SNMPv1

按照RFC1157的建議，給出SNMPv1的實現架構如圖1所示。使用SNMP的網絡中存在兩種角色「management station」和「network element」，同一個設備能夠既是Manager又是Element。兩種角色上安裝着各類屬於特定SNMP Community的Application entity，每種SNMP Community使用同一個認證機制。

圖1 實現架構

SNMPv1使用UDP報文傳輸，主題框架使用ASN.1描述以下：（首先要說明一下 Oct String -- A variable-length sequence of octets）

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
        }
-- the individual PDUs and commonly used
-- data types will be defined later
END

Trap使用與通常PDU不一樣的UDP端口號來標識。

    0   1   2   3

    + - - - - - + - - - - - - - - + - - - - - - + - - - - - - +

    |  version  |    community    |   PDU Type  |   Real Data |

    + - - - - - + - - - - - - - - + - - - - - - + - - - - - - +

5種不一樣PDU詳細描述：

其中「GetRequest-PDU」、「GetNextRequest-PDU」、「SetRequest-PDU」和「Trap-PDU」是主動由SNMP application entity發起，交由SNMP protocol entity代理產生報文。惟有「GetResponse-PDU」是被動的在接收到「GetRequest-PDU」、「GetNextRequest-PDU」以及「SetRequest-PDU」後產生的。

The form of the GetRequest-PDU is: GetRequest-PDU ::= [0] IMPLICIT SEQUENCE { request-id RequestID, error-status -- always 0 ErrorStatus, error-index -- always 0 ErrorIndex, variable-bindings VarBindList }

The form of the GetNextRequest-PDU is：

 GetNextRequest-PDU ::= [1] IMPLICIT SEQUENCE { request-id RequestID, error-status -- always 0 ErrorStatus, error-index -- always 0 ErrorIndex, variable-bindings VarBindList }

GetNextRequest的用法可能有點讓人費解，不要緊，RFC1157 4.1.3.1節中有個例子，看了就懂了，還能順便了解一下SNMP的使用方法。

The form of the GetResponse-PDU is

GetResponse-PDU ::=
                      [2]
                          IMPLICIT SEQUENCE {
                              request-id
                                  RequestID,
                              error-status
                                  ErrorStatus,

                              error-index
                                  ErrorIndex,

                              variable-bindings
                                  VarBindList
                          }

只有當SNMP Agent 收到「GetRequest-PDU」、「GetNextRequest-PDU」以及「SetRequest-PDU」時纔會產生。
SetRequest-PDU ::=
                      [3]
                          IMPLICIT SEQUENCE {
                              request-id
                                  RequestID,

                              error-status        -- always 0
                                  ErrorStatus,

                              error-index         -- always 0
                                  ErrorIndex,

                              variable-bindings
                                  VarBindList
                          }

The form of the Trap-PDU is:

     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
             }

關於「Trap-PDU」要說明的是，Trap-PDU的目的地址怎樣選定在協議中沒有說明，是由應用決定的。