On 10/21/2022 5:34 AM, Joshua C. Colp wrote:
On Thu, Oct 20, 2022 at 8:23 PM <[email protected]
<mailto:[email protected]>> wrote:
Hi, all,
Something I have been working on recently is adding support to
PJSIP for device feature key synchronization (the as-feature-event
Broadworks spec that many common IP phones, e.g. Polycom support) to
control server-side features from endpoints. It's using the PJSIP
pub/sub capabilities; I had to add the ability to execute a custom
module callback when a SUBSCRIBE is refreshed, but with that
addition,
it works as it's supposed to.
You should further elaborate on all of the server-side features you
expect to implement, if it extends beyond DND.
The other one is Call Forwarding (Always/Busy/No Answer, with number of
rings)
I wanted to solicit some input on what an ideal way of triggering
updates should be. Currently we have hints, which actually work
reasonably all right for Do Not Disturb, which is simply a boolean
on/off, easily represented with a hint and custom device state for
DND.
The PJSIP module emits an AMI event, the user can process it and
change
the device state if needed, which will trigger a NOTIFY to go out
to the
endpoint.
What user? An outside AMI application? An internal consumer in Asterisk?
The administrator of the Asterisk system, who can add AMI logic to
receive the event and then do something with it.
The reason this is needed is the phone isn't turning DND on directly,
for example. It's merely a request. The server can decide to not allow
it, for example if that phone isn't allowed to toggle DND. The server
will process it, and send it the updated status. This usually reflects
what the phone wanted, but not necessarily.
(I've elaborated on this more below)
This sort of came up about 12 years ago[1]. The actual SIP stuff
is not
complicated; it's the user interface to it that requires more
thought.
For call forwarding, there are more moving pieces and abusing
hints/custom device state for that is super clunky. You can't
communicate the call forwarding target, # of rings, etc. in a device
state, so additional hints are then needed for that. It works but
it's
super clunky and I don't think this is a great pipeline.
Okay, so this covers call forwarding as well.
Yes.
I'm wondering if people have thoughts on what an ideal mechanism
would
be for users, once they process a request to enable/disable a feature
from the phone, to communicate that to the PJSIP module. The problem
with abusing hints, especially for call forwarding, is it's not a
good
way to communicate details into the module. One option perhaps is to
have dialplan extensions, setup in a manner similar to use with
EVAL_EXTEN, where it returns the current value needed, as any
relevant
function, DB, ODBC, CURL, custom function, etc. could be used to
retrieve the current feature value. The clunky part is more
signaling to
the PJSIP module that it needs to send the phone the updated
status (by
checking those extensions, for example). The device state callback
happens to be convenient for this kind of signaling but not really
appropriate here. It would be better to push the info into the module
directly rather than the signaling it and making it retrieve the
updated
data in some arbitrary way.
So with this in mind, I'm currently leaning in the direction of a
dialplan function/AMI action that could be used to set the
appropriate
info for a subscription, which would trigger the NOTIFY, and then
nothing would actually need to be added to the dialplan at all
(inasmuch
as hints and things of that nature). One d I think starting purely
from that perspective
isadvantage of this is that
for every single update, unlike callbacks, we have to traverse the
entire list of subscriptions (though maybe that's not a big deal).
The
bigger problem is this is push only and the PJSIP module still
needs to
be able to "pull" feature statuses on demand, which is where the
hint/lookup model is useful. A potential middle ground solution is
use
the dialplan function/AMI action to push only, but cache all of
this in
AstDB (as subscriptions themselves are), so that we can retrieve the
latest/most current value at any point if needed. Then we don't
need to
be concerned at all with how the user is managing state as that is
fully
decoupled, although obviously this would lead to a little
duplication/redundancy. Any thoughts?
You've thrown a lot of lower level implementation details at us and to
be quite honest it's overwhelming. There's no full user facing
examples of how it would all work, beyond the bits and pieces in your
text that we'd then have to deduce and after reading a few times it
doesn't feel very friendly. To start off with: Is this a developer API
and interface, or is this also meant for the common everyday user? I
would hope it's for the common everyday user, in which case it should
be approached from that perspective first with implementation details
following.
Yes, it's for the common every day end user. A callback mechanism if
used would be more of a development one but that would be more a means
to an end.
Here's an example of what I have in my dialplan right now, in the
subscribe_context for the endpoint:
exten => dnd_Polycom5,hint,Custom:${EXTEN}
exten => callfwd_Polycom5,hint,Custom:${EXTEN}
exten => callfwdalways_Polycom5,1,${FOOBAR(callforward,2135)}
exten => callfwdbusy_Polycom5,1,${FOOBAR(callforwardbusy,2135)}
exten => callfwdnoanswer_Polycom5,1,${FOOBAR(callforwardnoanswer,2135)}
The user gets the AMI event, processes it with whatever processing is
needed (e.g. checking that the DND feature is available for that line,
setting it in AstDB, ODBC, or whatever is the source of truth for
feature statuses), and then updates the relevant hints above.
The module is currently hardcoded to use these extensions in the
subscribe context: the prefix + the endpoint name. Obviously that's also
inflexible.
Right now, the first 2 extensions the user will set to signal the module
to send an updated NOTIFY. The first hint by virtue of being binary
contains the DND status itself, and the 3 bottom extensions are needed
to retrieve the call forwarding numbers from the source of truth for
these features. (Here, FOOBAR is a custom - but any arbitrary - dialplan
function I have that retrieves the status).
I bring this up only to show the current implementation and how hacky it
is; I don't like this at all or think it's appropriate (except for DND,
possibly). It was more a proof of concept of testing the underlying SIP
technology.
A better implementation might look like:
User gets the AMI event and processes it as usual, and then simply does
Set(PJSIP_DEVICE_FEATURE_STATE(PJSIP/Polycom1,donotdisturb)=enabled) or
Set(PJSIP_DEVICE_FEATURE_STATE(PJSIP/Polycom1,callforwardingnoanswer)=8675309,4)
(forward on no answer to 8675309 after 4 rings).
Internally, PJSIP_DEVICE_FEATURE_STATE could also persist input to AstDB
so it's available to the module.
As an everyday user I'd expect not to have to deal with AMI or complex
dialplan. I'd expect to be able to set and get the information from
the dialplan using dialplan functions (or a single function) so I
could use that in the dialplan, and have it "just work" with my phones
that support the feature. I'd expect it to persist across Asterisk
restarts. For example ${EXTENSION_DND(alice)} for retrieving DND
status of Alice, and if Asterisk restarted then that should stay the same.
That's a good point. The problem is that means that Asterisk internally
is the source of truth of DND, and that may not necessarily be what
people want. For instance, that wouldn't meet my own requirements. The
way that Broadworks works is the phone is merely requesting a certain
disposition, but the server isn't under any obligation to carry it out.
So I think there needs to be some mechanism for the user to be involved
in that pipeline, to be able to deny something that a phone wants. Maybe
the user doesn't have DND, maybe certain phones aren't allowed to
toggle, whatever. Lots of people store their feature states in MySQL
databases and use them for Asterisk clusters. Some systems might have
specific requirements for that. So users should have flexibility to
reject it. On some systems, maybe the same DND status is used for
several lines and Asterisk internally would not have any idea about this.
There are lots of different scenarios that are beyond what I think
Asterisk itself should handle, hence the "two-part" process described
above: the user (system admin) can do whatever needs to be done, and
then just tell PJSIP what the new state is. PJSIP doesn't need to know
or care about where feature states are actually stored or what the logic
is or how they are mapped to endpoints.
I do see your point though and I think it would be nice to have a
"simple, default mode" where Asterisk will internally just "approve
everything" that the phone wants, and users can use that if that suits
their needs, but fundamentally I think users should be able to be
involved in the decision pipeline if they want/need to. I'm not yet sure
what that would look like: maybe a pjsip.conf option to emit AMI events
rather than auto-handling them? And then the PJSIP_DEVICE_FEATURE_STATE
function would have to be used to tell PJSIP what to do, and Asterisk
itself would not be the source of truth for feature statuses in this
case (but it would cache them as described above) (though in this case
reading the PJSIP_DEVICE_FEATURE_STATE could still return the cached
disposition)
For developers the same thing could apply except having AMI actions to
set and get.
Functionally this is a simple storage mechanism of data. The AstDB is
used to persist such information. It could use sorcery to then be
backed by a real database. The manipulation could be like I said
previously, using dialplan functions with also AMI actions. To get the
information to consumers it would be a defined API to allow
setting/getting/subscribing/anything else, and for pushing to
consumers it would use stasis which would also give AMI events when
things changed. It's sorta like device state, but with persistent data
and simpler.
This would also need explicit configuration to ensure that stale data
doesn't persist in the persistent storage.
Can you elaborate on what you mean by this? If something like
PJSIP_DEVICE_FEATURE_STATE was the only way of telling the PJSIP module
that a device state changed, then as long as the system administrator
maintains the invariant of always telling PJSIP about feature state
changes, this means it should have the latest feature data at all times,
and it shouldn't get stale.
For example, this all needs to work with feature codes too - assuming
*78 to enable DND, *79 to enable, and *72 for call forwarding variable, e.g:
exten => *72,1,Read(callfwdnum)
same => n,Set(ODBC_CALLFWD(${CALLERID(num)})=${callfwdnum}) ; not
sure if the syntax is right... it's been a hot second since I've used
ODBC stuff...
same =>
n,Set(PJSIP_DEVICE_FEATURE_STATE(${HINT(${CALLERID(num)})=${callfwdnum})
same => n,Return(0)
exten => *78,1,SetODBC_DND(${CALLERID(num)})=1)
same =>
n,Set(PJSIP_DEVICE_FEATURE_STATE(${HINT(${CALLERID(num)})=enabled)
same => n,Return(0)
exten => *78,1,SetODBC_DND(${CALLERID(num)})=0)
same =>
n,Set(PJSIP_DEVICE_FEATURE_STATE(${HINT(${CALLERID(num)})=disabled)
same => n,Return(0)
--
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