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RFC 2572
Network Working Group J. Case
Request for Comments: 2572 SNMP Research Inc.
Obsoletes: 2272 D. Harrington
Category: Standards Track Cabletron Systems, Inc.
R. Presuhn
BMC Software, Inc.
B. Wijnen
IBM T. J. Watson Research
April 1999
Message Processing and Dispatching for the
Simple Network Management Protocol (SNMP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This document describes the Message Processing and Dispatching for
SNMP messages within the SNMP architecture [RFC2571]. It defines the
procedures for dispatching potentially multiple versions of SNMP
messages to the proper SNMP Message Processing Models, and for
dispatching PDUs to SNMP applications. This document also describes
one Message Processing Model - the SNMPv3 Message Processing Model.
Table of Contents
1. Introduction ................................................ 3
2. Overview .................................................... 3
2.1. The Dispatcher. .......................................... 5
2.2. Message Processing Subsystem .............................. 5
3. Elements of Message Processing and Dispatching .............. 5
3.1. messageProcessingModel .................................... 6
3.2. pduVersion ................................................ 6
3.3. pduType ................................................... 7
3.4. sendPduHandle ............................................. 7
4. Dispatcher Elements of Procedure ............................ 7
4.1. Sending an SNMP Message to the Network .................... 7
SNMPv3 Working Group Standards Track [Page 1]
RFC 2572 Message Processing and Dispatching April 1999
4.1.1. Sending a Request or Notification ....................... 7
4.1.2. Sending a Response to the Network ....................... 9
4.2. Receiving an SNMP Message from the Network ................ 11
4.2.1. Message Dispatching of received SNMP Messages ........... 11
4.2.2. PDU Dispatching for Incoming Messages ................... 12
4.2.2.1. Incoming Requests and Notifications ................... 12
4.2.2.2. Incoming Responses .................................... 14
4.3. Application Registration for Handling PDU types ........... 15
4.4. Application Unregistration for Handling PDU Types ......... 16
5. Definitions ................................................. 16
5.1. Definitions for SNMP Message Processing and Dispatching ... 16
6. The SNMPv3 Message Format ................................... 20
6.1. msgVersion ................................................ 21
6.2. msgID ..................................................... 21
6.3. msgMaxSize ................................................ 21
6.4. msgFlags .................................................. 22
6.5. msgSecurityModel .......................................... 24
6.6. msgSecurityParameters ..................................... 24
6.7. scopedPduData ............................................. 24
6.8. scopedPDU ................................................. 25
6.8.1. contextEngineID ......................................... 25
6.8.2. contextName ............................................. 25
6.8.3. data .................................................... 25
7. Elements of Procedure for v3MP .............................. 25
7.1. Prepare an Outgoing SNMP Message .......................... 36
7.2. Prepare Data Elements from an Incoming SNMP Message ....... 31
8. Intellectual Property ....................................... 37
9. Acknowledgements ............................................ 37
10. Security Considerations .................................... 39
11. References ................................................. 40
12. Editors' Addresses ......................................... 41
13. Changes From RFC 2272 ...................................... 42
14. Full Copyright Statement ................................... 44
SNMPv3 Working Group Standards Track [Page 2]
RFC 2572 Message Processing and Dispatching April 1999
1. Introduction
The Architecture for describing Internet Management Frameworks
[RFC2571] describes that an SNMP engine is composed of:
1) a Dispatcher
2) a Message Processing Subsystem,
3) a Security Subsystem, and
4) an Access Control Subsystem.
Applications make use of the services of these subsystems.
It is important to understand the SNMP architecture and its
terminology to understand where the Message Processing Subsystem and
Dispatcher described in this document fit into the architecture and
interact with other subsystems within the architecture. The reader
is expected to have read and understood the description of the SNMP
architecture, defined in [RFC2571].
The Dispatcher in the SNMP engine sends and receives SNMP messages.
It also dispatches SNMP PDUs to SNMP applications. When an SNMP
message needs to be prepared or when data needs to be extracted from
an SNMP message, the Dispatcher delegates these tasks to a message
version-specific Message Processing Model within the Message
Processing Subsystem.
A Message Processing Model is responsible for processing a SNMP
version-specific message and for coordinating the interaction with
the Security Subsystem to ensure proper security is applied to the
SNMP message being handled.
Interactions between the Dispatcher, the Message Processing
Subsystem, and applications are modeled using abstract data elements
and abstract service interface primitives defined by the SNMP
architecture.
Similarly, interactions between the Message Processing Subsystem and
the Security Subsystem are modeled using abstract data elements and
abstract service interface primitives as defined by the SNMP
architecture.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
SNMPv3 Working Group Standards Track [Page 3]
RFC 2572 Message Processing and Dispatching April 1999
2. Overview
The following illustration depicts the Message Processing in relation
to SNMP applications, the Security Subsystem and Transport Mappings.
+-------------------------------------------------------------------+
| SNMP Entity |
| |
| +---------------------------------------------------------------+ |
| | Applications | |
| | +-----------+ +--------------+ | |
| | | Command | | Notification | | |
| | | Generator | | Originator | +-----------+ +--------------+| |
| | +-----------+ +--------------+ | Proxy | | Other | |
| | +-----------+ +--------------+ | Forwarder | |Application(s)|| |
| | | Command | | Notification | +-----------+ +--------------+| |
| | | Responder | | Receiver | | |
| | +-----------+ +--------------+ | |
| +---------------------------------------------------------------+ |
| ^ ^ ^ ^ |
| | | | | |
| v v v v |
| +--------+-------+---------------+-----------+ |
| ^ |
| | +---------------------+ +-----------------+ |
| | | Message Processing | | Security | |
| Dispatcher v | Subsystem | | Subsystem | |
| +------------------+ | +------------+ | | | |
| | PDU Dispatcher | | +->| v1MP * |<--->| +-------------+ | |
| | | | | +------------+ | | | Other | | |
| | | | | +------------+ | | | Security | | |
| | | | +->| v2cMP * |<--->| | Model | | |
| | Message | | | +------------+ | | +-------------+ | |
| | Dispatcher <-------->+ | | | |
| | | | | +------------+ | | +-------------+ | |
| | | | +->| v3MP * |<--->| | User-based | | |
| | Transport | | | +------------+ | | | Security | | |
| | Mapping | | | +------------+ | | | Model | | |
| | (e.g RFC 1906) | | +->| otherMP * |<--->| +-------------+ | |
| +------------------+ | +------------+ | | | |
| ^ +---------------------+ +-----------------+ |
| | |
+----------|--------------------------------------------------------+
v
+------------------+
| Network |
+------------------+
SNMPv3 Working Group Standards Track [Page 4]
RFC 2572 Message Processing and Dispatching April 1999
2.1. The Dispatcher.
The Dispatcher is a key piece of an SNMP engine. There is only one in
an SNMP engine, and its job is to dispatch tasks to the multiple
version-specific Message Processing Models, and to dispatch PDUs to
various applications.
For outgoing messages, an application provides a PDU to be sent, plus
the data needed to prepare and send the message, and the application
specifies which version-specific Message Processing Model will be
used to prepare the message with the desired security processing.
Once the message is prepared, the Dispatcher sends the message.
For incoming messages, the Dispatcher determines the SNMP version of
the incoming message and passes the message to the version-specific
Message Processing Model to extract the components of the message and
to coordinate the processing of security services for the message.
After version-specific processing, the PDU Dispatcher determines
which application, if any, should receive the PDU for processing and
forwards it accordingly.
The Dispatcher, while sending and receiving SNMP messages, collects
statistics about SNMP messages and the behavior of the SNMP engine in
managed objects to make them accessible to remote SNMP entities.
This document defines these managed objects, the MIB module which
contains them, and how these managed objects might be used to provide
useful management.
2.2. Message Processing Subsystem
The SNMP Message Processing Subsystem is the part of an SNMP engine
which interacts with the Dispatcher to handle the version-specific
SNMP messages. It contains one or more Message Processing Models.
This document describes one Message Processing Model, the SNMPv3
Message Processing Model, in Section 6. The SNMPv3 Message Processing
Model is defined in a separate section to show that multiple
(independent) Message Processing Models can exist at the same time
and that such Models can be described in different documents. The
SNMPv3 Message Processing Model can be replaced or supplemented with
other Message Processing Models in the future. Two Message Processing
Models which are expected to be developed in the future are the
SNMPv1 message format [RFC1157] and the SNMPv2c message format
[RFC1901]. Others may be developed as needed.
SNMPv3 Working Group Standards Track [Page 5]
RFC 2572 Message Processing and Dispatching April 1999
3. Elements of Message Processing and Dispatching
See [RFC2571] for the definitions of
contextEngineID
contextName
scopedPDU
maxSizeResponseScopedPDU
securityModel
securityName
securityLevel
messageProcessingModel
For incoming messages, a version-specific message processing module
provides these values to the Dispatcher. For outgoing messages, an
application provides these values to the Dispatcher.
For some version-specific processing, the values may be extracted
from received messages; for other versions, the values may be
determined by algorithm, or by an implementation-defined mechanism.
The mechanism by which the value is determined is irrelevant to the
Dispatcher.
The following additional or expanded definitions are for use within
the Dispatcher.
3.1. messageProcessingModel
The value of messageProcessingModel identifies a Message Processing
Model. A Message Processing Model describes the version-specific
procedures for extracting data from messages, generating messages,
calling upon a securityModel to apply its security services to
messages, for converting data from a version-specific message format
into a generic format usable by the Dispatcher, and for converting
data from Dispatcher format into a version-specific message format.
3.2. pduVersion
The value of pduVersion represents a specific version of protocol
operation and its associated PDU formats, such as SNMPv1 or SNMPv2
[RFC1905]. The values of pduVersion are specific to the version of
the PDU contained in a message, and the PDUs processed by
applications. The Dispatcher does not use the value of pduVersion
directly.
SNMPv3 Working Group Standards Track [Page 6]
RFC 2572 Message Processing and Dispatching April 1999
An application specifies the pduVersion when it requests the PDU
Dispatcher to send a PDU to another SNMP engine. The Dispatcher
passes the pduVersion to a Message Processing Model, so it knows how
to handle the PDU properly.
For incoming messages, pduVersion is provided to the Dispatcher by a
version-specific Message Processing module. The PDU Dispatcher passes
the pduVersion to the application so it knows how to handle the PDU
properly. For example, a command responder application needs to know
whether to use [RFC1905] elements of procedure and syntax instead of
those specified for SNMPv1.
3.3. pduType
A value of pduType represents a specific type of protocol operation.
The values of pduType are specific to the version of the PDU
contained in a message.
Applications register to support particular pduTypes for particular
contextEngineIDs.
For incoming messages, pduType is provided to the Dispatcher by a
version-specific Message Processing module. It is subsequently used
to dispatch the PDU to the application which registered for the
pduType for the contextEngineID of the associated scopedPDU.
3.4. sendPduHandle
This handle is generated for coordinating the processing of requests
and responses between the SNMP engine and an application. The handle
must be unique across all version-specific Message Processing Models,
and is of local significance only.
4. Dispatcher Elements of Procedure
This section describes the procedures followed by the Dispatcher when
generating and processing SNMP messages.
4.1. Sending an SNMP Message to the Network
This section describes the procedure followed by an SNMP engine
whenever it sends an SNMP message.
SNMPv3 Working Group Standards Track [Page 7]
RFC 2572 Message Processing and Dispatching April 1999
4.1.1. Sending a Request or Notification
The following procedures are followed by the Dispatcher when an
application wants to send an SNMP PDU to another (remote)
application, i.e., to initiate a communication by originating a
message, such as one containing a request or a trap.
1) The application requests this using the abstract service
primitive:
statusInformation = -- sendPduHandle if success
-- errorIndication if failure
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 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
)
2) If the messageProcessingModel value does not represent a Message
Processing Model known to the Dispatcher, then an errorIndication
(implementation-dependent) is returned to the calling application.
No further processing is performed.
3) The Dispatcher generates a sendPduHandle to coordinate subsequent
processing.
4) The Message Dispatcher sends the request to the version-specific
Message Processing module identified by messageProcessingModel
using the abstract service primitive:
SNMPv3 Working Group Standards Track [Page 8]
RFC 2572 Message Processing and Dispatching April 1999
statusInformation = - success or error indication
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 in step 3.
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- the message length
)
5) If the statusInformation indicates an error, the errorIndication
is returned to the calling application. No further processing is
performed.
6) If the statusInformation indicates success, the sendPduHandle is
returned to the application, and the outgoingMessage is sent via
the transport specified by the transportDomain to the address
specified by the transportAddress.
Outgoing Message Processing is complete.
4.1.2. Sending a Response to the Network
The following procedure is followed when an application wants to
return a response back to the originator of an SNMP Request.
1) An application can request this using the abstract service
primitive:
SNMPv3 Working Group Standards Track [Page 9]
RFC 2572 Message Processing and Dispatching April 1999
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 Response PDU
IN stateReference -- reference to state information
-- as presented with the request
IN statusInformation -- success or errorIndication
) -- (error counter OID and value
-- when errorIndication)
2) The Message Dispatcher sends the request to the appropriate
Message Processing Model indicated by the received value of
messageProcessingModel using the abstract service primitive:
result = -- SUCCESS or errorIndication
prepareResponseMessage(
IN messageProcessingModel -- specified by application
IN securityModel -- specified by application
IN securityName -- specified by application
IN securityLevel -- specified by application
IN contextEngineID -- specified by application
IN contextName -- specified by application
IN pduVersion -- specified by application
IN PDU -- specified by application
IN maxSizeResponseScopedPDU -- specified by application
IN stateReference -- specified by application
IN statusInformation -- specified by application
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- the message length
)
3) If the result is an errorIndication, the errorIndication is
returned to the calling application. No further processing is
performed.
4) If the result is success, the outgoingMessage is sent over the
transport specified by the transportDomain to the address
specified by the transportAddress.
SNMPv3 Working Group Standards Track [Page 10]
RFC 2572 Message Processing and Dispatching April 1999
Message Processing is complete.
4.2. Receiving an SNMP Message from the Network
This section describes the procedure followed by an SNMP engine
whenever it receives an SNMP message.
Please note, that for the sake of clarity and to prevent the text
from being even longer and more complicated, some details were
omitted from the steps below. In particular, The elements of
procedure do not always explicitly indicate when state information
needs to be released. The general rule is that if state information
is available when a message is to be "discarded without further
processing", then the state information must also be released at that
same time.
4.2.1. Message Dispatching of received SNMP Messages
1) The snmpInPkts counter [RFC1907] is incremented.
2) The version of the SNMP message is determined in an
implementation-dependent manner. If the packet cannot be
sufficiently parsed to determine the version of the SNMP message,
then the snmpInASNParseErrs [RFC1907] counter is incremented, and
the message is discarded without further processing. If the
version is not supported, then the snmpInBadVersions [RFC1907]
counter is incremented, and the message is discarded without
further processing.
3) The origin transportDomain and origin transportAddress are
determined.
4) The message is passed to the version-specific Message Processing
Model which returns the abstract data elements required by the
Dispatcher. This is performed using the abstract service
primitive:
SNMPv3 Working Group Standards Track [Page 11]
RFC 2572 Message Processing and Dispatching April 1999
result = -- SUCCESS or errorIndication
prepareDataElements(
IN transportDomain -- origin as determined in step 3.
IN transportAddress -- origin as determined in step 3.
IN wholeMsg -- as received from the network
IN wholeMsgLength -- as received from the network
OUT messageProcessingModel -- typically, SNMP version
OUT securityModel -- Security Model specified
OUT securityName -- on behalf of this principal
OUT securityLevel -- Level of Security specified
OUT contextEngineID -- data from/at this 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 a matched request
OUT maxSizeResponseScopedPDU -- maximum size of Response PDU
OUT statusInformation -- success or errorIndication
-- (error counter OID and value
-- when errorIndication)
OUT stateReference -- reference to state information
-- to be used for a possible
) -- Response
5) If the result is a FAILURE errorIndication, the message is
discarded without further processing.
6) At this point, the abstract data elements have been prepared and
processing continues as described in Section 4.2.2, PDU
Dispatching for Incoming Messages.
4.2.2. PDU Dispatching for Incoming Messages
The elements of procedure for the dispatching of PDUs depends on the
value of sendPduHandle. If the value of sendPduHandle is <none>,
then this is a request or notification and the procedures specified
in Section 4.2.2.1 apply. If the value of snmpPduHandle is not
<none>, then this is a response and the procedures specified in
Section 4.2.2.2 apply.
4.2.2.1. Incoming Requests and Notifications
The following procedures are followed for the dispatching of PDUs
when the value of sendPduHandle is <none>, indicating this is a
request or notification.
SNMPv3 Working Group Standards Track [Page 12]
RFC 2572 Message Processing and Dispatching April 1999
1) The combination of contextEngineID and pduType is used to
determine which application has registered for this request or
notification.
2) If no application has registered for the combination, then
a) The snmpUnknownPDUHandlers counter is incremented.
b) A Response message is generated using the abstract service
primitive:
result = -- SUCCESS or FAILURE
prepareResponseMessage(
IN messageProcessingModel -- as provided by MP module
IN securityModel -- as provided by MP module
IN securityName -- as provided by MP module
IN securityLevel -- as provided by MP module
IN contextEngineID -- as provided by MP module
IN contextName -- as provided by MP module
IN pduVersion -- as provided by MP module
IN PDU -- as provided by MP module
IN maxSizeResponseScopedPDU -- as provided by MP module
IN stateReference -- as provided by MP module
IN statusInformation -- errorIndication plus
-- snmpUnknownPDUHandlers OID
-- value pair.
OUT destTransportDomain -- destination transportDomain
OUT destTransportAddress -- destination transportAddress
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- its length
)
c) If the result is SUCCESS, then the prepared message is sent to
the originator of the request as identified by the
transportDomain and transportAddress.
d) The incoming message is discarded without further processing.
Message Processing for this message is complete.
3) The PDU is dispatched to the application, using the abstract
service primitive:
SNMPv3 Working Group Standards Track [Page 13]
RFC 2572 Message Processing and Dispatching April 1999
processPdu( -- process Request/Notification
IN messageProcessingModel -- as provided by MP module
IN securityModel -- as provided by MP module
IN securityName -- as provided by MP module
IN securityLevel -- as provided by MP module
IN contextEngineID -- as provided by MP module
IN contextName -- as provided by MP module
IN pduVersion -- as provided by MP module
IN PDU -- as provided by MP module
IN maxSizeResponseScopedPDU -- as provided by MP module
IN stateReference -- as provided by MP module
-- needed when sending response
)
Message processing for this message is complete.
4.2.2.2. Incoming Responses
The following procedures are followed for the dispatching of PDUs
when the value of sendPduHandle is not <none>, indicating this is a
response.
1) The value of sendPduHandle is used to determine, in an
implementation-defined manner, which application is waiting for
a response associated with this sendPduHandle.
2) If no waiting application is found, the message is discarded
without further processing, and the stateReference is released.
The snmpUnknownPDUHandlers counter is incremented. Message
Processing is complete for this message.
3) Any cached information, including stateReference, about the
message is discarded.
4) The response is dispatched to the application using the
abstract service primitive:
SNMPv3 Working Group Standards Track [Page 14]
RFC 2572 Message Processing and Dispatching April 1999
processResponsePdu( -- process Response PDU
IN messageProcessingModel -- provided by the MP module
IN securityModel -- provided by the MP module
IN securityName -- provided by the MP module
IN securityLevel -- provided by the MP module
IN contextEngineID -- provided by the MP module
IN contextName -- provided by the MP module
IN pduVersion -- provided by the MP module
IN PDU -- provided by the MP module
IN statusInformation -- provided by the MP module
IN sendPduHandle -- provided by the MP module
)
Message Processing is complete for this message.
4.3. Application Registration for Handling PDU types
Applications that want to process certain PDUs must register with the
PDU Dispatcher. Applications specify the combination of
contextEngineID and pduType(s) for which they want to take
responsibility
1) An application registers according to the abstract interface
primitive:
statusInformation = -- success or errorIndication
registerContextEngineID(
IN contextEngineID -- take responsibility for this one
IN pduType -- the pduType(s) to be registered
)
Note: implementations may provide a means of requesting
registration for simultaneous multiple contextEngineID values,
e.g., all contextEngineID values, and may also provide means for
requesting simultaneous registration for multiple values of
pduType.
2) The parameters may be checked for validity; if they are not, then
an errorIndication (invalidParameter) is returned to the
application.
3) Each combination of contextEngineID and pduType can be registered
only once. If another application has already registered for the
specified combination, then an errorIndication (alreadyRegistered)
is returned to the application.
4) Otherwise, the registration is saved so that SNMP PDUs can be
dispatched to this application.
SNMPv3 Working Group Standards Track [Page 15]
RFC 2572 Message Processing and Dispatching April 1999
4.4. Application Unregistration for Handling PDU Types
Applications that no longer want to process certain PDUs must
unregister with the PDU Dispatcher.
1) An application unregisters using the abstract service primitive:
unregisterContextEngineID(
IN contextEngineID -- give up responsibility for this
IN pduType -- the pduType(s) to be unregistered
)
Note: implementations may provide means for requesting
unregistration for simultaneous multiple contextEngineID values,
e.g., all contextEngineID values, and may also provide means for
requesting simultaneous unregistration for multiple values of
pduType.
2) If the contextEngineID and pduType combination has been
registered, then the registration is deleted.
If no such registration exists, then the request is ignored.
5. Definitions
5.1. Definitions for SNMP Message Processing and Dispatching
SNMP-MPD-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF
MODULE-IDENTITY, OBJECT-TYPE,
snmpModules, Counter32 FROM SNMPv2-SMI;
snmpMPDMIB MODULE-IDENTITY
LAST-UPDATED "9905041636Z" -- 4 April 1999
ORGANIZATION "SNMPv3 Working Group"
CONTACT-INFO "WG-EMail: snmpv3@lists.tislabs.com
Subscribe: majordomo@lists.tislabs.com
In message body: subscribe snmpv3
Chair: Russ Mundy
TIS Labs at Network Associates
postal: 3060 Washington Road
Glenwood, MD 21738
USA
EMail: mundy@tislabs.com
phone: +1 301-854-6889
SNMPv3 Working Group Standards Track [Page 16]
RFC 2572 Message Processing and Dispatching April 1999
Co-editor: Jeffrey Case
SNMP Research, Inc.
postal: 3001 Kimberlin Heights Road
Knoxville, TN 37920-9716
USA
EMail: case@snmp.com
phone: +1 423-573-1434
Co-editor Dave Harrington
Cabletron Systems, Inc.
postal: Post Office Box 5005
MailStop: Durham
35 Industrial Way
Rochester, NH 03867-5005
USA
EMail: dbh@ctron.com
phone: +1 603-337-7357
Co-editor: Randy Presuhn
BMC Software, Inc.
postal: 965 Stewart Drive
Sunnyvale, CA 94086
USA
EMail: randy_presuhn@bmc.com
phone: +1 408-616-3100
Co-editor: Bert Wijnen
IBM T. J. Watson Research
postal: Schagen 33
3461 GL Linschoten
Netherlands
EMail: wijnen@vnet.ibm.com
phone: +31 348-432-794
"
DESCRIPTION "The MIB for Message Processing and Dispatching"
REVISION "9905041636Z" -- 4 April 1999
DESCRIPTION "Updated addresses, published as RFC 2572."
REVISION "9709300000Z" -- 30 September 1997
DESCRIPTION "Original version, published as RFC 2272."
::= { snmpModules 11 }
-- Administrative assignments ***************************************
snmpMPDAdmin OBJECT IDENTIFIER ::= { snmpMPDMIB 1 }
snmpMPDMIBObjects OBJECT IDENTIFIER ::= { snmpMPDMIB 2 }
snmpMPDMIBConformance OBJECT IDENTIFIER ::= { snmpMPDMIB 3 }
SNMPv3 Working Group Standards Track [Page 17]
RFC 2572 Message Processing and Dispatching April 1999
-- Statistics for SNMP Messages *************************************
snmpMPDStats OBJECT IDENTIFIER ::= { snmpMPDMIBObjects 1 }
snmpUnknownSecurityModels OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they referenced a
securityModel that was not known to or supported by
the SNMP engine.
"
::= { snmpMPDStats 1 }
snmpInvalidMsgs OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because there were invalid
or inconsistent components in the SNMP message.
"
::= { snmpMPDStats 2 }
snmpUnknownPDUHandlers OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because the PDU contained
in the packet could not be passed to an application
responsible for handling the pduType, e.g. no SNMP
application had registered for the proper
combination of the contextEngineID and the pduType.
"
::= { snmpMPDStats 3 }
-- Conformance information ******************************************
snmpMPDMIBCompliances OBJECT IDENTIFIER ::= {snmpMPDMIBConformance 1}
snmpMPDMIBGroups OBJECT IDENTIFIER ::= {snmpMPDMIBConformance 2}
-- Compliance statements
snmpMPDCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION "The compliance statement for SNMP entities which
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RFC 2572 Message Processing and Dispatching April 1999
implement the SNMP-MPD-MIB.
"
MODULE -- this module
MANDATORY-GROUPS { snmpMPDGroup }
::= { snmpMPDMIBCompliances 1 }
snmpMPDGroup OBJECT-GROUP
OBJECTS {
snmpUnknownSecurityModels,
snmpInvalidMsgs,
snmpUnknownPDUHandlers
}
STATUS current
DESCRIPTION "A collection of objects providing for remote
monitoring of the SNMP Message Processing and
Dispatching process.
"
::= { snmpMPDMIBGroups 1 }
END
SNMPv3 Working Group Standards Track [Page 19]
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6. The SNMPv3 Message Format
This section defines the SNMPv3 message format and the corresponding
SNMP version 3 Message Processing Model (v3MP).
SNMPv3MessageSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
SNMPv3Message ::= SEQUENCE {
-- identify the layout of the SNMPv3Message
-- this element is in same position as in SNMPv1
-- and SNMPv2c, allowing recognition
-- the value 3 is used for snmpv3
msgVersion INTEGER ( 0 .. 2147483647 ),
-- administrative parameters
msgGlobalData HeaderData,
-- security model-specific parameters
-- format defined by Security Model
msgSecurityParameters OCTET STRING,
msgData ScopedPduData
}
HeaderData ::= SEQUENCE {
msgID INTEGER (0..2147483647),
msgMaxSize INTEGER (484..2147483647),
msgFlags OCTET STRING (SIZE(1)),
-- .... ...1 authFlag
-- .... ..1. privFlag
-- .... .1.. reportableFlag
-- Please observe:
-- .... ..00 is OK, means noAuthNoPriv
-- .... ..01 is OK, means authNoPriv
-- .... ..10 reserved, must NOT be used.
-- .... ..11 is OK, means authPriv
msgSecurityModel INTEGER (1..2147483647)
}
ScopedPduData ::= CHOICE {
plaintext ScopedPDU,
encryptedPDU OCTET STRING -- encrypted scopedPDU value
}
ScopedPDU ::= SEQUENCE {
contextEngineID OCTET STRING,
contextName OCTET STRING,
data ANY -- e.g., PDUs as defined in RFC 1905
}
END
SNMPv3 Working Group Standards Track [Page 20]
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6.1. msgVersion
The msgVersion field is set to snmpv3(3) and identifies the message
as an SNMP version 3 Message.
6.2. msgID
The msgID is used between two SNMP entities to coordinate request
messages and responses, and by the v3MP to coordinate the processing
of the message by different subsystem models within the architecture.
Values for msgID SHOULD be generated in a manner that avoids re-use
of any outstanding values. Doing so provides protection against some
replay attacks. One possible implementation strategy would be to use
the low-order bits of snmpEngineBoots [RFC2571] as the high-order
portion of the msgID value and a monotonically increasing integer for
the low-order portion of msgID.
Note that the request-id in a PDU may be used by SNMP applications to
identify the PDU; the msgID is used by the engine to identify the
message which carries a PDU. The engine needs to identify the message
even if decryption of the PDU (and request-id) fails. No assumption
should be made that the value of the msgID and the value of the
request-id are equivalent.
The value of the msgID field for a response takes the value of the
msgID field from the message to which it is a response. By use of
the msgID value, an engine can distinguish the (potentially multiple)
outstanding requests, and thereby correlate incoming responses with
outstanding requests. In cases where an unreliable datagram service
is used, the msgID also provides a simple means of identifying
messages duplicated by the network. If a request is retransmitted, a
new msgID value SHOULD be used for each retransmission.
6.3. msgMaxSize
The msgMaxSize field of the message conveys the maximum message size
supported by the sender of the message, i.e., the maximum message
size that the sender can accept when another SNMP engine sends an
SNMP message (be it a response or any other message) to the sender of
this message on the transport in use for this message.
When an SNMP message is being generated, the msgMaxSize is provided
by the SNMP engine which generates the message. At the receiving
SNMP engine, the msgMaxSize is used to determine the maximum message
size the sender can accommodate.
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6.4. msgFlags
The msgFlags field of the message contains several bit fields which
control processing of the message.
The reportableFlag is a secondary aid in determining whether a Report
PDU must be sent. It is only used in cases where the PDU portion of
a message cannot be decoded, due to, for example, an incorrect
encryption key. If the PDU can be decoded, the PDU type forms the
basis for decisions on sending Report PDUs.
When the reportableFlag is used, if its value is one, a Report PDU
MUST be returned to the sender under those conditions which can cause
the generation of Report PDUs. Similarly, when the reportableFlag is
used and its value is zero, then a Report PDU MUST NOT be sent. The
reportableFlag MUST always be zero when the message contains a PDU
from the Unconfirmed Class, such as a Report PDU, a response-type PDU
(such as a Response PDU), or an unacknowledged notification-type PDU
(such as an SNMPv2-trap PDU). The reportableFlag MUST always be one
for a PDU from the Confirmed Class, include request-type PDUs (such
as a Get PDU) and an acknowledged notification-type PDUs (such as an
Inform PDU).
If the reportableFlag is set to one for a message containing a PDU
from the Unconfirmed Class, such as a Report PDU, a response-type PDU
(such as a Response PDU), or an unacknowledged notification-type PDU
(such as an SNMPv2-trap PDU), then the receiver of that message MUST
process it as though the reportableFlag had been set to zero.
If the reportableFlag is set to zero for a message containing a
request-type PDU (such as a Get PDU) or an acknowledged
notification-type PDU (such as an Inform PDU), then the receiver of
that message must process it as though the reportableFlag had been
set to one.
Report PDUs are generated directly by the SNMPv3 Message Processing
Model, and support engine-to-engine communications, but may be passed
to applications for processing.
An SNMP engine that receives a reportPDU may use it to determine what
kind of problem was detected by the remote SNMP engine. It can do so
based on the error counter included as the first (and only) varBind
of the reportPDU. Based on the detected error, the SNMP engine may
try to send a corrected SNMP message. If that is not possible, it
may pass an indication of the error to the application on whose
behalf the failed SNMP request was issued.
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The authFlag and privFlag portions of the msgFlags field are set by
the sender to indicate the securityLevel that was applied to the
message before it was sent on the wire. The receiver of the message
MUST apply the same securityLevel when the message is received and
the contents are being processed.
There are three securityLevels, namely noAuthNoPriv, which is less
than authNoPriv, which is in turn less than authPriv. See the SNMP
architecture document [RFC2571] for details about the securityLevel.
a) authFlag
If the authFlag is set to one, then the securityModel used by the
SNMP engine which sent the message MUST identify the securityName
on whose behalf the SNMP message was generated and MUST provide,
in a securityModel-specific manner, sufficient data for the
receiver of the message to be able to authenticate that
identification. In general, this authentication will allow the
receiver to determine with reasonable certainty that the message
was:
- sent on behalf of the principal associated with the
securityName,
- was not redirected,
- was not modified in transit, and
- was not replayed.
If the authFlag is zero, then the securityModel used by the SNMP
engine which sent the message must identify the securityName on
whose behalf the SNMP message was generated but it does not need
to provide sufficient data for the receiver of the message to
authenticate the identification, as there is no need to
authenticate the message in this case.
b) privFlag
If the privFlag is set, then the securityModel used by the SNMP
engine which sent the message MUST also protect the scopedPDU in
an SNMP message from disclosure, i.e., it MUST encrypt/decrypt the
scopedPDU. If the privFlag is zero, then the securityModel in use
does not need to protect the data from disclosure.
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RFC 2572 Message Processing and Dispatching April 1999
It is an explicit requirement of the SNMP architecture that if
privacy is selected, then authentication is also required. That
means that if the privFlag is set, then the authFlag MUST also be
set to one.
The combination of the authFlag and the privFlag comprises a Level
of Security as follows:
authFlag zero, privFlag zero -> securityLevel is noAuthNoPriv
authFlag zero, privFlag one -> invalid combination, see below
authFlag one, privFlag zero -> securityLevel is authNoPriv
authFlag one, privFlag one -> securityLevel is authPriv
The elements of procedure (see below) describe the action to be taken
when the invalid combination of authFlag equal to zero and privFlag
equal to one is encountered.
The remaining bits in msgFlags are reserved, and MUST be set to zero
when sending a message and SHOULD be ignored when receiving a
message.
6.5. msgSecurityModel
The v3MP supports the concurrent existence of multiple Security
Models to provide security services for SNMPv3 messages. The
msgSecurityModel field in an SNMPv3 Message identifies which Security
Model was used by the sender to generate the message and therefore
which securityModel must be used by the receiver to perform security
processing for the message. The mapping to the appropriate
securityModel implementation within an SNMP engine is accomplished in
an implementation-dependent manner.
6.6. msgSecurityParameters
The msgSecurityParameters field of the SNMPv3 Message is used for
communication between the Security Model modules in the sending and
receiving SNMP engines. The data in the msgSecurityParameters field
is used exclusively by the Security Model, and the contents and
format of the data is defined by the Security Model. This OCTET
STRING is not interpreted by the v3MP, but is passed to the local
implementation of the Security Model indicated by the
msgSecurityModel field in the message.
6.7. scopedPduData
The scopedPduData field represents either the plain text scopedPDU if
the privFlag in the msgFlags is zero, or it represents an
encryptedPDU (encoded as an OCTET STRING) which must be decrypted by
the securityModel in use to produce a plaintext scopedPDU.
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RFC 2572 Message Processing and Dispatching April 1999
6.8. scopedPDU
The scopedPDU contains information to identify an administratively
unique context and a PDU. The object identifiers in the PDU refer to
managed objects which are (expected to be) accessible within the
specified context.
6.8.1. contextEngineID
The contextEngineID in the SNMPv3 message, uniquely identifies,
within an administrative domain, an SNMP entity that may realize an
instance of a context with a particular contextName.
For incoming messages, the contextEngineID is used in conjunction
with pduType to determine to which application the scopedPDU will be
sent for processing.
For outgoing messages, the v3MP sets the contextEngineID to the value
provided by the application in the request for a message to be sent.
6.8.2. contextName
The contextName field in an SNMPv3 message, in conjunction with the
contextEngineID field, identifies the particular context associated
with the management information contained in the PDU portion of the
message. The contextName is unique within the SNMP entity specified
by the contextEngineID, which may realize the managed objects
referenced within the PDU. An application which originates a message
provides the value for the contextName field and this value may be
used during processing by an application at the receiving SNMP
Engine.
6.8.3. data
The data field of the SNMPv3 Message contains the PDU. Among other
things, the PDU contains the PDU type that is used by the v3MP to
determine the type of the incoming SNMP message. The v3MP specifies
that the PDU must be one of those specified in [RFC1905].
7. Elements of Procedure for v3MP
This section describes the procedures followed by an SNMP engine when
generating and processing SNMP messages according to the SNMPv3
Message Processing Model.
Please note, that for the sake of clarity and to prevent the text
from being even longer and more complicated, some details were
omitted from the steps below.
SNMPv3 Working Group Standards Track [Page 25]
RFC 2572 Message Processing and Dispatching April 1999
a) Some steps specify that when some error conditions are
encountered when processing a received message, a message
containing a Report PDU is generated and the received message
is discarded without further processing. However, a Report-PDU
must not be generated unless the PDU causing generation of the
Report PDU can be determine to be a member of the Confirmed
Class, or the reportableFlag is set to one and the PDU class
cannot be determined.
b) The elements of procedure do not always explicitly indicate
when state information needs to be released. The general rule
is that if state information is available when a message is to
be "discarded without further processing", then the state
information should also be released at that same time.
7.1. Prepare an Outgoing SNMP Message
This section describes the procedure followed to prepare an SNMPv3
message from the data elements passed by the Message Dispatcher.
1) The Message Dispatcher may request that an SNMPv3 message
containing a Read Class, Write Class, or Notification Class PDU be
prepared for sending.
a) It makes such a request according to the abstract service
primitive:
statusInformation = -- success or errorIndication
prepareOutgoingMessage(
IN transportDomain -- requested transport domain
IN transportAddress -- requested destination 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 entity
IN contextName -- data from/in this context
IN pduVersion -- version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN expectResponse -- TRUE or FALSE *
IN sendPduHandle -- the handle for matching
-- incoming responses
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- the length of the message
)
SNMPv3 Working Group Standards Track [Page 26]
RFC 2572 Message Processing and Dispatching April 1999
* The SNMPv3 Message Processing Model does not use the values of
expectResponse or pduVersion.
b) A unique msgID is generated. The number used for msgID should
not have been used recently, and must not be the same as was
used for any outstanding request.
2) The Message Dispatcher may request that an SNMPv3 message
containing a Response Class or Internal Class PDU be prepared for
sending.
a) It makes such a request according to the abstract service
primitive:
result = -- SUCCESS or FAILURE
prepareResponseMessage(
IN messageProcessingModel -- typically, SNMP version
IN securityModel -- same as on incoming request
IN securityName -- same as on incoming request
IN securityLevel -- same as on incoming request
IN contextEngineID -- data from/at this SNMP entity
IN contextName -- data from/in this context
IN pduVersion -- version of the PDU
IN PDU -- SNMP Protocol Data Unit
IN maxSizeResponseScopedPDU -- maximum size sender can accept
IN stateReference -- reference to state
-- information presented with
-- the request
IN statusInformation -- success or errorIndication
-- error counter OID and value
-- when errorIndication
OUT destTransportDomain -- destination transport domain
OUT destTransportAddress -- destination transport address
OUT outgoingMessage -- the message to send
OUT outgoingMessageLength -- the length of the message
)
b) The cached information for the original request is retrieved
via the stateReference, including
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RFC 2572 Message Processing and Dispatching April 1999
- msgID,
- contextEngineID,
- contextName,
- securityModel,
- securityName,
- securityLevel,
- securityStateReference,
- reportableFlag,
- transportDomain, and
- transportAddress.
The SNMPv3 Message Processing Model does not allow cached data
to be overridden, except by error indications as detailed in
(3) below.
3) If statusInformation contains values for an OID/value combination
(potentially also containing a securityLevel value,
contextEngineID value, or contextName value), then
a) If reportableFlag is zero, then the original message is
discarded, and no further processing is done. A result of
FAILURE is returned. SNMPv3 Message Processing is complete.
b) If a PDU is provided, it is the PDU from the original request.
If possible, extract the request-id.
c) A Report PDU is prepared:
1) the varBindList is set to contain the OID and value from the
statusInformation
2) error-status is set to 0
3) error-index is set to 0.
4) request-id is set to the value extracted in step b)
Otherwise, request-id is set to 0
d) The errorIndication in statusInformation may be accompanied by
a securityLevel value, a contextEngineID value, or a
contextName value.
1) If statusInformation contains a value for securityLevel,
then securityLevel is set to that value, otherwise it is set
to noAuthNoPriv.
SNMPv3 Working Group Standards Track [Page 28]
RFC 2572 Message Processing and Dispatching April 1999
2) If statusInformation contains a value for contextEngineID,
then contextEngineID is set to that value, otherwise it is
set to the value of this entity's snmpEngineID.
3) If statusInformation contains a value for contextName, then
contextName is set to that value, otherwise it is set to the
default context of "" (zero-length string).
e) PDU is set to refer to the new Report-PDU. The old PDU is
discarded.
f) Processing continues with step 6) below.
4) If contextEngineID is not yet determined, then the contextEngineID
is determined, in an implementation-dependent manner, possibly
using the transportDomain and transportAddress.
5) If the contextName is not yet determined, the contextName is set
to the default context.
6) A scopedPDU is prepared from the contextEngineID, contextName, and
PDU.
7) msgGlobalData is constructed as follows
a) The msgVersion field is set to snmpv3(3).
b) msgID is set as determined in step 1 or 2 above.
c) msgMaxSize is set to an implementation-dependent value.
d) msgFlags are set as follows:
- If securityLevel specifies noAuthNoPriv, then authFlag and
privFlag are both set to zero.
- If securityLevel specifies authNoPriv, then authFlag is set
to one and privFlag is set to zero.
- If securityLevel specifies authPriv, then authFlag is set to
one and privFlag is set to one.
- If the PDU is from the Unconfirmed Class, then the
reportableFlag is set to zero.
- If the PDU is from the Confirmed Class then the
reportableFlag is set to one.
SNMPv3 Working Group Standards Track [Page 29]
RFC 2572 Message Processing and Dispatching April 1999
- All other msgFlags bits are set to zero.
e) msgSecurityModel is set to the value of securityModel
8) If the PDU is from the Response Class or the Internal Class, then
a) The specified Security Model is called to generate the message
according to the primitive:
statusInformation =
generateResponseMsg(
IN messageProcessingModel -- SNMPv3 Message Processing
-- Model
IN globalData -- msgGlobalData from step 7
IN maxMessageSize -- from msgMaxSize (step 7c)
IN securityModel -- as determined in step 7e
IN securityEngineID -- the value of snmpEngineID
IN securityName -- on behalf of this principal
IN securityLevel -- for the outgoing message
IN scopedPDU -- as prepared in step 6)
IN securityStateReference -- as determined in step 2
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
)
If, upon return from the Security Model, the statusInformation
includes an errorIndication, then any cached information about
the outstanding request message is discarded, and an
errorIndication is returned, so it can be returned to the
calling application. SNMPv3 Message Processing is complete.
b) A SUCCESS result is returned. SNMPv3 Message Processing is
complete.
9) If the PDU is from the Confirmed Class or the Notification Class,
then
a) If the PDU is from the Unconfirmed Class, then securityEngineID
is set to the value of this entity's snmpEngineID.
Otherwise, the snmpEngineID of the target entity is determined,
in an implementation-dependent manner, possibly using
transportDomain and transportAddress. The value of
securityEngineID is set to the value of the target entity's
snmpEngineID.
SNMPv3 Working Group Standards Track [Page 30]
RFC 2572 Message Processing and Dispatching April 1999
b) The specified Security Model is called to generate the message
according to the primitive:
statusInformation =
generateRequestMsg(
IN messageProcessingModel -- SNMPv3 Message Processing Model
IN globalData -- msgGlobalData, from step 7
IN maxMessageSize -- from msgMaxSize in step 7 c)
IN securityModel -- as provided by caller
IN securityEngineID -- authoritative SNMP entity
-- from step 9 a)
IN securityName -- as provided by caller
IN securityLevel -- as provided by caller
IN scopedPDU -- as prepared in step 6
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of the generated message
)
If, upon return from the Security Model, the statusInformation
includes an errorIndication, then the message is discarded, and
the errorIndication is returned, so it can be returned to the
calling application, and no further processing is done.
SNMPv3 Message Processing is complete.
c) If the PDU is from the Confirmed Class, information about the
outgoing message is cached, and a (implementation-specific)
stateReference is created. Information to be cached includes
the values of:
- sendPduHandle
- msgID
- snmpEngineID
- securityModel
- securityName
- securityLevel
- contextEngineID
- contextName
d) A SUCCESS result is returned. SNMPv3 Message Processing is
complete.
7.2. Prepare Data Elements from an Incoming SNMP Message
This section describes the procedure followed to extract data from an
SNMPv3 message, and to prepare the data elements required for further
processing of the message by the Message Dispatcher.
SNMPv3 Working Group Standards Track [Page 31]
RFC 2572 Message Processing and Dispatching April 1999
1) The message is passed in from the Message Dispatcher according to
the abstract service primitive:
result = -- SUCCESS or errorIndication
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 to use
OUT securityName -- on behalf of this principal
OUT securityLevel -- Level of Security requested
OUT contextEngineID -- data from/at this entity
OUT contextName -- data from/in this context
OUT pduVersion -- 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 sender can accept
OUT statusInformation -- success or errorIndication
-- error counter OID and value
-- when errorIndication
OUT stateReference -- reference to state information
-- to be used for a possible
) -- Response
2) If the received message is not the serialization (according to
the conventions of [RFC1906]) of an SNMPv3Message value, then the
snmpInASNParseErrs counter [RFC1907] is incremented, the message
is discarded without further processing, and a FAILURE result is
returned. SNMPv3 Message Processing is complete.
3) The values for msgVersion, msgID, msgMaxSize, msgFlags,
msgSecurityModel, msgSecurityParameters, and msgData are
extracted from the message.
4) If the value of the msgSecurityModel component does not match a
supported securityModel, then the snmpUnknownSecurityModels
counter is incremented, the message is discarded without further
processing, and a FAILURE result is returned. SNMPv3 Message
Processing is complete.
5) The securityLevel is determined from the authFlag and the
privFlag bits of the msgFlags component as follows:
a) If the authFlag is not set and the privFlag is not set, then
securityLevel is set to noAuthNoPriv.
SNMPv3 Working Group Standards Track [Page 32]
RFC 2572 Message Processing and Dispatching April 1999
b) If the authFlag is set and the privFlag is not set, then
securityLevel is set to authNoPriv.
c) If the authFlag is set and the privFlag is set, then
securityLevel is set to authPriv.
d) If the authFlag is not set and privFlag is set, then the
snmpInvalidMsgs counter is incremented, the message is
discarded without further processing, and a FAILURE result is
returned. SNMPv3 Message Processing is complete.
e) Any other bits in the msgFlags are ignored.
6) The security module implementing the Security Model as specified
by the securityModel component is called for authentication and
privacy services. This is done according to the abstract service
primitive:
statusInformation = -- errorIndication or success
-- error counter OID and
-- value if error
processIncomingMsg(
IN messageProcessingModel -- SNMPv3 Message Processing Model
IN maxMessageSize -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size sender can accept
OUT securityStateReference -- reference to security state
) -- information, needed for
-- response
If an errorIndication is returned by the security module, then
a) If statusInformation contains values for an OID/value pair,
then generation of a Report PDU is attempted (see step 3 in
section 7.1).
1) If the scopedPDU has been returned from processIncomingMsg
then determine contextEngineID, contextName, and PDU.
2) Information about the message is cached and a
stateReference is created (implementation-specific).
SNMPv3 Working Group Standards Track [Page 33]
RFC 2572 Message Processing and Dispatching April 1999
Information to be cached includes the values of:
msgVersion,
msgID,
securityLevel,
msgFlags,
msgMaxSize,
securityModel,
maxSizeResponseScopedPDU,
securityStateReference
3) Request that a Report-PDU be prepared and sent, according
to the abstract service primitive:
result = -- SUCCESS or FAILURE
returnResponsePdu(
IN messageProcessingModel -- SNMPv3(3)
IN securityModel -- same as on incoming request
IN securityName -- from processIncomingMsg
IN securityLevel -- same as on incoming request
IN contextEngineID -- from step 6 a) 1)
IN contextName -- from step 6 a) 1)
IN pduVersion -- SNMPv2-PDU
IN PDU -- from step 6 a) 1)
IN maxSizeResponseScopedPDU -- from processIncomingMsg
IN stateReference -- from step 6 a) 2)
IN statusInformation -- from processIncomingMsg
)
b) The incoming message is discarded without further processing,
and a FAILURE result is returned. SNMPv3 Message Processing is
complete.
7) The scopedPDU is parsed to extract the contextEngineID, the
contextName and the PDU. If any parse error occurs, then the
snmpInASNParseErrs counter [RFC1907] is incremented, the security
state information is discarded, the message is discarded without
further processing, and a FAILURE result is returned. SNMPv3
Message Processing is complete. Treating an unknown PDU type is
treated as a parse error is an implementation option.
8) The pduVersion is determined in an implementation-dependent
manner. For SNMPv3, the pduVersion would be an SNMPv2-PDU.
9) The pduType is determined, in an implementation-dependent manner.
For RFC 1905, the pduTypes include:
SNMPv3 Working Group Standards Track [Page 34]
RFC 2572 Message Processing and Dispatching April 1999
- GetRequest-PDU,
- GetNextRequest-PDU,
- GetBulkRequest-PDU,
- SetRequest-PDU,
- InformRequest-PDU,
- SNMPv2-Trap-PDU,
- Response-PDU,
- Report-PDU.
10) If the pduType is from the Response Class or the Internal Class,
then
a) The value of the msgID component is used to find the cached
information for a corresponding outstanding Request message.
If no such outstanding Request message is found, then the
security state information is discarded, the message is
discarded without further processing, and a FAILURE result is
returned. SNMPv3 Message Processing is complete.
b) sendPduHandle is retrieved from the cached information.
Otherwise, sendPduHandle is set to <none>, an implementation
defined value.
11) If the pduType is from the Internal Class, then
a) statusInformation is created using the contents of the
Report-PDU, in an implementation-dependent manner. This
statusInformation will be forwarded to the application
associated with the sendPduHandle.
b) The cached data for the outstanding message, referred to by
stateReference, is retrieved. If the securityModel or
securityLevel values differ from the cached ones, it is
important to recognize that Internal Class PDUs delivered at
the security level of noAuthNoPriv open a window of
opportunity for spoofing or replay attacks. If the receiver
of such messages is aware of these risks, the use of such
unauthenticated messages is acceptable and may provide a
useful function for discovering engine IDs or for detecting
misconfiguration at remote nodes.
When the securityModel or securityLevel values differ from
the cached ones, an implementation may retain the cached
information about the outstanding Request message, in
anticipation of the possibility that the Internal Class PDU
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RFC 2572 Message Processing and Dispatching April 1999
received might be illegitimate. Otherwise, any cached
information about the outstanding Request message message is
discarded.
c) The security state information for this incoming message is
discarded.
d) stateReference is set to <none>
e) A SUCCESS result is returned. SNMPv3 Message Processing is
complete.
12) If the pduType is from the Response Class, then
a) The cached data for the outstanding request, referred to by
stateReference, is retrieved, including
- snmpEngineID
- securityModel
- securityName
- securityLevel
- contextEngineID
- contextName
b) If the values extracted from the incoming message differ from
the cached data, then any cached information about the
outstanding Request message is discarded, the incoming
message is discarded without further processing, and a
FAILURE result is returned. SNMPv3 Message Processing is
complete.
When the securityModel or securityLevel values differ from
the cached ones, an implementation may retain the cached
information about the outstanding Request message, in
anticipation of the possibility that the Response Class PDU
received might be illegitimate.
c) Otherwise, any cached information about the outstanding
Request message is discarded, and stateReference is set to
<none>.
d) A SUCCESS result is returned. SNMPv3 Message Processing is
complete.
13) If the pduType is from the Confirmed Class, then
a) If the value of securityEngineID is not equal to the value of
snmpEngineID, then the security state information is
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discarded, any cached information about this message is
discarded, the incoming message is discarded without further
processing, and a FAILURE result is returned. SNMPv3 Message
Processing is complete.
b) Information about the message is cached and a stateReference
is created (implementation-specific). Information to be
cached includes the values of:
msgVersion,
msgID,
securityLevel,
msgFlags,
msgMaxSize,
securityModel,
maxSizeResponseScopedPDU,
securityStateReference
c) A SUCCESS result is returned. SNMPv3 Message Processing is
complete.
14) If the pduType is from the Unconfirmed Class, then A SUCCESS
result is returned. SNMPv3 Message Processing is complete.
8. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
SNMPv3 Working Group Standards Track [Page 37]
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9. Acknowledgements
This document is the result of the efforts of the SNMPv3 Working
Group. Some special thanks are in order to the following SNMPv3 WG
members:
Harald Tveit Alvestrand (Maxware)
Dave Battle (SNMP Research, Inc.)
Alan Beard (Disney Worldwide Services)
Paul Berrevoets (SWI Systemware/Halcyon Inc.)
Martin Bjorklund (Ericsson)
Uri Blumenthal (IBM T. J. Watson Research Center)
Jeff Case (SNMP Research, Inc.)
John Curran (BBN)
Mike Daniele (Compaq Computer Corporation)
T. Max Devlin (Eltrax Systems)
John Flick (Hewlett Packard)
Rob Frye (MCI)
Wes Hardaker (U.C.Davis, Information Technology - D.C.A.S.)
David Harrington (Cabletron Systems Inc.)
Lauren Heintz (BMC Software, Inc.)
N.C. Hien (IBM T. J. Watson Research Center)
Michael Kirkham (InterWorking Labs, Inc.)
Dave Levi (SNMP Research, Inc.)
Louis A Mamakos (UUNET Technologies Inc.)
Joe Marzot (Nortel Networks)
Paul Meyer (Secure Computing Corporation)
Keith McCloghrie (Cisco Systems)
Bob Moore (IBM)
Russ Mundy (TIS Labs at Network Associates)
Bob Natale (ACE*COMM Corporation)
Mike O'Dell (UUNET Technologies Inc.)
Dave Perkins (DeskTalk)
Peter Polkinghorne (Brunel University)
Randy Presuhn (BMC Software, Inc.)
David Reeder (TIS Labs at Network Associates)
David Reid (SNMP Research, Inc.)
Aleksey Romanov (Quality Quorum)
Shawn Routhier (Epilogue)
Juergen Schoenwaelder (TU Braunschweig)
Bob Stewart (Cisco Systems)
Mike Thatcher (Independent Consultant)
Bert Wijnen (IBM T. J. Watson Research Center)
The document is based on recommendations of the IETF Security and
Administrative Framework Evolution for SNMP Advisory Team. Members
of that Advisory Team were:
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David Harrington (Cabletron Systems Inc.)
Jeff Johnson (Cisco Systems)
David Levi (SNMP Research Inc.)
John Linn (Openvision)
Russ Mundy (Trusted Information Systems) chair
Shawn Routhier (Epilogue)
Glenn Waters (Nortel)
Bert Wijnen (IBM T. J. Watson Research Center)
As recommended by the Advisory Team and the SNMPv3 Working Group
Charter, the design incorporates as much as practical from previous
RFCs and drafts. As a result, special thanks are due to the authors
of previous designs known as SNMPv2u and SNMPv2*:
Jeff Case (SNMP Research, Inc.)
David Harrington (Cabletron Systems Inc.)
David Levi (SNMP Research, Inc.)
Keith McCloghrie (Cisco Systems)
Brian O'Keefe (Hewlett Packard)
Marshall T. Rose (Dover Beach Consulting)
Jon Saperia (BGS Systems Inc.)
Steve Waldbusser (International Network Services)
Glenn W. Waters (Bell-Northern Research Ltd.)
10. Security Considerations
The Dispatcher coordinates the processing of messages to provide a
level of security for management messages and to direct the SNMP PDUs
to the proper SNMP application(s).
A Message Processing Model, and in particular the V3MP defined in
this document, interacts as part of the Message Processing with
Security Models in the Security Subsystem via the abstract service
interface primitives defined in [RFC2571] and elaborated above.
The level of security actually provided is primarily determined by
the specific Security Model implementation(s) and the specific SNMP
application implementation(s) incorporated into this framework.
Applications have access to data which is not secured. Applications
should take reasonable steps to protect the data from disclosure, and
when they send data across the network, they should obey the
securityLevel and call upon the services of an Access Control Model
as they apply access control.
The values for the msgID element used in communication between SNMP
entities must be chosen to avoid replay attacks. The values do not
need to be unpredictable; it is sufficient that they not repeat.
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When exchanges are carried out over an insecure network, there is an
open opportunity for a third party to spoof or replay messages when
any message of an exchange is given at the security level of
noAuthNoPriv. For most exchanges, all messages exist at the same
security level. In the case where the final message is an Internal
Class PDU, this message may be delivered at a level of noAuthNoPriv
or authNoPriv, independent of the security level of the preceding
messages. Internal Class PDUs delivered at the level of authNoPriv
are not considered to pose a security hazard. Internal Class PDUs
delivered at the security level of noAuthNoPriv open a window of
opportunity for spoofing or replay attacks. If the receiver of such
messages is aware of these risks, the use of such unauthenticated
messages is acceptable and may provide a useful function for
discovering engine IDs or for detecting misconfiguration at remote
nodes.
This document also contains a MIB definition module. None of the
objects defined is writable, and the information they represent is
not deemed to be particularly sensitive. However, if they are deemed
sensitive in a particular environment, access to them should be
restricted through the use of appropriately configured Security and
Access Control models.
11. References
[RFC1901] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Introduction to
Community-based SNMPv2", RFC 1901, January 1996.
[RFC2578] McCloghrie, K., Perkins, D. and J. Schoenwaelder,
"Structure of Management Information Version 2 (SMIv2)",
STD 58, RFC 2578, April 1999.
[RFC1905] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Protocol Operations for
Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1905, January 1996.
[RFC1906] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Transport Mappings for
Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1906, January 1996.
[RFC1907] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Management Information Base
for Version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1907 January 1996.
SNMPv3 Working Group Standards Track [Page 40]
RFC 2572 Message Processing and Dispatching April 1999
[RFC1908] The SNMPv2 Working Group, Case, J., McCloghrie, K.,
Rose, M. and S. Waldbusser, "Coexistence between Version
1 and Version 2 of the Internet-standard Network
Management Framework", RFC 1908, January 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2028] Hovey, R. and S. Bradner, "The Organizations Involved in
the IETF Standards Process", BCP 11, RFC 2028, October
1996.
[RFC2571] Harrington, D., Presuhn, R. and B. Wijnen, "An
Architecture for describing SNMP Management Frameworks",
RFC 2571, April 1999.
[RFC2574] Blumenthal, U. and B. Wijnen, "The User-Based Security
Model for Version 3 of the Simple Network Management
Protocol (SNMPv3)", RFC 2574, April 1999.
[RFC2575] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based
Access Control Model for the Simple Network Management
Protocol (SNMP)", RFC 2575, April 1999.
[RFC2573] Levi, D., Meyer, P. and B. Stewart, "SNMP
Applications", RFC 2573, April 1999.
[RFC2570] Case, J., Mundy, R., Partain, D. and B. Stewart,
"Introduction to Version 3 of the Internet-standard
Network Management Framework", RFC 2570, April 1999.
12. Editors' Addresses
Jeffrey Case
SNMP Research, Inc.
3001 Kimberlin Heights Road
Knoxville, TN 37920-9716
USA
Phone: +1 423-573-1434
EMail: case@snmp.com
SNMPv3 Working Group Standards Track [Page 41]
RFC 2572 Message Processing and Dispatching April 1999
Dave Harrington
Cabletron Systems, Inc
Post Office Box 5005
Mail Stop: Durham
35 Industrial Way
Rochester, NH 03867-5005
USA
Phone: +1 603-337-7357
EMail: dbh@ctron.com
Randy Presuhn
BMC Software, Inc.
965 Stewart Drive
Sunnyvale, CA 94086
USA
Phone: +1 408-616-3100
EMail: randy_presuhn@bmc.com
Bert Wijnen
IBM T. J. Watson Research
Schagen 33
3461 GL Linschoten
Netherlands
Phone: +31 348-432-794
EMail: wijnen@vnet.ibm.com
13. Changes From RFC 2272
The following change log records major changes from the previous
version of this document, RFC 2272.
- Updated contact information for editors.
- Made parameter identification in prepareResponseMessage()
consistent, both internally and with architecture.
- Made references to processIncomingMsg() consistent, both
internally and with architecture.
- Deleted superfluous expectResponse parameter from
processIncomingMsg(), consistent with architecture.
SNMPv3 Working Group Standards Track [Page 42]
RFC 2572 Message Processing and Dispatching April 1999
- Fixed typos.
- Removed sending of a report PDU from step 4 on page 30 on RFC
2272.
- Use "PDU Class" terminology instead of directly using RFC 1905
PDU types, in order to potentially allow use of new PDU types
in the future.
- Added intro document to references.
- Made various clarifications to the text.
- The handling of the reportableFlag has been made consistent.
- The acknowledgement list has been updated.
SNMPv3 Working Group Standards Track [Page 43]
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14. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
SNMPv3 Working Group Standards Track [Page 44]
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