Subject: [Megaco] new version of H.248 Implementors Guide

From: Terry L Anderson <[email-masked]>
Date: Sat, 29 Jun 2002 21:16:47 -0400

A new version of the H.248 Implementors Guide was discussed here at the 
Bruges, Belgium ITU SG16 WP2 Rapporteurs meeting.  It contains 7 new 
items applying to both H.248.1 V1 and V2.  (Recall that the base 
recommendation containing the main body and Annexes A-E and Appendix A 
were renumbered after Feb 2002 as H.248.1 with other Annexes as H.248.x).

It can be accessed by anonymous ftp at
    ftp://standard.pictel.com/avc-site/0206_Bru/TD-36.zip
although if you look soon it may still be in:
    ftp://standard.pictel.com/avc-site/incoming/TD-36.zip

Note that there are two issues that readers should particularly note:

IG 6.6 changes the recommendation for documenting packages, that 
statistics should be specified as they actually have been in Annex E and 
include a specification of the type.  The item suggests that all the 
same types available for parameters be allowed.  The non-numeric types 
may not seem reasonable for "statistics" but if one considers that 
statistics are simply a way to query about the value of some readonly 
parameter (already available with audit) but one that is automatically 
reported on a Subtract.  Then non-numeric values may be quite useful 
(eg, the contents remaining in a dial string buffer, DMTF digits, value 
of some enumeration used by the package, etc.)  If the experts conclude 
that these non-numeric values are not useful we can remove them before 
final approval of the IG at the next Study Group meeting in Oct.

IG 6.3 resolves an issue raised some time ago on the list by Troy Cauble 
about a parsing conflict introduced by previous changes to allow empty 
Events lists.  He proposed this solution nearly a year ago but the 
thread ran out with no comments.  The issue was raised again recently 
and someone pointed out this solution.  We put the solution into the IG 
but on further discussion I am not convinced that the "conflict" needs 
to be fixed.  Let's discuss this and decide whether to keep the change.

Troy's point was an parsing ambiguity between an empty eventsDescriptor 
and  auditItem since in the location he illustrates either could be 
simply "Events".

Troy wrote:
This reminds me...
Last June I pointed out that the change to which you are referring (IG 
6.79) broke the ABNF in a fairly serious way.

Here's an example.  The EventsToken below could be one of two things.

MEGACO/1 [135.114.123.123]:1234
Reply = 5000 { 
    Context = 1 {   
        Add = t1 {     
            Events    ; this could be an empty eventsDescriptor OR an 
auditItem   
        } 
    }
}

It is absolutely true that this ambiguity exists now, but the question 
is whether it is a problem.  
I would assert that both mean the same thing.  It could be a reply to an 
Add with an empty Events Descriptor (This would be more likely in a 
Modify, but the same issue exists there).  Or it could be a reply to an 
Add including a AuditValue{Events} when the event list is empty.  But 
the meaning is the same (that the events descriptor is empty).  I 
suppose strictly speaking one could send an Add (or Modify) with an 
empty Events descriptor and also request Audit{Events} in which case one 
could get Events, Events in the response, but it is a little pointless 
for an MGC to do this unless it is simply testing the MG for complience 
(if it sets it to empty the Audit should always return empty).  So it 
certainly leads the parser down two choices and any automatic parser 
generator is likely to return an error, but since both mean the same 
thing it is no really an operational problem.  

The question is: do we want to fix it as specified here or leave the 
syntax unchanged and keep the ambiguity?  The item will remain in the 
IG, changing the syntax (after IG is approved), unless I get sufficient 
responses saying we'd rather leave it.

-- 

------------------------------------------------------------
Terry L Anderson              mailto:[email-masked]
Tel:732.949.5628   Fax:732.949.7016
Lucent Technologies/INS/Voice Over IP Access Networks
Rm 2G-219A, 101 Crawfords Corner Rd, Holmdel, NJ 07733-3030
http://its.lucent.com/~tla (Lucent internal) http://www.gti.net/tla

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