[asterisk-dev] BLF/SLA on SPA942 via "notifyexten"

[Eugene Grossi]

I am placing this previous private email (from the summer) onto this list to
get comments from developers. The issue (as summed up by Russell below) is
that:
some phones doesn't support sending a SUBSCRIBE, so there is no way to
receive the NOTIFY. We would create an option called something like
"notifyexten" that indicates that state changes for that extension should be
sent to this peer, regardless of whether there is a subscription for it or
not.

In particular this would enable the BLF/SLA for the SPA94x family.

Our previous email:
> I have modified chan_sip to deal with the issue of "non" subscription 
> of Sipura phones (working with SPA942's).
> 
> Specifically, the issue is that if a spa942 is programmed to "share" a 
> line, it sends a NOTIFY without prior creation of a subscription. My 
> current modifications allow acceptance of the NOTIFY and control the 
> "lamp state" by a "sip notify xxx xxx" to the given peer from the command
line interface.
> 
> I now am going to create a "subscription" such that SLA will work for 
> these phones. This issue is which way to go. Since I want to be able 
> to submit this code to the developer community I need your input.
> Should Asterisk take an inappropriate NOTIFY from the SIPURA and at 
> that point in time create the subscription or should a "SUBSCRIPTION" 
> field variable be added to the sip.conf entry for a peer, such that 
> when a peer registers, the SUBSCRIPTON is created using the 
> configuration value of the "SUBSCRIPTION" variable.
> I believe that the ASTERISK community would get great value out of 
> having BLF/SLA working on these phones, however I would like to 
> minimize the amount of "hack" since basically the reason it doesn't 
> work in the first place is a nonstandard approach by the Sipura software.

I apologize for the extreme delay in getting back to you.  The message got
lost in the depths of my inbox ...

I'm not sure that I really understand the problem.  Asterisk has no support
currently for *receiving* a NOTIFY message, as you may have already
discovered.
 So, the phone should not be sending any NOTIFY messages to Asterisk.

The SLA feature in Asterisk is not quite like other similar implementations
out there, which in my opinion, are really more like shared extensions.
(I'm referring to the SIP-B drafts).  Asterisk uses NOTIFY messages to
control the lamp on the phone to indicate the state of the shared line.

Now, if your problem is that the phone doesn't support sending a SUBSCRIBE,
so there is no way to receive the NOTIFY, then I think the sip.conf option
is a good idea.  I would create an option called something like
"notifyexten" that indicates that state changes for that extension should be
sent to this peer, regardless of whether there is a subscription for it or
not.

The tricky part here is that the code to do this in trunk is going to be
much different than 1.4.  However, we only merge new features into trunk,
and you likely need this for 1.4.

I would be happy to further discuss the details of what changes are
necessary to code this, but I'd like to move the discussion to the
asterisk-dev mailing list so that others may benefit from the discussion.
See lists.digium.com for the mailing list information.

Thanks,

--
Russell Bryant
Software Engineer
Digium, Inc.
-------------------------------      
Eugene A. Grossi, M.D.
Professor of Cardiothoracic Surgery
New York University School of Medicine
Suite 9-V; 530 First Avenue
New York, NY 10016
ph (212) 263-7452
fax(212) 263-5534
grossi at cv.med.nyu.edu