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rfc:rfc4910

Network Working Group S. Legg Request for Comments: 4910 eB2Bcom Category: Experimental D. Prager

                                                             July 2007
                Robust XML Encoding Rules (RXER) for
                Abstract Syntax Notation One (ASN.1)

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

Abstract

 This document defines a set of Abstract Syntax Notation One (ASN.1)
 encoding rules, called the Robust XML Encoding Rules or RXER, that
 produce an Extensible Markup Language (XML) representation for values
 of any given ASN.1 data type.  Rules for producing a canonical RXER
 encoding are also defined.

Legg & Prager Experimental [Page 1] RFC 4910 Robust XML Encoding Rules July 2007

Table of Contents

 1. Introduction ....................................................3
 2. Conventions .....................................................4
 3. Definitions .....................................................5
 4. Additional Basic Types ..........................................6
    4.1. The Markup Type ............................................6
         4.1.1. Self-Containment ....................................9
         4.1.2. Normalization for Canonical Encoding Rules .........12
    4.2. The AnyURI Type ...........................................13
    4.3. The NCName Type ...........................................14
    4.4. The Name Type .............................................14
    4.5. The QName Type ............................................14
 5. Expanded Names for ASN.1 Types .................................15
 6. Encoding Rules .................................................17
    6.1. Identifiers ...............................................19
    6.2. Component Encodings .......................................20
         6.2.1. Referenced Components ..............................20
         6.2.2. Element Components .................................20
                6.2.2.1. Namespace Properties for Elements .........22
                6.2.2.2. Namespace Prefixes for Element Names ......24
         6.2.3. Attribute Components ...............................25
                6.2.3.1. Namespace Prefixes for Attribute Names ....26
         6.2.4. Unencapsulated Components ..........................26
         6.2.5. Examples ...........................................27
    6.3. Standalone Encodings ......................................28
    6.4. Embedded ASN.1 Values .....................................28
    6.5. Type Referencing Notations ................................32
    6.6. TypeWithConstraint, SEQUENCE OF Type, and SET OF Type .....33
    6.7. Character Data Translations ...............................34
         6.7.1. Restricted Character String Types ..................35
         6.7.2. BIT STRING .........................................36
         6.7.3. BOOLEAN ............................................38
         6.7.4. ENUMERATED .........................................38
         6.7.5. GeneralizedTime ....................................39
         6.7.6. INTEGER ............................................41
         6.7.7. NULL ...............................................42
         6.7.8. ObjectDescriptor ...................................43
         6.7.9. OBJECT IDENTIFIER and RELATIVE-OID .................43
         6.7.10. OCTET STRING ......................................43
         6.7.11. QName .............................................44
                6.7.11.1. Namespace Prefixes for Qualified Names ...44
         6.7.12. REAL ..............................................45
         6.7.13. UTCTime ...........................................46
         6.7.14. CHOICE as UNION ...................................47
         6.7.15. SEQUENCE OF as LIST ...............................50
    6.8. Combining Types ...........................................50
         6.8.1. CHARACTER STRING ...................................51

Legg & Prager Experimental [Page 2] RFC 4910 Robust XML Encoding Rules July 2007

         6.8.2. CHOICE .............................................51
         6.8.3. EMBEDDED PDV .......................................52
         6.8.4. EXTERNAL ...........................................52
         6.8.5. INSTANCE OF ........................................52
         6.8.6. SEQUENCE and SET ...................................52
         6.8.7. SEQUENCE OF and SET OF .............................54
         6.8.8. Extensible Combining Types .........................55
                6.8.8.1. Unknown Elements in Extensions ............55
                6.8.8.2. Unknown Attributes in Extensions ..........59
    6.9. Open Type .................................................60
    6.10. Markup ...................................................61
    6.11. Namespace Prefixes for CRXER .............................63
    6.12. Serialization ............................................65
         6.12.1. Non-Canonical Serialization .......................65
         6.12.2. Canonical Serialization ...........................68
         6.12.3. Unicode Normalization in XML Version 1.1 ..........70
    6.13. Syntax-Based Canonicalization ............................70
 7. Transfer Syntax Identifiers ....................................71
    7.1. RXER Transfer Syntax ......................................71
    7.2. CRXER Transfer Syntax .....................................71
 8. Relationship to XER ............................................71
 9. Security Considerations ........................................73
 10. Acknowledgements ..............................................74
 11. IANA Considerations ...........................................75
 12. References ....................................................75
    12.1. Normative References .....................................75
    12.2. Informative References ...................................77
 Appendix A. Additional Basic Definitions Module ...................78

1. Introduction

 This document defines a set of Abstract Syntax Notation One (ASN.1)
 [X.680] encoding rules, called the Robust XML Encoding Rules or RXER,
 that produce an Extensible Markup Language (XML) [XML10][XML11]
 representation of ASN.1 values of any given ASN.1 type.
 An ASN.1 value is regarded as analogous to the content and attributes
 of an XML element, or in some cases, just an XML attribute value.
 The RXER encoding of an ASN.1 value is the well-formed and valid
 content and attributes of an element, or an attribute value, in an
 XML document [XML10][XML11] conforming to XML namespaces
 [XMLNS10][XMLNS11].  Simple ASN.1 data types such as PrintableString,
 INTEGER, and BOOLEAN define character data content or attribute
 values, while the ASN.1 combining types (i.e., SET, SEQUENCE, SET OF,
 SEQUENCE OF, and CHOICE) define element content and attributes.  The
 attribute and child element names are generally provided by the
 identifiers of the components in combining type definitions, i.e.,
 elements and attributes correspond to the NamedType notation.

Legg & Prager Experimental [Page 3] RFC 4910 Robust XML Encoding Rules July 2007

 RXER leaves some formatting details to the discretion of the encoder,
 so there is not a single unique RXER encoding for an ASN.1 value.
 However, this document also defines a restriction of RXER, called the
 Canonical Robust XML Encoding Rules (CRXER), which does produce a
 single unique encoding for an ASN.1 value.  Obviously, the CRXER
 encoding of a value is also a valid RXER encoding of that value.  The
 restrictions on RXER to produce the CRXER encoding are interspersed
 with the description of the rules for RXER.
 Note that "ASN.1 value" does not mean a Basic Encoding Rules (BER)
 [X.690] encoding.  The ASN.1 value is an abstract concept that is
 independent of any particular encoding.  BER is just one possible way
 to encode an ASN.1 value.  This document defines an alternative way
 to encode an ASN.1 value.
 A separate document [RXEREI] defines encoding instructions [X.680-1]
 that may be used in an ASN.1 specification to modify how values are
 encoded in RXER, for example, to encode a component of a combining
 ASN.1 type as an attribute rather than as a child element.  A
 pre-existing ASN.1 specification will not have RXER encoding
 instructions, so any mention of encoding instructions in this
 document can be ignored when dealing with such specifications.
 Encoding instructions for other encoding rules have no effect on RXER
 encodings.

2. Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document are
 to be interpreted as described in BCP 14, RFC 2119 [BCP14].  The key
 word "OPTIONAL" is exclusively used with its ASN.1 meaning.
 A reference to an ASN.1 production [X.680] (e.g., Type, NamedType) is
 a reference to the text in an ASN.1 specification corresponding to
 that production.
 The specification of RXER makes use of definitions from the XML
 Information Set (Infoset) [INFOSET].  In particular, information item
 property names follow the Infoset convention of being shown in square
 brackets, e.g., [local name].  Literal values of Infoset properties
 are enclosed in double quotes; however, the double quotes are not
 part of the property values.  In the sections that follow,
 "information item" will be abbreviated to "item", e.g., "element
 information item" is abbreviated to "element item".  The term
 "element" or "attribute" (without the "item") is referring to an
 element or attribute in an XML document, rather than an information
 item.

Legg & Prager Experimental [Page 4] RFC 4910 Robust XML Encoding Rules July 2007

 Literal character strings to be used in an RXER encoding appear
 within double quotes; however, the double quotes are not part of the
 literal value and do not appear in the encoding.
 This document uses the namespace prefix [XMLNS10][XMLNS11] "asnx:" to
 stand for the namespace name "urn:ietf:params:xml:ns:asnx", uses the
 namespace prefix "xs:" to stand for the namespace name
 "http://www.w3.org/2001/XMLSchema", and uses the namespace prefix
 "xsi:" to stand for the namespace name
 "http://www.w3.org/2001/XMLSchema-instance".  However, in practice,
 any valid namespace prefixes are permitted in non-canonical RXER
 encodings (namespace prefixes are deterministically generated for
 CRXER).
 The encoding instructions [X.680-1] referenced by name in this
 specification are encoding instructions for RXER [RXEREI].
 Throughout this document, references to the Markup, AnyURI, NCName,
 Name, and QName ASN.1 types are references to the types described in
 Section 4 and consolidated in the AdditionalBasicDefinitions module
 in Appendix A.  Any provisions associated with the reference do not
 apply to types defined in other ASN.1 modules that happen to have
 these same names.
 Code points for characters [UCS][UNICODE] are expressed using the
 Unicode convention U+n, where n is four to six hexadecimal digits,
 e.g., the space character is U+0020.

3. Definitions

 Definition (white space character): A white space character is a
 space (U+0020), tab (U+0009), carriage return (U+000D), or line feed
 (U+000A) character.
 Definition (white space):  White space is a sequence of one or more
 white space characters.
 Definition (line break):  A line break is any sequence of characters
 that is normalized to a line feed by XML End-of-Line Handling
 [XML10][XML11].
 Definition (serialized white space): Serialized white space is a
 sequence of one or more white space characters and/or line breaks.
 Definition (declaring the default namespace):  A namespace
 declaration attribute item is declaring the default namespace if the
 [prefix] of the attribute item has no value, the [local name] of the
 attribute item is "xmlns" and the [normalized value] is not empty.

Legg & Prager Experimental [Page 5] RFC 4910 Robust XML Encoding Rules July 2007

 Definition (undeclaring the default namespace):  A namespace
 declaration attribute item is undeclaring the default namespace if
 the [prefix] of the attribute item has no value, the [local name] of
 the attribute item is "xmlns" and the [normalized value] is empty
 (i.e., xmlns="").
 Definition (canonical namespace prefix): A canonical namespace prefix
 is an NCName [XMLNS10] beginning with the letter 'n' (U+006E)
 followed by a non-negative number string.  A non-negative number
 string is either the digit character '0' (U+0030), or a non-zero
 decimal digit character (U+0031-U+0039) followed by zero, one, or
 more of the decimal digit characters '0' to '9' (U+0030-U+0039).
 For convenience, a CHOICE type where the ChoiceType is subject to a
 UNION encoding instruction will be referred to as a UNION type, and a
 SEQUENCE OF type where the SequenceOfType is subject to a LIST
 encoding instruction will be referred to as a LIST type.

4. Additional Basic Types

 This section defines an ASN.1 type for representing markup in
 abstract values, as well as basic types that are useful in encoding
 instructions [RXEREI] and other related specifications [ASN.X].
 The ASN.1 definitions in this section are consolidated in the
 AdditionalBasicDefinitions ASN.1 module in Appendix A.

4.1. The Markup Type

 A value of the Markup ASN.1 type holds the [prefix], [attributes],
 [namespace attributes], and [children] of an element item, i.e., the
 content and attributes of an element.
 RXER has special provisions for encoding values of the Markup type
 (see Section 6.10).  For other encoding rules, a value of the Markup
 type is encoded according to the following ASN.1 type definition
 (with AUTOMATIC TAGS):
    Markup ::= CHOICE {
        text    SEQUENCE {
            prolog      UTF8String (SIZE(1..MAX)) OPTIONAL,
            prefix      NCName OPTIONAL,
            attributes  UTF8String (SIZE(1..MAX)) OPTIONAL,
            content     UTF8String (SIZE(1..MAX)) OPTIONAL
        }
    }

Legg & Prager Experimental [Page 6] RFC 4910 Robust XML Encoding Rules July 2007

 The text alternative of the Markup CHOICE type provides for the
 [prefix], [attributes], [namespace attributes], and [children] of an
 element item to be represented as serialized XML using the UTF-8
 character encoding [UTF-8].
    Aside: The CHOICE allows for one or more alternative compact
    representations of the content and attributes of an element to be
    supported in a future specification.
 With respect to some element item whose content and attributes are
 represented by a value of the text alternative of the Markup type:
 (1) the prolog component of the value contains text that, after line
     break normalization, conforms to the XML prolog production
     [XML10][XML11],
 (2) the prefix component is absent if the [prefix] of the element
     item has no value; otherwise, the prefix component contains the
     [prefix] of the element item,
 (3) the attributes component of the value contains an XML
     serialization of the [attributes] and [namespace attributes] of
     the element item, if any, with each attribute separated from the
     next by serialized white space, and
 (4) the content component is absent if the [children] property of the
     element item is empty; otherwise, the content component of the
     value contains an XML serialization of the [children] of the
     element item.
 All the components of a value of the Markup type MUST use the same
 version of XML, either version 1.0 [XML10] or version 1.1 [XML11].
 If XML version 1.1 is used, then the prolog component MUST be present
 and MUST have an XMLDecl for version 1.1.  If the prolog component is
 absent, then XML version 1.0 is assumed.
 If the prefix component is present, then there MUST be a namespace
 declaration attribute in the attributes component that defines that
 namespace prefix (since an element whose content and attributes are
 described by a value of Markup is required to be self-contained; see
 Section 4.1.1).
 Note that the prefix component is critically related to the NamedType
 that has Markup as its type.  If a Markup value is extracted from one
 enclosing abstract value and embedded in another enclosing abstract
 value (i.e., becomes associated with a different NamedType), then the
 prefix may no longer be appropriate, in which case it will need to be
 revised.  It may also be necessary to add another namespace

Legg & Prager Experimental [Page 7] RFC 4910 Robust XML Encoding Rules July 2007

 declaration attribute to the attributes component so as to declare a
 new namespace prefix.
 Leading and/or trailing serialized white space is permitted in the
 attributes component.  A value of the attributes component consisting
 only of serialized white space (i.e., no actual attributes) is
 permitted.
 The attributes and content components MAY contain entity references
 [XML10][XML11].  If any entity references are used (other than
 references to the predefined entities), then the prolog component
 MUST be present and MUST contain entity declarations for those
 entities in the internal or external subset of the document type
 definition.
 Example
    Given the following ASN.1 module:
       MyModule DEFINITIONS
       AUTOMATIC TAGS ::= BEGIN
       Message ::= SEQUENCE {
           messageType   INTEGER,
           messageValue  Markup
       }
       ENCODING-CONTROL RXER
           TARGET-NAMESPACE "http://example.com/ns/MyModule"
           COMPONENT message Message
               -- a top-level NamedType
       END
    consider the following XML document:
       <?xml version='1.0'?>
       <!DOCTYPE message [
           <!ENTITY TRUE 'true'>
       ]>
       <message>
        <messageType>1</messageType>
        <messageValue xmlns:ns="http://www.example.com/ABD"
                      ns:foo="1" bar="0">
         <this>&TRUE;</this>
         <that/>

Legg & Prager Experimental [Page 8] RFC 4910 Robust XML Encoding Rules July 2007

        </messageValue>
       </message>
    A Markup value corresponding to the content and attributes of the
    <messageValue> element is, in ASN.1 value notation [X.680] (where
    "lf" represents the line feed character):
       text:{
           prolog     { "<?xml version='1.0'?>", lf,
                        "<!DOCTYPE message [", lf,
                        "    <!ENTITY TRUE 'true'>", lf,
                        "]>", lf },
           attributes { " xmlns:ns=""http://www.example.com/ABD""",
                        lf,
                        "               ns:foo=""1"" bar=""0""" },
           content    { lf,
                        "  <this>&TRUE;</this>", lf,
                        "  <that/>", lf, " " }
       }
    The following Markup value is an equivalent representation of the
    content and attributes of the <messageValue> element:
       text:{
           attributes {
                        "bar=""0"" ns:foo=""1"" ",
                        "xmlns:ns=""http://www.example.com/ABD""" },
           content    { lf,
                        "  <this>true</this>", lf,
                        "  <that/>", lf, " " }
       }
 By itself, the Markup ASN.1 type imposes no data type restriction on
 the markup contained by its values and is therefore analogous to the
 XML Schema anyType [XSD1].
 There is no ASN.1 basic notation that can directly impose the
 constraint that the markup represented by a value of the Markup type
 must conform to the markup allowed by a specific type definition.
 However, certain encoding instructions (i.e., the reference encoding
 instructions [RXEREI]) have been defined to have this effect.

4.1.1. Self-Containment

 An element, its attributes and its content, including descendent
 elements, may contain qualified names [XMLNS10][XMLNS11] as the names
 of elements and attributes, in the values of attributes, and as
 character data content of elements.  The binding between namespace

Legg & Prager Experimental [Page 9] RFC 4910 Robust XML Encoding Rules July 2007

 prefix and namespace name for these qualified names is potentially
 determined by the namespace declaration attributes of ancestor
 elements (which in the Infoset representation are inherited as
 namespace items in the [in-scope namespaces]).
 In the absence of complete knowledge of the data type of an element
 item whose content and attributes are described by a value of the
 Markup type, it is not possible to determine with absolute certainty
 which of the namespace items inherited from the [in-scope namespaces]
 of the [parent] element item are significant in interpreting the
 Markup value.  The safe and easy option would be to assume that all
 the namespace items from the [in-scope namespaces] of the [parent]
 element item are significant and need to be retained within the
 Markup value.  When the Markup value is re-encoded, any of the
 retained namespace items that do not appear in the
 [in-scope namespaces] of the enclosing element item in the new
 encoding could be made to appear by outputting corresponding
 namespace declaration attribute items in the [namespace attributes]
 of the enclosing element item.
 From the perspective of the receiver of the new encoding, this
 enlarges the set of attribute items in the [namespace attributes]
 represented by the Markup value.
 In addition, there is no guarantee that the sender of the new
 encoding has recreated the original namespace declaration attributes
 on the ancestor elements, so the [in-scope namespaces] of the
 enclosing element item is likely to have new namespace declarations
 that the receiver will retain and pass on in the
 [namespace attributes] when it in turn re-encodes the Markup value.
 This unbounded growth in the set of attribute items in the
 [namespace attributes] defeats any attempt to produce a canonical
 encoding.
 The principle of self-containment is introduced to avoid this
 problem.  An element item (the subject element item) is
 self-contained if the constraints of Namespaces in XML 1.0 [XMLNS10]
 are satisfied (i.e., that prefixes are properly declared) and none of
 the following bindings are determined by a namespace declaration
 attribute item in the [namespace attributes] of an ancestor element
 item of the subject element item:
 (1) the binding between the [prefix] and [namespace name] of the
     subject element item,
 (2) the binding between the [prefix] and [namespace name] of any
     descendant element item of the subject element item,

Legg & Prager Experimental [Page 10] RFC 4910 Robust XML Encoding Rules July 2007

 (3) the binding between the [prefix] and [namespace name] of any
     attribute item in the [attributes] of the subject element item or
     the [attributes] of any descendant element item of the subject
     element item,
 (4) the binding between the namespace prefix and namespace name of
     any qualified name in the [normalized value] of any attribute
     item in the [attributes] of the subject element item or the
     [attributes] of any descendant element item of the subject
     element item, or
 (5) the binding between the namespace prefix and namespace name of
     any qualified name represented by a series of character items
     (ignoring processing instruction and comment items) in the
     [children] of the subject element item or the [children] of any
     descendant element item of the subject element item.
    Aside: If an element is self-contained, then separating the
    element from its parent does not change the semantic
    interpretation of its name and any names in its content and
    attributes.
 A supposedly self-contained element in a received RXER encoding that
 is in fact not self-contained SHALL be treated as an ASN.1 constraint
 violation.
    Aside: ASN.1 does not require an encoding with a constraint
    violation to be immediately rejected; however, the constraint
    violation must be reported at some point, possibly in a separate
    validation step.
 Implementors should note that an RXER decoder will be able to detect
 some, but not all, violations of self-containment.  For example, it
 can detect element and attribute names that depend on namespace
 declarations appearing in the ancestors of a supposedly
 self-contained element.  Similarly, where type information is
 available, it can detect qualified names in character data that
 depend on the namespace declarations of ancestor elements.  However,
 type information is not always available, so some qualified names
 will escape constraint checking.  Thus, the onus is on the creator of
 the original encoding to ensure that element items required to be
 self-contained really are completely self-contained.
 An element item whose content and attributes are described by a value
 of the Markup type MUST be self-contained.

Legg & Prager Experimental [Page 11] RFC 4910 Robust XML Encoding Rules July 2007

    Aside: The procedures in Section 6 take account of the
    requirements for self-containment so that an RXER encoder
    following these procedures will not create violations of
    self-containment.

4.1.2. Normalization for Canonical Encoding Rules

 Implementations are given some latitude in how the content and
 attributes of an element are represented as an abstract value of the
 Markup type, in part because an Infoset can have different equivalent
 serializations.  For example, the order of attributes and the amount
 and kind of white space characters between attributes are irrelevant
 to the Infoset representation.  The content can also include one or
 more elements corresponding to an ASN.1 top-level NamedType or having
 a data type that is an ASN.1 type.  It is only necessary to preserve
 the abstract value for such elements, and a particular abstract value
 can have different Infoset representations.
 These two characteristics mean that when an RXER encoded value of the
 Markup type is decoded, the components of the recovered Markup value
 may not be exactly the same, character for character, as the original
 value that was encoded, though the recovered value will be
 semantically equivalent.
 However, canonical ASN.1 encoding rules such as the Distinguished
 Encoding Rules (DER) and the Canonical Encoding Rules (CER) [X.690],
 which encode Markup values according to the ASN.1 definition of the
 Markup type, depend on character-for-character preservation of string
 values.  This requirement can be accommodated if values of the Markup
 type are normalized when they are encoded according to a set of
 canonical encoding rules.
    Aside: The RXER encoding and decoding of a Markup value might
    change the character string components of the value from the
    perspective of BER, but there will be a single, repeatable
    encoding for DER.
 A value of the Markup type will appear as the content and attributes
 of an element in an RXER encoding.  When the value is encoded using a
 set of ASN.1 canonical encoding rules other than CRXER, the
 components of the text alternative of the value MUST be normalized as
 follows, by reference to the element as it would appear in a CRXER
 encoding:
 (1) The value of the prolog component SHALL be the XMLDecl
     <?xml version="1.1"?> with no other leading or trailing
     characters.

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 (2) If the element's name is unprefixed in the CRXER encoding, then
     the prefix component SHALL be absent; otherwise, the value of the
     prefix component SHALL be the prefix of the element's name in the
     CRXER encoding.
 (3) Take the character string representing the element's attributes,
     including namespace declarations, in the CRXER encoding.  If the
     first attribute is a namespace declaration that undeclares the
     default namespace (i.e., xmlns=""), then remove it.  Remove any
     leading space characters.  If the resulting character string is
     empty, then the attributes component SHALL be absent; otherwise,
     the value of the attributes component SHALL be the resulting
     character string.
        Aside: Note that the attributes of an element can change if an
        RXER encoding is re-encoded in CRXER.
 (4) If the element has no characters between the start-tag and
     end-tag [XML11] in the CRXER encoding, then the content component
     SHALL be absent; otherwise, the value of the content component
     SHALL be identical to the character string in the CRXER encoding
     bounded by the element's start-tag and end-tag.
    Aside: A consequence of invoking the CRXER encoding is that any
    nested element corresponding to an ASN.1 top-level NamedType, or
    indeed the element itself, will be normalized according to its
    ASN.1 value rather than its Infoset representation.  Likewise for
    an element whose data type is an ASN.1 type.  Section 6.4
    describes how these situations can arise.
    Aside: It is only through values of the Markup type that
    processing instructions and comments can appear in CRXER
    encodings.
 If an application uses DER, but has no knowledge of RXER, then it
 will not know to normalize values of the Markup type.  If RXER is
 deployed into an environment containing such applications, then
 Markup values SHOULD be normalized, even when encoding using
 non-canonical encoding rules.

4.2. The AnyURI Type

 A value of the AnyURI ASN.1 type is a character string conforming to
 the format of a Uniform Resource Identifier (URI) [URI].
    AnyURI ::= UTF8String (CONSTRAINED BY
                { -- conforms to the format of a URI -- })

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4.3. The NCName Type

 A value of the NCName ASN.1 type is a character string conforming to
 the NCName production of Namespaces in XML 1.0 [XMLNS10].
    NCName ::= UTF8String (CONSTRAINED BY
                   { -- conforms to the NCName production of
                     -- Namespaces in XML 1.0 -- })
    Aside: The NCName production for Namespaces in XML 1.1 [XMLNS11]
    allows a wider range of characters than the NCName production for
    Namespaces in XML 1.0.  The NCName type for ASN.1 is currently
    restricted to the characters allowed by Namespaces in XML 1.0,
    though this may change in a future specification of RXER.

4.4. The Name Type

 A value of the Name ASN.1 type is a character string conforming to
 the Name production of XML version 1.0 [XML10].
    Name ::= UTF8String (CONSTRAINED BY
                   { -- conforms to the Name production of XML -- })

4.5. The QName Type

 A value of the QName ASN.1 type describes an expanded name [XMLNS10],
 which appears as a qualified name [XMLNS10] in an RXER encoding.
 RXER has special provisions for encoding values of the QName type
 (see Section 6.7.11).  For other encoding rules, a value of the Qname
 type is encoded according to the following ASN.1 type definition
 (with AUTOMATIC TAGS):
    QName ::= SEQUENCE {
        namespace-name  AnyURI OPTIONAL,
        local-name      NCName
    }
 The namespace-name component holds the namespace name of the expanded
 name.  If the namespace name of the expanded name has no value, then
 the namespace-name component is absent.
    Aside: A namespace name can be associated with ASN.1 types and
    top-level NamedType instances by using the TARGET-NAMESPACE
    encoding instruction.
 The local-name component holds the local name of the expanded name.

Legg & Prager Experimental [Page 14] RFC 4910 Robust XML Encoding Rules July 2007

5. Expanded Names for ASN.1 Types

 A TypeAssignment in ASN.1 associates a typereference with a Type.
 For RXER and Abstract Syntax Notation X (ASN.X) [ASN.X], a
 TypeAssignment is also regarded as associating an expanded name
 [XMLNS10] with the Type.  The local name of the expanded name is the
 typereference on the left-hand side of the TypeAssignment.  If the
 target namespace [RXEREI] of the ASN.1 module in which the
 TypeAssignment is defined is not absent, then the namespace name of
 the expanded name is that target namespace; otherwise, the namespace
 name of the expanded name has no value.
 A Type that is a BuiltinType or ReferencedType that is one of the
 productions in Table 1 is regarded as a reference to a built-in ASN.1
 type.  These built-in types also have expanded names.  In each case,
 the local name of the expanded name is as indicated in Table 1, and
 the namespace name of the expanded name is
 "urn:ietf:params:xml:ns:asnx".

Legg & Prager Experimental [Page 15] RFC 4910 Robust XML Encoding Rules July 2007

 Table 1: Local Names for Built-in Types
    +------------------------------------+-------------------+
    | ASN.1 Production                   | Local Name        |
    +====================================+===================+
    | BitStringType                      |                   |
    |    without a NamedBitList          | BIT-STRING        |
    +------------------------------------+-------------------+
    | BooleanType                        | BOOLEAN           |
    +------------------------------------+-------------------+
    | CharacterStringType                |                   |
    |    RestrictedCharacterStringType   |                   |
    |       BMPString                    | BMPString         |
    |       GeneralString                | GeneralString     |
    |       GraphicString                | GraphicString     |
    |       IA5String                    | IA5String         |
    |       ISO646String                 | ISO646String      |
    |       NumericString                | NumericString     |
    |       PrintableString              | PrintableString   |
    |       TeletexString                | TeletexString     |
    |       T61String                    | T61String         |
    |       UniversalString              | UniversalString   |
    |       UTF8String                   | UTF8String        |
    |       VideotexString               | VideotexString    |
    |       VisibleString                | VisibleString     |
    |    UnrestrictedCharacterStringType | CHARACTER-STRING  |
    +------------------------------------+-------------------+
    | EmbeddedPDVType                    | EMBEDDED-PDV      |
    | ExternalType                       | EXTERNAL          |
    +------------------------------------+-------------------+
    | IntegerType                        |                   |
    |    without a NamedNumberList       | INTEGER           |
    +------------------------------------+-------------------+
    | NullType                           | NULL              |
    | ObjectIdentifierType               | OBJECT-IDENTIFIER |
    | OctetStringType                    | OCTET-STRING      |
    | RealType                           | REAL              |
    | RelativeOIDType                    | RELATIVE-OID      |
    +------------------------------------+-------------------+
    | UsefulType                         |                   |
    |    GeneralizedTime                 | GeneralizedTime   |
    |    UTCTime                         | UTCTime           |
    |    ObjectDescriptor                | ObjectDescriptor  |
    +------------------------------------+-------------------+

Legg & Prager Experimental [Page 16] RFC 4910 Robust XML Encoding Rules July 2007

 When the expanded name for an ASN.1 type is used in an RXER encoding,
 it appears as a qualified name [XMLNS10][XMLNS11].  The namespace
 prefix for the qualified name is determined according to
 Section 6.7.11.1.
 If a compatible XML Schema translation of an ASN.1 specification is
 provided (see Section 6.4), then that schema SHOULD associate the
 same expanded name with the XML Schema translation of an ASN.1 type.
 Definition (namespace-qualified reference): An ASN.1 Type is a
 namespace-qualified reference if one of the following applies:
 (1) the Type is a typereference (not a DummyReference) or an
     ExternalTypeReference in a DefinedType in a ReferencedType, the
     ASN.1 module in which the referenced type is defined has a
     TARGET-NAMESPACE encoding instruction, the referenced type is not
     directly or indirectly an open type [X.681], and the referenced
     type is not directly or indirectly the Markup type (Section 4.1),
     or
 (2) the Type is a BuiltinType or ReferencedType that is one of the
     productions in Table 1.
 The type definition referenced by a namespace-qualified reference
 will have an expanded name with a value for the namespace name.

6. Encoding Rules

 With respect to RXER, ASN.1 abstract values are uniformly regarded as
 analogous to the content and attributes of an element, or just an
 attribute value, not complete elements or attributes in their own
 right.  Elements and attributes in an RXER encoding are defined by
 ASN.1 NamedType notation.  Since elements are the fundamental
 discrete structures of an XML document, the notion of a NamedType
 having a value that can be encoded is useful for descriptive purposes
 (particularly for describing the RXER encoding of values of the ASN.1
 combining types).  There is no conceptual basis in X.680 [X.680] for
 talking about the value of a NamedType, or its encoding, so the
 terminology is introduced here.
 Definition (value of a NamedType):  An abstract value of the Type in
 a NamedType is also a value of that NamedType.  The RXER encoding of
 the value of a NamedType is the RXER encoding of the abstract value
 of the Type encapsulated according to the definition of that
 NamedType.

Legg & Prager Experimental [Page 17] RFC 4910 Robust XML Encoding Rules July 2007

 This document does not refer to a value of a NamedType as being an
 abstract value so as to remain consistent with X.680.  An abstract
 value is exclusively a value of an ASN.1 type.
 A complete ASN.1 encoding is traditionally the encoding of an
 abstract value, but it is more natural to think of an XML document as
 being the RXER encoding of a value of a NamedType (because an XML
 document has a single root element that contains all the other
 elements and attributes).  The ASN.1 basic notation does not allow a
 NamedType to appear on its own, outside of an enclosing combining
 type.  That is, the basic notation does not have a concept analogous
 to a global element or attribute definition.  However, an ASN.1
 specification may use an RXER encoding control section [RXEREI] to
 define global elements and attributes using the NamedType notation.
 A NamedType that is not contained in an ASN.1 type definition is
 called a top-level NamedType [RXEREI].  Thus, an RXER encoding would
 typically be described as the encoding of a value of a top-level
 NamedType.
 Section 6.2 describes how a value of a NamedType is encoded.
 Section 6.3 defines an alternative method for encoding the document
 element of an XML document when a top-level NamedType is not
 specified.  Section 6.4 describes how the encodings of ASN.1 values
 can be embedded in an XML document where the other parts of the
 document are validated by an XML Schema.
 The RXER encoding of an abstract value, or the encoding of a value of
 a NamedType, is described as a translation into a synthetic Infoset,
 which is then serialized as XML.  This separation has been chosen for
 descriptive convenience and is not intended to impose any particular
 architecture on RXER implementations.  An RXER encoder is free to
 encode an ASN.1 value directly to XML provided the result is
 equivalent to following the two stage procedure described in this
 document.
 The process of translating an abstract value into an Infoset is
 described as producing either:
 (1) a string of characters that either becomes part of the
     [normalized value] of an attribute item or becomes character
     items among the [children] of an enclosing element item, or
 (2) a collection of zero or more attribute items contributing to the
     [attributes] of an enclosing element item, plus a series of zero
     or more character, element, processing instruction (PI), or
     comment items contributing to the [children] of the enclosing
     element item.

Legg & Prager Experimental [Page 18] RFC 4910 Robust XML Encoding Rules July 2007

 NamedType notation in the ASN.1 specification controls whether the
 translation of an abstract value is encapsulated in an element item
 or in an attribute item.
 Sections 6.5 to 6.10 describe the translation of abstract values into
 an Infoset for each of the ASN.1 type notations.
 Section 6.11 describes post-processing of namespace prefixes for
 CRXER encodings.
 Section 6.12 specifies how the Infoset translation is serialized as
 XML.
 This specification assumes that the COMPONENTS OF transformation
 specified in X.680, Clause 24.4 [X.680] has already been applied to
 all relevant types.
 Examples of RXER encodings in the following sections use a <value>
 start-tag and </value> end-tag to hold attributes and delimit the
 content.  These start-tags and end-tags are for illustration only and
 are not part of the encoding of an abstract value.  In normal use,
 the name of the enclosing element is provided by the context of the
 type of the abstract value, e.g., a NamedType in an enclosing
 SEQUENCE type.
 An RXER decoder is a conforming XML processor [XML10][XML11].

6.1. Identifiers

 An identifier, as defined in ASN.1 notation (Clause 11.3 of X.680
 [X.680]), is a character string that begins with a Latin lowercase
 letter (U+0061-U+007A) and is followed by zero, one or more Latin
 letters (U+0041-U+005A, U+0061-U+007A), decimal digits (U+0030-
 U+0039), and hyphens (U+002D).  A hyphen is not permitted to be the
 last character, and a hyphen is not permitted to be followed by
 another hyphen.  The case of letters in an identifier is always
 significant.
 ASN.1 identifiers are used for the [local name] of attribute and
 element items, and may also appear in the character data content of
 elements or the values of attributes.  RXER encoding instructions can
 be used to substitute an NCName [XMLNS10] for an identifier.

Legg & Prager Experimental [Page 19] RFC 4910 Robust XML Encoding Rules July 2007

6.2. Component Encodings

 The translation of the value of a NamedType is the translation of the
 abstract value of the Type of the NamedType encapsulated according to
 the definition of that NamedType.  This section specifies the form of
 this encapsulation.

6.2.1. Referenced Components

 A value of a NamedType that is subject to a COMPONENT-REF encoding
 instruction is translated as a value of the top-level NamedType
 referenced by the encoding instruction.

6.2.2. Element Components

 A value of a NamedType that is not subject to an ATTRIBUTE,
 ATTRIBUTE-REF, GROUP, or SIMPLE-CONTENT encoding instruction is
 translated as an element item, either as a child element item added
 to the [children] of the enclosing element item or as the document
 element item added to the [children] and [document element] of the
 document item.  If the element item is a child element item, then the
 [parent] is the enclosing element item; otherwise, the [parent] is
 the document item.
 The [local name] of the element item is the local name of the
 expanded name of the NamedType (see [RXEREI]).
    Aside: If there are no NAME, ATTRIBUTE-REF, COMPONENT-REF,
    ELEMENT-REF, or REF-AS-ELEMENT encoding instructions, then the
    local name of the expanded name of a NamedType is the same as the
    identifier of the NamedType.
 If the namespace name of the expanded name has no value, then the
 [namespace name] of the element item has no value (i.e., the
 element's name is not namespace qualified); otherwise, the
 [namespace name] is the namespace name of the expanded name.
 If the type of the NamedType is directly or indirectly the Markup
 type, then the [in-scope namespaces] and [namespace attributes] of
 the element item are constructed as specified in Section 6.10;
 otherwise, the [in-scope namespaces] and [namespace attributes] of
 the element item are constructed as specified in Section 6.2.2.1.
 If the [namespace name] of the element item has no value, then the
 [prefix] of the element item has no value; else if the type of the
 NamedType is not directly or indirectly the Markup type, then the

Legg & Prager Experimental [Page 20] RFC 4910 Robust XML Encoding Rules July 2007

 [prefix] of the element item is determined as specified in
 Section 6.2.2.2; otherwise, the [prefix] is determined by the Markup
 value as specified in Section 6.10.
 The element item becomes the enclosing element item for the
 translation of the value of the Type of the NamedType.
 For a non-canonical RXER encoding, if the type of the NamedType is
 not directly or indirectly the Markup type, then PI and comment items
 MAY be added to the [children] of the element item (before or after
 any other items).  The element item becomes the [parent] for each PI
 and comment item.  These particular PI and comment items in a
 received RXER encoding MAY be discarded by an application.
    Aside: There is no provision for representing comments and PIs in
    ASN.1 abstract values of types other than the Markup type.  These
    items will be lost if the abstract value is re-encoded using a
    different set of encoding rules.
 For a non-canonical RXER encoding, an attribute item with the
 [local name] "type" and the [namespace name]
 "http://www.w3.org/2001/XMLSchema-instance" (i.e., xsi:type [XSD1])
 SHOULD be added to the [attributes] of the element item if the
 corresponding NamedType is subject to a TYPE-AS-VERSION encoding
 instruction and MAY be added to the [attributes] of the element item
 if the Type of the corresponding NamedType is a namespace-qualified
 reference (see Section 5).  The [prefix] of this attribute item is
 determined as specified in Section 6.2.3.1.  The [normalized value]
 of this attribute item is a qualified name for the expanded name of
 the referenced type, with the namespace prefix determined as
 specified in Section 6.7.11.1.  The element item is the
 [owner element] for the attribute item.
    Aside: Where a compatible XML Schema translation of the ASN.1
    specification has been provided, the xsi:type attribute indicates
    to an XML Schema validator which type definition it should use for
    validating the RXER encoding.
    Aside: An xsi:type attribute is generally not permitted in a CRXER
    encoding.  Section 6.4 describes some circumstances where it is
    required in a CRXER encoding.  An xsi:type attribute might also
    appear in a CRXER encoding if it is contained in a value of the
    Markup type.
 For a non-canonical RXER encoding, if the type of the NamedType is
 not directly or indirectly the Markup type, then attribute items with
 the [local name] "schemaLocation" or "noNamespaceSchemaLocation" and
 the [namespace name] "http://www.w3.org/2001/XMLSchema-instance"

Legg & Prager Experimental [Page 21] RFC 4910 Robust XML Encoding Rules July 2007

 [XSD1] MAY be added to the [attributes] of the element item.  The
 [prefix] for each of these attribute items is determined as specified
 in Section 6.2.3.1.  The [normalized value] of these attribute items
 MUST reference a compatible XML Schema translation of the ASN.1
 specification.  The element item is the [owner element] for the
 attribute items.

6.2.2.1. Namespace Properties for Elements

 This section describes how the [in-scope namespaces] and
 [namespace attributes] of an element item are constructed when the
 content and attributes of the element item are not described by a
 value of the Markup type (otherwise, see Section 6.10).
 The [in-scope namespaces] property of the element item initially
 contains only the mandatory namespace item for the "xml" prefix
 [INFOSET].
 For a CRXER encoding, if the element item is not the
 [document element] of the document item and the [in-scope namespaces]
 property of the element item's [parent] contains a namespace item for
 the default namespace, then a namespace declaration attribute item
 that undeclares the default namespace (see Section 3) SHALL be added
 to the element item's [namespace attributes].
 Definition (default namespace restricted): With respect to an element
 item, the default namespace is restricted if:
 (1) the [namespace name] of the element item has no value (i.e., the
     element's name is not namespace qualified), or
 (2) the element item is the enclosing element item for a value of the
     UNION type where the member attribute will be required (see
     Section 6.7.14), or
 (3) the element item is the enclosing element item for a value of the
     QName type where the namespace-name component is absent (see
     Section 6.7.11).  This includes the case where the translation of
     the QName value is contained in the [normalized value] of an
     attribute item in the [attributes] of the element item.
 For a non-canonical RXER encoding, if the element item is not the
 [document element] of the document item and the [in-scope namespaces]
 property of the element item's [parent] contains a namespace item for
 the default namespace, then either:
 (1) that item is copied to the [in-scope namespaces] of the element
     item, or

Legg & Prager Experimental [Page 22] RFC 4910 Robust XML Encoding Rules July 2007

 (2) a namespace declaration attribute item that declares the default
     namespace is added to the element item's [namespace attributes]
     (the namespace name is the encoder's choice), and an equivalent
     namespace item is added to the [in-scope namespaces] of the
     element item, or
 (3) a namespace declaration attribute item that undeclares the
     default namespace is added to the element item's
     [namespace attributes].
 Options (1) and (2) SHALL NOT be used if the default namespace is
 restricted with respect to the element item.
 For a CRXER encoding, if the element item is not the
 [document element] of the document item and the element item is not
 required to be self-contained, then all the namespace items in the
 [in-scope namespaces] of the [parent], excluding the namespace item
 for the "xml" prefix and any namespace item for the default
 namespace, are copied to the [in-scope namespaces] of the element
 item.
 For a non-canonical RXER encoding, if the element item is not the
 [document element] of the document item and the element item is not
 required to be self-contained, then any subset (including none or
 all) of the namespace items in the [in-scope namespaces] of the
 [parent], excluding certain items, is copied to the
 [in-scope namespaces] of the element item.  The excluded items that
 MUST NOT be copied are:  the namespace item for the "xml" prefix, any
 namespace item for the default namespace, and any namespace item that
 matches the [prefix], but not the [namespace name], of a namespace
 item retained for the re-encoding of an unknown attribute item (see
 Section 6.8.8) or an unknown alternative of a UNION (see
 Section 6.7.14).
    Aside: The descriptive approach used by this document only allows
    a namespace prefix to be used by a new namespace item if it is not
    currently used by another namespace item in the
    [in-scope namespaces].  By not inheriting a namespace item, the
    prefix of that namespace is again available for reuse without fear
    of breaking an existing dependency on the prefix.
 Element items that are required to be self-contained inherit none of
 the namespace items in the [in-scope namespaces] of the [parent].
 Any namespace item that is retained for the re-encoding of an unknown
 attribute item (Section 6.8.8) or an unknown alternative of a UNION
 (Section 6.7.14) and which is not in the [in-scope namespaces] of the
 element item MUST be added to the [in-scope namespaces].  An

Legg & Prager Experimental [Page 23] RFC 4910 Robust XML Encoding Rules July 2007

 equivalent namespace declaration attribute item MUST be added to the
 [namespace attributes] of the element item.
 Definition (unused namespace prefix): A namespace prefix is unused if
 it does not match the [prefix] of any namespace item in the
 [in-scope namespaces] of the element item.
 For a non-canonical RXER encoding, if the type of the NamedType is
 not directly or indirectly the Markup type, then additional namespace
 declaration attribute items for currently unused namespace prefixes
 MAY be added to the [namespace attributes] of the element item.  An
 equivalent namespace item MUST be added to the [in-scope namespaces]
 of the element item for each additional namespace declaration
 attribute item.
 For a non-canonical RXER encoding, if the type of the NamedType is
 not directly or indirectly the Markup type, and the
 [in-scope namespaces] property of the element item does not contain a
 namespace item for the default namespace, and the default namespace
 is not restricted with respect to the element item, then a namespace
 declaration attribute item for the default namespace MAY be added to
 the [namespace attributes] of the element item, in which case an
 equivalent namespace item MUST be added to the [in-scope namespaces]
 of the element item.
 Whenever a namespace declaration attribute item is added to an
 element item's [namespace attributes], the [owner element] of the
 attribute item is set to the element item.

6.2.2.2. Namespace Prefixes for Element Names

 This section describes how the [prefix] of an element item is
 determined when the element item has a value for its [namespace name]
 and the content and attributes of the element item are not described
 by a value of the Markup type (otherwise, see Section 6.10).
 For a CRXER encoding, if the [namespace name] of the element item has
 a value, then the [prefix] of the element item is any unused
 non-canonical namespace prefix unless the [in-scope namespaces]
 property of the element item contains a namespace item with the same
 [namespace name] as the element item.  In that case, the [prefix] of
 that namespace item SHALL be used as the [prefix] of the element
 item.
    Aside: These prefixes will be rewritten to canonical namespace
    prefixes during the final step in producing the Infoset
    translation (see Section 6.11).  Canonical namespace prefixes are
    not used here in the first instance because canonicalization

Legg & Prager Experimental [Page 24] RFC 4910 Robust XML Encoding Rules July 2007

    depends on knowing the final [namespace attributes] produced by
    encoding the abstract value of the type of the NamedType.  If an
    implementation looks ahead to determine this final set prior to
    translating the abstract value, then it can assign the appropriate
    canonical namespace prefix in this step and skip the rewriting
    step.
 For a non-canonical RXER encoding, if the [namespace name] has a
 value, then the [prefix] of the element item is any unused namespace
 prefix unless the [in-scope namespaces] property of the element item
 contains a namespace item with the same [namespace name] as the
 element item.  In that case, the [prefix] of that namespace item MAY
 be used as the [prefix] of the element item.  Note that the [prefix]
 of a namespace item for the default namespace has no value.
 If the [prefix] of the element item is an unused namespace prefix,
 then a namespace declaration attribute item associating the namespace
 prefix with the namespace name MUST be added to the
 [namespace attributes] of the element item, and a corresponding
 namespace item MUST be added to the [in-scope namespaces] of the
 element item.
    Aside: The [local name] of the namespace declaration attribute
    item is the same as the [prefix] of the element item, the
    [namespace name] of the attribute item is
    "http://www.w3.org/2000/xmlns/", and the [normalized value] of the
    attribute item is the same as the [namespace name] of the element
    item.  The namespace item has the same [prefix] and
    [namespace name] as the element item.

6.2.3. Attribute Components

 A value of a NamedType subject to an ATTRIBUTE or ATTRIBUTE-REF
 encoding instruction is translated as an attribute item added to the
 [attributes] of the enclosing element item (which becomes the
 [owner element] of the attribute item).
 The [local name] of the attribute item is the local name of the
 expanded name of the NamedType (see [RXEREI]).
 If the namespace name of the expanded name has no value, then the
 [namespace name] of the attribute item has no value; otherwise, the
 [namespace name] is the namespace name of the expanded name.
 If the [namespace name] has a value, then the [prefix] of the
 attribute item is determined as specified in Section 6.2.3.1;
 otherwise, the [prefix] of the attribute item has no value.

Legg & Prager Experimental [Page 25] RFC 4910 Robust XML Encoding Rules July 2007

 The [normalized value] of the attribute item is the translation of
 the value of the Type of the NamedType.
 For completeness, the [specified] property is set to true, the
 [attribute type] has no value, and the value of the [references]
 property is set to unknown.

6.2.3.1. Namespace Prefixes for Attribute Names

 This section applies when an attribute item with a value for its
 [namespace name] is added to the [attributes] of an element item.
 For a CRXER encoding, the [prefix] of the attribute item is any
 unused non-canonical namespace prefix unless the
 [in-scope namespaces] property of the [owner element] contains a
 namespace item with a value for the [prefix] (i.e., is not a
 namespace item for the default namespace) and the same
 [namespace name] as the attribute item.  In that case, the [prefix]
 of that namespace item SHALL be used as the [prefix] of the attribute
 item.
 For a non-canonical RXER encoding, the [prefix] of the attribute item
 is any unused namespace prefix unless the [in-scope namespaces]
 property of the [owner element] contains a namespace item with a
 value for the [prefix] and the same [namespace name] as the attribute
 item.  In that case, the [prefix] of that namespace item MAY be used
 as the [prefix] of the attribute item.
 If the [prefix] of the attribute item is an unused namespace prefix,
 then a namespace declaration attribute item associating the namespace
 prefix with the namespace name MUST be added to the
 [namespace attributes] of the [owner element], and a corresponding
 namespace item MUST be added to the [in-scope namespaces] of the
 [owner element].

6.2.4. Unencapsulated Components

 A value of a NamedType subject to a GROUP or SIMPLE-CONTENT encoding
 instruction is translated as the value of the Type of the NamedType,
 i.e., without encapsulation in an element item or attribute item.
 Consequently, the enclosing element item for the translation of the
 value of the NamedType is also the enclosing element item for the
 translation of the value of the Type of the NamedType.

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6.2.5. Examples

 Consider this type definition:
    CHOICE {
        one    [0] BOOLEAN,
        two    [1] [RXER:ATTRIBUTE] INTEGER,
        three  [2] [RXER:NAME AS "THREE"] OBJECT IDENTIFIER,
        four   [3] [RXER:ATTRIBUTE-REF {
                       namespace-name "http://www.example.com",
                       local-name     "foo" }] UTF8String,
        five   [4] [RXER:ELEMENT-REF {
                       namespace-name "http://www.example.com",
                       local-name     "bar" }] Markup,
        six    [5] [RXER:GROUP] SEQUENCE {
            seven  [0] [RXER:ATTRIBUTE] INTEGER,
            eight  [1] INTEGER
        }
    }
 The content and attributes of each of the following <value> elements
 are the RXER encoding of a value of the above type:
    <value>
     <one>true</one>
    </value>
    <value two="100"/>
    <value>
     <THREE>2.5.4.3</THREE>
    </value>
    <value xmlns:ex="http://www.example.com"
           ex:foo="a string"/>
    <value>
     <ex:bar xmlns:ex="http://www.example.com">another string</ex:bar>
    </value>
    <value seven="200">
     <eight>300</eight>
    </value>

Legg & Prager Experimental [Page 27] RFC 4910 Robust XML Encoding Rules July 2007

6.3. Standalone Encodings

 A typical RXER encoding is the encoding of a value of a nominated
 top-level NamedType.  An abstract value MAY be encoded as an XML
 document without nominating an explicit top-level NamedType by
 invoking a Standalone RXER Encoding or Standalone CRXER Encoding.
 In a Standalone RXER Encoding or Standalone CRXER Encoding, the
 abstract value is encoded as the value of a notional NamedType where
 the identifier of the NamedType is "value" and the Type of the
 NamedType is the type of the abstract value.  The NamedType is
 assumed to be subject to no encoding instructions.
    Aside: Thus, the element item corresponding to the document
    element will have the [local name] "value" and no value for the
    [namespace name] and [prefix].
 If RXER is chosen as the transfer syntax in an EMBEDDED PDV value,
 then the data-value OCTET STRING SHALL contain a Standalone RXER
 encoding.
 If CRXER is chosen as the transfer syntax in an EMBEDDED PDV value,
 then the data-value OCTET STRING SHALL contain a Standalone CRXER
 encoding.
 If RXER is chosen as the transfer syntax in an EXTERNAL value, then
 the octet-aligned OCTET STRING or arbitrary BIT STRING SHALL contain
 a Standalone RXER encoding.
 If CRXER is chosen as the transfer syntax in an EXTERNAL value, then
 the octet-aligned OCTET STRING or arbitrary BIT STRING SHALL contain
 a Standalone CRXER encoding.

6.4. Embedded ASN.1 Values

 The reference encoding instructions [RXEREI] allow XML Schema
 definitions to be referenced from an ASN.1 specification.  It is also
 possible to reference an ASN.1 type or top-level NamedType from an
 XML Schema definition or from an information item validated by an
 XML Schema wildcard.  The manner in which an XML Schema definition
 references an ASN.1 type or top-level NamedType has an effect on the
 CRXER encoding of a value of the type or top-level NamedType.
 This section also applies to XML Schema definitions that validate
 information items that are contained in a value of the Markup type.

Legg & Prager Experimental [Page 28] RFC 4910 Robust XML Encoding Rules July 2007

    Aside: So the document element of an XML document might be
    described by an XML Schema definition that at some point
    references an ASN.1 definition that uses a reference encoding
    instruction to reference another XML Schema definition that then
    references another ASN.1 definition, and so on.
 In each of the following cases, an element or attribute item is only
 permitted to be, or to encapsulate, an RXER Infoset translation of an
 ASN.1 value if an XML Schema element declaration or ASN.1 NamedType
 is known for the [parent] element item ([owner element] in the case
 of an attribute declaration), for the [parent] of the [parent]
 element item, and so on, to the document element of the XML document.
 This condition is not satisfied by a NamedType where the Type is
 directly or indirectly the Markup type and the NamedType is not
 subject to a reference encoding instruction.
    Aside: An element declaration becomes known for an element item
    through assessment [XSD1].  A NamedType becomes known for an
    element item through decoding.
    Aside: If an XML Schema element declaration or ASN.1 NamedType is
    not known for an element item, then the type of the element item
    and the type of every nested element item are treated as unknown.
    Although an xsi:type attribute definitively identifies the type of
    an element item even if an element declaration for the element
    item is not known, this attribute is generally optional in an RXER
    encoding and so cannot be relied upon when seen in isolation from
    an element declaration.  Although only top-level NamedType
    instances can have namespace-qualified names in the current RXER
    specification, a future version may allow nested NamedType
    instances to also have namespace-qualified names, in which case it
    will not necessarily be possible to distinguish a nested NamedType
    from a top-level NamedType without knowledge of the type of the
    [parent] element item.
 An ASN.1 type with an expanded name (Section 5) MAY be referenced by
 the type attribute of an XML Schema element declaration.  The
 reference takes the form of a qualified name for the expanded name.
 An element item validated by such an element declaration encapsulates
 the Infoset translation of an abstract value of the ASN.1 type.  The
 [namespace name] and [local name] of the element item are determined
 by the XML Schema element declaration.  The remaining properties are
 determined according to RXER.  The element item MUST be
 self-contained for a CRXER encoding.
    Aside: The element item is not required to be self-contained for a
    non-canonical RXER encoding.

Legg & Prager Experimental [Page 29] RFC 4910 Robust XML Encoding Rules July 2007

 A top-level NamedType MAY be referenced by the ref attribute of an
 XML Schema element declaration if the NamedType is not subject to an
 ATTRIBUTE encoding instruction.  The reference takes the form of a
 qualified name for the expanded name of the top-level NamedType
 [RXEREI].  An element item validated by such an element declaration
 is the Infoset translation of a value of the referenced top-level
 NamedType.  All the properties of the element item are determined
 according to RXER.  The element item MUST be self-contained for a
 CRXER encoding.
 A top-level NamedType MAY be referenced by the ref attribute of an
 XML Schema attribute declaration if the NamedType is subject to an
 ATTRIBUTE encoding instruction and the definition of the type of the
 NamedType does not depend on the QName type in any way.  An attribute
 item validated by such an attribute declaration is the Infoset
 translation of a value of the referenced top-level NamedType, except
 that whatever valid [prefix] is initially chosen for the attribute
 item MUST be preserved in any re-encoding.  The remaining properties
 of the attribute item are determined according to RXER.
    Aside: The exclusion of the QName type means that the attribute
    value is not dependent upon any namespace declarations of its
    parent element item.
 An element item that is validated by an XML Schema element
 declaration that has the ur-type (i.e., anyType) as its type
 definition MAY encapsulate the Infoset translation of a value of an
 ASN.1 type with an expanded name.  The [namespace name] and
 [local name] of the element item are determined by the XML Schema
 element declaration.  The remaining properties of the element item
 are determined according to RXER.  The [attributes] of the element
 item SHALL contain an attribute item with the [local name] "type" and
 the [namespace name] "http://www.w3.org/2001/XMLSchema-instance"
 (i.e., an xsi:type attribute).  The [prefix] of this attribute item
 is determined as specified in Section 6.2.3.1.  The
 [normalized value] of this attribute item is a qualified name for the
 expanded name of the ASN.1 type, with the namespace prefix determined
 as specified in Section 6.7.11.1.  The element item MUST be
 self-contained for a CRXER encoding.
 An element item that is validated by an XML Schema wildcard (i.e.,
 <xs:any/>) MAY be the Infoset translation of a value of a top-level
 NamedType that is not subject to an ATTRIBUTE encoding instruction
 and comes from an ASN.1 module with a target namespace [RXEREI] that
 satisfies the namespace constraint of the wildcard.  All the
 properties of the element item are determined according to RXER.  The
 element item MUST be self-contained for a CRXER encoding.

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 An attribute item that is validated by an XML Schema wildcard (i.e.,
 <xs:anyAttribute/>) MAY be the Infoset translation of a value of a
 top-level NamedType if the NamedType is subject to an ATTRIBUTE
 encoding instruction, comes from an ASN.1 module with a target
 namespace that satisfies the namespace constraint of the wildcard,
 and has a type that does not depend on the QName type in any way.
 Whatever valid [prefix] is initially chosen for the attribute item
 MUST be preserved in any re-encoding.  The remaining properties of
 the attribute item are determined according to RXER.
 No other mechanisms for referencing an ASN.1 type or top-level
 NamedType from a different XML schema language are supported in this
 version of RXER.  In particular, this excludes an ASN.1 type being
 used as the base type in an XML Schema derivation by extension or
 restriction, as a member type for an XML Schema union type, as an
 item type for an XML Schema list type, or as the type in an
 XML Schema attribute declaration.
 A fully conformant RXER implementation will understand both ASN.1 and
 XML Schema and will recognize the transitions between information
 items controlled by ASN.1 definitions and those controlled by
 XML Schema definitions.  However, a purely XML Schema validator used
 to assess the validity of an RXER encoding will perceive any
 reference to an ASN.1 type or top-level NamedType as an unresolved
 reference.  In order to enable such assessment, it is desirable to
 provide an XML Schema translation of the ASN.1 definitions being
 referenced from an XML Schema.  Although XML Schema and ASN.1 are
 broadly similar, they each have unique features that cannot be
 adequately expressed in the other language, so a semantically
 equivalent translation is not possible in the general case.
 Fortunately, to simply achieve successful assessment it is sufficient
 for the XML Schema translation of an ASN.1 specification to be
 compatible with that ASN.1 specification.  That is, the XML Schema
 translation MUST be constructed such that every correct RXER encoding
 is assessed as valid.  Although not ideal, it is acceptable for the
 XML Schema to assess some incorrect RXER encodings as also being
 valid (a conformant RXER decoder will, of course, reject such an
 encoding).
 The simplest compatible XML Schema translation of an ASN.1 module is
 one in which every type is equivalent to the XML Schema ur-type.  For
 example, given an ASN.1 type with the reference name MyType, a
 sufficient compatible XML Schema type definition is:

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    <xs:complexType name="MyType" mixed="true">
     <xs:sequence>
      <xs:any processContents="lax"
               minOccurs="0" maxOccurs="unbounded"/>
     </xs:sequence>
     <xs:anyAttribute processContents="lax"/>
    </xs:complexType>
        OR
    <xs:complexType name="MyType">
     <xs:complexContent>
      <xs:extension base="xs:anyType"/>
     </xs:complexContent>
    </xs:complexType>
    Aside: Because of the possible presence of an asnx:context
    attribute (Section 6.8.8.1), it is easiest to assume that all
    ASN.1 types translate into XML Schema complex types.
 Given an ASN.1 top-level NamedType that is not subject to an
 ATTRIBUTE encoding instruction and has the reference name myElement,
 a sufficient compatible XML Schema element declaration is:
    <xs:element name="myElement"/>
 Given an ASN.1 top-level NamedType that is subject to an ATTRIBUTE
 encoding instruction and has the reference name myAttribute, a
 sufficient compatible XML Schema attribute declaration is:
    <xs:attribute name="myAttribute"/>
 An application specification that mixes ASN.1 and XML Schema is free
 to provide a stricter translation of its ASN.1 definitions; however,
 a more thorough treatment for translating an ASN.1 module into an
 XML Schema is out of scope for this document.

6.5. Type Referencing Notations

 A value of a type with a defined type name is translated according to
 the type definition on the right-hand side of the type assignment for
 the type name.
 A value of a type denoted by the use of a parameterized type with
 actual parameters is translated according to the parameterized type
 with the DummyReferences [X.683] substituted with the actual
 parameters.

Legg & Prager Experimental [Page 32] RFC 4910 Robust XML Encoding Rules July 2007

 A value of a constrained type is translated as a value of the type
 without the constraint.  See X.680 [X.680] and X.682 [X.682] for the
 details of ASN.1 constraint notation.
 A prefixed type [X.680-1] associates an encoding instruction with a
 type.  A value of a prefixed type is translated as a value of the
 type without the prefix.
    Aside: This does not mean that RXER encoding instructions are
    ignored.  It is simply easier to describe their effects in
    relation to specific built-in types, rather than as the
    translation of a value of a prefixed type.
 A tagged type is a special case of a prefixed type.  A value of a
 tagged type is translated as a value of the type without the tag.
 ASN.1 tags do not appear in the XML encodings defined by this
 document.
 A value of a fixed type denoted by an ObjectClassFieldType is
 translated according to that fixed type (see Section 6.9 for the case
 of an ObjectClassFieldType denoting an open type).
 A value of a selection type is translated according to the type
 referenced by the selection type.  Note that component encoding
 instructions are not inherited by the type referenced by a selection
 type [RXEREI].
 A value of a type described by TypeFromObject notation [X.681] is
 translated according to the denoted type.
 A value of a type described by ValueSetFromObjects notation [X.681]
 is translated according to the governing type.

6.6. TypeWithConstraint, SEQUENCE OF Type, and SET OF Type

 For the purposes of this document, a TypeWithConstraint is treated as
 if it were the parent type [X.680] (either a SEQUENCE OF or SET OF
 type).
 For example,
    SEQUENCE SIZE(1..MAX) OF SomeType
       is treated like
    SEQUENCE OF SomeType

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 Additionally, a "SEQUENCE OF Type" (including the case where it is
 the parent type for a TypeWithConstraint) is treated as if it were a
 "SEQUENCE OF NamedType", where the identifier of the NamedType is
 assumed to be "item".  Similarly, a "SET OF Type" (including the case
 where it is the parent type for a TypeWithConstraint) is treated as
 if it were a "SET OF NamedType", where the identifier of the
 NamedType is assumed to be "item".
 For example,
    SEQUENCE SIZE(1..MAX) OF SomeType
       is ultimately treated like
    SEQUENCE OF item SomeType

6.7. Character Data Translations

 For the majority of ASN.1 built-in types, encodings of values of
 those types never have element content.  The encoding of a value of
 an ASN.1 combining type (except a UNION or LIST type) typically has
 element content.
 For those types that do not produce element content, the translation
 of an abstract value is described as a character string of ISO 10646
 characters [UCS].  This character data translation will be destined
 to become either part of the [normalized value] of an attribute item,
 or a series of character items in the [children] of an element item
 (which becomes the [parent] for the character items).  The case that
 applies is determined in accordance with Section 6.2.
 For a non-canonical RXER encoding, if the type of the abstract value
 is not directly or indirectly a restricted character string type, the
 NULL type, or a UNION type, then leading and/or trailing white space
 characters MAY be added to the character data translation.
    Aside: White space characters are significant in the encoding of a
    value of a restricted character string type, and a restricted
    character string type can be a member type of a UNION type.  The
    encoding of a NULL value produces no character data.
    Aside: Optional white space characters are not permitted in a
    CRXER encoding.
 For a non-canonical RXER encoding, if the type of the abstract value
 is directly or indirectly the AnyURI, NCName, or Name type, then
 leading and trailing white space characters MAY be added to the
 character data translation.

Legg & Prager Experimental [Page 34] RFC 4910 Robust XML Encoding Rules July 2007

    Aside: These types are indirectly a restricted character string
    type (UTF8String); however, their definitions exclude white space
    characters, so any white space characters appearing in an encoding
    are not part of the abstract value and can be safely ignored.
    This exception does not apply to other subtypes of a restricted
    character string type that happen to exclude white space
    characters.

6.7.1. Restricted Character String Types

 The character data translation of a value of a restricted character
 string type is the sequence of characters in the string.
 Depending on the ASN.1 string type, and an application's internal
 representation of that string type, a character may need to be
 translated to or from the equivalent ISO 10646 character code [UCS].
 The NumericString, PrintableString, IA5String, VisibleString
 (ISO646String), BMPString, UniversalString, and UTF8String character
 encodings use the same character codes as ISO 10646.  For the
 remaining string types (GeneralString, GraphicString, TeletexString,
 T61String, and VideotexString), see X.680 [X.680].
 The null character (U+0000) is not a legal character for XML.  It is
 omitted from the character data translation of a string value.
 Certain other control characters are legal for XML version 1.1, but
 not for version 1.0.  If any string value contains these characters,
 then the RXER encoding must use XML version 1.1 (see Section 6.12).
 All white space characters in the RXER encoding of a value of a
 restricted character string type (excluding the AnyURI, NCName, and
 Name subtypes) are significant, i.e., part of the abstract value.
 Examples
    The content of each of the following <value> elements is the RXER
    encoding of an IA5String value:
       <value> Don't run with scissors! </value>
       <value>Markup (e.g., &lt;value&gt;) has to be escaped.</value>
       <value>Markup (e.g., <![CDATA[<value>]]>)
       has to be escaped. </value>

Legg & Prager Experimental [Page 35] RFC 4910 Robust XML Encoding Rules July 2007

6.7.2. BIT STRING

 The character data translation of a value of the BIT STRING type is
 either a binary digit string, a hexadecimal digit string, or a list
 of bit names.
 A binary digit string is a sequence of zero, one, or more of the
 binary digit characters '0' and '1' (i.e., U+0030 and U+0031).  Each
 bit in the BIT STRING value is encoded as a binary digit in order
 from the first bit to the last bit.
 For a non-canonical RXER encoding, if the BIT STRING type has a
 NamedBitList, then trailing zero bits MAY be omitted from a binary
 digit string.
 A hexadecimal digit string is permitted if and only if the number of
 bits in the BIT STRING value is zero or a multiple of eight and the
 character data translation is destined for the [children] of an
 element item.
 A hexadecimal digit string is a sequence of zero, one, or more pairs
 of the hexadecimal digit characters '0'-'9', 'A'-'F', and 'a'-'f'
 (i.e., U+0030-U+0039, U+0041-U+0046 and U+0061-U+0066).  Each group
 of eight bits in the BIT STRING value is encoded as a pair of
 hexadecimal digits where the first bit is the most significant.  An
 odd number of hexadecimal digits is not permitted.  The characters
 'a'-'f' (i.e., U+0061-U+0066) SHALL NOT be used in the CRXER encoding
 of a BIT STRING value.  If a hexadecimal digit string is used, then
 the enclosing element's [attributes] MUST contain an attribute item
 with the [local name] "format", the [namespace name]
 "urn:ietf:params:xml:ns:asnx", and the [normalized value] "hex"
 (i.e., asnx:format="hex").  The [prefix] of the attribute item is
 determined as specified in Section 6.2.3.1.
    Aside: The hexadecimal digit string is intended to conform to the
    lexical representation of the XML Schema [XSD2] hexBinary data
    type.
 For a non-canonical RXER encoding, if the preconditions for using a
 hexadecimal digit string are satisfied, then a hexadecimal digit
 string MAY be used.
 A list of bit names is permitted if and only if the BIT STRING type
 has a NamedBitList and each '1' bit in the BIT STRING value has a
 corresponding identifier in the NamedBitList.
    Aside: ASN.1 does not require that an identifier be assigned for
    every bit.

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 A list of bit names is a sequence of names for the '1' bits in the
 BIT STRING value, in any order, each separated from the next by at
 least one white space character.  If the BitStringType is not subject
 to a VALUES encoding instruction, then each '1' bit in the BIT STRING
 value is represented by its corresponding identifier from the
 NamedBitList.  If the BitStringType is subject to a VALUES encoding
 instruction, then each '1' bit in the BIT STRING value is represented
 by the replacement name [RXEREI] for its corresponding identifier.
 For a CRXER encoding, if the BIT STRING type has a NamedBitList, then
 a binary digit string MUST be used, and trailing zero bits MUST be
 omitted from the binary digit string; else if the number of bits in
 the BIT STRING value is greater than or equal to 64, and the
 preconditions for using a hexadecimal digit string are satisfied,
 then a hexadecimal digit string MUST be used; otherwise, a binary
 digit string MUST be used.
    Aside: Because the asnx:format attribute adds an overhead to a
    hexadecimal encoding (including a namespace declaration for the
    "asnx" prefix), a bit string of less than 64 bits is more
    compactly encoded as a binary digit string.
 Examples
    Consider this type definition:
       BIT STRING { black(0), red(1), orange(2), yellow(3),
           green(4), blue(5), indigo(6), violet(7) }
    The content and attributes of each of the following <value>
    elements are an RXER encoding of the same abstract value:
       <value>  green violet  orange</value>
       <value> 001<!--Orange-->01001 </value>
       <value xmlns:asnx="urn:ietf:params:xml:ns:asnx"
              asnx:format="hex">
        29
       </value>
       <value>00101001</value>
    The final case contains the CRXER encoding of the abstract value.

Legg & Prager Experimental [Page 37] RFC 4910 Robust XML Encoding Rules July 2007

6.7.3. BOOLEAN

 For a non-canonical RXER encoding, the character data translation of
 the BOOLEAN value TRUE is the string "true" or "1", at the encoder's
 discretion.  For a CRXER encoding, the character data translation of
 the BOOLEAN value TRUE is the string "true".
 For a non-canonical RXER encoding, the character data translation of
 the BOOLEAN value FALSE is the string "false" or "0", at the
 encoder's discretion.  For a CRXER encoding, the character data
 translation of the BOOLEAN value FALSE is the string "false".
    Aside: The RXER encoding of BOOLEAN values is intended to conform
    to the lexical representation of the XML Schema [XSD2] boolean
    data type.
 Examples
    The content of each of the following <value> elements is the RXER
    encoding of a BOOLEAN value:
       <value>1</value>
       <value>
           false
       </value>
       <value> fal<!-- a pesky comment -->se </value>

6.7.4. ENUMERATED

 The character data translation of a value of an ENUMERATED type where
 the EnumeratedType is not subject to a VALUES encoding instruction is
 the identifier corresponding to the actual value.
 Examples
    Consider this type definition:
       ENUMERATED { sunday, monday, tuesday,
           wednesday, thursday, friday, saturday }
    The content of both of the following <value> elements is the RXER
    encoding of a value of the above type:

Legg & Prager Experimental [Page 38] RFC 4910 Robust XML Encoding Rules July 2007

       <value>monday</value>
       <value>
           thursday
       </value>
 The character data translation of a value of an ENUMERATED type where
 the EnumeratedType is subject to a VALUES encoding instruction is the
 replacement name [RXEREI] for the identifier corresponding to the
 actual value.
 Examples
    Consider this type definition:
       [RXER:VALUES ALL CAPITALIZED,
               sunday AS "SUNDAY", saturday AS "SATURDAY"]
           ENUMERATED { sunday, monday, tuesday,
               wednesday, thursday, friday, saturday }
    The content of each of the following <value> elements is the RXER
    encoding of a value of the above type:
       <value>SUNDAY</value>
       <value>
           Monday
       </value>
       <value> Tuesday </value>

6.7.5. GeneralizedTime

 The character data translation of a value of the GeneralizedTime type
 is a date, the letter 'T' (U+0054), a time of day, optional
 fractional seconds, and an optional time zone.
 The date is two decimal digits representing the century, followed by
 two decimal digits representing the year, a hyphen ('-', U+002D), two
 decimal digits representing the month, a hyphen ('-', U+002D), and
 two decimal digits representing the day.
 The time of day is two decimal digits representing the hour, followed
 by a colon (':', U+003A), two decimal digits representing the
 minutes, a colon (':', U+003A), and two decimal digits representing
 the seconds.
 Note that the hours value "24" is disallowed [X.680].

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 A GeneralizedTime value with fractional hours or minutes is first
 converted to the equivalent time with whole minutes and seconds and,
 if necessary, fractional seconds.
 The minutes are encoded as "00" if the GeneralizedTime value omits
 minutes.  The seconds are encoded as "00" if the GeneralizedTime
 value omits seconds.
 The fractional seconds part is a full stop ('.', U+002E) followed by
 zero, one, or more decimal digits (U+0030-U+0039).  For a CRXER
 encoding, trailing zero digits (U+0030) in the fractional seconds
 SHALL be omitted, and the full stop SHALL be omitted if there are no
 following digits.
 The time zone, if present, is either the letter 'Z' (U+005A) to
 indicate Coordinated Universal Time, a plus sign ('+', U+002B)
 followed by a time zone differential, or a minus sign ('-', U+002D)
 followed by a time zone differential.
 A time zone differential indicates the difference between local time
 (the time specified by the preceding date and time of day) and
 Coordinated Universal Time.  Coordinated Universal Time can be
 calculated from the local time by subtracting the differential.
 For a CRXER encoding, a GeneralizedTime value with a time zone
 differential SHALL be encoded as the equivalent Coordinated Universal
 Time, i.e., the time zone will be "Z".
 A local time GeneralizedTime value is not converted to Coordinated
 Universal Time for a CRXER encoding.  Other canonical ASN.1 encoding
 rules specify that local times must be encoded as Coordinated
 Universal Time but do not specify a method to convert a local time to
 a Coordinated Universal Time.  Consequently, canonicalization of
 local time values is unreliable and applications SHOULD NOT use local
 time.
 A time zone differential is encoded as two decimal digits
 representing hours, a colon (':', U+003A), and two decimal digits
 representing minutes.  The minutes are encoded as "00" if the
 GeneralizedTime value omits minutes from the time zone differential.
    Aside: The RXER encoding of GeneralizedTime values is intended to
    conform to the lexical representation of the XML Schema [XSD2]
    dateTime data type.

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 Examples
    The content of each of the following <value> elements is the RXER
    encoding of a GeneralizedTime value:
       <value>2004-06-15T12:00:00Z</value>
       <value> 2004-06-15T02:00:00+10:00 </value>
       <value>
           2004-06-15T12:00:00.5
       </value>

6.7.6. INTEGER

 For a CRXER encoding, the character data translation of a value of an
 IntegerType is a canonical number string representing the integer
 value.
 A canonical number string is either the digit character '0' (U+0030),
 or an optional minus sign ('-', U+002D) followed by a non-zero
 decimal digit character (U+0031-U+0039) followed by zero, one, or
 more of the decimal digit characters '0' to '9' (U+0030-U+0039).
 For a non-canonical RXER encoding, the character data translation of
 a value of the IntegerType without a NamedNumberList is a number
 string representing the integer value.
 A number string is a sequence of one or more of the decimal digit
 characters '0' to '9' (U+0030-U+0039), with an optional leading sign,
 either '+' (U+002B) or '-' (U+002D).  Leading zero digits are
 permitted in a number string for a non-canonical RXER encoding.
    Aside: The RXER encoding of values of the IntegerType without a
    NamedNumberList is intended to conform to the lexical
    representation of the XML Schema [XSD2] integer data type.
 For a non-canonical RXER encoding, if the IntegerType has a
 NamedNumberList, and the NamedNumberList defines an identifier for
 the actual value, and the IntegerType is not subject to a VALUES
 encoding instruction, then the character data translation of the
 value is either a number string or the identifier.
 Examples
    Consider this type definition:
       INTEGER { zero(0), one(1) }

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    The content of each of the following <value> elements is the RXER
    encoding of a value of the above type:
       <value>0</value>
       <value> zero </value>
       <value> 2 <!-- This number doesn't have a name. --> </value>
       <value>00167</value>
 For a non-canonical RXER encoding, if the IntegerType is subject to a
 VALUES encoding instruction (it necessarily must have a
 NamedNumberList) and the NamedNumberList defines an identifier for
 the actual value, then the character data translation of the value is
 either a number string or the replacement name [RXEREI] for the
 identifier.
 Examples
    Consider this type definition:
       [RXER:VALUES ALL UPPERCASED] INTEGER { zero(0), one(1) }
    The content of both of the following <value> elements is the RXER
    encoding of a value of the above type:
       <value>0</value>
       <value> ZERO </value>

6.7.7. NULL

 The character data translation of a value of the NULL type is an
 empty character string.
 Examples
    <value/>
    <value><!-- Comments don't matter. --></value>
    <value></value>
    The final case is the CRXER encoding.

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6.7.8. ObjectDescriptor

 A value of the ObjectDescriptor type is translated according to the
 GraphicString type.

6.7.9. OBJECT IDENTIFIER and RELATIVE-OID

 The character data translation of a value of the OBJECT IDENTIFIER or
 RELATIVE-OID type is a full stop ('.', U+002E) separated list of the
 object identifier components of the value.
 Each object identifier component is translated as a non-negative
 number string.  A non-negative number string is either the digit
 character '0' (U+0030), or a non-zero decimal digit character
 (U+0031-U+0039) followed by zero, one, or more of the decimal digit
 characters '0' to '9' (U+0030-U+0039).
 Examples
    The content of each of the following <value> elements is the RXER
    encoding of an OBJECT IDENTIFIER value:
       <value>2.5.6.0</value>
       <value>
           2.5.4.10
       </value>
       <value> 2.5.4.3 <!-- commonName --> </value>

6.7.10. OCTET STRING

 The character data translation of a value of the OCTET STRING type is
 the hexadecimal digit string representation of the octets.
 The octets are encoded in order from the first octet to the last
 octet.  Each octet is encoded as a pair of the hexadecimal digit
 characters '0'-'9', 'A'-'F', and 'a'-'f' (i.e., U+0030-U+0039,
 U+0041-U+0046, and U+0061-U+0066) where the first digit in the pair
 corresponds to the four most significant bits of the octet.  An odd
 number of hexadecimal digits is not permitted.  The characters 'a'-
 'f' (i.e., U+0061-U+0066) SHALL NOT be used in the CRXER encoding of
 an OCTET STRING value.
    Aside: The RXER encoding of OCTET STRING values is intended to
    conform to the lexical representation of the XML Schema [XSD2]
    hexBinary data type.

Legg & Prager Experimental [Page 43] RFC 4910 Robust XML Encoding Rules July 2007

 Examples
    The content of each of the following <value> elements is the RXER
    encoding of an OCTET STRING value:
       <value>27F69A0300</value>
       <value>
           efA03bFF
       </value>

6.7.11. QName

 The character data translation of a value of the QName type
 (Section 4.5) is a qualified name conforming to the QName production
 of Namespaces in XML 1.0 [XMLNS10].
 The local part (i.e., LocalPart) of the qualified name SHALL be the
 value of the local-name component of the QName value.
 If the namespace-name component of the QName value is absent, then
 the namespace prefix (i.e., Prefix) of the qualified name SHALL be
 absent; otherwise, the namespace prefix is determined as specified in
 Section 6.7.11.1 using the value of the namespace-name component of
 the QName value as the namespace name.

6.7.11.1. Namespace Prefixes for Qualified Names

 This section describes how the namespace prefix of a qualified name
 is determined given the namespace name to which the namespace prefix
 must map.
 For a CRXER encoding, the namespace prefix of the qualified name is
 any unused non-canonical namespace prefix unless the
 [in-scope namespaces] property of the enclosing element item contains
 a namespace item with a [namespace name] that matches the namespace
 name.  In that case, the [prefix] of that namespace item SHALL be
 used as the namespace prefix of the qualified name.
    Aside: If the qualified name appears in the [normalized value] of
    an attribute item, then the enclosing element item is the
    [owner element] for that attribute item.
 For a non-canonical RXER encoding, the namespace prefix of the
 qualified name is any unused namespace prefix unless the
 [in-scope namespaces] property of the enclosing element item contains
 a namespace item with the same [namespace name] as the element item.
 In that case, the [prefix] of that namespace item MAY be used as the

Legg & Prager Experimental [Page 44] RFC 4910 Robust XML Encoding Rules July 2007

 namespace prefix of the qualified name.  Note that the [prefix] of a
 namespace item for the default namespace has no value.
 If the namespace prefix of the qualified name is an unused namespace
 prefix, then a namespace declaration attribute item associating the
 namespace prefix with the namespace name MUST be added to the
 [namespace attributes] of the enclosing element item, and a
 corresponding namespace item MUST be added to the
 [in-scope namespaces] of the enclosing element item.

6.7.12. REAL

 The character data translation of a value of the REAL type is the
 character string "0" if the value is positive zero, the character
 string "-0" if the value is negative zero, the character string "INF"
 if the value is positive infinity, the character string "-INF" if the
 value is negative infinity, the character string "NaN" if the value
 is not a number, or a real number otherwise.
 A real number is the mantissa followed by either the character 'E'
 (U+0045) or 'e' (U+0065) and the exponent.  The character 'e' SHALL
 NOT be used for a CRXER encoding.  If the exponent is zero, then the
 'E' or 'e' and exponent MAY be omitted for a non-canonical RXER
 encoding.
 The mantissa is a decimal number with an optional leading sign,
 either '+' (U+002B) or '-' (U+002D).  A decimal number is a sequence
 of one or more of the decimal digit characters '0' to '9'
 (U+0030-U+0039) optionally partitioned by a single full stop
 character ('.', U+002E) representing the decimal point.  Multiple
 leading zero digits are permitted for a non-canonical RXER encoding.
 The exponent is encoded as a number string (see Section 6.7.6).
    Aside: The RXER encoding of REAL values is intended to be
    compatible with the lexical representation of the XML Schema
    [XSD2] double data type, but allows real values outside the set
    permitted by double.
 For a CRXER encoding:
 (1) The real number MUST be normalized so that the mantissa has a
     single non-zero digit immediately to the left of the decimal
     point.
 (2) Leading zero digits SHALL NOT be used.

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 (3) A leading plus sign SHALL NOT be used in the mantissa or the
     exponent.
 (4) The fractional part of the mantissa (i.e., that part following
     the decimal point) MUST have at least one digit (which may be
     '0') and MUST NOT have any trailing zeroes after the first digit.
 (5) The exponent SHALL be present and SHALL be a canonical number
     string (see Section 6.7.6).
 Examples
    The content of each of the following <value> elements is the RXER
    encoding of a REAL value:
       <value>3.14159<!-- pi --></value>
       <value> 1.0e6 </value>
       <value> INF </value>
       <value>
           -01e-06
       </value>

6.7.13. UTCTime

 The character data translation of a value of the UTCTime type is a
 date, the letter 'T' (U+0054), a time of day, and a time zone.
 The date is two decimal digits representing the year (no century), a
 hyphen ('-', U+002D), two decimal digits representing the month, a
 hyphen ('-', U+002D), and two decimal digits representing the day.
 The time of day is two decimal digits representing the hour, followed
 by a colon (':', U+003A), two decimal digits representing the
 minutes, a colon (':', U+003A), and two decimal digits representing
 the seconds.
 Note that the hours value "24" is disallowed [X.680].
 The seconds are encoded as "00" if the UTCTime value omits seconds.
 The time zone is either the letter 'Z' (U+005A) to indicate
 Coordinated Universal Time, a plus sign ('+', U+002B) followed by a
 time zone differential, or a minus sign ('-', U+002D) followed by a
 time zone differential.

Legg & Prager Experimental [Page 46] RFC 4910 Robust XML Encoding Rules July 2007

 A time zone differential indicates the difference between local time
 (the time specified by the preceding date and time of day) and
 Coordinated Universal Time.  Coordinated Universal Time can be
 calculated from the local time by subtracting the differential.
 For a CRXER encoding, a UTCTime value with a time zone differential
 SHALL be encoded as the equivalent Coordinated Universal Time, i.e.,
 the time zone will be "Z".
 A time zone differential is encoded as two decimal digits
 representing hours, a colon (':', U+003A), and two decimal digits
 representing minutes.

6.7.14. CHOICE as UNION

 The chosen alternative of a value of a UNION type corresponds to some
 NamedType in the UNION type definition (a ChoiceType).
 The character data translation of a value of a UNION type is the
 character data translation of the value of the type of the chosen
 alternative, i.e., without any kind of encapsulation.
 Leading and trailing white space characters are not permitted to be
 added to the character data translation of a value of a UNION type
 (see Section 6.7); however, this does not preclude such white space
 being added to the character data translation of the value of the
 chosen alternative.
 The character data translation of a value of a UNION type is
 necessarily destined for the [children] of an enclosing element item.
    Aside: This is because the ATTRIBUTE encoding instruction cannot
    be applied to a NamedType with a type that is a UNION type.
 The chosen alternative can be identified by a member attribute item,
 i.e., an attribute item with the [local name] "member" and
 [namespace name] "urn:ietf:params:xml:ns:asnx", added to the
 [attributes] of the enclosing element item.  The [prefix] of this
 attribute item is determined as specified in Section 6.2.3.1.  The
 [normalized value] of the attribute item is a qualified name for the
 expanded name of the NamedType (see [RXEREI]) corresponding to the
 chosen alternative.
    Aside: It is not possible to associate a namespace name with a
    NamedType in a UNION type using the current specification for RXER
    encoding instructions.  Consequently, the [normalized value] of
    the member attribute item will always contain a qualified name
    without a namespace prefix.

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 For a CRXER encoding, the member attribute item MUST be used, and the
 [normalized value] of the attribute item MUST be the CRXER
 translation of the QName value equal to the expanded name.
 In the absence of a member attribute item, an RXER decoder MUST
 determine the chosen alternative by considering the alternatives of
 the choice in the order prescribed below and accepting the first
 alternative for which the encoding is valid.
 If the UNION encoding instruction has a PrecedenceList, then the
 alternatives of the ChoiceType referenced by the PrecedenceList are
 considered in the order identified by that PrecedenceList, then the
 remaining alternatives are considered in the order of their
 definition in the ChoiceType.  If the UNION 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 non-canonical RXER encoder MUST use the member attribute item if an
 RXER decoder would determine the chosen alternative to be something
 other than the actual chosen alternative of the CHOICE value being
 translated; otherwise, the member attribute item MAY be used.
 Examples
    Consider this type definition:
       [RXER:UNION PRECEDENCE serialNumber] CHOICE {
           name          [0] IA5String,
           serialNumber  [1] INTEGER
       }
    In the absence of a member attribute, an RXER decoder would first
    consider whether the received encoding was a valid serialNumber
    (an INTEGER) before considering whether it was a valid name (an
    IA5String).
    The content and attributes of each of the following <value>
    elements are the RXER encoding of a value of the above type:
       <value>Bob</value>
       <value xmlns:asnx="urn:ietf:params:xml:ns:asnx"
              asnx:member="name">Alice</value>
       <value>
        <!-- Don't have a name for this one! --> 344
       </value>

Legg & Prager Experimental [Page 48] RFC 4910 Robust XML Encoding Rules July 2007

       <value xmlns:asnx="urn:ietf:params:xml:ns:asnx"
              asnx:member="name"><!-- A strange name. -->100</value>
    The member attribute is required in the final case to prevent the
    value being interpreted as a serialNumber.
 If the UNION (i.e., CHOICE) type is extensible [X.680], then an
 application MUST accept and be prepared to re-encode (using the same
 encoding rules) any unknown extension in received encoded values of
 the type.  An unknown extension in a value of a UNION type (an
 unknown alternative) takes the form of an unknown name in the
 [normalized value] of the member attribute and/or character data in
 the [children] of the enclosing element item that do not conform to
 any of the known alternatives.
 To enable re-encoding of an unknown alternative, it is necessary to
 retain the [normalized value] of the member attribute, if present,
 and the [children] property of the enclosing element item.
 The character data for an unknown alternative may contain qualified
 names that depend on the [in-scope namespaces] of the enclosing
 element item for their interpretation.  Therefore, semantically
 faithful re-encoding of an unknown alternative may require
 reproduction of at least some part of the [in-scope namespaces] of
 the enclosing element item.  The problem is deciding which of the
 namespace items are actually needed.  In the absence of type
 information, it is not possible to discern whether anything that
 syntactically resembles a qualified name in the character data of the
 enclosing element item actually is a qualified name.  The simplest
 approach is to retain all the namespace items from the
 [in-scope namespaces] of the enclosing element item and output them
 as namespace declaration attribute items in the
 [namespace attributes] of the enclosing element item when re-encoding
 the unknown alternative.  At best, an application can omit the
 namespace items that do not define the namespace prefix of any
 potential qualified name.
 An application MUST retain the namespace items in the
 [in-scope namespaces] of the enclosing element item that define the
 namespace prefixes of all the potential qualified names in the
 [children] of the enclosing element item.  Other namespace items in
 the [in-scope namespaces] of the enclosing element item MAY be
 retained.  The effect of these retained namespace items on the
 [namespace attributes] and [in-scope namespaces] of the enclosing
 element item when re-encoding is considered in Section 6.2.2.1.

Legg & Prager Experimental [Page 49] RFC 4910 Robust XML Encoding Rules July 2007

    Aside: The context attribute (Section 6.8.8) is not added to the
    [attributes] of the enclosing element item when re-encoding an
    unknown alternative since the type of a NamedType in a UNION type
    cannot be the Markup type.

6.7.15. SEQUENCE OF as LIST

 The character data translation of a value of a LIST type (a
 SEQUENCE OF NamedType) is the concatenation of the character data
 translations of the component values, i.e., the abstract values of
 the type of the NamedType, each separated from the next by at least
 one white space character.  For a CRXER encoding, separating white
 space MUST be exactly one space character (U+0020).
 Example
    Consider this type definition:
       [LIST] SEQUENCE OF timeStamp GeneralizedTime
    The content of the following <value> element is the RXER encoding
    of a value of the above type:
       <value>
           2004-06-15T12:14:56Z
           2004-06-15T12:18:13Z
           2004-06-15T01:00:25Z
       </value>

6.8. Combining Types

 The encoding of a value of an ASN.1 combining type (except a UNION or
 LIST type) typically has element content.
 The Infoset translation of a value of a specific ASN.1 combining type
 (excluding a UNION or LIST type) contains zero or more attribute
 items to be added to the [attributes] of the enclosing element item
 and zero or more element items to be added to the [children] of the
 enclosing element item.  These translations are described in Sections
 6.8.1 to 6.8.7.
 For a non-canonical RXER encoding, white space character items MAY be
 added to the [children] of the enclosing element item (before or
 after any other items).

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 For a CRXER encoding, a character item with the [character code]
 U+000A (a line feed) MUST be inserted immediately before each element
 item in the [children] of the enclosing element item.  No other white
 space character items are permitted to be added to the [children] of
 the enclosing element item.
    Aside: Without the single line feed character before each child
    element, a typical CRXER encoding would be a single, very long
    line.

6.8.1. CHARACTER STRING

 A value of the unrestricted CHARACTER STRING type is translated
 according to the corresponding SEQUENCE type defined in Clause 40.5
 of X.680 [X.680].

6.8.2. CHOICE

 The chosen alternative of a value of a CHOICE type corresponds to,
 and is a value of (see Section 6), some NamedType in the CHOICE type
 definition.
 The translation of a value of a CHOICE type other than the Markup
 type or a UNION type (see Section 6.7.14) is the translation of the
 value of the NamedType corresponding to the actual chosen
 alternative.
 Examples
    Consider this type definition:
       CHOICE {
           name          [0] IA5String,
           serialNumber  [1] INTEGER
       }
    The content of each of the following <value> elements is the RXER
    encoding of a value of the above type:
       <value><name>Bob</name></value>
       <value>
        <name>Alice</name>
       </value>

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       <value>
        <!-- Don't have a name for this one! -->
        <serialNumber>
         344
        </serialNumber>
       </value>
       <value>
        <!-- A strange name. -->
        <name>100</name>
       </value>
 If the CHOICE type is extensible [X.680], then an application MUST
 accept, and be prepared to re-encode (in RXER), any attribute item or
 child element item with a name that is not recognized (see
 Section 6.8.8).

6.8.3. EMBEDDED PDV

 A value of the EMBEDDED PDV type is translated according to the
 corresponding SEQUENCE type defined in Clause 33.5 of X.680 [X.680].

6.8.4. EXTERNAL

 A value of the EXTERNAL type is translated according to the
 corresponding SEQUENCE type defined in Clause 8.18.1 of X.690
 [X.690].

6.8.5. INSTANCE OF

 A value of the INSTANCE OF type is translated according to the
 corresponding SEQUENCE type defined in Annex C of X.681 [X.681].

6.8.6. SEQUENCE and SET

 Each component value of a value of a SEQUENCE or SET type corresponds
 to, and is a value of (see Section 6), some NamedType in the SEQUENCE
 or SET type definition.
 A value of a SEQUENCE or SET type, other than the QName type
 (Section 4.5), is translated by translating in turn each component
 value actually present in the SEQUENCE or SET value and adding the
 resulting attribute items and/or element items to the [attributes]
 and/or [children] of the enclosing element item.  Attribute items may
 be added to the [attributes] of the enclosing element item in any
 order.  Element items resulting from the translation of component

Legg & Prager Experimental [Page 52] RFC 4910 Robust XML Encoding Rules July 2007

 values MUST be appended to the [children] of the enclosing element
 item in the order of the component values' corresponding NamedType
 definitions in the SEQUENCE or SET type definition.
    Aside: In the case of the SET type, this is a deliberate departure
    from BER [X.690], where the components of a SET can be encoded in
    any order.
 If a DEFAULT value is defined for a NamedType and the value of the
 NamedType is the same as the DEFAULT value, then the translation of
 the value of the NamedType SHALL be omitted for a CRXER encoding and
 MAY be omitted for a non-canonical RXER encoding.
 Examples
    Consider this type definition:
       SEQUENCE {
           name        [0] IA5String OPTIONAL,
           partNumber  [1] INTEGER,
           quantity    [2] INTEGER DEFAULT 0
       }
    The content of each of the following <value> elements is the RXER
    encoding of a value of the above type:
       <value>
        <partNumber>23</partNumber>
        <!-- The quantity defaults to zero. -->
       </value>
       <value>
        <name>chisel</name>
        <partNumber> 37 </partNumber>
        <quantity> 0 </quantity>
       </value>
       <value>
        <!-- The name component is optional. -->
        <partNumber>1543</partNumber>
        <quantity>29</quantity>
       </value>
 If the SEQUENCE or SET type is extensible [X.680], then an
 application MUST accept, and be prepared to re-encode (in RXER), any
 attribute item or child element item with a name that is not
 recognized (see Section 6.8.8).

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6.8.7. SEQUENCE OF and SET OF

 Each component value of a value of a type that is a SET OF NamedType
 or a SEQUENCE OF NamedType corresponds to, and is a value of (see
 Section 6), the NamedType in the type definition.
 A value of a type that is a SET OF NamedType, or a
 SEQUENCE OF NamedType other than a LIST type (see Section 6.7.15), is
 translated by adding the translation of each value of the NamedType
 to the [children] of the enclosing element item.
    Aside: An ATTRIBUTE encoding instruction cannot appear in the
    component type for a SEQUENCE OF or SET OF type, so there are no
    attribute items to add to the [attributes] of the enclosing
    element item.
 If the type is a SEQUENCE OF NamedType, then the values of the
 NamedType are translated in the order in which they appear in the
 value of the type.
 For a non-canonical RXER encoding, if the type is a SET OF NamedType,
 then the values of the NamedType may be translated in any order.
 For a CRXER encoding, if the type is a SET OF NamedType, then the
 values of the NamedType MUST be translated in ascending order where
 the order is determined by comparing the octets of their CRXER
 encodings (which will be UTF-8 encoded character strings; see
 Section 6.12.2).  A shorter encoding is ordered before a longer
 encoding that is identical up to the length of the shorter encoding.
 Examples
    Consider this type definition:
       SEQUENCE OF timeStamp GeneralizedTime
    The content of the following <value> element is the RXER encoding
    of a value of the above type:
       <value>
           <timeStamp>2004-06-15T12:14:56Z</timeStamp>
           <timeStamp>2004-06-15T12:18:13Z</timeStamp>
           <timeStamp>
               2004-06-15T01:00:25Z
           </timeStamp>
       </value>

Legg & Prager Experimental [Page 54] RFC 4910 Robust XML Encoding Rules July 2007

    Consider this type definition (also see Section 6.6):
       SEQUENCE OF INTEGER
    The content of the following <value> element is the RXER encoding
    of a value of the above type:
       <value>
        <item>12</item>
        <item>
         9
        </item>
        <item> 7 <!-- A prime number. --></item>
       </value>

6.8.8. Extensible Combining Types

 An application must accept and be prepared to re-encode (using the
 same encoding rules) any unknown extension appearing in the encoding
 of a value of an extensible CHOICE, SEQUENCE, or SET type.  An
 unknown extension in a value of an extensible combining type (except
 UNION types) takes the form of unknown element and/or attribute
 items.  Section 6.8.8.1 describes the processing of unknown element
 items and Section 6.8.8.2 describes the processing of unknown
 attribute items.
 An application cannot produce a canonical encoding if an abstract
 value contains unknown extensions.  However, the method for
 re-encoding unknown extensions does not prevent a receiving
 application with knowledge of the extension from producing the
 correct canonical encoding.

6.8.8.1. Unknown Elements in Extensions

 To enable re-encoding of an unknown element item it is necessary to
 retain the [prefix], [local name], [attributes],
 [namespace attributes], and [children] properties of the element
 item.
 Definition (inherited namespace item):  An inherited namespace item
 is a namespace item in the [in-scope namespaces] of an element item
 for which there is no corresponding namespace declaration attribute
 item in the [namespace attributes] of the element item.
 The content and attributes of an unknown element item may contain
 qualified names whose interpretation depends on inherited namespace
 items.  Semantically faithful re-encoding of the unknown item may
 require reproduction of at least some of the inherited namespace

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 items.  The problem is deciding which of the inherited namespace
 items are actually needed.  Qualified names as the names of elements
 and attributes are easily recognized, but in the absence of type
 information it is not possible to discern whether anything that
 syntactically resembles a qualified name in the value of an attribute
 or the character data of an element actually is a qualified name.
 The simplest approach is to retain all the inherited namespace items
 and output corresponding namespace declaration attribute items in the
 [namespace attributes] of the unknown element item when re-encoding
 the element item.  At best, an application can omit the inherited
 namespace items that do not define the namespace prefix of any
 definite or potential qualified name, though this requires examining
 the content and attributes of the unknown extension.
 Regardless of how clever an implementation tries to be, adding any
 namespace declaration attribute items to an unknown element item is
 harmful to canonicalization if the ASN.1 type for the element item
 turns out to be the Markup type.  To counter this problem, a special
 attribute is used to identify the namespace declaration attribute
 items added to an unknown element item so that they can be removed
 later, if it proves necessary.
 If the outermost element item in an unknown extension does not have
 an attribute item with the [local name] "context" and
 [namespace name] "urn:ietf:params:xml:ns:asnx" in its [attributes],
 then namespace declaration attribute items corresponding to the
 inherited namespace items that define the namespace prefixes of all
 the definite and potential qualified names in the content and
 attributes of the element item MUST be added to the retained
 [namespace attributes].  Other inherited namespace items MAY be added
 to the retained [namespace attributes].
 If there are one or more of these added namespace declaration
 attribute items, then an attribute item with the [local name]
 "context" and [namespace name] "urn:ietf:params:xml:ns:asnx" MUST be
 added to the retained [attributes].
 The [prefix] of the context attribute item is any namespace prefix
 that does not match the [local name] of any namespace declaration
 attribute item in the [namespace attributes] unless the
 [namespace attributes] property contains a namespace declaration
 attribute item with a non-empty [prefix] and a [normalized value] of
 "urn:ietf:params:xml:ns:asnx".  In that case, the [local name] of
 that namespace declaration attribute item MAY be used as the [prefix]
 of the context attribute item.

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 If the [prefix] of the context attribute item does not match the
 [local name] of any namespace declaration attribute item, then an
 attribute item with the [prefix] "xmlns", [namespace name]
 "urn:ietf:params:xml:ns:asnx", and [local name] equal to the [prefix]
 of the context attribute item MUST be added to the retained
 [namespace attributes] of the element item.
 The [normalized value] of the context attribute is the white-space-
 separated unordered list of the [local names] of the added namespace
 declaration attribute items (i.e., a list of the namespace prefixes),
 including any namespace declaration attribute item added to define
 the [prefix] of the context attribute.  Note that the [local name]
 for a namespace declaration attribute item declaring the default
 namespace is "xmlns".
    Aside: A receiver that knows about the extension will use the
    context attribute to strip out the added namespace declaration
    attributes if the type of the associated NamedType is the Markup
    type (Section 6.10), and will discard the context attribute
    otherwise.  A receiver that does not know about the extension will
    re-encode the extension as is.
 Adding the required namespace declaration attribute items to an
 element item effectively makes the element item self-contained.  A
 received encoding has an encoding error if it contains an element
 item that is not self-contained but has a context attribute item in
 its [attributes].
 An RXER encoder MUST NOT add the context attribute item to an element
 item corresponding to a NamedType that is known to it.
 An RXER decoder MUST accept the context attribute item on an element
 item corresponding to a NamedType that does not appear to be an
 extension.
    Aside: It is not uncommon for extension markers to be neglected in
    specifications traditionally using only BER, since extension
    markers do not alter BER encodings.  Consequently, it is not
    immediately obvious in later versions of the specification which
    instances of NamedType belong to extensions of the original base
    specification.
 Example
    Suppose there are three applications, A, B, and C.  Suppose that
    Application A uses the first edition of an ASN.1 specification
    containing the following type definition:

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       MyType ::= SEQUENCE {
               field1  INTEGER,  -- present in first edition
               ...
       }
    Suppose that Application B uses the second edition of the ASN.1
    specification:
       MyType ::= SEQUENCE {
               field1  INTEGER,  -- present in first edition
               ...,
               field2  QName     -- added in second edition
       }
    Suppose that Application C uses the third edition of the ASN.1
    specification:
       MyType ::= SEQUENCE {
               field1  INTEGER,  -- present in first edition
               ...,
               field2  QName,    -- added in second edition
               field3  Markup    -- added in third edition
       }
    Application C produces the following RXER encoding and sends it to
    Application B:
       <value xmlns:p2="http://example.com/ns2">
        <field1> 100 </field1>
        <field2> p2:foobar </field2>
        <field3 xmlns:p1="http://example.com/ns1"> p1:foobar </field3>
       </value>
    Application B doesn't know about <field3>, so it adds the
    asnx:context attribute to <field3> when it re-encodes the abstract
    value to send to Application A:

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       <value xmlns:p1="http://example.com/ns2">
        <!-- Application B knows the white space in field1 and
             field2 is optional and discards it. -->
        <field1>100</field1>
        <field2>p1:foobar</field2>
        <!-- Application B doesn't know about field3
             so it leaves the character data alone. -->
        <field3 asnx:context="asnx p2"
                xmlns:asnx="urn:ietf:params:xml:ns:asnx"
                xmlns:p1="http://example.com/ns1"
                xmlns:p2="http://example.com/ns2"> p1:foobar </field3>
       </value>
    Application A doesn't know about <field2> and <field3>, so it adds
    the asnx:context attribute to <field2> and leaves <field3> alone
    when it re-encodes the abstract value:
       <value>
        <!-- Application A knows about field1 and chooses
             to add some white space. -->
        <field1> 100 </field1>
        <!-- Application A doesn't know about field2 or field3
             so it leaves the character data alone. -->
        <field2 asnx:context="asnx p1"
                xmlns:asnx="urn:ietf:params:xml:ns:asnx"
                xmlns:p1="http://example.com/ns2">p1:foobar</field2>
        <field3 asnx:context="asnx p2"
                xmlns:asnx="urn:ietf:params:xml:ns:asnx"
                xmlns:p1="http://example.com/ns1"
                xmlns:p2="http://example.com/ns2"> p1:foobar </field3>
       </value>
    If Application C receives this final encoding, it has sufficient
    information to discard the asnx:context, xmlns:asnx, and xmlns:p2
    attributes from the received Markup value of <field3> to recover
    the original value.  Application C knows about <field2>, so it
    uses the namespace declaration for p1 when decoding the QName
    value and ignores the other declarations.

6.8.8.2. Unknown Attributes in Extensions

 To enable re-encoding of an unknown attribute item it is necessary to
 retain at least the [local name], [namespace name], and
 [normalized value] properties of the attribute item.
 The [normalized value] of an unknown attribute item may contain
 qualified names whose interpretation depends on the
 [in-scope namespaces] of the [owner element].  Semantically faithful

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 re-encoding of the unknown attribute item may require reproduction of
 at least some part of the [in-scope namespaces].  In the absence of
 type information, it is not possible to discern whether anything that
 syntactically resembles a qualified name in the [normalized value] of
 an unknown attribute item actually is a qualified name.
 The simplest approach is to retain all the namespace items of the
 [in-scope namespaces] and output corresponding namespace declaration
 attribute items in the [namespace attributes] of the [owner element]
 when re-encoding the extension.  At best, an application can omit the
 namespace items that do not define the namespace prefix of any
 potential qualified name in the [normalized value].
 An application MUST retain the namespace items in the
 [in-scope namespaces] of the [owner element] that define the
 namespace prefixes of all the potential qualified names in the
 [normalized value] of the unknown attribute item.  Other namespace
 items in the [in-scope namespaces] of the [owner element] MAY be
 retained.
    Aside: If the enclosing element item has more than one unknown
    attribute item, then it is sufficient to save the union of the
    retained namespace items with the element item, rather than saving
    the retained namespace items with each unknown attribute item.
 When the unknown attribute item is re-encoded, the retained namespace
 items affect the [namespace attributes] and [in-scope namespaces] of
 the enclosing element item as specified in Section 6.2.2.1, and the
 [prefix] of the attribute item is determined as specified in
 Section 6.2.3.1.
    Aside: The context attribute is not added to the [attributes] of
    the [owner element] when re-encoding an unknown attribute item
    because the type of a NamedType subject to an ATTRIBUTE or
    ATTRIBUTE-REF encoding instruction cannot be the Markup type.

6.9. Open Type

 A value of an open type denoted by an ObjectClassFieldType [X.681] is
 translated according to the specific Type of the value.
 If the specific Type of the value is directly or indirectly the
 Markup type, then the enclosing element item MUST be self-contained.
 For a non-canonical RXER encoding, if the translation of the value
 does not generate an attribute item with the [local name] "type" and
 the [namespace name] "http://www.w3.org/2001/XMLSchema-instance"
 (i.e., xsi:type) and the specific Type of the value is a

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 namespace-qualified reference (Section 5), then an attribute item
 with the [local name] "type" and the [namespace name]
 "http://www.w3.org/2001/XMLSchema-instance" (i.e., xsi:type) MAY be
 added to the [attributes] of the enclosing element item.  The
 [normalized value] of this attribute item is a qualified name for the
 expanded name of the referenced type, with the namespace prefix
 determined as specified in Section 6.7.11.1.
    Aside: The xsi:type attribute is added by RXER encoders for the
    benefit of XML Schema validators.  This attribute tells an
    XML Schema validator which type definition in a compatible
    XML Schema translation of the ASN.1 specification it should use
    for validating the content and attributes of the enclosing
    element.  For an RXER decoder, the actual type in an open type
    value is generally determined by an associated component relation
    constraint [X.682], so the xsi:type attribute can be ignored.
 Example
    The content and attributes of the following <value> element are
    the RXER encoding of an open type value containing a BOOLEAN
    value:
       <value xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xmlns:asnx="urn:ietf:params:xml:ns:asnx"
              xsi:type="asnx:BOOLEAN"> true </value>
 If the ObjectClassFieldType denoting an open type is not constrained
 by a TableConstraint, or is constrained by a TableConstraint where
 the constraining object set is extensible, then an application MUST
 accept and be prepared to re-encode (using the same encoding rules)
 any value of the open type where the specific Type of the value is
 unknown.  In such cases, the enclosing element item is treated like
 an unknown element item in the value of an extensible combining ASN.1
 type (see Section 6.8.8.1).

6.10. Markup

 Conceptually, a value of the Markup type holds the [prefix],
 [attributes], [namespace attributes], and [children] of an element
 item.  The Infoset translation of a value of the Markup type
 initially simply sets the [prefix], [attributes],
 [namespace attributes], and [children] of the enclosing element item
 to the corresponding properties represented by the Markup value.
 Recall that the enclosing element item for the translation of a
 Markup value is required to be self-contained (Section 4.1.1).

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 If the enclosing element item is not the [document element] of the
 document item, and the [in-scope namespaces] property of the
 enclosing element item's [parent] contains a namespace item for the
 default namespace, and the [namespace attributes] property
 represented by the Markup value does not contain a namespace item
 declaring or undeclaring the default namespace, then a namespace
 declaration attribute item that undeclares the default namespace
 SHALL be added to the enclosing element item's
 [namespace attributes].
 It is not necessary to populate the [in-scope namespaces] of the
 enclosing element item for encoding purposes (though it may be
 warranted for other purposes).
 An element item nested in the [children] is potentially the Infoset
 translation of a value of a top-level NamedType (as allowed by
 Section 6.4), and the entire Markup value can represent the content
 and attributes of an element item that is the translation of a value
 of a top-level NamedType.
    Aside: The latter case arises when an ELEMENT-REF encoding
    instruction references a top-level NamedType.
 The content and attributes of an element item nested in the
 [children] of a Markup value are potentially the Infoset translation
 of an abstract value of an ASN.1 type (as allowed by Section 6.4),
 and the entire Markup value can represent the translation of a single
 abstract value.
    Aside: The latter case arises when a TYPE-REF encoding instruction
    references an ASN.1 type.
 For a non-canonical RXER encoding, any element item, at any level of
 nesting (including the enclosing element item itself), that
 corresponds to the value of a top-level NamedType MAY be replaced
 with any valid translation of that value.
 For a non-canonical RXER encoding, any element item, at any level of
 nesting (including the enclosing element item itself), with content
 and attributes that correspond to an abstract value of an ASN.1 type
 MAY have that content and those attributes replaced with any valid
 translation of that abstract value.  If the content and attributes
 are replaced, then the [prefix], [in-scope namespaces], and
 [namespace attributes] of the element item are constructed as
 specified in Sections 6.2.2.1 and 6.2.2.2.  The enclosing element
 item for the Markup value is still required to be self-contained.

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    Aside: Insofar as a Markup value represents ASN.1 abstract values,
    it is sufficient for the RXER encoding of the Markup value to
    preserve the abstract values rather than preserve the exact
    Infoset representation.
 For a CRXER encoding, any element item, at any level of nesting
 (including the enclosing element item itself), that corresponds to a
 value of a top-level NamedType MUST be replaced with the CRXER
 translation of that value.
 For a CRXER encoding, any element item, at any level of nesting
 (including the enclosing element item itself), with content and
 attributes that correspond to an abstract value of an ASN.1 type MUST
 have that content and those attributes replaced with the CRXER
 translation of that abstract value.  The [prefix],
 [in-scope namespaces], and [namespace attributes] of the element item
 are constructed as specified in Sections 6.2.2.1 and 6.2.2.2.
 If the [attributes] property of the enclosing element item from a
 received RXER encoding contains an attribute item with the
 [local name] "context" and [namespace name]
 "urn:ietf:params:xml:ns:asnx" (i.e., asnx:context), then this
 attribute item MUST be omitted from the [attributes] represented by
 the Markup value, and each namespace declaration attribute item with
 a [local name] matching an NCName in the [normalized value] of the
 attribute item MUST be omitted from the [namespace attributes]
 represented by the Markup value.

6.11. Namespace Prefixes for CRXER

 The final step in translating the value of a top-level NamedType for
 a CRXER encoding, or an abstract value for a Standalone CRXER
 Encoding, is the replacement of the arbitrarily chosen namespace
 prefixes with algorithmically determined canonical namespace
 prefixes.  This procedure for prefix replacement applies to each
 element item where the [namespace attributes] have been constructed
 according to Section 6.2.2.1.  This includes any element item
 corresponding to a value of a top-level NamedType, or with content
 and attributes that correspond to an abstract value of an ASN.1 type,
 that is nested in a value of the Markup type.
 For each element item where prefix replacement applies, the following
 sequence of steps is repeated until there are no more eligible
 attribute items to select in step (1):

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 (1) Select the attribute item with the least [normalized value] from
     amongst the attribute items of the [namespace attributes] that
     have a [local name] that is not a canonical namespace prefix
     (i.e., select from the namespace declaration attribute items that
     have not already been processed).  A [normalized value] is less
     than another [normalized value] if the former appears before the
     latter in an ordering of the values determined by comparing the
     ISO 10646 code points [UCS] of their characters, from first to
     last.  A shorter string of characters is ordered before a longer
     string of characters that is identical up to the length of the
     shorter string.
        Aside: Note that when a namespace declaration (other than for
        the default namespace) is represented as an attribute item in
        the [namespace attributes], the attribute's [prefix] is
        "xmlns", its [local name] is the namespace prefix, and its
        [normalized value] is the namespace name.
 (2) A canonical namespace prefix is unused if it is not currently the
     [prefix] of any namespace item in the [in-scope namespaces] of
     the element item.  Replace the [local name] of the selected
     attribute item with the unused canonical namespace prefix that
     has the non-negative number string with the least integer value
     (e.g., n2 is less than n10).
 (3) The selected attribute item has a corresponding namespace item in
     the [in-scope namespaces] of the element.  Replace the [prefix]
     of this corresponding namespace item with the canonical namespace
     prefix determined in step (2).
 (4) The element item and its [attributes] property, and descendent
     element items and their [attributes] properties, may depend on
     the selected attribute item to determine the binding between
     their [prefix] and [namespace name].  Replace the [prefix] of any
     such dependent element items and attribute items with the
     canonical namespace prefix determined in step (2).
     Note that a namespace prefix can be redeclared (reused).
     Replacement of the prefix does not apply to an element item
     wherein the prefix is redeclared, or to the descendants of such
     an element item.
 (5) The character data translations for values of the QName ASN.1
     type may depend on the selected attribute item to determine the
     binding between their namespace prefix and namespace name.
     Replace the namespace prefix of any such dependent character data
     translation with the canonical namespace prefix determined in
     step (2).

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     Note that a character data translation can appear in the
     [normalized value] of an attribute item, or as a sequence of
     character items in the [children] of an element item.

6.12. Serialization

 The final RXER encoding is produced by serializing the Infoset
 translation as an XML document.  An implementation MUST serialize the
 Infoset translation as an XML document in such a way that the Infoset
 of the resulting XML document matches the Infoset translation, after
 ignoring the following properties:
 (1) all properties of the document item except the
     [document element],
 (2) the [base URI] of any item,
 (3) the [element content whitespace] of character items,
 (4) the [notation] of processing instruction items,
 (5) the [in-scope namespaces] of element items.
    Aside: The [in-scope namespaces] of a parent element item are only
    selectively inherited by its child element items in the Infoset
    translations of ASN.1 values.  This means that the Infoset
    reconstructed by parsing the XML document serialization of the
    original Infoset will generally have more namespace items in its
    [in-scope namespaces], but these extra namespace items will not be
    significant.
    Aside: A consequence of case (1) is that comments and PIs before
    and after the document element are permitted.
 In general, there is more than one possible serialization for any
 given Infoset translation.  Section 6.12.1 highlights some important
 considerations in producing a correct serialization and discusses
 some of the serialization options.
 Section 6.12.2 applies to CRXER encodings and limits the
 serialization options so that each distinct Infoset has only one
 possible serialization.

6.12.1. Non-Canonical Serialization

 This section discusses aspects of Infoset serialization for
 non-canonical RXER encodings, but is not an exhaustive list of the
 options for non-canonical serialization.

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 If one or more character items have a [character code] in the range
 U+0001 to U+0008, U+000B to U+000C, or U+000E to U+001F, or one or
 more characters in any attribute's [normalized value] are in the
 range U+0001 to U+0008, U+000B to U+000C, or U+000E to U+001F, then
 the Infoset translation MUST be serialized as an XML version 1.1
 document; otherwise, the Infoset translation is serialized as either
 an XML version 1.0 or version 1.1 document.
 A non-canonical RXER encoding may use any of the allowed character
 encoding schemes for XML.  RXER encoders and decoders MUST support
 the UTF-8 character encoding.
 An element item may be serialized as an empty-element tag if it has
 no items in its [children].
 Attributes of an element can appear in any order since the
 [attributes] and [namespace attributes] of an element item are
 unordered.
 Ampersand ('&', U+0026) and open angle bracket ('<', U+003C)
 characters in the [normalized value] of an attribute item must be
 escaped appropriately [XML10][XML11] (with a character reference or a
 predefined entity reference).  Double quote (U+0022) and single quote
 (U+0027) characters in an attribute item's [normalized value] may
 also need to be escaped.  Character items with the [character code]
 U+0026 (ampersand, '&') or U+003C (open angle bracket, '<') must be
 escaped appropriately (with a character reference, a predefined
 entity reference or a CDATA section).
 Line break normalization by XML processors allows some freedom in how
 a character item for a line feed character (U+000A) is serialized:
 (1) If XML version 1.0 is selected, then a character item with the
     [character code] U+000A (line feed) is serialized as either a
     line feed character (U+000A), a carriage return character
     (U+000D) followed by a line feed character (U+000A), or just a
     carriage return character (U+000D) provided the next item is not
     a character item that is serialized as a line feed character
     (U+000A).
 (2) If XML version 1.1 is selected, then a character item with the
     [character code] U+000A (line feed) is serialized as either a
     line feed character (U+000A), a next line character (U+0085), a
     line separator character (U+2028), a carriage return character
     (U+000D) followed by a line feed character (U+000A), a carriage
     return character (U+000D) followed by a next line character

Legg & Prager Experimental [Page 66] RFC 4910 Robust XML Encoding Rules July 2007

     (U+0085), or just a carriage return character (U+000D) provided
     the next item is not a character item that is serialized as a
     line feed (U+000A) or next line (U+0085) character.
       Aside: All these sequences will be normalized to a line feed
       character (U+000A) during decoding.
 A character item with the [character code] U+000D (carriage return),
 U+0085 (next line), or U+2028 (line separator) must be serialized as
 a character reference to protect the character from line break
 normalization during decoding.
 The attribute value normalization performed by XML processors allows
 some freedom in how a space character (U+0020) is serialized:
 (1) If XML version 1.0 is selected, then a space character (U+0020)
     in an attribute item's [normalized value] is serialized as either
     a space character (U+0020), a tab character (U+0009), a carriage
     return character (U+000D), a line feed character (U+000A), a
     carriage return character (U+000D) followed by a line feed
     character (U+000A), or just a carriage return character (U+000D)
     provided the next character in the [normalized value] is not
     serialized as a line feed character (U+000A).
 (2) If XML version 1.1 is selected, then a space character (U+0020)
     in an attribute item's [normalized value] is serialized as either
     a space character (U+0020), a tab character (U+0009), a carriage
     return character (U+000D), a line feed character (U+000A), a next
     line character (U+0085), a line separator character (U+2028), a
     carriage return character (U+000D) followed by a line feed
     character (U+000A), a carriage return character (U+000D) followed
     by a next line character (U+0085), or just a carriage return
     character (U+000D) provided the next character in the
     [normalized value] is not serialized as a line feed (U+000A) or
     next line (U+0085) character.
        Aside: All these sequences will be normalized to a space
        character (U+0020) during decoding, through a combination of
        line break normalization and attribute value normalization.
 Each tab (U+0009), line feed (U+000A), or carriage return (U+000D)
 character in an attribute item's [normalized value] must be
 serialized as a character reference to protect the character from
 attribute value normalization during decoding.  In addition, if XML
 version 1.1 is selected, then each next line (U+0085) or line
 separator (U+2028) character must be serialized as a character
 reference.

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 Parsed entity references may be used (unless the environment in which
 the RXER encoding is used disallows entity references).  If entity
 references to other than the predefined entities are used, then the
 XML document containing the RXER encoding must necessarily contain a
 document type declaration, and the internal or external subset of the
 document type definition must contain entity declarations for those
 entities.

6.12.2. Canonical Serialization

 This section discusses Infoset serialization for CRXER encodings.
 The serialization of an Infoset for a CRXER encoding is restricted so
 that each distinct Infoset has only one possible serialization as an
 XML document.
    Aside: These restrictions have been chosen so as to be consistent
    with Canonical XML [CXML], where possible.
 The document SHALL be encoded in UTF-8 without a leading Byte Order
 Mark [UCS].
 The XMLDecl of the document SHALL be <?xml version="1.1"?>.
 A document type declaration (doctypedecl) SHALL NOT be used.
    Aside: This has the effect of excluding entity references, except
    those for the predefined entities (e.g., &amp;).
 A single line feed character (U+000A) MUST be inserted immediately
 before the document element.
 No other white space characters are permitted before or after the
 document element.
 There SHALL NOT be any PIs or comments before or after the document
 element.
 An element item MUST NOT be serialized as an empty-element tag.
    Aside: If an element item has no items in its [children], then it
    is serialized as a start-tag followed by an end-tag.
 There SHALL NOT be any white space characters immediately before the
 closing '>' of an element's start-tag and end-tag.  The white space
 preceding each attribute SHALL be exactly one space character
 (U+0020).  There SHALL NOT be any white space characters immediately
 before or after the equals sign (U+003D) in an attribute.

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 The delimiter for attribute values SHALL be the double quote
 character (U+0022).
 Namespace declaration attributes MUST appear before any other
 attributes of an element.  A namespace declaration for the default
 namespace, if present, MUST appear as the first attribute.  The
 remaining namespace declaration attributes MUST appear in
 lexicographic order based on [local name].
    Aside: In particular, this means that xmlns:n10 comes before
    xmlns:n2.
 The attributes that are not namespace declarations MUST be
 lexicographically ordered on [namespace name] as the primary key and
 [local name] as the secondary key.
 CDATA sections SHALL NOT be used.
 Each ampersand character ('&', U+0026) in an attribute item's
 [normalized value] MUST be serialized as the entity reference &amp;.
 Each open angle bracket character ('<', U+003C) in an attribute
 item's [normalized value] MUST be serialized as the entity reference
 &lt;.  Each double quote character (U+0022) in an attribute item's
 [normalized value] MUST be serialized as the entity reference &quot;.
 Each character in the range U+0001 to U+001F or U+007F to U+009F in
 an attribute item's [normalized value] MUST be serialized as a
 character reference.  No other character in a [normalized value] is
 permitted to be serialized as an entity reference or character
 reference.
 Each character item with the [character code] U+0026 (the ampersand
 character) MUST be serialized as the entity reference &amp;.  Each
 character item with the [character code] U+003C (the open angle
 bracket character) MUST be serialized as the entity reference &lt;.
 Each character item with the [character code] U+003E (the closing
 angle bracket character) MUST be serialized as the entity reference
 &gt;.  Each character item with a [character code] in the range
 U+0001 to U+0008, U+000B to U+001F, or U+007F to U+009F MUST be
 serialized as a character reference.  No other character item is
 permitted to be serialized as an entity reference or character
 reference.
 Character references, where they are permitted, SHALL use uppercase
 hexadecimal with no leading zeroes.  For example, the carriage return
 character is represented as &#xD;.
 A space character (U+0020) in an attribute item's [normalized value]
 MUST be serialized as a single U+0020 character.

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 A character item with the [character code] U+000A MUST be serialized
 as a single U+000A character.
 The white space separating the [target] and [content] in the
 serialization of a processing instruction item SHALL be exactly one
 space character (U+0020).
    Aside: A processing instruction or comment can only appear in a
    CRXER encoding if it is embedded in a Markup value.

6.12.3. Unicode Normalization in XML Version 1.1

 XML Version 1.1 recommends, but does not absolutely require, that
 text be normalized according to Unicode Normalization Form C
 [UNICODE].  ASN.1 has no similar requirement on abstract values of
 string types, and ASN.1 canonical encoding rules depend on the code
 points of characters being preserved.
 To accommodate both requirements, applications SHOULD normalize
 abstract values of ASN.1 character string types according to Unicode
 Normalization Form C at the time the values are created, but MUST NOT
 normalize a previously decoded abstract value of an ASN.1 character
 string type prior to re-encoding it.  An application may, of course,
 normalize a decoded abstract value for other purposes, such as
 display to a user.

6.13. Syntax-Based Canonicalization

 ASN.1 encoding rules are designed to preserve abstract values, but
 not to preserve every detail of each transfer syntax that is used.
 In the case of RXER, this means that the Infoset representation of an
 abstract value is not necessarily preserved when the abstract value
 is decoded and re-encoded (regardless of the encoding rules used).
 However, syntax-based canonicalization for XML documents (e.g.,
 Canonical XML [CXML]) depends on the Infoset of an XML document being
 preserved.  The Infoset representation of an XML document containing
 the RXER encoding of an ASN.1 abstract value potentially changes if
 that value is decoded and re-encoded, disrupting the Canonical XML
 representation.  Extra normalization is required if RXER is to be
 usefully deployed in environments where syntax-based canonicalization
 is used.
 Prior to applying syntax-based canonicalization to an XML document,
 any element items in the Infoset representation of the document that
 correspond to the value of an ASN.1 top-level NamedType or have
 content and attributes that correspond to an ASN.1 abstract value
 MUST be replaced by the translation of the value according to CRXER.

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 If an application uses Canonical XML but has no knowledge of RXER,
 then it will not know to normalize RXER encodings.  If RXER is
 deployed into an environment containing such applications, then the
 Infoset translation for CRXER SHOULD be used for all RXER encodings.

7. Transfer Syntax Identifiers

7.1. RXER Transfer Syntax

 The following OBJECT IDENTIFIER has been assigned by xmled.org to
 identify the Robust XML Encoding Rules, under an arc assigned to
 xmled.org by the Internet Assigned Numbers Authority (IANA):
    { iso(1) identified-organization(3) dod(6)
      internet(1) private(4) enterprise(1)
      xmled(21472) asnx(1) encoding(1) rxer(0) }
 This OBJECT IDENTIFIER would be used, for example, to describe the
 transfer syntax for an RXER encoded data-value in an EMBEDDED PDV
 value.

7.2. CRXER Transfer Syntax

 The following OBJECT IDENTIFIER has been assigned by xmled.org to
 identify the Canonical Robust XML Encoding Rules, under an arc
 assigned to xmled.org by the IANA:
    { iso(1) identified-organization(3) dod(6)
      internet(1) private(4) enterprise(1)
      xmled(21472) asnx(1) encoding(1) crxer(1) }
 This OBJECT IDENTIFIER would be used, for example, to describe the
 transfer syntax for a CRXER encoded data-value in an EMBEDDED PDV
 value.

8. Relationship to XER

 The Robust XML Encoding Rules (RXER) and the XML Encoding Rules (XER)
 [X.693] are separate, distinctly different and incompatible ASN.1
 encoding rules for producing XML markup from ASN.1 abstract values.
 RXER is therefore unrelated to the XML value notation of X.680
 [X.680].
 This section describes some of the major differences between RXER and
 XER.

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 There are essentially two varieties of XER:  BASIC-XER (with a
 canonical form called CANONICAL-XER) and EXTENDED-XER.  The
 significant difference between the two varieties is that XER encoding
 instructions are used by EXTENDED-XER, but are ignored by BASIC-XER
 (and therefore by CANONICAL-XER).  There isn't a canonical variant of
 EXTENDED-XER.  Characteristics that are common to BASIC-XER and
 EXTENDED-XER will simply be noted as being characteristics of XER.
 Elements and attributes are the fundamental discrete structures of an
 XML document.  Not surprisingly, schema languages for XML typically
 have the means to describe, name, and reference global (i.e.,
 top-level) elements and attributes.  Global type definitions are seen
 more as a convenience for defining the contents of elements and
 attributes.  Traditional ASN.1 has the means to define global types
 (and other global constructs that support the definition of types)
 but nothing akin to a global element or attribute definition.  The
 fundamental difference between RXER and XER is in how this omission
 is addressed.
 With XER, type definitions are also regarded as being element
 definitions by default, or as attribute definitions in the presence
 of an XER ATTRIBUTE encoding instruction.  In some circumstances an
 anonymous Type is required to define an element, which leads to
 element names like <BOOLEAN> and <SEQUENCE>.  NamedType notation also
 defines local elements, and there are some curious cases in
 EXTENDED-XER where NamedType notation can define a global type.  So
 under XER, types can be defined by either Type or NamedType notation,
 and elements and attributes can also be defined by either Type or
 NamedType notation.
 With RXER, types are only defined by Type notation and elements and
 attributes are only defined by NamedType notation.  Global element
 and attribute definitions are made possible by top-level NamedType
 notation in an RXER encoding control section.
 RXER, with its clean separation of Type notation for types and
 NamedType notation for elements and attributes, is a better basis
 than XER for translating an ASN.1 specification into an XML
 representation (i.e., ASN.X [ASN.X]) or a compatible XML Schema,
 where type, element, and attribute definitions are also distinctly
 separate constructs.
 There is usually a requirement on applications specified in ASN.1 to
 maintain backward compatibility with the encodings generated by
 previous versions.  The encodings in question are typically BER.
 Even with the backward-compatibility constraint there is still
 considerable leeway for specification writers to rewrite the earlier
 specification.  For example, they could rename types, factor out an

Legg & Prager Experimental [Page 72] RFC 4910 Robust XML Encoding Rules July 2007

 in-line type definition as a defined type (or the reverse), or
 replace a type definition with an equivalent parameterized reference.
 These changes produce no change to BER, DER, CER [X.690], Packed
 Encoding Rules (PER) [X.691], or Generic String Encoding Rules (GSER)
 [GSER] encodings (so specification writers have felt free to make
 such changes to improve their specification), but can change the
 names of elements in the XER encoding because XER uses types as
 element definitions.  The RXER encoding is immune to this problem,
 thus RXER encodings are more stable than XER encodings over
 successive revisions of an ASN.1 specification (which explains the
 first 'R' in RXER).  This has an obvious benefit for
 interoperability.
 RXER has special provisions for encoding values of the QName and
 Markup types.  QName is used to hold qualified names and Markup can
 be used to hold arbitrary untyped markup.  XER doesn't recognize any
 special types like these, but it is possible to get the same effects
 as RXER's QName and Markup types by using XER encoding instructions.
 Since CANONICAL-XER ignores encoding instructions, this means that
 under XER an application can either support qualified names and
 untyped markup, or support canonical XML encodings, but not both.  In
 contrast, CRXER has canonicalization rules for qualified names and
 for Markup.  Furthermore, EXTENDED-XER does not address the issues of
 normalization of untyped data for other ASN.1 canonical encoding
 rules (e.g., for DER; see Section 4.1.2) or normalization of XML
 encodings for syntax-based canonicalization (e.g., for Canonical XML;
 see Section 6.13).
 Both EXTENDED-XER and RXER use encoding instructions to define
 attributes, union types, and list types, among other things.  Since
 CANONICAL-XER ignores encoding instructions, this means that under
 XER an application must choose between making use of attributes,
 union types, list types, etc., or supporting canonical XML encodings.
 In contrast, the canonicalization rules for CRXER encompass all the
 encoding instructions for RXER.

9. Security Considerations

 RXER does not necessarily enable the exact BER octet encoding of
 values of the TeletexString, VideotexString, GraphicString, or
 GeneralString types to be reconstructed, so a transformation from DER
 to RXER and back to DER may not reproduce the original DER encoding.
 This is a result of inadequate normalization of values of these
 string types in DER.  A character in a TeletexString value (for
 example) that corresponds to a specific ISO 10646 character can be
 encoded for BER in a variety of ways that are indistinguishable in an

Legg & Prager Experimental [Page 73] RFC 4910 Robust XML Encoding Rules July 2007

 RXER re-encoding of the TeletexString value.  DER does not mandate
 one of these possible character encodings in preference to all
 others.
 Because of the above, RXER MUST NOT be used to re-encode, whether for
 storage or transmission, ASN.1 abstract values whose original DER or
 CER encoding must be recoverable, and whose type definitions involve
 the TeletexString, VideotexString, GraphicString, or GeneralString
 type.  Such recovery is needed for the verification of digital
 signatures.  In such cases, protocols ought to use DER or a DER-
 reversible encoding.  In other cases where ASN.1 canonical encoding
 rules are used, values of the Markup type must be normalized as
 described in Section 4.1.2.
 A transformation from CRXER to BER and back to CRXER does reproduce
 the original CRXER encoding, therefore it is safe to use BER, DER, or
 CER to re-encode ASN.1 abstract values whose original CRXER encoding
 must be recoverable.
 Digital signatures may also be calculated on the Canonical XML
 representation of an XML document.  If RXER encodings appear in such
 documents, then applications must normalize the encodings as
 described in Section 6.13.
 The null character (U+0000) cannot be represented in XML and hence
 cannot be transmitted in an RXER encoding.  Null characters in
 abstract values of ASN.1 string types will be dropped if the values
 are RXER encoded; therefore, RXER MUST NOT be used by applications
 that attach significance to the null character.
 When interpreting security-sensitive fields, and in particular fields
 used to grant or deny access, implementations MUST ensure that any
 comparisons are done on the underlying abstract value, regardless of
 the particular encoding used.  Comparisons of Markup values MUST
 operate as though the values have been normalized as specified in
 Section 4.1.2.

10. Acknowledgements

 The technology described in this document is the product of a
 research project begun jointly by Adacel Technologies Limited and
 Deakin University, and subsequently refined and completed by eB2Bcom.

Legg & Prager Experimental [Page 74] RFC 4910 Robust XML Encoding Rules July 2007

11. IANA Considerations

 The IANA has registered a new XML namespace in accordance with RFC
 3688 [XMLREG].
 URI:  urn:ietf:params:xml:ns:asnx
 Registrant Contact:  Steven Legg <steven.legg@eb2bcom.com>
 XML:  None

12. References

12.1. Normative References

 [BCP14]    Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
            10646", RFC 3629, November 2003.
 [XMLREG]   Mealling, M., "The IETF XML Registry", RFC 3688, January
            2004.
 [URI]      Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
            Resource Identifiers (URI): Generic Syntax", STD 66, RFC
            3986, January 2005.
 [RXEREI]   Legg, S., "Encoding Instructions for the Robust XML
            Encoding Rules (RXER)", RFC 4911, July 2007.
 [ASN.X]    Legg, S., "Abstract Syntax Notation X (ASN.X)", RFC 4912,
            July 2007.
 [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.
 [X.681]    ITU-T Recommendation X.681 (07/02) | ISO/IEC 8824-2,
            Information technology - Abstract Syntax Notation One
            (ASN.1):  Information object specification.
 [X.682]    ITU-T Recommendation X.682 (07/02) | ISO/IEC 8824-3,
            Information technology - Abstract Syntax Notation One
            (ASN.1):  Constraint specification.

Legg & Prager Experimental [Page 75] RFC 4910 Robust XML Encoding Rules July 2007

 [X.683]    ITU-T Recommendation X.683 (07/02) | ISO/IEC 8824-4,
            Information technology - Abstract Syntax Notation One
            (ASN.1):  Parameterization of ASN.1 specifications.
 [X.690]    ITU-T Recommendation X.690 (07/02) | ISO/IEC 8825-1,
            Information technology - ASN.1 encoding rules:
            Specification of Basic Encoding Rules (BER), Canonical
            Encoding Rules (CER) and Distinguished Encoding Rules
            (DER).
 [UCS]      ISO/IEC 10646-1:2000, Information technology - Universal
            Multiple-Octet Coded Character Set (UCS) - Part 1:
            Architecture and Basic Multilingual Plane.
 [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
            4.0", Boston, MA, Addison-Wesley Developers Press, 2003.
            ISBN 0-321-18578-1.
 [XML10]    Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E. and
            F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
            Edition)", W3C Recommendation,
            http://www.w3.org/TR/2006/REC-xml-20060816, August 2006.
 [XML11]    Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
            Yergeau, F., and J. Cowan, "Extensible Markup Language
            (XML) 1.1 (Second Edition)", W3C Recommendation,
            http://www.w3.org/TR/2006/REC-xml11-20060816, August 2006.
 [XMLNS10]  Bray, T., Hollander, D., Layman, A., and R. Tobin,
            "Namespaces in XML 1.0 (Second Edition)", W3C
            Recommendation,
            http://www.w3.org/TR/2006/REC-xml-names-20060816, August
            2006.
 [XMLNS11]  Bray, T., Hollander, D., Layman, A. and R. Tobin,
            "Namespaces in XML 1.1 (Second Edition)", W3C
            Recommendation,
            http://www.w3.org/TR/2006/REC-xml-names11-20060816, August
            2006.
 [INFOSET]  Cowan, J. and R. Tobin, "XML Information Set (Second
            Edition)", W3C Recommendation,
            http://www.w3.org/TR/2004/REC-xml-infoset-20040204,
            February 2004.

Legg & Prager Experimental [Page 76] RFC 4910 Robust XML Encoding Rules July 2007

 [XSD1]     Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn,
            "XML Schema Part 1: Structures Second Edition", W3C
            Recommendation,
            http://www.w3.org/TR/2004/REC-xmlschema-1-20041028/,
            October 2004.

12.2. Informative References

 [GSER]     Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
            Types", RFC 3641, October 2003.
 [X.691]    ITU-T Recommendation X.691 (07/02) | ISO/IEC 8825-4:2002,
            Information technology - ASN.1 encoding rules:
            Specification of Packed Encoding Rules (PER).
 [X.693]    ITU-T Recommendation X.693 (12/01) | ISO/IEC 8825-4:2002,
            Information technology - ASN.1 encoding rules: XML
            encoding rules (XER).
 [XSD2]     Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes
            Second Edition", W3C Recommendation,
            http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/,
            October 2004.
 [CXML]     Boyer, J., "Canonical XML Version 1.0", W3C
            Recommendation,
            http://www.w3.org/TR/2001/REC-xml-c14n-20010315, March
            2001.

Legg & Prager Experimental [Page 77] RFC 4910 Robust XML Encoding Rules July 2007

Appendix A. Additional Basic Definitions Module

 This appendix is normative.
 AdditionalBasicDefinitions
     { iso(1) identified-organization(3) dod(6)
       internet(1) private(4) enterprise(1)
       xmled(21472) asnx(1) module(0) basic(0) }
  1. - Copyright (C) The IETF Trust (2007). This version of
  2. - this ASN.1 module is part of RFC 4910; see the RFC itself
  3. - for full legal notices.
  4. -
  5. - Regarding this ASN.1 module or any portion of it, the authors
  6. - make no guarantees and are not responsible for any damage
  7. - resulting from its use. The authors grant irrevocable permission
  8. - to anyone to use, modify, and distribute it in any way that does
  9. - not diminish the rights of anyone else to use, modify, and
  10. - distribute it, provided that redistributed derivative works do
  11. - not contain misleading author or version information.
  12. - Derivative works need not be licensed under similar terms.
 DEFINITIONS
 RXER INSTRUCTIONS
 AUTOMATIC TAGS
 EXTENSIBILITY IMPLIED ::= BEGIN
 Markup ::= CHOICE {
     text    SEQUENCE {
         prolog      UTF8String (SIZE(1..MAX)) OPTIONAL,
         prefix      NCName OPTIONAL,
         attributes  UTF8String (SIZE(1..MAX)) OPTIONAL,
         content     UTF8String (SIZE(1..MAX)) OPTIONAL
     }
 }
 AnyURI ::= UTF8String (CONSTRAINED BY
                { -- conforms to the format of a URI -- })
 NCName ::= UTF8String (CONSTRAINED BY
                { -- conforms to the NCName production of
                  -- Namespaces in XML 1.0 -- })
 Name ::= UTF8String (CONSTRAINED BY
                { -- conforms to the Name production of XML -- })

Legg & Prager Experimental [Page 78] RFC 4910 Robust XML Encoding Rules July 2007

 QName ::= SEQUENCE {
     namespace-name  AnyURI OPTIONAL,
     local-name      NCName
 }
 ENCODING-CONTROL RXER
     TARGET-NAMESPACE "urn:ietf:params:xml:ns:asnx" PREFIX "asnx"
     COMPONENT context [ATTRIBUTE] [LIST] SEQUENCE OF prefix NCName
 END

Authors' Addresses

 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
 Dr. Daniel Prager
 EMail: dap@austhink.com

Legg & Prager Experimental [Page 79] RFC 4910 Robust XML Encoding Rules July 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Legg & Prager Experimental [Page 80]

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