Network Working Group C. Vigano Internet-Draft Universitaet Bremen Intended status: Informational H. Birkholz Expires: January 6, 2016 Fraunhofer SIT July 05, 2015 CBOR data definition language: a notational convention to express CBOR data structures. draft-greevenbosch-appsawg-cbor-cddl-06 Abstract This document proposes a notational convention to express CBOR data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 6, 2016. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Vigano & Birkholz Expires January 6, 2016 [Page 1] Internet-Draft CBOR notation July 2015 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements notation . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. The Style of Data Structure Specification . . . . . . . . . . 4 2.1. Groups and Composition in CDDL . . . . . . . . . . . . . 5 2.1.1. Usage . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . 8 2.2. Types . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.1. Values . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.2. Choices . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.3. Representation Types . . . . . . . . . . . . . . . . 10 2.2.4. Root type . . . . . . . . . . . . . . . . . . . . . . 10 3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1. General conventions . . . . . . . . . . . . . . . . . . . 11 3.2. Occurrence . . . . . . . . . . . . . . . . . . . . . . . 12 3.3. Predefined names for types . . . . . . . . . . . . . . . 12 3.4. Arrays . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.5. Maps . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.5.1. Structs . . . . . . . . . . . . . . . . . . . . . . . 14 3.5.2. Tables . . . . . . . . . . . . . . . . . . . . . . . 17 3.6. Tags . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1. Moves in a computer game . . . . . . . . . . . . . . . . 18 4.2. Fruit . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.3. RFC 7071 . . . . . . . . . . . . . . . . . . . . . . . . 24 4.4. Examples from JSON Content Rules . . . . . . . . . . . . 28 5. Making Use of CDDL . . . . . . . . . . . . . . . . . . . . . 30 5.1. As a guide to a human user . . . . . . . . . . . . . . . 30 5.2. For automated checking of CBOR data structure . . . . . . 30 5.3. For data analysis tools . . . . . . . . . . . . . . . . . 30 6. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 31 7. Resolved Issues . . . . . . . . . . . . . . . . . . . . . . . 31 8. Security considerations . . . . . . . . . . . . . . . . . . . 32 9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 32 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 11.1. Normative References . . . . . . . . . . . . . . . . . . 32 11.2. Informative References . . . . . . . . . . . . . . . . . 33 Appendix A. Cemetery . . . . . . . . . . . . . . . . . . . . . . 33 Appendix B. Nursery . . . . . . . . . . . . . . . . . . . . . . 33 B.1. Annotations . . . . . . . . . . . . . . . . . . . . . . . 33 B.1.1. Annotation .size . . . . . . . . . . . . . . . . . . 34 B.1.2. Annotation .bits . . . . . . . . . . . . . . . . . . 34 Vigano & Birkholz Expires January 6, 2016 [Page 2] Internet-Draft CBOR notation July 2015 B.1.3. Annotation .regexp . . . . . . . . . . . . . . . . . 35 B.2. Socket/Plug . . . . . . . . . . . . . . . . . . . . . . . 35 B.3. Generics . . . . . . . . . . . . . . . . . . . . . . . . 36 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 37 Appendix D. ABNF grammar . . . . . . . . . . . . . . . . . . . . 38 Appendix E. Standard Prelude . . . . . . . . . . . . . . . . . . 40 Appendix F. The CDDL tool . . . . . . . . . . . . . . . . . . . 42 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43 1. Introduction In this document, a notational convention to express CBOR [RFC7049] data structures is defined. The main goal for the convention is to provide a unified notation that can be used when defining protocols that use CBOR. The CBOR notational convention has the following goals: (G1) Provide an unambiguous description of the overall structure of a CBOR data structure. (G2) Flexibility to express the freedoms of choice in the CBOR data format. (G3) Possibility to restrict format choices where appropriate [_format]. (G4) Able to express common CBOR datatypes and structures. (G5) Human and machine readable and processable. (G6) Automatic checking of data format compliance. (G7) Extraction of specific elements from CBOR data for further processing. This document has the following structure: The syntax of CDDL is defined in Section 3. Examples of CDDL and related CBOR data instances are defined in Section 4. Section 5 discusses usage of CDDL. Examples are provided early in the text to better illustrate concept definitions. A formal definition of CDDL using ABNF grammar is provided in Appendix D. Finally, a prelude of standard CDDL definitions available in every CBOR specification is listed in Appendix E. Vigano & Birkholz Expires January 6, 2016 [Page 3] Internet-Draft CBOR notation July 2015 1.1. Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119, BCP 14 [RFC2119]. 1.2. Terminology New terms are introduced in _cursive_. CDDL text in the running text is in "typewriter". 2. The Style of Data Structure Specification CDDL focuses on styles of specification that are in use in the community employing the data model as pioneered by JSON and now refined in CBOR. There are a number of more or less atomic elements of a CBOR data model, such as numbers, simple values (false, true, nil), strings; CDDL does not focus on specifying their structure [_reptype]. CDDL of course also allows adding a CBOR tag to a data item. The more important components of a data structure definition language are the data types used for composition: arrays and maps in CBOR (called arrays and objects in JSON). While these are only two representation formats, they are used to specify four loosely distinguishable styles of composition: o A _vector_, an array of elements that are mostly of the same semantics. The set of signatures associated with a signed data item is a typical application of a vector. o A _record_, an array the elements of which have different, positionally defined semantics, as detailed in the data structure definition. A 2D point, specified as an array of an x coordinate (which comes first) and a y coordinate (coming second) is an example of a record, as is the pair of exponent (first) and mantissa (second) in a CBOR decimal fraction. o A _table_, a map from a domain of map keys to a domain of map values, that are mostly of the same semantics. A set of language tags, each mapped to a string translated to that specific language, is an example of a table. The key domain is usually not limited to a specific set by the specification, but open for the application, e.g., in a table mapping IP addresses to MAC addresses, the specification does not attempt to foresee all possible IP addresses. Vigano & Birkholz Expires January 6, 2016 [Page 4] Internet-Draft CBOR notation July 2015 o A _struct_, a map from a domain of map keys as defined by the specification to a domain of map values the semantics of each of which is bound to a specific map key. This is what most people have in mind when they think about JSON objects; CBOR adds the ability to use map keys that are not just strings. Structs can be used to solve similar problems as records; the use of explicit map keys facilitates optionality and extensibility. Two important concepts provide the foundation for CDDL: 1. Instead of defining all four types of composition in CDDL separately, or even defining one kind for arrays (vectors and records) and one kind for maps (tables and structs), there is only one kind of composition in CDDL: the _group_ (Section 2.1). 2. The other important concept is that of a _type_. The entire CDDL specification defines a type (the one defined by its first _rule_), which formally is the set of CBOR instances that are acceptable for this specification. CDDL predefines a number of basic types such as "uint" (unsigned integer) or "tstr" (text string), often making use of a simple formal notation for CBOR data items. Each value that can be expressed as a CBOR data item also is a type in its own right, e.g. "1". A type can be built as a _choice_ of other types, e.g., an "int" is either a "uint" or a "nint" (negative integer). Finally, a type can be built as an array or a map from a group. 2.1. Groups and Composition in CDDL CDDL Groups are lists of name/value pairs (group _entries_). In an array context, only the value of the entry is represented; the name is annotation only (and can be left off if not needed). In a map context, the names become the map keys ("member keys"). In an array context, the sequence of elements in the group is important, as it is the information that allows associating actual array elements with entries in the group. In a map context, the sequence of entries in a group is not relevant (but there is still a need to write down group entries in a sequence). A group can be placed in (round) parentheses, and given a name by using it in a rule: Vigano & Birkholz Expires January 6, 2016 [Page 5] Internet-Draft CBOR notation July 2015 pii = ( age: int, name: tstr, employer: tstr, ) Figure 1: A basic group Or a group can just be used in the definition of something else: person = {( age: int, name: tstr, employer: tstr, )} Figure 2: Using a group in a map which, given the above rule for pii, is identical to: person = { pii } Figure 3: Using a group by name Note that the (curly) braces signify the creation of a map; the groups themselves are neutral as to whether they will be used in a map or an array. The parentheses for groups are optional, so it would be slightly more natural to express Figure 2 as: person = { age: int, name: tstr, employer: tstr, } Groups can be used to factor out common parts of structs, e.g., instead of writing: Vigano & Birkholz Expires January 6, 2016 [Page 6] Internet-Draft CBOR notation July 2015 person = { age: int, name: tstr, employer: tstr, } dog = { age: int, name: tstr, leash-length: float, } one can choose a name for the common subgroup and write: person = { identity, employer: tstr, } dog = { identity, leash-length: float, } identity = ( age: int, name: tstr, ) Figure 4: Using a group for factorization Note that the contents of the braces in the above definitions constitute (anonymous) groups, while "identity" is a named group. 2.1.1. Usage Groups are the instrument used in composing data structures with CDDL. It is a matter of style in defining those structures whether to define groups (anonymously) right in their contexts or whether to define them in a separate rule and to reference them with their respective name (possibly more than once). With this, one is allowed to define all small parts of their data structures and compose bigger protocol units with those or to have only one big protocol data unit that has all definitions ad hoc where needed. Vigano & Birkholz Expires January 6, 2016 [Page 7] Internet-Draft CBOR notation July 2015 2.1.2. Syntax The composition syntax intends to be concise and easy to read: o The start of a group can be marked by '(' o The end of a group can be marked by ')' o Definitions of entries inside of a group are noted as follows: _keytype => valuetype,_ (read "keytype maps to valuetype"). The comma is actually optional (not just in the final entry), but it is considered good style to set it. The double arrow can be replaced by a colon to optimize for the common case of using a string as a key (see Section 3.5.1). An entry consists of a _keytype_ and a _valuetype_: o _keytype_ is either an atom used as the actual key or a valuetype. This may be needed when using groups in a table context, where the actual keys are of lesser importance than the key types, e.g in contexts verifying incoming data. o _valuetype_ is either a valuetype derived from the major types defined in [RFC7049], a convenience valuetype defined in this document (Appendix E) or the name of a group defined in the protocol file. A group definition can also contain choices between groups, see Section 2.2.2. 2.2. Types 2.2.1. Values Values such as numbers and strings can be used in place of a type. (For instance, this is a very common thing to do for a keytype, common enough that CDDL provides additional convenience syntax for this.) 2.2.2. Choices Many places that allow a type also allow a choice between types, delimited by a "/" (slash). The entire choice construct can be put into parentheses if this is required to make the construction unambiguous (please see Appendix D for the details). Choices of values can be used to express enumerations: Vigano & Birkholz Expires January 6, 2016 [Page 8] Internet-Draft CBOR notation July 2015 attire = "bow tie" / "necktie" / "Internet attire" protocol = 6 / 17 Similarly as for types, CDDL also allows choices between groups, delimited by a "//" (double slash). address = { delivery } delivery = ( street: tstr, ? number: uint, city // po-box: uint, city // per-pickup: true ) city = ( name: tstr, zip-code: uint ) Both for type choices and for group choices, additional alternatives can be added to a rule later in separate rules by using "/=" and "//=", respectively, instead of "=": attire /= "swimwear" delivery //= ( lat: float, long: float, drone-type: tstr ) 2.2.2.1. Ranges Instead of naming all the values that make up a choice, CDDL allows building a _range_ out of two values that are in an ordering relationship [_range]. A range can be inclusive of both ends given (denoted by joining two values by ".."), or include the first and exclude the second (denoted by instead using "..."). device-address = byte max-byte = 255 byte = 0..max-byte ; inclusive range first-non-byte = 256 byte1 = 0...first-non-byte ; byte1 is equivalent to byte CDDL currently only allows ranges between numbers. 2.2.2.2. Turning a group into a choice Some choices are built out of large numbers of values, often integers, each of which is best given a semantic name in the specification. Instead of naming each of these integers and then Vigano & Birkholz Expires January 6, 2016 [Page 9] Internet-Draft CBOR notation July 2015 accumulating these into a choice, CDDL allows building a choice from a group by prefixing it with a "&" character: terminal-color = &basecolors basecolors = ( black: 0, red: 1, green: 2, yellow: 3, blue: 4, magenta: 5, cyan: 6, white: 7, ) extended-color = &( basecolors, orange: 8, pink: 9, purple: 10, brown: 11, ) As with the use of groups in arrays (Section 3.4), the membernames have only documentary value (in particular, they might be used by a tool when displaying integers that are taken from that choice). 2.2.3. Representation Types CDDL allows the specification of a data item type by referring to the CBOR representation (major and minor numbers). How this is used should be evident from the prelude (Appendix E). It may be necessary to make use of representation types outside the prelude, e.g., a specification could start by making use of an existing tag in a more specific way, or define a new tag not defined in the prelude: my_breakfast = #6.55799(breakfast) ; cbor-any is too general! breakfast = cereal / porridge cereal = #6.998(tstr) porridge = #6.999([liquid, solid]) liquid = milk / water milk = 0 water = 1 solid = tstr 2.2.4. Root type There is no special syntax to identify the root of a CDDL data structure definition: that role is simply taken by the first rule defined in the file. This is motivated by the usual top-down approach for defining data structures, decomposing a big data structure unit into smaller parts; however, except for the root type, there is no need to strictly follow this sequence. Vigano & Birkholz Expires January 6, 2016 [Page 10] Internet-Draft CBOR notation July 2015 3. Syntax In this section, the overall syntax of CDDL is shown, alongside some examples just illustrating syntax. (The definition will not attempt to be overly formal; refer to Appendix D for the details.) 3.1. General conventions The basic syntax is somewhat inspired by ABNF [RFC5234], with o rules, whether they define groups or types, are defined with a name, followed by an equals sign "=" and the actual definition according to the respective syntactic rules of that definition. o A name can consist of any of the characters from the set {'A', ..., 'Z', 'a', ..., 'z', '0', ..., '9', '_', '-', '@', '.', '$'}, starting with an alphabetic character (including '@', '_', '$') and ending in one or a digit. * Names are case sensitive. * It is preferred style to start a name with a lower case letter. * The hyphen is preferred over the underscore (except in a "bareword" (Section 3.5.1), where the semantics may actually require an underscore). * The period may be useful for larger specifications, to express some module structure (as in "tcp.throughput" vs. "udp.throughput"). * A number of names are predefined in the CDDL prelude, as listed in Appendix E. * Rule names (types or groups) do not appear in the actual CBOR encoding, but names used as "barewords" in member keys do. o Comments are started by a ';' (semicolon) character and finish at the end of a line (LF or CRLF). o outside strings, whitespace (spaces, newlines, and comments) is used to separate syntactic elements for readability (and to separate identifiers or numbers that follow each other); it is otherwise completely optional. o Hexadecimal numbers are preceded by '0x' (without quotes, lower case x), and are case insensitive. Similarly, binary numbers are preceded by '0b'. Vigano & Birkholz Expires January 6, 2016 [Page 11] Internet-Draft CBOR notation July 2015 o Strings are enclosed by double quotation '"' characters. They follow the conventions for strings as defined in [RFC7159], section 7. [_strings] o CDDL uses UTF-8 [RFC3629] for its encoding. Example: ; This is a comment person = { g } g = ( "name": tstr, age: int, ) 3.2. Occurrence An optional _occurrence_ indicator can be given in front of a group entry. It is either one of the characters '?' (optional), '*' (zero or more), or '+' (one or more), or is of the form n*m, where n and m are optional unsigned integers and n is the lower limit (default 0) and m is the upper limit (default no limit) of occurrences. If no occurrence indicator is specified, the group entry is to occur exactly once (as in 1*1). Note that CDDL, outside any directives/annotations that could possibly be defined, does not make any prescription as to whether arrays or maps use the definite length or indefinite length encoding. I.e., there is no correlation between leaving the size of an array "open" in the spec and the fact that it is then interchanged with definite or indefinite length. 3.3. Predefined names for types CDDL predefines a number of names. This subsection summarizes these names, but please see Appendix E for the exact definitions. The following keywords for primitive datatypes are defined: "bool" Boolean value (major type 7, additional information 20 or 21). "uint" An unsigned integer (major type 0). "nint" A negative integer (major type 1). Vigano & Birkholz Expires January 6, 2016 [Page 12] Internet-Draft CBOR notation July 2015 "int" An unsigned integer or a negative integer. "float16" IEEE 754 half-precision float (major type 7, additional information 25). "float32" IEEE 754 single-precision float (major type 7, additional information 26). "float64" IEEE 754 double-precision float (major type 7, additional information 27). "float" One of float16, float32, or float64. "bstr" or "bytes" A byte string (major type 2). "tstr" or "text" Text string (major type 3) (Note that there are no predefined names for arrays or maps; these are defined with the syntax given below.) In addition, a number of types are defined in the prelude that are associated with CBOR tags, such as "tdate", "bigint", "regexp" etc. 3.4. Arrays Array definitions surround a group with square brackets. For each entry, an occurrence indicator as specified in Section 3.2 is permitted. For example: unlimited-people = [* person] one-or-two-people = [1*2 person] at-least-two-people = [2* person] person = ( name: tstr, age: uint, ) The group "person" is defined in such a way that repeating it in the array each time generates alternating names and ages, so these are four valid values for a data item of type "unlimited-people": Vigano & Birkholz Expires January 6, 2016 [Page 13] Internet-Draft CBOR notation July 2015 ["roundlet", 1047, "psychurgy", 2204, "extrarhythmical", 2231] [] ["aluminize", 212, "climograph", 4124] ["penintime", 1513, "endocarditis", 4084, "impermeator", 1669, "coextension", 865] 3.5. Maps The syntax for specifying maps merits special attention, as well as a number of optimizations and conveniences, as it is likely to be the focal point of many specifications employing CDDL. While the syntax does not strictly distinguish struct and table usage of maps, it caters specifically to each of them. 3.5.1. Structs The "struct" usage of maps is similar to the way JSON objects are used in many JSON applications. A map is defined in the same way as defining an array (see Section 3.4), except for using curly braces "{}" instead of square brackets "[]". An occurrence indicator as specified in Section 3.2 is permitted for each group entry. The following is an example of a structure: Geography = [ city : tstr, gpsCoordinates : GpsCoordinates, ] GpsCoordinates = { longitude : uint, ; multiplied by 10^7 latitude : uint, ; multiplied by 10^7 } When encoding, the Geography structure is encoded using a CBOR array with two entries, whereas the GpsCoordinates are encoded as a CBOR map with two key-value pairs. Types used in a structure can be defined in separate rules or just in place (potentially placed inside parentheses, such as for choices). E.g.: Vigano & Birkholz Expires January 6, 2016 [Page 14] Internet-Draft CBOR notation July 2015 located-samples = { sample-point: int, samples: [+ float], } where "located-samples" is the datatype to be used when referring to the struct, and "sample-point" and "samples" are the keys to be used. This is actually a complete example: an identifier that is followed by a colon can be directly used as the text string for a member key (we speak of a "bareword" member key), as can a double-quoted string or a number. (When other types, in particular multi-valued ones, are used as keytypes, they are followed by a double arrow, see below.) If a text string key does not match the syntax for an identifier (or if the specifier just happens to prefer using double quotes), the text string syntax can also be used in the member key position, followed by a colon. The above example could therefore have been written with quoted strings in the member key positions. All the types defined can be used in a keytype position by following them with a double arrow. A string also is a (single-valued) type, so another form for this example is: located-samples = { "sample-point" => int, "samples" => [+ float], } A better way to demonstrate the double-arrow use may be: located-samples = { sample-point: int, samples: [+ float], * equipment-type => equipment-tolerances, } equipment-type = [name: tstr, manufacturer: tstr] equipment-tolerances = [+ [float, float]] The example below defines a struct with optional entries: display name (as a text string), the name components first name and family name (as a map of text strings), and age information (as an unsigned integer). Vigano & Birkholz Expires January 6, 2016 [Page 15] Internet-Draft CBOR notation July 2015 PersonalData = { ? displayName: tstr, NameComponents, ? age: uint, } NameComponents = ( ? firstName: tstr, ? familyName: tstr, ) Note that the group definition for NameComponents does not generate another map; instead, all four keys are directly in the struct built by PersonalData. In this example, all key/value pairs are optional from the perspective of CDDL. With no occurrence indicator, an entry is mandatory. If the addition of more entries not specified by the current specification is desired, one can add this possibility explicitly: PersonalData = { ? displayName: tstr, NameComponents, ? age: uint, * tstr => any } NameComponents = ( ? firstName: tstr, ? familyName: tstr, ) Figure 5: Personal Data: Example for extensibility The cddl tool (Appendix F) generated as one acceptable instance for this specification: {"familyName": "agust", "antiforeignism": "pretzel", "springbuck": "illuminatingly", "exuviae": "ephemeris", "kilometrage": "frogfish"} (See Appendix B.2 for one way to explicitly identify an extension point.) Vigano & Birkholz Expires January 6, 2016 [Page 16] Internet-Draft CBOR notation July 2015 3.5.2. Tables A table can be specified by defining a map with entries where the keytype is not single-valued, e.g.: square-roots = {* x => y} x = int y = float Here, the key in each key/value pair has datatype x (defined as int), and the value has datatype y (defined as float). If the specification does not need to restrict one of x or y (i.e. the application is free to choose per entry), it can be replaced by the predefined name "any". As another example, the following could be used as a conversion table converting from an integer or float to a string: tostring = {* x => tstr} x = int / float 3.6. Tags A type can make use of a CBOR tag (major type 6) by using the representation type notation, giving #6.nnn(type) where nnn is an unsigned integer giving the tag number and "type" is the type of the data item being tagged. For example, the following line from the CDDL prelude (Appendix E) defines "biguint" as a type name for a positive bignum N: biguint = #6.2(bstr) The tags defined by [RFC7049] are included in the prelude. Additional tags since registered need to be added to a CDDL specification as needed; e.g., a binary UUID tag could be referenced as "buuid" in a specification after defining buuid = #6.37(bstr) In the following example, usage of the tag 32 for URIs is optional: my_uri = #6.32(tstr) / tstr Vigano & Birkholz Expires January 6, 2016 [Page 17] Internet-Draft CBOR notation July 2015 4. Examples This section contains various examples of structures defined using the CBOR notational convention. 4.1. Moves in a computer game A multiplayer computer game uses CBOR to exchange moves between the players. To ensure a good gaming experience, the move information needs to be exchanged quickly and frequently. Therefore, the game uses CBOR to send its information in a compact format. Figure 6 shows definition of the CBOR information exchange format. Vigano & Birkholz Expires January 6, 2016 [Page 18] Internet-Draft CBOR notation July 2015 UpdateMsg = [* { move_no : uint, ; increases for each move player_info : PlayerInfo, ; general information moves : Moves, ; moves in this message }] PlayerInfo = { alias : tstr, player_id : uint, experience : uint, ; beginner: 0; expert: 3 gold : uint, supplies : Supplies, avg_strength : float16, } Supplies = { wood => uint iron => uint grain => uint } wood = 0 iron = 1 grain = 2 Moves = [* Move] Move = ( unit_id : uint, unit_strength : uint, ; between 0 and 100 2*2 source_pos : uint, ; (x,y) 2*2 target_pos : uint, ; (x,y) ) Figure 6: CDDL definition of an information exchange format for a computer game The CDDL tool generates this as a possible instance: Vigano & Birkholz Expires January 6, 2016 [Page 19] Internet-Draft CBOR notation July 2015 [{"move_no": 3985, "player_info": {"alias": "timbrologist", "player_id": 699, "experience": 2699, "gold": 328, "supplies": {0: 1768, 1: 3087, 2: 1401}, "avg_strength": 0.9712613869888417}, "moves": [[1702, 458, 38, 399, 327, 304], [3145, 4454, 1175, 3441, 74, 1542], [4099, 4062, 2808, 8, 3174, 3048], [367, 3649, 756, 3644, 3725, 2769]]}, {"move_no": 199, "player_info": {"alias": "cipo", "player_id": 4309, "experience": 4094, "gold": 4114, "supplies": {0: 873, 1: 4706, 2: 1733}, "avg_strength": 0.37808379403466696}, "moves": [[1977, 3129, 3890, 4000, 1555, 377], [2646, 286, 3363, 4381, 3815, 1039]]}, {"move_no": 2226, "player_info": {"alias": "Stacey", "player_id": 1055, "experience": 207, "gold": 285, "supplies": {0: 3325, 1: 1515, 2: 3304}, "avg_strength": 0.8590028130444863}, "moves": [[869, 4126, 2382, 3155, 1523, 2621]]}] Notice that the supplies have been encoded as a map with integer keys. In this example, using string keys would also have been suitable; the example just illustrates the possibility to use other datatypes for keys, leading to more efficient encoding. The tool-generated binary CBOR for the instance about cannot express yet that the floating point values are 16-bit: 83 # array(3) a3 # map(3) 67 # text(7) 6d6f76655f6e6f # "move_no" 19 0f91 # unsigned(3985) 6b # text(11) 706c617965725f696e666f # "player_info" a6 # map(6) 65 # text(5) 616c696173 # "alias" 6c # text(12) 74696d62726f6c6f67697374 # "timbrologist" 69 # text(9) 706c617965725f6964 # "player_id" 19 02bb # unsigned(699) 6a # text(10) 657870657269656e6365 # "experience" 19 0a8b # unsigned(2699) 64 # text(4) 676f6c64 # "gold" Vigano & Birkholz Expires January 6, 2016 [Page 20] Internet-Draft CBOR notation July 2015 19 0148 # unsigned(328) 68 # text(8) 737570706c696573 # "supplies" a3 # map(3) 00 # unsigned(0) 19 06e8 # unsigned(1768) 01 # unsigned(1) 19 0c0f # unsigned(3087) 02 # unsigned(2) 19 0579 # unsigned(1401) 6c # text(12) 6176675f737472656e677468 # "avg_strength" fb 3fef1492c29f8275 # primitive(4606923564386321013) 65 # text(5) 6d6f766573 # "moves" 84 # array(4) 86 # array(6) 19 06a6 # unsigned(1702) 19 01ca # unsigned(458) 18 26 # unsigned(38) 19 018f # unsigned(399) 19 0147 # unsigned(327) 19 0130 # unsigned(304) 86 # array(6) 19 0c49 # unsigned(3145) 19 1166 # unsigned(4454) 19 0497 # unsigned(1175) 19 0d71 # unsigned(3441) 18 4a # unsigned(74) 19 0606 # unsigned(1542) 86 # array(6) 19 1003 # unsigned(4099) 19 0fde # unsigned(4062) 19 0af8 # unsigned(2808) 08 # unsigned(8) 19 0c66 # unsigned(3174) 19 0be8 # unsigned(3048) 86 # array(6) 19 016f # unsigned(367) 19 0e41 # unsigned(3649) 19 02f4 # unsigned(756) 19 0e3c # unsigned(3644) 19 0e8d # unsigned(3725) 19 0ad1 # unsigned(2769) a3 # map(3) 67 # text(7) 6d6f76655f6e6f # "move_no" 18 c7 # unsigned(199) Vigano & Birkholz Expires January 6, 2016 [Page 21] Internet-Draft CBOR notation July 2015 6b # text(11) 706c617965725f696e666f # "player_info" a6 # map(6) 65 # text(5) 616c696173 # "alias" 64 # text(4) 6369706f # "cipo" 69 # text(9) 706c617965725f6964 # "player_id" 19 10d5 # unsigned(4309) 6a # text(10) 657870657269656e6365 # "experience" 19 0ffe # unsigned(4094) 64 # text(4) 676f6c64 # "gold" 19 1012 # unsigned(4114) 68 # text(8) 737570706c696573 # "supplies" a3 # map(3) 00 # unsigned(0) 19 0369 # unsigned(873) 01 # unsigned(1) 19 1262 # unsigned(4706) 02 # unsigned(2) 19 06c5 # unsigned(1733) 6c # text(12) 6176675f737472656e677468 # "avg_strength" fb 3fd832865ea1b216 # primitive(4600482572053623318) 65 # text(5) 6d6f766573 # "moves" 82 # array(2) 86 # array(6) 19 07b9 # unsigned(1977) 19 0c39 # unsigned(3129) 19 0f32 # unsigned(3890) 19 0fa0 # unsigned(4000) 19 0613 # unsigned(1555) 19 0179 # unsigned(377) 86 # array(6) 19 0a56 # unsigned(2646) 19 011e # unsigned(286) 19 0d23 # unsigned(3363) 19 111d # unsigned(4381) 19 0ee7 # unsigned(3815) 19 040f # unsigned(1039) a3 # map(3) 67 # text(7) 6d6f76655f6e6f # "move_no" Vigano & Birkholz Expires January 6, 2016 [Page 22] Internet-Draft CBOR notation July 2015 19 08b2 # unsigned(2226) 6b # text(11) 706c617965725f696e666f # "player_info" a6 # map(6) 65 # text(5) 616c696173 # "alias" 66 # text(6) 537461636579 # "Stacey" 69 # text(9) 706c617965725f6964 # "player_id" 19 041f # unsigned(1055) 6a # text(10) 657870657269656e6365 # "experience" 18 cf # unsigned(207) 64 # text(4) 676f6c64 # "gold" 19 011d # unsigned(285) 68 # text(8) 737570706c696573 # "supplies" a3 # map(3) 00 # unsigned(0) 19 0cfd # unsigned(3325) 01 # unsigned(1) 19 05eb # unsigned(1515) 02 # unsigned(2) 19 0ce8 # unsigned(3304) 6c # text(12) 6176675f737472656e677468 # "avg_strength" fb 3feb7cf377a65699 # primitive(4605912429042751129) 65 # text(5) 6d6f766573 # "moves" 81 # array(1) 86 # array(6) 19 0365 # unsigned(869) 19 101e # unsigned(4126) 19 094e # unsigned(2382) 19 0c53 # unsigned(3155) 19 05f3 # unsigned(1523) 19 0a3d # unsigned(2621) Figure 7: CBOR instance for game example 4.2. Fruit Figure 8 contains an example for a CBOR structure that contains information about fruit. Vigano & Birkholz Expires January 6, 2016 [Page 23] Internet-Draft CBOR notation July 2015 fruitlist = [* Fruit] Fruit = { name : tstr, colour : [* color], avg_weight : float16, price : uint, international_names : International, rfu : bstr, ; reserved for future use } International = { "DE" : tstr, ; German "EN" : tstr, ; English "FR" : tstr, ; French "NL" : tstr, ; Dutch "ZH-HANS" : tstr, ; Chinese } color = &( black: 0, red: 1, green: 2, yellow: 3, blue: 4, magenta: 5, cyan: 6, white: 7, ) Figure 8: Example CBOR structure 4.3. RFC 7071 [RFC7071] defines the Reputon structure for JSON using somewhat formalized English text. Here is a (somewhat verbose) equivalent definition using the same terms, but notated in CDDL: Vigano & Birkholz Expires January 6, 2016 [Page 24] Internet-Draft CBOR notation July 2015 reputation-object = { reputation-context, reputon-list } reputation-context = ( application: tstr ) reputon-list = ( reputons: reputon-array ) reputon-array = [* reputon] reputon = { rater-value, assertion-value, rated-value, rating-value, ? conf-value, ? normal-value, ? sample-value, ? gen-value, ? expire-value, * ext-value, } rater-value = ( rater: tstr ) assertion-value = ( assertion: tstr ) rated-value = ( rated: tstr ) rating-value = ( rating: float16 ) conf-value = ( confidence: float16 ) normal-value = ( normal-rating: float16 ) sample-value = ( sample-size: uint ) gen-value = ( generated: uint ) expire-value = ( expires: uint ) ext-value = ( tstr => any ) An equivalent, more compact form of this example would be: Vigano & Birkholz Expires January 6, 2016 [Page 25] Internet-Draft CBOR notation July 2015 reputation-object = { application: tstr reputons: [* reputon] } reputon = { rater: tstr assertion: tstr rated: tstr rating: float16 ? confidence: float16 ? normal-rating: float16 ? sample-size: uint ? generated: uint ? expires: uint * tstr => any } Note how this rather clearly delineates the structure somewhat shrouded by so many words in section 6.2.2. of [RFC7071]. Also, this definition makes it clear that several ext-values are allowed (by definition with different member names); RFC 7071 could be read to forbid the repetition of ext-value ("A specific reputon-element MUST NOT appear more than once" is ambiguous.) The CDDL tool (which hasn't quite been trained for polite conversation) says: Vigano & Birkholz Expires January 6, 2016 [Page 26] Internet-Draft CBOR notation July 2015 { "application": "tridentiferous", "reputons": [ { "rater": "loamily", "assertion": "Dasyprocta", "rated": "uncommensurableness", "rating": 0.05055809746548934, "confidence": 0.7484706448605812, "normal-rating": 0.8677887734049299, "sample-size": 4059, "expires": 3969, "bearer": "nitty", "faucal": "postulnar", "naturalism": "sarcotic" }, { "rater": "precreed", "assertion": "xanthosis", "rated": "balsamy", "rating": 0.36091333590593955, "confidence": 0.3700759808403371, "sample-size": 3904 }, { "rater": "urinosexual", "assertion": "malacostracous", "rated": "arenariae", "rating": 0.9210673488013762, "normal-rating": 0.4778762617112776, "sample-size": 4428, "generated": 3294, "backfurrow": "enterable", "fruitgrower": "flannelflower" }, { "rater": "pedologistically", "assertion": "unmetaphysical", "rated": "elocutionist", "rating": 0.42073613384304287, "misimagine": "retinaculum", "snobbish": "contradict", "Bosporanic": "periostotomy", "dayworker": "intragyral" } ] } Vigano & Birkholz Expires January 6, 2016 [Page 27] Internet-Draft CBOR notation July 2015 4.4. Examples from JSON Content Rules Although JSON Content Rules [I-D.newton-json-content-rules] seems to address a more general problem than CDDL, it is still a worthwhile resource to explore for examples (beyond all the inspiration the format itself has had for CDDL). Figure 2 of the JCR I-D looks very similar, if slightly less noisy, in CDDL: root = [2*2 { precision: tstr, Latitude: float, Longitude: float, Address: tstr, City: tstr, State: tstr, Zip: tstr, Country: tstr }] Figure 9: JCR, Figure 2, in CDDL Apart from the lack of a need to quote the member names, text strings are called "tstr" in CDDL. (As an alternative, the simple prelude rule "text = tstr" allows to stick with a more familiar name here.) The CDDL tool creates the below example instance for this: [{"precision": "pyrosphere", "Latitude": 0.5399712314350172, "Longitude": 0.5157523963028087, "Address": "resow", "City": "problemwise", "State": "martyrlike", "Zip": "preprove", "Country": "Pace"}, {"precision": "unrigging", "Latitude": 0.10422704368372193, "Longitude": 0.6279808663725834, "Address": "picturedom", "City": "decipherability", "State": "autometry", "Zip": "pout", "Country": "wimple"}] Figure 4 of the JCR I-D in CDDL: Vigano & Birkholz Expires January 6, 2016 [Page 28] Internet-Draft CBOR notation July 2015 root = { image } image = ( Image: { size, Title: tstr, thumbnail, IDs: [* int] } ) size = ( Width: 0..1280 Height: 0..1024 ) thumbnail = ( Thumbnail: { size, Url: uri } ) This shows how the group concept can be used to keep related elements (here: width, height) together, and to emulate the JCR style of specification. (It also shows using a tag from the prelude, "uri" - this could be done differently.) The more compact form of Figure 5 of the JCR I-D could be emulated like this: root = { Image: { size, Title: tstr, Thumbnail: { size, Url: uri }, IDs: [* int] } } size = ( Width: 0..1280, Height: 0..1024, ) The CDDL tool creates the below example instance for this: {"Image": {"Width": 566, "Height": 516, "Title": "leisterer", "Thumbnail": {"Width": 1111, "Height": 176, "Url": 32("scrog")}, "IDs": []}} Vigano & Birkholz Expires January 6, 2016 [Page 29] Internet-Draft CBOR notation July 2015 5. Making Use of CDDL In this section, we discuss several potential ways to employ CDDL. 5.1. As a guide to a human user CDDL can be used to efficiently define the layout of CBOR data, such that a human implementer can easily see how data is supposed to be encoded. Since CDDL maps parts of the CBOR data to human readable names, tools could be built that use CDDL to provide a human friendly representation of the CBOR data, and allow them to edit such data while remaining compliant to its CDDL definition. 5.2. For automated checking of CBOR data structure CDDL has been specified such that a machine can handle the CDDL definition and related CBOR data. For example, a machine could use CDDL to check whether or not CBOR data is compliant to its definition. The need for thoroughness of such compliance checking depends on the application. For example, an application may decide not to check the data structure at all, and use the CDDL definition solely as a means to indicate the structure of the data to the programmer. On the other end, the application may also implement a checking mechanism that goes as far as checking that all mandatory map pairs are available. The matter in how far the data description must be enforced by an application is left to the designers and implementers of that application, keeping in mind related security considerations. In no case the intention is that a CDDL tool would be "writing code" for an implementation. 5.3. For data analysis tools In the long run, it can be expected that more and more data will be stored using the CBOR data format. Where there is data, there is data analysis and the need to process such data automatically. CDDL can be used for such automated data processing, allowing tools to verify data, clean it, and extract particular parts of interest from it. Vigano & Birkholz Expires January 6, 2016 [Page 30] Internet-Draft CBOR notation July 2015 Since CBOR is designed with constrained devices in mind, a likely use of it would be small sensors. An interesting use would thus be automated analysis of sensor data. 6. Open Issues CDDL already is usable in its present form, as Section 4.3 should have demonstrated. However, additional examples should be developed, and some experience be gained with the usefulness of tools built around CDDL. At least the following issues need further work: o The precise semantics of occurrence indicators as defined in Section 3.2 needs to be explained in more detail. E.g., the exact semantics of an occurrence indicators on a group name in a map (which means the entire group can occur in this way). o Build good use cases that demonstrate vector, record, table and struct usage. o There probably are some security considerations. 7. Resolved Issues o To indicate optionality, occurrence indicators are now always used: Mandatory fields in structures are unadorned group entries; optional group entries are prefixed by the "?" occurrence indicator. o We used to use the '.' character as a wildcard for datatypes. This originated from the "map(tstr,.)" notation, but maybe hard to read in notations such as "aField: .;". We now have replaced '.' with a new prelude rule called "any", such that the example becomes "aField: any;", and the map notation "{tstr => any}". o The key/value pairs in maps have no fixed ordering. However, there may be situations where fixing the ordering may be of use. For example, an decoder could look for values related with integer keys 1, 3 and 7. If the order was fixed and the decoder encounters the key 4 without having encountered key 3, it can conclude that key 3 is not available without doing more complicated bookkeeping. Unfortunately, neither JSON nor CBOR support this, so no attempt was made to support this in CDDL either. o CDDL distinguishes the various CBOR number types, but there is only one number type in JSON. There is no effect in specifying a Vigano & Birkholz Expires January 6, 2016 [Page 31] Internet-Draft CBOR notation July 2015 precision (float16/float32/float64) when using CDDL for specifying JSON data structures. (The current validator implementation Appendix F does not handle this very well, either.) 8. Security considerations This document presents a content rules language for expressing CBOR data structures. As such, it does not bring any security issues on itself, although specification of protocols that use CBOR naturally need security analysis when defined. Topics that could be considered in a security considerations section that uses CDDL to define CBOR structures include the following: o Where could the language maybe cause confusion in a way that will enable security issues? 9. IANA considerations This document does not require any IANA registrations. 10. Acknowledgements CDDL was originally conceived by Bert Greevenbosch, who also wrote the original five versions of this document. Inspiration was taken from the C and Pascal languages, MPEG's conventions for describing structures in the ISO base media file format, Relax-NG and its compact syntax [RELAXNG], and in particular from Andrew Lee Newton's "JSON Content Rules" [I-D.newton-json-content-rules]. Useful feedback came from Carsten Bormann, Joe Hildebrand, Sean Leonard and Jim Schaad. The CDDL tool was written by Carsten Bormann, building on previous work by Troy Heninger and Tom Lord. 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. Vigano & Birkholz Expires January 6, 2016 [Page 32] Internet-Draft CBOR notation July 2015 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, October 2013. [RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, March 2014. 11.2. Informative References [RELAXNG] OASIS, "RELAX-NG Compact Syntax", November 2002, . [RFC7071] Borenstein, N. and M. Kucherawy, "A Media Type for Reputation Interchange", RFC 7071, November 2013. [I-D.newton-json-content-rules] Newton, A., "A Language for Rules Describing JSON Content", draft-newton-json-content-rules-04 (work in progress), December 2014. Appendix A. Cemetery The following ideas are buried for now: o <...> as syntax for enumerations. We view values to be very specific types, so that an enumeration can be denoted as a choice using "/" as the delimiter of choices. Because of this, no evidence is present that a separate syntax for enumerations is needed. Appendix B. Nursery This appendix describes advanced features that are still under heavy review. B.1. Annotations An _annotation_ allows to annotate a _target_ type with a _control_ type via an _annotator_. The syntax for an annotated type is "target .annotator control", where annotators are special identifiers prefixed by a dot. (Note that _target_ or _control_ might need to be parenthesized.) Vigano & Birkholz Expires January 6, 2016 [Page 33] Internet-Draft CBOR notation July 2015 Three annotators are defined at his point. Note that the CDDL tool does not currently support combining multiple annotations on a single target. B.1.1. Annotation .size A ".size" annotation controls the size of the target in bytes by the control type. Examples: full-address = [[+ label], ip4, ip6] ip4 = bstr .size 4 ip6 = bstr .size 16 label = bstr .size (1..63) Figure 10: Annotation for size in bytes In the CDDL tool, the target must be a byte string for now. B.1.2. Annotation .bits A ".bits" annotation on a byte string indicates that, in the target, only the bits numbered by a number in the control type are allowed to be set. (Bits are counted the usual way, bit number "n" being set in "str" meaning that "(str[n >> 3] & (1 << (n & 7))) != 0".) [_bitsendian] Similarly, a ".bits" annotation on an unsigned integer "i" indicates that for all unsigned integers "n" where "(i & (1 << n)) != 0", "n" is in the control type. tcpflagbytes = bstr .bits flags flags = &( fin: 8, syn: 9, rst: 10, psh: 11, ack: 12, urg: 13, ece: 14, cwr: 15, ns: 0, ) / (4..7) ; data offset bits rwxbits = uint .bits rwx rwx = &(r: 2, w: 1, x: 0) Figure 11: Annotation for what bits can be set Vigano & Birkholz Expires January 6, 2016 [Page 34] Internet-Draft CBOR notation July 2015 The CDDL tool generates the following ten example instances for "tcpflagbytes": h'906d' h'01fc' h'8145' h'01b7' h'013d' h'409f' h'018e' h'c05f' h'01fa' h'01fe' These examples do not illustrate that the above CDDL specification does not explicitly specify a size of two bytes: A valid all clear instance of flag bytes could be "h''" or "h'00'" or even "h'000000'" as well. B.1.3. Annotation .regexp A ".regexp" annotation indicates that the text string given as a target needs to match the PCRE regular expression given as a value in the control type, where that regular expression is anchored on both sides. (If anchoring is not desired for a side, ".*" needs to be inserted there.) nai = tstr .regexp "\\w+@\\w+(\\.\\w+)+" Figure 12: Annotation with a PCRE regexp The CDDL tool proposes: "N1@CH57HF.4Znqe0.dYJRN.igjf" B.2. Socket/Plug Both for type choices and group choices, a mechanism is defined that facilitates starting out with empty choices and assembling them later, potentially in separate files that are concatenated to build the full specification. tcp-header = {seq: uint, ack: uint, * $$tcp-option} ; later, in a different file $$tcp-option //= ( sack: [+(left: uint, right: uint)] ) ; and, maybe in another file $$tcp-option //= ( sack-permitted: true ) Vigano & Birkholz Expires January 6, 2016 [Page 35] Internet-Draft CBOR notation July 2015 Names that start with a single "$" are "type sockets", names with a double "$$" are "group sockets". It is not an error if there is no definition for a socket; this then means there is no way to satisfy the rule (i.e., the choice is empty). All definitions (plugs) for socket names must be augments, i.e., they must be using "/=" and "//=", respectively. To pick up the example illustrated in Figure 5, the socket/plug mechanism could be used as shown in Figure 13: PersonalData = { ? displayName: tstr, NameComponents, ? age: uint, * $$personaldata-extensions } NameComponents = ( ? firstName: tstr, ? familyName: tstr, ) ; The above already works as is. ; But then, we can add later: $$personaldata-extensions //= ( favorite-salsa: tstr, ) ; and again, somewhere else: $$personaldata-extensions //= ( shoesize: uint, ) Figure 13: Personal Data example: Using socket/plug extensibility B.3. Generics Using angle brackets, the left hand side of a rule can add formal parameters after the name being defined, as in: messages = message<"reboot", "now"> / message<"sleep", 1..100> message = {type: t, value: v} When using a generic rule, the formal parameters are bound to the actual arguments supplied (also using angle brackets), within the Vigano & Birkholz Expires January 6, 2016 [Page 36] Internet-Draft CBOR notation July 2015 scope of the generic rule (as if there were a rule of the form parameter = argument). (There are some limitations to nesting of generics in Appendix F at this time.) Appendix C. Change Log Changes from version 00 to version 01: o Removed constants o Updated the tag mechanism o Extended the map structure o Added examples Changes from version 01 to version 02: o Fixed example Changes from version 02 to version 03: o Added information about characters used in names o Added text about an overall data structure and order of definition of fields o Added text about encoding of keys o Added table with keywords o Strings and integer writing conventions o Added ABNF Changes from version 03 to version 04: o Removed optional fields for non-maps o Defined all key/value pairs in maps are considered optional from the CDDL perspective o Allow omission of type of keys for maps with only text string and integer keys o Changed order of definitions Vigano & Birkholz Expires January 6, 2016 [Page 37] Internet-Draft CBOR notation July 2015 o Updated fruit and moves examples o Renamed the "Philosophy" section to "Using CDDL", and added more text about CDDL usage o Several editorials Changes from version 04 to version 05: o Added text about alternative datatypes and any datatype o Fixed typos o Restructured syntax and semantics Changes from version 05 to version 05: o Fixed the ABNF for choices (no longer need to write a: (b/c)) o Added group choices (//) o Added /= and //= o Added experimental socket/plug o Added aliases text, bytes, null to prelude o Documented generics o Fixed more typos Appendix D. ABNF grammar The following is a formal definition of the CDDL syntax in Augmented Backus-Naur Form (ABNF, [RFC5234]). [_abnftodo] cddl = S 1*rule rule = typename [genericparm] S assign S type S / groupname [genericparm] S assign S grpent S typename = id groupname = id assign = "=" / "/=" / "//=" genericparm = "<" S id S *("," S id S ) ">" genericarg = "<" S type1 S *("," S type1 S ) ">" Vigano & Birkholz Expires January 6, 2016 [Page 38] Internet-Draft CBOR notation July 2015 type = type1 S *("/" S type1 S) type1 = type2 [S (rangeop / annotator) S type2] / "#" "6" ["." uint] "(" S type S ")" ; note no space! / "#" DIGIT ["." uint] ; major/ai / "#" ; any / "{" S group S "}" / "[" S group S "]" / "&" S "(" S group S ")" / "&" S groupname [genericarg] type2 = value / typename [genericarg] / "(" type ")" rangeop = "..." / ".." annotator = "." id group = grpchoice S *("//" S grpchoice S) grpchoice = *grpent grpent = [occur S] [memberkey S] type optcom / [occur S] groupname [genericarg] optcom ; preempted by above / [occur S] "(" S group S ")" optcom memberkey = type1 S "=>" / bareword S ":" / value S ":" bareword = id optcom = S ["," S] occur = [uint] "*" [uint] / "+" / "?" uint = ["0x" / "0b"] "0" / ["0x" / "0b"] DIGIT1 *DIGIT value = number / string int = ["-"] uint ; This is a float if it has fraction or exponent; int otherwise Vigano & Birkholz Expires January 6, 2016 [Page 39] Internet-Draft CBOR notation July 2015 number = int ["." fraction] ["e" exponent ] fraction = 1*DIGIT exponent = int string = %x22 *SCHAR %x22 SCHAR = %x20-21 / %x23-7E / SESC SESC = "\" %x20-7E id = EALPHA *(*("-" / ".") (EALPHA / DIGIT)) ALPHA = %x41-5A / %x61-7A EALPHA = %x41-5A / %x61-7A / "@" / "_" / "$" DIGIT = %x30-39 DIGIT1 = %x31-39 S = *WS WS = SP / NL SP = %x20 NL = COMMENT / CRLF COMMENT = ";" *(SP / VCHAR) CRLF VCHAR = %x21-7E CRLF = %x0A / %x0D.0A Figure 14: CDDL ABNF Appendix E. Standard Prelude The following prelude is automatically added to each CDDL file. (Note that technically, it is a postlude, as it does not disturb the selection of the first rule as the root of the definition.) Vigano & Birkholz Expires January 6, 2016 [Page 40] Internet-Draft CBOR notation July 2015 any = # uint = #0 nint = #1 int = uint / nint bstr = #2 bytes = bstr tstr = #3 text = tstr tdate = #6.0(tstr) time = #6.1(number) number = int / float biguint = #6.2(bstr) bignint = #6.3(bstr) bigint = biguint / bignint integer = int / bigint decfrac = #6.4([e10: int, m: integer]) bigfloat = #6.5([e2: int, m: integer]) eb64url = #6.21(any) eb64legacy = #6.21(any) eb16 = #6.21(any) encoded-cbor = #6.24(bstr) uri = #6.32(tstr) b64url = #6.33(tstr) b64legacy = #6.34(tstr) regexp = #6.35(tstr) mime-message = #6.36(tstr) cbor-any = #6.55799(any) float16 = #7.25 float32 = #7.26 float64 = #7.27 float16-32 = float16 / float32 float32-64 = float32 / float64 float = float16-32 / float64 false = #7.20 true = #7.21 bool = false / true nil = #7.22 null = nil undefined = #7.23 Figure 15: CDDL Prelude Vigano & Birkholz Expires January 6, 2016 [Page 41] Internet-Draft CBOR notation July 2015 Note that the prelude is deemed to be fixed. This means, for instance, that additional tags beyond [RFC7049], as registered, need to be defined in each CDDL file that is using them. Appendix F. The CDDL tool A rough CDDL tool is available. For CDDL specifications that do not use recursion, it can check the syntax, generate one or more instances (expressed in CBOR diagnostic notation or in pretty-printed JSON), and validate an existing instance against the specification: Usage: cddl spec.cddl generate [n] cddl spec.cddl json-generate [n] cddl spec.cddl validate instance.cbor cddl spec.cddl validate instance.json Figure 16: CDDL tool usage Install on a system with a modern Ruby via: gem install cddl Figure 17 The accompanying CBOR diagnostic tools (which are automatically installed by the above) are described in https://github.com/cabo/ cbor-diag ; they can be used to convert between binary CBOR, a pretty-printed form of that, CBOR diagnostic notation, JSON, and YAML. Editorial Comments [_format] This item requires some further discussion, i.e. we aren't there yet and/or we don't really know whether we want to be there. [_reptype] We don't have a way yet to qualify the representation of a value, e.g., whether it is float16, float32 or float64. TO DO: probably borrowing something from diagnostic notation (section 6.1 RFC 7049). [_range] TO DO: define precisely. This clearly includes integers and floats. Strings - as in "a".."z" - could be added if desired, but this would require adopting a definition of string ordering and possibly a successor function so "a".."z" does not include "bb". Vigano & Birkholz Expires January 6, 2016 [Page 42] Internet-Draft CBOR notation July 2015 [_strings] TO DO: This still needs to be fully realized in the ABNF and in the CDDL tool. [_bitsendian] How useful would it be to have another variant that counts bits like in RFC box notation? (Or at least per-byte? 32-bit words don't always perfectly mesh with byte strings.) [_abnftodo] TO DO: This doesn't allow non-ASCII characters in the text strings yet; there is no value notation for byte strings; representation indicators are missing as well. Authors' Addresses Christoph Vigano Universitaet Bremen Email: christoph.vigano@uni-bremen.de Henk Birkholz Fraunhofer SIT Rheinstrasse 75 Darmstadt 64295 Germany Email: henk.birkholz@sit.fraunhofer.de Vigano & Birkholz Expires January 6, 2016 [Page 43]