Subject: [Megaco] new Implementors Guide

From: Terry L Anderson <[email-masked]>
Date: Sun, 30 Jun 2002 21:30:50 -0400

I sent this earlier  but it apparently was not forwarded by the 
reflector since it was sent from my alternate email account.  I am 
sending it again.  I apologize if you get it twice.

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

_______________________________________________
Megaco mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/megaco