L2CAP을 이용한 Socket Programming Note

- MTU

아래와 같이 set/getsockeopt() 함수를 이용하면 MTU(Maximum Transmission Unit) 값을 변경할 수 있으며

해당 Local socket에서만 적용이 된다. Server or Client 각각 이를 설정해야 값을 지정할 수 있다.

기본 MTU 값은 672 Bytes이며 최소 48  Bytes에서 최대 65,535 Bytes까지 적용하 수 있다.

struct l2cap_options { uint16_t omtu; uint16_t imtu; uint16_t flush_to; uint8_t mode; };   int set_l2cap_mtu( int sock, uint16_t mtu ) { struct l2cap_options opts; int optlen = sizeof(opts), err; err = getsockopt( s, SOL_L2CAP, L2CAP_OPTIONS, &opts, &optlen ); if( ! err ) { opts.omtu = opts.imtu = mtu; err = setsockopt( s, SOL_L2CAP, L2CAP_OPTIONS, &opts, optlen ); } return err; };

* 용어 정리

PSM : Protocol Service Multiplexer

L2CAP socket에서 사용가능한 Port는 4097~32,767(0x1001-0x7FFF)까지 홀수 번호이다.

ex) L2CAP Server&Client

1) Server

#include <stdio.h> #include <string.h> #include <sys/socket.h> #include <bluetooth/bluetooth.h> #include <bluetooth/l2cap.h> int main(int argc, char **argv) {     struct sockaddr_l2 loc_addr = { 0 }, rem_addr = { 0 }; //  struct l2cap_options opts;     char buf[1024] = { 0 };     int s, client, bytes_read, optlen;     socklen_t opt = sizeof(rem_addr);     // allocate socket     s = socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);     // bind socket to port 0x1001 of the first available     // bluetooth adapter     loc_addr.l2_family = AF_BLUETOOTH;     loc_addr.l2_bdaddr = *BDADDR_ANY;     loc_addr.l2_psm = htobs(0x1001);     bind(s, (struct sockaddr *)&loc_addr, sizeof(loc_addr)); //  optlen = sizeof(opts); //  getsockopt(s, SOL_L2CAP, L2CAP_OPTIONS, &opts, &optlen); //  printf("before omtu = %d, imtu = %d\n", opts.omtu, opts.imtu);     // put socket into listening mode     listen(s, 1);     // accept one connection     client = accept(s, (struct sockaddr *)&rem_addr, &opt); //  optlen = sizeof(opts); //  getsockopt(s, SOL_L2CAP, L2CAP_OPTIONS, &opts, &optlen); //  printf("after omtu = %d, imtu = %d\n", opts.omtu, opts.imtu);     ba2str( &rem_addr.l2_bdaddr, buf );     fprintf(stderr, "accepted connection from %s\n", buf);     memset(buf, 0, sizeof(buf));     // read data from the client     bytes_read = read(client, buf, sizeof(buf));     if( bytes_read > 0 ) {         printf("received [%s]\n", buf);     }     // close connection     close(client);     close(s); }

2) Client

#include <string.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/socket.h> #include <bluetooth/bluetooth.h> #include <bluetooth/l2cap.h> int set_l2cap_mtu( int sock, uint16_t mtu ); int main(int argc, char **argv) {     struct sockaddr_l2 addr = { 0 };     int s, status;     char *message = "hello!";     char dest[18] = "01:23:45:67:89:AB";     struct l2cap_options opts;     int optlen;     if(argc < 2)     {         fprintf(stderr, "usage: %s <bt_addr>\n", argv[0]);         exit(2);     }     strncpy(dest, argv[1], 18);     // allocate a socket     s = socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);     set_l2cap_mtu(s, 65535);     // set the connection parameters (who to connect to)     addr.l2_family = AF_BLUETOOTH;     addr.l2_psm = htobs(0x1001);     str2ba( dest, &addr.l2_bdaddr );     // connect to server     status = connect(s, (struct sockaddr *)&addr, sizeof(addr));     optlen = sizeof(opts);     getsockopt(s, SOL_L2CAP, L2CAP_OPTIONS, &opts, &optlen);     printf("after omtu = %d, imtu = %d\n", opts.omtu, opts.imtu);     // send a message     if( status == 0 ) {         status = write(s, "hello!", 6);     }     if( status < 0 ) perror("uh oh");     close(s); } int set_l2cap_mtu( int sock, uint16_t mtu ) {     struct l2cap_options opts;     int optlen = sizeof(opts), err;     err = getsockopt( sock, SOL_L2CAP, L2CAP_OPTIONS, &opts, &optlen );     if( ! err ) {         printf(" current omtu = %d, imtu = %d\n", opts.omtu, opts.imtu);         opts.omtu = opts.imtu = mtu;         err = setsockopt( sock, SOL_L2CAP, L2CAP_OPTIONS, &opts, optlen );     }     return err; }

춮처 : Bluetooth Essentials