Study on Integrated Routing in IP over WDM Networks

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1 Study on Integrated Routing in IP over WDM Networks Tong Yet, Qingji Zeng, and Zhizhong Zhang tcenter of Broadband Optical Network Technology (CBONT), Shanghai Jiaotong University, Shanghai, China ABSTRACT The problem of integrated routing in multifiber IP over WDM networks is studied in this paper. To solve this problem, a layered-graph is constructed and an algorithm, called integrated cost-based shortest path (ICSP) algorithm, is then proposed. ICSP can not only balance the traffic uniformly but also make a dynamic tradeoff between all the links in the layered-graph. A parameter r1 is introduced to characterize the resource richness of an IP over WDM Network. Simulation results show that ICSP outperforms other algorithms significantly in terms of blocking probability in all the cases we studied, and the performances of other algorithms are affected by r1 greatly. Because a multifiber network can be functionally equivalent to a single fiber network with limited wavelength conversion (WC), we make the first known attempt to investigate the impact of WC on dynamic integrated routing by studying the multifiber networks. Results show that the effect of WC depends on the granularities of label switched path (LSP) requests. If the granularity of each request is large, WC will improve the network performance; if the granularity of each request is small, WC will worsen the performance. Keywords: IP over WDM, Integrated Routing, Wavelength Conversion, Logical Topology, Label Switched Path 1. INTRODUCTION The Internet transport infrastructure is moving towards a model of high-speed routers interconnected by optical core networks. There is a growing consensus that an major component of the next generation Internet will be.ip directly over WDM network.' In an IP-over-WDM network, optical cross-connects (OXC) are interconnected via optical fiber links in a mesh topology to construct an optical network, and IP routers are attached to OXCs through E-O interfaces. The optical iietwork provides point-to-point connectivity between IP routers in the form of lghtpaths. The topology of logical network interconnecting IP routers is defined by the collection of lightpaths. If the OXCs in optical core are incapable of converting the data on one wavelength to another wavelength (wavelength conversion2), each lightpath should be established with the same wavelength on all the links along its path. As an illustration, a sample IP over WDM network is shown in Fig.1. The IP and WDM layers can be combined in a peer-to-peer model.1 IP routers and OXCs are treated together as a single integrated network. For this model, an integrated rnnltiprotocol label switching (MPLS)3 based control plane is adopted. Integrated routing is an efficient routing approach for IP over WDM networks based on the peer model. There are several integrated routing algorithms had been M. Kodialam firstly proposed two dynamic integrated routing algorithms (IMH and MOCA) in Ref. 4. The integrated mm-hop (IMH) algorithm only considers the route with minimum hops, thus failing to result in a good performance. The maximum open capacity (MOCA) algorithm always picks a path for the arriving LSP request so that the residual capacities between the router pairs are maximized after this stream is routed. Though MOCA performs well, it yields a high complexity. C. Assi also proposed two algorithms.5 The first one attempts to establish a multi-hop connection on the logical topology for each traffic stream before it considers building up a direct lightpath on the physical topology which is called integrated logical first (ILF) routing here. On the contrary, the second one would like to first setup a direct lightpath on the physical topology for each traffic stream before it resorts to routing on the IP connectivity, which is called integrated physical first (IPF) routing. Tong Ye: yetong sjtu.edu.cn, Telephone: +86 (0) OptiComm 2003: Optical Networking and Communications, Arun K. Somani, Zhensheng Zhang, Editors, Proceedings of SPIE Vol (2003) 2003 SPIE X/03/$15.00

2 5) oh.) QVC030 0 x t ).))a) '--It, t an tjac (0) (a) Physical Topology (0 (b( Logical Topology tiglopath established with wavelength lighnpatb established with wavelength 0( Figure 1. A sample IP over WDM network. Each optical fiber link is bidirectional. All the OXCs can perrm WC. There are 2 fibers in each direction of link and 2 wavelengths on each fiber. (a) Physical topology, and (b) logical topology. Note that "LP' means "lightpath". II -So oxc ttl'sn II (a) Node eqaivalettce )b) LG for the network in its initial state )c) Updated LG after establishment of S LPs Figure 2. The layered-graph for the sample network shown in Fig.1: (a) Node equivalence; (b) LO for the network in its initial state; (c) updated LG after establishment of 6 lightpaths. Note that "LP" means 'lightpath" here, However, all of above-mentioned algorithms just only consider the integrated routing in single-fiber networks. Multifiber network has attracted more attentions because of the economic advantage of installing bundles of fibers for the purposes of fault tolerance and future network growth.6 Moreover, a network, with n fiber in each link and rn wavelengths on each fiber, is functionally equivalent to an nm-wavelength single fiber network with limited WC of degree ri. So like Ref. 6, we can investigate the impact of WC on dynamic integrated routing problem by studying the multifiber networks. In this paper the integrated routing in multifiber IP over WDM networks is considered. In Section 2, we describe the construction of layered-graph. In Section 3, a layered-graph based algorithm called ICSP is proposed. Simulation results and analyses will be given in Section 4. Some conclusions will be given in Section MODEL FOR IP OVER WDM NETWORKS There are two types of nodes involved in IP over WDM networks. The first type node is just an OXC. The Second type node consists of an OXC controlled by an IP router (called IF-Router simply in this paper). For example, there are 2 OXCs and 4 IP-Routers in the sample network. In this paper, we assume the OXC can't perform WC capability Layered-Graph (LG) for IP over WDM Networks Define a physical topology G(N, L, F, W) for a given IP over WDM network, where N is the set of nodes (including the OXCs and the IP-Routers), L is the set of bi-directed optical fiber links, F is the set of fibers in each direction of an optical fiber link, and W = {A, )2,, \iw} is the set of wavelengths on each fiber. According to the functions of nodes, we expand a node (IP-Router or OXC) into a number of sub-nodes. An IP-Router can be made up of an input sub-node (ISN), an output sub-node (OSN), and W optical sub-nodes (OPSN). An OXC can be composed of W( OPSNs. The OSN can demultiplex and even terminate low speed LSPs from lightpaths. The ISN can receive low speed traffic from upper layer clients or OSN, and then multiplex them onto lightpaths. The WI OPSNs of each node are labelled as A, )2,, Aw respectively. The OPSN Proc. of SPIE Vol

3 labelled as A can only perform the cross-connection for the lightpaths established with wavelength A. We use directed arcs from OPSNs to OSN to denote the function of demultiplexing, directed arcs from ISN to OPSNs to denote the function of multiplexing, and directed arc from OSN to the ISN to denote the function of IF electronic processing. All these arcs are called inside-links. If there exists an optical fiber link l EL (Vx, y N) in physical topology, then the OPSNs labelled with ) (for all ) W) of node x and y are connected by a directed arc WL (called wavelength link). Each wavelength link in the LG carries Fl equivalent channels. The LG for the sample network in its initial state is depicted in Fig.2 (b) Update of Layered-Graph The network state is changed dynamically. So the LG should be updated timely. Before explaining how to update the LG, we would like to follow some notations employed in Ref. 4: the total bandwidth of a lightpath is one unit; an LSP request is defined by a triple (s, d, b) where s and d denote the source and destination IP-Routers respectively, and b (0 < b < 1) is its bandwidth requirement; it is not permissible for the routing algorithm to split the traffic of an LSP request in an arbitrary manner. We use = {O3(m)} to denote the set of lightpaths between IP-Router i and For j. an arriving request, if there is a lightpath newly established between IP-Router i and j,then add it to the set of and subtract 1 from the number of free channels on the wavelength links which are used to establish the lightpath. On the contrary, if the reside capacity on O (m) reaches one unit on any LSP departure, then (ri-i) is removed from e3 and add 1 to the number of free channels on corresponding wavelength links. If e3, then introduce a directed arc LL23 (called logical link) to connect the ISN of IP-Router i to the OSN of IP-Router ; otherwise, the logical link is removed from the LG. The set is defined as an associated-set of LL. The updated LG for the sample network is shown in Fig.2 (c). There are 6 lightpaths in the sample network. The numbers of free channels on the wavelength links, which are used to set up these lightpaths, are changed from 2 to 1. An instance for logical link is the LL31. The associated-set of LL31 is e31 = {031 (1 ), O3 (2) }, where the O3 (1 ) and 931 (2) are corresponding to lightpath 5 and 6 in Fig.1 (b) respectively. 3. INTEGRATED ROUTING ALGORITHM Many route selection algorithms in networks are based on Dijkstra's algorithm, which selects a route with minimum cost from all potential routes. The assignment of link costs provides a strategy to control route selection. The cost of each inside-link in the LG is assumed to be ( k O) in this paper. So the LSP selection strategy for an arriving request is determined by the cost definitions of the wavelength links and logical links. This definition in our strategy is based on the number of available channels on the link, which will guide the routing algorithms to consider the links which have more available channels. For a wavelength link WL3 in the layered-graph, the cost function is defined as follows: C ifo<f<f WL iff=o where Lc is a constant. For a logical link LL3 in the LG, the cost of it is determined by its associated-set e13. Define Ijj as the number of lightpaths in whose residual capacity is larger than the bandwidth requirement of the arriving request, then there exists a value I = rnax(i3) (Vi, j E N). So the cost of LL3 can be given as follows: I /.\b1 + &i x (I I) if 0 <I < I CLL _ if = 0 (2) where & is a constant. For reasons of clarity, we set zc1=/.hb1 and &=zb. Here we also introduce a parameter v which is defined to make a tradeoff between the logical links and the wavelength links, i.e., Lb1 = vlhb. If v < 1, means the logical links will be used more possibly. On the contrary, the wavelength links will be selected more possibly. Based on our LSP selection strategy, the objective of ICSP is to find a minimum cost path in the LG. Suppose that an LG has been constructed, for an arriving LSP request A we do as follows. [Step 1] Determine the costs (1) 406 Proc. of SPIE Vol. 5285

4 of all the links in the LG using Eq. 1 and 2; [Step 2] Find a shortest path in the LG from the ISN of node S to the OSN of node d, using Dijkstra's algorithm. If the cost of the shortest path is infinite, then block A; otherwise, accept the request and allocate the resource along the path for it; [Step 3] Update the LG. 4. RESULTS AND DISCUSSIONS In this section, we evaluate the performances of ICSP, 1MB, IPF and ILF* in the networks shown in Fig.4. All optical fiber links in these networks are bi-directional. In our simulation, the LSP requests are assumed to arriving at the network according to an independent Poisson process with arriving rate 3. The LSP's holding time is exponentially distributed with mean 1/it. The sources and destinations of the requests are selected randomly among the IP-Routers according to a uniform distribution. There are two types of traffic modes considered in this paper. For a request (s, d, b) b is uniformly distributed between 0.2 and 0.4, and between 0.5 and 0.7 for traffic mode 1, and traffic mode 2 respectively. Herein we use a parameter TI = 2 JLI! [IRI (RI 1 )J to characterize the resource richness of an IP over WDM Network. Given the values of IF and WI, large TI means physical resource are relative rich to construct a logical topology, and small TI means the resource is scarce. We consider the blocking probabilities (BP) of different algorithms as a function of the Tl in Fig.4. The results show that IPF has better performance than IMH with large TI, while IMH outperforms IPF with small TI. Compared to IMH, the merit of IPF is that it can balance the network traffic distribution, and the demerit is that IPF are less "integrated". When the TI is large, the optical resource is rich enough to construct a dynamic logical topology with higher connectivity, thus IPF can use the logical links more optimally than IMH. However, when TI 15 small, only sparse logical connections can be constructed, which makes IPF lost more chances to accept the requests than IMH. ICSP always has the best performance for all the TI. The fact can be explained by the results shown in Fig.5. ICSP tries to balance the traffic, while IMH results in collective use of the resources. Moreover, the optical hops of a logical channel are greater than or equal to 1. Routing a request over the existing logical links will consume more resources than on wavelength links in general. So assigning higher cost to logical link is more consistent with the practice, which results in the decrease of BP again when v =1 1.4 and the dramatic increase of blocking performance when v < 1. Of course, if v is overestimated, BP will also increase. Because a network with IFI = and WI = m is equivalent to an nm-wavelength single fiber network with limited wavelength conversion of degree rt, we make the first known attempt to investigate the effects of WC on integrated routing by studying the multifiber networks. Fig.6 and Fig.7 compare the BPs of different algorithms for the CERNETLike under traffic mode 1 and 2 respectively. FIIWI are fixed at 16, and Ft is varied from 1 to 16. The network with Fl = 16 is equivalent to a 16-wavelength single fiber network with full WC, and F = 1 means no conversion. Simulation results show that the effect of WC depends on the traffic mode. Under traffic mode 2 (the granularity of each request is large), Fl = 16 brings lower BPs for the algorithms than Ft = 1, that is, WC improves the network performance. However, under traffic mode 1 (the granularity of each request is small), Fl = 16 brings higher BPs for the algorithms than Fl = 1, that is, WC worsens the performance. This scenario is explained as follows. In traffic mode 1, the bandwidth requirement of a request is no more than 0.4 units. Thus after a new lightpath is established, the residual capacity of the lightpath is greater than or equal to 0.6. This lightpath can still be used by many future requests again and again, as long as its residual capacity is less than one unit. Thus the holding time of a lightpath is much greater than 1/ti, that is, once a lightpath is established it will be a semi-permanent channel. This results in that the traffic load at the optical layer is very heavy. When the traffic load at the optical layer is very heavy, an abnormal scenario, where no conversion has lower BPL than full conversion at the optical layer, will emerge.2 So, in this case, the optical layer without WC can provide more such semi-permanent lightpaths to the IP layer, which directly leads to a lower BP. However, in traffic mode 2, the bandwidth requirement of a request is equal or greater than 0.5 units. Thus the lightpath can not be used again and again. So the holding time of a lightpath is almost equal to 1/ti. This does not result in a heavy load on the optical layer. So WC can improve the performance in this case. *There is a deference between IPF (or ILF) in this paper and IPF (or ILF) in Ref. 5. The first step of IPF (or ILF) in Ref. 5 tries to find an existing direct lightpath between s-d pair for the arriving request. The aim of this step is to save the transceivers. In this paper, this step is removed here in order to achieve traffic balance better. tthe traffic load at the optical layer is defined as ratio of A/B, where A is mean arrive rate of lightpath connection requests and 1/B is mean holding time of the lightpaths. Proc. of SPIE Vol

5 Ill IWI o cl700 ri rig ARPANET NSFNET MH CERNET_LIKE SI SlI Figure 3. Sample networks used in simulation. Each white node in the networks denotes an OXC, and each gray node denotes an IP-Router. Figure 4. BPs of different algorithms vs. parameter TI under traffic mode 1. ro Feffectdftratficbglarrca 1::. IF1=B41=4 Lad=6OQ eriang 1o o. :IFII W)16 oad=6oo jiang : I IFIIWI 16 Load 200 erlang v=r 4 ILF : V : : IPF I') (2) (4) (8) (18) 11 re 141 1') (6) IC IC Figure 5. BPs vs. parameter v for the Figure 6. BPs of different algorithms CERNET.Like with FL = WI = 4 and vs. F for the CERNETLike under p = 600 Erlang, under traffic mode 1. traffic mode CONCLUSION Figure 7. BPs of different algorithms vs. F for the CERNET.Like under traffic mode 2. In this paper, the dynamic integrated routing in multifiber IP over WDM networks is studied. Firstly, a layeredgraph is constructed and a resource assignment algorithm ICSP is proposed. Then a parameter r1 is introduced to characterize the resource richness of an IP over WDM network. At last, we investigate the effect of WC on integrated routing by studying the multifiber networks. We get some interesting results via simulation. ICSP outperforms other algorithms significantly in all the cases we studied. The blocking performances of IMH and IPF are influenced by the parameter TI. The effect of WC depends on the granularities of LSP requests. If the granularity of each LSP request is small, WC will worsen the performance. REFERENCES 1. J. Y. Wei, "Advances in the management and control of optical internet," 20, pp , May E. Karasan and E. Ayanoglu, "Effects of wavelength routing and selection algorithms on wavelength conversion gain in wdm optical networks," 6, pp , E. Rosen and et al. "MultiProtocol Label Switching architecture." IETF RFC 3031, Jan M. Kodialam and T. V. Lakshman, "Integrated dynamic IP and wavelength routing in IP over WDM networks," in Proc. IEEE INFOCOM, pp , C. Assi and et al., "Integrated routing algorithms for provisioning "sub-wavelength" connections in IF-over- WDM networks," 6. S. Xu, L. Li, and S. Wang, "Dynamic routing and assignment of wavelength algorithms in multifiber wavelength division multiplexing networks," 18, pp , Oct Proc. of SPIE Vol. 5285