routing scalability in multi-domain dwdm networksnghani/papers/pnet2009.pdf · routing scalability...

Photon Netw Commun (2009) 17:63–74DOI 10.1007/s11107-008-0143-0

Routing scalability in multi-domain DWDM networks

Qing Liu · Chongyang Xie · Tannous Frangieh ·Nasir Ghani · Ashwin Gumaste · Nageswara S. V. Rao

Received: 16 May 2008 / Accepted: 16 July 2008 / Published online: 18 September 2008© Springer Science+Business Media, LLC 2008

Abstract This paper studies routing scalability in multi-domain DWDM networks. Although inter-domain provision-ing has been well studied for packet/cell-switching networks,the wavelength dimension (along with wavelength conver-sion) poses many challenges in multi-domain DWDM set-tings. To address these concerns a detailed GMPLS-basedhierarchical routing framework is proposed for multi-domainDWDM networks with wavelength conversion. This solu-tion uses mesh topology abstraction schemes to hide domain-internal state. However related inter-domain routing loadscan be significant here, growing by the square of the num-ber of border nodes. To address these scalability limitations,improved inter-domain routing update strategies are also pro-posed and the associated performance of inter-domain light-path RWA and signaling schemes studied.

Keywords Multi-domain networks · Optical networks ·GMPLS · Inter-domain DWDM routing · Topologyabstraction · Full wavelength conversion

1 Introduction

In recent years dense wavelength division multiplexing(DWDM) has seen fast growth in long-haul and metro sec-tors [1]. DWDM exploits the huge unused spectrum in single

Q. Liu · C. Xie · T. Frangieh · N. Ghani (B)ECE Department, University of New Mexico, Albuquerque,NM 87111, USAe-mail: [email protected]

A. GumasteIIT Bombay, Mumbai, India

N. S. V. RaoOak Ridge National Laboratory, Oak Ridge, TN, USA

mode fibers (SMF) to transmit multiple DWDM channels,thus yielding unprecedented terabits per second level speeds.As this technology matures, related standards have alsoemerged. Most notably, the IETF has defined a general-ized multi-protocol label switching (GMPLS) framework foroptical network provisioning. Specifically GMPLS providesextensions for wavelength routing, signaling, and link dis-covery [2] by adapting packet-based multi-protocol labelswitching (MPLS) protocols for circuit-switched provision-ing. In addition the ITU-T has specified a comprehensiveautomatic switched transport network (ASTN) frameworkbased upon a multi-level routing hierarchy approach [2].Finally, the OIF has also standardized several optical inter-connection protocols, both at the user edge, i.e., user networkinterface (UNI), and between networks, i.e., network-to-net-work interface (NNI).

On the research side, DWDM routing and wavelengthassignment (RWA) algorithms have been extensively studied.The goal of RWA is to find an optimized route and wave-length so as to minimize the overall blocking rate. In partic-ular [3] presents a comprehensive performance analysis ofvarious wavelength assignment schemes, e.g., first-fit (FF),most-used (MU), least-used (LU). Many DWDM surviv-ability techniques have also been developed, with key typesincluding dedicated path protection, shared path protection,and shared segment protection [1,2]. However most DWDMstudies have focused on single-domain settings and assumethe availability of “globalized” domain-level state, i.e., viacentralized and/or distributed routing. In general, thisapproach is only applicable to small-scale networks, e.g.,tens of nodes. As the number of DWDM nodes increases,this “flat” topology approach poses many restrictions, suchas high storage cost, slow convergence time, low scalability.It is evident, therefore, that some effective form of domain-level partitioning and inter-domain/multi-domain routing is

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64 Photon Netw Commun (2009) 17:63–74

needed [4,5]. Furthermore commensurate inter-domain RWAalgorithms are also required to provision lightpath connec-tion requests.

Multi-domain DWDM networking is becoming a keyfocus area today [5–11]. From a standards perspective thereare advanced efforts within the OIF to define signaling androuting protocols across domain boundaries. In particular thelatest external-NNI (E-NNI) proposal details a hierarchicalrouting setup based upon modified open-shortest path first-traffic engineering (OSPF-TE) [2], i.e., link-state routing.Meanwhile on the research side a variety of schemes havebeen studied for lightpath provisioning in all-optical [8,9]and even mixed optical/opto-electronic conversion (regen-eration) [5,10] networks. The latter is an important consid-eration given the increased distance and coverage of suchnetworks. However the detailed routing scalability implica-tions of inter-domain DWDM networks have not yet beenstudied. This is a very crucial concern as related resourcestate information in optical networks (wavelength, convert-ers, etc.) is very different from more traditional data/packetrouting networks. Along these lines this paper addressesinter-domain routing scalability in distributed multi-domainDWDM networks with wavelength conversion. Specificallythe work develops new and improved link-state routingupdate/triggering schemes for multi-domain DWDM net-works.

This paper is organized as follow. First, Sect. 2 surveyssome of the recent work in inter-domain DWDM networkprovisioning. Subsequently, Sect. 3 outlines the proposedGMPLS-based inter-domain solution. Section 4 then extendsthis work by presenting improved inter-domain routing updatestrategies to boost scalability for high levels of virtual linkstate. Detailed performance evaluation studies are then con-ducted in Section V for a wide range of multi-domain networktopologies. Finally, conclusions and future research direc-tions are presented in Sect. 6.

2 Background

Various multi-domain DWDM studies have been conductedto date, with a primary focus on lightpath selection andresource reservation. For example, [5] presents a theoreticalanalysis of state aggregation in multi-domain DWDM net-works with border node conversion. Here various informa-tion models are studied and lightpath assessment is modeledas a Bayesian decision problem. However this treatment onlyconsiders (restricted) bus topologies; thus, the crucial issueof aggregated topology computation between domain bordernodes is not considered. Furthermore no routing protocols ordistributed lightpath setup signaling procedures are detailed.Meanwhile an earlier proposal in [6] presents an architectureusing route advertise/withdraw messaging between domains

running modified border gateway protocol (BGP). Here spe-cialized proxy lightpath route arbiters (LRA) are proposed tocompute routes between border optical cross-connect (OXC)nodes, which must maintain complete (alternate) route state.Nevertheless the detailed algorithmic study of this schemein the context of inter-domain DWDM routing/provision-ing algorithms is not done. Alternatively the authors in [7]present a domain-by-domain routing and signaling schemefor inter-domain lightpath setup in which domain gatewaysmaintain complete (alternate) route state. However relatedresource propagation issues, i.e., inter-domain routing, arenot considered and hence this setup is more favorable to BGP-type implementations. Meanwhile [8] tables a hierarchicalinter-domain solution for ASON based upon a simple node(coarse) abstraction policy. However no signaling extensionsare presented here and hence the scheme is best-suited forcentralized implementation. Additionally no provisions aremade in [8] for opto-electronic conversion/regeneration, acritical necessity in inter-domain settings. Furthermore [9]extends the work in [8] by presenting a new RWA strategybased upon the stochastic estimation of effective number ofavailable wavelengths (ENAW) along inter-domain paths.Particularly an adaptive Kalman filter is devised to furtherrefine the estimation. However detailed topology aggrega-tion schemes and/or signaling setup extensions are still notconsidered—key requirements in distributed operational set-tings.

More recently some novel solutions for inter-domain RWAare presented in [10] and [11]. Namely the authors detail ahierarchical link-state routing model to condense and prop-agate domain-level state using advanced topology abstrac-tions. Furthermore commensurate lightpath computation andsetup algorithms are also analyzed. Note that the use oflink-state routing between optical domains is in-line withthe overall NNI standards being developed within the OIF.Overall results in [10] and [11] show much improved inter-domain lightpath blocking performance with the incorpora-tion of full-mesh topology abstractions. However associatedinter-domain routing loads are notably higher and alternatestar-based topology abstraction tends to give mixed results[11]. This forms the main motivation of the work herein,i.e., to improve link-state routing scalability in multi-domainDWDM networks.

Note that the overall concept of topology abstraction isnot new and has been shown to give good improvementsin packet-switching networks with capacity and delay linkconstraints, see [12–14]. However it is generally difficult todirectly re-apply these algorithms in circuit-switchingDWDM networks owing to the specific considerations/limitations of the optical link state. Specifically the main con-straints are no longer bandwidth and delay, but instead, wave-lengths and converters. Moreover these DWDM resourceconstraints are also more integral/fixed in nature. Hence a

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Photon Netw Commun (2009) 17:63–74 65

novel framework is now detailed for adapting these method-ologies into multi-domain DWDM networks.

3 Distributed inter-domain lightpath provisioning

Distributed inter-domain RWA is very challenging, partic-ularly in the presence of opto-electronic conversion acrossdomains. Here the lack of global state coupled with the wave-length converter dimension poses many new complexitiesfor both resource abstraction (routing) and lightpath compu-tation/signaling. To address these challenges, [10] and [11]have presented a comprehensive GMPLS-based frameworkfor distributed multi-domain lightpath RWA. Specifically,the scheme features both hierarchical inter-domain routingand path computation capabilities. With regards to routing,OXC nodes are termed as either interior or border types,with the latter designating those nodes which interconnect toborder OXC nodes in other domains. Within a domain, allnodes (interior, border) run GMPLS OSPF-TE routing [2]to exchange intra-domain link state. Furthermore all borderOXC nodes also run an additional (second) level of OSPF-TE routing to exchange state on physical inter-domain links,i.e., link-state routing setup at the inter-domain level as well,akin to OIF NNI [2]. Moreover all nodes are assumed tobe wavelength-conversion-enabled based upon a share-per-node architecture 2. Although other wavelength conversionmodels can be considered, the above model is chosen in orderto maximize converter usage and simplify node design, i.e.,converters are shared among all physical output links on OXCnode.

A major contribution in [10] is the development of topol-ogy abstraction techniques to summarize and propagatedomain-internal DWDM state. Here specific border OXCnodes in each domain are assigned as routing area leaders(RAL) [2] and tasked with computing abstracted, i.e., com-pressed, resource level views of their domains. This is doneby transforming the physical resource topology into a virtualtopology with abstract links. The resource levels of thesevirtual links are then flooded to border OXC nodes in alldomains via the second level of OSPF-TE routing, i.e., inaddition to link state for physical inter-domain links. Over-all this hierarchical routing scheme gives border OXC nodesa synchronized “global” resource view and allows them toresolve skeletal inter-domain requests. Finally path compu-tation is also done in a hierarchical manner as well by lever-aging GMPLS RSVP-TE loose route (LR) [2] signaling andexpansion features, Sect. 3.1.

Now from the above discussions it is evident that inter-domain link-state dissemination strategies will have a directimpact on the quality and level of global state. In turn thesechoices will affect RWA blocking performance. Thereforethese contingencies mandate the careful design of commen-surate routing update/triggering policies to boost inter-domain

scalability without compromising blocking performance. Todate these concerns have not been studied in detail, especiallywithin the context of DWDM networks, and this forms thecore motivation of the work here. Along these lines, novelinter-domain routing strategies are presented in Sect. 4. Toproperly detail these schemes, the associated multi-domainrouting/topology abstraction and path computation solutionsare first presented.

3.1 Topology abstraction

Consider a multi-domain DWDM network of D domainswhere the i-th domain has ni nodes and bi border nodes,1≤i≤D. Each domain is represented as a sub-graph, Gi (V i ,Li ),1≤i≤D, where V i = {vi

1, . . . , vini } is the set of physical

domain OXC nodes and Li = {lijk} is the set of physical

intra-domain links (1 ≤ i ≤ D, 1 ≤ j, k ≤ ni ), i.e., lijk is the

link between OXC nodes vij and vi

k . All links are bi-direc-tional with W wavelengths each. The set of border nodeswith a domain i is given by Bi and it is assumed (with-out loss of generality) that these nodes are the first set ofdomain nodes, i.e., Bi = {vi

1, . . . , vibi }. Furthermore, inter-

domain routing also defines a higher-level topology com-prising border OXC nodes and inter-domain links, i.e., H(U,E), where U = ∑

i {Bi } is the set of global border nodes

and E = {ei jkm}is the set of physical inter-domain links,

i.e.,ei jkm interconnects vi

k in domain i with vjm in domain j ,

1 ≤ i, j ≤ D, 1 ≤ k ≤ bi , 1 ≤ m ≤ b j . This graph contains allphysical border nodes and inter-domain links but may nothave full connectivity—which is achieved via subsequenttopology abstraction. Note that DWDM links (physical, vir-tual) have associated binary wavelength availability vectors,λ

i jkm , i.e., λ

i jkm(n) = 1 if the n-th wavelength is available,

1 ≤ n ≤ W . All OXC nodes have C converters as per a shared-per-node architecture.

Before detailing full-mesh topology abstraction, it isinstructive to review more basic simple node abstraction,originally proposed in [10]. This solutions does not per-form any domain state compression and simply condenses adomain into a single virtual node, Fig. 1. Clearly inter-domainrouting overheads are much lower since link-state updates areonly sent for physical inter-domain links, i.e., O(|E |) stor-age/update complexity. The above transformation is repre-sented as H(U, E) → Hsn (Usn , E) where Usn = {vi } isthe condensed set of virtual nodes representing each domaini and E is the set of physical inter-domain links, Fig. 1

Conversely the full-mesh abstraction scheme computes aset of virtual links to summarize domain-level state. Spe-cifically the available wavelengths on multiple traversingintra-domain routes between borders pairs must be sum-marized to generate virtual link availability vectors. Nowwavelength conversion adds notable complexity as one must

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Fig. 1 Simple node andfull-mesh topology abstraction

Full Mesh AbstractionSimple Node Abstraction

Physical Multi-Domain Topology

Virtual

3 niamoD1 niamoD

Domain 2

G1(V1,L1)

G2(V2,L2)

G3(V3,L3)

Inter-domain graph H(U,E)

U={v11, v1

2, v22, v3

2,v13, v2

3}

Hsn(Usn,E)

Hmesh(U, E+Σ i Eimesh)

v11

v21

v31v4

1

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v12 v2

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E=

Usn={v1, v2, v3}

v1

v2

v3

Physical

{ }2232

2223

2231

2213

2221

2212 ,,,,, eeeeeeE2

mesh=

1 border node4 interior nodes

3 border nodes3 interior nodes

2 border nodes3 interior nodes

RAL

RAL

RAL

Border/gateway OXCInterior OXC

{ }3212

2321

3111

1311

2111

1211 ,,,,, eeeeee

{ }3321

3312 , eeE3

mesh=

E1mesh= {}

somehow convey wavelength converter locations and checkthe wavelength continuity constraint on each “sub-path,” i.e.,at least one common wavelength has to be available betweentwo adjacent conversion-enabled OXC nodes. To resolve thisissue a novel approach is proposed which precludes the needto decouple and separately advertise (domain-internal) con-verter state. Specifically only the available wavelengths onthe bottleneck sub-paths are summarized, indirectly captur-ing any conversion limitations between two respective bordernodes. For example, in Fig. 2 the most congested sub-path isused to represent the traversing cost between border nodes

vi1 and vi

2, e.g., path vi1 −vi

3 −vi4 −vi

2 becomes a feasible can-didate if conversion is enabled at node vi

3. Here the sub-pathvi

3 − vi4 − vi

2 with wavelength availability vector [101000]is the bottleneck segment and is therefore chosen to repre-sent the cost of the whole path, i.e., the virtual link betweenborder nodes vi

1 and vi2. The exact details of the full-mesh

algorithm are as follows.The detailed flowchart for the mesh-abstraction algorithm

is presented in Fig. 3. The scheme loops through each bordernode pair (indices j , k) and computes the associated wave-length availability vector for the corresponding virtual link.

Fig. 2 Full-mesh abstraction inthe presence of full wavelengthconversion

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End

Given domain-level sub-graph Gi(Vi,Li)

No

Initialize counter j=1

If k>ni

Initialize counter k=j+1

Yes

No

Yes

Compute the ì all-optical” K shortest path vectors pi

jkm from vij to vi

k in Gi(Vi,Li), 1≤m≤K

k=k+1

j=j+1

If j>ni

Set the final virtual link wavelength usage vectors: iikj

iijk λλ ,

At leastone “all-optical” path

is found

Yes

No

Compute the K shortest path vectors pijkm

from vij to vi

k in Gi(Vi,Li), 1≤m≤K

Enumerate all wavelength converter placements along pi

jkm . Select the one minimizing the number of used converters

Choose either the minimum hop count path or least loaded path pi

jkm*

Fig. 3 Mesh topology abstraction algorithm

The scheme first runs the K -shortest path algorithm to gen-erate a set of paths between each border node pair, denoted as{pi

jkm} where pijkm is the m-th path vector (node-sequence)

between border nodes vij and vi

k in domain i , 1 ≤ m ≤ K .Here only paths which satisfy wavelength continuity con-straint are considered in this step, i.e., at least one commonwavelength has to be available along all links of a given path.This can be done by modifying the generic Dijkstra algorithmto track the wavelength availability vector, i.e., a newly foundnode is the closest node that also satisfies wavelength con-tinuity constraint. This is denoted as “all-optical” K -short-est path (Fig. 3). Note if no such path can be found, then

opto-electronic paths must be computed as follows. Namely,the K -shortest path algorithm is first run to generate a setof candidate paths between the border node pairs. All con-verter combinations along these paths are then searched tominimize the number of used converters. Finally, either theminimum hop count path or least loaded path is selected, andthe resultant wavelength availability vector is computed forthe virtual link between the border nodes, λi i

jk ,1 ≤ j, k ≤ bi ,Fig. 3.

Overall full-mesh abstraction is represented as H(U, E) →Hmesh(U, EU

∑i {Ei

mesh}), where Eimesh is the above-com-

puted set of virtual links (1 ≤ i ≤ D), Fig. 1. This approachprovides good domain visibility, albeit at the cost of sig-nificant RAL compute complexity and inter-domain routingloads. Namely, border OXC nodes must maintain/propagatestate for O(

∑i n

i (ni − 1)) = O(∑

i (ni )2) virtual links in

addition to physical inter-domain links. This yields a totalquadratic storage/update complexity of O(|E| + ∑

i (ni )2)

across all domains, a key scalability limitation for largedomains with many border OXC nodes. To address these lim-itations, some novel routing update strategies are developedin Sect. 4.

3.2 Inter-domain lightpath provisioning

Distributed inter-domain RWA uses the above-generated statealong with RSVP-TE loose route (LR) [2] signaling to expandall route links and wavelengths. Specifically a hierarchicalcomputation approach is developed in which a source nodefirst queries its closest border node (or RAL) to compute aLR domain sequence to the destination domain, i.e., “skel-eton path.” Now the LR algorithm runs on the inter-domaingraph, i.e., Hsn(Usn, E) or Hmesh(U, EU

∑i {Ei

mesh}), andconsiders all possible source/destination domain border nodepairs to derive the “best” path. Specifically for each such pair,the associated K -shortest paths are first searched to find theminimum hop-count path. Next, all converter combinationsalong this candidate LR are checked to minimize the numberof used converters. This step identifies the exact border nodelocations for wavelength conversion. Overall this schemeexploits wavelength converters to setup shorter inter-domainpaths, loosely akin to a widest-shortest approach. Given thatinter-domain links will generally have much higher resourcecosts than intra-domain links, minimizing inter-domain linkresources can be more beneficial for blocking reduction.Alternatively, future studies can also consider shortest-wid-est path strategies.

The sourcing node uses the above-computed LR sequencesto generate a RSVP-TE PATH signaling message to resolveexplicit end-to-end paths, as shown in Fig. 4. This messagecontains an initialized “all-ones” path availability vector andalso explicitly identifies all wavelength conversion locationsin the LR sequence. Foremost, downstream border nodes

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Fig. 4 Inter-domain opto-electronic lightpath setup

receiving these PATH messages perform explicit route (ER)expansion on the incoming LR sequence to resolve the explicitintra-domain OXC node sequence across their domains. Thisroute computation is again done using a widest-shortestapproach on the intra-domain topology database, see [10].Additionally, all receiving nodes must process the PATH mes-sage in one of two ways. Namely, if the receiving node is notdesignated as a wavelength conversion site, it simply per-forms a logical AND of the incoming path availability vec-tor with the wavelength availability vector on its outbounddownstream link, i.e., λ

i jkm , e.g., node B1, Fig. 4. This opera-

tion effectively tracks available end-to-end wavelengths, andthe PATH message is only propagated if the resultant vectoris not null. Conversely, if the node is designated as a wave-length conversion point, it must first select a wavelength forall previous links in the expanded PATH LR sequence up tothe last conversion OXC node (or source OXC) using most-used (MU) [3] selection, i.e., node B2, Fig. 4. Additionallythis conversion node must also re-set the path availabilityvector to “all-ones” and check for a non-zero nodal wave-length converter count, i.e., C > 0. Only if a converter isavailable is the PATH message propagated downstream. Thiseffectively “re-generates” a lightpath. The destination OXCis then responsible for wavelength selection up to the lastconversion node in the expanded PATH sequence.

4 Inter-domain routing & scalability

As described in Sect. 3, the proposed routing scheme usesa two-level hierarchical link-state routing approach to dis-seminate intra- and inter-domain state. Note that the use oflink-state routing in multi-domain optical settings is quitegermane as the number of domains will be drastically lowerthan the number of autonomous systems (AS) domains in thebroader Internet (which instead uses distance-vector routingfor “best-effort” packet routing). The first routing level runsthe OSPF-TE protocol between all domain-internal OXC

nodes to exchange full wavelength-level state for all links.Specifically, link-state advertisement (LSA) update messagesare generated using a basic significance change factor (SCF)triggering policy, where updates are flooded to all neigh-boring nodes if the relative change in free wavelengths ona node’s link exceeds a given SCF threshold and the dura-tion since the last update exceeds a hold-down timer (HT)[15]. These LSA updates contain wavelength vectors indi-cating the free/reserved wavelengths on the link, as definedin OSPF-TE extensions for DWDM links [2]. Meanwhilethe second (hierarchical) level of link-state routing also usesOSPF-TE between border OXC nodes and exchanges bothinter-domain physical link state and virtual/abstract intra-domain resource state.

Now as mentioned earlier, full-mesh abstraction generatesO((ni )2) virtual links per domain, i.e., to aggregate the tra-versing cost between all potential border node pairs (Sect.3.1). Clearly as the number of border nodes or the numberof domains increases, this abstraction scheme will generatesignificant inter-domain routing loads, thus resulting in poornetwork scalability, e.g., i.e., O(|E| + ∑

i (ni )2) updates. In

general this scalability issue is more a result of the abstrac-tion scheme and not the nature of the underlying link DWDMlink types. To resolve this limitation some studies have pro-posed alternate topology abstraction schemes with the over-riding aim of designing better virtual topologies with fewervirtual links. For example [11] adapts star abstraction to all-optical DWDM networks by generating a virtual node foreach domain. Although this reduces routing load notably,the resulting blocking performance results are not as goodas full-mesh abstraction, e.g., approaching those of basicsimple node abstraction at lighter loads. In addition com-pute complexity (at RAL node) is also much higher as thestar abstraction scheme sources off the full-mesh abstractionschemes. Further considerations for opto-electronic multi-domain settings are bound to further increase compute com-plexities. Although [12–14] have also researched variousother abstracted topologies, these studies are done within

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Fig. 5 Maximum relativechange (MRC) virtual linkupdate strategy

the context of packet-switching networks and will entail sig-nificant further complexities for DWDM settings.

In light of the above, the work here tables two new schemesto reduce inter-domain DWDM link-state routing complex-ity. In the proposed solutions, rather than computing newabstracted topologies, the inherent triggering mechanismsfor propagating virtual link state for full-mesh abstraction aremodified. First consider a basic “SCF-only” routing updatepolicy which uses relative-change threshold triggering noti-fication for inter-domain links—both virtual and physical.Namely, the RAL node uses an inter-domain hold-off timer(IHT) to periodically compute full-mesh abstractions for itsdomain and propagates link-state updates for any virtuallink whose available wavelengths change by over SCF. Thisscheme is used as a baseline for comparison and will yieldhigh virtual link update loads under heavy connection loads(low scalability). In order to address this shortcoming, twonovel proposals are developed:

(1) Maximum Relative Change (MRC): This scheme onlyadvertises updates for those virtual links that see the maxi-mum relative change in available wavelengths, i.e., up to hin total, where h � (ni )2. In other words, upon expiry of theIHT, a full-mesh abstraction is still computed by the RALand new virtual links generated. However only those virtuallinks whose change in available wavelengths exceeds the SCFvalue are short-listed and the top h values are advertised at theinter-domain level. The detailed MRC pseudo-code is shownin Fig. 5. Note that step 4 in Fig. 5 is used to ensure routingupdate policy adherence to the basic SCF rule as well. Inessence the h parameter hard limits virtual state growth in alinear fashion with the number of domains, O(hD), a notabledecrease from full-mesh abstraction overheads.

(2) Most Used-Maximum Relative Change (MU-MRC):This scheme further incorporates most-used (MU) [3] wave-length state with the MRC scheme, i.e., updates are trig-gered based upon the actual wavelengths that have changedstatus since the last update (and not just the number, as inMRC). In other words only those virtual links experiencingthe most number of changes in the most MU wavelengthsare updated. The overall rationale of this approach is pre-

mised upon extensive RWA research studies which indicatethat using MU state generally gives lower blocking as com-pared to other wavelength selection strategies [3,16]. Spe-cifically, the updates are generated as follows. First, as perthe MRC scheme, a full-mesh abstraction is computed by theRAL upon expiry of the IHT. Upon this timer’s expiry, alldomain-level wavelengths are also listed in decreasing orderof usage, e.g., by consulting the local intra-domain OSPF-TEdatabase. Next the top m wavelengths are chosen and all vir-tual links scanned for changes therein, i.e., m � W . Finally,updates are transmitted for the subset of virtual links whichexperience the maximum change in the (above-selected) mMU wavelengths. The detailed pseudo-code listing for thismore selective update scheme is shown in Fig. 6. As per theMRC scheme, the above scheme also adheres to the SCF-based update policy. The maximum number of updates isagain limited to h, where h � (ni )2..

5 Performance evaluation

Detailed performance analysis is conducted to study theeffectiveness of the improved routing update strategies ofSect. 4. Here all evaluation is done via discrete event sim-ulation using the OPNET ModelerT M tool. Specifically alllightpath requests are generated between randomly selecteddomains using a 70–30% intra/inter-domain ratio. This ischosen to reflect practical networks which will likely fieldmore intra-domain requests. Furthermore the mean light-path connection holding times are set to 600 s (exponential)and commensurate request inter-arrival times are varied withloading (exponential). Within a given domain the source-des-tination OXC nodes are chosen randomly using a uniformdistribution. Furthermore intra/inter-domain routing timers(HT, IHT) are set to 500 s and all SCF update values set to10% (0.1), i.e., for both levels of OSPF-TE routing. Finallyall runs are averaged over 500,000 connections and the MUwavelength selection scheme is adopted as it is shown togive superior performance in both intra- and inter-domainDWDM settings, see [3,10,16].

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Fig. 6 Most-used maximumrelative change (MU-MRC)virtual link update strategy

Fig. 7 9-Domain topology withsmall domain sizes

All the simulation runs are conducted on a sample nine-domain topology with 38 uni-directional inter-domain links,as shown in Fig. 7. First, inter-domain lightpath blockingperformance results are presented, comparing the baselineSCF-only and improved MRC and MU-MRC routing updatepolicies. Namely, Figs. 8and 9 present findings for W = 8and W = 16 wavelengths, respectively. As expected, full-mesh abstraction with SCF-only updates yields notably lowerblocking than simple node abstraction (i.e., physical inter-domain link updates only), averaging about 30–50% lower(logarithmic plots). This is due to the very accurate vir-tual link state flooded, which allows intelligent inter-domainRWA schemes (Sect. 3.2) to effectively bypass congesteddomain/border nodes.

0.0001

0.001

0.01

0.1

1

20 40 60 80

Load (Erlang)

Inte

r-d

om

ain

Pb

Simple NodeFull Mesh - SCF onlyFull Mesh - MRC (h = 3)Full Mesh - MRC (h = 5)Full Mesh - MUMRC (3,3)Full Mesh - MUMRC (5,5)

Fig. 8 Inter-domain lightpath blocking (W = 8)

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0.0001

0.001

0.01

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80 120 160 200 240

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Fig. 9 Inter-domain lightpath blocking (W = 16)

-500

500

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ra f

aile

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ll-m

esh

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F-o

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Simple NodeFull Mesh - MRC (h = 3)

Full Mesh - MRC (h = 5)Full Mesh - MUMRC (3,3)Full Mesh - MUMRC (5,5)

Fig. 10 Difference in failed inter-domain connections (W = 8)

More importantly the results in Figs. 8 and 9 also show thatcoupling full-mesh abstraction with improved MRC updates(h = 3) yields very comparable blocking performance to full-mesh with SCF-only updates. The more selective MU-MRCupdate policy (e.g., h = 3, m = 3 and h = 5, m = 5) gives verygood blocking gains here, i.e., almost on par with full-meshabstraction and notably better than single node abstraction.In fact these improved routing strategies closely track theblocking performance of the SCF-only scheme across thefull range of tested loads. To illustrate this more closely,the difference in failed inter-domain lightpaths (versus thebaseline full-mesh scheme with SCF-only updates) is plot-ted in Fig. 10. At lower loads it is seen that this discrepancyis almost negligible, albeit it does rise slightly at extremelyhigh connection loads, i.e., 10–20% blocking range. Thesefindings validate the general hypothesis that propagating asubset of virtual link-state updates can be very beneficialin summarizing domain state change, particularly at low-to-medium blocking regimes (i.e., corresponding tomost operational settings). In addition the results alsoconfirm the effectiveness of using MU wavelength state forinter-domain routing, particularly for achieving domain statecompression. As such these findings extend upon earlier stud-ies on the efficacy of MU state for RWA in general, see surveyin [3].

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In addition to blocking performance, inter-domain rout-ing loads are also tested, i.e., measured as the number ofOSPF-TE messages exchanged per second (LSA/s). Theseresults are shown in Fig. 11 (W = 8) and Fig. 12 (W = 16),respectively. As expected, full-mesh abstraction with SCF-only updates yields the highest routing loads, almost 4–5times higher than simple node abstraction (which yields thelowest). Conversely, the improved MRC scheme reducesOSPF-TE update messaging loads anywhere from 20% to40% (h = 5) while still achieving comparable inter-domainblocking. More importantly, the more-selective MU-MRCstrategy yields even more significant routing load reduc-tions, particularly at low-medium load levels (typical of mostnormal operating settings). For, example for W = 16 and 83Erlang loading, Fig. 12, MU-MRC with h = m = 3 gives aninter-domain routing load of 41 LSA/s, about 46% lowerthan SCF-only (76 LSA/s) and 12% lower than MRC withh = 3 (50 LSA/s). An even more pronounced reduction isshown in Fig. 11 for MU-MRC with W = 8 wavelengths and30 Erlang loading, i.e., over 50% lower than full-mesh withSCF-only. Note that MU-MRC strategy approaches MRC athigher loads as IHT timers quickly limit overall LSA updaterates, i.e., “clamp-down” effect [15].

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Fig. 14 Inter-domain routing loads (2% inter-domain blocking)

Finally, additional tests are done to gauge network perfor-mance at nominal, i.e., low blocking, operational regimes.Such tests are very relevant to carriers as they indicate theload-carrying (i.e., revenue generation) capacity about a typ-ical operational point. Specifically, the carried load is mea-sured for an inter-domain lightpath blocking probability of2% for varying numbers of link wavelengths. Here Fig. 13plots the normalized carried load for differing wavelengthcounts and Fig. 14 plots the corresponding inter-domain LSArouting loads. Specifically, all loads are normalized versusthat for W = 8 wavelengths (which is set to unity). Theresults here confirm good gains with full-mesh abstraction,about 8–15% higher carried load across all wavelengthcounts. More importantly the MU-MRC scheme gives nearlyidentical carried load performance as compared with SCF-only updates, albeit with notably lower inter-domain routingloads, see Fig. 14 (20–25% lower).

6 Conclusions

This paper studies the important topic of inter-domain rout-ing scalability in multi-domain DWDM networks. A detailed

GMPLS-based framework is first developed for lightpathprovisioning in realistic multi-domain DWDM networks withwavelength conversion. This scheme uses a hierarchical (two-level) OSPF-TE link-state routing setup along with full-meshtopology abstraction. To improve inter-domain routing scala-bility, two novel update/triggering policies are developed tolimit virtualized link-state overhead between domains, i.e.,maximum relative change (MRC) and most-used maximumrelative change (MU-MRC). Detailed performance evalua-tion studies show notable reductions in inter-domain rout-ing overheads versus more basic “SCF-only” update strat-egies, ranging from 20% to 45%. More importantly thesegains come without any significant increase in inter-domainlightpath blocking. Overall these results confirm the viabilityof topology abstraction in multi-domain DWDM networks.Future efforts will study the extension of strategies in broadermulti-domain, multi-layer networks such as IP-DWDM andSONET-DWDM.

References

[1] Ghani, N., et al.: Metropolitan Optical Networks. Optical FiberTelecommunications IV, pp. 329–403. Academic Press (March2002)

[2] Bernstein, G., Rajagopalan, B., Saha, D.: Optical Net-work Control-Architecture, Protocols and Standards. AddisonWesley, Boston (2003)

[3] Zang, H., Jue, J., Mukherjee, B.: A review of routing andwavelength assignment approaches for wavelength-routed opti-cal WDM networks. Opt. Netw. Mag. 1(1), 47–60 (2000)

[4] Zhang, R., Vasseur, J.: MPLS inter-autonomous system (AS)traffic engineering (TE) requirements. RFC 4216, November2005

[5] Liu, G., Ji, C., Chan, V.W.S.: On the scalability of network man-agement information for inter-domain light-path assessment.IEEE Trans. Network. 13(1), 160–172 (2005)

[6] St. Arnaud, B. et al.: BGP Optical Switches and Lightpath RouteArbiter. Opt. Netw. Mag. 2(2), 73–81 (2001)

[7] Yang, X., Ramamurthy, B.: Inter-domain dynamic routing inmulti-layer optical transport networks. In: IEEE GLOBECOM2003. San Francisco, Dec. 2003

[8] Sanchez-Lopez, S., et al.: A Hierarchical routing approach forgmpls-based control plane for ASON. In: IEEE ICC 2005. Seoul,Korea, June 2005

[9] Yannuzzi, M., et al.: Interdomain RWA based on stochastic esti-mation methods and adaptive filtering for optical networks. In:IEEE GLOBECOM, Nov. 2006

[10] Liu, Q., et al.: Distributed inter-domain lightpath provisioning inthe presence of wavelength conversion. Computer Communica-tions, December 2007

[11] Liu, Q., et al.: Application of topology abstraction techniques inmulti-domain optical networks. In: IEEE ICCCN 2006. Arling-ton, October 2006

[12] Hao, F., Zegura, E.: On scalable QoS routing: performance eval-uation of topology aggregation. In: IEEE INFOCOM 2000, pp.147–156

[13] Awerbuch, B., Shavitt, Y.: Topology aggregation for directedgraphs. IEEE/ACM Trans. Network. 9(2), 82–90 (2001)

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[14] Liu, K., Nahrstedt, K., Chen, S.: Routing with topologyabstraction in delay-bandwidth sensitive networks. IEEE/ACMTrans. Network. 12(1), 17–29 (2004)

[15] Alnuweri, R., et al.: Performance of new link state advertise-ment mechanisms in routing protocols with traffic engineeringextensions. IEEE Commun. Mag. 42(5), 151–162 (2004). doi:10.1109/MCOM.2004.1299361

[16] Assi, C., Shami, A., Zhang, Z. Ali, M.: impact of wavelengthconverters on the performance of optical networks. SPIE Opt.Netw. 3(2), March/April 2002

Author Biographies

Qing Liu received a B.E. degreein telecom engineering and an M.E.degree in computer science fromNanjing University of Post and Tele-communications, China, in 2001 and2004, respectively. He is currentlycompleting his Ph.D. degree in theDepartment of Electrical and Com-puter Engineering at the Univer-sity of New Mexico, Albuquerque.In the past he has worked as asummer intern at Motorola Chinaand Acadia Optronics LLC, USA,and as a software engineer at BellLabs/Lucent Technologies China.

He has authored over 15 publications, and his research interests includeoptical networks, multi-domain networks, traffic engineering, multime-dia communications, and wireless networks.

Chongyang Xie received theB.E and M.E degrees bothin Electronic Engineering fromSouth China University of Tech-nology, Guangzhou, China, res-pectively in 2004 and 2007. Heis currently pursuing a Ph.D.degree in the Department ofElectrical and Computer Engi-neering at the University of NewMexico, US. In the past he hasworked as a network engineerin Shanda Entertainment, Shang-hai, China, in China Mobile,Shenzhen, China, and a R&D

engineer in Innovasic Semiconductor, USA. His research interestscover traffic engineering, advance reservation, multi-layer and multi-domain networking.

Tannous Frangieh receivedhis B.E. degree in ComputerEngineering from the LebaneseAmerican University, Lebanon,in 2005. He is currently pursu-ing a Ph.D. degree in the Depart-ment of Electrical and Com-puter Engineering at the Uni-versity of New Mexico, Albu-querque. He has several years ofindustry experience, mainly as asoftware engineer for web appli-cations and reconfigurable sys-tems programming. His research

interests include optical networks, multi-domain/multi-layer networks,traffic engineering, and reconfigurable and embedded systems.

Nasir Ghani received a Bach-elor’s degree in computer engi-neering from the Universityof Waterloo, Canada, in 1991,a Master’s degree in electri-cal engineering from McMas-ter University, Canada, in 1992,and a Ph.D. degree in electricaland computer engineering (ECE)from the University of Waterlooin 1997. Currently, he is an asso-ciate professor in ECE at the Uni-versity of New Mexico, wherehe is involved in various fundedresearch projects. He gained a

wide range of industrial and academic experience in the networkingarea, and has held senior positions at Nokia, IBM, Motorola, SorrentoNetworks, and Tennessee Tech University. He is a recipient of the NSFCAREER Award and has authored over 100 publications (includingjournal and conference papers, book chapters, industry magazine arti-cles, and standardization drafts), as well as two patents. He is a co-chairof the IEEE INFOCOM High-Speed Network (HSN) series and hasserved as a co-chair for the optical symposia for IEEE ICC 2006 andIEEE GLOBECOM 2006. He is also a program committee member forIEE/OSA OFC and numerous other conferences, and has served regu-larly on NSF, DOE, and international panels. He is an associate editorof IEEE Communications Letters and has guest edited IEEE Network,IEEE Communications Magazine, and Cluster Computing.

Ashwin Gumaste is currently a fac-ulty member in the Department ofComputer Science and Engineeringat the Indian Institute of Technol-ogy, Bombay, and a visiting scholarat the Massachusetts Institute ofTechnology. He was previously withFujitsu Laboratories (USA) Inc. asa member of the research staff inthe Photonics Networking Labora-tory (2001–2005). Prior to this heworked in Fujitsu Network Com-munications R&D and prior to that,with Cisco Systems in the OpticalNetworking Group (ONG). He has

over 40 pending U.S. and EU patents, and has published close to 60 arti-cles in referred conferences and journals. He has authored three bookson broadband networks: DWDM Network Designs and EngineeringSolutions (a networking bestseller), First-Mile Access Networks andEnabling Technologies (Pearson Education/Cisco Press), and Broad-band Services: User Needs, Business Models and Technologies (Wiley).He is also an active consultant to industry, and has worked with bothservice providers and vendors. In addition, he has served as programchair, co-chair, publicity chair, and workshop chair for various IEEEconferences, and has been a technical program committee memberfor IEEE ICC, IEEE GLOBECOM, IEEE Broadnets, IEEE ICCCN,Gridnets, and so on. He is also a guest editor for IEEE Communica-tions Magazine and was General Chair of the 1st International Sympo-sium on Advanced Networks and Telecommunication Systems, held inBombay, India.

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http://dx.doi.org/10.1109/MCOM.2004.1299361

http://dx.doi.org/10.1109/MCOM.2004.1299361


Nageswara S. V. Rao received aB. Tech from the National Insti-tute of Technology, Warangal, Indiain 1982, an M.E. from the Schoolof Automation, Indian Institute ofScience, Bangalore in 1984, anda Ph.D. in computer science fromLouisiana State University in 1988.He is currently UT-Battelle Corpo-rate Fellow at Oak Ridge NationalLaboratory, which he joined in1993. He was an assistant professorof computer science at Old Domin-ion University from 1988 to 1993.His research interests include high-

performance networking, network transport dynamics, and informationand sensor fusion. He has published more than 250 technical journaland conference papers. He is an Associate Editor for Pattern Recog-nition Letters, International Journal of Distributed Sensor Networks,and Information Fusion. He received the 2005 IEEE Computer SocietyTechnical Achievement Award for his contributions in the informationfusion area.

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