MobCast: Overlay Architecture for Seamless IP
Mobility using Scalable Anycast Proxies
Christopher P. Lee, Keshav Attrey, Carlos Caballero, Nick Feamster, Milena Mihail, John A. Copeland
Schools of Electrical and Computer Engineering and Computer Science
Georgia Institute of Technology
Atlanta, Georgia 30332–0250
{chrislee@, keshav@, feamster@cc., mihail@cc., copeland@ece.}gatech.edu, ccaballero@iss.net
Abstract— We propose a routing overlay system, MobCast,
for simple and efficient routing to mobile hosts. Mobcast nodes
advertise the same address space at each proxy location, and
each mobile host is assigned a “universal” IP address from this
address space, so packets sent to a mobile host’s universal IP
address automatically go to the nearest proxy on the overlay.
The overlay then delivers the packets to the mobile host.
Our architecture enables seamless mobility for both micro
and macro mobility. While our initial design is not as mature
as Mobile IP, it shows great promise to solve the traditional
problems of ingress routing, firewalls, NATs, and rapid mobility
with much lower complexity. We present our design as a scalable
and deployable alternative to Mobile IP.
In this paper, we focus on describing the MobCast system
architecture. We form our arguments for scalability, handoff-
speed, and simplicity, and give our initial results for scalability.
We postpone a detailed discussion of MobCast’s security model
for future work.
I. INTRODUCTION
Ubiquitous wireless access has changed the way we live and
work, but most common internet applications, such as email
and web browsing, would not benefit from seamless mobility.
A person can travel between wireless access points at different
coffee shops, changing IP addresses each time, and still access
his email at each one. However, seamlessness is required
when using time-sensitive media streaming applications such
as voice over IP (VoIP) and video conferencing. As technology
enables seamlessness for these application, other applications
will arise that utilize this technology. The basic problem of
seamless mobility is that a TCP connection is not defined
between hosts but between IP addresses that identify points of
connectivity to the Internet, and changing IP addresses breaks
active TCP connections.
A. Mobility Analogy
Let’s provide an analogy using the postal mail system.
Bob’s firm is in Atlanta, but he has customers internationally
that correspond with him through the postal service. Without
constantly updating his customers on his address, how can Bob
exchange mail with his customers efficiently while traveling?
Mobile IP’s solution is to have Bob’s customers (the corre-
sponding nodes) to mail him at his Atlanta address, where his
firm (his home agent) receives his mail and forwards it to his
current location (his care-of address). Bob simply has to keep
his firm up-to-date on his whereabouts.
A frequent problem is that when Bob is in Hong Kong, his
customers in Hong Kong mail him via his firm in Atlanta,
which takes too long. Bob would like to mail back these
customers directly, but then he must use a Hong Kong “from”
address rather than his permanent Atlanta address, or else
the Hong Kong postal system will reject the mail (ingress
filtering). If Bob’s customers receive mail from a Hong Kong
address (instead of his Atlanta address), they cannot be sure
it is really him. But he does not want to mail his customers
via his Atlanta office.
Our solution addresses these problems. With MobCast,
Bob’s firm purchases a permanent forwarding address for Bob
at International Mail, a postal mail service with branches
around the world. Bob supplies all his customers with his per-
manent forwarding address, e.g., ”Member #525, International
Mail,” and he informs International Mail whenever he moves.
His customers can reach him by sending mail to their nearest
International Mail branch, which forwards the mail to Bob,
wherever he is. So when Bob is in Hong Kong, mail from
Bob’s Hong Kong customers reach him via the Hong Kong
branch of International Mail. When Bob replies to them, he
also sends his mail via the Hong Kong branch of International
Mail. Since the mail comes from ”International Mail,” his
customers know that it is still the same person.
II. RELATED WORK
Mobile IP [7] is the foremost solution for enabling seamless
mobility without altering the protocol stack. The mobile node
associates with its home agent and uses an address allocated
by the home agent to form connections. When the mobile
node moves to a foreign network, it needs a care-of address to
continue communications. A foreign agent forms a tunnel with
the home agent and decapsulates the packets before delivering
them to the mobile node, as seen in Figure 1. Packets from
the corresponding node to the mobile node must always pass
through the home agent. Packets in the opposite direction,
from the mobile node to the corresponding node, can be sent
directly without passing through the home agent (triangle
routing).
Mobile IP is not without its limitations. First, all mobile
nodes must have a home agent [7]. The current network
must act as a home agent for them, or they must use a pre-
designated home agent from another network. Furthermore,

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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2007 proceedings.

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