Network Working Group J. Korhonen, Ed.

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
Huawei
December 2009

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

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
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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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RFC 5729 Diameter Realm Routing Clarifications December 2009

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.

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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
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

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RFC 5729 Diameter Realm Routing Clarifications December 2009

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
[email protected]. 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
[email protected]. 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.

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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
granularity.

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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RFC 5729 Diameter Realm Routing Clarifications December 2009

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
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.

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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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RFC 5729 Diameter Realm Routing Clarifications December 2009

1) Roaming Terminal -> NAS/NAP
Identity/NAI = [email protected]

2) NAS/NAP -> z.example.com
User-Name = [email protected]
Destination-Realm = z.example.com

3) Realm-Z -> x.example.com
User-Name = [email protected]
Destination-Realm = x.example.com

4) Realm-X -> h.example.com
User-Name = [email protected]
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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RFC 5729 Diameter Realm Routing Clarifications December 2009

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.

Korhonen, et al. Standards Track [Page 9]

RFC 5729 Diameter Realm Routing Clarifications December 2009

[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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RFC 5729 Diameter Realm Routing Clarifications December 2009

Authors' Addresses

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

EMail: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

Korhonen, et al. Standards Track [Page 11]