Network Working Group D. Harrington Request for Comments: 5590 Huawei Technologies (USA) Updates: 3411, 3412, 3414, 3417 J. Schoenwaelder Category: Standards Track Jacobs University Bremen June 2009 Transport Subsystem 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) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
AbstractThis document defines a Transport Subsystem, extending the Simple Network Management Protocol (SNMP) architecture defined in RFC 3411. This document defines a subsystem to contain Transport Models that is comparable to other subsystems in the RFC 3411 architecture. As work is being done to expand the transports to include secure transports, such as the Secure Shell (SSH) Protocol and Transport Layer Security
(TLS), using a subsystem will enable consistent design and modularity of such Transport Models. This document identifies and describes some key aspects that need to be considered for any Transport Model for SNMP. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. The Internet-Standard Management Framework . . . . . . . . 3 1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 3 1.3. Where This Extension Fits . . . . . . . . . . . . . . . . 4 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Requirements of a Transport Model . . . . . . . . . . . . . . 7 3.1. Message Security Requirements . . . . . . . . . . . . . . 7 3.1.1. Security Protocol Requirements . . . . . . . . . . . . 7 3.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 8 3.2.1. Architectural Modularity Requirements . . . . . . . . 8 3.2.2. Access Control Requirements . . . . . . . . . . . . . 11 3.2.3. Security Parameter Passing Requirements . . . . . . . 12 3.2.4. Separation of Authentication and Authorization . . . . 12 3.3. Session Requirements . . . . . . . . . . . . . . . . . . . 13 3.3.1. No SNMP Sessions . . . . . . . . . . . . . . . . . . . 13 3.3.2. Session Establishment Requirements . . . . . . . . . . 14 3.3.3. Session Maintenance Requirements . . . . . . . . . . . 15 3.3.4. Message Security versus Session Security . . . . . . . 15 4. Scenario Diagrams and the Transport Subsystem . . . . . . . . 16 5. Cached Information and References . . . . . . . . . . . . . . 17 5.1. securityStateReference . . . . . . . . . . . . . . . . . . 17 5.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 17 5.2.1. Transport Information . . . . . . . . . . . . . . . . 18 5.2.2. securityName . . . . . . . . . . . . . . . . . . . . . 19 5.2.3. securityLevel . . . . . . . . . . . . . . . . . . . . 20 5.2.4. Session Information . . . . . . . . . . . . . . . . . 20 6. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 21 6.1. sendMessage ASI . . . . . . . . . . . . . . . . . . . . . 21 6.2. Changes to RFC 3411 Outgoing ASIs . . . . . . . . . . . . 22 6.2.1. Message Processing Subsystem Primitives . . . . . . . 22 6.2.2. Security Subsystem Primitives . . . . . . . . . . . . 23 6.3. The receiveMessage ASI . . . . . . . . . . . . . . . . . . 24 6.4. Changes to RFC 3411 Incoming ASIs . . . . . . . . . . . . 25 6.4.1. Message Processing Subsystem Primitive . . . . . . . . 25 6.4.2. Security Subsystem Primitive . . . . . . . . . . . . . 26 7. Security Considerations . . . . . . . . . . . . . . . . . . . 27 7.1. Coexistence, Security Parameters, and Access Control . . . 27 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 10.1. Normative References . . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . . 30 Appendix A. Why tmStateReference? . . . . . . . . . . . . . . . . 32 A.1. Define an Abstract Service Interface . . . . . . . . . . . 32 A.2. Using an Encapsulating Header . . . . . . . . . . . . . . 32 A.3. Modifying Existing Fields in an SNMP Message . . . . . . . 32 A.4. Using a Cache . . . . . . . . . . . . . . . . . . . . . . 33 RFC3411]. This document identifies and describes some key aspects that need to be considered for any Transport Model for SNMP. Section 7 of RFC 3410 [RFC3410]. RFC2119]. Lowercase versions of the keywords should be read as in normal English. They will usually, but not always, be used in a context that relates to compatibility with the RFC 3411 architecture or the subsystem defined here but that might have no impact on on-the-wire compatibility. These terms are used as guidance for designers of proposed IETF models to make the designs compatible with RFC 3411 subsystems and Abstract Service Interfaces (ASIs). Implementers are free to implement differently. Some usages of these lowercase terms are simply normal English usage. For consistency with SNMP-related specifications, this document favors terminology as defined in STD 62, rather than favoring terminology that is consistent with non-SNMP specifications that use different variations of the same terminology. This is consistent with the IESG decision to not require the SNMPv3 terminology be modified to match the usage of other non-SNMP specifications when SNMPv3 was advanced to Full Standard. This document discusses an extension to the modular RFC 3411 architecture; this is not a protocol document. An architectural "MUST" is a really sharp constraint; to allow for the evolution of technology and to not unnecessarily constrain future models, often a
"SHOULD" or a "should" is more appropriate than a "MUST" in an architecture. Future models MAY express tighter requirements for their own model-specific processing. 3410 and 3411, and have a general understanding of the functionality defined in RFCs 3412-3418. The "Transport Subsystem" is an additional component for the SNMP Engine depicted in RFC 3411, Section 3.1. The following diagram depicts its place in the RFC 3411 architecture. +-------------------------------------------------------------------+ | SNMP entity | | | | +-------------------------------------------------------------+ | | | SNMP engine (identified by snmpEngineID) | | | | | | | | +------------+ | | | | | Transport | | | | | | Subsystem | | | | | +------------+ | | | | | | | | +------------+ +------------+ +-----------+ +-----------+ | | | | | Dispatcher | | Message | | Security | | Access | | | | | | | | Processing | | Subsystem | | Control | | | | | | | | Subsystem | | | | Subsystem | | | | | +------------+ +------------+ +-----------+ +-----------+ | | | +-------------------------------------------------------------+ | | | | +-------------------------------------------------------------+ | | | Application(s) | | | | | | | | +-------------+ +--------------+ +--------------+ | | | | | Command | | Notification | | Proxy | | | | | | Generator | | Receiver | | Forwarder | | | | | +-------------+ +--------------+ +--------------+ | | | | | | | | +-------------+ +--------------+ +--------------+ | | | | | Command | | Notification | | Other | | | | | | Responder | | Originator | | | | | | | +-------------+ +--------------+ +--------------+ | | | +-------------------------------------------------------------+ | | | +-------------------------------------------------------------------+
The transport mappings defined in RFC 3417 do not provide lower-layer security functionality, and thus do not provide transport-specific security parameters. This document updates RFC 3411 and RFC 3417 by defining an architectural extension and modifying the ASIs that transport mappings (hereafter called "Transport Models") can use to pass transport-specific security parameters to other subsystems, including transport-specific security parameters that are translated into the transport-independent securityName and securityLevel parameters. The Transport Security Model [RFC5591] and the Secure Shell Transport Model [RFC5592] utilize the Transport Subsystem. The Transport Security Model is an alternative to the existing SNMPv1 Security Model [RFC3584], the SNMPv2c Security Model [RFC3584], and the User- based Security Model [RFC3414]. The Secure Shell Transport Model is an alternative to existing transport mappings as described in [RFC3417]. RFC3414] largely uses the first approach -- it provides its own security. It utilizes existing mechanisms (e.g., SHA), but provides all the coordination. USM provides for the authentication of a principal, message encryption, data integrity checking, timeliness checking, etc. USM was designed to be independent of other existing security infrastructures. USM therefore uses a separate principal and key management infrastructure. Operators have reported that deploying another principal and key management infrastructure in order to use
SNMPv3 is a deterrent to deploying SNMPv3. It is possible to use external mechanisms to handle the distribution of keys for use by USM. The more important issue is that operators wanted to leverage existing user management infrastructures that were not specific to SNMP. A USM-compliant architecture might combine the authentication mechanism with an external mechanism, such as RADIUS [RFC2865], to provide the authentication service. Similarly, it might be possible to utilize an external protocol to encrypt a message, to check timeliness, to check data integrity, etc. However, this corresponds to the second approach -- requiring the coordination of a number of differently subcontracted services. Building solid security between the various services is difficult, and there is a significant potential for gaps in security. An alternative approach might be to utilize one or more lower-layer security mechanisms to provide the message-oriented security services required. These would include authentication of the sender, encryption, timeliness checking, and data integrity checking. This corresponds to the third approach described above. There are a number of IETF standards available or in development to address these problems through security layers at the transport layer or application layer, among them are TLS [RFC5246], Simple Authentication and Security Layer (SASL) [RFC4422], and SSH [RFC4251] From an operational perspective, it is highly desirable to use security mechanisms that can unify the administrative security management for SNMPv3, command line interfaces (CLIs), and other management interfaces. The use of security services provided by lower layers is the approach commonly used for the CLI, and is also the approach being proposed for other network management protocols, such as syslog [RFC5424] and NETCONF [RFC4741]. This document defines a Transport Subsystem extension to the RFC 3411 architecture that is based on the third approach. This extension specifies how other lower-layer protocols with common security infrastructures can be used underneath the SNMP protocol and the desired goal of unified administrative security can be met. This extension allows security to be provided by an external protocol connected to the SNMP engine through an SNMP Transport Model [RFC3417]. Such a Transport Model would then enable the use of existing security mechanisms, such as TLS [RFC5246] or SSH [RFC4251], within the RFC 3411 architecture.
There are a number of Internet security protocols and mechanisms that are in widespread use. Many of them try to provide a generic infrastructure to be used by many different application-layer protocols. The motivation behind the Transport Subsystem is to leverage these protocols where it seems useful. There are a number of challenges to be addressed to map the security provided by a secure transport into the SNMP architecture so that SNMP continues to provide interoperability with existing implementations. These challenges are described in detail in this document. For some key issues, design choices are described that might be made to provide a workable solution that meets operational requirements and fits into the SNMP architecture defined in [RFC3411]. Section 1.4 of [RFC3411]. The security requirements outlined there do not require protection against denial of service or traffic analysis; however, transport security protocols should not make those threats significantly worse.
A Transport Model SHOULD NOT require modifications to the underlying protocol. Modifying the protocol might change its security characteristics in ways that could impact other existing usages. If a change is necessary, the change SHOULD be an extension that has no impact on the existing usages. Any Transport Model specification should include a description of potential impact on other usages of the protocol. Since multiple Transport Models can exist simultaneously within the Transport Subsystem, Transport Models MUST be able to coexist with each other. Section 3 of [RFC3411]) to allow the evolution of the SNMP protocol standards over time and to minimize the side effects between subsystems when changes are made. The RFC 3411 architecture includes a Message Processing Subsystem for permitting different message versions to be handled by a single engine, a Security Subsystem for enabling different methods of providing security services, Applications to support different types of Application processors, and an Access Control Subsystem for allowing multiple approaches to access control. The RFC 3411 architecture does not include a subsystem for Transport Models, despite the fact there are multiple transport mappings already defined for SNMP [RFC3417]. This document describes a Transport Subsystem that is compatible with the RFC 3411 architecture. As work is being done to use secure transports such as SSH and TLS, using a subsystem will enable consistent design and modularity of such Transport Models. The design of this Transport Subsystem accepts the goals of the RFC 3411 architecture that are defined in Section 1.5 of [RFC3411]. This Transport Subsystem uses a modular design that permits Transport Models (which might or might not be security-aware) to be "plugged into" the RFC 3411 architecture. Such Transport Models would be independent of other modular SNMP components as much as possible. This design also permits Transport Models to be advanced through the standards process independently of other Transport Models. The following diagram depicts the SNMPv3 architecture, including the new Transport Subsystem defined in this document and a new Transport Security Model defined in [RFC5591].
+------------------------------+ | Network | +------------------------------+ ^ ^ ^ | | | v v v +-------------------------------------------------------------------+ | +--------------------------------------------------+ | | | Transport Subsystem | | | | +-----+ +-----+ +-----+ +-----+ +-------+ | | | | | UDP | | TCP | | SSH | | TLS | . . . | other | | | | | +-----+ +-----+ +-----+ +-----+ +-------+ | | | +--------------------------------------------------+ | | ^ | | | | | Dispatcher v | | +-------------------+ +---------------------+ +----------------+ | | | Transport | | Message Processing | | Security | | | | Dispatch | | Subsystem | | Subsystem | | | | | | +------------+ | | +------------+ | | | | | | +->| v1MP |<--->| | USM | | | | | | | | +------------+ | | +------------+ | | | | | | | +------------+ | | +------------+ | | | | | | +->| v2cMP |<--->| | Transport | | | | | Message | | | +------------+ | | | Security | | | | | Dispatch <--------->| +------------+ | | | Model | | | | | | | +->| v3MP |<--->| +------------+ | | | | | | | +------------+ | | +------------+ | | | | PDU Dispatch | | | +------------+ | | | Other | | | | +-------------------+ | +->| otherMP |<--->| | Model(s) | | | | ^ | +------------+ | | +------------+ | | | | +---------------------+ +----------------+ | | v | | +-------+-------------------------+---------------+ | | ^ ^ ^ | | | | | | | v v v | | +-------------+ +---------+ +--------------+ +-------------+ | | | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | | | | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | | | | Application | | | | Applications | | Application | | | +-------------+ +---------+ +--------------+ +-------------+ | | ^ ^ | | | | | | v v | | +----------------------------------------------+ | | | MIB instrumentation | SNMP entity | +-------------------------------------------------------------------+
RFC 3411 architecture and the Security Subsystem assume that a Security Model is called by a Message Processing Model and will perform multiple security functions within the Security Subsystem. A Transport Model that supports a secure transport protocol might perform similar security functions within the Transport Subsystem, including the translation of transport-security parameters to/from Security-Model-independent parameters. To accommodate this, an implementation-specific cache of transport- specific information will be described (not shown), and the data flows on this path will be extended to pass Security-Model- independent values. This document amends some of the ASIs defined in RFC 3411; these changes are covered in Section 6 of this document. New Security Models might be defined that understand how to work with these modified ASIs and the transport-information cache. One such Security Model, the Transport Security Model, is defined in [RFC5591]. RFC 3411 architecture. While the steps are the same, they might occur in a different order, and might be done by different subsystems. With the existing RFC 3411 architecture, security processing starts when the Message Processing Model decodes portions of the encoded message to extract parameters that identify which Security Model MUST handle the security-related tasks. A secure transport performs those security functions on the message, before the message is decoded. Some of these functions might then be repeated by the selected Security Model. RFC3584] and the User-based Security Model establish a security association for each SNMP message, newer Transport Models MAY support sending multiple SNMP messages through the same tunnel to amortize the costs of establishing a security association.
RFC 3411 made some design decisions related to the support of an Access Control Subsystem. These include establishing and passing in a model-independent manner the securityModel, securityName, and securityLevel parameters, and separating message authentication from data-access authorization.
from "alice" authenticated by TLS. Transport domains and their corresponding prefixes are coordinated via the IANA registry "SNMP Transport Domains". Section 5) into which the Transport Model puts information about the transport and security parameters applied to a transport connection or an incoming message; a Security Model might extract that information from the cache. A tmStateReference is passed as an extra parameter in the ASIs between the Transport Subsystem and the Message Processing and Security Subsystems in order to identify the relevant cache. This approach of passing a model-independent reference is consistent with the securityStateReference cache already being passed around in the RFC 3411 ASIs. RFC 3411 architecture defines a separation of authentication and the authorization to access and/or modify MIB data. A set of model- independent parameters (securityModel, securityName, and securityLevel) are passed between the Security Subsystem, the Applications, and the Access Control Subsystem. This separation was a deliberate decision of the SNMPv3 WG, in order to allow support for authentication protocols that do not provide data-access authorization capabilities, and in order to support data- access authorization schemes, such as the View-based access Control Model (VACM), that do not perform their own authentication. A Message Processing Model determines which Security Model is used, either based on the message version (e.g., SNMPv1 and SNMPv2c) or possibly by a value specified in the message (e.g., msgSecurityModel field in SNMPv3). The Security Model makes the decision which securityName and securityLevel values are passed as model-independent parameters to an Application, which then passes them via the isAccessAllowed ASI to the Access Control Subsystem.
An Access Control Model performs the mapping from the model- independent security parameters to a policy within the Access Control Model that is Access-Control-Model-dependent. A Transport Model does not know which Security Model will be used for an incoming message, and so cannot know how the securityName and securityLevel parameters will be determined. It can propose an authenticated identity (via the tmSecurityName field), but there is no guarantee that this value will be used by the Security Model. For example, non-transport-aware Security Models will typically determine the securityName (and securityLevel) based on the contents of the SNMP message itself. Such Security Models will simply not know that the tmStateReference cache exists. Further, even if the Transport Model can influence the choice of securityName, it cannot directly determine the authorization allowed to this identity. If two different Transport Models each authenticate a transport principal that are then both mapped to the same securityName, then these two identities will typically be afforded exactly the same authorization by the Access Control Model. The only way for the Access Control Model to differentiate between identities based on the underlying Transport Model would be for such transport-authenticated identities to be mapped to distinct securityNames. How and if this is done is Security-Model-dependent. RFC3411] and the Transport Subsystem defined in this document do not support SNMP sessions or include a session selector in the Abstract Service Interfaces.
The Transport Subsystem might support transport sessions. However, the Transport Subsystem does not have access to the pduType (i.e., the SNMP operation type), and so cannot select a given transport session for particular types of traffic. Certain parameters of the Abstract Service Interfaces might be used to guide the selection of an appropriate transport session to use for a given request by an Application. The transportDomain and transportAddress identify the transport connection to a remote network node. Elements of the transport address (such as the port number) might be used by an Application to send a particular PDU type to a particular transport address. For example, the SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are used to configure notification originators with the destination port to which SNMPv2-Trap PDUs or Inform PDUs are to be sent, but the Transport Subsystem never looks inside the PDU. The securityName identifies which security principal to communicate with at that address (e.g., different Network Management System (NMS) applications), and the securityLevel might permit selection of different sets of security properties for different purposes (e.g., encrypted SET vs. non-encrypted GET operations). However, because the handling of transport sessions is specific to each Transport Model, some Transport Models MAY restrict selecting a particular transport session. A user application might use a unique combination of transportDomain, transportAddress, securityModel, securityName, and securityLevel to try to force the selection of a given transport session. This usage is NOT RECOMMENDED because it is not guaranteed to be interoperable across implementations and across models. Implementations SHOULD be able to maintain some reasonable number of concurrent transport sessions, and MAY provide non-standard internal mechanisms to select transport sessions.
Transport session establishment might require provisioning authentication credentials at an engine, either statically or dynamically. How this is done is dependent on the Transport Model and the implementation.
If a secure transport session is closed between the time a request message is received and the corresponding response message is sent, then the response message SHOULD be discarded, even if a new session has been established. The SNMPv3 WG decided that this should be a "SHOULD" architecturally, and it is a Security-Model-specific decision whether to REQUIRE this. The architecture does not mandate this requirement in order to allow for future Security Models where this might make sense; however, not requiring this could lead to added complexity and security vulnerabilities, so most Security Models SHOULD require this. SNMPv3 was designed to support multiple levels of security, selectable on a per-message basis by an SNMP Application, because, for example, there is not much value in using encryption for a command generator to poll for potentially non-sensitive performance data on thousands of interfaces every ten minutes; such encryption might add significant overhead to processing of the messages. Some Transport Models might support only specific authentication and encryption services, such as requiring all messages to be carried using both authentication and encryption, regardless of the security level requested by an SNMP Application. A Transport Model MAY upgrade the security level requested by a transport-aware Security Model, i.e., noAuthNoPriv and authNoPriv might be sent over an authenticated and encrypted session. A Transport Model MUST NOT downgrade the security level requested by a transport-aware Security Model, and SHOULD discard any message where this would occur. This is a SHOULD rather than a MUST only to permit the potential development of models that can perform error-handling in a manner that is less severe than discarding the message. However, any model that does not discard the message in this circumstance should have a clear justification for why not discarding will not create a security vulnerability. Sections 4.6.1 and 4.6.2 of RFC 3411 provide scenario diagrams to illustrate how an outgoing message is created and how an incoming message is processed. RFC 3411 does not define ASIs for the "Send SNMP Request Message to Network", "Receive SNMP Response Message from Network", "Receive SNMP Message from Network" and "Send SNMP message to Network" arrows in these diagrams. This document defines two ASIs corresponding to these arrows: a sendMessage ASI to send SNMP messages to the network and a receiveMessage ASI to receive SNMP messages from the network. These ASIs are used for all SNMP messages, regardless of pduType.
RFC 3411 architecture uses caches to maintain the short-term message state, and uses references in the ASIs to pass this information between subsystems. This document defines the requirements for a cache to handle additional short-term message state and longer-term transport state information, using a tmStateReference parameter to pass this information between subsystems. To simplify the elements of procedure, the release of state information is not always explicitly specified. As a general rule, if state information is available when a message being processed gets discarded, the state related to that message should also be discarded. If state information is available when a relationship between engines is severed, such as the closing of a transport session, the state information for that relationship should also be discarded. Since the contents of a cache are meaningful only within an implementation, and not on-the-wire, the format of the cache is implementation-specific. RFC 3411. Its primary purpose is to provide a mapping between a request and the corresponding response. This cache is not accessible to Transport Models, and an entry is typically only retained for the lifetime of a request-response pair of messages.
might have incomplete information, and the missing information might be populated by the other model when the information becomes available. The tmState might contain both long-term and short-term information. The session information typically remains valid for the duration of the transport session, might be used for several messages, and might be stored in a local configuration datastore. Some information has a shorter lifespan, such as tmSameSecurity and tmRequestedSecurityLevel, which are associated with a specific message. Since this cache is only used within an implementation, and not on- the-wire, the precise contents and format of the cache are implementation-dependent. For architectural modularity between Transport Models and transport-aware Security Models, a fully-defined tmState MUST conceptually include at least the following fields: tmTransportDomain tmTransportAddress tmSecurityName tmRequestedSecurityLevel tmTransportSecurityLevel tmSameSecurity tmSessionID The details of these fields are described in the following subsections. RFC 3411; hence, this information is not directly available to the Security Model. A transport-aware Security Model might wish to take account of the transport protocol and originating address when authenticating the request and setting up the authorization parameters. It is therefore
necessary for the Transport Model to include this information in the tmStateReference cache so that it is accessible to the Security Model. o tmTransportDomain: the transport protocol (and hence the Transport Model) used to receive the incoming message. o tmTransportAddress: the source of the incoming message. The ASIs used for processing an outgoing message all include explicit transportDomain and transportAddress parameters. The values within the securityStateReference cache might override these parameters for outgoing messages.
RFC 3411 to describe the conceptual data flows between the various subsystems within an SNMP entity and to help keep the subsystems independent of each other except for the common parameters. This document introduces a couple of new ASIs to define the interface between the Transport and Dispatcher Subsystems; it also extends some of the ASIs defined in RFC 3411 to include transport-related information. This document follows the example of RFC 3411 regarding the release of state information and regarding error indications. 1) The release of state information is not always explicitly specified in a Transport Model. As a general rule, if state information is available when a message gets discarded, the message- state information should also be released, and if state information is available when a session is closed, the session-state information should also be released. Keeping sensitive security information longer than necessary might introduce potential vulnerabilities to an implementation. 2)An error indication in statusInformation will typically include the Object Identifier (OID) and value for an incremented error counter. This might be accompanied by values for contextEngineID and contextName for this counter, a value for securityLevel, and the appropriate state reference if the information is available at the point where the error is detected. Section 4.6.1 of RFC 3411 and the text "Send SNMP Message to Network" that appears in Section 4.6.2 of RFC 3411. If present and valid, the tmStateReference refers to a cache containing Transport-Model-specific parameters for the transport and transport security. How a tmStateReference is determined to be present and valid is implementation-dependent. How the information in the cache is used is Transport-Model-dependent and implementation- dependent.
This might sound underspecified, but a Transport Model might be something like SNMP over UDP over IPv6, where no security is provided, so it might have no mechanisms for utilizing a tmStateReference cache. statusInformation = sendMessage( IN destTransportDomain -- transport domain to be used IN destTransportAddress -- transport address to be used IN outgoingMessage -- the message to send IN outgoingMessageLength -- its length IN tmStateReference -- reference to transport state ) RFC 3411 that are concerned with communication between the Dispatcher and Message Processing Subsystems, and between the Message Processing and Security Subsystems.
OUT destTransportDomain -- destination transport domain OUT destTransportAddress -- destination transport address OUT outgoingMessage -- the message to send OUT outgoingMessageLength -- its length OUT tmStateReference -- (NEW) reference to transport state ) statusInformation = -- success or errorIndication prepareResponseMessage( 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 maxSizeResponseScopedPDU -- maximum size able to accept IN stateReference -- reference to state information -- as presented with the request IN statusInformation -- success or errorIndication -- error counter OID/value if error OUT destTransportDomain -- destination transport domain OUT destTransportAddress -- destination transport address OUT outgoingMessage -- the message to send OUT outgoingMessageLength -- its length OUT tmStateReference -- (NEW) reference to transport state )
statusInformation = generateRequestMsg( IN transportDomain -- (NEW) destination transport domain IN transportAddress -- (NEW) destination transport address IN messageProcessingModel -- typically, SNMP version IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN securityModel -- for the outgoing message IN securityEngineID -- authoritative SNMP entity IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested IN scopedPDU -- message (plaintext) payload OUT securityParameters -- filled in by Security Module OUT wholeMsg -- complete generated message OUT wholeMsgLength -- length of generated message OUT tmStateReference -- (NEW) reference to transport state ) statusInformation = generateResponseMsg( IN transportDomain -- (NEW) destination transport domain IN transportAddress -- (NEW) destination transport address IN messageProcessingModel -- Message Processing Model IN globalData -- msgGlobalData IN maxMessageSize -- from msgMaxSize IN securityModel -- as determined by MPM IN securityEngineID -- the value of snmpEngineID IN securityName -- on behalf of this principal IN securityLevel -- for the outgoing message IN scopedPDU -- as provided by MPM IN securityStateReference -- as provided by MPM OUT securityParameters -- filled in by Security Module OUT wholeMsg -- complete generated message OUT wholeMsgLength -- length of generated message OUT tmStateReference -- (NEW) reference to transport state ) Section 4.6.1 of RFC 3411 and the text "Receive SNMP Message from Network" from Section 4.6.2 of RFC3411. When a message is received on a given transport session, if a cache does not already exist for that session, the Transport Model might create one, referenced by tmStateReference. The contents of this
cache are discussed in Section 5. How this information is determined is implementation- and Transport-Model-specific. "Might create one" might sound underspecified, but a Transport Model might be something like SNMP over UDP over IPv6, where transport security is not provided, so it might not create a cache. The Transport Model does not know the securityModel for an incoming message; this will be determined by the Message Processing Model in a Message-Processing-Model-dependent manner. statusInformation = receiveMessage( IN transportDomain -- origin transport domain IN transportAddress -- origin transport address IN incomingMessage -- the message received IN incomingMessageLength -- its length IN tmStateReference -- reference to transport state ) RFC 3411. How or if a Message Processing Model or Security Model uses tmStateReference is message-processing- and Security-Model-specific. This might sound underspecified, but a Message Processing Model might have access to all the information from the cache and from the message. The Message Processing Model might determine that the USM Security Model is specified in an SNMPv3 message header; the USM Security Model has no need of values in the tmStateReference cache to authenticate and secure the SNMP message, but an Application might have specified to use a secure transport such as that provided by the SSH Transport Model to send the message to its destination.
IN wholeMsgLength -- as received from the network IN tmStateReference -- (NEW) from the Transport Model 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 -- the version of the PDU OUT PDU -- SNMP Protocol Data Unit OUT pduType -- SNMP PDU type OUT sendPduHandle -- handle for matched request OUT maxSizeResponseScopedPDU -- maximum size sender can accept OUT statusInformation -- success or errorIndication -- error counter OID/value if error OUT stateReference -- reference to state information -- to be used for possible Response )
RFC 3411 architecture, the Message Processing Model makes the decision about which Security Model to use. The architectural change described by this document does not alter that.
The architecture change described by this document does, however, allow SNMP to support two different approaches to security -- message-driven security and transport-driven security. With message- driven security, SNMP provides its own security and passes security parameters within the SNMP message; with transport-driven security, SNMP depends on an external entity to provide security during transport by "wrapping" the SNMP message. Using a non-transport-aware Security Model with a secure Transport Model is NOT RECOMMENDED for the following reasons. Security Models defined before the Transport Security Model (i.e., SNMPv1, SNMPv2c, and USM) do not support transport-based security and only have access to the security parameters contained within the SNMP message. They do not know about the security parameters associated with a secure transport. As a result, the Access Control Subsystem bases its decisions on the security parameters extracted from the SNMP message, not on transport-based security parameters. Implications of combining older Security Models with Secure Transport Models are known. The securityName used for access control decisions is based on the message-driven identity, which might be unauthenticated, and not on the transport-driven, authenticated identity: o An SNMPv1 message will always be paired with an SNMPv1 Security Model (per RFC 3584), regardless of the transport mapping or Transport Model used, and access controls will be based on the unauthenticated community name. o An SNMPv2c message will always be paired with an SNMPv2c Security Model (per RFC 3584), regardless of the transport mapping or Transport Model used, and access controls will be based on the unauthenticated community name. o An SNMPv3 message will always be paired with the securityModel specified in the msgSecurityParameters field of the message (per RFC 3412), regardless of the transport mapping or Transport Model used. If the SNMPv3 message specifies the User-based Security Model (USM) with noAuthNoPriv, then the access controls will be based on the unauthenticated USM user. o For outgoing messages, if a Secure Transport Model is selected in combination with a Security Model that does not populate a tmStateReference, the Secure Transport Model SHOULD detect the lack of a valid tmStateReference and fail.
In times of network stress, a Secure Transport Model might not work properly if its underlying security mechanisms (e.g., Network Time Protocol (NTP) or Authentication, Authorization, and Accounting (AAA) protocols or certificate authorities) are not reachable. The User- based Security Model was explicitly designed to not depend upon external network services, and provides its own security services. It is RECOMMENDED that operators provision authPriv USM as a fallback mechanism to supplement any Security Model or Transport Model that has external dependencies, so that secure SNMP communications can continue when the external network service is not available. RFC2578]. Values are to be assigned via [RFC5226] "Specification Required". The registry has been populated with the following initial entries: Registry Name: SNMP Transport Domains Reference: [RFC2578] [RFC3417] [RFC5590] Registration Procedures: Specification Required Each domain is assigned a MIB-defined OID under snmpDomains Prefix snmpDomains Reference ------- ----------------------------- --------- udp snmpUDPDomain [RFC3417] [RFC5590] clns snmpCLNSDomain [RFC3417] [RFC5590] cons snmpCONSDomain [RFC3417] [RFC5590] ddp snmpDDPDomain [RFC3417] [RFC5590] ipx snmpIPXDomain [RFC3417] [RFC5590] prxy rfc1157Domain [RFC3417] [RFC5590]
WG members who performed detailed reviews: Wes Hardaker, Jeffrey Hutzelman, Tom Petch, Dave Shield, and Bert Wijnen. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., "Structure of Management Information Version 2 (SMIv2)", STD 58, RFC 2578, April 1999. [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. [RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network Management Protocol (SNMP) Applications", STD 62, RFC 3413, 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. [RFC3417] Presuhn, R., "Transport Mappings for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3417, December 2002. [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart, "Introduction and Applicability Statements for Internet- Standard Management Framework", RFC 3410, December 2002.
[RFC3584] Frye, R., Levi, D., Routhier, S., and B. Wijnen, "Coexistence between Version 1, Version 2, and Version 3 of the Internet-standard Network Management Framework", BCP 74, RFC 3584, August 2003. [RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006. [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006. [RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741, December 2006. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009. [RFC5591] Harrington, D. and W. Hardaker, "Transport Security Model for the Simple Network Management Protocol (SNMP)", RFC 5591, June 2009. [RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)", RFC 5592, June 2009.
RFC 3411/3412 practice; it also helps to ensure that any Transport Model proposals pass the necessary data and do not cause side effects by creating model-specific dependencies between itself and models or subsystems other than those that are clearly defined by an ASI. RFC3412] defines the SNMPv3 message, which contains fields to pass security-related parameters. The Transport Subsystem could use these fields in an SNMPv3 message (or comparable fields in other message
formats) to pass information between Transport Models in different SNMP engines and to pass information between a Transport Model and a corresponding Message Processing Model. If the fields in an incoming SNMPv3 message are changed by the Transport Model before passing it to the Security Model, then the Transport Model will need to decode the ASN.1 message, modify the fields, and re-encode the message in ASN.1 before passing the message on to the Message Dispatcher or to the transport layer. This would require an intimate knowledge of the message format and message versions in order for the Transport Model to know which fields could be modified. This would seriously violate the modularity of the architecture. RFC 3411 ASIs.