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What is ASN.1

What is ASN.1. Internationally standardised, vendor, platform & language independent notation for specifying data structures at a high level of abstraction

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What is ASN.1

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  1. What is ASN.1 • Internationally standardised, vendor, platform & language independent notation for specifying data structures at a high level of abstraction • It is supported by encoding rules that determine the precise bit patterns to represent values of these data-structures that will be transferred over computer network. • It is supported by tools which map ASN.1 notation into data structure definition in a computer language of choice.

  2. ASN.1 - Outline • ASN.1 Specification • ASN.1 Basic Encoding Rules • ASN.1 Compilers • ASN.1 & SDL

  3. Abstract Syntax Notation 1 (ASN.1) • ASN.1 was defined by ISO, as generic means of allowing differing computer systems, with different internal data representations to interchange data. • ASN.1 is now extensively used by the ITU & ISO to define protocol headers and formats. • In OSI, ASN.1 is used to define presentation layer applications.

  4. ASN.1 Standards • X.680 ASN.1: Specification of basic notation • X.681 ASN.1: Information object specification • X.682 ASN.1: Constraint specification • X.683 ASN.1: Parameterization of ASN.1 specifications • X.690 ASN.1 encoding rules - Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) • X.691 ASN.1 encoding rules - Specification of Packed Encoding Rules (PER)

  5. ASN.1 Introduction • Basic Data Types • INTEGER, • BOOLEAN, • UFT8String and other • Production Mechanisms for constructing more complex data types • SEQUENCE, SEQUENCE OF • SET, SET OF • CHOICE and other • Elements for Structuring the ASN.1 specification • Modules, Module Headers • Exports/Imports Statements

  6. ASN.1 Character Set • An ASN.1 item shall consist of a sequence of the characters: • A to Z • a to z • 0 to 9 • : = , { } < . @ ( ) [ ] - ' " | & ^ * ; ! • ASN.1 is case sensitive ! • Font type (bold, italic, size …) is not significant

  7. Naming Rules • Names of fields, elements & items can be arbitrarily long. Quite long names are common • Names of Types must start with uppercase • Names of Identifiers must start with lowercase Production Mechanism Type Name Person-type ::= SEQUENCE {name VisibleString age INTEGER, employed BOOLEAN} Identifier Built-in, Simple Type

  8. Built-In Types • BOOLEAN Type • Has two values: TRUE and FALSE • INTEGER Type • Integer numbers … -2,-1, 0, 1, 2 … • ENUMERATED Type - Enumeration is way to assign name (identifier) to integers • Example: LeadTime-type ::= ENUMERATED { tomorrow (0), three-day (1), week (2)} Latest ASN.1 doesn’t require numbers

  9. More Built-In Types • REAL Type • values: {mantissa 3.14159, base 10, exponent -5} , 0 , PLUS-INIFINITY, MINUS-INFINITY • BITSTRING Type • value example: ‘101100110001’B or ‘B31’H (in Hex) • NULL Type • NULL Type has just one value: NULL • It is most often used in CHOICE constructs • E.G. Toys ::= CHOICE { cars Cars, dolls Dolls, none NULL}

  10. ANY Type • A type of type ANY can be set to any type • e.g. MyThing ::= ANY • This states that the definition of the type is beyond the scope of ASN.1 • Sometimes is referring to items that are to be defined in future.

  11. Object Identifier Type • Object identifiers are defining a globally unique naming space • Object identifier name space is defined by hierarchically allocated tree-structure • Each arc of the tree is labeled by a numeric value. • Global authorities are responsible for allocation of top level arcs • More local authorities are responsible for the lower-level arcs

  12. Example Object Identifier • {iso standard 8571 pci (1)} or {1 0 8571 1} • Would each identify an object, “pci”, defined in ISO 8571. • With the following additional definition:ftam OBJECT IDENTIFIER ::= {iso standard 8571} • Above defined pci object may be then also uniquely identified as {ftam pci(1)}

  13. Built-In Character String Types • PrintableString • old “telex” character set (not very useful today) • VisibleString • all printiable ASCII characters plus “space” • UTF8String • any character (form Egyptian hieroglyphs to ASCII) • Other like: • NumericString, TeletexString, VideotexString, IA5String, GraphicString, GeneralString, UniversalString, BMPString • Value Notation • simple: “This is example of value notation” • or more complex: {null,{0,0,4,29} cyrillicCapitalLetterIe, “ABC”}

  14. Built-In Date/Time Types • UTCTime • two digits for year (+49, -50 from the current year) • value example: yymmddhhmmZ : 0003221830Z (Z is for GMT) • GeneralizedTime • same as UTCTime but with four digits for year

  15. Production • Each production consists of the following parts, on one or several lines, in order: • a name for the new collection of production sequences; • the characters ::= • one or more alternative collections of production sequences (defined later) Example: • ClassMember ::= CHOICE { • student Student, • lecturer Lecturer • }

  16. SEQUENCE Production Mechanism • A SEQUENCE is a sequence of elements of differing type where the order is important. • Example Type notation: SEQUENCE {name IA5String, age INTEGER} • Value Notation: {name “John”, age 26} • A SEQUENCE OF is a sequence of elements of the same type where order is important. • Example Type notation: SEQUENCE OF INTEGER • Value Notation: {12,13,14}

  17. SET Production Mechanism • A SET is a sequence of elements of differing type where the order is not important. • Example Type notation: SET { name IA5String, age INTEGER } • Value Notation: {age 26, name “John”,age 23, age 6} • A SET OF is a sequence of elements of the same type where the order is not important. • Example Type Notation: SET OF INTEGER • Example Value Notation: {12,13,14} • SETs are more expensive for encoding so SEQUENCES are preferable

  18. Default & Optional • Keyword DEFAULT • Identifies a default value for an element of a SEQUENCE or SET to be assumed if value for that element is not included • Keyword OPTIONAL • Identifies an element for which a value can be omitted • Example: Person::= SEQUENCE { name UTF8String, number-of-parents INTEGER DEFAULT 2, spouse-name UTF8String OPTIONAL }

  19. CHOICE Production Mechanism • A CHOICE is an element that can be constructed as one of a finite set of types. • Example of type notation: CHOICE {name IA5String, nickname IA5String , TFN INTEGER} • value notation : {name:“Brad”} or {nickname:“Braddles” } or {TFN:123456}

  20. Tagged types • A tagged type is a new type which is decoded and encoded identically to the type it derives from, but has a different tag • Tags are used to differentiate between items of the same base type • Example: Flight ::= SEQUENCE {origin IA5String, stop1 [0] IA5String OPTIONAL, stop2 [1] IA5String OPTIONAL, destination IA5String }

  21. Subtyping • Simple Subtyping • INTEGER (1..56) subset of integers in the range of 1 to 56 inclusive • It is used to improve encoding efficiency (subtypes require less bytes for encoding then full length types) • Complex Subtyping involves using construct like: • SIZE, FROM • INTERSECTION, UNION, EXCEPT, ALL • Example of Complex Subtyping • INTEGER (ALL EXCEPT (1..20)) • (“yes” UNION “no” UNION “maybe”) • PrintableString (SIZE (10)) limit to exactly 10 characters • PrintableString (SIZE (1..10))

  22. Soft Typing • The value of ANY types can be constrained using the DEFINED BY syntax. Eg X.500 or CMIP; Attribute ::= SEQUENCE { type OBJECT IDENTIFIER, value ANY DEFINED BY type } This is sometimes called Soft Typing.

  23. Soft typing • When receiving a message containing these messages, a presentation entity should in theory look in an "attribute catalogue", using the specific value of the "type" as a key. • The attribute catalogue will specify, among other things, contain the ASN.1 specification of the attribute values.

  24. Soft Typing Example TextBook ::= SEQUENCE {author IA5String, CitationType INTEGER, reference ANY DEFINED BY CitationType }, • Type TextBook requires a list of CitationTypes defined elsewhere that specifies the ASN.1 type for each permitted value of INTEGER • Possible Values are:{ author ”Milan Kundera", CitationType 0, reference IA5String "ISBN0571178170" } • or { author "Milan Kundera", CitationType 1, reference INTEGER 1995 } • or { author "Milan Kundera", CitationType 2, reference PrintableString ”Slowness" }

  25. ASN.1 Macros • Macros are a mechanism for the user to define a new notation with which they can then construct and reference ASN.1 types or specify values of types. • Shorthand notation.

  26. Example Macro • Suppose it is desired to have a notation for type definition of the form • PAIR TYPEX = . . . . TYPEY = . . . . • with a corresponding value notation allowing • (X = ----, Y = ----) • Where the . . . . and the ---- refer to any ASN.1 type and corresponding value respectively. * Example from X.208

  27. Production Rules • A macro definition consists of a set of "production rules", described in "Bakus-Naur" like form. • Several alternatives separated by "|" • Each alternative described by a list of "tokens", which can be: • place holders for ASN.1 types or values, • keywords enclosed within quotes, like "TYPEX", "=" or "=" in our example, • place holders for identifiers, or strings

  28. Macro Definitions • A macro definition must always contain at least two production rules, called "TYPE NOTATION" and "VALUE NOTATION". • The "TYPE NOTATION", preceded by the MACRO name, can be used wherever a "type definition" is expected within the ASN.1 language, • the syntax defined in the "VALUE NOTATION" can be used to define a value for a type defined by a macro

  29. Macro Type Notation PAIR MACRO ::= BEGIN TYPE NOTATION ::= “TYPEX” “=” type (Local-type-1) -- Expects any ASN.1 type and assigns it -- to the variable Local-type-1; “TYPEY” “=” type (Local-type-2) -- Expects a second ASN.1 type and assigns --it to the variable Local-type-2;

  30. Macro Value Notation VALUE NOTATION ::= “(” “X” “=” value (Local-value-1 Local-type-1) -- Expects a value for the type in Local-type-1, and assigns it -- to the variable Local-value-1; “,” “Y” “=” value (Local-value-2 Local-type-2) -- Expects a value for the type in Local-type-2 and assigns it -- to the variable Local-value-2; <VALUE SEQUENCE {Local-type-1, Local-type-2} ::= {Local-value-1, Local-value-2}> -- This “embedded definition” returns -- the final value as the value -- of a sequence of the two types. “)” END

  31. Example Macro T1 ::= PAIR TYPEX = INTEGER TYPEY = BOOLEAN T2 ::= PAIR TYPEX = VisibleString TYPEY = T1 Then a value of type T1 might be: (X = 3, Y = TRUE) and a value of type T2 might be: (X = “Name”, Y = (X = 4, Y = FALSE))

  32. X.500 Directory Databases ROSE Remote operations TCAP IN protocol X.509 Security Protocol X.400 Messaging Q.932 ISDN Supplementary Services USSD Supplementary Service Data for GSM CMIP Network Management Use of Macros in Protocol Definition • Its usually more important to know how to use a particular Macro than how to define new Macros. • The following protocols use ASN.1 Macros. *This is list is by no way complete.

  33. Structuring an ASN.1 Specification • ASN.1 definitions are defined in modules. • Modules contain a complete set of type definitions that are self contained. • Modules can import definitions from other modules, and export definitions to other modules.

  34. EXPORTS/IMPORTS Statements • IMPORTS - specifies list of types defined in other modules • EXPORTS - specifies list of types defined in this module that are available for use in other modules • if EXPORTS statements is omitted all types are available • EXPORTS ; (empty list of types) means that nothing is available • IMPORTS/EXPORTS Statements are optional and if exist they should be located at the head of the module after BEGIN statement and before any type definition • Semicolon is used to terminate EXPORT/IMPORT statements

  35. Example ASN.1 Module • ExampleASN.1 DEFINITIONS ::= • -- This is a comment • -- Start of module • BEGIN • EXPORTS Student; • -- This can be re-used in other definitions • Student ::= SEQUENCE { • -- A sequence order is important • age INTEGER, • name IA5String • } • ClassRoom ::= SET OF Student • -- order is not important in a SET • END

  36. Basic Encoding Rules • An abstract syntax is converted into a transfer syntax using an encoding rule. • ASN.1 has a set of basic encoding rules defined in ISO 8825/ X.690. • X.690 is very inefficient, so Packed Encoding Rules (X.691) are defined which are more compact

  37. Basic Encoding Rules • Each data item is encoded to contain: • an identifier (tag or type) • a length indicating the size of the data field • a data, which contains the actual contents of the object. • an optional end-of-content flag if data length is unknown • Aligned on octet boundaries

  38. IDENTIFIER LENGTH CONTENTS OCTETS OCTETS OCTETS The number of octets in the contents octets. Encoding Length - Definite Form • If length < 128, use a single byte, with the high bit set to zero. • if high bit is set to one, the low seven bits set to the length of length field. • The length is then encoded in that many bytes.

  39. Encoding length - Indefinite Form. • If length field has a value of 0 - i.e. [1000|0000]. • The object is ended by sending two zero bytes. IDENTIFIER LENGTH CONTENTS END OF OCTETS OCTETS OCTETS CONTENTS Indicates that the there are no more contents octets. Indicates that the end of contents, is used instead of length.

  40. IDENTIFIER-A LENGTH-A IDENTIFIER-B LENGTH-B CONTENTS Value B is a part of structure A. Identifier and length of B is carried in content part of A. Basic Encoding RulesConstructor Types • Types contained within types, are sent in the content portion of the message.

  41. ASN.1 Example Types • PersonnelRecord ::= SET { • Name, • title [0] VisibleString, • children [1] SEQUENCE OF • ChildInformation DEFAULT {} • } • ChildInformation ::= SET { • Name, dateOfBirth [0] Date} • Name:: = SEQUENCE { • givenName VisibleString, • familyName VisibleString}

  42. ASN.1 Example Value A set of records with one value (John Smiths might be as follows); • { {givenName "John",familyName "Smith"}, • title "Director", • children • {{{givenName "Ralph",familyName "Smith"}, • dateOfBirth "19571111"}, • {{givenName "Susan",familyName "Jones"}, • dateOfBirth "19590717"}} • }

  43. ASN.1 Example Encoding • Personnel • Record Length Contents • 60 8185 Set Length Contents • 31 8183 Name Length Contents • 61 0D Visible- Length Contents • String • 1A 04 "John" • Visible- Length Contents • String • 1A 05 "Smith" • Title Length Contents • A0 0A Visible- Length Contents • String • 1A 08 "Director' • .... (removed for brevity) ...

  44. ASN.1 Example Encoding (cont) [1] Length Contents A1 42 Set Length Contents 31 1F Name Length Contents 61 0E Visible- Length Contents String 1A 05 "Ralph" Visible- Length Contents String 1A 05 "Smith" .... (removed for brevity) ...

  45. Packed Encoding Rules • Specialised encoding based on the data type. • Only generates tags when they are needed to prevent ambiguity. • Only generates lengths when the size of an object can vary. • lengths represented in compact form.

  46. Packed Encoding Rules • Encoding not always aligned on byte boundaries • unless 'aligned' variant of the rules is used • Optional elements in a sequence are indicated by a list of single bit flags placed at the start of a sequence .

  47. To implement a protocol defined in ASN.1 • Hand code • or feed the ASN.1 specification in an "ASN.1 compiler", that will automatically generate the conversion routines.

  48. ASN.1 Compilers • Compiled Manual • Develop an application Days Months • Update a protocol Hours Weeks • Achieve conformance Correct Test and • Input fail • Reliability of code Good Dubious • Debugging tools Yes ??? • Speed of decoding Good Excellent... • ...or poor • Respect of layering Natural Depends • on discipline

  49. Using ASN.1 for Protocol Specification & Implementation. • ASN.1 compilers can be used to create C structures or C++ classes from ASN.1 specifications. • Public domain ASN.1 compilers available unfortunately, very few implement MACROs or SoftTyping required for most applications that use ASN.1

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