SNMP MIB Implementation

This wiki describe how to generate a MIB (Management Information Base) for SNMP agent.

Steps

Create a ASN.1 MIB script foo.mib (an ASCII text file) for the tree structure.
Convert foo.mib to binary file using mib2bin
Build snmp's PDU use BER (Base encoding rules) encoder and decoder library to process data that's transfer between NMS and agents.
Build snmp API use uIP-stack to communicate between NMS and Agents (open two ports: The manager speak to agents on one port, the agent responds manager on the other port).
Build binary MIB file reader library.
Build functions service oid tree.
Merge MIB ANS.1 file to NMS.

Create ASN.1 MIB Script

Build MIB file's written in ANS.1 notation.
Tutorial: Understanding SNMP Stack to create ASN.1 MIB Script.
Tools to create ASN.1 MIB Script
- ASN.1 Editor plugin for Eclipse (Now using).
- Emacs mode for editing ASN.1 files.

Abstract Syntax Notation

Each MIB variable contains several attributes, such as data type, access type and object identifier.
Abstract Syntax Notation version 1 (ASN.1) is a language to define these attributes in SNMP.

Convert MIB to Binary File

mib2bin tool is modified from net-snmp to convert ASN.1 format file to three files, because the microchip mib2bib converter only supports upto 255 OIDs.

                                MIB compiler tools: mib2bin
 foo.mib (ANS.1 format)   -----------------------------------> foo.bin + foo.h + foo_data.h

Syntax to use mib2bin tool:

  mib2bin <MIBfile>...

where MIBfile file is ASN.1 format file. MIBfile = <name>.<type>
- <name>.bin is the binary file storing information of OID tree. This file can be placed on an SD media card to be read by the FAT16 file system.
- <name>_data.h is C header file storing information of OID tree. This file is generated by converting mchip.bin file to the C header file. It's only used when a system don't have system file and place on program memory.
- <name>.h is C header file storing ID that's reference to function service of OID.
Note:
- Subfolder mibs containing the basics MIB files (e.g.: RFC1155-SMI, RFC1213-MIB, RFC-1215, SNMPv2-MIB ... for us MIB file), must be present under the directory of execution.
- If the three files exist, mib2bin tool will overwrite the files.

Binary File Format

The binary file is an image of MIB file. It is generated by mib2bin tool. Agents will read binary file to respond NMS request.
In the binary file, A parent is stored first, followed by its last-child to first-child. Next, the structure of next this parent is stored. This structure is repeated until the entire tree is stored.
A parent or child is a record. Single record of binary file have format:

 <oid>, <nodeInfo>, 
 [id], [siblingOffset]/[distantSiblingOffset], [dataType], [dataLen], [data], 
 [{<IndexNumber>},{<IndexCount>, <IndexNodeInfo>, <IndexDataType>} ...]

where:
- fields indicated by angle brackets (< >) are always present
- fields in square brackets ([ ]) are optional depending on characteristics of the current node.
- fields in braces ({ }) are optional but always occur together.

<oid> field

The microchip format only supports OIDs upto 255. The following is an workaround to store OID greater than 255.
Format of OID:

  oid_1 <0x10> oid_2 <0x10> ....

if (oid_i < 0xFF): oid_i = BYTE (oid_i)
else: oid_i = <0xFF> + BYTE (length) + BYTE (oid_i.B0) + BYTE (oid_i.B1) + ...
where
- BYTE (x) mean the byte representation of x.
- <0x10> is the OID separator.
- <0xFF> is used to indicate that the OID is greater than 254.
- length is the number of bytes of oid_i that follows.
- oid_i.Bj is j-th byte of oid_i in hexadecimal representation, j = 0 corresponds to the Least Significant Byte (LSB), i.e. little-endian format.
Example:

   The OID for a node is 4.1.17095. After compiling to binary, the OID is represented as:
      The OID:      4   .     1   .               17095    .
      The result: 0x04 0x10 0x10 0x10 0xFF 0x02 0xC7 0x42 0x10
      where:
          oid_1 = 4  =>  BYTE (oid_1) = 0x04
          oid_2 = 2  =>  BYTE (oid_2) = 0x01
          oid_3 = 17095 = 0x42C7  => 0xFF BYTE (length) = 0x02 BYTE (oid_3.B0) BYTE (oid_3.B1) = 0xC7 0x42 (In little-endian format)
   Note: When a node have no sibling and only has one child, the node's OID will be merged with its child's OID (reduce data's stored). 
   For example, in case of private (4) -> enterprises (1) -> microchip (17095), where private has no sibling and only has one child, 
   and enterprises also has no sibling and only has one child, they can combined to a record has oid field is 4.1.17095.

<nodeInfo> field

information of node

  bit        when (set = 1)
  0          Node is a parent (0: node is a leaf)
  1          Node is sequence
  2          Node is readable
  3          Node is writable
  4          Node is able to create
  5          Node has default data
  6          (if node is sequence, this is mean implied index node)
  7          always set 1

[id] field

If this record is leaf, id that's reference to function services the record.

[distantSiblingOffset] field

If this record is a node [distantSiblingOffset] is enabled. Point to next node sibling.
In little-endian format.
The last node's distant offset is set to 0x00000000.

[siblingOffset] field

If this record is a leaf [siblingOffset] is enabled. Point to next leaf sibling.

  siblingOffset and distantSiblingOffset in this format is same function (point to next node sibling). 
  They're only differences:
     siblingOffset use with leaf node.
     distantSiblingOffset use with node.
  so it's same value.

[dataType], [dataLen], [data] fields

If this record is a leaf and has default data
- [dataType] is type of leaf's data.
- [dataLen] is length of data.
- [data] is data on string.
The tool supports the following base data types defined in SNMPv1:
- INTEGER: The integer data type is a signed integer in the range of -2,147,483,648 to 2,147,483,647.
- OCTETSTRING: Octet strings are ordered sequences of 0 to 65,535 octets.
- Gauge: Nonnegative integers that can increase or decrease but retain the maximum value reached. The limit of 2^32 -1.
- TimeTicks: A hundredth of a second since some event. The limit of 2^32 -1.
- Counter: Nonnegative integers that increase until they reach a maximum value (2^32 -1); then, the integers return to zero.
- DisplayString: a special case of the octet string type where all the bytes are printable ASCII characters, include formatting characters such as CR and LF, and the C programming language string terminator character zero.
- IpAddress: A four byte octet string in network order.
- NetworkAddress: Used to indicate an address choice from one of the possible protocol families. Currently, only IP addresses are supported.
- Opaque: An arbitrary encoding that is used to pass arbitrary information strings that do not conform to the strict data typing used by the mib.
- SEQUENCE: An ordered list of objects, somewhat like a struct in the C language. Type of objects in sequence is same type of node.

[{<IndexNumber>}] and [{<IndexCount>, <IndexNodeInfo>, <IndexDataType>}] fields

If this record is sequence (an order list of objects),
- <IndexNumber> is the number of INDEXes in sequence.
- <IndexCount>: is OID of index node in table
- <IndexNodeInfo>: is info of index node
- <IndexDataType>: is data type of index node
See example of accessing data in a table
Example:
- trap node is a sequence to inform the NMS of a significant event (an extraordinary event has occurred at an agent) asynchronously. This sequence has two INDEXes, so we have:

     <IndexNumber> = 0x02
     with the 1st INDEX:
         <IndexCount> = 0x04
         <IndexNodeInfo> = 0xCC
         <IndexDataType> = 0x02
     with the 2nd INDEX:
         <IndexCount> = 0x01
         <IndexNodeInfo> = 0x80
         <IndexDataType> = 0x03
  In this example, trap is a table which has 4 columns: 
     trapReceiverNumber (1), trapEnabled (2), trapReceiverIPAddress (3), trapCommunity(4). 
     trapReceiverNumber (1), trapEnabled (2), trapReceiverIPAddress (3), trapCommunity(4). 
     trapReceiverNumber (1), trapEnabled (2), trapReceiverIPAddress (3), trapCommunity(4). 
  Each significant event will be a row defined in the trap table. 
  
  This example has two INDEXes: the 1st INDEX node is trapCommunity (4) and the 2nd INDEX node trapReceiverNumber (1) 
  Each INDEX is a node, so it has OID, info, data type.
  The 1st INDEX node is trapCommunity, which has OID = 4, info is 0xCC and data type is INTEGER (0x02). so
         IndexCount = 0x04
         IndexNodeInfo = 0xCC
         IndexDataType = 0x02
  The 2nd INDEX node is trapReceiverNumber, which has OID = 1, info is 0x80 and data type is DisplayString (0x03). so
         IndexCount = 0x01
         IndexNodeInfo = 0x80
         IndexDataType = 0x03

Example

OID Tree

An example OID tree is given below:

Binary File

The corresponding binary file is mchip.bin and it has data describe in table below:
- oid is <oid> fields.
- info is <nodeInfo> fields.
- dist is [distantSiblingOffset]/[siblingOffset] fields.
- id is [id] fields.
- defval is data fields, include [dataType], [dataLen], [data] fields.
- index is index fields of sequence, include [{index_number} , {<IndexCount>, <IndexNodeInfo>, <indexDataType>, ...}].

The detail description of mchip.bin is illustrated in the diagram below:
- The black arrow lines represent the OID tree structure.
- The red and green lines represent pointers to data.
- The dist field points to next sibling record. After parent record is it's children.

SNMP MIB Implementation

Contents

Steps

Create ASN.1 MIB Script

Abstract Syntax Notation

Convert MIB to Binary File

Binary File Format

<oid> field

<nodeInfo> field

[id] field

[distantSiblingOffset] field

[siblingOffset] field

[dataType], [dataLen], [data] fields

[{<IndexNumber>}] and [{<IndexCount>, <IndexNodeInfo>, <IndexDataType>}] fields

Example

OID Tree

Binary File

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