MOBILE IP PERFORMANCE IN HIGH MOBILITY ENVIRONMENTS

M. Femminella(1), F. Pugini(2)

(1) University of Perugia, D.I.E.I. – femminella@diei.unipg.it (2) University of Rome “La Sapienza”, Dept. of InfoCom – pugini@infocom.uniroma1.it

1 Introduction........................................................................................................................................................... 2 2 Radio link characteristics ...................................................................................................................................... 3 3 The handover mechanism ..................................................................................................................................... 3

3.1 Techniques to reduce the impact of THC........................................................................................................ 4 3.2 Movement Detection algorithms................................................................................................................... 7 3.3 MD algorithms performance in overlapping zones: registration oscillations.............................................. 10 3.4 MIP performance entering overlapping zones: registration coherence ....................................................... 11 3.5 MD algorithms performance leaving overlapping zones: absence of smooth handovers ........................... 16 3.6 Handovers classification ............................................................................................................................. 18

4 QoS setup procedure upon handover .................................................................................................................. 20 References................................................................................................................................................................... 21 Acronyms.................................................................................................................................................................... 22

1 INTRODUCTION IP protocol was not developed with user mobility in mind. IP addresses are primarily used to identify a particular end-system but are also used to route datagrams to the relevant host. Therefore a contradiction arises: a mobile end-system needs a stable IP address in order to be universally identified by any other IP system but, at the same time, if this address is stable the routing to the mobile host is defined (i.e. datagrams are always routed to the same point of attachment) and no mobility can be provided. This problem is overcome by the Mobile IP protocol which provides the user with an additional address (Care-of Address, CoA) as the mobile host migrates to a new sub-network.

In the following we will briefly summarise the way of operation of Mobile IPv4 [PER96], with reference to Fig. 1.

When an IP mobile host moves to a Visited (or Foreign) Network a new Care-of Address is assigned to it and notified to the Home Network. Here, the Home Agent, a node which is in charge of managing the traffic of mobile nodes, creates a soft state “binding” between the Home Address of the relevant mobile host and its actual CoA. Each packet sent by Correspondent Nodes towards the Home Address (other end-systems are unaware of Mobile Host position or movements) is captured by the Home Agent, encapsulated with IP-in-IP technique and tunnelled to the Foreign Network (that is towards the CoA). Here, a suitable network node, the so called the Foreign Agent, is committed to intercept these packets, to de-capsulate them, and finally to send them to the Mobile Host (by means of its Layer 2/MAC address).

Home agentForeign agent

Correspondent Node

Visited Network

Mobile Node

S: Home AddressD: Foreign Agent

Home Network

D: Home AddressS: Corresp. Node

S: Home AddressD: Corresp. Node

Tunnell

Via F.A.

Fig. 1 – Mobile IP principles

Many considerations arise from this brief description. First of all it must be noticed that Mobile IP has been developed to provide the user with portability features, that is to allow the mobile host to disconnect from its Home Network and reconnect to the Internet in any sub-network adopting Mobile IP protocol without loosing its identity profile. Seamless roaming in wireless environments is not the main goal of Mobile IP and handover procedures may considerably impact the performance of the perceived service.

In fact, each handover implies a new CoA assignment and an end-to-end signalling procedure to notify the new position to the Home Agent.

In the time interval elapsed from new CoA assignment to the mobile node till binding refresh in the Home Agent cache, data loss occurs, since packets are still addressed to the old CoA.

The enhanced Mobile IP version is Mobile IPv6 [MIPv6] (not yet a standard) which is enriched by many new features, and lots of problems of Mobile IPv4 are overcome. For example the triangle routing problem. As depicted in Fig. 1 the uplink path (from Mobile Node to Correspondent Node) and downlink path can be considerably

different, for example when Mobile Node and Correspondent Node are quite near, while the Home Agent is far away. Thus the delay difference (and resource waste) between uplink and downlink data flows may be not negligible. This problem, critical for real time applications, is avoided by using the “routing header” option in Mobile IPv6 packet headers, which allows the correspondent node (now aware of Mobile Host CoA) to send data to the mobile node directly. Foreign Agents are then useless in Mobile IPv6 and tunnelling is generally avoided. However, these new features do not solve the basic problem of Mobile IP when deployed over wireless cellular environments, especially when handover frequency increases (for example when cell dimension decreases or when mobile host mobility increases). The signalling burden, latency and losses implied by frequent end-to-end signalling lead to the conclusion that plain Mobile IPv4/6 are not suitable for this kind of environments.

In the following sections we want to analyse more in detail the impact of radio access and mobility on IP services. In particular three different aspects must be considered: radio link characteristics, handover mechanism, QoS setup procedure upon handover.

2 RADIO LINK CHARACTERISTICS As far as the first issue is concerned, we can affirm that the intrinsic instability of wireless links characteristics (such as loss rate or signal to noise ratio) renders the QoS provision concept itself less defined. Network resources (mainly capacity) varies in time and other disruptive effects (such as shadowing) may occur. The lower layer are committed to hide these fluctuations to Network layer and upper Layers by means, for example, of retransmissions but this is not always enough to overcome all the impairments and therefore service performance, even on a single radio link, can vary in time accordingly to these aforementioned effects. However, overcoming or concealing link characteristics variations is not a duty of the QoS providing framework while it should tackle only network procedures effectiveness. Therefore, in the following, we will deal only with impairments due to handover events and QoS framework structure.

3 THE HANDOVER MECHANISM We have already mentioned that Mobile IP (MIP) protocols [RFC2002], [MIPv6] have been developed mainly with portability features in mind. The handover procedures are suitable to be executed once per session that is as the Mobile Node (MN) turns on in a new access network (not necessarily a wireless one). Therefore the delay involved by registration requests and confirmation messages is not of primary importance, and the migration in a new access network can be easily detected or maybe even triggered by the human user. When this protocol suite is adopted in wireless environments and roaming among different access networks is aimed at, the handover procedures are executed each time a migration occurs, and the impact of this frequent operation may be not negligible and even become intolerable for real time services. In this section we will analyse the service performance decay involved by MIP protocols operation in wireless access networks, and also micro-pico cellular networks, where handover frequency is higher, will be considered. In order to correctly address the service impairments due to handover procedures when a MN migrates from the current cell (we call cell the radio coverage area of a wireless access node) to a new cell, we will consider the Handover Delay, TH, that is the time interval that elapses between the reception of the last packet in the previous cell and the first received packet in the new cell, upon migration. It is composed by two main components:

• The Movement Detection Delay (TMD); it is the time interval is necessary to detect migrations, i.e. that the MN is no more in the coverage area of the radio Access Node (AN) it is still registered with.

• The Handover Completion Delay (THC), that is the time interval that elapses between migration detection by MN and the reception of the first packet in the new cell.

Thus, we have:

TH = TMD + THC. ( Eq. 1 )

Let us now focus our attention on the handover procedure. As soon as the MN leaves the current AN coverage area it can no more receive data packets from it. In the ideal case in which the MN can be instantaneously aware of the

migration and get the new CoA (TMD=0), it immediately issues a registration message (Binding Update, BU) to the HAg (and CN if it is the case) to notify it. Before BU reaches the relevant end-systems, packets are wrongly routed to the old AN and then lost. Due to the end-to-end scope of this signaling procedure, this time interval may be not negligible.

As soon as the BU is accomplished, packets are correctly routed again, but an additional end-to-end delay has to elapse before packets are delivered to the MN. In the more realistic perspective in which TMD is greater than zero, a new delay component, and further losses, are introduced.

We want to stress that the overall impact of handover events on service performance is naturally related to migration frequency and may become disruptive for real time applications even when this frequency is below the proposed limit of one per second [RFC2002].

0 10 20 30 40 50 60 70 80 90 1000

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Fig. 2 – Plain Mobile IP performance in a simple CBR case study

Fig. 2 shows the results of the simulation of a simple Mobile IP scenario. A Constant Bit Rate (CBR) data source emits a 64 Kbit/s UDP flow addressed to a MN, which is linearly roaming, with an average speed of 10 m/s, among adjacent cells of radius 100 meters. The Round Trip Time (RTT) is fixed to 400 ms to simulate a WAN. In the mentioned figure, packet losses (silence periods) are due only to handover procedures. The simulation shows an overall loss in the order of 15%, while the mean Handover Delay is about 3 seconds. The mean handover frequency is 0,1 hand/sec, i.e. ten times smaller than the aforementioned proposed limit, and it is easy to foresee that, if it increases (for example when cell dimension decreases and/or MN speed increases), losses will increase proportionally. This result supports the currently growing opinion that plain Mobile IP protocol can not be sufficient to support user mobility with Quality of Service requirements and suitable enhancements should be developed.

In the following of this work we will discuss some possible solutions to reduce TMD and THC and therefore to improve Mobile IP performance. More in detail, we will show that the recently proposed approach of reducing THC is necessary but not sufficient to overcome the above discussed limitations of MIP.

3.1 Techniques to reduce the impact of THC Most of the efforts spent towards the improvement of Mobile IP protocols are aimed at reducing THC . This component of Handover Delay is mainly due to the end-to-end delay introduced by registration messages upon handover. Therefore, to reduce THC , it would be better to reduce the amount of this kind of signalling and, possibly,

to reduce its scope. To this aim Micro-mobility architectures have been proposed (for example, Regional Registration [REG00], Hierarchical Mobile IP [HMIP6], Cellular IP [CEL00], IDMP [MIS00], Hawaii [RAM00]).

To our aims micro-mobility can be defined as the ability for a mobile node to move without notifying its home agent while macro-mobility is when the mobile node's home agent is notified of any movement, as it happens in the conventional Mobile IP.

Besides the specific implementation details, micro-mobility solutions share the fundamental idea of hiding, to the rest of the network, the actual migration of a mobile node when it takes place in a regional domain. Thus the localisation of the handover procedure has the goal of reducing the end-to-end plain Mobile IP handover limitations when the migration frequency increases.

In order to explain their common approach to solve the problem, we will refer to a general micro-mobility architecture, common to all the above-referred proposals.

With reference to Fig. 3, a regional domain is called Local Mobility Network (LMN). Within an LMN, mobility is managed by a Local Mobility Gateway (LMG, usually the sub-network gateway, also called in some proposals “anchor” router), but also other routers in the local domain could be involved. In these proposals, handover procedures are divided into two classes: Global Handover and Local Handover.

A Global Handover occurs when the Mobile Node (MN) first enters the LMN. A plain Mobile IP Handover has to be accomplished. Thus, the new Care-of Address is signalled to the Correspondent Node(s) and to the Home Agent by means of Binding Updates messages. Clearly, given the possibly large distances involved, the handover latency may be large, thus leading to a potentially large packet loss and severe QoS impairments for real time flows.

The idea of micro-mobility management architectures is to perform Global Handovers only when a MN migrates from a LMN to a new one. When the Mobile Node connects to a different Access Node within the same LMN, a Local Handover is performed. The idea is that the Local Mobility Gateway (but this function may be furthermore distributed within the LMN) provides a further level of indirection, by storing a binding between the Care-of Address, advertised to the rest of the network, and a supplementary local IP address, assigned by the new access node upon handover. Upon local handover and change of mobile node address, no changes in the Care-of Address are needed (thus no messages are sent to the Correspondent Node or to the Home Agent and the handover procedure is transparent to the outer domains). It is only necessary to update the binding stored at the LMG (and in the involved local routers, if it is the case) via a Local Binding Update procedure. To avoid the need for explicit signalling, and for protocol roburstness, the LMG manages soft states, i.e., the bindings stored in the LMG have a limited lifetime and must be periodically refreshed. Note that local mobility management, as proposed in [REGR, CELL], could be performed at the first common point between old and new path. In this paper, for convenience of presentation, we assume that this functionality is located only in the LMG.

Mobile Node

Correspondent Node

Home Agent Internet

Local Mobility Gateway

Access Router 1

Local Mobility Network

BU

BU

BU BU

LBU

Local Handover

BU

BU= Binding Update LBU = Local Binding Update

Previous Local

Mobility Network

Global Handover

Access Router 2

LBU

Mobile Node

Fig. 3 – Global and Local Handover

It is important to underline that a micro-mobility solution has to be added to plain macro-mobility, that is they do not substitute Mobile IP but strictly inter-work with it. Substantially macro and micro mobility architectures have different goals, the former offers portability and is used to manage migration among LMNs, while the latter provides light-weight mobility within local domains. Therefore if they are used jointly they can provide an enhanced mobility service.

Another approach to reduce the handover impact upon service performance, is to recover data loss by means of suitable procedures, such as Smooth Handover [PER99], which do not modify the MIP way of operation, that is they leave untouched the scope of the registration messages.

The basic principles of Smooth Handover is to notify the new CoA, assigned to the MN after migrations, to the old AN too. This network node is, then, in charge of buffering and forwarding all data packets, addressed to the MN, to the new visited network. However this way of operation:

1. Is critical, at least as regards delay, for real time services and it can aggravate problems due to out of sequence packet delivery.

2. Implies a buffering operation during the whole time spent in a cell (the MN is unaware of the time instant in which the handover will occur) thus a resource waste is introduced.

3. Introduces the new problem of correct buffer dimensioning which can not be solved once and for all since it depends on the nature of the relevant data flow (burstiness, mean rate, peak rate, etc.).

Therefore it can not be considered as a comprehensive solution, while these two approaches can be merged together to obtain a more effective framework.

3.2 Movement Detection algorithms The second component of TH is the Movement Detection (MD) delay. The detection of migrations is necessary to trigger the handover procedures and can be performed at the lower layers of the MN structural model i.e. Physical and Data Link/MAC layers. However TMD refers to the migration detection at IP layer and, to the best of our knowledge, its impact on service performance has not been deeply focused on so far. Probably this is due to the fact that the Mobile IP suite is supposed to be supported by lower layers in detecting movements by means of, for example, signal strength or end-systems position information provision.

This approach can be highly effective, but impoverishes the IP layer intrinsic independence by data-link and physical layers. In other words the Mobile IP Movement Detection (MD) algorithm should be as efficient and effective as possible, avoiding to necessarily rely on the eventually available information coming from “smart” radio access systems beneath. In addition, MIP has been proposed to perform the so-called multi-segment handovers, that is handovers between different access systems (e.g. UMTS and satellite systems). In this case, the procedures can be much more simple and efficient if executed at the IP layer.

As a consequence, only IP packets should be used to detect migrations, as suggested by proposed standards (see [RFC2002]).

Only few algorithms adopting this philosophy have been proposed so far, among them Lazy Cell Switching, Early Cell Switching, Eager Cell Switching. We describe these protocols in the following, assuming that the MN leaves the current cell (the terms relevant to such cell are addressed as “old”) and enters a new one (whose terms are addressed as “new”).

Lazy Cell Switching (LCS) uses the current registration lifetime to detect migrations. This implies that the MN must miss a number N of advertisement signals from ANold, before switching to ANnew. In other words:

adv_oldMDadv_old NTT)T(N- ≤≤1 ( Eq. 2 )

while the mean value of TMD, (assuming that the time instant in which the MN crosses the cell boundaries is uniformly distributed within 0 and Tadv_old,) is

adv_oldMD )T/(N- T 21= ( Eq. 3 )

where Tadv is the time interval between two advertisement packets issued from the ANs. Commonly, Tadv is suggested to be no lower than 1 second, while N is assumed to be equal to 3. With these assumptions we obtain

sec52, TMD ≥ ( Eq. 4 )

Early Cell Switching (EyCS) has an opposite approach to the one proposed by LCS. While the latter tries to be attached to the current AN as long as possible, according to EyCS, the MN switches as soon as it intercepts an advertisement packet from ANnew. It is easy to conclude that the following relationship holds (with self-explaining terminology):

adv_newMD TT ≤≤0 ( Eq. 5 )

and, with the same numerical assumptions made above, it results:

sec502/1 _ ,T T newadvMD ≥= ( Eq. 6 )

It is easy to see that, while EyCS is faster in detecting migrations, it can introduce heavier signalling burdens and oscillations in the registration procedures due to, for example, continuous crossing, back and forth across the same border line.

Eager Cell Switching (ErCS) principles are similar to EyCS, but this proposal avoids the increasing of the signalling burden and of frequent registration oscillations. The ErCS mechanism forces the MN to switch as soon as it intercepts an advertisement packet from ANnew, but no more frequently than once per second. We can conclude that, in this case, the following relationship holds:

) ,T( T adv_newMD sec1max0 ≤≤ ( Eq. 7 )

and, with the numerical assumptions made in the case of LCS, together with a Tadv_new ≥ 1 sec, we obtain:

sec502/1 _ ,T T newadvMD ≥= ( Eq. 8 )

The mean value of TMD, whichever the MD method, is never lower than half a second and it may result disruptive for real time traffic, especially if migration frequency increases i.e. in micro-pico cellular environments.

These considerations lead us to conclude that the MD delay is not a negligible component of TH and therefore Micro-mobility frameworks and Smooth Handovers mechanisms do not represent a comprehensive solution to the provision of high performance services to mobile IP users. The MD issue has to be considered too.

Works appeared, (e.g., [FE199, FE299]), which give quantitative examples of the impact of LSC and ErCS on service performance of UDP and TCP data flows, but they primarily focus on a single handover, that is they put in evidence the consequences on the aforementioned communications when the MN leaves a cells and enters a new one, and they consider only the case of adjacent cells (no overlap); MNs can be engaged only with one AN at a time. Moreover to the best of our knowledge, no satisfying alternatives to the aforementioned algorithms are given.

Thus, in the following we will provide some simulation results in order to confirm the aforementioned expected values for TMD and we will also try to give some guidelines to face the above issue.

In the simulation results showed in Fig. 2, the LCS method was used and the mean TMD value was 2.6 sec, in conformance to what stated about this protocol. We run other simulations with the same network topology and the same values for the mean parameters (cell dimension, average MN speed, Advertisement interval) as in Fig. 2 while adopting ErCS or EyCS MD algorithms. The simulation results are reported in Fig. 4 and the mean TMD value in these cases was equal to 0.6 sec. The readers will easily convince themselves that these two MD algorithms show the same delay performance in this case, since the limitation introduced by ErCS to handover frequency never occurs due to the cell radius and the MN speed which involve handovers approximately every 10 sec.

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Fig. 4 – Plain Mobile IP performance with ErCS or EyCS MD algorithms.

Since TMD values are not negligible it would be useful to look for some performance improvement.

A possible approach to decrease TMD could be to reduce Tadv i.e. to increase the advertisement message frequency, but this results in (precious) radio resource waste, since such higher advertisement frequency is useful for MD algorithms only upon handover, while it is not exploited in the rest of the time.

Another possibility, at least for LCS, is to reduce N, but, in this case, the method has drawbacks when advertisements are accidentally lost (false movements are detected).

It must be noticed, however, that the delay introduced by MD algorithms is not the only drawback of Mobile IP protocols in wireless environments. More in detail, we have considered, so far, the simple case in which radio coverage cells are adjacent, i.e. when no overlapping among cells occurs. In this case, the MN can intercept advertisements from one AN only, at a time, and the reception of an advertisement packet from a new AN means, with no uncertainty, that the MN has moved into a new cell.

The MD algorithm, in this simple situation, is only in charge of deciding whether and when the handover has to be performed (for example, continuous hops among two cells are possible and the MN waits a certain time interval in the new cell, before the handover).

While the absence of cell overlapping is an assumption that can be accepted in some topology architectures, there are situations in which this assumption can not be easily satisfied.

The reader can consider, for example, two autonomous radio access sub-networks that are deployed one near the other. The coverage area planning is feasible within the ANs of each sub-network but probably not among ANs belonging to different owners.

Another example is represented by wireless Ad-Hoc networks with more than one Internet Access Point, where ANs are free to move and no coverage area planning is possible.

When cell overlapping can occur, more considerations are needed. The Whyless.com environment belongs to this network typology and, therefore, the involved effects have to be deeply analysed.

In the following of this section we will, then, consider some other aspects that have to be taken into account when coverage areas of different access nodes overlap.

3.3 MD algorithms performance in overlapping zones: registration oscillations When the MN is in a overlapping zone of different coverage areas, the MN itself could receive advertisement messages from two or more ANs, depending on the time interval spent in this border region (we assume that the MN has the technical capability to intercept advertisements from more than one AN, and to handle more than one CoA). In the following we will analyse the case of two overlapping cells, as shown in

Fig. 5, where the MN moves from cell A towards cell B.

The behaviour in overlapping zones depends on the adopted MD algorithm. In particular the Oscillation Region can be defined as the region in which the relevant MD algorithm performs handovers with regularity i.e. the registration of the MN oscillates among the involved Access Nodes in a more or less predictable way. The wider this region the longer the registration oscillation period. In order to understand what can happen in the Oscillation Region let us consider the case in which the MN performs a handover from Access Node A to Access Node B. While waiting for the confirmation message for the registration with Node B the MN should be able to keep on receiving data from Node A.

If this is not the case during the registration period the MN is unable to receive packets and a silence period (i.e. a loss period) in introduced. Even in the case in which the MN is able to handle to simultaneous connections with Node A and Node B, registration oscillations may introduce a service decay. In fact, packets are periodically sent on two different paths, i.e. from HAg/CN to Node A and B, and this can introduce severe out of sequence problems due to the possibly different end-to-end delay along the two data paths. In conclusion we can affirm that registration oscillation is an undesired phenomenon and should be avoided as much as possible. The registration oscillation frequency is related to the adopted MD algorithm.

In the case of LCS the Oscillation Region vanishes since in the overlapping zone the MN still receives the advertisement messages from the Access Node it is currently registered with.

If EyCS is adopted, each new advertisement reception implies a handover procedure and therefore this is obviously the worst case for registration oscillation phenomenon. The oscillation frequency, in this case is related to the advertisement frequency of the overlapping Access Nodes. Both in the case of ErCS and EyCS methods the Oscillation Region coincides with the overlapping area of the Access Nodes, but in the ErCS case, the oscillation frequency is limited by the protocol itself to be no more than one per second.

We may state that, as far as Registration Oscillation is regarded, the LCS offers the best performance, i.e. laziness in changing point of attachment results in a higher roburstness of service performance to coverage area displacement.

Some numerical results are provided in

Fig. 5 to give an idea of the oscillation entity in overlapping zones. The number of registrations is reported, for the mentioned three MD algorithms, for a 30 second long stay in the overlapping zone of two Access Nodes. In this simulation both the advertisement frequencies in the two cells were equal to one per second.

Foreign Agent A Foreign Agent B

HA--CN

Mobile Node

Da Db

Eager Cell Switching 18

Early Cell Switching 29

Lazy Cell Switching 0

Fig. 5 – Number of registrations due to MD algorithms

The number of handovers performed in the EyCS case clearly shows that this MD algorithm is extremely sensible to overlapping zones.

3.4 MIP performance entering overlapping zones: registration coherence Another important aspect that has to be taken into account, in case overlap among coverage areas is possible, is the coherence of registrations that can introduce additional service decay.

In this section we will consider the case in which the MN is not registered with any Access Node (AN) when it enters the overlapping zone (for example the MN is turned on in this region, or it enters this region after a period of lack of connectivity or when the confirmation of the registration with a node has not been received yet). In the following we will refer to the scheme of

Fig. 5 for the sake of simplicity. The MN will receive advertisements from both the ANs and the involved phenomenon is independent by the adopted MD method.

To correctly understand what happens, it must be noticed that the MN, upon registration, is unable to detect if a registration reply is lost or it is still on the way back. Thus, for the sake of roburstness and effectiveness, it issues a new registration request upon reception of each new advertisement signal till it is correctly registered with one AN (not necessarily the one relevant to the first advertisement the MN received). As a consequence, after the first registration is accomplished and even if the MN does not move at all from (or in) the overlapping zone, delay and/or loss phenomena occur, depending on: i) the time interval, δ, elapsing between the time instants in which the MN receives the advertisements from the Access Node A (TA) and B (TB): δ=TA-TB; ii) the difference, D, between the

End-to-End delays of the two different paths from AN B (DB) and A (DA) to the HAg/CN: D=DB-DA (we assume DB>DA). Let us consider all the possibilities with respect to δ.

1. δδδδ=0. In this (ideal) case the MN receives advertisements from AN A and B simultaneously, say at time t0. The registration requests relevant to both ANs are issued (ideally) at the same time towards HAg/CN. The implications of this event are reported in Fig. 6. Due to the different end-to-end delay on the two data paths a silence (no loss) period occurs of duration D seconds. Even if no losses are introduced in this case, the amount of the absence of data reception at MN side may be large or intolerable for real time applications. Note that this event occurs after the first registration and with no respect to the adopted MD algorithm.

2. 0<δδδδ<D. In this case the MN receives first the advertisement from the AN endowed with the higher End-to-End delay, while the second advertisement (the one from AN A) is received within a time interval of D seconds. Being this temporal separation lower than the delay difference of the two paths the advertisement received first by HAg/CN is still the one from AN A. Also in this case a silence interval of duration D occurs after the first registration. The situation is reported in Fig. 7.

3. D<δδδδ<2D. Also in this case the MN receives first the advertisement from the AN endowed with the higher End-to-End delay, while the second advertisement (the one from AN A) is received within a time interval no greater than 2D seconds but no lower than D seconds. This implies that the advertisement received first at HAg/CN side is the one relevant to AN B (the one endowed with the higher End-to-End delay) but the reply received first at MN side is the one relevant to AN A. These events are shown in Fig. 8. From the time instant t0 + 2 DB the MN is registered with AN B but data packets are routed to AN A. This disruptive effect is due to the lack of consistency between the two registration information at HAg/CN and MN side. This absence of connectivity can be avoided by timestamping confirmation messages issued by HAg/CN. In this case the MN is committed to ignore those registration confirmation messages which are older than the timestamp of the current registration.

4. δ>δ>δ>δ>2D. In this case the MN receives first the advertisement from the AN endowed with the higher End-to-End delay, while the second advertisement (the one from AN A) is received after a time interval greater than 2D seconds. The registration message relevant to node B is received first and is confirmation message arrives before the one relevant to AN A to the MN. This time losses phenomena are introduced for D seconds as shown in Fig. 9

5. δδδδ<0. This is the case in which the advertisement relevant to the AN endowed with the smallest End-to-End is received first. Therefore a delay of D seconds is introduced as shown in Fig. 10.

Silence = D

Reg. B

t0

t0 + 2DA

MN HA

Reg. A

t0

MN HA

t0 + 2DB

t0 + DB + DA

time

Messages relevant to AN B Messages relevant to AN A

Fig. 6 – Time scheme for the case 1

Silence = D

Reg. B t0 +δδδδ+ 2DA

MN HA

Reg. A

MN HA

t0 + 2DB

t0 + DB + DA

time

t0

t0 + δδδδ

Messages relevant to AN B Messages relevant to AN A

Fig. 7 - Time scheme for the case 2

Reg. B

t0 +δδδδ+ 2DA

M N H A

Reg. A

M N H A

t0 + 2D B

t0 +δδδδ + DB + D A

tim e

t0

t0 + δδδδ

M essages relevant to AN B M essages relevant to AN A

No connectivity

Fig. 8 - Time scheme for the case 3

R eg. B

t0 + δδδδ+ 2D A

H A

Reg. A

M N

t0 + 2D B

t0 + δδδδ + D B + D A tim e

t0

t0 + δδδδ

M essages relevant to A N B M essages relevant to A N A

Loss for D sec.

Fig. 9 - Time scheme for the case 4

Reg. B

t0 +δδδδ+ 2DB

HA

Reg. A

MN

t0 + δδδδ + DA + DB

time

t0

t0 + δδδδ

Messages relevant to AN B Messages relevant to AN A

Silence for D sec.

Fig. 10 - Time scheme for the case 5

We can conclude that, whichever the mutual position of the two advertisements (i.e. the value of δ) a service decay is introduced by the way of operation of MIP protocol (irrespective of the adopted MD algorithm). More in detail a loss or a delay of D seconds (whose value may be not negligible) is involved and, in the particular case of D<δ<2D suitable modification to the standard MIP protocols have to be adopted in order to avoid a complete service interruption. We want to underline that the above consideration have been presented here since they represent another important aspect of MIP protocol when used in environments where coverage area planning is difficult or unfeasible. The mentioned service decay can not be avoided unless MIP protocol is modified, and therefore this problem has to be taken into account while analysing the MIP performance in high mobility radio access networks.

3.5 MD algorithms performance leaving overlapping zones: absence of smooth handovers In the previous sections we analysed the performance of Mobile IP protocol also in the case the MN enters a zone in which different coverage areas of two or more ANs overlap being nor registered with any ANs. We showed that some service decay (delay or loss) occurs in this case, with no respect of the adopted Movement Detection (MD) method, after the first registration is accomplished. We also analysed the performance of MD algorithms while the MN is in the overlapping zoned, underlining that, when ErCS or EyCS is adopted, registration oscillations occur which may cause service decay. In this section we will analyse the MD method performance leaving the overlapping zone. A first consideration is that, since a gradual migration among cells is available, performance should be improved, with respect to the case of adjacent cells, and smooth handovers could be performed. In other words, since the MN can detect advertisements from the next cell it should be able to detect the migration before losing connection with the current AN. We show that this is not always the case.

Let us assume that the MN moves from cell A to Cell B (the reader can consider the scheme in

Fig. 5 as a reference), and that it is registered with AN A when it enters the overlapping zone. The following events depend on the MD methods.

Case of LCS. Since the MN is already registered with the AN A, it will ignore all the other advertisements: the MN will keep on detecting the advertisements from Node A. When the MN leaves the overlapping zone, the algorithm behaves as the two cells were adjacent. No improvement (i.e. a TMD reduction) is then gained for the MD process by having a gradual migration among cells.

Case of EyCS/ErCS. In these cases, the MN will start oscillating, that is it will alternatively register with Node A and B continuously, in a way depending on the relative time position of advertisements packets (i.e. their phase δ). ErCS oscillates with a lower frequency than EyCS. As we said before, these oscillations do not endanger performance too much (except for out of sequence events) if MN is able to keep on receiving data packets from the previous AN while waiting for the confirmation message for the BU relevant to the new AN. When the MN eventually exits this region it can be: i) registered with A (and yet no improvement for the MD process is obtained by the gradual migration among cells); ii) registered with B (this case is the best one since TMD =0); iii) in a transient situation in which registration with AN B is incomplete, i.e. a registration message has been sent to HAg/CN but the relevant reply has not arrived yet. In this case only a small delay is introduced and some improvement is obtained by the gradual migration, since TMD is reduced. iv) in a transient situation in which registration with AN A is incomplete. In this case the MN will not be able to receive the reply, since AN A is no more reachable. Therefore a service disruption will occur. In fact, HAg/CN will receive a registration request relevant to AN A and therefore will route packets towards AN A while the MN is no more in its coverage area. Upon receiving advertisements from B, the MN will not issue any BU, since it is still registered with it (in fact no confirmation relevant to AN A will arrive). In other words registrations at MN side and at HAg/CN side are not coherent. The MN will wait for the confirmation relevant to AN A for a fixed time interval and then a registration request, relevant to AN B, will be issued, thus re-establishing the connectivity. Being this time interval long, we assumed a duration equal to the registration expiration time (3 seconds), the performance decay is noticeable.

The four possibilities for the MN registration status, upon leaving the overlapping zone, can then be considered random variables with respect to:

1. The parameter ρ = Tover / Tadv, where Tover is the time spent in the overlapping zone (we chose this parameter because it includes two contrasting parameters: the MN speed and the overlapping zone dimension). The greater ρ the greater the number of advertisements the MN receives in the overlapping zone (we assume, for the sake of simplicity, Tadv_A=Tadv_B=Tadv).

2. The parameter σ = RTT/Tadv (we assume, for the sake of simplicity, DA = DB = ½ RTT). The greater σ the greater the number of BUs that can be accomplished between the reception of two consecutive advertisements from the same AN.

3. The phase δ of the two advertisements signals (0<δ<Tadv).

We analytically evaluated, for the ErCS case, the mean delay (with respect to δ), as a function of ρ, due to MD with an overlapping area with fixed σ = 0.4 (RTT=400ms, Tavd=1sec). This delay is reported in Fig. 11, (with ρ varying within a range of 0-8). If we average such delay also with respect to the selected values of ρ, we obtain 0.70 seconds; thus the overall mean delay is increased, with respect to the no-overlapping mean value of TMD, of about 40%.

2 4 6 8

0.1

0.6

1.1

1.6

Sile

nce

Perio

d in

B (d

elay

in se

c)ErCS

ErCS (mean)

σ = 0.4ρ

Fig. 11 - Mean delay due to overlap area crossing

These results lead us to conclude that laziness in leaving the current AN is desirable, because MD algorithms performance worsening can be avoided, even if this implies the impossibility of taking advantage of the gradual migration among cells and TMD can not be reduced.

On the other side oscillations of MD methods imply a worsening of the average delay due to movement detection.

We can conclude that, at least, two main features should be analysed to characterise a MN method:

1. The rapidity of detection when a migration occurs, which is strictly related to the false detection probability.

2. The roburstness to oscillations, which is strictly related to the performance in overlapping zones. 3.6 Handovers classification

In this section we want to classify the handover events in order to correctly address the issues implied by QoS-provision in wireless micro-pico cellular Mobile IP environments, and we also introduce some further basic terminology. Some concepts have been discussed already in the previous sections but it is useful to have a brief recap in order to give a complete perspective of the mobility aspects. Handovers may be classified according to several parameters. The first parameter that can be used to classify handovers is performance, so we can distinguish among:

1. Fast Handover. A handover procedure which does not result in any excessive packet transfer delay.

2. Smooth Handover. A handover procedure which does not result in any packet loss.

3. Seamless Handover. A seamless handover is a fast and smooth handover.

As we said before, if micro-mobility frameworks are adopted, handovers can be also divided into:

1. Global Handover, among different LMN.

2. Local Handover, within the same LMN.

We also propose to assume that two types of handover may occur:

1. Forced Handover. It is due to a link connectivity failure, it can be unpredictable and could imply a connection abortion. In this case, no QoS maintenance can be assured.

2. Voluntary Handover. It is due to a voluntary change in the point of attachment triggered by mobile user protocols. This kind of handover is possible when the coverage areas of different access routers considerably overlaps.

Even though Voluntary handover may seem unlikely in homogeneous access networks, it could be common:

In a multiple access technology network perspective. For example, UMTS, satellite and IEEE 802.11 wireless accesses could be available at the same time in the same area and mobile users could decide to switch from one to another for convenience considerations (e.g. with respect to QoS performance, charging criteria or, in general, according to a preference list of access interfaces).

In a multi-hop environment where mobile terminals are network nodes at the same time (hence the usual name “terminodes”) and then there is no base station coverage area planning.

The Voluntary handover mechanism is developed to assure QoS preservation during the path switching operation. To allow the user to get admission control confirmation on the new access path, the Mobile Node must be able to uphold two distinct connections per each original flow (a MN may be involved in multiple QoS-aware sessions at the same time). On the "old" link, the Mobile Node keeps on receiving QoS data traffic while on the "new" one, it triggers new flow setup procedures in order to establish new QoS flows to switch to. To this aim the mobile user must be able to handle two different IP addresses, to communicate with two different access routers and, eventually, to switch its current sessions to the new path. Note that this double address assignment is not adopted in the case of the plain Global Handover.

We want to underline that Forced/Voluntary and Global/Local characteristics are independent. This implies that Voluntary handovers, where QoS parameters may be preserved by suitably setup connections before switching, can be either Global or Local and therefore QoS maintenance upon migration does not depend on its scope. On the other side we would like to obtain appreciable QoS parameters preservation when Local handover occurs (that is most times) being it Forced or Voluntary, i.e. independently of the reasons that cause the switching.

The conclusion are briefly summarised in Tab. 1. Voluntary handover preserves QoS parameters when successful and its duration is related to its scope. Forced handover can not guarantee QoS maintenance but Local handovers should be QoS-oriented anyway. Global Forced handovers, since they imply plain Mobile IP end-to-end procedures, can deeply impact QoS and will not be developed any further.

Some more consideration can be given about Forced/Voluntary Handovers and service performance. More in detail the performance level/price trade-off should be considered. We do not want to analyse these issues in detail but it will be sufficient to note that if a Voluntary Handover is feasible than the migration decision should be taken according to price and/or performance consideration. In particular a change of the point of access to the Internet could be done if it is possible to maintain the same service level with a lower cost, or if a better service with the same cost is available. These consideration could be formalised by means of a decision function implemented in the end-user terminal. This algorithm could be set a priori on the basis of the customer preferences in terms of desired QoS and willingness to pay and it should fulfil two main constraints: the perceived performance should be better than a minimum reference level, and the price has to be lower than a maximum value. According to these rules Voluntary Handovers could be seamless, i.e. transparent to user applications since they are performed autonomously by this module. If a better connection is available a switch will take place otherwise the current point of attachment will be kept.

Upon Forced Handover, this operation could be performed as well, maybe by means of the same decision module. However in this case it is not possible to guarantee a seamless handover since Forced Handovers are characterised by a drop of connectivity and the service level/price negotiation can not be performed while still using the previous Access Point. A variable service decay is necessarily introduced. The minimisation of setup and negotiation procedures in this case, therefore, should be aimed at.

Forced Voluntary

Local To be improved QoS Preservation (fast)

Global Not considered QoS Preservation (slow)

Tab. 1 – Handover type and QoS considerations

4 QOS SETUP PROCEDURE UPON HANDOVER

Each mobile user migration changes the actual data path and therefore it implies a new CAC procedure and a new resource estimation and reservation. In order to completely localise the handover procedure we suggest to develop a localised QoS setup phase. This means that CAC procedure and resource reservation signalling should involve only network elements included in the new part of the data path. We’ll show that this assumption, even though it may seem trivial, can not be introduced in every QoS provisioning architecture for wired networks.

It is quite interesting to note, in fact, that there is a subtle reason that renders RSVP not easily adaptable to micro-mobility strategies. In RSVP, the bandwidth to be reserved is evaluated in the receiver end system by means of a sum of error terms collected by the PATH message in each crossed router. Hence, it represents an overall characteristic of the path and a new PATH message has to be sent from the source to the receiver end-point each time the path changes. Thus, end-to-end signalling is necessary even when a local handover occurs, that is, when the path varies locally in the micro-mobility domain.

In order to provide a local reservation phase, it would seem sufficient to endow the LMG with additional RSVP capabilities, that is to address the PATH massage to the LMG when a local handover is performed, to proxy the Correspondent Node. In this case, the LMG should: i) recognise PATH messages sent by different addresses as belonging to the same user (as a consequence of the Handover procedures) and distinguishing the ones sent before Local Handover by the ones sent afterwards; ii) avoid forwarding towards the Correspondent Node the PATH messages generated after Local Handovers; iii) be aware of the error term(s) of the external (i.e. from the LMG to the Correspondent Node) part of the data path, in order to add to this term the information carried by the local PATH message; iv) keep track of the error term(s) of the previous local path of the data flow. At this point, two different events can occur:

If the new overall error term is smaller than the old one, a smaller bandwidth is needed. In this case, since no reservation refresh occurs in the external part of the path (i.e. from the LMG to the Correspondent Node) the resource waste could be significant in this part.

If the new overall error term is greater than the old one, a greater bandwidth is needed along the entire path (that is from the Mobile Node up to the Correspondent Node). In this case, the scope of reservation procedure following the handover can not be limited to the local domain.

We may conclude that, when RSVP is used to provide QoS over micro-mobility networks, each intra-domain user migration cannot be managed in a localised manner (as micro-mobility management suggests), but requires RSVP signalling information to travel back and forth to the destination node. This happens even if we modify the RSVP way of operation by adding to the LMG the above functionality. In addition, this solution would break the RSVP

loop, thus causing all the foreseeable and unforeseeable problems that arise when a protocol is changed with respect to its original specification.

A trivial way to keep the reservation procedures as local as the handover is, would be to adopt a worst case approach:

• the LMG should know the characteristics of all possible local paths;

• the LMG should intercept all the outgoing PATH messages and properly replace the therein contained error term with the worst case error term.

However, this would imply i) complexity of the LMG; ii) waste of resources; iii) un-necessary denial of connection requests (due to the worst case evaluation).

The aforementioned considerations lead us to conclude that RSVP/IntServ approach is not suitable for micro-mobility and IP QoS oriented networks. The rationale should be to have localised procedures, that is a hop-by-hop strategy in which each network element is completely independent from others (and most of all from end-systems) in measuring and monitoring resources and in deciding for admission control. In this perspective DiffServ architectures and EAC CAC algorithms seem to be the favourite candidates.

Another important consideration deals with QoS provision impact on the handover procedure itself. Plain Mobile IP and micro-mobility proposals are not intended to provide strict QoS parameters maintenance upon migrations, they simply aim at obtaining as much as they can when handovers occur. We can say that a “best effort” handover is performed. The provision of QoS changes this point of view. Handover procedures become a crucial point in maintaining QoS parameters unchanged while migrating from an access node to a new one.

When a Forced Handover occurs, a QoS setup phase should be performed after the handover in order to conclude this procedure as soon as possible. In this case no QoS information about the new point of attachment are available before the handover starts (otherwise a Voluntary handover would occur) and collecting these information could delay the handover procedure. In this case it is more urgent to obtain plain connectivity in order to establish a new wireless link over which CAC procedures can be carried out. On the other side, Voluntary Handover should be performed in order to switch session on QoS provisioning links. Therefore it must include a QoS setup phase and it is successfully completed only when (and if) a new QoS providing data path is established.

Moreover it must be said that handover procedures can be triggered by the Mobile Node or by some network element which is aware of mobile host movements and cells position/coverage. We assume Mobile Controlled HandOver (MCHO, [POL96]), i.e., triggered by the MN according to some internal procedures.

REFERENCES

[BFP01] WP4 internal report, DIEI department, University of Perugia, N. Blefari-Melazzi, M. Femminella, F. Pugini, “QoS and Mobility support in wireless Mobile IP environments”, May 2001.

[BRE00] L. Breslau, E. W. Knightly, S. Schenker, I. Stoica, H. Zhang: "Endpoint Admission Control: Architectural Issues and Performance", ACM SIGCOMM 2000, Stockholm, Sweden, August 2000.

[CEL00] A. Campbell, J. Gomez et alii: “Cellular IP”. Draft-ietf-mobileip-cellularip-00.txt. Work in progress.

[DIS98] S. Blade, D. Black, M. Carlson, E. Davies, Z. Wang, W. Weiss, "An Architecture for Differentiated Services", RFC2475, December 1998.

[HMIP6] H. Soliman, C. Castelluccia, K. El-Malki, L. Bellier: “Hierarchical MIPv6 mobility management”. Draft-ietf-mobileip-hmipv6-02.txt. Work in progress.

[MIPV6] D. B. Johnson, C. Perkins: “Mobility Support in IPv6”, draft-ietf-mobileip-ipv6-13.txt, Work in progress.

[MIS00] A.Misra, S.Das, A.Mcauley, A.Dutta, S.K.Das, “IDMP”: An Intra-Domain Mobility Management protocol using Mobility Agents”, Internet draft, draft-misra-mobileip-idmp-00.txt, July 2000, work in progress.

[PER96] C. Perkins, “IP Mobility Support”, RFC 2002, October 1996.

[POL96] G. P. Pollini, “Trends in Handover Design”, IEEE Communication Magazine, March 1996,Vol. 34, No. 3.

[RAM00] R.Ramjee, S.Thuel, T.La Porta, K.Varadhan, “IP Micro-Mobility support using HAWAII”, Internet draft, draft-ietf-mobileip-hawaii-01.txt, July 2000, work in progress.

[REG00] J. Malinen, C. Perkins: “Mobile Ipv6 Regional Registrations”. Draft-malinen-mobileip-regereg6-00.txt. Work in progress.

[RSV97] J. Wroclawsky, "The use of RSVP with IETF Integrated Services", RFC2210, September 1997.

ACRONYMS

AN Access Node

AS Advertisement Solicitation

CAC Connection Admission Control

CBR Constant Bit Rate

CN Correspondent Node

CoA Care-of Address

EAC Endpoint Admission Control

ErCS Eager Cell Switching

EyCS Early Cell Switching

FAg Foreign Agent

HAg Home Agent

LCS Lazy Cell Switching

LMG Local Mobility Gateway

LMN Local Mobility Network

MAC Medium Access Control

MD Movement Detection

MIP Mobile IP

MN Mobile Node

QoS Quality of Service

RSVP Resource Reservation Protocol

RTT Round Trip Time

WAN Wide Area Network