTU must be between RFCOMM_MIN_MTU and RFCOMM_MAX_MTU, as defined in the Ws2bth.h header file.

For the SO_BTH_MTU_MIN socket option, optval is a pointer to ULONG min_mtu; optlen is equivalent to "sizeof(ULONG)".

Related topics

Windows Sockets
getsockopt
setsockopt
BluetoothAuthenticateDevice
connect
accept

 

 

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