# x25

## NAME



           x25 - ITU-T X.25 / ISO/IEC 8208 protocol interface
    


## SYNOPSIS



           **#include <sys/socket.h>**
           **#include <linux/x25.h>**
    
           _x25_socket_ **= socket(AF_X25, SOCK_SEQPACKET, 0);**
    


## DESCRIPTION



           X25 sockets provide an interface to the X.25 packet layer
           protocol.  This allows applications to communicate over a public
           X.25 data network as standardized by International
           Telecommunication Union's recommendation X.25 (X.25 DTE-DCE mode).
           X25 sockets can also be used for communication without an
           intermediate X.25 network (X.25 DTE-DTE mode) as described in
           ISO/IEC 8208.
    
           Message boundaries are preserved — a read(2) from a socket will
           retrieve the same chunk of data as output with the corresponding
           write(2) to the peer socket.  When necessary, the kernel takes
           care of segmenting and reassembling long messages by means of the
           X.25 M-bit.  There is no hard-coded upper limit for the message
           size.  However, reassembling of a long message might fail if there
           is a temporary lack of system resources or when other constraints
           (such as socket memory or buffer size limits) become effective.
           If that occurs, the X.25 connection will be reset.
    
       **Socket addresses**
           The **AF_X25** socket address family uses the _struct sockaddr_x25_ for
           representing network addresses as defined in ITU-T recommendation
           X.121.
    
               struct sockaddr_x25 {
                   sa_family_t sx25_family;    /* must be AF_X25 */
                   x25_address sx25_addr;      /* X.121 Address */
               };
    
           _sx25_addr_ contains a char array _x25_addr[]_ to be interpreted as a
           null-terminated string.  _sx25_addr.x25_addr[]_ consists of up to 15
           (not counting the terminating null byte) ASCII characters forming
           the X.121 address.  Only the decimal digit characters from '0' to
           '9' are allowed.
    
       **Socket options**
           The following X.25-specific socket options can be set by using
           setsockopt(2) and read with getsockopt(2) with the _level_ argument
           set to **SOL_X25**.
    
           **X25_QBITINCL**
                  Controls whether the X.25 Q-bit (Qualified Data Bit) is
                  accessible by the user.  It expects an integer argument.
                  If set to 0 (default), the Q-bit is never set for outgoing
                  packets and the Q-bit of incoming packets is ignored.  If
                  set to 1, an additional first byte is prepended to each
                  message read from or written to the socket.  For data read
                  from the socket, a 0 first byte indicates that the Q-bits
                  of the corresponding incoming data packets were not set.  A
                  first byte with value 1 indicates that the Q-bit of the
                  corresponding incoming data packets was set.  If the first
                  byte of the data written to the socket is 1, the Q-bit of
                  the corresponding outgoing data packets will be set.  If
                  the first byte is 0, the Q-bit will not be set.
    


## VERSIONS



           The AF_X25 protocol family is a new feature of Linux 2.2.
    


## BUGS



           Plenty, as the X.25 PLP implementation is **CONFIG_EXPERIMENTAL**.
    
           This man page is incomplete.
    
           There is no dedicated application programmer's header file yet;
           you need to include the kernel header file _< linux/x25.h>_.
           **CONFIG_EXPERIMENTAL** might also imply that future versions of the
           interface are not binary compatible.
    
           X.25 N-Reset events are not propagated to the user process yet.
           Thus, if a reset occurred, data might be lost without notice.
    


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