Clarifications on the Routing of Diameter

Requests Based on the Username and the Realm

Abstract

This specification defines the behavior required of Diameter agents to route requests when the User-Name Attribute Value Pair contains a Network Access Identifier formatted with multiple realms. These multi-realm, or "Decorated", Network Access Identifiers are used in order to force the routing of request messages through a predefined list of mediating realms.

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology and Abbreviations ...................................2
   3. Problem Overview ................................................3
   4. Solution Overview ...............................................5
      4.1. Interpretation of Decorated NAIs ...........................5
      4.2. Internationalization of Decorated NAIs .....................5
      4.3. Ensuring Backwards Compatibility ...........................6
      4.4. Enhanced Request Routing Solution ..........................7
   5. Security Considerations .........................................8
   6. Acknowledgements ................................................8
   7. References ......................................................9
      7.1. Normative References .......................................9
      7.2. Informative References .....................................9

1. Introduction

This specification defines the behavior required of Diameter agents to route requests when the User-Name Attribute Value Pair (AVP) contains a Network Access Identifier (NAI) formatted with multiple realms (hereafter referred to as a Decorated NAI). Decorated NAIs are used in order to force the routing of request messages through a predefined list of mediating realms. This specification does not define a new Diameter application but instead defines behaviour that would be common across all new Diameter applications that require request routing based on Decorated NAI.

The Diameter Base Protocol [RFC3588] NAI usage is originally based on [RFC2486], which has since been revised to [RFC4282]. While the use of multiple realms is generally discouraged, RFC 4282 does allow multiple realms. The use of this facility appears in, for instance, [RFC4284]. However, RFC 4282 does not define how the Decorated NAIs should be handled by Diameter agents, so this specification was written to capture those requirements.

2. Terminology and Abbreviations

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].

Network Access Identifier (NAI):

The user identity submitted by the client during access authentication. In roaming, the purpose of the NAI is to identify the user as well as to assist in the routing of the authentication request.

Decorated NAI:

An NAI containing multiple realms used to specify a source route and formatted according to Section 2.7 in RFC 4282.

Network Access Provider (NAP):

A business entity that provides network access infrastructure to one or more realms. A NAP infrastructure comprises one or more network access servers.

Network Access Server (NAS):

The device to which peers connect in order to obtain access to the network.

3. Problem Overview

Section 6.1 of "The Diameter Base Protocol" (RFC 3588) defines the request routing in detail. That specification concerns only the cases where a Destination-Realm AVP is included in a Diameter request message. As described in RFC 3588 Section 6.1, a Diameter peer originating a request message MAY retrieve the realm information from the User-Name AVP and use that realm to populate the Destination- Realm AVP. In that case, the User-Name AVP is in form of an NAI including the realm part.

Decorated NAIs are used to force routing of messages through a predefined list of realms and, in that way, force certain inter-realm roaming arrangements; see Section 2.7 of RFC 4282. For example, a terminal (e.g., a mobile host) may learn, based on some application- or implementation-specific manner, that its network access authentication signaling must traverse certain realms in order to reach the home realm. In this case, the terminal would decorate its NAI during the network access authentication with the list of intermediating realms and the home realm. As a result, the network access server (NAS) and intermediating Diameter agents would make sure that all Diameter request messages traverse through the desired realms as long as the request messages contain the User-Name AVP with a Decorated NAI.

   NAI decoration has previously been used in RADIUS-based [RFC2865]
   roaming networks, using RFC 2486 NAIs in a proprietary manner.  There
   is a need to replicate the same NAI-based routing enforcement
   functionality in Diameter-based roaming networks.  Moreover, there
   are publicly available specifications (e.g., see [3GPP.23.234],
   [3GPP.24.234], [3GPP.23.003], [3GPP.29.273], and [WiMAX]) that assume
   NAI-decoration-based request routing enforcement is fully supported

by RFC 3588. The same assumption is carried over to Network Server Application Requirements (NASREQ) [RFC4005] and Extensible Authentication Protocol (EAP) [RFC4072] Diameter applications.

Figure 1 illustrates an example deployment scenario where Decorated NAIs would be used to force a certain route through desired realms. A roaming terminal (e.g., a mobile host) discovers a number of Network Access Providers (NAP): NAP A and NAP B. None of the NAPs are able to provide direct connectivity to the roaming terminal's home realm (i.e., h.example.com). However, the roaming terminal learns, somehow, that NAP B is able to provide connectivity to h.example.com through x.example.com (i.e., the visited realm from the roaming terminal point of view). During the network access authentication, the roaming terminal would decorate its NAI as h.example.com!username@x.example.com. The roaming terminal has also an alternative route to its home realm through NAP A: z.example.com and x.example.com. If the roaming terminal were to choose to use NAP A, then it would decorate its NAI as x.example.com!h.example.com!username@z.example.com. Diameter agents would now be able to route the request message through desired realms using the Decorated NAI originally found in the User-Name AVP.

         .--.                  .--.                    .--.
       _(.   `)              _(.   `)                _(.   `)
     _( Visited`)_         _( Visited`)_           _(  Home  `)_
    (z.example.com`)<---->(x.example.com`)<------>(h.example.com`)
   ( `  .        )  )    ( `  .        )  )      ( `  .        )  )
    `--(_______)---'      `--(_______)---'        `--(_______)---'
          |                 __ /
          |               /
         .--.          .--.
       _(    `.      _(    `.
      (  NAP A )    (  NAP B )
     ( `  .  )  )  ( `  .  )  )
      `--(___.-'    `--(___.-'
                     )
            (  (   )
              (  |
                 +-+
                 |M|
                 +-+

Figure 1: Example roaming scenario with intermediating realms. The

mobile host authenticates to the home realm through one or more

visited realms.

NAI decoration is not limited to the network access authentication and authorization procedures. It can be used with any Diameter application whose commands are proxiable and include the User-Name AVP with an NAI. Generally, the NAI decoration can be used to force a certain route for all Diameter request messages at a realm granularity.

As a problem summary, we have two main issues:

• Updating both Destination-Realm and User-Name AVPs based on the Decorated NAI extracted from the User-Name AVP. The update would be done by intermediating Diameter agents that participate in realm-based request routing. Specifically, this would concern Diameter proxies.

• How Diameter agents could implement the handling of the NAI- decoration-based routing enforcement in a way that is still backwards compatible with RFC 3588.

Section 2.3 of [RFC5113] also discusses NAI-decoration-related issues with EAP [RFC3748] in general.

4. Solution Overview

This specification defines a solution for Diameter realm-based request routing with routing enforcement using the User-Name AVP NAI decoration. Diameter proxy agent implementations can claim compliance using the solution described in this specification. The Diameter answer processing is left unmodified and follows the procedures described in Section 6.2 of RFC 3588.

4.1. Interpretation of Decorated NAIs

Implementations compliant to this specification MUST have a uniform way of interpreting decorated NAIs. That is, in the case of decoration, the character '!' (hexadecimal 0x21) is used to separate realms in the list of decorated realms in the NAI (as shown in examples in [RFC4282]).

4.2. Internationalization of Decorated NAIs

RFC 3588, Section 1.3 states that NAI realm names are required to be unique and are piggybacked on the administration of the Domain Name System (DNS) ([RFC1034], [RFC1035]) namespace. However, an NAI, with or without decoration, may contain international characters as allowed by RFC 4282. This causes problems, as international characters as such are not supported by RFC 1034 and RFC 1035. The conversion of International Domain Names (IDN), which in this document's scope are NAI realms, are discussed in [RFC3490] and are further to be revised in [IDNA].

The general guidance for handling NAI realms with international characters is described in Section 2.4 of RFC 4282 and discussed more in [NAI] based on recent operational experiences. This specification does not attempt to fix the issue with the internationalization of NAIs, as the problem space is large and concerns much more than just NAI realms and NAI decoration. However, this specification has the following assumptions:

• The conversion from a realm name that includes international characters to ASCII-compatible encoding should only take place when DNS infrastructure needs to be queried and not, for example, during the realm-placement processing of Decorated NAIs. The conversion is normally handled by a DNS resolver library on the local Diameter agent or, when not available in the resolver library, by the Diameter agent. In both cases, the realm in the NAI remains unchanged.

• It is the responsibility of the operators administrating their realm and domain name spaces to ensure that the DNS is provisioned properly for all realms that may appear in Decorated NAIs. This implicitly requires that the conversion from any realm with international characters to a domain name cannot fail (i.e., the realms conform to the preconditions for internationalized domain names).

From the above discussion, it can be concluded that avoiding international characters in realms contained in NAIs is the best way to avoid problems with internationalized domain names and Decorated NAI handling in general.

4.3. Ensuring Backwards Compatibility

The handling of the NAI-decoration-based routing enforcement as described in this specification will be supported by any new Diameter application. Therefore, backwards compatibility with existing Diameter implementations, applications, and deployments will be guaranteed. Existing Diameter agents not compliant with this specification will not advertise support for these new applications that implement the enhanced routing solution based on Decorated NAIs, and will therefore be bypassed.

4.4. Enhanced Request Routing Solution

When a Diameter client originates a request message, the Destination-Realm AVP is populated with the realm part of the NAI available in the User-Name AVP (the realm given after the '@' character of the NAI). The NAI in the User-Name AVP may or may not be decorated.

When a Diameter agent receives a request message containing the Destination-Realm AVP with a realm that the agent is configured to process locally (and, in the case of proxies, the Diameter application is locally supported), it MUST do the following further processing before handling the message locally:

• If the User-Name AVP is available in the request message, then the Diameter agent MUST inspect whether the User-Name AVP contains a Decorated NAI. If the NAI is not decorated, then the Diameter agent proceeds with a normal RFC 3588 message processing.

• If the User-Name AVP contains a Decorated NAI, then the Diameter agent MUST process the NAI as defined in RFC 4282 and update the value of the User-Name AVP accordingly. Furthermore, the Diameter agent MUST update the Destination-Realm AVP to match the new realm in the User-Name AVP.

• The request message is then sent to the next hop using the normal request routing rules as defined in RFC 3588.

Figure 2 illustrates an example of a roaming terminal that originates signaling with the home realm (h.example.com), through a NAP and two intermediating realms (z.example.com, x.example.com) before reaching the home realm (h.example.com). The example shows how the User-Name AVP and the Destination-Realm AVP change at each realm before reaching the final destination. If the signaling were originated from the NAS/NAP only, then step 1 can be omitted.

   1) Roaming Terminal -> NAS/NAP
   
       Identity/NAI = x.example.com!h.example.com!username@z.example.com
   
   2) NAS/NAP -> z.example.com
       User-Name = x.example.com!h.example.com!username@z.example.com
       Destination-Realm = z.example.com
   
   3) Realm-Z -> x.example.com
       User-Name = h.example.com!username@x.example.com
       Destination-Realm = x.example.com
   
   4) Realm-X -> h.example.com
       User-Name = username@h.example.com
       Destination-Realm = h.example.com
   
     Figure 2: The roaming terminal decides that the Diameter messages
   must be routed via z.example.com and x.example.com to h.example.com.

5. Security Considerations

A malicious node initiating (or indirectly causing initiation of) a Diameter request may purposely create a malformed list of realms in the NAI. This may cause the routing of requests through realms that would normally have nothing to do with the initiated Diameter message exchange. Furthermore, a malformed list of realms may contain non- existing realms, causing the routing of Diameter messages that cannot ultimately be routed anywhere. However, the request message might get routed several hops before such non-existent realms are discovered, thus creating unnecessary overhead to the routing system in general.

The NAI decoration is used in Authentication, Authorization, and Accounting (AAA) infrastructures where the Diameter messages are transported between the NAS and the Diameter server via one or more AAA brokers or Diameter proxies. In this case, the NAS to Diameter server AAA communication relies on the security properties of the intermediate AAA brokers and Diameter proxies.

6. Acknowledgements

The authors would like to thank Victor Fajardo, Stefan Winter, Jari Arkko, and Avi Lior for their detailed comments on this document.

Jouni Korhonen would like to thank the TEKES WISEciti project for providing funding to work on this document while he was at TeliaSonera's employ.

7. References

7.1. Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3588]     Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and
                 J. Arkko, "Diameter Base Protocol", RFC 3588, September
                 2003.
   
   [RFC4282]     Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
                 Network Access Identifier", RFC 4282, December 2005.

7.2. Informative References

   [3GPP.23.003] 3GPP, "Numbering, addressing and identification", 3GPP
                 TS 23.003 8.5.0, June 2009.
   
   [3GPP.23.234] 3GPP, "3GPP system to Wireless Local Area Network
                 (WLAN) interworking; System description", 3GPP TS
                 23.234 6.10.0, October 2006.
   
   [3GPP.24.234] 3GPP, "3GPP system to Wireless Local Area Network
                 (WLAN) interworking; WLAN User Equipment (WLAN UE) to
                 network protocols; Stage 3", 3GPP TS 24.234 6.7.0,
                 October 2006.
   
   [3GPP.29.273] 3GPP, "Evolved Packet System (EPS); 3GPP EPS AAA
                 interfaces", 3GPP TS 29.273 8.3.0, September 2009.
   
   [NAI]         DeKok, A., "The Network Access Identifier", Work in
                 Progress, September 2009.
   
   [IDNA]        Klensin, J., "Internationalized Domain Names in
                 Applications (IDNA): Protocol", Work in Progress,
                 October 2009.
   
   [RFC1034]     Mockapetris, P., "Domain names - concepts and
                 facilities", STD 13, RFC 1034, November 1987.
   
   [RFC1035]     Mockapetris, P., "Domain names - implementation and
                 specification", STD 13, RFC 1035, November 1987.
   
   [RFC2486]     Aboba, B. and M. Beadles, "The Network Access
                 Identifier", RFC 2486, January 1999.
   
   [RFC2865]     Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                 "Remote Authentication Dial In User Service (RADIUS)",
                 RFC 2865, June 2000.
   
   [RFC3490]     Faltstrom, P., Hoffman, P., and A. Costello,
                 "Internationalizing Domain Names in Applications
                 (IDNA)", RFC 3490, March 2003.
   
   [RFC3748]     Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
                 H. Levkowetz, Ed., "Extensible Authentication Protocol
                 (EAP)", RFC 3748, June 2004.
   
   [RFC4005]     Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
                 "Diameter Network Access Server Application", RFC 4005,
                 August 2005.
   
   [RFC4072]     Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter
                 Extensible Authentication Protocol (EAP) Application",
                 RFC 4072, August 2005.
   
   [RFC4284]     Adrangi, F., Lortz, V., Bari, F., and P. Eronen,
                 "Identity Selection Hints for the Extensible
                 Authentication Protocol (EAP)", RFC 4284, January 2006.
   
   [RFC5113]     Arkko, J., Aboba, B., Korhonen, J., Ed., and F. Bari,
                 "Network Discovery and Selection Problem", RFC 5113,
                 January 2008.
   
   [WiMAX]       WiMAX Forum, "WiMAX Forum Network Architecture (Stage
                 2: Architecture Tenets, Reference Model and Reference
                 Points)", Release 1 Version 1.2, January 2008.

Authors' Addresses

Jouni Korhonen (editor)
Nokia Siemens Networks
Linnoitustie 6
Espoo FIN-02600
Finland

   EMail: jouni.nospam@gmail.com
   
   Mark Jones
   Bridgewater Systems
   303 Terry Fox Drive
   Ottawa,  Ontario  K2K 3J1
   Canada
   
   EMail: Mark.Jones@bridgewatersystems.com
   
   Lionel Morand
   Orange Labs
   38-40 rue du general Leclerc
   Issy-moulineaux Cedex 9,  92794
   France
   
   EMail: Lionel.morand@orange-ftgroup.com
   
   Tina Tsou (Ting ZOU)
   Huawei
   R&D Center, Huawei Technologies Co., Ltd
   Bantian,  Shenzhen
   P.R. China

EMail:

          tena@huawei.com