AX25(4)             Linux Programmer's Manual             AX25(4)



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. There
       is no SOCK_PACKET support under AX.25.  Instead an AF_INET
       socket of type SOCK_PACKET should 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_sock-
       addr_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 arbitary 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 'infi-
       nite  retry' option supported however. The method of back-
       off for retries is  configurable  via  the  socket  option
       AX25_BACKOFF,  a  value of true indicates the use of expo-
       nential 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),  axparms(5), axassociate(8), axparms(8), kissat-
       tach(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>



Linux                    15 October 1996                  AX25(4)