Subject: [Megaco] Ambiguity in MEGACO textual grammar ?

From: "Angelo Contardi" <[email-masked]>
Date: Wed, 13 Oct 2004 18:51:00 +0200

Hello Megaco - IETF Team,

during my study about MEGACO V2 Textual syntax (Annex B) i have probably
discover TWO (similar) ambiguties in "auditReply" productions. In fact,

auditReply = (AuditValueToken / AuditCapToken ) ( contextTerminationAudit  / auditOther)

and may be "rewritten" as:

auditReply =  AuditValueToken  contextTerminationAudit /    (1)
              AuditValueToken  auditOther              /    (2)
              AuditCapToken    contextTerminationAudit /    (3) 
              AuditCapToken    auditOther                   (4)
                                                                    
Let me consider the First two productions (1) and (2) (but what i will say is valid
for the last couple too, (3) and (4)).

auditOther              = EQUAL TerminationID [LBRKT terminationAudit RBRKT]
contextTerminationAudit = EQUAL CtxToken LBRKT errorDescriptor RBRKT / ...
terminationAudit        = auditReturnParameter *(COMMA auditReturnParameter)
auditReturnParameter    = ... / ... / errorDescriptor

So, production   (1) may genarate a "string" (sentencial form) of this kind:

		AuditValueToken EQUAL CtxToken      LBRKT errorDescriptor RBRKT

While production (2) may generate a "string" (sentencial form) of this kind:

        AuditValueToken EQUAL TerminationID LBRKT errorDescriptor RBRKT

Well, if i choose a TerminationID in pathNAME form, that simulate the
CtxToken value, "c" or "context", the two production are the same and it's
not possible to distinguish between them.
Changing AuditValueToken with AuditCapToken, demostrate that production
(3) and (4) may generate the same sequence (are ambiguous, in some cases).

I know NOBODY probably choose TerminationID value = CtxToken value, but
theorically this choice is allowed.

Best regards

Angelo Contardi
[email-masked]
2004-10-13

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