AXTCP(9) XR32 REFERENCE MANUAL 9/2/2013
NAME
AXTCP -- AX25-over-TCP Tunnelling.
DESCRIPTION
AXTCP is AX25 "encapsulated" within TCP datagrams. This
enables AX25 systems to communicate with each other via TCP/IP
networks (e.g. the Internet) in a similar way to AXIP and
AXUDP. The frame structure is as follows:
.-----------.------------.------------------------.
| IP header | TCP header | AX25 frame |
'-----------'------------'------------------------'
(20 bytes) (20 bytes) (Typically 15-340 bytes)
AXTCP is particularly useful when it isn't possible to route
AXIP or AXUDP, as detailed below:
The Problem
~~~~~~~~~~~
Many people in the UK use mobile broadband "dongles" to
provide their Internet connection, either because it's a lower
cost option than fixed-line broadband, or because the latter
is not available in their area, or because they're "roving".
As an example of the latter, a "mobile" node may be
established in a vehicle and used to provide a local packet
"hot-spot" for special events, emergency datacomms etc. It may
be using mobile broadband, WiFi or other available Internet
connections, and it is highly unlikely that such a connection
could be configured to route AXIP or AXUDP.
The characteristics of mobile Internet which prevent the use
of AXIP or AXUDP are that (a) the subscriber's IP address is
volatile (so you can't set IPLINK), and (b) the subscriber is
on a private network behind a NAT firewall, which doesn't pass
incoming AXIP or AXUDP traffic!
The solution
~~~~~~~~~~~~
AXTCP offers a solution to these problems. It differs from
AXIP and AXUDP in three respects:
- Firstly, it uses a bidirectional stream protocol (TCP/IP)
which guarantees that the packets can flow in both
directions, even through several stages of NAT and
firewalling.
- Secondly, it uses a client-server approach where the fixed
server has no need to know the mobile client's IP address
and port number. The connection is initiated by the client,
and the server simply responds via that connection.
- And thirdly, the TCP protocol ensures that no AX25 packets
are lost.
The AX25 links created using AXTCP can in turn support NetRom
and amateur TCP/IP, just like real radio links.
To use AXTCP, the "static" node is configured as an AXTCP
server, and the "mobile" node as an AXTCP client.
Configuring XR32 for AXTCP
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
AXTCP is enabled by declaring an INTERFACE with TYPE=AXTCP. A
single PORT is then attached to that interface.
AXTCP interfaces can act as server, client, or both at once,
depending on which keywords are included.
An AXTCP server can support several simultaneous client
connections, and a client can connect to several servers
simultaneously.
Note: When configuring a server, you must ensure that incoming
TCP/IP connections are directed to the server's TCP port, i.e.
there must be suitable NAT entries or "port forwarding" set
up in the Internet "front end" router.
Example AXTCP Server Interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The following configures an AXTCP server, listening for
clients on TCP port 9393:
INTERFACE=5 <-- change to suit yourself
TYPE=AXTCP
MTU=256
INTNUM=9393 <- Server port
ENDINTERFACE
The INTNUM parameter activates the server and specifies the
TCP port that it listens on. If this is zero, or not
specified, the server is disabled.
Example AXTCP Client Interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This specifies a client which connects with the KIDDER node,
whose address is g8pzt.ath.cx, port 9393:
INTERFACE=5
TYPE=AXTCP
MTU=256
CONFIG=KIDDER g8pzt.ath.cx 9393
ENDINTERFACE
The CONFIG directive is used to specify a server, thereby
activating client mode. The format is as follows:
CONFIG=<servname:> <hostname | ipaddr> <tcp_port>
You may specify additional servers, by including a CONFIG for
each one. There is no limit to the number of CONFIG
directives that can be used with a single interface.
Combined AXTCP Client/Server Interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A combined interface is specified by including both INTNUM and
CONFIG directives. For example, this combines a server
listening on TCP port 9393 with 3 client connections:
INTERFACE=5
TYPE=AXTCP
INTNUM=9393
MTU=256
CONFIG=DORSET 216.23.45.91 10093
CONFIG=WALES gw3xr.dyndns.org 9394
CONFIG=Brum bm23.ath.cx 7507
ENDINTERFACE
This is just an example, and it could only be used at a
"static" node, in which case the client connections could
be replaced by AXIP or AXUDP anyway.
Such a configuration *couldn't* be used for "mobile" node,
because there's no way of directing incoming TCP port 9393 to
a mobile server!
AXTCP PORT Settings
~~~~~~~~~~~~~~~~~~~
A PORT attached to an AXTCP interface needs no special
configuration, and should simply be configured with
parameters (FRACK, RESPTIME, RFBAUDS) approriate to a high
speed connection, for example:
PORT=8
ID=AXTCP Operations
INTERFACENUM=5
FRACK=2000
RESPTIME=200
RFBAUDS=56000
ENDPORT
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.
By default, all AXTCP connections use the same PORT, but if
you prefer to use one port per neighbour, see the following
section. You must NOT attach more than one PORT to an AXTCP
interface.
Multiple Interfaces / Ports
~~~~~~~~~~~~~~~~~~~~~~~~~~~
A single AXTCP server can accommodate many clients, so there
is no practical need to use more than one AXTCP interface and
port.
This is analogous to having several nodes on the same radio
frequency, except that the nodes don't see each other's
transmissions, and there is no "hidden station" effect.
You may however prefer to assign each AXTCP partner to a
seperate port, analogous to having a dedicated radio
frequency for each neighbour. In this case you may configure
several interfaces, each with a single PORT attached.
The only proviso is that if INTNUM is used, no two AXTCP
interfaces may use the same INTNUM.
Transaction Logging
~~~~~~~~~~~~~~~~~~~
If LOGFLAGS is non-zero, XR32 logs AXTCP connections and
disconnections, otherwise nothing is logged.
Notes:
~~~~~~
When a client connects to a server, it should immediately be
possible to force an AX25 level 2 connection from either
client or server, by entering the command "C n ALIAS-1",
where n is the AXTCP port number, and ALIAS is the node alias
of the peer.
However, it may take a few minutes for the client and server
to commence NetRom operations, because each end must first
receive a nodes broadcast from the other. The link-up time
will be shorter if a prior link has been made recently. It
is hoped to speed up the linking time in a later version.
SEE ALSO
AXIP(9) -- AX25-over-IP Encapsulation
AXUDP(9) -- AX25-over-UDP Encapsulation
XROUTER.CFG(8) -- Main Configuration File
AXTCP(9) END OF DOCUMENT
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