APC-1711, 1712 review

John Segers jsegers at LUCENT.COM
Fri Oct 15 20:19:39 EDT 1999


It's good to see that someone had a good look at APC-1711 and 1712.
Below you find some responses to your observations.

I'm x-posting to SG16 list, as the contributions will be discussed in
the Red Bank meeting.


Abdallah Rayhan wrote:
> I took a pass through APC-1711 and APC-1712 and I would like to post
> my analysis
> on the mailing list. I don't claim my analysis to be complete, however
> I tried to point
> out the main problems I came across while reading APC-1711 ABNF. My
> primary
> goal is the final ABNF and not the ASN.1 for two reasons. First, I am
> not an ASN.1
> expert, so I would leave critiquing it to the ASN.1 experts. Second,
> it would
> be a big distraction to the IETF having the MEGACO ABNF at an advanced
> stage with a set of testbenches (ANNEX G). Hence, I will try to focus
> on APC-1711 ABNF
> which I call ABNF1711 and MEGACO/H.248 ABNF which I call ABNF248.
> To start with I have noted that the ABNF1711 does not take the ABNF248
> as is
> but modifies it to suite ASN.1 encoding ( is ASN.1 so restrictive that
> it does not
> allow ABNF248 syntax ? )

In developing an ASN.1 to ABNF translation mechanism, you have to take
into account that the ABNF that is generated must be unambiguous.  One
of the dangers of omitting tags (field names) is introducing ambiguity,
for instance because a string value may happen to be the same as a tag.
If you do an ABNF spec by hand, you can verify that this does not lead
to ambiguity _for that ABNF specification_ but in a mechanical
translation process this is much harder.  That's why the mechanism
proposed in APC-1712 puts restrictions on the use of "AS ?"

>                          These discrepancies are found in: (I started
> noting
> the different  PRs but then found myself marking almost every one.)

Well, one grammar can be specified by different sets of PRs.  So
different sets of PRs I don't find worrying.  Differences in the grammar
is what's important.

> - Tagging every value.
> ABNF1711:
> PkgdName = LBRKT  PkgItemID [ PkgName ]  RBRKT
> PkgName = LBRKT  "/"  ( ( "PE" EQUAL Name )  / "*" ) RBRKT
> PkgItemID = ( ( "PI" EQUAL Name )  /   "*"  )
> ABNF248:
> pkgdName  = [ (PackageName | "*")  SLASH ] (ItemID | "*" )
> PackageName  = NAME
> ItemID = NAME
> You need "PE=" before package name and "PI=" before ItemID.
> These differences appear in many production rules.

Yes, this is true.  But I don't think it is a major change because the
structure isn't changed very much.

> - Extra brackets as illustrated in the previous example. In ABNF1711,
> there is a curly bracket before and after PkgdName while ABNF248 does
> not
> have that. This is all over ABNF1711.

There are indeed many instances where the generated ABNF has more
braces.  This is because braces are used to delimit sequences of
parameters, consistently throughout the entire syntax.  A packaged name
is a sequence of two parameters: package name and item name.  Hence it
is surrounded by braces.

> - Extensions do not appear any where in ABNF1711 while they are a
> major
> feature of ABNF248.

The ASN.1 has extension markers (the ellipses).  These are not
translated into ABNF, because the indicate that optional parameters may
be added.  If it is important, every extension marker in the ASN.1 of
APC-1711 can be preceded by a sequence to produce the extension fields
in the ABNF.

> -ABNF1711 changes the syntax of the MEGACO constructs.
> ABNF1711:
>  RequestedActions = LBRKT RequestedAction [ "E" EQUAL
> SecondEventsDescriptor ]
>     [ "SG" EQUAL SignalsDescriptor ]  [ "IC" ]   RBRKT
> ABNF248:
> requestedActions     = requestedAction LWSP [COLON LWSP
> embeddedSignalEvents ] [COLON LWSP InterceptToken ]
> embeddedSignalEvents = firstEmbedding [COMMA firstEmbedding]
> as you can see the first level of embedding (firstEmbedding) is
> chopped in ABNF1711.
> As a matter of fact, it changes the syntax into something different.

There's no more production for firstEmbedding, but in ABNF248, the
syntax continues:
firstEmbedding = (secondEvent | signalsDescriptor)
secondEvent = EventsDescriptorToken EQUAL [...]
signalsDescriptor = SignalsDescriptorToken [...]

Noting that the EventsDescriptorToken is an "E" and that the
SignalsDescriptorToken is the string "SG", you see that the differences
are very minor.  ABNF1711 does not use colons for separators, but I
think that's not important.

> -ABNF1711 enforces ordering of descriptors and parameters while the
> ABNF248
> does not. This issue was raised before and the resolution was made to
> enforce no order
> since there is no relationship between descriptors, e.g., modems or
> muxes, whichever comes
> first does not affect the other.

True.  This is not a problem of the approach per se, but my personal
stubborness.  ABNF1711 enforces ordering, but the syntax specifies that
every descriptor can only occurs once in a list, while ABNF248 puts that
in a comment (i.e., specifies it as a semantic constraint).  It's a
small operation to write ASN.1 that will produce ABNF that contains
"descriptor *(COMMA descriptor)" constructs instead of the
"[descr1][descr2][descr3]" constructs.

> - ABNF248 uses COMMA as a separator for commands, descriptors
> and parameters. This concept is gone in ABNF1711.

If we rewrite as explained above, you'll have your commas back.

> - ABNF1711 changes the syntax and the semantics of MEGACO, e.g.,
> ABNF1711:
> NotifyReply = LBRKT TerminationID  [ "ER" EQUAL ErrorDescriptor ]
> You always have TerminationID regardless of whether it is NotifyReply
> error or not,
> and ErrorDescriptor is tagged.
> ABNF248
> notifyReply       = NotifyToken ( commandError | [EQUAL
> TerminationID])
> commandError = EQUAL TerminationID LBRKT errorDescriptor RBRKT
> TerminationID is optional in pure notify reply and mandatory in notify
> error.
> ErrorDescriptor is not tagged.

I explained about the tagging above, so let that go.  The optional
TerminationID presented a bit of a problem for the ASN.1.  Because I
couldn't for the life of me remember why it's important to be able to
leave TerminationIDs out in NotifyReply, I made them mandatory.

> Also, ABNF1711:
> SecurityParmIndex = ( 1( BASE64 ) BASE64END )
> SequenceNum = ( 1( BASE64 ) BASE64END )
> AuthData = ( 5( BASE64 ) BASE64END )
> BASE64    = 4( BASE64CHAR )
> BASE64END  = 4( BASE64CHAR ) / ( 3( BASE64CHAR ) "?" ) / ( 2(
> BASE64CHAR ) "??" )
> Where  did "?" and "??" come from ?

They ensure that the number of characters in the BASE64 encoded octet
string is a multiple of 4.  I was told that BASE64 works that way.

> ABNF248:
> SecurityParmIndex    = 8(HEXDIG)
> SequenceNum          = 8(HEXDIG)
> AuthData             = 32*64(HEXDIG)
> These are some of the differences. If we review each PR, no PR in
> ABNF1711
> will be saved. So I will stop here.

Once again, the PRs aren't the most important, the differences in
grammar are.

> I dont know if the kind of constraints ABNF1711 enforces on the ABNF
> are due to
> to the ASN.1 encoding constraints or because of ASN.12ABNF.
> Whatever the reasoning is, APC1711 should focus on completing the
> ASN.1
> description and generate similar testbenches as in the MEGACO I-D
> either
> binary or text, or both. The binary will be definitely different than
> the text because of
> tagging, however the text should be same as the test benches written
> so far.
> In theory, bin and text should reflect the same protocol and should
> introduce the least
> entropy into the syntax.
> My recommendation is that rather than writing ASN.12ABNF tool, APC1711
> should write examples that use the ASN.1 and bin2text and text2bin
> tools in order to
> test the ASN.1 encoding. Once that is out of the way and we know that
> the ASN.1 is ok,
> we could look into tools like ASN.12ABNF presuming that it would
> generate ABNF
> that is equivalent to ABNF248 syntactical and semantical wise.
> Otherwise, ABNF1711
> approach is unproductive and waste of effort trying to verify every PR
> of ABNF1711.

I agree that examples are useful, but this comment is counter

> In addition to that, if we assume that the ASN.1 is ok and the
> ASN.12ABNF is
> ok, from what I saw both are not,

Well, if you define ABNF248 as being right and anything that's different
as being wrong, then obviously your right, right?  Otherwise, your're
wrong, right?

As far as I can tell, the ASN.1 is ok, but it definitely needs review.
I'm glad you took the effort to be the first to comment. And I'm glad
that you didn't find any semantic errors (barring the mandatory
TerminationID, which I don't consider an error but a difference).

>                                   is there any guarantee that when
> they change
> the ASN.1, that will not introduce problems into the ABNF ? My feeling
> is that
> it would be a source of confusion and we would need to review the ABNF
> and
> probably hand craft some PRs to make it consistent and ambiguous free.
> If that is that is the case, why should we throw out ABNF248 which
> we have tested and have a high degree of confidence in. We are very
> familiar
> with ABNF248; we know the ins and outs of it; and we know how to fix
> any problem
> without disturbing the rest of the grammar. How would ABNF1711 do that
> ?

Using the mechanical translation mechanism gives me a higher degree of
confidence.  I don't have to worry anymore about having accidentally
omitted a closing brace, or being inconsistent with the use of

> As for ASN.1, it is not compatible with the standard. This means that
> ASN.1 compilers
> can not produce a correct code.

APC-1712 addresses this.  The extensions that were added to deal with
the different types of separators and brackets in ABNF248 can be put
into special comments in the ASN.1.  They take the form "--& <extension>
--"  A tool that does the text encoding as described, recognizes these
comments, a normal ASN.1 compiler sees them as comments, ignores them
and produces code (correctness is up to the compiler writer :-).

> regards
> Abdallah Rayhan

John Segers                                  email: jsegers at lucent.com
Lucent Technologies                                        Room HE 306
Dept. Forward Looking Work                      phone: +31 35 687 4724
P.O. Box 18, 1270 AA  Huizen                      fax: +31 35 687 5954

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