Subject: [Megaco] more problems with AVD-2313 (and draft-boyle-megaco-alerting-03.txt)

From: Troy Cauble <[email-masked]>
Date: Wed, 30 Apr 2003 13:58:27 -0400

Consider the following two parameters.

> ParameterID: ddb (0x0001)
> Type: String
> Possible values: See description
> Description: The Display Data Block contains the message data 
> to be sent to the set. Binary encoding is the binary data itself, 
> while text encoding shall carry the data as a hex string.

> ParameterID: db (0x0001)
> Type: String
> Possible values: See description
> Description: The Data Block contains the message data to be 
> sent to the set. Binary encoding is the binary data itself, 
> while text encoding shall carry the data as a hex string.  

The descriptions of the encondings (and the examples in the document)
conflict with the meaning of "Type: String" as defined
in H.248.1 v1 and v2 which state in Annex A that a "String"
is encoded as an IA5String or UTF8String, and in Annex B that
a "String" MUST use the quotedString form.

In my opinion, the intent of the sections in Annex A and B
that specify the encodings of VALUEs was to create encoding
rules that would allow the translation of VALUEs between
ASN.1 and text forms knowing only the Type (String, etc.) and
not requiring detailed package knowledge.
(At least that was my intent when I helped write those sections.)

This means that packages MUST NOT specify things like it's
a String and in text form it will contain *this* and in
binary form it will contain *that*.  Simply say it's a
String containing *this*; H.248.1 specifies how to
encode for the text and ASN.1 cases.

The parameters in question could remain type "String" (containing
the Hex encoding of binary data), but that would require quotes
in the examples and, more significantly, lead to a silly
ASN.1 encoding of the hex string within IA5String form.

A better match is the type "Octet String" which is defined for ASN.1
in Annex A, but unfortunately ommited in Annex B.  I guess it would
require a (modification/clarification/IG entry?) to Annex B describing
how the "Octet String" Types are encoded for the text form (hex encoding).

Note that Annex B does define Hexadecimal octet coding, but it
1) doesn't tell us where it might be used, and
2) specifies LSB first (!?!).

-troy

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