AXUDP(9) XR32 REFERENCE MANUAL 14/2/2013
NAME
AXUDP -- AX25-over-UDP Tunnelling.
DESCRIPTION
AXUDP is AX25 "encapsulated" within UDP datagrams. This
enables AX25 systems to communicate with each other via TCP/IP
networks (e.g. the Internet). The frame structure is as
follows:
.-----------.------------.------------------------.
| IP header | UDP header | AX25 frame |
'-----------'------------'------------------------'
(20 bytes) (8 bytes) (Typically 15-340 bytes)
It is slightly less efficient than AXIP, because there is an
extra 8 byte UDP header between the IP header and AX25 portion
of the frame, but the difference is not significant.
One of the problems with AXIP is that many domestic routers
cannot be configured to route AXIP to a specific PC. And even
if they can, it can only be routed to one PC. This means you
can only have one AXIP node per public IP address.
The major advantage of AXUDP over AXIP is that it can usually
be handled by domestic Internet routers without problem. And
since UDP is a "ported" protocol (AXIP is "portless") you may
have more than one AXUDP node on the same public IP address,
by assigning them different UDP ports. The domestic router is
then able to route incoming packets according to UDP port.
The AX25 links created using AXUDP can in turn support NetRom
and amateur TCP/IP, just like real radio links.
Setting Up AXUDP Links
~~~~~~~~~~~~~~~~~~~~~~
Assuming you have a prospective AXUDP partner, you would set
up an AXUDP link as follows:
1) Configure and test IP routing between you and your
partner. If you don't have reliable IP routing there's no
point in proceeding!
If you are linking via the Internet, it makes sense to use
the Internet IP addresses for this purpose, rather than
the amateur (44.x.x.x) ones, because the routing is more
reliable and the throughput is faster.
2) If your partner has a dynamic IP address, they must have
an account with a "dynamic DNS" provider, and you must use
the hostname thus provided for all operations. If you use
the IP address instead, the link will stop working when
the address changes.
3) If you wish to use your prospective partner's hostname
(e.g. "g8pzt.ath.cx") instead of their IP address, your
system needs access to a Domain Name Server (DNS). This
would usually be provided by Windows nowadays, so you may
remove all "DNS=" lines from XROUTER.CFG.
4) If using the partner's hostname, verify that
"PING <hostname>" resolves the address correctly.
5) Add an AXUDP interface to XROUTER.CFG as follows:
INTERFACE=9
TYPE=AXUDP
MTU=256
ENDINTERFACE
(Choose the interface number to suit yourself).
This interface can support an unlimited number of AXUDP
PORTs. You may define multiple interfaces if your ports
need different MTU's.
Only TYPE=AXUDP and MTU= are required, all other
parameters are ignored (at present).
6) For each AXUDP partner, add an AXUDP port similar to this:
PORT=8
ID=AXUDP link with VK1UDP
INTERFACENUM=9
IPLINK=27.69.88.73
UDPLOCAL=93
UDPREMOTE=93
FRACK=2000
RESPTIME=200
ENDPORT
You must specify at least ID, INTERFACENUM, and IPLINK.
IPLINK is the remote host's IP address or hostname. It is
more efficient to use IP addresses, if you are able to do
so, because it removes the need to resolve the hostnames,
but see (2) above.
UDPLOCAL and UDPREMOTE are the UDP "service port" numbers
for each end of the link, and if omitted they default to
93 (don't confuse these with *protocol number* 93, which
is AXIP). The numbers are independent, e.g. you may use
93 for UDPLOCAL and 10093 for UDPREMOTE).
MAC parameters such as TXDELAY, TXTAIL, SLOTTIME, PERSIST,
FULLDUP, SOFTDCD etc. are meaningless for AXUDP, but
FRACK, RESPTIME, PACLEN, MAXFRAME, QUALITY etc. operate
as normal.
On fast internet links you may wish to use a much lower
FRACK, say 2000ms, than on radio. It is not recommended
that you reduce it much below 1000ms, as it needs to be
*at least* twice the worst round-trip time plus the other
end's RESPTIME.
RESPTIME is probably the one which will have most effect
on the responsiveness of the AX25 link, because it
controls the time delay between receiving a packet and
sending an ACK. It should be just a little more than the
time it takes to receive a maximum length packet. For
example, at a data rate of 56Kbits/sec, a 256 byte packet
lasts less than 50msec, so RESPTIME=50 would be adequate.
However the timing jitter due to operating under Windows
means that RESPTIME should be more like 200ms.
7) If XR32 is indirectly connected to the Internet via an
intermediate router, that router will probably be using
some form of NAT (Network Address Translation) to share
one "public" IP address between several systems on your
LAN. The "front end" router will probably route outgoing
AXUDP without problem, but it will not know where to send
incoming AXUDP unless explicitly configured.
Configuring such a router for AXUDP usually involves
specifying a UDP port number (your UDPLOCAL as specified
above), and the LAN IP address of a machine to which it
should be routed, i.e. Xrouter's LAN IP address. This is
sometimes called "port forwarding", and there are websites
(e.g. portforward.com) dedicated to showing you how to do
this for most makes of router.
Some routers don't have the facility to open specific UDP
ports, but at the very least should allow you to direct
all UDP traffic to a specified IP address.
8) Your link partner must set up a reciprocal arrangement,
i.e. their UDPREMOTE must match your UDPLOCAL and vice
versa.
If everything has been set up correctly, you should be able
to connect with your new neighbour node immediately, at least
at AX25 layer 2. You can test this by entering the command
"C n ALIAS-1", where n is the PORT number of your link, and
ALIAS is the node alias of your link partner. If this doesn't
work, you or your partner have made a mistake somewhere in the
configuration.
Even if everything is configured correctly, it may take a while
for NetRom to configure itself for the new link, as the nodes
need to exchange NODES brodcasts first. Once they have done
so, there should be no delays in future.
Notes
~~~~~
You may of course use AXUDP to communicate between nodes on
the LAN, or even on the same machine.
If you have more than one node on your LAN using AXUDP, your
UDPLOCAL must not be the same as the UDPLOCAL (or equivalent
thereof) of any other node on your LAN. There are two
reasons for this; Firstly, for a given UDP port, NAT routers
cannot direct direct incoming traffic to more than one LAN IP
address at a time. Secondly, only one application on a PC
may "own" a given UDP port number.
Unlike other software, you *DO NOT* need a different UDPLOCAL
for each AXUDP port. It is quite common for link partners to
specify that they will transmit AXUDP to you on a UDP port
that is different to your other UDPLOCAL settings. There is
absolutely *NO* valid reason for this! It makes life more
complicated for you, and you have to set up extra "port
forwarding" entries in your NAT router. For these reasons
you are strongly advised to use the same UDPLOCAL for all
AXUDP partners on a given node. As a rule, you may NOT tell
a link partner what his UDPLOCAL should be, no more than he
should dictate what your UDPLOCAL should be. Instead, he
should specify which UDP port he is listening on (which
becomes your UDPREMOTE), and in return you tell him which UDP
port you are listening on (which becomes his UDPREMOTE).
DO NOT set up an AXUDP link to a link partner with whom you
already have an AXIP link. This is a common mistake, and is
likely to cause problems!
IP Routing
~~~~~~~~~~
As mentioned earlier, you may route amateur IP (44.x.x.x) over
your new AXUDP link, and are encouraged to do so. Whilst the
amprnet purists will argue that this is not as efficient as
IP-over-IP (since it uses a few more bytes), it is a LOT
easier to set up, and doesn't require that your domestic
router and operating system can route IP-over-IP (many
routers are not able to route incoming IP-over-IP to a
specific PC, and Windows' IP stack totally blocks IP-over-IP).
To route amateur IP over an AXUDP link, simply add an IP route
entry directing the required subnet to your neighbour's IP
address on the AXUDP port using datagram mode. For example, if
the AXUDP port is port 8, and the link partner (44.136.20.2)
is able to accept all amprnet traffic for Australia, the entry
would look like this:
IP ROUTE ADD 44.136.0.0/16 44.136.20.2 8 d
The source IP address for this mode of routing is the
IPADDRESS of the AXUDP port. Therefore, if XR32's main
IPADDRESS (in XROUTER.CFG) is not a 44-net address, you must
override it with a 44-net address on the AXUDP port. If the
main IPADDRESS is a 44-net address, which is the recommended
configuration, do not specify IPADDRESS in the PORT
configuration block.
SEE ALSO
AXIP(9) -- AX25-over-IP Encapsulation
IP(1) -- IP Routing / Configuration Commands.
IPENCAP(9) -- IP-in-IP Encapsulation.
IPLINK(1) -- Display / Set a Port's IPLINK.
IP-PRIMER -- IP Addressing / Routing Primer.
IP-ROUTE.SYS(8) -- IP Routing / Configuration File.
IP-ROUTING(9) -- IP Routing With XR32.
UDPLOCAL(1) -- Display / Set a Port's UDPLOCAL.
UDPREMOTE(1) -- Display / Set a Port's UDPREMOTE.
XROUTER.CFG(8) -- Main Configuration File
AXUDP(9) END OF DOCUMENT
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