The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  TCP/IP Network Management Framework and Protocols (SNMP and RMON)                 9  TCP/IP Simple Network Management Protocol (SNMP) Protocol                      9  SNMP Protocol Operations

SNMP Protocol General Operation, Communication Methods and Message Classes 1 SNMP Protocol Table Traversal Using GetNextRequest and GetBulkRequest Messages

The obvious place to begin our detailed look at SNMP protocol operations is with the simplest type of information exchange. This would be a simple poll operation to read one or more management information variables, used by one SNMP entity (typically an SNMP manager) to request or read information from another entity (normally an SNMP agent on a managed device). SNMP implements this as a simple two-message request/response protocol exchange, similar the request/reply processes found in so many TCP/IP protocols.

This information request process typically begins with the user of an application wanting to check the status of a device or look at information about it. As we've seen, all this information is stored on the device in the form of MIB objects. The communication, therefore, takes the form of a request for particular MIB objects and a reply from the device containing those objects' values. In simplified form, the steps in the process are as follows (and as shown in Figure 274):

1. SNMP Manager Creates GetRequest-PDU: Based on the information required by the application and user, the SNMP software on the network management station creates a GetRequest-PDU message. It contains the names of the MIB objects whose values the application wants to retrieve.
   
   
2. SNMP Manager Sends GetRequest-PDU: The SNMP manager sends the PDU to the device that is being polled.
   
   
3. SNMP Agent Receives and Processes GetRequest-PDU: The SNMP agent receives and processes the request. It looks at the list of MIB object names contained in the message and checks to see if they are valid (ones the agent actually implements). It looks up the value of each variable that was correctly specified.
   
   
4. SNMP Agent Creates Response-PDU: The agent creates a Response-PDU to send back to the SNMP Manager. This message contains the values of the MIB objects requested and/or error codes to indicate any problems with the request, such as an invalid object name.
   
   
5. SNMP Agent Sends Response-PDU: The agent sends the response back to the SNMP Manager.
   
   
6. SNMP Manager Processes Response-PDU: The manager processes the information in the Response-PDU received from the agent.
   

   Figure 274: SNMP Information Poll Process

   The basic SNMP information polling process involves a simple exchange of a GetRequest-PDU sent by an SNMP manager and a Response-PDU returned by an SNMP agent.

   
   

The Response-PDU message is called GetResponse-PDU in SNMPv1. Presumably, this name was chosen from the fact that it was a response to a get operation, to make the names GetRequest-PDU and GetResponse-PDU somewhat symmetric. The problem is that this name is confusing, for two reasons. First, it sounds to some people like the purpose of the PDU is to “get a response”. Second, the GetResponse-PDU was also defined as the response message for operations other than “gets”, including the reply message for SetRequest-PDU. Having a “GetResponse” message be sent in reply to a “SetRequest” is disconcerting; the new name is more generic and avoids these problems.

SNMP Protocol General Operation, Communication Methods and Message Classes 1 SNMP Protocol Table Traversal Using GetNextRequest and GetBulkRequest Messages