MOBOPTS Research Group A. Dutta (Ed.)
Internet-Draft V. Fajardo
Intended status: Informational Telcordia
Expires: October 18, 2010 Y. Ohba
K. Taniuchi
Toshiba
H. Schulzrinne
Columbia Univ.
April 16, 2010
A Framework of Media-Independent Pre-Authentication (MPA) for Inter-
domain Handover Optimization
draft-irtf-mobopts-mpa-framework-07
Abstract
This document describes a framework of Media-independent Pre-
Authentication (MPA), a new handover optimization mechanism that
addresses the issues on existing mobility management protocols and
mobility optimization mechanisms to support inter-domain handover.
MPA is a mobile-assisted, secure handover optimization scheme that
works over any link-layer and with any mobility management protocol
and is best applicable to support optimization during inter-domain
handover. MPA's pre-authentication, pre-configuration, and proactive
handover techniques allow many of the handoff related operations to
take place before the mobile has moved to the new network. We
describe the details of all the associated techniques and its
applicability for different scenarios involving various mobility
protocols during inter-domain handover.
This document is a product of the IP Mobility Optimizations (MobOpts)
Research Group.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 18, 2010.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Specification of Requirements . . . . . . . . . . . . . . 7
1.2. Performance Requirements . . . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Handover Taxonomy . . . . . . . . . . . . . . . . . . . . . . 9
4. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Applicability of MPA . . . . . . . . . . . . . . . . . . . . . 13
6. MPA Framework . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Functional Elements . . . . . . . . . . . . . . . . . . . 15
6.3. Basic Communication Flow . . . . . . . . . . . . . . . . . 17
7. MPA Operations . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Pre-authentication in multiple CTN environment . . . . . . 21
7.3. Proactive IP address acquisition . . . . . . . . . . . . . 22
7.3.1. PANA-assisted proactive IP address acquisition . . . . 23
7.3.2. IKEv2-assisted proactive IP address acquisition . . . 23
7.3.3. Proactive IP address acquisition using DHCP only . . . 24
7.3.4. Proactive IP address acquisition using stateless
autoconfiguration . . . . . . . . . . . . . . . . . . 25
7.4. Tunnel management . . . . . . . . . . . . . . . . . . . . 25
7.5. Binding Update . . . . . . . . . . . . . . . . . . . . . . 27
7.6. Preventing packet loss . . . . . . . . . . . . . . . . . . 28
7.6.1. Packet loss prevention in single interface MPA . . . . 28
7.6.2. Preventing packet losses for multiple interfaces . . . 28
7.6.3. Reachability test . . . . . . . . . . . . . . . . . . 29
7.7. Security and mobility . . . . . . . . . . . . . . . . . . 30
7.7.1. Link-layer security and mobility . . . . . . . . . . . 30
7.7.2. IP layer security and mobility . . . . . . . . . . . . 31
7.8. Authentication in initial network attachment . . . . . . . 32
8. Security Considerations . . . . . . . . . . . . . . . . . . . 32
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1. Normative References . . . . . . . . . . . . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Proactive duplicate address detection . . . . . . . . 38
Appendix B. Address resolution . . . . . . . . . . . . . . . . . 39
Appendix C. MPA Deployment Issues . . . . . . . . . . . . . . . . 40
C.1. Considerations for failed switching and switch-back . . . 40
C.2. Authentication state management . . . . . . . . . . . . . 41
C.3. Pre-allocation of QoS resources . . . . . . . . . . . . . 42
C.4. Resource allocation issue during pre-authentication . . . 43
C.5. Systems evaluation and performance results . . . . . . . . 44
C.5.1. Intra-technology, Intra-domain . . . . . . . . . . . . 44
C.5.2. Inter-technology, Inter-domain . . . . . . . . . . . . 47
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C.5.3. MPA-assisted Layer 2 pre-authentication . . . . . . . 47
C.6. Guidelines for handover preparation . . . . . . . . . . . 52
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53
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1. Introduction
As wireless technologies including cellular and wireless LAN are
beginning to get popular, supporting terminal handovers across
different types of access networks, such as from a wireless LAN to
CDMA or to GPRS is considered a clear challenge. On the other hand,
supporting seamless terminal handovers between access networks of the
same type is still more challenging, especially when the handovers
are across IP subnets or administrative domains. To address those
challenges, it is important to provide terminal mobility that is
agnostic to link-layer technologies in an optimized and secure
fashion without incurring unreasonable complexity. In this document
we discuss a framework to support terminal mobility that provides
seamless handovers with low latency and low loss. Seamless handovers
are characterized in terms of performance requirements as described
in Section 1.2. [mpa-wireless] is an accompanying document which
describes implementation of a few MPA-based systems including
performance results to show how existing protocols could be leveraged
to realize the functionalities of MPA.
Terminal mobility is accomplished by a mobility management protocol
that maintains a binding between a locator and an identifier of a
mobile node, where the binding is referred to as the mobility
binding. The locator of the mobile node may dynamically change when
there is a movement of the mobile node. The movement that causes a
change of the locator may occur when there is a change in attachment
point due to physical movement or network change. A mobility
management protocol may be defined at any layer. In the rest of this
document, the term "mobility management protocol" refers to a
mobility management protocol which operates at the network layer or
higher.
There are several mobility management protocols at different layers.
Mobile IP [RFC3344] and Mobile IPv6 [RFC3775] are mobility management
protocols that operate at the network layer. Similarly, MOBIKE
(IKEv2 Mobility and Multihoming) [RFC4555] is an extension to IKEv2
that provides the ability to deal with a change of an IP address of
an IKEv2 end-point. There are several ongoing activities in the IETF
to define mobility management protocols at layers higher than network
layer. HIP (the Host Identity Protocol) [RFC5201] defines a new
protocol layer between network layer and transport layer to provide
terminal mobility in a way that is transparent to both network layer
and transport layer. Also, SIP-based mobility is an extension to SIP
to maintain the mobility binding of a SIP user agent [SIPMM].
While mobility management protocols maintain mobility bindings, these
cannot provide seamless handover if used in their current form. An
additional optimization mechanism is needed to prevent the loss of
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inflight packets transmitted during mobile's binding update procedure
and to achieve seamless handovers. Such a mechanism is referred to
as a mobility optimization mechanism. For example, mobility
optimization mechanisms [RFC4881] and [RFC5268] are defined for
Mobile IPv4 and Mobile IPv6, respectively, by allowing neighboring
access routers to communicate and carry information about mobile
terminals. There are protocols that are considered as "helpers" of
mobility optimization mechanisms. The CARD (Candidate Access Router
Discovery Mechanism) protocol [RFC4065] is designed to discover
neighboring access routers. The CTP (Context Transfer Protocol)
[RFC4066] is designed to carry state that is associated with the
services provided for the mobile node, or context, among access
routers. We describe some of the fast-handover scheme that attempt
to reduce the handover delay in Section 4.
There are several issues in existing mobility optimization
mechanisms. First, existing mobility optimization mechanisms are
tightly coupled with specific mobility management protocols. For
example, it is not possible to use mobility optimization mechanisms
designed for Mobile IPv4 or Mobile IPv6 for MOBIKE. What is strongly
desired is a single, unified mobility optimization mechanism that
works with any mobility management protocol. Second, there is no
existing mobility optimization mechanism that easily supports
handovers across administrative domains without assuming a pre-
established security association between administrative domains. A
mobility optimization mechanism should work across administrative
domains in a secure manner only based on a trust relationship between
a mobile node and each administrative domain. Third, a mobility
optimization mechanism needs to support not only terminals with
multiple-interfaces where simultaneous connectivity through multiple
interfaces or connectivity through single interface can be expected,
but also terminals with single-interface.
This document describes a framework of Media-independent Pre-
Authentication (MPA), a new handover optimization mechanism that
addresses all those issues. MPA is a mobile-assisted, secure
handover optimization scheme that works over any link-layer and with
any mobility management protocol including Mobile IPv4, Mobile IPv6,
MOBIKE, HIP, SIP mobility. In cases of multiple operators without
roaming relationship or without agreement to participate in a key
management scheme, MPA provides a framework that can perform pre-
authentication to establish the security mechanisms without assuming
a common source of trust. In MPA, the notion of IEEE 802.11i pre-
authentication is extended to work at higher layer, with additional
mechanisms to perform early acquisition of IP address from a network
where the mobile node may move as well as proactive handover to the
network while the mobile node is still attached to the current
network. Since this document focuses on the MPA framework, it is
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left to future work to choose the protocols for MPA and define
detailed operations. The accompanying document [mpa-wireless]
provides one method that describes usage and interactions between
existing protocols to accomplish MPA functionality.
This document represents the consensus of the (MobOpts) Research
Group. It has been reviewed by Research Group members active in the
specific area of work.
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].
1.2. Performance Requirements
In order to provide desirable quality of service for interactive VoIP
and streaming traffic, one needs to limit the value of end-to-end
delay, jitter and packet loss to a certain threshold level. ITU-T
and ITU-E standards define the acceptable values for these
parameters. For example for one-way delay, ITU-T G.114 [RG98]
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 [ITU98].
Also if an out-of-order packet is received after a certain threshold,
it is considered lost. According to ETSI TR 101 [ETSI], a normal
voice conversation can tolerate up to 2% packet loss. But this is
the mean packet loss probability and may be applicable to a scenario
when the mobile is subjected to repeated handoff during a normal
conversation. Measurement techniques for delay and jitter are
described in [RFC2679], [RFC2680] and [RFC2681].
In case of interactive VoIP traffic, end-to-end delay affects the
jitter value, and thus is an important issue to consider. An end-to-
end delay consists of several components, such as network delay,
operating system (OS) delay, codec delay and application delay. A
complete analysis of these delays can be found in [Wenyu]. During a
mobile's handover, in-flight transient traffic cannot reach the
mobile because of the associated handover delay. These in-flight
packets could either be lost or buffered. If the in-flight packets
are lost, then it contributes to jitter between the last packet
before handoff and first packet after handoff. If these packets are
buffered, packet loss is minimized, but there is additional jitter
for the in-flight packets when these are flushed after the handoff.
Buffering during handoff avoids the packet loss, but at the cost of
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additional one-way-delay. A trade-off between one-way-delay and
packet loss is desired based on the type of application.
The handover delay is attributed due to several factors, such as
discovery, configuration, authentication, binding update and media
delivery. Many of the security related procedures such as handover
keying and re-authentication procedures deal with cases where there
is a single source of trust at the top and the underlying AAA domain
elements trust the top source of trust and the keys it generates and
distributes. In this scenario, there is an appreciable delay in re-
establishing link security related parameters, such as
authentication, link key management and access authorization during
inter-domain handover. The focus of this draft is the design of a
framework that can reduce the delay due to authentication and other
handoff related operations such as configuration and binding update.
2. Terminology
Mobility Binding: A binding between a locator and an identifier of a
mobile terminal.
Mobility Management Protocol (MMP): A protocol that operates at
network layer or above to maintain a binding between a locator and
an identifier of a mobile node.
Binding Update: A procedure to update a mobility binding.
Media-independent Pre-Authentication Mobile Node (MN): A mobile node
of media-independent pre-authentication (MPA) which is a mobile-
assisted, secure handover optimization scheme that works over any
link-layer and with any mobility management protocol. An MPA
mobile node is an IP node. In this document, the term "mobile
node" or "MN" without a modifier refers to "MPA mobile node". An
MPA mobile node usually has a functionality of a mobile node of a
mobility management protocol as well.
Candidate Target Network (CTN):
A network to which the mobile may move in the near future.
Target Network (TN): The network to which the mobile has decided to
move. The target network is selected from one or more candidate
target network.
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Proactive Handover Tunnel (PHT): A bidirectional IP tunnel [RFC1853]
that is established between the MPA mobile node and an access
router of a candidate target network. In this document, the term
"tunnel" without a modifier refers to "proactive handover tunnel.
Point of Attachment (PoA): A link-layer device (e.g., a switch, an
access point or a base station) that functions as a link-layer
attachment point for the MPA mobile node to a network.
Care-of Address (CoA): An IP address used by a mobility management
protocol as a locator of the MPA mobile node.
3. Handover Taxonomy
Based on the type of movement, type of access network, and underlying
mobility support, one can primarily define the handover as inter-
technology, intra-technology, inter-domain, and intra-domain. We
describe briefly each of these handover processes. However, our
focus of the dicussion is on Inter-domain handover.
Inter-technology: A mobile may be equipped with multiple interfaces,
where each interface can support different access technology (802.11,
CDMA). A mobile may communicate with one interface at any time in
order to conserve the power. During the handover the mobile may move
out of the footprint of one access technology (e.g., 802.11) and move
into the footprint of a different access technology (e.g., CDMA).
This will warrant switching of the communicating interface on the
mobile as well. This type of Inter-technology handover is often
called as Vertical Handover since the mobile makes movement between
two different cell sizes.
Intra-technology: An intra-technology handover is defined when a
mobile moves between the same type of access technology such as
between 802.11[a,b,n] and 802.11 [a,b,n] or between CDMA1XRTT and
CDMA1EVDO. In this scenario a mobile may be equipped with a single
interface (with multiple PHY types of the same technology) or with
multiple interfaces. An Intra-technology handover may involve intra-
subnet or inter-subnet movement and thus may need to change its L3
locator depending upon type of movement.
Inter-domain: A domain can be defined in several ways. But for the
purposes of roaming we define domain as an administrative domain
which consists of networks that are managed by a single
administrative entity which authenticates and authorizes a mobile for
accessing the networks. An administrative entity may be a service
provider, an enterprise and any organization. Thus an Inter-domain
handover will by-default be subjected to inter-subnet handover and in
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addition it may be subjected to either inter-technology or intra-
technology handover. Inter-domain handover will be subjected to all
the transition steps a subnet handover goes through and in addition
it will be subjected to authentication and authorization process as
well. It is also likely that the type of mobility support in each
administrative domain will be different. For example, administrative
domain A may have MIPv6 support, while administrative domain B may
use Proxy MIPv6.
Intra-domain: When a mobile's movement is confined to movement within
an administrative domain it is called intra-domain movement. An
intra-domain movement may involve intra-subnet, inter-subnet, intra-
technology and inter-technology as well.
Both inter-domain and intra-domain handovers can be subjected to
either inter-technology or intra-technology handover based on the
network access characteristics. Inter-domain handover 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 inter-domain handover follows an authentication procedure that
also requires interaction with a central authority in a domain.
Thus, security associations between the network entities such as
routers in the neighboring administrative domains need to be
established before any interaction takes place between these
entities. Similarly, an inter-domain mobility may involve different
mobility protocols in each of its domains, such as MIPv6 and Proxy-
MIPv6. In that case, one needs a generalized framework to achieve
the optimization during inter-domain handover. Figure 1 shows a
typical example of inter-domain mobility involving two domains, such
as domain A and domain B. It illustrates several important components
such as AAA Home server (AAAH), AAA visited servers (e.g., AAAV1 and
AAAV2), Authentication Agent (AA), Layer 3 point of attachment, such
as Access Router (AR) and layer 2 point of attachment, such as Access
Point. Any mobile maybe using a specific mobility protocol and
associated mobility optimization technique during intra-domain
movement in either domain. But the same optimization technique may
not be suitable to support inter-domain handover independent of
whether it uses the same or different mobility protocol in either
domain.
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+-----------------------------+
| +--------+ |
| | | |
| | AAAH --------------------|
| | | | |
| +|-------+ | |
| | | |
| | Home Domain | |
| | | |
+-------|---------------------+ |
| |
| |
| |
+----------------------------|-----------+ +-------------|------------+
| | | | +|-------+ |
| Domain A +-------|+ | | +-----+ | | |
| | | | | | ------ AAAV2 | |
| | AAAV1 | | | | AA | | | |
| +-------------- | | | +|----+ +--------+ |
| | | +--------+ | | | |
| |AA | | | |--- ---- |
| +--|--+ | | / \ / \ |
| | /----\ | || AR |-----| AR | |
| -|-- / \ | | \ / \ / |
| / \ | AR | | | -|-- --|- |
| | AR ----------- / | |+--|---+ +------|------+ |
| \ / \--|-/ | || AP4 | | L2 Switch | |
| -/-- +-----|------+ | || | +-|---------|-+ |
| / | L2 Switch | | |+------+ | | |
| / +-|-------|--+ | | +---|--+ +----|-+ |
| +----/-+ +----|-+ +-|----+ | |Domain B| | | | |
| | | | | | | | | | AP5 | |AP6 | |
| | AP1 | | AP2 | | AP3 | | | +--|---+ +------+ |
| +------+ +------+ +--|---+ | | | |
+--------------------------------|-------+ +-----------|--------------+
--|--------- |
//// \\\\ -----|-----
// +------+ //// +------+ \\\\
| | MN ------------->|MN | \\\
| | | | | | | |
| +------+ | | +------+ |
\\ | // |
\\\\ \\\/ ///
------------ \\\\------------- ////
Figure 1: Inter-domain Mobility
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4. Related Work
While basic mobility management protocols such as Mobile IP
[RFC3344], Mobile IPv6 [RFC3775], SIP-Mobility [SIPMM] provide
continuity to TCP and RTP traffic, these are not optimized to reduce
the handover latency during mobile's movement between subnets and
domains. In general these mobility management protocols introduce
handover delays incurred at several layers such as, layer 3 and
application layer for updating the mobile's mobility binding. These
protocols also get affected due to underlying layer 2 delay as well.
As a result, applications using these mobility protocols suffer from
performance degradation.
There have been several optimization techniques that apply to current
mobility management schemes that try to reduce handover delay and
packet loss during a mobile's movement between cells, subnets and
domain. Micro-mobility management schemes [CELLIP], [HAWAII], and
intra-domain mobility management schemes such as [IDMP],
[I-D.ietf-mobileip-reg-tunnel] and [RFC5380] provide fast-handover by
limiting the signaling updates within a domain. Fast Mobile IP
protocols for IPv4 and IPv6 networks [RFC4881], [RFC5268] utilize
mobility information made available by link layer triggers. Yokota
et al. [YOKOTA] propose joint use of access point and a dedicated
MAC bridge to provide fast-handover without altering the MIPv4
specification. Shin et al. [MACD] propose a scheme reducing the
delay due to MAC layer handoff by providing a cache-based algorithm.
In this scheme, the mobile caches the neighboring channels that it
has already visited and thus uses a selective scanning method. This
helps to reduce the associated scanning time.
Some mobility management schemes use dual interfaces thus providing
make-before-break [SUM]. In a make-before-break situation,
communication usually continues with one interface, when the
secondary interface is in the process of getting connected. The IEEE
802.21 working group is discussing these scenarios in details
[802.21]. Providing fast-handover using a single interface needs
more careful design than for a client with multiple interfaces.
Dutta et al [SIPFAST] provide an optimized handover scheme for SIP-
based mobility management, where the transient traffic is forwarded
from the old subnet to the new one by using an application layer
forwarding scheme. [MITH] provides a fast handover scheme for the
single interface case that uses mobile-initiated tunneling between
the old foreign agent and new foreign agent. [MITH] defines two
types of handover schemes such as Pre-MIT (Mobile Initiated
Tunneling) and Post-MIT (Media Initiated Tunneling). The proposed
MPA scheme is very similar to MITH's predictive scheme where the
mobile communicates with the foreign agent before actually moving to
the new network. However, the MPA scheme is not limited to MIP type
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mobility protocol only and in addition, this scheme takes care of
movement between domains and performs pre-authentication in addition
to proactive handover. Thus, MPA reduces the overall delay to close
to link-layer handover delay. Most of the mobility optimization
techniques developed so far are restricted to a specific type of
mobility protocol only. While supporting optimization for inter-
domain mobility, these protocols assume that there is a pre-
established security arrangement between two administrative domains.
But this assumption may not be viable always. Thus, there is a need
to develop an optimization framework that can support inter-domain
mobility without any underlying constraints or security related
assumption.
Recently, the HOKEY WG within IETF is defining the ways to expedite
the authentication process. In particular, it has defined pre-
authentication [I-D.ietf-hokey-preauth-ps] and fast re-authentication
[RFC5169] mechanisms to expedite the authentication and security
association process.
5. Applicability of MPA
MPA is more applicable where an accurate prediction of movement can
be easily made. For other environments, special care must be taken
to deal with issues such as pre-authentication to multiple CTNs
(Candidate Target Networks) and failed switching and switching back
as described in [mpa-wireless]. However, addressing those issues in
actual deployments may not be easier. Some of the deployment issues
are described in Appendix C.
The effectiveness of MPA may be relatively reduced if the network
employs network-controlled localized mobility management in which the
MN does not need to change its IP address while moving within the
network. The effectiveness of MPA may also be relatively reduced if
signaling for network access authentication is already optimized for
movements within the network, e.g., when simultaneous use of multiple
interfaces during handover is allowed. In other words, MPA is most
viable solution for inter-administrative domain predictive handover
without the simultaneous use of multiple interfaces. Since MPA is
not tied to a specific mobility protocol, it is also applicable to
support optimization for inter-domain handover where each domain
maybe equipped with different mobility protocol. Figure 1 shows an
example of inter-domain mobility where MPA could be applied. For
example, domain A may support just Proxy MIPv6, whereas domain B may
support Client Mobile IPv6. MPA's different functional components
can provide the desired optimization techniques proactively.
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6. MPA Framework
6.1. Overview
Media-independent Pre-Authentication (MPA) is a mobile-assisted,
secure handover optimization scheme that works over any link layer
and with any mobility management protocol. With MPA, a mobile node
is not only able to securely obtain an IP address and other
configuration parameters for a CTN, but also able to send and receive
IP packets using the IP address obtained before it actually attaches
to the CTN. This makes it possible for the mobile node to complete
the binding update of any mobility management protocol and use the
new CoA before performing a handover at link-layer.
MPA provides three basic procedures to provide this functionality.
The first procedure is referred to as "pre-authentication", the
second procedure is referred to as "pre-configuration", the
combination of the third and fourth procedures are referred to as
"secure proactive handover". The security association established
through pre-authentication is referred to as an "MPA-SA".
This functionality is provided by allowing a mobile node which has
connectivity to the current network but is not yet attached to a CTN,
to (i) establish a security association with the CTN to secure the
subsequent protocol signaling, then (ii) securely execute a
configuration protocol to obtain an IP address and other parameters
from the CTN as well as execute a tunnel management protocol to
establish a Proactive Handover Tunnel (PHT) [RFC1853] between the
mobile node and an access router of the CTN, then (iii) send and
receive IP packets, including signaling messages for binding update
of an MMP and data packets transmitted after completion of binding
update, over the PHT using the obtained IP address as the tunnel
inner address, and finally (iv) deleting or disabling the PHT
immediately before attaching to the CTN when it becomes the target
network and then re-assigning the inner address of the deleted or
disabled tunnel to its physical interface immediately after the
mobile node is attached to the target network through the interface.
Instead of deleting or disabling the tunnel before attaching to the
target network, the tunnel may be deleted or disabled immediately
after being attached to the target network.
Especially, the third procedure described above (i.e., binding update
procedure) makes it possible for the mobile to complete the higher-
layer handover before starting link-layer handover. This means that
the mobile is able to send and receive data packets transmitted after
completing the binding update over the tunnel, while data packets
transmitted before completion of binding update do not use the
tunnel.
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6.2. Functional Elements
In the MPA framework, the following functional elements are expected
to reside in each CTN to communicate with a mobile node:
Authentication Agent (AA), Configuration Agent (CA) and Access Router
(AR). These elements can reside in one or more network devices.
An authentication agent is responsible for pre-authentication. An
authentication protocol is executed between the mobile node and the
authentication agent to establish an MPA-SA. The authentication
protocol MUST be able to derive a key between the mobile node and the
authentication agent and SHOULD be able to provide mutual
authentication. The authentication protocol SHOULD be able to
interact with a AAA protocol such as RADIUS and Diameter to carry
authentication credentials to an appropriate authentication server in
the AAA infrastructure. The derived key is used for further deriving
keys used for protecting message exchanges used for pre-configuration
and secure proactive handover. Other keys that are used for
bootstrapping link-layer and/or network-layer ciphers MAY also be
derived from the MPA-SA. A protocol that can carry EAP [RFC3748]
would be suitable as an authentication protocol for MPA.
A configuration agent is responsible for one part of pre-
configuration, namely securely executing a configuration protocol to
deliver an IP address and other configuration parameters to the
mobile node. The signaling messages of the configuration protocol
(e.g., DHCP) MUST be protected using a key derived from the key
corresponding to the MPA-SA.
An access router is a router that is responsible for the other part
of pre-configuration, i.e., securely executing a tunnel management
protocol to establish a proactive handover tunnel to the mobile node.
IP packets transmitted over the proactive handover tunnel SHOULD be
protected using a key derived from the key corresponding to the
MPA-SA. Details of this procedure are described in Section 6.3.
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+----+
| |
| CN |
*----+
//
/--------\ /
|// \\//
| Core / |
|\X Network \
/ \--------/ \\
/ \
/ \
/ \\
+-------------/-----------+ +--------\-------------+
| +-----+ | |+-----+ |
| | | +-----+ | || | +-----+ |
| | AA | |CA | | ||AA | | CA | |
| +--+--+ +--+--+ | |+--+--+ +--+--+ |
| | +------+ | | | | +-----+ | |
| | | pAR | | | | | |nAR | | |
| ---+---+ +---+-----+----+---+-+ +-----+ |
| +---+--+ | | +-----+ |
| | | | |
| | | | |
| | | | |
+------------+------------+ +--------|--------------+
Current | Candidate| Target Network
Network | |
| |
| |
| |
| |
---+-------- --------|-----
///// \\\\\///// \\\\\
// //\\ \\
| +-+----+ | | |
| oPoA | MN | | | |
| | | | | |
| +------+ \\ | //
\\ XX\\\ nPoA /////
\\\\\ ///// -------------
------------
Figure 2: MPA Functional Components
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6.3. Basic Communication Flow
Assume that the mobile node is already connected to a point of
attachment, say oPoA (old point of attachment), and assigned a
care-of address, say oCoA (old care-of address). The communication
flow of MPA is described as follows. Throughout the communication
flow, data packet loss should not occur except for the period during
the switching procedure in Step 5, and it is the responsibility of
link-layer handover to minimize packet loss during this period.
Step 1 (pre-authentication phase): The mobile node finds a CTN
through some discovery process such as IEEE 802.21 and obtains the IP
addresses of an authentication agent, a configuration agent and an
access router in the CTN (Candidate Target Network) by some means.
Details of discovery mechanism are discussed in Section 7.1. The
mobile node performs pre-authentication with the authentication
agent. As discussed in Section 7.2, the mobile may need to pre-
authenticate with multiple candidate target networks. The decision
regarding which candidate network the mobile needs to pre-
authenticate with will depend upon several policies, such as
signaling overhead, bandwidth requirement (QoS), mobile's location,
communication cost, and handover robustness etc. Determining the
policy that decides the target network the mobile should pre-
authenticate with is out of scope for this document.
If the pre-authentication is successful, an MPA-SA is created between
the mobile node and the authentication agent. Two keys are derived
from the MPA-SA, namely an MN-CA key and an MN-AR key, which are used
to protect subsequent signaling messages of a configuration protocol
and a tunnel management protocol, respectively. The MN-CA key and
the MN-AR key are then securely delivered to the configuration agent
and the access router, respectively.
Step 2 (pre-configuration phase): The mobile node realizes that its
point of attachment is likely to change from oPoA to a new one, say
nPoA (new point of attachment). It then performs pre-configuration
with the configuration agent using the configuration protocol to
obtain several configuration parameters such as an IP address, say
nCoA (new care-of address), and default router from the CTN. The
mobile then communicates with the access router using the tunnel
management protocol to establish a proactive handover tunnel. In the
tunnel management protocol, the mobile node registers oCoA and nCoA
as the tunnel outer address and the tunnel inner address,
respectively. The signaling messages of the pre-configuration
protocol are protected using the MN-CA key and the MN-AR key. When
the configuration and the access router are co-located in the same
device, the two protocols may be integrated into a single protocol
like IKEv2. After completion of the tunnel establishment, the mobile
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node is able to communicate using both oCoA and nCoA by the end of
Step 4. A configuration protocol and a tunnel management protocol
may be combined in a single protocol or executed in different orders
depending on the actual protocol(s) used for configuration and tunnel
management.
Step 3 (secure proactive handover main phase): The mobile node
decides to switch to the new point of attachment by some means.
Before the mobile node switches to the new point of attachment, it
starts secure proactive handover by executing the binding update
operation of a mobility management protocol and transmitting
subsequent data traffic over the tunnel (main phase). This proactive
binding update could be triggered based on certain local policy at
the mobile node end, after the pre-configuration phase is over. This
local policy could be signal-to-noise ratio, location of the mobile
etc. In some cases, it may cache multiple nCOA addresses and perform
simultaneous binding with the CN or HA.
Step 4 (secure proactive handover pre-switching phase): The mobile
node completes the binding update and becomes ready to switch to the
new point of attachment. The mobile may execute the tunnel
management protocol to delete or disable the proactive handover
tunnel and cache nCoA after deletion or disabling of the tunnel.
This transient tunnel can be deleted prior to or after the handover.
The buffering module at the next access router buffers the packets
once the tunnel interface is deleted. The decision as to when the
mobile node is ready to switch to the new point of attachment depends
on the handover policy.
Step 5 (switching): It is expected that a link-layer handover occurs
in this step.
Step 6 (secure proactive handover post-switching phase): The mobile
node executes the switching procedure. Upon successful completion of
the switching procedure, the mobile node immediately restores the
cached nCoA and assigns it to the physical interface attached to the
new point of attachment. If the proactive handover tunnel was not
deleted or disabled in Step 4, the tunnel is deleted or disabled as
well. After this, direct transmission of data packets using nCoA is
possible without using a proactive handover tunnel.
Call flow for MPA is shown in Figure 3 and Figure 4.
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IP address(es)
Available for
Use by MN
|
+-----------------------------------+ |
| Candidate Target Network | |
| (Future Target Network) | |
MN oPoA | nPoA AA CA AR | |
| | | | | | | | |
| | +-----------------------------------+ |
| | | | | | .
+---------------+ | | | | | .
|(1) Found a CTN| | | | | | .
+---------------+ | | | | | |
| Pre-authentication | | | |
| [authentication protocol] | | |
|<--------+------------->|MN-CA key| | |
| | | |-------->|MN-AR key| |
+-----------------+ | | |------------------>| |
|(2) Increased | | | | | | [oCoA]
|chance to switch | | | | | | |
| to CTN | | | | | | |
+-----------------+ | | | | | |
| | | | | | |
| Pre-configuration | | | |
| [configuration protocol to get nCoA] | |
|<--------+----------------------->| | |
| Pre-configuration | | | |
| [tunnel management protocol to establish PHT] V
|<--------+--------------------------------->|
| | | | | | ^
+-----------------+ | | | | | |
|(3) Determined | | | | | | |
|to switch to CTN | | | | | | |
+-----------------+ | | | | | |
| | | | | | |
| Secure proactive handover main phase | |
| [execution of binding update of MMP and | |
| transmission of data packets through AR | [oCoA, nCoA]
| based on nCoA over the PHT] | | |
|<<=======+================================>+--->... |
. . . . . . .
. . . . . . .
. . . . . . .
Figure 3: Example Communication Flow (1/2)
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| | | | | | |
+----------------+ | | | | | |
|(4) Completion | | | | | | |
|of MMP BU and | | | | | | |
|ready to switch | | | | | | |
+----------------+ | | | | | |
| Secure proactive handover pre-switching phase |
| [tunnel management protocol to delete PHT] V
|<--------+--------------------------------->|
+---------------+ | | | |
|(5)Switching | | | | |
+---------------+ | | | |
| | | | |
+---------------+ | | | |
|(6) Completion | | | | |
|of switching | | | | |
+---------------+ | | | |
o<- Secure proactive handover post-switching phase ^
| [Re-assignment of TIA to the physical I/F] |
| | | | | |
| Transmission of data packets through AR | [nCoA]
| based on nCoA| | | | |
|<---------------+---------------------------+-->... |
| | | | | .
Figure 4: Example Communication Flow (2/2)
7. MPA Operations
In order to provide an optimized handover for a mobile experiencing
rapid subnet and domain handover, one needs to look into several
operations. These issues include discovery of neighboring networking
elements, choosing the right network to connect to based on certain
policy, changing the layer 2 point of attachment, obtaining an IP
address from a DHCP or PPP server, confirming the uniqueness of the
IP address, pre-authenticating with the authentication agent, sending
the binding update to the correspondent host and obtaining the
redirected streaming traffic to the new point of attachment, ping-
pong effect, probability of moving to more than one network and
associating with multiple target networks. We describe these issues
in details in the following paragraphs and describe how we have
optimized these in case of MPA-based secure proactive handover.
7.1. Discovery
Discovery of neighboring networking elements such as access points,
access routers, authentication servers help expedite the handover
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process during a mobile's rapid movement between networks. After
discovering the network neighborhood with a desired set of
coordinates, capabilities and parameters the mobile can perform many
of the operation such as pre-authentication, proactive IP address
acquisition, proactive address resolution, and binding update while
in the previous network.
There are several ways a mobile can discover neighboring networks.
The Candidate Access Router Discovery protocol [RFC4066] helps
discover the candidate access routers in the neighboring networks.
Given a certain network domain SLP (Service Location Protocol) and
DNS help provide addresses of the networking components for a given
set of services in the specific domain. In some cases many of the
network layer and upper layer parameters may be sent over link layer
management frames such as beacons when the mobile approaches the
vicinity of the neighboring networks. IEEE 802.11u is considering
issues such as discovering neighborhood using information contained
in link layer. However, if the link-layer management frames are
encrypted by some link layer security mechanism, then the mobile node
may not be able to obtain the requisite information before
establishing link layer connectivity to the access point. In
addition this may add burden to the bandwidth constrained wireless
medium. In such cases a higher layer protocol is preferred to obtain
the information regarding the neighboring elements. There is some
proposal such as [802.21] that helps obtain information about the
neighboring networks from a mobility server. When the mobile's
movement is imminent, it starts the discovery process by querying a
specific server and obtains the required parameters such as the IP
address of the access point, its characteristics, routers, SIP
servers or authentication servers of the neighboring networks. In
the event of multiple networks, it may obtain the required parameters
from more than one neighboring networks and keep these in a cache.
At some point the mobile finds out several CTNs out of many probable
networks and starts the pre-authentication process by communicating
with the required entities in the CTNs. Further details of this
scenario are in Section 7.2.
7.2. Pre-authentication in multiple CTN environment
In some cases, although a mobile decides a specific network to be the
target network, it may actually end up with moving into a neighboring
network other than the target network due to factors that are beyond
the mobile's control. Thus it may be useful to perform the pre-
authentication with a few probable candidate target networks and
establish time-bound tunnels with the respective access routers in
those networks. Thus, in the event of a mobile moving to a candidate
target network other than that was chosen as the target network, it
will not be subjected to packet loss due to authentication and IP
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address acquisition delay that could incur if the mobile did not pre-
authenticate with that candidate target network. It may appear that
by pre-authenticating with a number of candidate target networks and
reserving the IP addresses, the mobile is provisioning resources that
could be used otherwise. But since this happens for a time-limited
period it should not be a big problem. However, it depends upon the
mobility pattern and duration. The mobile uses pre-authentication
procedure to obtain an IP address proactively and to set up the time
bound tunnels with the access routers of the candidate target
networks. Also, MN may hold some or all of the nCoAs for future
movement.
The mobile may choose one of these addresses as the binding update
address and send it to the CN (Correspondent Node) or HA (Home
Agent), and will thus receive the tunneled traffic via the target
network while in the previous network. But in some instances, the
mobile may eventually end up moving to a network that is other than
the target network. Thus, there will be a disruption in traffic as
the mobile moves to the new network since the mobile has to go
through the process of assigning the new IP address and sending the
binding update again. Two solutions can take care of this problem.
The mobile can take advantage of the simultaneous mobility binding
and send multiple binding updates to the corresponding host or HA.
Thus, the corresponding host or HA forwards the traffic to multiple
IP addresses assigned to the virtual interfaces for a specific period
of time. This binding update gets refreshed at the CH after the
mobile moves to the new network, thus stopping the flow to the other
candidate networks. Wakikawa [I-D.wakikawa-mobileip-multiplecoa]
discusses different scenarios of mobility binding with multiple care-
of-addresses. In case simultaneous binding is not supported in a
specific mobility scheme, forwarding of traffic from the previous
target network will help take care of the transient traffic until the
new binding update is sent from the new network.
7.3. Proactive IP address acquisition
In general a mobility management protocol works in conjunction with
the Foreign Agent or in co-located address mode. The MPA approach
can use both co-located address mode and foreign agent address mode.
We discuss here the address assignment component that is used in co-
located address mode. There are several ways a mobile node can
obtain an IP address and configure itself. Most commonly a mobile
can configure itself statically in the absence of any configuration
element such as a server or router in the network. The IETF Zeroconf
working group defines auto-IP mechanism where a mobile is configured
in an ad-hoc manner and picks a unique address from a specified range
such as 169.254.0.0/16. In a LAN environment the mobile can obtain
an IP address from DHCP servers. In case of IPv6 networks, a mobile
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has the option of obtaining the IP address using stateless auto-
configuration or DHCPv6. In a wide area networking environment, the
mobile uses PPP to obtain the IP address by communicating with a NAS.
Each of these processes takes of the order of few hundred
milliseconds to few seconds depending upon the type of IP address
acquisition process and operating system of the clients and servers.
Since IP address acquisition is part of the handover process, it adds
to the handover delay and thus it is desirable to reduce this delay
as much as possible. There are few optimized techniques such as DHCP
Rapid Commit [RFC4039], GPS-coordinate based IP address [GPSIP]
available that attempt to reduce the handover time due to IP address
acquisition time. However, in all these cases the mobile also
obtains the IP address after it moves to the new subnet and incurs
some delay because of the signaling handshake between the mobile node
and the DHCP server.
In the following paragraph we describe few ways a mobile node can
obtain the IP address proactively from the CTN and the associated
tunnel setup procedure. These can broadly be divided into four
categories such as PANA-assisted proactive IP address acquisition,
IKE-assisted proactive IP address acquisition, proactive IP address
acquisition using DHCP only and stateless autoconfiguration.
7.3.1. PANA-assisted proactive IP address acquisition
In case of PANA-assisted proactive IP address acquisition, the mobile
node obtains an IP address proactively from a CTN. The mobile node
makes use of PANA [RFC5191] messages to trigger the address
acquisition process on the DHCP relay agent [RFC3046] that is
colocated with the PANA authentication agent in the access router in
the CTN. Upon receiving a PANA message from the mobile node, the
DHCP relay agent performs normal DHCP message exchanges to obtain the
IP address from the DHCP server in the CTN. This address is piggy-
backed in a PANA message and is delivered to the client. In case of
MIPv6 with stateless autoconfiguration, the router advertisement from
the new target network is passed to the client as part of PANA
message. The mobile uses this prefix and its MAC address to
construct the unique IPv6 address as it would have done in the new
network. Mobile IPv6 in stateful mode works very similar to DHCPv4.
7.3.2. IKEv2-assisted proactive IP address acquisition
IKEv2-assisted proactive IP address acquisition works when an IPsec
gateway and a DHCP relay agent are resident within each access router
in the CTN. In this case, the IPsec gateway and DHCP relay agent in
a CTN help the mobile node acquire the IP address from the DHCP
server in the CTN. The MN-AR key established during the pre-
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authentication phase is used as the IKEv2 pre-shared secret needed to
run IKEv2 between the mobile node and the access router. The IP
address from the CTN is obtained as part of standard IKEv2 procedure,
with using the co-located DHCP relay agent for obtaining the IP
address from the DHCP server in the target network using standard
DHCP. The obtained IP address is sent back to the client in the
IKEv2 Configuration Payload exchange. In this case, IKEv2 is also
used as the tunnel management protocol for a proactive handover
tunnel (see Section 7.4). Alternatively VPN-GW can itself dispense
the IP address from its IP address pool.
7.3.3. Proactive IP address acquisition using DHCP only
As another alternative, DHCP may be used for proactively obtaining an
IP address from a CTN without relying on PANA or IKEv2-based
approaches by allowing direct DHCP communication between the mobile
node and the DHCP relay or DHCP server in the CTN. In this case, the
mobile node sends a unicast DHCP message to the DHCP relay agent or
DHCP server in the CTN requesting an address, while using the address
associated with the current physical interface as the source address
of the request.
When the message is sent to the DHCP relay agent, the DHCP relay
agent relays the DHCP messages back and forth between the mobile node
and the DHCP server. In the absence of a DHCP relay agent the mobile
can also directly communicate with the DHCP server in the target
network. The broadcast option in the client's unicast DISCOVER
message should be set to 0 so that the relay agent or the DHCP server
can send the reply directly back to the mobile using the mobile
node's source address. This mechanism also works for an IPv6 node
using stateful configuration.
In order to prevent malicious nodes from obtaining an IP address from
the DHCP server, DHCP authentication should be used or the access
router should install a filter to block unicast DHCP message sent to
the remote DHCP server from mobile nodes that are not pre-
authenticated. When DHCP authentication is used, the DHCP
authentication key may be derived from the MPA-SA established between
the mobile node and the authentication agent in the candidate target
network.
The proactively obtained IP address is not assigned to the mobile
node's physical interface until the mobile has moved to the new
network. The IP address thus obtained proactively from the target
network should not be assigned to the physical interface but rather
to a virtual interface of the client. Thus, such a proactively
acquired IP address via direct DHCP communication between the mobile
node and the DHCP relay or the DHCP server in the CTN may be carried
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with additional information that is used to distinguish it from other
addresses as assigned to the physical interface.
Upon the mobile's entry to the new network, the mobile node can
perform DHCP over the physical interface to the new network to get
other configuration parameters such as the SIP server, DNS server by
using DHCP INFORM. This should not affect the ongoing communication
between the mobile and correspondent host. Also, the mobile node can
perform DHCP over the physical interface to the new network to extend
the lease of the address that was proactively obtained before
entering the new network.
In order to maintain the DHCP binding for the mobile node and keep
track of the dispensed IP address before and after the secure
proactive handover, the same DHCP client identifier needs to be used
for the mobile node for both DHCP for proactive IP address
acquisition and DHCP performed after the mobile node enters the
target network. The DHCP client identifier may be the MAC address of
the mobile node or some other identifier.
7.3.4. Proactive IP address acquisition using stateless
autoconfiguration
For IPv6, a network address is configured either using DHCPv6 or
stateless autoconfiguration. In order to obtain the new IP address
proactively, the router advertisement of the next hop router can be
sent over the established tunnel, and a new IPv6 address is generated
based on the prefix and MAC address of the mobile. Generating a COA
from the new network will avoid the time needed to obtain an IP
address and perform Duplicate Address Detection.
Duplicate address detection and address resolution are part of the IP
address acquisition process. As part of the proactive configuration
these two processes can be done ahead of time. Details of how these
two processes can be done proactively are described in Appendix A and
Appendix B, respectively.
In case of stateless autoconfiguration, the mobile checks to see the
prefix of the router advertisement in the new network and matches it
with the prefix of newly assigned IP address. If these turn out to
be the same then the mobile does not go through the IP address
acquisition phase again.
7.4. Tunnel management
After an IP address is proactively acquired from the DHCP server in a
CTN or via stateless autoconfiguration in case of IPv6, a proactive
handover tunnel is established between the mobile node and the access
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router in the CTN. The mobile node uses the acquired IP address as
the tunnel's inner address.
There are several reasons why this transient tunnel is established
between the NAR and the mobile in the old PoA, unlike transient
tunnel in FMIPv6 (Fast MIPv6) [RFC5268], where it is set up between
mobile's new point of attachment and the old access router.
In case of inter-domain handoff, it is important that any signaling
message between nPoA and the mobile needs to be secured. This
transient secured tunnel provides the desired functionality including
the securing the proactive binding update and transient data between
the end-points before the handover has taken place. Unlike proactive
mode of FMIPv6, transient handover packets are not sent to PAR, and
thus a tunnel between mobile's new point of attachment and old access
router is not needed.
In case of inter-domain handoff, PAR and NAR could logically be far
from each other. Thus, the signaling and data during pre-
authentication period will take a longer route, and thus, may be
subjected to longer one-way-delay. Hence, MPA provides a tradeoff
between larger packet loss or larger one-way-packet delay for a
transient period, when the mobile is preparing to handoff.
The proactive handover tunnel is established using a tunnel
management protocol. When IKEv2 is used for proactive IP address
acquisition, IKEv2 is also used as the tunnel management protocol.
Alternatively, when PANA is used for proactive IP address
acquisition, PANA may be used as the secure tunnel management
protocol.
Once the proactive handover tunnel is established between the mobile
node and the access router in the candidate target network, the
access router also needs to perform proxy address resolution (Proxy
ARP) on behalf of the mobile node so that it can capture any packets
destined to the mobile node's new address.
Since the mobile needs to be able to communicate with the
correspondent node while in the previous network some or all parts of
binding update and data from the correspondent node to mobile node
need to be sent back to the mobile node over a proactive handover
tunnel. Details of these binding update procedure are described in
Section 7.5.
In order for the traffic to be directed to the mobile node after the
mobile node attaches to the target network, the proactive handover
tunnel needs to be deleted or disabled. The tunnel management
protocol used for establishing the tunnel is used for this purpose.
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Alternatively, when PANA is used as the authentication protocol the
tunnel deletion or disabling at the access router can be triggered by
means of PANA update mechanism as soon as the mobile moves to the
target network. A link-layer trigger ensures that the mobile node is
indeed connected to the target network and can also be used as the
trigger to delete or disable the tunnel. A tunnel management
protocol also triggers the router advertisement (RA) the from next
access router to be sent over the tunnel, as soon as the tunnel
creation is complete.
7.5. Binding Update
There are several kinds of binding update mechanisms for different
mobility management schemes.
In case of Mobile IPv4 and Mobile IPv6, the mobile performs a binding
update with the home agent only, if route optimization is not used.
Otherwise, the mobile performs binding update with both the home
agent (HA) and corresponding node (CN).
In case of SIP-based terminal mobility, the mobile sends binding
update using INVITE to the correspondent node and REGISTER message to
the Registrar. Based on the distance between the mobile and the
correspondent node, the binding update may contribute to the handover
delay. SIP-fast handover [SIPFAST] provides several ways of reducing
the handover delay due to binding update. In case of secure
proactive handover using SIP-based mobility management we do not
encounter the delay due to binding update completely, as it takes
place in the previous network.
Thus, this proactive binding update scheme looks more attractive when
the correspondent node is too far from the communicating mobile node.
Similarly, in case of Mobile IPv6, the mobile sends the newly
acquired CoA from the target network as the binding update to the HA
and CN. Also all signaling messages between MN and HA and between MN
and CN are passed through this proactive tunnel that is set up.
These messages include Binding Update (BU), Binding Acknowledgement
(BA) and the associated return routability messages such as Home Test
Init (HoTI), Home Test (HoT), Care-of Test Init (CoTI), Care-of Test
(COT). In Mobile IPv6, since the receipt of on-link router
advertisement is mandatory for the mobile to detect the movement and
trigger the binding update, router advertisement from next access
router needs to be advertised over the tunnel. By proper
configuration on NAR, router advertisement can be sent over the
tunnel interface to trigger the proactive binding update. The mobile
also needs to make the tunnel interface the active interface, so that
it can send the binding update using this interface as soon as it
receives the router advertisement.
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If the proactive handover tunnel is realized as an IPsec tunnel, it
will also protect these signaling messages between the tunnel end
points and will make the return routability test secured as well.
Any subsequent data will also be tunneled through as long as the
mobile is in the previous network. The accompanying document
[mpa-wireless] talks about the details of how binding updates and
signaling for return routability are sent over the secured tunnel.
7.6. Preventing packet loss
7.6.1. Packet loss prevention in single interface MPA
For single interface MPA, there may be some transient packets during
link-layer handover that are directed to the mobile node at the old
point of attachment before the mobile node is able to attach to the
target network. Those transient packets can be lost. Buffering
these packets at the access router of the old point of attachment can
eliminate packet loss. Dynamic buffering signals that are signalled
from the MN can temporarily hold transient traffic during handover
and then these packets can be forwarded to the MN once it attaches to
the target network. A detailed analysis of buffering technique can
be found in [PIMRC06].
An alternative method is to use bicasting. Bicasting helps to
forward the traffic to two destinations at the same time. However,
it does not eliminate packet loss if link-layer handover is not
seamlessly performed. On the other hand, buffering does not reduce
packet delay. While packet delay can be compensated by a playout
buffer at the receiver side for streaming application, a playout
buffer does not help much for interactive VoIP application that
cannot tolerate for large delay jitters. Thus it is still important
to optimize the link-layer handover anyway.
7.6.2. Preventing packet losses for multiple interfaces
MPA usage in multi-interface handover scenarios involves preparing
the second interface for use via the current active interface. This
preparation involves pre-authentication and provisioning at a target
network where the second interface would be the eventual active
interface. For example, during inter-technology handover from a
Wi-Fi to a CDMA network, pre-authentication at the CDMA network can
be performed via the Wi-Fi interface. The actual handover occurs
when the CDMA interface becomes the active interface for the MN.
In such scenarios, if handover occurs while both interfaces are
active, there is generally no packet loss since transient packets
directed towards the old interface will still reach the MN. However,
if sudden disconnection of the current active interface is used to
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initiate handover to the prepared interface then transient packets
for the disconnected interface will be lost while the MN attempts to
be reachable at the prepared interface. In such cases, a specialized
form of buffering can be used to eliminate packet loss where packets
are merely copied at an access router in the current active network
prior to disconnection. If sudden disconnection does occur, copied
packets can be forwarded to the MN once the prepared interface
becomes the active reachable interface. The copy-and-foward
mechanism is not limited to multi-interface handover.
A notable side-effect of this process is the possible duplication of
packets during forwarding to the new active interface. Several
approaches can be employed to minimize this effect. Relying on upper
layer protocols such as TCP to detect and eliminate duplicates is the
most common approach. Customized duplicate detection and handling
techniques can also be used. In general, packet duplication is a
well known issue that can also be handled locally by the MN.
If the mobile takes a longer amount of time to detect the
disconnection event of the current active interface, it can also have
an adverse effect on the length of the handover process. Thus it
becomes necessary to use an optimized scheme of detecting interface
disconnection in such scenarios. Use of the current interface to
perform pre-authentication instead of the new interface is desirable
in certain circumstances, such as to save battery power or in cases
where the adjacent cells (e.g., WiFi, and CDMA) are non-overlapping
or in cases when the carrier does not allow simultaneous use of both
interfaces. However, in certain circumstances, depending upon the
type of target network, only parts of MPA operations can be performed
(e.g., pre-authentication, pre-configuration, proactive binding
update). In a specific scenario involving handoff between WiFi and
CDMA network, some of the PPP context can be set up during the pre-
authentication period, thus reducing the time for PPP activation.
7.6.3. Reachability test
In addition to previous techniques, the MN may also want to ensure
reachability of the new point of attachment before switching from the
old one. This can be done by exchanging link-layer management frames
with the new point of attachment. This reachability check should be
performed as quickly as possible. In order to prevent packet loss
during this reachability check, transmission of packets over the link
between the MN and old point of attachment should be suspended by
buffering the packets at both ends of the link during the
reachability check. How to perform this buffering is out of scope of
this document. Some of the results using this buffering scheme are
explained in the accompanying implementation document.
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7.7. Security and mobility
7.7.1. Link-layer security and mobility
Using the MPA-SA established between the mobile node and the
authentication agent for a CTN, during the pre-authentication phase,
it is possible to bootstrap link-layer security in the CTN while the
mobile node is in the current network in the following way. Figure 5
shows the sequence of operation.
(1) The authentication agent and the mobile node derives a PMK (Pair-
wise Master Key) [RFC5247] using the MPA-SA that is established as a
result of successful pre-authentication. Successful operation of EAP
and an AAA protocol may be involved during pre-authentication to
establish the MPA-SA. From the PMK, distinct TSKs (Transient Session
Keys) [RFC5247] for the mobile node are directly or indirectly
derived for each point of attachment of the CTN.
(2) The authentication agent may install the keys derived from the
PMK and used for secure association to points of attachment. The
derived keys may be TSKs or intermediary keys from which TSKs are
derived.
(3) After the mobile node chooses a CTN as the target network and
switches to a point of attachment in the target network (which now
becomes the new network for the mobile node), it executes a secure
association protocol such as the IEEE 802.11i 4-way handshake
[802.11] using the PMK in order to establish PTKs (Pair-wise
Transient Keys) and GTKs (Group Transient Keys) [RFC5247] used for
protecting link-layer packets between the mobile node and the point
of attachment. No additional execution of EAP authentication is
needed here.
(4) While the mobile node is roaming in the new network, the mobile
node only needs to perform a secure association protocol with its
point of attachment point and no additional execution of EAP
authentication is needed either. Integration of MPA with link-layer
handover optimization mechanisms such as 802.11r can be archived this
way.
The mobile node may need to know the link-layer identities of the
point of attachments in the CTN to derive TSKs.
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_________________ ____________________________
| Current Network | | CTN |
| ____ | | ____ |
| | | (1) pre-authentication | | |
| | MN |<------------------------------->| AA | |
| |____| | | |____| |
| . | | | |
| . | | | |
|____.____________| | | |
.movement | |(2) Keys |
____.___________________| | |
| _v__ _____ | |
| | |(3) secure assoc. | | | |
| | MN |<------------------>| AP1 |<-------+ |
| |____| |_____| | |
| . | |
| .movement | |
| . | |
| . | |
| _v__ _____ | |
| | |(4) secure assoc. | | | |
| | MN |<------------------>| AP2 |<-------+ |
| |____| |_____| |
|_____________________________________________________|
Figure 5: Bootstrapping Link-layer Security
7.7.2. IP layer security and mobility
IP layer security is typically maintained between the mobile and
first hop router or any other network element such as SIP proxy by
means of IPsec. This IPSec SA can be set up either in tunnel mode or
in ESP mode. However, as the mobile moves, the IP address of the
router and outbound proxy will change in the new network. The
mobile's IP address may or may not change depending upon the mobility
protocol being used. This will warrant re-establishing a new
security association between the mobile and the desired network
entity. In some cases such as in 3GPP/3GPP2 IMS/MMD environment data
traffic is not allowed to pass through unless there is an IPsec SA
established between the mobile and outbound proxy. This will of
course add unreasonable delay to the existing real-time communication
during mobile's movement. In this scenario, key exchange is done as
part of SIP registration that follows a key exchange procedure called
AKA (Authentication and Key Agreement).
MPA can be used to bootstrap this security association as part of
pre-authentication via the new outbound proxy. Prior to the
movement, if the mobile can pre-register via the new outbound proxy
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in the target network and completes the pre-authentication procedure,
then the new SA state between the mobile and new outbound proxy can
be established prior to the movement to the new network. A similar
approach can also be applied if a key exchange mechanism other than
AKA is used or the network element with which the security
association has to be established is different than an outbound
proxy.
By having the security association established ahead of time, the
mobile does not need to involve in any exchange to set up the new
security association after the movement. Any further key exchange
will be limited to renew the expiry time. This will also reduce the
delay for real-time communication as well.
7.8. Authentication in initial network attachment
When the mobile node initially attaches to a network, network access
authentication would occur regardless of the use of MPA. The
protocol used for network access authentication when MPA is used for
handover optimization can be a link-layer network access
authentication protocol such as IEEE 802.1X or a higher-layer network
access authentication protocol such as PANA.
8. Security Considerations
This document describes a framework of a secure handover optimization
mechanism based on performing handover-related signaling between a
mobile node and one or more candidate target networks to which the
mobile node may move in the future. This framework involves
acquisition of the resources from the CTN as well as data packet
redirection from the CTN to the mobile node in the current network
before the mobile node physically connects to one of those CTN.
Acquisition of the resources from the candidate target networks must
be done with appropriate authentication and authorization procedures
in order to prevent an unauthorized mobile node from obtaining the
resources. For this reason, it is important for the MPA framework to
perform pre-authentication between the mobile node and the candidate
target networks. The MN-CA key and the MN-AR key generated as a
result of successful pre-authentication can protect subsequent
handover signaling packets and data packets exchanged between the
mobile node and the MPA functional elements in the CTN's.
The MPA framework also addresses security issues when the handover is
performed across multiple administrative domains. With MPA, it is
possible for handover signaling to be performed based on direct
communication between the mobile node and routers or mobility agents
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in the candidate target networks. This eliminates the need for a
context transfer protocol [RFC5247] for which known limitations exist
in terms of security and authorization. For this reason, the MPA
framework does not require trust relationship among administrative
domains or access routers, which makes the framework more deployable
in the Internet without compromising the security in mobile
environments.
9. IANA Considerations
This document has no actions for IANA.
10. Acknowledgments
We would like to thank Farooq Anjum and Raziq Yaqub for their review
of this document, and Subir Das for standardization support in the
IEEE 802.21 WG.
Authors would like to acknowledge Christian Vogt, Rajeev Koodli,
Marco Liebsch and Juergen Schoenwaelder for their thorough review of
the draft and useful feedback.
11. References
11.1. Normative References
[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC5380] Soliman, H., Castelluccia, C., ElMalki, K., and L.
Bellier, "Hierarchical Mobile IPv6 (HMIPv6) Mobility
Management", RFC 5380, October 2008.
[RFC5268] Koodli, R., "Mobile IPv6 Fast Handovers", RFC 5268,
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June 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006.
[RFC4881] El Malki, K., "Low-Latency Handoffs in Mobile IPv4",
RFC 4881, June 2007.
[RFC4066] Liebsch, M., Singh, A., Chaskar, H., Funato, D., and E.
Shim, "Candidate Access Router Discovery (CARD)",
RFC 4066, July 2005.
[RFC4830] Kempf, J., "Problem Statement for Network-Based Localized
Mobility Management (NETLMM)", RFC 4830, April 2007.
[RFC4831] Kempf, J., "Goals for Network-Based Localized Mobility
Management (NETLMM)", RFC 4831, April 2007.
[RFC4065] Kempf, J., "Instructions for Seamoby and Experimental
Mobility Protocol IANA Allocations", RFC 4065, July 2005.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, August 2008.
[RFC5191] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
Yegin, "Protocol for Carrying Authentication for Network
Access (PANA)", RFC 5191, May 2008.
[RG98] ITU-T, "General Characteristics of International Telephone
Connections and International Telephone Circuits: One-Way
Transmission Time", ITU-T Recommendation G.114 1998.
[ITU98] ITU-T, "The E-Model, a computational model for use in
transmission planning", ITU-T Recommendation G.107 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.
11.2. Informative References
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", RFC 5201, April 2008.
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[RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Delay Metric for IPPM", RFC 2679, September 1999.
[RFC2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Packet Loss Metric for IPPM", RFC 2680, September 1999.
[RFC2681] Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
Delay Metric for IPPM", RFC 2681, September 1999.
[RFC1853] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
[RFC4039] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for
the Dynamic Host Configuration Protocol version 4
(DHCPv4)", RFC 4039, March 2005.
[RFC5172] Varada, S., "Negotiation for IPv6 Datagram Compression
Using IPv6 Control Protocol", RFC 5172, March 2008.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, April 2006.
[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[I-D.wakikawa-mobileip-multiplecoa]
Wakikawa, R., "Multiple Care-of Addresses Registration",
draft-wakikawa-mobileip-multiplecoa-05 (work in progress),
March 2006.
[I-D.ietf-nsis-qos-nslp]
Manner, J., Karagiannis, G., and A. McDonald, "NSLP for
Quality-of-Service Signaling", draft-ietf-nsis-qos-nslp-18
(work in progress), January 2010.
[I-D.ietf-hokey-preauth-ps]
Ohba, Y. and G. Zorn, "Extensible Authentication Protocol
(EAP) Early Authentication Problem Statement",
draft-ietf-hokey-preauth-ps-12 (work in progress),
January 2010.
[RFC5169] Clancy, T., Nakhjiri, M., Narayanan, V., and L. Dondeti,
"Handover Key Management and Re-Authentication Problem
Statement", RFC 5169, March 2008.
[SIPMM] Schulzrinne, H. and E. Wedlund, "Application Layer
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Mobility Using SIP", ACM MC2R.
[CELLIP] Cambell, A., Gomez, J., Kim, S., Valko, A., and C. Wan,
"Design, Implementation, and Evaluation of Cellular IP",
IEEE Personal communication Auguest 2000.
[MOBIQUIT07]
Lopez, R., Dutta, A., Ohba, Y., Schulzrinne, H., and A.
Skarmeta, "Network-layer assisted mechanism to optimize
authentication delay during handoff in 802.11 networks",
IEEE Mobiquitous June 2007.
[IEEE-03-084]
Mishra, A., Shin, M., Arbaugh, W., Lee, I., and K. Jang,
"Proactive Key Distribution to support fast and secure
roaming, IEEE 802.11 Working Group, IEEE-03-084r1-I,
"www.ieee802.org/11/Documents/DocumentHolder/3-084.zip"",
IEEE June 2003.
[SPRINGER07]
Dutta, A., Das, S., Famolari, D., Ohba, Y., Taniuchi, K.,
Fajardo, V., Schulzrinne, H., Lopez, R., Kodama, T., and
A. Skarmeta, "Seamless proactive handover across
heterogeneous access networks", Wireless Personal
Communication February 2007.
[HAWAII] Ramjee, R., Porta, T., Thuel, S., Varadhan, K., and S.
Wang, "HAWAII: A Domain-based Approach for Supporting
Mobility in Wide-area Wireless networks", International
Conference on Network Protocols ICNP'99.
[IDMP] Das, S., Dutta, A., Misra, A., and S. Das, "IDMP: An
Intra-Domain Mobility Management Protocol for Next
Generation Wireless Networks", IEEE Wireless Communication
Magazine October 2000.
[I-D.ietf-mobileip-reg-tunnel]
Calhoun, P., Montenegro, G., Perkins, C., and E.
Gustafsson, "Mobile IPv4 Regional Registration",
draft-ietf-mobileip-reg-tunnel-09 (work in progress),
July 2004.
[YOKOTA] Yokota, H., Idoue, A., and T. Hasegawa, "Link Layer
Assisted Mobile IP Fast Handoff Method over Wireless LAN
Networks", Proceedings of ACM Mobicom 2002.
[MACD] Shin, S., "Reducing MAC Layer Handoff Latency in IEEE
802.11 Wireless LANs", MOBIWAC Workshop .
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[SUM] Dutta, A., Zhang, T., Madhani, S., Taniuchi, K., Ohba, Y.,
and H. Schulzrinne, "Secured Universal Mobility",
WMASH 2004.
[SIPFAST] Dutta, A., Madhani, S., and H. Schulzrinne, "Fast handoff
Schemes for Application Layer Mobility Management",
PIMRC 2004.
[PIMRC06] Dutta, A., Ohba, Y., and H. Schulzrinne, "Dynamic
Buffering Protocol for Mobile", PIMRC 2006.
[MITH] Gwon, Y., Fu, G., and R. Jain, "Fast Handoffs in Wireless
LAN Networks using Mobile initiated Tunneling Handoff
Protocol for IPv4 (MITHv4)", Wireless Communications and
Networking 2003, January 2005.
[Wenyu] Jiang, W. and H. Schulzrinne, "Modeling of Packet Loss and
Delay and their Effect on Real-Time Multimedia Service
Quality", NOSSDAV 2000, June 2000.
[Romdhani]
Romdhani, I., Kellil, M., Lach, H., and A. Bouabdallah,
"IP Mobile Multicast Challenges and Solutions", IEEE
Communication Magazine 2004, March 2000.
[802.21] "Draft IEEE Standard for Local and Metropolitan Area
Networks: Media Independent Handover Services, IEEE
P802.21/D00.01,", A contribution to IEEE 802.21 WG ,
July 2005.
[802.11] "IEEE Wireless LAN Edition A compilation based on IEEE Std
802.11-1999(R2003)", Institute of Electrical and
Electronics Engineers September 2003.
[GPSIP] Dutta, A., "GPS-IP based fast-handoff for Mobiles", IEEE
Sarnoff Symposium 2006.
[MAGUIRE] Vatn, J. and G. Maguire, "The effect of using co-located
care-of-address on macro handover latency", 14th Nordic
Teletraffic Seminar 1998.
[mpa-mobike]
Mghazli, Y. and J. Bournelle, "MPA using IKEv2 and
MOBIKE", draft-yacine-preauth-ipsec-00 IETF.
[FMIP-results]
Cabellos-Apaicio, A., Nunez-Martinez, J., Julian-Bertomeu,
H., Jakab, L., Serral-Gracia, R., and J. Domingo-Pascual,
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"Evaluation of Fast Handover Implementation for Mobile
IPv6 in a Real Testbed", IPOM 2005 LNCS 3751.
[mpa-wireless]
Dutta, A., Famolari, D., Das, S., Ohba, Y., Fajardo, V.,
Taniuchi, K., Lopez, R., and H. Schulzrinne, "Media-
Independent Pre-authentication Supporting Secure
Interdomain Handover Optimization", IEEE Wireless
Magazine April 2008.
Appendix A. Proactive duplicate address detection
When the DHCP server dispenses an IP address, it updates its lease
table, so that this same address is not given to another client for
that specific period of time. At the same time the client also keeps
a lease table locally so that it can renew when needed. In some
cases where a network consists of both DHCP and non-DHCP enabled
clients, there is a probability that another client in the LAN may
have been configured with an IP address from the DHCP address pool.
In such scenario the server detects a duplicate address based on ARP
(Address Resolution Protocol) or IPv6 Neighbor Discovery before
assigning the IP address. This detection procedure may take from 4
sec to 15 sec [MAGUIRE] and will thus contribute to a larger handover
delay. In case of a proactive IP address acquisition process, this
detection is performed ahead of time and thus, does not affect the
handover delay at all. By performing the duplicate address detection
ahead of time, we reduce the IP address acquisition time.
The proactive duplicate address detection (DAD) over the candidate
target network should be performed by the PAR on behalf of the mobile
at the time of proactive handover tunnel establishment since
duplicate address detection over a tunnel is not always performed.
For example, in the case of IPv6, DAD over an IP-IP tunnel interface
is turned off in an existing implementation. In the case of IPv6
over PPP [RFC5172], IPCPv6 negotiates the link local addresses and
hence DAD over the tunnel is not needed. After the mobile has moved
to the target network, a DAD procedure may be started because of
reassignment of the nCoA to the physical interface to the target
network. In that case, the mobile should use optimistic DAD
[RFC4429] over the physical interface so that the nCoA that was used
inside the proactive handover tunnel before handover can be
immediately used over that physical interface after handover. The
schemes used for the proactive DAD and optimistic DAD are applicable
to both stateless and stateful address autoconfiguration schemes used
for obtaining a nCoA.
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Appendix B. Address resolution
Address resolution involves updating next access router's neighbor
cache. We briefly describe these two operations below.
During the process of pre-configuration, the MAC address resolution
mappings needed by the mobile node to communicate with nodes in the
target network after attaching to the target network can also be
known, where the communicating nodes maybe the access router,
authentication agent, configuration agent and correspondent node.
There are several possible ways of performing such proactive MAC
address resolution.
o Use an information service mechanism [802.21] to resolve the MAC
addresses of the nodes. This might require each node in the
target network to be involved in the information service so that
the server of the information service can construct the database
for proactive MAC address resolution.
o Extend the authentication protocol used for pre-authentication or
the configuration protocol used for pre-configuration to support
proactive MAC address resolution. For example, if PANA is used as
the authentication protocol for pre-authentication, PANA messages
may carry AVPs used for proactive address resolution. In this
case, the PANA authentication agent in the target network may
perform address resolution for on behalf of the mobile node.
o One can also make use of DNS to map the MAC address of the
specific interface associated with a specific IP address of the
network element in the target network. One may define a new DNS
resource record (RR) to proactively resolve the MAC addresses of
the nodes in the target network. But this approach may have its
own limitations since a MAC address is a resource that is bound to
an IP address, not directly to a domain name.
When the mobile node attaches to the target network, it installs the
proactively obtained address resolution mappings without necessarily
performing address resolution queries for the nodes in the target
network.
On the other hand, the nodes that reside in the target network and
are communicating with the mobile node should also update their
address resolution mappings for the mobile node as soon as the mobile
node attaches to the target network. The above proactive address
resolution methods could also be used for those nodes to proactively
resolve the MAC address of the mobile node before the mobile node
attaches to the target network. However, this is not useful since
those nodes need to detect the attachment of the mobile node to the
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target network before adopting the proactively resolved address
resolution mapping. A better approach would be integration of
attachment detection and address resolution mapping update. This is
based on gratuitously performing address resolution [RFC3344],
[RFC3775] in which the mobile node sends an ARP Request or an ARP
Reply in the case of IPv4 or a Neighbor Advertisement in the case of
IPv6 immediately after the mobile node attaches to the new network so
that the nodes in the target network can quickly update the address
resolution mapping for the mobile node.
Appendix C. MPA Deployment Issues
In this section we describe some of the deployment issues related to
MPA.
C.1. Considerations for failed switching and switch-back
The ping-Pong effect is one of the common problems found during
handover. The Ping-pong effect arises when a mobile is located at
the borderline of the cell or decision point and a handover procedure
is frequently executed. This results in higher call drop
probability, lower connection quality, increased signaling traffic
and waste of resources. All of these affect mobility optimization.
Handoff algorithms are the deciding factors for performing the
handoff between the networks. Traditionally these algorithms employ
a threshold to compare the values of different metrics to decide on
the handoff. These metrics include signal strength, path loss,
carrier-to-interference ratios (CIR), Signal to Interference Ratios
(SIR), Bit Error Rate (BER), power budget. In order to avoid the
ping-pong effect, some additional parameters are employed by the
decision algorithm such as hystereris margin, dwell timers, and
averaging window. For a vehicle moving with a high speed, other
parameters such as distance between the mobile node and the point of
attachment, velocity of the mobile, location of the mobile, traffic
and bandwidth characteristics are also taken into account to reduce
the ping-pong effect. Most recently there are other handoff
algorithms that help reduce the ping-pong effect in a heterogeneous
network environment that are based on techniques such as hypothesis
testing, dynamic programming and pattern recognition techniques.
While it is important to devise smart handoff algorithms to reduce
the ping-pong effect, it is also important to devise methods to
recover from this effect.
In the case of the MPA framework, the ping-pong effect will result in
the back-and-forth movement of the mobile between the current network
and target network and between the candidate target networks. MPA in
its current form will be affected because of a number of tunnels
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setup between the mobile and neighboring access routers, number of
binding updates and associated handoff latency resulting out of ping-
pong situation. The mobile's handoff rate may also contribute to
delay and packet loss. We propose few techniques that will help
reduce the probability of ping-pong and propose several methods for
the MPA framework so that it can recover from the packet loss
resulting out of the ping-pong effect.
The MPA framework can take advantage of the mobile's geo-location
with respect to APs in the neighboring networks using GPS. In order
to avoid the oscillation between the networks, a location-aware
algorithm can be derived by using a co-relation between user's
location and cached data from the previous handover attempts. In
some cases only location may not be the only indicator for a handoff
decision. For example in Manhattan type grid networks, although a
mobile is close to an AP, it may not have enough SNR (Signal to Noise
Ration) to make a good connection. Thus knowledge of mobility
pattern, dwell time in a call and path identification will help avoid
the ping-pong problem to a great extent.
In the absence of a good handoff algorithm that can avoid ping-pong
effect, it may be required to put in place a good recovery mechanism
so as to mitigate the effect of ping-pong. It may be necessary to
keep the established context in the current network for a period of
time, so that it can be quickly recovered when the mobile comes back
to the network where the context was last used. This context may
include security association, IP address used, tunnels established.
Bicasting the data to both the previous network and the new network
for a predefined period will also help the mobile to take care of the
lost packets in case the mobile moves back and forth between the
networks. The mobile can also take certain action, after it
determines that it is in a stable state with respect to a ping-pong
situation.
When the MPA framework takes advantage of a combination of IKEv2 and
MOBIKE, the ping-pong effect can be reduced further [mpa-mobike].
C.2. Authentication state management
In case of pre-authentication with multiple target networks, it is
useful to maintain the state in the authentication agent of each of
the neighboring networks for certain time. Thus, if the mobile does
move back and forth between neighboring networks, already maintained
authentication state can be helpful. We provide some highlights on
multiple security association state management below.
A mobile node that has pre-authenticated with an authentication agent
in a candidate target network and has a MPA-SA, may need to continue
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to keep the MPA-SA while it continues to stay in the current network
or even after it does handover to a network that is different from
the candidate target network.
When an MN that has been authenticated and authorized by an
authentication agent in the current network makes a handover to a
target network, it may want to hold the SA that has been established
between the MN and the authentication agent for a certain time period
so that it does not have to go through the entire authentication
signaling to create an SA from scratch in case it returns to the
previous network. Such an SA being held at the authentication agent
after the MN's handover to other network is considered as an MPA-SA.
In this case, the authentication agent should change the fully
authorized state for the MN to an unauthorized state. The
unauthorized state can be changed to the fully authorized state only
when the MN comes back to the network and provides a proof of
possession of a key associated with the MPA-SA.
While an MPA-SA is being held at an authentication agent, the MN will
need to keep updating the authentication agent when an IP address of
the MN changes due to a handover to re-establish the new SA.
C.3. Pre-allocation of QoS resources
In the pre-configuration phase, it is also possible to pre-allocate
QoS resources that may be used by the mobile node not only after
handover but also before handover. When pre-allocated QoS resources
are used before handover, it is used for application traffic carried
over a proactive handover tunnel.
It is possible that QoS resources are pre-allocated in an end-to-end
fashion. One method to achieve this proactive end-to-end QoS
reservation is to execute NSLP [I-D.ietf-nsis-qos-nslp] or RSVP
[RFC2205] over a proactive handover tunnel where pre-authentication
can be used for bootstrapping a security association for the
proactive handover tunnel to protect the QoS signaling. In this
case, QoS resources are pre-allocated on the path between the
correspondent node and a target access router can be used
continuously before and after handover. On the other hand, duplicate
pre-allocation of QoS resources between the target access router and
the mobile node is necessary when using pre-allocated QoS resources
before handover due to difference in paths between the target access
router and the mobile node before and after handover. QoS resources
to be used for the path between the target access router and the
mobile node after handover may be pre-allocated by extending NSLP to
work for off-path signaling (Note: this path can be viewed as off-
path before handover) or by media-specific QoS signaling at layer 2.
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C.4. Resource allocation issue during pre-authentication
In case of multiple CTNs, establishing multiple tunnels with the
neighboring target networks provides some additional benefits. But
it also contributes to some resource utilization issues as well. A
pre-authentication process with multiple candidate target networks
can happen in several ways.
The very basic scheme involves authenticating the mobile with the
multiple authentication agents in the neighboring networks, but
actual pre-configuration and binding update take place only after
layer 2 movement to a specific network is complete.
Similarly, in addition to pre-authentication, the mobile can also
complete the pre-configuration while in the previous network, but can
postpone the binding update until after the mobile has moved. Like
the previous case, in this case the mobile also does not need to set
up the pre-configured tunnels. While the pre-authentication process
and part of the pre-configuration process are taken care of before
the mobile has moved to the new network, binding update is actually
done after the mobile has moved.
The third type of multiple pre-authentication involves all the three
steps while the mobile is in the previous networks, such as
authentication, configuration and binding update. But, this specific
process utilizes the most amount of resources. Some of the resources
that get used during this process are as follows:
1)Additional signaling for pre-authentication in the neighboring
networks
2)Holding the IP address of the neighboring networks in mobiles cache
for certain amount of time. It needs additional processing in the
mobile for storing these IP addresses. In addition it also uses up
the temporary IP addresses from the neighboring routers.
3)There is an additional cost associated with setting up additional
transient tunnels with the target routers in the neighboring networks
and mobile.
4) In case of binding update with multiple IP addresses obtained from
the neighboring networks, multiple transient streams flow between the
CN and mobile using these transient tunnels.
When only pre-authentication and pre-configuration are done ahead of
time with multiple networks, the mobile sends one binding update to
the CN. In this case it is important to find out when to send the
binding update after the layer 2 handoff.
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In case binding update with multiple contact addresses is sent,
multiple media streams stem out of CN using the transient tunnels.
But in that case one needs to send another Binding Update after the
handover with the contact address set to the new address (only one
address) where the mobile has moved. This way the mobile stops
sending media to other neighboring networks where the mobile did not
move.
The following is an illustration of this specific case that takes
care of multiple binding streams, when the mobile moves only to a
specific network, but sends multiple binding updates in the previous
network. MN sends a binding update to CH with multiple contact
addresses such as c1,c2, and c3 that were obtained from three
neighboring networks. This allows the CN to send transient multiple
streams to the mobile over the pre-established tunnels. After the
mobile moves to the actual network, it sends another binding update
to the CN with the care-of-address of the mobile in the network where
the mobile has moved in. Some of the issues with multiple streams
are consumption of extra bandwidth for a small period of time.
Alternatively, one can apply the buffering technique at the target
access router or at the home agent. Transient data can be forwarded
to the mobile after it has moved in. Forwarding of data can be
triggered by the mobile either as part of Mobile IP registration or
as a separate buffering protocol.
C.5. Systems evaluation and performance results
In this Section, we present some of the results from MPA
implementation when applied to different handover scenarios. We
present the summary of results from our experiments using MPA
techniques for two types of handovers I) Intra-technology and Intra-
domain, II) Inter-technology and Inter-domain. We also present the
results from how MPA can bootstrap layer 2 security for both roaming
and non-roaming cases. Detailed procedure and results are explained
in [MOBIQUIT07] and [SPRINGER07].
C.5.1. Intra-technology, Intra-domain
The results for MIPv6 and SIP mobility involving intra-domain
mobility are shown in Figure 6 and Figure 7, respectively.
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Buffering Buffering Buffering Buffering
(disabled) (enabled) (disabled) (enabled)
& RO & RO & RO & RO
(disabled) (disabled) (enabled) (enabled)
-------------------------------------------------------------------
L2 handoff (ms) 4.00 4.33 4.00 4.00
L3 handoff (ms) 1.00 1.00 1.00 1.00
Avg. packet loss 1.33 0 0.66 0
Avg. inter-packet 16.00 16.00 16.00 16.00
arrival interval
(ms)
Avg. inter-packet n/a 45.33 n/a 66.60
arrival time during
handover
(ms)
Avg. packet jitter n/a 29.33 n/a 50.60
(ms)
Buffering Period n/a 50.00 n/a 50.00
(ms)
Buffered Packets n/a 2.00 n/a 3.00
Figure 6: Mobile IPv6 with MPA Results
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Buffering Buffering
disabled enabled
-----------------------------------------------
L2 handoff (ms) 4.00 5.00
L3 handoff (ms) 1.00 1.00
Avg. packet loss 1.50 0
Avg. inter-packet 16.00 16.00
arrival interval
(ms)
Avg. inter-packet n/a 29.00
arrival time during
handover
(ms)
Avg. packet jitter n/a 13.00
(ms)
Buffering Period n/a 20.00
(ms)
Buffered Packets n/a 3.00
Figure 7: SIP Mobility with MPA Results
For all measurement, we did not experience any performance
degradation during handover in terms of the audio quality of the
voice traffic.
With the use of buffering during handover, packet loss during the
actual L2 and L3 handover is eliminated with an appropriate and
reasonable settings of the buffering period for both MIP6 and SIP
mobility. In the case of MIP6, there is not a significant difference
in results with and without route optimization. It should be noted
that results with more samples would be necessary for a more detailed
analysis.
In case of non-MPA assisted handover, handover delay and associated
packet loss occurs from the moment the link-layer handover procedure
begins up to successful processing of the binding update. During
this process, IP address acquisitions via DHCP incurs the longest
delay. This is due to the detection of duplicate IP address in the
network before DHCP request completes. Binding update exchange also
experiences long delay if the CN is too far from the MN. As a
result, the Non-MPA assisted handover took an average of 4 seconds to
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complete with an approximate packet loss of about 200 packets. The
measurement is based on the same traffic rate and traffic source as
the MPA assisted handover.
C.5.2. Inter-technology, Inter-domain
Handoff involving heterogeneous access can take place in many
different ways. We limit the experiment to two interfaces and
therefore results in several possible setup scenarios depending upon
the activity of the second interface. In one scenario, the second
interface comes up when the link to the first interface goes down.
This is a reactive scenario and usually gives rise to undesirable
packet loss and handoff delay. In a second scenario, the second
interface is being prepared while the mobile still communicates using
the old interface. Preparation of the second interface should
include setup of all the required state and security associations
(e.g., PPP state, LCP, CHAP). If such lengthly process is
established ahead of time, it reduces the time taken for the
secondary interface to be attached to the network. After
preparation, the mobile decides to use the second interface as the
active interface. This results in less packet loss as it uses make-
before-break techniques. This is a proactive scenario and can have
two flavors. The first is where both interfaces are up and the
second is when only the old interface is up the prepared interface is
brought up only when handoff is about to occur. This scenario may be
beneficial from a battery management standpoint. Devices that
operate two interfaces simultaneously can rapidly deplete their
batteries. However, by activating the second interface only after an
appropriate network has been selected the client may utilize battery
effectively.
As compared to non-optimized handover that may result in delay up to
18 sec and 1000 packet loss during handover from WLAN to CDMA, we
observed 0 packet loss, and 50 ms handoff delay between the last pre-
handoff packet and first in-handoff packet. This handoff delay
includes the time due to link down detection and time needed to
delete the tunnel after the mobile has moved. However, we observed
about 10 duplicate packets because of the copy-and-forward mechanism
at the access routers. But these duplicate packets are usually
handled easily by the upper layer protocols.
C.5.3. MPA-assisted Layer 2 pre-authentication
In this section, we discuss the results obtained from MPA-assisted
layer 2 pre-authentication and compare these with EAP authentication
and IEEE 802.11i's pre-authentication techniques. Figure 12 shows
the experimental testbed where we have conducted the MPA-assisted
pre-authentication experiment for bootstrapping layer 2 security as
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explained in Section 7. By pre-authenticating and pre-configuring
the link, the security association procedure during handoff reduces
to a 4-way handshake only. Then MN moves to the AP and, after
association, runs a 4-way handshake by using the PSKap (Pre-shared
Key at AP) generated during PANA pre-authentication. At this point
the handoff is complete. Details of this experimental testbed can be
found in [MOBIQUIT07].
+----------------------------+-----------+ +-------------+------------+
| | | |
| Home Domain +-------++ | | |
| | | | | |
| |AAAHome | | | |
| + | | | |
| +-----+--+ | | |
| | | | Network B |
| Network A | | | |
| /----\ | | /---\ |
| /nAR \ | | / \ |
| | PAA |--------+-+----------+ pAR | |
| \ / | | \ / |
| \----/ | | \-+-/ |
| | | | | |
| +-------------------| | | | |
| | IEEE 802.11i| | | | |
| +------+ +------+ | | +---+--+ |
| | | | | | | | | |
| |AP2 | |AP1 | | | |AP0 | |
| +------+ +------+ | | +------+ |
| +------+ +-----+ | | +-----+ |
| | | | | | | | | |
| |MN +----------->|MN |<+------------- |MN | |
| +------+ +-----+ | | ++----+ |
|-----------------------------------------+-+------------+-------------+
Figure 8: Experimental Testbed for MPA-assisted L2 Pre-authentication
(Non-roaming)
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+-----------------------------+
| +--------+ |
| | | |
| | AAAH + |
| | | |
| ++-------+ |
| | |
| | Home AAA Domain |
| | |
+-------+---------------------+
|
|
|
Radius/ |
Diameter |
|
|
+----------------------------+-----------+ +-------------+------------+
| | | | |
| Roaming +-------++ | | |
| AAA Domain A | | | | |
| | AAAV | | | |
| + | | | |
| Network A +-----+--+ | | Network B |
| | | | |
| | | | |
| /----\ | | /---\ |
| /nAR \ | | / \ |
| | PAA |--------+-+----------+ pAR | |
| \ / | | \ / |
| \----/ | | \-+-/ |
| | | | | |
| +-------------------| | | | |
| | IEEE 802.11i| | | | |
| +------+ +------+ | | +---+--+ |
| | | | | | | | | |
| |AP2 | |AP1 | | | |AP0 | |
| +------+ +------+ | | +------+ |
| +------+ +-----+ | | +-----+ |
| | | | | | | | | |
| |MN +----------->|MN |<---------------| MN | |
| +------+ +-----+ | | ++----+ |
----------------------- -----------------+ +------------+-------------+
Figure 9: Experimental Testbed for MPA-assisted L2 Pre-authentication
(Roaming)
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We have experimented with three types of movement scenarios involving
both non-roaming and roaming cases using the testbeds shown in
figures 12 and 13, respectively. In the roaming case, MN is visiting
in a domain different than its home domain. Consequently, the AAAh
needs to be contacted which is placed in a location far from the
visiting domain. For the non-roaming case, we assume the MN is
moving within its home domain and only the local AAA server (AAAHome)
is contacted which is the home AAA server for the mobile.
The first scenario does not involve any pre-authentication. The MN
is initially connected to AP0 and moves to AP1. Because neither
network-layer authentication is enabled nor IEEE 802.11i pre-
authentication is used, the MN needs to engage in a full EAP
authentication with AP1 to gain access to the network after the move
(post-authentication). This experiment shows the effect of absence
of any kind of pre-authentication.
The second scenario involves 802.11i pre-authentication and involves
movement between AP1 and AP2. In this scenario, the MN is initially
connected to AP2, and starts IEEE 802.11i pre-authentication with
AP1. This is an ideal scenario to compare the values obtained from
802.11i pre-authentication with that of network-layer assisted pre-
authentication. Both scenarios use RADIUS as AAA protocol (APs
implement a RADIUS client). The third scenario takes advantage of
network layer assisted link-layer pre-authentication. It involves
movement between two APs (e.g., between AP0 and AP1) that belong to
two different subnets where 802.11i pre-authentication is not
possible. Here, Diameter is used as AAA protocol (PAA implements a
Diameter client).
In this third movement scenario, the MN is initially connected to
AP0. The MN starts PANA pre-authentication with the PAA which is co-
located on the AR in the new candidate target network (nAR in network
A) from the current associated network (network B). After
authentication, PAA proactively installs two keys, PSKap1 and PSKap2
in both AP1 and AP2 respectively. By doing the key installations
proactively, it preempts the process of communicating with AAA server
for the keys after the mobile moves to the new network. Finally,
because PSKap1 is already installed, AP1 starts immediately the 4-way
handshake. We have used measurement tools such as ethereal and
kismet to analyze the measurements for the 4-way handshake and PANA
authentication. These measurements reflect different operations
involved during network-layer pre-authentication.
In our experiment, as part of the discovery phase, we assume that the
MN is able to retrieve PAA's IP address and all required information
about AP1 and AP2 (e.g. channel, security-related parameters, etc.)
at some point before the handover. This avoids the scanning during
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link-layer handoff. We have applied this assumption to all three
scenarios. Because our focus is on reducing the time spent on
authentication part during handoff, we do not discuss the details of
how we avoid the scanning.
====================================================================
Types |802.11i | 802.11i | MPA-assisted
|Post | Pre | Layer 2
|Authentication | Authentication | Preauthentication
====================================================================
Operation| Non | Roaming | Non | Roaming |Non | Roaming|
| Roaming | | Roaming | |Roaming| |
===================================================================
Tauth | 61 ms | 599 ms | 99 ms | 638 ms | 177 ms| 831 ms |
-------------------------------------------------------------------
Tconf | -- | -- | -- | -- | 16 ms | 17ms |
-------------------------------------------------------------------
Tassoc+4 | | | | | | |
way | 18 ms | 17 ms | 16 ms | 17 ms | 16 ms | 17 ms |
------------------------------------------------------------------|
Total | 79 ms | 616 ms | 115 ms | 655 ms | 208 ms| 865 ms |
------------------------------------------------------------------|
Time | | | | | | |
affecting| 79 ms | 616 ms | 16 ms | 17 ms | 15 ms |17 ms |
handover | | | | | | |
------------------------------------------------------------------|
Figure 10: Results of MPA-assisted Layer 2 results
Figure 14 shows the timing (rounded off to the most significant
number) associated with some of the handoff operations we have
measured in the testbed. We describe each of the timing below.
Tauth refers to the execution of EAP-TLS authentication. This time
does not distinguish whether this authentication was performed during
pre-authentication or a typical post-authentication.
Tconf refers to time spent during PSK generation and installation
after EAP authentication is complete. When network-layer pre-
authentication is not used, this time is not considered.
Tassoc+4way refers to the time dedicated to the completion of
association and the 4-way handshake with the target AP after the
handoff.
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C.6. Guidelines for handover preparation
In this section, we provide some guidelines for the roaming clients
that use pre-authentication mechanisms to reduce the handoff delay.
These guidelines can help determine the extent of pre-authentication
operation that is needed based on a specific type of movement of the
client. IEEE 802.11i and 802.11r take advantage of preauthentication
mechanism at layer 2. Thus, many of the guidelines observed for
802.11i-based pre-authentication and 802.11r-based fast roaming could
also be applicable to the clients that use MPA-based pre-
authentication techniques. However, since MPA operations are not
limited to a specific subnet and involve inter-subnet and inter-
domain handover the guidelines need to take into account other
factors such as movement pattern of the mobile, cell size etc.
The time needed to complete pre-authentication mechanism is an
important parameter since the mobile node needs to determine how much
ahead of time the mobile needs to start the pre-authentication
process so that it can finish the desired operations before the
handover to the target network starts. The pre-authentication time
will vary depending upon the speed of the mobile (e.g., pedestrian,
vs. vehicular) and cell sizes (e.g., WiFi, Cellular). Cell residence
time is defined as the average time the mobile stays in the cell
before the next handoff takes place. Cell residence time is
dependent upon the coverage area and velocity of the mobile. Thus,
cell residence time is an important factor in determining the
desirable pre-authentication time that a mobile should consider.
Since pre-authentication operation involves six sub-operations as
described in Section 7.2 and each sub-operation takes some discrete
amount of time, only part of these sub-operations may be completed
before handoff depending upon the available delay budget.
For example, a mobile could complete only network discovery and
network layer authentication process before the handoff and postpone
the rest of the operations to until after the handover is complete.
On the other hand if it is a slow moving vehicle and the adjacent
cells are sparsely spaced, a mobile could complete all the desired
MPA related operations. Finishing all the MPA related operations
ahead of time reduces the handoff delay but adds other constraints
such as cell residence time.
We give a numerical example here similar to [IEEE-03-084].
D= Coverage diameter,
v= Mobile's velocity,
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RTT = round trip time from AP to AAA server including processing time
for authentication Tauth
Tpsk = Time spent to install keys proactively on the target APs
If for a given value of D = 100ft, Tpsk = 10 ms, and RTT = 100 ms, a
mobile needs to execute only the pre-authentication procedure
associated with MPA, then the following can be calculated for a
successful MPA procedure before the handoff is complete.
2RTT+Tpsk < D/v
v = 100 ft/(200 ms +10 ms) = ~500 ft/sec
Similarly, for a similar cell size, if the mobile is involved in both
pre-authentication and pre-configuration operations as part of the
MPA procedure, and it takes an amount of time Tconfig= 190 ms to
complete the layer 3 configuration including IP address
configuration, then for a successful MPA operation,
2RTT+Tpsk+Tconfig < D/v
v = 100 ft /(200 ms + 10 ms + 190 ms) = ~250 ft/sec
Thus, compared to only pre-authentication part of MPA operation, in
order to be able to complete both pre-autentication and pre-
configuration operations successfully, either the mobile needs to
move at a slower pace or it needs to expedite these operations for
this given cell size. Thus, types of MPA operations will be
constrained by the velocity of the mobile.
As an alternative if a mobile does complete all the pre-
authentication procedure much ahead of time, it uses up the resources
accordingly by way of extra IP address, tunnel and extra bandwidth.
Thus, there is always a tradeoff between the performance benefit
obtained from pre-authentication mechanism and network
characteristics, such as movement speed, cell size, and resources
utilized.
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Authors' Addresses
Ashutosh Dutta
Telcordia Technologies
1 Telcordia Drive
Piscataway, NJ 08854
USA
Phone: +1 732 699 3130
Email: adutta@research.telcordia.com
Victor Fajardo
Telcordia Technologies
1 Telcordia Drive
Piscataway, NJ 08854
USA
Phone:
Email: vf0213@gmail.com
Yoshihiro Ohba
Corporate R&D Center, Toshiba Corporation
1 Komukai-Toshiba-cho, Saiwai-ku
Kawasaki, Kanagawa 212-0001
Japan
Phone:
Email: yoshihiro.ohba@toshiba.co.jp
Kenichi Taniuchi
Toshiba Corporation
2-9 Suehiro-cho
Ome, Tokyo 198-8710
Japan
Phone:
Email: kenichi.taniuchi@toshiba.co.jp
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Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
USA
Phone: +1 212 939 7004
Email: hgs@cs.columbia.edu
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