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					               An Evaluation of Mobile IP Movement Detection
                                              Albert Hasson, Neco Ventura
                              Department of Electrical Engineering, University of Cape Town,
                                         Rondebosch, Cape Town, South Africa

                                                 {ahasson, neco}@crg.ee.uct.ac.za

Abstract—Mobile IP allows a mobile node to maintain                  It will be shown that this movement detection may last up to
IP connectivity as it migrates through different                     three seconds using certain algorithms. This delay is in
networks. When the mobile node moves from one                        addition to the delays caused by other handoff processes.
network to another, a Mobile IP handoff occurs.                      While a mobile node is undergoing movement detection, it
Although its upper-layer sessions will not be broken, this           is unable to receive IP packets. Consequently, the resulting
handoff introduces an interruption in the mobile node’s              handoff latency introduces packet losses in the mobile
IP traffic. The movement detection process, whereby a                node’s IP traffic. Such latency may be acceptable when
mobile node becomes aware that it is on a new network,               considering web traffic and when nodes are not highly
contributes significantly to the total Mobile IP handover            mobile. However, if traffic with stricter requirements such
latency and resulting packet loss.                                   as VoIP is to be supported, this movement detection delay
                                                                     and subsequent packet loss must be reduced considerably.
Recent recommendations that aim to minimise
movement detection delay are explored along with their               This paper considers Mobile IPv4[2] horizontal handoffs
implications. This paper discusses the design of a testing           (involving homogeneous media) between different 802.11b
framework that will be used to perform IP handoffs                   wireless LANs. The performance of these handoffs may be
between 802.11b wireless LANs. This framework will                   improved if the movement detection algorithms are
allow different movement detection algorithms to be                  optimised. One way of achieving this is to allow 802.11
evaluated. It is expected that by passing 802.11 link-               specific information and events to be available to the Mobile
layer information to Mobile IP, faster movement                      IP layer. Some of these optimisations are explored in this
detection can be achieved.                                           paper.

                                                                     Mobile IPv6[7] incorporates many enhancements to Mobile
                        I. INTRODUCTION                              IPv4. However, in many respects, their behaviour and
                                                                     functionality is very much the same. Mobile IPv4 will form
The IEEE 802.11 standard[3] enables the creation of highly           the basis of this study. Although many of movement
flexible wireless LANs. This has led to the recent popularity        detection schemes discussed in this paper have been
of consumer 802.11 devices. As a result, there has been a            proposed within the context of Mobile IPv6, they are
rapid deployment of small-scale wireless access networks in          equally applicable to Mobile IPv4.
airports or busy urban areas. A mobile node wishing to
connect to these wireless networks has to perform an IP-             Section 2 introduces the Mobile IPv4 protocol along with an
level configuration including the assignment of a valid IP           overview of the stages of a Mobile IP handoff. Special
address to its wireless network interface and the                    focus is placed on 802.11 link-layer handoffs and how they
configuration of a default router.                                   are incorporated into the total network handoff. In the
                                                                     following section, different Mobile IP movement detection
This IP configuration is only valid on a particular network.         algorithms are discussed. Both generic and optimised
Should the mobile node’s wireless interface connect to an            schemes presented in the literature are introduced. The last
adjacent network, the existing IP configuration becomes              section discusses the design and development of an 802.11-
invalid. Even though a physical connection to the adjacent           enabled experimental framework. The framework allows
network has been established, the mobile node will not be            handoff latency and packet loss to be evaluated. Different
able to send or receive IP packets.                                  Mobile IP movement detection algorithms will be
                                                                     implemented and evaluated based on these parameters.
Mobile IP allows a mobile node to migrate between
different IP networks without breaking any upper-layer
sessions. When a mobile node moves from one network-                                  II. MOBILE IP PROTOCOL
level point of attachment to another, a Mobile IP handoff
takes place. This handoff is composed of a sequence of               A. Mobile IPv4
stages that includes the detection of a mobile node’s
movement to the new network.                                         The Mobile IP protocol allows IP packets to be
                                                                     transparently routed, typically through the Internet, to a
                                                                     mobile node’s current point of attachment. This is achieved
The authors would like to thank Telkom SA, Siemens, the National
                                                                     through the interaction of a home agent and a foreign agent,
Research Foundation (NRF) and the Department of Trade and Industry
(DTI) for supporting this research project.                          collectively called mobility agents. While on its home
                                                                     network, a mobile node (MN) uses a “permanent” home IP
                                                                     address to send and receive IP packets. When a mobile
node connects to a foreign IP network, it is assigned an                  B. Mobile IP Handoff
additional temporary care-of address (CoA). A mobile
node’s home address reflects its identity while the CoA                   A Mobile IP handoff occurs when a MN moves from one IP
reflects its current network point of attachment.                         network to another. A Mobile IP handoff may be divided
                                                                          into the following three stages [5]:
When on its home network, a MN operates without any
mobility mechanisms. However, while a MN is visiting a                        1.   Link-layer handoff
foreign network, the home agent and the foreign agent                         2.   Mobile IP movement detection
cooperate to deliver IP packets addressed to the mobile                       3.   Mobile IP registration
node. A corresponding node uses the home address to send
packets to the MN. Any IP packets addressed to the mobile                 As these stages are performed in order, the total handoff
node’s home address are routed to the home network. If                    latency is the sum of the delays associated with each stage.
these packets arrive on the home network while the MN is                  It is assumed that a foreign agent CoA is used to configure
away, the home agent forwards these packets to the MN’s                   the MN CoA. It is also assumed that the configuration of a
current location [2].                                                     CoA does not represent a significant stage in the handoff
                                                                          process.
In order to function correctly on a visited network, a mobile
node must configure its CoA before it attempts to send or                 The first step in a Mobile IP handoff is that the MN must
receive IP packets. A mobile node may obtain a CoA from                   establish a physical connection to the new IP network. This
a foreign network using one of two modes. In the first                    is termed link-layer handoff because it involves only
mode, the MN receives a “foreign agent care-of address”                   physical and data link (layer 1 and 2) entities. Although this
directly from the foreign agent. The foreign agent supplies               stage is not strictly part of Mobile IP handoff, it can
the MN with its own IP address to use as a CoA. Using the                 significantly affect subsequent stages. When using IEEE
second mode, a MN relies on an external mechanism such                    802.11b devices to provide link-layer connectivity, a
as DHCP to receive a local IP address. This type of address               number of details need to be taken into account when
is called a “co-located CoA”.                                             analysing the handoff stages listed above.

A home agent intercepts any IP packets destined for a                     The IEEE 802.11 standard[3] was developed essentially as a
mobile node’s home address on its behalf. These packets                   wireless form of the 802.3 Ethernet standard. 802.11b
are then tunnelled by the home agent to the MN’s current                  devices offer relatively high bandwidth, up to 11Mbps
point of attachment. This is usually done by encapsulating                depending on distance and attenuation. There are two
the original packet in a new IP packet with the MN’s CoA                  operational modes offered by 802.11b devices. In ad-hoc
as destination address. The tunnelled packet is routed to the             mode, each 802.11b station can communicate directly with
foreign network. In the foreign agent CoA mode, a foreign                 any other station within range. This communication is
agent receives the tunnelled packet, extracts the original                carried out on a peer-to-peer basis. In infrastructure mode,
packet and delivers it to the mobile node. In the co-located              stations connect to a central access point (AP). An access
CoA mode, the MN receives tunnelled packets, and                          point is connected to a Distribution System (DS) usually via
decapsulates them itself. In both modes packets sent by the               a wired interface and to 802.11b stations via its 802.11b
mobile node are routed using normal IP routing                            wireless interface. The AP bridges data between its wired
mechanisms. These steps are depicted in Figure 1 below.                   and wireless interface for its associated stations. These
                                                                          entities and their interconnection are shown below.
                                                 Corresponding
                                                     Node
      Home Agent
 Binding Cache                                                                                     Distribution
  HAddr1 – CoA1                                                                         Access     System (DS)
                                                                                                                   Access
  HAddr2 – CoA2                                                                          Point                      Point
                               Internet

            Tunnel                                                                                     ESS
                                                        Dest: CoA
      Encapsulated Packet                                                                BSS                        BSS
                                                        Src: HA Addr
                               Foreign
                                                        Dest: Home Addr
                                Agent
     Mobile                                             Src: CN Addr
     Node                                                                 Fig. 2. IEEE 802.11 BSS, DS and ESS
Fig. 1. Mobile IP tunnelling (foreign agent CoA mode)
                                                                          A group of two or more communicating 802.11b stations is
The home agent maintains a mapping between a MN’s                         called a Basic Service Set (BSS). A BSS may consist of an
home address and CoA called a binding. An MN’s binding                    AP along with its associated stations. When two or more
must be updated as the MN moves through different IP                      BSSs are connected via a DS (usually an Ethernet link), an
networks to ensure that packets are tunnelled to the correct              Extended Service Set (ESS) is formed. An ESS is assigned
foreign network.                                                          an identifier called an Extended Service Set Identifier
                                                                          (ESSID) or just SSID.
An 802.11 link-layer handoff occurs when a station               In the first Mobile IPv4 RFC [1], the recommended
disassociates itself from one AP and associates itself with      maximum rate for broadcasting advertisements is once per
another. This occurs for example, when a station associated      second. This RFC was rendered obsolete by RFC 3220[2]
with the AP on the left in Figure 2 associates with the AP on    which does not recommend a particular rate. The guideline
the right. It is important to note that a station may only be    offered is that the advertisement broadcast rate should be
associated with one AP at a time[3].                             limited so as to avoid consuming significant network
                                                                 bandwidth. If an MN does not wish to wait for a periodic
Mobile IP operates on a different layer to the 802.11 link-      advertisement, it may broadcast an agent solicitation to
layer mechanisms described above. Mobile IP was designed         which mobility agents will reply with an appropriate
to be suitable for mobility over heterogeneous media and it      advertisement.
functions independently from the lower layers. Mobile IP is
therefore not aware of link-layer handoffs. This is why          There are two standard algorithms offered by Mobile IPv4
movement detection generally relies solely on network-layer      [2]. The first algorithm is based on the lifetime field
(IP) mechanisms.                                                 included in the standard ICMP portion of an agent
                                                                 advertisement. The advertisement lifetime is the maximum
The movement detection stage of a Mobile IP handoff is           time that an advertisement may be considered valid in the
entered after a mobile node attaches itself the new network.     absence of subsequent advertisements. Therefore, if a MN
In this stage a MN detects that it has moved to a new            does not receive an advertisement within the specified
network. The MN also receives relevant information from          lifetime, the MN should consider the current agent
the network that will allow it to configure its CoA and other    unreachable. This may imply that the MN has moved to a
network settings.                                                new network. If the MN has not cached any recent
                                                                 advertisements from other agents, it should begin actively
The last stage of a Mobile IP handoff is registration which is   searching for a new agent by sending an agent solicitation.
started after a MN configures its IP settings and CoA. In        The interval between successive advertisements is, at most,
this stage, the MN sends a binding update containing the         1/3 of the advertisement lifetime. In other words, a MN will
mobile’s new CoA back to the home agent. This ensures            miss three advertisements before the current foreign agent is
that the home agent is always aware of the mobile node’s         assumed unreachable. For example, when using the
current network point of attachment.                             maximum rate recommended by RFC 2002 [1] of one
                                                                 advertisement per second, the MN will only assume that
                                                                 movement has occurred after 3 seconds.
                III. MOVEMENT DETECTION
                                                                 In IPv6 networks, the Neighbour Discovery [8] protocol
IEEE 802.11b mechanisms are implemented in the hardware          allows routers to periodically broadcast their presence at an
or firmware of a particular wireless device and may not be       interval of at least 3 seconds. In order to allow faster
easily modified. These mechanisms also have to conform to        movement detection, this rate may be significantly increased
the IEEE standard. Mobile IP registration delays may be          for routers that support Mobile IPv6 nodes. The minimum
reduced by defining a hierarchical access network that           allowed interval used by these routers is between 0.03 and
handles node mobility within a particular domain[9]. Thus,       0.07 seconds [7].
an effective way of decreasing the interruption caused by a
Mobile IP handoff is to consider what optimisations may be       The second Mobile IPv4 movement detection algorithm
made to the movement detection process. This section will        makes use of optional network prefix information that may
present the standard movement detection algorithms of            be included in agent advertisements. If an advertisement is
Mobile IPv4 along with some optimisations presented in the       received from a new foreign agent advertising a new prefix,
literature. Movement detection mechanisms may be broadly         then the MN should assume it has moved to a new network.
divided into advertisement-based and hint-based [5]-[6].
                                                                 In general, Mobile IPv4 tries to delay a layer-3 handoff until
A. Advertisement-Based Mechanisms                                it is absolutely necessary. This is because of the significant
                                                                 disruption it causes to the mobile node’s IP traffic. Such
Mobile IP movement detection relies on the periodic              behaviour has been termed “lazy cell switching” (LCS) [6].
broadcasting of agent advertisements by all mobility agents.     Another movement detection algorithm is the eager-binding
These agent advertisements extend ICMP Router                    policy or “eager-cell switching” (ECS) [6]. In this case, a
Advertisement [15] messages to include mobility-specific         MN will consider any previously unheard router
information. In addition to the standard ICMP fields such        advertisement to be an indication that a network-layer
as on-link router address, an agent advertisement includes a     handoff has taken place. Instead of waiting for an interval
registration lifetime. A foreign agent may also include a list   timeout, movement is detected after the first new
of foreign agent CoAs available to the MN. The registration      advertisement. This policy assumes frequent location
lifetime is the maximum time that the MN is allowed to           changes and movement in straight lines. Eager-binding may
remain registered to this mobility agent. A mobility agent       significantly improve handoff performance over the lazy-
may also include a prefix-length extension in these agent        binding policy. However, if a mobile node is in an overlap
advertisements. This indicates the network prefix part of        region serviced by two agents, it may lead to situations
any supplied router addresses.                                   where the MN oscillates between the two.
B. Link-Layer Hint-Based Mechanisms                                 solicitation scheme above except that the need to
                                                                    perform the solicitation/advertisement signalling pair is
In all the above-mentioned advertisement-based schemes,             avoided.
there is an inherent trade-off between the bandwidth wasted
by advertisements and the movement detection                    Mobile IP registration delays can be made negligible by
performance.       The higher the rate that periodic            employing hierarchical network architectures as mentioned
advertisements are broadcast, the more bandwidth is wasted      previously. These architectures and their functionality are
by these messages. However, handoff latency and packet          beyond the scope of this paper. Nevertheless, if registration
loss are both reduced at higher rates. This is the main         delays are ignored, schemes 1 and 3 reduce a Mobile IP
reason that RFC 3220 and Mobile IPv6 allow shorter              (network layer) handoff to an 802.11 (link-layer) handoff.
intervals between advertisements. These considerations are
especially pertinent when considering wireless networks
such as 802.11b due to the reduced bandwidth relative to          IV. EVALUATION OF MOVEMENT DETECTION SCHEMES
wired interfaces.
                                                                An evaluation framework is currently being developed that
Movement detection optimisations aim to bypass this trade-      will be used to evaluate the movement detection schemes
off by compromising the layer independence between              listed above. The framework consists of a home and a
Mobile IP and the link-layer. This allows information from      foreign network interconnected by a Linux router shown
the link-layer to be passed up to Mobile IP, enabling faster    below. A mobile node connects to a particular network via
movement detection to be performed. As a result, lower          an 802.11 AP located on that network. A Mobile IP handoff
advertisement rates can be employed, improving bandwidth        can be performed by forcing a MN to disconnect from one
efficiency. Listed below are some movement detection            network and connect to another as illustrated in Figure 3.
mechanisms that allow link-layer hints to be gathered by        The results of some tests performed on this framework are
Mobile IP. A typical link-layer hint used by all these          discussed in this section.
schemes is the “attachment to a new AP” or “link-up” event.

1) Advertisement caching in an 802.11 AP [13]
                                                                           Home                     Linux        Foreign
   A modified 802.11 access point can cache recent                         Agent                    Router       Network
   unsolicited advertisements. The most recent                                                                (10.32.0.0/16)

   advertisement can be delivered as soon as a mobile
                                                                     802.11 AP
   node associates with the AP. The AP uses link-layer                                             Foreign
   hints to signal the arrival of a new MN. If this is the                                          Agent

   case, the MN will receive configuration information as            Home Network
                                                                     (10.128.0.0/16)
   soon as it attaches to the network. A MN will have to
   use an eager-binding policy to configure itself as soon
   as it receives a new advertisement. Therefore, all
                                                                      MN
   802.11 access points in a particular network will have
   to support this scheme.
                                                                Fig. 3. Evaluation framework
2) Solicitation on link-layer handover hint [6]
   If a MN’s Mobile IP layer were to be made aware of           A. Dynamics
   link-layer handoffs, a solicitation could be broadcast in
   response to such handoffs. This would allow a MN to          Dynamics [11] is a Mobile IPv4 implementation developed
   send an agent solicitation as soon as its 802.11 interface   at the Helsinki University of Technology (HUT). Dynamics
   connects to a new AP. Mobility agents will respond           is an open-source software package written for the Linux
   with an advertisement. As in the case above, the MN          operating system. Home and foreign agent entities are
   may have to use an eager-binding policy.                     implemented along with Mobile IPv4 functionality such as
                                                                tunnelling and registration.
    This scheme leads to increased signalling, especially in
    networks containing large numbers of access points and      The main reasons for using the Dynamics software are that
    mobile nodes. The bandwidth wasted by this signalling       it is open-source and may thus be modified to support
    may become significant because solicitation and             movement detection optimisations. Dynamics also does not
    advertisement messages are sent after every 802.11          use any kernel level code but runs entirely in user-space.
    handoff, even when there is no Mobile IP handoff.           This fact makes it easier to modify certain functions. Lastly,
                                                                Dynamics is a mature and complete package that is easy to
3) Including network configuration information in 802.11        install and configure compared to other implementations.
   parameters [5]
   When associating with an 802.11 access point, a MN           B. Bandwidth Used By Agent Advertisements
   will have the same SSID as the access point. If an
   access point includes the on-link router or agent            The following graph shows the trade-off between bandwidth
   information in the SSID, any MN that associates with         used by agent advertisements and movement detection
   that access point will immediately have access to this       performance. As the advertisement interval decreases,
   information.    This mechanism is similar to the
resulting in faster movement detection, the bandwidth                                  Orinoco 802.11b PCMCIA card. In order to determine the
“wasted” by agent advertisements increases. Typical values                             link-layer latency, a neighbouring host sends the MN a
for the advertisement interval were chosen. These values                               stream of UDP datagrams at a rate of one per 10ms. At the
begin from the Mobile IPv6 minimum (about 0.05 seconds)                                same time, the MN performs several link-layer handoffs.
and include the RFC 2002 minimum (1 second) and the                                    The following graph illustrates the delay between a packet
Neighbour Discovery router advertisement minimum (3                                    and the previously received packet at the MN. Also shown
seconds).                                                                              is the number of packets lost.

                                           Advertisement Bandwidth                                                                                 Packet Delay

                          18000                                                                            0.3
                          16000
                                                                                                          0.25
                          14000
                                                                                                           0.2
         Bandwith (bps)




                          12000




                                                                                              Delay (s)
                          10000                                                                           0.15
                           8000
                                                                                                           0.1
                           6000
                           4000                                                                           0.05
                           2000
                                                                                                            0
                              0




                                                                                                                 2
                                                                                                                     58
                                                                                                                          114
                                                                                                                                170
                                                                                                                                      226
                                                                                                                                            282
                                                                                                                                                  338
                                                                                                                                                        394
                                                                                                                                                              450
                                                                                                                                                                    506
                                                                                                                                                                          562
                                                                                                                                                                                618
                                                                                                                                                                                      674
                                                                                                                                                                                            730
                                                                                                                                                                                                  786
                                                                                                                                                                                                        842
                                                                                                                                                                                                              898
                                                                                                                                                                                                                    954
                                                                                                                                                                                                                          1010
                                                                                                                                                                                                                                 1066
                                  0.05   0.1    0.2     0.5      1       2     3   5
                                                Advertisement Interval (sec)                                                                             Packet Number

Fig. 4. Advertisement bandwidth                                                        Fig. 5. Packet loss caused by L2 handoff

These calculations consider a typical Mobile IPv4 foreign                              Each gap in the figure above indicates that about 20-25
agent advertisement to be 106 Bytes in length. The                                     packets are lost every handoff. After each gap, the delay
advertisements contain one foreign agent CoA and do not                                between the last packet received before handoff and the first
contain prefix information. Ethereal [14] was used to                                  packet received after handoff is about 200-250ms. Thus, it
capture these advertisements and measure their size over the                           can be inferred that this is approximately the time taken to
physical medium. At the Mobile IPv6 minimum, nearly                                    perform an 802.11b handoff.
17kbps is used to broadcast advertisements.           When
considering one MN connected at 11Mbps, this is a small                                D. Advertisement-Based Results
percentage (about 0.15%) of the raw bandwidth.
                                                                                       Two movement detection mechanisms have been evaluated.
However, when considering larger numbers of MNs                                        The first is the generic Mobile IPv4 advertisement lifetime
connected to the same AP, each station’s useable data rate                             (lazy cell switching) algorithm. The second is the eager cell
decreases significantly [10]. In addition, larger networks                             switching algorithm. Different advertisement intervals are
may deploy more than one foreign agent which increases the                             chosen and the different stages of a Mobile IP handoff are
number of broadcast advertisements. Lastly, an 802.11b                                 timed. Mobile IP handoffs were induced by forcing the MN
wireless device does not always provide the maximum                                    to move from the home network to the foreign network.
11Mbps bandwidth. The 802.11b device will offer less                                   Thus far, movement between foreign networks has not been
bandwidth, moving from 11, 5, and 2 to 1Mbps, as the MN                                considered.
moves further from the APa. When all these factors are
considered, the bandwidth wasted by advertisements may                                 Tables 1 and 2 below summarise the results obtained when
become significant.                                                                    the advertisement lifetime (LCS) and eager binding (ECS)
                                                                                       algorithms were used respectively. They list the time in
C. Link-Layer Handoff Latency                                                          seconds between the different stages of the algorithm. The
                                                                                       “Advertisement Received” column lists the average time
Before a Mobile IP handoff can be thoroughly investigated,                             elapsed between the start of an 802.11b handoff and the
the link-layer handoff characteristics must be investigated.                           receipt of the first foreign advertisement.               The
In this test, numerous 802.11b handoffs were performed                                 “Registration” column lists the average time elapsed
between two access points in the same network.                                         between the reception of the first advertisement and the start
                                                                                       of registration. As registration takes place immediately after
An extensive study has been carried out on 802.11b                                     movement detection, the start of registration signifies the
handoffs using different hardware [4]. The results of the                              end of movement detection.
study show that link-layer handoff can take a significant
amount of time to perform. Delays also vary significantly                              Adv. Interval                       Adv. Received                                    Registration                                  Total
depending on the equipment manufacturer. It is reported                                      1                              0.7410554                                        3.5009047                                 4.2419601
                                                                                             3                              1.9006444                                        8.453091                                 10.3537354
that generally, an 802.11b handoff takes between 100-                                        5                              2.6527861                                       11.2803803                                13.9331664
400ms to complete [4].                                                                      10                              5.9368708                                       25.2995982                                31.236469
                                                                                       Table 1. Advertisement lifetime algorithm
In the evaluation framework, DLink 802.11a/b/g access
points are used. The MN is equipped with a Lucent                                      Adv. Interval                       Adv. Received                                   Registration                                  Total
                                                                                             1                              0.7660191                                      0.0003463                                  0.7663654
                                                                                             3                              1.9130985                                      0.0001842                                  1.9132827
  a                                                                                          5                              2.7872856                                      0.0003736                                  2.7876592
      This behaviour was revealed through experimentation
      10           5.6352937        0.0003327      5.6356264      [5]    Fikouras N, Könsgen A, and Görg C, “Accelerating
Table 2. Eager binding algorithm                                         Mobile IP hand-offs through Link-layer Information,
                                                                         an Experimental Investigation with 802.11b and
One would expect a MN, on average, to arrive on a new link               Internet Audio,” Proceedings of the International
halfway through the advertisement interval. The results                  Multiconference on Measurement, Modelling, and
from Table 1 and 2 above agree with this and a MN                        Evaluation of Computer-Communication Systems
generally has to wait for half the interval before receiving an          (MMB), Germany, September 2001.
advertisement.                                                    [6]    Fikouras N, Görg C, “A Complete Comparison of
                                                                         Algorithms for Mobile IP Hand-Offs With Complex
As can be seen from the total movement detection times, the              Movement        Patterns    and    Internet   Audio,”
ECS algorithm is about 5.5 times faster that LCS. The main               Proceedings of the Fourth International Symposium
difference between these two algorithms is that LCS first                on Wireless Personal Multimedia Communications
waits for a timeout, shown in the “Registration” column,                 (WPMC), Denmark, September 2001.
before handing off to the new foreign agent. ECS however,         [7]    D. Johnson and C. Perkins, “Mobility Support in
performs the handoff as soon as a new advertisement is                   IPv6,” IETF draft, draft-ietf-mobileip-ipv6-24.txt,
received and thus the time delay between received                        February 2003.
advertisements and registration is very small. These small        [8]    T. Narten, E. Nordmark and W. Simpson, Neighbor
delays in Table 2 also confirm that the time taken to                    discovery for IP version 6, Internet Engineering Task
configure the foreign agent CoA is insignificant.                        Force (IETF), RFC 2461, (December 1998).
                                                                  [9]    Soliman H, Castelluccia C, El-Malki K, Bellier L,
The time between receiving an advertisement and the end of               “Hierarchical Mobile IPv6 mobility management
movement detection in Table 1 should always be                           (HMIPv6),” IETF draft draft-ietf-mobileip-hmipv6-
approximately three times the advertisement interval. It is              07.txt, October 2002.
important to note that the interval is slightly randomised to     [10]   Montavont N, Noel T, “Analysis and Evaluation of
prevent synchronisation with broadcasts from other agents                Mobile IPv6 Handovers over Wireless LAN,”
or routers. This is why the timeout values are not exactly               Mobile Networks and Applications 8, pp 643–653,
three times the interval.                                                November 2003.
                                                                  [11]    “Dynamics           Mobile       IP,”       [Online]
However, it was found that in practice the MN would                      http://dynamics.sourceforge.net/
sometimes wait only two intervals before registration. This       [12]   Daley G, “Movement Detection Optimization in
may be an optimisation built into the Dynamics software but              Mobile IPv6,” IETF draft, draft-daley-mobileip-
the exact cause of this anomaly has not been determined.                 movedetect-01.txt, May 2003.
                                                                  [13]   Choi J, “Fast Router Discovery with AP
                                                                         Notification,” IETF draft, draft-jinchoi-l2trigger-
                       V. CONCLUSION                                     fastrd-01.txt, June 2002.
                                                                  [14]   “Ethereal,” [Online] http://www.ethereal.com/
This paper has introduced the Mobile IP protocol and has          [15]   Deering S et al, “ICMP Router Discovery
presented an overview of the Mobile IP handoff procedure.                Messages,” Internet Engineering Task Force (IETF),
Movement detection has been isolated as a major source of                RFC 1256 September 1991.
delay in these handoffs. An eager binding algorithm allows
for faster movement detection over the generic schemes. If
information and events from the 802.11 link-layer are made
accessible to Mobile IP, even less handover latency and
packet loss can be expected.


                         REFERENCES

[1]    Perkins C et al, “IP Mobility Support,” Internet
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       1996.
[2]    Perkins C et al, “IP Mobility Support for IPv4,”
       Internet Engineering Task Force (IETF), RFC 3220,
       January 2002.
[3]    IEEE. Part 11: Wireless LAN Medium Access
       Control (MAC) and Physical Layer (PHY)
       Specifications. IEEE Standard 802.11, 1999.
[4]    Mishra A, Shin M, Arbaugh W, “An Empirical
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