Definition 4-1. The ASN.1 definition of SNMP

    RFC1157-SNMP DEFINITIONS ::= BEGIN
    IMPORTS
       ObjectName, ObjectSyntax, NetworkAddress, IpAddress, TimeTicks
          FROM RFC1155-SMI;

    -- top-level message

        Message ::=
           SEQUENCE {
              version             -- version-1 for this RFC
                 INTEGER {
                    version-1(0)
                 },
              community           -- community name
                 OCTET STRING,
              data                -- e.g., PDUs if trivial
                 ANY              -- authentication is being used
         }
    -- protocol data units

    PDUs ::=
      CHOICE {
            get-request
                GetRequest-PDU,

            get-next-request
                GetNextRequest-PDU,

            get-response
                GetResponse-PDU,

            set-request
                SetRequest-PDU,

            trap
               Trap-PDU
            }

    -- PDUs
    GetRequest-PDU ::=
       [0]
          IMPLICIT PDU
    GetNextRequest-PDU ::=
       [1]
          IMPLICIT PDU
    GetResponse-PDU ::=
       [2]
          IMPLICIT PDU
    SetRequest-PDU ::=
       [3]
          IMPLICIT PDU
    PDU ::=
          SEQUENCE {
             request-id
                INTEGER,
             error-status         -- sometimes ignored
                INTEGER {
                   noError(0),
                   tooBig(1),
                   noSuchName(2),
                   badValue(3),
                   readOnly(4),
                   genErr(5)
                },
             error-index          -- sometimes ignored
                INTEGER,
             variable-bindings    -- values are sometimes ignored
                VarBindList
            }
      Trap-PDU ::=
          [4]
          IMPLICIT SEQUENCE {
             enterprise           -- type of object generating
                                  -- trap, see sysObjectID in [5]

             OBJECT IDENTIFIER,
             agent-addr           -- address of object generating
                NetworkAddress,   -- trap
             generic-trap                -- generic trap type
               INTEGER {
                  coldStart(0),
                  warmStart(1),
                  linkDown(2),
                  linkUp(3),
                  authenticationFailure(4),
                  egpNeighborLoss(5),
                  enterpriseSpecific(6)
               },
             specific-trap        -- specific code, present even
                INTEGER,          -- if generic-trap is not
                                 -- enterpriseSpecific
             time-stamp           -- time elapsed between the last
                TimeTicks,        -- (re-)initialization of the
                                     network
                                  -- entity and the generation of
                                     the trap
             variable-bindings     -- “interesting” information
                VarBindList
      }

  -- variable bindings
  VarBind ::=
     SEQUENCE {
        name
          ObjectName,
        value
          ObjectSyntax
     }
  VarBindList ::=
     SEQUENCE OF
        VarBind
END

This chapter has explored the structure of the SNMP messages and looked at some examples of various SNMP PDUs. The Internet Engineering Task Force (IETF) has enhanced SNMP by adding support for new PDUs, security and other features. These enhancements are part of SNMP version 2, which will be discussed in the next chapter.

4.6 References

[4-1] Case, J.D., M. Fedor, M.L. Schoffstall, and C. Davin. “Simple Network Management Protocol (SNMP).” RFC 1157, May 1990.

[4-2] Case, Jeffrey D., et al. “Network Management and the Design of SNMP.” ConneXions (March 1989): 22-26.

[4-3] Kastenholz, F., ed. “SNMP Communication Services.” RFC 1270, October 1991.

[4-4] Davin, J., J. Galvin, and K. Mcloghrie. “SNMP Administrative Model.” RFC 1351, July 1992.

[4-5] Rose, M.T. ed. “A Convention for Defining Traps for Use with the SNMP.” RFC 1215, March 1991.