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(P2-23) A Novel Seamless Vertical Handoff Solution
Wei-Cheng Xiao, Shih-Hsuan Tang, Ling-Jyh Chen, and Cheng-Fu Chou
Abstract--In this work we propose a seamless vertical handoff
solution. We realize the seamlessness by adding a virtualinterface module into current Linux kernel. Different from the
mobile-IP based-vertical-handoff systems, there is no need of
extra agents or handoff servers in our system. In the experiment,
we show that the handoff latency is so short that it is
unnoticeable to the applications. Rebuilding transport layer
connections is also not required during the period of the handoff.
Our system is simple and can be applied to current IP network.
Therefore, it is instantly ready for real-world deployment.
I. INTRODUCTION
Nowadays and in the future, portable devices tend to be
equipped with multiple network interfaces. On most laptops,
wired and wireless LAN NICs have become basic components.
In addition, there are also some popular techniques such as
Bluetooth and general packet radio service (GPRS) supported
by some mobile devices. If these devices are to connect to
various networks and perform a continuous data transmission
whatever location they roam to, technique of vertical handoff
is needed. That is, different from horizontal handoff in which
handoff occurs among networks of the same type, in vertical
handoff, handoff can occur among networks with different
types.
In vertical handoff, an important design principle is
seamlessness. "Seamlessness" is achieved in the sense that
handoff is unnoticeable. First, in our system, the handoff
latency can be significantly short, i.e., 2ms 9ms, which is
unnoticeable to users and many applications. Second, different
from other handoff solutions in which some agents or handoff
servers are required, only with a mobile host can the handoff
process be done in our solution. It has an advantage of
preventing overloading on the agents or handoff servers.
Through experiments, we show that the handoff process can
be done in a very short time and that the current TCP
connections can still be kept.
Our system is based on the assumption that handoff only
occurs on networks with multiple network access methods
(i.e., soft handoff), in which the waiting time in bringing up an
interface can be ignored.
We integrate our handoff procedure directly into the Linux
kernel; that is, the handoff procedure can help the kernel
determine which network interface should be used to send
data packets. In addition, we create a virtual interface and set
it to be the default interface in the routing table. All packets
passing the virtual interface are then forwarded to the physical
interface chosen by the decision module, which will be
introduced later. Because we only modify the link layer
functions, the vertical handoff is network-layer-independent.
Therefore, the mobile host can keep all its connections and IP
address after handoff. Moreover, all the processes are run in
the kernel level, the handoff latency can be significantly short.
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II. RELATED WORK
A. Mobile-IP based vertical handoff
The handoff problem in IP networks can be considered as a
special issue of the broader mobility management problem. In
previous research [1][2][3][4][5], mobile IP is the most
general solution. In mobile IP, there are home and foreign
agents running on different networks. Every mobile host
roaming among these networks has it own home address and
care-of-address. The former address is to identify which
network the mobile host belongs to while the latter is related
to the network in which it is currently located. When a mobile
host is roaming to a network with physical link property
different from current network, vertical handoff occurs. Then,
the home agent acts as a proxy server to relay all the data
packets intended for the mobile host to the foreign agent
through an IP tunnel (IP-in-IP encapsulation) established
between these two agents. It is an intuitive and simple way to
migrate the idea of mobile IP to vertical handoff. However, it
needs help of the agents to keep the data transmission, and this
may cause overloading on the agents.
B. Handoff-server Based vertical handoff
Another solution is to introduce handoff servers to realized
vertical handoff. In the USHA [6] system, all mobile hosts
connect to the Internet with the help of a handoff server,
which is equipped with multiple network interfaces with
heterogeneous physical properties. Hence, mobile hosts can
communicate with the handoff server in various physical
connections. The IP tunneling technique (IP encapsulation) is
used in USHA with the handoff server functioning as one end
and the mobile host as the other. Upper layer communications
are bounded to a virtual interface - the tunnel interface,
instead of physical interfaces. All data packets are transmitted
through this IP tunnel. When the handoff event occurs, the
underlying physical connection of the virtual tunnel is
automatically switched to the new physical interface. In the
meantime, the mobile host also notifies the handoff server of
its change in physical connection. However, USHA tries to
keep the upper layer TCP connection during the vertical
handoff, and then the data packets can only be transmitted
using UDP connections between mobile hosts and the handoff
server. This causes new problems. In addition, similar to the
mobile IP scheme, the handoff server may be overloaded as
the network scale is getting large.
III. SEAMLESS VERTICAL HANDOFF
In this work, we assume that the IP address of the virtual
interface can be accepted in all the networks a mobile host
connects. In current Internet, this assumption can only be
realized in a local area network; that is, all these
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Fig. 1. The concept of our system design in the OSI network architecture.
Fig. 2. Throughput in MB/s before and after handoff (Handoff occurs at 5.Os.)
heterogeneous networks share the same gateway.
The concept of our system design is shown in Fig. 1. We
introduce a virtual interface layer between the network layer
and the data link layer and integrate this layer into the Linux
kernel. The virtual interface layer includes two components the virtual interface and the kernel interface selector. The
virtual interface is assigned a unique and static IP address.
Additionally, it's the only network interface that upper layer
applications can see. All kernel routing rules are associated
with the virtual interface. The kernel interface selector is
responsible for deciding which physical interface that should
be used in reality to send and receive data packets. In packet
transmission, all packets generated from the applications are
directed to the virtual interface and encapsulated with an IP
header, which contains the IP address of the virtual interface.
Then, these packets are forwarded to one of the physical
interfaces, according to the selection result of the kernel
interface selector. Finally, they are sent out via the selected
physical interface. In packets reception, packets received from
a physical interface are forwarded to the virtual interface,
decapsulated the IP headers, and then passed to upper layers.
When handoff occurs, the kernel interface selector sends an
ARP message, which contains the mapping of IP address and
the MAC address of the newly selected physical interface, to
the next hop host - probably the default gateway or another
host in current local area network. The host receiving this
ARP message then updates it ARP table so that it can use the
correct path to transmit data to the mobile host. Because the
handoff procedure is completed in the kernel, the handoff
latency is quite small, i.e., 2 9ms.
interfaces: one is a PCI Gigabit Ethernet NIC and another is a
D-Link DWL-AG660 802.1 l ab g Wireless NIC.
Next, we describe the scenario of the testing experiment as
follows. In the beginning, the mobile host downloads a file
from the FTP server via the 802.1 lb wireless link. At 5.Os, the
virtual-interface module detects a better path for the system
and initiates a vertical handoff from 802.1 lb wireless link to
the gigabit link. After that, the mobile host uses the gigabit
link to continue the FTP downloading. The ethereal is used to
collect the packets of the FTP connection.
Fig. 2 shows the results of the FTP connection, where the
X-axis is the experimental time (in second) and the Y-axis is
throughput (in MB;s). We note that when the vertical handoff
occurs at 5.Os, the FTP connection still runs smoothly without
noticing the change of the interface. Moreover, the handoff
latency is very small, i.e., from 2 to 9ms. That is, our virtualinterface module not only can provide a seamless solution for
the vertical handoff but also is transparent to the applications.
Another important feature of our approach is that the virtualinterface module can perform the vertical handoff well
without the assistance of the additional agents or handoff
servers. Therefore, we believe that such virtual-interface
approach is a promising method to provide the solution for the
vertical handoff problem and immediately ready for realworld deployment.
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IV. EXPERIMENTAL RESULTS
In this section, we use an experiment to illustrate how our
virtual-interface scheme helps a FTP connection smoothly and
seamlessly handover from one interface to another interface.
Of course, other type of connections can still benefit from our
virtual-interface scheme as well.
We implement the virtual-interface module in a laptop with
Intel Pentium M 1.73 GHz CPU and 512MB Memory. The
operating system of the laptop is Debian Linux with the
2.6.12.6 Kernel Version. There are two types of network
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