Network Working Group Y. Ohba (Editor)
Internet-Draft Toshiba
Expires: September 4, 2007 A. Dutta
Telcordia
S. Sreemanthula
Nokia
A. Yegin
Samsung
M. Mani
Avaya
March 3, 2007
EAP Pre-authentication Problem Statement
draft-ohba-preauth-ps-01
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Abstract
EAP pre-authentication is defined as the utilization of EAP to pre-
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establish EAP keying material on an authenticator prior to arrival of
the peer at the access network managed by that authenticator. This
draft discusses EAP pre-authentication problems in details.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Direct Pre-authentication . . . . . . . . . . . . . . . . 7
3.2. Indirect Pre-authentication . . . . . . . . . . . . . . . 7
4. Architectural Considerations . . . . . . . . . . . . . . . . . 9
4.1. Authenticator Discovery . . . . . . . . . . . . . . . . . 9
4.2. Context Binding . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Performance Requirements . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
When a mobile 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. During the handover
process, when the mobile changes its point-of-attachment in the
network, it may change its subnet or administrative domain it is
connected to. We provide in Appendix A some performance requirement
that are needed to support an interactive real-time communication
such as VoIP and thus can serve as the guidelines for handover
optimization.
Handover often requires authorization for acquisition or modification
of resources assigned to a mobile and the authorization needs
interaction with a central authority in a domain. In many cases an
authorization procedure during a handover procedure follows an
authentication procedure that also requires interaction with a
central authority in a domain. The delay introduced due to such an
authentication and authorization procedure adds to the handover
latency and consequently affects the ongoing multimedia sessions.
The authentication and authorization procedure may include EAP
authentication [RFC3748] where an AAA server may be involved in EAP
messaging during the handover. Depending upon the type of
architecture, in some cases the AAA signals traverse all the way to
the AAA server in the home domain of the mobile as well before the
network service is granted to the mobile in the new network.
Real-time communication and interactive traffic such as VoIP is very
sensitive to the delay. Thus it is desirable that interactions
between the mobile and AAA servers must be avoided or be reduced
during the handover.
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 authenticator prior to arrival of
the peer at the access network managed 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. Problem Statement
Basic mechanism of handover is a three-step procedure involving i)
discovery of potential 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
of L2 and L3 connectivity to the target network point of attachment.
Currently, security mechanisms for authentication and authorization
is performed as part of the third step directly with the target
network. For example, in basic IEEE 802.11b based wireless networks,
the security mechanism 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
authentication, a four-way handshake with the wireless station
derives a new set of the session keys for use in data communications.
Unless PMK (Pairwise Master Key) is not cached in the target AP, this
mechanism is same as the initial setup to the AP with no particular
optimizations for the handover scenario. The handover latency
introduced by this security mechanism has proven to be larger than
what is acceptable for some handover scenarios. Hence, improvement
in the handover latency performance due to security procedures is a
necessary objective for such scenarios.
For example, if a mobile only requires 250 ms for "fast reconnect"
then if it is moving at 60 mph (87 feet/second), then the mobile will
have moved roughly 22 feet during the EAP authentication process.
This is larger than the average coverage overlap of a wireless LAN
(WLAN).
There is relevant work undertaken by various standards organizations.
But these efforts are scoped to a specific access technology. IEEE
802.11f has defined context transfer between APs. IEEE 802.11i
defines a pre-authentication mechanism for use in 802.11 variant
wireless networks. This mechanism allows mobile devices to pre-
authenticate using EAP to one or more target authenticators over the
wired medium, by way of the current authenticator. Presently, IEEE
802.11r WG has been working to define Fast BSS transition mechanisms
involving a definition of key management hierarchy and setup of
session keys before the re-association to the target AP. These
mechanisms, as indicated before, are defined for IEEE 802.11
technologies and are only applicable within a certain access domain
and fall short when it comes to inter-access technology handovers.
They also require L2 (e.g., Ethernet) connectivity for transfer of
encapsulated signaling to the target AP. Especially, a solution is
needed to enable EAP pre-authentication in IEEE 802.11 to work even
if the STA and AP are not members of the same VLAN.
As various flavors of wireless technologies are increasingly
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available, there is a growing demand for seamless inter-access
technology mobility and handovers. This is particularly beneficial
in the presence of high bandwidth wireless technologies (e.g., IEEE
802.11a/b/g) with only hotspot like coverages while the overlay
licensed wireless/cellular coverages are expensive and relatively
lower bandwidth. There is a strong motivation to allow seamless
inter-technology handovers for all kinds of data communications.
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.
Solutions for inter-authenticator mobility security optimizations can
be largely seen as security context transfer, handover keying or EAP
pre-authentication. Security context transfer involves transfer of
reusable key context in the new point of attachment. However, the
recent AAA key management requirement [I-D.housley-aaa-key-mgmt] does
not recommend horizontal context transfer of reusable key context
because of domino effect in which a compromise of an authenticator
will lead to a compromise of another authenticator. Nakhjiri et al
[I-D.nakhjiri-aaa-hokey-ps] discusses handover keying. Handover
keying uses an existing EAP-generated key for deriving a key to be
used for a target authenticator in order to reduce the handover
delay, which eliminates the need for running EAP for each inter-
authenticator handover. On the other hand, there are certain cases
where an EAP-generated key does not exist or is not usable for
handover keying at the time of handover and an EAP run is not
avoidable to generate a key for the target authenticator. One case
is an inter-domain handover without any trust relationship between
domains. Another case is a handover to an existing technology that
does not support handover keying.
EAP pre-authentication discussed in this document is mainly to deal
with an environment where the mobile device and target authenticators
are not in the same subnet or of the same link-layer technology.
Such use of EAP pre-authentication would enable the mobile device to
authenticate and setup keys prior to connecting to the target
authenticator.
This framework has general applicability to various deployment
scenarios in which proactive signaling can take effect. In other
words, applicability of EAP pre-authentication is limited to the
scenarios where target authenticators can be easily 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 or where there is
sufficient radio-coverage overlap among different technologies.
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Note that EAP pre-authentication problem for intra-technology intra-
subnet handover could be solved by each link-layer and is thus out of
the scope of this document while a general solution developed at IETF
can be used for intra-technology and intra-subnet scenarios as well.
In EAP pre-authentication, AAA authentication and authorization for a
target authenticator is performed while application sessions are in
progress via the serving network. The goal of EAP pre-authentication
is to avoid AAA signaling for EAP when or soon after the device
moves. There are several AAA issues related to EAP pre-
authentication. The pre-authentication AAA issues are described in
another document [I-D.nakhjiri-preauth-aaa-req].
Figure 1 shows the functional elements that are related to EAP pre-
authentication.
+------+ +-------------+ +------+
|Mobile|---------| Serving | / \
| Node | |Authenticator|---/ \
+------+ +-------------+ / \
. / \ +----------+
. Move + Internet +---|AAA Server|
. \ / +----------+
v +-------------+ \ /
| Target |---\ /
|Authenticator| \ /
+-------------+ +------+
Figure 1: EAP Pre-authentication Functional Elements
A mobile node is attached to the serving access network. Before the
mobile node performs handover from the serving access network to a
target access network, it performs EAP pre-authentication with a
target authenticator, an authenticator in the target access network,
via the serving access network. The mobile node may perform EAP pre-
authentication with one or more target authenticators. It is assumed
that each authenticator has an IP address when authenticators are on
different IP links. It is assumed that there is at least one target
authenticator in each target access network while the serving access
network may or may not have a serving authenticator. The serving and
target access networks may use different link-layer technologies.
Each authenticator has the functionality of EAP authenticator which
is either standalone EAP authenticator or pass-through EAP
authenticator. When an authenticator acts as a standalone EAP
authenticator, it also has the functionality of EAP server. On the
other hand, when an authenticator acts as a pass-through EAP
authenticator, it communicates with EAP server typically implemented
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on a AAA server using a AAA protocol such as RADIUS and Diameter.
If the target authenticator is of an existing link-layer technology
that uses an MSK (Master Session Key) [I-D.ietf-eap-keying] for
generating lower-layer ciphering keys, EAP pre-authentication is used
for proactively generating the MSK for the target authenticator.
3. Usage Scenarios
There are two scenarios on how EAP pre-authentication signaling can
happen among a mobile node, a serving authenticator, a target
authenticator and a AAA server, depending on how the serving
authenticator is involved in the EAP pre-authentication signaling.
3.1. Direct Pre-authentication
Direct pre-authentication signaling is shown in Figure 2.
Mobile Serving Target AAA
Node Authenticator Authenticator Server
(MN) (SA) (TA)
| | | |
| | | |
| MN-TA Signaling (L2 or 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
pre-authentication.
3.2. Indirect Pre-authentication
Indirect pre-authentication signaling is shown in Figure 3.
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Mobile Serving Target AAA
Node Authenticator Authenticator Server
(MN) (SA) (TA)
| | | |
| | | |
| MN-SA Signaling | SA-TA 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 MN cannot discover the TA's IP
address or if IP communication is not allowed between the target
authenticator and unauthorized nodes for security reasons.
Indirect pre-authentication signaling is spliced into mobile node to
serving authenticator signaling (MN-SA signaling) and serving
authenticator to target authenticator signaling (SA-TA signaling).
SA-TA signaling is performed over L3.
MN-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 mobile
node and the target authenticator and not to act as an EAP
authenticator, while it acts as an EAP authenticator for normal
authentication signaling. This is illustrated in Figure 4.
Mobile Serving Target
Node Authenticator Authenticator
(MN) (SA) (TA)
+-----------+ +-----------+
| |<- - - - - - - - - - - - - - - - - - ->| |
| EAP Peer | +-----------------------------+ | EAP Auth- |
| | |Pre-authentication Forwarding| | enticator |
+-----------+ +-----------+-----+-----------+ +-----------+
| MN-SA | | MN-SA | | SA-TA | | SA-TA |
| Signaling |<-->| Signaling | | Signaling |<-->| Signaling |
| Layer | | Layer | | Layer | | Layer |
+-----------+ +-----------+ +-----------+ +-----------+
Figure 4: Indirect Pre-authentication Layering Model
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4. Architectural Considerations
There are two architectural issues relating to pre-authentication,
i.e., authenticator discovery and context binding.
4.1. Authenticator Discovery
In general, pre-authentication requires an address of a target
authenticator to be discovered either by a mobile node or by a
serving authenticator prior to handover. An authenticator discovery
protocol is typically defined as a separated protocol from a pre-
authentication protocol as described below.
When a target authenticator uses link-layer EAP transport for both
normal authentication and pre-authentication, a mechanism for target
authenticator discovery is typically defined in each link-layer
technology. For other cases, a mechanism for discovering an IP
address of a target authenticator is needed. 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
discovering an IP address of a target authenticator.
4.2. Context Binding
When a target authenticator uses different EAP transport protocols
for normal authentication and pre-authentication, a mechanisms is
needed to bind link-layer independent context carried over pre-
authentication signaling to the link-layer specific context of the
link to be established between the mobile node and the target
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
mobile node and the target authenticator.
There are two possible approaches to address the context binding
issue. The first approach is based on communicating the lower-layer
context as opaque data via pre-authentication signaling and perform
the link-layer specific secure association procedure after handover.
The second approach is based on use of normal EAP authentication
after handover with using the same link-layer independent context for
both pre-authentication and normal authentication and then perform
the link-layer specific secure association procedure.
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5. Security Considerations
Since pre-authentication described in this document needs to work
across multiple authenticators, any solution for this problem needs
considerations on the following security threats.
First, a possible resource consumption denial of service attack where
an attacker that is not on the same IP link as the mobile node or the
target authenticator may send unprotected pre-authentication messages
to the mobile node or the target authenticator to let the legitimate
mobile node and target authenticator spend their computational and
bandwidth resources.
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 would
be 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. A simple solution
would be to let the peer always initiate EAP pre-authentication and
not allow EAP pre-authentication initiation from authenticator side.
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgments
The authors would like to thank Bernard Aboba, Jari Arkko and Madjid
Nakhjiri for their valuable input.
8. References
8.1. 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.
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[I-D.ietf-eap-keying]
Aboba, B., "Extensible Authentication Protocol (EAP) Key
Management Framework", draft-ietf-eap-keying-18 (work in
progress), February 2007.
[I-D.ietf-pana-preauth]
Ohba, Y., "Pre-authentication Support for PANA",
draft-ietf-pana-preauth-01 (work in progress), March 2006.
[I-D.nakhjiri-preauth-aaa-req]
Nakhjiri, M. and Y. Ohba, "Pre-Authentication AAA
requirements", draft-nakhjiri-preauth-aaa-req-00 (work in
progress), September 2006.
8.2. Informative References
[I-D.nakhjiri-aaa-hokey-ps]
Nakhjiri, M., "AAA based Keying for Wireless Handovers:
Problem Statement", draft-nakhjiri-aaa-hokey-ps-03 (work
in progress), June 2006.
[I-D.housley-aaa-key-mgmt]
Housley, R. and B. Aboba, "Guidance for AAA Key
Management", draft-housley-aaa-key-mgmt-09 (work in
progress), February 2007.
[802.21] IEEE, "Draft Standard for Local and Metropolitan Area
Networks: Media Independent Handover Services", LAN MAN
Standards Committee of the IEEE Computer Society 2007.
[ITU] ITU-T, "General Characteristics of International Telephone
Connections and International Telephone Circuits: One-Way
Transmission Time", ITU-T Recommendation G.114 1998.
[ETSI] ETSI, "Telecommunications and Internet Protocol
Harmonization Over Networks (TIPHON) Release 3: End-to-end
Quality of Service in TIPHON systems; Part 1: General
aspects of Quality of Service.", ETSI TR 101 329-6 V2.1.1.
[WPA] The Wi-Fi Alliance, "WPA (Wi-Fi Protected Access)", Wi-
Fi WPA v3.1, 2004.
Appendix A. Performance Requirements
In order to provide the desirable quality of service for interactive
VoIP and streaming traffic during handoff, one needs to limit the
value of end-to-end delay, jitter and packet loss to a certain
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threshold level. ITU-T and ITU-R standards define the acceptable
values for these parameters. For example for one-way delay, ITU-T
G.114 [ITU] recommends 150 ms as the upper limit for most of the
applications, and 400 ms as generally unacceptable delay. One way
delay tolerance for video conferencing is in the range of 200 to 300
ms. Also if an out-of-order packet is received after a certain
threshold, it is considered lost. The performance requirement will
vary based on the type of application and its characteristics such as
delay tolerance and loss tolerance limit. Interactive traffic such
as VoIP and streaming traffic will have different tolerance for delay
and packet loss. For example, according to ETSI TR 101 [ETSI] a
normal voice conversation can tolerate up to 2% packet loss.
Similarly there are other factors such as Transmission Rating Factor
(R) standardized within ITU-T G.107, End to End delay (one way mouth-
to-ear) and call blocking ratio that determine the QoS metrics. An R
value of 50 is considered to be poor and a value of 90 can be
considered as the best that provides most user satisfaction. As an
example, a class B QoS which is equivalent to cellular telephony has
a R factor that is greater than 70, E2E delay of less than 150 ms and
call blocking ratio which is less than or equal to 0.15. Class A QoS
that is the highest and is equivalent to fixed phone quality has an R
value that is more than 80 and an end-to-end delay that is less than
100 ms. Similarly, 3GPP TS23.107 defines 4 application classes:
conversational, streaming, interactive and background each with
different set of end-to-end delay and QoS requirement. The streaming
class has the tolerable packet (SDU) error rates ranging from 0.1 to
0.00001 and the transfer delay of less than 300ms. In short, the
delay and packet loss tolerance value will depend upon the type of
application and different standard bodies and vendors provide
different specification for each type of application and thus any
optimized handoff mechanism will need to take these values into
consideration.
It is desirable to support a heterogeneous handover that is agnostic
to link-layer technologies in an optimized and secure fashion without
incurring unreasonable complexity while providing seamless handover
experience to the user. As a mobile goes through a handover process,
it is subjected to handover delay because of the rebinding of
properties at several layers of the protocol stack, such as layer 2,
layer 3 and application layer. There are several common properties
that contribute to the re-establishment or modification of these
layers during handover. These properties can mostly be attributed to
things such as access characteristics (e.g., bandwidth, channel
characteristics, channel scan, access point association), access
mechanism (e.g., CDMA, CSMA/CA, TDMA), configuration of layer 3
parameters such as IP address acquisition, re-authentication, re-
authorization, rebinding of security association at all layers,
binding update etc. Although each of the components during the
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handover process that contributes to the handover delay needs to be
optimized, we focus our discussion on optimizing the delay due to
authentication and authorization.
Authors' Addresses
Yoshihiro Ohba
Toshiba America Research, Inc.
1 Telcordia Drive
Piscataway, NJ 08854
USA
Phone: +1 732 699 5365
Email: yohba@tari.toshiba.com
Ashutosh Dutta
Telcordia
1 Telcordia Drive
Piscataway, NJ 08854
USA
Phone: +1 732 699 3130
Email: adutta@research.telcordia.com
Srivinas Sreemanthula
Nokia Research Center
6000 Connection Dr.
Irving, TX 75028
USA
Email: srinivas.sreemanthula@nokia.com
Alper E. Yegin
Samsung Advanced Institute of Technology
Istanbul,
Turkey
Phone: +90 538 719 0181
Email: alper01.yegin@partner.samsung.com
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Mahalingam Mani
Avaya
Email: mmani@avaya.com
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