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Army Research Laboratory 

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ARL-TR-758 February 2001 

Mobile IP LAN for ARL Mobile 

Communications/Nettiorking 

Testbed 

Brian B. Luu 

Computational and Information Sciences Directorate 

Approved for public release: distribution unlimited. 
Abstract 

This report describes the protocol and implementation of a Mobile IP 

(Internet protocol) LAN (local area network) for the LAN of the U.S. 

Army Research Laboratory (ARL) Mobile Communications/Networking 

Testbed. Mobile IP LAN protocol is developed to allow a LAN to be 

mobile with IP mobility. The mobility of a LAN provides mobility for its 

nodes, which are fixed relative to the LAN and are not mobile IP in 

nature. This report includes discussions and comparisons of Mobile IP 

LAN versus Mobile IP for mobile nodes. 
Contents 

1. Introduction 

2. Mobile II’ LAN 

2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

2.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

2.3 Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

3. Implementation on Linux 

4. Discussion 

5. Conclusion 

References 

Distribution 

Report Documentation Page 

4 

8 

10 

11 

13 

15 

Figures 

1. LAN when connecting normally at home network ..................... 5 

2. LAN when moving and connecting at foreign network ............... 6 

. . . 

111 
1. Introduction 

The U.S. Army Research Laboratory (ARL) Mobile Communications/ 

Networking Testbed has demonstrated the usefulness and necessity of mobility for the U.S. Army digital battlefield. However, the vast majority of 

software and network applications for the Army either rely on commercial 

off-the-shelf products or are developed and based on standard protocols 

that are not generally mobile. In particular, the local area network (LAN) of 

the ARL Testbed is Internet protocol (II’)-based and comprises wired workstations, such as Sun (Sun Microsystems, Inc.), SGI (Silicon Graphics, Inc.), 

Dell (Dell Computer Corporation), and wireless portable laptop personal 

computers. 

Recently, there have been experiments and protocols that support II’ mobility, especially Mobile IP (ME’) specified by the RFC (request for comment) 

2002 [l]. But so far;MIP has been implemented only on individual mobile 

nodes, and in some implementations, it can be resource demanding. For 

example, Dixit and Gupta’s [2] or MosquitoNet [3] implementation requires 

an IP tunnel for every mobile node. Moreover, the II? version 6 (IPv6) is 

designed to accommodate mobility. Therefore, the industry will not likely 

accept and adopt MIP for use in the current IPv4. As a result, most of the 

implementations for MIP are just experimental and available only on an 

open-source platform, such as Linux. 

On the other hand, the mobility for the standard II? nodes, which are not 

mobile IP in nature, can be supported through the mobility of the LAN. The 

Communications and Computer Science Division of the Information Science and Technology Directorate (now called the Computational and Information Sciences Directorate) of ARL has studied, designed, implemented, 

and tested Mobile II’ LAN protocol in the ARL Testbed. Mobile IP LAN is 

designed to provide mobility for II’ LANs on the Internet but still support 

Internet connectivity for nonmobile IP nodes on LANs. 

For the remainder of this report, I will discuss LANs that are P-based. For 

simplicity, the aspects of registration (the “handshaking” to establish a temporary connection change of a mobile LAN at a foreign network), 

deregistration (the handshaking to disconnect a temporary connection 

change of a mobile LAN), and security are not discussed in this report. The 

registration and deregistration are needed to improve accountability and 

network security for mobile LANs. 
2. Mobile IP LAN 

2.1 Overview 

Normally, a LAN accesses a gateway to the Internet by at least one connection through a router on the LAN. When the LAN moves to a different location with a different connection to the Internet, all the connectivity within 

the LAN still behaves normally, but the inbound traffic for the LAN continues to be routed on the Internet to its home network (the autonomous system where the LAN belongs). To reestablish the link, a special computer 

node in its home network (called home agent) accepts the traffic for the 

LAN and routes the inbound LAN traffic through an IP tunnel with the 

destination address at the new Internet connection of the LAN router. The 

outbound traffic of the LAN can be routed normally through the foreign 

network connection to the Internet. In brief, when the LAN is away from 

the home network, the traffic of a mobile LAN is redirected to an IP tunnel 

whose end nodes are the home agent and LAN router. 

2.2 Requirements 

Because of the complexity of LAN connections, not all LANs can adopt 

Mobile II’ LAN implementation. Striving for simplicity and flexibility, I 

designed Mobile II’ LAN protocol to support LANs meeting the following 

criteria: 

l Single network address determined based on netmask. 

l Ending LAN that does not route other LANs’ traffic through the LAN. 

l Single gateway connection that provides a single entering and exiting of traffic from the LAN. This implies a single default gateway for 

all nodes in the LAN. 

l All LAN nodes must be moved together as a single entity. 

2.3 Details 

The network routing of Mobile II’ LAN protocol is modeled based on the 

concept of network routing of MIP [l]. However, instead of considering IP 

mobility for individual nodes, I apply IP mobility implementation to a LAN 

as a whole, which, in turn, makes LAN nodes IP mobile. 

Basically, Mobile II? LAN protocol requires a minimum of two nodes furnished with mobility software, a home agent, and a mobile router. The mobile 

router is usually the router of a mobile LAN. A foreign agent [l] can be 

implemented, but it just complicates the network routing and reduces the 

network efficiency. Generally, the mobile router should have two network 

interface cards (NIC): one that connects to the mobile LAN, called LAN 

NIC, and the other that connects to the gateway LAN (home network), called 

gateway NIC. 

2 
Mobile IP LAN operation happens when a LAN is moved and the gateway 

NIC of the LAN is connected to a new point of attachment on the Internet. 

Through its gateway NIC, the LAN’s mobile router tries to acquire an II’ 

address (a care-of address) at the new location through Dynamic Host Configuration Protocol (DHCP) mechanism or preassignment. The mobile router 

then configures the gateway NIC with all the acquired information (such as 

II’ address, netmask, broadcast address, and default gateway for routing) 

to be able to communicate on the Internet. The mobile router uses the careof address to communicate with a home agent in its home network to request the home agent to reroute all the inbound traffic of the LAN through 

an IF’ tunnel, which uses an encapsulation mechanism [4], to the care-ofaddress destination. The home agent should use an II’ routing software, 

such as Routing Information Protocol (RIP) or Open Shortest Path First 

(OSPF), to advertise the network reachability of the LAN through it (home 

agent) in its home network and spread on to the Internet. Concisely, the 

inbound traffic for the LAN is received and tunneled by the home agent to 

the care-of address of the mobile router. Receiving encapsulated packets 

from the II’ tunnel, the mobile router decapsulates (“detunnels”) and routes 

the packets to the LAN. 

On the other hand, the outbound traffic of the LAN is channeled through 

the mobile router and through a default gateway of the foreign network. A 

second IP tunnel can be created to redirect the outbound traffic of the LAN 

from the mobile router to the home agent and to let the home agent route all 

inbound and outbound traffic of the LAN. The second II? tunnel should be 

used if the foreign network environment blocks outbound traffic of unknown 

networks [5] or if the new LAN connection to the Internet is not to be 

disclosed. 

In short, for mobility, not one of the nodes in a mobile LAN, except for the 

mobile router, requires mobility software or changes in the nodes’ network 

configuration as long as the nodes have the mobile router set as the default 

gateway for their network configuration. Only the mobile router and home 

agent, which are the two end nodes of the II’ tunnel(s), need to be equipped 

with additional software to do routing, encapsulating, and decapsulating. 

3 
3. Implementation on Linux 

In this section, I describe the implementation of Mobile IP LAN on the ARL 

Testbed. For this demonstration (the feasibility of Mobile II’ LAN), there are 

no registration and deregistration of Mobile IP LAN. The Linux operating 

system (O/S) distributed by Red Hat, Inc., version 5.2, with kernel 2.0.36 

level, is chosen to implement Mobile IP LAN, since it provides all the needed 

software, such as IF’ tunneling, RIP, and OSPF. Be aware that those software 

features are not automatically built in the Linux kernel; a kernel rebuilt needs 

to be done to ensure those software modules are available for use. No additional software besides the Linux operating system is needed to implement 

Mobile II’ LAN. All connections and configuration are done manually. I 

use a preassigned care-of address at a new location for the mobile LAN. 

The LAN of the ARL Testbed mainly consists of an Ethernet network, with 

a wireless extension. The mobile router of the LAN is a laptop computer 

equipped with one wired Ethernet NIC and one wireless Ethernet NIC. The 

wireless Ethernet NIC is configured to connect to the LAN, and the wired 

NIC is configured as a gateway NIC for the LAN. The LAN NIC is wireless 

to provide flexibility in positioning the mobile router near an Internet connection point, which can be a distance away from the ARL Testbed. The 

home agent for the LAN is a desktop personal computer that is configured 

with one NIC connected to the ARL network at Adelphi Laboratory Center 

(ALC). 

For security reasons, simulated II’ addresses that conformed to the RFC 

1918 [6] are used in this report instead of the actual IP addresses used in the 

ARL Testbed. Figure 1 depicts the normal connection of the LAN to its home 

network. The mobile LAN with network identification 192.168.23.0 has a 

gateway to the home network 172.16.0.0 through the mobile router that has 

an II’ address 192.168.23.1 assigned to the LAN NIC and 172.16.27.211 assigned to the gateway NIC. The home agent resides on the home network 

with the II’ address 172.16.27.215. When the LAN is at the home network, 

its mobile router advertises to the home network its presence and its routing path with the gateway address 172.16.27.211. 

Figure 2 shows that the LAN moves and connects to a different.point of 

attachment on the Internet. The mobile router keeps the configuration of 

LAN NIC intact and configures the gateway NIC with the care-of-address 

172.30.250.99. Also the new default gateway is specified for the mobile router 

at the foreign network. The following commands are used to achieve the 

above setting on the mobile router: 

ifconfig ethl down 

ifconfig ethl 172.30.250.99 netmask 255.255.255.0 broadcast\ 

172.30.250.255 

route add default gw 172.30.250.1 dev ethl 

4 
Home agent 

172.16.27.215 

172.16.27.211 

Mobile router 

192.168.23.1 

Figure 1. LAN when connecting normally at home network. 

5 
r 

Home agent I 

192.168.23.1 

Mobile router 

172.30.250.99 

A /\ 

Figure 2. LAN when moving and connecting at foreign network. 

6 
At the home network, the home agent configures an IP tunnel with the destination address to the care-of-address 172.30.250.99 and routes the network 

traffic destined to the LAN through the tunnel. Supposedly, the routing 

software, such as RIP or OSPF, on the home agent should advertise the routing path of the LAN with the gateway II’ address of the home agent 

172.16.27.215. Unfortunately, I was unable to configure the routing software 

on the home agent to perform that routing advertisement. To overcome the 

problem, one has to create on the main gateway of the home network a 

static routing that points the routing path of the LAN to the home agent. 

The following commands are used to achieve the II? tunnel on the home 

agent: 

ifconfig tunl0 172.16.27.215 netmask 255.255.0.0 point to point\ 

172.30.250.99 

route add -net 192.168.23.0 netmask 255.255.255.0 dev tunl0 

When the LAN returns to the original attachment point at the home network, the home agent removes the tunnel; also the static routing on the 

main gateway should be removed. As a consequence, the routing path to 

the LAN at the foreign network connection is automatically removed. The 

mobile router reconfigures its gateway NIC as before and readvertises the 

routing path of the LAN at the home network. 

7 
4. Discussion 

As described in the previous section, one can implement the whole Mobile 

II’ LAN protocol just by using two Linux computer systems with an exception of a static route on the main router of a home network. I believe that the 

RIP or OSPF software on the Linux is not designed to advertise the network 

reachability of a LAN with the routing path through an II? tunnel, which is 

not a true NIC device, to avoid any routing loops. To solve this problem, 

one should modify the routing software to allow this network advertisement, especially, since the software source code is available on the Linux 

platform. 

In theory, a LAN can be moved on the Internet as long as routers on the 

Internet propagate the LAN’s routing reachability advertised by the LAN’s 

router at any new location. But for the Internet routing to be manageable, 

workable, and secure, all border gateway routers are configured to advertise only the network reachability of the networks, which belong to the organization or affiliated organizations of the routers. Exceptions might be 

made to accommodate a permanent move of a LAN but not for unpredictable moves of a LAN. For a LAN to move from one organization to another, 

border gateway routers and possibly some internal routers need numerous 

adjustments. Most likely, network administrators for those routers will adamantly oppose these requests. For example, imagine that an ARL II’ LAN is 

connected to an Internet location in Japan; ARL would have had to request 

coordination of each of the network administrators from the Defense Research Engineer Network, the military domain, intermediate domains (domains that link the military domain and the Japan domain), the Japan domain, and local Japanese organizations. It would be an impractical and 

hardly achievable attempt. Besides, this time-consuming process must be 

repeated whenever the LAN is moved. 

When a mobile LAN is connected to a foreign network without the second 

II’ tunnel, a mobile LAN can improve the LAN connectivity with the nodes 

on the foreign network by having its mobile router advertising the routing 

path of the mobile LAN on the foreign network environment. As mentioned 

previously, filtering policies on the foreign network routers filter routing 

advertisement of the mobile router, but usually only the main gateway routers or the border gateway protocol (BGP) routers perform this special filtering. Moreover, any computer nodes on the foreign network equipped with 

routing software can receive the routing advertisement of the mobile router 

directly rather than from a router. As a result, nodes in the mobile LAN are 

just one or a few network hops away from nodes on the foreign network 

instead of being numerous hops away if the mobile LAN were at its home 

network. 

At a foreign network connection, the routing path of network traffic of a 

mobile LAN with only one II’ tunnel .is asymmetric because the inbound 

traffic is tunneled from its home agent to its mobile router. The asymmetric 

8 
routing path causes dissimilar transmission time of inbound and outbound 

traffic. Because of the II’ tunnel, usually the outbound traffic’s transmission 

time is shorter than the inbound traffic’s time. To have the symmetric routing path for the traffic of the mobile LAN, the second IF’ tunnel should be 

created. A similar asymmetric routing problem occurred in the implementation of MIP, but Montenegro proposed a similar resolution in RFC 2344 

[7]. Unfortunately, I have not succeeded in implementing the second II’ tunnel for mobile IP LAN in the Red Hat Linux-kernel 2.0.36 platform. The 

problem arises in directing the outbound traffic of the LAN through a 

tunnel. 

Since all nodes of a mobile LAN are moved together, broadcast traffic within 

the mobile LAN should have no effect. At the time of this report, no full 

investigation has been done to determine any effects on multicasting in a 

mobile LAN. I believe that multicasting to the Internet should function properly on a mobile LAN as long as its mobile router supports multicast routing. With the use of a care-of address, a mobile router can participate to 

multicasting traffic at the foreign network and reroute multicast information to nodes in its mobile LAN. 

Although I have not yet tested MIP for individual mobile nodes in a mobile 

LAN that implements Mobile IP LAN, it should function properly as long 

as a foreign agent or care-of addresses are available in the LAN. 

In contrast to MIP, the home agent of a mobile LAN using Mobile II’ LAN 

does not have to be a node on the same network with the gateway network 

of the mobile LAN at the home network. Any nodes on any LANs within 

the home network environment furnished with the routing, encapsulating, 

and decapsulating software can be the home agent of the mobile LAN. 

When a mobile LAN using Mobile IP LAN is away from its home network, 

it still keeps its LAN traffic to itself. Therefore, two nodes in a mobile LAN 

still communicate normally regardless where the mobile LAN is located. 

Conversely, two mobile nodes using MIP from the same LAN must tunnel 

their traffic back and forth to their home network to communicate to each 

other even though they may be on the same foreign network. Furthermore, 

all nodes (except the mobile router) in a mobile LAN using Mobile II’ LAN 

require no additional software and reconfiguration for IP mobility, whereas 

currently, MIP can work only on a Linux platform. For example, on the 

mobile LAN of the ARL Testbed, an SGI workstation running IRIX O/S, a 

Sun workstation running Solaris 0 /S, and Dell workstations running Window 95 or Linux 0 /S functioned and communicated properly without any 

network adjustments or reconfiguration to the workstations. 

9 
5. Conclusion 

I have demonstrated the feasibility and ease of the implementation of Mobile I!? LAN. In fact, the mobile LAN for the ARL Testbed was successfully 

tested at the Internet connection at the University of Maryland University 

College. I also encountered less software development, routing management, network overhead, and computer resources for the implementation 

of Mobile II’ LAN compared to MII? Although all the nodes of the mobile 

LAN have to move together to have II’ mobility, I believe that by implementing network subnetting, a number of LAN nodes can be mobile together at a same foreign network without requiring the entire LAN to be 

mobile. 

10 
References 

1. C. Perkins, “II’ Mobility Support,” XFC 2002, October 1996. 

2. A. Dixit and V. Gupta, “Mobile IP for Linux (version l.OO),” May 1996. 

3. Computer Science Department of Stanford University, “Stanford 

MosquitoNet Project Mobile IP v4 Distribution Users Manual (release 1.0.5),” 

April 1999. 

4. C. Perkins, “II’ Encapsulation within II’,” XFC 2003, October 1996. 

5. I? Ferguson and D. Senie, “Network Ingress Filtering: Defeating Denial of 

Service Attacks Which Employ II’ Source Address Spoofing,” XFC 2267, January 1998. 

6. Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and E. Lear, “Address Allocation for Private Internets,” RFC 1928, February 1996. 

7. G. Montenegro, “Reverse Tunneling for Mobile IR” RFC 2344, May 1998. 

13 
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TITLE AND SUBTITLE Mobile IP LAN for ARL Mobile 5. FUNDING NUMBERS 

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13. ABSTRACT (Maximum ZOO words) 

This report describes the protocol and implementation of a Mobile IP (Internet protocol) LAN 

(local area network) for the LAN of the U.S. Army Research Laboratory (ARL) Mobile 

Communications/Networking Testbed. Mobile IP LAN protocol is developed to allow a LAN to be 

mobile with IP mobility. The mobility of a LAN provides mobility for its nodes, which are fixed 

relative to the LAN and are not mobile IP in nature. This report includes discussions and 

comparisons of Mobile IP LAN versus Mobile IP for mobile nodes. 

14.SUBJECTTERMS MIP, tunneling, routing 15. NUMBER OF PAGES 

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