Diameter User-Name and Realm Based Request Routing Clarifications
draft-ietf-dime-nai-routing-03.txt

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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.

Table of Contents

1.  Introduction
2.  Terminology and Abbreviations
3.  Problem Overview
4.  Solution Overview
    4.1.  Interpretation of Decorated NAIs
    4.2.  Ensuring Backwards Compatibility
    4.3.  Enhanced Request Routing Solution
5.  IANA Considerations
6.  Security Considerations
7.  Acknowledgements
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Authors' Addresses

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 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 Diameter applications which require request routing based on Decorated NAI.

At the time of publication of the Diameter Base Protocol [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.), the NAI definition was based on [RFC2486] (Aboba, B. and M. Beadles, “The Network Access Identifier,” January 1999.) in which a NAI could only contain a single realm. The NAI definition has since been updated in [RFC4282] (Aboba, B., Beadles, M., Arkko, J., and P. Eronen, “The Network Access Identifier,” December 2005.) to define Decorated NAIs that contain multiple realms. 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

Network Access Identifier (NAI):

The Network Access Identifier (NAI) is 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:

A 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 constitutes of one or more NASes.

Network Access Server (NAS):

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

3.  Problem Overview

The Diameter Base Protocol RFC 3588 Section 6.1 defines the request routing in detail. This specification concerns only those cases where a Destination-Realm AVP is included in a request message. 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 a NAI including the realm part.

Decorated NAIs are used to force routing of messages through a predefined list of realms and in that way e.g., 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 through 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 subsequent 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 [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) based roaming networks using RFC 2486 NAIs in a proprietary manner. There is a need to replicate the same NAI based routing enforcement functionality also in Diameter based roaming networks. There are also publicly available specifications (e.g., see [3GPP.23.234] (3GPP, “3GPP system to Wireless Local Area Network (WLAN) interworking; System description,” 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,” October 2006.), [3GPP.23.003] (3GPP, “Numbering, addressing and identification,” June 2009.), [3GPP.29.273] (3GPP, “Evolved Packet System (EPS); 3GPP EPS AAA interfaces,” April 2010.) and [WiMAX] (WiMAX Forum, “WiMAX Forum Network Architecture (Stage 2: Architecture Tenets, Reference Model and Reference Points),” January 2008.)) that assume NAI decoration based request routing enforcement is fully supported by RFC 3588. The same assumption is carried over to NASREQ [RFC4005] (Calhoun, P., Zorn, G., Spence, D., and D. Mitton, “Diameter Network Access Server Application,” August 2005.) and EAP [RFC4072] (Eronen, P., Hiller, T., and G. Zorn, “Diameter Extensible Authentication Protocol (EAP) Application,” August 2005.) Diameter applications.

Figure 1 (Example roaming scenario with intermediating realms. The mobile host authenticates to the home realm through one or more visited realms.) 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 roaming terminals home realm (i.e. Realm-H). However, the roaming terminal learns, somehow, that NAP B is able to provide connectivity to the Realm-H through the Realm-X (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 Realm-H!username@Realm-X. The roaming terminal has also an alternative route to its home realm through NAP A, Realm-Z and Realm-X. If the roaming terminal were to choose to use NAP A, then it would decorate its NAI as Realm-X!Realm-H!username@Realm-Z. Diameter agents should 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 `)_
 (   Realm-Z  `)<---->(   Realm-X  `)<------>(   Realm-H  `)
( `  .       )  )    ( `  .       )  )      ( `  .       )  )
 `--(_______)--'      `--(_______)--'        `--(_______)--'
       |                 __ /
       |               /
      .--.          .--.
    _(    `.      _(    `.
   (  NAP A )    (  NAP B )
  ( `  .  )  )  ( `  .  )  )
   `--(___.-'    `--(___.-'
                  )
         (  (   )
           (  |
              +-+
              |M|
              +-+

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 a NAI. Generally, the NAI decoration can be used to force a certain route for all 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 to 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] (Arkko, J., Aboba, B., Korhonen, J., and F. Bari, “Network Discovery and Selection Problem,” January 2008.) also discusses NAI decoration related issues with EAP [RFC3748] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) 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 '!' is used to separate realms in the list of decorated realms in the NAI (as shown in examples in [RFC4282] (Aboba, B., Beadles, M., Arkko, J., and P. Eronen, “The Network Access Identifier,” December 2005.)).

4.2.  Ensuring Backwards Compatibility

Implementations compliant to this specification MUST define a new Diameter application. This requirement is set to guarantee backwards compatibility with existing Diameter implementations, applications and deployments. 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.

4.3.  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 (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 (The roaming terminal decides that the Diameter messages must be routed via Realm-Z, Realm- X and Realm-H.) illustrates an example of a roaming terminal originated signaling with the home realm (Realm-H) through a NAP and two intermediating realms (Realm-Z, Realm-X) before reaching the home realm (Realm-H). 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 the step 1) can be omitted.

1) Roaming Terminal -> NAS/NAP
       Identity/NAI = realm-X!realm-H!username@realm-Z

2) NAS/NAP -> Realm-Z
       User-Name = realm-X!realm-H!username@realm-Z
       Destination-Realm = realm-Z

3) Realm-Z -> realm-X
       User-Name = realm-H!username@realm-X
       Destination-Realm = realm-X

4) Realm-X -> Realm-H
       User-Name = username@realm-H
       Destination-Realm = realm-H

5.  IANA Considerations

This specification has no actions to IANA.

6.  Security Considerations

A malicious node initiating (or indirectly causing initiation of) a Diameter request may purposely create 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 and thus creating unnecessary overhead to the routing system in general.

The NAI decoration is used in 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 the Diameter server AAA communication relies on the security properties of the intermediate AAA brokers and Diameter proxies.

7.  Acknowledgements

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

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

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses