L2CAP(7) Linux System Administration L2CAP(7)

l2cap - L2CAP protocol

#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.

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;

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.

Channel security level, possible values:

Value Security Level Link Key Type Encryption
BT_SECURITY_SDP 0 (SDP Only) None Not required
BT_SECURITY_LOW 1 (Low) Unauthenticated Not required
BT_SECURITY_MEDIUM 2 (Medium - default) Unauthenticated Desired
BT_SECURITY_HIGH 3 (High) Authenticated Required
BT_SECURITY_FIPS (since Linux 3.15) 4 (Secure Only) Authenticated (P-256 based Secure Simple Pairing and Secure Authentication) Required

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;
}

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:

Value Description Authorization
0 Disable (default) Not required
1 Enable Required

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;
    }
}

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

Define Value Description
BT_FLUSHABLE_OFF 0x00 (default) Do not flush data
BT_FLUSHABLE_ON 0x01 Flush data

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

Define Value Description
BT_POWER_FORCE_ACTIVE_OFF 0x00 (default) Don't force exit of sniff mode
BT_POWER_FORCE_ACTIVE_ON 0x01 Force exit of sniff mode

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

Define Value Description
BT_CHANNEL_POLICY_BREDR_ONLY 0 (default) BR/EDR only
BT_CHANNEL_POLICY_BREDR_PREFERRED 1 BR/EDR Preferred
BT_CHANNEL_POLICY_BREDR_PREFERRED 2 AMP Preferred

Channel supported PHY(s), possible values:

Define Value Description
BT_PHY_BR_1M_1SLOT BIT 0 BR 1Mbps 1SLOT
BT_PHY_BR_1M_3SLOT BIT 1 BR 1Mbps 3SLOT
BT_PHY_BR_1M_5SLOT BIT 2 BR 1Mbps 5SLOT
BT_PHY_BR_2M_1SLOT BIT 3 EDR 2Mbps 1SLOT
BT_PHY_BR_2M_3SLOT BIT 4 EDR 2Mbps 3SLOT
BT_PHY_BR_2M_5SLOT BIT 5 EDR 2Mbps 5SLOT
BT_PHY_BR_3M_1SLOT BIT 6 EDR 3Mbps 1SLOT
BT_PHY_BR_3M_3SLOT BIT 7 EDR 3Mbps 3SLOT
BT_PHY_BR_3M_5SLOT BIT 8 EDR 3Mbps 5SLOT
BT_PHY_LE_1M_TX BIT 9 LE 1Mbps TX
BT_PHY_LE_1M_RX BIT 10 LE 1Mbps RX
BT_PHY_LE_2M_TX BIT 11 LE 2Mbps TX
BT_PHY_LE_2M_RX BIT 12 LE 2Mbps RX
BT_PHY_LE_CODED_TX BIT 13 LE Coded TX
BT_PHY_LE_CODED_RX BIT 14 LE Coded RX

Channel Mode, possible values:

Define Value Description Link
BT_MODE_BASIC 0x00 (default) Basic mode Any
BT_MODE_ERTM 0x01 Enhanced Retransmission mode BR/EDR
BT_MODE_STREAM 0x02 Stream mode BR/EDR
BT_MODE_LE_FLOWCTL 0x03 Credit based flow control mode LE
BT_MODE_EXT_FLOWCTL 0x04 Extended Credit based flow control mode Any


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<linux-bluetooth@vger.kernel.org>

socket(7), l2test(1)

Free use of this software is granted under ther terms of the GNU Lesser General Public Licenses (LGPL).

May 2024 BlueZ