SNMPv1

按照RFC1157的建议，给出SNMPv1的实现架构如图1所示。使用SNMP的网络中存在两种角色“management station”和“network element”，同一个设备能够既是Manager又是Element。两种角色上安装着各类属于特定SNMP Community的Application entity，每种SNMP Community使用同一个认证机制。

图1 实现架构

SNMPv1使用UDP报文传输，主题框架使用ASN.1描述以下：（首先要说明一下 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使用与通常PDU不一样的UDP端口号来标识。

    0   1   2   3

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

    |  version  |    community    |   PDU Type  |   Real Data |

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

5种不一样PDU详细描述：

其中“GetRequest-PDU”、“GetNextRequest-PDU”、“SetRequest-PDU”和“Trap-PDU”是主动由SNMP application entity发起，交由SNMP protocol entity代理产生报文。惟有“GetResponse-PDU”是被动的在接收到“GetRequest-PDU”、“GetNextRequest-PDU”以及“SetRequest-PDU”后产生的。

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的用法可能有点让人费解，不要紧，RFC1157 4.1.3.1节中有个例子，看了就懂了，还能顺便了解一下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
                          }

只有当SNMP Agent 收到“GetRequest-PDU”、“GetNextRequest-PDU”以及“SetRequest-PDU”时才会产生。
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
             }

关于“Trap-PDU”要说明的是，Trap-PDU的目的地址怎样选定在协议中没有说明，是由应用决定的。