ISMS W. Hardaker Internet-Draft Sparta, Inc. Intended status: Standards Track May 8, 2009 Expires: November 9, 2009 Datagram Transport Layer Security Transport Model for SNMP draft-hardaker-isms-dtls-tm-04.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. 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 November 9, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Hardaker Expires November 9, 2009 [Page 1] Internet-Draft SNMP over DTLS May 2009 Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document describes a Transport Model for the Simple Network Management Protocol (SNMP), that uses the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides authentication and privacy services for SNMP applications. This document describes how the DTLS Transport Model (DTLSTM) implements the needed features of a SNMP Transport Subsystem to make this protection possible in an interoperable way. This transport model is designed to meet the security and operational needs of network administrators, operate in environments where a connectionless (e.g. UDP or SCTP) transport is preferred, and integrates well into existing public keying infrastructures. This document also defines a portion of the Management Information Base (MIB) for monitoring and managing the DTLS Transport Model for SNMP. Hardaker Expires November 9, 2009 [Page 2] Internet-Draft SNMP over DTLS May 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 7 2. The Datagram Transport Layer Security Protocol . . . . . . . . 8 2.1. The DTLS Record Protocol . . . . . . . . . . . . . . . . . 8 2.2. The DTLS Handshake Protocol . . . . . . . . . . . . . . . 9 2.3. SNMP requirements of DTLS . . . . . . . . . . . . . . . . 9 3. How the DTLSTM fits into the Transport Subsystem . . . . . . . 10 3.1. Security Capabilities of this Model . . . . . . . . . . . 11 3.1.1. Threats . . . . . . . . . . . . . . . . . . . . . . . 11 3.1.2. Message Protection . . . . . . . . . . . . . . . . . . 14 3.1.3. DTLS Sessions . . . . . . . . . . . . . . . . . . . . 14 3.2. Security Parameter Passing . . . . . . . . . . . . . . . . 15 3.3. Notifications and Proxy . . . . . . . . . . . . . . . . . 15 4. Elements of the Model . . . . . . . . . . . . . . . . . . . . 16 4.1. Certificates . . . . . . . . . . . . . . . . . . . . . . . 16 4.1.1. The Certificate Infrastructure . . . . . . . . . . . . 16 4.1.2. Provisioning for the Certificate . . . . . . . . . . . 18 4.2. Messages . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.3. SNMP Services . . . . . . . . . . . . . . . . . . . . . . 19 4.3.1. SNMP Services for an Outgoing Message . . . . . . . . 19 4.3.2. SNMP Services for an Incoming Message . . . . . . . . 20 4.4. DTLS Services . . . . . . . . . . . . . . . . . . . . . . 21 4.4.1. Services for Establishing a Session . . . . . . . . . 21 4.4.2. DTLS Services for an Incoming Message . . . . . . . . 22 4.4.3. DTLS Services for an Outgoing Message . . . . . . . . 23 4.5. Cached Information and References . . . . . . . . . . . . 24 4.5.1. DTLS Transport Model Cached Information . . . . . . . 24 5. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 24 5.1. Procedures for an Incoming Message . . . . . . . . . . . . 25 5.1.1. DTLS Processing for Incoming Messages . . . . . . . . 25 5.1.2. Transport Processing for Incoming Messages . . . . . . 26 5.2. Procedures for an Outgoing Message . . . . . . . . . . . . 27 5.3. Establishing a Session . . . . . . . . . . . . . . . . . . 28 5.4. Closing a Session . . . . . . . . . . . . . . . . . . . . 31 6. MIB Module Overview . . . . . . . . . . . . . . . . . . . . . 31 6.1. Structure of the MIB Module . . . . . . . . . . . . . . . 31 6.2. Textual Conventions . . . . . . . . . . . . . . . . . . . 32 6.3. Statistical Counters . . . . . . . . . . . . . . . . . . . 32 6.4. Configuration Tables . . . . . . . . . . . . . . . . . . . 32 6.5. Relationship to Other MIB Modules . . . . . . . . . . . . 32 6.5.1. MIB Modules Required for IMPORTS . . . . . . . . . . . 32 7. MIB Module Definition . . . . . . . . . . . . . . . . . . . . 32 8. Operational Considerations . . . . . . . . . . . . . . . . . . 47 8.1. Sessions . . . . . . . . . . . . . . . . . . . . . . . . . 47 8.2. Notification Receiver Credential Selection . . . . . . . . 48 8.3. contextEngineID Discovery . . . . . . . . . . . . . . . . 48 Hardaker Expires November 9, 2009 [Page 3] Internet-Draft SNMP over DTLS May 2009 9. Security Considerations . . . . . . . . . . . . . . . . . . . 49 9.1. Certificates, Authentication, and Authorization . . . . . 49 9.2. Use with SNMPv1/SNMPv2c Messages . . . . . . . . . . . . . 50 9.3. MIB Module Security . . . . . . . . . . . . . . . . . . . 50 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 51 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 52 12.1. Normative References . . . . . . . . . . . . . . . . . . . 52 12.2. Informative References . . . . . . . . . . . . . . . . . . 54 Appendix A. Target and Notificaton Configuration Example . . . . 54 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 56 Hardaker Expires November 9, 2009 [Page 4] Internet-Draft SNMP over DTLS May 2009 1. Introduction It is important to understand the modular SNMPv3 architecture as defined by [RFC3411] and enhanced by the Transport Subsystem [I-D.ietf-isms-tmsm]. It is also important to understand the terminology of the SNMPv3 architecture in order to understand where the Transport Model described in this document fits into the architecture and how it interacts with the other architecture subsystems. For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to Section 7 of [RFC3410]. This document describes a Transport Model that makes use of the Datagram Transport Layer Security (DTLS) Protocol [RFC4347], the datagram variant of the existing and commonly deployed Transport Layer Security (TLS) protocol [RFC5246], within a transport subsystem [I-D.ietf-isms-tmsm]. The Transport Model in this document is referred to as the Datagram Transport Layer Security Transport Model (DTLSTM). DTLS takes advantage of the X.509 public keying infrastructure [X509]. This transport model is designed to meet the security and operational needs of network administrators, operate in environments where a connectionless (e.g. UDP or SCTP) transport is preferred, and integrate well into existing public keying infrastructures. This document also specifies a portion of the Management Information Base (MIB) to define objects for monitoring and managing the DTLS Transport Model for SNMP. 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]. The diagram shown below gives a conceptual overview of two SNMP entities communicating using the DTLS Transport Model. One entity contains a Command Responder and Notification Originator application, and the other a Command Generator and Notification Responder application. It should be understood that this particular mix of application types is an example only and other combinations are equally as legitimate. +----------------------------------------------------------------+ Hardaker Expires November 9, 2009 [Page 5] Internet-Draft SNMP over DTLS May 2009 | Network | +----------------------------------------------------------------+ ^ ^ ^ ^ |Notifications |Commands |Commands |Notifications +---|---------------------|--------+ +--|---------------|-------------+ | V V | | V V | | +------------+ +------------+ | | +-----------+ +----------+ | | | DTLS | | DTLS | | | | DTLS | | DTLS | | | | Service | | Service | | | | Service | | Service | | | | (Client) | | (Server) | | | | (Client) | | (Server)| | | +------------+ +------------+ | | +-----------+ +----------+ | | ^ ^ | | ^ ^ | | | | | | | | | | +--+----------+ | | +-+--------------+ | | +-----|---------+----+ | | +---|--------+----+ | | | V |LCD | +-------+ | | | V |LCD | +--------+ | | | +------+ +----+ | | | | | +------+ +----+ | | | | | | DTLS | <---------->| Cache | | | | | DTLS | <---->| Cache | | | | | TM | | | | | | | | TM | | | | | | | +------+ | +-------+ | | | +------+ | +--------+ | | |Transport Subsystem | ^ | | |Transport Sub. | ^ | | +--------------------+ | | | +-----------------+ | | | ^ +----+ | | ^ | | | | | | | | | | | v | | | V | | | +-------+ +----------+ +-----+ | | | +-----+ +------+ +-----+ | | | | | |Message | |Sec. | | | | | | | MP | |Sec. | | | | | Disp. | |Processing| |Sub- | | | | |Disp.| | Sub- | |Sub- | | | | | | |Subsystem | |sys. | | | | | | |system| |sys. | | | | | | | | | | | | | | | | | | | | | | | | | | |+---+| | | | | | | | |+---+| | | | | | | +-----+ | || || | | | | | |+----+| || || | | | | <--->|v3MP |<-->||TSM|<-+ | | | <-->|v3MP|<->|TSM|<-+ | | | | | +-----+ | || || | | | | |+----+| || || | | +-------+ | | |+---+| | | +-----+ | | |+---+| | | ^ | | | | | | ^ | | | | | | | +----------+ +-----+ | | | +------+ +-----+ | | +-+------------+ | | +-+------------+ | | ^ ^ | | ^ ^ | | | | | | | | | | v v | | V V | | +-------------+ +--------------+ | | +-----------+ +--------------+ | | | COMMAND | | NOTIFICATION | | | | COMMAND | | NOTIFICATION | | | | RESPONDER | | ORIGINATOR | | | | GENERATOR | | RESPONDER | | | | application | | applications | | | |application| | application | | | +-------------+ +--------------+ | | +-----------+ +--------------+ | | SNMP entity | | SNMP entity | +----------------------------------+ +--------------------------------+ Hardaker Expires November 9, 2009 [Page 6] Internet-Draft SNMP over DTLS May 2009 1.1. Conventions For consistency with SNMP-related specifications, this document favors terminology as defined in STD62 rather than favoring terminology that is consistent with non-SNMP specifications. This is consistent with the IESG decision to not require the SNMPv3 terminology be modified to match the usage of other non-SNMP specifications when SNMPv3 was advanced to Full Standard. Authentication in this document typically refers to the English meaning of "serving to prove the authenticity of" the message, not data source authentication or peer identity authentication. The terms "manager" and "agent" are not used in this document, because in the RFC 3411 architecture [RFC3411], all SNMP entities have the capability of acting in either manager or agent or in both roles depending on the SNMP application types supported in the implementation. Where distinction is required, the application names of Command Generator, Command Responder, Notification Originator, Notification Receiver, and Proxy Forwarder are used. See "SNMP Applications" [RFC3413] for further information. Throughout this document, the terms "client" and "server" are used to refer to the two ends of the DTLS transport connection. The client actively opens the DTLS connection, and the server passively listens for the incoming DTLS connection. Either SNMP entity may act as client or as server, as discussed further below. The User-Based Security Model (USM) [RFC3414] is a mandatory-to- implement Security Model in STD 62. While DTLS and USM frequently refer to a user, the terminology preferred in RFC3411 [RFC3411] and in this memo is "principal". A principal is the "who" on whose behalf services are provided or processing takes place. A principal can be, among other things, an individual acting in a particular role; a set of individuals, with each acting in a particular role; an application or a set of applications, or a combination of these within an administrative domain. Throughout this document, the term "session" is used to refer to a secure association between two DTLS Transport Models that permits the transmission of one or more SNMP messages within the lifetime of the session. 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 [RFC2119]. Hardaker Expires November 9, 2009 [Page 7] Internet-Draft SNMP over DTLS May 2009 2. The Datagram Transport Layer Security Protocol The DTLS protocol is a datagram-compatible variant of the commonly used Transport Layer Security (TLS) protocol. DTLS provides authentication, data message integrity, and privacy at the transport layer. (See [RFC4347]) The primary goals of the DTLS Transport Model are to provide privacy, source authentication and data integrity between two communicating SNMP entities. The DTLS protocol is composed of two layers: the DTLS Record Protocol and the DTLS Handshake Protocol. The following sections provide an overview of these two layers. Please refer to [RFC4347] for a complete description of the protocol. Readers familiar with DTLS can skip Section 2 except for section Section 2.3. 2.1. The DTLS Record Protocol At the lowest layer, layered on top of a datagram transport protocol (e.g. UDP or SCTP) is the DTLS Record Protocol. The DTLS Record Protocol provides security that has three basic properties: o The session can be confidential. Symmetric cryptography is used for data encryption (e.g., AES [AES], DES [DES] etc.). The keys for this symmetric encryption are generated uniquely for each session and are based on a secret negotiated by another protocol (such as the DTLS Handshake Protocol). The Record Protocol can also be used without encryption. o Messages can have data integrity. Message transport includes a message integrity check using a keyed MAC. Secure hash functions (e.g., SHA, MD5, etc.) are used for MAC computations. The Record Protocol can operate without a MAC, but is generally only used in this mode while another protocol is using the Record Protocol as a transport for negotiating security parameters. o Messages are protected against replay. DTLS uses explicit sequence numbers, integrity checks, and a sliding window to protect against replay of messages within a session. DTLS also provides protection against replay of entire sessions. In a properly-implemented keying material exchange, both sides will generate new random numbers for each exchange. This results in different encryption and integrity keys for every session. Hardaker Expires November 9, 2009 [Page 8] Internet-Draft SNMP over DTLS May 2009 2.2. The DTLS Handshake Protocol The DTLS Record Protocol is used for encapsulation of various higher- level protocols. One such encapsulated protocol, the DTLS Handshake Protocol, allows the server and client to authenticate each other and to negotiate an integrity algorithm, an encryption algorithm and cryptographic keys before the application protocol transmits or receives its first octet of data. Only the DTLS client can initiate the handshake protocol. The DTLS Handshake Protocol provides security that has three basic properties: o The peer's identity can be authenticated using asymmetric (public key) cryptography (e.g., RSA [RSA], DSS [DSS], etc.). This authentication can be made optional, but is generally required by at least one of the peers. DTLS supports three authentication modes: authentication of both the server and the client, server authentication with an unauthenticated client, and total anonymity. For authentication of both entities, each entity provides a valid certificate chain leading to an acceptable certificate authority. Each entity is responsible for verifying that the other's certificate is valid and has not expired or been revoked. See [I-D.hodges-server-ident-check] for further details on standardized processing when checking Server certificate identities. o The negotiation of a shared secret is secure: the negotiated secret is unavailable to eavesdroppers, and for any authenticated handshake the secret cannot be obtained, even by an attacker who can place himself in the middle of the session. o The negotiation is not vulnerable to malicious modification: it is infeasible for an attacker to modify negotiation communication without being detected by the parties to the communication. o DTLS uses a stateless cookie exchange to protect against anonymous denial of service attacks and has retransmission timers, sequence numbers, and counters to handle message loss, reordering, and fragmentation. 2.3. SNMP requirements of DTLS To properly support the SNMP over DTLS Transport Model, the DTLS implementation requires the following: o The DTLS Transport Model SHOULD always use authentication of both the server and the client. Hardaker Expires November 9, 2009 [Page 9] Internet-Draft SNMP over DTLS May 2009 o At a minimum the DTLS Transport MUST support authentication of the Command Generator principals to guarantee the authenticity of the securityName. o The DTLS Transport SHOULD support the message encryption to protect sensitive data from eavesdropping attacks. 3. How the DTLSTM fits into the Transport Subsystem A transport model is a component of the Transport Subsystem. The DTLS Transport Model thus fits between the underlying DTLS transport layer and the message dispatcher [RFC3411] component of the SNMP engine and the Transport Subsystem. The DTLS Transport Model will establish a session between itself and the DTLS Transport Model of another SNMP engine. The sending transport model passes unprotected messages from the dispatcher to DTLS to be protected, and the receiving transport model accepts decrypted and authenticated/integrity-checked incoming messages from DTLS and passes them to the dispatcher. After a DTLS Transport model session is established, SNMP messages can conceptually be sent through the session from one SNMP message dispatcher to another SNMP message dispatcher. If multiple SNMP messages are needed to be passed between two SNMP applications they SHOULD be passed through the same session. A DTLSTM implementation engine MAY choose to close a DTLS session to conserve resources. The DTLS Transport Model of an SNMP engine will perform the translation between DTLS-specific security parameters and SNMP- specific, model-independent parameters. The diagram below depicts where the DTLS Transport Model fits into the architecture described in RFC3411 and the Transport Subsystem: +------------------------------+ | Network | +------------------------------+ ^ ^ ^ | | | v v v +-------------------------------------------------------------------+ | +--------------------------------------------------+ | | | Transport Subsystem | +--------+ | | | +-----+ +-----+ +-----+ +-----+ +-------+ | | | | | | | UDP | | TCP | | SSH | |DTLS | . . . | other |<--->| Cache | | Hardaker Expires November 9, 2009 [Page 10] Internet-Draft SNMP over DTLS May 2009 | | | | | | | TM | TM | | | | | | | | | +-----+ +-----+ +-----+ +-----+ +-------+ | +--------+ | | +--------------------------------------------------+ ^ | | ^ | | | | | | | Dispatcher v | | | +--------------+ +---------------------+ +----------------+ | | | | Transport | | Message Processing | | Security | | | | | Dispatch | | Subsystem | | Subsystem | | | | | | | +------------+ | | +------------+ | | | | | | | +->| v1MP |<--->| | USM | | | | | | | | | +------------+ | | +------------+ | | | | | | | | +------------+ | | +------------+ | | | | | | | +->| v2cMP |<--->| | Transport | | | | | | Message | | | +------------+ | | | Security |<--+ | | | Dispatch <---->| +------------+ | | | Model | | | | | | | +->| v3MP |<--->| +------------+ | | | | | | | +------------+ | | +------------+ | | | | PDU Dispatch | | | +------------+ | | | Other | | | | +--------------+ | +->| otherMP |<--->| | Model(s) | | | | ^ | +------------+ | | +------------+ | | | | +---------------------+ +----------------+ | | v | | +-------+-------------------------+---------------+ | | ^ ^ ^ | | | | | | | v v v | | +-------------+ +---------+ +--------------+ +-------------+ | | | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | | | | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | | | | application | | | | applications | | application | | | +-------------+ +---------+ +--------------+ +-------------+ | | ^ ^ | | | | | | v v | | +----------------------------------------------+ | | | MIB instrumentation | SNMP entity | +-------------------------------------------------------------------+ 3.1. Security Capabilities of this Model 3.1.1. Threats The DTLS Transport Model provides protection against the threats identified by the RFC 3411 architecture [RFC3411]: Hardaker Expires November 9, 2009 [Page 11] Internet-Draft SNMP over DTLS May 2009 1. Modification of Information - The modification threat is the danger that some unauthorized entity may alter in-transit SNMP messages generated on behalf of an authorized principal in such a way as to effect unauthorized management operations, including falsifying the value of an object. DTLS provides verification that the content of each received message has not been modified during its transmission through the network, data has not been altered or destroyed in an unauthorized manner, and data sequences have not been altered to an extent greater than can occur non-maliciously. 2. Masquerade - The masquerade threat is the danger that management operations unauthorized for a given principal may be attempted by assuming the identity of another principal that has the appropriate authorizations. The DTLSTM provides for authentication of the Command Generator, Command Responder, Notification Generator, Notification Responder and Proxy Forwarder through the use of X.509 certificates. The masquerade threat can be mitigated against by using an appropriate Access Control Model (ACM) such as the View-based Access Control Module (VACM) [RFC3415]. In addition, it is important to authenticate and verify both the authenticated identity of the DTLS client and the DTLS server to protect against this threat. (See Section 9 for more detail.) 3. Message stream modification - The re-ordering, delay or replay of messages can and does occur through the natural operation of many connectionless transport services. The message stream modification threat is the danger that messages may be maliciously re-ordered, delayed or replayed to an extent which is greater than can occur through the natural operation of connectionless transport services, in order to effect unauthorized management operations. DTLS provides replay protection with a MAC that includes a sequence number. DTLS uses a sliding window protocol with the sequence number for replay protection, see [RFC4347]. The technique used is the same as in IPsec AH/ESP [RFC4302] [RFC4303], by maintaining a bitmap window of received records. Records that are too old to fit in the window and records that have previously been received are silently discarded. The replay detection feature is optional, since packet duplication can also occur naturally due to routing errors and does not necessarily indicate an active attack. Applications may conceivably detect duplicate packets and accordingly modify their data transmission Hardaker Expires November 9, 2009 [Page 12] Internet-Draft SNMP over DTLS May 2009 strategy. 4. Disclosure - The disclosure threat is the danger of eavesdropping on the exchanges between SNMP engines. Protecting against this threat may be required by local policy at the deployment site. Symmetric cryptography (e.g., AES [AES], DES [DES] etc.) can be used by DTLS for data privacy. The keys for this symmetric encryption are generated uniquely for each session and are based on a secret negotiated by another protocol (such as the DTLS Handshake Protocol). 5. Denial of Service - the RFC 3411 architecture [RFC3411] states that denial of service (DoS) attacks need not be addressed by an SNMP security protocol. However, datagram security protocols are susceptible to a variety of denial of service attacks. Two attacks are of particular concern: o An attacker can consume excessive resources on the server by transmitting a series of handshake initiation requests, causing the server to allocate state and potentially to perform expensive cryptographic operations. o An attacker can use the server as an amplifier by sending session initiation messages with a forged source of the victim. The server then sends its next message (in DTLS, a Certificate message, which can be quite large) to the victim machine, thus flooding it. In order to counter both of these attacks, DTLS borrows the stateless cookie technique used by Photuris [RFC2522] and IKEv2 [RFC4306]. When the client sends its ClientHello message to the server, the server MAY respond with a HelloVerifyRequest message. This message contains a stateless cookie generated using the technique of [RFC2522]. The client MUST retransmit the ClientHello with the cookie added. The server then verifies the cookie and proceeds with the handshake only if it is valid. This mechanism forces the attacker/client to be able to receive the cookie, which makes DoS attacks with spoofed IP addresses difficult. This mechanism does not provide any defense against denial of service attacks mounted from valid IP addresses. Implementations are not required to perform the stateless cookie exchange for every DTLS handshakes but in environments where amplification could be an issue it is RECOMMENDED that the cookie exchange is utilized. Hardaker Expires November 9, 2009 [Page 13] Internet-Draft SNMP over DTLS May 2009 3.1.2. Message Protection The RFC 3411 architecture recognizes three levels of security: o without authentication and without privacy (noAuthNoPriv) o with authentication but without privacy (authNoPriv) o with authentication and with privacy (authPriv) The DTLS Transport Model determines from DTLS the identity of the authenticated principal, and the type and address associated with an incoming message, and the DTLS Transport Model provides this information to DTLS for an outgoing message. When an application requests a session for a message, through the cache, the application requests a security level for that session. The DTLS Transport Model MUST ensure that the DTLS session provides security at least as high as the requested level of security. How the security level is translated into the algorithms used to provide data integrity and privacy is implementation-dependent. However, the NULL integrity and encryption algorithms MUST NOT be used to fulfill security level requests for authentication or privacy. Implementations MAY choose to force DTLS to only allow cipher_suites that provide both authentication and privacy to guarantee this assertion. If a suitable interface between the DTLS Transport Model and the DTLS Handshake Protocol is implemented to allow the selection of security level dependent algorithms, for example a security level to cipher_suites mapping table, then different security levels may be utilized by the application. However, different port numbers will need to be used by at least one side of the connection to differentiate between the DTLS sessions. This is the only way to ensured proper selection of a session ID for an incoming DTLS message. The authentication, integrity and privacy algorithms used by the DTLS Protocol [RFC4347] may vary over time as the science of cryptography continues to evolve and the development of DTLS continues over time. Implementers are encouraged to plan for changes in operator trust of particular algorithms and implementations should offer configuration settings for mapping algorithms to SNMPv3 security levels. 3.1.3. DTLS Sessions DTLS sessions are opened by the DTLS Transport Model during the elements of procedure for an outgoing SNMP message. Since the sender Hardaker Expires November 9, 2009 [Page 14] Internet-Draft SNMP over DTLS May 2009 of a message initiates the creation of a DTLS session if needed, the DTLS session will already exist for an incoming message. Implementations MAY choose to instantiate DTLS sessions in anticipation of outgoing messages. This approach might be useful to ensure that a DTLS session to a given target can be established before it becomes important to send a message over the DTLS session. Of course, there is no guarantee that a pre-established session will still be valid when needed. DTLS sessions are uniquely identified within the DTLS Transport Model by the combination of transportDomain, transportAddress, securityName, and requestedSecurityLevel associated with each session. Each unique combination of these parameters MUST have a locally-chosen unique dtlsSessionID associated for active sessions. For further information see Section 4.4 and Section 5. 3.2. Security Parameter Passing For the DTLS server-side, DTLS-specific security parameters (i.e., cipher_suites, X.509 certificate fields, IP address and port) are translated by the DTLS Transport Model into security parameters for the DTLS Transport Model and security model (i.e., securityLevel, securityName, transportDomain, transportAddress). The transport- related and DTLS-security-related information, including the authenticated identity, are stored in a cache referenced by tmStateReference. For the DTLS client-side, the DTLS Transport Model takes input provided by the dispatcher in the sendMessage() Abstract Service Interface (ASI) and input from the tmStateReference cache. The DTLS Transport Model converts that information into suitable security parameters for DTLS and establishes sessions as needed. The elements of procedure in Section 5 discuss these concepts in much greater detail. 3.3. Notifications and Proxy DTLS sessions may be initiated by DTLS clients on behalf of command generators or notification originators. Command generators are frequently operated by a human, but notification originators are usually unmanned automated processes. The targets to whom notifications should be sent is typically determined and configured by a network administrator. The SNMP-TARGET-MIB module [RFC3413] contains objects for defining management targets, including transportDomain, transportAddress, Hardaker Expires November 9, 2009 [Page 15] Internet-Draft SNMP over DTLS May 2009 securityName, securityModel, and securityLevel parameters, for Notification Generator, Proxy Forwarder, and SNMP-controllable Command Generator applications. Transport domains and transport addresses are configured in the snmpTargetAddrTable, and the securityModel, securityName, and securityLevel parameters are configured in the snmpTargetParamsTable. This document defines a MIB module that extends the SNMP-TARGET-MIB's snmpTargetParamsTable to specify a DTLS client-side certificate to use for the connection. When configuring a DTLS target, the snmpTargetAddrTDomain and snmpTargetAddrTAddress parameters in snmpTargetAddrTable should be set to the snmpDTLSUDPDomain or snmpDTLSSCTPDomain object and an appropriate snmpDTLSUDPAddress or snmpDTLSSCTPAddress value respectively. The snmpTargetParamsMPModel column of the snmpTargetParamsTable should be set to a value of 3 to indicate the SNMPv3 message processing model. The snmpTargetParamsSecurityName should be set to an appropriate securityName value and the dtlstmParamsHashType and dtlstmParamsHashValue parameters of the dtlstmParamsTable should be set to values that refer to a locally held certificate to be used. Other parameters, for example cryptographic configuration such as cipher suites to use, must come from configuration mechanisms not defined in this document. The other needed configuration may be configured using SNMP or other implementation-dependent mechanisms (for example, via a CLI). This securityName defined in the snmpTargetParamsSecurityName column will be used by the access control model to authorize any notifications that need to be sent. 4. Elements of the Model This section contains definitions required to realize the DTLS Transport Model defined by this document. Readers familiar with X.509 certificates can skip this section until Section 4.1.2. 4.1. Certificates DTLS makes use of X.509 certificates for authentication of both sides of the transport. This section discusses the use of certificates in DTLS and the its effects on SNMP over DTLS. 4.1.1. The Certificate Infrastructure Users of a public key SHALL be confident that the associated private key is owned by the correct remote subject (person or system) with which an encryption or digital signature mechanism will be used. This confidence is obtained through the use of public key certificates, which are data structures that bind public key values Hardaker Expires November 9, 2009 [Page 16] Internet-Draft SNMP over DTLS May 2009 to subjects. The binding is asserted by having a trusted CA digitally sign each certificate. The CA may base this assertion upon technical means (i.e., proof of possession through a challenge- response protocol), presentation of the private key, or on an assertion by the subject. A certificate has a limited valid lifetime which is indicated in its signed contents. Because a certificate's signature and timeliness can be independently checked by a certificate-using client, certificates can be distributed via untrusted communications and server systems, and can be cached in unsecured storage in certificate-using systems. ITU-T X.509 (formerly CCITT X.509) or ISO/IEC/ITU 9594-8, which was first published in 1988 as part of the X.500 Directory recommendations, defines a standard certificate format [X509] which is a certificate which binds a subject (principal) to a public key value. This was later further documented in [RFC5280]. A X.509 certificate is a sequence of three required fields: tbsCertificate: The field contains the names of the subject and issuer, a public key associated with the subject, a validity period, and other associated information. This field may also contain extension components. signatureAlgorithm: The signatureAlgorithm field contains the identifier for the cryptographic algorithm used by the certificate authority (CA) to sign this certificate. signatureValue: The signatureValue field contains a digital signature computed upon the ASN.1 DER encoded tbsCertificate field. The ASN.1 DER encoded tbsCertificate is used as the input to the signature function. This signature value is then ASN.1 DER encoded as a BIT STRING and included in the Certificate's signature field. By generating this signature, a CA certifies the validity of the information in the tbsCertificate field. In particular, the CA certifies the binding between the public key material and the subject of the certificate. The basic X.509 authentication procedure is as follows: A system is initialized with a number of root certificates that contain the public keys of a number of trusted CAs. When a system receives a X.509 certificate, signed by one of those CAs, the certificate has to be verified. It first checks the signatureValue field by using the public key of the corresponding trusted CA. Then it compares the decrypted information with a digest of the tbsCertificate field. If they match, then the subject in the tbsCertificate field is authenticated. Hardaker Expires November 9, 2009 [Page 17] Internet-Draft SNMP over DTLS May 2009 4.1.2. Provisioning for the Certificate Authentication using DTLS will require that SNMP entities are provisioned with certificates, which are signed by trusted certificate authorities. Furthermore, SNMP entities will most commonly need to be provisioned with root certificates which represent the list of trusted certificate authorities that an SNMP entity can use for certificate verification. SNMP entities MAY also be provisioned with a X.509 certificate revocation mechanism which can be used to verify that a certificate has not been revoked. The authenticated tmSecurityName of the principal is looked up using the dtlstmCertificateToSNTable. This table either: o Maps a certificate's fingerprint hash type and value to a directly specified tmSecurityName. o Identifies a certificate issuer's fingerprint hash type and value and allows child certificate's subjectAltName or CommonName to directly used as the tmSecurityNome. The certificate trust anchors, being either CA certificates or public keys for use by self-signed certificates, must be installed through an out of band trusted mechanism into the server and its authenticity MUST be verified before access is granted. Implementations SHOULD choose to discard any connections for which no potential dtlstmCertificateToSNTable mapping exists before performing certificate verification to avoid expending computational resources associated with certificate verification. The typical enterprise configuration will map the "subjectAltName" component of the tbsCertificate to the DTLSSM specific tmSecurityName. Thus, the authenticated identity can be obtained by the DTLS Transport Model by extracting the subjectAltName from the peer's certificate and the receiving application will have an appropriate tmSecurityName for use by components like an access control model. This setup requires very little configuration: a single row in the dtlstmCertificateToSNTable referencing a certificate authority. An example mapping setup can be found in Appendix A This tmSecurityName may be later translated from a DTLSSM specific tmSecurityName to a SNMP engine securityName by the security model. A security model, like the TSM security model, may perform an identity mapping or a more complex mapping to derive the securityName from the tmSecurityName offered by the DTLS Transport Model. Hardaker Expires November 9, 2009 [Page 18] Internet-Draft SNMP over DTLS May 2009 4.2. Messages As stated in Section 4.1.1 of [RFC4347], each DTLS record must fit within a single DTLS datagram. The DTLSTM SHOULD prohibit SNMP messages from being sent that exceeds the maximum DTLS message. The DTLSTM implementation SHOULD return an error when the DTLS message size would be exceeded and the message won't be sent. 4.3. SNMP Services This section describes the services provided by the DTLS Transport Model with their inputs and outputs. The services are between the Transport Model and the dispatcher. The services are described as primitives of an abstract service interface (ASI) and the inputs and outputs are described as abstract data elements as they are passed in these abstract service primitives. 4.3.1. SNMP Services for an Outgoing Message The dispatcher passes the information to the DTLS Transport Model using the ASI defined in the transport subsystem: statusInformation = sendMessage( IN destTransportDomain -- transport domain to be used IN destTransportAddress -- transport address to be used IN outgoingMessage -- the message to send IN outgoingMessageLength -- its length IN tmStateReference -- reference to transport state ) The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation: An indication of whether the passing of the message was successful. If not it is an indication of the problem. destTransportDomain: The transport domain for the associated destTransportAddress. The Transport Model uses this parameter to determine the transport type of the associated destTransportAddress. This parameter may also be used by the transport subsystem to route the message to the appropriate Transport Model. This document specifies two DTLS based Transport Domains for use: the snmpDTLSUDPDomain and the snmpDTLSSCTPDomain. Hardaker Expires November 9, 2009 [Page 19] Internet-Draft SNMP over DTLS May 2009 destTransportAddress: The transport address of the destination DTLS Transport Model in a format specified by the SnmpDTLSUDPAddress or the SnmpDTLSSCTPAddress TEXTUAL-CONVENTIONs. outgoingMessage: The outgoing message to send to DTLS for encapsulation. outgoingMessageLength: The length of the outgoing message. tmStateReference: A handle/reference to tmSecurityData to be used when securing outgoing messages. 4.3.2. SNMP Services for an Incoming Message The DTLS Transport Model processes the received message from the network using the DTLS service and then passes it to the dispatcher using the following ASI: statusInformation = receiveMessage( IN transportDomain -- origin transport domain IN transportAddress -- origin transport address IN incomingMessage -- the message received IN incomingMessageLength -- its length IN tmStateReference -- reference to transport state ) The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation: An indication of whether the passing of the message was successful. If not it is an indication of the problem. transportDomain: The transport domain for the associated transportAddress. This document specifies two DTLS based Transport Domains for use: the snmpDTLSUDPDomain and the snmpDTLSSCTPDomain. transportAddress: The transport address of the source of the received message in a format specified by the SnmpDTLSUDPAddress or the SnmpDTLSSCTPAddress TEXTUAL-CONVENTION. incomingMessage: The whole SNMP message stripped of all DTLS protection data. Hardaker Expires November 9, 2009 [Page 20] Internet-Draft SNMP over DTLS May 2009 incomingMessageLength: The length of the SNMP message after being processed by DTLS. tmStateReference: A handle/reference to tmSecurityData to be used by the security model. 4.4. DTLS Services This section describes the services provided by the DTLS Transport Model with their inputs and outputs. These services are between the DTLS Transport Model and the DTLS transport layer. The following sections describe services for establishing and closing a session and for passing messages between the DTLS transport layer and the DTLS Transport Model. 4.4.1. Services for Establishing a Session The DTLS Transport Model provides the following ASI to describe the data passed between the Transport Model and the DTLS transport layer for session establishment. statusInformation = -- errorIndication or success openSession( IN destTransportDomain -- transport domain to be used IN destTransportAddress -- transport address to be used IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested OUT dtlsSessionID -- Session identifier for DTLS ) The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation: An indication of whether the process was successful or not. If not, then the status information will include the error indication provided by DTLS. destTransportDomain: The transport domain for the associated destTransportAddress. The DTLS Transport Model uses this parameter to determine the transport type of the associated destTransportAddress. This document specifies two DTLS based Transport Domains for use: the snmpDTLSUDPDomain and the snmpDTLSSCTPDomain. Hardaker Expires November 9, 2009 [Page 21] Internet-Draft SNMP over DTLS May 2009 destTransportAddress: The transport address of the destination DTLS Transport Model in a format specified by the SnmpDTLSUDPAddress or the SnmpDTLSSCTPAddress TEXTUAL-CONVENTION. securityName: The security name representing the principal on whose behalf the message will be sent. securityLevel: The level of security requested by the application. dtlsSessionID: An implementation-dependent session identifier to reference the specific DTLS session. DTLS and UDP do not provide a session de-multiplexing mechanism and it is possible that implementations will only be able to identify a unique session based on a unique combination of source address, destination address, source UDP port number and destination UDP port number. Because of this, when establishing a new sessions implementations MUST use a different UDP source port number for each connection to a remote destination IP-address/port-number combination to ensure the remote entity can properly disambiguate between multiple sessions from a host to the same port on a server. SCTP does provide session de-multiplexing so this restriction is not needed for DTLS/SCTP implementations. The procedural details for establishing a session are further described in Section 5.3. Upon completion of the process the DTLS Transport Model returns status information and, if the process was successful the dtlsSessionID. Other implementation-dependent data from DTLS are also returned. The dtlsSessionID is stored in an implementation- dependent manner and tied to the tmSecurityData for future use of this session. 4.4.2. DTLS Services for an Incoming Message When the DTLS Transport Model invokes the DTLS record layer to verify proper security for the incoming message, it must use the following ASI: statusInformation = -- errorIndication or success dtlsRead( IN dtlsSessionID -- Session identifier for DTLS IN wholeDtlsMsg -- as received on the wire IN wholeDtlsMsgLength -- length as received on the wire OUT incomingMessage -- the whole SNMP message from DTLS OUT incomingMessageLength -- the length of the SNMP message Hardaker Expires November 9, 2009 [Page 22] Internet-Draft SNMP over DTLS May 2009 ) The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation: An indication of whether the process was successful or not. If not, then the status information will include the error indication provided by DTLS. dtlsSessionID: An implementation-dependent session identifier to reference the specific DTLS session. How the DTLS session ID is obtained for each message is implementation-dependent. As an implementation hint, the DTLS Transport Model can examine incoming messages to determine the source IP address, source port number, destination IP address, and destination port number and use these values to look up the local DTLS session ID in the list of active sessions. wholeDtlsMsg: The whole message as received on the wire. wholeDtlsMsgLength: The length of the message as it was received on the wire. incomingMessage: The whole SNMP message stripped of all DTLS privacy and integrity data. incomingMessageLength: The length of the SNMP message stripped of all DTLS privacy and integrity data. 4.4.3. DTLS Services for an Outgoing Message When the DTLS Transport Model invokes the DTLS record layer to encapsulate and transmit a SNMP message, it must use the following ASI. statusInformation = -- errorIndication or success dtlsWrite( IN dtlsSessionID -- Session identifier for DTLS IN outgoingMessage -- the message to send IN outgoingMessageLength -- its length ) The abstract data elements passed as parameters in the abstract service primitives are as follows: Hardaker Expires November 9, 2009 [Page 23] Internet-Draft SNMP over DTLS May 2009 statusInformation: An indication of whether the process was successful or not. If not, then the status information will include the error indication provided by DTLS. dtlsSessionID: An implementation-dependent session identifier to reference the specific DTLS session that the message should be sent using. outgoingMessage: The outgoing message to send to DTLS for encapsulation. outgoingMessageLength: The length of the outgoing message. 4.5. Cached Information and References When performing SNMP processing, there are two levels of state information that may need to be retained: the immediate state linking a request-response pair, and potentially longer-term state relating to transport and security. "Transport Subsystem for the Simple Network Management Protocol" [I-D.ietf-isms-tmsm] defines general requirements for caches and references. 4.5.1. DTLS Transport Model Cached Information The DTLSTM has no specific responsibilities regarding the cached information beyond those discussed in "Transport Subsystem for the Simple Network Management Protocol" [I-D.ietf-isms-tmsm] 5. Elements of Procedure Abstract service interfaces have been defined by RFC 3411 to describe the conceptual data flows between the various subsystems within an SNMP entity. The DTLSTM uses some of these conceptual data flows when communicating between subsystems. These RFC 3411-defined data flows are referred to here as public interfaces. To simplify the elements of procedure, the release of state information is not always explicitly specified. As a general rule, if state information is available when a message gets discarded, the message-state information should also be released. If state information is available when a session is closed, the session state information should also be released. Sensitive information, like cryptographic keys, should be overwritten with zero value or random value data prior to being released. An error indication may return an OID and value for an incremented counter if the information is available at the point where the error Hardaker Expires November 9, 2009 [Page 24] Internet-Draft SNMP over DTLS May 2009 is detected. 5.1. Procedures for an Incoming Message The following section describes the procedures followed by the DTLS Transport Model when it receives a DTLS protected packet. The steps are broken into two different sections. The first section describes the needed steps for de-multiplexing multiple DTLS sessions and the second section describes the steps which are specific to transport processing once the DTLS processing has been completed. 5.1.1. DTLS Processing for Incoming Messages DTLS is significantly different in terms of session handling than SSH, TLS or other TCP-based session streams. The DTLS protocol, which is datagram-based, does not have a session identifier when run over UDP that allows implementations to determine through which session a packet is arriving like SCTP-based and TCP-based streams have. Thus, a process for de-multiplexing sessions when used over UDP must be incorporated into the procedures for an incoming message. The steps in this section describe how this can be accomplished, although any implementation dependent method for doing so should be suitable as long as the results are consistently deterministic. The important results from the steps in this section are the transportDomain, the transportAddress, the wholeMessage, the wholeMessageLength, and a unique implementation-dependent session identifier. This procedure assumes that upon session establishment, an entry in a local transport mapping table is created in the Transport Model's LCD. This transport mapping table entry should be able to map a unique combination of the remote address, remote port number, local address and local port number to a implementation-dependent dtlsSessionID. 1) The DTLS Transport Model examines the raw UDP message, in an implementation-dependent manner. If the message is not a DTLS message then it should be discarded. If the message is not a (D)TLS Application Data message then the message should be processed by the underlying DTLS framework as it is (for example) a session initialization or session modification message and no further steps below should be taken by the DTLS Transport. 2) The DTLS Transport Model queries the LCD using the transport parameters to determine if a session already exists and its dtlsSessionID. As noted previously, the source and destination addresses and ports of the message should uniquely assign the message to a specific session identifier. However, another Hardaker Expires November 9, 2009 [Page 25] Internet-Draft SNMP over DTLS May 2009 implementation-dependent method may be used if so desired. 3) If a matching entry in the LCD does not exist then the message is discarded. Increment the dtlstmSessionNoAvailableSessions counter and stop processing the message. Note that an entry would already exist if the client and server's session establishment procedures had been successfully completed (as described both above and in Section 5.3) even if no message had yet been sent through the newly established session. An entry may not exist, however, if a "rogue" message was routed to the SNMP entity by mistake. An entry might also be missing because of a "broken" session (see operational considerations). 4) Retrieve the dtlsSessionID from the LCD. 5) The dtlsWholeMsg, and the dtlsSessionID are passed to DTLS for integrity checking and decryption using the dtlsRead() ASI. 6) If the message fails integrity checks or other DTLS security processing then the dtlstmDTLSProtectionErrors counter is incremented, the message is discarded and processing of the message is stopped. 7) The output of the dtlsRead results in an incomingMessage and an incomingMessageLength. These results and the dtlsSessionID are used below in the Section 5.1.2 to complete the processing of the incoming message. 5.1.2. Transport Processing for Incoming Messages The procedures in this section describe how the DTLS Transport should process messages that have already been properly extracted from the DTLS stream, as described in Section 5.1.1. 1) Create a tmStateReference cache for the subsequent reference and assign the following values within it: tmTransportDomain = snmpDTLSUDPDomain or snmpDTLSSCTPDomain as appropriate. tmTransportAddress = The address the message originated from, determined in an implementation dependent way. tmSecurityLevel = The derived tmSecurityLevel for the session, as discussed in Section 3.1.2 and Section 5.3. Hardaker Expires November 9, 2009 [Page 26] Internet-Draft SNMP over DTLS May 2009 tmSecurityName = The derived tmSecurityName for the session as discussed in and Section 5.3. This value MUST be constant during the lifetime of the DTLS session. tmSessionID = The dtlsSessionID, which MUST be A unique session identifier for this DTLS session. The contents and format of this identifier are implementation dependent as long as it is unique to the session. A session identifier MUST NOT be reused until all references to it are no longer in use. The tmSessionID is equal to the dtlsSessionID discussed in Section 5.1.1. tmSessionID refers to the session identifier when stored in the tmStateReference and dtlsSessionID refers to the session identifier when stored in the LCD. They MUST always be equal when processing a given session's traffic. 2) The wholeMessage and the wholeMessageLength are assigned values from the incomingMessage and incomingMessageLength values from the DTLS processing. 3) The DTLS Transport Model passes the transportDomain, transportAddress, wholeMessage, and wholeMessageLength to the dispatcher using the receiveMessage ASI: statusInformation = receiveMessage( IN transportDomain -- snmpDTLSUDPDomain or snmpDTLSSCTPDomain IN transportAddress -- address for the received message IN wholeMessage -- the whole SNMP message from DTLS IN wholeMessageLength -- the length of the SNMP message IN tmStateReference -- (NEW) transport info ) 5.2. Procedures for an Outgoing Message The dispatcher sends a message to the DTLS Transport Model using the following ASI: 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 -- (NEW) transport info ) This section describes the procedure followed by the DTLS Transport Hardaker Expires November 9, 2009 [Page 27] Internet-Draft SNMP over DTLS May 2009 Model whenever it is requested through this ASI to send a message. 1) Extract tmSessionID, tmTransportAddress, tmSecurityName, tmRequestedSecurityLevel. and tmSameSecurity from the tmStateReference. Note: The tmSessionID value may be undefined if session exists yet. 2) If tmSameSecurity is true and either tmSessionID is undefined or refers to a session that is no longer open then increment the dtlstmSessionNoAvailableSessions counter, discard the message and return the error indication in the statusInformation. Processing of this message stops. 3) If tmSameSecurity is false and tmSessionID refers to a session that is no longer available then an implementation SHOULD open a new session using the openSession() ASI as described below in step 4b. An implementation MAY choose to return an error to the calling module. 4) If tmSessionID is undefined, then use tmTransportAddress, tmSecurityName and tmRequestedSecurityLevel to see if there is a corresponding entry in the LCD suitable to send the message over. 4a) If there is a corresponding LCD entry, then this session will be used to send the message. 4b) If there is not a corresponding LCD entry, then open a session using the openSession() ASI (discussed further in Section 4.4.1). Implementations MAY wish to offer message buffering to prevent redundant openSession() calls for the same cache entry. If an error is returned from OpenSession(), then discard the message, increment the dtlstmSessionOpenErrors, and return an error indication to the calling module. 5) Using either the session indicated by the tmSessionID if there was one or the session resulting in the previous step, pass the outgoingMessage to DTLS for encapsulation and transmission. 5.3. Establishing a Session The DTLS Transport Model provides the following primitive to establish a new DTLS session (previously discussed in Section 4.4.1): Hardaker Expires November 9, 2009 [Page 28] Internet-Draft SNMP over DTLS May 2009 statusInformation = -- errorIndication or success openSession( IN destTransportDomain -- transport domain to be used IN destTransportAddress -- transport address to be used IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested OUT dtlsSessionID -- Session identifier for DTLS ) The following sections describe the procedures followed by a DTLS Transport Model when establishing a session as a Command Generator, a Notification Originator or as part of a Proxy Forwarder. The following describes the procedure to follow to establish a session between SNMP engines to exchange SNMP messages. This process is followed by any SNMP engine establishing a session for subsequent use. This MAY be done automatically for SNMP messages which are not Response or Report messages. DTLS provides no explicit manner for transmitting an identity the client wishes to connect to during or prior to key exchange to facilitate certificate selection at the server (e.g. at a Notification Receiver). I.E., there is no available mechanism for sending notifications to a specific principal at a given UDP or SCTP port. Therefore, implementations MAY support responding with multiple identities using separate UDP or SCTP port numbers to indicate the desired principal or some other implementation-dependent solution. 1) The client selects the appropriate certificate and cipher_suites for the key agreement based on the tmSecurityName and the tmRequestedSecurityLevel for the session. For sessions being established as a result of a SNMP-TARGET-MIB based operation, the certificate will potentially have been identified via the dtlstmParamsTable mapping and the cipher_suites will have to be taken from system-wide or implementation-specific configuration. Otherwise, the certificate and appropriate cipher_suites will need to be passed to the openSession() ASI as supplemental information or configured through an implementation-dependent mechanism. It is also implementation-dependent and possibly policy-dependent how tmRequestedSecurityLevel will be used to influence the security capabilities provided by the DTLS session. However this is done, the security capabilities provided by DTLS MUST be at least as high as the level of security indicated by the tmRequestedSecurityLevel parameter. The actual security level of the session should be reported in the tmStateReference Hardaker Expires November 9, 2009 [Page 29] Internet-Draft SNMP over DTLS May 2009 cache as tmSecurityLevel. For DTLS to provide strong authentication, each principal acting as a Command Generator SHOULD have its own certificate. 2) Using the destTransportDomain and destTransportAddress values, the client will initiate the DTLS handshake protocol to establish session keys for message integrity and encryption. If the attempt to establish a session is unsuccessful, then dtlstmSessionOpenErrors is incremented, an error indication is returned, and session establishment processing stops. 3) Once the secure session is established and both sides have been authenticated, certificate validation and identity expectations are performed. a) The DTLS server side of the connection identifies the authenticated identity from the DTLS client's principal certificate using the dtlstmCertificateToSNTable mapping table and records this in the tmStateReference cache as tmSecurityName. The details of the lookup process are fully described in the DESCRIPTION clause of the dtlstmCertificateToSNTable MIB object. If this verification fails in any way (for example because of failures in cryptographic verification or the lack of an appropriate row in the dtlstmCertificateToSNTable) then the session establishment MUST fail, the dtlstmSessionInvalidClientCertificates object is incremented and processing is stopped. b) The DTLS client side of the connection SHOULD verify that authenticated identity of the DTLS server's certificate is the expected identity and MUST do so if the client application is a Notification Generator. If strong authentication is desired then the DTLS server certificate MUST always be verified and checked against the expected identity. Methods for doing this are described in [I-D.hodges-server-ident-check]. DTLS provides assurance that the authenticated identity has been signed by a trusted configured certificate authority. If verification of the server's certificate fails in any way (for example because of failures in cryptographic verification or the presented identity was not the expected identity) then the session establishment MUST fail, the dtlstmSessionInvalidServerCertificates object is incremented and processing is stopped. Hardaker Expires November 9, 2009 [Page 30] Internet-Draft SNMP over DTLS May 2009 4) The DTLS-specific session identifier is passed to the DTLS Transport Model and associated with the tmStateReference cache entry to indicate that the session has been established successfully and to point to a specific DTLS session for future use. 5.4. Closing a Session The DTLS Transport Model provides the following primitive to close a session: statusInformation = closeSession( IN tmStateReference -- transport info ) The following describes the procedure to follow to close a session between a client and server. This process is followed by any SNMP engine closing the corresponding SNMP session. 1) Look up the session in the cache and the LCD using the tmStateReference. 2) If there is no session open associated with the tmStateReference, then closeSession processing is completed. 3) Delete the entry from the cache and any other implementation- dependent information in the LCD. 4) Have DTLS close the specified session. This SHOULD include sending a close_notify TLS Alert to inform the other side that session cleanup may be performed. 6. MIB Module Overview This MIB module provides management of the DTLS Transport Model. It defines needed textual conventions, statistical counters and configuration infrastructure necessary for session establishment. Example usage of the configuration tables can be found in Appendix A. 6.1. 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. Hardaker Expires November 9, 2009 [Page 31] Internet-Draft SNMP over DTLS May 2009 6.2. Textual Conventions Generic and Common Textual Conventions used in this module can be found summarized at http://www.ops.ietf.org/mib-common-tcs.html This module defines two new Textual Conventions: a new TransportDomain and TransportAddress format for describing DTLS connection addressing requirements. 6.3. Statistical Counters The DTLSTM-MIB defines some statical counters that can provide network managers with feedback about DTLS session usage and potential errors that a MIB-instrumented device may be experiencing. 6.4. Configuration Tables The DTLSTM-MIB defines configuration tables that a manager can use for help in configuring a MIB-instrumented device for sending and receiving SNMP messages over DTLS. In particular, there is a MIB table that extends the SNMP-TARGET-MIB for configuring certificates to be used and a MIB table for mapping incoming DTLS client certificates to securityNames. 6.5. Relationship to Other MIB Modules Some management objects defined in other MIB modules are applicable to an entity implementing the DTLS Transport Model. In particular, it is assumed that an entity implementing the DTLSTM-MIB will implement the SNMPv2-MIB [RFC3418], the SNMP-FRAMEWORK-MIB [RFC3411], the SNMP-TARGET-MIB [RFC3413], the SNMP-NOTIFICATION-MIB [RFC3413] and the SNMP-VIEW-BASED-ACM-MIB [RFC3415]. This MIB module is for managing DTLS Transport Model information. 6.5.1. MIB Modules Required for IMPORTS The following MIB module imports items from SNMPV2-SMI [RFC2578], SNMPV2-TC [RFC2579], SNMP-FRAMEWORK-MIB [RFC3411], SNMP-TARGET-MIB [RFC3413] and SNMP-CONF [RFC2580]. 7. MIB Module Definition DTLSTM-MIB DEFINITIONS ::= BEGIN IMPORTS Hardaker Expires November 9, 2009 [Page 32] Internet-Draft SNMP over DTLS May 2009 MODULE-IDENTITY, OBJECT-TYPE, OBJECT-IDENTITY, snmpModules, snmpDomains, Counter32, Unsigned32 FROM SNMPv2-SMI TEXTUAL-CONVENTION, TimeStamp, RowStatus, StorageType FROM SNMPv2-TC MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF SnmpAdminString FROM SNMP-FRAMEWORK-MIB snmpTargetParamsEntry FROM SNMP-TARGET-MIB ; dtlstmMIB MODULE-IDENTITY LAST-UPDATED "200807070000Z" ORGANIZATION " " CONTACT-INFO "WG-EMail: Subscribe: Chairs: Co-editors: " DESCRIPTION "The DTLS Transport Model MIB Copyright (C) The IETF Trust (2008). 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 "200807070000Z" DESCRIPTION "The initial version, published in RFC XXXX." -- NOTE to RFC editor: replace XXXX with actual RFC number -- for this document and remove this note ::= { snmpModules xxxx } -- RFC Ed.: replace xxxx with IANA-assigned number and -- remove this note -- ************************************************ -- subtrees of the SNMP-DTLS-TM-MIB -- ************************************************ dtlstmNotifications OBJECT IDENTIFIER ::= { dtlstmMIB 0 } dtlstmObjects OBJECT IDENTIFIER ::= { dtlstmMIB 1 } dtlstmConformance OBJECT IDENTIFIER ::= { dtlstmMIB 2 } Hardaker Expires November 9, 2009 [Page 33] Internet-Draft SNMP over DTLS May 2009 -- ************************************************ -- Objects -- ************************************************ snmpDTLSUDPDomain OBJECT-IDENTITY STATUS current DESCRIPTION "The SNMP over DTLS transport domain. The corresponding transport address is of type SnmpDTLSUDPAddress. When an SNMP entity uses the snmpDTLSUDPDomain transport model, it must be capable of accepting messages up to the maximum MTU size for an interface it supports, minus the needed IP, UDP, DTLS and other protocol overheads. The securityName prefix to be associated with the snmpDTLSUDPDomain is 'dudp'. This prefix may be used by security models or other components to identify what secure transport infrastructure authenticated a securityName." ::= { snmpDomains yy } -- RFC Ed.: replace yy with IANA-assigned number and -- remove this note -- RFC Ed.: replace 'dudp' with the actual IANA assigned prefix string -- if 'dtls' is not assigned to this document. snmpDTLSSCTPDomain OBJECT-IDENTITY STATUS current DESCRIPTION "The SNMP over DTLS transport domain. The corresponding transport address is of type SnmpDTLSSCTPAddress. When an SNMP entity uses the snmpDTLSSCTPDomain transport model, it must be capable of accepting messages up to the maximum MTU size for an interface it supports, minus the needed IP, SCTP, DTLS and other protocol overheads. The securityName prefix to be associated with the snmpDTLSSCTPDomain is 'dsct'. This prefix may be used by security models or other components to identify what secure transport infrastructure authenticated a securityName." ::= { snmpDomains zz } Hardaker Expires November 9, 2009 [Page 34] Internet-Draft SNMP over DTLS May 2009 -- RFC Ed.: replace zz with IANA-assigned number and -- remove this note -- RFC Ed.: replace 'dsct' with the actual IANA assigned prefix string -- if 'dtls' is not assigned to this document. SnmpDTLSUDPAddress ::= TEXTUAL-CONVENTION DISPLAY-HINT "1a" STATUS current DESCRIPTION "Represents a UDP connection address for an IPv4 address, an IPv6 address or an ASCII encoded host name and port number. The hostname must be encoded in ASCII, as specified in RFC3490 (Internationalizing Domain Names in Applications) followed by a colon ':' (ASCII character 0x3A) and a decimal port number in ASCII. The name SHOULD be fully qualified whenever possible. An IPv4 address must be a dotted decimal format followed by a colon ':' (ASCII character 0x3A) and a decimal port number in ASCII. An IPv6 address must be a colon separated format, surrounded by square brackets (ASCII characters 0x5B and 0x5D), followed by a colon ':' (ASCII character 0x3A) and a decimal port number in ASCII. Values of this textual convention may not be directly usable as transport-layer addressing information, and may require run-time resolution. As such, applications that write them must be prepared for handling errors if such values are not supported, or cannot be resolved (if resolution occurs at the time of the management operation). The DESCRIPTION clause of TransportAddress objects that may have snmpDTLSUDPAddress values must fully describe how (and when) such names are to be resolved to IP addresses and vice versa. This textual convention SHOULD NOT be used directly in object definitions since it restricts addresses to a specific format. However, if it is used, it MAY be used either on its own or in conjunction with TransportAddressType or TransportDomain as a pair. When this textual convention is used as a syntax of an index object, there may be issues with the limit of 128 Hardaker Expires November 9, 2009 [Page 35] Internet-Draft SNMP over DTLS May 2009 sub-identifiers specified in SMIv2, STD 58. It is RECOMMENDED that all MIB documents using this textual convention make explicit any limitations on index component lengths that management software must observe. This may be done either by including SIZE constraints on the index components or by specifying applicable constraints in the conceptual row DESCRIPTION clause or in the surrounding documentation." SYNTAX OCTET STRING (SIZE (1..255)) SnmpDTLSSCTPAddress ::= TEXTUAL-CONVENTION DISPLAY-HINT "1a" STATUS current DESCRIPTION "Represents a SCTP connection address for an IPv4 address, an IPv6 address or an ASCII encoded host name and port number. The hostname must be encoded in ASCII, as specified in RFC3490 (Internationalizing Domain Names in Applications) followed by a colon ':' (ASCII character 0x3A) and a decimal port number in ASCII. The name SHOULD be fully qualified whenever possible. An IPv4 address must be a dotted decimal format followed by a colon ':' (ASCII character 0x3A) and a decimal port number in ASCII. An IPv6 address must be a colon separated format, surrounded by square brackets (ASCII characters 0x5B and 0x5D), followed by a colon ':' (ASCII character 0x3A) and a decimal port number in ASCII. Values of this textual convention may not be directly usable as transport-layer addressing information, and may require run-time resolution. As such, applications that write them must be prepared for handling errors if such values are not supported, or cannot be resolved (if resolution occurs at the time of the management operation). The DESCRIPTION clause of TransportAddress objects that may have snmpDTLSSCTPAddress values must fully describe how (and when) such names are to be resolved to IP addresses and vice versa. This textual convention SHOULD NOT be used directly in object definitions since it restricts addresses to a specific format. However, if it is used, it MAY be used either on its own or in conjunction with TransportAddressType or TransportDomain as a pair. Hardaker Expires November 9, 2009 [Page 36] Internet-Draft SNMP over DTLS May 2009 When this textual convention is used as a syntax of an index object, there may be issues with the limit of 128 sub-identifiers specified in SMIv2, STD 58. It is RECOMMENDED that all MIB documents using this textual convention make explicit any limitations on index component lengths that management software must observe. This may be done either by including SIZE constraints on the index components or by specifying applicable constraints in the conceptual row DESCRIPTION clause or in the surrounding documentation." SYNTAX OCTET STRING (SIZE (1..255)) X509IdentifierHashType ::= TEXTUAL-CONVENTION STATUS current DESCRIPTION "Identifies a hashing algorithm type that will be used for identifying an X.509 certificate. The md5(1) value SHOULD NOT be used." SYNTAX INTEGER { md5(1), sha1(2), sha256(3) } X509IdentifierHash ::= TEXTUAL-CONVENTION STATUS current DESCRIPTION "A hash value that uniquely identifies a certificate within a systems local certificate store. The length of the value stored in an object of type X509IdentifierHash is dependent on the hashing algorithm that produced the hash. MIB structures making use of this textual convention should have an accompanying object of type X509IdentifierHashType. " SYNTAX OCTET STRING -- The dtlstmSession Group dtlstmSession OBJECT IDENTIFIER ::= { dtlstmObjects 1 } dtlstmSessionOpens OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times an openSession() request has been executed as an DTLS client, whether it succeeded or failed." ::= { dtlstmSession 1 } dtlstmSessionCloses OBJECT-TYPE SYNTAX Counter32 Hardaker Expires November 9, 2009 [Page 37] Internet-Draft SNMP over DTLS May 2009 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times a closeSession() request has been executed as an DTLS client, whether it succeeded or failed." ::= { dtlstmSession 2 } dtlstmSessionOpenErrors OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times an openSession() request failed to open a session as a DTLS client, for any reason." ::= { dtlstmSession 3 } dtlstmSessionNoAvailableSessions OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times an outgoing message was dropped because the session associated with the passed tmStateReference was no longer (or was never) available." ::= { dtlstmSession 4 } dtlstmSessionInvalidClientCertificates OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times an incoming session was not established on an DTLS server because the presented client certificate was invalid. Reasons for invalidation includes, but is not limited to, cryptographic validation failures and lack of a suitable mapping row in the dtlstmCertificateToSNTable." ::= { dtlstmSession 5 } dtlstmSessionInvalidServerCertificates OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times an outgoing session was not established on an DTLS client because the presented server certificate was invalid. Reasons for invalidation includes, but is not limited to, cryptographic validation failures and an unexpected Hardaker Expires November 9, 2009 [Page 38] Internet-Draft SNMP over DTLS May 2009 presented certificate identity." ::= { dtlstmSession 6 } dtlstmDTLSProtectionErrors OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The number of times DTLS processing resulted in a message being discarded because it failed its integrity test, decryption processing or other DTLS processing." ::= { dtlstmSession 7 } -- Configuration Objects dtlstmConfig OBJECT IDENTIFIER ::= { dtlstmObjects 2 } -- Certificate mapping dtlstmCertificateMapping OBJECT IDENTIFIER ::= { dtlstmConfig 1 } dtlstmCertificateToSNCount OBJECT-TYPE SYNTAX Unsigned32 MAX-ACCESS read-only STATUS current DESCRIPTION "A count of the number of entries in the dtlstmCertificateToSNTable" ::= { dtlstmCertificateMapping 1 } dtlstmCertificateToSNTableLastChanged OBJECT-TYPE SYNTAX TimeStamp MAX-ACCESS read-only STATUS current DESCRIPTION "The value of sysUpTime.0 when the dtlstmCertificateToSNTable was last modified through any means, or 0 if it has not been modified since the command responder was started." ::= { dtlstmCertificateMapping 2 } dtlstmCertificateToSNTable OBJECT-TYPE SYNTAX SEQUENCE OF DtlstmCertificateToSNEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A table listing the X.509 certificates known to the entity and the associated method for determining the SNMPv3 security name from a certificate. Hardaker Expires November 9, 2009 [Page 39] Internet-Draft SNMP over DTLS May 2009 On an incoming DTLS/SNMP connection the client's presented certificate should be examined and validated based on an established trusted CA certificate or self-signed public certificate. This table does not provide a mechanism for uploading the certificates as that is expected to occur through an out-of-band transfer. Once the authenticity of the certificate has been verified, this table can be consulted to determine the appropriate securityName to identify the remote connection. This is done by comparing the issuer's fingerprint hash type and value and the certificate's fingerprint hash type and value against the dtlstmCertHashType and dtlstmCertHashValue values in each entry of this table. If a matching entry is found then the securityName is selected based on the dtlstmCertMapType, dtlstmCertHashType, dtlstmCertHashValue and dtlstmCertSecurityName fields and the resulting securityName is used to identify the other side of the DTLS connection. This table should be treated as an ordered list of mapping rules to check. The first mapping rule appropriately matching a certificate in the local certificate store with a corresponding hash type (dtlstmCertHashType) and hash value (dtlstmCertHashValue) will be used to perform the mapping from X.509 certificate values to a securityName. If, after a matching row is found but the mapping can not succeed for some other reason then further attempts to perform the mapping MUST NOT be taken. For example, if the entry being checked contains a dtlstmCertMapType of bySubjectAltName(2) and an incoming connection uses a certificate with an issuer certificate matching the dtlstmCertHashType and dtlstmCertHashValue fields but the connecting certificate does not contain a subjectAltName field then the lookup operation must be treated as a failure. No further rows are examined for other potential mappings. Missing values of dtlstmCertID are acceptable and implementations should treat missing entries as a failed match and should continue to the next highest numbered row. E.G., the table may legally contain only two rows with dtlstmCertID values of 10 and 20. Users are encouraged to make use of certificates with subjectAltName fields that can be used as securityNames so that a single root CA certificate can allow all child certificate's subjectAltName to map directly to a securityName via a 1:1 transformation. However, this table is flexible enough to allow for situations where existing deployed Hardaker Expires November 9, 2009 [Page 40] Internet-Draft SNMP over DTLS May 2009 certificate infrastructures do not provide adequate subjectAltName values for use as SNMPv3 securityNames. Certificates may also be mapped to securityNames using the CommonName portion of the Subject field which is also a scalable method of mapping certificate components to securityNames. Finally, direct mapping from each individual certificate fingerprint to a securityName is possible but requires one entry in the table per securityName." ::= { dtlstmCertificateMapping 3 } dtlstmCertificateToSNEntry OBJECT-TYPE SYNTAX DtlstmCertificateToSNEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A row in the dtlstmCertificateToSNTable that specifies a mapping for an incoming DTLS certificate to a securityName to use for the connection." INDEX { dtlstmCertID } ::= { dtlstmCertificateToSNTable 1 } DtlstmCertificateToSNEntry ::= SEQUENCE { dtlstmCertID Unsigned32, dtlstmCertHashType X509IdentifierHashType, dtlstmCertHashValue X509IdentifierHash, dtlstmCertMapType INTEGER, dtlstmCertSecurityName SnmpAdminString, dtlstmCertStorageType StorageType, dtlstmCertRowStatus RowStatus } dtlstmCertID OBJECT-TYPE SYNTAX Unsigned32 MAX-ACCESS not-accessible STATUS current DESCRIPTION "A unique arbitrary number index for a given certificate entry." ::= { dtlstmCertificateToSNEntry 1 } dtlstmCertHashType OBJECT-TYPE SYNTAX X509IdentifierHashType MAX-ACCESS read-create STATUS current DESCRIPTION "The hash algorithm to use when applying a hash to a X.509 certificate for purposes of referring to it from the dtlstmCertHashValue column. Hardaker Expires November 9, 2009 [Page 41] Internet-Draft SNMP over DTLS May 2009 The md5(1) value SHOULD NOT be used." DEFVAL { sha256 } ::= { dtlstmCertificateToSNEntry 2 } dtlstmCertHashValue OBJECT-TYPE SYNTAX X509IdentifierHash MAX-ACCESS read-create STATUS current DESCRIPTION "A cryptographic hash of a X.509 certificate. The use of this hash is dictated by the dtlstmCertMapType column. " ::= { dtlstmCertificateToSNEntry 3 } dtlstmCertMapType OBJECT-TYPE SYNTAX INTEGER { specified(1), bySubjectAltName(2), byCN(3) } MAX-ACCESS read-create STATUS current DESCRIPTION "The mapping type used to obtain the securityName from the certificate. The possible values of use and their usage methods are defined as follows: specified(1): The securityName that should be used locally to identify the remote entity is directly specified in the dtlstmCertSecurityName column from this table. The dtlstmCertHashValue MUST refer to a X.509 client certificate that will be mapped directly to the securityName specified in the dtlstmCertSecurityName column. bySubjectAltName(2): The securityName that should be used locally to identify the remote entity should be taken from the subjectAltName portion of the X.509 certificate. The dtlstmCertHashValue MUST refer to a trust anchor certificate that is responsible for issuing certificates with carefully controlled subjectAltName fields. byCN(3): The securityName that should be used locally to identify the remote entity should be taken from the CommonName portion of the Subject field from the X.509 certificate. The dtlstmCertHashValue MUST refer to a trust anchor certificate that is responsible for issuing certificates with carefully controlled CommonName fields." Hardaker Expires November 9, 2009 [Page 42] Internet-Draft SNMP over DTLS May 2009 DEFVAL { specified } ::= { dtlstmCertificateToSNEntry 4 } dtlstmCertSecurityName OBJECT-TYPE SYNTAX SnmpAdminString (SIZE(0..32)) MAX-ACCESS read-create STATUS current DESCRIPTION "The securityName that the session should use if the dtlstmCertMapType is set to specified(1), otherwise the value in this column should be ignored. If dtlstmCertMapType is set to specifed(1) and this column contains a zero-length string (which is not a legal securityName value) this row is effectively disabled and the match will not be considered successful." DEFVAL { "" } ::= { dtlstmCertificateToSNEntry 5 } dtlstmCertStorageType OBJECT-TYPE SYNTAX StorageType MAX-ACCESS read-create STATUS current DESCRIPTION "The storage type for this conceptual row. Conceptual rows having the value 'permanent' need not allow write-access to any columnar objects in the row." DEFVAL { nonVolatile } ::= { dtlstmCertificateToSNEntry 6 } dtlstmCertRowStatus OBJECT-TYPE SYNTAX RowStatus MAX-ACCESS read-create STATUS current DESCRIPTION "The status of this conceptual row. This object may be used to create or remove rows from this table. The value of this object has no effect on whether other objects in this conceptual row can be modified." ::= { dtlstmCertificateToSNEntry 7 } -- Maps securityNames to certificates for use by the SNMP-TARGET-MIB dtlstmParamsCount OBJECT-TYPE SYNTAX Unsigned32 MAX-ACCESS read-only STATUS current Hardaker Expires November 9, 2009 [Page 43] Internet-Draft SNMP over DTLS May 2009 DESCRIPTION "A count of the number of entries in the dtlstmParamsTable" ::= { dtlstmCertificateMapping 4 } dtlstmParamsTableLastChanged OBJECT-TYPE SYNTAX TimeStamp MAX-ACCESS read-only STATUS current DESCRIPTION "The value of sysUpTime.0 when the dtlstmParamsTable was last modified through any means, or 0 if it has not been modified since the command responder was started." ::= { dtlstmCertificateMapping 5 } dtlstmParamsTable OBJECT-TYPE SYNTAX SEQUENCE OF DtlstmParamsEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "This table augments the SNMP-TARGET-MIB's snmpTargetParamsTable with an additional DTLS client-side certificate certificate identifier to use when establishing new DTLS connections." ::= { dtlstmCertificateMapping 6 } dtlstmParamsEntry OBJECT-TYPE SYNTAX DtlstmParamsEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A conceptual row containing a locally held certificate's hash type and hash value for a given snmpTargetParamsEntry. The values in this row should be ignored if the connection that needs to be established, as indicated by the SNMP-TARGET-MIB infrastructure, is not a DTLS based connection." AUGMENTS { snmpTargetParamsEntry } ::= { dtlstmParamsTable 1 } DtlstmParamsEntry ::= SEQUENCE { dtlstmParamsHashType X509IdentifierHashType, dtlstmParamsHashValue X509IdentifierHash, dtlstmParamsStorageType StorageType, dtlstmParamsRowStatus RowStatus } dtlstmParamsHashType OBJECT-TYPE Hardaker Expires November 9, 2009 [Page 44] Internet-Draft SNMP over DTLS May 2009 SYNTAX X509IdentifierHashType MAX-ACCESS read-create STATUS current DESCRIPTION "The hash algorithm type for the hash stored in the dtlstmParamsHash column to identify a locally-held X.509 certificate that should be used when initiating a DTLS connection as a DTLS client." DEFVAL { sha256 } ::= { dtlstmParamsEntry 1 } dtlstmParamsHashValue OBJECT-TYPE SYNTAX X509IdentifierHash MAX-ACCESS read-create STATUS current DESCRIPTION "A cryptographic hash of a X.509 certificate. This object should store the hash of a locally held X.509 certificate that should be used when initiating a DTLS connection as a DTLS client." ::= { dtlstmParamsEntry 2 } dtlstmParamsStorageType OBJECT-TYPE SYNTAX StorageType MAX-ACCESS read-create STATUS current DESCRIPTION "The storage type for this conceptual row. Conceptual rows having the value 'permanent' need not allow write-access to any columnar objects in the row." DEFVAL { nonVolatile } ::= { dtlstmParamsEntry 3 } dtlstmParamsRowStatus OBJECT-TYPE SYNTAX RowStatus MAX-ACCESS read-create STATUS current DESCRIPTION "The status of this conceptual row. This object may be used to create or remove rows from this table. The value of this object has no effect on whether other objects in this conceptual row can be modified." ::= { dtlstmParamsEntry 4 } -- ************************************************ -- dtlstmMIB - Conformance Information Hardaker Expires November 9, 2009 [Page 45] Internet-Draft SNMP over DTLS May 2009 -- ************************************************ dtlstmCompliances OBJECT IDENTIFIER ::= { dtlstmConformance 1 } dtlstmGroups OBJECT IDENTIFIER ::= { dtlstmConformance 2 } -- ************************************************ -- Compliance statements -- ************************************************ dtlstmCompliance MODULE-COMPLIANCE STATUS current DESCRIPTION "The compliance statement for SNMP engines that support the SNMP-DTLS-TM-MIB" MODULE MANDATORY-GROUPS { dtlstmStatsGroup, dtlstmIncomingGroup, dtlstmOutgoingGroup } ::= { dtlstmCompliances 1 } -- ************************************************ -- Units of conformance -- ************************************************ dtlstmStatsGroup OBJECT-GROUP OBJECTS { dtlstmSessionOpens, dtlstmSessionCloses, dtlstmSessionOpenErrors, dtlstmSessionNoAvailableSessions, dtlstmSessionInvalidClientCertificates, dtlstmSessionInvalidServerCertificates, dtlstmDTLSProtectionErrors } STATUS current DESCRIPTION "A collection of objects for maintaining statistical information of an SNMP engine which implements the SNMP DTLS Transport Model." ::= { dtlstmGroups 1 } dtlstmIncomingGroup OBJECT-GROUP OBJECTS { dtlstmCertificateToSNCount, dtlstmCertificateToSNTableLastChanged, dtlstmCertHashType, dtlstmCertHashValue, Hardaker Expires November 9, 2009 [Page 46] Internet-Draft SNMP over DTLS May 2009 dtlstmCertMapType, dtlstmCertSecurityName, dtlstmCertStorageType, dtlstmCertRowStatus } STATUS current DESCRIPTION "A collection of objects for maintaining incoming connection certificate mappings to securityNames of an SNMP engine which implements the SNMP DTLS Transport Model." ::= { dtlstmGroups 2 } dtlstmOutgoingGroup OBJECT-GROUP OBJECTS { dtlstmParamsCount, dtlstmParamsTableLastChanged, dtlstmParamsHashType, dtlstmParamsHashValue, dtlstmParamsStorageType, dtlstmParamsRowStatus } STATUS current DESCRIPTION "A collection of objects for maintaining outgoing connection certificates to use when opening connections as a result of SNMP-TARGET-MIB settings." ::= { dtlstmGroups 3 } END 8. Operational Considerations This section discusses various operational aspects of the solution 8.1. Sessions A session is discussed throughout this document as meaning a security association between the DTLS client and the DTLS server. State information for the sessions are maintained in each DTLSTM and this information is created and destroyed as sessions are opened and closed. Because of the connectionless nature of UDP, a "broken" session, one side up one side down, could result if one side of a session is brought down abruptly (i.e., reboot, power outage, etc.). Whenever possible, implementations SHOULD provide graceful session termination through the use of disconnect messages. Implementations SHOULD also have a system in place for dealing with "broken" Hardaker Expires November 9, 2009 [Page 47] Internet-Draft SNMP over DTLS May 2009 sessions. Implementations SHOULD support the session resumption feature of TLS. To simplify session management it is RECOMMENDED that implementations utilize two separate ports, one for Notification sessions and one for Command sessions. If this implementation recommendation is followed, DTLS clients will always send REQUEST messages and DTLS servers will always send RESPONSE messages. With this assertion, implementations may be able to simplify "broken" session handling, session resumption, and other aspects of session management such as guaranteeing that Request- Response pairs use the same session. Implementations SHOULD limit the lifetime of established sessions depending on the algorithms used for generation of the master session secret, the privacy and integrity algorithms used to protect messages, the environment of the session, the amount of data transferred, and the sensitivity of the data. 8.2. Notification Receiver Credential Selection When an SNMP engine needs to establish an outgoing session for notifications, the snmpTargetParamsTable includes an entry for the snmpTargetParamsSecurityName of the target. However, the receiving SNMP engine (Server) does not know which DTLS certificate to offer to the Client so that the tmSecurityName identity-authentication will be successful. The best solution would be to maintain a one-to-one mapping between certificates and incoming ports for notification receivers, although other implementation dependent mechanisms may be used instead. This can be handled at the Notification Originator by configuring the snmpTargetAddrTable (snmpTargetAddrTDomain and snmpTargetAddrTAddress) and then requiring the receiving SNMP engine to monitor multiple incoming static ports based on which principals are capable of receiving notifications. Implementations MAY also choose to designate a single Notification Receiver Principal to receive all incoming TRAPS and INFORMS. 8.3. contextEngineID Discovery Because most Command Responders have contextEngineIDs that are identical to the USM securityEngineID, the USM provides Command Generators with the ability to discover a default contextEngineID to use. Because the DTLS transport does not make use of a discoverable securityEngineID like the USM does, it may be difficult for Command Generators to discover a suitable default contextEngineID. Implementations should consider offering another engineID discovery mechanism to continue providing Command Generators with a contextEngineID discovery mechanism. A recommended discovery solution is documented in [RFC5343]. Hardaker Expires November 9, 2009 [Page 48] Internet-Draft SNMP over DTLS May 2009 9. Security Considerations This document describes a transport model that permits SNMP to utilize DTLS security services. The security threats and how the DTLS transport model mitigates these threats are covered in detail throughout this document. Security considerations for DTLS are covered in [RFC4347] and security considerations for TLS are described in Section 11 and Appendices D, E, and F of TLS 1.2 [RFC5246]. DTLS adds to the security considerations of TLS only because it is more vulnerable to denial of service attacks. A random cookie exchange was added to the handshake to prevent anonymous denial of service attacks. RFC 4347 recommends that the cookie exchange is utilized for all handshakes and therefore it is RECOMMENDED that implementers also support this cookie exchange. 9.1. Certificates, Authentication, and Authorization Implementations are responsible for providing a security certificate configuration installation . Implementations SHOULD support certificate revocation lists and expiration of certificates or other access control mechanisms. DTLS provides for both authentication of the identity of the DTLS server and authentication of the identity of the DTLS client. Access to MIB objects for the authenticated principal MUST be enforced by an access control subsystem (e.g. the VACM). Authentication of the Command Generator principal's identity is important for use with the SNMP access control subsystem to ensure that only authorized principals have access to potentially sensitive data. The authenticated identity of the Command Generator principal's certificate is mapped to an SNMP model-independent securityName for use with SNMP access control. Furthermore, the DTLS handshake only provides assurance that the certificate of the authenticated identity has been signed by an configured accepted Certificate Authority. DTLS has no way to further authorize or reject access based on the authenticated identity. An Access Control Model (such as the VACM) provides access control and authorization of a Command Generator's requests to a Command Responder and a Notification Responder's authorization to receive Notifications from a Notification Originator. However to avoid man-in-the-middle attacks both ends of the DTLS based connection MUST check the certificate presented by the other side against what was expected. For example, Command Generators must check that the Command Responder presented and authenticated itself with a X.509 certificate that was expected. Not doing so would allow an impostor, at a minimum, to present false data, receive sensitive Hardaker Expires November 9, 2009 [Page 49] Internet-Draft SNMP over DTLS May 2009 information and/or provide a false-positive belief that configuration was actually received and acted upon. Authenticating and verifying the identity of the DTLS server and the DTLS client for all operations ensures the authenticity of the SNMP engine that provides MIB data. The instructions found in the DESCRIPTION clause of the dtlstmCertificateToSNTable object must be followed exactly. Specifically, it is important that if a row matching a certificate or a certificate's issuer is found but the translation to a securityName using the row fails that the lookup process stops and no further rows are consulted. It is also important that the rows of the table be search in order starting with the row containing the lowest numbered dtlstmCertID value. 9.2. Use with SNMPv1/SNMPv2c Messages The SNMPv1 and SNMPv2c message processing described in RFC3484 (BCP 74) [RFC3584] always selects the SNMPv1(1) Security Model for an SNMPv1 message, or the SNMPv2c(2) Security Model for an SNMPv2c message. When running SNMPv1/SNMPv2c over a secure transport like the DTLS Transport Model, the securityName and securityLevel used for access control decisions are then derived from the community string, not the authenticated identity and securityLevel provided by the DTLS Transport Model. 9.3. MIB Module Security The MIB objects in this document must be protected with an adequate level of at least integrity protection, especially those objects which are writable. Since knowledge of authorization rules and certificate usage mechanisms may be considered sensitive, protection from disclosure of the SNMP traffic via encryption is also highly recommended. SNMP versions prior to SNMPv3 did not include adequate security. Even if the network itself is secure (for example by using IPSec or DTLS) 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 section 8 of [RFC3410]), including full support for the USM (see [RFC3414]) and the DTLS Transport Model cryptographic mechanisms (for authentication and privacy). Hardaker Expires November 9, 2009 [Page 50] Internet-Draft SNMP over DTLS May 2009 10. IANA Considerations IANA is requested to assign: 1. a UDP port number in the range 1..1023 in the http://www.iana.org/assignments/port-numbers registry which will be the default port for SNMP command messages over a DTLS/UDP Transport Model as defined in this document, 2. a UDP port number in the range 1..1023 in the http://www.iana.org/assignments/port-numbers registry which will be the default port for SNMP notification messages over a DTLS/ UDP Transport Model as defined in this document, 3. a SCTP port number in the range 1..1023 in the http://www.iana.org/assignments/port-numbers registry which will be the default port for SNMP command messages over a DTLS/SCTP Transport Model as defined in this document, 4. a SCTP port number in the range 1..1023 in the http://www.iana.org/assignments/port-numbers registry which will be the default port for SNMP notification messages over a DTLS/ SCTP Transport Model as defined in this document, 5. an SMI number under snmpDomains for the snmpDTLSUDPDomain object identifier, 6. an SMI number under snmpDomains for the snmpDTLSSCTPDomain object identifier, 7. a SMI number under snmpModules, for the MIB module in this document, 8. "dudp" as the corresponding prefix for the snmpDTLSUDPDomain in the SNMP Transport Model registry, 9. "dsct" as the corresponding prefix for the snmpDTLSSCTPDomain in the SNMP Transport Model registry; 11. Acknowledgements This document closely follows and copies the Secure Shell Transport Model for SNMP defined by David Harrington and Joseph Salowey in [I-D.ietf-isms-secshell]. This document was reviewed by the following people who helped provide useful comments: David Harrington, Alan Luchuk, Ray Purvis. Hardaker Expires November 9, 2009 [Page 51] Internet-Draft SNMP over DTLS May 2009 This work was supported in part by the United States Department of Defense. Large portions of this document are based on work by General Dynamics C4 Systems and the following individuals: Brian Baril, Kim Bryant, Dana Deluca, Dan Hanson, Tim Huemiller, John Holzhauer, Colin Hoogeboom, Dave Kornbau, Chris Knaian, Dan Knaul, Charles Limoges, Steve Moccaldi, Gerardo Orlando, and Brandon Yip. 12. References 12.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2522] Karn, P. and W. Simpson, "Photuris: Session-Key Management Protocol", RFC 2522, March 1999. [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. [RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart, "Introduction and Applicability Statements for Internet- Standard Management Framework", RFC 3410, December 2002. [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. [RFC3413] Levi, D., Meyer, P., and B. Stewart, "Simple Network Management Protocol (SNMP) Applications", STD 62, RFC 3413, December 2002. [RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)", STD 62, RFC 3414, December 2002. [RFC3415] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Hardaker Expires November 9, 2009 [Page 52] Internet-Draft SNMP over DTLS May 2009 Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3415, December 2002. [RFC3418] Presuhn, R., "Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3418, December 2002. [RFC3584] Frye, R., Levi, D., Routhier, S., and B. Wijnen, "Coexistence between Version 1, Version 2, and Version 3 of the Internet-standard Network Management Framework", BCP 74, RFC 3584, August 2003. [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, April 2006. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. [I-D.ietf-isms-transport-security-model] Harington, D., "Transport Security Model for SNMP". [I-D.ietf-isms-tmsm] Harington, D. and J. Schoenwaelder, "Transport Subsystem for the Simple Network Management Protocol (SNMP)". [X509] Rivest, R., Shamir, A., and L. M. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems". [AES] National Institute of Standards, "Specification for the Advanced Encryption Standard (AES)". [DES] National Institute of Standards, "American National Standard for Information Systems-Data Link Encryption". [DSS] National Institute of Standards, "Digital Signature Standard". [RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems". Hardaker Expires November 9, 2009 [Page 53] Internet-Draft SNMP over DTLS May 2009 12.2. Informative References [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [I-D.ietf-isms-secshell] Harington, D. and J. Salowey, "Secure Shell Transport Model for SNMP". [RFC5343] Schoenwaelder, J., "Simple Network Management Protocol (SNMP) Context EngineID Discovery". [I-D.hodges-server-ident-check] Hodges, J. and B. Morgan, "Generic Server Identity Check". Appendix A. Target and Notificaton Configuration Example Configuring the SNMP-TARGET-MIB and NOTIFICATION-MIB along with access control settings for the SNMP-VIEW-BASED-ACM-MIB can be a daunting task without an example to follow. The following section describes an example of what pieces must be in place to accomplish this configuration. The isAccessAllowed() ASI requires configuration to exist in the following SNMP-VIEW-BASED-ACM-MIB tables: vacmSecurityToGroupTable vacmAccessTable vacmViewTreeFamilyTable The only table that needs to be discussed as particularly different here is the vacmSecurityToGroupTable. This table is indexed by both the SNMPv3 security model and the security name. The security model, when DTLSTM is in use, should be set to the value of XXX corresponding to the TSM [I-D.ietf-isms-transport-security-model]. An example vacmSecurityToGroupTable row might be filled out as follows (using a single SNMP SET request): Note to RFC editor: replace XXX in the previous paragraph above with the actual IANA-assigned number for the TSM security model and remove this note. Hardaker Expires November 9, 2009 [Page 54] Internet-Draft SNMP over DTLS May 2009 vacmSecurityModel = XXX (TSM) vacmSecurityName = "blueberry" vacmGroupaName = "administrators" vacmSecurityToGroupStorageType = 3 (nonVolatile) vacmSecurityToGroupStatus = 4 (createAndGo) Note to RFC editor: replace XXX in the vacmSecurityModel line above with the actual IANA-assigned number for the TSM security model and remove this note. This example will assume that the "administrators" group has been given proper permissions via rows in the vacmAccessTable and vacmViewTreeFamilyTable. Depending on whether this VACM configuration is for a Command Responder or a Command Generator the security name "blueberry" will come from a few different locations. For Notification Generator's performing authorization checks, the server's certificate must be verified against the expected certificate before proceeding to send the notification. The securityName be set by the SNMP-TARGET-MIB's snmpTargetParamsSecurityName column or other configuration mechanism and the certificate to use would be taken from the appropriate entry in the dtlstmParamsTable. The dtlstmParamsTable augments the SNMP- TARGET-MIB's snmpTargetParamsTable with client-side certificate information. For Command Responder applications, the vacmSecurityName "blueberry" value is a value that needs to come from an incoming DTLS session. The mapping from a recevied DTLS client certificate to a securityName is done with the dtlstmCertificateToSNTable. The certificates must be loaded into the device so that a dtlstmCertificateToSNEntry may refer to it. As an example, consider the following entry which will provide a mapping from a X.509's hash fingerprint directly to the "blueberry" securityName: dtlstmCertID = 1 (arbitrarily chosen) dtlstmCertHashType = sha256 dtlstmCertHashValue = (appropriate sha256 fingerprint) dtlstmCertMapType = specified(1) dtlstmCertSecurityName = "blueberry" dtlstmCertStorageType = 3 (nonVolatile) dtlstmCertRowStatus = 4 (createAndGo) The above is an example of how to map a particular certificate to a particular securityName. It is recommended that users make use of direct subjectAltName or CommonName mappings where possible since it Hardaker Expires November 9, 2009 [Page 55] Internet-Draft SNMP over DTLS May 2009 will provide a more scalable approach to certificate management. This entry provides an example of using a subjectAltName mapping: dtlstmCertID = 1 (arbitrarily chosen) dtlstmCertHashType = sha256 dtlstmCertHashValue = (appropriate sha256 fingerprint) dtlstmCertMapType = bySubjectAltName(2) dtlstmCertStorageType = 3 (nonVolatile) dtlstmCertRowStatus = 4 (createAndGo) The above entry indicates the subjectAltName field for certificates created by an Issuing certificate with a corresponding hash type and value will be trusted to always produce common names that are directly 1 to 1 mappable into SNMPv3 securityNames. This type of configuration should only be used when the certificate authorities naming conventions are carefully controlled. For the example, if the incoming DTLS client provided certificate contained a subjectAltName of "blueberry" and the certificate was signed by a certificate matching the dtlstmCertHashType and dtlstmCertHashValue values above and the CA's certificate was properly installed on the device then the CommonName of "blueberry" would be used as the securityName for the session. Author's Address Wes Hardaker Sparta, Inc. P.O. Box 382 Davis, CA 95617 US Phone: +1 530 792 1913 Email: ietf@hardakers.net Hardaker Expires November 9, 2009 [Page 56]