HANDOVER OPTIMIZATION FOR HOST

AND NETWORK MOBILITY

Hai Lin, Houda Labiod

Ecole Nationale Supérieure des Tétécommunication (ENST),46 rue Barrault, 75634 Paris Cedex, France

Guo Zhi Wei, Anne Wei

University of Paris XII, 61 Avenue du General de Gaulle, 94010 Créteil Cedex, France

Keywords: Mobile IP, NEMO, handover, multihomed, multiple mobile routers.

Abstract: Mobile IP and NEMO (network mobility) provide continuous connectivity to the Internet to a node or a

mobile network when moving from one access router to another. Because of link switching delay and IP

protocol operation, packets destined to mobile nodes or mobile networks can be delayed or lost during the

handover period. This paper proposes solutions to improve the performance of handover in the context of

Mobile IPv6 and NEMO. We introduce a new control entity for both MIPv6 and NEMO to manage the

traffic between access routers and mobile nodes or mobile routers, to provide low-latency and low packet

loss for real-time services during the handover.

1 INTRODUCTION

The need of keeping connection with Internet in

everywhere and at every time is more and more

demanded in recently years. We can find two kinds

of mobility scenarios. In the first one, only mobile

node moves and attaches to different locations in the

Internet topology through immovable routers, using

MIPv4 (Perkins, 2002) or MIPv6 (Johnson, 2004)

mechanisms, we call it Host Mobility. The second

one concerns the scenario where a set of hosts move

collectively as a unit, we call it Network Mobility.

There are many situations where an entire network

might move and attach to the Internet topology from

“anywhere” at “anytime”, like a mobile network that

can be contained in a train or a ship. In both cases,

continuous connectivity must be supported. Hence,

recently, several extensions to MIPv6 have been

proposed aiming to reduce the handover latency and

packet loss to improve the handover performance for

real-time applications support.

In the case of network mobility, host mobility

support protocols may produce enormous signaling

which is not suited to the whole network movement.

Moreover, not all nodes in a large mobile network

may be sophisticated enough to run such mobility

support protocols. Network Mobility support

protocols have then been proposed in the context of

a recent IETF working group called NEMO. In

mobile networks, the weakest part comprises mainly

the mobile router. If the MR is down, all connections

between Internet and mobile node are disrupted.

Hence, multihomed architecture with multiple

mobile routers which offers multiple connections to

the Internet is proposed for mobile networks. This

architecture enables connections to be maintained

even if one of the mobile routers fails. Our work

related to handover optimization for network

mobility focuses on this specific NEMO

architecture.

The goal of this paper is to optimize handover

process both in host and network mobility but

considered separetely. Firstly, we propose a new

scheme to achieve MIPv6 fast handover by

introducing a component called Handover Control

Function (HCF) in Hierarchical Mobile IPv6. This

function allows us to predict a new attachment point

while a mobile node moves. Secondly, we enhance

the handover performance in the case of multiple

mobile routers installed in mobile network, by using

a new component called Intelligent Control Entity

(ICE).

The remainder of the paper is organized as

follows. Section 2 presents background and related

work of mobile IP and NEMO. Section 3 presents

our HCF Based Handover Function for a novel

MIPv6 scheme and the detailed protocol operation.

Lin H., Labiod H., Zhi Wei G. and Wei A. (2006).

HANDOVER OPTIMIZATION FOR HOST AND NETWORK MOBILITY.

In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 59-64

 SciTePress

Section 4 deals with NEMO with multiple mobile

routers via a general architecture and a new scheme.

Finally, conclusion and future work are mentioned

in section 5.

2 BACKGROUND AND RELATED

WORK

As our paper deals with both host and network

mobility, we describe the several proposals related

to these two kinds of mobility separately.

2.1 Host Mobility with Mobile IPv6

Many kinds of wireless technologies, such as GSM,

UMTS, Wireless LAN, using the Host Mobility

architecture currently co-exist and are likely that their

number will further increase in the near future. R

ecently,

as the WLAN technologies, especially the IEEE

802.11 standards have got great attention, a growing

number of WLANs have been set up in public

buildings or corporate environments as access

networks to the Internet. In this paper, we focus on

improving the handover performance of Mobile

IPv6 over Wireless LAN.

Actually, the main proposals accepted by IETF

are Hierarchical Mobile IPv6 (HMIPv6) (Soliman

2005) and Fast Handover for MIPv6 (FHMIPv6)

(Koodli, 2005). HMIPv6 introduces Mobility

Anchor Point (MAP) (a special node located in the

network visited by a Mobile Node (MN)) who acts

somewhat like a local Home Agent (HA) for the

visiting MN. Moreover, HMIPv6 separates MN

mobility into micro-mobility (within one domain or

within the same MAP) and macro-mobility (between

domains or between MAPs). With this hierarchical

network structure, MAP can limit the amount of

signaling required outside the MAP's domain.

Therefore, the amount and latency of signaling

between a MN, HA and one or more Correspondent

Nodes (CNs) decrease.

FHMIPv6 reduces packets loss by providing fast

IP connectivity as soon as a new link is established.

The network uses layer 2 triggers to launch either

Pre-Registration or Post-Registration handover

scheme. In Pre-Registration scheme, the network

provides support for pre-configuration of link

information (such as the subnet prefix) in the new

subnet while MN is still attached to the old subnet.

By reducing the pre-configuration time on the new

subnet, it enables IP connectivity to be restored at

the new point of attachment sooner than would

otherwise be possible. In Post-Registration scheme,

by tunneling data between the previous point of

attachment and a new point of attachment, the

packets delivered to the old Care-of-Address (CoA)

are forwarded to the new CoA during link

configuration and Binding Update. So it is possible

to provide IP connectivity in advance contrarily to

the actual Mobile IP registration with the HA or CN.

Besides the main proposals, there have been

numerous approaches for providing lossless

handover and minimizing the handover delay. In

(Chaouchi, 2004), a Pre-Handover Signaling (PHS)

protocol is proposed to support the triggering of a

predictive handover and to allow the network to

achieve accurate handover decisions. In (Bi, 2004), a

Hierarchical Network-layer Mobility Management

(HNMM) framework is described in which an

integrated IP-layer handover solution is proposed to

provide optimized network connectivity. Also, a

Competition based Soft Handover Management

(CSHM) protocol (Kristiansson, 2004), and the

Multi-path Transmission Algorithm (Kashihara,

2002) are proposed to decrease packet loss during

handover.

2.2 Network Mobility

Network Mobility (NEMO) provides continuous

connectivity to the Internet to a set of nodes within a

mobile network. As illustrated in figure 1, a mobile

network is composed of one or more mobile IP-

subnets (NEMO-link) and is viewed as a single unit.

This network unit is connected to the Internet by

means of one or more Mobile Routers (MRs). Three

types of nodes behind the MR are defined : Local

Fixed Nodes, Local Mobile Nodes and Visiting

Mobile Nodes.

At a home link, an entity named Home Agent

(HA) is presented (figure 1), with which the mobile

router will register its care-of address and prefix.

While the mobile network is away from home, the

home agent intercepts packets on the home link

destined to the mobile network, encapsulates them,

and tunnels them to the MR’s registered care-of

address. At the foreign link, MRs get network layer

access to the global Internet from the Access

Router(s) (AR) (figure 1), via which packets from/to

Internet are transported.

The NEMO basic protocol (Devarapalli, 2005)

requires the MR to act on behalf of the nodes within

its mobile network. When an MR configures a new

CoA at a foreign link, it sends a Binding Update

message to its home agent, which contains its CoA

and its prefixes (in the case where the MR’s prefix

can be determined by home agent, prefix is not

included in Binding Update message). These

prefixes are then used by the HA to intercept packets

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Figure 1: Architectural Components of NEMO.

addressed to the mobile nodes and these packets are

tunnelled to the MR. In response to the Binding

Update message, HA returns a Binding

Acknowledgement message to MR, which indicates

whether the HA has successfully processed the

Binding Update and has set up forwarding for the

Mobile network. If the procedure of Binding Update

is successful, a bidirectional tunnel between the MR

and the HA should be established. From now, the

MR encapsulates the packets from the mobile

network and tunnels them to HA, the latter

decapsulates the packets and forwards them to the

Correspondent Nodes (CNs). In the opposite

direction, the HA encapsulates the packets to the

mobile network and tunnels to the MR, the MR

decapsulates the packets and forwards them to the

mobile nodes.

As described above, the packets from/to mobile

network must go through the bidirectional tunnel,

which increase the packet’s size (add an additional

tunnel encapsulation) and the end-to-end delay. A

straight-forward approach to route optimization in

NEMO is proposed. Instead of preceding the

Binding Update with HA, MR sends Binding

Updates containing one or more Mobile Network

Prefix options to the CN. The CN having received

the Binding Update, can then set up a bi-directional

tunnel with the MR at the current care-of address of

the MR, and inject a route to its routing table so that

packets destined for addresses in the mobile network

prefix will be routed through the bidirectional tunnel

between the CN and the MR (Ng, 2005). But

establishing this kind of tunnel can be difficult to

perform, especially in the case where the CN

belongs to a mobile network node, and this mobile

network is away from its home link. However, we

can establish a tunnel to CN’s MR. To extend this

idea, if a tunnel can be established between the MR

and an entity which is located closer to the CN than

the HA, the route between mobile network and the

CN can be said to be optimized.

2.3 Open Research Issues

Besides the problems of handover delay and packet

loss in wireless LANs, there are many important

issues related to handover such as handover

architectures and handover decision algorithms that

should be studied. The handover architectures

concern mobility management and admission control

while the handover decision algorithms decide some

parameter adoptions and user preferences. In

addition of handover issues, quality-of-service

(QoS) and security are also the open research issues

that are addressed at current and future wireless

networks.

The most attractive feature for NEMO is the

simplicity of its NEMO basic protocol. Since it is an

extension of the Mobile IPv6 operation at the mobile

routers and home agents, the effective deployment

of mobile networks will strongly depend on how we

can overcome some critical issues such as sub-

optimal routing, handoff optimization, QoS

management, multihomed configurations, security

issues especially in nested configurations,

compatibility with Mobile IPv6, access control,

billing, and scalability. Some solutions have been

proposed to tackle some of these issues but some

other remain open and need further research like

handover, security and QoS support.

When moving from an AR to another, because of

a handover situation, traffic can be lost due to link

switching delay and IP protocol operation (CoA

configuration, Duplicate Address Detection, etc). Up

to now, there are no proposals and we aim to

propose some mechanisms to enhance NEMO

handover performance in the case of multiple MRs

installed in the mobile network (see section 4). This

type of mobile network is one kind of multihomed

mobile network; the latter can be configured into

serveral types (Ng, Paik, 2005): when a MR has

multiple egress interfaces, the mobile network has

multiple MRs, the mobile network is associated with

multiple HAs, and multiple global prefixes are

available in the mobile network. Advantages of such

architectures are listed in (Ernst, 2005) including

permanent and ubiquitous access, reliability, load

balancing, and preference settings.

3 HANDOVER CONTROL

FUNCTION (HCF) BASED

HANDOVER FOR MOBILE

IPV6

As mentioned in Section 2.1, MIPv6 handover has

been studied in some paper. We introduce a local

HANDOVER OPTIMIZATION FOR HOST AND NETWORK MOBILITY

intelligent entity called Handover Control Function

(HCF) which should be capable of controlling

several ARs and MNs.

3.1 HCF General Architecture

Linking with ARs, Handover Control Function

(HCF) enables to decide the MN’s new attachment.

We define four new messages: Handover Request

(HOReq), Handover Reply (HORep), Connection

Establish Information (CEInf) and Handover Finish

Confirmation (HFCon) messages in Mobile IPv6. It

is necessary to mention that HCF can manage

handover, resources distribution and security. Figure

2 illustrates the considered architecture.

Based on Figure 2, MN surveys periodically the

received signal strength. Once the signal strength

drops below the threshold predefined, MN will

begin to scan and discover the new available AP. It

reports to HCF the APs’ BSSID (Basic Service Set

Identifier) and signal strengths that it can probe.

Based upon the reported information, AR/AP’s

loading, and MN’s QoS demand, by using a

predefined algorithm, HCF decides whether or

which AP MN shall associate with and notifies MN

about the new AR/AP's information, such as AP's

BSSID, AR interface address, and sub-network

prefix. HCF decides which AR's interface MN

should move to as well. Consequently, the new

network prefix of MN will be notified by HCF

through HCFRep message accordingly.

The “IPv6 address allocation and assignment

policy" issued by RIPE (Ripe, 2003) provides the

guidelines for allocation and distribution of IPv6

addresses. This draft reveals that in an IPv6 access

network as MN moves across the subnets, the only

change in its IPv6 address occurs in subnet identifier

field of the address. The remaining portion of the

address, including 48 bit global routing prefix and

the 64 bit interface identifier remains unchanged.

Moreover, in our proposal, MN's interface identifier

is allocated according to the norm of EUI-64. It

ensures that the MN’s new CoA is unique in Mobile

Figure 2: HCF Based Handover for Mobile IPv6.

IPv6. Consequently, MN could configure its new

CoA and launches the Binding Update process even

if it is still attached with previous AR/AP. HCF also

knows MN's new CoA according to MN's old CoA

and MN's new network prefix. Furthermore,

Duplicated Address Detection (DAD) can be

omitted during handover.

After HCF sends the HCFRep message, HCF

will intercept all packets sent to the MN’s previous

CoA, and buffers these packets. MN will send CEInf

to HCF as soon as it finishes its handover process at

the layer 2. HCF then encapsulates these packets and

sends to MN with the HFCon message. Figure 3

shows messages exchange during the handover

procedure.

Figure 3: Protocol for HCF based handover.

3.2 HCF Procedure

HCF procedure is detailed as follows:

1) When MN registers to HA at the first time after

it attaches to a new AR/AP, it sends Binding Update

message to HA and HA responses with Binding

Acknowledgement.

2) Moving at sub-network, if the threshold of

received signal strength is overstepped, MN begins

to probe all neighbor AP's information, including

signal strength once. Then MN sends HCFReq

message directly to HCF to report the information of

its neighbor AP.

3) Receiving the HCFReq message, HCF decides

whether or which AR/AP MN shall associate with.

The choice of AR/AP is based mostly on the signal

strength that MN receives and AR/AP’s loading. For

example, if the number of registered MNs in one AR

or AP has reached a limit, HCF will not accept MN

to move to that network. After making the decision,

HCF sends the HCFRep message to MN. HCF

notifies MN about new AR/AP's information, such

as link prefix and AR's address. The information

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will help MN to obtain a new CoA before it attaches

to the new AR/AP.

4) After MN receives the HCFRep message, it

knows that which AR/AP it will associate with and

will configure its new CoA. Once MN attaches to

the new AP and finishes the new CoA configuration,

it sends the CEInf message to HCF.

5) After receiving this message, HCF begins to

send the buffered packets to MN’s new CoA. Thus,

HCF sends the message HFCon to end the handover

procedure.

As shown in HCF procedure, a MN can obtain

its new CoA before it really attaches to the new

AR/AP, so handover is optimized to reduce

handover latency and packet loss.

4 HANDOVER FOR NEMO WITH

MULTIPLE MOBILE ROUTERS

To support handover optimization for network

mobility, we define a new architecture (figure 4). In

this architecture, we introduce a local Intelligent

Control Entity called ICE which should be capable

of controlling several ARs and the MRs attaching to

these ARs. In our case, the ICE can manage

handover, but it can also be used in the future for

managing connections, resources, QoS, security, etc.

An ICE domain contains an ICE and several ARs.

The ICE should possess the information of each its

ARs, like capacity, preferences, etc. Once an MR

attaches to an AR within an ICE domain, the ICE

should also collect this MR’s information, this

information is sent by the MR which knows the

ICE’s address by AR’s router advertisement

message. The information includes MR’s home

address, MR’s CoA, address of the attached AR,

capacity, preference, etc.

Our mechanism for handover optimization is

conceived for the case of multiple mobile routers

installed in mobile network. As illustrated in figure

4, if MRs are located separately in the mobile

network, as the mobile network moves, the MR at

the right end of the mobile network (MR1) will

perform handover firstly, when two others connect

always to previous AR (AR1). When the behind

MRs begin to perform handover, the MR at the right

end of the mobile network should complete its

handover. So in any time, it can exist at least one

MR which does not perform handover, and which

can transport the traffic addressed to the MRs being

performing handover. However, if the distance

between the MRs is not long enough, all MRs can

perform handover overlapping in time, so the

distance impacts the performance of handover

proposed by our mechanism.

When the MR1 (in figure 5) begins to perform

handover, as a response to some link-specific event

Figure 4: Our new NEMO architecture with 3 MRs.

(L2 “trigger”), which anticipates the handover, the

MR1 sends the handover initiate message to its ICE.

Upon receiving this message, the ICE knows that

MR1 will begin the handover, it should, according to

the collected information (capacity, preference, etc),

choose another MR which is at the same mobile link

and which is the best candidate to transport the

MR1’s traffic. If ICE can not find any candidate

MR, it returns a handover response message

indicating that the handover procedure has failed.

After having chosen the candidate MR (for example,

MR2 in figure 5), ICE sends a traffic transfer

message to MR1’s previous AR (AR1), to inform it

to transfer the packets addressed to MR1 to MR2.

From now on, AR1 encapsulates the packets

addressed to MR1 and sends them to MR2, the latter

decapsulates these packets and sends them to MR1

through the mobile link. To indicate whether the

procedure of establishment of the tunnel is achieved,

AR1 sends a handover response message to MR1. If

AR1 succeeds the establishment, this message must

contain the identity of the candidate MR (the

identity of MR2), this identity enables the MR1 to

know which MR transports its traffics. If not, the

MR1 knows that the procedure of establishment of

the tunnel has failed, and NEMO basic operation is

performed. If MR1 does not receive this message

over a certain time period, MR1 considers that the

establishment of the tunnel fails. In the case where

MR1 has disconnected to AR1, this message should

be sent to MR2, MR2 then transmits it to MR1.

Figure 5: MR1 performs handover.

HANDOVER OPTIMIZATION FOR HOST AND NETWORK MOBILITY

In the opposite direction, MR1 should send the

packets from the mobile network to Internet to MR2

until it completes the Binding Update, MR2 should

forward these packets to AR1. This procedure

enables all packets from the mobile network to

Internet to be transmitted without buffering even

MR1 at the handover state. Additionally, this

procedure ensures that packets are not dropped due

to ingress filtering. When MR1 has established the

connection with the new AR (AR2), i.e. it obtains

the new CoA, it involves the procedure of Binding

Update, which similarly as in NEMO. At the same

time, it performs the information registration

procedure as described above. Once the MR1

receives the Binding Update Acknowledgement

message, it stops transmitting the packets from

mobile network to Internet via MR2.

As described above, if it exists at least an MR at

non handover state (does not perform handover) at

any given time, the packets can always be

transported without being buffered during handover.

5 CONCLUSION AND

PROSPECTS

In this paper, we propose mechanisms to enhance

handover operation both in host and network

mobility. In the case of host mobility, HCF scheme

is proposed without modifying the part of classical

Access Router and Access Point in MIPv6. This new

HCF scheme allows MN to get the new CoA and to

launch Binding Update procedure before moving to

the new AR/AP. Moreover, the omission of DAD

process optimizes greatly handover performance.

Furthermore, by the means of buffering the traffic

during the layer 2 handover processes, then

resending to the new AR/AP attachment by HCF

after the layer 2 handover, the packet loss could be

minimized and overcome.

For NEMO mobility, our proposition provides

not only the minimization of delay and packet loss,

but takes into consideration the resource

management as well during handover. Additionally,

our protocol is compatible with the ingress filter

policy. However, since during handover, one MR

may transport other MRs’ traffics, the capacity of

the MRs should be designed to be large enough.

This paper details our primary concepts and

mechanisms. In the next step, our mechanisms will

be simulated and evaluated by using simulation

tools. In the future, we intend to investigate the other

MIPv6 or NEMO issues, such as cross-layer

solution, security aspects and QoS support taking

benefit of using the introduced new intelligent

entities to better manage the mobility and the

network resource.

ACKNOWLEDGEMENTS

This work is supported by the international project

PRA-SIP under Grant SIP 04-03.

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