Routing Information Exchange in Large-Scale WDM Networks

Transcription

1 Routing Information Exchange in arge-scale WDM Networs Yong Zhu, Admela Juan and Mostafa Ammar Georgia Institute of echnology, Atlanta, GA 30332, USA {yongzhu, ajuan, Abstract In this paper, we focus on issues of routing information exchange in large-scale WDM networs. Specifically, we present two schemes, i.e., direct routing information exchange and routing information broer. We then analyze and compare these schemes based on their scalability, complexity and communication overhead. Finally, on examples of metro alloptical networs interconnected over an all-optical WDM bacbone, we present the related numerical results based on simulations. I. INRODUCION In large-scale optical networs that generally include multiple domains, such as bacbone and metro networs based on WDM, reliable routing information exchange among different parts of the networ is an important issue. For multidomain networs or for networs with multiple segments [1], i.e., where an optical networ is composed of a set of networing segments, each representing a sub-networ that requires separate consideration for wavelength routing and administration, this function involves both intra-segment and inter-segment routing information exchange. here is very little existing wor addressing and evaluating routing information exchange, in particular for multi-segment networs. Pendarais et al., considered routing information exchange across the UNI and NNI [2]. In [3], OSPF was extended to support carrying lin state information for Generalized Multi-Protocol abel Switching (GMPS). Wang et al., proposed OSPF extensions for metro/core interworing when routing a connection across multiple subnetwors [4]. However, little wor has been done to evaluate the performance of different routing information exchange schemes. In this paper, we first present two solutions for routing information exchange: direct routing information exchange and routing information broer, differing in the location where the routing information is maintained and routing decision is made. In combination with various information refreshing approaches, e.g. periodical or event-driven, these schemes are used to handle dynamic networ conditions with low overhead. Furthermore, they are adapted to multi-segment networs in a hybrid way to allow gradual upgrading. We analyze and compare these schemes based on their scalability, complexity and communication overhead. Finally, we present related numerical results on examples of metro all-optical networs interconnected over an all-optical WDM bacbone. II. ROUING INFORMAION EXCHANGE ARCHIECURE A. Networ model he networ we are using for evaluation is composed of nodes and lins, which are collaborating in the optical control plane to provide routing information and to mae the routing decision. he networ under control is modeled using the multi-segment model [1], which is capable of representing the networ interconnections and capturing the following three critical dimensions: i) heterogeneous, segment-specific topologies and routing objectives; ii) global, intra-segment, and local, inter-segment, traffic; iii) gateway adaptation on segment boundaries. Clustering into segments can reflect any consideration for wavelength routing such as administrative domain, management policy, traffic aggregation, or vendor specification. In this paper, we assume the existence of multiple segments, e.g. metro segments interconnected over the optical bacbone (Fig. 1). he above model is generic, since every networ that is not segmented can be represented as a segmented networ with a single segment. nal in S 2 Fig. 1. Metropolitan networ S 3 S 4 Metropolitan networ S 1 Bacbone networ Gateway in Metropolitan networ S 5 Metropolitan networ Metro/Bacbone multi-segment networ Wavelength routing in optical networs, i.e., path selection and wavelength allocation, requires the control plane to provide the following routing information regarding current optical networ states: opology: includes both connectivity within segments and segment interconnections through gateways. Resource availability: number of wavelengths on lins (i.e., wavelength capacity), wavelength continuity constraints. Gateway adaptation information: number of gateways, their placements and corresponding adaptation properties (wavelength conversion, waveband interchangeability). Administrative information: such as choices of segment specific routing algorithms, gateway selection rules [1], and protection/restoration considerations. Based on how the routing information is maintained and exchanged, we present two schemes, namely direct routing information exchange and routing information broer. B. Direct routing information exchange (DRIE) In the Direct Routing Information Exchange (DRIE) scheme, routing information is exchanged directly among nodes, either through in-band or out-of-band channels. DRIE can be accomplished through either flooding or hierarchical

2 distribution. In flooding based DRIE (Fig. 2.a), each node builds a consistent view of the whole networ by advertising its local information and forwarding any received information to all its neighbors, except for the node that the information comes from. his involves excessive amount of information exchange and does not scale well. A more scalable approach is hierarchical information exchange (Fig. 2.b), where the routing information is flooded within the local segment and only the aggregated information (such as segment connectivity) is distributed to other segments. he tradeoff of this approach is that each node has detailed local information and only aggregated information regarding other segments. RIB have the flexibility of providing rich functionality in terms of routing and signaling, while at the same time hiding segment-specific implementations or administrative policies. D. Hybrid routing information exchange In a heterogeneous optical networ, different segments can have different properties such as traffic pattern, wavelength conversion capability, and optical technology. Specifically, each segment can have its own solutions of routing information exchange, either direct exchange or through broers. Fig. 4 shows an example of hybrid architecture, where segment A has a RIB and the other two segments implement hierarchical based direct information exchange. his flexibility is important such that it allows networ carriers a smooth upgrade. (a) (b) A Networing Segment Gateway and Gateway in Routing Information Exchange Fig. 2. Direct routing information exchange: (a) flooding based DRIE (b) hierarchical DRIE. C. Routing information broer (RIB) he idea of broer has been proposed in differentiated services [5], where the bandwidth broer (BB) is an agent responsible for allocating preferred service to users as requested, and for configuring the networ routers with the correct forwarding behavior for the defined service. We use this idea to overcome the problem of the DRIE scheme, where the basic assumption is that all segments can directly communicate with each other. his might be hard to achieve among heterogeneous segments from different vendors. his motivates us to consider the Routing Information Broer (RIB), a separate entity sitting on top of the networ collecting and maintaining routing information and accomplishing a variety of routing functions. Networing Segment (a) Gateway and Gateway in RIB (b) Broer-Broer Routing Information face Exchange Fig. 3. Options of optical routing information broer: (a) central RIB (b) segment specific RIB. wo RIB architectures are possible: central broer (Fig. 3.a) and segment-specific broer (Fig.3.b). he central RIB collects all the routing information of the whole networ through some information exchange channels, maintains the routing information database and maes the routing decision. On the other hand, segment specific RIB architecture has multiple broers and each of them perform local wavelength routing functions for a single segment, e.g. based on vendor-specific implementations or administrative policies. Broers also exchange global information with each other. Segment specific Fig. 4. B Hybrid routing information exchange. E. Refreshing the routing informaiton he state of optical networ resources is not expected to be static. It can be dynamic in case of lin/node up/down, gateway up/down, or based on resource availability change (wavelength capacity, free wavelengths). Outdated routing information tends to deteriorate wavelength routing performances and may even result in a sub-optimal or resource-inefficient routing decision. Keeping the routing information up-to-date is therefore of paramount importance. he refreshing of the routing information can be periodical or event-driven. Periodical refreshing may be suitable when the networ information tends to be more static, in which case the information can be relatively infrequently refreshed, to reduce communication overhead. Another way is to trigger refreshing by events such as changes in topology, wavelength capacity, and wavelength utilization. For example, when wavelength utilization reaches a threshold, the lin is required to advertise the current available wavelengths. Since only the lin information related to the change is refreshed, event-driven refreshing has the advantage of smaller routing information exchange overhead. A special event can be a connection triggered refreshing, where some important connections (such as long term, high bandwidth connections) will solicit the networ for the current routing information. In the eventdriven approach, the refreshing time depends on the traffic pattern and the routing algorithm, i.e. bursty traffic will result in more frequent refreshing of the routing information. he periodical and event-driven approaches can be also combined to achieve a more flexible solution in terms of low communication overhead and accuracy. For example, using infrequent periodical refreshing with event-driven updates can provide quic responds to bursty local networ state changes and capture smoother fluctuations with low communication overhead. C

3 F. actions with wavelength routing Routing in the optical control plane includes two critical dimensions: routing information exchange and wavelength routing (i.e., path selection and wavelength allocation). On the one dimension, the routing algorithm performs the path selection and wavelength allocation based on the available routing information. On the other dimension, wavelength routed networs need to provide up-to-date and consistent routing information to the routing algorithm. We have developed three wavelength routing algorithms for multi-segment optical networs: end to end shortest path (E2E), concatenated shortest path routing (CSR) and hierarchical routing (HIR), differing in the way the global traffic is accommodated and the routing information is maintained [1]. E2E routing assumes every node maintains full information of the networ and selects the end-to-end path using global shortest path algorithm. In CSR routing, each segment decides the route and allocates wavelengths only based on local information. Gateways, on the other hand, mae the decision regarding to the next segment towards the destination based on the segment interconnection information. HIR routing is between E2E and CSR in the sense that all nodes maintain local information and some inter-segment connectivity information such that they can directly choose the right gateway towards the next segment to the destination. Routing Algorithm E2E CSR HIR PNNI Flooding DRIE HIR DRIE Central RIB Segment RIB Routing Information Exchange Schemes Fig. 5. Relationship between routing algorithms and routing information exchange scheme. Both flooding based DRIE and central RIB schemes can provide the routing algorithm full routing information of the whole networ, therefore, they can accommodate any routing algorithms including E2E, CSR and HIR. On the other hand, both hierarchical DRIE and segment specific routing schemes can only provide detailed routing information regarding the local segment and aggregated information regarding other segments. his is sufficient for both CSR and HIR routing but inadequate for end-to-end path selection and wavelength allocation. PNNI-based routing is similar to the CSR routing since it requires internal nodes to maintain local information and peer group leaders maintain global information. herefore, PNNI can be accommodated by all the routing information exchange schemes we proposed. Fig. 5 summarizes the relationship of routing algorithms and routing information exchange schemes. Wavelength routing is based on the approximate information (due to dynamic conditions) given by routing information exchange schemes and the traffic pattern and routing algorithms will affect the fluctuation of resource utilization which will in turn trigger routing information refreshing. herefore, problems of routing information exchange and the wavelength routing are coupled. In this scenario, how to evaluate the performance of the routing function as a whole is an interesting research issue. III. ANAYSIS OF ROUING INFORMAION EXCHANGE SCHEMES o be able to quantitatively analyze how different routing information exchange schemes will perform and scale, we identify the following parameters related to the scalability, complexity and communication overhead. Bandwidth requirements (B R ): In all routing information exchange schemes, the routing information is disseminated in forms of Routing Information Advertisement pacets (RIA pacets). Since the RIA pacets are handled by individual nodes (or broers) and transmitted through lins, the amount of RIA pacets determines requirements of both the process power at individual nodes and bandwidth on lins in the control plane. Memory requirements (M R ): All routing information exchange schemes need to maintain databases regarding current networ states. he size of the database determines the memory requirements at networ nodes and broers. In the rest of this section, we will analyze each routing information exchange scheme based on these issues. As described previously, routing information can be refreshed periodically or triggered by events. he bandwidth requirement analysis here is for periodical refreshing. For the event-driven refreshing, bandwidth requirements cannot be estimated without the nowledge of the traffic pattern and wavelength routing algorithms. A. Analysis of the direct information exchange In flooding based DRIE, since the routing information regarding each optical lin is flooded throughout the control plane, any control plane lin carries at least one RIA pacets for each optical lin during any refreshing period. his determines the lower bound of the lin bandwidth requirement. If a node has seen the incoming pacet before, it silently discards the pacet. herefore, there are at most two copies of the same RIA pacet carried by any single lin, this refers to the situation where both ends of the lin forward the same RIA pacet at simultaneously. Assume the networ under control has a total of lins, the size of the RIA pacet is S, and the refreshing period is, then the lin bandwidth requirement can be expressed as follows: BR < 2 (1) he flooding based DRIE scheme results in synchronized routing information database at each node. he size of such database is given by: R (2) R is the size of routing information record for a single lin. In hierarchical DRIE, detailed intra-segment information is flooded within the local segment and inter-segment information is exchanged among all segments in the networ. We differentiate between two inds of lins: internal lin (connecting nodes within the same segment) and gateway lin (connecting nodes from consecutive segments). nal lins carry both intra-segment RIA pacets and inter-segment RIA pacets and gateway lins carry only inter-segment RIA

4 pacets. For the th segment, the bandwidth needed for intrasegment RIA is: (3) B < 2 where is the total number of lins in the th segment, S and are intra-segment RIA pacet size and intra-segment refreshing period of the th segment respectively. he bandwidth required for inter-segment advertisement is: G G (4) BR _ < 2 where S and are inter-segment RIA pacet size and inter-segment refreshing period respectively, G is the total number of gateway lins. herefore, the bandwidth requirement for an internal lin in the th segment is: G G (5) + BR < 2 ( + ) And the bandwidth requirement on the gateway lin is B R_ given in (4). In hierarchical DRIE, every node is only required to store intra-segment information and aggregated inter-segment information. herefore, memory requirements for nodes in the th segment is: R + G R (6) In which R is the routing information record size of the th segment and R is the inter-segment routing information record size. Comparing results of hierarchical DRIE and flooding based DRIE, we can see that by using multiple segments (i. e., <), hierarchical exchange can significantly reduce the bandwidth requirements and memory requirements. he improvement depends on the clustering granularity such that using small segments can achieve low intra-segment communication overhead and database size. However, this will increase the total number of segments for the same networ, which will in turn increase inter-segment exchanges. Furthermore, small segment means more and more connections will travel across multiple segments and this will increase the blocing probability [1][6]. Equations (1) and (3) also show that the bandwidth requirements are inversely proportion to the refreshing period, which is consistent with the refreshing analysis in the previous section. It should be noted that the information exchanged through hierarchical DRIE can be tuned to accommodate different routing algorithms, for example, in the CSR routing [1] and PNNI routing, the inter-segment information only needs to be maintained at the gateway or the peer group leader (PG) instead of having the inter-segment information disseminated into the segment. Consequently, the second part of (5) should be removed and (6) becomes the requirements for the gateway/pg. B. Analysis of the routing information broer scheme In the routing information broer scheme, the routing information is exchanged through channels between the networ nodes and the broer. Bandwidth requirements on these channels depend on the physical location of the broer and the topology of the networ. herefore, instead of evaluating the bandwidth requirement on a specific lin, we estimate the total bandwidth required to perform routing information exchange. In central RIB, all lins periodically report their states to the central broer. herefore, only one RIA pacet per lin is needed during any refreshing period and the total bandwidth requirement to the broer is: B R = (7) Unlie the DRIE scheme, which has duplicated routing information databases, a single database is maintained by the broer. he size of the database at the central broer is the same as that of the flooding based DRIE at individual node in (2). In segment specific RIB, routing information in each segment is directly reported to the local broer and we assume inter-segment information is flooded among broers through broer-to-broer channels. herefore, there are bandwidth requirements for both collecting local information and broerto-broer communications, and is given by: Q Q G S S G S (8) + G BR < + 2 G = 1 = 1 Where Q is the total number of segments and G is the number of gateway lins in the control plane. Memory requirement of the th broer is the same as that of a single node in the th segment in the hierarchical DRIE scheme given in (6). Comparing above results with corresponding DRIE results, we can see that the total bandwidth for RIB scheme is close to the bandwidth on a single lin in DRIE scheme. Furthermore, RIB has memory requirements only on broers, the number of which is usually much smaller than the total number of nodes. herefore, we can significantly improve both bandwidth requirements and memory requirements by using broer. C. Effects of wavelength capacity he existence of wavelengths differs the routing information exchange in the WDM networs to IP networs. Wavelength capacity affects the scalability of routing information exchange in the following aspects. Associated with each wavelength, there is a set of routing information, such as service type, active connections carried by the wavelength and wavelength conversion set. All of them need to be advertised throughout the networ. herefore, it is reasonable to assume that the size of the RIA pacet for any lin is proportional to the wavelengths capacity at that lin. Also, under dynamic networ conditions, information refreshing can be triggered by the change of wavelength utilization. herefore, given the same data traffic load, higher wavelength capacity can tolerant more changes and result in less frequent refreshing. IV. EXPERIMENS AND NUMERICA RESUS We perform our simulation on a multi-segment networ where a 6 6 mesh-torus bacbone connects 4 equal-sized metro networs with ring topology (Fig. 1). In the following

5 experiments, we assume all segments have the same pacet size and refreshing period and measure the number of pacets instead of bandwidth to avoid selections of these parameters. he first experiment illustrates the scalability of different schemes by showing the relationship between the total number of RIA pacets and the networ size. While fixing the bacbone networ as well as gateway locations, we increase the networ size by increasing the metro segment size. As we can see from Fig. 6, the number of pacets goes up with the increasing metro size in all four schemes. DRIE schemes disseminate routing information through flooding so that they generate much more pacets than RIB schemes. Hierarchical DRIE is more scalable than the flooding based DRIE since the flooding is limited within the segment instead of the whole networ. Since there are only 4 segments, the segment specific RIB requires slightly more pacets than the central RIB for inter-segment exchange. Fig. 6 also shows results for hybrid information exchange where the bacbone is assumed to use RIB scheme (e.g. Hybrid 2 DRIE means 2 metro segments perform DRIE and others use routing information broer). he total number of pacets for hybrid exchange is between that of DRIE and RIB schemes. considered together, both the bacbone lin change and metro lin change require the same number of RIA pacets and this number is increasing with the increasing metro size. However, in hierarchical DRIE, the amount of RIA pacets for bacbone lin change is fixed regardless the size of metro segment. his shows an advantage of the multi-segment model where effects of state change is restricted within the local segment and does not affect other segments. Compared with DRIE schemes, RIB schemes are more scalable to the networ change such that only one pacet is needed to report single lin change to the broer (not shown here). he last experiment shows the effect of clustering (Fig. 8). he simulation is performed on a 200-node bi-directional ring. All nodes are clustered into a number of equal-sized segments from 2 nodes/segment (100 segments) to 200 nodes/segment (1 segment). Since both flooding based DRIE and central RIB treat the multiple segments as a whole networ, clustering does not have any effect on them. In segment specific RIB scheme, the total intra-segment bandwidth is fixed, but the number of segments is decreasing with the increasing segment size. herefore, the total number of pacets is decreasing due to reduced inter-segment exchange. For hierarchical DRIE, as we explained in previous section, increasing the segment size will increase the intra-segment exchange but decrease intersegment exchange. herefore, there is an optimal segment size where the best tradeoff between intra-segment flooding and inter-segment exchange can be achieved and the total number of pacets is minimized Fig otal number of pacets vs. metro segment size Flooding Metro in Change HIR Metro in Change Flooding Bacbone in Change HIR Bacbone in Change otal Number of Pacets Flooding DRIE HIR DRIE Central RIB Segment RIB Num of RIA pacets Metro size Fig. 7. Number of RIA for single lin change vs. metro segment size o quicly respond to dynamic networ conditions, information regarding changes should be timely refreshed. Fig. 7 shows the number of pacets to advertise the state change (e.g. lin up/down, wavelength capacity/utilization change) of a single lin (metro lin or bacbone lin). o illustrate the scalability, we plot results under increasing metro segment sizes while eeping the bacbone fixed. In flooding based DRIE, where both the bacbone and metro networ are Segment Size Fig. 8. Effects of segment granularity V. CONCUSIONS We have presented and compared two routing information exchange schemes in this paper: direct routing information exchange and routing information broer. o handle dynamic networ conditions, routing information needs to be refreshed which can be either periodically or triggered by events. Both analytical and simulation results showed that using the multisegment routing information exchange can reduce the communication overhead and memory requirements. Furthermore, using multiple segments allows isolation of networ changes and providing flexible segment specific control functions. Finally, our simulation also gives the guideline for choosing suitable segment size for a networ to reduce the routing information exchange overhead.

6 REFERENCES [1] Y. Zhu, A. Juan and M. Ammar, Multi-segment Wavelength Routing in arge-scale Optical Networs, ICC2003. [2] D. Pendarais, B. Rajagopalan, amd D. Saha, Routing Information Exchange in Optical Networs, IEF net draft. [3] K. Kompella et al., OSPF extensiions in support of generalized MPS, IEF net draft. [4] D. Wang, J. Strand and J. Yates, OSPF for Routing Information Exchange Across Metro/Core Optical Networs, Optical Networ Magazine, Sept [5] K. Nichols, V. Jacobson and. Zhang, A wo-bit Differentiated Services Architecture for the net, RFC2638, July [6] Y. Zhu, A. Juan and M. Ammar, "Performance analy-sis of multisegment wavelength routing", IEEE/EOS Summer opical Meetings, 2002, Quebec, Canada.