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Internet Engineering Task Force (IETF)                     A. DeKok, Ed.Request for Comments: 6158                                    FreeRADIUSBCP: 158                                                        G. WeberCategory: Best Current Practice                   Individual ContributorISSN: 2070-1721                                               March 2011RADIUS Design GuidelinesAbstract   This document provides guidelines for the design of attributes used   by the Remote Authentication Dial In User Service (RADIUS) protocol.   It is expected that these guidelines will prove useful to authors and   reviewers of future RADIUS attribute specifications, within the IETF   as well as other Standards Development Organizations (SDOs).Status of This Memo   This memo documents an Internet Best Current Practice.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   BCPs is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6158.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.DeKok & Weber             Best Current Practice                 [Page 1]

RFC 6158                RADIUS Design Guidelines              March 2011   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1. Introduction ....................................................31.1. Terminology ................................................41.2. Requirements Language ......................................41.3. Applicability ..............................................51.3.1. Reviews .............................................52. Guidelines ......................................................62.1. Data Types .................................................82.2. Vendor Space ...............................................92.3. Service Definitions and RADIUS .............................92.4. Translation of Vendor Specifications ......................103. Rationale ......................................................113.1. RADIUS Operational Model ..................................113.2. Data Model Issues .........................................143.2.1. Issues with Definitions of Types ...................153.2.2. Tagging Mechanism ..................................163.2.3. Complex Data Types .................................163.2.4. Complex Data Type Exceptions .......................183.3. Vendor Space ..............................................193.3.1. Interoperability Considerations ....................203.3.2. Vendor Allocations .................................203.3.3. SDO Allocations ....................................203.4. Polymorphic Attributes ....................................214. IANA Considerations ............................................225. Security Considerations ........................................225.1. New Data Types and Complex Attributes .....................236. References .....................................................246.1. Normative References ......................................246.2. Informative References ....................................24Appendix A.  Design Guidelines Checklist ..........................27A.1. Types Matching the RADIUS Data Model ......................27A.1.1. Transport of Basic Data Types ........................27A.1.2. Transport of Authentication and Security Data ........27A.1.3. Opaque Data Types ....................................27A.1.4. Pre-existing Data Types ..............................28DeKok & Weber             Best Current Practice                 [Page 2]

RFC 6158                RADIUS Design Guidelines              March 2011A.2. Improper Data Types .......................................28A.2.1. Simple Data Types ....................................28A.2.2. More Complex Data Types ..............................29A.3. Vendor-Specific Formats ...................................29A.4. Changes to the RADIUS Operational Model ...................30A.5. Allocation of Attributes ..................................31Appendix B.  Complex Attributes ...................................32B.1. CHAP-Password .............................................32B.2. CHAP-Challenge ............................................32B.3. Tunnel-Password ...........................................33B.4. ARAP-Password .............................................33B.5. ARAP-Features .............................................34B.6. Connect-Info ..............................................34B.7. Framed-IPv6-Prefix ........................................35B.8. Egress-VLANID .............................................36B.9. Egress-VLAN-Name ..........................................37B.10. Digest-* .................................................37   Acknowledgments ...................................................371.  Introduction   This document provides guidelines for the design of Remote   Authentication Dial In User Service (RADIUS) attributes within the   IETF as well as within other Standards Development Organizations   (SDOs).  By articulating RADIUS design guidelines, it is hoped that   this document will encourage the development and publication of high-   quality RADIUS attribute specifications.   However, the advice in this document will not be helpful unless it is   put to use.  As with "Guidelines for Authors and Reviewers of MIB   Documents" [RFC4181], it is expected that authors will check their   document against the guidelines in this document prior to publication   or requesting review (such as an "Expert Review" described in   [RFC3575]).  Similarly, it is expected that this document will be   used by reviewers (such as WG participants or the Authentication,   Authorization, and Accounting (AAA) Doctors [DOCTORS]), resulting in   an improvement in the consistency of reviews.   In order to meet these objectives, this document needs to cover not   only the science of attribute design but also the art.  Therefore, in   addition to covering the most frequently encountered issues, this   document explains some of the considerations motivating the   guidelines.  These considerations include complexity trade-offs that   make it difficult to provide "hard and fast" rules for attribute   design.  This document explains those trade-offs through reviews of   current attribute usage.DeKok & Weber             Best Current Practice                 [Page 3]

RFC 6158                RADIUS Design Guidelines              March 2011   The rest of the document is organized as follows.Section 1   discusses the applicability of the guidelines and defines a   recommended review process for RADIUS specifications.Section 2   defines the design guidelines in terms of what is "RECOMMENDED" and   "NOT RECOMMENDED".Section 3 gives a longer explanation of the   rationale behind the guidelines given in the previous section.Appendix A repeats the guidelines in a "checklist" format.AppendixB discusses previously defined attributes that do not follow the   guidelines.   Authors of new RADIUS specifications can be compliant with the design   guidelines by working through the checklists given inAppendix A.   Reviewers of RADIUS specifications are expected to be familiar with   the entire document.1.1.  Terminology   This document uses the following terms:   Network Access Server (NAS)      A device that provides an access service for a user to a network.   RADIUS server      A RADIUS authentication, authorization, and accounting (AAA)      server is an entity that provides one or more AAA services to a      NAS.   Standard space      Codes in the RADIUS Attribute Type Space that are allocated by      IANA and that follow the format defined inSection 5 of RFC 2865      [RFC2865].   Vendor space      The contents of the Vendor-Specific Attribute (VSA), as defined in[RFC2865], Section 5.26.  These attributes provide a unique      attribute type space in the "String" field for each vendor      (identified by the Vendor-Type field), which they can self-      allocate.1.2.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].DeKok & Weber             Best Current Practice                 [Page 4]

RFC 6158                RADIUS Design Guidelines              March 20111.3.  Applicability   The advice in this document applies to RADIUS attributes used to   encode service-provisioning, authentication, or accounting data based   on the attribute encodings and data formats defined inRFC 2865   [RFC2865],RFC 2866 [RFC2866], and subsequent RADIUS RFCs.   Since this document represents a Best Current Practice, it does not   update or deprecate existing standards.  As a result, uses of the   terms "MUST" and "MUST NOT" are limited to requirements already   present in existing documents.   It is RECOMMENDED that these guidelines be followed for all new   RADIUS specifications, whether they originate from a vendor, an SDO,   or the IETF.  Doing so will ensure the widest possible applicability   and interoperability of the specifications, while requiring minimal   changes to existing systems.  In particular, it is expected that   RADIUS specifications requesting allocation within the standard space   will follow these guidelines and will explain why this is not   possible if they cannot.   However, there are situations in which vendors or SDOs can choose not   to follow these guidelines without major consequences.  As noted inSection 5.26 of [RFC2865], Vendor-Specific Attributes (VSAs) are   "available to allow vendors to support their own extended Attributes   not suitable for general usage".  Where vendors or SDOs develop   specifications "not suitable for general usage", limited   interoperability and inability to use existing implementations may be   acceptable, and, in these situations, vendors and SDOs MAY choose not   to conform to these guidelines.   Note that the RADEXT WG is currently (as of 2011) involved in   developing updates to RADIUS.  Those updates will provide their own   usage guidelines that may modify some of the guidelines defined here,   such as defining new data types, practices, etc.   RADIUS protocol changes, or specification of attributes (such as   Service-Type), that can, in effect, provide new RADIUS commands   require greater expertise and deeper review, as do changes to the   RADIUS operational model.  As a result, such changes are outside the   scope of this document and MUST NOT be undertaken outside the IETF.1.3.1.  Reviews   For specifications utilizing attributes within the standard space,   conformance with the design guidelines in this document is expected   unless a good case can be made for an exception.  Reviewers SHOULD   use the design guidelines as a review checklist.DeKok & Weber             Best Current Practice                 [Page 5]

RFC 6158                RADIUS Design Guidelines              March 2011   While not required, IETF review may also be beneficial for   specifications utilizing the vendor space.  Experience has shown that   attributes not originally designed for general usage can subsequently   garner wide-spread deployment.  An example is the Vendor-Specific   Attributes defined in [RFC2548], which have been widely implemented   within IEEE 802.11 Access Points.   In order to assist in the development of specifications conforming to   these guidelines, authors can request review by sending an email to   the AAA Doctors [DOCTORS] or equivalent mailing list.  The IETF   Operations & Management Area Directors will then arrange for the   review to be completed and posted to the AAA Doctors mailing list   [DOCTORS], RADEXT WG mailing list, or other IETF mailing lists.   Since reviews are handled by volunteers, responses are provided on a   best-effort basis, with no service-level guarantees.  Authors are   encouraged to seek review as early as possible, so as to avoid   potential delays.   As reviewers require access to the specification, vendors and SDOs   are encouraged to make it publicly available.  Where the RADIUS   specification is embedded within a larger document that cannot be   made public, the RADIUS attribute and value definitions can be made   available on a public web site or can be published as an   Informational RFC, as with [RFC4679].   The review process requires neither allocation of attributes within   the standard space nor publication of an RFC.  Requiring SDOs or   vendors to rehost VSAs into the standard space solely for the purpose   of obtaining review would put pressure on the standard space and may   be harmful to interoperability since it would create two ways to   provision the same service.  Rehosting may also require changes to   the RADIUS data model, which will affect implementations that do not   intend to support the SDO or vendor specifications.   Similarly, vendors are encouraged to make their specifications   publicly available, for maximum interoperability.  However, it is not   necessary for a vendor to request publication of a VSA specification   as an RFC.2.  Guidelines   The RADIUS protocol as defined in [RFC2865] and [RFC2866] uses   elements known as attributes in order to represent authentication,   authorization, and accounting data.DeKok & Weber             Best Current Practice                 [Page 6]

RFC 6158                RADIUS Design Guidelines              March 2011   Unlike Simple Network Management Protocol (SNMP), first defined in   [RFC1157] and [RFC1155], RADIUS does not define a formal data   definition language.  The data type of RADIUS attributes is not   transported on the wire.  Rather, the data type of a RADIUS attribute   is fixed when an attribute is defined.  Based on the RADIUS attribute   type code, RADIUS clients and servers can determine the data type   based on pre-configured entries within a data dictionary.   To explain the implications of this early RADIUS design decision, we   distinguish two kinds of data types, namely "basic" and "complex".   Basic data types use one of the existing RADIUS data types as defined   inSection 2.1, encapsulated in a [RFC2865] RADIUS attribute or in a   [RFC2865] RADIUS VSA.  All other data formats are "complex types".   RADIUS attributes can be classified into one of three broad   categories:      * Attributes that are of interest to a single vendor, e.g., for a        product or product line.  Minimal cross-vendor interoperability        is needed.        Vendor-Specific Attributes (VSAs) are appropriate for use in        this situation.  Code-point allocation is managed by the vendor        with the vendor space defined by their Private Enterprise Number        (PEN), as given in the Vendor-Id field.      * Attributes that are of interest to an industry segment, where an        SDO defines the attributes for that industry.  Multi-vendor        interoperability within an industry segment is expected.        Vendor-Specific Attributes (VSAs) MUST be used.  Code-point        allocation is managed by the SDO with the vendor space defined        by the SDO's PEN rather than the PEN of an individual vendor.      * Attributes that are of broad interest to the Internet community.        Multi-vendor interoperability is expected.        Attributes within the standard space are appropriate for this        purpose and are allocated via IANA as described in [RFC3575].        Since the standard space represents a finite resource, and is        the only attribute space available for use by IETF working        groups, vendors, and SDOs are encouraged to utilize the vendor        space rather than request allocation of attributes from the        standard space.  Usage of attribute type codes reserved for        standard attributes is considered antisocial behavior and is        strongly discouraged.DeKok & Weber             Best Current Practice                 [Page 7]

RFC 6158                RADIUS Design Guidelines              March 20112.1.  Data Types   RADIUS defines a limited set of data types, defined as "basic data   types".  The following data qualifies as "basic data types":      * 32-bit unsigned integer in network byte order.      * Enumerated data types, represented as a 32-bit unsigned integer        with a list of name to value mappings (e.g., Service-Type).      * IPv4 address in network byte order.      * Time as a 32-bit unsigned value in network byte order and in        seconds since 00:00:00 UTC, January 1, 1970.      * IPv6 address in network byte order.      * Interface-Id (8-octet string in network byte order).      * IPv6 prefix.      * String (i.e., binary data), totaling 253 octets or less in        length.  This includes the opaque encapsulation of data        structures defined outside of RADIUS.  See alsoAppendix A.1.3        for additional discussion.      * UTF-8 text [RFC3629], totaling 253 octets or less in length.   Note that the length limitations for VSAs of type String and Text are   less than 253 octets, due to the additional overhead of the Vendor-   Specific encoding.   The following data also qualifies as "basic data types":      * Attributes grouped into a logical container using the [RFC2868]        tagging mechanism.  This approach is NOT RECOMMENDED (seeSection 3.2.2) but is permissible where the alternatives are        worse.      * Attributes requiring the transport of more than 253 octets of        Text or String data.  This includes the opaque encapsulation of        data structures defined outside of RADIUS, e.g., EAP-Message.   All other data formats (including nested attributes) are defined to   be "complex data types" and are NOT RECOMMENDED for normal use.   Complex data types MAY be used in situations where they reduce   complexity in non-RADIUS systems or where using the basic data types   would be awkward (such as where grouping would be required in orderDeKok & Weber             Best Current Practice                 [Page 8]

RFC 6158                RADIUS Design Guidelines              March 2011   to link related attributes).  Since there are no "hard and fast"   rules for where complexity is best located, each situation has to be   decided on a case-by-case basis.  Examples of this trade-off are   discussed inAppendix B.  Where a complex data type is selected, an   explanation SHOULD be offered as to why this was necessary.2.2.  Vendor Space   The Vendor space is defined to be the contents of the Vendor-Specific   Attribute ([RFC2865], Section 5.26) where the Vendor-Id defines the   space for a particular vendor, and the contents of the "String" field   define a unique attribute type space for that vendor.  As discussed   there, it is intended for vendors and SDOs to support their own   attributes not suitable for general use.   While the encoding of attributes within the vendor space is under the   control of vendors and SDOs, following the guidelines described here   is advantageous since it enables maximum interoperability with   minimal changes to existing systems.   For example, RADIUS server support for new attributes using "basic   data types" can typically be accomplished by editing a RADIUS   dictionary, whereas "complex data types" typically require RADIUS   server code changes, which can add complexity and delays in   implementation.   Vendor RADIUS Attribute specifications SHOULD self-allocate   attributes from the vendor space rather than request an allocation   from within the standard space.   VSA encodings that do not follow the[RFC2865], Section 5.26 encoding   scheme are NOT RECOMMENDED.  Although [RFC2865] does not mandate it,   implementations commonly assume that the Vendor Id can be used as a   key to determine the on-the-wire encoding of a VSA.  Vendors   therefore SHOULD NOT use multiple encodings for VSAs that are   associated with a particular Vendor Id.  A vendor wishing to use   multiple VSA encodings SHOULD request one Vendor Id for each VSA   encoding that they will use.2.3.  Service Definitions and RADIUS   RADIUS specifications define how an existing service or protocol can   be provisioned using RADIUS, usually via the Service-Type Attribute.   Therefore, it is expected that a RADIUS attribute specification will   reference documents defining the protocol or service to be   provisioned.  Within the IETF, a RADIUS attribute specificationDeKok & Weber             Best Current Practice                 [Page 9]

RFC 6158                RADIUS Design Guidelines              March 2011   SHOULD NOT be used to define the protocol or service being   provisioned.  New services using RADIUS for provisioning SHOULD be   defined elsewhere and referenced in the RADIUS specification.   New attributes, or new values of existing attributes, SHOULD NOT be   used to define new RADIUS commands.  RADIUS attributes are intended   to:      * authenticate users      * authorize users (i.e., service provisioning or changes to        provisioning)      * account for user activity (i.e., logging of session activity)   Requirements for allocation of new commands (i.e., the Code field in   the packet header) and new attributes within the standard space are   described in[RFC3575], Section 2.1.2.4.  Translation of Vendor Specifications[RFC2865], Section 5.26 defines Vendor-Specific Attributes as   follows:      This Attribute is available to allow vendors to support their own      extended Attributes not suitable for general usage.  It MUST NOT      affect the operation of the RADIUS protocol.      Servers not equipped to interpret the vendor-specific information      sent by a client MUST ignore it (although it may be reported).      Clients which do not receive desired vendor-specific information      SHOULD make an attempt to operate without it, although they may do      so (and report they are doing so) in a degraded mode.   The limitation on changes to the RADIUS protocol effectively   prohibits VSAs from changing fundamental aspects of RADIUS operation,   such as modifying RADIUS packet sequences or adding new commands.   However, the requirement for clients and servers to be able to   operate in the absence of VSAs has proven to be less of a constraint   since it is still possible for a RADIUS client and server to mutually   indicate support for VSAs, after which behavior expectations can be   reset.   Therefore,RFC 2865 provides considerable latitude for development of   new attributes within the vendor space, while prohibiting development   of protocol variants.  This flexibility implies that RADIUS   attributes can often be developed within the vendor space without   loss (and possibly even with gain) in functionality.DeKok & Weber             Best Current Practice                [Page 10]

RFC 6158                RADIUS Design Guidelines              March 2011   As a result, translation of RADIUS attributes developed within the   vendor space into the standard space may provide only modest   benefits, while accelerating the exhaustion of the standard space.   We do not expect that all RADIUS attribute specifications requiring   interoperability will be developed within the IETF, and allocated   from the standard space.  A more scalable approach is to recognize   the flexibility of the vendor space, while working toward   improvements in the quality and availability of RADIUS attribute   specifications, regardless of where they are developed.   It is therefore NOT RECOMMENDED that specifications intended solely   for use by a vendor or SDO be translated into the standard space.3.  Rationale   This section outlines the rationale behind the above recommendations.3.1.  RADIUS Operational Model   The RADIUS operational model includes several assumptions:      * The RADIUS protocol is stateless.      * Provisioning of services is not possible within an Access-Reject        or Disconnect-Request.      * There is a distinction between authorization checks and user        authentication.      * The protocol provides for authentication and integrity        protection of packets.      * The RADIUS protocol is a Request/Response protocol.      * The protocol defines packet length restrictions.   While RADIUS server implementations may keep state, the RADIUS   protocol is stateless, although information may be passed from one   protocol transaction to another via the State Attribute.  As a   result, documents that require stateful protocol behavior without use   of the State Attribute are inherently incompatible with RADIUS as   defined in [RFC2865] and MUST be redesigned.  See [RFC5080],Section2.1.1 for additional discussion surrounding the use of the State   Attribute.   As noted in[RFC5080], Section 2.6, the intent of an Access-Reject is   to deny access to the requested service.  As a result, RADIUS does   not allow the provisioning of services within an Access-Reject orDeKok & Weber             Best Current Practice                [Page 11]

RFC 6158                RADIUS Design Guidelines              March 2011   Disconnect-Request.  Documents that include provisioning of services   within an Access-Reject or Disconnect-Request are inherently   incompatible with RADIUS and need to be redesigned.[RFC5176], Section 3 notes the following:      A Disconnect-Request MUST contain only NAS and session      identification attributes.  If other attributes are included in a      Disconnect-Request, implementations MUST send a Disconnect-NAK; an      Error-Cause Attribute with value "Unsupported Attribute" MAY be      included.   As a result, documents that include provisioning of services within a   Disconnect-Request are inherently incompatible with RADIUS and need   to be redesigned.   As noted in[RFC5080], Section 2.1.1, a RADIUS Access-Request may not   contain user authentication attributes or a State Attribute linking   the Access-Request to an earlier user authentication.  Such an   Access-Request, known as an authorization check, provides no   assurance that it corresponds to a live user.  RADIUS specifications   defining attributes containing confidential information (such as   Tunnel-Password) should be careful to prohibit such attributes from   being returned in response to an authorization check.  Also,[RFC5080], Section 2.1.1 notes that authentication mechanisms need to   tie a sequence of Access-Request/Access-Challenge packets together   into one authentication session.  The State Attribute is RECOMMENDED   for this purpose.   While [RFC2865] did not require authentication and integrity   protection of RADIUS Access-Request packets, subsequent   authentication mechanism specifications, such as RADIUS/EAP [RFC3579]   and Digest Authentication [RFC5090], have mandated authentication and   integrity protection for certain RADIUS packets.  [RFC5080],Section2.1.1 makes this behavior RECOMMENDED for all Access-Request packets,   including Access-Request packets performing authorization checks.  It   is expected that specifications for new RADIUS authentication   mechanisms will continue this practice.   The RADIUS protocol as defined in [RFC2865] is a request-response   protocol spoken between RADIUS clients and servers.  A single RADIUS   request packet ([RFC2865], [RFC2866], or [RFC5176]) will solicit in   response at most a single response packet, sent to the IP address and   port of the RADIUS client that originated the request.  Changes to   this model are likely to require major revisions to existing   implementations, and this practice is NOT RECOMMENDED.DeKok & Weber             Best Current Practice                [Page 12]

RFC 6158                RADIUS Design Guidelines              March 2011   The Length field in the RADIUS packet header is defined in[RFC2865]   Section 3.  It is noted there that the maximum length of a RADIUS   packet is 4096 octets.  As a result, attribute designers SHOULD NOT   assume that a RADIUS implementation can successfully process RADIUS   packets larger than 4096 octets.   Even when packets are less than 4096 octets, they may be larger than   the Path Maximum Transmission Unit (PMTU).  Any packet larger than   the PMTU will be fragmented, making communications more brittle as   firewalls and filtering devices often discard fragments.  Transport   of fragmented UDP packets appears to be a poorly tested code path on   network devices.  Some devices appear to be incapable of transporting   fragmented UDP packets, making it difficult to deploy RADIUS in a   network where those devices are deployed.  We RECOMMEND that RADIUS   messages be kept as small possible.   If a situation is envisaged where it may be necessary to carry   authentication, authorization, or accounting data in a packet larger   than 4096 octets, then one of the following approaches is   RECOMMENDED:      1.  Utilization of a sequence of packets.          For RADIUS authentication, a sequence of Access-          Request/Access-Challenge packets would be used.  For this to          be feasible, attribute designers need to enable inclusion of          attributes that can consume considerable space within Access-          Challenge packets.  To maintain compatibility with existing          NASes, either the use of Access-Challenge packets needs to be          permissible (as with RADIUS/EAP, defined in [RFC3579]) or          support for receipt of an Access-Challenge needs to be          indicated by the NAS (as in RADIUS Location [RFC5580]).  Also,          the specification needs to clearly describe how attribute          splitting is to be signaled and how attributes included within          the sequence are to be interpreted, without requiring stateful          operation.  Unfortunately, previous specifications have not          always exhibited the required foresight.  For example, even          though very large filter rules are conceivable, the NAS-          Filter-Rule Attribute defined in [RFC4849] is not permitted in          an Access-Challenge packet, nor is a mechanism specified to          allow a set of NAS-Filter-Rule Attributes to be split across          an Access-Request/Access-Challenge sequence.          In the case of RADIUS accounting, transporting large amounts          of data would require a sequence of Accounting-Request          packets.  This is a non-trivial change to RADIUS, since RADIUS          accounting clients would need to be modified to split theDeKok & Weber             Best Current Practice                [Page 13]

RFC 6158                RADIUS Design Guidelines              March 2011          attribute stream across multiple Accounting-Requests, and          billing servers would need to be modified to reassemble and          interpret the attribute stream.      2.  Utilization of names rather than values.          Where an attribute relates to a policy that could conceivably          be pre-provisioned on the NAS, then the name of the pre-          provisioned policy can be transmitted in an attribute rather          than the policy itself, which could be quite large.  An          example of this is the Filter-Id Attribute defined in[RFC2865], Section 5.11, which enables a set of pre-          provisioned filter rules to be referenced by name.      3.  Utilization of Packetization Layer Path MTU Discovery          techniques, as specified in [RFC4821].          As a last resort, where the above techniques cannot be made to          work, it may be possible to apply the techniques described in          [RFC4821] to discover the maximum supported RADIUS packet size          on the path between a RADIUS client and a home server.  While          such an approach can avoid the complexity of utilization of a          sequence of packets, dynamic discovery is likely to be time          consuming and cannot be guaranteed to work with existing          RADIUS implementations.  As a result, this technique is not          generally applicable.3.2.  Data Model Issues   While[RFC2865], Section 5 defines basic data types, later   specifications did not follow this practice.  This problem has led   implementations to define their own names for data types, resulting   in non-standard names for those types.   In addition, the number of vendors and SDOs creating new attributes   within the vendor space has grown, and this has led to some   divergence in approaches to RADIUS attribute design.  For example,   vendors and SDOs have evolved the data model to support functions   such as new data types along with attribute grouping and attribute   fragmentation, with different groups taking different approaches.   These approaches are often incompatible, leading to additional   complexity in RADIUS implementations.   In order to avoid repeating old mistakes, this section describes the   history of the RADIUS data model and attempts to codify existing   practices.DeKok & Weber             Best Current Practice                [Page 14]

RFC 6158                RADIUS Design Guidelines              March 20113.2.1.  Issues with Definitions of Types[RFC2865], Section 5 explicitly defines five data types: text,   string, address, integer, and time.  Both the names and   interpretations of the types are given.   Subsequent RADIUS specifications defined attributes by using type   names not defined in [RFC2865], without defining the new names as   done in [RFC2865].  They did not consistently indicate the format of   the value field using the same conventions as [RFC2865].  As a   result, the data type is ambiguous in some cases and may not be   consistent among different implementations.   It is out of the scope of this document to resolve all potential   ambiguities within existing RADIUS specifications.  However, in order   to prevent future ambiguities, it is RECOMMENDED that future RADIUS   attribute specifications explicitly define newly created data types   at the beginning of the document and indicate clearly the data type   to be used for each attribute.   For example, [RFC3162] utilizes, but does not explicitly define, a   type that encapsulates an IPv6 address (Sections2.1 and2.4) and   another type that encapsulates an IPv6 prefix (Section 2.3).  The   IPv6 address attributes confusingly are referenced as type "Address"   in the document.  This is a similar name as the "address" type   defined in [RFC2865], which was defined to refer solely to IPv4   addresses.   While the Framed-Interface-Id Attribute defined in [RFC3162],Section2.2 included a value field of 8 octets, the data type was not   explicitly indicated; therefore, there is controversy over whether   the format of the data was intended to be an 8-octet String or   whether a special Interface-Id type was intended.   Given that attributes encapsulating an IPv6 address and an IPv6   prefix are already in use, it is RECOMMENDED that RADIUS server   implementations include support for these as basic types, in addition   to the types defined in [RFC2865].  Where the intent is to represent   a specific IPv6 address, an "IPv6 address" type SHOULD be used.   Although it is possible to use an "IPv6 Prefix" type with a prefix   length of 128 to represent an IPv6 address, this usage is NOT   RECOMMENDED.  Implementations supporting the Framed-Interface-Id   Attribute may select a data type of their choosing (most likely an   8-octet String or a special "Interface Id" data type).DeKok & Weber             Best Current Practice                [Page 15]

RFC 6158                RADIUS Design Guidelines              March 2011   It is worth noting that since RADIUS only supports unsigned integers   of 32 bits, attributes using signed integer data types or unsigned   integer types of other sizes will require code changes and SHOULD be   avoided.   For [RFC2865] RADIUS VSAs, the length limitation of the String and   Text types is 247 octets instead of 253 octets, due to the additional   overhead of the Vendor-Specific Attribute.3.2.2.  Tagging Mechanism   [RFC2868] defines an attribute grouping mechanism based on the use of   a one-octet tag value.  Tunnel attributes that refer to the same   tunnel are grouped together by virtue of using the same tag value.   This tagging mechanism has some drawbacks.  There are a limited   number of unique tags (31).  The tags are not well suited for use   with arbitrary binary data values because it is not always possible   to tell if the first byte after the Length is the tag or the first   byte of the untagged value (assuming the tag is optional).   Other limitations of the tagging mechanism are that when integer   values are tagged, the value portion is reduced to three bytes,   meaning only 24-bit numbers can be represented.  The tagging   mechanism does not offer an ability to create nested groups of   attributes.  Some RADIUS implementations treat tagged attributes as   having the additional data types tagged-string and tagged-integer.   These types increase the complexity of implementing and managing   RADIUS systems.   For these reasons, the tagging scheme described inRFC 2868 is NOT   RECOMMENDED for use as a generic grouping mechanism.3.2.3.  Complex Data Types   As described in this section, the creation of complex types can lead   to interoperability and deployment issues, so they need to be   introduced with care.  For example, the RADIUS attribute encoding is   summarized in [RFC2865]:    0                   1                   2    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   |     Type      |    Length     |  Value ...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-DeKok & Weber             Best Current Practice                [Page 16]

RFC 6158                RADIUS Design Guidelines              March 2011   However, some standard attributes pack multiple sub-fields into the   "Value" field, resulting in the creation a non-standard, i.e.,   complex, type.  Separating these sub-fields into different   attributes, each with its own type and length, would have the   following benefits:      * When manual data entry is required, it is easier for an        administrator to enter the data as well-known types rather than        as complex structures.      * It enables additional error checking by leveraging the parsing        and validation routines for well-known types.      * It simplifies implementations by eliminating special-case,        attribute-specific parsing.   One of the fundamental goals of the RADIUS protocol design was to   allow RADIUS servers to be configured to support new attributes,   without requiring server code changes.  RADIUS server implementations   typically provide support for basic data types and define attributes   in a data dictionary.  This architecture enables a new attribute to   be supported by the addition of a dictionary entry, without requiring   other RADIUS server code changes.   Code changes can also be required in policy management systems and in   the RADIUS server's receive path.  These changes are due to   limitations in RADIUS server policy languages, which commonly provide   for limited operations (such as comparisons or arithmetic operations)   on the existing data types.  Many existing RADIUS policy languages   typically are not capable of parsing sub-elements or providing more   sophisticated matching functionality.   On the RADIUS client, code changes are typically required in order to   implement a new attribute.  The RADIUS client typically has to   compose the attribute dynamically when sending.  When receiving, a   RADIUS client needs to be able to parse the attribute and carry out   the requested service.  As a result, a detailed understanding of the   new attribute is required on clients, and data dictionaries are less   useful on clients than on servers.   Given these limitations, the introduction of new types can require   code changes on the RADIUS server, which would be unnecessary if   basic data types had been used instead.  In addition, if "ad hoc"   types are used, attribute-specific parsing is required, which means   more complex software to develop and maintain.  More complexity can   lead to more error-prone implementations, interoperability problems,DeKok & Weber             Best Current Practice                [Page 17]

RFC 6158                RADIUS Design Guidelines              March 2011   and even security vulnerabilities.  These issues can increase costs   to network administrators as well as reduce reliability and introduce   deployment barriers.3.2.4.  Complex Data Type Exceptions   As described inSection 2.1, the introduction of complex data types   is discouraged where viable alternatives are available.  A potential   exception is attributes that inherently require code changes on both   the client and server.  For example, as described inAppendix B,   complex attributes have been used in situations involving   authentication and security attributes, which need to be dynamically   computed and verified.  Supporting this functionality requires code   changes on both the RADIUS client and server, regardless of the   attribute format.  As a result, in most cases, the use of complex   attributes to represent these methods is acceptable and does not   create additional interoperability or deployment issues.   Another exception to the recommendation against complex types is for   types that can be treated as opaque data by the RADIUS server.  For   example, the EAP-Message Attribute, defined in [RFC3579],Section3.1, contains a complex data type that is an Extensible   Authentication Protocol (EAP) packet.  Since these complex types do   not need to be parsed by the RADIUS server, the issues arising from   server limitations do not arise.  Similarly, since attributes of   these complex types can be configured on the server using a data type   of String, dictionary limitations are also not encountered.AppendixA.1 includes a series of checklists that may be used to analyze a   design for RECOMMENDED and NOT RECOMMENDED behavior in relation to   complex types.   If the RADIUS Server simply passes the contents of an attribute to   some non-RADIUS portion of the network, then the data is opaque to   RADIUS and SHOULD be defined to be of type String.  A concrete way of   judging this requirement is whether or not the attribute definition   in the RADIUS document contains delineated fields for sub-parts of   the data.  If those fields need to be delineated in RADIUS, then the   data is not opaque to RADIUS, and it SHOULD be separated into   individual RADIUS attributes.   An examination of existing RADIUS RFCs discloses a number of complex   attributes that have already been defined.Appendix B includes a   listing of complex attributes used within [RFC2865], [RFC2868],   [RFC2869], [RFC3162], [RFC4818], and [RFC4675].  The discussion of   these attributes includes reasons why a complex type is acceptable or   suggestions for how the attribute could have been defined to follow   the RADIUS data model.DeKok & Weber             Best Current Practice                [Page 18]

RFC 6158                RADIUS Design Guidelines              March 2011   In other cases, the data in the complex type are described textually   in a specification.  This is possible because the data types are not   sent within the attributes but are a matter for endpoint   interpretation.  An implementation can define additional data types   and use these data types today by matching them to the attribute's   textual definition.3.3.  Vendor Space   The usage model for RADIUS VSAs is described in [RFC2865],Section6.2:      Note that RADIUS defines a mechanism for Vendor-Specific      extensions (Attribute 26) and the use of that should be encouraged      instead of allocation of global attribute types, for functions      specific only to one vendor's implementation of RADIUS, where no      interoperability is deemed useful.   Nevertheless, many new attributes have been defined in the vendor   space in situations where interoperability is not only useful but is   required.  For example, SDOs outside the IETF (such as the IEEE 802   and the 3rd Generation Partnership Project (3GPP)) have been assigned   Vendor-Ids, enabling them to define their own VSA encoding and assign   Vendor types within their own vendor space, as defined by their   unique Vendor-Id.   The use of VSAs by SDOs outside the IETF has gained in popularity for   several reasons:   Efficiency      As with SNMP, which defines an "Enterprise" Object Identifier      (OID) space suitable for use by vendors as well as other SDOs, the      definition of Vendor-Specific Attributes has become a common      occurrence as part of standards activity outside the IETF.  For      reasons of efficiency, it is easiest if the RADIUS attributes      required to manage a standard are developed within the same SDO      that develops the standard itself.  As noted in "Transferring MIB      Work from IETF Bridge MIB WG to IEEE 802.1 WG" [RFC4663], today      few vendors are willing to simultaneously fund individuals to      participate within an SDO to complete a standard as well as to      participate in the IETF in order to complete the associated RADIUS      attributes specification.   Attribute scarcity      The standard space is limited to 255 unique attributes.  Of these,      only about half remain available for allocation.  In the vendor      space, the number of attributes available is a function of the      encoding of the attribute (the size of the Vendor type field).DeKok & Weber             Best Current Practice                [Page 19]

RFC 6158                RADIUS Design Guidelines              March 20113.3.1.  Interoperability Considerations   Vendors and SDOs are reminded that the standard space and the   enumerated value space for enumerated attributes are reserved for   allocation through work published via the IETF, as noted in[RFC3575], Section 2.1.  In the past, some vendors and SDOs have   assigned vendor-specific meaning to "unused" values from the standard   space.  This process results in interoperability issues and is   counterproductive.  Similarly, the vendor-specific enumeration   practice discussed in[RFC2882], Section 2.2.1 is NOT RECOMMENDED.   If it is not possible to follow the IETF process, vendors and SDOs   SHOULD self-allocate an attribute, which MUST be in their own vendor   space as defined by their unique Vendor-Id, as discussed in Sections   3.3.2 and 3.3.3.   The design and specification of VSAs for multi-vendor usage SHOULD be   undertaken with the same level of care as standard RADIUS attributes.   Specifically, the provisions of this document that apply to standard   RADIUS attributes also apply to VSAs for multi-vendor usage.3.3.2.  Vendor Allocations   As noted in[RFC3575], Section 2.1, vendors are encouraged to utilize   VSAs to define functions "specific only to one vendor's   implementation of RADIUS, where no interoperability is deemed useful.   For functions specific only to one vendor's implementation of RADIUS,   the use of that should be encouraged instead of the allocation of   global attribute types".   The recommendation for vendors to allocate attributes from a vendor   space rather than via the IETF process is a recognition that vendors   desire to assert change control over their own RADIUS specifications.   This change control can be obtained by requesting a PEN from the   Internet Assigned Number Authority (IANA) for use as a Vendor-Id   within a Vendor-Specific Attribute.  The vendor can then allocate   attributes within the vendor space defined by that Vendor-Id at their   sole discretion.  Similarly, the use of data types (complex or   otherwise) within that vendor space is solely under the discretion of   the vendor.3.3.3.  SDO Allocations   Given the expanded utilization of RADIUS, it has become apparent that   requiring SDOs to accomplish all their RADIUS work within the IETF is   inherently inefficient and unscalable.  It is therefore RECOMMENDEDDeKok & Weber             Best Current Practice                [Page 20]

RFC 6158                RADIUS Design Guidelines              March 2011   that SDO RADIUS Attribute specifications allocate attributes from the   vendor space rather than request an allocation from the RADIUS   standard space for attributes matching any of the following criteria:      * Attributes relying on data types not defined within RADIUS      * Attributes intended primarily for use within an SDO      * Attributes intended primarily for use within a group of SDOs   Any new RADIUS attributes or values intended for interoperable use   across a broad spectrum of the Internet community SHOULD follow the   allocation process defined in [RFC3575].   The recommendation for SDOs to allocate attributes from a vendor   space rather than via the IETF process is a recognition that SDOs   desire to assert change control over their own RADIUS specifications.   This change control can be obtained by requesting a PEN from the   Internet Assigned Number Authority (IANA) for use as a Vendor-Id   within a Vendor-Specific Attribute.  The SDO can then allocate   attributes within the vendor space defined by that Vendor-Id at their   sole discretion.  Similarly, the use of data types (complex or   otherwise) within that vendor space is solely under the discretion of   the SDO.3.4.  Polymorphic Attributes   A polymorphic attribute is one whose format or meaning is dynamic.   For example, rather than using a fixed data format, an attribute's   format might change based on the contents of another attribute.  Or,   the meaning of an attribute may depend on earlier packets in a   sequence.   RADIUS server dictionary entries are typically static, enabling the   user to enter the contents of an attribute without support for   changing the format based on dynamic conditions.  However, this   limitation on static types does not prevent implementations from   implementing policies that return different attributes based on the   contents of received attributes; this is a common feature of existing   RADIUS implementations.   In general, polymorphism is NOT RECOMMENDED.  Polymorphism rarely   enables capabilities that would not be available through use of   multiple attributes.  Polymorphism requires code changes in the   RADIUS server in situations where attributes with fixed formats would   not require such changes.  Thus, polymorphism increases complexity   while decreasing generality, without delivering any corresponding   benefits.DeKok & Weber             Best Current Practice                [Page 21]

RFC 6158                RADIUS Design Guidelines              March 2011   Note that changing an attribute's format dynamically is not the same   thing as using a fixed format and computing the attribute itself   dynamically.  RADIUS authentication attributes, such as User-   Password, EAP-Message, etc., while being computed dynamically, use a   fixed format.4.  IANA Considerations   This document has no action items for IANA.  However, it does provide   guidelines for Expert Reviewers appointed as described in [RFC3575].5.  Security Considerations   This specification provides guidelines for the design of RADIUS   attributes used in authentication, authorization, and accounting.   Threats and security issues for this application are described in   [RFC3579] and [RFC3580]; security issues encountered in roaming are   described in [RFC2607].   Obfuscation of RADIUS attributes on a per-attribute basis is   necessary in some cases.  The current standard mechanism for this is   described in[RFC2865], Section 5.2 (for obscuring User-Password   values) and is based on the MD5 algorithm specified in [RFC1321].   The MD5 and SHA-1 algorithms have recently become a focus of scrutiny   and concern in security circles, and as a result, the use of these   algorithms in new attributes is NOT RECOMMENDED.  In addition,   previous documents referred to this method as generating "encrypted"   data.  This terminology is no longer accepted within the   cryptographic community.   Where new RADIUS attributes use cryptographic algorithms, algorithm   negotiation SHOULD be supported.  Specification of a mandatory-to-   implement algorithm is REQUIRED, and it is RECOMMENDED that the   mandatory-to-implement algorithm be certifiable under FIPS 140   [FIPS].   Where new RADIUS attributes encapsulate complex data types, or   transport opaque data, the security considerations discussed inSection 5.1 SHOULD be addressed.   Message authentication in RADIUS is provided largely via the Message-   Authenticator attribute.  SeeSection 3.2 of [RFC3579] and alsoSection 2.2.2 of [RFC5080], which say that client implementations   SHOULD include a Message-Authenticator Attribute in every Access-   Request.DeKok & Weber             Best Current Practice                [Page 22]

RFC 6158                RADIUS Design Guidelines              March 2011   In general, the security of the RADIUS protocol is poor.  Robust   deployments SHOULD support a secure communications protocol such as   IPsec.  SeeSection 4 of [RFC3579] andSection 5 of [RFC3580] for a   more in-depth explanation of these issues.   Implementations not following the suggestions outlined in this   document may be subject to problems such as ambiguous protocol   decoding, packet loss leading to loss of billing information, and   denial-of-service attacks.5.1.  New Data Types and Complex Attributes   The introduction of complex data types brings the potential for the   introduction of new security vulnerabilities.  Experience shows that   the common data types have few security vulnerabilities, or else that   all known issues have been found and fixed.  New data types require   new code, which may introduce new bugs and therefore new attack   vectors.   Some systems permit complex attributes to be defined via a method   that is more capable than traditional RADIUS dictionaries.  These   systems can reduce the security threat of new types significantly,   but they do not remove it entirely.   RADIUS servers are highly valued targets, as they control network   access and interact with databases that store usernames and   passwords.  An extreme outcome of a vulnerability due to a new,   complex type would be that an attacker is capable of taking complete   control over the RADIUS server.   The use of attributes representing opaque data does not reduce this   threat.  The threat merely moves from the RADIUS server to the system   that consumes that opaque data.  The threat is particularly severe   when the opaque data originates from the user and is not validated by   the NAS.  In those cases, the RADIUS server is potentially exposed to   attack by malware residing on an unauthenticated host.   Any system consuming opaque data that originates from a RADIUS system   SHOULD be properly isolated from that RADIUS system and SHOULD run   with minimal privileges.  Any potential vulnerabilities in the non-   RADIUS system will then have minimal impact on the security of the   system as a whole.DeKok & Weber             Best Current Practice                [Page 23]

RFC 6158                RADIUS Design Guidelines              March 20116.  References6.1.  Normative References   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate                 Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2865]     Rigney, C., Willens, S., Rubens, A., and W. Simpson,                 "Remote Authentication Dial In User Service (RADIUS)",RFC 2865, June 2000.   [RFC3575]     Aboba, B., "IANA Considerations for RADIUS (Remote                 Authentication Dial In User Service)",RFC 3575, July                 2003.6.2.  Informative References   [RFC1155]     Rose, M. and K. McCloghrie, "Structure and                 identification of management information for TCP/IP-                 based internets", STD 16,RFC 1155, May 1990.   [RFC1157]     Case, J., Fedor, M., Schoffstall, M., and J. Davin,                 "Simple Network Management Protocol (SNMP)",RFC 1157,                 May 1990.   [RFC1321]     Rivest, R., "The MD5 Message-Digest Algorithm",RFC1321, April 1992.   [RFC2548]     Zorn, G., "Microsoft Vendor-specific RADIUS                 Attributes",RFC 2548, March 1999.   [RFC2607]     Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy                 Implementation in Roaming",RFC 2607, June 1999.   [RFC2866]     Rigney, C., "RADIUS Accounting",RFC 2866, June 2000.   [RFC2868]     Zorn, G., Leifer, D., Rubens, A., Shriver, J.,                 Holdrege, M., and I. Goyret, "RADIUS Attributes for                 Tunnel Protocol Support",RFC 2868, June 2000.   [RFC2869]     Rigney, C., Willats, W., and P. Calhoun, "RADIUS                 Extensions",RFC 2869, June 2000.   [RFC2882]     Mitton, D., "Network Access Servers Requirements:                 Extended RADIUS Practices",RFC 2882, July 2000.   [RFC3162]     Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",RFC 3162, August 2001.DeKok & Weber             Best Current Practice                [Page 24]

RFC 6158                RADIUS Design Guidelines              March 2011   [RFC3579]     Aboba, B. and P. Calhoun, "RADIUS (Remote                 Authentication Dial In User Service) Support For                 Extensible Authentication Protocol (EAP)",RFC 3579,                 September 2003.   [RFC3580]     Congdon, P., Aboba, B., Smith, A., Zorn, G., and J.                 Roese, "IEEE 802.1X Remote Authentication Dial In User                 Service (RADIUS) Usage Guidelines",RFC 3580, September                 2003.   [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO                 10646", STD 63,RFC 3629, November 2003.   [RFC4181]     Heard, C., Ed., "Guidelines for Authors and Reviewers                 of MIB Documents",BCP 111,RFC 4181, September 2005.   [RFC4663]     Harrington, D., "Transferring MIB Work from IETF Bridge                 MIB WG to IEEE 802.1 WG",RFC 4663, September 2006.   [RFC4675]     Congdon, P., Sanchez, M., and B. Aboba, "RADIUS                 Attributes for Virtual LAN and Priority Support",RFC4675, September 2006.   [RFC4679]     Mammoliti, V., Zorn, G., Arberg, P., and R. Rennison,                 "DSL Forum Vendor-Specific RADIUS Attributes",RFC4679, September 2006.   [RFC4818]     Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix                 Attribute",RFC 4818, April 2007.   [RFC4821]     Mathis, M. and J. Heffner, "Packetization Layer Path                 MTU Discovery",RFC 4821, March 2007.   [RFC4849]     Congdon, P., Sanchez, M., and B. Aboba, "RADIUS Filter                 Rule Attribute",RFC 4849, April 2007.   [RFC5080]     Nelson, D. and A. DeKok, "Common Remote Authentication                 Dial In User Service (RADIUS) Implementation Issues and                 Suggested Fixes",RFC 5080, December 2007.   [RFC5090]     Sterman, B., Sadolevsky, D., Schwartz, D., Williams,                 D., and W. Beck, "RADIUS Extension for Digest                 Authentication",RFC 5090, February 2008.   [RFC5176]     Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.                 Aboba, "Dynamic Authorization Extensions to Remote                 Authentication Dial In User Service (RADIUS)",RFC5176, January 2008.DeKok & Weber             Best Current Practice                [Page 25]

RFC 6158                RADIUS Design Guidelines              March 2011   [DOCTORS]     AAA Doctors Mailing List, www.ietf.org/mail-                 archive/web/aaa-doctors.   [FIPS]        FIPS 140-3 (DRAFT), "Security Requirements for                 Cryptographic Modules",http://csrc.nist.gov/publications/PubsFIPS.html.   [IEEE-802.1Q] IEEE Standards for Local and Metropolitan Area                 Networks: Draft Standard for Virtual Bridged Local Area                 Networks, P802.1Q-2003, January 2003.   [RFC5580]     Tschofenig, H., Ed., Adrangi, F., Jones, M., Lior, A.,                 and B. Aboba, "Carrying Location Objects in RADIUS and                 Diameter",RFC 5580, August 2009.   [AAA-SIP]     Sterman, B., Sadolevsky, D., Schwartz, D., Williams,                 D., and W. Beck, "RADIUS Extension for Digest                 Authentication", Work in Progress, November 2004.DeKok & Weber             Best Current Practice                [Page 26]

RFC 6158                RADIUS Design Guidelines              March 2011Appendix A.  Design Guidelines Checklist   The following text provides guidelines for the design of attributes   used by the RADIUS protocol.  Specifications that follow these   guidelines are expected to achieve maximum interoperability with   minimal changes to existing systems.A.1. Types Matching the RADIUS Data ModelA.1.1. Transport of Basic Data Types   Does the data fit within the basic data types described inSection2.1?  If so, it SHOULD be encapsulated in a [RFC2865] format RADIUS   attribute or in a [RFC2865] format RADIUS VSA that uses one of the   existing RADIUS data types.A.1.2. Transport of Authentication and Security Data   Does the data provide authentication and/or security capabilities for   the RADIUS protocol as outlined below?  If so, use of a complex data   type is acceptable under the following circumstances:      * Complex data types that carry authentication methods that RADIUS        servers are expected to parse and verify as part of an        authentication process.      * Complex data types that carry security information intended to        increase the security of the RADIUS protocol itself.   Any data type carrying authentication and/or security data that is   not meant to be parsed by a RADIUS server is an "opaque data type",   as defined in Section A.1.3.A.1.3. Opaque Data Types   Does the attribute encapsulate an existing data structure defined   outside of the RADIUS specifications?  Can the attribute be treated   as opaque data by RADIUS servers (including proxies)?  If both   questions can be answered affirmatively, a complex structure MAY be   used in a RADIUS specification.   The specification of the attribute SHOULD define the encapsulating   attribute to be of type String.  The specification SHOULD refer to an   external document defining the structure.  The specification SHOULD   NOT define or describe the structure, for reasons discussed inSection 3.2.3.DeKok & Weber             Best Current Practice                [Page 27]

RFC 6158                RADIUS Design Guidelines              March 2011A.1.4. Pre-Existing Data Types   There is a trade-off in design between reusing existing formats for   historical compatibility or choosing new formats for a "better"   design.  This trade-off does not always require the "better" design   to be used.  As a result, pre-existing complex data types described   inAppendix B MAY be used.A.2. Improper Data Types   This section suggests alternatives to data types that do not fall   within the "basic data type" definition.  Section A.2.1 describes   simple data types, which should be replaced by basic data types.   Section A.2.2 describes more complex data types, which should be   replaced by multiple attributes using the basic data types.A.2.1. Simple Data Types   Does the attribute use any of the following data types?  If so, the   data type SHOULD be replaced with the suggested alternatives, or it   SHOULD NOT be used at all.      * Signed integers of any size.        SHOULD NOT be used.  SHOULD be replaced with one or more        unsigned integer attributes.  The definition of the attribute        can contain information that would otherwise go into the sign        value of the integer.      * 8-bit unsigned integers.        SHOULD be replaced with 32-bit unsigned integer.  There is        insufficient justification to save three bytes.      * 16-bit unsigned integers.        SHOULD be replaced with 32-bit unsigned integer.  There is        insufficient justification to save two bytes.      * Unsigned integers of size other than 32 bits.        SHOULD be replaced by an unsigned integer of 32 bits.  There is        insufficient justification to define a new size of integer.      * Integers of any size in non-network byte order.        SHOULD be replaced by unsigned integer of 32 bits in network.        There is no reason to transport integers in any format other        than network byte order.      * Multi-field text strings.        Each field SHOULD be encapsulated in a separate attribute.DeKok & Weber             Best Current Practice                [Page 28]

RFC 6158                RADIUS Design Guidelines              March 2011      * Polymorphic attributes.        Multiple attributes, each with a static data type, SHOULD be        defined instead.      * Nested attribute-value pairs (AVPs).        Attributes should be defined in a flat typespace.A.2.2. More Complex Data Types   Does the attribute:      * define a complex data type not described inAppendix B?      * that a RADIUS server and/or client is expected to parse,        validate, or create the contents of via a dynamic computation        (i.e., a type that cannot be treated as opaque data (Section        A.1.3))?      * involve functionality that could be implemented without code        changes on both the client and server (i.e., a type that doesn't        require dynamic computation and verification, such as those        performed for authentication or security attributes)?   If so, this data type SHOULD be replaced with simpler types, as   discussed inAppendix A.2.1.  See alsoSection 2.1 for a discussion   of why complex types are problematic.A.3. Vendor-Specific Formats   Does the specification contain Vendor-Specific Attributes that match   any of the following criteria?  If so, the VSA encoding should be   replaced with the[RFC2865], Section 5.26 encoding or should not be   used at all.      * Vendor types of more than 8 bits.        SHOULD NOT be used.  Vendor types of 8 bits SHOULD be used        instead.      * Vendor lengths of less than 8 bits (i.e., zero bits).        SHOULD NOT be used.  Vendor lengths of 8 bits SHOULD be used        instead.      * Vendor lengths of more than 8 bits.        SHOULD NOT be used.  Vendor lengths of 8 bits SHOULD be used        instead.DeKok & Weber             Best Current Practice                [Page 29]

RFC 6158                RADIUS Design Guidelines              March 2011      * Vendor-specific contents that are not in Type-Length-Value        format.        SHOULD NOT be used.  Vendor-Specific Attributes SHOULD be in        Type-Length-Value format.   In general, Vendor-Specific Attributes SHOULD follow the encoding   suggested inSection 5.26 of [RFC2865].  Vendor extensions to non-   standard encodings are NOT RECOMMENDED as they can negatively affect   interoperability.A.4. Changes to the RADIUS Operational Model   Does the specification change the RADIUS operation model as outlined   in the list below?  If so, then another method of achieving the   design objectives SHOULD be used.  Potential problem areas include   the following:      * Defining new commands in RADIUS using attributes.        The addition of new commands to RADIUS MUST be handled via        allocation of a new Code and not by the use of an attribute.        This restriction includes new commands created by overloading        the Service-Type Attribute to define new values that modify the        functionality of Access-Request packets.      * Using RADIUS as a transport protocol for data unrelated to        authentication, authorization, or accounting.        Using RADIUS to transport authentication methods such as EAP is        explicitly permitted, even if those methods require the        transport of relatively large amounts of data.  Transport of        opaque data relating to AAA is also permitted, as discussed inSection 3.2.3. However, if the specification does not relate to        AAA, then RADIUS SHOULD NOT be used.      * Assuming support for packet lengths greater than 4096 octets.        Attribute designers cannot assume that RADIUS implementations        can successfully handle packets larger than 4096 octets.  If a        specification could lead to a RADIUS packet larger than 4096        octets, then the alternatives described inSection 3.3 SHOULD be        considered.      * Stateless operation.        The RADIUS protocol is stateless, and documents that require        stateful protocol behavior without the use of the State        Attribute need to be redesigned.DeKok & Weber             Best Current Practice                [Page 30]

RFC 6158                RADIUS Design Guidelines              March 2011      * Provisioning of service in an Access-Reject.        Such provisioning is not permitted, and MUST NOT be used.  If        limited access needs to be provided, then an Access-Accept with        appropriate authorizations can be used instead.      * Provisioning of service in a Disconnect-Request.        Such provisioning is not permitted and MUST NOT be used.  If        limited access needs to be provided, then a CoA-Request        [RFC5176] with appropriate authorizations can be used instead.      * Lack of user authentication or authorization restrictions.        In an authorization check, where there is no demonstration of a        live user, confidential data cannot be returned.  Where there is        a link to a previous user authentication, the State Attribute        SHOULD be present.      * Lack of per-packet integrity and authentication.        It is expected that documents will support per-packet integrity        and authentication.      * Modification of RADIUS packet sequences.        In RADIUS, each request is encapsulated in its own packet and        elicits a single response that is sent to the requester.  Since        changes to this paradigm are likely to require major        modifications to RADIUS client and server implementations, they        SHOULD be avoided if possible.   For further details, seeSection 3.1.A.5. Allocation of Attributes   Does the attribute have a limited scope of applicability as outlined   below?  If so, then the attributes SHOULD be allocated from the   vendor space rather than requesting allocation from the standard   space.      * attributes intended for a vendor to support their own systems        and not suitable for general usage      * attributes relying on data types not defined within RADIUS      * attributes intended primarily for use within an SDO      * attributes intended primarily for use within a group of SDOs   Note that the points listed above do not relax the recommendations   discussed in this document.  Instead, they recognize that the RADIUS   data model has limitations.  In certain situations whereDeKok & Weber             Best Current Practice                [Page 31]

RFC 6158                RADIUS Design Guidelines              March 2011   interoperability can be strongly constrained by the SDO or vendor, an   expanded data model MAY be used.  It is RECOMMENDED, however, that   the RADIUS data model be used, even when it is marginally less   efficient than alternatives.   When attributes are used primarily within a group of SDOs, and are   not applicable to the wider Internet community, we expect that one   SDO will be responsible for allocation from their own private vendor   space.Appendix B.  Complex Attributes   This appendix summarizes RADIUS attributes with complex data types   that are defined in existing RFCs.   This appendix is published for informational purposes only and   reflects the usage of attributes with complex data types at the time   of the publication of this document.B.1. CHAP-Password[RFC2865], Section 5.3 defines the CHAP-Password Attribute, which is   sent from the RADIUS client to the RADIUS server in an Access-   Request.  The data type of the CHAP Identifier is not given, only the   one-octet length:    0                   1                   2    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   |     Type      |    Length     |  CHAP Ident   |  String ...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   Since this is an authentication attribute, code changes are required   on the RADIUS client and server to support it, regardless of the   attribute format.  Therefore, this complex data type is acceptable in   this situation.B.2. CHAP-Challenge[RFC2865], Section 5.40 defines the CHAP-Challenge Attribute, which   is sent from the RADIUS client to the RADIUS server in an Access-   Request.  While the data type of the CHAP Identifier is given, the   text also says:      If the CHAP challenge value is 16 octets long it MAY be placed in      the Request Authenticator field instead of using this attribute.DeKok & Weber             Best Current Practice                [Page 32]

RFC 6158                RADIUS Design Guidelines              March 2011   Defining attributes to contain values taken from the RADIUS packet   header is NOT RECOMMENDED.  Attributes should have values that are   packed into a RADIUS AVP.B.3. Tunnel-Password[RFC2868], Section 3.5 defines the Tunnel-Password Attribute, which   is sent from the RADIUS server to the client in an Access-Accept.   This attribute includes Tag and Salt fields, as well as a String   field that consists of three logical sub-fields: the Data-Length   (required and one octet), Password sub-fields (required), and the   optional Padding sub-field.  The attribute appears as follows:    0                   1                   2                   3    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |     Tag       |   Salt   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      Salt (cont)  |   String ...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Since this is a security attribute, code changes are required on the   RADIUS client and server to support it, regardless of the attribute   format.  However, while use of a complex data type is acceptable in   this situation, the design of the Tunnel-Password Attribute is   problematic from a security perspective since it uses MD5 as a cipher   and provides a password to a NAS, potentially without proper   authorization.B.4. ARAP-Password[RFC2869], Section 5.4 defines the ARAP-Password Attribute, which is   sent from the RADIUS client to the server in an Access-Request.  It   contains four 4-octet values instead of having a single Value field:    0                   1                   2                   3    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |             Value1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |             Value2   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |             Value3   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |             Value4   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+DeKok & Weber             Best Current Practice                [Page 33]

RFC 6158                RADIUS Design Guidelines              March 2011   As with the CHAP-Password Attribute, this is an authentication   attribute that would have required code changes on the RADIUS client   and server, regardless of format.B.5. ARAP-Features[RFC2869], Section 5.5 defines the ARAP-Features Attribute, which is   sent from the RADIUS server to the client in an Access-Accept or   Access-Challenge.  It contains a compound string of two single octet   values, plus three 4-octet values, which the RADIUS client   encapsulates in a feature flags packet in the Apple Remote Access   Protocol (ARAP):   0                   1                   2                   3    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |     Value1    |    Value2     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                           Value3                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                           Value4                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                           Value5                              |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Unlike the previous attributes, this attribute contains no encrypted   component, nor is it directly involved in authentication.  The   individual sub-fields therefore could have been encapsulated in   separate attributes.   While the contents of this attribute are intended to be placed in an   ARAP packet, the fields need to be set by the RADIUS server.  Using   standard RADIUS data types would have simplified RADIUS server   implementations and subsequent management.  The current form of the   attribute requires either the RADIUS server implementation or the   RADIUS server administrator to understand the internals of the ARAP   protocol.B.6. Connect-Info[RFC2869], Section 5.11 defines the Connect-Info Attribute, which is   used to indicate the nature of the connection.    0                   1                   2    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |     Text...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+DeKok & Weber             Best Current Practice                [Page 34]

RFC 6158                RADIUS Design Guidelines              March 2011   Even though the type is Text, the rest of the description indicates   that it is a complex attribute:      The Text field consists of UTF-8 encoded 10646 [8] characters.      The connection speed SHOULD be included at the beginning of the      first Connect-Info attribute in the packet.  If the transmit and      receive connection speeds differ, they may both be included in the      first attribute with the transmit speed first (the speed the NAS      modem transmits at), a slash (/), the receive speed, then      optionally other information.      For example, "28800 V42BIS/LAPM" or "52000/31200 V90"      More than one Connect-Info attribute may be present in an      Accounting-Request packet to accommodate expected efforts by ITU      to have modems report more connection information in a standard      format that might exceed 252 octets.   This attribute contains no encrypted component and is not directly   involved in authentication.  The individual sub-fields could   therefore have been encapsulated in separate attributes.   However, since the definition refers to potential standardization   activity within ITU, the Connect-Info Attribute can also be thought   of as opaque data whose definition is provided elsewhere.  The   Connect-Info Attribute could therefore qualify for an exception as   described inSection 3.2.4.B.7. Framed-IPv6-PrefixSection 2.3 of [RFC3162] defines the Framed-IPv6-Prefix Attribute,   andSection 3 of [RFC4818] reuses this format for the Delegated-   IPv6-Prefix Attribute; these attributes are sent from the RADIUS   server to the client in an Access-Accept.    0                   1                   2                   3    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |  Reserved     | Prefix-Length |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                Prefix   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                Prefix   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                Prefix   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                Prefix                             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+DeKok & Weber             Best Current Practice                [Page 35]

RFC 6158                RADIUS Design Guidelines              March 2011   The sub-fields encoded in these attributes are strongly related, and   there was no previous definition of this data structure that could be   referenced.  Support for this attribute requires code changes on both   the client and server, due to a new data type being defined.  In this   case, it appears to be acceptable to encode them in one attribute.B.8. Egress-VLANID[RFC4675], Section 2.1 defines the Egress-VLANID Attribute, which can   be sent by a RADIUS client or server.    0                   1                   2                   3    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |            Value   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+           Value (cont)            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   While it appears superficially to be of type Integer, the Value field   is actually a packed structure, as follows:    0                   1                   2                   3    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Tag Indic.   |        Pad            |       VLANID          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The length of the VLANID field is defined by the [IEEE-802.1Q]   specification.  The Tag Indicator field is either 0x31 or 0x32, for   compatibility with the Egress-VLAN-Name, as discussed below.  The   complex structure of Egress-VLANID overlaps with that of the base   Integer data type, meaning that no code changes are required for a   RADIUS server to support this attribute.  Code changes are required   on the NAS, if only to implement the VLAN ID enforcement.   Given the IEEE VLAN requirements and the limited data model of   RADIUS, the chosen method is likely the best of the possible   alternatives.DeKok & Weber             Best Current Practice                [Page 36]

RFC 6158                RADIUS Design Guidelines              March 2011B.9. Egress-VLAN-Name[RFC4675], Section 2.3 defines the Egress-VLAN-Name Attribute, which   can be sent by a RADIUS client or server.    0                   1                   2                   3    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |   Tag Indic.  |   String...   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The Tag Indicator is either the character '1' or '2', which in ASCII   map to the identical values for Tag Indicator in Egress-VLANID above.   The complex structure of this attribute is acceptable for reasons   identical to those given for Egress-VLANID.B.10. Digest-*   [RFC5090] attempts to standardize the functionality provided by an   expired Internet-Draft [AAA-SIP], which improperly uses two   attributes from the standard space without having been assigned them   by IANA.  This self-allocation is forbidden, as described inSection2.  In addition, the document uses nested attributes, which are   discouraged inSection 2.1.  The updated document uses basic data   types and allocates nearly 20 attributes in the process.   However, the document has seen wide-spread implementation, but   [RFC5090] has not.  One explanation may be that implementors   disagreed with the trade-offs made in the updated specification.  It   may have been better to simply document the existing format and   request IANA allocation of two attributes.  The resulting design   would have used nested attributes but may have gained more wide-   spread implementation.Acknowledgments   We would like to acknowledge David Nelson, Bernard Aboba, Emile van   Bergen, Barney Wolff, Glen Zorn, Avi Lior, and Hannes Tschofenig for   contributions to this document.DeKok & Weber             Best Current Practice                [Page 37]

RFC 6158                RADIUS Design Guidelines              March 2011Authors' Addresses   Alan DeKok (editor)   The FreeRADIUS Server Projecthttp://freeradius.org/   EMail: aland@freeradius.org   Greg Weber   Knoxville, TN 37932   USA   EMail: gdweber@gmail.comDeKok & Weber             Best Current Practice                [Page 38]