Network Working Group                                   J. Korhonen, Ed.
Request for Comments: 5729                        Nokia Siemens Networks
Updates: 3588                                                   M. Jones
Category: Standards Track                            Bridgewater Systems
                                                               L. Morand
                                                             Orange Labs
                                                                 T. Tsou
                                                           December 2009

               Clarifications on the Routing of Diameter
              Requests Based on the Username and the Realm


   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

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

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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",
   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

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   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

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

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   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  `)_
   ( `  .        )  )    ( `  .        )  )      ( `  .        )  )
    `--(_______)---'      `--(_______)---'        `--(_______)---'
          |                 __ /
          |               /
         .--.          .--.
       _(    `.      _(    `.
      (  NAP A )    (  NAP B )
     ( `  .  )  )  ( `  .  )  )
      `--(___.-'    `--(___.-'
            (  (   )
              (  |

    Figure 1: Example roaming scenario with intermediating realms.  The
      mobile host authenticates to the home realm through one or more
                              visited realms.

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   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

   As a problem summary, we have two main issues:

   o  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.

   o  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

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   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:

   o  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.

   o  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

   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.

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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:

   o  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.

   o  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.

   o  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.

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   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.

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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

   [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.

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   [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.

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Authors' Addresses

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

   EMail: jouni.nospam@gmail.com

   Mark Jones
   Bridgewater Systems
   303 Terry Fox Drive
   Ottawa,  Ontario  K2K 3J1

   EMail: Mark.Jones@bridgewatersystems.com

   Lionel Morand
   Orange Labs
   38-40 rue du general Leclerc
   Issy-moulineaux Cedex 9,  92794

   EMail: Lionel.morand@orange-ftgroup.com

   Tina Tsou (Ting ZOU)
   R&D Center, Huawei Technologies Co., Ltd
   Bantian,  Shenzhen
   P.R. China

   EMail: tena@huawei.com

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