Subject: [Megaco] Question on the semantics of Subtract command; Question regarding Digitmap Completion Events

From: "Kamitses, Jerry" <[email-masked]>
Date: Thu, 16 Dec 2004 18:28:26 -0500

Reference RFC 3525    Gateway Control Protocol Version 1

Question 1) I would appreciate some clarification regarding precise effect 
of the Subtract command on physical terminations.  In Section  7.2.3 Subtract, 
the final paragraph states:

   When a provisioned Termination is Subtracted from a Context, its
   property values shall revert to:
   -  the default value, if specified for the property and not
      overridden by provisioning;
   -  otherwise, the provisioned value.

1a) I assume "provisioned Termination" in the 1st line should read "physical Termination"
since the concept of "provisioned Termination" does not appear elsewhere in the RFC.
Is this correct, or is there some other meaning for "provisioned Termination" that is 
intended here?

(1b) It is not clear to me from the context exactly what is meant by "property"? Is 
"property" used to refer strictly to the formal concept of termination properties, 
that is,  parameters that are assigned values via the 
        propertyParm         = pkgdName parmValue
productions in the formal syntax using the localControlDescriptor and/or terminationStateDescriptor.  

Or is "property" here used in some more generic sense to possibly refer to other 
aspects of the termination state?

For example, if a subtract is sent to a termination with a currently active signal then 
does the signal stop as a side effect of receiving the subtract, or should the signal 
be unaffected by the subtract.?

Question 2) Digitmap Completion Events 

2a)  Section 7.1.9 Events Descriptor contains the text:
    The default streamID is 0, indicating that the event to be
    detected is not related to a particular media stream.

We have assumed that streamID 0 also could mean the event is related to all 
media streams. That is, for those events which apply to media streams, then 
when there are one or more streams on the context, and the MGC wants to enable 
an event concurrently on all streams, must the MGC send one instance of the 
event with streamID 0, or must the MGC send multiple instances each a separate 
non-0 streamID?

2b) This section also contains the text: 
  If a digit map completion event is present or implied in the
   EventsDescriptor, the EventDM parameter is used to carry either the
   name or the value of the associated digit map.  

Could you please explain "implied" digitmap completion event and provide a 
pointer to RFC text that discusses this concept? 

2c) Regarding the statement in section 7.1.14.6 DigitMap Activation
   A digit map completion event that does not contain an eventDM field
   in its requested actions field is considered an error.  
We have encountered instances of the following message syntax

    ... Events = 1 { dd/ce }, DigitMap = { (0 | xxxxxxxx|x.#) } ...

Since the digitMapDescriptor in the above example omits the digitMapName, 
then clearly it has no possible use except in the context of the associated 
completion event.  Perhaps  this could be an instance of an "implied" digitMapDescriptor 
for the completion event,  and be allowed even though it seems to technically violate 
the correct syntax for the ce event, which should be 

    ... Events = 1 { dd/ce{ DigitMap = { (0 | xxxxxxxx|x.#) } }  }, ...

Is the 1st Events example above allowable ?

Thanks.

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---

From: Tom Taylor <[email-masked]>
Date: Thu, 16 Dec 2004 20:33:02 -0500

Answers below, marked by [PTT].

> Is this correct, or is there some other meaning for "provisioned Termination" that is 
[PTT] I may have introduced this phrase once upon a time.  It's probably a red 
herring, but I have always believed that one could provision terminations that 
would otherwise be ephemerals -- PVCs in effect.  That's why the term, but I 
expect in normal practice this is identical with "physical termination".

> be unaffected by the subtract.?

[PTT] "Property" here is intended in the generic sense that all descriptor values 
are properties of the termination.  Thus a Subtract definitely ends an on-going 
signal, and may possibly install an alternative if a non-empty Signals descriptor 
was provisioned as the default for the termination.
> event with streamID 0, or must the MGC send multiple instances each a separate 
[PTT] One instance with streamID 0 will pick up digitmap completion events from 
all active streams.  If it is important to relate the event to the originating 
stream, you might want to enable the events against the individual streams.

> pointer to RFC text that discusses this concept? 

[PTT] I believe this was meant to refer to the use of the "ALL" wildcard as 
described in the preceding paragraph.  I would have to look at the Megaco 
correspondance between February and May of 2000 to verify that.  (The text first 
appears in the RFC 2886 corrigendum to RFC 2885.)
> Is the 1st Events example above allowable ?
[PTT] I think I'll leave this one for Kevin or Christian to comment on.

-- 
Tom Taylor
Carrier VoIP Standards Development
Nortel Networks
Phone +1 613 763 1496  (ESN 393-1496)
E-mail: [email-masked]