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Internet Engineering Task Force (IETF)
Request for Comments: 7966
Category: Informational
ISSN: 2070-1721
H. Tschofenig
J. Korhonen, Ed.
Broadcom Limited
G. Zorn
Network Zen
K. Pillay
Internet Solutions
September 2016

Security at the Attribute-Value Pair (AVP) Level for

Non-neighboring Diameter Nodes: Scenarios and Requirements

Abstract

This specification specifies requirements for providing Diameter security at the level of individual Attribute-Value Pairs (AVPs).

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7966.

Copyright Notice

Copyright © 2016 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 Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Security Threats  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Scenarios for Diameter AVP-Level Protection . . . . . . . . .   7
   5.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1. Introduction

The Diameter base protocol specification [2] defines security protection between neighboring Diameter peers. Diameter mandates that peer connections must be protected by Transport Layer Security (TLS) [6] for TCP, by Datagram TLS (DTLS) [7] for the Stream Control Transmission Protocol (SCTP), or by security mechanisms that are independent of Diameter (such as IPsec [5]). These security protocols offer a wide range of security properties, including entity authentication, data-origin authentication, integrity protection, confidentiality protection, and replay protection. They also support a large number of cryptographic algorithms, algorithm negotiation, and different types of credentials. It should be understood that TLS/DTLS/IPsec in the Diameter context does not provide end-to-end security unless the Diameter nodes are direct peers, i.e., neighboring Diameter nodes. The current Diameter security is realized hop by hop.

The need to also offer additional security protection of AVPs between non-neighboring Diameter nodes was recognized very early in the work on Diameter. This led to work on Diameter security using the Cryptographic Message Syntax (CMS) [3]. However, due to the lack of deployment interest at that time (and the complexity of the developed solution), the specification was never completed.

In the meanwhile, Diameter had received a lot of deployment interest from the cellular operator community, and because of the sophistication of those deployments, the need for protecting Diameter AVPs between non-neighboring nodes resurfaced. Since the early 2000s (when the work on [3] was discontinued), the Internet community has seen advances in cryptographic algorithms (for example, authenticated encryption algorithms), and new security building blocks have been developed.

This document specifies requirements for developing a solution to protect Diameter AVPs between non-neighboring Diameter nodes.

2. Terminology

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 RFC 2119 [1].

This document reuses terminology from the Diameter base specification [2].

In the figures below, AVP refers to an unprotected AVP, and {AVP}k refers to an AVP that experiences security protection (using key "k") without further distinguishing between integrity and confidentiality protection.

The following terms are also used in this document: 

   AAA broker

An entity that manages Authentication, Authorization, and Accounting (AAA) traffic between roaming partner networks. 

   AAA broker network

A network operated by a AAA broker, which consists of necessary AAA functions to provide AAA brokering services for its customer AAA networks. 

   Diameter firewall

A Diameter firewall is a proxy (or a relay) agent that acts similarly to conventional IP traffic firewalls but only at the Diameter AVP and command level. A Diameter firewall may, for example, discard AVPs that violate security policy, thus preventing them from traversing the firewall. The Diameter firewall may even discard entire Diameter messages based on the security policy.

3. Security Threats

This section describes various security threats that raise the need for protecting Diameter Attribute-Value Pairs (AVPs). Figure 1 illustrates an example of a Diameter-based roaming architecture in which Diameter clients within the visited networks need to interact with Diameter servers in the home domain. AAA domains are interconnected using a Diameter-based AAA interconnection network labeled as "AAA broker network". 

      +oooooooooooooooooo+              +====================+
      |   Example.net    |              |                    |
      |                  |              |                    |
   +--------+      +--------+        +--------+        +--------+
   |Diameter|      |Diameter+--------+Diameter|        |Diameter|
   |Client 1|      |Proxy A1|        |Proxy B |        |Proxy C |
   | (NAS)  +------+        | +------+        +--------+        |----+
   +--------+      +--------+ |      +--------+        +--------+    |
      |                  |    |         |                    |       |
      | Visited Domain 1 |    |         | AAA Broker Network |       |
      +oooooooooooooooooo+    |         +====================+       |
                              |                                      |
                              |                                      |
                              |                                      |
                              |            +\\\\\\\\\\\\\\\\\\\\+    |
                              |     +--------+  Example.com     |    |
                              |     |Diameter|                  |    |
      +oooooooooooooooooo+    |     |Server X+--+         +--------+ |
      |   Example.org    |    |     +--------+  |         |Diameter| |
      |                  |    |     +--------+  +---------+Proxy D |-+
   +--------+      +--------+ |     |Diameter|  |         +--------+
   |Diameter|      |Diameter| |     |Server Y+--+               |
   |Client 2+------+Proxy A2+-+     +--------+    Home Domain   |
   | (NAS)  |      |        |              +////////////////////+
   +--------+      +--------+
      |                  |
      | Visited Domain 2 |
      +oooooooooooooooooo+

Figure 1: Example Diameter Deployment

   Eavesdropping:  Some Diameter applications carry information that is
   
      only intended for consumption by end points, either by the
      Diameter client or by the Diameter server but not by
      intermediaries.  As an example, consider the Diameter Extensible
      Authentication Protocol (EAP) application [4] that allows the

transport of keying material between the Diameter server to the Diameter client (using the EAP-Master-Session-Key AVP) for the protection of the air interface (i.e., the wireless link) between the end device (such as a mobile phone; not shown in the figure) and the Network Access Server (NAS). The content of the EAP- Master-Session-Key AVP should benefit from protection against eavesdropping by intermediaries. Other AVPs (for example, those listed in Section 13.3 of [2]) might also carry sensitive personal data that, when collected by intermediaries, allow for traffic analysis.

In the context of the deployment shown in Figure 1, the adversary could, for example, be in the AAA broker network. 

   Injection and Manipulation:  The Diameter base protocol specification
      mandates security protection between neighboring nodes, but
      Diameter agents may be compromised or misconfigured and inject or
      manipulate AVPs.  To detect such actions, additional security
      protection needs to be applied at the Diameter layer.

Nodes that could launch such an attack are any Diameter agents along the end-to-end communication path. 

   Impersonation:  Imagine a case where a Diameter message from
      Example.net contains information claiming to be from Example.org.
      This would either require strict verification at the edge of the
      AAA broker network or cryptographic assurance at the Diameter
      layer to prevent a successful impersonation attack.

Any Diameter realm could launch such an attack aiming for financial benefits or to disrupt service availability.

4. Scenarios for Diameter AVP-Level Protection

This scenario outlines a number of cases for deploying security protection of individual Diameter AVPs.

In the first scenario, shown in Figure 2, end-to-end security protection is provided between the Diameter client and the Diameter server with any number of intermediate Diameter agents. Diameter AVPs exchanged between these two Diameter nodes may be protected end to end (notation '{AVP}k') or unprotected (notation 'AVP'). 

   +--------+                                                +--------+
   |Diameter| AVP, {AVP}k                                    |Diameter|
   |Client  +-----------------........... -------------------+Server  |
   +--------+                                                +--------+

Figure 2: End-to-End Diameter AVP Security Protection

In the second scenario, shown in Figure 3, a Diameter proxy acts on behalf of the Diameter client with regard to security protection. It applies security protection to outgoing Diameter AVPs and verifies incoming AVPs. Typically, the proxy enforcing the security protection belongs to the same domain as the Diameter client/server without end-to-end security features. 

   +--------+     +--------+                                 +--------+
   |Diameter| AVP |Diameter|   AVP, {AVP}k                   |Diameter|
   |Client  +-----+Proxy A +---------- .......... -----------+Server  |
   +--------+     +--------+                                 +--------+

Figure 3: Middle-to-End Diameter AVP Security Protection

In the third scenario, shown in Figure 4, a Diameter proxy acts on behalf of the Diameter server. 

   +--------+                                 +--------+     +--------+
   |Diameter| AVP, {AVP}k                     |Diameter| AVP |Diameter|
   |Client  +-----------------........... ----+Proxy D +-----+Server  |
   +--------+                                 +--------+     +--------+

Figure 4: End-to-Middle Diameter AVP Security Protection

The fourth and the final scenario (see Figure 5) is a combination of the middle-to-end and the end-to-middle scenarios shown in Figures 3 and 4. From a deployment point of view, this scenario is easier to accomplish for two reasons. First, Diameter clients and Diameter servers remain unmodified. This ensures that no modifications are needed to the installed Diameter infrastructure, except for the security-enabled proxies, obviously. Second, the key management is also simplified since a fewer number of keys need to be negotiated and provisioned. The assumption here is that the number of security- enabled proxies would be significantly less than unprotected Diameter nodes in the installed base. 

   +--------+     +--------+                  +--------+     +--------+
   |Diameter| AVP |Diameter|   AVP, {AVP}k    |Diameter| AVP |Diameter|
   |Client  +-----+Proxy A +-- .......... ----+Proxy D +-----+Server  |
   +--------+     +--------+                  +--------+     +--------+

Figure 5: Middle-to-Middle Diameter AVP Security Protection

5. Requirements

   Requirement #1:  The solution MUST support an extensible set of
      cryptographic algorithms.

Motivation: Solutions MUST be able to evolve to adapt to evolving cryptographic algorithms and security requirements. This may include the provision of a modular mechanism to allow cryptographic algorithms to be updated without substantial disruption to deployed implementations. 

   Requirement #2:  The solution MUST support confidentiality,
      integrity, and data-origin authentication.  Solutions for
      integrity protection MUST work in a backwards-compatible way with
      existing Diameter applications and therefore be able to traverse
      legacy proxy and relay agents.
   
   Requirement #3:  The solution MUST support replay protection.
   
   Requirement #4:  The solution MUST support the ability to delegate
      security functionality to another entity.

Motivation: As described in Section 4, the ability to let a Diameter proxy perform security services on behalf of all clients within the same administrative domain is important for incremental deployability. The same applies to the other communication side where a load balancer terminates security services for the servers it interfaces. 

   Requirement #5:  The solution MUST be able to selectively apply its
      cryptographic protection to certain Diameter AVPs.

Motivation: Some Diameter applications assume that certain AVPs are added, removed, or modified by intermediaries. As such, it must be possible to apply security protection selectively.

Furthermore, there are AVPs that must not be confidentiality protected but may still be integrity protected, such as those required for Diameter message routing. 

   Requirement #6:  The solution MUST define a mandatory-to-implement
      cryptographic algorithm.

Motivation: For interoperability purposes, it is beneficial to have a mandatory-to-implement cryptographic algorithm specified (unless profiles for specific usage environments specify otherwise). 

   Requirement #7:  The solution MUST support symmetric keys and
      asymmetric keys.

Motivation: Symmetric and asymmetric cryptographic algorithms provide different security services. Asymmetric algorithms, for example, allow non-repudiation services to be offered. 

   Requirement #8:  A solution for dynamic key management MUST be
      included in the overall solution framework.

However, it is assumed that no "new" key management protocol needs to be developed; instead, existing ones are reused, if at all possible. Rekeying could be triggered by (a) management actions and (b) expiring keying material.

6. Security Considerations

This entire document focuses on the discussion of new functionality for securing Diameter AVPs selectively between non-neighboring nodes.

Various security threats are mitigated by selectively applying security protection for individual Diameter AVPs. Without protection, there is the possibility for password sniffing, confidentiality violation, and AVP insertion, deletion, or modification. Additionally, applying a digital signature offers non- repudiation capabilities, a feature not yet available in today's Diameter deployment. Modification of certain Diameter AVPs may not necessarily be the act of malicious behavior but could also be the result of misconfiguration. An over-aggressively configured firewalling Diameter proxy may also remove certain AVPs. In most cases, data-origin authentication and integrity protection of AVPs will provide the most benefits for existing deployments with minimal overhead and (potentially) operate in a full-backwards compatible manner.

7. References

7.1. Normative References

   [1]        Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.
   
   [2]        Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733,
              DOI 10.17487/RFC6733, October 2012,
              <http://www.rfc-editor.org/info/rfc6733>.

7.2. Informative References

   [3]        Calhoun, P., Farrell, S., and W. Bulley, "Diameter CMS
              Security Application", Work in Progress,
              draft-ietf-aaa-diameter-cms-sec-04, March 2002.
   
   [4]        Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter
              Extensible Authentication Protocol (EAP) Application",
              RFC 4072, DOI 10.17487/RFC4072, August 2005,
              <http://www.rfc-editor.org/info/rfc4072>.
   
   [5]        Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.
   
   [6]        Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.
   
   [7]        Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083,
              DOI 10.17487/RFC6083, January 2011,
              <http://www.rfc-editor.org/info/rfc6083>.

Acknowledgments

We would like to thank Guenther Horn, Martin Dolly, Steve Donovan, Lionel Morand, and Tom Taylor (rest in peace, Tom) for their review comments.

The authors also thank Qin Wu, Christer Holmberg, Ben Campbell, and Radia Perlman, who provided additional reviews during the Last Call.

Authors' Addresses

Hannes Tschofenig
Hall in Tirol 6060
Austria 

   Email: Hannes.tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at
   
   Jouni Korhonen (editor)
   Broadcom Limited
   3151 Zanker Rd.
   San Jose, CA  95134
   United States of America
   
   Email: jouni.nospam@gmail.com
   
   Glen Zorn
   Network Zen
   227/358 Thanon Sanphawut
   Bang Na, Bangkok  10260
   Thailand

Email: 

          glenzorn@gmail.com

Kervin Pillay
Internet Solutions
South Africa 

   Email: kervin.pillay@gmail.com