1. Dept. of Computer Science and Engineering
ASN.1 & BER
J. Won-Ki Hong
Dept. of Computer Science and Engineering
POSTECH
Tel: 1 1

2. Table of Contents Abstract Syntax Notation One (ASN.1)
Overview
Properties & Restrictions
Type and Value Definitions
ASN.1 Simple Types
ASN.1 Structured Types
ASN.1 Macro Definitions
Basic Encoding Rules (BER)
Tags, Lengths & Values
Encoding Examples 3

3. Overview of ASN.1 a machine independent data description language
CCITT (X.208) and ISO (ISO 8824) standard
define abstract syntax of application data
define the structure of application and presentation protocol data units (PDUs)
define SNMP and OSI Management Information Base (MIB) 4

4. ASN.1 Terminology Abstract Syntax Data Type Encoding Encoding Rules
describes the generic structure of data
allows data types and values to be defined
Data Type
a named set of values -- may be simple or structured
Encoding
sequence of octets used to represent a data value
Encoding Rules
specifies the mapping from one syntax to another
Transfer Syntax
describes how data are actually represented in terms of bit patterns while in transit 4

5. Abstract & Transfer Syntaxes
Application
component
data transfer
(e.g, TCP, OSI
session)
Local storage
(e.g, MIB)
User
local
mapping
user presentation
Abstract
Syntax
(e.g., ASN.1)
encoding
rules
Transfer
(e.g., BER) 4

6. ASN.1 Module Definition <modulereference> DEFINITIONS ::= BEGIN
The basic building block of an ASN.1 specification is the module
See Figure B.2 (BNF grammar for ASN.1)
Modules have the following basic form
<modulereference> DEFINITIONS ::=
BEGIN
EXPORTS
IMPORTS
AssignmentList
END
Definitions in this module that may be imported by other modules.
Definitions that are to be imported from other modules.
Type assignments, value assignments, and macro definitions that are defined in this module. 4

7. Lexical Conventions
Comments begin with two hyphens (“--”) and terminated either by another set (“--”) or the end of line character
Identifiers begin with a letter, and may contain letters, digits, and hyphens, but may not end with a hyphen or contain two consecutive hyphens
The type identifier must start with an uppercase letter
The value identifier must start with a lowercase letter
Reserved keywords are all uppercase
Multiple spaces and blank lines can be considered as a single space 4

8. Categories of ASN.1 Data Types
Simple (Primitive)
atomic types, with no components
Structured
types with components
Tagged
types derived from other types
Other
CHOICE and ANY types
Every ASN.1 data type, with the exception of CHOICE and ANY types, has an associated TAG, which consists of a class name and nonnegative integer tag number 4

9. Classes of TAG UNIVERSAL APPLICATION CONTEXT-SPECIFIC PRIVATE
Built-in types, application independent types
See Table B.2
APPLICATION
Application specific types
CONTEXT-SPECIFIC
limited to a context within an application
PRIVATE
defined by users and not covered by any standard 4

10. ASN.1 Simple Types INTEGER OCTET STRING OBJECT IDENTIFIER (OID) NULL
the positive and negative whole numbers, including zero
OCTET STRING
a sequence of zero or more octets (8-bit bytes)
OBJECT IDENTIFIER (OID)
the set of values associated with information objects allocated by the standard
NULL
the single value NULL
Other ASN.1 simple types include boolean, bit string, real, enumerated, PrintableString, etc. 4

11. ASN.1 Structured Types SET SET OF SEQUENCE SEQUENCE OF CHOICE
a collection of one or more types
SET OF
a collection of zero or more occurrences of a given type
SEQUENCE
an ordered collection of one or more types
SEQUENCE OF
an ordered collection of zero or more occurrences of a given type
CHOICE
a list of alternatives 4

12. Informal Description of Personnel Record
ASN.1 Example
Informal Description of Personnel Record
Name: James W Hong
Title: Associate Professor
Employee Number:
Date of Hire: May 26, 1995
Name of Spouse: In-Young B Hong
Number of Children: 2
Child Information
Name: Suk D Hong
Date of Birth: 29 March 1988
Name: Myungdo M Hong
Date of Birth: 10 August 1994 4

13. ASN.1 Description of the Record Structure
PersonalRecord ::= [APPLICATION 0] IMPLICIT SET {
Name,
title [0] VisibleString,
number EmployeeNo,
dateOfHire [1] Date,
nameOfSpouse [2] Name,
children [3] IMPLICIT SEQUENCE OF ChildInfo
DEFAULT {} }
ChildInfo ::= SET {
dateOfBirth [0] Date}
Name ::= [APPLICATION 1] IMPLICIT SEQUENCE {
givenName VisibleString,
initial VisibleString,
familyName VisibleString}
EmployeeNo ::= [APPLICATION 2] IMPLICIT INTEGER
Date ::= [APPLICATION 3] IMPLICIT VisibleString 4

14. ASN.1 Description of a Record Value
{ {givenName “James”, initial “W”, familyName “Hong”},
title “Associate Professor”
number
dateOfHire “ ”
nameOfSpouse {givenName “In-Young”, initial “B”,
familyName “Hong”},
children
{ { {givenName “Suk”, initial “D”,
dateOfBirth “ ”},
{ {givenName “Myungdo”, initial “M”,
dateOfBirth “ ”} } 4

15. ASN.1 Macro Definitions
ASN.1 macro notation can be used to extend the syntax of ASN.1 to define new types and values
a macro definition is expressed in the macro notation and used to define a set of macro instances
a macro instance is generated from a macro definition by substituting values for variables
the macro is used to extend the ASN.1 syntax but does not extend the encoding 4

16. Macro Definition Format
<macroname> MACRO ::=
BEGIN
TYPE NOTATION ::= <new-type-syntax>
VALUE NOTATION ::= <new-value-syntax>
<supporting-productions>
END 4

17. Macro Definition Example
OBJECT-TYPE MACRO ::=
BEGIN
TYPE NOTATION ::= “SYNTAX” type (TYPE ObjectSyntax)
“ACCESS” Access
“STATUS” Status
VALUE NOTATION ::= value (VALUE ObjectName)
Access ::= “read-only” | “read-write” | “write-only” |
“not-accessible”
Status ::= “mandatory” | “optional” | “obsolete”
END 4

18. Fields of a BER encoded ASN.1 value
Overview of BER
BER – Basic Encoding Rules
an encoding specification
CCITT (X.209) and ISO (ISO 8825) standard
describes a method for encoding values of each ASN.1 type as a string of octets
based on the use of a type-length-value (TLV) structure
Type
Length
Value
Fields of a BER encoded ASN.1 value 4

19. Not used in the SNMP protocol
BER Type Field
Class
BIT 8 7 6 5 4 3 2 1
Constructed
Most significant
Least significant
Tag number
CLASS
Bit 8
Bit 7
Description
Universal
Built-in types
Application 1
SNMP defined types
Context-Specific 1
Used in context
Private 1 1
Not used in the SNMP protocol 4

20. Tag Values for SNMP Types
SNMPv1
protocol
SNMPv2
protocol
Tag
Number
Tag
Value
Type
ASN.1 Tag
INTEGER/Integer32
UNIVERSAL 2
0x02
0x02
OCTET STRING
UNIVERSAL 4
0x04
0x04
NULL
UNIVERSAL 5
0x05
0x05
OBJECT IDENTIFIER
UNIVERSAL 6
0x06
0x06
SEQUENCE
UNIVERSAL 16
0x10
0x30
IpAddress
APPLICATION 0
0x00
0x40
Counter/Counter32
APPLICATION 1
0x01
0x41
Gauge/Gauge32
APPLICATION 2
0x02
0x42
TimeTicks
APPLICATION 3
0x03
0x43
Opaque
APPLICATION 4
0x04
0x44
Counter64
APPLICATION 6
0x06
0x46 4

21. Encoding Methods
The TLV structure is recursive – for any ASN.1 value that consists of one or more components, the “value” portion of its TLV encoding itself consists of one or more TLV structures
Three methods for encoding an ASN.1 value:
Primitive, definite-length encoding
Constructed, definite-length encoding
Constructed, indefinite-length encoding
The method chosen depends on the ASN.1 type of the value to be encoded and whether or not the length of the value is known based on the type (see Table B.4) 4

22. Short(0)/Long(1) form indicator
BER Length Field
two forms of length field exist:
short form: specified in a single octet
long form: specified in multiple octets
Value
= 102 1
Short(0)/Long(1) form indicator
Value 1 1 1 1 1 1 =
Short/Long form indicator
Length of length
Length value 4

23. BER Examples - Integers1
Tag Universal 2
Length 1
Value 0
What value was encoded? 1
Tag Universal 2
Length 2
Value (1 of 2)
Value (2 of 2)
What value was encoded? 4

24. BER Example - Octet String1
Tag Universal 4
Length 4
1st octet
2nd octet
3rd octet
4th octet
Overall Length = 6
Value of Octet String encoded is ‘EB069937’ 4

25. BER Example - SEQUENCE Message ::= SEQUENCE {
version INTEGER { version-1(0) },
community OCTET STRING }
Given the above definition,
what is the BER encoding of
sampleMessage ::= { 0, ‘EB069937’h } ? 4

26. ... and its BER encoding is 1 Value (6 of 9) OCTET STRING1
Value (6 of 9) OCTET STRING
Value (7 of 9) OCTET STRING
Value (8 of 9) OCTET STRING
Value (1 of 9) integer
Value (2 of 9) integer
Value (3 of 9) integer
Tag universal 16
Length 9
Value (4 of 9) OCTET STRING
Value (5 of 9) OCTET STRING
Value (9 of 9) OCTET STRING 4

27. Summary
We have covered a subset of ASN.1 and BER which are used in SNMP and OSI Management Frameworks
ASN.1 is widely used in defining application data and protocol data units
BER is widely used in defining transfer syntaxes
READ:
Stallings, SNMP, SNMPv2, SNMPv3 and RMON 1 and 2, 3rd Edition, Addison-Wesley, Appendix B 4