Internet Engineering Task Force (IETF) H. Tschofenig

Internet Engineering Task Force (IETF) H. Tschofenig
Request for Comments: 7966
Category: Informational J. Korhonen, Ed.
ISSN: 2070-1721 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.

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Copyright Notice

Copyright (c) 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
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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

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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.

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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.

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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

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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.

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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

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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.

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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.

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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.

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Authors' Addresses

Hannes Tschofenig
Hall in Tirol 6060
Austria

Email: [email protected]
URI: http://www.tschofenig.priv.at

Jouni Korhonen (editor)
Broadcom Limited
3151 Zanker Rd.
San Jose, CA 95134
United States of America

Email: [email protected]

Glen Zorn
Network Zen
227/358 Thanon Sanphawut
Bang Na, Bangkok 10260
Thailand

Email: [email protected]

Kervin Pillay
Internet Solutions
South Africa

Email: [email protected]

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