SNMP MIB Implementation

SNMP's PDU using BER

• SNMP is the protocol that allows communicate between NMS and agents by exchanging SNMP messages. the SNMP message is a single field, of the Sequence type. SNMP message use data types specified by ASN.1 and use Basic Encoding Rules (BER) to encode data. The entire SNMP message is a Sequence of three smaller fields: the SNMP Version (Integer), the SNMP Community String (Octet String), and the SNMP PDU.
• The SNMP's PDU is reference to SNMP version 1 (SNMPv1) PDU.
• SNMPv1 PDU have five different PDU types:GetRequest, GetNextRequest, GetResponse, SetRequest and Trap.
• Get Request, GetNext Request, Get Response, Set Request are same format PDU. Trap use other format PDU.

ASN.1 data types

• ASN.1 data types fall into two categories: primitive and complex.
• ASN.1 data types is used to build SNMP messages.
  • ASN.1 primitive data types include Integer, Octet (byte, character) String, Null, Boolean and Object Identifier.
  • ASN.1 complex data types are used to build SNMP message are Sequence type, Sequence-of type.

Encoding Rules (BER)

• BER has three parts: Type, Length and Data field.

  BER format: 
     +------+--------+------+
     | Type | Length | Data |
     +------+--------+------+

• Type field is single byte identifier.

  Constructing byte Data type.
  Format of byte Data type
     +--+-+-----+
     |  | |     +
     +--+-+-----+
      2  1   5
  Bits 8 and 7 shall be encoded to represent the class of Data type follow table:
     +------------------+-------+-------+
     |      Class       | Bit 8 | Bit 7 |
     +------------------+-------+-------+
     | Universal        |   0   |   0   |
     | Application      |   0   |   1   |
     | Context-specific |   1   |   0   |
     | Private          |   1   |   1   |
     +------------------+-------+-------+
  Bit 6 is encoded data is Primitive or Constructed follow table:
     +-------+-------------+
     | Bit 6 |   Type      |
     +-------+-------------+
     |   0   | Primitive   |
     |   1   | Constructed |
     +-------+-------------+ 
  Bits 5 to 1 encode the number of Data type as a integer number.     

  Data type identifier in SNMP
     Data type                 Identifier      Note
     Integer                      0x02         Primitive ASN.1 types
     Octet String                 0x04         Primitive ASN.1 types  
     Null                         0x05         Primitive ASN.1 types
     Object identifier            0x06         Primitive ASN.1 types
     Sequence                     0x30         Constructed ASN.1 types
     IpAddress                    0x40         Primitive SNMP application types
     Counter                      0x41         Primitive SNMP application types
     Gauge                        0x42         Primitive SNMP application types
     TimeTicks                    0x43         Primitive SNMP application types 
     Opaque                       0x44         Primitive SNMP application types
     NsapAddress                  0x45         Primitive SNMP application types
     GetRequest PDU               0xA0         Context-specific Constructed SNMP types
     GetNextRequest PDU           0xA1         Context-specific Constructed SNMP types
     GetResponse PDU              0xA2         Context-specific Constructed SNMP types
     SetRequest PDU               0xA3         Context-specific Constructed SNMP types
     Trap PDU                     0xA4         Context-specific Constructed SNMP types

• Length field is the number of bytes in Data field.
  • Length field is used either the short form or the long form as a option depend on Data field.
    • The short form, Length field is a single octet in which bit 8 is zero and bits 7 to 1 encode the number of bytes in Data field, as an unsigned binary integer with bit 7 as the most significant bit.
    • The long form, Length field shall consists of an initial octet and one or more subsequent octets.
      • The initial octet is encoded as follows:
        • Bit 8 shall be one.
        • Bits 7 to 1 shall encode the number of subsequent octets in the length field, as an unsigned binary integer with bit 7 as the most significant bit.
        • The value 0xFF shall not be used.
      • Subsequent octets:
        • From the first subsequent octet to the last subsequent octet, shall be the encoding of an unsigned binary integer equal to the number bytes in Data field, with bit 8 of the first subsequent octet as the most significant bit.
• Data field is actual data content.

• Example:

  Actual Data is an integer, the value 100 can be encode as:
  The short form:
     +------+--------+-------+
     | Type | Length | Data  |
     +------+--------+-------+
     | 0x02 |  0x01  | 0x64  |
     +------+--------+-------+
  The long form:
     +------+-----------+-------+
     | Type |  Length   | Data  |
     +------+-----------+-------+
     | 0x02 | 0x81 0x01 | 0x64  |
     +------+-----------+-------+

SNMP Message Format

• SNMP Message is a Sequence of three smaller fields: the SNMP Version (Integer), the SNMP Community String (Octet String), and the SNMP PDU.

  SNMP Message Format use BER.
     +------------------------------------------------------------------------+
     |                      SNMP Message (Sequence type)                      |
     +------+-----------------------------------------------------------------+
     | Type | Length of Data |                      Data                      |
     +------+----------------+-------------+-----------------------+----------+
     | 0x30 |     Length     |SNMP Version | SNMP Community String | SNMP PDU |                      
     |      |                |  (Integer)  |    (Octet String)     |          |
     +------+----------------+-------------+-----------------------+----------+

• Length is bytes of Data field (SNMP Version, SNMP Community String and SNMP PDU).
• SNMP Version is an integer that identifies the version of SNMP, SNMP version 1 = 0.
• SNMP Community String is an Octet String to add security to Agents.
• SNMP PDU is SNMP verion 1 (SNMPv1) PDU.