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X25(7)				    Linux Programmer's Manual				   X25(7)

NAME
       x25 - ITU-T X.25 / ISO-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 applica-
       tions 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-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 con-
       straints (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 correspond-
	      ing 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  ver-
       sions 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.

SEE ALSO
       socket(2), socket(7)

       Jonathan Simon Naylor: "The Re-Analysis	and  Re-Implementation	of  X.25."   The  URL  is
       <ftp://ftp.pspt.fi/pub/ham/linux/ax25/x25doc.tgz>.

COLOPHON
       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2012-08-05					   X25(7)
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