NAME

       AF_AX25 - AX.25 amateur packet radio protocol family

DESCRIPTION

       AX.25 is a protocol used extensively by radio amateurs. The Linux AX.25 protocol family permits access to
       these protocols via the standard networking socket metaphor.

       The AX.25 protocol layer supports both connected mode and datagram (UI) frame modes. IP  traffic  may  be
       stacked on top of AX.25 frames for IP transmission over the AX.25 medium.

       The  primary  mode  of  operation  is  connected  mode  which  is  the  mode  used  for  a socket of type
       SOCK_SEQPACKET (stream sockets are not available in AX.25).  This requires that the user  ensures  output
       data  is  suitably  packetised,  and that input data is read a packet at a time into a buffer of suitable
       size.  The Linux AX.25 protocol layer can operate in standard AX.25 mode with three bit sequence  numbers
       or in PE1CHL extended AX.25 mode which uses seven bit sequence numbers. The protocol passed to the socket
       is used for all outgoing frames. Passing 0 causes the normal AX.25 Text PID to be used.

       SOCK_DGRAM gives access to AX.25 UI frames. For access to special frames (of any form)  SOCK_RAW  can  be
       used.

       AX.25  addresses  consist  of  6 ASCII characters and a number called the SSID.  These are encoded into a
       sockaddr_ax25 structure which is provided to the relevant system calls. When digipeaters are  included  a
       callsign  path  can  be  much  more  complex. When this is the case a struct full_sockaddr_ax25 should be
       passed to the system calls.

       AX.25 has some unusual properties. Notably in a multi-user system an AX.25 address  is  often  associated
       with a user, and some users may not have such an association. a set of ioctl calls are provided to manage
       an association table, and in addition the superuser may use an  arbitrary  callsign  by  binding  to  the
       callsign desired and specifying the port to use as a first digipeated hop.

       AX.25  supports  the  following  socket  options  for  SOL_AX25.  AX25_T1 is the T1 timer in 1/10ths of a
       second, AX25_T2 is the T2 timer in 1/10ths of a second, AX25_T3 is the T3 timer. The window  is  settable
       with  AX25_WINDOW.   AX25_N2, the retry counter is also configurable. There is no 'infinite retry' option
       supported however. The method of backoff for retries is configurable via the socket option  AX25_BACKOFF,
       a  value  of true indicates the use of exponential backoff and false simple linear backoff. The mode of a
       connection made be altered to be either standard AX.25 or extended AX.25 via AX25_EXTSEQ.  It is possible
       to  have  the complete AX.25 header returned to the application by setting AX25_HDRINCL to true, programs
       must be aware of the internal structure  of  AX.25  frames  to  use  this  option.  Note  that  if  AX.25
       fragmentation  is encountered, only the control information of the first frame is returned along with the
       defragmented data.

SEE ALSO

       call(1),  socket(2),  setsockopt(2),  getsockopt(2),   axctl(8),   axports(5),   axparms(8),   packet(7),
       kissattach(8).

BUGS

       Too numerous to list in full currently.

       •  Minor protocol violations exist.

AUTHOR

       Alan Cox GW4PTS <alan@cymru.net>
       Jonathan Naylor G4KLX <g4klx@g4klx.demon.co.uk>