I found it rather easy to implement when I did it.
Only field c) is redundant, but I found the implementation simpler
when each value was preceded by a length because I already had code
that picked up a length n and then the value as the next n bytes.
It is then easy to check later that the sum of these bytes equals the
value length of the whole compound value.
Bob Herriot
> From ipp-owner at pwg.org Mon Jan 12 13:35:48 1998
> Date: Mon, 12 Jan 1998 14:18:42 -0700
> From: Carl Kugler <kugler at us.ibm.com>
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> To: ipp at pwg.org> Subject: IPP Mail Archive: Re: IPP>MOD add another issue [encoding of CompoundValue]
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>> > Although compoundValue can be made to work, its complexity will lead to
> > bugs. Also its type is determined by looking at all of the tags of
> > values that it contains. This suggests that we should look for a
> > simpler-to-implement option.
> >
> > The most obvious solution is to add two new types text-language and
> > name-language which are the langauge constrained versions of text and
> > name.
> >
> Since we still have 'compound-attribute' in the protocol, couldn't we represent
> 'textWithLanguage' and 'nameWithLanguage' using that mechanism?
>> (The 'textWithLanguage' attribute syntax is a compound attribute syntax, represented as an
> OCTET_STRING consisting of 4 fields: a) a SIGNED-SHORT which is the number of octets in the
> following field b) a value of type natural-language, c) a SIGNED-SHORT which is the number
> of octets in the following field, d) a value of type text. The length of a
> textWithLanguage value SHALL be 4 + the value of field a + the value of field c.).
>> I'm finding all these embedded lengths to be a pain to implement, and it seems redundant with
> the logic we already have for compound-attribute.
>> -Carl Kugler
>> ======================Original message=========================
>> > Re: IPP>MOD add another issue [encoding of CompoundValue]
> >
> > Robert Herriot (Robert.Herriot at Eng.Sun.COM)
> > Tue, 25 Nov 1997 18:54:45 -0800
> >
> > * Messages sorted by: [ date ][ thread ][ subject ][ author ]
> > * Next message: Carl-Uno Manros: "Re: IPP>IPP Last Call - need advance list of issues"
> > * Previous message: Marcia Beaulieu: "IPP> Re: IPP conflict with TLS in DC"
> >
> > I will try to explain the issue by giving more detail.
> >
> > The compoundValue has an integer value which specifies the number of
> > following values that compose the compound value. There are two
> > obvious ways to implement compoundValue in a general way:
> >
> > 1) recurse looking for additional values until the correct number
> > is found or until a non-null attribute name is found or a delimiter
> > tag is found. The latter two conditions are errors. This method
> > works, but is tricky the "nested" values are really at the same
> > level as other values in the protocol.
> >
> > 2) continue picking up values, but make a note that a compoundValue
> > is being built. In this case, there must be a check when a
> > non-null name is encountered, and when a delimiter tag is found
> > for the error of a compoundValue is still being built.
> > At first glance, this seems simpler, but it is easy to forget the
> > checks mentioned above.
> >
> > Although compoundValue can be made to work, its complexity will lead to
> > bugs. Also its type is determined by looking at all of the tags of
> > values that it contains. This suggests that we should look for a
> > simpler-to-implement option.
> >
> > The most obvious solution is to add two new types text-language and
> > name-language which are the langauge constrained versions of text and
> > name. Attributes with text and name values could also have a value of
> > type text-language or name-language. Tom and others have suggested
> > that language and text/name be separated by a single-quote character.
> > It would work, but is not in the spirit of the current protocol which
> > uses lengths instead of delimiting characters. So I suggest the value
> > be <language length> <language string> <text/name string>. The length
> > of the text/name string is length of the value minus ( language-length
> > + 2).
> >
> > This solution is easier to parse because the components are contained
> > with a single value.
> >
> > If we believe that tags are in short supply and that we don't want to
> > allocate two values for such obscure types, we could create a tag type
> > of "typed-octets" where the first byte of the value contains the
> > sub-tag value which in our case would be either text-language or
> > name-language. We could also have 2 bytes for the sub-tag rather than
> > 1.
> >
> > > From hastings at cp10.es.xerox.com Mon Nov 24 10:46:48 1997
> > >
> > > As long as you've re-opened this issue, I'd like to add several
> > > other alternatives into the mix. (A committee is better able to
> > > pick between alternatives, than to design one on the fly).
> > >
> > > On the other hand, it may be better to live with the current scheme
> > > than to try to pick a new one.
> > >
> > > At 19:48 11/21/1997 PST, Robert Herriot wrote:
> > > >
> > > >As I am implementing the CompoundValue, I am finding problems that make
> > > >me think it should be changed. It requires too much special-casing and
> > > >in its current form it will introduce bugs where the value of the
> > > >CompoundValue exceeds the number of remaining attributes for the
> > > >attribute name or attribute group. To avoid those bugs, checks have to
> > > >be made in several places.
> > >
> > > Please explain this problem more.
> > >
> > > >
> > > >I suggest we reexamine the other possible solutions, one simple with
> > > >no room for extension, the other with room for extension.
> > > >
> > > > a) add two new value types: text-language and name-language each of which
> > > > is a single value in the protocol but which consists of 4 subfields:
> > > > a text/name length field, a text/name field, a language length field,
> > > > and a language field, .
> > > >
> > > > b) add a single new type: compound-value which consists of a single value
> > > > in the protocol but which consists of a value-tag field followed by
> > > > any number of groups-of-three subfields. Each group-of-three
> > > > consists of a value tag, a value length and a value. The text-language
> > > > solution of a) is represented by a text-language tag, a text tag, a
> > > > text length, a text value, a natural-language-tag, a natural-language
> > > > length and a natural-language value.
> > > >
> > > >I prefer b) because it offers room for extension and an implementation can
> > > >determine if it supports the compound value by examining the initial
> > > >tag in the compoundValue.
> > >
> > > Here are my additional alternatives:
> > >
> > >
> > > c) Amplify the 'text' and 'name' attribute syntaxes to allow a second
> > > natural language override value to precede the actual value which indicates
> > > the language of the immediately following value. The attribute syntax of
> > > the first value, when present, is: 'naturalLanguage' as defined in the
> > > current spec.
> > >
> > > Advantages: simple
> > >
> > > Disadvantages: a single-valued attribute sometimes has two values, making
> > > the validation of single-valued attributes more adhoc. Also counting
> > > attribute values is more adhoc.
> > >
> > >
> > > d) have two data types for each of 'text' and 'name':
> > > 'text' (same as current) and 'taggedText'
> > > 'name' (same as current) and 'taggedName'
> > >
> > > The 'taggedText' and 'taggedName' data types use the RFC 2184 tagging
> > > in the beginning of the data (but for language only, not charset)
> > > to indicate natural language override:
> > >
> > > en'...
> > > en-us'...
> > >
> > > to indicate English and U.S. English
> > >
> > > Those attributes which currently have 'text' and 'name' would
> > > be changed to require support of both 'text' and 'taggedText'
> > > and 'name' and 'taggedName'
> > >
> > > For example:
> > >
> > > job-name (name | taggedName)
> > >
> > > Advantages: most request/response instances would not need to use the
> > > taggedText and taggedName in most interchanges.
> > >
> > > Disadvantages: clients and IPP objects would still have to support both
> > > forms.
> > >
> > >
> > > e) Change the spec for 'text' and 'name' to always require the RFC 2184
> > > natural language prefix (but not charset).
> > >
> > > Advantages: simple, natural language tag is always stored with the data.
> > > Only one protocol value for each attribute value.
> > >
> > > Disadvantages: tag has to be skipped over when processing or displaying
> > > the data.
> > >
> > >
> > > f) Same as e) except include the charset tag as well, so in full compliance
> > > with RFC 2184 (same as we had in the Model document after the Atlanta
> > > meeting). Example:
> > >
> > > us-ascii'en'...
> > > utf-8'en-us'...
> > >
> > > Advantages: simple, charset and natural language tag is always stored
> > > with the data. Only one protocol value for each attribute value.
> > > IPP object doesn't need to charset convert data to a single charset.
> > >
> > > Disadvantage: tags have to be skipped over when processing or displaying
> > > the data.
> > >
> > >
> > > g) Add the dictionary attribute syntax that we postponed.
> > >
> > > Advantages: It is even more general than your alternative b) and is
> > > something we have agreed is something we want. I'd hate to see us
> > > put in something that is half a dictionary. I think that the dictionary
> > > also fixes the length checking problem that the current CompoundValue
> > > has, correct?
> > >
> > > Disadvantages: None.
> > >
> > > Tom
> > >
> > >
> > >
> > >
> >
> > * Next message: Carl-Uno Manros: "Re: IPP>IPP Last Call - need advance list of issues"
> > * Previous message: Marcia Beaulieu: "IPP> Re: IPP conflict with TLS in DC"
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