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Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The draft header indicates that this document updates RFC3411, but the abstract doesn't seem to directly say this. It does mention RFC3411 though, so this could be OK. -- The draft header indicates that this document updates RFC3412, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document updates RFC3414, but the abstract doesn't seem to mention this, which it should. -- The draft header indicates that this document updates RFC3417, but the abstract doesn't seem to mention this, which it should. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust Copyright Line does not match the current year == Line 404 has weird spacing: '...patcher v ...' 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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 4346 (Obsoleted by RFC 5246) -- Obsolete informational reference (is this intentional?): RFC 4741 (Obsoleted by RFC 6241) == Outdated reference: A later version (-18) exists of draft-ietf-isms-secshell-09 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 13 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Harrington 3 Internet-Draft Huawei Technologies (USA) 4 Updates: 3411,3412,3414,3417 J. Schoenwaelder 5 (if approved) Jacobs University Bremen 6 Intended status: Standards Track November 18, 2007 7 Expires: May 21, 2008 9 Transport Subsystem for the Simple Network Management Protocol (SNMP) 10 draft-ietf-isms-tmsm-11 12 Status of This Memo 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on May 21, 2008. 37 Copyright Notice 39 Copyright (C) The IETF Trust (2007). 41 Abstract 43 This document defines a Transport Subsystem, extending the Simple 44 Network Management Protocol (SNMP) architecture defined in RFC 3411. 45 This document defines a subsystem to contain Transport Models, 46 comparable to other subsystems in the RFC3411 architecture. As work 47 is being done to expand the transport to include secure transport 48 such as SSH and TLS, using a subsystem will enable consistent design 49 and modularity of such Transport Models. This document identifies 50 and describes some key aspects that need to be considered for any 51 Transport Model for SNMP. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 1.1. The Internet-Standard Management Framework . . . . . . . . 3 57 1.2. Where this Extension Fits . . . . . . . . . . . . . . . . 3 58 1.3. Conventions . . . . . . . . . . . . . . . . . . . . . . . 5 59 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 5 60 3. Requirements of a Transport Model . . . . . . . . . . . . . . 7 61 3.1. Message Security Requirements . . . . . . . . . . . . . . 7 62 3.1.1. Security Protocol Requirements . . . . . . . . . . . . 7 63 3.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 8 64 3.2.1. Architectural Modularity Requirements . . . . . . . . 8 65 3.2.2. Access Control Requirements . . . . . . . . . . . . . 11 66 3.2.3. Security Parameter Passing Requirements . . . . . . . 12 67 3.2.4. Separation of Authentication and Authorization . . . . 13 68 3.3. Session Requirements . . . . . . . . . . . . . . . . . . . 14 69 3.3.1. Session Establishment Requirements . . . . . . . . . . 14 70 3.3.2. Session Maintenance Requirements . . . . . . . . . . . 15 71 3.3.3. Message security versus session security . . . . . . . 16 72 4. Scenario Diagrams and the Transport Subsystem . . . . . . . . 17 73 5. Cached Information and References . . . . . . . . . . . . . . 17 74 5.1. securityStateReference . . . . . . . . . . . . . . . . . . 18 75 5.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 18 76 6. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 19 77 6.1. sendMessage ASI . . . . . . . . . . . . . . . . . . . . . 19 78 6.2. Other Outgoing ASIs . . . . . . . . . . . . . . . . . . . 20 79 6.3. The receiveMessage ASI . . . . . . . . . . . . . . . . . . 22 80 6.4. Other Incoming ASIs . . . . . . . . . . . . . . . . . . . 22 81 7. Security Considerations . . . . . . . . . . . . . . . . . . . 24 82 7.1. Coexistence, Security Parameters, and Access Control . . . 25 83 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 84 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26 85 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 86 10.1. Normative References . . . . . . . . . . . . . . . . . . . 26 87 10.2. Informative References . . . . . . . . . . . . . . . . . . 27 88 Appendix A. Why tmStateReference? . . . . . . . . . . . . . . . . 28 89 A.1. Define an Abstract Service Interface . . . . . . . . . . . 28 90 A.2. Using an Encapsulating Header . . . . . . . . . . . . . . 29 91 A.3. Modifying Existing Fields in an SNMP Message . . . . . . . 29 92 A.4. Using a Cache . . . . . . . . . . . . . . . . . . . . . . 29 93 Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . . 30 94 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 30 96 1. Introduction 98 This document defines a Transport Subsystem, extending the Simple 99 Network Management Protocol (SNMP) architecture defined in [RFC3411]. 100 This document identifies and describes some key aspects that need to 101 be considered for any Transport Model for SNMP. 103 1.1. The Internet-Standard Management Framework 105 For a detailed overview of the documents that describe the current 106 Internet-Standard Management Framework, please refer to section 7 of 107 RFC 3410 [RFC3410]. 109 1.2. Where this Extension Fits 111 It is expected that readers of this document will have read RFC3410 112 and RFC3411, and have a general understanding of the functionality 113 defined in RFCs 3412-3418. 115 The "Transport Subsystem" is an additional component for the SNMP 116 Engine depicted in RFC3411, section 3.1. 118 The following diagram depicts its place in the RFC3411 architecture.: 120 +-------------------------------------------------------------------+ 121 | SNMP entity | 122 | | 123 | +-------------------------------------------------------------+ | 124 | | SNMP engine (identified by snmpEngineID) | | 125 | | | | 126 | | +------------+ | | 127 | | | Transport | | | 128 | | | Subsystem | | | 129 | | +------------+ | | 130 | | | | 131 | | +------------+ +------------+ +-----------+ +-----------+ | | 132 | | | Dispatcher | | Message | | Security | | Access | | | 133 | | | | | Processing | | Subsystem | | Control | | | 134 | | | | | Subsystem | | | | Subsystem | | | 135 | | +------------+ +------------+ +-----------+ +-----------+ | | 136 | +-------------------------------------------------------------+ | 137 | | 138 | +-------------------------------------------------------------+ | 139 | | Application(s) | | 140 | | | | 141 | | +-------------+ +--------------+ +--------------+ | | 142 | | | Command | | Notification | | Proxy | | | 143 | | | Generator | | Receiver | | Forwarder | | | 144 | | +-------------+ +--------------+ +--------------+ | | 145 | | | | 146 | | +-------------+ +--------------+ +--------------+ | | 147 | | | Command | | Notification | | Other | | | 148 | | | Responder | | Originator | | | | | 149 | | +-------------+ +--------------+ +--------------+ | | 150 | +-------------------------------------------------------------+ | 151 | | 152 +-------------------------------------------------------------------+ 154 The transport mappings defined in RFC3417 do not provide lower-layer 155 security functionality, and thus do not provide transport-specific 156 security parameters. This document updates RFC3411 and RFC3417 by 157 defining an architectural extension and ASIs that transport mappings 158 (models) can use to pass transport-specific security parameters to 159 other subsystems, including transport-specific security parameters 160 translated into transport-independent securityName and securityLevel 161 parameters 163 The Transport Security Model [I-D.ietf-isms-transport-security-model] 164 and the Secure Shell Transport Model [I-D.ietf-isms-secshell] utilize 165 the Transport Subsystem. The Transport Security Model is an 166 alternative to the existing SNMPv1 Security Model [RFC3584], the 167 SNMPv2c Security Model [RFC3584], and the User-based Secutiry Model 168 [RFC3414]. The Secure Shell Transport Model is an alternative to 169 existing transport mappings (or models) as described in [RFC3417]. 171 1.3. Conventions 173 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 174 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 175 document are to be interpreted as described in RFC 2119 [RFC2119]. 177 Non uppercased versions of the keywords should be read as in normal 178 English. They will usually, but not always, be used in a context 179 relating to compatibility with the RFC3411 architecture or the 180 subsystem defined here, but which might have no impact on on-the-wire 181 compatibility. These terms are used as guidance for designers of 182 proposed IETF models to make the designs compatible with RFC3411 183 subsystems and Abstract Service Interfaces (see section 3.2). 184 Implementers are free to implement differently. Some usages of these 185 lowercase terms are simply normal English usage. 187 For consistency with SNMP-related specifications, this document 188 favors terminology as defined in STD62 rather than favoring 189 terminology that is consistent with non-SNMP specifications that use 190 different variations of the same terminology. This is consistent 191 with the IESG decision to not require the SNMPv3 terminology be 192 modified to match the usage of other non-SNMP specifications when 193 SNMPv3 was advanced to Full Standard. 195 2. Motivation 197 Just as there are multiple ways to secure one's home or business, in 198 a continuum of alternatives, there are multiple ways to secure a 199 network management protocol. Let's consider three general 200 approaches. 202 In the first approach, an individual could sit on his front porch 203 waiting for intruders. In the second approach, he could hire an 204 employee , schedule the employee, position the employee to guard what 205 he wants protected, hire a second guard to cover if the first gets 206 sick, and so on. In the third approach, he could hire a security 207 company, tell them what he wants protected, and they could hire 208 employees, train them, position the guards, schedule the guards, send 209 a replacement when a guard cannot make it, etc., thus providing the 210 desired security, with no significant effort on his part other than 211 identifying requirements and verifying the quality of the service 212 being provided. 214 The User-based Security Model (USM) as defined in [RFC3414] largely 215 uses the first approach - it provides its own security. It utilizes 216 existing mechanisms (e.g., SHA), but provides all the coordination. 217 USM provides for the authentication of a principal, message 218 encryption, data integrity checking, timeliness checking, etc. 220 USM was designed to be independent of other existing security 221 infrastructures. USM therefore requires a separate principal and key 222 management infrastructure. Operators have reported that deploying 223 another principal and key management infrastructure in order to use 224 SNMPv3 is a deterrent to deploying SNMPv3. It is possible to use 225 external mechanisms to handle the distribution of keys for use by 226 USM. The more important issue is that operators wanted to leverage a 227 single user base that wasn't specific to SNMP. 229 A solution based on the second approach might use a USM-compliant 230 architecture, but combine the authentication mechanism with an 231 external mechanism, such as RADIUS [RFC2865], to provide the 232 authentication service. It might be possible to utilize an external 233 protocol to encrypt a message, to check timeliness, to check data 234 integrity, etc. It is difficult to cobble together a number of 235 subcontracted services and coordinate them however, because it is 236 difficult to build solid security bindings between the various 237 services, and potential for gaps in the security is significant. 239 A solution based on the third approach might utilize one or more 240 lower-layer security mechanisms to provide the message-oriented 241 security services required. These would include authentication of 242 the sender, encryption, timeliness checking, and data integrity 243 checking. There are a number of IETF standards available or in 244 development to address these problems through security layers at the 245 transport layer or application layer, among them TLS [RFC4346], SASL 246 [RFC4422], and SSH [RFC4251]. 248 From an operational perspective, it is highly desirable to use 249 security mechanisms that can unify the administrative security 250 management for SNMPv3, command line interfaces (CLIs) and other 251 management interfaces. The use of security services provided by 252 lower layers is the approach commonly used for the CLI, and is also 253 the approach being proposed for NETCONF [RFC4741]. 255 This document defines a Transport Subsystem extension to the RFC3411 256 architecture based on the third approach. This extension specifies 257 how other lower layer protocols with common security infrastructures 258 can be used underneath the SNMP protocol and the desired goal of 259 unified administrative security can be met. 261 This extension allows security to be provided by an external protocol 262 connected to the SNMP engine through an SNMP Transport Model 263 [RFC3417]. Such a Transport Model would then enable the use of 264 existing security mechanisms such as (TLS) [RFC4346] or SSH [RFC4251] 265 within the RFC3411 architecture. 267 There are a number of Internet security protocols and mechanisms that 268 are in wide spread use. Many of them try to provide a generic 269 infrastructure to be used by many different application layer 270 protocols. The motivation behind the Transport Subsystem is to 271 leverage these protocols where it seems useful. 273 There are a number of challenges to be addressed to map the security 274 provided by a secure transport into the SNMP architecture so that 275 SNMP continues to provide interoperability with existing 276 implementations. These challenges are described in detail in this 277 document. For some key issues, design choices are described that 278 might be made to provide a workable solution that meets operational 279 requirements and fits into the SNMP architecture defined in 280 [RFC3411]. 282 3. Requirements of a Transport Model 284 3.1. Message Security Requirements 286 Transport security protocols SHOULD provide protection against the 287 following message-oriented threats [RFC3411]: 289 1. modification of information 290 2. masquerade 291 3. message stream modification 292 4. disclosure 294 These threats are described in section 1.4 of [RFC3411]. It is not 295 required to protect against denial of service or traffic analysis, 296 but it should not make those threats significantly worse. 298 3.1.1. Security Protocol Requirements 300 There are a number of standard protocols that could be proposed as 301 possible solutions within the Transport Subsystem. Some factors 302 SHOULD be considered when selecting a protocol. 304 Using a protocol in a manner for which it was not designed has 305 numerous problems. The advertised security characteristics of a 306 protocol might depend on it being used as designed; when used in 307 other ways, it might not deliver the expected security 308 characteristics. It is recommended that any proposed model include a 309 description of the applicability of the Transport Model. 311 A Transport Model SHOULD require no modifications to the underlying 312 protocol. Modifying the protocol might change its security 313 characteristics in ways that would impact other existing usages. If 314 a change is necessary, the change SHOULD be an extension that has no 315 impact on the existing usages. Any Transport Model SHOULD include a 316 description of potential impact on other usages of the protocol. 318 Transport Models MUST be able to coexist with each other. 320 3.2. SNMP Requirements 322 3.2.1. Architectural Modularity Requirements 324 SNMP version 3 (SNMPv3) is based on a modular architecture (defined 325 in [RFC3411] section 3) to allow the evolution of the SNMP protocol 326 standards over time, and to minimize side effects between subsystems 327 when changes are made. 329 The RFC3411 architecture includes a Security Subsystem for enabling 330 different methods of providing security services, a Message 331 Processing Subsystem permitting different message versions to be 332 handled by a single engine, Applications(s) to support different 333 types of application processors, and an Access Control Subsystem for 334 allowing multiple approaches to access control. The RFC3411 335 architecture does not include a subsystem for Transport Models, 336 despite the fact there are multiple transport mappings already 337 defined for SNMP. This document addresses the need for a Transport 338 Subsystem compatible with the RFC3411 architecture. As work is being 339 done to expand the transport to include secure transport such as SSH 340 and TLS, using a subsystem will enable consistent design and 341 modularity of such Transport Models. 343 The design of this Transport Subsystem accepts the goals of the 344 RFC3411 architecture defined in section 1.5 of [RFC3411]. This 345 Transport Subsystem uses a modular design that will permit Transport 346 Models to be advanced through the standards process independently of 347 other Transport Models, and independent of other modular SNMP 348 components as much as possible. 350 Parameters have been added to the ASIs to pass model-independent 351 transport address information. 353 IETF standards typically require one mandatory to implement solution, 354 with the capability of adding new mechanisms in the future. Part of 355 the motivation of developing Transport Models is to develop support 356 for secure transport protocols, such as a Transport Model that 357 utilizes the Secure Shell protocol. Any Transport Model SHOULD 358 define one minimum-compliance security mechanism, such as 359 certificates, to ensure a basic level of interoperability, but should 360 also be able to support additional existing and new mechanisms. 362 The Transport Subsystem permits multiple transport protocols to be 363 "plugged into" the RFC3411 architecture, supported by corresponding 364 Transport Models, including models that are security-aware. 366 The RFC3411 architecture and the Security Subsystem assume that a 367 Security Model is called by a Message Processing Model and will 368 perform multiple security functions within the Security Subsystem. A 369 Transport Model that supports a secure transport protocol might 370 perform similar security functions within the Transport Subsystem. A 371 Transport Model might perform the translation of transport security 372 parameters to/from security-model-independent parameters. 374 To accommodate this, an implementation-specific cache of transport- 375 specific information will be described (not shown), and the data 376 flows between the Transport Subsystem and the Transport Dispatch, 377 between the Message Dispatch and the Message Processing Subsystem, 378 and between the Message Processing Subsystem and the Security 379 Subsystem will be extended to pass security-model-independent values. 380 New Security Models may also be defined that understand how to work 381 with the modified ASIs and the cache. One such Security Model, the 382 Transport Security Model, is defined in 383 [I-D.ietf-isms-transport-security-model] 385 The following diagram depicts the SNMPv3 architecture including the 386 new Transport Subsystem defined in this document, and a new Transport 387 Security Model defined in [I-D.ietf-isms-transport-security-model]. 389 +------------------------------+ 390 | Network | 391 +------------------------------+ 392 ^ ^ ^ 393 | | | 394 v v v 395 +-------------------------------------------------------------------+ 396 | +--------------------------------------------------+ | 397 | | Transport Subsystem | | 398 | | +-----+ +-----+ +-----+ +-----+ +-------+ | | 399 | | | UDP | | TCP | | SSH | | TLS | . . . | other | | | 400 | | +-----+ +-----+ +-----+ +-----+ +-------+ | | 401 | +--------------------------------------------------+ | 402 | ^ | 403 | | | 404 | Dispatcher v | 405 | +-------------------+ +---------------------+ +----------------+ | 406 | | Transport | | Message Processing | | Security | | 407 | | Dispatch | | Subsystem | | Subsystem | | 408 | | | | +------------+ | | +------------+ | | 409 | | | | +->| v1MP |<--->| | USM | | | 410 | | | | | +------------+ | | +------------+ | | 411 | | | | | +------------+ | | +------------+ | | 412 | | | | +->| v2cMP |<--->| | Transport | | | 413 | | Message | | | +------------+ | | | Security | | | 414 | | Dispatch <--------->| +------------+ | | | Model | | | 415 | | | | +->| v3MP |<--->| +------------+ | | 416 | | | | | +------------+ | | +------------+ | | 417 | | PDU Dispatch | | | +------------+ | | | Other | | | 418 | +-------------------+ | +->| otherMP |<--->| | Model(s) | | | 419 | ^ | +------------+ | | +------------+ | | 420 | | +---------------------+ +----------------+ | 421 | v | 422 | +-------+-------------------------+---------------+ | 423 | ^ ^ ^ | 424 | | | | | 425 | v v v | 426 | +-------------+ +---------+ +--------------+ +-------------+ | 427 | | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | | 428 | | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | | 429 | | application | | | | applications | | application | | 430 | +-------------+ +---------+ +--------------+ +-------------+ | 431 | ^ ^ | 432 | | | | 433 | v v | 434 | +----------------------------------------------+ | 435 | | MIB instrumentation | SNMP entity | 436 +-------------------------------------------------------------------+ 438 3.2.1.1. Processing Differences between USM and Secure Transport 440 USM and secure transports differ in the processing order and 441 responsibilities within the RFC3411 architecture. While the steps 442 are the same, they occur in a different order, and may be done by 443 different subsystems. With USM and some other Security Models, 444 security processing starts when the Message Processing Model decodes 445 portions of the encoded message to extract security parameters and 446 header parameters that identify which Security Model should process 447 the message to perform authentication, decryption, timeliness 448 checking, integrity checking, and translation of parameters to model- 449 independent parameters. A secure transport performs those security 450 functions on the message, before the message is decoded. 452 3.2.1.2. Passing Information between Engines 454 A secure Transport Model will establish an authenticated and/or 455 encrypted tunnel between the Transport Models of two SNMP engines. 456 After a transport layer tunnel is established, then SNMP messages can 457 be sent through the tunnel from one SNMP engine to the other SNMP 458 engine. Transport Models MAY support sending multiple SNMP messages 459 through the same tunnel. 461 3.2.2. Access Control Requirements 463 RFC3411 made some design decisions related to the support of an 464 Access Control Subsystem. These include establishing and passing in 465 a model-independent manner the securityModel, securityName and 466 securityLevel parameters, and separating message authentication from 467 data access authorization. 469 3.2.2.1. securityName and securityLevel Mapping 471 SNMP data access controls are expected to work on the basis of who 472 can perform what operations on which subsets of data, and based on 473 the security services that will be provided to secure the data in 474 transit. The securityModel and securityLevel parameters establish 475 the protections for transit - whether authentication and privacy 476 services will be or have been applied to the message. The 477 securityName is a model-independent identifier of the security 478 "principal", 480 The Message Processing Subsystem relies on a Security Model, such as 481 USM, to play a role in security that goes beyond protecting the 482 message - it provides a mapping between the security-model-specific 483 principal for an incoming message to a security-model independent 484 securityName which can be used for subsequent processing, such as for 485 access control. The securityName is mapped from a mechanism-specific 486 identity, and this mapping must be done for incoming messages by the 487 Security Model before it passes securityName to the Message 488 Processing Model via the processIncoming ASI. 490 A Security Model is also responsible to specify, via the 491 securityLevel parameter, whether incoming messages have been 492 authenticated and/or encrypted, and to ensure that outgoing messages 493 are authenticated and/or encrypted based on the value of 494 securityLevel. 496 A translation from a mechanism-specific identity to a securityName 497 might be done by a Transport Model, and the proposed securityName and 498 a proposed securityLevel might then be made available to a Security 499 Model via the tmStateReference. A Security Model may have multiple 500 sources for determining the principal and desired security services, 501 and a particular Security Model may or may not utilize the 502 securityName mapping and securityLevel made available by the 503 Transport Model when deciding the value of the securityName and 504 securityLevel to be passed to the Message Processing Model. 506 3.2.3. Security Parameter Passing Requirements 508 RFC3411 section 4 describes abstract data flows between the 509 subsystems, models and applications within the architecture. 510 Abstract Service Interfaces describe the flow of data, passing model- 511 independent information between subsystems within an engine. The 512 RFC3411 architecture has no ASI parameters for passing security 513 information between the Transport Subsystem and the dispatcher, or 514 between the dispatcher and the Message Processing Model. This 515 document defines or modifies ASIs for this purpose. 517 A Message Processing Model might unpack SNMP-specific security 518 parameters from an incoming message before calling a specific 519 Security Model to authenticate and decrypt an incoming message, 520 perform integrity checking, and translate security-model-specific 521 parameters into model-independent parameters. When using a secure 522 Transport Model, some security parameters might be provided through 523 means other than carrying them in the SNMP message; some of the 524 parameters for incoming messages might be extracted from the 525 transport layer by the Transport Model before the message is passed 526 to the Message Processing Subsystem. 528 This document describes a cache mechanism (see Section 5), into which 529 the Transport Model puts information about the transport and security 530 parameters applied to a transport connection or an incoming message, 531 and a Security Model may extract that information from the cache. A 532 tmStateReference is passed as an extra parameter in the ASIs of the 533 Transport Subsystem and the Message Processing and Security 534 Subsystems, to identify the relevant cache. This approach of passing 535 a model-independent reference is consistent with the 536 securityStateReference cache already being passed around in the 537 RFC3411 ASIs. 539 For outgoing messages, even when a secure Transport Model will 540 provide the security services, a Message Processing Model might have 541 a Security Model actually create the message from its component 542 parts. Whether there are any security services provided by the 543 Security Model for an outgoing message is security-model-dependent. 544 For incoming messages, even when a secure Transport Model provides 545 security services, a Security Model might provide some security 546 functionality that can only be provided after the message version or 547 other parameters are extracted from the message. 549 3.2.4. Separation of Authentication and Authorization 551 The RFC3411 architecture defines a separation of authentication and 552 the authorization to access and/or modify MIB data. A set of model- 553 independent parameters (securityModel, securityName, and 554 securityLevel) are passed between the Security Subsystem, the 555 applications, and the Access Control Subsystem. 557 This separation was a deliberate decision of the SNMPv3 WG, to allow 558 support for authentication protocols which did not provide data 559 access authorization capabilities, and to support data access 560 authorization schemes, such as VACM, that do not perform their own 561 authentication. This decision also permits different types of data 562 access policies, such as one built on UNIX groups or Windows domains. 563 The VACM approach is based on administrator-defined groups of users. 565 A Message Processing Model determines which Security Model is used, 566 either based on the message version, e.g., SNMPv1 and SNMPv2c, and 567 possibly by a value specified in the message, e.g., SNMPv3. 569 The Security Model makes the decision which securityName and 570 securityLevel values are passed as model-independent parameters to an 571 application, which then passes them via the isAccessAllowed ASI to 572 the Access Control Subsystem. 574 An Access Control Model performs the mapping from the model- 575 independent security parameters to a policy within the Access Control 576 Model that is access-control-model-dependent. 578 A Transport Model does not know which securityModel will be used for 579 an incoming message, so a Transport Model cannot know how the 580 securityName and securityLevel parameters are determined. A 581 Transport Model can provide a mapping from a transport-specific 582 identity and provide candidate values for the securityName and 583 securityLevel, but there is no guarantee the transport-provided 584 values will be used by the Security Model. 586 For example, the SNMPv1 Message Processing Model described in RFC3584 587 always selects the SNMPv1 Security Model. This is true even if the 588 SNMPv1 message was protected in transit using a secure Transport 589 Model, such as one based on SSH or TLS. The SNMPv1 Security Model 590 does not know the tmStateReference exists. 592 3.3. Session Requirements 594 Some secure transports might have a notion of sessions, while other 595 secure transports might provide channels or other session-like 596 mechanism. Throughout this document, the term session is used in a 597 broad sense to cover sessions, channels, and session-like mechanisms. 598 Session refers to an association between two SNMP engines that 599 permits the transmission of one or more SNMP messages within the 600 lifetime of the session. How the session is actually established, 601 opened, closed, or maintained is specific to a particular Transport 602 Model. 604 Sessions are not part of the SNMP architecture defined in [RFC3411], 605 but are considered desirable because the cost of authentication can 606 be amortized over potentially many transactions. 608 The architecture defined in [RFC3411] does not include a session 609 selector in the Abstract Service Interfaces, and neither is that done 610 for the Transport Subsystem, so an SNMP application has no mechanism 611 to select a session using the ASIs except by passing a unique 612 combination of transportDomain, transportAddress, securityName, and 613 securityLevel. Implementers, of course, might provide non-standard 614 mechanisms to select sessions. The transportDomain and 615 transportAddress identify the transport connection to a remote 616 network node; the securityName identifies which security principal to 617 communicate with at that address (e.g., different NMS applications), 618 and the securityLevel might permit selection of different sets of 619 security properties for different purposes (e.g., encrypted SETs vs. 620 non-encrypted GETs). 622 To reduce redundancy, this document describes aspects that are 623 expected to be common to all Transport Model sessions. 625 3.3.1. Session Establishment Requirements 627 SNMP has no mechanism to specify a transport session using the ASIs 628 except by passing a unique combination transportDomain, 629 transportAddress, securityName, and securityLevel to be used to 630 identify a session in a transport-independent manner. SNMP 631 applications provide the transportDomain, transportAddress, 632 securityName, and securityLevel to be used to create a session. 634 For an outgoing message, securityLevel is the requested security for 635 the message, passed in the ASIs. If the Transport Model cannot 636 provide at least the requested level of security, the Transport Model 637 SHOULD discard the message and notify the dispatcher that 638 establishing a session and sending the message failed. 640 A Transport Model determines whether an appropriate session exists 641 (transportDomain, transportAddress, securityName, and securityLevel) 642 for an outgoing message. If an appropriate session does not yet 643 exist, the Transport Model attempts to establish a session for 644 delivery . If a session cannot be established then the message is 645 discarded and the dispatcher should be notified that sending the 646 message failed. 648 Transport session establishment might require provisioning 649 authentication credentials at an engine, either statically or 650 dynamically. How this is done is dependent on the transport model 651 and the implementation. 653 The Transport Subsystem has no knowledge of pduType, so cannot 654 distinguish between a session created to carry different pduTypes. 655 To differentiate a session established for different purposes, such 656 as a notification session versus a request-response session, an 657 application can use different securityNames or transport addresses. 658 For example, in SNMPv1, UDP ports 161 and 162 were used to 659 differentiate types of traffic. New transport models may define a 660 single well-known port for all traffic types. Administrators might 661 choose to define one port for SNMP request-response traffic, but 662 configure notifications to be sent to a different port. 664 3.3.2. Session Maintenance Requirements 666 A Transport Model can tear down sessions as needed. It might be 667 necessary for some implementations to tear down sessions as the 668 result of resource constraints, for example. 670 The decision to tear down a session is implementation-dependent. 671 While it is possible for an implementation to automatically tear down 672 each session once an operation has completed, this is not recommended 673 for anticipated performance reasons. How an implementation 674 determines that an operation has completed, including all potential 675 error paths, is implementation-dependent. 677 The elements of procedure describe when cached information can be 678 discarded, in some circumstances, and the timing of cache cleanup 679 might have security implications, but cache memory management is an 680 implementation issue. 682 If a Transport Model defines MIB module objects to maintain session 683 state information, then the Transport Model MUST define what SHOULD 684 happen to the objects when a related session is torn down, since this 685 will impact interoperability of the MIB module. 687 3.3.3. Message security versus session security 689 A Transport Model session is associated with state information that 690 is maintained for its lifetime. This state information allows for 691 the application of various security services to multiple messages. 692 Cryptographic keys associated with the transport session SHOULD be 693 used to provide authentication, integrity checking, and encryption 694 services, as needed, for data that is communicated during the 695 session. The cryptographic protocols used to establish keys for a 696 Transport Model session SHOULD ensure that fresh new session keys are 697 generated for each session. In addition sequence information might 698 be maintained in the session which can be used to prevent the replay 699 and reordering of messages within a session. If each session uses 700 new keys, then a cross-session replay attack will be unsuccessful; 701 that is, an attacker cannot successfully replay on one session a 702 message he observed from another session. A good security protocol 703 will also protect against replay attacks _within_ a session; that is, 704 an attacker cannot successfully replay a message observed earlier in 705 the same session. 707 A Transport Model session will have a single transportDomain, 708 transportAddress, securityName and securityLevel associated with it. 709 If an exchange between communicating engines requires a different 710 securityLevel or is on behalf of a different securityName, then 711 another session would be needed. An immediate consequence of this is 712 that implementations SHOULD be able to maintain some reasonable 713 number of concurrent sessions. 715 For Transport Models, securityName should be specified during session 716 setup, and associated with the session identifier. 718 SNMPv3 was designed to support multiple levels of security, 719 selectable on a per-message basis by an SNMP application, because, 720 for example, there is not much value in using encryption for a 721 Commander Generator to poll for potentially non-sensitive performance 722 data on thousands of interfaces every ten minutes; the encryption 723 might add significant overhead to processing of the messages. 725 Some Transport Models might support only specific authentication and 726 encryption services, such as requiring all messages to be carried 727 using both authentication and encryption, regardless of the security 728 level requested by an SNMP application. A Transport Model may 729 upgrade the requested security level, i.e. noAuthNoPriv and 730 authNoPriv MAY be sent over an authenticated and encrypted session. 732 4. Scenario Diagrams and the Transport Subsystem 734 RFC3411 section 4.6.1 and 4.6.2 provide scenario diagrams to 735 illustrate how an outgoing message is created, and how an incoming 736 message is processed. RFC3411 does not define ASIs for "Send SNMP 737 Request Message to Network" or "Receive SNMP Response Message from 738 Network", and does not define ASIs for "Receive SNMP Message from 739 Network" or "Send SNMP message to Network". 741 This document defines a sendMessage ASI to send SNMP messages to the 742 network, regardless of pduType, and a receiveMessage ASI to receive 743 SNMP messages from the network, regardless of pduType. 745 5. Cached Information and References 747 The RFC3411 architecture uses caches to store dynamic model-specific 748 information, and uses references in the ASIs to indicate in a model- 749 independent manner which cached information flows between subsystems. 751 There are two levels of state that might need to be maintained: the 752 security state in a request-response pair, and potentially long-term 753 state relating to transport and security. 755 This state is maintained in caches. To simplify the elements of 756 procedure, the release of state information is not always explicitly 757 specified. As a general rule, if state information is available when 758 a message being processed gets discarded, the state related to that 759 message should also be discarded, and if state information is 760 available when a relationship between engines is severed, such as the 761 closing of a transport session, the state information for that 762 relationship might also be discarded. 764 This document differentiates the tmStateReference from the 765 securityStateReference. This document does not specify an 766 implementation strategy, only an abstract description of the data 767 that flows between subsystems. An implementation might use one cache 768 and one reference to serve both functions, but an implementer must be 769 aware of the cache-release issues to prevent the cache from being 770 released before a security or Transport Model has had an opportunity 771 to extract the information it needs. 773 5.1. securityStateReference 775 The securityStateReference parameter is defined in RFC3411. 776 securityStateReference is not accessible to models of the Transport 777 Subsystem. 779 5.2. tmStateReference 781 For each transport session, information about the message security is 782 stored in a cache to pass model- and mechanism-specific parameters. 783 The state referenced by tmStateReference may be saved across multiple 784 messages, in a Local Configuration Datastore (LCD), as compared to 785 securityStateReference which is usually only saved for the life of a 786 request-response pair of messages. 788 For security reasons, if a secure transport session is closed between 789 the time a request message is received and the corresponding response 790 message is sent, then the response message SHOULD be discarded, even 791 if a new session has been established. The SNMPv3 WG decided that 792 this should be a SHOULD architecturally, and it is a security-model- 793 specific decision whether to REQUIRE this. 795 Since a transport model does not know whether a message contains a 796 response, and transport session information is transport-model- 797 specific, the tmStateReference contains two pieces of information for 798 performing the request-response transport session pairing. 800 Each transport model that supports sessions and supports the 801 tmStateReference cache SHOULD include a transport-specific session 802 identifier in the cache for an incoming message, so that if a 803 security model requests the same session, the transport model can 804 determine whether the current existing session is the same as the 805 session used for the incoming request. 807 Each Security Model that supports the tmStateReference cache SHOULD 808 pass a tmSameSession parameter in the tmStateReference cache for 809 outgoing messages to indicate whether the same session MUST be used 810 for the outgoing message as was used for the corresponding incoming 811 message. 813 If the same session requirement is indicated by the security model, 814 but the session identified in the tmStateReference does not match the 815 current established transport session, i.e., it is not the same 816 session, then the message MUST be discarded, and the dispatcher 817 should be notified the sending of the message failed. 819 Since the contents of a cache are meaningful only within an 820 implementation, and not on-the-wire, the format of the cache and the 821 LCD are implementation-specific. 823 6. Abstract Service Interfaces 825 Abstract service interfaces have been defined by RFC 3411 to describe 826 the conceptual data flows between the various subsystems within an 827 SNMP entity, and to help keep the subsystems independent of each 828 other except for the common parameters. 830 This document follows the example of RFC3411 regarding the release of 831 state information, and regarding error indications. 833 1) The release of state information is not always explicitly 834 specified in a transport model. As a general rule, if state 835 information is available when a message gets discarded, the message- 836 state information should also be released, and if state information 837 is available when a session is closed, the session state information 838 should also be released. Note that keeping sensitive security 839 information longer than necessary might introduce potential 840 vulnerabilities to an implementation. 842 2) An error indication in statusInformation may include an OID and 843 value for an incremented counter and a value for securityLevel, and 844 values for contextEngineID and contextName for the counter, and the 845 securityStateReference if the information is available at the point 846 where the error is detected. 848 6.1. sendMessage ASI 850 The sendMessage ASI is used to pass a message from the Dispatcher to 851 the appropriate Transport Model for sending. 853 In the diagram in section 4.6.1 of RFC 3411, the sendMessage ASI 854 replaces the text "Send SNMP Request Message to Network". In section 855 4.6.2, the sendMessage ASI replaces the text "Send SNMP Message to 856 Network" 858 If present and valid, the tmStateReference refers to a cache 859 containing transport-model-specific parameters for the transport and 860 transport security. How the information in the cache is used is 861 transport-model-dependent and implementation-dependent. How a 862 tmStateReference is determined to be present and valid is 863 implementation-dependent. 865 This may sound underspecified, but a transport model might be 866 something like SNMP over UDP over IPv6, where no security is 867 provided, so it might have no mechanisms for utilizing a securityName 868 and securityLevel. 870 statusInformation = 871 sendMessage( 872 IN destTransportDomain -- transport domain to be used 873 IN destTransportAddress -- transport address to be used 874 IN outgoingMessage -- the message to send 875 IN outgoingMessageLength -- its length 876 IN tmStateReference -- reference to transport state 877 ) 879 6.2. Other Outgoing ASIs 881 A tmStateReference parameter has been added to the 882 prepareOutgoingMessage, prepareResponseMessage, generateRequestMsg, 883 and generateResponseMsg ASIs as an OUT parameter. The 884 transportDomain and transportAddress parameters have been added to 885 the generateRequestMsg, and generateResponseMsg ASIs as IN parameters 886 (not shown). 888 statusInformation = -- success or errorIndication 889 prepareOutgoingMessage( 890 IN transportDomain -- transport domain to be used 891 IN transportAddress -- transport address to be used 892 IN messageProcessingModel -- typically, SNMP version 893 IN securityModel -- Security Model to use 894 IN securityName -- on behalf of this principal 895 IN securityLevel -- Level of Security requested 896 IN contextEngineID -- data from/at this entity 897 IN contextName -- data from/in this context 898 IN pduVersion -- the version of the PDU 899 IN PDU -- SNMP Protocol Data Unit 900 IN expectResponse -- TRUE or FALSE 901 IN sendPduHandle -- the handle for matching 902 incoming responses 903 OUT destTransportDomain -- destination transport domain 904 OUT destTransportAddress -- destination transport address 905 OUT outgoingMessage -- the message to send 906 OUT outgoingMessageLength -- its length 907 OUT tmStateReference -- (NEW) reference to transport state 908 ) 910 statusInformation = -- success or errorIndication 911 prepareResponseMessage( 912 IN messageProcessingModel -- typically, SNMP version 913 IN securityModel -- Security Model to use 914 IN securityName -- on behalf of this principal 915 IN securityLevel -- Level of Security requested 916 IN contextEngineID -- data from/at this entity 917 IN contextName -- data from/in this context 918 IN pduVersion -- the version of the PDU 919 IN PDU -- SNMP Protocol Data Unit 920 IN maxSizeResponseScopedPDU -- maximum size able to accept 921 IN stateReference -- reference to state information 922 -- as presented with the request 923 IN statusInformation -- success or errorIndication 924 -- error counter OID/value if error 925 OUT destTransportDomain -- destination transport domain 926 OUT destTransportAddress -- destination transport address 927 OUT outgoingMessage -- the message to send 928 OUT outgoingMessageLength -- its length 929 OUT tmStateReference -- (NEW) reference to transport state 930 ) 932 The tmStateReference parameter of generateRequestMsg or 933 generateResponseMsg is passed in the OUT parameters of the Security 934 Subsystem to the Message Processing Subsystem. If a cache exists for 935 a session identifiable from transportDomain, transportAddress, 936 securityModel, securityName, and securityLevel, then an appropriate 937 Security Model might create a tmStateReference to the cache and pass 938 that as an OUT parameter. 940 If one does not exist, the Security Model might create a cache 941 referenced by tmStateReference. This information might include 942 transportDomain, transportAddress, the securityLevel, and the 943 securityName, plus any model or mechanism-specific details. The 944 contents of the cache may be incomplete until the Transport Model has 945 established a session. What information is passed, and how this 946 information is determined, is implementation and security-model- 947 specific. 949 The prepareOutgoingMessage ASI passes tmStateReference from the 950 Message Processing Subsystem to the dispatcher. How or if the 951 Message Processing Subsystem modifies or utilizes the contents of the 952 cache is message-processing-model-specific. 954 This may sound underspecified, but a message processing model might 955 have access to all the information from the cache and from the 956 message, and an application might specify a Security Model such as 957 USM to authenticate and secure the SNMP message, but also specify a 958 secure transport such as that provided by the SSH Transport Model to 959 send the message to its destination. 961 6.3. The receiveMessage ASI 963 If one does not exist, the Transport Model might create a cache 964 referenced by tmStateReference. If present, this information might 965 include transportDomain, transportAddress, securityLevel, and 966 securityName, plus model or mechanism-specific details. How this 967 information is determined is implementation and transport-model- 968 specific. 970 In the diagram in section 4.6.1 of RFC 3411, the receiveMessage ASI 971 replaces the text "Receive SNMP Response Message from Network". In 972 section 4.6.2, the receiveMessage ASI replaces the text "Receive SNMP 973 Message from Network" 975 This may sound underspecified, but a transport model might be 976 something like SNMP over UDP over IPv6, where no security is 977 provided, so it might have no mechanisms for determining a 978 securityName and securityLevel. 980 The Transport Model does not know the securityModel for an incoming 981 message; this will be determined by the Message Processing Model in a 982 message-processing-model-dependent manner. 984 The receiveMessage ASI is used to pass a message from the Transport 985 Subsystem to the Dispatcher. 987 statusInformation = 988 receiveMessage( 989 IN transportDomain -- origin transport domain 990 IN transportAddress -- origin transport address 991 IN incomingMessage -- the message received 992 IN incomingMessageLength -- its length 993 IN tmStateReference -- reference to transport state 994 ) 996 6.4. Other Incoming ASIs 998 To support the Transport Subsystem, the tmStateReference is added to 999 the prepareDataElements ASI (from the Dispatcher to the Message 1000 Processing Subsystem), and to the processIncomingMsg ASI (from the 1001 Message Processing Subsystem to the Security Model Subsystem). How 1002 or if a Message Processing Model or Security Model uses 1003 tmStateReference is message-processing-model-dependent and security- 1004 model-dependent. 1006 result = -- SUCCESS or errorIndication 1007 prepareDataElements( 1008 IN transportDomain -- origin transport domain 1009 IN transportAddress -- origin transport address 1010 IN wholeMsg -- as received from the network 1011 IN wholeMsgLength -- as received from the network 1012 IN tmStateReference -- (NEW) from the Transport Model 1013 OUT messageProcessingModel -- typically, SNMP version 1014 OUT securityModel -- Security Model to use 1015 OUT securityName -- on behalf of this principal 1016 OUT securityLevel -- Level of Security requested 1017 OUT contextEngineID -- data from/at this entity 1018 OUT contextName -- data from/in this context 1019 OUT pduVersion -- the version of the PDU 1020 OUT PDU -- SNMP Protocol Data Unit 1021 OUT pduType -- SNMP PDU type 1022 OUT sendPduHandle -- handle for matched request 1023 OUT maxSizeResponseScopedPDU -- maximum size sender can accept 1024 OUT statusInformation -- success or errorIndication 1025 -- error counter OID/value if error 1026 OUT stateReference -- reference to state information 1027 -- to be used for possible Response 1028 ) 1030 statusInformation = -- errorIndication or success 1031 -- error counter OID/value if error 1032 processIncomingMsg( 1033 IN messageProcessingModel -- typically, SNMP version 1034 IN maxMessageSize -- of the sending SNMP entity 1035 IN securityParameters -- for the received message 1036 IN securityModel -- for the received message 1037 IN securityLevel -- Level of Security 1038 IN wholeMsg -- as received on the wire 1039 IN wholeMsgLength -- length as received on the wire 1040 IN tmStateReference -- (NEW) from the Transport Model 1041 OUT securityEngineID -- authoritative SNMP entity 1042 OUT securityName -- identification of the principal 1043 OUT scopedPDU, -- message (plaintext) payload 1044 OUT maxSizeResponseScopedPDU -- maximum size sender can handle 1045 OUT securityStateReference -- reference to security state 1046 ) -- information, needed for response 1048 The tmStateReference parameter of prepareDataElements is passed from 1049 the dispatcher to the Message Processing Subsystem. How or if the 1050 Message Processing Subsystem modifies or utilizes the contents of the 1051 cache is message-processing-model-specific. 1053 The processIncomingMessage ASI passes tmStateReference from the 1054 Message Processing Subsystem to the Security Subsystem. 1056 If tmStateReference is present and valid, an appropriate Security 1057 Model might utilize the information in the cache. How or if the 1058 Security Subsystem utilizes the information in the cache is security- 1059 model-specific. 1061 This may sound underspecified, but a message processing model might 1062 have access to all the information from the cache and from the 1063 message. The Message Processing Model might determine that the USM 1064 Security Model is specified in an SNMPv3 message header; the USM 1065 Security Model has no need of values in the tmStateReference cache to 1066 authenticate and secure the SNMP message, but an application might 1067 have specified to use a secure transport such as that provided by the 1068 SSH Transport Model to send the message to its destination. 1070 7. Security Considerations 1072 This document defines an architectural approach that permits SNMP to 1073 utilize transport layer security services. Each proposed Transport 1074 Model should discuss the security considerations of the Transport 1075 Model. 1077 It is considered desirable by some industry segments that SNMP 1078 Transport Models should utilize transport layer security that 1079 addresses perfect forward secrecy at least for encryption keys. 1080 Perfect forward secrecy guarantees that compromise of long term 1081 secret keys does not result in disclosure of past session keys. Each 1082 proposed Transport Model should include a discussion in its security 1083 considerations of whether perfect forward security is appropriate for 1084 the Transport Model. 1086 Since the cache and LCD will contain security-related parameters, 1087 implementers should store this information (in memory or in 1088 persistent storage) in a manner to protect it from unauthorized 1089 disclosure and/or modification. 1091 Care must be taken to ensure that a SNMP engine is sending packets 1092 out over a transport using credentials that are legal for that engine 1093 to use on behalf of that user. Otherwise an engine that has multiple 1094 transports open might be "tricked" into sending a message through the 1095 wrong transport. 1097 A Security Model may have multiple sources from which to define the 1098 securityName and securityLevel. The use of a secure Transport Model 1099 does not imply that the securityName and securityLevel chosen by the 1100 Security Model represent the transport-authenticated identity or the 1101 transport-provided security services. The securityModel, 1102 securityName, and securityLevel parameters are a related set, and an 1103 administrator should understand how the specified securityModel 1104 selects the corresponding securityName and securityLevel. 1106 7.1. Coexistence, Security Parameters, and Access Control 1108 In the RFC3411 architecture, the Message Processing Model makes the 1109 decision about which Security Model to use. The architectural change 1110 described by this document does not alter that. 1112 The architecture change described by this document does however, 1113 allow SNMP to support two different approaches to security - message- 1114 driven security and transport-driven security. With message-driven 1115 security, SNMP provides its own security, and passes security 1116 parameters within the SNMP message; with transport-driven security, 1117 SNMP depends on an external entity to provide security during 1118 transport by "wrapping" the SNMP message. 1120 Security models defined before the Transport Security Model (i.e., 1121 SNMPv1, SNMPv2c, and USM) do not support transport-based security, 1122 and only have access to the security parameters contained within the 1123 SNMP message. They do not know about the security parameters 1124 associated with a secure transport. As a result, the Access Control 1125 Subsystem bases its decisions on the security parameters extracted 1126 from the SNMP message, not on transport-based security parameters. 1128 Implications of coexistence of older security models with secure 1129 transport models are known. The securityName used for access control 1130 decisions represents an SNMP-authenticated identity, not the 1131 transport-authenticated identity. (I can transport-authenticate as 1132 guest and then simply use a community name for root, or a USM non- 1133 authenticated identity.) 1134 o An SNMPv1 message will always be paired with an SNMPv1 Security 1135 Model (per RFC3584), regardless of the transport mapping or 1136 transport model used, and access controls will be based on the 1137 community name. 1138 o An SNMPv2c message will always be paired with an SNMPv2c Security 1139 Model (per RFC3584), regardless of the transport mapping or 1140 transport model used, and access controls will be based on the 1141 community name. 1142 o An SNMPv3 message will always be paired with the securityModel 1143 specified in the msgSecurityParameters field of the message (per 1144 RFC3412), regardless of the transport mappng or transport model 1145 used. If the SNMPv3 message specifies the User-based Security 1146 Model (USM), access controls will be based on the USM user.If the 1147 SNMPv3 message specifies the Transport Security Model (TSM), 1148 access controls will be based on the principal authenticated by 1149 the transport. 1151 8. IANA Considerations 1153 This document requires no action by IANA. 1155 9. Acknowledgments 1157 The Integrated Security for SNMP WG would like to thank the following 1158 people for their contributions to the process: 1160 The authors of submitted Security Model proposals: Chris Elliot, Wes 1161 Hardaker, David Harrington, Keith McCloghrie, Kaushik Narayan, David 1162 Perkins, Joseph Salowey, and Juergen Schoenwaelder. 1164 The members of the Protocol Evaluation Team: Uri Blumenthal, 1165 Lakshminath Dondeti, Randy Presuhn, and Eric Rescorla. 1167 WG members who performed detailed reviews: Jeffrey Hutzelman, Bert 1168 Wijnen, Tom Petch. 1170 10. References 1172 10.1. Normative References 1174 [RFC2119] Bradner, S., "Key words for 1175 use in RFCs to Indicate 1176 Requirement Levels", 1177 BCP 14, RFC 2119, 1178 March 1997. 1180 [RFC3411] Harrington, D., Presuhn, 1181 R., and B. Wijnen, "An 1182 Architecture for Describing 1183 Simple Network Management 1184 Protocol (SNMP) Management 1185 Frameworks", STD 62, 1186 RFC 3411, December 2002. 1188 [RFC3412] Case, J., Harrington, D., 1189 Presuhn, R., and B. Wijnen, 1190 "Message Processing and 1191 Dispatching for the Simple 1192 Network Management Protocol 1193 (SNMP)", STD 62, RFC 3412, 1194 December 2002. 1196 [RFC3414] Blumenthal, U. and B. 1198 Wijnen, "User-based 1199 Security Model (USM) for 1200 version 3 of the Simple 1201 Network Management Protocol 1202 (SNMPv3)", STD 62, 1203 RFC 3414, December 2002. 1205 [RFC3417] Presuhn, R., "Transport 1206 Mappings for the Simple 1207 Network Management Protocol 1208 (SNMP)", STD 62, RFC 3417, 1209 December 2002. 1211 10.2. Informative References 1213 [RFC2865] Rigney, C., Willens, S., 1214 Rubens, A., and W. Simpson, 1215 "Remote Authentication Dial 1216 In User Service (RADIUS)", 1217 RFC 2865, June 2000. 1219 [RFC3410] Case, J., Mundy, R., 1220 Partain, D., and B. 1221 Stewart, "Introduction and 1222 Applicability Statements 1223 for Internet-Standard 1224 Management Framework", 1225 RFC 3410, December 2002. 1227 [RFC3584] Frye, R., Levi, D., 1228 Routhier, S., and B. 1229 Wijnen, "Coexistence 1230 between Version 1, Version 1231 2, and Version 3 of the 1232 Internet-standard Network 1233 Management Framework", 1234 BCP 74, RFC 3584, 1235 August 2003. 1237 [RFC4346] Dierks, T. and E. Rescorla, 1238 "The Transport Layer 1239 Security (TLS) Protocol 1240 Version 1.1", RFC 4346, 1241 April 2006. 1243 [RFC4422] Melnikov, A. and K. 1244 Zeilenga, "Simple 1245 Authentication and Security 1246 Layer (SASL)", RFC 4422, 1247 June 2006. 1249 [RFC4251] Ylonen, T. and C. Lonvick, 1250 "The Secure Shell (SSH) 1251 Protocol Architecture", 1252 RFC 4251, January 2006. 1254 [RFC4741] Enns, R., "NETCONF 1255 Configuration Protocol", 1256 RFC 4741, December 2006. 1258 [I-D.ietf-isms-transport-security-model] Harrington, D., "Transport 1259 Security Model for SNMP", d 1260 raft-ietf-isms-transport- 1261 security-model-06 (work in 1262 progress), September 2007. 1264 [I-D.ietf-isms-secshell] Harrington, D. and J. 1265 Salowey, "Secure Shell 1266 Transport Model for SNMP", 1267 draft-ietf-isms-secshell-09 1268 (work in progress), 1269 November 2007. 1271 Appendix A. Why tmStateReference? 1273 This appendix considers why a cache-based approach was selected for 1274 passing parameters. 1276 There are four approaches that could be used for passing information 1277 between the Transport Model and a Security Model. 1279 1. one could define an ASI to supplement the existing ASIs, or 1280 2. one could add a header to encapsulate the SNMP message, 1281 3. one could utilize fields already defined in the existing SNMPv3 1282 message, or 1283 4. one could pass the information in an implementation-specific 1284 cache or via a MIB module. 1286 A.1. Define an Abstract Service Interface 1288 Abstract Service Interfaces (ASIs) are defined by a set of primitives 1289 that specify the services provided and the abstract data elements 1290 that are to be passed when the services are invoked. Defining 1291 additional ASIs to pass the security and transport information from 1292 the Transport Subsystem to Security Subsystem has the advantage of 1293 being consistent with existing RFC3411/3412 practice, and helps to 1294 ensure that any Transport Model proposals pass the necessary data, 1295 and do not cause side effects by creating model-specific dependencies 1296 between itself and other models or other subsystems other than those 1297 that are clearly defined by an ASI. 1299 A.2. Using an Encapsulating Header 1301 A header could encapsulate the SNMP message to pass necessary 1302 information from the Transport Model to the dispatcher and then to a 1303 Message Processing Model. The message header would be included in 1304 the wholeMessage ASI parameter, and would be removed by a 1305 corresponding Message Processing Model. This would imply the (one 1306 and only) messaging dispatcher would need to be modified to determine 1307 which SNMP message version was involved, and a new Message Processing 1308 Model would need to be developed that knew how to extract the header 1309 from the message and pass it to the Security Model. 1311 A.3. Modifying Existing Fields in an SNMP Message 1313 [RFC3412] defines the SNMPv3 message, which contains fields to pass 1314 security related parameters. The Transport Subsystem could use these 1315 fields in an SNMPv3 message, or comparable fields in other message 1316 formats to pass information between Transport Models in different 1317 SNMP engines, and to pass information between a Transport Model and a 1318 corresponding Message Processing Model. 1320 If the fields in an incoming SNMPv3 message are changed by the 1321 Transport Model before passing it to the Security Model, then the 1322 Transport Model will need to decode the ASN.1 message, modify the 1323 fields, and re-encode the message in ASN.1 before passing the message 1324 on to the message dispatcher or to the transport layer. This would 1325 require an intimate knowledge of the message format and message 1326 versions so the Transport Model knew which fields could be modified. 1327 This would seriously violate the modularity of the architecture. 1329 A.4. Using a Cache 1331 This document describes a cache, into which the Transport Model puts 1332 information about the security applied to an incoming message, and a 1333 Security Model can extract that information from the cache. Given 1334 that there might be multiple TM-security caches, a tmStateReference 1335 is passed as an extra parameter in the ASIs between the Transport 1336 Subsystem and the Security Subsystem, so the Security Model knows 1337 which cache of information to consult. 1339 This approach does create dependencies between a specific Transport 1340 Model and a corresponding specific Security Model. However, the 1341 approach of passing a model-independent reference to a model- 1342 dependent cache is consistent with the securityStateReference already 1343 being passed around in the RFC3411 ASIs. 1345 Appendix B. Open Issues 1347 NOTE to RFC editor: If this section is empty, then please remove this 1348 open issues section before publishing this document as an RFC. (If 1349 it is not empty, please send it back to the editor to resolve. 1350 o 1352 Appendix C. Change Log 1354 NOTE to RFC editor: Please remove this change log before publishing 1355 this document as an RFC. 1357 Changes from -09- to -10- 1359 o Pointed to companion documents 1360 o Wordsmithed extensively 1361 o Modified the note about SNMPv3-consistent terminology 1362 o Modified the note about RFC2119 terminology. 1363 o Modified discussion of cryptographic key generation. 1364 o Added security considerations about coexistence with older 1365 security models 1366 o Expanded discussion of same session functionality 1367 o Described how sendMessage and receiveMessage fit into RFC3411 1368 diagrams 1369 o Modified prepareResponseMessage ASI 1370 o 1372 Changes from -08- to -09- 1374 o A question was raised that notifications would not work properly, 1375 but we could never find the circumstances where this was true. 1376 o removed appendix with parameter matrix 1377 o Added a note about terminology, for consistency with SNMPv3 rather 1378 than with RFC2828. 1380 Changes from -07- to -08- 1382 o Identified new parameters in ASIs. 1383 o Added discussion about well-known ports. 1385 Changes from -06- to -07- 1387 o Removed discussion of double authentication 1388 o Removed all direct and indirect references to pduType by Transport 1389 Subsystem 1390 o Added warning regarding keeping sensitive security information 1391 available longer than needed. 1392 o Removed knowledge of securityStateReference from Transport 1393 Subsystem. 1394 o Changed transport session identifier to not include securityModel, 1395 since this is not known for incoming messages until the message 1396 processing model. 1398 Changes from revision -05- to -06- 1400 mostly editorial changes 1401 removed some paragraphs considered unnecessary 1402 added Updates to header 1403 modified some text to get the security details right 1404 modified text re: ASIs so they are not API-like 1405 cleaned up some diagrams 1406 cleaned up RFC2119 language 1407 added section numbers to citations to RFC3411 1408 removed gun for political correctness 1410 Changes from revision -04- to -05- 1412 removed all objects from the MIB module. 1413 changed document status to "Standard" rather than the xml2rfc 1414 default of informational. 1416 changed mention of MD5 to SHA 1417 moved addressing style to TDomain and TAddress 1418 modified the diagrams as requested 1419 removed the "layered stack" diagrams that compared USM and a 1420 Transport Model processing 1421 removed discussion of speculative features that might exist in 1422 future Transport Models 1423 removed openSession and closeSession ASIs, since those are model- 1424 dependent 1425 removed the MIB module 1426 removed the MIB boilerplate intro (this memo defines a SMIv2 MIB 1427 ...) 1428 removed IANA considerations related to the now-gone MIB module 1429 removed security considerations related to the MIB module 1430 removed references needed for the MIB module 1431 changed receiveMessage ASI to use origin transport domain/address 1432 updated Parameter CSV appendix 1433 Changes from revision -03- to -04- 1434 changed title from Transport Mapping Security Model Architectural 1435 Extension to Transport Subsystem 1436 modified the abstract and introduction 1437 changed TMSM to TMS 1438 changed MPSP to simply Security Model 1439 changed SMSP to simply Security Model 1440 changed TMSP to Transport Model 1441 removed MPSP and TMSP and SMSP from Acronyms section 1442 modified diagrams 1443 removed most references to dispatcher functionality 1444 worked to remove dependencies between transport and security 1445 models. 1446 defined snmpTransportModel enumeration similar to 1447 snmpSecurityModel, etc. 1448 eliminated all reference to SNMPv3 msgXXXX fields 1449 changed tmSessionReference back to tmStateReference 1451 Changes from revision -02- to -03- 1453 o removed session table from MIB module 1454 o removed sessionID from ASIs 1455 o reorganized to put ASI discussions in EOP section, as was done in 1456 SSHSM 1457 o changed user auth to client auth 1458 o changed tmStateReference to tmSessionReference 1459 o modified document to meet consensus positions published by JS 1460 * authoritative is model-specific 1461 * msgSecurityParameters usage is model-specific 1462 * msgFlags vs. securityLevel is model/implementation-specific 1463 * notifications must be able to cause creation of a session 1464 * security considerations must be model-specific 1465 * TDomain and TAddress are model-specific 1466 * MPSP changed to SMSP (Security Model security processing) 1468 Changes from revision -01- to -02- 1470 o wrote text for session establishment requirements section. 1471 o wrote text for session maintenance requirements section. 1472 o removed section on relation to SNMPv2-MIB 1473 o updated MIB module to pass smilint 1474 o Added Structure of the MIB module, and other expected MIB-related 1475 sections. 1476 o updated author address 1477 o corrected spelling 1478 o removed msgFlags appendix 1479 o Removed section on implementation considerations. 1481 o started modifying the security boilerplate to address TMS and MIB 1482 security issues 1483 o reorganized slightly to better separate requirements from proposed 1484 solution. This probably needs additional work. 1485 o removed section with sample protocols and sample 1486 tmSessionReference. 1487 o Added section for acronyms 1488 o moved section comparing parameter passing techniques to appendix. 1489 o Removed section on notification requirements. 1491 Changes from revision -00- 1492 o changed SSH references from I-Ds to RFCs 1493 o removed parameters from tmSessionReference for DTLS that revealed 1494 lower layer info. 1495 o Added TMS-MIB module 1496 o Added Internet-Standard Management Framework boilerplate 1497 o Added Structure of the MIB Module 1498 o Added MIB security considerations boilerplate (to be completed) 1499 o Added IANA Considerations 1500 o Added ASI Parameter table 1501 o Added discussion of Sessions 1502 o Added Open issues and Change Log 1503 o Rearranged sections 1505 Authors' Addresses 1507 David Harrington 1508 Huawei Technologies (USA) 1509 1700 Alma Dr. Suite 100 1510 Plano, TX 75075 1511 USA 1513 Phone: +1 603 436 8634 1514 EMail: dharrington@huawei.com 1516 Juergen Schoenwaelder 1517 Jacobs University Bremen 1518 Campus Ring 1 1519 28725 Bremen 1520 Germany 1522 Phone: +49 421 200-3587 1523 EMail: j.schoenwaelder@iu-bremen.de 1525 Full Copyright Statement 1527 Copyright (C) The IETF Trust (2007). 1529 This document is subject to the rights, licenses and restrictions 1530 contained in BCP 78, and except as set forth therein, the authors 1531 retain all their rights. 1533 This document and the information contained herein are provided on an 1534 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1535 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 1536 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 1537 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 1538 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1539 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1541 Intellectual Property 1543 The IETF takes no position regarding the validity or scope of any 1544 Intellectual Property Rights or other rights that might be claimed to 1545 pertain to the implementation or use of the technology described in 1546 this document or the extent to which any license under such rights 1547 might or might not be available; nor does it represent that it has 1548 made any independent effort to identify any such rights. Information 1549 on the procedures with respect to rights in RFC documents can be 1550 found in BCP 78 and BCP 79. 1552 Copies of IPR disclosures made to the IETF Secretariat and any 1553 assurances of licenses to be made available, or the result of an 1554 attempt made to obtain a general license or permission for the use of 1555 such proprietary rights by implementers or users of this 1556 specification can be obtained from the IETF on-line IPR repository at 1557 http://www.ietf.org/ipr. 1559 The IETF invites any interested party to bring to its attention any 1560 copyrights, patents or patent applications, or other proprietary 1561 rights that may cover technology that may be required to implement 1562 this standard. Please address the information to the IETF at 1563 ietf-ipr@ietf.org. 1565 Acknowledgement 1567 Funding for the RFC Editor function is provided by the IETF 1568 Administrative Support Activity (IASA).