SNMP PDU Structure

PDUs allow network managers using SNMP to monitor and configure networked devices.

SNMP is a networking protocol allowing users to monitor and adjust managed objects throughout the network, designed with simplicity in mind. Protocol Data Units are the commands that provide the means for network managers to perform network tasks and receive information, and have a relatively consistent structure in SNMP versions 1 and 2. SNMPv3 introduced greater security, along with a modified PDU structure to adapt to the changes that this more robust version introduced.

V1 & V2 Identification and Authentication

A PDU's Authentication Header combines the Version Identifier and Community Name fields, which confirms to the PDU recipient that the issued request or generated response is authorized. The Version Identifier field identifies the SNMP version used, while the Community Name serves as the network's password. The RequestID field follows, identifying the network node issuing or responding to a PDU.

V1 & V2 Error Handling

Error Status and Error Index are the next fields in the PDU's structure. Error Status returns a non-zero value if a transmission error occurs and, when this happens, the Error Index returns an integer value corresponding to a pre-defined type of error. Non-zero values indicate that there's something wrong with the values in the Variable Bindings field, so the node receiving the PDU will return no data pertaining to those values.

Variable Bindings

The Variable Bindings field couples object ID names with their data values, allowing the network management station issuing the PDU to associate requests and responses with specific network nodes. SNMPv2's "GetBulkRequest" improves upon SNMPv1's "GetRequest" and "GetNextRequest" PDUs by adding the Non-Repeaters and Max Repetitions fields, which contain values for the first non-repeating data value retrieved and maximum number of repetitions for the remaining data requested by the NMS. This allows the PDU to return a batch of data in the Variable Bindings field, rather than having to return values one at a time during repeated iterations of "GetNextRequest."

Traps

Network agents autonomously send Traps to an NMS to provide information on events such as device rebootings, failed network connections or receipt of messages containing improper authentication. Trap PDUs incorporate additional fields in their structure that provide information about device or network events to their recipients. The Generic Trap and Specific Trap fields contain integers corresponding to specific Trap types, each of which describes a specific event. A Trap's Time Stamp field indicates the amount of time between a node's last re-initialization and the time it sent out the Trap.

SNMPv3 PDUs

SNMPv3's PDUs have a Message ID field that matches requests with responses, but unlike the Request ID field, it identifies the specific message containing the PDU, not the PDU itself. Version 3's Message Size field indicates the maximum message size the node issuing the PDU is able to process, while the Message Flag field contains information about the security measures used on the message and whether the issuing node requires a response from the PDU's recipient. The Security Model and Message Security Parameter fields contain further information about the encryption used on the PDU and how network security measures are to be employed during data exchange. SNMPv3 introduces Scoped PDUs, which are used in a specific context. SNMPv3 defines context as particular data that a network node can access. Scoped PDUs include the Context Engine ID and Context Name fields, which respectively identify the network nodes and specific information that a context references.

References

• Fundamentals of Telecommunications; Roger L. Freeman
• TCP/IP - The Ultimate Protocol Guide: Complete 2 Volume Set; Philip M. Miller
• Internet Engineering Task Force; Request for Comments: 1157; J. Case, et. al.
• Data Networks, IP, and the Internet: Protocols, Design, and Operation; Martin P. Clark
• Data Storage Fundamentals; Alan Johnson

Resources

• Pulse: What is SNMP?