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4. Dispatcher 过程元素

4.1. 向网络发送 SNMP 消息​

本节描述当应用希望向另一个 SNMP 实体发送 SNMP PDU 时 Dispatcher 遵循的过程.

4.1.1. 发送请求或通知​

当应用希望向网络发送 SNMP Request 或 Notification 时, 遵循以下过程.

过程:

  1. 应用通过抽象 sendPdu 服务接口原语调用 Dispatcher, 请求 Dispatcher 发送 SNMP PDU:
statusInformation = sendPdu(
    IN transportDomain,           -- transport domain to be used
    IN transportAddress,          -- destination network address
    IN messageProcessingModel,    -- typically, SNMP version
    IN securityModel,             -- Security Model to use
    IN securityName,              -- on behalf of this principal
    IN securityLevel,             -- Level of Security requested
    IN contextEngineID,           -- data from/at this SNMP entity
    IN contextName,               -- data from/in this context
    IN pduVersion,                -- the version of the PDU
    IN PDU,                       -- SNMP Protocol Data Unit
    IN expectResponse,            -- TRUE or FALSE
    IN sendPduHandle              -- the handle for matching incoming replies
)

  1. 如果 messageProcessingModel 值不表示 Dispatcher 已知的 Message Processing Model, 则向调用应用返回 errorIndication (unsupportedMsgProcessingModel). 该消息处理停止.

  2. Dispatcher 请求适当的 Message Processing Model 根据其消息格式规范准备消息:

statusInformation = prepareOutgoingMessage(
    IN transportDomain,           -- as specified by application
    IN transportAddress,          -- as specified by application
    IN messageProcessingModel,    -- as specified by application
    IN securityModel,             -- as specified by application
    IN securityName,              -- as specified by application
    IN securityLevel,             -- as specified by application
    IN contextEngineID,           -- as specified by application
    IN contextName,               -- as specified by application
    IN pduVersion,                -- as specified by application
    IN PDU,                       -- as specified by application
    IN expectResponse,            -- as specified by application
    IN sendPduHandle,             -- as determined by application
    OUT destTransportDomain,      -- destination transport domain
    OUT destTransportAddress,     -- destination transport address
    OUT outgoingMessage,          -- the message to send
    OUT outgoingMessageLength     -- its length
)

  1. 如果 statusInformation 参数中的 statusInformation 不是 success, Dispatcher 将 statusInformation 返回给调用应用. 该消息处理停止.

  2. 如果 statusInformation 为 success, Dispatcher 通过 transportDomain 和 transportAddress 指定的传输向网络发送准备好的消息.

  3. Dispatcher 向调用应用返回 success.

4.1.2. 向网络发送响应​

当应用希望向网络发送响应时, 遵循以下过程.

过程:

  1. 应用使用 returnResponsePdu 抽象服务接口调用 Dispatcher, 请求 Dispatcher 发送 SNMP PDU:
result = returnResponsePdu(
    IN messageProcessingModel,    -- typically, SNMP version
    IN securityModel,             -- Security Model in use
    IN securityName,              -- on behalf of this principal
    IN securityLevel,             -- same as on incoming request
    IN contextEngineID,           -- data from/at this SNMP entity
    IN contextName,               -- data from/in this context
    IN pduVersion,                -- the version of the PDU
    IN PDU,                       -- SNMP Protocol Data Unit
    IN maxSizeResponseScopedPDU,  -- maximum size of the Response PDU
    IN stateReference,            -- reference to state information
    IN statusInformation          -- success or errorIndication
)

注意: 抽象数据元素 securityModel, securityName, securityLevel, contextEngineID, contextName 和 stateReference 应与最初接收请求并传递给应用时确定的值相同. 可以使用 stateReference 作为键从缓存中取回这些值.

  1. 如果 messageProcessingModel 值不表示 Dispatcher 已知的 Message Processing Model, 则向调用应用返回 errorIndication (unsupportedMsgProcessingModel). 该消息处理停止.

  2. Dispatcher 请求适当的 Message Processing Model 根据其消息格式规范准备响应消息:

result = prepareResponseMessage(
    IN messageProcessingModel,    -- as provided by application
    IN securityModel,             -- as provided by application
    IN securityName,              -- as provided by application
    IN securityLevel,             -- as provided by application
    IN contextEngineID,           -- as provided by application
    IN contextName,               -- as provided by application
    IN pduVersion,                -- as provided by application
    IN PDU,                       -- as provided by application
    IN maxSizeResponseScopedPDU,  -- as provided by application
    IN stateReference,            -- as provided by application
    IN statusInformation,         -- as provided by application
    OUT destTransportDomain,      -- destination transport domain
    OUT destTransportAddress,     -- destination transport address
    OUT outgoingMessage,          -- the message to send
    OUT outgoingMessageLength     -- its length
)

  1. 如果 result 参数中的 result 不是 success, Dispatcher 将 errorIndication 返回给调用应用. 该消息处理停止.

  2. 如果 result 为 success, Dispatcher 通过 destTransportDomain 和 destTransportAddress 指定的传输向网络发送准备好的消息.

  3. Dispatcher 向调用应用返回 success.

4.2. 从网络接收 SNMP 消息​

本节描述 Dispatcher 从网络接收 SNMP 消息时遵循的过程.

4.2.1. 收到 SNMP 消息的消息分派​

Dispatcher 从网络接收 SNMP Message 时, 遵循以下过程:

  1. Dispatcher 从网络接收消息.

  2. Dispatcher 确定 SNMP 消息版本. 如果数据包无法充分解析以确定版本, 则递增 snmpInASNParseErrs 计数器 [RFC3418], 并丢弃该消息且不再进一步处理. 该消息处理停止.

  3. Dispatcher 根据 SNMP 消息版本确定适当的 Message Processing Model.

  4. 如果 Dispatcher 不知道该 SNMP Message 版本, 则递增 snmpInBadVersions 计数器 [RFC3418], 并丢弃该消息且不再进一步处理. 该消息处理停止.

4.2.2. 传入消息的 PDU 分派​

消息分派确定 Message Processing Model 后, 使用以下抽象服务接口请求 Message Processing Model 从消息中提取 PDU 和数据:

result = prepareDataElements(
    IN transportDomain,           -- origin transport domain
    IN transportAddress,          -- origin transport address
    IN wholeMsg,                  -- as received from the network
    IN wholeMsgLength,            -- as received from the network
    OUT messageProcessingModel,   -- typically, SNMP version
    OUT securityModel,            -- Security Model in use
    OUT securityName,             -- on behalf of this principal
    OUT securityLevel,            -- Level of Security requested
    OUT contextEngineID,          -- data from/at this SNMP entity
    OUT contextName,              -- data from/in this context
    OUT pduVersion,               -- the version of the PDU
    OUT PDU,                      -- SNMP Protocol Data Unit
    OUT pduType,                  -- SNMP PDU type
    OUT sendPduHandle,            -- handle for matched request
    OUT maxSizeResponseScopedPDU, -- maximum size of Response PDU
    OUT statusInformation,        -- success or errorIndication
    OUT stateReference            -- reference to state information
)

如果 result errorIndication 被设置为 noError 以外的值, 则丢弃该消息且不再进一步处理. 该消息处理停止.

4.2.2.1. 传入请求和通知​

当 pduType 的值指示消息包含 Request 或 Notification 时, 遵循以下过程分派 PDU:

  1. 根据 pduType, contextEngineID 和 contextName, 选择适当的应用.

    a. 如果没有应用为 pduType, contextEngineID 和 contextName 的组合注册, 且没有应用为 pduType 与 contextEngineID 且 contextName 为空的组合注册, 则:

    • 如果 pduType 是 confirmed, 则向消息发起者发送 Report PDU. Dispatcher 调用适当的 Message Processing Model, 使用 reportPDU 抽象服务接口原语生成错误报告:
result = reportPDU(
    IN messageProcessingModel,    -- typically, SNMP version
    IN securityModel,             -- Security Model in use
    IN securityName,              -- on behalf of this principal
    IN securityLevel,             -- same as on incoming request
    IN contextEngineID,           -- data from/at this SNMP entity
    IN contextName,               -- data from/in this context
    IN pduVersion,                -- the version of the PDU
    IN PDU,                       -- SNMP Protocol Data Unit
    IN maxSizeResponseScopedPDU,  -- maximum size of the Response PDU
    IN stateReference,            -- reference to state information
    IN statusInformation          -- error indication
)
  • 丢弃该消息. 该消息处理停止.

b. 如果 pduType 是 unconfirmed, 则丢弃该消息. 该消息处理停止.

  1. 使用 processPdu 抽象服务接口将 PDU 分派给应用:
processPdu(
    IN messageProcessingModel,    -- typically, SNMP version
    IN securityModel,             -- Security Model in use
    IN securityName,              -- on behalf of this principal
    IN securityLevel,             -- Level of Security
    IN contextEngineID,           -- data from/at this SNMP entity
    IN contextName,               -- data from/in this context
    IN pduVersion,                -- the version of the PDU
    IN PDU,                       -- SNMP Protocol Data Unit
    IN maxSizeResponseScopedPDU,  -- maximum size of Response PDU
    IN stateReference             -- reference to state information
)

4.2.2.2. 传入响应​

当 pduType 指示 Response, Report 或其他 Class 2 类型 PDU 时, 遵循以下过程分派 PDU:

  1. sendPduHandle 中的值用于确定应将 PDU 分派给哪个应用.

  2. 如果未找到应用注册, 则递增 snmpUnknownPDUHandlers 计数器 [RFC3418], 并丢弃该消息. 该消息处理停止.

  3. 否则, 使用 processResponsePdu 抽象服务接口将消息分派给应用:

processResponsePdu(
    IN messageProcessingModel,    -- typically, SNMP version
    IN securityModel,             -- Security Model in use
    IN securityName,              -- on behalf of this principal
    IN securityLevel,             -- Level of Security
    IN contextEngineID,           -- data from/at this SNMP entity
    IN contextName,               -- data from/in this context
    IN pduVersion,                -- the version of the PDU
    IN PDU,                       -- SNMP Protocol Data Unit
    IN statusInformation,         -- success or errorIndication
    IN sendPduHandle              -- handle for matched request
)

4.3. 处理 PDU 类型的应用注册​

希望处理某些 PDU 的应用必须向 PDU Dispatcher 注册. 应用指定它希望负责的 pduType, contextEngineID 和 contextName 组合.

应用注册遵循以下过程:

  1. 应用使用 registerContextEngineID 抽象服务接口原语调用 PDU Dispatcher:
statusInformation = registerContextEngineID(
    IN contextEngineID,           -- take responsibility for this one
    IN pduType                    -- the pduType(s) to be registered
)

  1. 如果另一个应用已经为指定的 contextEngineID 和 pduType 注册, 则向调用应用返回 errorIndication (duplicateRegistration). 注册处理停止.

  2. 否则, PDU Dispatcher 保存该注册并返回 success.

4.4. 处理 PDU 类型的应用注销​

不再希望处理某些类型 PDU 的应用必须向 PDU Dispatcher 注销. 应用必须指定它不再希望负责的 contextEngineID 和 pduType 组合.

应用注销遵循以下过程:

  1. 应用使用 unregisterContextEngineID 抽象服务接口原语调用 PDU Dispatcher:
unregisterContextEngineID(
    IN contextEngineID,           -- give up responsibility for this
    IN pduType                    -- the pduType(s) to be unregistered
)

  1. 如果找到应用注册, 则删除该注册.

  2. 如果未找到此类注册, 则忽略该请求.

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