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Weber 5 Individual Contributor 6 Expires: February 9, 2011 7 9 July 2010 9 RADIUS Design Guidelines 10 draft-ietf-radext-design-16 12 Abstract 14 This document provides guidelines for the design of attributes used 15 by the Remote Authentication Dial In User Service (RADIUS) protocol. 16 It is expected that these guidelines will prove useful to authors and 17 reviewers of future RADIUS attribute specifications, both within the 18 IETF as well as other Standards Development Organizations (SDOs). 20 Status of this Memo 22 This Internet-Draft is submitted to IETF in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF), its areas, and its working groups. Note that 27 other groups may also distribute working documents as Internet- 28 Drafts. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 The list of current Internet-Drafts can be accessed at 36 http://www.ietf.org/ietf/1id-abstracts.txt. 38 The list of Internet-Draft Shadow Directories can be accessed at 39 http://www.ietf.org/shadow.html. 41 This Internet-Draft will expire on February 9, 2011. 43 Copyright Notice 45 Copyright (c) 2010 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info/) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 This document may contain material from IETF Documents or IETF 59 Contributions published or made publicly available before November 60 10, 2008. The person(s) controlling the copyright in some of this 61 material may not have granted the IETF Trust the right to allow 62 modifications of such material outside the IETF Standards Process. 63 Without obtaining an adequate license from the person(s) controlling 64 the copyright in such materials, this document may not be modified 65 outside the IETF Standards Process, and derivative works of it may 66 not be created outside the IETF Standards Process, except to format 67 it for publication as an RFC or to translate it into languages other 68 than English. 70 Table of Contents 72 1. Introduction ............................................. 5 73 1.1. Terminology ......................................... 5 74 1.2. Requirements Language ............................... 6 75 1.3. Applicability ....................................... 6 76 1.3.1. Reviews ........................................ 7 77 2. Guidelines ............................................... 8 78 2.1. Data Types .......................................... 9 79 2.2. Vendor-Specific Attribute Space ..................... 10 80 2.3. Service definitions and RADIUS ...................... 10 81 2.4. Translation of Vendor Specifications ................ 11 82 3. Rationale ................................................ 12 83 3.1. RADIUS Operational Model ............................ 12 84 3.2. Data Model Issues ................................... 15 85 3.2.1. Issues with Definitions of Types ............... 15 86 3.2.2. Tagging Mechanism .............................. 16 87 3.2.3. Complex Data Types ............................. 17 88 3.3. Vendor Space ........................................ 20 89 3.3.1. Interoperability Considerations ................ 21 90 3.3.2. Vendor Allocations ............................. 21 91 3.3.3. SDO Allocations ................................ 21 92 3.4. Polymorphic Attributes .............................. 22 93 4. IANA Considerations ...................................... 23 94 5. Security Considerations .................................. 23 95 5.1. New Data Types and Complex Attributes ............... 24 96 6. References ............................................... 24 97 6.1. Normative References ................................ 24 98 6.2. Informative References .............................. 25 99 Appendix A - Design Guidelines ............................... 28 100 A.1. Types matching the RADIUS data model ................. 28 101 A.1.1. Transport of basic data types ................... 28 102 A.1.2. Transport of Authentication and Security Data ... 28 103 A.1.3. Opaque data types ............................... 28 104 A.1.4. Pre-existing data types ......................... 28 105 A.2. Improper Data Types .................................. 29 106 A.2.1. Basic Data Types ................................ 29 107 A.2.2. Complex Data Types .............................. 30 108 A.3. Vendor-Specific formats .............................. 30 109 A.4. Changes to the RADIUS Operational Model .............. 31 110 A.5. Allocation of attributes ............................. 32 111 Appendix B - Complex Attributes .............................. 33 112 B.1. CHAP-Password ........................................ 33 113 B.2. CHAP-Challenge ....................................... 33 114 B.3. Tunnel-Password ...................................... 33 115 B.4. ARAP-Password ........................................ 34 116 B.5. ARAP-Features ........................................ 34 117 B.6. Connect-Info ......................................... 35 118 B.7. Framed-IPv6-Prefix ................................... 36 119 B.8. Egress-VLANID ........................................ 36 120 B.9. Egress-VLAN-Name ..................................... 37 121 B.10. Digest-* ............................................ 37 123 1. Introduction 125 This document provides guidelines for the design of RADIUS attributes 126 both within the IETF as well as within other SDOs. By articulating 127 RADIUS design guidelines, it is hoped that this document will 128 encourage the development and publication of high quality RADIUS 129 attribute specifications. 131 However, the advice in this document will not be helpful unless it is 132 put to use. As with "Guidelines for Authors and Reviewers of MIB 133 Documents" [RFC4181], it is expected that this document will be used 134 by authors to check their document against the guidelines prior to 135 publication, or requesting review (such as an "Expert Review" 136 described in [RFC3575]). Similarly, it is expected that this 137 document will used by reviewers (such as WG participants or the AAA 138 Doctors [DOCTORS]), resulting in an improvement in the consistency of 139 reviews. 141 In order to meet these objectives, this document needs to cover not 142 only the science of attribute design, but also the art. Therefore, 143 in addition to covering the most frequently encountered issues, this 144 document explains some of the considerations motivating the 145 guidelines. These considerations include complexity trade-offs that 146 make it difficult to provide "hard and fast" rules for attribute 147 design. This document explains those trade-offs through reviews of 148 current attribute usage. 150 1.1. Terminology 152 This document uses the following terms: 154 Network Access Server (NAS) 155 A device that provides an access service for a user to a network. 157 RADIUS server 158 A RADIUS authentication, authorization, and/or accounting (AAA) 159 server is an entity that provides one or more AAA services to a 160 NAS. 162 standard space 163 RADIUS attributes which are allocated by IANA and which follow the 164 format defined in RFC 2865 [RFC2865] Section 5. 166 vendor space 167 The contents of the "String" field of a Vendor-Specific Attribute 168 (VSA), as defined in [RFC2865] Section 5.26. These attributes 169 provide a unique attribute space for each vendor (identified by the 170 Vendor-Type field) which they can self-allocate. 172 1.2. Requirements Language 174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 176 document are to be interpreted as described in [RFC2119]. 178 1.3. Applicability 180 The advice in this document applies to attributes used to encode 181 service-provisioning, authentication, or accounting data, based on 182 the attribute encodings and data formats defined in RFC 2865 183 [RFC2865], RFC 2866 [RFC2866] and subsequent RADIUS RFCs. 185 Since this document represents a Best Current Practice, it does not 186 update or deprecate existing standards. As a result, uses of the 187 terms "MUST" and "MUST NOT" are limited to requirements already 188 present in existing documents. 190 It is RECOMMENDED that these guidelines be followed for all new 191 RADIUS specifications, whether they originate from a vendor, an SDO, 192 or the IETF. Doing so will ensure the widest possible applicability 193 and interoperability of the specifications, while requiring minimal 194 changes to existing systems. In particular, it is expected that 195 RADIUS specifications requesting allocation within the standards 196 space will follow these guidelines, and will explain why this is not 197 possible if they cannot. 199 However, there are situations in which vendors or SDOs can choose not 200 to follow these guidelines without major consequences. As noted in 201 [RFC2865] Section 5.26, Vendor-Specific Attributes (VSAs) are 202 "available to allow vendors to support their own extended Attributes 203 not suitable for general usage." Where vendors or SDOs develop 204 specifications "not suitable for general usage", limited 205 interoperability and inability to use existing implementations may be 206 acceptable and in these situations, vendors and SDOs MAY choose to 207 not conform to these guidelines. 209 Note that the RADEXT WG is currently (as of 2010) involved in 210 developing updates to RADIUS. Those updates will provide their own 211 usage guidelines that may over-ride some of the guidelines discussed 212 here. 214 RADIUS protocol changes, or specification of attributes (such as 215 Service-Type) that can, in effect, provide new RADIUS commands 216 require greater expertise and deeper review, as do changes to the 217 RADIUS operational model. As a result, such changes are outside the 218 scope of this document and MUST NOT be undertaken outside the IETF. 220 1.3.1. Reviews 222 For specifications utilizing attributes within the standards space, 223 conformance with the design guidelines in this document is expected 224 unless a good case can be made for an exception. Reviewers SHOULD 225 use the design guidelines as a review checklist. 227 While not required, IETF review may also be beneficial for 228 specifications utilizing the Vendor-Specific space. Experience has 229 shown that attributes not originally designed for general usage can 230 subsequently garner wide-spread deployment. An example is the 231 vendor-specific attributes defined in [RFC2548], which have been 232 widely implemented within IEEE 802.11 Access Points. 234 In order to assist in the development of specifications conforming to 235 these guidelines, authors can request review by sending email to the 236 AAA Doctors [DOCTORS] or equivalent mailing list. The IETF 237 Operations & Management Area Directors will then arrange for the 238 review to be completed and posted to the AAA Doctors mailing list 239 [DOCTORS], RADEXT WG mailing list, or other IETF mailing list. Since 240 reviews are handled by volunteers, responses are provided on a best- 241 effort basis, with no service level guarantees. Authors are 242 encouraged to seek review as early as possible, so as to avoid 243 potential delays. 245 As reviewers require access to the specification, vendors and SDOs 246 are encouraged to make them publicly available. Where the RADIUS 247 specification is embedded within a larger document which cannot be 248 made public, the RADIUS attribute and value definitions can be made 249 available on a public web site or can be published as an 250 Informational RFC, as with [RFC4679]. 252 The review process requires neither allocation of attributes within 253 the IETF standard attribute space nor publication of an IETF RFC. 254 Requiring SDOs or vendors to rehost VSAs into the IETF standards 255 attribute space solely for the purpose of obtaining review would put 256 pressure on the standards space, and may be harmful to 257 interoperability, since would create two ways to provision the same 258 service. Rehosting may also require changes to the RADIUS data model 259 which will affect implementations that do not intend to support the 260 SDO or vendor specifications. 262 Similarly, vendors are encouraged to make their specifications 263 publicly available, for maximum interoperability. However, it is not 264 necessary for a vendor to request publication of a VSA specification 265 as an Informational RFC by the IETF. 267 2. Guidelines 269 The Remote Authentication Dial In User Service (RADIUS) defined in 270 [RFC2865] and [RFC2866] uses elements known as attributes in order to 271 represent authentication, authorization and accounting data. 273 Unlike SNMP, first defined in [RFC1157] and [RFC1155], RADIUS does 274 not define a formal data definition language. The data type of 275 RADIUS attributes is not transported on the wire. Rather, the data 276 type of a RADIUS attribute is fixed when an attribute is defined. 277 Based on the RADIUS attribute type code, RADIUS clients and servers 278 can determine the data type based on preconfigured entries within a 279 data dictionary. 281 To explain the implications of this early RADIUS design decision we 282 distinguish two types of data types, namely "basic" and "complex". 283 Basic data types use one of the existing RADIUS data types defined in 284 Section 2.1, encapsulated in a [RFC2865] RADIUS attribute, or in a 285 [RFC2865] RADIUS VSA. All other data formats are "complex types". 287 RADIUS attributes can be classified into one of three broad 288 categories: 290 * Attributes that are of interest to a single vendor, e.g., for a 291 product or product line. Minimal cross-vendor interoperability 292 is needed. 294 Vendor-Specific Attributes (VSAs) are appropriate for use in 295 this situation.. Code-point allocation is managed by the vendor 296 with the number space defined by their Private Enterprise Number 297 (PEN). 299 * Attributes that are of interest to an industry segment, where an 300 SDO defines the attributes for that industry. Multi-vendor 301 interoperability within an industry segment is expected. 303 Vendor-Specific Attributes (VSAs) MUST be used. Code-point 304 allocation is managed by the SDO with the number space defined 305 by the SDOs PEN, rather then the PEN of an individual vendor. 307 * Attributes that are of broad interest to the Internet Community. 308 Multi-vendor interoperability is expected. 310 Attributes within the standards space are appropriate for this 311 purpose, and are allocated via IANA as described in [RFC3575]. 312 Since the standards space represents a finite resource, and is 313 the only attribute space available for use by IETF working 314 groups, vendors and SDOs are encouraged to utilize the VSA 315 space, rather than requesting allocation of attributes from the 316 standards space. Self-allocation of standards attributes is 317 considered anti-social behavior and is strongly discouraged. 319 2.1. Data Types 321 RADIUS defines a limited set of data types, defined as "basic data 322 types". The following data qualifies as "basic data types": 324 * 32-bit unsigned integer, in network byte order. 326 * Enumerated data types, represented as a 32-bit unsigned integer 327 with a list of name to value mappings. (e.g. Service-Type) 329 * IPv4 address in network byte order. 331 * time as 32 bit unsigned value, in network byte order, and in 332 seconds since 00:00:00 UTC, January 1, 1970. 334 * IPv6 address in network byte order. 336 * Interface-Id (8 octet string in network byte order) 338 * IPv6 prefix. 340 * string (i.e., binary data), totalling 253 octets or less in 341 length. This includes the opaque encapsulation of data 342 structures defined outside of RADIUS. See also Appendix A.1.3, 343 below, for additional discussion. 345 * UTF-8 text [RFC3629], totalling 253 octets or less in length. 347 Note that the length limitations for VSAs of type String and Text are 348 less than 253 octets, due to the additional overhead of the Vendor- 349 Specific encoding. 351 The following data also qualifies as "basic data types": 353 * Attributes grouped into a logical container, using the 354 [RFC2868] tagging mechanism. This approach is NOT RECOMMENDED 355 (see Section 3.2.2), but is permissible where the alternatives 356 are worse. 358 * Attributes requiring the transport of more than 253 octets of 359 Text or String data. This includes the opaque encapsulation 360 of data structures defined outside of RADIUS. 361 (e.g. EAP-Message) 363 All other data formats (including nested attributes) are defined to 364 be "complex data types", and are NOT RECOMMENDED for normal use. 365 Complex data types MAY be used in situations where they reduce 366 complexity in non-RADIUS systems, or where using the basic data types 367 would be awkward (such as where grouping would be required in order 368 to link related attributes). Since there are no "hard and fast" 369 rules for where complexity is best located, each situation has to be 370 decided on a case-by-case basis. Examples of this tradeoff are 371 discussed in Appendix B. Where a complex data type is selected, an 372 explanation SHOULD be offered as to why this was necessary. 374 2.2. Vendor-Specific Attribute Space 376 The Vendor-Specific Attribute space is defined to be the contents of 377 the "String" field of the Vendor-Specific Attribute ([RFC2865] 378 Section 5.26). As discussed there, it is intended for vendors and 379 SDOs to support their own Attributes not suitable for general use. 381 While the encoding of attributes within the vendor space is under the 382 control of vendors and SDOs, following the guidelines described here 383 is advantageous since it enables maximum interoperability with 384 minimal changes to existing systems. 386 For example, RADIUS server support for new attributes using "basic 387 data types" can typically be accomplished by editing a RADIUS 388 dictionary, whereas "complex data types" typically require RADIUS 389 server code changes, which can add complexity and delays in 390 implementation. 392 Vendor RADIUS Attribute specifications SHOULD self-allocate 393 attributes from the vendor space, rather than requesting an 394 allocation (or self-allocating) from within the RADIUS standard 395 attribute space. 397 VSA encodings that do not follow the [RFC2865] Section 5.26 scheme 398 are NOT RECOMMENDED. Although [RFC2865] does not mandate it, 399 implementations commonly assume that the Vendor Id can be used as a 400 key to determine the on-the-wire encoding of a VSA. Vendors 401 therefore SHOULD NOT use multiple encodings for VSAs that are 402 associated with a particular Vendor Id. A vendor wishing to use 403 multiple VSA encodings SHOULD request one Vendor Id for each VSA 404 encoding that they will use. 406 2.3. Service definitions and RADIUS 408 RADIUS specifications define how an existing service or protocol can 409 be provisioned using RADIUS. Therefore, it is expected that a RADIUS 410 attribute specification will reference documents defining the 411 protocol or service to be provisioned. Within the IETF, a RADIUS 412 attribute specification SHOULD NOT be used to define the protocol or 413 service being provisioned. New services using RADIUS for 414 provisioning SHOULD be defined elsewhere and referenced in the RADIUS 415 specification. 417 New attributes, or new values of existing attributes, SHOULD NOT be 418 used to define new RADIUS commands. RADIUS attributes are intended 419 to: 421 * authenticate users 423 * authorize users (i.e., service provisioning or changes to 424 provisioning) 426 * account for user activity (i.e., logging of session activity) 428 Requirements for allocation of new commands (i.e. the Code field in 429 the packet header) and new attributes within the standards space are 430 described in [RFC3575] Section 2.1. 432 2.4. Translation of Vendor Specifications 434 [RFC2865] Section 5.26 defines Vendor-Specific attributes as follows: 436 This Attribute is available to allow vendors to support their own 437 extended Attributes not suitable for general usage. It MUST NOT 438 affect the operation of the RADIUS protocol. 440 Servers not equipped to interpret the vendor-specific information 441 sent by a client MUST ignore it (although it may be reported). 442 Clients which do not receive desired vendor-specific information 443 SHOULD make an attempt to operate without it, although they may do 444 so (and report they are doing so) in a degraded mode. 446 The limitation on changes to the RADIUS protocol effectively 447 prohibits VSAs from changing fundamental aspects of RADIUS operation, 448 such as modifying RADIUS packet sequences, or adding new commands. 449 However, the requirement for clients and servers to be able to 450 operate in the absence of VSAs has proven to be less of a constraint, 451 since it is still possible for a RADIUS client and server to mutually 452 indicate support for VSAs, after which behavior expectations can be 453 reset. 455 Therefore, RFC 2865 provides considerable latitude for development of 456 new attributes within the vendor space, while prohibiting development 457 of protocol variants. This flexibility implies that RADIUS 458 attributes can often be developed within the vendor space without 459 loss (and possibly even with gain) in functionality. 461 As a result, translation of RADIUS attributes developed within the 462 vendor space into the standard space may provide only modest 463 benefits, while accelerating the exhaustion of the standard attribute 464 space. We do not expect that all RADIUS attribute specifications 465 requiring interoperability will be developed within the IETF, and 466 allocated from the standards space. A more scalable approach is to 467 recognize the flexibility of the vendor space, while working toward 468 improvements in the quality and availability of RADIUS attribute 469 specifications, regardless of where they are developed. 471 It is therefore NOT RECOMMENDED that specifications intended solely 472 for use by a vendor or SDO use be translated into the standard space. 474 3. Rationale 476 This section outlines the rationale behind the above recommendations. 478 3.1. RADIUS Operational Model 480 The RADIUS operational model includes several assumptions: 482 * The RADIUS protocol is stateless; 484 * Provisioning of services is not possible within an 485 Access-Reject; 487 * There is a distinction between authorization checks and user 488 authentication; 490 * The protocol provides for authentication and integrity 491 protection of packets; 493 * The RADIUS protocol is a Request/Response protocol; 495 * The protocol defines packet length restrictions. 497 While RADIUS server implementations may keep state, the RADIUS 498 protocol is stateless, although information may be passed from one 499 protocol transaction to another via the State Attribute. As a 500 result, documents which require stateful protocol behavior without 501 use of the State Attribute are inherently incompatible with RADIUS as 502 defined in [RFC2865], and SHOULD be redesigned. See [RFC5080] 503 Section 2.1.1 for additional discussion surrounding the use of the 504 State Attribute. 506 As noted in [RFC5080] Section 2.6, the intent of an Access-Reject is 507 to deny access to the requested service. As a result, RADIUS does 508 not allow the provisioning of services within an Access-Reject. 509 Documents which include provisioning of services within an Access- 510 Reject are inherently incompatible with RADIUS, and SHOULD be 511 redesigned. 513 As noted in [RFC5080] Section 2.1.1, a RADIUS Access-Request may not 514 contain user authentication attributes or a State Attribute linking 515 the Access-Request to an earlier user authentication. Such an 516 Access-Request, known as an authorization check, provides no 517 assurance that it corresponds to a live user. RADIUS specifications 518 defining attributes containing confidential information (such as 519 Tunnel-Password) should be careful to prohibit such attributes from 520 being returned in response to an authorization check. Also, 521 [RFC5080] Section 2.1.1 notes that authentication mechanisms need to 522 tie a sequence of Access-Request/Access-Challenge packets together 523 into one authentication session. The State Attribute is RECOMMENDED 524 for this purpose. 526 While [RFC2865] did not require authentication and integrity 527 protection of RADIUS Access-Request packets, subsequent 528 authentication mechanism specifications such as RADIUS/EAP [RFC3579] 529 and Digest Authentication [RFC5090] have mandated authentication and 530 integrity protection for certain RADIUS packets. [RFC5080] Section 531 2.1.1 makes this behavior RECOMMENDED for all Access-Request packets, 532 including Access-Request packets performing authorization checks. It 533 is expected that specifications for new RADIUS authentication 534 mechanisms will continue this practice. 536 The RADIUS protocol as defined in [RFC2865] is a request-response 537 protocol spoken between RADIUS clients and servers. A single RADIUS 538 Access-Request packet will solicit in response at most a single 539 Access-Accept, Access-Reject or Access-Challenge packet, sent to the 540 IP address and port of the RADIUS Client that originated the Access- 541 Request. Similarly, a single Change-of-Authorization (CoA)-Request 542 packet [RFC5176] will solicit in response at most a single CoA-ACK or 543 CoA-NAK packet, sent to the IP address and port of the Dynamic 544 Authorization Client (DAC) that originated the CoA-Request. A single 545 Disconnect-Request packet will solicit in response at most a single 546 Disconnect-ACK or Disconnect-NAK packet, sent to the IP address and 547 port of the Dynamic Authorization Client (DAC) that originated the 548 Disconnect-Request. Changes to this model are likely to require 549 major revisions to existing implementations and so this practice is 550 NOT RECOMMENDED. 552 The Length field in the RADIUS packet header is defined in [RFC2865] 553 Section 3. It is noted there that the maximum length of a RADIUS 554 packet is 4096 octets. As a result, attribute designers SHOULD NOT 555 assume that a RADIUS implementation can successfully process RADIUS 556 packets larger than 4096 octets. 558 Even when packets are less than 4096 octets, they may be larger than 559 the Path Maximum Transmission Unit (PMTU). Any packet larger than 560 the PMTU will be fragmented, making communications more brittle as 561 firewalls and filtering devices often discard fragments. Transport 562 of fragmented UDP packets appears to be a poorly tested code path on 563 network devices. Some devices appear to be incapable of transporting 564 fragmented UDP packets, making it difficult to deploy RADIUS in a 565 network where those devices are deployed. We RECOMMEND that RADIUS 566 messages be kept as small possible. 568 If a situation is envisaged where it may be necessary to carry 569 authentication, authorization or accounting data in a packet larger 570 than 4096 octets, then one of the following approaches is 571 RECOMMENDED: 573 1. Utilization of a sequence of packets. 574 For RADIUS authentication, a sequence of Access-Request/ Access- 575 Challenge packets would be used. For this to be feasible, 576 attribute designers need to enable inclusion of attributes that 577 can consume considerable space within Access-Challenge packets. 578 To maintain compatibility with existing NASes, either the use of 579 Access-Challenge packets needs to be permissible (as with 580 RADIUS/EAP, defined in [RFC3579]), or support for receipt of an 581 Access-Challenge needs to be indicated by the NAS (as in RADIUS 582 Location [RFC5580]). Also, the specification needs to clearly 583 describe how attribute splitting is to be signalled and how 584 attributes included within the sequence are to be interpreted, 585 without requiring stateful operation. Unfortunately, previous 586 specifications have not always exhibited the required foresight. 587 For example, even though very large filter rules are 588 conceivable, the NAS-Filter-Rule Attribute defined in [RFC4849] 589 is not permitted in an Access-Challenge packet, nor is a 590 mechanism specified to allow a set of NAS-Filter-Rule attributes 591 to be split across an Access-Request/Access-Challenge sequence. 593 In the case of RADIUS accounting, transporting large amounts of 594 data would require a sequence of Accounting-Request packets. 595 This is a non-trivial change to RADIUS, since RADIUS accounting 596 clients would need to be modified to split the attribute stream 597 across multiple Accounting-Requests, and billing servers would 598 need to be modified to re-assemble and interpret the attribute 599 stream. 601 2. Utilization of names rather than values. 602 Where an attribute relates to a policy that could conceivably be 603 pre-provisioned on the NAS, then the name of the pre-provisioned 604 policy can be transmitted in an attribute, rather than the 605 policy itself, which could be quite large. An example of this 606 is the Filter-Id Attribute defined in [RFC2865] Section 5.11, 607 which enables a set of pre-provisioned filter rules to be 608 referenced by name. 610 3. Utilization of Packetization Layer Path MTU Discovery 611 techniques, as specified in [RFC4821]. As a last resort, where 612 the above techniques cannot be made to work, it may be possible 613 to apply the techniques described in [RFC4821] to discover the 614 maximum supported RADIUS packet size on the path between a 615 RADIUS client and a home server. While such an approach can 616 avoid the complexity of utilization of a sequence of packets, 617 dynamic discovery is likely to be time consuming and cannot be 618 guaranteed to work with existing RADIUS implementations. As a 619 result, this technique is not generally applicable. 621 3.2. Data Model Issues 623 While [RFC2865] Section 5 defines basic data types, later 624 specifications did not follow this practice. This problem has led 625 implementations to define their own names for data types, resulting 626 in non-standard names for those types. 628 In addition, the number of vendors and SDOs creating new attributes 629 within the Vendor-Specific attribute space has grown, and this has 630 lead to some divergence in approaches to RADIUS attribute design. 631 For example, vendors and SDOs have evolved the data model to support 632 functions such as new data types, along with attribute grouping and 633 attribute fragmentation, with different groups taking different 634 approaches. These approaches are often incompatible, leading to 635 additional complexity in RADIUS implementations. 637 In order to avoid repeating old mistakes, this section describes the 638 history of the RADIUS data model, and attempts to codify existing 639 practices. 641 3.2.1. Issues with Definitions of Types 643 [RFC2865] Section 5 explicitly defines five data types: text, string, 644 address, integer and time. Both the names and interpretations of the 645 types are given. 647 Subsequent RADIUS specifications defined attributes by using type 648 names not defined in [RFC2865], without defining the new names as was 649 done in [RFC2865]. They did not consistently indicate the format of 650 the value field using the same conventions as [RFC2865]. As a 651 result, the data type is ambiguous in some cases, and may not be 652 consistent among different implementations. 654 It is out of the scope of this document to resolve all potential 655 ambiguities within existing RADIUS specifications. However in order 656 to prevent future ambiguities, it is recommended that future RADIUS 657 attribute specifications explicitly define newly created data types 658 at the beginning of the document, and indicate clearly the data type 659 to be used for each attribute. 661 For example, [RFC3162] utilizes but does not explicitly define a type 662 which encapsulates an IPv6 address (Section 2.1 and 2.4), and another 663 type which encapsulates an IPv6 prefix (Section 2.3). The IPv6 664 address attributes confusingly are referenced as type "Address" in 665 the document. This is a similar name as the "address" type defined 666 in [RFC2865], which was defined to refer solely to IPv4 addresses. 668 While the Framed-Interface-Id attribute defined in [RFC3162] Section 669 2.2 included a value field of 8 octets, the data type was not 670 explicitly indicated, and therefore there is controversy over whether 671 the format of the data was intended to be an 8 octet String or 672 whether a special Interface-Id type was intended. 674 Given that attributes encapsulating an IPv6 address and an IPv6 675 prefix are already in use, it is RECOMMENDED that RADIUS server 676 implementations include support for these as basic types, in addition 677 to the types defined in [RFC2865]. Where the intent is to represent 678 a specific IPv6 address, an "IPv6 address" type SHOULD be used. 679 Although it is possible to use an "IPv6 Prefix" type with a prefix 680 length of 128 to represent an IPv6 address, this usage is NOT 681 RECOMMENDED. Implementations supporting the Framed-Interface-Id 682 attribute may select a data type of their choosing (most likely an 8 683 octet String or a special "Interface Id" data type). 685 It is worth noting that since RADIUS only supports unsigned integers 686 of 32 bits, attributes using signed integer data types or unsigned 687 integer types of other sizes will require code changes, and SHOULD be 688 avoided. 690 For [RFC2865] RADIUS VSAs, the length limitation of the String and 691 Text types is 247 octets instead of 253 octets, due to the additional 692 overhead of the Vendor-Specific Attribute. 694 3.2.2. Tagging Mechanism 696 [RFC2868] defines an attribute grouping mechanism based on the use of 697 a one octet tag value. Tunnel attributes that refer to the same 698 tunnel are grouped together by virtue of using the same tag value. 700 This tagging mechanism has some drawbacks. There are a limited 701 number of unique tags (31). The tags are not well suited for use 702 with arbitrary binary data values, because it is not always possible 703 to tell if the first byte after the Length is the tag or the first 704 byte of the untagged value (assuming the tag is optional). 706 Other limitations of the tagging mechanism are that when integer 707 values are tagged, the value portion is reduced to three bytes 708 meaning only 24-bit numbers can be represented. The tagging 709 mechanism does not offer an ability to create nested groups of 710 attributes. Some RADIUS implementations treat tagged attributes as 711 having additional data types tagged-string and tagged-integer. These 712 types increase the complexity of implementing and managing RADIUS 713 systems. 715 For these reasons, the tagging scheme described in RFC 2868 is NOT 716 RECOMMENDED for use as a generic grouping mechanism. 718 3.2.3. Complex Data Types 720 The RADIUS attribute encoding is summarized in [RFC2865]: 722 0 1 2 723 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 724 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 725 | Type | Length | Value ... 726 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 728 However, some standard attributes do not follow this encoding. 729 Attributes that use an encoding other than the basic data types as 730 discussed above are defined to be "complex types". As described 731 below in this section, the creation of complex types can lead to 732 interoperability and deployment issues, so they need to be introduced 733 with care. 735 In general, complex types containing multiple sub-fields can be 736 supported by concatenating the sub-fields into a String data type 737 field. However, separating these sub-fields into different 738 attributes, each with its own type and length, would have the 739 following benefits: 741 * it is easier for an administator to enter the data as well-known 742 types, rather than complex structures; 744 * it enables additional error checking by leveraging the 745 parsing and validation routines for well-known types; 747 * it simplifies implementations by eliminating special-case 748 attribute-specific parsing. 750 One of the fundamental goals of the RADIUS protocol design was to 751 allow RADIUS servers to be configured to support new attributes 752 without requiring server code changes. RADIUS server implementations 753 typically provide support for basic data types, and define attributes 754 in a data dictionary. This architecture enables a new attribute to 755 be supported by the addition of a dictionary entry, without requiring 756 other RADIUS server code changes. 758 On the RADIUS client, code changes are typically required in order to 759 implement a new attribute. The RADIUS client typically has to 760 compose the attribute dynamically when sending. When receiving, a 761 RADIUS client needs to be able to parse the attribute and carry out 762 the requested service. As a result, a detailed understanding of the 763 new attribute is required on clients, and data dictionaries are less 764 useful on clients than on servers. 766 Given these considerations, the introduction of a new basic or 767 complex attribute will typically require code changes on the RADIUS 768 client. The magnitude of changes for the complex attribute could be 769 greater, due to the potential need for custom parsing logic. 771 The RADIUS server can be configured to send a new static attribute by 772 entering its type and data format in the RADIUS server dictionary, 773 and then filling in the value within a policy based on the attribute 774 name, data type and type-specific value. For data types not 775 supported by current RADIUS server dictionaries, changes to the 776 dictionary code can be required in order to allow the new type to be 777 supported by and configured on the RADIUS server. 779 Code changes can also be required in policy management and in the 780 RADIUS server's receive path. These changes are due to limitations 781 in RADIUS server policy languages, which typically only provide for 782 limited operations (such as comparisons or arithmetic operations) on 783 the existing data types. Many existing RADIUS policy languages 784 typically are not capable of parsing sub-elements, or providing 785 sophisticated matching functionality. 787 Given these limitations, the introduction of new types can require 788 code changes on the RADIUS server which would be unnecessary if basic 789 data types had been used instead. In addition if "ad-hoc" types are 790 used, attribute-specific parsing means more complex software to 791 develop and maintain. More complexity can lead to more error prone 792 implementations, interoperability problems, and even security 793 vulnerabilities. These issues can increase costs to network 794 administrators as well as reducing reliability and introducing 795 deployment barriers. 797 As described in Section 2.1, the introduction of complex data types 798 is discouraged where viable alternatives are available. A potential 799 exception is attributes that inherently require code changes on both 800 the client and server. For example, as described in Appendix B, 801 complex attributes have been used in situations involving 802 authentication and security attributes that need to be dynamically 803 computed and verified. 805 As can be seen in Appendix B, most of the existing complex attributes 806 involve authentication or security functionality. Supporting this 807 functionality requires code changes on both the RADIUS client and 808 server, regardless of the attribute format. As a result, in most 809 cases, the use of complex attributes to represent these methods is 810 acceptable, and does not create additional interoperability or 811 deployment issues. 813 Another exception to the recommendation against complex types is for 814 types that can be treated as opaque data by the RADIUS server. For 815 example, the EAP-Message attribute, defined in [RFC3579] Section 3.1 816 contains a complex data type that is an EAP packet. Since these 817 complex types do not need to be parsed by the RADIUS server, the 818 issues arising from policy language limitations do not arise. 819 Similarly, since attributes of these complex types can be configured 820 on the server using a data type of String, dictionary limitations are 821 also not encountered. Appendix A.1 below includes a series of 822 checklists that may be used to analyze a design for RECOMMENDED and 823 NOT RECOMMENDED behavior in relation to complex types. 825 If the RADIUS Server simply passes the contents of an attribute to 826 some non-RADIUS portion of the network, then the data is opaque, and 827 SHOULD be defined to be of type String. A concrete way of judging 828 this requirement is whether or not the attribute definition in the 829 RADIUS document contains delineated fields for sub-parts of the data. 830 If those fields need to be delineated in RADIUS, then the data is not 831 opaque, and it SHOULD be separated into individual RADIUS attributes. 833 An examination of existing RADIUS RFCs discloses a number of complex 834 attributes that have already been defined. Appendix B includes a 835 listing of complex attributes used within [RFC2865], [RFC2868], 836 [RFC2869], [RFC3162], [RFC4818], and [RFC4675]. The discussion of 837 these attributes includes reasons why a complex type is acceptable, 838 or suggestions for how the attribute could have been defined to 839 follow the RADIUS data model. 841 In other cases, the data in the complex type are described textually. 842 This is possible because the data types are not sent within the 843 attributes, but are a matter for endpoint interpretation. An 844 implementation can define additional data types, and use these data 845 types today by matching them to the attribute's textual description. 847 3.3. Vendor Space 849 The usage model for RADIUS VSAs is described in [RFC2865] Section 850 6.2: 852 Note that RADIUS defines a mechanism for Vendor-Specific 853 extensions (Attribute 26) and the use of that should be encouraged 854 instead of allocation of global attribute types, for functions 855 specific only to one vendor's implementation of RADIUS, where no 856 interoperability is deemed useful. 858 Nevertheless, many new attributes have been defined in the vendor 859 specific space in situations where interoperability is not only 860 useful, but is required. For example, SDOs outside the IETF (such as 861 the IEEE 802 and the 3rd Generation Partnership Project (3GPP)) have 862 been assigned Vendor-Ids, enabling them to define their own VSA 863 encoding and assign Vendor types within their own space. 865 The use of VSAs by SDOs outside the IETF has gained in popularity for 866 several reasons: 868 Efficiency 869 As with SNMP, which defines an "Enterprise" Object Identifier (OID) 870 space suitable for use by vendors as well as other SDOs, the 871 definition of Vendor-Specific RADIUS attributes has become a common 872 occurrence as part of standards activity outside the IETF. For 873 reasons of efficiency, it is easiest if the RADIUS attributes 874 required to manage a standard are developed within the same SDO 875 that develops the standard itself. As noted in "Transferring MIB 876 Work from IETF Bridge MIB WG to IEEE 802.1 WG" [RFC4663], today few 877 vendors are willing to simultaneously fund individuals to 878 participate within an SDO to complete a standard, as well as to 879 participate in the IETF in order to complete the associated RADIUS 880 attributes specification. 882 Attribute scarcity 883 The standard RADIUS attribute space is limited to 255 unique 884 attributes. Of these, only about half remain available for 885 allocation. In the vendor space, the number of attributes 886 available is a function of the encoding of the attribute (the size 887 of the Vendor type field). 889 3.3.1. Interoperability Considerations 891 Vendors and SDOs are reminded that the standard RADIUS attribute 892 space, and the enumerated value space for enumerated attributes, are 893 reserved for allocation through work published via the IETF, as noted 894 in [RFC3575] Section 2.1. Some vendors and SDOs have in the past 895 performed self-allocation by assigning vendor-specific meaning to 896 "unused" values from the standard RADIUS attribute ID or enumerated 897 value space. This self-allocation results in interoperability 898 issues, and is counter-productive. Similarly, the Vendor-Specific 899 enumeration practice discussed in [RFC2882] Section 2.2.1 is NOT 900 RECOMMENDED. 902 If it is not possible to follow the IETF process, vendors and SDOs 903 SHOULD self-allocate an attribute, which MUST be in vendor space, as 904 discussed in Sections 3.3.2 and 3.3.3, below. 906 The design and specification of VSAs for multi-vendor usage SHOULD be 907 undertaken with the same level of care as standard RADIUS attributes. 908 Specifically, the provisions of this document that apply to standard 909 RADIUS attributes also apply to VSAs for multi-vendor usage. 911 3.3.2. Vendor Allocations 913 As noted in [RFC3575] Section 2.1, vendors are encouraged to utilize 914 VSAs to define functions "specific only to one vendor's 915 implementation of RADIUS, where no interoperability is deemed useful. 916 For functions specific only to one vendor's implementation of RADIUS, 917 the use of that should be encouraged instead of the allocation of 918 global attribute types." 920 The recommendation for vendors to allocate attributes from a vendor 921 space rather than via the IETF process is a recognition that vendors 922 desire to assert change control over their own RADIUS specifications. 923 This change control can be obtained by requesting a PEC from the 924 Internet Assigned Number Authority (IANA), for use as a Vendor-Id 925 within a Vendor-Specific attribute. The vendor can then allocate 926 attributes within the VSA space defined by that Vendor-Id, at their 927 sole discretion. Similarly, the use of data types (complex or 928 otherwise) within that VSA space is solely under the discretion of 929 the vendor. 931 3.3.3. SDO Allocations 933 Given the expanded utilization of RADIUS, it has become apparent that 934 requiring SDOs to accomplish all their RADIUS work within the IETF is 935 inherently inefficient and unscalable. Is is therefore RECOMMENDED 936 that SDO RADIUS Attribute specifications allocate attributes from the 937 vendor space, rather than requesting an allocation from the RADIUS 938 standard attribute space, for attributes matching any of the 939 following criteria: 941 * attributes relying on data types not defined within RADIUS 943 * attributes intended primarily for use within an SDO 945 * attributes intended primarily for use within a group of SDOs. 947 Any new RADIUS attributes or values intended for interoperable use 948 across a broad spectrum of the Internet Community SHOULD follow the 949 allocation process defined in [RFC3575]. 951 The recommendation for SDOs to allocate attributes from a vendor 952 space rather than via the IETF process is a recognition that SDOs 953 desire to assert change control over their own RADIUS specifications. 954 This change control can be obtained by requesting a PEC from the 955 Internet Assigned Number Authority (IANA), for use as a Vendor-Id 956 within a Vendor-Specific attribute. The SDO can then allocate 957 attributes within the VSA space defined by that Vendor-Id, at their 958 sole discretion. Similarly, the use of data types (complex or 959 otherwise) within that VSA space is solely under the discretion of 960 the SDO. 962 3.4. Polymorphic Attributes 964 A polymorphic attribute is one whose format or meaning is dynamic. 965 For example, rather than using a fixed data format, an attribute's 966 format might change based on the contents of another attribute. Or, 967 the meaning of an attribute may depend on earlier packets in a 968 sequence. 970 RADIUS server dictionary entries are typically static, enabling the 971 user to enter the contents of an attribute without support for 972 changing the format based on dynamic conditions. However, this 973 limitation on static types does not prevent implementations from 974 implementing policies that return different attributes based on the 975 contents of received attributes; this is a common feature of existing 976 RADIUS implementations. 978 In general, polymorphism is NOT RECOMMENDED. Polymorphism rarely 979 enables capabilities that would not be available through use of 980 multiple attributes. Polymorphism requires code changes in the 981 RADIUS server in situations where attributes with fixed formats would 982 not require such changes. Thus, polymorphism increases complexity 983 while decreasing generality, without delivering any corresponding 984 benefits. 986 Note that changing an attribute's format dynamically is not the same 987 thing as using a fixed format and computing the attribute itself 988 dynamically. RADIUS authentication attributes such as User-Password, 989 EAP-Message, etc. while being computed dynamically, use a fixed 990 format. 992 4. IANA Considerations 994 This document has no action items for IANA. However, it does provide 995 guidelines for Expert Reviewers appointed as described in [RFC3575]. 997 5. Security Considerations 999 This specification provides guidelines for the design of RADIUS 1000 attributes used in authentication, authorization and accounting. 1001 Threats and security issues for this application are described in 1002 [RFC3579] and [RFC3580]; security issues encountered in roaming are 1003 described in [RFC2607]. 1005 Obfuscation of RADIUS attributes on a per-attribute basis is 1006 necessary in some cases. The current standard mechanism for this is 1007 described in [RFC2865] Section 5.2 (for obscuring User-Password 1008 values) and is based on the MD5 algorithm specified in [RFC1321]. 1009 The MD5 and SHA-1 algorithms have recently become a focus of scrutiny 1010 and concern in security circles, and as a result, the use of these 1011 algorithms in new attributes is NOT RECOMMENDED. In addition, 1012 previous documents referred to this method as generating "encrypted" 1013 data. This terminology is no longer accepted within the 1014 cryptographic community. 1016 Where new RADIUS attributes use cryptographic algorithms, algorithm 1017 negotiation SHOULD be supported. Specification of a mandatory-to- 1018 implement algorithm is REQUIRED, and it is RECOMMENDED that the 1019 mandatory-to-implement algorithm be certifiable under FIPS 140 1020 [FIPS]. 1022 Where new RADIUS attributes encapsulate complex data types, or 1023 transport opaque data, the security considerations discussed in 1024 Section 5.1 SHOULD be addressed. 1026 Message authentication in RADIUS is provided largely via the Message- 1027 Authenticator attribute. See [RFC3579] Section 3.2, and also 1028 [RFC5080] 2.2.2, which says that client implementations SHOULD 1029 include a Message-Authenticator attribute in every Access-Request. 1031 In general, the security of the RADIUS protocol is poor. Robust 1032 deployments SHOULD support a secure communications protocol such as 1033 IPSec. See [RFC3579] Section 4, and [RFC3580] Section 5 for a more 1034 in-depth explanation of these issues. 1036 Implementations not following the suggestions outlined in this 1037 document may be subject to a problems such as ambiguous protocol 1038 decoding, packet loss leading to loss of billing information, and 1039 denial of service attacks. 1041 5.1. New Data Types and Complex Attributes 1043 The introduction of complex data types brings the potential for the 1044 introduction of new security vulnerabilities. Experience shows that 1045 the common data types have few security vulnerabilities, or else that 1046 all known issues have been found and fixed. New data types require 1047 new code, which may introduce new bugs, and therefore new attack 1048 vectors. 1050 Some systems permit complex attributes to be defined via a method 1051 that is more capable than traditional RADIUS dictionaries. These 1052 systems can reduce the security threat of new types significantly, 1053 but they do not remove it entirely. 1055 RADIUS servers are highly valued targets, as they control network 1056 access and interact with databases that store usernames and 1057 passwords. An extreme outcome of a vulnerability due to a new, 1058 complex type would be that an attacker is capable of taking complete 1059 control over the RADIUS server. 1061 The use of attributes representing opaque data does not reduce this 1062 threat. The threat merely moves from the RADIUS server to the system 1063 that consumes that opaque data. 1065 The threat is particularly severe when the opaque data originates 1066 from the user, and is not validated by the NAS. In those cases, the 1067 RADIUS server is potentially exposed to attack by malware residing on 1068 an unauthenticated host. 1070 Any system consuming opaque data that originates from a RADIUS system 1071 SHOULD be properly isolated from that RADIUS system, and SHOULD run 1072 with minimal privileges. Any potential vulnerabilities in the non- 1073 RADIUS system will then have minimal impact on the security of the 1074 system as a whole. 1076 6. References 1078 6.1. Normative References 1080 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1081 Requirement Levels", BCP 14, RFC 2119, March 1997. 1083 [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote 1084 Authentication Dial In User Service (RADIUS)", RFC 2865, June 1085 2000. 1087 [RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote 1088 Authentication Dial In User Service)", RFC 3575, July 2003. 1090 6.2. Informative References 1092 [RFC1155] Rose, M. and K. McCloghrie, "Structure and identification of 1093 management information for TCP/IP-based internets", STD 16, 1094 RFC 1155, May 1990. 1096 [RFC1157] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple 1097 Network Management Protocol (SNMP)", STD 15, RFC 1157, May 1098 1990. 1100 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 1101 April 1992. 1103 [RFC2548] Zorn, Glen, "Microsoft Vendor-specific RADIUS Attributes", RFC 1104 2548, March 1999. 1106 [RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy 1107 Implementation in Roaming", RFC 2607, June 1999. 1109 [RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000. 1111 [RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M., 1112 and I. Goyret, "RADIUS Attributes for Tunnel Protocol 1113 Support", RFC 2868, June 2000. 1115 [RFC2869] Rigney, C., Willats, W., and P. Calhoun, "RADIUS Extensions", 1116 RFC 2869, June 2000. 1118 [RFC2882] Mitton, D, "Network Access Servers Requirements: Extended 1119 RADIUS Practices", RFC 2882, July 2000. 1121 [RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC 1122 3162, August 2001. 1124 [RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial 1125 In User Service) Support For Extensible Authentication 1126 Protocol (EAP)", RFC 3579, September 2003. 1128 [RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., Roese, J., "IEEE 1129 802.1X Remote Authentication Dial In User Service (RADIUS) 1130 Usage Guidelines", RFC3580, September 2003. 1132 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", 1133 RFC 3629, November 2003. 1135 [RFC4181] Heard, C., "Guidelines for Authors and Reviewers of MIB 1136 Documents", RFC 4181, September 2005. 1138 [RFC4663] Harrington, D., "Transferring MIB Work from IETF Bridge MIB WG 1139 to IEEE 802.1 WG", RFC 4663, September 2006. 1141 [RFC4675] Congdon, P., Sanchez, M. and B. Aboba, "RADIUS Attributes for 1142 Virtual LAN and Priority Support", RFC 4675, September 2006. 1144 [RFC4679] Mammoliti, V., et al., "DSL Forum Vendor-Specific RADIUS 1145 Attributes", RFC 4679, September 2006. 1147 [RFC4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix 1148 Attribute", RFC 4818, April 2007. 1150 [RFC4821] Mathis, M. and Heffner, J, "Packetization Layer Path MTU 1151 Discovery", RFC 4821, March 2007. 1153 [RFC4849] Congdon, P. et al, "RADIUS Filter-Rule Attribute", RFC 4849, 1154 April 2007. 1156 [RFC5080] Nelson, D. and DeKok, A, "Common Remote Authentication Dial In 1157 User Service (RADIUS) Implementation Issues and Suggested 1158 Fixes", RFC 5080, December 2007. 1160 [RFC5090] Sterman, B. et al., "RADIUS Extension for Digest 1161 Authentication", RFC 5090, February 2008. 1163 [RFC5176] Chiba, M. et al., "Dynamic Authorization Extensions to Remote 1164 Authentication Dial In User Service (RADIUS)", RFC 5176, 1165 January 2008. 1167 [DOCTORS] AAA Doctors Mailing list 1169 [FIPS] FIPS 140-3 (DRAFT), "Security Requirements for Cryptographic 1170 Modules", http://csrc.nist.gov/publications/fips/fips140-3/ 1172 [IEEE-802.1Q] 1173 IEEE Standards for Local and Metropolitan Area Networks: Draft 1174 Standard for Virtual Bridged Local Area Networks, 1175 P802.1Q-2003, January 2003. 1177 [RFC5580] Tschofenig, H. (Ed.), "Carrying Location Objects in RADIUS and 1178 Diameter", RFC 5580, August 2009. 1180 [AAA-SIP] Sterman, B. et al., "RADIUS Extension for Digest 1181 Authentication", draft-sterman-sip-aaa-00.txt, November 2001 1182 (expired). 1184 Appendix A - Design Guidelines 1186 The following text provides guidelines for the design of attributes 1187 used by the RADIUS protocol. Specifications that follow these 1188 guidelines are expected to achieve maximum interoperability with 1189 minimal changes to existing systems. 1191 A.1. Types matching the RADIUS data model 1193 A.1.1. Transport of basic data types 1195 Does the data fit within the basic data types described in Section 1196 2.1? If so, it SHOULD be encapsulated in a [RFC2865] format RADIUS 1197 attribute, or in a [RFC2865] format RADIUS VSA that uses one of the 1198 existing RADIUS data types. 1200 A.1.2. Transport of Authentication and Security Data 1202 Does the data provide authentication and/or security capabilities for 1203 the RADIUS protocol, as outlined below? If so, use of a complex data 1204 type is acceptable, under the following circumstances: 1206 * Complex data types that carry authentication methods which 1207 RADIUS servers are expected to parse and verify as part of 1208 an authentication process. 1210 * Complex data types that carry security information intended 1211 to increase the security of the RADIUS protocol itself. 1213 Any data type carrying authentication and/or security data that is 1214 not meant to be parsed by a RADIUS server is an "opaque data type", 1215 as defined below. 1217 A.1.3. Opaque data types 1219 Does the attribute encapsulate an existing data structure defined 1220 outside of the RADIUS specifications? Can the attribute be treated 1221 as opaque data by RADIUS servers (including proxies?) If both 1222 questions can be answered affirmatively, a complex structure MAY be 1223 used in a RADIUS specification. 1225 The specification of the attribute SHOULD define the encapsulating 1226 attribute to be of type String. The specification SHOULD refer to an 1227 external document defining the structure. The specification SHOULD 1228 NOT define or describe the structure, for reasons discussed above in 1229 Section 3.2.3. 1231 A.1.4. Pre-existing data types 1232 There is a trade-off in design between re-using existing formats for 1233 historical compatibility, or choosing new formats for a "better" 1234 design. This trade-off does not always require the "better" design 1235 to be used. As a result. pre-existing complex data types described 1236 in Appendix B MAY be used. 1238 A.2. Improper Data Types 1240 This section suggests alternatives to data types which do not fall 1241 within the "basic data type" definition. 1243 A.2.1. Basic Data Types 1245 Does the attribute use any of the following data types? If so, the 1246 data type SHOULD be replaced with the suggested alternatives, or it 1247 SHOULD NOT be used at all. 1249 * Signed integers of any size. 1250 SHOULD NOT be used. SHOULD be replaced with one or more 1251 unsigned integer attributes. The definition of the attribute 1252 can contain information that would otherwise go into the sign 1253 value of the integer. 1255 * 8 bit unsigned integers. 1256 SHOULD be replaced with 32-bit unsigned integer. There is 1257 insufficient justification to save three bytes. 1259 * 16 bit unsigned integers. 1260 SHOULD be replaced with 32-bit unsigned integer. There is 1261 insufficient justification to save two bytes. 1263 * Unsigned integers of size other than 32 bits. 1264 SHOULD be replaced by an unsigned integer of 32 bits. 1265 There is insufficient justification to define a new size of 1266 integer. 1268 * Integers of any size in non-network byte order 1269 SHOULD be replaced by unsigned integer of 32 bits in network 1270 There is no reason to transport integers in any format other 1271 than network byte order. 1273 * Multi-field text strings. 1274 Each field SHOULD be encapsulated in a separate attribute. 1276 * Polymorphic attributes. 1277 Multiple attributes, each with a static data type SHOULD be 1278 defined instead. 1280 * Nested AVPs. 1281 Attributes should be defined in a flat typespace. 1283 A.2.2. Complex Data Types 1285 Does the attribute: 1287 * define a complex data type not described in Appendix B, 1289 * that a RADIUS server and/or client is expected to parse, 1290 validate, or create the contents of via a dynamic computation? 1291 i.e. A type that cannot be treated as opaque data (Section A.1.3) 1293 * involve functionality that could be implemented without code 1294 changes on both the client and server? (i.e. a type that doesn't 1295 require dynamic computation and verification, such as those 1296 performed for authentication or security attributes) 1298 If so, this data type SHOULD be replaced with simpler types, as 1299 discussed above in Appendix A.2.1. See also Section 2.1 for a 1300 discussion of why complex types are problematic. 1302 A.3. Vendor-Specific formats 1304 Does the specification contain Vendor-Specific attributes that match 1305 any of the following criteria? If so, the VSA encoding should be 1306 replaced with the [RFC2865] Section 5.26 encoding, or should not be 1307 used at all. 1309 * Vendor types of more than 8 bits. 1310 SHOULD NOT be used. Vendor types of 8 bits SHOULD be used 1311 instead. 1313 * Vendor lengths of less than 8 bits. (i.e., zero bits) 1314 SHOULD NOT be used. Vendor lengths of 8 bits SHOULD be used 1315 instead. 1317 * Vendor lengths of more than 8 bits. 1318 SHOULD NOT be used. Vendor lengths of 8 bits SHOULD be used 1319 instead. 1321 * Vendor-Specific contents that are not in Type-Length-Value 1322 format. 1323 SHOULD NOT be used. Vendor-Specific attributes SHOULD be in 1324 Type-Length-Value format. 1326 In general, Vendor-Specific attributes SHOULD follow the [RFC2865] 1327 Section 5.26 suggested encoding. Vendor extensions to non-standard 1328 encodings are NOT RECOMMENDED as they can negatively affect 1329 interoperability. 1331 A.4. Changes to the RADIUS Operational Model 1333 Does the specification change the RADIUS operation model, as outlined 1334 in the list below? If so, then another method of achieving the 1335 design objectives SHOULD be used. Potential problem areas include: 1337 * Defining new commands in RADIUS using attributes. 1338 The addition of new commands to RADIUS MUST be handled via 1339 allocation of a new Code, and not by the use of an attribute. 1340 This restriction includes new commands created by overloading 1341 the Service-Type attribute to define new values that modify 1342 the functionality of Access-Request packets. 1344 * Using RADIUS as a transport protocol for data unrelated to 1345 authentication, authorization, or accounting. Using RADIUS to 1346 transport authentication methods such as EAP is explicitly 1347 permitted, even if those methods require the transport of 1348 relatively large amounts of data. Transport of opaque data 1349 relating to AAA is also permitted, as discussed above in 1350 Section 3.2.3. However, if the specification does not relate 1351 to AAA, then RADIUS SHOULD NOT be used. 1353 * Assuming support for packet lengths greater than 4096 octets. 1354 Attribute designers cannot assume that RADIUS implementations 1355 can successfully handle packets larger than 4096 octets. 1356 If a specification could lead to a RADIUS packet larger than 1357 4096 octets, then the alternatives described in Section 3.3 1358 SHOULD be considered. 1360 * Stateless operation. The RADIUS protocol is stateless, and 1361 documents which require stateful protocol behavior without the 1362 use of the State Attribute need to be redesigned. 1364 * Provisioning of service in an Access-Reject. Such provisioning 1365 is not permitted, and MUST NOT be used. If limited access needs 1366 to be provided, then an Access-Accept with appropriate 1367 authorizations can be used instead. 1369 * Lack of user authentication or authorization restrictions. 1370 In an authorization check, where there is no demonstration of a 1371 live user, confidential data cannot be returned. Where there 1372 is a link to a previous user authentication, the State attribute 1373 SHOULD be present. 1375 * Lack of per-packet integrity and authentication. 1377 It is expected that documents will support per-packet 1378 integrity and authentication. 1380 * Modification of RADIUS packet sequences. 1381 In RADIUS, each request is encapsulated in its own packet, and 1382 elicits a single response that is sent to the requester. Since 1383 changes to this paradigm are likely to require major 1384 modifications to RADIUS client and server implementations, they 1385 SHOULD be avoided if possible. 1386 For further details, see Section 3.1. 1388 A.5. Allocation of attributes 1390 Does the attribute have a limited scope of applicability, as outlined 1391 below? If so, then the attributes SHOULD be allocated from the 1392 vendor space, rather than requesting allocation from the standard 1393 space. 1395 * attributes intended for a vendor to support their own systems, 1396 and not suitable for general usage 1398 * attributes relying on data types not defined within RADIUS 1400 * attributes intended primarily for use within an SDO 1402 * attributes intended primarily for use within a group of SDOs. 1404 Note that the points listed above do not relax the recommendations 1405 discussed in this document. Instead, they recognize that the RADIUS 1406 data model has limitations. In certain situations where 1407 interoperability can be strongly constrained by the SDO or vendor, an 1408 expanded data model MAY be used. It is RECOMMENDED, however, that 1409 the RADIUS data model be used, even when it is marginally less 1410 efficient than alternatives. 1412 When attributes are used primarily within a group of SDOs, and are 1413 not applicable to the wider Internet community, we expect that one 1414 SDO will be responsible for allocation from their own private space. 1416 Appendix B - Complex Attributes 1418 This section summarizes RADIUS attributes with complex data types 1419 that are defined in existing RFCs. 1421 This appendix is published for informational purposes only, and 1422 reflects the usage of attributes with complex data types at the time 1423 of the publication of this document. 1425 B.1. CHAP-Password 1427 [RFC2865] Section 5.3 defines the CHAP-Password Attribute which is 1428 sent from the RADIUS client to the RADIUS server in an Access- 1429 Request. The data type of the CHAP Identifier is not given, only the 1430 one octet length: 1432 0 1 2 1433 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 1434 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 1435 | Type | Length | CHAP Ident | String ... 1436 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 1438 Since this is an authentication attribute, code changes are required 1439 on the RADIUS client and server to support it, regardless of the 1440 attribute format. Therefore, this complex data type is acceptable in 1441 this situation. 1443 B.2. CHAP-Challenge 1445 [RFC2865] Section 5.40 defines the CHAP-Challenge Attribute which is 1446 sent from the RADIUS client to the RADIUS server in an Access- 1447 Request. While the data type of the CHAP Identifier is given, the 1448 text also says: 1450 If the CHAP challenge value is 16 octets long it MAY be placed in 1451 the Request Authenticator field instead of using this attribute. 1453 Defining attributes to contain values taken from the RADIUS packet 1454 header is NOT RECOMMENDED. Attributes should have values that are 1455 packed into a RADIUS AVP. 1457 B.3. Tunnel-Password 1459 [RFC2868] Section 3.5 defines the Tunnel-Password Attribute, which is 1460 sent from the RADIUS server to the client in an Access-Accept. This 1461 attribute includes Tag and Salt fields, as well as a string field 1462 which consists of three logical sub-fields: the Data-Length (one 1463 octet) and Password sub-fields (both of which are required), and the 1464 optional Padding sub-field. The attribute appears as follows: 1466 0 1 2 3 1467 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1468 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1469 | Type | Length | Tag | Salt 1470 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1471 Salt (cont) | String ... 1472 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1474 Since this is a security attribute, code changes are required on the 1475 RADIUS client and server to support it, regardless of the attribute 1476 format. However, while use of a complex data type is acceptable in 1477 this situation, the design of the Tunnel-Password attribute is 1478 problematic from a security perspective, since it uses MD5 as a 1479 cipher, and provides a password to a NAS, potentially without proper 1480 authorization. 1482 B.4. ARAP-Password 1484 [RFC2869] Section 5.4 defines the ARAP-Password attribute, which is 1485 sent from the RADIUS client to the server in an Access-Request. It 1486 contains four 4 octet values, instead of having a single Value field: 1488 0 1 2 3 1489 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1490 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1491 | Type | Length | Value1 1492 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1493 | Value2 1494 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1495 | Value3 1496 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1497 | Value4 1498 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1499 | 1500 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1502 As with the CHAP-Password attribute, this is an authentication 1503 attribute which would have required code changes on the RADIUS client 1504 and server regardless of format. 1506 B.5. ARAP-Features 1508 [RFC2869] Section 5.5 defines the ARAP-Features Attribute, which is 1509 sent from the RADIUS server to the client in an Access-Accept or 1510 Access-Challenge. It contains a compound string of two single octet 1511 values, plus three 4-octet values, which the RADIUS client 1512 encapsulates in a feature flags packet in the ARAP protocol: 1514 0 1 2 3 1515 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1516 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1517 | Type | Length | Value1 | Value2 | 1518 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1519 | Value3 | 1520 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1521 | Value4 | 1522 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1523 | Value5 | 1524 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1526 Unlike the previous attributes, this attribute contains no encrypted 1527 component, nor is it directly involved in authentication. The 1528 individual sub-fields therefore could have been encapsulated in 1529 separate attributes. 1531 While the contents of this attribute is intended to be placed in an 1532 ARAP packet, the fields need to be set by the RADIUS server. Using 1533 standard RADIUS data types would have simplified RADIUS server 1534 implementations, and subsequent management. The current form of the 1535 attribute requires either the RADIUS server implementation, or the 1536 RADIUS server administrator, to understand the internals of the ARAP 1537 protocol. 1539 B.6. Connect-Info 1541 [RFC2869] Section 5.11 defines the Connect-Info attribute, which is 1542 used to indicate the nature of the connection. 1544 0 1 2 1545 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 1546 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1547 | Type | Length | Text... 1548 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1550 Even though the type is Text, the rest of the description indicates 1551 that it is a complex attribute: 1553 The Text field consists of UTF-8 encoded 10646 _8_ characters. 1554 The connection speed SHOULD be included at the beginning of the 1555 first Connect-Info attribute in the packet. If the transmit and 1556 receive connection speeds differ, they may both be included in the 1557 first attribute with the transmit speed first (the speed the NAS 1558 modem transmits at), a slash (/), the receive speed, then 1559 optionally other information. 1561 For example, "28800 V42BIS/LAPM" or "52000/31200 V90" 1563 More than one Connect-Info attribute may be present in an 1564 Accounting-Request packet to accommodate expected efforts by ITU 1565 to have modems report more connection information in a standard 1566 format that might exceed 252 octets. 1568 This attribute contains no encrypted component, and is it not 1569 directly involved in authentication. The individual sub-fields could 1570 therefore have been encapsulated in separate attributes. 1572 However, since the definition refers to potential standardization 1573 activity within ITU, the Connect-Info attribute can also be thought 1574 of opaque data whose definition is provided elsewhere. The Connect- 1575 Info attribute could therefore qualify for an exception as described 1576 in Section 3.2.3. 1578 B.7. Framed-IPv6-Prefix 1580 [RFC3162] Section 2.3 defines the Framed-IPv6-Prefix Attribute and 1581 [RFC4818] Section 3 reuses this format for the Delegated-IPv6-Prefix 1582 Attribute; these attributes are sent from the RADIUS server to the 1583 client in an Access-Accept. 1585 0 1 2 3 1586 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1587 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1588 | Type | Length | Reserved | Prefix-Length | 1589 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1590 Prefix 1591 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1592 Prefix 1593 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1594 Prefix 1595 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1596 Prefix | 1597 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1599 The sub-fields encoded in these attributes are strongly related, and 1600 there was no previous definition of this data structure that could be 1601 referenced. Support for this attribute requires code changes on both 1602 the client and server, due to a new data type being defined. In this 1603 case it appears to be acceptable to encode them in one attribute. 1605 B.8. Egress-VLANID 1607 [RFC4675] Section 2.1 defines the Egress-VLANID Attribute which can 1608 be sent by a RADIUS client or server. 1610 0 1 2 3 1611 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1612 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1613 | Type | Length | Value 1614 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1615 Value (cont) | 1616 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1618 While it appears superficially to be of type Integer, the Value field 1619 is actually a packed structure, as follows: 1621 0 1 2 3 1622 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1623 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1624 | Tag Indic. | Pad | VLANID | 1625 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1627 The length of the VLANID field is defined by the [IEEE-802.1Q] 1628 specification. The Tag indicator field is either 0x31 or 0x32, for 1629 compatibility with the Egress-VLAN-Name, as discussed below. The 1630 complex structure of Egress-VLANID overlaps with that of the base 1631 Integer data type, meaning that no code changes are required for a 1632 RADIUS server to support this attribute. Code changes are required 1633 on the NAS, if only to implement the VLAN ID enforcement. 1635 Given the IEEE VLAN requirements and the limited data model of 1636 RADIUS, the chosen method is likely the best of the possible 1637 alternatives. 1639 B.9. Egress-VLAN-Name 1641 [RFC4675] Section 2.3 defines the Egress-VLAN-Name Attribute which 1642 can be sent by a RADIUS client or server. 1644 0 1 2 3 1645 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1646 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1647 | Type | Length | Tag Indic. | String... 1648 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1650 The Tag Indicator is either the character '1' or '2', which in ASCII 1651 map to the identical values for Tag Indicator in Egress-VLANID, 1652 above. The complex structure of this attribute is acceptable for 1653 reasons identical to those given for Egress-VLANID. 1654 B.10. Digest-* 1656 [RFC5090] attempts to standardize the functionality provided by an 1657 expired internet-draft [AAA-SIP] which improperly used two attributes 1658 from the standard space without being assigned them by IANA. This 1659 self-allocation is forbidden, as described above in Section 2. In 1660 addition, the draft uses nested attributes, which are discouraged in 1661 Section 2.1. The updated document uses basic data types, and 1662 allocates nearly 20 attributes in the process. 1664 However, the draft has seen wide-spread implementation, where 1665 [RFC5090] has not. One explanation may be that implementors 1666 disagreed with the trade-offs made in the updated specification. It 1667 may have been better to simply document the existing format, and 1668 request IANA allocation of two attributes. The resulting design 1669 would have used nested attributes, but may have gained more wide- 1670 spread implementation. 1672 Acknowledgments 1674 We would like to acknowledge David Nelson, Bernard Aboba, Emile van 1675 Bergen, Barney Wolff, Glen Zorn, Avi Lior, and Hannes Tschofenig for 1676 contributions to this document. 1678 Authors' Addresses 1680 Greg Weber 1681 Knoxville, TN 37932 1682 USA 1684 Email: gdweber@gmail.com 1686 Alan DeKok 1687 The FreeRADIUS Server Project 1688 http://freeradius.org/ 1690 Email: aland@freeradius.org