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Network Working Group                                   J. Schoenwaelder
Request for Comments: 5343                      Jacobs University Bremen
Updates: 3411                                             September 2008
Category: Standards Track

  Simple Network Management Protocol (SNMP) Context EngineID Discovery

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.


   The Simple Network Management Protocol (SNMP) version three (SNMPv3)
   requires that an application know the identifier (snmpEngineID) of
   the remote SNMP protocol engine in order to retrieve or manipulate
   objects maintained on the remote SNMP entity.

   This document introduces a well-known localEngineID and a discovery
   mechanism that can be used to learn the snmpEngineID of a remote SNMP
   protocol engine.  The proposed mechanism is independent of the
   features provided by SNMP security models and may also be used by
   other protocol interfaces providing access to managed objects.

   This document updates RFC 3411.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   3.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Local EngineID  . . . . . . . . . . . . . . . . . . . . . . 4
     3.2.  EngineID Discovery  . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

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RFC 5343            SNMP Context EngineID Discovery       September 2008

1.  Introduction

   To retrieve or manipulate management information using the third
   version of the Simple Network Management Protocol (SNMPv3) [RFC3410],
   it is necessary to know the identifier of the remote SNMP protocol
   engine, the so-called snmpEngineID [RFC3411].  While an appropriate
   snmpEngineID can in principle be configured on each management
   application for each SNMP agent, it is often desirable to discover
   the snmpEngineID automatically.

   This document introduces a discovery mechanism that can be used to
   learn the snmpEngineID of a remote SNMP protocol engine.  The
   proposed mechanism is independent of the features provided by SNMP
   security models.  The mechanism has been designed to coexist with
   discovery mechanisms that may exist in SNMP security models, such as
   the authoritative engine identifier discovery of the User-based
   Security Model (USM) of SNMP [RFC3414].

   This document updates RFC 3411 [RFC3411] by clarifying the IANA rules
   for the maintenance of the SnmpEngineID format registry.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Background

   Within an administrative domain, an SNMP engine is uniquely
   identified by an snmpEngineID value [RFC3411].  An SNMP entity, which
   consists of an SNMP engine and several SNMP applications, may provide
   access to multiple contexts.

   An SNMP context is a collection of management information accessible
   by an SNMP entity.  An item of management information may exist in
   more than one context and an SNMP entity potentially has access to
   many contexts [RFC3411].  A context is identified by the snmpEngineID
   value of the entity hosting the management information (also called a
   contextEngineID) and a context name that identifies the specific
   context (also called a contextName).

   To identify an individual item of management information within an
   administrative domain, a four tuple is used consisting of

   1.  a contextEngineID,

   2.  a contextName,

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RFC 5343            SNMP Context EngineID Discovery       September 2008

   3.  an object type, and

   4.  its instance identification.

   The last two elements are encoded in an object identifier (OID)
   value.  The contextName is a character string (following the
   SnmpAdminString textual convention of the SNMP-FRAMEWORK-MIB
   [RFC3411]) while the contextEngineID is an octet string constructed
   according to the rules defined as part of the SnmpEngineID textual
   convention of the SNMP-FRAMEWORK-MIB [RFC3411].

   The SNMP protocol operations and the protocol data units (PDUs)
   operate on OIDs and thus deal with object types and instances
   [RFC3416].  The SNMP architecture [RFC3411] introduces the concept of
   a scopedPDU as a data structure containing a contextEngineID, a
   contextName, and a PDU.  The SNMP version 3 (SNMPv3) message format
   uses ScopedPDUs to exchange management information [RFC3412].

   Within the SNMP framework, contextEngineIDs serve as end-to-end
   identifiers.  This becomes important in situations where SNMP proxies
   are deployed to translate between protocol versions or to cross
   middleboxes such as network address translators.  In addition,
   snmpEngineIDs separate the identification of an SNMP engine from the
   transport addresses used to communicate with an SNMP engine.  This
   property can be used to correlate management information easily, even
   in situations where multiple different transports were used to
   retrieve the information or where transport addresses can change

   To retrieve data from an SNMPv3 agent, it is necessary to know the
   appropriate contextEngineID.  The User-based Security Model (USM) of
   SNMPv3 provides a mechanism to discover the snmpEngineID of the
   remote SNMP engine, since this is needed for security processing
   reasons.  The discovered snmpEngineID can subsequently be used as a
   contextEngineID in a ScopedPDU to access management information local
   to the remote SNMP engine.  Other security models, such as the
   Transport Security Model (TSM) [TSM], lack such a procedure and may
   use the discovery mechanism defined in this memo.

3.  Procedure

   The proposed discovery mechanism consists of two parts, namely (i)
   the definition of a special well-known snmpEngineID value, called the
   localEngineID, which always refers to a local default context, and
   (ii) the definition of a procedure to acquire the snmpEngineID scalar
   of the SNMP-FRAMEWORK-MIB [RFC3411] using the special well-known
   local localEngineID value.

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3.1.  Local EngineID

   An SNMP command responder implementing this specification MUST
   register their pduTypes using the localEngineID snmpEngineID value
   (defined below) by invoking the registerContextEngineID() Abstract
   Service Interface (ASI) defined in RFC 3412 [RFC3412].  This
   registration is done in addition to the normal registration under the
   SNMP engine's snmpEngineID.  This is consistent with the SNMPv3
   specifications since they explicitly allow registration of multiple
   engineIDs and multiple pduTypes [RFC3412].

   The SnmpEngineID textual convention [RFC3411] defines that an
   snmpEngineID value MUST be between 5 and 32 octets long.  This
   specification proposes to use the variable length format 3) of the
   SnmpEngineID textual convention and to allocate the reserved, unused
   format value 6, using the enterprise ID 0 for the localEngineID.  An
   ASN.1 definition for localEngineID would look like this:

               localEngineID OCTET STRING ::= '8000000006'H

   The localEngineID value always provides access to the default context
   of an SNMP engine.  Note that the localEngineID value is intended to
   be used as a special value for the contextEngineID field in the
   ScopedPDU.  It MUST NOT be used as a value to identify an SNMP
   engine; that is, this value MUST NOT be used in the snmpEngineID.0
   scalar [RFC3418] or in the msgAuthoritativeEngineID field in the
   securityParameters of the User-based Security Model (USM) [RFC3414].

3.2.  EngineID Discovery

   Discovery of the snmpEngineID is done by sending a Read Class
   protocol operation (see Section 2.8 of [RFC3411]) to retrieve the
   snmpEngineID scalar using the localEngineID defined above as a
   contextEngineID value.  Implementations SHOULD only perform this
   discovery step when it is needed.  In particular, if security models
   are used that already discover the remote snmpEngineID (such as USM),
   then no further discovery is necessary.  The same is true in
   situations where the application already knows a suitable
   snmpEngineID value.

   The procedure to discover the snmpEngineID of a remote SNMP engine
   can be described as follows:

   1.  Check whether a suitable contextEngineID value is already known.
       If yes, use the provided contextEngineID value and stop the
       discovery procedure.

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   2.  Check whether the selected security model supports discovery of
       the remote snmpEngineID (e.g., USM with its discovery mechanism).
       If yes, let the security model perform the discovery.  If the
       remote snmpEngineID value has been successfully determined,
       assign it to the contextEngineID and stop the discovery

   3.  Send a Read Class operation to the remote SNMP engine using the
       localEngineID value as the contextEngineID in order to retrieve
       the scalar snmpEngineID.0 of the SNMP-FRAMEWORK-MIB [RFC3411].
       If successful, set the contextEngineID to the retrieved value and
       stop the discovery procedure.

   4.  Return an error indication that a suitable contextEngineID could
       not be discovered.

   The procedure outlined above is an example and can be modified to
   retrieve more variables in step 3, such as the sysObjectID.0 scalar
   or the snmpSetSerialNo.0 scalar of the SNMPv2-MIB [RFC3418].

4.  IANA Considerations

   RFC 3411 requested that IANA create a registry for SnmpEngineID
   formats.  However, RFC 3411 did not ask IANA to record the initial
   assignments made by RFC 3411 nor did RFC 3411 spell out the precise
   allocation rules.  To address this issue, the following rules are
   hereby established.

   IANA maintains a registry for SnmpEngineID formats.  The first four
   octets of an SnmpEngineID carry an enterprise number, while the fifth
   octet in a variable length SnmpEngineID value, called the format
   octet, indicates how the following octets are formed.  The following
   format values were allocated in [RFC3411]:

     Format    Description                     References
     -------   -----------                     ----------
          0    reserved, unused                 [RFC3411]
          1    IPv4 address                     [RFC3411]
          2    IPv6 address                     [RFC3411]
          3    MAC address                      [RFC3411]
          4    administratively assigned text   [RFC3411]
          5    administratively assigned octets [RFC3411]
       6-127   reserved, unused                 [RFC3411]
     128-255   enterprise specific              [RFC3411]

   IANA can assign new format values out of the originally assigned and
   reserved number space 1-127.  For new assignments in this number

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   space, a specification is required as per [RFC5226].  The number
   space 128-255 is enterprise specific and is not controlled by IANA.

   Per this document, IANA has made the following assignment:

     Format    Description                     References
     -------   -----------                     ----------
          6    local engine                     [RFC5343]

5.  Security Considerations

   SNMP version 3 (SNMPv3) provides cryptographic security to protect
   devices from unauthorized access.  This specification recommends use
   of the security services provided by SNMPv3.  In particular, it is
   RECOMMENDED to protect the discovery exchange.

   An snmpEngineID can contain information such as a device's MAC
   address, IPv4 address, IPv6 address, or administratively assigned
   text.  An attacker located behind a router / firewall / network
   address translator may not be able to obtain this information
   directly, and he therefore might discover snmpEngineID values in
   order to obtain this kind of device information.

   In many environments, making snmpEngineID values accessible via a
   security level of noAuthNoPriv will benefit legitimate tools that try
   to algorithmically determine some basic information about a device.
   For this reason, the default View-based Access Control Model (VACM)
   configuration in Appendix A of RFC 3415 [RFC3415] gives noAuthNoPriv
   read access to the snmpEngineID.  Furthermore, the USM discovery
   mechanism defined in RFC 3414 [RFC3414] uses unprotected messages and
   reveals snmpEngineID values.

   In highly secure environments, snmpEngineID values can be protected
   by using the discovery mechanism described in this document together
   with a security model that does not exchange cleartext SNMP messages,
   such as the Transport Security Model (TSM) [TSM].

   The isAccessAllowed() abstract service primitive of the SNMP access
   control subsystem does not take the contextEngineID into account when
   checking access rights [RFC3411].  As a consequence, it is not
   possible to define a special view for context engineID discovery.  A
   request with a localEngineID is thus treated like a request with the
   correct snmpEngineID by the access control subsystem.  This is inline
   with the SNMPv3 design where the authenticated identity is the
   securityName (together with the securityModel and securityLevel
   information), and transport addresses or knowledge of contextEngineID
   values do not impact the access-control decision.

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6.  Acknowledgments

   Dave Perkins suggested the introduction of a "local" contextEngineID
   during the interim meeting of the ISMS (Integrated Security Model for
   SNMP) working group in Boston, 2006.  Joe Fernandez, David
   Harrington, Dan Romascanu, and Bert Wijnen provided helpful review
   and feedback, which helped to improve this document.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [RFC3412]  Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
              "Message Processing and Dispatching for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3412,
              December 2002.

   [RFC3414]  Blumenthal, U. and B. Wijnen, "User-based Security Model
              (USM) for version 3 of the Simple Network Management
              Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.

   [RFC3416]  Presuhn, R., "Version 2 of the Protocol Operations for the
              Simple Network Management Protocol (SNMP)", STD 62,
              RFC 3416, December 2002.

   [RFC3418]  Presuhn, R., "Management Information Base (MIB) for the
              Simple Network Management Protocol (SNMP)", STD 62,
              RFC 3418, December 2002.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

7.2.  Informative References

   [RFC3410]  Case, J., Mundy, R., Partain, D., and B. Stewart,
              "Introduction and Applicability Statements for Internet-
              Standard Management Framework", RFC 3410, December 2002.

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RFC 5343            SNMP Context EngineID Discovery       September 2008

   [RFC3415]  Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
              Access Control Model (VACM) for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3415,
              December 2002.

   [TSM]      Harrington, D., "Transport Security Model for SNMP", Work
              in Progress, July 2008.

Author's Address

   Juergen Schoenwaelder
   Jacobs University Bremen
   Campus Ring 1
   28725 Bremen

   Phone: +49 421 200-3587
   EMail: j.schoenwaelder@jacobs-university.de

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RFC 5343            SNMP Context EngineID Discovery       September 2008

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