Network Working Group Y. Ohba (Editor)
Internet-Draft Toshiba
Expires: December 6, 2008 June 4, 2008
EAP Pre-authentication Problem Statement
draft-ietf-hokey-preauth-ps-03
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Abstract
EAP pre-authentication is defined as the use of EAP to pre-establish
EAP keying material on a target authenticator prior to arrival of the
peer at the access network managed by that authenticator. This draft
discusses EAP pre-authentication problems in details.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
4. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Direct Pre-authentication . . . . . . . . . . . . . . . . 9
4.2. Indirect Pre-authentication . . . . . . . . . . . . . . . 10
5. Architectural Considerations . . . . . . . . . . . . . . . . . 11
5.1. Authenticator Discovery . . . . . . . . . . . . . . . . . 11
5.2. Context Binding . . . . . . . . . . . . . . . . . . . . . 11
6. AAA Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
When a mobile device during an active communication session moves
from one access network to another access network and changes its
point of attachment it is subjected to disruption in the continuity
of service because of the associated handover operation. The
performance requirement of a real-time application will vary based on
the type of application and its characteristics such as delay
tolerance and loss tolerance limit. ITU-T G.114 [ITU] recommends 150
ms as the upper limit for VoIP applications and 400 ms as generally
unacceptable delay. Similarly, a streaming application has the
tolerable packet (SDU) error rates ranging from 0.1 to 0.00001 and
the transfer delay of less than 300 ms. Thus, any optimized handoff
mechanism will need to take care of these factors into consideration
in order to be able to support a heterogeneous handover that is
agnostic to link-layer technologies.
As a mobile device goes through a handover process, it is subjected
to delay because of the rebinding of its association at several
layers of the protocol stack. Delays incurred within each of these
layers affect the ongoing multimedia application and data traffic
within the client [WCM].
Handover often requires authentication and authorization for
acquisition or modification of resources assigned to a mobile device
and the authentication and authorization needs interaction with a
central authority in a domain in most cases. In some cases the
central authority may be placed far away from the mobile device. The
delay introduced due to such an authentication and authorization
procedure adds to the handover latency and consequently affects
ongoing application sessions [MQ7]. We focus our discussion
highlighting the factors that affect the performance due to network
access authentication and authorization where EAP [RFC3748] is used
for network access authentication.
This draft discusses EAP pre-authentication problems in details where
EAP pre-authentication is defined as the utilization of EAP to pre-
establish EAP keying material on an EAP authenticator prior to
arrival of the mobile device that acts as an EAP peer, at the access
network served by that authenticator.
1.1. Specification of Requirements
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].
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2. Terminology
Peer
The end of the link that responds to the authenticator [RFC3748].
EAP Authenticator
The end of the link initiating EAP authentication. [RFC3748].
EAP Server
The entity that terminates the EAP authentication method with the
peer [RFC3748].
Serving Authenticator
An authenticator that is currently serving the peer.
Target Authenticator
An authenticator that has been chosen to be the new serving
authenticator for a peer.
Candidate Authenticator
An authenticator that can potentially become the target
authenticator for a peer. There can be multiple candidate
authenticators for the peer.
Master Session Key (MSK)
Keying material that is derived between the peer and EAP server
and exported by an EAP authentication method.
Access Point (AP)
A network point of attachment in IEEE 802.11 wireless LAN
[802.11].
Basic Service Set (BSS)
The basic building block of an IEEE 802.11 wireless LAN [802.11].
The BSS consists of a group of any number of 802.11 stations.
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Extended Service Set (ESS)
A set of infrastructure BSSs in IEEE 802.11 wireless LAN [802.11],
where the access points communicate amongst themselves to forward
traffic from one BSS to another to facilitate movement of stations
between BSSs.
Access Domain
A set of access networks of a specific link layer technology among
which a peer is allowed to change its network points of attachment
without changing its serving authenticator. An IEEE 802.11r
mobility domain [802.11r] is an access domain.
Inter-Access-Domain Handover
A handover across multiple access domains.
Inter-ESS Handover
An 802.11 handover across multiple ESSs.
Intra-AAA-Domain Handover (Intra-Domain Handover)
A handover within the same AAA domain.
Inter-AAA-Domain Handover (Inter-Domain Handover)
A handover across multiple AAA domains.
Intra-Technology Handover
A handover within the same link layer technology.
Inter-Technology Handover
A handover across different link layer technologies.
Inter-Authenticator Handover
A handover across multiple authenticators. An inter-authenticator
handover includes an inter-access-domain handover, an inter-ESS
handover, an inter-AAA-domain handover, an inter-technology
handover, and any possible combination of them.
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ERP (EAP Extensions for EAP Re-authentication Protocol)
Extensions to EAP and EAP keying hierarchy to support an EAP
method-independent protocol for efficient re-authentication
between the peer and an EAP re-authentication server defined in
[I-D.ietf-hokey-erx].
3. Problem Statement
Basic mechanism of handover is a three-step procedure involving i)
discovery of candidate network points of attachment and their
authenticators, ii) network selection procedure to determine the
appropriate candidate network point of attachment and iii) handover
or setting up L2 and L3 connectivity to the target network point of
attachment. Currently, network access authentication and
authorization are performed as part of the third step directly with
the target network. For example, in IEEE 802.11 wireless LANs
[802.11], the network access authentication and authorization
involves performing a new IEEE 802.1X message exchange with the
authenticator in the target AP to initiate an EAP exchange to the
authentication server [WPA]. Following a successful EAP
authentication, a secure association procedure is performed between
the peer and the target authenticator to derive a new set of link-
layer ciphering keys from EAP keying material such as MSK. The third
step may require full EAP authentication in the absence of any
particular optimization for handover key management. The handover
latency introduced by full EAP authentication has proven to be larger
than that is acceptable for real-time applications scenarios as
described in [MQ7]. Hence, improvement in the handover latency
performance due to EAP is a necessary objective for such scenarios.
There is relevant work undertaken by various standards organizations,
but these efforts are scoped to a specific link layer technology.
IEEE 802.11F [802.11f], a trial use document has defined context
transfer and caching mechanism to transfer some IEEE 802.11 keying
material between the neighboring APs. However, it has been
administratively withdrawn since 2006. IEEE 802.11 [802.11] defines
a pre-authentication mechanism for use in 802.11 wireless networks.
This mechanism allows peers to pre-authenticate to one or more
candidate authenticators over the wired medium, by way of the serving
authenticator. IEEE 802.11r [802.11r] defines Fast BSS transition
mechanisms involving a definition of key management hierarchy and
setup of session keys before the re-association to the target AP in
the same 802.11r mobility domain. These mechanisms, as indicated
before, are defined for IEEE 802.11 technologies and are only
applicable for intra-access-domain handovers and fall short when it
comes to inter-technology handovers. They also require L2 (e.g.,
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Ethernet) connectivity for transfer of key management signaling to a
candidate or the target AP. Especially, a solution is needed to
enable EAP pre-authentication for inter-access-domain or inter-ESS
handovers in IEEE 802.11.
As various flavors of wireless technologies are increasingly
available, there is a growing demand for seamless inter-technology
mobility and handovers. This is particularly beneficial in the
presence of high bandwidth, wireless technologies (e.g., IEEE 802.11)
with only hotspot-like coverages while the overlay licensed wireless/
cellular coverages are expensive and relatively low bandwidth.
Hence, the security optimization mechanisms for better handover
performance must be looked at from the IP level so as to make it a
common method over different access technologies.
Optimized solutions for secure inter-authenticator handovers can be
largely seen as security context transfer, ERP [I-D.ietf-hokey-erx],
or EAP pre-authentication. Security context transfer involves
transfer of reusable key context to the new point of attachment.
However, the recent AAA key management requirement [RFC4962] does not
recommend horizontal context transfer of reusable key context because
of the domino effect in which the compromise of an authenticator will
lead to the compromise of another authenticator. ERP uses existing
EAP keying material for deriving a re-authentication key to be
distributed to an ERP server in a visited domain in order to reduce
the handover delay, which eliminates the need for running full EAP
authentication with the EAP server in the home domain for intra-
domain handovers. On the other hand, there are certain cases where
ERP is not applicable or an additional optimization mechanism is
needed to establish a key for the candidate authenticator:
o One case is an inter-domain handover with or without any trust
relationship between the home and visited AAA domains. If there
is no trust relationship between the two AAA domains, ERP cannot
be used in the visited AAA domain, and the EAP server in the home
AAA domain is the only entity that can authenticate the peer.
Even if there is a trust relationship between the two AAA domains
and the visited AAA domain supports ERP, full EAP authentication
with the EAP server in the home AAA domain is still needed when
entering the visited AAA domain unless the security policy of the
home AAA domain allows the same re-authentication root key to be
shared with the visited AAA domain.
o Another case is an intra-domain, inter-authenticator handover
where the target authenticator or AAA domain do not support ERP,
or ERP needs to be performed proactively before the peer arrives
at the target authenticator.
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EAP pre-authentication discussed in this document is mainly to deal
with these cases while a general solution for the EAP pre-
authentication problem can be used for other handover cases.
EAP pre-authentication has general applicability to various
deployment scenarios of the above handover cases in which proactive
signaling can take effect. In other words, applicability of EAP pre-
authentication is limited to the scenarios where candidate
authenticators can be discovered, an accurate prediction of movement
can be easily made. Also the effectiveness of EAP pre-authentication
may be less significant for particular inter-technology handover
scenarios where simultaneous use of multiple technologies is not a
major concern.
In EAP pre-authentication, AAA-based authentication and authorization
for a candidate authenticator is performed while ongoing data
communication is in progress via the serving network. The goal of
EAP pre-authentication is to avoid AAA signaling for EAP when or soon
after the peer moves. There are several AAA issues related to EAP
pre-authentication. The pre-authentication AAA issues are described
in Section 6.
Figure 1 shows the functional elements that are related to EAP pre-
authentication.
+------+ +-------------+ +------+
| Peer |---------| Serving | / \
| | |Authenticator|---/ \
+------+ +-------------+ / \
. / \ +--------------+
. Move + IP Network +---|AAA/EAP Server|
. \ / +--------------+
v +-------------+ \ /
| Candidate |---\ /
|Authenticator| \ /
+-------------+ +------+
Figure 1: EAP Pre-authentication Functional Elements
A peer is attached to the serving access network. Before the peer
performs handover from the serving access network to a candidate
access network, it performs EAP pre-authentication with a candidate
authenticator via the serving access network. The peer may perform
EAP pre-authentication with one or more candidate authenticators. It
is assumed that each authenticator has an IP address. It is assumed
that there is at least one candidate authenticator in each candidate
access network while the serving access network may or may not have a
serving authenticator. The serving and candidate access networks may
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use different link layer technologies.
Each authenticator is either a standalone authenticator or pass-
through authenticator [RFC3748]. When an authenticator acts as a
standalone authenticator, it also has the functionality of an EAP
server. When an authenticator acts as a pass-through authenticator,
it communicates with the EAP server typically implemented on a AAA
server using a AAA transport protocol such as RADIUS [RFC2865] and
Diameter [RFC3588].
If the candidate authenticator uses an MSK [I-D.ietf-eap-keying] for
generating lower-layer ciphering keys, EAP pre-authentication is used
for proactively generating an MSK for the candidate authenticator.
4. Usage Scenarios
There are two scenarios for how EAP pre-authentication signaling can
happen among a peer, serving authenticator, candidate authenticator
and AAA server, depending on how the serving authenticator is
involved in the EAP pre-authentication signaling. It is assumed in
both scenarios that there is no direct L2 connectivity between a peer
and a CA. No security association between the serving authenticator
and the candidate authenticator is required for either pre-
authentication scenario (see Section 7 for more detailed discussion).
4.1. Direct Pre-authentication
Direct pre-authentication signaling is shown in Figure 2.
Peer Serving Candidate AAA/EAP
Authenticator Authenticator Server
(SA) (CA)
| | | |
| | | |
| Peer-CA Signaling (L3) | AAA |
|<------------------+------------------->|<----------------->|
| | | |
| | | |
Figure 2: Direct Pre-authentication
In this type of pre-authentication, the serving authenticator
forwards the EAP pre-authentication traffic as it would any other
data traffic or there may be no serving authenticator at all in the
serving access network.
[I-D.ietf-pana-preauth] is identified as a protocol to realize direct
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pre-authentication.
4.2. Indirect Pre-authentication
In indirect pre-authentication, it is assumed that a trust
relationship exists between the serving network (or serving AAA
domain) and candidate network (or candidate AAA domain). Indirect
pre-authentication signaling is shown in Figure 3.
Peer Serving Candidate AAA/EAP
Authenticator Authenticator Server
(SA) (CA)
| | | |
| | | |
| Peer-SA Signaling | SA-CA Signaling | AAA |
| (L2 or L3) | (L3) | |
|<----------------->|<------------------>|<----------------->|
| | | |
| | | |
Figure 3: Indirect Pre-authentication
With indirect pre-authentication, the serving authenticator is
involved in EAP pre-authentication signaling. Indirect pre-
authentication is needed if the peer cannot discover the CA's IP
address or if IP communication is not available due to security or
network topology reasons.
Indirect pre-authentication signaling between a peer and a candidate
authenticator consists of peer to serving authenticator signaling
(Peer-SA signaling) and serving authenticator to candidate
authenticator signaling (SA-CA signaling).
SA-CA signaling is performed over L3.
Peer-SA signaling is performed over L2 or L3.
The role of the serving authenticator in indirect pre-authentication
is to forward EAP pre-authentication signaling between the peer and
the candidate authenticator and not to act as an authenticator, while
it acts as an authenticator for normal authentication signaling.
This is illustrated in Figure 4.
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Peer Serving Candidate
Authenticator Authenticator
(SA) (CA)
+-----------+ +-----------+
| |<- - - - - - - - - - - - - - - - - - ->| |
| EAP Peer | +-----------------------------+ | EAP Auth- |
| | |Pre-authentication Forwarding| | enticator |
+-----------+ +-----------+-----+-----------+ +-----------+
| Peer-SA | | Peer-SA | | SA-CA | | SA-CA |
| Signaling |<-->| Signaling | | Signaling |<-->| Signaling |
| Layer | | Layer | | Layer | | Layer |
+-----------+ +-----------+ +-----------+ +-----------+
Figure 4: Indirect Pre-authentication Layering Model
5. Architectural Considerations
There are two architectural issues relating to pre-authentication:
authenticator discovery and context binding.
5.1. Authenticator Discovery
In general, pre-authentication requires an address of a candidate
authenticator to be discovered either by a peer or by a serving
authenticator prior to handover. An authenticator discovery protocol
is typically defined as a separate protocol from a pre-authentication
protocol. For both intra-domain and inter-domain handover, the IP
address of a candidate authenticator must be reachable by the peer or
the serving authenticator that is performing the pre-authentication.
For example, IEEE 802.21 Information Service (IS) [802.21] provides a
link layer independent mechanism for obtaining neighboring network
information by defining a set of Information Elements (IEs), where
one of the IEs is defined to contain an IP address of a point of
attachment. IEEE 802.21 IS queries for such an IE may be used as a
method for authenticator discovery.
If IEEE 802.21 IS or a similar mechanism is used, an authenticator
discovery requires a database on the neighboring network information.
Provisioning of a server with such a database is another issue.
5.2. Context Binding
When a candidate authenticator uses different EAP transport protocols
for normal authentication and pre-authentication, a mechanism is
needed to bind link layer independent context carried over pre-
authentication signaling to the link layer specific context of the
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link to be established between the peer and the candidate
authenticator. The link layer independent context includes the
identities of the peer and authenticator as well as the MSK. The
link layer specific context includes link layer addresses of the peer
and the candidate authenticator. Such context binding can happen
before or after the peer changes its point of attachment.
There are at least two possible approaches to address the context
binding issue. The first approach is based on communicating the link
layer context as opaque data via pre-authentication signaling. The
second approach is based on running EAP over the link layer of the
candidate authenticator after the peer arrives at the authenticator
using short-term credentials generated via pre-authentication. In
this case, the short-term credentials are shared between the peer and
the candidate authenticator, and hence the EAP server for the EAP
performed after the peer arrives at the target authenticator resides
in the authenticator. In both approaches, context binding needs to
be securely made between the peer and the candidate authenticator.
Also, the peer is not fully authorized by the candidate authenticator
until the peer completes the link layer specific secure association
procedure with the authenticator using the link layer signaling.
6. AAA Issues
Most of the AAA documents today do not distinguish between a normal
authentication and a pre-authentication and this creates a set of
open issues:
Pre-authentication authorization: Many users may not be allowed to
have more than one logon session at the time. This means that
when such users actively engage in an active session (as a result
of a previously valid authentication), they will not be able to
perform pre-authentication. The AAA server currently has no way
of distinguishing between a normal authentication request and a
pre-authentication request.
Pre-authentication lifetime: Currently AAA protocols define
attributes carrying lifetime information for a normal
authentication session. Even when a user profile and the AAA
server support pre-authentication, the lifetime for a pre-
authentication session is typically valid only for a short amount
of time because the peer has not completed its authentication at
the target link layer. It is currently not possible for a AAA
server to indicate to the AAA client or a peer the lifetime of the
pre-authenticated session unless AAA protocols are extended to
carry pre-authentication session lifetime information. In other
words, it is not clear to the peer or the authenticator when the
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pre-authentication session will expire.
Pre-authentication retries: It is typically expected that shortly
following the pre-authentication process, the peer moves to the
new point of attachment and converts the pre-authentication state
to a normal authentication state (the procedure for which is not
the topic of this particular subsection). However, if the peer
has not yet moved to the new location and realizes that the pre-
authentication is expiring, it may perform another pre-
authentication. Some limiting mechanism is needed to avoid
unlimited number of pre-authentication tries.
Completion of network attachment: Once the peer has successfully
attached to the new point of attachment, it needs to convert its
authentication state from pre-authenticated to fully attached and
authorized. There may need to be a mechanism within the AAA
protocol to provide this indication to the AAA server if the AAA
server needs to differentiate between pre-authentication and
normal authentication.
Session Resumption: In case the peer cycles between a network N1
with which it has a normal authentication state to another network
N2 and then back to N1, it should be possible to simply convert
the full authentication state to a pre-authenticated state. The
problems around handling session lifetime and keying material
caching needs to be dealt with.
Multiple candidate authenticators: There may be situations where
the peer may need to make a selection between a number of
candidate authenticators. In such cases, it is desirable for the
peer to perform pre-authentication with multiple candidate
authenticators. In such cases the AAA server may need to be aware
of the situation.
Inter-technology support: Current specifications on pre-
authentication mostly deal with homogeneous 802.11 networks. The
AAA attributes such as Calling-Station-ID [I-D.aboba-radext-wlan]
may need to be expanded to cover other access technologies.
Furthermore, inter-technology handovers may require a change of
the peer identifier as part of the handover. Investigation on the
best type of identifiers for peers that support multiple access
technologies is required.
7. Security Considerations
Since pre-authentication described in this document needs to work
across multiple authenticators, any solution needs to consider the
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following security threats.
First, a resource consumption denial of service attack is possible,
where an attacker that is not on the same IP link as the legitimate
peer or the candidate authenticator may send unprotected pre-
authentication messages to the legitimate peer or the candidate
authenticator. As a result, they may spend their computational and
bandwidth resources for processing pre-authentication messages sent
by the attacker. This attack is possible for both direct and
indirect pre-authentication scenarios. To mitigate this attack, the
candidate network or authenticator may apply non-cryptographic packet
filtering so that pre-authentication messages received from only a
specific set of serving networks or authenticators are processed. In
addition, a simple solution for the peer side would be to let the
peer always initiate EAP pre-authentication and not allow EAP pre-
authentication initiation from authenticator side.
Second, consideration for the Channel Binding problem described in
[I-D.ietf-eap-keying] is needed as lack of Channel Binding may enable
an authenticator to impersonate another authenticator or communicate
incorrect information via out-of-band mechanisms (such as via a AAA
or lower layer protocol) [RFC3748]. It should be noted that it is
relatively easier to launch such an impersonation attack for pre-
authentication than normal authentication because an attacker does
not need to be physically on the same link as the legitimate peer to
send a pre-authentication trigger to the peer.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgments
The authors would like to thank Bernard Aboba, Jari Arkko, Ajay
Rajkumar, Maryna Komarova, Charles Clancy, Glen Zorn, Subir Das,
Shubhranshu Singh, Preetida Vinayakray and Rafa Marin Lopez for their
valuable input.
10. Contributors
The following people contributed to this document.
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Yoshihiro Ohba (yohba@tari.toshiba.com)
Ashutosh Dutta (adutta@research.telcordia.com)
Srinivas Sreemanthula (srinivas.sreemanthula@nokia.com)
Alper E. Yegin (alper.yegin@yegin.org)
Madjid Nakhjiri (madjid.nakhjiri@motorola.com)
Mahalingam Mani (mmani@avaya.com)
11. References
11.1. 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, June 2004.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management",
BCP 132, RFC 4962, July 2007.
[I-D.ietf-eap-keying]
Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
draft-ietf-eap-keying-22 (work in progress),
November 2007.
11.2. Informative References
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[I-D.ietf-hokey-erx]
Narayanan, V. and L. Dondeti, "EAP Extensions for EAP Re-
authentication Protocol (ERP)", draft-ietf-hokey-erx-14
(work in progress), March 2008.
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[I-D.aboba-radext-wlan]
Aboba, B., Malinen, J., Congdon, P., and J. Salowey,
"RADIUS Attributes for IEEE 802 Networks",
draft-aboba-radext-wlan-08 (work in progress), June 2008.
[I-D.ietf-pana-preauth]
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Author's Address
Yoshihiro Ohba
Toshiba America Research, Inc.
1 Telcordia Drive
Piscataway, NJ 08854
USA
Phone: +1 732 699 5365
Email: yohba@tari.toshiba.com
Ohba (Editor) Expires December 6, 2008 [Page 17]
Internet-Draft EAP Pre-authentication Problem Statement June 2008
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Ohba (Editor) Expires December 6, 2008 [Page 18]
