path: root/pysnmp/proto/mpmod/rfc3412.py

#
# This file is part of pysnmp software.
#
# Copyright (c) 2005-2017, Ilya Etingof <etingof@gmail.com>
# License: http://pysnmp.sf.net/license.html
#
import sys
from pysnmp.proto.mpmod.base import AbstractMessageProcessingModel
from pysnmp.proto import rfc1905, rfc3411, api, errind, error
from pyasn1.type import univ, namedtype, constraint
from pyasn1.codec.ber import decoder, eoo
from pyasn1.error import PyAsn1Error
from pysnmp import debug

# API to rfc1905 protocol objects
pMod = api.protoModules[api.protoVersion2c]


# SNMPv3 message format

class ScopedPDU(univ.Sequence):
    componentType = namedtype.NamedTypes(
        namedtype.NamedType('contextEngineId', univ.OctetString()),
        namedtype.NamedType('contextName', univ.OctetString()),
        namedtype.NamedType('data', rfc1905.PDUs())
    )


class ScopedPduData(univ.Choice):
    componentType = namedtype.NamedTypes(
        namedtype.NamedType('plaintext', ScopedPDU()),
        namedtype.NamedType('encryptedPDU', univ.OctetString()),
    )


class HeaderData(univ.Sequence):
    componentType = namedtype.NamedTypes(
        namedtype.NamedType('msgID',
                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(0, 2147483647))),
        namedtype.NamedType('msgMaxSize',
                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(484, 2147483647))),
        namedtype.NamedType('msgFlags', univ.OctetString().subtype(subtypeSpec=constraint.ValueSizeConstraint(1, 1))),
        namedtype.NamedType('msgSecurityModel',
                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(1, 2147483647)))
    )


class SNMPv3Message(univ.Sequence):
    componentType = namedtype.NamedTypes(
        namedtype.NamedType('msgVersion',
                            univ.Integer().subtype(subtypeSpec=constraint.ValueRangeConstraint(0, 2147483647))),
        namedtype.NamedType('msgGlobalData', HeaderData()),
        namedtype.NamedType('msgSecurityParameters', univ.OctetString()),
        namedtype.NamedType('msgData', ScopedPduData())
    )


# XXX move somewhere?
_snmpErrors = {(1, 3, 6, 1, 6, 3, 15, 1, 1, 1, 0): 'unsupportedSecLevel',
               (1, 3, 6, 1, 6, 3, 15, 1, 1, 2, 0): 'notInTimeWindow',
               (1, 3, 6, 1, 6, 3, 15, 1, 1, 3, 0): 'unknownUserName',
               (1, 3, 6, 1, 6, 3, 15, 1, 1, 4, 0): 'unknownEngineID',
               (1, 3, 6, 1, 6, 3, 15, 1, 1, 5, 0): 'wrongDigest',
               (1, 3, 6, 1, 6, 3, 15, 1, 1, 6, 0): 'decryptionError'}


class SnmpV3MessageProcessingModel(AbstractMessageProcessingModel):
    messageProcessingModelID = univ.Integer(3)  # SNMPv3
    snmpMsgSpec = SNMPv3Message
    _emptyStr = univ.OctetString('')
    _msgFlags = {0: univ.OctetString('\x00'),
                 1: univ.OctetString('\x01'),
                 3: univ.OctetString('\x03'),
                 4: univ.OctetString('\x04'),
                 5: univ.OctetString('\x05'),
                 7: univ.OctetString('\x07')}

    def __init__(self):
        AbstractMessageProcessingModel.__init__(self)
        self.__scopedPDU = ScopedPDU()
        self.__engineIdCache = {}
        self.__engineIdCacheExpQueue = {}
        self.__expirationTimer = 0

    def getPeerEngineInfo(self, transportDomain, transportAddress):
        k = transportDomain, transportAddress
        if k in self.__engineIdCache:
            return (self.__engineIdCache[k]['securityEngineId'],
                    self.__engineIdCache[k]['contextEngineId'],
                    self.__engineIdCache[k]['contextName'])
        else:
            return None, None, None

    # 7.1.1a
    def prepareOutgoingMessage(self, snmpEngine, transportDomain,
                               transportAddress, messageProcessingModel,
                               securityModel, securityName, securityLevel,
                               contextEngineId, contextName, pduVersion,
                               pdu, expectResponse, sendPduHandle):
        snmpEngineID, = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols('__SNMP-FRAMEWORK-MIB',
                                                                                              'snmpEngineID')
        snmpEngineID = snmpEngineID.syntax

        # 7.1.1b
        msgID = self._cache.newMsgID()

        debug.logger & debug.flagMP and debug.logger('prepareOutgoingMessage: new msgID %s' % msgID)

        k = (transportDomain, transportAddress)
        if k in self.__engineIdCache:
            peerSnmpEngineData = self.__engineIdCache[k]
        else:
            peerSnmpEngineData = None

        debug.logger & debug.flagMP and debug.logger(
            'prepareOutgoingMessage: peer SNMP engine data %s for transport %s, address %s' % (
                peerSnmpEngineData, transportDomain, transportAddress))

        # 7.1.4
        if contextEngineId is None:
            if peerSnmpEngineData is None:
                contextEngineId = snmpEngineID
            else:
                contextEngineId = peerSnmpEngineData['contextEngineId']
                # Defaulting contextEngineID to securityEngineId should
                # probably be done on Agent side (see 7.1.3.d.2,) so this
                # is a sort of workaround.
                if not contextEngineId:
                    contextEngineId = peerSnmpEngineData['securityEngineId']
        # 7.1.5
        if not contextName:
            contextName = self._emptyStr

        debug.logger & debug.flagMP and debug.logger(
            'prepareOutgoingMessage: using contextEngineId %r, contextName %r' % (contextEngineId, contextName))

        # 7.1.6
        scopedPDU = self.__scopedPDU
        scopedPDU.setComponentByPosition(0, contextEngineId)
        scopedPDU.setComponentByPosition(1, contextName)
        scopedPDU.setComponentByPosition(2)
        scopedPDU.getComponentByPosition(2).setComponentByType(
            pdu.tagSet, pdu, verifyConstraints=False
        )

        # 7.1.7
        msg = self._snmpMsgSpec

        # 7.1.7a
        msg.setComponentByPosition(
            0, self.messageProcessingModelID, verifyConstraints=False
        )
        headerData = msg.setComponentByPosition(1).getComponentByPosition(1)

        # 7.1.7b
        headerData.setComponentByPosition(0, msgID, verifyConstraints=False)

        snmpEngineMaxMessageSize, = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols(
            '__SNMP-FRAMEWORK-MIB', 'snmpEngineMaxMessageSize')

        # 7.1.7c
        # XXX need to coerce MIB value as it has incompatible constraints set
        headerData.setComponentByPosition(
            1, snmpEngineMaxMessageSize.syntax, verifyConstraints=False
        )

        # 7.1.7d
        msgFlags = 0
        if securityLevel == 1:
            pass
        elif securityLevel == 2:
            msgFlags |= 0x01
        elif securityLevel == 3:
            msgFlags |= 0x03
        else:
            raise error.ProtocolError(
                'Unknown securityLevel %s' % securityLevel
            )

        if pdu.tagSet in rfc3411.confirmedClassPDUs:
            msgFlags |= 0x04

        headerData.setComponentByPosition(
            2, self._msgFlags[msgFlags], verifyConstraints=False
        )

        # 7.1.7e
        # XXX need to coerce MIB value as it has incompatible constraints set
        headerData.setComponentByPosition(3, int(securityModel))

        debug.logger & debug.flagMP and debug.logger('prepareOutgoingMessage: %s' % (msg.prettyPrint(),))

        if securityModel in snmpEngine.securityModels:
            smHandler = snmpEngine.securityModels[securityModel]
        else:
            raise error.StatusInformation(
                errorIndication=errind.unsupportedSecurityModel
            )

        # 7.1.9.a
        if pdu.tagSet in rfc3411.unconfirmedClassPDUs:
            securityEngineId = snmpEngineID
        else:
            if peerSnmpEngineData is None:
                # Force engineID discovery (rfc3414, 4)
                securityEngineId = securityName = self._emptyStr
                securityLevel = 1
                # Clear possible auth&priv flags
                headerData.setComponentByPosition(
                    2, self._msgFlags[msgFlags & 0xfc], verifyConstraints=False
                )
                # XXX
                scopedPDU = self.__scopedPDU
                scopedPDU.setComponentByPosition(
                    0, self._emptyStr, verifyConstraints=False
                )
                scopedPDU.setComponentByPosition(1, contextName)
                scopedPDU.setComponentByPosition(2)

                # Use dead-empty PDU for engine-discovery report
                emptyPdu = pdu.clone()
                pMod.apiPDU.setDefaults(emptyPdu)

                scopedPDU.getComponentByPosition(2).setComponentByType(
                    emptyPdu.tagSet, emptyPdu, verifyConstraints=False
                )
                debug.logger & debug.flagMP and debug.logger('prepareOutgoingMessage: force engineID discovery')
            else:
                securityEngineId = peerSnmpEngineData['securityEngineId']

        debug.logger & debug.flagMP and debug.logger(
            'prepareOutgoingMessage: securityModel %r, securityEngineId %r, securityName %r, securityLevel %r' % (
                securityModel, securityEngineId, securityName, securityLevel))

        # 7.1.9.b
        (securityParameters,
         wholeMsg) = smHandler.generateRequestMsg(
            snmpEngine, self.messageProcessingModelID, msg,
            snmpEngineMaxMessageSize.syntax, securityModel,
            securityEngineId, securityName, securityLevel, scopedPDU
        )

        # Message size constraint verification
        if len(wholeMsg) > snmpEngineMaxMessageSize.syntax:
            raise error.StatusInformation(errorIndication=errind.tooBig)

        # 7.1.9.c
        if pdu.tagSet in rfc3411.confirmedClassPDUs:
            # XXX rfc bug? why stateReference should be created?
            self._cache.pushByMsgId(msgID, sendPduHandle=sendPduHandle,
                                    msgID=msgID, snmpEngineID=snmpEngineID,
                                    securityModel=securityModel,
                                    securityName=securityName,
                                    securityLevel=securityLevel,
                                    contextEngineId=contextEngineId,
                                    contextName=contextName,
                                    transportDomain=transportDomain,
                                    transportAddress=transportAddress)

        snmpEngine.observer.storeExecutionContext(
            snmpEngine, 'rfc3412.prepareOutgoingMessage',
            dict(transportDomain=transportDomain,
                 transportAddress=transportAddress,
                 wholeMsg=wholeMsg,
                 securityModel=securityModel,
                 securityName=securityName,
                 securityLevel=securityLevel,
                 contextEngineId=contextEngineId,
                 contextName=contextName,
                 pdu=pdu)
        )
        snmpEngine.observer.clearExecutionContext(
            snmpEngine, 'rfc3412.prepareOutgoingMessage'
        )

        return transportDomain, transportAddress, wholeMsg

    def prepareResponseMessage(self, snmpEngine, messageProcessingModel,
                               securityModel, securityName, securityLevel,
                               contextEngineId, contextName, pduVersion,
                               pdu, maxSizeResponseScopedPDU, stateReference,
                               statusInformation):
        snmpEngineID, = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols('__SNMP-FRAMEWORK-MIB',
                                                                                              'snmpEngineID')
        snmpEngineID = snmpEngineID.syntax

        # 7.1.2.b
        cachedParams = self._cache.popByStateRef(stateReference)
        msgID = cachedParams['msgID']
        contextEngineId = cachedParams['contextEngineId']
        contextName = cachedParams['contextName']
        securityModel = cachedParams['securityModel']
        securityName = cachedParams['securityName']
        securityLevel = cachedParams['securityLevel']
        securityStateReference = cachedParams['securityStateReference']
        reportableFlag = cachedParams['reportableFlag']
        maxMessageSize = cachedParams['msgMaxSize']
        transportDomain = cachedParams['transportDomain']
        transportAddress = cachedParams['transportAddress']

        debug.logger & debug.flagMP and debug.logger('prepareResponseMessage: stateReference %s' % stateReference)

        # 7.1.3
        if statusInformation is not None and 'oid' in statusInformation:
            # 7.1.3a
            if pdu is None:
                pduType = None
            else:
                requestID = pdu.getComponentByPosition(0)
                pduType = pdu.tagSet

            # 7.1.3b
            if pdu is None and not reportableFlag or \
                    pduType is not None and \
                    pduType not in rfc3411.confirmedClassPDUs:
                raise error.StatusInformation(
                    errorIndication=errind.loopTerminated
                )

            # 7.1.3c
            reportPDU = rfc1905.ReportPDU()
            pMod.apiPDU.setVarBinds(reportPDU, ((statusInformation['oid'], statusInformation['val']),))
            pMod.apiPDU.setErrorStatus(reportPDU, 0)
            pMod.apiPDU.setErrorIndex(reportPDU, 0)
            if pdu is None:
                pMod.apiPDU.setRequestID(reportPDU, 0)
            else:
                # noinspection PyUnboundLocalVariable
                pMod.apiPDU.setRequestID(reportPDU, requestID)

            # 7.1.3d.1
            if 'securityLevel' in statusInformation:
                securityLevel = statusInformation['securityLevel']
            else:
                securityLevel = 1

            # 7.1.3d.2
            if 'contextEngineId' in statusInformation:
                contextEngineId = statusInformation['contextEngineId']
            else:
                contextEngineId = snmpEngineID

            # 7.1.3d.3
            if 'contextName' in statusInformation:
                contextName = statusInformation['contextName']
            else:
                contextName = ""

            # 7.1.3e
            pdu = reportPDU

            debug.logger & debug.flagMP and debug.logger(
                'prepareResponseMessage: prepare report PDU for statusInformation %s' % statusInformation)
        # 7.1.4
        if not contextEngineId:
            contextEngineId = snmpEngineID  # XXX impl-dep manner

        # 7.1.5
        if not contextName:
            contextName = self._emptyStr

        debug.logger & debug.flagMP and debug.logger(
            'prepareResponseMessage: using contextEngineId %r, contextName %r' % (contextEngineId, contextName))

        # 7.1.6
        scopedPDU = self.__scopedPDU
        scopedPDU.setComponentByPosition(0, contextEngineId)
        scopedPDU.setComponentByPosition(1, contextName)
        scopedPDU.setComponentByPosition(2)
        scopedPDU.getComponentByPosition(2).setComponentByType(
            pdu.tagSet, pdu, verifyConstraints=False
        )

        # 7.1.7
        msg = self._snmpMsgSpec

        # 7.1.7a
        msg.setComponentByPosition(0, self.messageProcessingModelID,
                                   verifyConstraints=False)

        headerData = msg.setComponentByPosition(1).getComponentByPosition(1)

        # 7.1.7b
        headerData.setComponentByPosition(0, msgID, verifyConstraints=False)

        snmpEngineMaxMessageSize, = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols(
            '__SNMP-FRAMEWORK-MIB', 'snmpEngineMaxMessageSize')

        # 7.1.7c
        # XXX need to coerce MIB value as it has incompatible constraints set
        headerData.setComponentByPosition(
            1, snmpEngineMaxMessageSize.syntax, verifyConstraints=False
        )

        # 7.1.7d
        msgFlags = 0
        if securityLevel == 1:
            pass
        elif securityLevel == 2:
            msgFlags |= 0x01
        elif securityLevel == 3:
            msgFlags |= 0x03
        else:
            raise error.ProtocolError('Unknown securityLevel %s' % securityLevel)

        if pdu.tagSet in rfc3411.confirmedClassPDUs:  # XXX not needed?
            msgFlags |= 0x04

        headerData.setComponentByPosition(
            2, self._msgFlags[msgFlags], verifyConstraints=False
        )

        # 7.1.7e
        headerData.setComponentByPosition(
            3, securityModel, verifyConstraints=False
        )

        debug.logger & debug.flagMP and debug.logger('prepareResponseMessage: %s' % (msg.prettyPrint(),))

        if securityModel in snmpEngine.securityModels:
            smHandler = snmpEngine.securityModels[securityModel]
        else:
            raise error.StatusInformation(errorIndication=errind.unsupportedSecurityModel)

        debug.logger & debug.flagMP and debug.logger(
            'prepareResponseMessage: securityModel %r, securityEngineId %r, securityName %r, securityLevel %r' % (
                securityModel, snmpEngineID, securityName, securityLevel))

        # 7.1.8a
        try:
            (securityParameters,
             wholeMsg) = smHandler.generateResponseMsg(
                snmpEngine, self.messageProcessingModelID, msg,
                snmpEngineMaxMessageSize.syntax, securityModel,
                snmpEngineID, securityName, securityLevel, scopedPDU,
                securityStateReference
            )
        except error.StatusInformation:
            # 7.1.8.b
            raise

        debug.logger & debug.flagMP and debug.logger('prepareResponseMessage: SM finished')

        # Message size constraint verification
        if len(wholeMsg) > min(snmpEngineMaxMessageSize.syntax, maxMessageSize):
            raise error.StatusInformation(errorIndication=errind.tooBig)

        snmpEngine.observer.storeExecutionContext(
            snmpEngine,
            'rfc3412.prepareResponseMessage',
            dict(transportDomain=transportDomain,
                 transportAddress=transportAddress,
                 securityModel=securityModel,
                 securityName=securityName,
                 securityLevel=securityLevel,
                 contextEngineId=contextEngineId,
                 contextName=contextName,
                 securityEngineId=snmpEngineID,
                 pdu=pdu)
        )
        snmpEngine.observer.clearExecutionContext(
            snmpEngine, 'rfc3412.prepareResponseMessage'
        )

        return transportDomain, transportAddress, wholeMsg

    # 7.2.1

    def prepareDataElements(self, snmpEngine, transportDomain,
                            transportAddress, wholeMsg):
        # 7.2.2
        try:
            msg, restOfwholeMsg = decoder.decode(wholeMsg, asn1Spec=self._snmpMsgSpec)

        except PyAsn1Error:
            debug.logger & debug.flagMP and debug.logger('prepareDataElements: %s' % (sys.exc_info()[1],))
            snmpInASNParseErrs, = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols('__SNMPv2-MIB',
                                                                                                        'snmpInASNParseErrs')
            snmpInASNParseErrs.syntax += 1
            raise error.StatusInformation(errorIndication=errind.parseError)

        debug.logger & debug.flagMP and debug.logger('prepareDataElements: %s' % (msg.prettyPrint(),))

        if eoo.endOfOctets.isSameTypeWith(msg):
            raise error.StatusInformation(errorIndication=errind.parseError)

        # 7.2.3
        headerData = msg.getComponentByPosition(1)
        msgVersion = messageProcessingModel = msg.getComponentByPosition(0)
        msgID = headerData.getComponentByPosition(0)
        msgFlags, = headerData.getComponentByPosition(2).asNumbers()
        maxMessageSize = headerData.getComponentByPosition(1)
        securityModel = headerData.getComponentByPosition(3)
        securityParameters = msg.getComponentByPosition(2)

        debug.logger & debug.flagMP and debug.logger(
            'prepareDataElements: msg data msgVersion %s msgID %s securityModel %s' % (
                msgVersion, msgID, securityModel))

        # 7.2.4
        if securityModel not in snmpEngine.securityModels:
            snmpUnknownSecurityModels, = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols(
                '__SNMP-MPD-MIB', 'snmpUnknownSecurityModels')
            snmpUnknownSecurityModels.syntax += 1
            raise error.StatusInformation(errorIndication=errind.unsupportedSecurityModel)

        # 7.2.5
        if msgFlags & 0x03 == 0x00:
            securityLevel = 1
        elif (msgFlags & 0x03) == 0x01:
            securityLevel = 2
        elif (msgFlags & 0x03) == 0x03:
            securityLevel = 3
        else:
            snmpInvalidMsgs = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols('__SNMP-MPD-MIB',
                                                                                                    'snmpInvalidMsgs')
            snmpInvalidMsgs.syntax += 1
            raise error.StatusInformation(errorIndication=errind.invalidMsg)

        if msgFlags & 0x04:
            reportableFlag = 1
        else:
            reportableFlag = 0

        # 7.2.6
        smHandler = snmpEngine.securityModels[securityModel]
        try:
            (securityEngineId, securityName,
             scopedPDU, maxSizeResponseScopedPDU,
             securityStateReference) = smHandler.processIncomingMsg(
                snmpEngine, messageProcessingModel, maxMessageSize,
                securityParameters, securityModel, securityLevel,
                wholeMsg, msg
            )
            debug.logger & debug.flagMP and debug.logger('prepareDataElements: SM succeeded')

        except error.StatusInformation:
            statusInformation, origTraceback = sys.exc_info()[1:3]
            debug.logger & debug.flagMP and debug.logger(
                'prepareDataElements: SM failed, statusInformation %s' % statusInformation)
            if 'errorIndication' in statusInformation:
                # 7.2.6a
                if 'oid' in statusInformation:
                    # 7.2.6a1
                    securityStateReference = statusInformation['securityStateReference']
                    contextEngineId = statusInformation['contextEngineId']
                    contextName = statusInformation['contextName']
                    if 'scopedPDU' in statusInformation:
                        scopedPDU = statusInformation['scopedPDU']
                        pdu = scopedPDU.getComponentByPosition(2).getComponent()
                    else:
                        pdu = None
                    maxSizeResponseScopedPDU = statusInformation['maxSizeResponseScopedPDU']
                    securityName = None  # XXX secmod cache used

                    # 7.2.6a2
                    stateReference = self._cache.newStateReference()
                    self._cache.pushByStateRef(
                        stateReference, msgVersion=messageProcessingModel,
                        msgID=msgID, contextEngineId=contextEngineId,
                        contextName=contextName, securityModel=securityModel,
                        securityName=securityName, securityLevel=securityLevel,
                        securityStateReference=securityStateReference,
                        reportableFlag=reportableFlag,
                        msgMaxSize=maxMessageSize,
                        maxSizeResponseScopedPDU=maxSizeResponseScopedPDU,
                        transportDomain=transportDomain,
                        transportAddress=transportAddress
                    )

                    # 7.2.6a3
                    try:
                        snmpEngine.msgAndPduDsp.returnResponsePdu(
                            snmpEngine, 3, securityModel, securityName,
                            securityLevel, contextEngineId, contextName,
                            1, pdu, maxSizeResponseScopedPDU, stateReference,
                            statusInformation
                        )
                    except error.StatusInformation:
                        pass

                    debug.logger & debug.flagMP and debug.logger('prepareDataElements: error reported')
            # 7.2.6b
            if sys.version_info[0] <= 2:
                raise statusInformation
            else:
                try:
                    raise statusInformation.with_traceback(origTraceback)
                finally:
                    # Break cycle between locals and traceback object
                    # (seems to be irrelevant on Py3 but just in case)
                    del origTraceback
        else:
            # Sniff for engineIdCache
            k = (transportDomain, transportAddress)
            if k not in self.__engineIdCache:
                contextEngineId, contextName, pdus = scopedPDU
                pdu = pdus.getComponent()
                # Here we assume that authentic/default EngineIDs
                # come only in the course of engine-to-engine communication.
                if pdu.tagSet in rfc3411.internalClassPDUs:
                    self.__engineIdCache[k] = {
                        'securityEngineId': securityEngineId,
                        'contextEngineId': contextEngineId,
                        'contextName': contextName
                    }

                    expireAt = int(self.__expirationTimer + 300 / snmpEngine.transportDispatcher.getTimerResolution())
                    if expireAt not in self.__engineIdCacheExpQueue:
                        self.__engineIdCacheExpQueue[expireAt] = []
                    self.__engineIdCacheExpQueue[expireAt].append(k)

                    debug.logger & debug.flagMP and debug.logger(
                        'prepareDataElements: cache securityEngineId %r for %r %r' % (
                            securityEngineId, transportDomain, transportAddress))

        snmpEngineID, = snmpEngine.msgAndPduDsp.mibInstrumController.mibBuilder.importSymbols('__SNMP-FRAMEWORK-MIB',
                                                                                              'snmpEngineID')
        snmpEngineID = snmpEngineID.syntax

        # 7.2.7 XXX PDU would be parsed here?
        contextEngineId, contextName, pdu = scopedPDU
        pdu = pdu.getComponent()  # PDUs

        # 7.2.8
        pduVersion = api.protoVersion2c

        # 7.2.9
        pduType = pdu.tagSet

        # 7.2.10
        if pduType in rfc3411.responseClassPDUs or \
                pduType in rfc3411.internalClassPDUs:
            # 7.2.10a
            try:
                cachedReqParams = self._cache.popByMsgId(msgID)
            except error.ProtocolError:
                smHandler.releaseStateInformation(securityStateReference)
                raise error.StatusInformation(
                    errorIndication=errind.dataMismatch
                )
            # 7.2.10b
            sendPduHandle = cachedReqParams['sendPduHandle']
        else:
            sendPduHandle = None

        debug.logger & debug.flagMP and debug.logger(
            'prepareDataElements: using sendPduHandle %s for msgID %s' % (sendPduHandle, msgID))

        # 7.2.11
        if pduType in rfc3411.internalClassPDUs:
            # 7.2.11a
            varBinds = pMod.apiPDU.getVarBinds(pdu)
            if varBinds:
                statusInformation = error.StatusInformation(
                    errorIndication=_snmpErrors.get(varBinds[0][0],
                                                    'errorReportReceived'),
                    oid=varBinds[0][0], val=varBinds[0][1],
                    sendPduHandle=sendPduHandle
                )
            else:
                statusInformation = error.StatusInformation(
                    sendPduHandle=sendPduHandle
                )

            # 7.2.11b (incomplete implementation)

            snmpEngine.observer.storeExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:internal',
                dict(transportDomain=transportDomain,
                     transportAddress=transportAddress,
                     securityModel=securityModel,
                     securityName=securityName,
                     securityLevel=securityLevel,
                     contextEngineId=contextEngineId,
                     contextName=contextName,
                     securityEngineId=securityEngineId,
                     pdu=pdu)
            )
            snmpEngine.observer.clearExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:internal'
            )

            # 7.2.11c
            smHandler.releaseStateInformation(securityStateReference)

            # 7.2.11d
            # no-op

            # 7.2.11e XXX may need to pass Reports up to app in some cases...
            raise statusInformation

        statusInformation = None  # no errors ahead

        # 7.2.12
        if pduType in rfc3411.responseClassPDUs:
            # 7.2.12a -> noop

            # 7.2.12b
            # noinspection PyUnboundLocalVariable
            if securityModel != cachedReqParams['securityModel'] or \
                    securityName != cachedReqParams['securityName'] or \
                    securityLevel != cachedReqParams['securityLevel'] or \
                    contextEngineId != cachedReqParams['contextEngineId'] or \
                    contextName != cachedReqParams['contextName']:
                smHandler.releaseStateInformation(securityStateReference)
                raise error.StatusInformation(
                    errorIndication=errind.dataMismatch
                )

            snmpEngine.observer.storeExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:response',
                dict(transportDomain=transportDomain,
                     transportAddress=transportAddress,
                     securityModel=securityModel,
                     securityName=securityName,
                     securityLevel=securityLevel,
                     contextEngineId=contextEngineId,
                     contextName=contextName,
                     securityEngineId=securityEngineId,
                     pdu=pdu)
            )
            snmpEngine.observer.clearExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:response'
            )

            # 7.2.12c
            smHandler.releaseStateInformation(securityStateReference)
            stateReference = None

            # 7.2.12d
            return (messageProcessingModel, securityModel, securityName,
                    securityLevel, contextEngineId, contextName,
                    pduVersion, pdu, pduType, sendPduHandle,
                    maxSizeResponseScopedPDU, statusInformation,
                    stateReference)

        # 7.2.13
        if pduType in rfc3411.confirmedClassPDUs:
            # 7.2.13a
            if securityEngineId != snmpEngineID:
                smHandler.releaseStateInformation(securityStateReference)
                raise error.StatusInformation(
                    errorIndication=errind.engineIDMismatch
                )

            # 7.2.13b
            stateReference = self._cache.newStateReference()
            self._cache.pushByStateRef(
                stateReference, msgVersion=messageProcessingModel,
                msgID=msgID, contextEngineId=contextEngineId,
                contextName=contextName, securityModel=securityModel,
                securityName=securityName, securityLevel=securityLevel,
                securityStateReference=securityStateReference,
                reportableFlag=reportableFlag, msgMaxSize=maxMessageSize,
                maxSizeResponseScopedPDU=maxSizeResponseScopedPDU,
                transportDomain=transportDomain,
                transportAddress=transportAddress
            )

            debug.logger & debug.flagMP and debug.logger('prepareDataElements: new stateReference %s' % stateReference)

            snmpEngine.observer.storeExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:confirmed',
                dict(transportDomain=transportDomain,
                     transportAddress=transportAddress,
                     securityModel=securityModel,
                     securityName=securityName,
                     securityLevel=securityLevel,
                     contextEngineId=contextEngineId,
                     contextName=contextName,
                     securityEngineId=securityEngineId,
                     pdu=pdu)
            )
            snmpEngine.observer.clearExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:confirmed'
            )

            # 7.2.13c
            return (messageProcessingModel, securityModel, securityName,
                    securityLevel, contextEngineId, contextName,
                    pduVersion, pdu, pduType, sendPduHandle,
                    maxSizeResponseScopedPDU, statusInformation,
                    stateReference)

        # 7.2.14
        if pduType in rfc3411.unconfirmedClassPDUs:
            # Pass new stateReference to let app browse request details
            stateReference = self._cache.newStateReference()

            snmpEngine.observer.storeExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:unconfirmed',
                dict(transportDomain=transportDomain,
                     transportAddress=transportAddress,
                     securityModel=securityModel,
                     securityName=securityName,
                     securityLevel=securityLevel,
                     contextEngineId=contextEngineId,
                     contextName=contextName,
                     securityEngineId=securityEngineId,
                     pdu=pdu)
            )
            snmpEngine.observer.clearExecutionContext(
                snmpEngine, 'rfc3412.prepareDataElements:unconfirmed'
            )

            # This is not specified explicitly in RFC
            smHandler.releaseStateInformation(securityStateReference)

            return (messageProcessingModel, securityModel, securityName,
                    securityLevel, contextEngineId, contextName,
                    pduVersion, pdu, pduType, sendPduHandle,
                    maxSizeResponseScopedPDU, statusInformation,
                    stateReference)

        smHandler.releaseStateInformation(securityStateReference)
        raise error.StatusInformation(
            errorIndication=errind.unsupportedPDUtype
        )

    def __expireEnginesInfo(self):
        if self.__expirationTimer in self.__engineIdCacheExpQueue:
            for engineKey in self.__engineIdCacheExpQueue[self.__expirationTimer]:
                del self.__engineIdCache[engineKey]
                debug.logger & debug.flagMP and debug.logger('__expireEnginesInfo: expiring %r' % (engineKey,))
            del self.__engineIdCacheExpQueue[self.__expirationTimer]
        self.__expirationTimer += 1

    def receiveTimerTick(self, snmpEngine, timeNow):
        self.__expireEnginesInfo()
        AbstractMessageProcessingModel.receiveTimerTick(self, snmpEngine, timeNow)