PERFORMANCE ISSUES FOR LOCALISED IP MOBILITY MANAGEMENT
Theodoros Pagtzis and Peter Kirstein
Dept. of Computer Science
University College London, U.K.
ft.pagtzis,p.kirstein
g@cs.ucl.ac.uk
Charles Perkins
Nokia Research Center
California, USA
[email protected]
ABSTRACT
Base IP mobility presents performance limitations when the mo-bile host increases its handoff rate between successive wireless points of attachment as a result of its mobility pattern. These limi-tations are augmented when latency externalities such as the round trip time between the MN and its peers, add to the total of latency and signalling overhead, impeding the performance of the mobil-ity function. To alleviate that, IP mobilmobil-ity management has been extended by localised IP mobility management functions focusing within the context of micromobility.
This paper presents an investigation on performance issues for localised IP mobility management (LMM). Based on generic prin-ciples derived from the effect of localising mobility control sig-nalling, it explores the behaviour of the localisation effect of the mobility management entity.
1. INTRODUCTION
The paradigm shift in network access practices from wireline to wireless infrastructures instigates expectations for IP-mobile ser-vice quality similar to wireline access. Such vision, however, poses a number of technical issues in performance and scalabil-ity for base IP mobilscalabil-ity management protocol standards [5]. The majority of such issues stem from the distribution of mobility man-agement signalling between the mobile node (MN) and its commu-nication peers, namely Home Agent (HA)1and its
Correspond-ing Nodes (CNs). They manifest themselves as increased latencies
in the control signalling between MN and its peers.
Real-time application services, however, impose stringent de-lay boundaries in IP traffic delivery [6, 8, 4]. IP mobility-related latency beyond these boundaries, will cause real-time IP traffic to experience noticeable degradation in quality, as the MN moves be-tween different IP-enabled coverage areas. This is due to the re-quirement to provide its peers with valid mobility bindings2 that yield a reachable IP destination through a new Access Router
(AR) within the visited domain.
As the round-trip time (RTT) between the MN and its peers ex-ceeds the aformentioned delay boundaries3, IP mobility control signalling injects latency in the communicated traffic of the MN, during handoff. This introduces a strong possibility for packet loss4 or in worst-case scenarios loss of the call session for that
class of IP applications.
1For the sake of clarity and robustness, terms identified in this paper
and their abbreviations will be used interchangeably
2through a cross-domain mobility Binding Update.
3on average above 150-200 msec for one way delay
4late packets are considered lost in real-time communications
Latency is further exacerbated as the transition rate of the MN increases, between different networks or IP links effected over small cell radius. The rate at which mobility bindings are effected at peers is essentially tracked by the RTT between the MN and each of these peers. This is, for instance, the case with MNs com-municating with globally distributed HTTP servers as their peers. These CN entities will experience increased signalling overheads the effect of which is dependent on RTT between the MN and that HTTP server.
To alleviate the above IP mobility management issues, a number of Localised Mobility Management (LMM) protocols have been proposed in the Mobile IP WG for next generation Internet (IPv6), in the greater context of micromobility [3]. These LMM proposals [9, 2] currently address only part of the space pertaining to the per-formance of the LMM function. Analysing further the underlying design considerations of the core LMM mechanism is important in confering on quantifiable metrics of the effect or efficiency of localisation on IPv65mobility management.
This paper is organized as follows: Section 2 presents the prin-ciples behind localised mobility management schemes. Section 3 presents an analysis of the signalling overhead metrics devised to quantify the performance of the LMM function in terms of sig-nalling. Section 3 describes our simulation results. We conclude with a summary on insights about comparative performance be-tween base IP mobility and the LMM function in Section 5.
2. PRINCIPLES OF CORE LOCALISED MOBILITY MANAGEMENT
A localised mobility management scheme does not attempt to eliminate propagation of mobility bindings to the HA6. It strives to minimize excessive IP mobility management signalling (BUs) towards its peers, caused by frequent change of care-of address (CoA).
This is achieved by establishing an entity instance of the home
domain, similar to the Home Agent, into a visited administrative
domain accomodating the MN; such entity is generally identified as Localised Mobility Management Agent (LMM agent). By ensconcing the LMM agent closer to the MN as shown in figure 1, MN transitions are now managed according to the topological lo-cality of the visited domain as opposed to the total number of hops between the MN and its Home Agent. Network transitions for an MN can now be characterised as:
5This paper focuses on Localised mobility management over next
gen-eration IPv6 networks. For this reason the terms IPv6 and IP will be used interchangeably
intra-domain transitions. The MN transits within a set of
subnets/sub-domains that define the topology of a single ad-ministrative network domain. Base Mobile IP does not dis-tinguish between intra and inter-domain transitions.
inter-domain transitions. The MN transits between different
administrative domains.
The distinction between types of transitions by an LMM scheme, extends standard7IP mobility by identifying a separation of two classes in control signalling messages with respect to the
locality of the notion of an adminstrative domain:
localised or regional mobility signals (intra-domain):
bound within a single administrative domain and primarily sent towards some LMM agent. Localised mobility signals are generally referred as Regional Binding Updates (RBU).
global mobility signals (inter-domain): communicated
across different administrative domain with ultimate desti-nation the true peer entities, namely the HA and CNs of the MN. Global mobility signals are commonly known as
Bind-ing Updates (BUs).
: RBU−ACK : Access Network Router (ANR) : Access Router (AR)
Scope of visibility for RCoA
Scope of visibility for Backbone RTT component (variable)
Home Domain signaling RTT LMM base MIPv6 (BU) mobility management LMM agent : Global BU−ACK : Regional BU (RBU) LCoA : Global BU (BU) CN Internet domain edge HA V−CA H−CA MN 00 00 00 11 11 11 000 1110011 0 0 0 0 0 0 1 1 1 1 1 1 00 00 11 11 0 0 0 0 0 0 1 1 1 1 1 1 000 000 111 111 00000 00000 11111 11111
Figure 1: Localised mobility management. Multiple intra-domain RBUs vs. single inter-domain BU
This separation in IP mobility signalling by means of an LMM agent provides effectively for a level of indirection in two respects:
signalling and addressing of the MN. This design approach is
supported by measuments in [7] identifying that 69% of the mo-bility of a user remains local with respect to a reference location.
With respect to signalling, the end-to-end message overhead ex-perienced by the communication peers is reduced for the time pe-riod that the MN remains mobile within some visiting domain. As illustrated in figure 1, the MN informs its peers of its change of domain-wide CoA when it moves from coverage areaH CA
toV CA by means of a global BU while the LMM agent is
informed accordingly through an R-BU signal. For the remaining time that the MN transits through the particular network domain no global BU signals are sent towards the peers; instead an RBU signal is sent to the edge LMM agent.
The addressing indirection supports signalling by allocating to the MN a mobility global or regional care-of address (RCoA) for its entire mobility pattern within the visited domain, and a
7the terms standard or base IP mobility refers to the Mobile IPv6
pro-tocol standard [5] and will be used interchangeably
mobility-local or on-link care-of address (LCoA) to the MN. While the RCoA address is visible globally to peers and thus lo-cates the MN with domain-wide accuracy, the LCoA address is visible explicitly only within the visited domain.
Localizing the mobility bindings carries further associated ben-efits on the communicated packet flows between the MN and its peers. These are manifested primarily as reduction of signalling latencies and accompanied packet loss. It is thus, possible that an LMM protocol may contribute in sustaining stringent packet de-livery constraints for real-time dede-livery of IP traffic such as inter-active multimedia; nevertheless, existence of an LMM mechanism by itself does not warrant real-time performance in such classes of applications. Real-time performance depends on other factors such as IP connectivity latencies [11, 10] with respect to the LCoA; such factors are typically outside the scope of core LMM mecha-nisms.
3. CORE LMM PERFORMANCE
Studying the perfomance of the signalling and addressing indirec-tion effected by an LMM scheme is important in deriving a robust set of requirements or boundaries that must be set as a lowest com-mon denominator in any LMM protocol design. The importance pertains to the need of controling the signalling overhead towards the communicating peers as the handoff rate of an MN increases, such that, IP mobility signalling (BUs) can scale for very large number of MNs. In the following sections we present a qualita-tive as well as quantitaqualita-tive treatise on signalling performance that encompasses essential considerations for the design of an LMM protocol mechanism.
The reduction of control messaging towards the peers of the MN has been the fundamental design consideration for LMM mecha-nisms in the light of providing scaleable IP mobility. In this sec-tion we investigate the signalling overhead of the LMM funcsec-tion against standard IP mobility and boundaries for simulated random variables under signalling stress conditions.
3.1. LMM performance metrics
The efficiency of an LMM mechanism depends primarily on the amount of signalling that is sustained locally for an MN before the MN exits visited domain; such localisation factor may be identi-fied as the frequency of intra-domain transitions against the total number of transitions in the travel path of an MN. To quantify this relation, we introduce the metric LMM gain, denoted as(t)in
the following form:
(t)= P i=0 i(t) P i=0 i (t)+(t) [0;1℄ (1) where i
(t)is the frequency of intra-domain transitions within the i
th
visting domain and(t)is the frequency of inter-domain
tran-sitions per unit time. tis considered to be time spent within the
visited domain. If(t)=1that implies zero frequency of
inter-domain transitions. As(t)approaches zero, the LMM scheme
employed provides no benefit compared to standard IP mobility. The LMM gain reflects the impact of the LMM function on sig-nalling overheads. Its components, namely the frequency of intra-or inter-domain transitions, are directly related to the number of LMM signals required to maintain mobility bindings for an MN. To compare against standard IP-MM signalling, the related cost of creating and maintaining mobility binding at a set of k CNs
need also be derived. For that a random travel path originating at the home link and ending at some destination link within a visited domain, is considered. The travel path is represented as the total number of link transitionsnhbetween home and destination links
met in the movement trajectory of the MN. We analyse the num-ber of cross-link transitions versus the separation of between intra-and inter- domain hintra-andoffs.
To aid the analysis, the lifetime8of a BU at the peers, is taken
to be always longer than the period the MN remains attached to some link/AR; this factors out the number of BUs sent to refresh the existing mobility bindings (BU lifetime expiry) at the peers since it is common to both base IP mobility or LMM extensions and as such will only inflate the derived results. This simplification without loss of substance in the metric observations, yields a single global BU sent towards each of the peers of an MN per link/AR transition.
In standard IP mobility, for nh number of handoffs the MN
will generate a minimum9 signalling cost, denoted as
C mip, of the form: C mip = ( (k+2)n h if Previous HA BU included ; (k+1)n h otherwise (2) withkthe number of CNs communicating with the MN; core IP
mobility does not distinguish between domains. For an LMM scheme, the associated signalling cost per MN, denoted asC
lmm becomes: C lmm = 8 > < > : r(2+k)+2 P r l=0 nl Previous HA BU; r(2+k)+ P r l=0 n l otherwise and n h =r+ P r l=0 n l (3)
wherenlis the number of intra-domain handoffs forrvisited
ad-ministrative domains. The number of visited domains corresponds also to the number of inter-domain handoffs, while the number of signalled peers increases by one, since an R-BU is required to be sent towards the LMM agent per inter-domain handoff for LMM registration.
Equation (2) shows that forn
htransitions an equal number of
BUs10has to be sent by the MN towards all peers over the Internet backbone. In the case of an LMM scheme (3), the MN signals only one R-BU to the LMM agent per intra-domain handoff, indepen-dent of the number of peers; only a single global BU is required to each peer per inter-domain handoff. From (3) we may deduce the following:
Lemma 1 The frequency of global BUs for a single MN over
LMM mechanisms, is equal to one per peer (HA or CN) per ad-ministrative domain, for any number of transitions in that domain.
Lemma 2 The frequency of R-BUs for a single MN over LMM
mechanisms, is equal to one per transition for any number of com-municating peers.
Lemma 3 The LMM function yields signalling benefits over base
IP mobility if and only the number of intra-domain handoffs per domain in the travel path of the MN exceeds a minimum threshold value
8in practice the MN must refresh the lifetime of the binding upon its
expiry (binding request).
9minimum because bindings have lifetimes that need to be refreshed
regardless whether the node moves or not constantly
10optionally, the previous link HA may receive a regional BU.
Lemma 1 may be proved trivially by settingn l
=0in (3), while
Lemma 2 is shown by settingr=0in (3), whilen0is the number
of intra-domain handoffs of the first domain dwelling for the MN. Lemma 3 may be proved for the case where no previous HA BU is employed in equation 3, as follows:
Clmm=(nh+ r X l=0 nl)(2+k)+ r X l=0 nl 2n h r X l=0 n l +kn h k r X l=0 n l (k+1)nh+[nh (k+1) r X l=0 nl℄ (4)
If the LMM function is to preserve superiority in signalling cost over base IP mobility then equation (4) becomes:
C lmm <Cmip (k+1)nh+[nh (k+1) r X l=0 nl℄<(k+1)nh [n h (k+1) r X l=0 n l ℄<0 (k+1) r X l=0 nl>nh r X l=0 n l > nh k+1 n l > n h r(k+1) (5) wheren
lis the average number of intra-domain handoffs by
the MN within a visited domain, and thus the minimum threshold value satisfying Lemma 3.
From the minimum valuen
lof (5), occurs that the signalling
cost of both IP mobility and LMM extension must consider two factors: the rate of arrival of new CNs at the MN and the rate of change of the visited domain in the travel path of that MN; the latter is modelled as single inter-domain transitions randomly dis-tributed over multiple intra-domain handoffs. To quantify the mea-sure of intra-domain handoffs we introduce the metric domain
crossing; it is defined as the number of wireless links traversed
within a visited domain before an inter-domain transition is en-countered; it is denoted asTr
, and comprises part of the travel
path of an MN towards a destination.
Both arrival rates were modelled as Poisson processes in the NS-2 simulator [1], with monotonically increasing upper bound-aries of CNs and IP mobility links per domain. The entire travel path of the MN comprized of 5000 handoffs. Each scenario of varied number of CNs and domain crossing was iterated for a min-imum of 40 times with averages computed for a 95% confidence interval.
4. SIMULATION RESULTS
The travel path of the MN was simulated as monotonically increas-ing straight-line trajectories passincreas-ing through coverage areas within a random area defining an administrative domain.
Variable Domain Crossing Discrete
Real MN mobility trajectories can be modelled as straight line trajectories when studying the underlying domain crossing
MN
MN mobility trajectories (direction independent)
Figure 2: MN mobility trajectories modelled as straight lines when focusing on domain crossing
To stress the signalling savings from the LMM function against base IP mobility, the number of CNs was varied in relation to the number of IP mobility links in thei
th
domain crossing component of the MN’s trajectory. The simulations are insensitive to direc-tion, cell size, or velocity of the MN since the measurements are targeting signalling behaviour as a function of the domain crossing and number of CNs.
Figure 3 shows the lower boundary ofTr that an MN must
maintain during its travel path as a function of the number of CNs. Graph (a) shows that the LMM function offers signalling savings over base IP mobility forCN =2only when the MN maintains
a domain crossing ofTr 6. As the number of CNs increases,
the LMM function establishes signalling savings at lower domain crossing values; forCN = 3, IP-LMM performs better when Tr 5, while forCN =5andCN = 10or above IP-LMM
signalling scales better than IP-MM forTr
4andTr
3
respectively. BeyondCN =10the LMM function does not
of-fer any further signalling savings for boundaries any lower than
Tr
3
11
We experimented further with the behaviour of LMM BU sig-nalling as the number of CNs increases (doubles) with respect to the domain crossing in the travel path of the MN. We observed that while the BU signalling for base mobility grows as the domain crossing and number of CNs increase, in the case of the LMM function the BU overhead remains approximately constant at about 21000-22000 BU signals whileTr 8. For larger domain
cross-ingsTr
[16;40℄the BU overhead stabilises at about 17000 BU
signals while the number of CNs continues to grow up to CN=100. The simulations were iterated while increasing12 the average domain crossing against the average number of CNs communicat-ing with the MN in the respective Poisson processes. We observed that the number of BU signals peak for the LMM function at about 10000 for small domain crossing (Tr
[0;5℄) and two
communi-cating CNs. As theTr and CNs increase, BU signalling for the
LMM function reaches a lower bound of around 8000-7500. For larger domain crossingsTr
[40;100℄with half the no of CNs,
BU signalling peaks for LMM at about the same lower bound, that is 7500-8000 signals.
It was further important to understand and establish perfor-mance boundaries for the core LMM function against standard IP
11This is derived by computing the domain crossing boundary for very
large number of CNs(CN=12000). 12double 1 10 100 1000 10000 No. of transitions 0 5000 10000 15000 20000 No. of BU signals std. MIP BU signals LMM BU signals (Tr=1) LMM BU signals (Tr=3) LMM BU signals (Tr=4) LMM BU signals (Tr=6) LMM BU signals (Tr=7) Tr=1 Tr=6 Tr=7 std MIP signals (a) CN=2 1 10 100 1000 10000 No. of transitions 0 5000 10000 15000 20000 No. of BU signals std. MIP BU signals LMM BU signals (Tr=1) LMM BU signals (Tr=2) LMM BU signals (Tr=3) LMM BU signals (Tr=4) LMM BU signals (Tr=5) LMM BU signals (Tr=6) Tr=1 Tr=6 Tr=5 std MIP signals (b) CN=3 1 10 100 1000 10000 No. of transitions 0 5000 10000 15000 20000 25000 30000 No. of BU signals std. MIP BU signals LMM BU signals (Tr=1) LMM BU signals (Tr=2) LMM BU signals (Tr=3) LMM BU signals (Tr=4) LMM BU signals (Tr=5) Tr=1 Tr=5 Tr=4 std MIP signals (c) CN=5 1 10 100 1000 10000 No. of transitions 0 10000 20000 30000 40000 No. of BU signals std. MIP BU signals LMM BU signals (Tr=1) LMM BU signals (Tr=2) LMM BU signals (Tr=3) LMM BU signals (Tr=4) Tr=1 Tr=4 Tr=3 std MIP signals (d) CN=10
Figure 3: Signaling gains from the core LMM function are de-pendent on Domain crossingTr
. Hower, Tr
reaches the lowest
value as the number of CNs increases.
mobility. For this purpose we devised in the simulations two met-rics: the aforementioned LMM gain and signalling efficiency ratio
(SER) with respect to the number of communicating CNs as well
as the size of the domain crossing potentially effected in the mo-bility trajectory of the MN. The SER metric is different from the gain of an LMM scheme since the former accounts for the fac-tor achieved in signalling efficiency of the LMM function against standard mobility. In contrast the LMM gain accounts for the sig-nalling gains achieved by the LMM function in the face of variable domain crossing.
With respect to the gain factor achieved from the LMM function in the presence of variable domain crossing in the travel path of the MN we observed an exponential increase such that the gain factor rises sharply (Figure 4.a) when the number of CNs remains be-tween 1-4 and the domain crossingTr
20. The rate of increase
of the exponent drops in that CN range considerably for higher val-ues ofTr , which implies that for small numbers of CNs a
mod-erate domain crossing is sufficient to achieve high signalling gains in an LMM scheme against base IP mobility. For larger number of communicating CNs the LMM gain curve reaches considerably smaller rate of increase effecting a gain of 0.8 at aboutTr =60
whenCNs =10. We can deduce that for an upper bound of 5
CNs, high signalling gains are attainable for Tr
20. As the
number of CNs grows beyond this figure the signalling gain be-gins to drop; the drop can be compensated with an increase of the domain crossing. However, for large number of CNs the LMM gain will be considerably lower (than for small CN); this is only marginaly compensated with larger domain crossing values.
As a function of the number of CNs, the LMM gain presents an almost symmetric behaviour against the domain crossing size.
0 20 40 60 80 100 Domain Crossing (Tr_c) 0 0.2 0.4 0.6 0.8 1 LMM gain CN=1 CN=2 CN=3 CN=5 CN=10 CN=25 CN=50 CN=100 (a) Gain w.r.tTr 0 20 40 60 80 100 Domain Crossing (Tr_c) -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Signaling efficiency ratio (LMM/MIP)
CN=1 CN=2 CN=3 CN=4 CN=5 CN=10 CN=20 CN=50 CN=100 (b) SER w.r.tTr 0 20 40 60 80 100 No. of CNs 0 0.2 0.4 0.6 0.8 1 LMM gain Tr_c=1 Tr_c=3 Tr_c=5 Tr_c=10 Tr_c=50 Tr_c=100 (c) Gain w.r.t CNs 0 20 40 60 80 100 No. of CNs -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Signaling efficiency ratio (LMM/MIP)
Tr_c=1 Tr_c=2 Tr_c=3 Tr_c=4 Tr_c=5 Tr_c=10 Tr_c=50 Tr_c=100 (d) SER w.r.t CNs
Figure 4: LMM gain and signalling efficiency ratio per MN as a function of the number of CNs and the domain crossingrin the
travel path of the MN
Sharp drops were observed in the signalling gain from the LMM function for small domain crossing as the number of CNs in-creases. The LMM gain diminishes as the number of CNs con-tinues to grow faster than the domain crossing. Best LMM gain values are attained forCN10andTr 1.
With respect to signalling efficiency ratio the LMM function becomes superior against IP-MM forTr
3; the efficiency of
the LMM BU signals improve dramaticaly whenTr [3;21℄.
Figure (5.a) shows that the majority of the domain crossing distri-butions for different number of CNs follows the same pattern and for -approximately- the same peak values. By means of non-linear regression, the residuals of each set of points from each distribu-tion (per CN sample) were analysed and a curve fit describing the behaviour of these distributions was derived; such curve results maximalSER=0:82forTr
21. The result maintains a 95%
confidence interval with the extreme outliers removed from the fit, as they mislead the regressor.
The above fit shows that the LMM function yields significant signalling reductions as the size of the domain crossing increases in the travel path of an MN, (approximately) irrespective of the number of the CNs. This is not because the LMM function yields higher savings as the number of CNs increases but because base IP mobility induces excessive signalling for a growing number of CNs. This can also be seen from the PDF of the LMM gain which is far lower and more spread out over the domain crossing when accounted against the number of CNs.
AsTrincreases per visited domain, in the travel path of the MN,
the regional BU signalling component of an LMM mechanism re-lies more on the number and size of signalling interactions between the MN and LMM agent; this is because the MN requires short re-sponse times by the edge LMM agent, for an increased number
0 8 16 24 32 40 Domain Crossing (Tr_c) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 PDF(x) -(SER) (a) PDF(x) - SER 0 20 40 60 80 100 Domain Crossing (Tr_c) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 PDF(x) - LMM gain CN=1 CN=10 CN=100 CN=25 CN=50 (b) PDF(x) - LMM gain
Figure 5: Probability Distribution Function for Signalling Effi-ciency Ratio and LMM gain as a function of the domain crossing
Tr
of transitions within consequtive visited links (size ofTr) within
that domain. If the LMM mechanism introduces further signalling interactions in this path, that may impede the signalling efficiency of localising the update of the mobility binding; this can detract from the benefit of employing an LMM mechanism as opposed to base IP mobility. Signaling overhead may be manifested as in-creased latency in acknowledging the regional BU (RBU-ACK) due to congestion and potentially subsequent packet loss of mobil-ity signals. Thus, the core LMM function must avoid introducing
signalling overheads on the path between the mobile and the LMM agent(s) that can detract from the signalling efficiency of the IP
LMM mechanism.
However, from the perspective of added-value features, elim-ination of new signalling overheads transposes to performance tradeoffs against feature-richness in LMM schemes; each LMM approach may tackle localisation in mobility management from a different angle/perspective and thus, present different signalling requirements. For instance, tight coupling of LMM extensions with context transfer considerations could yield feature-rich LMM schemes that unavoidably require byte-wise or packet-wise more than a single regional BU interaction between MN and LMM agent; for instance, header compession context relocation between two ARs, requires two signalling interactions (and accordingly RTTs) before it can establish compression context at the new point of attachement. Furthermore,with respect to security, introduction of at least one level of signalling as well as traffic forwarding in-direction, establishes a need for security of these LMM functions, through establishment of security context association state at the LMM agent(s). This presents a realistic perspective where sig-nalling must be augmented, unavoidably, to support robustness of the LMM function.
In addition, while there is growing support that a single BU/RBU message must suffice for signalling between an LMM agent and the MN, the restriction on the number of BU/RBU sig-nals is only apparent. The rationale behind such argument is that by design, a BU message can be easily inflated by extending it with BU suboptions. This design approach allows extensibility of the base IPv6 mobility protocol which is typically inherited to most IP mobility extensions such as an LMM scheme. Thus the restriction of signalling overhead elimination/reduction becomes in principle void since the protocol designer has other means of introducing/extending his signalling needs by tailoring the BU to
his signalling needs. This is also expected to be the case for the configuration task of the core LMM function with respect to the registrant MNs hosted by some LMM agent within the visited do-main: Router Advertisments and subsequent BU/RBUs are pop-ulated with signal suboptions. Although a neuralgic spot for any LMM scheme, the configuration issue of the core LMM function is considered by many as of secondary importance; however scala-bility concerns as we see further, promote it amongst the essential requirements, if the LMM protocol design is to address realistic deployment over scalable network infrastructures.
It is for this reason that elimination/reduction of signalling over-heads, in the scope of this paper, pertains to the core LMM func-tionality as opposed to diversity in feature-rich LMM mechanims. However to effect some control on LMM-related signalling over-head we propose that the elimination/reduction of BU signalling is considered from the perspective of bytes with respect to the MTU size supported by the individual access link. This is in addition to the discrete number of mobility signal which primarily affect the RTT between LMM agent and MN and can thus degrade the LMM function from the perspective of signalling latencies.
5. CONCLUSIONS
In this paper we presented an investigation on the signalling per-formance of the core localised IP mobility management function. In particular, we analysed the signalling cost in terms of binding update signals that are expended during inter/intra domain transi-tions by a mobile node and we compared the BU signalling per-formance of MIP and core LMM function. We concluded that for a small number of peers (i.e.CN2) communicating with some
MN, an LMM mechanism does not offer significant signalling sav-ings unless the domain crossing of the MN has a minimum size of
Tr6.
This above has been derived analyticaly by means of identify-ing the minimum threshold value that the domain crossidentify-ing must maintain, before the LMM function can yield signalling savings over base IP mobility. Simulations have in fact shown that if the movement trajectory of the MN is tangental over the span of some visited domain (small domain crossing), the LMM function is ex-pected to perform worse during mobility bindings signalling than base IP mobility.
6. ACKNOWLEDGMENTS
This work has been a collaborative effort between University Col-lege London (UCL) and Nokia Research Center (NRC), CA/USA; we would like to acknowledge Nokia Research Center, CA/USA, for supporting this work through its internship programme.
We would further like to thank Hannu Flinck (NRC/Mountain View), Jari Malinen (NRC/Mountain View), Sandro Grech (NRC/Helsinki) for their support and valuable discussions during the writing of this paper. Thanks also to Carl Williams (NTT-Docomo) and James Kempf (NTT-(NTT-Docomo) for their feedback in improving the presentation of this paper.
The authors would also like to acknowledge the constituents of the IETF Mobile IP WG and the existence of the respective mailing list that assisted in the faster assimilation of issues regarding our contributions on requirements for localised mobility management schemes.
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