Paper Submitted on “Mobility Management” at the ACE04 Technical conference conducted in Larsen & Toubro InfoTech Limited.

1. Mobility Management
ABSTRACT
Rajasekaran.S, Vivek P Communications & Embedded Systems, L&T Infotech
Mobile devices will become the predominant user terminal in near future. Several problems
will arise in offering mobile services to a host roaming from one domain to another. Many
protocols have been proposed over the years to provide seamless services to mobile users
targeting different layers in the protocol stack.. We discuss the strengths and weaknesses of
mobility at three different layers (Transport layer, Network Layer and Application layer) of
the protocol stack, concluding that the Network layer mobility scheme is likely to suit mobile
Internet users best
INTRODUCTION As shown in figure 1, the USER’s
IP address known to HOST is ZZ when
Nowadays more and more people the USER was in Subnet A. Now when the
enjoy the advantages of Internet services user moves to subnet B the USER has
by carrying their mobile workstations acquired a new IP address YY. The HOST
(Laptops, mobile phones, and personal is unaware of this new IP Address. So the
digital assistant (PDA)) thus overcoming USER has to inform the HOST about his
the barriers of time and distance. Indeed, it new IP address, practically this is not a
is easy to foresee that a mobile user would feasible solution. So some mechanism
like to access Internet technology anytime should be provided for the USER to be
and anywhere, and also connect to fixed reachable.
broadband networks, wireless LANs, or In IP networks, routing is based on
mixture of 2G, 3G and 4G technologies. stationary IP addresses, similar to how a
postal letter is delivered to a fixed address
on an envelope. A device on a network is
reachable through normal IP routing by
HOST the IP address it is assigned on the
network. However, problems occur when a
device roams away from its home network
and is no longer reachable using normal IP
Subnet A
routing. This causes the active sessions of
the device to be terminated.
Meaning of Mobility
Subnet B
Mobility means the ability of a
Moves to SubnetB Host to overcome the location dependent
nature of IP addresses by a suitable
USER USER
translation mechanism, and to send and
IP ADDR : ZZ IP ADDR : YY receive data efficiently from any location.
The user should not be required to restart
applications or in the worst-case reboot,
Figure 1: when visiting a new network
Illustrating Non- Reach ability when USER
moves to a new Subnet

2. Why do we need mobility? Network Access Identifier. The user is not
mandated to use a specific terminal in
Mobile Users have to face several order to connect her “home” network.
problems that makes roaming with Internet Instead the user may change terminal from
enabled mobile devices difficult. Problems time to time and still get access to the
arise when somebody disconnects his same network services without being
mobile device from the Internet in order to required to go through annoying and
connect elsewhere. Normally she would troublesome configuration procedures
not be able to continue communication each time she changes terminal equipment.
until she configures the system with the Service Mobility: Allowing users to
new IP address, the correct netmask, and a maintain access to their services even
new default router. while moving or changing devices and
network service providers is termed as
Flavors of Mobility Service Mobility
There are two flavors of Mobility:
Micro Mobility: Micro Mobility refers to APPLICATION L AYER MOBILITY
movement of nodes within the same
subnet. Application Layer Mobility
Micro Mobility: Macro Mobility refers to provides mobility of users. i.e. users can
movement of nodes between different use different terminals. Application Layer
subnet Mobility is based on SIP (Session
Initiation Protocol). SIP supports personal
Forms of mobility mobility as part of its signalling
mechanism and provide adequate means of
We shall see the different terminal, and service mobility.
definitions of host mobility, not all of
them will be useful for our discussion. Application layer mobility
Mobility (Micro / Macro Mobility) can be mechanism doesn’t require any changes in
divided, rather seen in different forms as the Mobile Hosts kernel as compared to
network layer and transport layer
Terminal Mobility: The ability of a mechanisms. However, Application layer
terminal, while in motion, to access mobility introduces Handoff Delay and
telecommunication services from different Signaling Overhead. Application layer
locations, and the capability of the mobility is being considered for next
network to identify and locate that generation heterogeneous networks
terminal is referred to as Terminal because of the fact that application layer
Mobility. being the highest layer, it can function
Session Mobility: Session Mobility refers across cross networks. Also introducing
to maintaining sessions even while mobility at this layer will force all user
changing terminals moving parts of the application to become mobility conscious,
session to new devices which is not desirable.
Personal Mobility: The ability of a user
to access services at any terminal on the The detailed discussion of
basis of a personal identifier is termed as Application Layer Mobility protocol is
Personal Mobility beyond the scope of this document.
Personal (user) mobility makes it possible
for a user to use any terminal in order to
get access to IP services. The user is
identified by means of a NAI [15],

3. Like TCP, SCTP is reliable and
TRANSPORT LAYER MOBILITY offers new features such as “Multi-
Streaming” and “Multi-homing”. In
Movement between networks will particular, the multi-homing feature of
break transport layer connections. These SCTP enables it to be used for mobility
broken connections must be made to support, without any special router agents
resume transparently from higher layers. in the network.
Introducing mobility at this layer will Other features included in SCTP
result in a modification of the existing are error-free and non-duplicated data
protocols. transfer, network-level fault tolerance
through supporting of multi-homing, and
Transport layer mobility will resistance to flooding or masquerade
minimize the packet losses during attacks.
handovers by holding transmissions during As mentioned earlier, the multi-
handoffs. There is no requirement of any homing ability enables SCTP to support
new infrastructure compared to network mobility. A host is called Multihomed if it
layer mobility where deployments of HA has multiple network layer addresses (e.g.
(Home Agent), FA (Foreign Agent) and IP addresses).A transport protocol
routers for fast and smooth handover is supports multi-homing if the endpoint can
required. have more than one transport layer
addresses, as is the case with SCTP. The
The transport layer is handling the mobility comes here from the ability to
congestion control. Good Congestion change the endpoints (e.g. IP addresses)
control requires keeping data on the end- while keeping the end-to-end connection
to-end path between the hosts. Whenever intact.
the path between the hosts changes, then it
is better if the transport layer is aware The problem in SCTP is to perform
about the change so that it takes suitable these address recon-figurations
steps to adjust to the new path. For dynamically. The solution is to use the
example the transmission rate may require Dynamic address Reconfiguration
modification to avoid the packet loss. (ADDIP) [11] extension for SCTP, which
enables the SCTP to add, delete, and
The applications will have better change the IP Addresses during an active
control over their sessions, by allowing the connection.
applications to specify the network The SCTP with the ADDIP
interface to be used for transmission of extension is called mobile SCTP (mSCTP)
different traffic. SCTP provides support [10], and it provides a seamless handover
for multiple addresses per host for mobile hosts that are roaming between
(Multihoming), which can be very useful IP networks.
in providing transport layer mobility.
When the mobile node (MN)
initiates an SCTP association with the
Overview of SCTP and mSCTP corresponding node (CN). The resulting
association consists of MN’s IP address
Stream Control Transmission and CN’s IP address(the primary path).
Protocol (SCTP) [9] is an IETF proposed After a while, MN decides to move to a
standard protocol for the transport layer. It different network. The following steps [6]
is designed to eventually replace TCP and are repeated every time MN moves into a
perhaps also UDP. new location.

4. Step 1: Obtaining an IP address for new NETWORK LAYER MOBILITY
location. As MN is moving towards a new
network , at some point it reaches the Mobility can be seen as a “Address
overlapping region. Then MN obtains the translation”, i.e the IP changes from ZZ to
new IP address from the Access router YY (see figure 1) when the user moves
with the help of DHCP [5] or IPv6 address from one location to another. So the
auto-configuration. problem can be best resolved at Network
layer by defining mechanisms to hide this
Step 2: Adding the new IP address to the problem from higher layers. This is the
SCTP association .MN informs CN of the type of mobility, which has received most
new address by sending an Address attention from the research community,
Configuration Change (ASCONF) chunk. particularly with the development of
As a reply the ASCONF-ACK is sent. MobileIP in the IETF (Internet
Engineering Task Force).
Step 3: Changing the primary IP address. Mobile IP is a proposed standard
While MN further continues to move protocol that builds on the Internet
towards a new Access router , it needs to Protocol by making mobility transparent to
set the new address as its primary address. applications and higher-level protocols
The changing of addresses is done like TCP. There are two variations of
according to specific rules, for example as Mobile IP, Mobile IPV4, based on IPV4
soon as a new IP address is detected. and Mobile IPV6, based on IPV6.
However, the configuration of this change-
triggering rule is a challenging issue for Overview of Mobile IP
mSCTP.
Mobile IP enables users to keep the
Step 4: Deleting the old IP address. As same IP address while traveling to a
MN has moved to a new network , the old different network (which may even be
IP address becomes inactive, and it is operated by a different wireless operator),
deleted from the address list. The thus ensuring that a roaming individual
knowledge from underlying layers can be can continue communication without
used to determine when the address sessions or connections being dropped.
becomes inactive. It is worth noting that if Mobile IP is designed to solve the problem
CN is initiating the association towards the of Macro Mobility Management. It is less
MN, a location management scheme is well suited for micro mobility
needed. Mobile IP can be used, for management. For our further discussion
example, for CN to find the current we will consider only Mobile IPV4.
location of MN and to establish an SCTP
association. After the association is Mobile IP Terminology
successfully setup, the mSCTP will be
used for providing seamless handover as Mobile Node(MN)
discussed above. But this approach is not A host or router that changes its point of
independent by itself. i.e. the transport attachment from one Network or sub
layer approach requires more co-operation network to another.
from other layers. The Mobile Node is a device such as a cell
phone, PDA, or laptop whose software
Enables network roaming capabilities.
Home Agent (HA)
A router on a mobile node’s home network
which tunnels datagram’s for delivery to

5. the mobile node when it is away from protocol. Diameter provides the
home, and maintains current location functionalities such as NAI based user
information for the mobile node. authentication, Dynamic Home Agent
Allocation in etc.
Foreign Agent (FA)
A router on a mobile node’s visited AAAH
network, which provides routing services Diameter Server in Home Network
to the mobile node while, registered. The
Foreign agent detunnels and delivers AAAF
datagram’s to the mobile node that were Diameter Server in Foreign Network
tunneled by the mobile node’s home agent.
For datagram’s sent by a mobile node, the Mobile IPV4 operation
foreign agent may serve as a default router
for registered mobile nodes. Mobility agents ( HA and FA)
advertise their services in response to
Correspondent Node (CN) agent solicitation message from MN and /
A peer node with which MN or periodically advertise their services on
communicates. CN may be stationary or the local subnet. Based on these
Mobile. advertisements MN determines its location
( Home Network or Foreign Network). If
Home Link it is at home network, it will operate
The link on which the nodes’ Home subnet without the mobility services. When the
prefix is defined. Standard IP routing MN moves into a foreign network it
Mechanisms will deliver packets destined obtains a new IP address (“Care-of
for a node to its home link. Address”) on the foreign Network. The
COA can be obtained in two ways:
Mobility Agent
Either a Home Agent or a Foreign Agent. 1. From FA’s Advertisement:
The IP address advertised by the
Agent Advertisement FA (through Agent advertisement) can be
An agent advertisement is an ICMP router used by the MN for registering with the
advertisement that has been extended to Home Network. This is nothing but the IP
also carry a mobility agent advertisement address of the FA.
extension.
2. External assignment:
A “Co-located Care-of Address”
can be acquired by the MN by external
Home Network mechanisms such as DHCP [5], or may be
A network, possibly virtual, having a owned by the MN as a long-term address
network prefix matching that of a mobile for its use only while visiting some foreign
node’s home address. network.
Foreign Network Using Co-located Care-of Address
A network other than the MNs Home has the advantage of removing FA from
network. the infrastructure. However, IPV4
addresses are limited in number there by
Diameter placing constraint on the number of Co-
AAA (Authentication, authorization, located Care-of Addresses.
accounting) protocol [8]. Diameter is the
successor of the well know RADIUS

6. serving the MN, the AAAH. The AAAH
authenticates and authorizes the MN and
forwards the registration request to the HA
Home Netowk serving the MN. The HA responds with a
CN
Registration reply which takes the same
HA
path backwards. The AAAH can
T optionally allocate a Home Agent for an
U MN, which has not been configured with
1 MN requests services
N 2 FA relays it to HA
the HA IP address. Otherwise the HA
N 3 HA Accepts or denies itself can handle the home agent allocation
2 3 4 FA relays reply to to such MN’s.
E
MN
L
2. The FA can directly send the
registration request to the HA. The HA
contacts the AAAH for authenticating the
MN and relays the reply to FA.
FA 4
MN
The detailed discussion related the
1 AAAH and AAAF are beyond the scope
of this document. The Registration reply
will contains the status of the registration
Figure 2: MN in Foreign Network (Success / Failure, and the reason for
failure). The FA relays the registration
If the MN is using Co-Located reply to the MN. On Successful
Care-of Address, it can interact with the registration the MN is granted a
home HA directly. (Except when the FA “Lifetime”, the period for which the MN
has not forced the MN to send messages can use the services from the foreign
via FA. This is done in some networks to network.
enforce local policies on the visiting node)
The HA will maintain the list of
After acquiring the new IP address MNs roaming outside the home network
as mentioned above, the MN must register and information which includes the
this new address with the HA. The MN registered COA and services granted to the
send a Registration Request to the HA. MN. On expiry of the lifetime the MN has
The registration request message is used to to issue a new registration to the HA if it
register the MN’s COA with the Home wishes to continue using the services.
network and also to request services like
reception of Broadcast packets from the Mobile IP uses protocol tunneling
Home Network, Reverse Tunneling, to hide the MN’s address from
Encapsulation type, etc. intermediate routers between its home
For our discussion let us assume the MN is network and its current location. Now any
using FA COA. Now the MN sends the datagram’s destined to the MN will be
registration request to the FA and intercepted by the HA in the home
depending on the network configuration, network. The HA tunnels the datagram’s
the FA can relay it in two directions. to the MN’s COA. This mechanism is
referred to as “Forward Tunneling”.
1. The FA relays it to the AAA server in
its domain (AAAF) which based on the
realm part of the MN’s NAI(Network
Access Identifier) contacts the AAA server

7. How does the HA intercept Packets home network update their ARP cache
destined to MN? with the hardware address of the HA
against the IP address of the MN. Thus the
HA makes use of Address Resolution HA fools other nodes about the location of
Protocol (ARP). ARP is used to resolve a the MN. Any node wanting to send the
target nodes link-layer address from its IP packet to the MN will send it to HA. The
– address. In general every node maintains HA then tunnels the packets to the MN.
an ARP table containing IP addresses and
the corresponding Hardware addresses When the MN is away from the
(MAC addresses). So when it has to send a home network any ARP requests seeking
packet on the home link it sends it over the MN’s MAC address is answered by the
Ethernet by looking up the ARP table for HA on behalf of MN. This is called
MAC address of the required destination “Proxy ARP”.( Figure 3)
IP address. When the MN returns back to the home
If there is no entry for the concerned IP network, it issues a de-Registration
address in the ARP table the node Request to the HA and broadcasts a
broadcasts an ARP request packet, only Gratuitous ARP packet on the home Link.
the concerned node whose MAC address The HA also broadcasts the gratuitous
is requested replies for the request. Now ARP packet, this time with the original
the Packets are delivered to this Hardware MAC address of MN. This will be useful
address. in wireless networks where the coverage
Range Of HA will be more compared to
MN.
Why tunneling?
The application must always see a
single IP address at all times, inorder to
exist TCP connections alive while
handovers. Therefore IP tunnels are used.
The tunneling mechanisms used are
Figure 3: Home Agent sending Proxy ARP 1. IP encapsulation within IP
2. Generic Routing Encapsulation
3. Minimal Encapsulation within IP.
Tunneling includes adding an outer
header to the existing packet, which as a
whole becomes the payload for this new
header as shown in the figure 5
Figure 4: Home Agent Broadcasting Gratuitous
ARP
On accepting the registration
request from the MN, the HA broadcasts
Gratuitous ARP packet(with HA’s MAC
address and MN’s IP address, Figure 4)
the home link, so that all nodes in the

8. HA CN
IPHEADER Original
SRC: MNs Home Address Packet
DST: CNs IP address
PAYLOAD
IPHEADER
SRC: MNs COA Encapsulated
FA
DST: HA IP address Packet
PAYLOAD
IPHEADER
SRC: MNs Home Address MN
DST : CNs IP address
PAYLOAD
Figure 6: Tunneling in MIPV4
Forward Tunnel
Figure 5: Encapsulated Packet
Reverse Tunnel
When the MN needs to send the
packets to CN, the MN will use its Home Triangular Routing
IP address as source address and CN’s IP
address as destination. The MN's Home
address prefix is not valid in the foreign Route Optimization
network. Usually routers are configured to
drop packets originating from a The triangular routing is due to
topologically incorrect subnet. So packets (from CN) being sent through the
“Reverse Tunneling” is employed where HA and HA tunneling it to the MN.
in any packets from MN to CN follows the Instead the HA can update the CN about
path: MN FA HA CN. the MN’s Location so that it can directly
send packets to MN. This is not preferred
The Path Followed by the because MN’s privacy is being touched
datagram’s is shown in figure 6. The path upon.
takes the shape of a triangle. The There is a problem in using Route
“Triangular routing“ problem in Mobile optimization, i.e. when the MN moves out
IPV4 is of serious concern as the distance of the Foreign Network before the expiry
between the HA and FA increases. To of the “Lifetime” the HA is unaware about
solve this problem “Route Optimization” its movement. During this time if CN
can be used wants to communicate with the MN, it will
not be possible for CN's to exchange
packets with MN.

9. Open issues in Mobile IPV4 The major upside to implementing
mobility support in the IP layer is that
Inefficient Routing since it is at the waist of the protocol stack
We have discussed the triangular routing hourglass model, it is the one place where
and route optimization above. But Route mobility support can benefit every higher
optimization requires changes in the CN layer. This is not only beneficial from the
which is not desirable. standpoint of minimizing reproduction
Of effort, but also in limiting potential
Security issues bugs or security concerns
A great deal of attention is being focused Table1 gives a comparison of
on making Mobile IP coexist with the mobility on transport layer and Network
security features coming into use within layer.
the Internet. Firewalls, in particular, cause Having see the Pros and Cons of mobility
difficulty for Mobile IP because they at various layers, we conclude that
block all classes of incoming packets that implementing mobility at network layer is
do not meet specified criteria. Enterprise the best option.
firewalls are typically configured to block
packets from entering via the Internet that Transport/Application Network Layer
appear to emanate from internal Layer
computers. Although this permits Network layer needs to Same IP
management of internal Internet nodes be identified with a new address during
without great attention to security, it IP address at every the lifetime of
presents difficulties for mobile nodes point of new one IP level
wishing to communicate with other nodes attachment session
within their home enterprise networks. Simple Internet Routing Complexity in
Such communications, originating from (No Tunnels) network layer
the mobile node, carry the mobile node's End user Applications Not required
home address, and would thus be blocked have to be mobility Slight
by the firewall. aware fluctuations
when changing
CONCLUSION to a new access
technology or
Introducing mobility at application access network,
layer has the disadvantage that end user but commodity
applications must be aware of mobility. applications and
So keeping mobility out of application TCP sessions
layer will simplify end user application will survive.
design
The transport layer approach Table 1: Comparison of Mobility at Transport layer
requires more cooperation between layers and Network Layer.
than the network layer approach, as the
location management functions are
handled separately. In some sense, this is
more of a cross-layer or interlayer
approach than purely a transport layer
approach. The transport layer, however, is
the only place where protocols may
require significant modifications.

10. REFERENCES [12] Perkins, C., “IP Encapsulation within
IP”, RFC2003, October 1996
[1]. Wesley M. Eddy, “At What Layer
Does Mobility Belong?” IEEE [13] Perkins, C., “Minimal Encapsulation
Communications Magazine. October 2004 within IP”, RFC2004, October 1996
[2]. H. Schulzrinne and E. Wedlund, [14] Hanks, S., Li, T., Farinacci, D. and P.
"Application-Layer Mobility using SIP", Traina, "Generic Routing Encapsulation
ACM Mobile Computing and (GRE)", RFC 2784 March 2000.
Communications Review, Vol. 4, No. 3,
July 2000, pp. 47-57.
[15]B.Aboba, M. Beadles “The Network
[3] C. Perkins, Ed., "IP Mobility Support Access Identifier.” RFC2486, January
for IPv4", IETF RFC 3344 (Proposed 1999
Standard), Aug 2002.
[4] http://www.its.bldrdoc.gov/
[5] Droms, R. “Dynamic Host Biography of the author(s).
Configuration Protocol”, RFC2131, March
1997 Rajasekaran. S (Project Leader)
Larsen and Toubro InfoTech,
[6] Nilanjan Banerjee, Wei Wu, and Sajal Communications Department
K. Das. “Mobility Support in Wireless Vivek Purushotham, (SET), Larsen and
Internet” IEEE wireless Communication Toubro InfoTech,
October 2003 Communications Department
[7] http:// www.ipunplugged.com
[8] P. Calhoun, H. Akhtar, J. Arkko,
E.Guttman, A. Rubens, “Diameter Base
Protocol”, RFC 3588, September 2003.
[9] Stewart, R., Xie, Q., Mornmeault, K.,
Sharp, H., Taylor, T.,Rytina, I., Kalla, M.,
Zhang, L. and V. Paxson, "Stream Control
Transport Protocol", RFC 2960, October
2000.
[10] Mika Ratola, “Which Layer for
Mobility? - Comparing Mobile IPv6, HIP
and SCTP” Seminar on InterNetworking
[11] [AddIP] Stewart, R., et al., "Stream
Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", Work
in Progress.