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 rules The 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? ASN.1 for the data syntax PDU PDU BER for theI 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 N e t w o r k
BER
for the
(sequential)
transfer syntax 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. ASN.1 Use PDU PDU ASN.1 ASN.1 BER Encode Decode
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
Encode
Decode
C++, Java,...
C++, Java,... 1 |

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 DEFINITIONS DEFINITIONS::= 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. Assignments Type assignment Value assignment W h e a t r R p o : = S
EQUENCE{ . } N m f y n c d i
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 { }
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
timeOfReport “ Z” }

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 }
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 Type Length Value
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
Basic Encoding Rules BER
Packed Encoding Rules PER
Canonical Encoding Rules CER
Disting. Encoding Rules DER

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

23. 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 |
Class:
00 UNIVERSAL
01 APPLICATION
10 Context specific
11 Private f:
0 primitive encoding
1 constructed encoding
number:
<31 directly
>=31 as below:
last
...
class f 11111
1 …
1 …
0 …

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

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>
...
<value>
<value>

26. 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 | | | | | 1 | | || | | | | 1 | | | | | | | 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 ) I N T E G E R E x . 1 ( d e c i m a l ) | c l a s f n u m b e r 1 | l e n g t h 1 | 1 | 1 | | | 1 | | | 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 ... class f 11111 1 … 1 … 0 …
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 …