Network Working Group T. Clancy
Request for Comments: 5169 LTS
Category: Informational M. Nakhjiri
V. NarayananL. Dondeti
March 2008 Handover Key Management and Re-Authentication Problem Statement
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
This document describes the Handover Keying (HOKEY) re-authentication
problem statement. The current Extensible Authentication Protocol
(EAP) keying framework is not designed to support re-authentication
and handovers without re-executing an EAP method. This often causes
unacceptable latency in various mobile wireless environments. This
document details the problem and defines design goals for a generic
mechanism to reuse derived EAP keying material for handover.
The Extensible Authentication Protocol (EAP), specified in RFC 3748
[RFC3748] is a generic framework supporting multiple authentication
methods. The primary purpose of EAP is network access control. It
also supports exporting session keys derived during the
authentication. The EAP keying hierarchy defines two keys that are
derived at the top level, the Master Session Key (MSK) and the
Extended Master Session Key (EMSK).
In many common deployment scenarios, an EAP peer and EAP server
authenticate each other through a third party known as the pass-
through authenticator (hereafter referred to as simply
"authenticator"). The authenticator is responsible for encapsulating
EAP packets from a network-access technology lower layer within the
Authentication, Authorization, and Accounting (AAA) protocol. The
authenticator does not directly participate in the EAP exchange, and
simply acts as a gateway during the EAP method execution.
After successful authentication, the EAP server transports the MSK to
the authenticator. Note that this is performed using AAA protocols,
not EAP itself. The underlying L2 or L3 protocol uses the MSK to
derive additional keys, including the transient session keys (TSKs)
used for per-packet encryption and authentication.
Note that while the authenticator is one logical device, there can be
multiple physical devices involved. For example, the CAPWAP model
[RFC3990] splits authenticators into two logical devices: Wireless
Termination Points (WTPs) and Access Controllers (ACs). Depending on
the configuration, authenticator features can be split in a variety
of ways between physical devices; however, from the EAP perspective,
there is only one logical authenticator.
Wireless handover between access points or base stations is typically
a complex process that involves several layers of protocol execution.
Often times executing these protocols results in unacceptable delays
for many real-time applications such as voice [MSA03]. One part of
the handover process is EAP re-authentication, which can contribute
significantly to the overall handover time [MSPCA04]. Thus, in many
environments we can lower overall handover time by lowering EAP re-
In EAP existing implementations, when a peer arrives at the new
authenticator, it runs an EAP method irrespective of whether it has
been authenticated to the network recently and has unexpired keying
material. This typically involves an EAP-Response/Identity message
from the peer to the server, followed by one or more round trips
between the EAP server and peer to perform the authentication,
followed by the EAP-Success or EAP-Failure message from the EAP
server to the peer. At a minimum, the EAP exchange consists of 1.5
round trips. However, given the way EAP interacts with AAA, and
given that an EAP identity exchange is typically employed, at least 2
round trips are required to the EAP server. An even higher number of
round trips is required by the most commonly used EAP methods. For
instance, EAP-TLS (Extensible Authentication Protocol - Transport
Layer Security) requires at least 3, but typically 4 or more, round
There have been attempts to solve the problem of efficient re-
authentication in various ways. However, those solutions are either
EAP-method specific or EAP lower-layer specific. Furthermore, these
solutions do not deal with scenarios involving handovers to new
authenticators, or they do not conform to the AAA keying requirements
specified in [RFC4962].
This document provides a detailed description of efficient EAP-based
re-authentication protocol design goals. The scope of this protocol
is specifically re-authentication and handover between authenticators
within a single administrative domain. While the design goals
presented in this document may facilitate inter-technology handover
and inter-administrative-domain handover, they are outside the scope
of this protocol.
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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], with the
qualification that, unless otherwise stated, they apply to the design
of the re-authentication protocol, not its implementation or
With respect to EAP, this document follows the terminology that has
been defined in [RFC3748] and [EAP-KEYING].
3. Problem Statement
Under the existing model, any re-authentication requires a full EAP
exchange with the EAP server to which the peer initially
authenticated [RFC3748]. This introduces handover latency from both
network transit time and processing delay. In service provider
networks, the home EAP server for a peer could be on the other side
of the world, and typical intercontinental latencies across the
Internet are 100 to 300 milliseconds per round trip [LGS07].
Processing delays average a couple of milliseconds for symmetric-key
operations and hundreds of milliseconds for public-key operations.
An EAP conversation with a full EAP method run can take two or more
round trips to complete, causing delays in re-authentication and
handover times. Some methods specify the use of keys and state from
the initial authentication to finish subsequent authentications in
fewer round trips and without using public-key operations (detailed
in Section 6.1). However, even in those cases, multiple round trips
to the EAP server are required, resulting in unacceptable handover
In summary, it is undesirable to run an EAP Identity and complete EAP
method exchange each time a peer authenticates to a new authenticator
or needs to extend its current authentication with the same
authenticator. Furthermore, it is desirable to specify a method-
independent, efficient, re-authentication protocol. Keying material
from the initial authentication can be used to enable efficient re-
authentication. It is also desirable to have a local server with
low-latency connectivity to the peer that can facilitate re-
authentication. Lastly, a re-authentication protocol should also be
capable of supporting scenarios where an EAP server passes
authentication information to a remote re-authentication server,
allowing a peer to re-authenticate locally, without having to
communicate with its home re-authentication server.
These problems are the primary issues to be resolved. In solving
them, there are a number of constraints to conform to, and those
result in some additional work to be done in the area of EAP keying.
4. Design Goals
The following are the goals and constraints in designing the EAP re-
authentication and key management protocol:
Lower-latency operation: The protocol MUST be responsive to handover
and re-authentication latency performance objectives within a
mobile access network. A solution that reduces latency as
compared to a full EAP authentication will be most favorable,
since in networks that rely on reactive re-authentication this
will directly impact handover times.
EAP lower-layer independence: Any keying hierarchy and protocol
defined MUST be lower-layer independent in order to provide
capabilities over heterogeneous technologies. The defined
protocols MAY require some additional support from the lower
layers that use it, but should not require any particular lower
EAP method independence: Changes to existing EAP methods MUST NOT be
required as a result of the re-authentication protocol. There
MUST be no requirements imposed on future EAP methods, provided
they satisfy [EAP-KEYING] and [RFC4017]. Note that the only EAP
methods for which independence is required are those that
currently conform to the specifications of [EAP-KEYING] and
[RFC4017]. In particular, methods that do not generate the keys
required by [EAP-KEYING] need not be supported by the re-
AAA protocol compatibility and keying: Any modifications to EAP and
EAP keying MUST be compatible with RADIUS [RADEXT-DESIGN] and
Diameter [DIME-APP-DESIGN]. Extensions to both RADIUS and
Diameter to support these EAP modifications are acceptable. The
designs and protocols must be configurable to satisfy the AAA key
management requirements specified in RFC 4962 [RFC4962].
Compatibility: Compatibility and coexistence with compliant
([RFC3748] [EAP-KEYING]) EAP deployments MUST be provided.
Specifically, the protocol should be designed such that a peer not
supporting fast re-reauthentication should still function in a
network supporting fast re-authentication, and also a peer
supporting fast re-authentication should still function in a
network not supporting fast re-authentication.
Cryptographic Agility: Any re-authentication protocol MUST support
cryptographic algorithm agility, to avoid hard-coded primitives
whose security may eventually prove to be compromised. The
protocol MAY support cryptographic algorithm negotiation, provided
it does not adversely affect overall performance (i.e., by
requiring additional round trips).
Impact to Existing Deployments: Any re-authentication protocol MAY
make changes to the peer, authenticator, and EAP server, as
necessary to meet the aforementioned design goals. In order to
facilitate protocol deployment, protocols should seek to minimize
the necessary changes, without sacrificing performance.
5. Security Goals
This section draws from the guidance provided in [RFC4962] to further
define the security goals to be achieved by a complete re-
authentication keying solution.
5.1. Key Context and Domino Effect
Any key must have a well-defined scope and must be used in a specific
context and for the intended use. This specifically means the
lifetime and scope of each key must be defined clearly so that all
entities that are authorized to have access to the key have the same
context during the validity period. In a hierarchical key structure,
the lifetime of lower-level keys must not exceed the lifetime of
higher-level keys. This requirement may imply that the context and
the scope parameters have to be exchanged. Furthermore, the
semantics of these parameters must be defined to provide proper
channel binding specifications. The definition of exact parameter
syntax definition is part of the design of the transport protocol
used for the parameter exchange, and that may be outside scope of
If a key hierarchy is deployed, compromising lower-level keys must
not result in a compromise of higher-level keys that were used to
derive the lower-level keys. The compromise of keys at each level
must not result in compromise of other keys at the same level. The
same principle applies to entities that hold and manage a particular
key defined in the key hierarchy. Compromising keys on one
authenticator must not reveal the keys of another authenticator.
Note that the compromise of higher-level keys has security
implications on lower levels.
Guidance on parameters required, caching, and storage and deletion
procedures to ensure adequate security and authorization provisioning
for keying procedures must be defined in a solution document.
All the keying material must be uniquely named so that it can be
5.2. Key Freshness
As [RFC4962] defines, a fresh key is one that is generated for the
intended use. This would mean the key hierarchy must provide for
creation of multiple cryptographically separate child keys from a
root key at higher level. Furthermore, the keying solution needs to
provide mechanisms for refreshing each of the keys within the key
Each handover keying participant must be authenticated to any other
party with whom it communicates to the extent it is necessary to
ensure proper key scoping, and securely provide its identity to any
other entity that may require the identity for defining the key
The EAP Key management document [EAP-KEYING] discusses several
vulnerabilities that are common to handover mechanisms. One
important issue arises from the way the authorization decisions might
be handled at the AAA server during network access authentication.
Furthermore, the reasons for making a particular authorization
decision are not communicated to the authenticator. In fact, the
authenticator only knows the final authorization result. The
proposed solution must make efforts to document and mitigate
5.5. Channel Binding
Channel Binding procedures are needed to avoid a compromised
intermediate authenticator providing unverified and conflicting
service information to each of the peer and the EAP server. To
support fast re-authentication, there will be intermediate entities
between the peer and the back-end EAP server. Various keys need to
be established and scoped between these parties and some of these
keys may be parents to other keys. Hence, the channel binding for
this architecture will need to consider layering intermediate
entities at each level to make sure that an entity with a higher
level of trust can examine the truthfulness of the claims made by
5.6. Transport Aspects
Depending on the physical architecture and the functionality of the
elements involved, there may be a need for multiple protocols to
perform the key transport between entities involved in the handover
keying architecture. Thus, a set of requirements for each of these
protocols, and the parameters they will carry, must be developed.
The use of existing AAA protocols for carrying EAP messages and
keying material between the AAA server and AAA clients that have a
role within the architecture considered for the keying problem will
be carefully examined. Definition of specific parameters, required
for keying procedures and for being transferred over any of the links
in the architecture, are part of the scope. The relation between the
identities used by the transport protocol and the identities used for
keying also needs to be explored.
6. Use Cases and Related Work
In order to further clarify the items listed in scope of the proposed
work, this section provides some background on related work and the
use cases envisioned for the proposed work.
6.1. Method-Specific EAP Re-Authentication
A number of EAP methods support fast re-authentication. In this
section, we examine their properties in further detail.
EAP-SIM [RFC4186] and EAP-AKA [RFC4187] support fast re-
authentication, bootstrapped by the keys generated during an initial
full authentication. In response to the typical EAP-Request/
Identity, the peer sends a specially formatted identity indicating a
desire to perform a fast re-authentication. A single round-trip
occurs to verify knowledge of the existing keys and provide fresh
nonces for generating new keys. This is followed by an EAP success.
In the end, it requires a single local round trip between the peer
and authenticator, followed by another round trip between the peer
and EAP server. AKA is based on symmetric-key cryptography, so
processing latency is minimal.
EAP-TTLS [EAP-TTLS] and PEAP (Protected EAP Protocol)
[JOSEFSSON-PPPEXT] support using TLS session resumption for fast re-
authentication. During the TLS handshake, the client includes the
message ID of the previous session he wishes to resume, and the
server can echo that ID back if it agrees to resume the session.
EAP-FAST [RFC4851] also supports TLS session resumption, but
additionally allows stateless session resumption as defined in
[RFC5077]. Overall, for all three protocols, there are still two
round trips between the peer and EAP server, in addition to the local
round trip for the Identity request and response.
To improve performance, fast re-authentication needs to reduce the
number of overall round trips. Optimal performance could result from
eliminating the EAP-Request/Identity and EAP-Response/Identity
messages observed in typical EAP method execution, and allowing a
single round trip between the peer and a local re-authentication
6.2. IEEE 802.11r Applicability
One of the EAP lower layers, IEEE 802.11 [IEEE.802-11R-D9.0], is in
the process of specifying a fast handover mechanism. Access Points
(APs) are grouped into mobility domains. Initial authentication to
any AP in a mobility domain requires execution of EAP, but handover
between APs within the mobility domain does not require the use of
Internal to the mobility domain are sets of security associations to
support key transfers between APs. In one model, relatively few
devices, called R0-KHs, act as authenticators. All EAP traffic
traverses an R0-KH, and it derives the initial IEEE 802.11 keys. It
then distributes cryptographically separate keys to APs in the
mobility domain, as necessary, to support the client mobility. For a
deployment with M designated R0-KHs and N APs, this requires M*N
security associations. For small M, this approach scales reasonably.
Another approach allows any AP to act as an R0-KH, necessitating a
full mesh of N2 security associations, which scales poorly.
The model that utilizes designated R0-KHs is architecturally similar
to the fast re-authentication model proposed by HOKEY. HOKEY,
however, allows for handover between authenticators. This would
allow an IEEE 802.11r-enabled peer to handover from one mobility
domain to another without performing an EAP authentication.
6.3. CAPWAP Applicability
The CAPWAP (Control and Provisioning of Wireless Access Points)
protocol [CAPWAP-PROTOCOL-SPEC] allows the functionality of an IEEE
802.11 access point to be split into two physical devices in
enterprise deployments. Wireless Termination Points (WTPs) implement
the physical and low-level Media Access Control (MAC) layers, while a
centralized Access Controller (AC) provides higher-level management
and protocol execution. Client authentication is handled by the AC,
which acts as the AAA authenticator.
One of the many features provided by CAPWAP is the ability to roam
between WTPs without executing an EAP authentication. To accomplish
this, the AC caches the MSK from an initial EAP authentication, and
uses it to execute a separate four-way handshake with the station as
it moves between WTPs. The keys resulting from the four-way
handshake are then distributed to the WTP to which the station is
associated. CAPWAP is transparent to the station.
CAPWAP currently has no means to support roaming between ACs in an
enterprise network. The proposed work on EAP efficient re-
authentication addresses is an inter-authenticator handover problem
from an EAP perspective, which applies during handover between ACs.
Inter-AC handover is a topic yet to be addressed in great detail and
the re-authentication work can potentially address it in an effective
7. Security Considerations
This document details the HOKEY problem statement. Since HOKEY is an
authentication protocol, there is a myriad of security-related issues
surrounding its development and deployment.
In this document, we have detailed a variety of security properties
inferred from [RFC4962] to which HOKEY must conform, including the
management of key context, scope, freshness, and transport;
resistance to attacks based on the domino effect; and authentication
and authorization. See Section 5 for further details.
This document represents the synthesis of two problem statement
documents. In this section, we acknowledge their contributions, and
involvement in the early documents.
France Telecom R&D
Rafael Marin Lopez
Universidad de Murcia
The authors would like to thank the participants of the HOKEY working
group for their review and comments including: Glen Zorn, Dan
Harkins, Joe Salowey, and Yoshi Ohba. The authors would also like to
thank those that provided last-call reviews including: Bernard Aboba,
Alan DeKok, Jari Arkko, and Hannes Tschofenig.
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J.,
Carlson, J., and H. Levkowetz, "Extensible
Authentication Protocol (EAP)", RFC 3748,
[RFC4017] Stanley, D., Walker, J., and B. Aboba,
"Extensible Authentication Protocol (EAP)
Method Requirements for Wireless LANs",
RFC 4017, March 2005.
[RFC4962] Housley, R. and B. Aboba, "Guidance for
Authentication, Authorization, and Accounting
(AAA) Key Management", BCP 132, RFC 4962,
10.2. Informative References
[CAPWAP-PROTOCOL-SPEC] Calhoun, P., Montemurro, M., and D. Stanely,
"CAPWAP Protocol Specification", Work
in Progress, March 2008.
[DIME-APP-DESIGN] Fajardo, V., Asveren, T., Tschofenig, H.,
McGregor, G., and J. Loughney, "Diameter
Applications Design Guidelines", Work
in Progress, January 2008.
[EAP-KEYING] Aboba, B., Simon, D., and P. Eronen,
"Extensible Authentication Protocol (EAP) Key
Management Framework", Work in Progress,
[EAP-TTLS] Funk, P. and S. Blake-Wilson, "EAP Tunneled
TLS Authentication Protocol Version 0 (EAP-
TTLSv0)", Work in Progress, March 2008.
[IEEE.802-11R-D9.0] "Information technology - Telecommunications
and information exchange between systems -
Local and metropolitan area networks -
Specific requirements - Part 11: Wireless LAN
Medium Access Control (MAC) and Physical
Layer (PHY) specifications - Amendment 2:
Fast BSS Transition", IEEE Standard 802.11r,
[JOSEFSSON-PPPEXT] Josefsson, S., Palekar, A., Simon, D., and G.
Zorn, "Protected EAP Protocol (PEAP) Version
2", Work in Progress, October 2004.
[LGS07] Ledlie, J., Gardner, P., and M. Selter,
"Network Coordinates in the Wild",
USENIX Symposium on Networked System Design
and Implementation, April 2007.
[MSA03] Mishra, A., Shin, M., and W. Arbaugh, "An
Empirical Analysis of the IEEE 802.11 MAC-
Layer Handoff Process", ACM SIGCOMM Computer
and Communications Review, April 2003.
[MSPCA04] Mishra, A., Shin, M., Petroni, N., Clancy,
T., and W. Arbaugh, "Proactive Key
Distribution using Neighbor Graphs",
IEEE Wireless Communications, February 2004.
[RADEXT-DESIGN] Weber, G. and A. DeKok, "RADIUS Design
Guidelines", Work in Progress, December 2007.
[RFC3990] O'Hara, B., Calhoun, P., and J. Kempf,
"Configuration and Provisioning for Wireless
Access Points (CAPWAP) Problem Statement",
RFC 3990, February 2005.
[RFC4186] Haverinen, H. and J. Salowey, "Extensible
Authentication Protocol Method for Global
System for Mobile Communications (GSM)
Subscriber Identity Modules (EAP-SIM)",
RFC 4186, January 2006.
[RFC4187] Arkko, J. and H. Haverinen, "Extensible
Authentication Protocol Method for 3rd
Generation Authentication and Key Agreement
(EAP-AKA)", RFC 4187, January 2006.
[RFC4851] Cam-Winget, N., McGrew, D., Salowey, J., and
H. Zhou, "The Flexible Authentication via
Secure Tunneling Extensible Authentication
Protocol Method (EAP-FAST)", RFC 4851,
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H.
Tschofenig, "Transport Layer Security (TLS)
Session Resumption without Server-Side
State", RFC 5077, January 2008.
T. Charles Clancy, Editor
Laboratory for Telecommunications Sciences
US Department of Defense
College Park, MD
San Diego, CA
San Diego, CA
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