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IP-MTU.X1H
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1992-11-02
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Using TheNet X-1H with large packets
The basic version of TheNet X-1H has an AX.25 paclen limit of 256
bytes and IP router MTU sizes of 256 bytes for each port. It will
also accept AX.25 datagrams with packet sizes greater than 256
bytes and fragment them in the IP router.
This is sometimes inadequate. To cater for this, the code has
been modified to allow it to work with larger frames, but will
need to be patched to enable this.
The patching must be done manually.
IT MUST BE STRESSED THAT IT IS NOT A SENSIBLE THING TO DO, TO
SIGNIFICANTLY INCREASE THE PACKET SIZES. This is because the node
will run out of buffers much quicker, but if you want to
experiment, be my guest. I suspect that crashes may become more
frequent, but would greatly appreciate reports from people who
try it.
The details follow.
There are 3 basic parts to patch.
1. The maxframe level 2 limit.
This parameter sets a limit on the overall level 2 packet
size. It is set currently to 328, ie enough space for a 256
bytes of L2 data plus a full digipeat header address ( 70
bytes ) and 2 bytes for control data.
The parameter is located at offset 08ee hex into BOTH parts
of the rom image ( ie thenet1.x1h AND thenet2.x1h ). Both
must be patched. It is stored as a 16 bit word with least
significant byte at the lowest address, so at offsets 08ee
and 08ef respectively you will find the values 48 and 01.
This corresponds to 0148 hex, ie 328 decimal. Patch these to
whatever limit you want. For example for IP frames of 512
bytes, set it to 5c 02 hex ( ie a value of 025c hex,
corresponding to 512 data bytes, 20 IP header, 70 AX.25
address and 2 control.
The software does not always check this value !.
2. The I field limit
The second value to patch is the maximum number of data
bytes permitted in an AX.25 I field. More than this will
result in an FRMR error response.
As for the above parameter, it is also a 2 byte entry
located at offset 14a8 hex into the file thenet1.x1h. Note
that only the first part needs patching.
It is currently set to 0101 hex ( ie 257 decimal ). Increase
it to whatever size you want.
3. The IP router MTU entries
The final two entries that are patchable are the level 2
MTUs. These are located in the file thenet1.x1h, at offsets
of 7f5e and 7f66 hex respectively for ports 0 ( the radio )
and 1 ( the RS232 port ). They are currently set to 00 01 (
ie 0100 hex, ie 256 decimal ). Set them to whatever you
want, BUT DO NOT EXCEED THE NODE'S LEVEL 2 LIMITS !.
Note that 8 bytes earlier you will find the netrom MTU, but
only increase this if you wish to cause trouble.
4. Closing comments
Whilst I believe the changes work, you are on your own. I
have checked the code out with 512 byte packets, and it
appears to work but that's your lot!.
Personally, I think the overhead saved is a small gain and
can be saved better in other more elegant ways.