Message Types

There are five type of messages defined by SNMP,Get-Request,Get-Response,Get-Next-Request,Set-request,Trap.

NMS uses Get-Request messages to retrieve data from a agent, and agent responses with Get-Response message.NMS uses Get-Request and Get-Next-Request message to retrieve a Dataset which prepared by a agent.

NMS uses Set-Request to maintenance and configure a agent, even to de-active or delete a device.

Agent uses Trap message to send information which is not requested by NMS, such as event, to a NMS.

Trap send to port 162 on NMS, NMS sent request to the agent on port 161,agents send response to NMS on port 162.

SNMP communicates

An agent is configured with three community names: read-only, read-write and trap. The community strings are essentially passwords. The read-only community means that NMS only can retrieves data from an agent, but can not write or set information to an agent; the read-write community not only can query information from an agent but also can set or write data to. The trap community means that the NMS can receive trap message from an agent.

The structure of management information

You can refer to RFC 1155 (SMI) to get more information about structure of management information.

I have found some web pages in Chinese related to SNMP, I hope it will be of use.

1.       http://www.nmscommunications.com/manuals/6744-13/chap1.htm

2.       http://net-snmp.cn/snmp/snmp2.html

3.       http://www.cnpaf.net/class/snmp/