Logical Link Control and Adaptation Protocol (L2CAP)

L2CAP layer enables connection-oriented channels which can be enable with the configuration option: CONFIG_BT_L2CAP_DYNAMIC_CHANNEL. This channels support segmentation and reassembly transparently, they also support credit based flow control making it suitable for data streams.

Channels instances are represented by the bt_l2cap_chan struct which contains the callbacks in the bt_l2cap_chan_ops struct to inform when the channel has been connected, disconnected or when the encryption has changed. In addition to that it also contains the recv callback which is called whenever an incoming data has been received. Data received this way can be marked as processed by returning 0 or using bt_l2cap_chan_recv_complete() API if processing is asynchronous.

Note

The recv callback is called directly from RX Thread thus it is not recommended to block for long periods of time.

For sending data the bt_l2cap_chan_send() API can be used noting that it may block if no credits are available, and resuming as soon as more credits are available.

Servers can be registered using bt_l2cap_server_register() API passing the bt_l2cap_server struct which informs what psm it should listen to, the required security level sec_level, and the callback accept which is called to authorize incoming connection requests and allocate channel instances.

Client channels can be initiated with use of bt_l2cap_chan_connect() API and can be disconnected with the bt_l2cap_chan_disconnect() API. Note that the later can also disconnect channel instances created by servers.

API Reference

group bt_l2cap

L2CAP.

Defines

BT_L2CAP_HDR_SIZE

L2CAP PDU header size, used for buffer size calculations

BT_L2CAP_TX_MTU

Maximum Transmission Unit (MTU) for an outgoing L2CAP PDU.

BT_L2CAP_RX_MTU

Maximum Transmission Unit (MTU) for an incoming L2CAP PDU.

BT_L2CAP_BUF_SIZE(mtu)

Helper to calculate needed buffer size for L2CAP PDUs. Useful for creating buffer pools.

Parameters

• mtu – Needed L2CAP PDU MTU.

Returns

Needed buffer size to match the requested L2CAP PDU MTU.

BT_L2CAP_SDU_HDR_SIZE

L2CAP SDU header size, used for buffer size calculations

BT_L2CAP_SDU_TX_MTU

Maximum Transmission Unit for an unsegmented outgoing L2CAP SDU.

The Maximum Transmission Unit for an outgoing L2CAP SDU when sent without segmentation, i.e. a single L2CAP SDU will fit inside a single L2CAP PDU.

The MTU for outgoing L2CAP SDUs with segmentation is defined by the size of the application buffer pool.

BT_L2CAP_SDU_RX_MTU

Maximum Transmission Unit for an unsegmented incoming L2CAP SDU.

The Maximum Transmission Unit for an incoming L2CAP SDU when sent without segmentation, i.e. a single L2CAP SDU will fit inside a single L2CAP PDU.

The MTU for incoming L2CAP SDUs with segmentation is defined by the size of the application buffer pool. The application will have to define an alloc_buf callback for the channel in order to support receiving segmented L2CAP SDUs.

BT_L2CAP_SDU_BUF_SIZE(mtu)

Helper to calculate needed buffer size for L2CAP SDUs. Useful for creating buffer pools.

Parameters

• mtu – Required BT_L2CAP_*_SDU.

Returns

Needed buffer size to match the requested L2CAP SDU MTU.

BT_L2CAP_LE_CHAN(_ch)

Helper macro getting container object of type bt_l2cap_le_chan address having the same container chan member address as object in question.

Parameters

• _ch – Address of object of bt_l2cap_chan type

Returns

Address of in memory bt_l2cap_le_chan object type containing the address of in question object.

BT_L2CAP_CHAN_SEND_RESERVE

Headroom needed for outgoing L2CAP PDUs.

BT_L2CAP_SDU_CHAN_SEND_RESERVE

Headroom needed for outgoing L2CAP SDUs.

Typedefs

typedef void (*bt_l2cap_chan_destroy_t)(struct bt_l2cap_chan *chan)

Channel destroy callback.

Param chan

Channel object.

typedef enum bt_l2cap_chan_state bt_l2cap_chan_state_t

Life-span states of L2CAP CoC channel.

Used only by internal APIs dealing with setting channel to proper state depending on operational context.

typedef enum bt_l2cap_chan_status bt_l2cap_chan_status_t

Status of L2CAP channel.

Enums

enum bt_l2cap_chan_state

Life-span states of L2CAP CoC channel.

Used only by internal APIs dealing with setting channel to proper state depending on operational context.

Values:

enumerator BT_L2CAP_DISCONNECTED

Channel disconnected

enumerator BT_L2CAP_CONNECTING

Channel in connecting state

enumerator BT_L2CAP_CONFIG

Channel in config state, BR/EDR specific

enumerator BT_L2CAP_CONNECTED

Channel ready for upper layer traffic on it

enumerator BT_L2CAP_DISCONNECTING

Channel in disconnecting state

enum bt_l2cap_chan_status

Status of L2CAP channel.

Values:

enumerator BT_L2CAP_STATUS_OUT

Channel output status

enumerator BT_L2CAP_STATUS_SHUTDOWN

Channel shutdown status.

   Once this status is notified it means the channel will no longer be
   able to transmit or receive data.

enumerator BT_L2CAP_STATUS_ENCRYPT_PENDING

Channel encryption pending status.

enumerator BT_L2CAP_NUM_STATUS

Functions

int bt_l2cap_server_register(struct bt_l2cap_server *server)

Register L2CAP server.

Register L2CAP server for a PSM, each new connection is authorized using the accept() callback which in case of success shall allocate the channel structure to be used by the new connection.

For fixed, SIG-assigned PSMs (in the range 0x0001-0x007f) the PSM should be assigned to server->psm before calling this API. For dynamic PSMs (in the range 0x0080-0x00ff) server->psm may be pre-set to a given value (this is however not recommended) or be left as 0, in which case upon return a newly allocated value will have been assigned to it. For dynamically allocated values the expectation is that it’s exposed through a GATT service, and that’s how L2CAP clients discover how to connect to the server.

Parameters

• server – Server structure.

Returns

0 in case of success or negative value in case of error.

int bt_l2cap_br_server_register(struct bt_l2cap_server *server)

Register L2CAP server on BR/EDR oriented connection.

Register L2CAP server for a PSM, each new connection is authorized using the accept() callback which in case of success shall allocate the channel structure to be used by the new connection.

Parameters

• server – Server structure.

Returns

0 in case of success or negative value in case of error.

int bt_l2cap_ecred_chan_connect(struct bt_conn *conn, struct bt_l2cap_chan **chans, uint16_t psm)

Connect Enhanced Credit Based L2CAP channels.

Connect up to 5 L2CAP channels by PSM, once the connection is completed each channel connected() callback will be called. If the connection is rejected disconnected() callback is called instead.

Parameters

• conn – Connection object.

• chans – Array of channel objects.

• psm – Channel PSM to connect to.

Returns

0 in case of success or negative value in case of error.

int bt_l2cap_ecred_chan_reconfigure(struct bt_l2cap_chan **chans, uint16_t mtu)

Reconfigure Enhanced Credit Based L2CAP channels.

Reconfigure up to 5 L2CAP channels. Channels must be from the same bt_conn. Once reconfiguration is completed each channel reconfigured() callback will be called. MTU cannot be decreased on any of provided channels.

Parameters

• chans – Array of channel objects. Null-terminated. Elements after the first 5 are silently ignored.

• mtu – Channel MTU to reconfigure to.

Returns

0 in case of success or negative value in case of error.

int bt_l2cap_chan_connect(struct bt_conn *conn, struct bt_l2cap_chan *chan, uint16_t psm)

Connect L2CAP channel.

Connect L2CAP channel by PSM, once the connection is completed channel connected() callback will be called. If the connection is rejected disconnected() callback is called instead. Channel object passed (over an address of it) as second parameter shouldn’t be instantiated in application as standalone. Instead of, application should create transport dedicated L2CAP objects, i.e. type of bt_l2cap_le_chan for LE and/or type of bt_l2cap_br_chan for BR/EDR. Then pass to this API the location (address) of bt_l2cap_chan type object which is a member of both transport dedicated objects.

Parameters

• conn – Connection object.

• chan – Channel object.

• psm – Channel PSM to connect to.

Returns

0 in case of success or negative value in case of error.

int bt_l2cap_chan_disconnect(struct bt_l2cap_chan *chan)

Disconnect L2CAP channel.

Disconnect L2CAP channel, if the connection is pending it will be canceled and as a result the channel disconnected() callback is called. Regarding to input parameter, to get details see reference description to bt_l2cap_chan_connect() API above.

Parameters

• chan – Channel object.

Returns

0 in case of success or negative value in case of error.

int bt_l2cap_chan_send(struct bt_l2cap_chan *chan, struct net_buf *buf)

Send data to L2CAP channel.

Send data from buffer to the channel. If credits are not available, buf will be queued and sent as and when credits are received from peer. Regarding to first input parameter, to get details see reference description to bt_l2cap_chan_connect() API above.

When sending L2CAP data over an BR/EDR connection the application is sending L2CAP PDUs. The application is required to have reserved BT_L2CAP_CHAN_SEND_RESERVE bytes in the buffer before sending. The application should use the BT_L2CAP_BUF_SIZE() helper to correctly size the buffers for the for the outgoing buffer pool.

When sending L2CAP data over an LE connection the application is sending L2CAP SDUs. The application can optionally reserve BT_L2CAP_SDU_CHAN_SEND_RESERVE bytes in the buffer before sending. By reserving bytes in the buffer the stack can use this buffer as a segment directly, otherwise it will have to allocate a new segment for the first segment. If the application is reserving the bytes it should use the BT_L2CAP_BUF_SIZE() helper to correctly size the buffers for the for the outgoing buffer pool. When segmenting an L2CAP SDU into L2CAP PDUs the stack will first attempt to allocate buffers from the original buffer pool of the L2CAP SDU before using the stacks own buffer pool.

Note

Buffer ownership is transferred to the stack in case of success, in case of an error the caller retains the ownership of the buffer.

Returns

Bytes sent in case of success or negative value in case of error.

int bt_l2cap_chan_recv_complete(struct bt_l2cap_chan *chan, struct net_buf *buf)

Complete receiving L2CAP channel data.

Complete the reception of incoming data. This shall only be called if the channel recv callback has returned -EINPROGRESS to process some incoming data. The buffer shall contain the original user_data as that is used for storing the credits/segments used by the packet.

Parameters

• chan – Channel object.

• buf – Buffer containing the data.

Returns

0 in case of success or negative value in case of error.

struct bt_l2cap_chan

#include <l2cap.h>

L2CAP Channel structure.

Public Members

struct bt_conn *conn

Channel connection reference

const struct bt_l2cap_chan_ops *ops

Channel operations reference

struct bt_l2cap_le_endpoint

#include <l2cap.h>

LE L2CAP Endpoint structure.

Public Members

uint16_t cid

Endpoint Channel Identifier (CID)

uint16_t mtu

Endpoint Maximum Transmission Unit

uint16_t mps

Endpoint Maximum PDU payload Size

uint16_t init_credits

Endpoint initial credits

atomic_t credits

Endpoint credits

struct bt_l2cap_le_chan

#include <l2cap.h>

LE L2CAP Channel structure.

Public Members

struct bt_l2cap_chan chan

Common L2CAP channel reference object

struct bt_l2cap_le_endpoint rx

Channel Receiving Endpoint.

If the application has set an alloc_buf channel callback for the channel to support receiving segmented L2CAP SDUs the application should inititalize the MTU of the Receiving Endpoint. Otherwise the MTU of the receiving endpoint will be initialized to BT_L2CAP_SDU_RX_MTU by the stack.

uint16_t pending_rx_mtu

Pending RX MTU on ECFC reconfigure, used internally by stack

struct bt_l2cap_le_endpoint tx

Channel Transmission Endpoint

struct k_fifo tx_queue

Channel Transmission queue

struct net_buf *tx_buf

Channel Pending Transmission buffer

struct k_work tx_work

Channel Transmission work

struct bt_l2cap_br_endpoint

#include <l2cap.h>

BREDR L2CAP Endpoint structure.

Public Members

uint16_t cid

Endpoint Channel Identifier (CID)

uint16_t mtu

Endpoint Maximum Transmission Unit

struct bt_l2cap_br_chan

#include <l2cap.h>

BREDR L2CAP Channel structure.

Public Members

struct bt_l2cap_chan chan

Common L2CAP channel reference object

struct bt_l2cap_br_endpoint rx

Channel Receiving Endpoint

struct bt_l2cap_br_endpoint tx

Channel Transmission Endpoint

uint16_t psm

Remote PSM to be connected

uint8_t ident

Helps match request context during CoC

struct bt_l2cap_chan_ops

#include <l2cap.h>

L2CAP Channel operations structure.

Public Members

void (*connected)(struct bt_l2cap_chan *chan)

Channel connected callback.

If this callback is provided it will be called whenever the connection completes.

Param chan

The channel that has been connected

void (*disconnected)(struct bt_l2cap_chan *chan)

Channel disconnected callback.

If this callback is provided it will be called whenever the channel is disconnected, including when a connection gets rejected.

Param chan

The channel that has been Disconnected

void (*encrypt_change)(struct bt_l2cap_chan *chan, uint8_t hci_status)

Channel encrypt_change callback.

If this callback is provided it will be called whenever the security level changed (indirectly link encryption done) or authentication procedure fails. In both cases security initiator and responder got the final status (HCI status) passed by related to encryption and authentication events from local host’s controller.

Param chan

The channel which has made encryption status changed.

Param status

HCI status of performed security procedure caused by channel security requirements. The value is populated by HCI layer and set to 0 when success and to non-zero (reference to HCI Error Codes) when security/authentication failed.

struct net_buf *(*alloc_seg)(struct bt_l2cap_chan *chan)

Channel alloc_seg callback.

If this callback is provided the channel will use it to allocate buffers to store segments. This avoids wasting big SDU buffers with potentially much smaller PDUs. If this callback is supplied, it must return a valid buffer.

Param chan

The channel requesting a buffer.

Return

Allocated buffer.

struct net_buf *(*alloc_buf)(struct bt_l2cap_chan *chan)

Channel alloc_buf callback.

If this callback is provided the channel will use it to allocate buffers to store incoming data. Channels that requires segmentation must set this callback. If the application has not set a callback the L2CAP SDU MTU will be truncated to BT_L2CAP_SDU_RX_MTU.

Param chan

The channel requesting a buffer.

Return

Allocated buffer.

int (*recv)(struct bt_l2cap_chan *chan, struct net_buf *buf)

Channel recv callback.

Param chan

The channel receiving data.

Param buf

Buffer containing incoming data.

Return

0 in case of success or negative value in case of error.

Return

-EINPROGRESS in case where user has to confirm once the data has been processed by calling bt_l2cap_chan_recv_complete passing back the buffer received with its original user_data which contains the number of segments/credits used by the packet.

void (*sent)(struct bt_l2cap_chan *chan)

Channel sent callback.

If this callback is provided it will be called whenever a SDU has been completely sent.

Param chan

The channel which has sent data.

void (*status)(struct bt_l2cap_chan *chan, atomic_t *status)

Channel status callback.

If this callback is provided it will be called whenever the channel status changes.

Param chan

The channel which status changed

Param status

The channel status

void (*reconfigured)(struct bt_l2cap_chan *chan)

Channel reconfigured callback.

If this callback is provided it will be called whenever peer or local device requested reconfiguration. Application may check updated MTU and MPS values by inspecting chan->le endpoints.

Param chan

The channel which was reconfigured

struct bt_l2cap_server

#include <l2cap.h>

L2CAP Server structure.

Public Members

uint16_t psm

Server PSM.

Possible values: 0 A dynamic value will be auto-allocated when bt_l2cap_server_register() is called.

0x0001-0x007f Standard, Bluetooth SIG-assigned fixed values.

0x0080-0x00ff Dynamically allocated. May be pre-set by the application before server registration (not recommended however), or auto-allocated by the stack if the app gave 0 as the value.

bt_security_t sec_level

Required minimum security level

int (*accept)(struct bt_conn *conn, struct bt_l2cap_chan **chan)

Server accept callback.

This callback is called whenever a new incoming connection requires authorization.

Param conn

The connection that is requesting authorization

Param chan

Pointer to received the allocated channel

Return

0 in case of success or negative value in case of error.

Return

-ENOMEM if no available space for new channel.

Return

-EACCES if application did not authorize the connection.

Return

-EPERM if encryption key size is too short.