Network Working Group                                            S. Legg
Request for Comments: 4792                                       eB2Bcom
Updates: 3641                                               January 2007
Category: Standards Track

Encoding Instructions for the
Generic String Encoding Rules (GSER)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2007).

Abstract

   Abstract Syntax Notation One (ASN.1) defines a general framework for
   annotating types in an ASN.1 specification with encoding instructions
   that alter how values of those types are encoded according to ASN.1
   encoding rules.  This document defines the supporting notation for
   encoding instructions that apply to the Generic String Encoding Rules
   (GSER) and, in particular, defines an encoding instruction to provide
   a machine-processable representation for the declaration of a GSER
   ChoiceOfStrings type.

   1. Introduction ....................................................2
   2. Conventions .....................................................2
   3. Notation for GSER Encoding Instructions .........................2
   4. The CHOICE-OF-STRINGS Encoding Instruction ......................3
      4.1. Effect on GSER Encodings ...................................5
      4.2. Replacement of Existing ChoiceOfStrings Declarations .......6
   5. Security Considerations .........................................7
   6. Normative References ............................................7

1. Introduction

   Abstract Syntax Notation One (ASN.1) [X.680] defines a general
   framework for annotating types in an ASN.1 specification with
   encoding instructions [X.680-1] that alter how values of those types
   are encoded according to ASN.1 encoding rules.  This document defines
   the supporting notation for encoding instructions that apply to the
   Generic String Encoding Rules (GSER) [GSER], and in particular
   defines an encoding instruction, the CHOICE-OF-STRINGS encoding
   instruction, to provide a machine-processable representation for the
   declaration of a GSER ChoiceOfStrings type.

   The CHOICE-OF-STRINGS encoding instruction SHOULD be used instead of
   simply declaring a ChoiceOfStrings type.

2. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [BCP14].

   Throughout this document, "type" shall be taken to mean an ASN.1
   type, and "value" shall be taken to mean an ASN.1 abstract value,
   unless qualified otherwise.

   A reference to an ASN.1 production [X.680] (e.g., Type, NamedType) is
   a reference to text in an ASN.1 specification corresponding to that
   production.

3. Notation for GSER Encoding Instructions

   The grammar of ASN.1 permits the application of encoding instructions
   [X.680-1], through type prefixes and encoding control sections, that
   modify how abstract values are encoded by nominated encoding rules.

   The generic notation for type prefixes and encoding control sections
   is defined by the ASN.1 basic notation [X.680] [X.680-1], and
   includes an encoding reference to identify the specific encoding
   rules that are affected by the encoding instruction.

   The encoding reference that identifies the Generic String Encoding
   Rules is literally GSER.

   The specific notation for an encoding instruction for a particular
   set of encoding rules is left to the specification of those encoding
   rules.  Consequently, this companion document to the GSER
   specification [GSER] defines the notation for GSER encoding

   instructions.  Specifically, it elaborates the EncodingInstruction
   and EncodingInstructionAssignmentList placeholder productions of the
   ASN.1 basic notation.

   In the context of the GSER encoding reference the EncodingInstruction
   production is defined as follows, using the conventions of the ASN.1
   basic notation:

      EncodingInstruction ::=
          ChoiceOfStringsInstruction

   In the context of the GSER encoding reference the
   EncodingInstructionAssignmentList production (which only appears in
   an encoding control section) is empty:

      EncodingInstructionAssignmentList ::= empty

4. The CHOICE-OF-STRINGS Encoding Instruction

   The CHOICE-OF-STRINGS encoding instruction allows a GSER encoder to
   encode the alternative of a CHOICE (of restricted string types)
   without the leading identifier.  The optional PrecedenceList also
   allows a specification writer to alter the order in which a GSER
   decoder will consider the alternatives of the CHOICE as it determines
   which alternative has been encoded when the identifier is absent.

   The notation for a CHOICE-OF-STRINGS encoding instruction is defined
   as follows:

      UnionInstruction ::= "CHOICE-OF-STRINGS" AlternativesPrecedence ?

      AlternativesPrecedence ::= "PRECEDENCE" PrecedenceList

      PrecedenceList ::= identifier PrecedenceList ?

   The Type in the EncodingPrefixedType for a CHOICE-OF-STRINGS encoding
   instruction SHALL be:

   (a) a BuiltinType that is a ChoiceType, or

   (b) a ConstrainedType that is not a TypeWithConstraint where the Type
       in the ConstrainedType is one of (a) to (d), or

   (c) a BuiltinType that is a PrefixedType that is a TaggedType where
       the Type in the TaggedType is one of (a) to (d), or

   (d) a BuiltinType that is a PrefixedType that is an
       EncodingPrefixedType where the Type in the EncodingPrefixedType
       is one of (a) to (d).

   The effect of this condition is to force the CHOICE-OF-STRINGS
   encoding instruction to be textually co-located with the CHOICE type
   definition to which it applies.  This makes it clear to a reader that
   the encoding instruction applies to every use of the CHOICE type no
   matter how it might be referenced.

   The ChoiceType in case (a) is said to be "subject to" the CHOICE-OF-
   STRINGS encoding instruction.

   The Type of each NamedType of the ChoiceType in case (a) MUST be:

   (1) the NumericString, PrintableString, TeletexString (T61String),
       VideotexString, IA5String, GraphicString, VisibleString
       (ISO646String), GeneralString, BMPString, UniversalString, or
       UTF8String type, or

   (2) a type notation that references a type that is one of (1) to (4),
       or

   (3) a constrained type where the type that is constrained is one of
       (1) to (4), or

   (4) a prefixed type where the type that is prefixed is one of (1) to
       (4).

      ASIDE: A tagged type is a special case of a prefixed type.  An
      effect of case (4) is that tagging is not significant.

   The ASN.1 restricted string type in case (1) MUST be different for
   each NamedType in the ChoiceType, i.e., no two alternatives have the
   same restricted string type.

   If case (3) applies to any NamedType, then the constraint in case (3)
   MUST be the same for each NamedType, i.e., either none of the
   alternatives has a constraint, or all of the alternatives have
   exactly the same constraint.

   Each identifier in the PrecedenceList MUST be the identifier of a
   NamedType of the ChoiceType.

   A particular identifier SHALL NOT appear more than once in the same
   PrecedenceList.

4.1. Effect on GSER Encodings

   A value of a CHOICE type is encoded according to the <ChoiceValue>
   [GSER] Augmented Backus-Naur Form [ABNF] rule.  The ABNF for
   <ChoiceValue> is reproduced here for convenience:

      ChoiceValue           = IdentifiedChoiceValue /
                              ChoiceOfStringsValue

      IdentifiedChoiceValue = identifier ":" Value
      ChoiceOfStringsValue  = StringValue

   The <IdentifiedChoiceValue> rule MUST be used to encode values of a
   CHOICE type where the ChoiceType is not subject to a CHOICE-OF-
   STRINGS encoding instruction.

   The chosen alternative of a value of a CHOICE type corresponds to
   some NamedType in the definition of the type.  The <identifier> in
   the <IdentifiedChoiceValue> is the identifier of this NamedType.

   Either the <IdentifiedChoiceValue> rule or the <ChoiceOfStringsValue>
   rule is used to encode values of a CHOICE type where the ChoiceType
   is subject to a CHOICE-OF-STRINGS encoding instruction.

   If <ChoiceOfStringsValue> has been used, then a GSER decoder MUST
   determine the chosen alternative by considering the alternatives of
   the CHOICE in the order prescribed below and accepting the first
   alternative that allows all of the characters in the <StringValue>.

   If the CHOICE-OF-STRINGS encoding instruction has a PrecedenceList,
   then the alternatives of the ChoiceType referenced by the
   PrecedenceList are considered in the order identified by that
   PrecedenceList, and then the remaining alternatives are considered in
   the order of their definition in the ChoiceType.  If the CHOICE-OF-
   STRINGS encoding instruction does not have a PrecedenceList, then all
   the alternatives of the ChoiceType are considered in the order of
   their definition in the ChoiceType.

   A GSER encoder MUST use <IdentifiedChoiceValue> if a GSER decoder
   would determine the chosen alternative to be something other than the
   chosen alternative of the CHOICE value being encoded; otherwise,
   <ChoiceOfStringsValue> MAY be used.

   Example

      Consider this type definition:

         [GSER:CHOICE-OF-STRINGS PRECEDENCE basicName] CHOICE {
             extendedName  UTF8String,
             basicName     PrintableString
         }

      If a <ChoiceOfStringsValue> has been used, then a GSER decoder
      would first consider whether the <StringValue> was a valid
      basicName (a PrintableString) before considering whether it was a
      valid extendedName (a UTF8String).

4.2. Replacement of Existing ChoiceOfStrings Declarations

   In line with the previous declaration [GSER] of the DirectoryString
   type as a ChoiceOfStrings type, applications using GSER MUST add this
   encoding instruction:

      [GSER:CHOICE-OF-STRINGS PRECEDENCE printableString uTF8String]

   immediately before the "CHOICE" keyword in the definition of the
   DirectoryString type in the third and every subsequent edition of the
   SelectedAttributeTypes ASN.1 module of X.520 [X.520-3] [X.520-4]
   [X.520-5].

   For example, this is how the DirectoryString definition would appear
   in the third, fourth and fifth editions:

      DirectoryString{INTEGER:maxSize} ::=
      [GSER:CHOICE-OF-STRINGS PRECEDENCE printableString uTF8String]
      CHOICE {
          teletexString     TeletexString(SIZE (1..maxSize)),
          printableString   PrintableString(SIZE (1..maxSize)),
          universalString   UniversalString(SIZE (1..maxSize)),
          bmpString         BMPString(SIZE (1..maxSize)),
          uTF8String        UTF8String(SIZE (1..maxSize))
      }

   The uTF8String alternative did not appear in the second edition of
   the SelectedAttributeTypes ASN.1 module of X.520 [X.520-2].  For
   compatibility, applications using GSER with the second edition of
   X.520 MUST add this encoding instruction:

      [GSER:CHOICE-OF-STRINGS PRECEDENCE printableString]

   immediately before the "CHOICE" keyword in the definition of the
   DirectoryString type.

   For example, this is how the DirectoryString definition would appear
   in the second edition:

      DirectoryString{INTEGER:maxSize} ::=
      [GSER:CHOICE-OF-STRINGS PRECEDENCE printableString]
      CHOICE {
          teletexString     TeletexString(SIZE (1..maxSize)),
          printableString   PrintableString(SIZE (1..maxSize)),
          universalString   UniversalString(SIZE (1..maxSize))
      }

5. Security Considerations

   This specification changes the manner in which ChoiceOfStrings types
   are declared but does not alter the existing behaviour of GSER
   implementations.  The security considerations for GSER are unchanged
   (see [GSER]).

6. Normative References

   [BCP14]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [GSER]     Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
              Types", RFC 3641, October 2003.

   [ABNF]     Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", RFC 4234, October 2005.

   [X.520-2]  ITU-T Recommendation X.520 (1993) | ISO/IEC 9594-6:1994,
              Information Technology - Open Systems Interconnection -
              The Directory: Selected attribute types

   [X.520-3]  ITU-T Recommendation X.520 (08/97) | ISO/IEC 9594-6:1998,
              Information Technology - Open Systems Interconnection -
              The Directory: Selected attribute types

   [X.520-4]  ITU-T Recommendation X.520 (02/01) | ISO/IEC 9594-6:2001,
              Information technology - Open Systems Interconnection -
              The Directory: Selected attribute types

   [X.520-5]  ITU-T Recommendation X.520 (08/05) | ISO/IEC 9594-6:2005,
              Information technology - Open Systems Interconnection -
              The Directory: Selected attribute types

   [X.680]    ITU-T Recommendation X.680 (07/02) | ISO/IEC 8824-1,
              Information technology - Abstract Syntax Notation One
              (ASN.1): Specification of basic notation.

   [X.680-1]  ITU-T Recommendation X.680 (2002) Amendment 1 (10/03) |
              ISO/IEC 8824-1:2002/Amd 1:2004, Support for EXTENDED-XER.

Author's Address

   Dr. Steven Legg
   eB2Bcom
   Suite 3, Woodhouse Corporate Centre
   935 Station Street
   Box Hill North, Victoria 3129
   AUSTRALIA

   Phone: +61 3 9896 7830
   Fax:   +61 3 9896 7801
   EMail: steven.legg@eb2bcom.com

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