Network Working Group D. Harrington Internet-Draft Futurewei Technologies Expires: September 5, 2006 J. Schoenwaelder International University Bremen March 4, 2006 Transport Mapping Security Model (TMSM) Architectural Extension for the Simple Network Management Protocol (SNMP) draft-ietf-isms-tmsm-01.txt Status of This Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 5, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document describes a Transport Mapping Security Model (TMSM) subsystem for the Simple Network Management Protocol (SNMP) architecture defined in RFC 3411. This document identifies and discusses some key aspects that need to be considered for any transport-mapping-based security model for SNMP. Harrington & Schoenwaelder Expires September 5, 2006 [Page 1] Internet-Draft SNMP Transport Mapping Security Model March 2006 This memo also defines a portion of the Management Information Base (MIB) for managing the Transport Mapping Security Model Subsystem. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. The Internet-Standard Management Framework . . . . . . . . 4 1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Requirements of a Transport Mapping Security Model . . . . . . 6 2.1. Security Requirements . . . . . . . . . . . . . . . . . . 6 2.1.1. Security Protocol Requirements . . . . . . . . . . . . 6 2.2. Session Requirements . . . . . . . . . . . . . . . . . . . 7 2.2.1. Session Establishment Requirements . . . . . . . . . . 8 2.2.2. Session Maintenance Requirements . . . . . . . . . . . 8 2.2.3. Message security versus session security . . . . . . . 8 2.3. Architectural Modularity Requirements . . . . . . . . . . 9 2.3.1. USM and the RFC3411 Architecture . . . . . . . . . . . 12 2.3.2. TMSM and the RFC3411 Architecture . . . . . . . . . . 13 2.4. Passing Messages between Subsystems . . . . . . . . . . . 15 2.5. Security Parameter Passing Requirement . . . . . . . . . . 16 2.5.1. Define an Abstract Service Interface . . . . . . . . . 17 2.5.2. Using an Encapsulating Header . . . . . . . . . . . . 17 2.5.3. Modifying Existing Fields in an SNMP Message . . . . . 17 2.5.4. Using a Cache . . . . . . . . . . . . . . . . . . . . 18 2.6. Architectural Requirements for Access Control . . . . . . 18 2.6.1. securityName Binding . . . . . . . . . . . . . . . . . 18 2.6.2. Separation of Authentication and Authorization . . . . 19 2.7. Requirements for Notifications . . . . . . . . . . . . . . 20 3. Scenario Diagrams . . . . . . . . . . . . . . . . . . . . . . 21 3.1. Command Generator or Notification Originator . . . . . . . 21 3.2. Command Responder . . . . . . . . . . . . . . . . . . . . 22 4. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 23 5. TMSM Abstract Service Interfaces . . . . . . . . . . . . . . . 24 6. Integration with the SNMPv3 Message Format . . . . . . . . . . 26 6.1. msgVersion . . . . . . . . . . . . . . . . . . . . . . . . 26 6.2. msgGlobalData . . . . . . . . . . . . . . . . . . . . . . 27 6.3. securityLevel and msgFlags . . . . . . . . . . . . . . . . 27 7. The tmStateReference for Passing Security Parameters . . . . . 28 8. securityStateReference Cached Security Data . . . . . . . . . 29 9. Prepare an Outgoing SNMP Message . . . . . . . . . . . . . . . 29 10. Prepare Data Elements from an Incoming SNMP Message . . . . . 30 11. Notifications . . . . . . . . . . . . . . . . . . . . . . . . 31 12. Transport Mapping Security Model Samples . . . . . . . . . . . 31 12.1. TLS/TCP Transport Mapping Security Model . . . . . . . . . 31 12.1.1. tmStateReference for TLS . . . . . . . . . . . . . . . 32 12.1.2. MPSP for TLS TM-Security Model . . . . . . . . . . . . 32 Harrington & Schoenwaelder Expires September 5, 2006 [Page 2] Internet-Draft SNMP Transport Mapping Security Model March 2006 12.1.3. MIB Module for TLS Security . . . . . . . . . . . . . 32 12.2. DTLS/UDP Transport Mapping Security Model . . . . . . . . 32 12.2.1. tmStateReference for DTLS . . . . . . . . . . . . . . 33 12.3. SASL Transport Mapping Security Model . . . . . . . . . . 34 12.3.1. tmStateReference for SASL DIGEST-MD5 . . . . . . . . 34 13. The TMSM MIB Module . . . . . . . . . . . . . . . . . . . . . 35 13.1. Structure of the MIB Module . . . . . . . . . . . . . . . 35 13.1.1. Textual Conventions . . . . . . . . . . . . . . . . . 35 13.1.2. The tmsmStats Subtree . . . . . . . . . . . . . . . . 35 13.1.3. The tmsmsSession Subtree . . . . . . . . . . . . . . . 35 13.1.4. The Notifications Subtree . . . . . . . . . . . . . . 35 13.2. Relationship to Other MIB Modules . . . . . . . . . . . . 36 13.2.1. Relationship to the SNMPv2-MIB . . . . . . . . . . . . 36 13.2.2. MIB Modules Required for IMPORTS . . . . . . . . . . . 36 14. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 36 15. Implementation Considerations . . . . . . . . . . . . . . . . 42 15.1. Applications that Benefit from Sessions . . . . . . . . . 42 15.2. Applications that Suffer from Sessions . . . . . . . . . . 43 15.2.1. Troubleshooting . . . . . . . . . . . . . . . . . . . 43 16. Security Considerations . . . . . . . . . . . . . . . . . . . 43 17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44 18. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45 19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45 19.1. Normative References . . . . . . . . . . . . . . . . . . . 45 19.2. Informative References . . . . . . . . . . . . . . . . . . 47 Appendix A. Questions about msgFlags: . . . . . . . . . . . . . . 47 A.1. msgFlags versus actual security . . . . . . . . . . . . . 48 Appendix B. Parameter Table . . . . . . . . . . . . . . . . . . . 49 B.1. ParameterList.csv . . . . . . . . . . . . . . . . . . . . 49 Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 50 Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 51 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 51 Intellectual Property and Copyright Statements . . . . . . . . . . 51 Harrington & Schoenwaelder Expires September 5, 2006 [Page 3] Internet-Draft SNMP Transport Mapping Security Model March 2006 1. Introduction This document describes a Transport Mapping Security Model (TMSM) subsystem for the Simple Network Management Protocol (SNMP) architecture defined in [RFC3411]. This document identifies and discusses some key aspects that need to be considered for any transport-mapping-based security model for SNMP. 1.1. The Internet-Standard Management Framework For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to section 7 of RFC 3410 [RFC3410]. Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. MIB objects are generally accessed through the Simple Network Management Protocol (SNMP). Objects in the MIB are defined using the mechanisms defined in the Structure of Management Information (SMI). This memo specifies a MIB module that is compliant to the SMIv2, which is described in STD 58, RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580 [RFC2580]. 1.2. Conventions 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 [RFC2119]. Some points requiring further WG research and discussion are identified by [discuss] markers in the text. Some points requiring further editing by the editors are marked [todo] in the text. 1.3. Motivation There are multiple ways to secure one's home or business, but they largely boil down to a continuum of alternatives. Let's consider three general approaches. In the first approach, an individual could buy a gun, learn to use it, and sit on your front porch waiting for intruders. In the second approach, one could hire an employee with a gun, schedule the employee, position the employee to guard what you want protected, hire a second guard to cover if the first gets sick, and so on. In the third approach, you could hire a security company, tell them what you want protected, and they could hire employees, train them, buy the guns, position the guards, schedule the guards, send a replacement when a guard cannot make it, etc., thus providing the security you want, with no significant effort on your part other than identifying requirements and verifying the quality of the Harrington & Schoenwaelder Expires September 5, 2006 [Page 4] Internet-Draft SNMP Transport Mapping Security Model March 2006 service being provided. The User-based Security Model (USM) as defined in [RFC3414] largely uses the first approach - it provides its own security. It utilizes existing mechanisms (MD5=the gun), 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 requires a separate user and key management infrastructure. Operators have reported that deploying another user and key management infrastructure in order to use SNMPv3 is a deterrent to deploying SNMPv3. It is possible but difficult to define external mechanisms that handle the distribution of keys for use by the USM approach. A solution based on the second approach might use a USM-compliant architecture, but combine the authentication mechanism with an external mechanism, such as RADIUS [RFC2865], to provide the authentication service. It might be possible to utilize an external protocol to encrypt a message, to check timeliness, to check data integrity, etc. It is difficult to cobble together a number of subcontracted services and coordinate them however, because it is difficult to build solid security bindings between the various services, and potential for gaps in the security is significant. A solution based on the third approach might 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. 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 TLS [RFC2246], SASL [RFC2222], 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 NETCONF [I-D.ietf-netconf-ssh]. This document proposes a Transport Mapping Security Model (TMSM) subsystem, as an extension of the RFC3411 architecture, that allows security to be provided by an external protocol connected to the SNMP engine through an SNMP transport-mapping. Such a TMSM would then enable the use of existing security mechanisms such as (TLS) [RFC2246] or SSH [RFC4251] within the RFC3411 architecture. Harrington & Schoenwaelder Expires September 5, 2006 [Page 5] Internet-Draft SNMP Transport Mapping Security Model March 2006 There are a number of Internet security protocols and mechanisms that are in wide spread use. Many of them try to provide a generic infrastructure to be used by many different application layer protocols. The motivation behind TMSM 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 work without any surprises. These challenges are discussed in detail in this document. For some key issues, design choices are discussed that may be made to provide a workable solution that meets operational requirements and fits into the SNMP architecture defined in [RFC3411] . 2. Requirements of a Transport Mapping Security Model 2.1. Security Requirements Transport mapping security protocols SHOULD ideally provide the protection against the following message-oriented threats [RFC3411]: 1. modification of information 2. masquerade 3. message stream modification 4. disclosure According to [RFC3411], it is not required to protect against denial of service or traffic analysis. 2.1.1. Security Protocol Requirements There are a number of standard protocols that could be proposed as possible solutions within the TMSM framework. Some factors should be considered when selecting a protocol for use within this framework. Using a protocol in a manner for which it was not designed has numerous problems. The advertised security characteristics of a protocol may depend on its being used as designed; when used in other ways, it may not deliver the expected security characteristics. It is recommended that any proposed model include a discussion of the applicability statement of the protocols to be used. A protocol used for the TMSM framework should ideally require no modifications to the protocol. Modifying the protocol may change its security characteristics in ways that would impact other existing usages. If a change is necessary, the change should be an extension that has no impact on the existing usages. It is recommended that Harrington & Schoenwaelder Expires September 5, 2006 [Page 6] Internet-Draft SNMP Transport Mapping Security Model March 2006 any proposed model include a discussion of potential impact on other usages of the protocol. It has been a long-standing requirement that SNMP be able to work when the network is unstable, to enable network troubleshooting and repair. The UDP approach has been considered to meet that need well, with an assumption that getting small messages through, even if out of order, is better than getting no messages through. There has been a long debate about whether UDP actually offers better support than TCP when the underlying IP or lower layers are unstable. There has been recent discussion of whether operators actually use SNMP to troubleshoot and repair unstable networks. There has been discussion of ways SNMP could be extended to better support management/monitoring needs when a network is running just fine. Use of a TCP transport, for example, could enable larger message sizes and more efficient table retrievals. TMSM models MUST be able to coexist with other protocol models, and may be designed to utilize either TCP or UDP, depending on the transport. 2.2. Session Requirements Throughout this document, the term session is used. Some underlying secure transports will have a notion of session. Some underlying secure transports might enable the use of channels or other session- like thing. In this document the term session refers to an association between two SNMP engines, that permits the secure transmission of one or more SNMP messages within the lifetime of the session. How the session is actually established, opened, closed, or maintained is specific to a particular security model. Sessions are not part of the SNMP architecture described in [RFC3411], but are considered desirable because the cost of authentication can be amortized over potentially many transactions. It is important to note that the architecture described in [RFC3411] does not include a session selector in the Abstract Service Interfaces, and neither is that done for this architectural extension, so an SNMP application cannot select the session except by passing a unique combination of securityName, securityModel, and securityLevel. All TMSM-based security models should discuss the impact of sessions on SNMP usage, including how to establish/open a TMSM session (i.e. how it maps to the concepts of session-like things of the underlying protocol), how to behave when a TMSM session cannot be established, Harrington & Schoenwaelder Expires September 5, 2006 [Page 7] Internet-Draft SNMP Transport Mapping Security Model March 2006 how to close a TMSM session (and the underlying protocol equivalent) properly, how to behave when a TMSM session is closed improperly, the session security properties, session establishment overhead, and session maintenance overhead. To reduce redundancy, this document will discuss aspects that are expected to be common to all TMSM-based security model sessions. 2.2.1. Session Establishment Requirements [todo] contributions welcome. 2.2.2. Session Maintenance Requirements [todo] contributions welcome. 2.2.3. Message security versus session security A TMSM session is associated with state information that is maintained for its lifetime. This state information allows for the application of various security services to TMSM-based security models. Cryptographic keys established at the beginning of the session SHOULD be used to provide authentication, integrity checking, and encryption services for data that is communicated during the session. The cryptographic protocols used to establish keys for a TMSM-based security model session SHOULD ensure that fresh new session keys are generated for each session. If each session uses new session keys, then messages cannot be replayed from one session to another. In addition sequence information MAY be maintained in the session which can be used to prevent the replay and reordering of messages within a session. A TMSM session will typically have a single securityName and securityLevel associated with it. If an exchange between communicating engines would require a different securityLevel or would be on behalf of a different securityName, then another session would be needed. An immediate consequence of this is that implementations should be able to maintain some reasonable number of concurrent sessions. For TMSM models, securityName is typically specified during session setup, and associated with the session identifier. SNMPv3 was designed to support multiple levels of security, selectable on a per-message basis by an SNMP application, because there is not much value in using encryption for a Commander Generator to poll for non-sensitive performance data on thousands of interfaces every ten minutes; the encryption adds significant overhead to Harrington & Schoenwaelder Expires September 5, 2006 [Page 8] Internet-Draft SNMP Transport Mapping Security Model March 2006 processing of the messages. Some TMSM-based security models MAY 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. Some security models may use an underlying transport that provides a per-message requested level of authentication and encryption services. For example, if a session is created as 'authPriv', then keys for encryption could still be negotiated once at the beginning of the session. But if a message is presented to the session with a security level of authNoPriv, then that message could simply be authenticated and not encrypted within the same transport session. Whether this is possible depends on the security model and the secure transport used. If the underlying transport layer security was configurable on a per- message basis, a TMSM-based security model could have a security- model-specific MIB module with configurable maxSecurityLevel and a minSecurityLevel objects to identify the range of possible levels. A session's maxSecurityLevel would identify the maximum security it could provide, and a session created with a minSecurityLevel of authPriv would reject an attempt to send an authNoPriv message. The elements of procedure of the security model would need to describe the procedures to enable this determination. For security models that do not support variable security services in one session, multiple sessions could be established, with different security levels, and for every packet the SNMP engine could select the appropriate session based on the requested securityLevel. Some SNMP entities are resource-constrained. Adding sessions increases the need for resources, but so does encrypting unnecessarily. Designers of security models should consider the tradeoffs for resource-constrained devices. 2.3. Architectural Modularity Requirements SNMP version 3 (SNMPv3) is based on a modular architecture (described in [RFC3411] section 3) to allow the evolution of the SNMP protocol standards over time, and to minimize side effects between subsystems when changes are made. This architecture includes a Security Subsystem which is responsible for realizing security services. In SNMPv2, there were many problems of side effects between subsystems caused by the manipulation of MIB objects, especially those related to authentication and authorization, because many of the parameters were stored in shared MIB objects, and different Harrington & Schoenwaelder Expires September 5, 2006 [Page 9] Internet-Draft SNMP Transport Mapping Security Model March 2006 models and protocols could assign different values to the objects. Contributors assumed slightly different shades of meaning depending on the models and protocols being used. As the shared MIB module design was modified to accommodate a specific model, other models which used the same MIB objects were broken. Abstract Service Interfaces (ASIs) were developed to pass model- independent parameters. The models were required to translate from their model-dependent formats into a model-independent format, defined using model-independent semantics, which would not impact other models. Parameters have been provided in the ASIs to pass model-independent information about the authentication that has been provided. These parameters include a model-independent identifier of the security "principal", the security model used to perform the authentication, and which SNMP-specific security features were applied to the message (authentication and/or privacy). The design of a transport mapping security model must abide the goals of the RFC3411 architecture defined in [RFC3411]. To that end, this transport mapping security model proposal focuses on a modular subsystem that can be advanced through the standards process independently of other proposals, and independent of other subsystems as much as possible. There has been some discussion of maintaining multiple sessions for different security levels or for different applications. The ability to have an application select different sessions or connections on a per-message basis would likely require a modification to the SNMP architecture to provide new ASIs, which is out of scope for this document. [discuss] I am not sure whether the previous paragraph is still correct - I think we need to solve at least some of the session problem space. IETF standards typically require one mandatory-to-implement solution, with the capability of adding new security mechanisms in the future. Any transport mapping security model should define one minimum- compliance mechanism, preferably one which is already widely deployed within the transport layer security protocol used. The TMSM subsystem is designed as an architectural extension that permits additional transport security protocols to be "plugged into" the RFC3411 architecture, supported by corresponding transport- security-aware transport mapping models. Harrington & Schoenwaelder Expires September 5, 2006 [Page 10] Internet-Draft SNMP Transport Mapping Security Model March 2006 The RFC3411 architecture, and the USM approach, assume that a security model is called by a message-processing model and will perform multiple security functions. The TMSM approach performs similar functions but performs them in different places within the architecture, so we need to distinguish the two locations for security processing. Transport mapping security is by its very nature a security layer which is plugged into the RFC3411 architecture between the transport layer and the message dispatcher. Conceptually, transport mapping security processing will be called from within the Transport Mapping functionality of an SNMP engine dispatcher to perform the translation of transport security parameters to/from security-model-independent parameters. This transport mapping security processor will be referred to in this document as TMSP. Additional functionality may be performed as part of the message processing function, i.e. in the security subsystem of the RFC3411 architecture. This document will refer to message processor's security processor as the MPSP. Thus a TMSM is composed of both a TPSP and an MPSP. +------------------------------+ | Network | +------------------------------+ ^ ^ ^ | | | v v v +-----+ +-----+ +-------+ | UDP | | TCP | . . . | other | +-----+ +-----+ +-------+ ^ ^ ^ | | | v v v +-----+ +-----+ +-------+ | SSH | | TLS | . . . | other | +-----+ +-----+ +-------+ (traditional SNMP agent) +-------------------------------------------------------------------+ | ^ | | | | | Dispatcher v | | +-------------------+ | | | Transport | +--------------+ | | | Mapping |<---> | TMSM | | | | (e.g., RFC 3417) | | TMSP | | | | | +--------------+ | Harrington & Schoenwaelder Expires September 5, 2006 [Page 11] Internet-Draft SNMP Transport Mapping Security Model March 2006 | | | | | | | +---------------------+ +----------------+ | | | | | Message Processing | | Security | | | | | | Subsystem | | Subsystem | | | | | | +------------+ | | | | | | | | +->| v1MP * |<--->| +------------+ | | | | | | | +------------+ | | | Other | | | | | | | | +------------+ | | | Security | | | | | | | +->| v2cMP * |<--->| | Model | | | | | Message | | | +------------+ | | +------------+ | | | | Dispatcher <--------->| +------------+ | | +------------+ | | | | | | +->| v3MP * |<--->| | TMSM | | | | | | | | +------------+ | | | MPSP | | | | | PDU Dispatcher | | | +------------+ | | | | | | | +-------------------+ | +->| otherMP * |<--->| +------------+ | | | ^ | +------------+ | | | | | | +---------------------+ +----------------+ | | v | | +-------+-------------------------+---------------+ | | ^ ^ ^ | | | | | | | v v v | | +-------------+ +---------+ +--------------+ +-------------+ | | | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | | | | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | | | | application | | | | applications | | application | | | +-------------+ +---------+ +--------------+ +-------------+ | | ^ ^ | | | | | | v v | | +----------------------------------------------+ | | | MIB instrumentation | SNMP entity | +-------------------------------------------------------------------+ 2.3.1. USM and the RFC3411 Architecture The following diagrams illustrate the difference in the security processing done by the USM model and the security processing done by a TMSM model. The USM security model is encapsulated by the messaging model, because the messaging model needs to perform the following steps (for incoming messages) 1) decode the ASN.1 (messaging model) Harrington & Schoenwaelder Expires September 5, 2006 [Page 12] Internet-Draft SNMP Transport Mapping Security Model March 2006 2) determine the SNMP security model and parameters (messaging model) 3) decrypt the encrypted portions of the message (security model) 4) translate parameters to model-independent parameters (security model) 5) determine which application should get the decrypted portions (messaging model), and 6) pass on the decrypted portions with model-independent parameters. The USM approach uses SNMP-specific message security and parameters. | -----------------------------------------------| | transport layer | | -----------------------------------------------| ^ | v -------------------------------------------------- | -----------------------------------------------| | | transport mapping | | -----------------------------------------------| | ^ | | | v | --------------------------------------------- | | --------------------- ------------------ | | SNMP messaging <--> | decryption + | | | | translation | | | --------------------- ------------------ | | ^ | | | v | --------------------- ------------------ | | | SNMP applications | <--> | access control | | | --------------------- ------------------ | | --------------------------------------------- | 2.3.2. TMSM and the RFC3411 Architecture In the TMSM approach, the order of the steps differ and may be handled by different subsystems: Harrington & Schoenwaelder Expires September 5, 2006 [Page 13] Internet-Draft SNMP Transport Mapping Security Model March 2006 1) decrypt the encrypted portions of the message (transport layer) 2) determine the SNMP security model and parameters (transport mapping) 3*) translate parameters to model-independent parameters (transport mapping) 4) decode the ASN.1 (messaging model) 5) determine which application should get the decrypted portions (messaging model) 6*) translate parameters to model-independent parameters (security model) 7) pass on the decrypted portions with model-independent security parameters This is largely based on having non-SNMP-specific message security and parameters. The transport mapping model might provide the translation from e.g., an SSH user name to the securityName in step 3, OR the SSH user might be passed to the messaging model to pass to a TMSM security model to do the translation in step 6, if the WG decides all translations should use the same translation table (e.g., the USM MIB). Harrington & Schoenwaelder Expires September 5, 2006 [Page 14] Internet-Draft SNMP Transport Mapping Security Model March 2006 | -----------------------------------------------| | ------------------ | | transport layer <--> | decryption | | | ------------------ | | -----------------------------------------------| ^ | v -------------------------------------------------- | -----------------------------------------------| | ------------------ | | transport mapping <--> | translation* | | | ------------------ | | -----------------------------------------------| | ^ | | | v | --------------------------------------------- | | ------------------ | | SNMP messaging <--> | translation* | | | ------------------ | | --------------------- ------------------ | | ^ | | | v | --------------------- ------------------ | | | SNMP applications | <--> | access control | | | --------------------- ------------------ | | --------------------------------------------- | 2.4. Passing Messages between Subsystems RFC3411 defines ASIs that describe the passing of messages between subsystem within an engine, and the parameters which are expected to be passed between the subsystems. The ASIs generally pass model- independent information. A TMSM model will establish an encrypted tunnel between the transport mappings of two SNMP engines. One transport mapping security model instance encrypts all messages, and the other transport mapping security model instance decrypts the messages. After the transport layer tunnel is established, then SNMP messages can conceptually be sent through the tunnel from one SNMP message dispatcher to another SNMP message dispatcher. Once the tunnel is Harrington & Schoenwaelder Expires September 5, 2006 [Page 15] Internet-Draft SNMP Transport Mapping Security Model March 2006 established, multiple SNMP messages may be able to be passed through the same tunnel. Within an engine, outgoing SNMP messages are passed unencrypted from the message dispatcher to the transport mapping, and incoming messages are passed unencrypted from the transport mapping to the message dispatcher. 2.5. Security Parameter Passing Requirement RFC3411 section 4 describes primitives to describe the abstract service interfaces used to conceptually pass information between the various subsystems, models and applications within the architecture. The security parameters include a model-independent identifier of the security "principal", the security model used to perform the authentication, and which SNMP-specific security services were (should be) applied to the message (authentication and/or privacy). In the RFC3411 architecture, the messaging model must unpack SNMP- specific security parameters from the message before calling a security model to authenticate and decrypt an incoming message, perform integrity checking, and translate model-specific security parameters into model-independent parameters. In the TMSM approach, the security-model specific parameters are not all carried in the SNMP message, and can be determined from the transport layer by the transport mapping, before the message processing begins. [discuss] For outgoing messages, it is necessary to have an MPSP because it is the MPSP that actually creates the message from its component parts. Does the MPSP need to know the transport address or the actual transport security capabilities, or can this be handled in the TMSP, given the model-independent (and message-version- independent) parameters? Are there any security services provided by the MPSP for an outgoing message? [discuss] For incoming messages, is there security functionality that can only be handled after the message version is known, such as the comparison of transport security capabilities and msgFlags? Does that functionality need to know the transport address and session or just the model-independent security parameters (securityName, model, level)? Are there any SNMP-specific parameters that need to be unpacked from the message for MPSP handling? msgFlags, securityLevel, etc.? The RFC3411 architecture has no ASI parameters for passing security information between the transport mapping and the dispatcher, and between the dispatcher and the message processing model. If there is Harrington & Schoenwaelder Expires September 5, 2006 [Page 16] Internet-Draft SNMP Transport Mapping Security Model March 2006 a need to have an MPSP called from the message processing model to, for example, verify that msgFlags and the transport security are consistent, then it will be necessary to pass the model-independent security parameters from the TPSP through to the MPSP. There are four approaches that could be used for passing information between the TMSP and an MPSP. 1. one could define an ASI to supplement the existing ASIs, or 2. the TMSM could add a header to encapsulate the SNMP message, 3. the TMSM could utilize fields already defined in the existing SNMPv3 message, or 4. the TMSM could pass the information in an implementation-specific cache or via a MIB module. 2.5.1. Define an Abstract Service Interface Abstract Service Interfaces (ASIs) [RFC3411] are defined by a set of primitives that specify the services provided and the abstract data elements that are to be passed when the services are invoked. Defining additional ASIs to pass the security and transport information from the transport mapping to a messaging security model has the advantage of being consistent with existing RFC3411/3412 practice, and helps to ensure that any TMSM proposals pass the necessary data, and do not cause side effects by creating model- specific dependencies between itself and other models or other subsystems other than those that are clearly defined by an ASI. 2.5.2. Using an Encapsulating Header A header could encapsulate the SNMP message to pass necessary information from the TMSP to the dispatcher and then to a messaging security model. The message header would be included in the wholeMessage ASI parameter, and would be removed by a corresponding messaging model. This would imply the (one and only) messaging dispatcher would need to be modified to determine which SNMP message version was involved, and a new message processing model would need to be developed that knew how to extract the header from the message and pass it to the MPSP. 2.5.3. Modifying Existing Fields in an SNMP Message [RFC3412] describes the SNMPv3 message, which contains fields to pass security related parameters. The TMSM could use these fields in an SNMPv3 message, or comparable fields in other message formats to pass information between transport mapping security models in different SNMP engines, and to pass information between a transport mapping security model and a corresponding messaging security model. Harrington & Schoenwaelder Expires September 5, 2006 [Page 17] Internet-Draft SNMP Transport Mapping Security Model March 2006 If the fields in an incoming SNMPv3 message are changed by the TMSP before passing it to the MPSP, then the TMSP 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 so the TMSP knew which fields could be modified. This would seriously violate the modularity of the architecture. 2.5.4. Using a Cache A cache mechanism could be used, into which the TMSP puts information about the security applied to an incoming message, and an MPSP extracts that information from the cache. Given that there may be multiple TM-security caches, a cache ID would need to be passed through an ASI so the MPSP knows which cache of information to consult. The cache reference could be thought of as an additional parameter in the ASIs between the transport mapping and the messaging security model. The RFC3411 ASIs would not need to be changed since the SNMPv3 WG expected that additional parameters could be passed for value-add features of specific implementations. This approach does create dependencies between a model-specific TPSP and a corresponding specific MPSP. If a TMSM-model-independent ASI parameter is passed, this approach would be consistent with the securityStateReference cache already being passed around in the ASI. This document will describe a cache-based approach. 2.6. Architectural Requirements for Access Control 2.6.1. securityName Binding For SNMP access control to function properly, the security mechanism must establish a securityModel identifier, a securityLevel, and a securityName, which is the security model independent identifier for a principal. The SNMPv3 message processing architecture subsystem relies on a security model, such as USM, to play a role in security that goes beyond protecting the message - it provides a mapping between the USM-specific principal to a security-model independent securityName which can be used for subsequent processing, such as for access control. The TMSM is a two-stage security model, with a transport mapping security process (TMSP) and a message processing security process (MPSP). Depending on the design of the specific TMSM model, i.e. Harrington & Schoenwaelder Expires September 5, 2006 [Page 18] Internet-Draft SNMP Transport Mapping Security Model March 2006 which transport layer protocol is used, different features might be provided by the TMSP or by the MPSP. For example, the translation from a mechanism-specific authenticated identity to a securityName might be done by the TMSP or by the MPSP. [discuss] It may be possible to define a consistent division of stages regardless of the transport layer protocol used, and a consistent division of functionality would be preferred. The SNMP architecture distinguishes between messages with no authentication and no privacy (noAuthNoPriv), authentication without privacy (authNoPriv) and authentication with privacy (authPriv). Hence, the authentication of a transport layer identity plays an important role and must be considered by any TMSM, and user authentication must be available via the transport layer security protocol. If the type of authentication provided by the transport layer (e.g. host-based or anonymous) is considered adequate to secure and/or encrypt the message, but inadequate to provide the desired granularity of access control (e.g. user-based), a second authentication, e.g. one provided by a AAA server, may be used to provide the authentication identity which is bound to the securityName. This approach would require a good analysis of the potential for man-in-the-middle attacks or masquerade possibilities. 2.6.2. Separation of Authentication and Authorization A TMSM security model should take care to not violate the separation of authentication and authorization in the RFC3411 architecture.. The isAccessAllowed() primitive is used for passing security-model independent parameters between the subsystems of the architecture. Mapping of (securityModel, securityName) to an access control policy should be handled within the access control subsystem, not the security subsystem, to be consistent with the modularity of the RFC3411 architecture. This separation was a deliberate decision of the SNMPv3 WG, to allow support for authentication protocols which did not provide authorization capabilities, and to support authorization schemes, such as VACM, that do not perform their own authentication. An authorization model MAY require authentication by certain securityModels and a minimum securityLevel to allow access to the data. TMSM is an enhancement for the SNMPv3 privacy and authentication provisions, but it is not a significant improvement for the Harrington & Schoenwaelder Expires September 5, 2006 [Page 19] Internet-Draft SNMP Transport Mapping Security Model March 2006 authorization needs of SNMPv3. TMSM provides all the model- independent parameters for the isAccessAllowed() primitive [RFC3411]. TMSM does not specify how the securityModel and securityName could be dynamically mapped to a VACM-style groupName. The mapping of (securityModel, securityName) to a groupName is a VACM-specific mechanism for naming an access control policy, and for tying the named policy to the addressing capabilities of the data modeling language (e.g. SMIv2 [RFC2578]), the operations supported, and other factors. Providing a binding outside the Access Control subsystem might create dependencies that could make it harder to develop alternate models of access control, such as one built on UNIX groups, Windows domains, XML hierarchies, or task-based controls. The preferred approach is to pass the model-independent security parameters via the isAccessAllowed() ASI, and perform the mapping within the access control model. To provide support for protocols which simultaneously send information for authentication and authorization, such as RADIUS [RFC2865], model-specific authorization information MAY be cached or otherwise made available to the access control subsystem, e.g. via a MIB table similar to the vacmSecurityToGroupTable, so the access control subsystem can create an appropriate binding between the model-independent securityModel and securityName and a model-specific access control policy. This may be highly undesirable, however, if it creates a dependency between a security model and an access control model, just as it is undesirable that the TMSM approach creates a dependency between a TMSP and an MPSP. 2.7. Requirements for Notifications [todo] cleanup this section RFC 3430 (SNMP over TCP) suggests that TCP connections are initiated by notification originators in case there is no currently established connection that can be used to send the notification. Following this approach with SSH would require to provision authentication credentials on the agent so that agents can successfully authenticate to a notification receiver. There might be other approaches, like the reuse of manager initiated secure transport connections for notifications. There is some text in Appendix A in RFC 3430 which captures some of these discussions when RFC 3430 was written. [todo] merge this text and text from RFC 3430 into the section dealing with sessions? This seems to be the right place for this discussion. Harrington & Schoenwaelder Expires September 5, 2006 [Page 20] Internet-Draft SNMP Transport Mapping Security Model March 2006 3. Scenario Diagrams RFC3411 section 4.6 provides scenario diagrams to illustrate how an outgoing message is created, and how an incoming message is processed. Both diagrams are incomplete, however. In section 4.6.1, the diagram doesn't show the ASI for sending an SNMP request to the network or receiving an SNMP response message from the network. In section 4.6.2, the diagram doesn't illustrate the interfaces required to receive an SNMP message from the network, or to send an SNMP message to the network. 3.1. Command Generator or Notification Originator This diagram from RFC3411 4.6.1 shows how a Command Generator or Notification Originator application [RFC3413]requests that a PDU be sent, and how the response is returned (asynchronously) to that application. Harrington & Schoenwaelder Expires September 5, 2006 [Page 21] Internet-Draft SNMP Transport Mapping Security Model March 2006 Command Dispatcher Message Security Generator | Processing Model | | Model | | sendPdu | | | |------------------->| | | | | prepareOutgoingMessage | | : |----------------------->| | : | | generateRequestMsg | : | |-------------------->| : | | | : | |<--------------------| : | | | : |<-----------------------| | : | | | : |------------------+ | | : | Send SNMP | | | : | Request Message | | | : | to Network | | | : | v | | : : : : : : : : : : : : : : : : | | | | : | Receive SNMP | | | : | Response Message | | | : | from Network | | | : |<-----------------+ | | : | | | : | prepareDataElements | | : |----------------------->| | : | | processIncomingMsg | : | |-------------------->| : | | | : | |<--------------------| : | | | : |<-----------------------| | | processResponsePdu | | | |<-------------------| | | | | | | 3.2. Command Responder This diagram shows how a Command Responder or Notification Receiver application registers for handling a pduType, how a PDU is dispatched to the application after an SNMP message is received, and how the Response is (asynchronously) send back to the network. Harrington & Schoenwaelder Expires September 5, 2006 [Page 22] Internet-Draft SNMP Transport Mapping Security Model March 2006 Command Dispatcher Message Security Responder | Processing Model | | Model | | | | | | registerContextEngineID | | | |------------------------>| | | |<------------------------| | | | | | Receive SNMP | | | : | Message | | | : | from Network | | | : |<-------------+ | | : | | | : |prepareDataElements | | : |------------------->| | : | | processIncomingMsg | : | |------------------->| : | | | : | |<-------------------| : | | | : |<-------------------| | | processPdu | | | |<------------------------| | | | | | | : : : : : : : : | returnResponsePdu | | | |------------------------>| | | : | prepareResponseMsg | | : |------------------->| | : | |generateResponseMsg | : | |------------------->| : | | | : | |<-------------------| : | | | : |<-------------------| | : | | | : |--------------+ | | : | Send SNMP | | | : | Message | | | : | to Network | | | : | v | | 4. Abstract Service Interfaces The OUT parameters of the prepareOutgoingMessage() ASI are used to pass information from the message processing model to the dispatcher Harrington & Schoenwaelder Expires September 5, 2006 [Page 23] Internet-Draft SNMP Transport Mapping Security Model March 2006 and on to the transport mapping: statusInformation = -- success or errorIndication prepareOutgoingMessage( IN transportDomain -- transport domain to be used IN transportAddress -- transport address to be used 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 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 -- its length ) 5. TMSM Abstract Service Interfaces A set of abstract service interfaces have been defined within this document to describe the conceptual data flows between the Transport Mapping Security Models and adjacent components in the system.. The SendMessage ASI is used to pass a message from the Dispatcher to the transport mapping security model subsystem for sending. 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 -- OUT sessionID ) The RecvMessage ASI is used to pass a message from the transport mapping security model subsystem to the Dispatcher. Harrington & Schoenwaelder Expires September 5, 2006 [Page 24] Internet-Draft SNMP Transport Mapping Security Model March 2006 statusInformation RecvMessage( IN destTransportDomain -- transport domain to be used IN destTransportAddress -- transport address to be used IN incomingMessage -- the message received IN incomingMessageLength -- its length OUT tmStateReference -- OUT sessionID ) The Transport Mapping Security Model provides the following primitives to pass data back and forth between the TMSM and specific TMSM-based security models, which provide the interface to the underlying secure transport service. Each TMSM-based security model should define the security-model-specific elements of procedure for the establishSession(), closeSession(), TxMessage(), and RxMessage() interfaces. Harrington & Schoenwaelder Expires September 5, 2006 [Page 25] Internet-Draft SNMP Transport Mapping Security Model March 2006 statusInformation TxMessage( 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 -- OUT sessionID ) statusInformation RxMessage( IN destTransportDomain -- transport domain to be used IN destTransportAddress -- transport address to be used IN incomingMessage -- the message to send IN incomingMessageLength -- its length OUT tmStateReference -- ) statusInformation establishSession( IN transportDomain -- transport domain to be used IN transportAddress -- transport address to be used IN tmStateReference -- OUT sessionID ) statusInformation closeSession( IN sessionID ) 6. Integration with the SNMPv3 Message Format TMSM proposals can use the SNMPv3 message format, defined in RFC3412, section 6. This section discusses how the fields could be reused. 6.1. msgVersion For proposals that reuse the SNMPv3 message format, this field should contain the value 3. Harrington & Schoenwaelder Expires September 5, 2006 [Page 26] Internet-Draft SNMP Transport Mapping Security Model March 2006 6.2. msgGlobalData The fields msgID and msgMaxSize are used identically for the TMSM models as for the USM model. The msgSecurityModel field should be set to a value from the SnmpSecurityModel enumeration [RFC3411] to identify the specific TMSM model. Each standards-track TMSM model should have an enumeration assigned by IANA. Each enterprise-specific security model should have an enumeration assigned following instructions in the description of the SnmpSecurityModel TEXTUAL-CONVENTION from RFC3411. The msgSecurityParameters field would carry security information required for message security processing. It is unclear whether this field would be useful or what parameters would be carried to support security, since message security is provided by an external process, and msgSecurityParameters are not used by the access control subsystem. RFC3412 defines two primitives, generateRequestMsg() and processIncomingMsg() which require the specification of an authoritative SNMP entity. [discuss] We need to discuss what the meaning of authoritative would be in a TMSM environment, whether the specific services provided in USM security from msgSecurityParameters still are needed, and how the Message Processing model provides this information to the security model via generateRequestMsg() and processIncomingMsg() primitives. RFC3412 specifies that "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." The msgFlags have the same values for the TMSM models as for the USM model. "The authFlag and privFlag fields indicate the securityLevel that was applied to the message before it was sent on the wire." 6.3. securityLevel and msgFlags For an outgoing message, msgFlags is the requested security for the message; if a TMSM cannot provide the requested securityLevel, the model MUST describe a standard behavior that is followed for that situation. If the TMSM cannot provide at least the requested level of security, the TMSM MUST discard the request and SHOULD notify the message processing model that the request failed. [discuss] how is yet to be determined, and may be model-specific or implementation-specific. Harrington & Schoenwaelder Expires September 5, 2006 [Page 27] Internet-Draft SNMP Transport Mapping Security Model March 2006 For an outgoing message, if the TMSM is able to provide stronger than requested security, that may be acceptable. The transport layer protocol would need to indicate to the receiver what security has been applied to the actual message. To avoid the need to mess with the ASN.1 encoding, the SNMPv3 message carries the requested msgFlags, not the actual securityLevel applied to the message. If a message format other than SNMPv3 is used, then the new message may carry the more accurate securityLevel in the SNMP message. For an incoming message, the receiving TMSM knows what must be done to process the message based on the transport layer mechanisms. If the underlying transport security mechanisms for the receiver cannot provide the matching securityLevel, then the message should follow the standard behaviors for the transport security mechanism, or be discarded silently. Part of the responsibility of the TMSM is to ensure that the actual security provided by the underlying transport layer security mechanisms is configured to meet or exceed the securityLevel required by the msgFlags in the SNMP message. When the MPSP processes the incoming message, it should compare the msgFlags field to the securityLevel actually provided for the message by the transport layer security. If they differ, the MPSP should determine whether the changed securityLevel is acceptable. If not, it should discard the message. Depending on the model, the MPSP may issue a reportPDU with the XXXXXXX model-specific counter. 7. The tmStateReference for Passing Security Parameters A tmStateReference is used to pass data between the TMSP and the MPSP, similar to the securityStateReference described in RFC3412. This can be envisioned as being appended to the ASIs between the TM and the MP or as being passed in an encapsulating header. The TMSP may provide only some aspects of security, and leave some aspects to the MPSP. tmStateReference should be used to pass any parameters, in a model- and mechanism-specific format, that will be needed to coordinate the activities of the TMSP and MPSP, and the parameters subsequently passed in securityStateReference. For example, the TMSP may provide privacy and data integrity and authentication and authorization policy retrievals, or some subset of these features, depending on the features available in the transport mechanisms. A field in tmStateReference should identify which services were provided for each received message by the TMSP, the securityLevel applied to the received message, the model-specific security identity, the session identifier for session based transport security, and so on. Harrington & Schoenwaelder Expires September 5, 2006 [Page 28] Internet-Draft SNMP Transport Mapping Security Model March 2006 8. securityStateReference Cached Security Data From RFC3411: "For each message received, the Security Model caches the state information such that a Response message can be generated using the same security information, even if the Local Configuration Datastore is altered between the time of the incoming request and the outgoing response. A Message Processing Model has the responsibility for explicitly releasing the cached data if such data is no longer needed. To enable this, an abstract securityStateReference data element is passed from the Security Model to the Message Processing Model. The cached security data may be implicitly released via the generation of a response, or explicitly released by using the stateRelease primitive, as described in RFC3411 section 4.5.1." For the TMSM approach, the TMSP may need to provide information to the message processing model, such as the security-model-independent securityName, securityLevel, and securityModel parameters, and for responses, the messaging model may need to pass the parameters back to the TMSP. To differentiate what information needs to be provided to the message processing model by the TMSP, and vice-versa, this document will differentiate the tmStateReference provide by the TMSP from the securityStateReference provided by the MPSP. An implementation MAY use one cache and one reference to serve both functions, but an implementer must be aware of the cache-release issues to prevent the cache from being released before the transport mapping has had an opportunity to extract the information it needs. 9. Prepare an Outgoing SNMP Message Following RFC3412, section 7.1, the SNMPv3 message processing model uses the generateResponseMsg() or generateRequestMsg() primitives, to call the MPSP. The message processing model, or the MPSP it calls, may need to put information into the tmStateReference cache for use by the TMSP, such as: tmSecurityStateReference - the unique identifier for the cached information tmTransportDomain tmTransportAddress tmSecurityModel - an indicator of which mechanisms to use tmSecurityName - a model-specific identifier of the security principal tmSecurityLevel - an indicator of which security services are requested and may contain additional information such as Harrington & Schoenwaelder Expires September 5, 2006 [Page 29] Internet-Draft SNMP Transport Mapping Security Model March 2006 tmSessionID tmSessionKey tmSessionMsgID According to RFC3411, section 4.1.1, the application provides the transportDomain and transportAddress to the PDU dispatcher via the sendPDU() primitive. If we permit multiple sessions per transportAddress, then we would need to define how session identifiers get passed from the application to the PDU dispatcher (and then to the MP model). The SNMP over TCP Transport Mapping document [RFC3430] says that TCP connections can be recreated dynamically or kept for future use and actually leaves all that to the transport mapping. [discuss] we might define a new transportDomain and transportAddress, which includes the address and session identifier. For situations where a session has not yet been established, we could pass a 0x0000 session identifier (or whatever) to indicate that a session should be established. Well, this won't work with the current TAddress definitions and is probably too ugly to do. We might have a MIB module that records the session information for subsequent use by the applications and other subsystems, or it might be passed in the tmStateReference cache. For notifications, I assume the SNMPv3 notification tables would be a place to find the address, but I'm not sure how to identify the presumably-dynamic session identifiers. The MIB module could identify whether the session was initiated by the remote engine or initiated by the current engine, and possibly assigned a purpose (incoming request/response or outgoing notifications). First we need to decide whether to handle notifications and requests in one or two (or more) sessions, which might depend on the transport protocol we choose (the same problem netconf faced). 10. Prepare Data Elements from an Incoming SNMP Message For an incoming message, the TMSP will need to put information from the transport mechanisms used into the tmStateReference so the MPSP can extract the information and add it conceptually to the securityStateReference. The tmStateReference cache will likely contain at least the following information: tmStateReference - a unique identifier for the cached information Harrington & Schoenwaelder Expires September 5, 2006 [Page 30] Internet-Draft SNMP Transport Mapping Security Model March 2006 tmSecurityStateReference - the unique identifier for the cached information tmTransportDomain tmTransportAddress tmSecurityModel - an indicator of which mechanisms to use tmSecurityName - a model-specific identifier of the security principal tmSecurityLevel - an indicator of which security services are requested tmAuthProtocol tmPrivProtocol and may contain additional information such as tmSessionID tmSessionKey tmSessionMsgID 11. Notifications For notifications, if the cache has been released and then session closed, then the MPSP will request the TMSP to establish a session, populate the cache, and pass the securityStateReference to the MPSP. [discuss] We need to determine what state needs to be saved here. 12. Transport Mapping Security Model Samples There are a number of standard protocols that could be proposed as possible solutions within the TMSM framework. Some factors should be considered when selecting a protocol for use within this framework. Using a protocol in a manner for which is was not designed has numerous problems. The advertised security characteristics of a protocol may depend on its being used as designed; when used in other ways, it may not deliver the expected security characteristics. It is recommended that any proposed model include a discussion of the applicability statement of the protocols to be used. 12.1. TLS/TCP Transport Mapping Security Model SNMP supports multiple transports. The preferred transport for SNMP over IP is UDP [RFC3417]. An experimental transport for SNMP over TCP is defined in [RFC3430]. TLS/TCP will create an association between the TMSM of one SNMP entity and the TMSM of another SNMP entity. The created "tunnel" may provide encryption and data integrity. Both encryption and data Harrington & Schoenwaelder Expires September 5, 2006 [Page 31] Internet-Draft SNMP Transport Mapping Security Model March 2006 integrity are optional features in TLS. The TLS TMSP MUST provide authentication if auth is requested in the securityLevel of the SNMP message request (RFC3412 4.1.1). The TLS TM-security model MUST specify that the messages be encrypted if priv is requested in the securityLevel parameter of the SNMP message request (RFC3412 4.1.1). The TLS TM-security model MUST support the TLS Handshake Protocol with mutual authentication. 12.1.1. tmStateReference for TLS Upon establishment of a TLS session, the TMSP will cache the state information. A unique tmStateReference will be passed to the corresponding MPSP. The MPSP will pass the securityStateReference to the Message Processing Model for memory management. The tmStateReference cache: tmStateReference tmSecurityStateReference tmTransportDomain = TCP/IPv4 tmTransportAddress = x.x.x.x:y tmSecurityModel - TLS TMSM tmSecurityName = "dbharrington" tmSecurityLevel = "authPriv" 12.1.2. MPSP for TLS TM-Security Model messageProcessingModel = SNMPv3 securityModel = TLS TMSM securityName = tmSecurityName securityLevel = msgSecurityLevel 12.1.3. MIB Module for TLS Security Each security model should use its own MIB module, rather than utilizing the USM MIB, to eliminate dependencies on a model that could be replaced some day. See RFC3411 section 4.1.1. The TLS MIB module needs to provide the mapping from model-specific identity to a model independent securityName. [todo] Module needs to be worked out once things become stable... 12.2. DTLS/UDP Transport Mapping Security Model DTLS has been proposed as a UDP-based TLS. Transport Layer Security (TLS) [RFC2246] traditionally requires a connection-oriented transport and is usually used over TCP. Datagram Transport Layer Harrington & Schoenwaelder Expires September 5, 2006 [Page 32] Internet-Draft SNMP Transport Mapping Security Model March 2006 Security (DTLS) [I-D.rescorla-dtls] provides security services equivalent to TLS for connection-less transports such as UDP. DTLS provides all the security services needed from an SNMP architectural point of view. Although it is possible to derive a securityName from the public key certificates (e.g. the subject field), this approach requires installing certificates on all SNMP entities, leading to a certificate management problem which does not integrate well with established AAA systems. [discuss] why does this not integrate well with existing AAA systems? Another option is to run an authentication exchange which is integrated with TLS, such as Secure Remote Password with TLS [I-D.ietf-tls-srp]. A similar option would be to use Kerberos authentication with TLS as defined in [RFC2712]. It is important to stress that the authentication exchange must be integrated into the TLS mechanism to prevent man-in-the-middle attacks. While SASL [RFC2222] is often used on top of a TLS encrypted channel to authenticate users, this choice seems to be problematic until the mechanism to cryptographically bind SASL into the TLS mechanism has been defined. DTLS will create an association between the TMSM of one SNMP entity and the TMSM of another SNMP entity. The created "tunnel" may provide encryption and data integrity. Both encryption and data integrity are optional features in DTLS. The DTLS TM-security model MUST provide authentication if auth is requested in the securityLevel of the SNMP message request (RFC3412 4.1.1). The TLS TM-security model MUST specify that the messages be encrypted if priv is requested in the securityLevel parameter of the SNMP message request (RFC3412 4.1.1). The DTLS TM-security model MUST support the TLS Handshake Protocol with mutual authentication. 12.2.1. tmStateReference for DTLS DTLS has been suggested as a possible secure transport. It is not clear whether DTLS is a reasonable choice for SNMP interactions. It is mentioned here only as an example. Upon establishment of a DTLS session, the TMSP will cache the state information. A unique tmStateReference will be passed to the corresponding MPSP. The MPSP will pass the securityStateReference to the Message Processing Model for memory management. The tmStateReference cache: Harrington & Schoenwaelder Expires September 5, 2006 [Page 33] Internet-Draft SNMP Transport Mapping Security Model March 2006 tmStateReference tmSecurityStateReference tmTransportDomain = UDP/IPv4 tmTransportAddress = x.x.x.x:y tmSecurityModel - DTLS TMSM tmSecurityName = "dbharrington" tmSecurityLevel = "authPriv" 12.3. SASL Transport Mapping Security Model The Simple Authentication and Security Layer (SASL) [RFC2222] provides a hook for authentication and security mechanisms to be used in application protocols. SASL supports a number of authentication and security mechanisms, among them Kerberos via the GSSAPI mechanism [RFC4121]. This sample will use DIGEST-MD5 because it supports authentication, integrity checking, and confidentiality. DIGEST-MD5 supports auth, auth with integrity, and auth with confidentiality. Since SNMPv3 assumes integrity checking is part of authentication, if msgFlags is set to authNoPriv, the qop-value should be set to auth-int; if msgFlags is authPriv, then qop-value should be auth-conf. Realm is optional, but can be utilized by the securityModel if desired. SNMP does not use this value, but a TMSM could map the realm into SNMP processing in various ways. For example, realm and username could be concatenated to be the securityName value, e.g. helpdesk::username", or the realm could be used to specify a groupName to use in the VACM access control. This would be similar to having the securityName-to-group mapping done by the external AAA server. 12.3.1. tmStateReference for SASL DIGEST-MD5 The tmStateReference cache: tmStateReference tmSecurityStateReference tmTransportDomain = TCP/IPv4 tmTransportAddress = x.x.x.x:y tmSecurityModel - SASL TMSM tmSecurityName = username tmSecurityLevel = [auth-conf] tmAuthProtocol = md5-sess tmPrivProtocol = 3des Harrington & Schoenwaelder Expires September 5, 2006 [Page 34] Internet-Draft SNMP Transport Mapping Security Model March 2006 tmServicesProvided mutual authentication, reauthentication, integrity, encryption tmParameters = "realm=helpdesk, serv-type=SNMP 13. The TMSM MIB Module This memo defines a portion of the Management Information Base (MIB) for managing the Transport Mapping Security Model Subsystem. 13.1. Structure of the MIB Module Objects in this MIB module are arranged into subtrees. Each subtree is organized as a set of related objects. The overall structure and assignment of objects to their subtrees, and the intended purpose of each subtree, is shown below. 13.1.1. Textual Conventions Generic and Common Textual Conventions used in this document can be found summarized at http://www.ops.ietf.org/mib-common-tcs.html 13.1.2. The tmsmStats Subtree This subtree contains security-model-independent counters which are applicable to all security models based on the .Transport Mapping Security Model Subsystem. This subtree provides information for identifying fault conditions and performance degradation. 13.1.3. The tmsmsSession Subtree This subtree contains security-model-independent information about sessions which are applicable to all security models based on the Transport Mapping Security Model Subsystem. This subtree provides information for managing sessions for any security model based on the Transport Mapping Security Model Subsystem. 13.1.4. The Notifications Subtree This subtree contains notifications to alert other entities to events which could alter the operational behavior of the entity in a network Harrington & Schoenwaelder Expires September 5, 2006 [Page 35] Internet-Draft SNMP Transport Mapping Security Model March 2006 utilizing the SAMPLE Protocol. 13.2. Relationship to Other MIB Modules Some management objects defined in other MIB modules are applicable to an entity implementing this MIB. In particular, it is assumed that an entity implementing the TMSM-MIB module will also implement the SNMPv2-MIB [RFC3418]. This MIB module is expected to be used with the MIB modules defined for managing specific security models that are based on the TMSM subsystem. This MIB module is designed to be security-model independent, and conatins objects useful for managing common aspects of any TMSM-based security model. Specific security models may define a MIB module to contain security-model-dependent information. 13.2.1. Relationship to the SNMPv2-MIB The 'system' subtree in the SNMPv2-MIB [RFC3418] is defined as being mandatory for all systems, and the objects apply to the entity as a whole. The 'system' subtree provides identification of the management entity and certain other system-wide data. The TMSM-MIB utilizes, but does not dupicate, some of those objects. [todo] do we actually use any of the objects, since we don't have any elements of procedure? 13.2.2. MIB Modules Required for IMPORTS The following MIB module imports items from [RFC2578], [RFC2579], [RFC2580], [RFC3411], and [RFC3419] 14. Definitions TMSM-MIB DEFINITIONS ::= BEGIN IMPORTS MODULE-IDENTITY, OBJECT-TYPE, mib-2, Integer32, Unsigned32, Gauge32 FROM SNMPv2-SMI TestAndIncr FROM SNMPv2-TC MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF SnmpSecurityModel, SnmpAdminString, SnmpSecurityLevel, SnmpEngineID FROM SNMP-FRAMEWORK-MIB TransportAddress, TransportAddressType Harrington & Schoenwaelder Expires September 5, 2006 [Page 36] Internet-Draft SNMP Transport Mapping Security Model March 2006 FROM TRANSPORT-ADDRESS-MIB ; tmsmMIB MODULE-IDENTITY LAST-UPDATED "200602270000Z" ORGANIZATION "ISMS Working Group" CONTACT-INFO "WG-EMail: isms@lists.ietf.org Subscribe: isms-request@lists.ietf.org Chairs: Juergen Quittek NEC Europe Ltd. Network Laboratories Kurfuersten-Anlage 36 69115 Heidelberg Germany +49 6221 90511-15 quittek@netlab.nec.de Juergen Schoenwaelder International University Bremen Campus Ring 1 28725 Bremen Germany +49 421 200-3587 j.schoenwaelder@iu-bremen.de Editor: David Harrington Effective Software 50 Harding Rd Portsmouth, New Hampshire 03801 USA +1 603-436-8634 ietfdbh@comcast.net " DESCRIPTION "The Transport Mapping Security Model Subsystem MIB Copyright (C) The Internet Society (2006). This version of this MIB module is part of RFC XXXX; see the RFC itself for full legal notices. -- NOTE to RFC editor: replace XXXX with actual RFC number -- for this document and remove this note " REVISION "200602270000Z" -- 27 February 2006 DESCRIPTION "The initial version, published in RFC XXXX. Harrington & Schoenwaelder Expires September 5, 2006 [Page 37] Internet-Draft SNMP Transport Mapping Security Model March 2006 -- NOTE to RFC editor: replace XXXX with actual RFC number -- for this document and remove this note " ::= { mib-2 xxxx } -- RFC Ed.: replace xxxx with IANA-assigned number and -- remove this note -- ---------------------------------------------------------- -- -- subtrees in the TMSM-MIB -- ---------------------------------------------------------- -- tmsmNotifications OBJECT IDENTIFIER ::= { tmsmMIB 0 } tmsmObjects OBJECT IDENTIFIER ::= { tmsmMIB 1 } tmsmConformance OBJECT IDENTIFIER ::= { tmsmMIB 2 } -- ------------------------------------------------------------- -- Objects -- ------------------------------------------------------------- -- Statistics for the Transport Model Security Model Subsystem tmsmStats OBJECT IDENTIFIER ::= { tmsmObjects 1 } -- [discuss] do we need any tmsm stats? -- these should be for interoperability, not local debug. -- we could probably track session establishment failures -- although this really belongs in an SSH-MIB, not TMSM-MIB -- The tmsmSession Group tmsmSession OBJECT IDENTIFIER ::= { tmsmObjects 2 } tmsmSessionSpinLock OBJECT-TYPE SYNTAX TestAndIncr MAX-ACCESS read-write STATUS current DESCRIPTION "An advisory lock used to allow several cooperating TMSM security models to coordinate their use of facilities to create sessions in the tmsmSessionTable. " ::= { tmsmSession 1 } tmsmSessionCurrent OBJECT-TYPE SYNTAX Gauge32 Harrington & Schoenwaelder Expires September 5, 2006 [Page 38] Internet-Draft SNMP Transport Mapping Security Model March 2006 MAX-ACCESS read-only STATUS current DESCRIPTION "The current number of established sessions. " ::= { tmsmSession 2 } tmsmSessionMaxSupported OBJECT-TYPE SYNTAX Unsigned32 MAX-ACCESS read-only STATUS current DESCRIPTION "The maximum number of open sessions allowed. " ::= { tmsmSession 3 } tmsmSessionTable OBJECT-TYPE SYNTAX SEQUENCE OF TmsmSessionEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "The table of currently available sessions configured in the SNMP engine's Local Configuration Datastore (LCD). Sessions are created as needed, and do not persist across network management system reboots. " ::= { tmsmSession 4 } tmsmSessionEntry OBJECT-TYPE SYNTAX TmsmSessionEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A session configured in the SNMP engine's Local Configuration Datastore (LCD) for Transport Mapping Security Models. " INDEX { tmsmSessionID } ::= { tmsmSessionTable 1 } TmsmSessionEntry ::= SEQUENCE { tmsmSessionID Integer32, tmsmSessionTransport TransportAddressType, tmsmSessionAddress TransportAddress, tmsmSessionSecurityModel SnmpSecurityModel, tmsmSessionSecurityName SnmpAdminString, tmsmSessionSecurityLevel SnmpSecurityLevel, tmsmSessionEngineID SnmpEngineID Harrington & Schoenwaelder Expires September 5, 2006 [Page 39] Internet-Draft SNMP Transport Mapping Security Model March 2006 } tmsmSessionID OBJECT-TYPE SYNTAX Integer32 (1..65535) MAX-ACCESS not-accessible STATUS current DESCRIPTION "A locally-unique identifier for a session. " ::= { tmsmSessionEntry 1 } tmsmSessionTransport OBJECT-TYPE SYNTAX TransportAddressType MAX-ACCESS read-only STATUS current DESCRIPTION "The transport domain associated with this session. " ::= { tmsmSessionEntry 2 } tmsmSessionAddress OBJECT-TYPE SYNTAX TransportAddress MAX-ACCESS read-only STATUS current DESCRIPTION "The transport address associated with this session. " ::= { tmsmSessionEntry 3 } tmsmSessionSecurityModel OBJECT-TYPE SYNTAX SnmpSecurityModel MAX-ACCESS read-only STATUS current DESCRIPTION "The Security Model associated with this session." ::= { tmsmSessionEntry 4 } tmsmSessionSecurityName OBJECT-TYPE SYNTAX SnmpAdminString MAX-ACCESS read-only STATUS current DESCRIPTION "A human readable string representing the principal in Security Model independent format. The default transformation of the Secure Shell Security Model dependent security ID to the securityName and vice versa is the identity function so that the securityName is the same as the SSH user name. " ::= { tmsmSessionEntry 5 } Harrington & Schoenwaelder Expires September 5, 2006 [Page 40] Internet-Draft SNMP Transport Mapping Security Model March 2006 tmsmSessionSecurityLevel OBJECT-TYPE SYNTAX SnmpSecurityLevel MAX-ACCESS read-only STATUS current DESCRIPTION "The Level of Security at which SNMP messages can be sent using this session, in particular, one of: noAuthNoPriv - without authentication and without privacy, authNoPriv - with authentication but without privacy, authPriv - with authentication and with privacy. " DEFVAL { authPriv } ::= { tmsmSessionEntry 6 } tmsmSessionEngineID OBJECT-TYPE SYNTAX SnmpEngineID MAX-ACCESS read-only STATUS current DESCRIPTION "The administratively-unique identifier for the remote SNMP engine associated with this session. " ::= { tmsmSessionEntry 7 } -- ------------------------------------------------------------- -- tmsmMIB - Conformance Information -- ------------------------------------------------------------- tmsmGroups OBJECT IDENTIFIER ::= { tmsmConformance 1 } tmsmCompliances OBJECT IDENTIFIER ::= { tmsmConformance 2 } -- ------------------------------------------------------------- -- Units of conformance -- ------------------------------------------------------------- tmsmGroup OBJECT-GROUP OBJECTS { tmsmSessionCurrent, tmsmSessionMaxSupported, tmsmSessionTransport, tmsmSessionAddress, tmsmSessionSecurityModel, tmsmSessionSecurityName, tmsmSessionSecurityLevel, tmsmSessionEngineID, tmsmSessionSpinLock Harrington & Schoenwaelder Expires September 5, 2006 [Page 41] Internet-Draft SNMP Transport Mapping Security Model March 2006 } STATUS current DESCRIPTION "A collection of objects for maintaining session information of an SNMP engine which implements the SNMP Secure Shell Security Model. " ::= { tmsmGroups 2 } -- ------------------------------------------------------------- -- Compliance statements -- ------------------------------------------------------------- tmsmCompliance MODULE-COMPLIANCE STATUS current DESCRIPTION "The compliance statement for SNMP engines that support the TMSM-MIB" MODULE MANDATORY-GROUPS { tmsmGroup } ::= { tmsmCompliances 1 } END 15. Implementation Considerations 15.1. Applications that Benefit from Sessions [todo] contributions welcome. There has been discussion of ways SNMP could be extended to better support management/monitoring needs when a network is running just fine. Use of a TCP transport, for example, could enable larger message sizes and more efficient table retrievals. Discussing how to improve SNMP once you have less strict message size constraints is beyond the scope of this document, or that of TMSM- based security models. Applications utilizing TMSM-based security models may want to take advantage of the increased message sizes by sending larger requests and utilizing existing SNMP operations (e.g. getbulk) effectively. However, doing so might have negative impacts on existing SNMP management and the networks that contain them. Harrington & Schoenwaelder Expires September 5, 2006 [Page 42] Internet-Draft SNMP Transport Mapping Security Model March 2006 15.2. Applications that Suffer from Sessions [todo] contributions welcome. 15.2.1. Troubleshooting It has been a long-standing requirement that SNMP be able to work when the network is unstable, to enable network troubleshooting and repair. The UDP approach has been considered to meet that need well, with an assumption that getting small messages through, even if out of order, is better than gettting no messages through. There has been a long debate about whether UDP actually offers better support than TCP when the underlying IP or lower layers are unstable. There has been recent discussion of whether operators actually use SNMP to troubleshoot and repair unstable networks. The need to establish a session before using SNMP to troubleshoot a device may prove problematic in practice. TMSM-based security models should include discussion of how troubleshooting applications might be impacted by the use of the specific security model, and recommend workarounds. This document RECOMMENDS that all TMSM-based security models include a fallback approach, triggered by multiple failed attempts to establish sessions. The default fallback should be to utilize the IETF-Standard USM security model to send a notification, so an administrator can attempt to manually correct the problem. 16. Security Considerations This document describes an architectural approach and multiple proposed configurations that would permit SNMP to utilize transport layer security services. Each section containing a proposal should discuss the security considerations of that approach. [discuss] expand as needed. It is considered desirable by some industry segments that SNMP security models should utilize transport layer security that addresses perfect forward secrecy at least for encryption keys. Perfect forward secrecy guarantees that compromise of long term secret keys does not result in disclosure of past session keys. There are a number of management objects defined in this MIB module with a MAX-ACCESS clause of read-write and/or read-create. Such objects may be considered sensitive or vulnerable in some network environments. The support for SET operations in a non-secure environment without proper protection can have a negative effect on Harrington & Schoenwaelder Expires September 5, 2006 [Page 43] Internet-Draft SNMP Transport Mapping Security Model March 2006 network operations. These are the tables and objects and their sensitivity/vulnerability: o [todo] list the tables and objects and state why they are sensitive. There are no management objects defined in this MIB module that have a MAX-ACCESS clause of read-write and/or read-create. So, if this MIB module is implemented correctly, then there is no risk that an intruder can alter or create any management objects of this MIB module via direct SNMP SET operations. Some of the readable objects in this MIB module (i.e., objects with a MAX-ACCESS other than not-accessible) may be considered sensitive or vulnerable in some network environments. It is thus important to control even GET and/or NOTIFY access to these objects and possibly to even encrypt the values of these objects when sending them over the network via SNMP. These are the tables and objects and their sensitivity/vulnerability: o [todo] list the tables and objects and state why they are sensitive. SNMP versions prior to SNMPv3 did not include adequate security. Even if the network itself is secure (for example by using IPSec), even then, there is no control as to who on the secure network is allowed to access and GET/SET (read/change/create/delete) the objects in this MIB module. It is RECOMMENDED that implementers consider the security features as provided by the SNMPv3 framework (see [RFC3410], section 8), including full support for the SNMPv3 cryptographic mechanisms (for authentication and privacy). Further, deployment of SNMP versions prior to SNMPv3 is NOT RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to enable cryptographic security. It is then a customer/operator responsibility to ensure that the SNMP entity giving access to an instance of this MIB module is properly configured to give access to the objects only to those principals (users) that have legitimate rights to indeed GET or SET (change/create/delete) them. 17. IANA Considerations Harrington & Schoenwaelder Expires September 5, 2006 [Page 44] Internet-Draft SNMP Transport Mapping Security Model March 2006 The MIB module in this document uses the following IANA-assigned OBJECT IDENTIFIER values recorded in the SMI Numbers registry: Descriptor OBJECT IDENTIFIER value ---------- ----------------------- tmsmMIB { mib-2 XXXX } Editor's Note (to be removed prior to publication): the IANA is requested to assign a value for "XXXX" under the 'mib-2' subtree and to record the assignment in the SMI Numbers registry. When the assignment has been made, the RFC Editor is asked to replace "XXXX" (here and in the MIB module) with the assigned value and to remove this note. [discuss] How do we add a new TransportType? 18. Acknowledgments The Integrated Security for SNMP WG would like to thank the following people for their contributions to the process: The authors of submitted security model proposals: Chris Elliot, Wes Hardaker, Dave Harrington, Keith McCloghrie, Kaushik Narayan, Dave Perkins, Joseph Salowey, and Juergen Schoenwaelder. The members of the Protocol Evaluation Team: Uri Blumenthal, Lakshminath Dondeti, Randy Presuhn, and Eric Rescorla. WG members who committed to and performed detailed reviews: Jeffrey Hutzelman 19. References 19.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2222] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. Harrington & Schoenwaelder Expires September 5, 2006 [Page 45] Internet-Draft SNMP Transport Mapping Security Model March 2006 [RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., "Structure of Management Information Version 2 (SMIv2)", STD 58, RFC 2578, April 1999. [RFC2579] McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., "Textual Conventions for SMIv2", STD 58, RFC 2579, April 1999. [RFC2580] McCloghrie, K., Perkins, D., and J. Schoenwaelder, "Conformance Statements for SMIv2", STD 58, RFC 2580, April 1999. [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [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. [RFC3417] Presuhn, R., "Transport Mappings for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3417, December 2002. [RFC3418] Presuhn, R., "Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3418, December 2002. [RFC3419] Daniele, M. and J. Schoenwaelder, "Textual Conventions for Transport Addresses", RFC 3419, December 2002. [RFC3430] Schoenwaelder, J., "Simple Network Management Protocol Over Transmission Control Protocol Transport Mapping", RFC 3430, December 2002. [RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006. Harrington & Schoenwaelder Expires September 5, 2006 [Page 46] Internet-Draft SNMP Transport Mapping Security Model March 2006 [I-D.rescorla-dtls] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", draft-rescorla-dtls-05 (work in progress), June 2005. 19.2. Informative References [RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)", RFC 2712, October 1999. [RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart, "Introduction and Applicability Statements for Internet- Standard Management Framework", RFC 3410, December 2002. [RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network Management Protocol (SNMP) Applications", STD 62, RFC 3413, December 2002. [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2", RFC 4121, July 2005. [I-D.ietf-netconf-ssh] Wasserman, M. and T. Goddard, "Using the NETCONF Configuration Protocol over Secure Shell (SSH)", draft-ietf-netconf-ssh-05 (work in progress), October 2005. [I-D.ietf-tls-srp] Taylor, D., "Using SRP for TLS Authentication", draft-ietf-tls-srp-10 (work in progress), October 2005. Appendix A. Questions about msgFlags: [discuss] many of these questions can be resolved by deciding whether the TMSP or MPSP provides the service of comparing msgFlags (from inside the message) to actual capabilities of the transport layer security (external to the message). It may however be necessary to provide this service for two slightly different purposes depending on whether the message is outgoing (and may need to be checked by the TMSP when a new transport session might be created) or the message is incoming ( the capabilities of the transport layer session are already known, but msgFlags has not been unpacked yet at the TMSP, so the comparison must be done at the MPSP). Of course, we really only need to compare the authflag and the privflag, i.e. the Harrington & Schoenwaelder Expires September 5, 2006 [Page 47] Internet-Draft SNMP Transport Mapping Security Model March 2006 securityLevel, so if we pass the securityLevel between the two stages, then they each have the info they need to do their respective comparisons. There have been a large number of questions about msgFlags in the TMSM approach, mostly concerning the msgFlags value and the actual security provided, and whether msgFlags can be used to initiate per- message or per-session security. A.1. msgFlags versus actual security Using IPSEC, SSH, or SSL/TLS to provide security services "below" the SNMP message, the use of securityName and securityLevel will differ from the USM/VACM approach to SNMP access control. VACM uses the "securityName" and the "securityLevel" to determine if access is allowed. With the SNMPv3 message and USM security model, both securityLevel and securityName are contained in every SNMPv3 message. Any proposal for a security model using IPSEC, SSH, or SSL/TLS needs to specify how this info is made available to the SNMPv3 message processing, and how it is used. One specific case to consider is the relationship between the msgFlags of an SNMPv3 message, and the actual services provided by the lower layer security. For example, if a session is set up with encryption, is the priv bit always (or never) set in the msgFlags field, and is the PDU never (or always) encrypted? Do msgFlags have to match the security services provided by the lower layer, or are the msgFlags ignored and the values from the lower layer used? Is the securityLevel looked at before the security model gets to it.? No. the security model has two parts - the TMSP and the MPSP. The securityLevel is looked at by the TMSP before it gets to the MPSP, but both are parts of the same security model. Would it be legal for the security model to ignore the incoming flags and change them before passing them back up? If it changed them, it wouldn't necessarily be ignoring them. The TMSP should pass both an actual securityLevel applied to the message, and the msgFlags in the SNMP message to the MPSP for consideration related to access control.. The msgFlags parameter in the SNMP message is never changed when processing an incoming message. Would it be legal for the security model to ignore the outgoing flags and change them before passing them out? no; because the two stages are parts of the same security model, either the MPSP should recognize that a securityLevel cannot be met or exceeded, and reject the message during the message-build phase, or the TMSP should determine if it is possible to honor the request. It is possible to apply an increased securityLevel for an outgoing Harrington & Schoenwaelder Expires September 5, 2006 [Page 48] Internet-Draft SNMP Transport Mapping Security Model March 2006 request, but the procedure to do so must be spelled out clearly in the model design. The security model MUST check the incoming security level flags to make sure they matched the transport session setup. and if not drop the message. Yes, mostly. Depending on the model, either the TMSP or the MPSP MUST verify that the actual processing met or exceeded the securityLevel requested by the msgFlags and that it is acceptable to the specific-model processing (or operator configuration) for this different securityLevel to be applied to the message. This is also true (especially) for outgoing messages. You might legally be able to have a authNoPriv message that is actually encrypted via the transport (but not the other way around of course). Yes, a TMSM could define that as the behavior (or permit an operator to specify that is acceptable behavior) when a requested securityLevel cannot be provided, but a stronger securityLevel can be provided. Appendix B. Parameter Table Following is a CSV-formatted matrix useful for tracking data flows into and out of the dispatcher, message, and security subsystems. Import this into your favorite spreadsheet or other CSV-compatible application. You wil need to remove lines feeds from the second and thrid lines, which needed to be wrapped to fit into RFC limits. B.1. ParameterList.csv ,Dispatcher,,,,Messaging,,,Security,, ,sendPDU,returnResponse,processPDU,processResponse ,prepareOutgoingMessage,prepareResponseMessage,prepareDataElements ,generateRequest,processIncoming,generateResponse transportDomain,In,,,,In,,In,,, transportAddress,In,,,,In,,In,,, destTransportDomain,,,,,Out,Out,,,, destTransportAddress,,,,,Out,Out,,,, messageProcessingModel,In,In,In,In,In,In,Out,In,In,In securityModel,In,In,In,In,In,In,Out,In,In,In securityName,In,In,In,In,In,In,Out,In,Out,In Harrington & Schoenwaelder Expires September 5, 2006 [Page 49] Internet-Draft SNMP Transport Mapping Security Model March 2006 securityLevel,In,In,In,In,In,In,Out,In,In,In contextEngineID,In,In,In,In,In,In,Out,,, contextName,In,In,In,In,In,In,Out,,, expectResponse,In,,,,In,,,,, PDU,In,In,In,In,In,In,Out,,, pduVersion,In,In,In,In,In,In,Out,,, statusInfo,Out,In,,In,,In,Out,Out,Out,Out errorIndication,Out,Out,,,,,Out,,, sendPduHandle,Out,,,In,In,,Out,,, maxSizeResponsePDU,,In,In,,,In,Out,,Out, stateReference,,In,In,,,In,Out,,, wholeMessage,,,,,Out,Out,,Out,In,Out messageLength,,,,,Out,Out,,Out,In,Out maxMessageSize,,,,,,,,In,In,In globalData,,,,,,,,In,,In securityEngineID,,,,,,,,In,Out,In scopedPDU,,,,,,,,In,Out,In securityParameters,,,,,,,,Out,,Out securityStateReference,,,,,,,,,Out,In pduType,,,,,,,Out,,, tmStateReference,,,,,,Out,In,,In, Appendix C. Open Issues Harrington & Schoenwaelder Expires September 5, 2006 [Page 50] Internet-Draft SNMP Transport Mapping Security Model March 2006 Appendix D. Change Log NOTE to RFC editor: Please remove this change log before publishing this document as an RFC. Changes from revison -00- changed SSH references from I-Ds to RFCs removed parameters from tmState Reference for DTLS that revealed lower layer info. Added TMSM-MIB module Added Internet-Standard Management Framework boilerplate Added Structure of the MIB Module Added MIB security considerations boilerplate (to be completed) Added IANA Considerations Added ASI Parameter table Added discussion of Sessions Added Open issues and Change Log Rearranged sections Authors' Addresses David Harrington Futurewei Technologies 1700 Alma Dr. Suite 100 Plano, TX 75075 USA Phone: +1 603 436 8634 EMail: dharrington@huawei.com Juergen Schoenwaelder International University Bremen Campus Ring 1 28725 Bremen Germany Phone: +49 421 200-3587 EMail: j.schoenwaelder@iu-bremen.de Full Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors Harrington & Schoenwaelder Expires September 5, 2006 [Page 51] Internet-Draft SNMP Transport Mapping Security Model March 2006 retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Harrington & Schoenwaelder Expires September 5, 2006 [Page 52]