From: Riley Williams (vvz.mluznajt@centro-net.cz)
Date: Sun Jun 29 2003 - 10:57:56 EEST
Hi Phil.
> I am trying to improve the performance of my ax25 system and
> one of the few debugging tools is "listen". Unfortunately,
> "man listen" only provides a bare outline of the command line
> options and I can't find a detailed description of how to
> interpret the output.
I'm no expert on the listen command, but I do know the basic
AX.25 protocol, and I'm offering reasonable guesses as to what
each means. I could easily be wrong about some aspects of it,
so don't treat this as gospel.
> For example, the following output from "listen -ar" contains
> a "^" after the "UI". What does that indicate?
>
> uhf: fm VK6DLX-9 to APRS via VK6BBS-10 ctl UI^ pid=F0(Text)
> len 88
Presumably that the P/F bit was set. I read that line as "From
VK6DLX-9 digipeat via VK6BBS-10 to APRS, frame UI with POLL set,
Protocol ID F) (straight text), length 88 bytes".
> In this line, what does the "+" at the end of the line
> indicate?
>
> axip: fm WA7V-2 to VK6DLX-7 ctl RR0+
Presumably also that the P/F bit was set, but this is a response
frame. I read that line as "From WA7V-2 to VK6DLX-7 Receiver
Ready acknowledging all outstanding frames up to and including
frame 0".
> In this line I guess the I17 means it is an information frame
> with sequence ID of 17. What does the "+" after the 17
> indicate?
>
> uhf: fm VK6BBS to VK6AD ctl I17+ pid=F0(Text) len 5
I read that as "From VK6BBS to VK6AD information of length 5
bytes using Protocol ID F0 (Straight Text), frame number 1 with
the POLL bit set, acknowledging receipt of all outstanding
frames up to and including frame 7".
> According to the ax25 specification, modulo 8 sequence
> numbers should be 0 to 7. Since I am seeing a sequence number
> of 74 here I assume this link has negotiated modulo 128
> operation.
>
> uhf: fm VK6BBS to VK6AD ctl I74+ pid=F0(Text) len 250
I read that as "From VK6BBS to VK6AD information of length 250
bytes using protocol ID F0 (straight text), frame number 7 with
the POLL bit set, acknowledging receipt of all outstanding
frames up to and including frame 4".
> A moment later however, on a fresh connection, I saw the
> sequence number count up to 63 and cycle back to 0.
Remember that I-frames have TWO sequence numbers, one for the
frame itself, the second for frames acknowledged as received.
You appear to be confusing them.
> Here are some reject commands:
>
> uhf: fm VK6AD to VK6BBS ctl REJ2-
Read as "From VK6AD to VK6BBS acknowledging reception of all
outstanding frames up to and including frame 1, and requesting
retransmission of frame 2 onwards, with the FINAL bit NOT set".
> uhf: fm VK6AD to VK6BBS ctl REJ0-
Read as "From VK6AD to VK6BBS acknowledging reception of all
outstanding frames up to and including frame 7, and requesting
retransmission of frame 0 onwards, with the FINAL bit NOT set".
> They don't seem to specify what frames have been rejected.
> The first reject was sent after sequence number 24 arrived
> from VK6BBS. And what does the "-" indicate?
See above analysis of each one.
> In summary, does anyone know if there is a comprehensive
> write-up of the "listen" command?
That I can't help you with, but hopefully the above will give
some clues.
Best wishes from Riley.
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