The GetRequest-PDU, GetNextRequest-PDU,and GetBulkRequest-PDU messages are the three members of the SNMP “Read” class of PDUs—they are used to let an SNMP manager read MIB objects from an MIB agent. The opposite function is represented by the SNMP “Write” class, which contains a single member: the SNMP SetRequest-PDU message.

The use of this PDU is fairly obvious; where one of the three Get PDUs specifies a variable whose value is to be retrieved, the SetRequest-PDU message contains a specification for variables whose values are to be modified by the network administrator. Remember that SNMP does not include specific commands to let a network administrator control a managed device. This is in fact the “control method”, by setting variables that affect the operation of the managed device.

The set process is the complement of the get process; the same basic idea, pretty much, but a reversal in how the object values “travel” and what is done with them. The process follows these steps (see Figure 275):

1. SNMP Manager Creates SetRequest-PDU: Based on the information changes specified by the user through the SNMP application, the SNMP software on the network management station creates a SetRequest-PDU message. It contains a set of MIB object names and the values to which they are to be set.
   
   
2. SNMP Manager Sends SetRequest-PDU: The SNMP manager sends the PDU to the device being controlled.
   
   
3. SNMP Agent Receives and Processes SetRequest-PDU: The SNMP agent receives and processes the set request. It examines each object in the request along with the value to which the object is to be set, and determines if the request should or should not be honored.
   
   
4. SNMP Agent Makes Changes and Creates Response-PDU: Assuming that the information in the request was correct (and any security provisions have been satisfied), the SNMP agent makes changes to its internal variables. The agent creates a Response-PDU to send back to the SNMP Manager, which either indicates that the request succeeded, or contains error codes to indicate any problems with the request found during processing.
   
   
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 to see the results of the set.
   

   Figure 275: SNMP Object Modification Process

   The communication process for setting a MIB object value is very similar to that used for reading one. The main difference is that, of course, the object values are sent from the SNMP Manager to the SNMP Agent, carried in the SetRequest-PDU message.

    

   
   

Obviously, telling a device to change a variable's value is a more significant request than just asking the device to read the value. For this reason, the managed device must very carefully analyze and verify the information in the request to ensure that the request is valid. The checks performed include:

• Verifying the names of the objects to be changed.
  
  
• Verifying that the objects are allowed to be modified (based on their Access or Max-Access object characteristic.)
  
  
• Checking the value included in the request to ensure that its type and size are valid for the object to be changed.

This is also a place where general protocol security issues become more important.