4.1.1. Common Constructs

Before introducing the six PDU types of the protocol, it is appropriate to consider some of the ASN.1 constructs used frequently:

                  -- request/response information

                  RequestID ::=
                          INTEGER

                  ErrorStatus ::=
                          INTEGER {
                              noError(0),
                              tooBig(1),
                              noSuchName(2),
                              badValue(3),
                              readOnly(4)
                              genErr(5)
                          }

                  ErrorIndex ::=
                          INTEGER

                  -- variable bindings

                  VarBind ::=
                          SEQUENCE {
                              name
                                  ObjectName,

                              value
                                  ObjectSyntax
                          }

                  VarBindList ::=
                          SEQUENCE OF
                              VarBind

RequestIDs are used to distinguish among outstanding requests. By use of the RequestID, an SNMP application entity can correlate incoming responses with outstanding requests. In cases where an unreliable datagram service is being used, the RequestID also provides a simple means of identifying messages duplicated by the network.

A non-zero instance of ErrorStatus is used to indicate that an exception occurred while processing a request. In these cases, ErrorIndex may provide additional information by indicating which variable in a list caused the exception.

The term variable refers to an instance of a managed object. A variable binding, or VarBind, refers to the pairing of the name of a variable to the variable's value. A VarBindList is a simple list of variable names and corresponding values. Some PDUs are concerned only with the name of a variable and not its value (e.g., the GetRequest-PDU). In this case, the value portion of the binding is ignored by the protocol entity. However, the value portion must still have valid ASN.1 syntax and encoding. It is recommended that the ASN.1 value NULL be used for the value portion of such bindings.