Network Working Group G. Weber INTERNET-DRAFT Individual Contributor Category: Best Current Practice Alan DeKok (ed.) <draft-ietf-radext-design-05.txt> FreeRADIUS Expires: February 26, 2009 26 August 2008 RADIUS Design Guidelines draft-ietf-radext-design-05.txt By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 7, 2009. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract 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, both within the IETF as well as other Standards Development Organizations (SDOs). Weber, et al. Best Current Practice [Page 1]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Table of Contents 1. Introduction ............................................. 4 1.1. Applicability ....................................... 4 1.2. Terminology ......................................... 5 1.3. Requirements Language ............................... 5 2. RADIUS Data Model ........................................ 5 2.1. Standard Space ...................................... 6 2.1.1. Basic Data Types ............................... 6 2.1.2. Tagging Mechanism .............................. 7 2.1.3. Complex Attribute Usage ........................ 8 2.1.4. Complex Attributes and Security ................ 10 2.1.5. Service definitions and RADIUS ................. 11 2.2. Extended RADIUS Attributes .......................... 11 2.3. Vendor Space ........................................ 12 3. Data Model Issues ........................................ 13 3.1. Vendor Space ........................................ 14 3.1.1. Interoperability Considerations ................ 16 3.1.2. Vendor Allocations ............................. 16 3.1.3. SDO Allocations ................................ 17 3.1.4. Publication of specifications .................. 17 3.2. Polymorphic Attributes .............................. 18 3.3. RADIUS Operational Model ............................ 18 4. IANA Considerations ...................................... 21 5. Security Considerations .................................. 21 6. References ............................................... 22 6.1. Normative References ................................ 22 6.2. Informative References .............................. 22 Appendix A - Design Guidelines ............................... 25 A.1. Types matching the RADIUS data model ................. 25 A.1.1. Transport of simple data ........................ 25 A.1.2. Extended data types ............................. 25 A.1.3. Opaque data types ............................... 26 A.2. Improper Data Types .................................. 26 A.2.1. Simple Data Types ............................... 26 A.2.2. Complex Data Types .............................. 27 A.3. Vendor-Specific formats .............................. 27 A.4. Changes to the RADIUS Operational Model .............. 28 A.5. Allocation of attributes ............................. 29 Appendix B - Complex Attributes .............................. 30 B.1. CHAP-Password ........................................ 30 B.2. CHAP-Challenge ....................................... 30 B.3. Tunnel-Password ...................................... 30 B.4. ARAP-Password ........................................ 31 B.5. ARAP-Features ........................................ 31 B.6. Connect-Info ......................................... 32 B.7. Framed-IPv6-Prefix ................................... 32 B.8. Egress-VLANID ........................................ 33 Weber, et al. Best Current Practice [Page 2]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 B.9. Egress-VLAN-Name ..................................... 34 Full Copyright Statement ..................................... 35 Weber, et al. Best Current Practice [Page 3]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 1. Introduction This document provides guidelines for the design of RADIUS attributes both 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 this document will be used by authors to check their document against the guidelines prior to requesting review (such as an "Expert Review" described in [RFC3575]). Similarly, it is expected that this document will used by reviewers (such as WG participants or the 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. As a result, in addition to covering the most frequently encountered issues, this document attempts to provide some of the considerations motivating the guidelines. In order to characterize current attribute usage, both the basic and complex data types defined in the existing RADIUS RFCs are reviewed, together with the ad-hoc extensions to that data model that have been used in Vendor-Specific Attributes. 1.1. Applicability As RADIUS has become more widely accepted as a management protocol, its usage has become more prevalent, both within the IETF as well as within other SDOs. 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. By articulating guidelines for RADIUS attribute design, this document enables SDOs to design their own RADIUS attributes within the Vendor- Specific Attribute (VSA) space, seeking review from the IETF. In order to enable IETF review of SDO RADIUS attribute specifications, the authors recommend that: * SDOs make their RADIUS attribute specifications publicly available; * SDOs request review of RADIUS attribute specifications by sending email to the AAA Doctors [DOCTORS] or equivalent mailing list; Weber, et al. Best Current Practice [Page 4]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 * IETF and SDO RADIUS attribute specifications are reviewed according to the guidelines proposed in this document; * Reviews of specifications are posted to the RADEXT WG mailing list, the AAA Doctors mailing list [DOCTORS] or another IETF mailing list suggested by the Operations & Management Area Directors of the IETF. The advice in this document applies to attributes used to encode service-provisioning or authentication data. RADIUS protocol changes, or specification of attributes (such as Service-Type) that can be used to, in effect, provide new RADIUS commands require greater expertise and deeper review, as do changes to the RADIUS operational model, as described in Section 3.3 . Such changes should not be undertaken outside the IETF and when handled within the IETF require "IETF Consensus" for adoption, as noted in [RFC3575] Section 2.1. 1.2. 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/or accounting (AAA) server is an entity that provides one or more AAA services to a NAS. RADIUS proxy A RADIUS proxy acts as a RADIUS server to the NAS, and a RADIUS client to the RADIUS server. 1.3. 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]. 2. RADIUS Data Model The Remote Authentication Dial In User Service (RADIUS) defined in [RFC2865] and [RFC2866] uses elements known as attributes in order to represent authentication, authorization and accounting data. Unlike SNMP, first defined in [RFC1157] and [RFC1155], RADIUS does not define a formal data definition language. A handful of basic Weber, et al. Best Current Practice [Page 5]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 data types are provided, and a data type is associated with an attribute when the attribute is defined. Two distinct attribute spaces are defined: the standard space, and a Vendor-Specific space. Attributes in the standard space generally are composed of a type, length, value (TLV) triplet, although complex attributes have also been defined. The Vendor-Specific space is encapsulated within a single attribute type (Vendor-Specific Attribute). The format of this space is defined by individual vendors, but the same TLV encoding used by the standard space is recommended in [RFC2865] Section 5.26. The similarity between attribute formats has enabled implementations to leverage common parsing functionality, although in some cases the attributes in the Vendor-Specific space have begun to diverge from the common format. 2.1. Standard Space The following subsections describe common data types and formats within the RADIUS standard attribute space. Common exceptions are identified. 2.1.1. Basic Data Types The data type of RADIUS attributes is not transported on the wire. Rather, the data type of a RADIUS attribute is fixed when that 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. [RFC2865] defines the following data types: text 1-253 octets containing UTF-8 encoded 10646 [RFC3629] characters. Text of length zero (0) MUST NOT be sent; omit the entire attribute instead. string 1-253 octets containing binary data (values 0 through 255 decimal, inclusive). Strings of length zero (0) MUST NOT be sent; omit the entire attribute instead. IPv4 address 32 bit value, in network byte order. integer 32 bit unsigned value, most significant octet first. time 32 bit unsigned value, most significant octet first -- seconds since 00:00:00 UTC, January 1, 1970. In addition to these data types, follow-on RADIUS specifications define attributes using the following additional types: IPv6 address 128 bit value, most significant octet first. IPv6 prefix 8 bits of reserved, 8 bits of prefix length, up to 128 bits of value, in network byte order. Weber, et al. Best Current Practice [Page 6]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 integer64 64 bit unsigned value, most significant octet first. This type has also been used to represent an IPv6 interface identifier. Examples of the IPv6 address type include NAS-IPv6-Address defined in [RFC3162] Section 2.1 and Login-IPv6-Host defined in [RFC3162] Section 2.4. The IPv6 prefix type is used in [RFC3162] Section 2.3, and in [RFC4818] Section 3. The integer64 type is used for the ARAP- Challenge-Response Attribute defined in [RFC2869] Section 5.15, and the Framed-Interface-Id Attribute defined in [RFC3162] Section 2.2. [RFC4675] Section 2.4 defines User-Priority-Table as 64-bits in length, but denotes it as type String. Given that attributes of type IPv6 address, IPv6 prefix, and integer64 are already in use, it is RECOMMENDED that RADIUS server implementations include support for these additional basic types, in addition to the types defined in [RFC2865]. Where the intent is to represent a specific IPv6 address, the IPv6 address type SHOULD be used. Although it is possible to use the IPv6 IPv6 Prefix type with a prefix length of 128 to represent an IPv6 address, this usage is NOT RECOMMENDED. It is worth noting that since RADIUS only supports unsigned integers of 32 or 64 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. 2.1.2. 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 mechaism 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 Weber, et al. Best Current Practice [Page 7]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 attributes. Some RADIUS implementations treat tagged attributes as having additional data types tagged-string and tagged-integer. These types increase the complexity of implementing and managing RADIUS systems. New attributes SHOULD NOT use this tagging method because of the limitations described above. New attributes SHOULD use the grouping method described in [EXTEN]. 2.1.3. Complex Attribute Usage 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 ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- However, some standard attributes do not follow this format. Attributes that use sub-fields instead of using a basic data type are known as "complex attributes". As described below, the definition of complex attributes can lead to interoperability and deployment issues, so they need to be introduced with care. In general, complex attributes sent from the RADIUS server to the client can be supported by concatenating the values into a String data type field. However, separating these values into different attributes, each with its own type and length, would have the following benefits: * it is easier for the user to enter the data as well-known types, rather than 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 use 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 RADIUS server code changes. On the RADIUS client, code changes are typically required in order to implement a new attribute. The RADIUS client typically has to Weber, et al. Best Current Practice [Page 8]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 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 considerations, the introduction of a new basic or complex attribute will typically require code changes on the RADIUS client. The magnitude of changes for the complex attribute could be greater, due to the potential need for custom parsing logic. The RADIUS server can be configured to send a new static attribute by entering its type and data format in the RADIUS server dictionary, and then filling in the value within a policy based on the attribute name, data type and type-specific value. For complex attribute types not supported by RADIUS server dictionaries, changes to the dictionary code can be required in order to allow the new attribute to be supported by and configured on the RADIUS server. Code changes can also be required in policy management and in the RADIUS server's receive path. These changes are due to limitations in RADIUS server policy languages, which typically only provide for limited operations (such as comparisons or arithmetic operations) on the basic data types. Many existing RADIUS policy languages typically are not capable of parsing sub-elements, or providing sophisticated matching functionality. Given these limitations, the introduction of complex attributes can require code changes on the RADIUS server which would be unnecessary if basic data types had been used instead. In addition, attribute- specific parsing means more complex software to develop and maintain. More complexity can lead to more error prone implementations, interoperatibility problems, and even security vulnerabilities. These issues can increase costs to network administrators as well as reducing reliability and introducing deployment barriers. As a result, the introduction of new complex data types within RADIUS attribute specifications SHOULD be avoided, except in the case of complex attributes involving authentication or security functionality. As can be seen in Appendix B, most of the existing complex attributes involve authentication or security functionality. 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. Weber, et al. Best Current Practice [Page 9]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 The only other 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] Section 3.1 contains a complex data type that is an EAP packet. Since these complex types do not need to be parsed by the RADIUS server, the issues arising from policy language 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. Section A.1 below 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, 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, 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. In other cases, the data in the complex type are described textually. 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 description. 2.1.4. Complex Attributes and Security 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. 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. Weber, et al. Best Current Practice [Page 10]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 The use of attributes representing opaque data does not reduce this threat. The threat merely moves from the RADIUS server to the application 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. Applications consuming opaque data that reside on the RADIUS server SHOULD be properly isolated from the RADIUS server, and SHOULD run with minimal privileges. Any potential vulnerabilities in that application will then have minimal impact on the security of the system as a whole. 2.1.5. Service definitions and RADIUS RADIUS specifications define how an existing service or protocol can be provisioned using RADIUS. 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 specification 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) New commands (i.e., the Code field in the packet header) are allocated only through "IETF Consensus" as noted in [RFC3575] Section 2.1. Specifications also SHOULD NOT use new attributes to modify the interpretation of existing RADIUS commands. 2.2. Extended RADIUS Attributes The extended RADIUS attribute document [EXTEN] defines a number of extensions to RADIUS. The standard attribute space is extended by permitting standard allocations from within a specific subset of the VSA space; the format of extended attributes is defined; and extended data types are defined within that format. New specifications seeking to extend the standard RADIUS data model SHOULD examine [EXTEN] to see if their needs fit within the extended Weber, et al. Best Current Practice [Page 11]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 RADIUS data model. 2.3. Vendor Space As noted in [RFC2865] Section 5.26, the VSA format is defined 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 | Vendor-Id +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Vendor-Id (cont) | String... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- The high-order octet of the Vendor-Id field is 0 and the low-order 3 octets are the Structure of Management Information (SMI) Network Management Private Enterprise Code (PEC) of the Vendor in network byte order. While the format of the String field is defined by the vendor, [RFC2865] Section 5.26 notes: It SHOULD be encoded as a sequence of vendor type / vendor length / value fields, as follows. The Attribute-Specific field is dependent on the vendor's definition of that attribute. An example encoding of the Vendor-Specific attribute using this method 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 | Vendor-Id +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Vendor-Id (cont) | Vendor type | Vendor length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute-Specific... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Multiple sub-attributes MAY be encoded within a single Vendor- Specific attribute, although they do not have to be. Note that the Vendor type field in the recommended VSA format is only a single octet, like the RADIUS type field. While this limitation results in an efficient encoding, there are situations in which a vendor or SDO will eventually wish to define more than 255 attributes. Also, an SDO can be comprised of multiple subgroups, each of whom can desire autonomy over the definition of attributes Weber, et al. Best Current Practice [Page 12]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 within their group. In such a situation, a 16-bit Vendor type field would be more appropriate: 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 | Vendor-Id +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Vendor-Id (cont) | Vendor type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor length | Attribute-Specific... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ If additional functionality is required, the format defined in [EXTEN] SHOULD be used. Other attribute formats are NOT RECOMMENDED. Examples of NOT RECOMMENDED formats include Vendor types of more than 16 bits, Vendor lengths of less than 8 bits, Vendor lengths of more than 8 bits, and Vendor-Specific contents that are not in Type-Length-Value format. In order to be compatible with the above recommendations for attribute definitions, it is RECOMMENDED that RADIUS servers support attributes using a 16-bit Vendor type field. 3. Data Model Issues Since the closure of the RADIUS Working Group, the popularity and prevalence of RADIUS has continued to grow. In addition to increasing demand for allocation of attributes within the RADIUS standard attribute space, the number of vendors and SDOs creating new attributes within the Vendor-Specific attribute space has grown, and this has lead to some divergence in approaches to RADIUS attribute design. In general, standard RADIUS attributes have a more constrained data model than attributes within the vendor space. For example, vendors and SDOs have evolved the data model to support new functions such as attribute grouping and attribute fragmentation, with different groups taking different approaches. Given these enhancements, it has become difficult for vendors or SDOs to translate attributes from the vendor space to the more stringent standards space. For example, a Vendor-Specific attribute using sub- elements could require allocation of several standard space attributes using basic data types. In this case not only would translation require substantial additional work, it would further deplete the RADIUS standard attribute space. Given these Weber, et al. Best Current Practice [Page 13]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 limitations, translation of vendor attributes to the standards space is not necessarily desirable, particularly if the VSA specification is publicly available and can be implemented within existing RADIUS clients and servers. In such situations, the costs may substantially outweigh the benefits. It is possible that some of the enhancements made within the vendor space may eventually become available within the standard attribute space. However, the divergence of the standard and vendor attribute spaces is most likely a permanent feature, and should be recognized as such. Recent extensions to the RADIUS data model such as [EXTEN] make it possible to minimize the use of complex attributes. New specifications seeking to extend the standard RADIUS data model SHOULD examine [EXTEN] to see if their needs fit within the extended RADIUS data model. 3.1. Vendor Space The usage model for RADIUS VSAs is described in [RFC2865] Section 6.2: 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 specific space in situations where interoperability is not only useful, but is required. For example, Standards Development Organizations (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 format and assign Vendor types within their own space. 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 RADIUS 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 Weber, et al. Best Current Practice [Page 14]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 participate within an SDO to complete a standard, as well as to participate in IETF in order to complete the associated RADIUS attributes specification. Attribute scarcity The standard RADIUS attribute space is limited to 255 unique attributes. Of these, only about half remain available for allocation. In the Vendor-Specific space, the number of attributes available is a function of the format of the attribute (the size of the Vendor type field). Along with these advantages, some limitations of VSA usage are noted in [RFC2865] Section 5.26: 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 gain) in functionality. 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 attribute space. We do not expect that all RADIUS attribute specifications requiring interoperability will be developed within the IETF, and allocated from the standards 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. Weber, et al. Best Current Practice [Page 15]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 3.1.1. Interoperability Considerations Vendors and SDOs are reminded that the standard RADIUS attribute 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. Some vendors and SDOs have in the past performed self-allocation by assigning vendor-specific meaning to "unused" values from the standard RADIUS attribute ID or enumerated value space. This self-allocation results in interoperability issues, and is counter-productive. Similarly, the Vendor-Specific enumeration practice discussed in [RFC2882] Section 2.2.1 is NOT RECOMMENDED. If it is not possible to follow the above procedure, vendors and SDOs SHOULD self-allocate an attribute from their Vendor-Specific space, and define an appropriate value for it. As a side note, [RFC2865] Section 5.26 uses the term "Vendor-Specific Attribute" to refer to an encoding format which can be used by individual vendors to define attributes not suitable for general usage. However, since then VSAs have also become widely used by SDOs defining attributes intended for multi-vendor interoperability. As such, these attributes are not specific to any single vendor, and the term "Vendor-Specific" may be misleading. An alternate term which better describes this use case is SDO-Specific Attribute (SSA). 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 SSAs or VSAs for multi-vendor usage. 3.1.2. Vendor Allocations Vendor RADIUS Attribute specifications SHOULD allocate attributes from the vendor space, rather than requesting an allocation from the RADIUS standard attribute space. As discussed in [RFC2865] Section 5.26, the vendor space is intended for vendors to support their own extended attributes not suitable for general use. However, it is RECOMMENDED that vendors follow the guidelines outlined here, which are intended to enable maximum interoperability with minimal changes to existing systems. Following these guidelines means that RADIUS servers can be updated to support the vendor's equipment by editing a RADIUS dictionary. If these guidelines are not followed, then the vendor's equipment can only be supported via code changes in RADIUS server implementations. Such code changes add complexity and delay to implementations. Weber, et al. Best Current Practice [Page 16]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 3.1.3. SDO Allocations SDO RADIUS Attribute specifications SHOULD allocate attributes from the vendor space, rather than requesting an allocation from the RADIUS standard attribute 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 normal IETF process, and SHOULD result in allocations from the RADIUS standard space. The recommendation for SDOs to allocate attributes from a vendor space rather than via the IETF process is a recognition that SDOs may desire to assert change control over their own RADIUS specifications. This change control can be obtained by requesting a PEC from the Internet Assigned Number Authority (IANA), for use as a Vendor-Id within a Vendor-Specific attribute. Further allocation of attributes inside of the VSA space defined by that Vendor-Id is subject solely to the discretion of the SDO. Similarly, the use of data types (complex or not) within that VSA space is solely under the discretion of the SDO. It is RECOMMENDED that SDOs follow the guidelines outlined here, which are intended to enable maximum interoperability with minimal changes to existing systems. It should be understood that SDOs do not have to rehost VSAs into the standards space solely for the purpose of obtaining IETF review. Rehosting puts pressure on the standards space, and may be harmful to interoperability, since it can 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 specifications. 3.1.4. Publication of specifications SDOs are encouraged to seek early review of SSA specifications by the IETF. This review may be initiated by sending mail to the AAA Doctors list [DOCTORS]. Since the IETF is not a membership organization, in order to enable the RADIUS SSA specification to be reviewed, it is RECOMMENDED that it be made publicly available; this also encourages interoperability. Where the RADIUS SSA specification is embedded within a larger document which cannot be made public, the RADIUS attribute and value definitions SHOULD be published instead as an Informational RFC, as with [RFC4679]. This process SHOULD be Weber, et al. Best Current Practice [Page 17]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 followed instead of requesting IANA allocations from within the standard RADIUS attribute space. Similarly, vendors are encouraged to make their specifications publicly available, for maximum interoperability. However, it is not necessary for them to request publication of their VSA specifications as Informational RFCs by the IETF. All other specifications, including new authentication and/or security mechanisms SHOULD be allocated via the standard RADIUS space, as noted in [RFC3575] Section 2.1. 3.2. 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. 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. 3.3. RADIUS Operational Model The RADIUS operational model includes several assumptions: * The RADIUS protocol is stateless; * Provisioning of services is not possible within an Weber, et al. Best Current Practice [Page 18]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Access-Reject; * Distinction between authorization checks and user authentication; * Authentication and integrity protection of RADIUS packets; * The RADIUS protocol is a Request/Response protocol. * Packet length restriction. 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 which require stateful protocol behavior without use of the State Attribute are inherently incompatible with RADIUS as defined in [RFC2865], and need to be redesigned. See [RFC5080] Section 2.1.1 for a more in-depth discussion of 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. Documents which include provisioning of services within an Access- Reject 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] Section 2.1.1 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 Weber, et al. Best Current Practice [Page 19]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Access-Request packet will solicit in response at most a single Access-Accept, Access-Reject or Access-Challenge packet, sent to the IP address and port of the RADIUS Client that originated the Access- Request. Similarly, a single Change-of-Authorization (CoA)-Request packet [RFC5176] will solicit in response at most a single CoA-ACK or CoA-NAK packet, sent to the IP address and port of the Dynamic Authorization Client (DAC) that originated the CoA-Request. A single Disconnect-Request packet will solicit in response at most a single Disconnect-ACK or Disconnect-NAK packet, sent to the IP address and port of the Dynamic Authorization Client (DAC) that originated the CoA-Request. Changes to this model are likely to require major revisions to existing implementations and so this practice is NOT RECOMMENDED. 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. 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 [RADIUSLOC]). Also, the specification needs to clearly describe how attribute splitting is to be signalled 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 the attribute stream across multiple Accounting-Requests, and billing servers would Weber, et al. Best Current Practice [Page 20]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 need to be modified to re-assemble 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 discovery of of 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. 4. IANA Considerations This document requires no action by IANA. 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]. Encryption 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. 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]. Weber, et al. Best Current Practice [Page 21]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Where new RADIUS attributes encapsulate complex data types, or transport opaque data, the security considerations discussed in Section 2.1.4 SHOULD be addressed. 6. References 6.1. 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)", STD 15, RFC 1157, May 1990. [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [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. Weber, et al. Best Current Practice [Page 22]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 [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., Roese, J., "IEEE 802.1X Remote Authentication Dial In User Service (RADIUS) Usage Guidelines", RFC3580, September 2003. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 3629, November 2003. [RFC4181] Heard, C., "Guidelines for Authors and Reviewers of MIB Documents", 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", RFC 4675, September 2006. [RFC4679] Mammoliti, V., et al., "DSL Forum Vendor-Specific RADIUS Attributes", RFC 4679, September 2006. [RFC4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix Attribute", RFC 4818, April 2007. [RFC4821] Mathis, M. and Heffner, J, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. [RFC4849] Congdon, P. et al, "RADIUS Filter-Rule Attribute", RFC 4849, April 2007. [RFC5080] Nelson, D. and DeKok, A, "Common Remote Authentication Dial In User Service (RADIUS) Implementation Issues and Suggested Fixes", RFC 5080, December 2007. [RFC5090] Sterman, B. et al., "RADIUS Extension for Digest Authentication", RFC 5090, February 2008. [RFC5176] Chiba, M. et al., "Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)", RFC 5176, January 2008. [DOCTORS] AAA Doctors Mailing list <aaa-doctors@ops.ietf.org> [EXTEN] Li, Y., et al., "Extended Remote Authentication Dial In User Service (RADIUS) Attributes", draft-ietf-radext-extended- attributes-04.txt, (work in progress). Weber, et al. Best Current Practice [Page 23]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 [FIPS] FIPS 140-3 (DRAFT), "Security Requirements for Cryptographic Modules", http://csrc.nist.gov/publications/fips/fips140-3/ [IEEE-802.1Q] IEEE Standards for Local and Metropolitan Area Networks: Draft Standard for Virtual Bridged Local Area Networks, P802.1Q-2003, January 2003. [RADIUSLOC] Tschofenig, H. (Ed.), "Carrying Location Objects in RADIUS and Diameter", draft-ietf-geopriv-radius-lo-19.txt, (work in progress) Weber, et al. Best Current Practice [Page 24]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Appendix A - Design Guidelines 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 model A.1.1. Transport of simple data Does the data fit within the existing RADIUS data model, as outlined below? If so, it SHOULD be encapsulated in a [RFC2865] format RADIUS attribute, or in a [RFC2865] format RADIUS VSA. * 32-bit unsigned integer, most significant octet first. * Enumerated data types, represented as a 32-bit unsigned integer with a list of name to value mappings. (e.g., Service-Type) * 64-bit unsigned integer, most significant octet first. * IPv4 address in network byte order. * IPv6 address in network byte order. * IPv6 prefix. * time as 32 bit unsigned value, most significant octet first, in seconds since 00:00:00 UTC, January 1, 1970. * string (i.e., binary data), totalling 253 octets or less in length. This includes the opaque encapsulation of data structures defined outside of RADIUS. See also Section A.1.3, below. * UTF-8 text, totalling 253 octets or less in length. * Complex data types for authentication and/or security. These attributes SHOULD be defined only within the RADIUS attribute space, and SHOULD NOT be defined within the VSA space. 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 format. A.1.2. Extended data types Where possible, the data types defined above in Section A.1.1 SHOULD be used. The extended data types defined in [EXTEN] SHOULD be used only where there is no clear method of expressing the data using existing types. Does the data fit within the extended RADIUS data model, as outlined below? If so, it SHOULD be encapsulated in a [EXTEN] format RADIUS VSA. Weber, et al. Best Current Practice [Page 25]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 * Attributes grouped into a logical container. This does not include nested groups. * Attributes requiring the transport of more than 247 octets of Text or String data. This includes the opaque encapsulation of data structures defined outside of RADIUS. See also Section A.1.3, below. 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, as discussed above in Section 2.1.3. A.2. Improper Data Types All data types other than the ones described above in Section A.1 SHOULD NOT be used. This section describes in detail a number of data types that are NOT RECOMMENDED in new RADIUS specifications. Where possible, replacement data types are suggested. 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 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 or 64. SHOULD be replaced by an unsigned integer of 32 or 64 bits. There is insufficient justification to define a new size of Weber, et al. Best Current Practice [Page 26]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 integer. * Integers of any size in non-network byte order SHOULD be replaced by unsigned integer of 32 or 64 bits, in network byte order. There is no reason to transport integers in any format other than network byte order. * Tagged data types as described in [RFC2868]. These data types SHOULD NOT be used in new specifications. The attribute grouping method defined in [EXTEN] SHOULD be used instead. * Complex data structures defined only within RADIUS. The additional functionality defined in [EXTEN] SHOULD be used instead. This recommendation does not apply to new attributes for authentication or security, as described below in Section A.2.2. * Multi-field text strings. Each field SHOULD be encapsulated in a separate attribute. Where grouping of fields is desired, the additional functionality defined in [EXTEN] SHOULD be used instead. * Polymorphic attributes. Multiple attributes, each with a static data type SHOULD be defined instead. A.2.2. Complex Data Types Does the attribute define a complex data type for purposes other than authentication or security? If so, this data type SHOULD be replaced with simpler types, as discussed above in Section A.2.1. Also see Section 2.1.3 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 data type should be replaced with the suggested alternatives, or should not be used at all. * Vendor types of more than 16 bits. SHOULD NOT be used. Vendor types of 8 or 16 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. * Vendor-Specific contents that are not in Type-Length-Value format. SHOULD NOT be used. Vendor-Specific attributes SHOULD be in Weber, et al. Best Current Practice [Page 27]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Type-Length-Value format. We recognize that SDOs may require more than 256 attributes, which is the limit of the 8-bit [RFC2865] Vendor-Specific space. Those SDOs SHOULD use Vendor types of 16 bits, as described in [EXTEN]. However, SDOs SHOULD NOT use Vendor types of 16 bits unless there are immediate requirements. Future-proofing a specification is insufficient grounds for using 16-bit Vendor types. In general, Vendor-Specific attributes SHOULD follow the [RFC2865] suggested format, or the [EXTEN] format if more functionality or a larger attribute space is necessary. A.4. Changes to the RADIUS Operational Model Does the specification extend 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: * 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 above in Section 2.1.3. 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 in Section 3.3 SHOULD be considered. * Stateless operation. The RADIUS protocol is stateless, and documents which require stateful protocol behavior without the use of the State Attribute need to be redesigned. * 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. * Lack of user authentication or authorization restrictions. Weber, et al. Best Current Practice [Page 28]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 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 needs to 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 it's 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, see Section 3.3. 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-Specific 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 where interoperability can be strongly constrained, a data model extended by the SDO or vendor MAY be used. We recommend, however, that the RADIUS data model SHOULD be used, even if 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 space. Weber, et al. Best Current Practice [Page 29]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Appendix B - Complex Attributes This section summarizes RADIUS attributes with complex data types that are defined in existing RFCs. 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 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. 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 which consists of three logical sub-fields: the Data-Length (one octet) and Password sub-fields (both of which are 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 Weber, et al. Best Current Practice [Page 30]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Tag | Salt +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Salt (cont) | String ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Since this is a security attribute and is encrypted, 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.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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ As with the CHAP-Password attribute, this is an authentication attribute which 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 ARAP protocol: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Weber, et al. Best Current Practice [Page 31]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 | 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. 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... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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 it not directly involved in authentication. The individual sub-fields could therefore have been encapsulated in separate attributes. B.7. Framed-IPv6-Prefix [RFC3162] Section 2.3 defines the Framed-IPv6-Prefix Attribute and [RFC4818] Section 3 reuses this format for the Delegated-IPv6-Prefix Weber, et al. Best Current Practice [Page 32]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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 Weber, et al. Best Current Practice [Page 33]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 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. Future specifications that attempt to obtain similar functionality SHOULD use the extended types from [EXTEN]. B.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. Future specifications that attempt to obtain similar functionality SHOULD use the extended types from [EXTEN]. Acknowledgments We would like to acknowledge David Nelson, Bernard Aboba, Emile van Bergen, Barney Wolff and Glen Zorn for contributions to this document. Authors' Addresses Greg Weber Knoxville, TN 37932 USA Email: gdweber@gmail.com Alan DeKok The FreeRADIUS Server Project http://freeradius.org/ Email: aland@freeradius.org Weber, et al. Best Current Practice [Page 34]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Weber, et al. Best Current Practice [Page 35]
INTERNET-DRAFT RADIUS Design Guidelines 26 August 2008 Open issues Open issues relating to this document are tracked on the following web site: http://www.drizzle.com/~aboba/RADEXT/ Weber, et al. Best Current Practice [Page 36]
