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Keywords: Mobility, Optimization, Proactive handoff, Link-layer security, Handover taxonomy, Layer 2 handoff, Layer 3 handoff, Network discovery, Handover delay, Packet loss, Proactive binding update, Multi-interface, IP address acquisition, Tunnel management
Internet Research Task Force (IRTF) A. Dutta, Ed.
Request for Comments: 6252 V. Fajardo
Category: Informational NIKSUN
ISSN: 2070-1721 Y. Ohba
K. Taniuchi
Toshiba
H. Schulzrinne
Columbia Univ.
June 2011
A Framework of Media-Independent Pre-Authentication (MPA) for
Inter-Domain Handover Optimization
Abstract
This document describes 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 most
applicable to supporting 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 node has moved to the new network. We describe the
details of all the associated techniques and their applicability for
different scenarios involving various mobility protocols during
inter-domain handover. We have implemented the MPA mechanism for
various network-layer and application-layer mobility protocols, and
we report a summary of experimental performance results in this
document.
This document is a product of the IP Mobility Optimizations (MOBOPTS)
Research Group.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Research Task Force
(IRTF). The IRTF publishes the results of Internet-related research
and development activities. These results might not be suitable for
deployment. This RFC represents the consensus of the MOBOPTS
Research Group of the Internet Research Task Force (IRTF). Documents
approved for publication by the IRSG are not a candidate for any
level of Internet Standard; see Section 2 of RFC 5741.
Dutta, et al. Informational [Page 1]
RFC 6252 MPA Framework June 2011
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6252.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................3
1.1. Specification of Requirements ..............................5
1.2. Performance Requirements ...................................5
2. Terminology .....................................................7
3. Handover Taxonomy ...............................................7
4. Related Work ...................................................11
5. Applicability of MPA ...........................................12
6. MPA Framework ..................................................13
6.1. Overview ..................................................13
6.2. Functional Elements .......................................14
6.3. Basic Communication Flow ..................................16
7. MPA Operations .................................................20
7.1. Discovery .................................................21
7.2. Pre-Authentication in Multiple-CTN Environment ............22
7.3. Proactive IP Address Acquisition ..........................23
7.3.1. PANA-Assisted Proactive IP Address Acquisition .....24
7.3.2. IKEv2-Assisted Proactive IP Address Acquisition ....24
7.3.3. Proactive IP Address Acquisition Using
DHCPv4 Only ........................................24
7.3.4. Proactive IP Address Acquisition Using Stateless
Autoconfiguration ..................................26
7.4. Tunnel Management .........................................26
7.5. Binding Update ............................................28
7.6. Preventing Packet Loss ....................................29
7.6.1. Packet Loss Prevention in Single-Interface MPA .....29
7.6.2. Preventing Packet Losses for Multiple Interfaces ...29
7.6.3. Reachability Test ..................................30
Dutta, et al. Informational [Page 2]
RFC 6252 MPA Framework June 2011
7.7. Security and Mobility .....................................31
7.7.1. Link-Layer Security and Mobility ...................31
7.7.2. IP-Layer Security and Mobility .....................32
7.8. Authentication in Initial Network Attachment ..............33
8. Security Considerations ........................................33
9. Acknowledgments ................................................34
10. References ....................................................34
10.1. Normative References .....................................34
10.2. Informative References ...................................36
Appendix A. Proactive Duplicate Address Detection .................40
Appendix B. Address Resolution ....................................41
Appendix C. MPA Deployment Issues .................................42
C.1. Considerations for Failed Switching and Switch-Back ........42
C.2. Authentication State Management ............................43
C.3. Pre-Allocation of QoS Resources ............................44
C.4. Resource Allocation Issue during Pre-Authentication ........45
C.5. Systems Evaluation and Performance Results .................47
C.5.1. Intra-Technology, Intra-Domain .........................47
C.5.2. Inter-Technology, Inter-Domain .........................49
C.5.3. MPA-Assisted Layer 2 Pre-Authentication ................49
C.6. Guidelines for Handover Preparation ........................54
1. Introduction
As wireless technologies, including cellular and wireless LANs, are
becoming popular, supporting terminal handovers across different
types of access networks, such as from a wireless LAN to CDMA or to
General Packet Radio Service (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 that 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
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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 that operates at the network layer or
higher.
There are several mobility management protocols at different layers.
Mobile IP [RFC5944] 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 the
Internet Key Exchange Protocol (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 the network layer. HIP (Host
Identity Protocol) [RFC5201] defines a new protocol layer between the
network layer and transport layer to provide terminal mobility in a
way that is transparent to both the 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
in-flight packets transmitted during the mobile node'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 for Mobile IPv4 [RFC4881] and Mobile
IPv6 [RFC5568] are defined to allow 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) protocol
[RFC4066] is designed to discover neighboring access routers. CXTP
(Context Transfer Protocol) [RFC4067] is designed to carry state that
is associated with the services provided for the mobile node, or
context, among access routers. In Section 4, we describe some of the
fast-handover schemes that attempt to reduce the handover delay.
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 with 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.
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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 a single interface can be
expected, but also terminals with a 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, and SIP mobility. In cases of multiple operators
without a roaming relationship or without an 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 a higher layer,
with additional mechanisms to perform early acquisition of an 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 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 IP Mobility
Optimizations (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
Voice over IP (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
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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 node is subjected to
repeated handoff during a normal conversation. Measurement
techniques for delay and jitter are described in [RFC2679],
[RFC2680], and [RFC2681].
In the 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 node's handover, in-flight transient traffic cannot reach the
mobile node because of the associated handover delay. These
in-flight packets could either be lost or buffered. If the in-flight
packets are lost, this packet loss will contribute to jitter between
the last packet before handoff and the 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 additional one-way delay. A tradeoff between one-
way delay and packet loss is desired based on the type of
application. For example, for a streaming application, packet loss
can be reduced by increasing the playout buffer, resulting in longer
one-way packet delay.
The handover delay is attributed 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
Authentication, Authorization, and Accounting (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 document 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.
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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 the
network layer or above to maintain a binding between a locator and
an identifier of a mobile node.
Binding Update (BU): A procedure to update a mobility binding.
Media-independent Pre-Authentication Mobile Node (MN): A mobile node
using Media-independent Pre-Authentication (MPA). MPA 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 node
may move in the near future.
Target Network (TN): The network to which the mobile node has
decided to move. The target network is selected from one or more
candidate target networks.
Proactive Handover Tunnel (PHT): A bidirectional IP tunnel [RFC2003]
[RFC2473] 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 discussion is on inter-domain handover.
Dutta, et al. Informational [Page 7]
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Inter-technology: A mobile node may be equipped with multiple
interfaces, where each interface can support a different access
technology (e.g., 802.11, CDMA). A mobile node may communicate
with one interface at any time in order to conserve power. During
the handover, the mobile node 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 node as
well. This type of inter-technology handover is often called
"vertical handover", since the mobile node moves between two
different cell sizes.
Intra-technology: An intra-technology handover is defined as when a
mobile node moves within 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 node 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 the 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 that consists of networks managed by a single
administrative entity that authenticates and authorizes a mobile
node for accessing the networks. An administrative entity may be
a service provider, an enterprise, or any organization. Thus, an
inter-domain handover will by default be subjected to inter-subnet
handover, and in addition it may be subjected to either inter-
technology or intra-technology handover. A mobile node is
subjected to inter-subnet handover when it moves from one subnet
(broadcast domain) to another subnet (broadcast domain). Inter-
domain handover will be subjected to all the transition steps a
subnet handover goes through, and it will be subjected to
authentication and authorization processes 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 Mobile IP version 6 (MIPv6) support, while administrative
domain B may use Proxy MIPv6 [RFC5213].
Intra-domain: When a mobile node'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.
Dutta, et al. Informational [Page 8]
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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 node, 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, such as MIPv6 and Proxy MIPv6, in each of its
domains. 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,
domain A and domain B. It illustrates several important components,
such as a AAA Home server (AAAH); AAA visited servers (e.g., AAAV1
and AAAV2); an Authentication Agent (AA); a layer 3 point of
attachment, such as an Access Router (AR); and a layer 2 point of
attachment, such as an Access Point (AP). Any mobile node may be
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 a
different mobility protocol in either domain.
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+-----------------------------+
| +--------+ |
| | | |
| | AAAH ------------------|
| | | | |
| +|-------+ | |
| | | |
| | Home Domain | |
| | | |
+-------|---------------------+ |
| |
| |
| |
+----------------------------|---------+ +-------------|------------+
| Domain A | | | Domain B | |
| | | | +|-------+ |
| +-------|+ | | +-----+ | | |
| | | | | | ------ AAAV2 | |
| | AAAV1 | | | | AA | | | |
| +-------------- | | | +|----+ +--------+ |
| | | +--------+ | | | |
| |AA | | | |--- ---- |
| +--|--+ | | / \ / \ |
| | /----\ | || AR |-----| AR | |
| -|-- / \ | | \ / \ / |
| / \ | AR | | | -|-- --|- |
| | AR ----------- / | |+--|---+ +------|------+ |
| \ / \--|-/ | || AP4 | | L2 Switch | |
| -/-- +-----|------+ | || | +-|---------|-+ |
| / | L2 Switch | | |+------+ | | |
| / +-|-------|--+ | | +---|--+ +----|-+ |
| +----/-+ +----|-+ +-|----+ | | | | | | |
| | | | | | | | | | 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
[RFC5944], Mobile IPv6 [RFC3775], and SIP-Mobility [SIPMM] provide
continuity to TCP and RTP traffic, these are not optimized to reduce
the handover latency during a mobile node's movement between subnets
and domains. In general, these mobility management protocols
introduce handover delays incurred at several layers, such as layer 3
and the application layer, for updating the mobile node's mobility
binding. These protocols are affected by 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 node's movement between cells, subnets,
and domains. Micro-mobility management schemes such as [CELLIP] and
[HAWAII], and intra-domain mobility management schemes such as
[IDMP], [MOBIP-REG], and [RFC5380], provide fast handover by limiting
the signaling updates within a domain. Fast Mobile IP protocols for
IPv4 and IPv6 networks [RFC4881] [RFC5568] utilize mobility
information made available by link-layer triggers. Yokota et
al. [YOKOTA] propose the joint use of an access point and a dedicated
Media Access Control (MAC) bridge to provide fast handover without
altering the MIPv4 specification. Shin et al. [MACD] propose a
scheme that reduces the delay due to MAC-layer handoff by providing a
cache-based algorithm. In this scheme, the mobile node 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 detail [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 a new Foreign Agent. [MITH] defines two
types of handover schemes: Pre-MIT (Mobile Initiated Tunneling) and
Post-MIT (Media Initiated Tunneling). The proposed MPA scheme is
very similar to Mobile Initiated Tunneling Handoff's (MITH's)
predictive scheme, where the mobile node communicates with the
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Foreign Agent before actually moving to the new network. However,
the MPA scheme is not limited to MIP; this scheme takes care of
movement between domains and performs pre-authentication in addition
to proactive handover. Thus, MPA reduces the overall delay to a
period close to that of 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 always be
viable. Thus, there is a need to develop an optimization mechanism
that can support inter-domain mobility without any underlying
constraints or security-related assumptions.
Recently, the HOKEY working group within the IETF has been defining
ways to expedite the authentication process. In particular, it has
defined pre-authentication [RFC5836] 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 authors of the accompanying document [MPA-WIRELESS] have cited
several use cases of how MPA can be used to optimize several network-
layer and application-layer mobility protocols. 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 more
viable as a 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 may be equipped with a different mobility protocol.
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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.
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 the link layer.
MPA adopts the following basic procedures to provide this
functionality. The first procedure is referred to as
"pre-authentication", the second procedure is referred to as
"pre-configuration", and the combination of the third and fourth
procedures is 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 that 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) [RFC2003] between the mobile node and an access
router of the CTN, then
(iii) send and receive IP packets, including signaling messages
for the binding update of an MMP and data packets transmitted
after completion of the binding update, over the PHT, using the
obtained IP address as the tunnel inner address, and finally
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(iv) delete or disable the PHT immediately before attaching to the
CTN when it becomes the target network, and then re-assign 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.
Step (iii) above (i.e., the binding update procedure), in particular,
makes it possible for the mobile node to complete the higher-layer
handover before starting a link-layer handover. This means that the
mobile node is able to send and receive data packets transmitted
after completing the binding update over the tunnel, while data
packets transmitted before completion of the binding update do not
use the tunnel.
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: an
Authentication Agent (AA), a Configuration Agent (CA), and an 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 establish a shared 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 or Diameter, to carry
authentication credentials to an appropriate authentication server in
the AAA infrastructure. This interaction happens through the
authentication agent, such as the PANA Authentication Agent (PAA).
In turn, the derived key is used to derive additional keys that will
be applied to 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 the Extensible
Authentication Protocol (EAP) [RFC3748] would be suitable as an
authentication protocol for MPA.
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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 in the MPA framework 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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Figure 2 shows the basic functional components of MPA.
+----+
| CN |
+----+
/
(Core Network)
/ \
/ \
+----------------/--------+ +----\-----------------+
| +-----+ | |+-----+ |
| | | +-----+ | || | +-----+ |
| | AA | |CA | | ||AA | | CA | |
| +--+--+ +--+--+ | |+--+--+ +--+--+ |
| | +------+ | | | | +-----+ | |
| | | pAR | | | | | |nAR | | |
| ---+---+ +---+-----+----+---+-+ +-----+ |
| +---+--+ | | +-----+ |
| | | | |
| | | | |
| | | | |
+------------+------------+ +--------|-------------+
Current | Candidate| Target Network
Network | |
+------+ +------+
| oPoA | | nPoA |
+--.---+ +--.---+
. .
. .
+------+
| MN | ---------->
+------+
Figure 2: MPA Functional Components
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 below, and it is the responsibility
of link-layer handover to minimize packet loss during this period.
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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 about discovery mechanisms are discussed
in Section 7.1. The mobile node performs pre-authentication with
the authentication agent. As discussed in Section 7.2, the mobile
node may need to pre-authenticate with multiple candidate target
networks. The decision regarding with which candidate network the
mobile node needs to pre-authenticate will depend upon several
factors, such as signaling overhead, bandwidth requirement
(Quality of Service (QoS)), the mobile node's location,
communication cost, handover robustness, etc. Determining the
policy that decides the target network with which the mobile node
should pre-authenticate 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 the 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 a
default router from the CTN. The mobile node 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 the oCoA and the 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 agent and the access router are co-located in the
same device, the two protocols may be integrated into a single
protocol, such as IKEv2. After completion of the tunnel
establishment, the mobile node is able to communicate using both
the oCoA and the 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.
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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 node, etc. In some cases, it may cache
multiple nCoA addresses and perform simultaneous binding with the
Correspondent Node (CN) or Home Agent (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 node may execute the
tunnel management protocol to delete or disable the proactive
handover tunnel and cache the 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 the nCoA is possible without using a proactive handover
tunnel.
Call flow for MPA is shown in Figures 3 and 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 Tunnel Inner Address | |
| 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 node
experiencing rapid movement between subnets and/or domains, one needs
to look into several operations. These issues include:
i) discovery of neighboring networking elements,
ii) connecting to the right network based on certain policy,
iii) changing the layer 2 point of attachment,
iv) obtaining an IP address from a DHCP or PPP server,
v) confirming the uniqueness of the IP address,
vi) pre-authenticating with the authentication agent,
vii) sending the binding update to the Correspondent Host (CH),
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viii) obtaining the redirected streaming traffic to the new point
of attachment,
ix) ping-pong effect, and
x) probability of moving to more than one network and associating
with multiple target networks.
We describe these issues in detail in the following paragraphs and
describe how we have optimized these issues in the case of MPA-based
secure proactive handover.
7.1. Discovery
Discovery of neighboring networking elements such as access points,
access routers, and authentication servers helps expedite the
handover process during a mobile node's movement between networks.
After discovering the network neighborhood with a desired set of
coordinates, capabilities, and parameters, the mobile node can
perform many of the operations, 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 node 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) [RFC2608] 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 node approaches the vicinity of the neighboring networks.
IEEE 802.11u is considering issues such as discovering the
neighborhood using information contained in the 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. Some proposals, such as [802.21], help obtain
information about the neighboring networks from a mobility server.
When the movement is imminent, the mobile node 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 network
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and keep these in a cache. At some point, the mobile node finds
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 node selects a specific network to
be the target network, it may actually end up moving into a
neighboring network other than the target network, due to factors
that are beyond the mobile node's control. Thus, it may be useful to
perform the pre-authentication with a few probable candidate target
networks and establish time-bound transient tunnels with the
respective access routers in those networks. Thus, in the event of a
mobile node moving to a candidate target network other than that
chosen as the target network, it will not be subjected to packet loss
due to authentication and IP address acquisition delay that could
occur if the mobile node 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 node is reserving resources that could be used otherwise.
But since this happens for a time-limited period, it should not be a
big problem; it depends upon the mobility pattern and duration. The
mobile node uses a 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, the MN may
retain some or all of the nCoAs for future movement.
The mobile node 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 node 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 node moves to the new network, since the mobile node
has to go through the process of assigning the new IP address and
sending the binding update again. There are two solutions to this
problem. As one solution to the problem, the mobile node can take
advantage of the simultaneous mobility binding and send multiple
binding updates to the Correspondent Host or HA. Thus, the
Correspondent 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
node moves to the new network, thus stopping the flow to the other
candidate networks. RFC 5648 [RFC5648] discusses different scenarios
of mobility binding with multiple care-of-addresses. As the second
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solution, 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 the co-located address mode. The MPA
approach can use both the co-located address mode and the Foreign
Agent address mode. We discuss here the address assignment component
that is used in the co-located address mode. There are several ways
a mobile node can obtain an IP address and configure itself. In some
cases, a mobile node can configure itself statically in the absence
of any configuration element such as a server or router in the
network. In a LAN environment, the mobile node can obtain an IP
address from DHCP servers. In the case of IPv6 networks, a mobile
node has the option of obtaining the IP address using stateless
autoconfiguration or DHCPv6. In some wide-area networking
environments, the mobile node uses PPP (Point-to-Point Protocol) to
obtain the IP address by communicating with a NAS (Network Access
Server).
Each of these processes takes on the order of few hundred
milliseconds to a 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 a few optimized techniques available,
such as DHCP Rapid Commit [RFC4039] and GPS-coordinate-based IP
address [GPSIP], that attempt to reduce the handover delay due to IP
address acquisition time. However, in all these cases, the mobile
node 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 Fast MIPv6 [RFC5568], through the RtSolPr and PrRtAdv messages,
the MN also formulates a prospective new CoA (nCoA) when it is still
present on the Previous Access Router's (pAR's) link. Hence, the
latency due to new prefix discovery subsequent to handover is
eliminated. However, in this case, both the pAR and the Next Access
Router (nAR) need to cooperate with each other to be able to retrieve
the prefix from the target network.
In the following paragraph, we describe a few ways that 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: PANA-assisted proactive IP address acquisition,
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IKE-assisted proactive IP address acquisition, proactive IP address
acquisition using DHCP only, and stateless autoconfiguration. When
DHCP is used for address configuration, a DHCP server is assumed to
be serving one subnet.
7.3.1. PANA-Assisted Proactive IP Address Acquisition
In the 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 IP address
acquisition process via a DHCP client that is co-located with the
PANA authentication agent in the access router in the CTN acting on
behalf of the mobile node. Upon receiving a PANA message from the
mobile node, the DHCP client on the authentication 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 mobile node. In the case of IPv6, a Router
Advertisement (RA) is carried as part of the PANA message. In the
case of stateless autoconfiguration, the mobile node uses the
prefix(es) obtained as part of the RA and its MAC address to
construct the unique IPv6 address(es) as it would have done in the
new network. In the case of stateful address autoconfiguration, a
procedure similar to DHCPv4 can be applied.
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 [RFC3046] 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-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 the standard IKEv2
procedure, 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, a VPN gateway can dispense
the IP address from its IP address pool.
7.3.3. Proactive IP Address Acquisition Using DHCPv4 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 agent or DHCP server in the CTN. The
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mechanism described in this section is applicable to DHCPv4 only.
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 node 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 node using
the mobile node's source address.
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 be configured with a filter to block unicast DHCP
messages 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 node 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 agent or the DHCP server in the CTN may be
carried with additional information that is used to distinguish it
from other addresses as assigned to the physical interface.
Upon the mobile node'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 or DNS server,
by using DHCP INFORM. This should not affect the ongoing
communication between the mobile node 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
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for the mobile node for both DHCP for proactive IP address
acquisition and for 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 node. 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 the case of stateless autoconfiguration, the mobile node checks to
see the prefix of the router advertisement in the new network and
matches it with the prefix of the newly assigned IP address. If
these turn out to be the same, then the mobile node 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 the case of IPv6, a
proactive handover tunnel is established between the mobile node and
the access 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 node in the old PoA, unlike the
transient tunnel in FMIPv6 (Fast MIPv6) [RFC5568], where it is set up
between the mobile node's new point of attachment and the old access
router.
In the case of inter-domain handoff, it is important that any
signaling message between the nPoA and the mobile node needs to be
secured. This transient secured tunnel provides the desired
functionality, including securing the proactive binding update and
transient data between the end-points before the handover has taken
place. Unlike the proactive mode of FMIPv6, transient handover
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packets are not sent to the pAR, and thus a tunnel between the mobile
node's new point of attachment and the old access router is not
needed.
In the case of inter-domain handoff, the pAR and nAR could logically
be far from each other. Thus, the signaling and data during the
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 node is preparing for 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 node needs to be able to communicate with the
Correspondent Node while in the previous network, some or all parts
of the binding update and data from the Correspondent Node to the
mobile node need to be sent back to the mobile node over a proactive
handover tunnel. Details of these binding update procedures 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.
Alternatively, when PANA is used as the authentication protocol, the
tunnel deletion or disabling at the access router can be triggered by
means of the PANA update mechanism as soon as the mobile node 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) from the next
access router to be sent over the tunnel, as soon as the tunnel
creation is complete.
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7.5. Binding Update
There are several kinds of binding update mechanisms for different
mobility management schemes.
In the case of Mobile IPv4 and Mobile IPv6, the mobile node performs
a binding update with the Home Agent only, if route optimization is
not used. Otherwise, the mobile node performs the binding update
with both the Home Agent (HA) and Correspondent Node (CN).
In the case of SIP-based terminal mobility, the mobile node sends a
binding update using an INVITE to the Correspondent Node and a
REGISTER message to the Registrar. Based on the distance between the
mobile node 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 the case of secure proactive handover using SIP-based
mobility management, we do not encounter the delay due to the binding
update at all, 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 the case of Mobile IPv6, the mobile node sends the
newly acquired CoA from the target network as the binding update to
the HA and CN. Also, all signaling messages between the MN and HA
and between the 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), and Care-of Test (CoT). In Mobile IPv6, since the receipt of
an on-link router advertisement is mandatory for the mobile node to
detect the movement and trigger the binding update, a router
advertisement from the next access router needs to be advertised over
the tunnel. By proper configuration on the nAR, the router
advertisement can be sent over the tunnel interface to trigger the
proactive binding update. The mobile node 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.
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 node 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.
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7.6. Preventing Packet Loss
In the MPA case, packet loss due to IP address acquisition, secured
authentication, and binding update does not occur. However,
transient packets during link-layer handover can be lost. Possible
scenarios of packet loss and its prevention are described below.
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 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 the 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 a streaming application, a playout
buffer does not help much for interactive VoIP applications that
cannot tolerate 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 WiFi
to a CDMA network, pre-authentication at the CDMA network can be
performed via the WiFi 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
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
Dutta, et al. Informational [Page 29]
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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-forward
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 node takes a longer amount of time to detect the
disconnection event of the current active interface, this 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 or CDMA) are
non-overlapping, or in cases when the carrier does not allow the
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, or proactive binding update). In a specific
scenario involving handoff between WiFi and CDMA networks, 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 the 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 of using this buffering scheme
are explained in the accompanying document [MPA-WIRELESS].
Dutta, et al. Informational [Page 30]
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7.7. Security and Mobility
This section describes how MPA can help establish layer 2 and layer 3
security association in the target networks while the mobile node is
in the previous network.
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, as described in the following
steps. Figure 5 shows the sequence of operation.
(1) The authentication agent and the mobile node derive 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 a 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 group 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, 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
points of attachment in the CTN to derive TSKs.
Dutta, et al. Informational [Page 31]
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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 node and
the 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 node moves, the IP address of
the router and outbound proxy will change in the new network. The
mobile node'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 node and the desired
network entity. In some cases, such as in a 3GPP/3GPP2 IMS/MMD
environment, data traffic is not allowed to pass through unless there
is an IPsec SA established between the mobile node and outbound
proxy. This will of course add unreasonable delay to the existing
real-time communication during a mobile node's movement. In this
scenario, key exchange is done as part of a SIP registration that
follows a key exchange procedure called AKA (Authentication and Key
Agreement).
Dutta, et al. Informational [Page 32]
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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 node can pre-register via the new outbound
proxy in the target network and completes the pre-authentication
procedure, then the new SA state between the mobile node 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 node does not need to be involved 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 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 for 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
CTNs.
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 CTNs.
Dutta, et al. Informational [Page 33]
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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
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 relationships among administrative
domains or access routers, which makes the framework more deployable
in the Internet without compromising the security in mobile
environments.
9. 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 working group.
The authors would like to acknowledge Christian Vogt, Rajeev Koodli,
Marco Liebsch, Juergen Schoenwaelder, and Charles Perkins for their
thorough review of the document and useful feedback.
Author and Editor Ashutosh Dutta would like to thank Telcordia
Technologies, and author Victor Fajardo would like to thank Toshiba
America Research and Telcordia Technologies, for supporting the
development of their document while they were employed in their
respective organizations.
10. References
10.1. Normative References
[RFC5944] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised",
RFC 5944, November 2010.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "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, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
Dutta, et al. Informational [Page 34]
RFC 6252 MPA Framework June 2011
[RFC5380] Soliman, H., Castelluccia, C., El Malki, K., and L.
Bellier, "Hierarchical Mobile IPv6 (HMIPv6) Mobility
Management", RFC 5380, October 2008.
[RFC5568] Koodli, R., Ed., "Mobile IPv6 Fast Handovers", RFC 5568,
July 2009.
[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., Ed., "Low-Latency Handoffs in Mobile IPv4",
RFC 4881, June 2007.
[RFC4066] Liebsch, M., Ed., Singh, A., Ed., Chaskar, H., Funato, D.,
and E. Shim, "Candidate Access Router Discovery (CARD)",
RFC 4066, July 2005.
[RFC4067] Loughney, J., Nakhjiri, M., Perkins, C., and R. Koodli,
"Context Transfer Protocol (CXTP)", RFC 4067, 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., Ed., 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 (QoS)", ETSI TR 101
329-1 V3.1.2, 2002.
Dutta, et al. Informational [Page 35]
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10.2. Informative References
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
Henderson, "Host Identity Protocol", RFC 5201,
April 2008.
[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.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2608] Guttman, E., Perkins, C., Veizades, J., and M. Day,
"Service Location Protocol, Version 2", RFC 2608,
June 1999.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[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., Ed., "Negotiation for IPv6 Datagram
Compression Using IPv6 Control Protocol", RFC 5172,
March 2008.
[RFC5648] Wakikawa, R., Ed., Devarapalli, V., Tsirtsis, G.,
Ernst, T., and K. Nagami, "Multiple Care-of Addresses
Registration", RFC 5648, October 2009.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection
(DAD) for IPv6", RFC 4429, April 2006.
Dutta, et al. Informational [Page 36]
RFC 6252 MPA Framework June 2011
[RFC5836] Ohba, Y., Ed., Wu, Q., Ed., and G. Zorn, Ed.,
"Extensible Authentication Protocol (EAP) Early
Authentication Problem Statement", RFC 5836,
April 2010.
[RFC5213] Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
RFC 5213, August 2008.
[RFC5974] Manner, J., Karagiannis, G., and A. McDonald, "NSIS
Signaling Layer Protocol (NSLP) for Quality-of-Service
Signaling", RFC 5974, October 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
Mobility Using SIP", ACM MC2R, July 2000.
[CELLIP] Campbell, A., Gomez, J., Kim, S., Valko, A., Wan, C.,
and Z. Turanyi, "Design, Implementation, and
Evaluation of Cellular IP", IEEE Personal
Communications, August 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.
[MISHRA04] Mishra, A., Shin, M., Petroni, N., Clancy, T., and W.
Arbaugh, "Proactive key distribution using neighbor
graphs", IEEE Wireless Communications Magazine,
February 2004.
[SPRINGER07] Dutta, A., Das, S., Famolari, D., Ohba, Y., Taniuchi,
K., Fajardo, V., Lopez, R., Kodama, T., Schulzrinne,
H., and A. Skarmeta, "Seamless proactive handover
across heterogeneous access networks", Wireless
Personal Communications, November 2007.
[HAWAII] Ramjee, R., La 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.
Dutta, et al. Informational [Page 37]
RFC 6252 MPA Framework June 2011
[IDMP] Das, S., McAuley, A., Dutta, A., Misra, A.,
Chakraborty, K., and S. Das, "IDMP: An Intra-Domain
Mobility Management Protocol for Next Generation
Wireless Networks", IEEE Wireless Communications
Magazine, October 2000.
[MOBIP-REG] Gustafsson, E., Jonsson, A., and C. Perkins, "Mobile
IPv4 Regional Registration", Work in Progress,
June 2004.
[YOKOTA] Yokota, H., Idoue, A., Hasegawa, T., and T. Kato,
"Link Layer Assisted Mobile IP Fast Handoff Method
over Wireless LAN Networks", Proceedings of ACM
MobiCom02, 2002.
[MACD] Shin, S., Forte, A., Rawat, A., and H. Schulzrinne,
"Reducing MAC Layer Handoff Latency in IEEE 802.11
Wireless LANs", MobiWac Workshop, 2004.
[SUM] Dutta, A., Zhang, T., Madhani, S., Taniuchi, K.,
Fujimoto, K., Katsube, Y., Ohba, Y., and H.
Schulzrinne, "Secured Universal Mobility for Wireless
Internet", WMASH'04, October 2004.
[SIPFAST] Dutta, A., Madhani, S., Chen, W., Altintas, O., and H.
Schulzrinne, "Fast-handoff Schemes for Application
Layer Mobility Management", PIMRC 2004.
[PIMRC06] Dutta, A., Berg, E., Famolari, D., Fajardo, V., Ohba,
Y., Taniuchi, K., Kodama, T., and H. Schulzrinne,
"Dynamic Buffering Control Scheme for Mobile Handoff",
Proceedings of PIMRC 2006, 1-11.
[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.
[802.21] "IEEE Standard for Local and Metropolitan Area
Networks: Media Independent Handover Services, IEEE
802.21-2008", a contribution to IEEE 802.21 WG,
January 2009.
Dutta, et al. Informational [Page 38]
RFC 6252 MPA Framework June 2011
[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., Madhani, S., Chen, W., Altintas, O., and H.
Schulzrinne, "GPS-IP based fast-handoff approaches for
Mobiles", IEEE Sarnoff Symposium 2006.
[MAGUIRE] Vatn, J. and G. Maguire, "The effect of using
co-located care-of addresses on macro handover
latency", 14th Nordic Teletraffic Seminar 1998.
[MPA-MOBIKE] El Mghazli, Y., Bournelle, J., and J. Laganier, "MPA
using IKEv2 and MOBIKE", Work in Progress, June 2006.
[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 Communications Magazine, April 2008.
Dutta, et al. Informational [Page 39]
RFC 6252 MPA Framework June 2011
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 a 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 the 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 (DAD) ahead of time, we reduce the IP address acquisition
time.
The proactive DAD over the candidate target network should be
performed by the nAR on behalf of the mobile node at the time of
proactive handover tunnel establishment, since DAD 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], the IP Control Protocol
(IPCPv6) negotiates the link-local addresses, and hence DAD over the
tunnel is not needed. After the mobile node 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 node 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 the 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 may be the access router,
authentication agent, configuration agent, or Correspondent Node.
There are several possible ways of performing such proactive MAC
address resolution.
o One can 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 One can 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 attribute-value pairs
(AVPs) used for proactive address resolution. In this case, the
PANA authentication agent in the target network may perform
address resolution 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, and 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
that 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
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those nodes need to detect the attachment of the mobile node to the
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 [RFC5944],
[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 node 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 Ratio (CIR), Signal-to-Interference Ratio
(SIR), Bit Error Rate (BER), and power budget. In order to avoid the
ping-pong effect, some additional parameters are employed by the
decision algorithm, such as hysteresis margin, dwell timers, and
averaging window. For a vehicle moving at a high speed, other
parameters, such as the distance between the mobile node and the
point of attachment, velocity of the mobile node, location of the
mobile node, traffic, and bandwidth characteristics are also taken
into account to reduce the ping-pong effect. More recently, there
are other handoff algorithms available that help reduce the ping-pong
effect in a heterogeneous network environment and 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 node between the current
network and target network, and between the candidate target
networks. MPA in its current form will be affected because of the
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number of tunnels set up between the mobile node and neighboring
access routers, the number of binding updates, and associated handoff
latency resulting from the ping-pong situation. The mobile node's
handoff rate may also contribute to delay and packet loss. We
propose a few techniques that will help reduce the probability of the
ping-pong effect and propose several methods for the MPA framework so
that it can recover from the packet loss resulting from the ping-pong
effect.
The MPA framework can take advantage of the mobile node'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 the
user's location and cached data from the previous handover attempts.
In some cases, location may not be the only indicator for a handoff
decision. For example, in Manhattan-type grid networks, although a
mobile node is close to an AP, it may not have enough SNR (Signal-to-
Noise Ratio) to make a good connection. Thus, knowledge of the
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 the 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
node comes back to the network where the context was last used. This
context may include security association, IP address used, and
tunnels established. Bicasting the data to both the previous network
and the new network for a predefined period will also help the mobile
node to take care of the lost packets in case the mobile node moves
back and forth between the networks. The mobile node 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 the 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 a certain time period. Thus, if the
mobile node 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.
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A mobile node that has pre-authenticated with an authentication agent
in a candidate target network and has an MPA-SA may need to continue
to keep the MPA-SA while it continues to stay in the current network
or even after it makes a 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 another 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 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, they are 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 the NSIS Signaling Layer Protocol (NSLP)
[RFC5974] or the Resource Reservation Protocol (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 and 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 differences in
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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.
C.4. Resource Allocation Issue during Pre-Authentication
In the case of multiple CTNs, establishing multiple tunnels with the
neighboring target networks provides some additional benefits. But
it 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 node 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 node can
also complete the pre-configuration while in the previous network,
but can postpone the binding update until after the mobile node has
moved. Like the previous case, in this case the mobile node 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 node has moved to the new network,
the binding update is actually done after the mobile node has moved.
The third type of multiple pre-authentication involves all the three
steps while the mobile node is in the previous networks, such as
authentication, configuration, and binding update. But, this
specific process utilizes the highest 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 the mobile
node's cache for a certain amount of time. Additional
processing in the mobile node is needed for storing these IP
addresses. In addition, this caching of addresses 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 the mobile node.
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(4) In the case of a binding update with multiple IP addresses
obtained from the neighboring networks, multiple transient
streams flow between the CN and mobile node using these
transient tunnels.
However, there are pros and cons related to sending the binding
update after the handover. If the binding update is sent after the
mobile node has moved to the new network, this will contribute to the
delay if the CH or HA is far from the MN. Multiple binding updates
can be taken care of in many different ways. We describe a few of
these update mechanisms below.
When only pre-authentication and pre-configuration are done ahead of
time with multiple networks, the mobile node 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.
In case a binding update with multiple contact addresses is sent,
multiple media streams stem out of the 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 node has moved. This way, the
mobile node stops sending media to other neighboring networks where
the mobile node did not move.
The following is an illustration of this specific case that takes
care of multiple binding streams, when the mobile node moves only to
a specific network, but sends multiple binding updates in the
previous network. The MN sends a binding update to the 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 node over the pre-
established tunnels. After the mobile node moves to the actual
network, it sends another binding update to the CN with the care-of
address of the mobile node in the network where the mobile node has
moved. One issue with multiple streams is 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 node after it has moved. Forwarding of data can be
triggered by the mobile node either as part of Mobile IP registration
or as a separate buffering protocol.
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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, and ii) inter-technology and inter-domain. We also
present the results of how the MPA can bootstrap layer 2 security for
both roaming and non-roaming cases. Detailed procedures 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 Figures 6 and 7, respectively.
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
RO = Router Optimization
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 measurements, 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 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 the case of non-MPA-assisted handover, handover delay and
associated packet loss occur 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 incur the
longest delay. This is due to the detection of duplicate IP
addresses in the network before the DHCP request completes. The
binding update exchange also experiences a long delay if the CN is
too far from the MN. As a result, the non-MPA-assisted handover took
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an average of 4 seconds to 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 node 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, the Link Control Protocol (LCP), the Challenge
Handshake Authentication Protocol (CHAP)). If such a lengthy 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 node 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; the
second is when only the old interface is up and 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 power effectively.
As compared to non-optimized handover that may result in a delay of
up to 18 sec and loss of 1000 or more packets during the handover
from the wireless LAN (WLAN) to CDMA, we observed 0 packet loss and a
50-ms handoff delay between the last pre-handoff packet and the 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
node 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 8 shows the
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experimental testbed where we have conducted the MPA-assisted
pre-authentication experiment for bootstrapping layer 2 security as
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 the 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 8 and 9, respectively. In the roaming case, the MN is
visiting in a domain different than its home domain. Consequently,
the MN needs to contact the AAA server in the home domain (AAAH) from
its new domain. For the non-roaming case, we assume the MN is moving
within its home domain, and only the local AAA server (AAAHome),
which is the home AAA server for the mobile node, is contacted.
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 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 the 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 the 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 the AAA protocol (PAA implements
a Diameter client).
In the 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, the PAA proactively installs two keys, PSKap1 and
PSKap2, in AP1 and AP2, respectively. By doing the key installations
proactively, the PAA preempts the process of communicating with the
AAA server for the keys after the mobile node moves to the new
network. Finally, because PSKap1 is already installed, AP1
immediately starts 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 the PAA's IP address and all required
information about AP1 and AP2 (e.g., channel, security-related
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parameters, etc.) at some point before the handover. This avoids the
scanning during link-layer handoff. We have applied this assumption
to all three scenarios. Because our focus is on reducing the time
spent on the authentication phase 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 | Pre-authentication
=====================================================================
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+ | | | | | | |
4way | 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
Pre- and Post-Authentication
Figure 10 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 parameters
below.
"Tauth" refers to the execution of EAP-Transport Layer Security (TLS)
authentication. This time does not distinguish whether this
authentication was performed during pre-authentication or a typical
post-authentication.
"Tconf" refers to the 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 the
association and the 4-way handshake with the target AP after the
handoff.
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The first two columns in the figure show the results for non-roaming
and roaming cases, respectively, when no pre-authentication is used
at all. The second two columns depict the same cases when IEEE
802.11i pre-authentication is used. The last two columns show when
we used network-layer-assisted layer 2 pre-authentication. When pre-
authentication is used, only the factor Tassoc+4way affects the
handoff time. When no pre-authentication is used, the time affecting
the handoff includes Tauth (the complete EAP-TLS authentication) plus
Tassoc+4way.
That is precisely the time affecting the handoff in the case where
the MN moves from AP0 to AP1 in the absence of pre-authentication.
As it is seen, these delays are not suitable for real-time
applications. Indeed, for the non-roaming case, we obtained a ~80-ms
delay for re-establishing the connection with target AP1. It takes
about 600 ms to complete the handoff when the MN moves to a visited
domain and the home AAA server is located far away. However,
network-layer pre-authentication is only affected by Tassoc+4way
(~17 ms) involving any kind of handoff authentication. As is
evident, IEEE 802.11i pre-authentication provides a comparable
benefit (~16 ms) in terms of handoff but is limited to cases when APs
are in the same Distribution System (DS). Additionally, network-
layer pre-authentication leverages a single EAP authentication to
bootstrap security in several target APs by allowing the MN to move
among APs under the same PAA without running EAP and consequently
without contacting the AAA server. In this sense, it extends IEEE
802.11r advantages over IEEE 802.11i by allowing inter-subnet and
inter-domain and even inter-technology handoffs.
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 the
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 the pre-authentication 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 node,
cell size, etc.
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The time needed to complete the pre-authentication mechanism is an
important parameter, since the mobile node needs to determine how
much ahead of time the mobile node 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 node (e.g., pedestrian vs. vehicular) and cell sizes (e.g.,
WiFi, Cellular). Cell residence time is defined as the average time
the mobile node stays in the cell before the next handoff takes
place. Cell residence time is dependent upon the coverage area and
velocity of the mobile node. Thus, cell residence time is an
important factor in determining the desirable pre-authentication time
that a mobile node should consider.
Since the pre-authentication operation involves six steps as
described in Section 6.3 and each step 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 node could complete only network discovery and
the network-layer authentication process before the handoff and
postpone the rest of the operations 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 node 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 [MISHRA04].
D = Coverage diameter
v = Mobile node's velocity
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 = 100 ft, Tpsk = 10 ms, and RTT = 100 ms, a
mobile node 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
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Similarly, for a similar cell size, if the mobile node is involved in
both pre-authentication and pre-configuration operations as part of
the MPA procedure, and it takes an amount of time Tconf = 190 ms to
complete the layer 3 configuration including IP address
configuration, then for a successful MPA operation,
2RTT + Tpsk + Tconf < D/v
v = 100 ft/(200 ms + 10 ms + 190 ms) = ~250 ft/sec
Thus, compared to only the pre-authentication part of the MPA
operation, in order to be able to complete both pre-authentication
and pre-configuration operations successfully, either the mobile node
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 node.
As an alternative, if a mobile node does complete all of the
pre-authentication procedure well ahead of time, it uses up the
resources accordingly by way of an extra IP address, tunnel, and
extra bandwidth. Thus, there is always a tradeoff between the
performance benefit obtained from the pre-authentication mechanism
and network characteristics, such as movement speed, cell size, and
resources utilized.
Dutta, et al. Informational [Page 56]
RFC 6252 MPA Framework June 2011
Authors' Addresses
Ashutosh Dutta (editor)
NIKSUN
100 Nassau Park Blvd.
Princeton, NJ 08540
USA
EMail: ashutosh.dutta@ieee.org
Victor Fajardo
NIKSUN
100 Nassau Park Blvd.
Princeton, NJ 08540
USA
EMail: vf0213@gmail.com
Yoshihiro Ohba
Corporate R&D Center, Toshiba Corporation
1 Komukai-Toshiba-cho, Saiwai-ku
Kawasaki, Kanagawa 212-0001
Japan
EMail: yoshihiro.ohba@toshiba.co.jp
Kenichi Taniuchi
Toshiba Corporation
2-9 Suehiro-cho
Ome, Tokyo 198-8710
Japan
EMail: kenichi.taniuchi@toshiba.co.jp
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
Dutta, et al. Informational [Page 57]