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An Introduction to Abstract Syntax Notation 1 (ASN.1)

An Introduction to Abstract Syntax Notation 1 (ASN.1). Steinar Andresen/Rolv Bræk/Finn Arve Aagesen Norwegian University of Science and Technology, Trondheim. The need of a “common language” In order to cooperate the following needs arise.

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An Introduction to Abstract Syntax Notation 1 (ASN.1)

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  1. An Introduction to Abstract Syntax Notation 1 (ASN.1) Steinar Andresen/Rolv Bræk/Finn Arve Aagesen Norwegian University of Science and Technology, Trondheim

  2. The need of a “common language”In order to cooperate the following needs arise A shared interpretation of what the data means and a common protocol for interchange • A shared view on data structure The ASN.1 Notation • Transfer coding rulesThe ASN.1 Encoding schemes: BER, PER, .. L A N - O r g A P U B L I C W A N L A N O r g C L A N O r g B

  3. The history of ASN.1 General purpose notation and encoding scheme: • Developed to be applied to the MHS protocols (X.400) (“born” in 1982) • Generalised as general tools ISO 8824 Notation and ISO 8825 Encoding rules in 1990, Revised in 1995 • General use in many application fields today

  4. The original “realm” of ASN • Applied first to the Presentation layer in the “Open Systems Interconnection”. • Soon used to define the protocols of the Applications layer:FTAM, ROSE, MHS, etc. • Also used for lower layers

  5. Why ASN.1 and BER? O S I P r o t o c o l S t a c k O S I P r o t o c o l S t a c k A p p l i c a t i o n A p p l i c a t i o n ASN.1 for the data syntax P r e s e n t a t i o n P r e s e n t a t i o n PDU PDU s S e s s i o n S e s s i o n n o T r a n s p o r t T r a n s p o r t BER for the (sequential) transfer syntax N e t w o r k N e t w o r k D a t a l i n k D a t a l i n k P h y s i c a l P h y s i c a l

  6. 0 1 | 0 | 1 | 0 | 0 | 1 | 1 ASN.1 Use A PDU can be a complex element (letter, document, …) and: • specified using datatypes of SDL, LOTOS, UML, ... • implemented using datatypes of CHILL, C++, Java, ... ASN.1 provides a language independent syntax and ASN.1 compilers take care of the mapping C++, Java, SDL... PDU PDU C++, Java, SDL... ASN.1 ASN.1 BER Decode Encode C++, Java,... C++, Java,...

  7. ASN.1 Module • A module is a set of ASN.1 definitions assembled for a specific purpose. • The structure of a module is:ModuleDefinition::= ModuleIdentifier DEFINITIONSDEFINITIONS::= BEGIN ModuleBody END

  8. ASN.1 Module Identifier • ModuleIdentifer is an element of type Object Identifiers • Object Identifiers are adminstrered by ISO, ITU-T, etc. A Module Identifier represents an official reference to the Module.

  9. Module Body ModuleBody::= Exports -- definitions that may be exported to other modules Imports -- definitions that are imported from other modules AssignmentList --this modules definitions | -- “|” means “or” empty

  10. W h e a t h e r R e p o r t : : = S EQUENCE{ . . . . . . } N a m e o f a t y p e r e f e r e n c e t o d e f i n e d t y p e Assignments Type assignment Value assignment s a m p l e W R e p o r t W h e a t h e r R e p o r t : : = { . . . . . . } N a m e o f a v a l u e T h e t y p e o f t h i s v a l u e A c t u a l v a l u e s p e c .

  11. Type assignment A type assignment has three syntactic elements: • the type reference (the name allocated to it), • the symbol “::=“ (means defined as) and • the appropriate type notation WheatherReport ::= SEQUENCE { stationNumber INTEGER {1..99999} timeOfReport UTCTime ......... }

  12. Value assignment A value assignment has four syntactic elements: • the value reference (the name allocated), • the type to which the value belongs • the symbol “::=“ (means defined as) and • the appropriate value notation Sample value assignment: sampleReport WheatherReport ::= { stationNumber 73290 timeOfReport “900102125703Z” ......... }

  13. Sequence Letter ::= SEQUENCE { opening OCTETSTRING, body OCTETSTRING, closing OCTETSTRING, address AddressType } AddressType ::= SEQUENCE { name OCTETSTRING, number INTEGER, street OCTETSTRING, postOffice OCTETSTRING, state OCTETSTRING, zipCode INTEGER }

  14. Optional AddressType ::= SEQUENCE { name OCTETSTRING, number INTEGER, street OCTETSTRING, apartNumber INTEGER OPTIONAL, postOffice OCTETSTRING, state OCTETSTRING, zipCode INTEGER }

  15. Tags Letter ::= SEQUENCE { opening OCTETSTRING, body OCTETSTRING, closing OCTETSTRING, receiverAddr AddressType OPTIONAL, senderAddr AddressType OPTIONAL } Letter ::= SEQUENCE { opening OCTETSTRING, body OCTETSTRING, closing OCTETSTRING, receiverAddr [0] AddressType OPTIONAL, senderAddr [1] AddressType OPTIONAL }

  16. Spelling Conventions • Reserved words: only CAPITAL letters (INTEGER, CHOICE,…) • Types: first letter Capital (TransactionID) • Value reference: first letter Lowercase (application(0)) • Identifier: first letter Lowercase (destination, responder) • Macro: only CAPITAL letters

  17. ASN.1 Tagged Types Four classes: 1. UNIVERSAL (ASN.1 In-build types) 2. Application specific types (non-ambiguous within module) 3. Context specific types (non-ambiguous within construct) 3. Private (non-ambiguous within some ”enterprise”) UNIVERSAL: • Simple types (BOOLEAN, INTEGER, ENUMERATED, …) • Constructor types (SEQUENCE, SEQUENCE OF, SET, SETOF) • Additional predefined (NumericString, Teletxstring, GeneralizedTime, …) • OBJECT IDENTIFIER • Tagging is needed to recognise the types in the transfer syntaxt (when decoding a received PDU)

  18. Universal Types Universal Tag ASN.1 Type • 1 BOOLEAN • 2 INTEGER • 3 BITSTRING • 4 OCTETSTRING • 5 NULL • 6 OBJECTIDENTIFIER • 7 ObjectDescriptor • 8 EXTERNAL • 9 REAL • 10 ENUMERATED • 11-15 reserved for addenda • 16 SEQUENCE, SEQUENCE OF • 17 SET, SET OF • … ...

  19. Example Example DEFINITIONS::= BEGIN TransactionID ::= INTEGER Component ::= INTEGER {application(0), networkManager(1),systemManager(2)} Request ::= PrintableString EntityIdentifier ::= [PRIVATE 1] IMPLICIT OCTETSTRING RequestPDU ::= SEQUENCE {responseRequired BOOLEAN, TransactionID, requestor EntityIdentifier,request CHOICE { command [0] IMPLICIT SET {destination Component, Request} shutdown [1] IMPLICIT NULL}} ResponsePDU ::= SEQUENCE {TransactionID, responder EntityIdentifier OPTIONAL, result INTEGER {success(0), failure(1)}} END

  20. A Value assignment example aRequestPDU RequestPDU ::= {ResponseRequired TRUE, 45, requestor “MySystem”,request { command {destination systemManager, ‘status’} }} aResponsePDU ResponsePDU::= {45, result INTEGER {success}}

  21. ASN.1 Transfer Syntax • Basic Encoding Rules BER • Packed Encoding Rules PER • Canonical Encoding Rules CER • Disting. Encoding Rules DER • Defines how to encode the values before sending over the line • Basically it is a Type, Length, Value encoding scheme with one or more octets for each of the fields. Type Length Value identified by tagging

  22. TLV encoding scheme ... Octet 1 Octet 2 Octet 3 Octet n bit8 bit 7 ...bit1 Type Length Type Length Value Type Length Value … Data Element Data Element Data Element

  23. Class: 00 UNIVERSAL 01 APPLICATION 10 Context specific 11 Private f: 0 primitive encoding 1 constructed encoding number: <31 directly >=31 as below: ASN.1 Transfer Syntax T A G F I E L D (=Type identifier field) class f n u m b e r | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | last ... class f 11111 1 … 1 … 0 …

  24. B O O L E A N 0 0 | 0 | 0 | 0 | 1 0 | 0 | c l a s s | f | n u m b e r | 0 0 | 0 | 0 | 1 | 0 0 | 0 I N T E G E R | c l a s s | f | n u m b e r | Examples • BOOLEAN: UNIVERSAL 1 = 00000001 hex • INTEGER: UNIVERSAL 2 = 00000002 hex • SEQUENCE: UNIVERSAL 16 constructed = 00110000 = 30 hex • [PRIVATE 35] IMPLICIT INTEGER = 11011111 00100011

  25. Length field Definite form • short: length < 128 • long: length > 128 Indefinite form: only for constructed types 0 <length> ... 1 <length of length field> <length field> <length field> ... 10000000 <value> <value> 00000000 00000000

  26. 0 0 | 0 | 0 | 1 | 0 0 | 0 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | c l a s s | f | n u m b e r | | l e n g t h | V A L U E F I E L D S B O O L E A N E x . F A L S E 0 0 | 0 | 0 | 0 | 1 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 0 | 0 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | c l a s s | f | n u m b e r | | l e n g t h | | c o n t e n t s | ( T R U E w i t h c o n t e n t v a l u e d i f f e r e n t f r o m 0 ) I N T E G E R E x . 1 0 0 ( d e c i m a l ) 0 | 1 | 1 | 0 | 0 | 1 | 0 | 0 | c o n t e n t s |

  27. How to encode RequestPDU ::= SEQUENCE {responseRequired BOOLEAN, TransactionID, requestor EntityIdentifier,request CHOICE { command [0] IMPLICIT SET {destination Component, Request} shutdown [1] IMPLICIT NULL}} where: ResponseRequired = FALSE, TransactionID = 10, requestor = “MySystem”,request = command { destination = networkManager, Request = ‘status’} ??

  28. Encoding summary • Universal Tag ASN.1 Type • 1 BOOLEAN • 2 INTEGER • 3 BITSTRING • 4 OCTETSTRING • 5 NULL • 6 OBJECTIDENTIIER • 7 ObjectDescriptor • 8 EXTERNAL • 9 REAL • 10 ENUMERATED • 11-15 reserved for addenda • 16 SEQUENCE, SEQUENCE OF • 17 SET, SET OF • … ... • Class: • 00 UNIVERSAL • 01 APPLICATION • 10 Context specific • 11 Private • f: • 0 primitive encoding • 1 constructed encoding class f n u m b e r | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | • number: • <31 directly • >=31 as below: ... class f 11111 1 … 1 … 0 …

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