[nos-bbs] Maybe for the next standards revision?

[Steven Stimpson (N1OHX)]

----- Original Message ----- 
From: "George (Skip) VerDuin" <k8rra at ameritech.net>
To: "TAPR xNOS - Mail List" <nos-bbs at lists.tapr.org>
Sent: Sunday, February 19, 2006 7:20 PM
Subject: [nos-bbs] Maybe for the next standards revision?


> Greetings,
>
> I have a situation that seems like a condition not addressed by the 1998
> AX.25 Link Access Protocol Standard V2.2 nor programmed into jnos.
> Consider the following exchange extracted from a packet trace:
>
> Wed Feb 15 19:31:02 2006 - vhf sent:        > fourth request
> AX25: K8RRA-15->N8YJT SABM(P)
>
> Wed Feb 15 19:31:42 2006 - vhf recv:    ERR?> in frame
> AX25: N8YJT->K8RRA-15 RR(P) NR=0
>
> [ANALYSIS: This RR(P) should not arrive - it implies an already open
> connection.

 Hi Skip. There is an open connection, k8rra-15 sent an SABM to n8yjt.
In all probability k8rra didn't recieve the UA, there is welcome text to
send,
and so n8yjt is (P)olling k8rra-15 to sync the link. Or, k8rra-15 got the
UA,
but didn't get the first frame. the fact that NR=0 clues that this is a new
connection.


>   The two ends of the link are out-of-sync at this moment.
>   The cause may be that N8YJT did not receive the UA(F) frame sent
> 19:28:30
>      to acknowledge the DISC from YJT at 19:28:30.  The DISC frame at
> 19:32:29
>      seems to confirm the cause.]


Hmmm.Hard to comment on that, there is no 19:28:30 traced here.

> Wed Feb 15 19:32:23 2006 - vhf sent:        > fifth request
> AX25: K8RRA-15->N8YJT SABM(P)
>
> Wed Feb 15 19:32:29 2006 - vhf recv:        > disconnect - connection=
> #3 finish
> AX25: N8YJT->K8RRA-15 DISC(P)
>
> Wed Feb 15 19:32:29 2006 - vhf sent:    ERR?> busy - connection= #3
> terminated
> AX25: K8RRA-15->N8YJT DM(F)
>
> It seems to me that the RR(P) frame needs to be treated as an
> "unexpected response error" in the Data-Link State Machine (appendix C4
> paragraph C4.3. under Error Codes) standards and in jnos.  It seems
> certain that jnos ignored the RR frame and continued with the SABM
> sequence.

 AN (P)oll frame, the first one sent out by the station that is being
connected TO, should be ignored by the station that send the SABM
if it has not yet recieved an UA. SO Jnos ignoring the RR(P) and
going on with the connect attempt is proper. Most of my packet
activity is on H.F., where supervisory frames are missed constantly.
A third party will monitor the following frequently:

n1ohx>n1llu SABM
n1llu>n1ohx UA
n1llu>n1ohx welcome to blah blah blah
n1llu>n1ohx RR(P) NR=0
n1ohx>n1llu SABM

 Assume n1ohx didn't recieve the UA or welcome message, but
DID recieve the (P)oll frame. If this frame was not ignored, n1ohx would
need
to send a DM to indicate that there is no connection between the two yet.
This could cause problems. It's better to just ignore all responses
during a connect request unless they are a UA, DISC, or DM
I have never seen a DISC sent as a response to a SABM, if
a connection is not prudent then usually a DM is sent.


> As a secondary thing, when the DISC frame arrived at 19:32:19, the
> respnse of UA seems correct rather than the DM.  This is a lesser issue
> because both the UA and DM cause the same result...
>
> The workaround for this situation might be to wait until all timers have
> expired and both ends have reset prior to beginning a SABM sequence.
>
> 73
> de Skip k8rra k
>
 Msys resets the connection, sets the frame numbers to 0, and issues a UA
when an SABM is recieved from the same station when there is already
an connection between them. this seems to work fine. I can't remember
how jnos handles an incoming SABM when a link is already established,
but you could test by having someone connect to you, then disable TX,
do an ax25 reset, enable tx, and then connect again.
73,
steven n1ohx