Network Working Group Y. Ohba (Editor)
Internet-Draft Toshiba
Expires: March 9, 2008 September 6, 2007
EAP Pre-authentication Problem Statement
draft-ietf-hokey-preauth-ps-00
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Abstract
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. This
draft discusses EAP pre-authentication problems in details.
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Table of Contents
1. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Specification of Requirements . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Direct Pre-authentication . . . . . . . . . . . . . . . . 7
4.2. Indirect Pre-authentication . . . . . . . . . . . . . . . 8
5. Architectural Considerations . . . . . . . . . . . . . . . . . 9
5.1. Authenticator Discovery . . . . . . . . . . . . . . . . . 9
5.2. Context Binding . . . . . . . . . . . . . . . . . . . . . 9
6. AAA Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Performance Requirements . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Contributors
The following people contributed to this document.
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 (mnakhjiri@huawei.com)
Mahalingam Mani (mmani@avaya.com)
2. 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
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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.
2.1. 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].
3. 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 (Access Point) 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
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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 candidate authenticators over
the wired medium, by way of the serving 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 a candidate or the target AP. Especially,
a solution is needed to enable EAP pre-authentication in IEEE 802.11
to work even if the station and AP are not members of the same VLAN.
As various flavors of wireless technologies are increasingly
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 [RFC4962] 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 candidate 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 candidate 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
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with an environment where the mobile device and candidate
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
one of the candidate authenticators.
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 candidate 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.
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
candidate 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
Section 6.
Figure 1 shows the functional elements that are related to EAP pre-
authentication.
+------+ +-------------+ +------+
|Mobile|---------| Serving | / \
| Node | |Authenticator|---/ \
+------+ +-------------+ / \
. / \ +----------+
. Move + Internet +---|AAA Server|
. \ / +----------+
v +-------------+ \ /
| Candidate |---\ /
|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
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candidate access network, it performs EAP pre-authentication with a
candidate authenticator, an authenticator in the candidate access
network, via the serving access network. The mobile node may perform
EAP pre-authentication with one or more candidate 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 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
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
on a AAA server using a AAA protocol such as RADIUS and Diameter.
If the candidate 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
candidate authenticator.
4. Usage Scenarios
There are two scenarios on how EAP pre-authentication signaling can
happen among a mobile node, a serving authenticator, a candidate
authenticator and a AAA server, depending on how the serving
authenticator is involved in the EAP pre-authentication signaling.
4.1. Direct Pre-authentication
Direct pre-authentication signaling is shown in Figure 2.
Mobile Serving Candidate AAA
Node Authenticator Authenticator Server
(MN) (SA) (CA)
| | | |
| | | |
| MN-CA Signaling (L2 or L3) | AAA |
|<------------------+------------------->|<----------------->|
| | | |
| | | |
Figure 2: Direct Pre-authentication
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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.
4.2. Indirect Pre-authentication
Indirect pre-authentication signaling is shown in Figure 3.
Mobile Serving Candidate AAA
Node Authenticator Authenticator Server
(MN) (SA) (CA)
| | | |
| | | |
| MN-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 MN cannot discover the CA's IP
address or if IP communication is not allowed between the candidate
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 candidate authenticator signaling (SA-CA signaling).
SA-CA 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 candidate 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.
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Mobile Serving Candidate
Node Authenticator Authenticator
(MN) (SA) (CA)
+-----------+ +-----------+
| |<- - - - - - - - - - - - - - - - - - ->| |
| EAP Peer | +-----------------------------+ | EAP Auth- |
| | |Pre-authentication Forwarding| | enticator |
+-----------+ +-----------+-----+-----------+ +-----------+
| MN-SA | | MN-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,
i.e., 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 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 candidate authenticator uses link-layer EAP transport for both
normal authentication and pre-authentication, a mechanism for
candidate authenticator discovery is typically defined in each link-
layer technology. For other cases, a mechanism for discovering an IP
address of a candidate 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 candidate authenticator.
5.2. Context Binding
When a candidate 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
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link to be established between the mobile node 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
mobile node and the candidate 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.
6. AAA Issues
Most of the AAA documentations today do not distinguish between a
full 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, 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 full authentication request and a pre-
authentication request.
Pre-authentication life time: Currently AAA protocols define
attributes (AVPs) carrying life time information for a full
authentication session. Even when a user profile and the AAA
server support pre-authentication function, after the pre-
authentication of a peer is complete, since the pre-authentication
may be accompanied with a pre-authorization, the pre-
authentication is typically valid only for a short amount of time.
It is currently not possible for a AAA server to indicate to the
AAA client or a peer what the life time of the pre- authenticated
session is. In other words, it is not clear to the peer or the
NAS, when the pre-authentication will expire.
Pre-authentication retries: It is typically expected that shortly
following the pre-authentication process, the mobile entity moves
to the new point of attachment and converts the pre-authentication
state to a full authentication state (the procedure for which is
not the topic of this particular subsection). However, if the
peer has yet not moved to the new location and realizes that the
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pre-authentication is expiring, it may perform another pre-
authentication. In order to avoid unlimited number of pre-
authentication tries, it is quite possible that the network policy
sets a limit on the maximum number of pre-authentication attempts.
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.
Session Resumption: In case the peer ping pongs between a network
N1 with which it has a full authentication state to another
network N2 and then back to N1, it should possible to simply
convert the full authentication state to a pre-authenticated
state. The problems around handling session life time and keying
material caching needs to be dealt with.
Multiple candidate authenticators: There may be situations where
the mobile node may need to make a selection between a number of
candidate attachment points. In such cases it is desirable for
the mobile to perform pre-authentication with multiple
authenticators. In such cases the AAA server may need to be aware
of the situation.
Roaming support: In case the pre-authentication is being performed
through a serving network that is foreign to the MN's home AAA
server, the AAA server needs to obtain the information about the
serving network in addition to the information about the candidate
network, so that the AAA server can make authorization decisions
accordingly, e.g., depending on the authorization policy, the home
AAA server may not allow pre-authentication via a particular
serving network.
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, heterogeneous handovers may require a change of the
MN identifier as part of the handover. Investigation on the best
type of identifiers for MNs that support multiple access
technologies is required.
Network controlled handovers: It is becoming quite common for the
network operators to maintain the control over the handovers for
various reasons including load balancing and performance. Hence
the network may need to direct the MN to perform pre-
authentication to a set of candidate authenticators in an
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anticipation for a handover. The AAA protocol extensions for
carrying out such procedures needs to be provided.
7. 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
candidate authenticator may send unprotected pre-authentication
messages to the mobile node or the candidate authenticator to let the
legitimate mobile node and candidate 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.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgments
The authors would like to thank Bernard Aboba and Jari Arkko for
their valuable input.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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., "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.
10.2. 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.aboba-radext-wlan]
Malinen, J. and B. Aboba, "RADIUS Attributes for IEEE 802
Networks", draft-aboba-radext-wlan-06 (work in progress),
July 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.
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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
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
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Internet-Draft EAP Pre-authentication Problem Statement September 2007
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
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.
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 March 9, 2008 [Page 15]
Internet-Draft EAP Pre-authentication Problem Statement September 2007
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Ohba (Editor) Expires March 9, 2008 [Page 16]
