MTU is a contraction of "Maximum Transmission
This is the maximum amount of TCP/IP data that is permitted in an AX.25
packet. It can be thought of as kind of a "Paclen" for IP. In TheNet X-1JR4,
the default is 256 bytes.
There is a lot of thought that must go into changing this value. Here are some things to know if you think you may wish to change this setting:
1) Short packets get through best.
2) The node sends an error message if it is asked to handle an
IP packet that is larger than it's permitted
3) Some users may set their MTU incorrectly, and you may get
flooded with mail from those users asking
why they are not able to use the node.
4) The node is more likely to crash if it is asked to handle
packets that are substantially larger than the
5) Most digipeaters in TNCs will not resend a packet larger than the standard 256 bytes.
6) The calculation used to determine MTU is a little different
than most may be accustomed to. Most IP
stations set the MTU to 236, subtracting the 40 bytes in the IP header from 256. In the X-1J, they ask
you to include all of the AX.25 call-sign and digi data, which is 38 Bytes plus 2 control bytes.
The bottom line is that 256 is a good default value, and it's best to keep it. However, if you have a user who can't seem to be able to send IP through the node, you may want him or her to check their MTU setting (and perhaps change it to 236 if it's a NOS or Linux station, 256 if it's another X-1J node).
If you do find that you need to change the MTU on the node, you'll find that it operates similar to the MODE command. Like MODE, you have a choice of syntax you can use - the old style, or the newer one. Here's a list of the MTU changes you can make with either:
Parameter Default Function
Sets MTU for the Radio Port (Port 0)
2 256 Sets MTU for the Serial Port (Port 1)
3 236 Sets MTU for Net/Rom IP routing
4 257 Sets the MTU that triggers an error response
5 328 Sets the MTU level governing the discard of packets
(they'll be ignored)
The first form you can use to enter data has the * character as a filler until you get to the parameter you want to change. Let's say you want to change the MTU on the Serial Port to 512. Here's how that command would look:
MTU * 512
Or the newer syntax makes it look like this:
MTU / 2 512
Both of the above examples tell the node to change the MTU on the serial port to 512 bytes.
This is really both a user and a Sysop command. It not only lets the user and the Sysop know how this node gets to another node, it's how the Sysop sets up a node's static routing table.
"Neighbour nodes" are ones that you can reach directly, and it's a good idea to have your reliable neighbour nodes "locked in" your node's routing table. It's also not a bad idea to enter known reliable nodes that are easily reachable through these neighbour nodes.
Each node is entered into the table with both an alias and call-sign, assigned a route quality, Obsolescence Count, Port number and digipeaters used to reach it, if necessary.
Here's how a neighbour node, WX9APR-2 with an alias of WAPR2, a route quality of 192, Obsolesence count of 6 on port 0 is added to the routing table:
NODE WX9APR-2 + 192 6 0
Note that this node is a neighbour node - we can reach it directly. If it weren't, we would have added the neighbour node call-sign at the end of that line.
To remove the same node from the routing table:
NODE WX9APR-2 - 192 6 0
Of course you can check routes by using the long form of the user NODE command. Let's say we have a node WAPR3 that we reach from WAPR2, the node shown in the above examples. If we send this to the node:
You'll see this in response:
Routes to WAPR3:
WX9APR-3 192 6 0
The first number, 192 is thr route quality, the second is the obsolescence count, the third is the port number.
The PARMS command is used for reading and setting various basic node parameters. When the PARMS command is issued by itself, the node responds with a string of 26 sets of numbers that tell what each value us. You'll need a key, such as this one below, to figure out what each set of numbers is telling you. Count the 5th number set from the left, for example, and you'll see the current setting of the initial obsolescence count. Here's the list of parameter numbers and what they indicate:
1 Size of destination node table
2 Minimum auto update quality
3 HDLC (radio port) default quality
4 RS-232 (Crosslink) default quality
5 Initial obsolescence count
6 Min Obs to broadcast
7 Nodes broadcast interval
8 Initial time-to-live
9 Transport FRACK timeout (seconds)
10 Transport RETRY counter
11 Transport acknowledgement delay
12 Transport busy delay
13 Transport window size
14 Transport overfill limit (frames)
15 No-Activity time-out (seconds)
16 Persistance (n/256)
18 FRACK (T1) time
19 AX.25 windowsize (MAXFRAME)
20 AX.25 retries
21 ACK (T2) time (L2 RESPTIME)
22 Activity check (T3)
24 Callsign validation
25 Beacon mode control
26 CQ broadcasts
These numbers can also be used to set various parameters by the Sysop. The Syntax can use either of two types of syntax, just like the MODE and MTU commands.
Let's say you want to change the Persistence to 128. Persistence is the 16th set of numbers that you can read or manipulate. In the older style syntax, this would look like:
PARMS * * * * * * * * * * * * * * * 128
In the newer style syntax, it would look like:
PARMS / 16 128
The PARMS command, when used by a non-sysop user, will only allow the display of paramters. The node can be set to not display the parms to users with the command:
PARMS - D
It can be enabled with:
PARMS + D
That's all for this part. Next time, we'll continue our alphabetical exploration of the Sysop commands for these nodes.
Until then, 73 from Andy.
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