l2cap - L2CAP protocol

NAME SYNOPSIS DESCRIPTION SOCKET ADDRESS SOCKET OPTIONS (SOL_BLUETOOTH) BT_SECURITY (since Linux 2.6.30) BT_DEFER_SETUP (since Linux 2.6.30) BT_FLUSHABLE (since Linux 2.6.39) BT_POWER (since Linux 3.1) BT_CHANNEL_POLICY (since Linux 3.10) BT_PHY (since Linux 5.10) BT_MODE (since Linux 5.10) SOCKET OPTIONS (SOL_SOCKET) SO_TIMESTAMPING, SO_TIMESTAMP, SO_TIMESTAMPNS IOCTLS SIOCETHTOOL (since Linux 6.16−rc1) RESOURCES REPORTING BUGS SEE ALSO COPYRIGHT

SYNOPSIS

#include <sys/socket.h>
#include <bluetooth/bluetooth.h>
#include <bluetooth/l2cap.h>

l2cap_socket = socket(PF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);

L2CAP is a protocol that provides an interface for higher−level protocols to send and receive data over a Bluetooth connection. L2CAP sits on top of the Bluetooth Host Controller Interface (HCI) and provides a set of channels that can be used by higher−level protocols to transmit data.

L2CAP provides a number of services to higher−level protocols, including segmentation and reassembly of large data packets and flow control to prevent overloading of the receiver. L2CAP also supports multiple channels per connection, allowing for concurrent data transmission using different protocols.

SOCKET ADDRESS

struct sockaddr_l2 {
sa_family_t l2_family;
unsigned short l2_psm;
bdaddr_t l2_bdaddr;
unsigned short l2_cid;
uint8_t l2_bdaddr_type;
};

Example:

struct sockaddr_l2 addr;

memset(&addr, 0, sizeof(addr));
addr.l2_family = AF_BLUETOOTH;
bacpy(&addr.l2_bdaddr, bdaddr);

if (cid)
addr.l2_cid = htobs(cid);
else
addr.l2_psm = htobs(psm);

addr.l2_bdaddr_type = bdaddr_type;

SOCKET OPTIONS (SOL_BLUETOOTH)

The socket options listed below can be set by using setsockopt(2) and read with getsockopt(2) with the socket level set to SOL_BLUETOOTH.

BT_SECURITY (since Linux 2.6.30)

Channel security level, possible values:

Example:

int level = BT_SECURITY_HIGH;
int err = setsockopt(l2cap_socket, SOL_BLUETOOTH, BT_SECURITY, &level,
sizeof(level));
if (err == −1) {
perror("setsockopt");
return 1;
}

BT_DEFER_SETUP (since Linux 2.6.30)

Channel defer connection setup, this control if the connection procedure needs to be authorized by userspace before responding which allows authorization at profile level, possible values:

Example:

int defer_setup = 1;
int err = setsockopt(l2cap_socket, SOL_BLUETOOTH, BT_DEFER_SETUP,
&defer_setup, sizeof(defer_setup));
if (err == −1) {
perror("setsockopt");
return err;
}

err = listen(l2cap_socket, 5);
if (err) {
perror("listen");
return err;
}

struct sockaddr_l2 remote_addr = {0};
socklen_t addr_len = sizeof(remote_addr);
int new_socket = accept(l2cap_socket, (struct sockaddr*)&remote_addr,
&addr_len);
if (new_socket < 0) {
perror("accept");
return new_socket;
}

/* To complete the connection setup of new_socket read 1 byte */
char c;
struct pollfd pfd;

memset(&pfd, 0, sizeof(pfd));
pfd.fd = new_socket;
pfd.events = POLLOUT;

err = poll(&pfd, 1, 0);
if (err) {
perror("poll");
return err;
}

if (!(pfd.revents & POLLOUT)) {
err = read(sk, &c, 1);
if (err < 0) {
perror("read");
return err;
}
}

BT_FLUSHABLE (since Linux 2.6.39)

Channel flushable flag, this control if the channel data can be flushed or not, possible values:

BT_POWER (since Linux 3.1)

Channel power policy, this control if the channel shall force exit of sniff mode or not, possible values:

BT_CHANNEL_POLICY (since Linux 3.10)

High−speed (AMP) channel policy, possible values:

BT_PHY (since Linux 5.10)

Channel supported PHY(s), possible values:

BT_MODE (since Linux 5.10)

Channel Mode, possible values:

SOCKET OPTIONS (SOL_SOCKET)

SOL_SOCKET level socket options that modify generic socket features (SO_SNDBUF, SO_RCVBUF, etc.) have their usual meaning, see socket(7).

The SOL_SOCKET level L2CAP socket options that have Bluetooth−specific handling in kernel are listed below.

SO_TIMESTAMPING, SO_TIMESTAMP, SO_TIMESTAMPNS

See <https://docs.kernel.org/networking/timestamping.html>

For L2CAP sockets, software RX timestamps are supported. Software TX timestamps (SOF_TIMESTAMPING_TX_SOFTWARE, SOF_TIMESTAMPING_TX_COMPLETION) are supported since Linux 6.15.

The software RX timestamp is the time when the kernel received the packet from the controller driver.

The SCM_TSTAMP_SND timestamp is emitted when packet is sent to the controller driver. The SCM_TSTAMP_COMPLETION timestamp is emitted when controller reports the packet completed. Other TX timestamp types are not supported.

You can use SIOCETHTOOL to query supported flags.

The timestamps are in CLOCK_REALTIME time.

Example (Enable RX timestamping):

int flags = SOF_TIMESTAMPING_SOFTWARE |
SOF_TIMESTAMPING_RX_SOFTWARE;
setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &flags, sizeof(flags));

Example (Read packet and its RX timestamp):

char data_buf[256];
union {
char buf[CMSG_SPACE(sizeof(struct scm_timestamping))];
struct cmsghdr align;
} control;
struct iovec data = {
.iov_base = data_buf,
.iov_len = sizeof(data_buf),
};
struct msghdr msg = {
.msg_iov = &data,
.msg_iovlen = 1,
.msg_control = control.buf,
.msg_controllen = sizeof(control.buf),
};
struct scm_timestamping tss;

res = recvmsg(fd, &msg, MSG_ERRQUEUE | MSG_DONTWAIT);
if (res < 0)
goto error;

for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
if (cmsg−>cmsg_level == SOL_SOCKET && cmsg−>cmsg_type == SCM_TIMESTAMPING)
memcpy(&tss, CMSG_DATA(cmsg), sizeof(tss));
}

tstamp_clock_realtime = tss.ts[0];

Example (Enable TX timestamping):

int flags = SOF_TIMESTAMPING_SOFTWARE |
SOF_TIMESTAMPING_TX_SOFTWARE |
SOF_TIMESTAMPING_TX_COMPLETION |
SOF_TIMESTAMPING_OPT_ID;
setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPING, &flags, sizeof(flags));

Example (Read TX timestamps):

union {
char buf[2 * CMSG_SPACE(sizeof(struct scm_timestamping))];
struct cmsghdr align;
} control;
struct iovec data = {
.iov_base = NULL,
.iov_len = 0
};
struct msghdr msg = {
.msg_iov = &data,
.msg_iovlen = 1,
.msg_control = control.buf,
.msg_controllen = sizeof(control.buf),
};
struct cmsghdr *cmsg;
struct scm_timestamping tss;
struct sock_extended_err serr;
int res;

res = recvmsg(fd, &msg, MSG_ERRQUEUE | MSG_DONTWAIT);
if (res < 0)
goto error;

for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
if (cmsg−>cmsg_level == SOL_SOCKET && cmsg−>cmsg_type == SCM_TIMESTAMPING)
memcpy(&tss, CMSG_DATA(cmsg), sizeof(tss));
else if (cmsg−>cmsg_level == SOL_BLUETOOTH && cmsg−>cmsg_type == BT_SCM_ERROR)
memcpy(&serr, CMSG_DATA(cmsg), sizeof(serr));
}

tstamp_clock_realtime = tss.ts[0];
tstamp_type = serr−>ee_info; /* SCM_TSTAMP_SND or SCM_TSTAMP_COMPLETION */
tstamp_seqnum = serr−>ee_data;

IOCTLS

The following ioctls with operation specific for L2CAP sockets are available.

SIOCETHTOOL (since Linux 6.16−rc1)

Supports only command ETHTOOL_GET_TS_INFO, which may be used to query supported SOF_TIMESTAMPING_* flags. The SOF_TIMESTAMPING_OPT_* flags are always available as applicable.

Example:

#include <linux/ethtool.h>
#include <linux/sockios.h>
#include <net/if.h>
#include <sys/socket.h>
#include <sys/ioctl.h>

...

struct ifreq ifr = {};
struct ethtool_ts_info cmd = {};
int sk;

snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "hci0");
ifr.ifr_data = (void *)&cmd;
cmd.cmd = ETHTOOL_GET_TS_INFO;

sk = socket(PF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);
if (sk < 0)
goto error;
if (ioctl(sk, SIOCETHTOOL, &ifr))
goto error;

sof_available = cmd.so_timestamping;