AXIP - AX25 over IP

AXIP is AX25 "encapsulated" within IP 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 |        AX25 frame      |
	'-----------'------------------------'
	(20 bytes) (Typically 15-340 bytes)

The AX25 links created using AXIP can in turn support NetRom and amateur TCP/IP, just like real radio links.

Setting Up AXIP Links

Assuming you have a prospective AXIP partner, you would set up an AXIP link as follows:

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.
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.
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 Linux nowadays, so you may remove all "DNS=" lines from XROUTER.CFG.
If using the partner's hostname, verify that "PING <hostname>" resolves the address correctly.
Add an AXIP interface to XROUTER.CFG as follows:
```
	INTERFACE=7
		TYPE=AXIP
		MTU=256
	ENDINTERFACE
```
Choose the interface number to suit yourself).
This interface can support an unlimited number of AXIP PORTs. You may define multiple interfaces if your ports need different MTU's.
Only TYPE=AXIP and MTU= are required, all other parameters are ignored (at present).
For each AXIP partner, add a PORT similar to this:
```
	PORT=8
		ID=AXIP link with WA3IP
		INTERFACENUM=7     <-- Points to the INTERFACE above
		IPLINK=55.73.88.69 <-- Partner's IP address/hostname
		etc...
	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.
To link with an AXIP partner on the SAME machine use a "localhost" address, e.g. "IPLINK=127.0.0.2".
The assigned "protocol number" for AXIP is 93 (decimal).
MAC parameters such as TXDELAY, TXTAIL, SLOTTIME, PERSIST, FULLDUP, SOFTDCD etc. are meaningless for AXIP, 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 a multi- -tasking O/S means that RESPTIME should be more like 200ms.
If XRPi 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. This "front end" router will probably route outgoing AXIP without problem, but it will not know where to send incoming AXIP unless explicitly configured.
Configuring such a router for AXIP usually involves specifying a protocol number (93 for AXIP), and the LAN IP address of a machine to which it should be routed, i.e. XRPi's LAN IP address.
Not all domestic routers can be configured to route incoming AXIP as it is not a commercially recognised protocol. Some routers only allow TCP and UDP port redirection, with no provision for any other protocol. If you or your link partner have such a router, you may need to consider AXUDP instead - see later.
Your link partner must set up a reciprocal arrangement, i.e. their IPLINK must be set to your IP address or hostname.

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 AXIP to communicate between nodes on the LAN, or even on the same machine, but only one of them can receive AXIP from outside the LAN.

If you have more than one node on your LAN using AXIP, your Internet router is only able to direct incoming AXIP to *one* of them. This means you can only have one AXIP node per public IP address.

DO NOT set up an AXIP link to a link partner if you already have an AXUDP link to that partner. This is a common mistake, and is likely to cause problems!

Using one port per neighbour may seem wasteful, but it is a reliable method, and it allows you to monitor exactly what is going on.

IP Routing Over AXIP

As mentioned earlier, you may route amateur IP (44.x.x.x) over your new AXIP 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 AXIP link, simply add an IP route entry directing the required subnet to your neighbour's IP address on the AXIP port using datagram mode. For example, if the AXIP 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 AXIP port. Therefore, if XRPi's main IPADDRESS (in XROUTER.CFG) is not a 44-net address, you must override it with a 44-net address on the AXIP port. If the main IPADDRESS is a 44-net address, which is the recommended configuration, do not specify IPADDRESS in the PORT configuration block.

XRPi Documentation - Protocols

AXIP - AX25 over IP

Setting Up AXIP Links

Notes

IP Routing Over AXIP

See Also: