Optical Packet Switching: A Network Perspective

Transcription

1 Optical Packet Switching: A Network Perspective Franco Callegati, Walter Cerroni, Giorgio Corazza, Carla Raffaelli D.E.I.S. - University of Bologna Viale Risorgimento, Bologna - ITALY {fcallegati,wcerroni,gcorazza,craffaelli}@deis.unibo.it Abstract In the last few years research on WDM optical packet-switching has mainly concentrated on issues at the single node level. The goal of this paper is to discuss about the problems arising when the focus is moved toward a network-wide scope. In particular, an overview of routing techniques that may be adopted in an optical packet-switched backbone is presented, showing the effects of adaptive multi-path routing strategies on the network performance. Furthermore, performance differentiation based on different routing and contention resolution strategies is proposed and analyzed in a simple QoS-aware scenario. Then, the application of adaptive routing strategies to network recovery in case of single link failure and the effects of dynamic multi-wavelength management and multi-path routing on packet sequence are also discussed. I. INTRODUCTION The advances experienced in the last decade by photonic technology have made optical networking a very good candidate to implement the very high-capacity backbone of future communication networks. The WDM optical circuit-switching paradigm (at either fiber or wavelength level) is a technique to realize such optical backbone with some flexibility in terms of resource provisioning, and offers huge bandwidth capacity to the end-user. Nonetheless, this approach provides access to bandwidth with a very coarse granularity and therefore with limited QoS management capability. Optical Burst Switching (OBS) [] and Optical Packet Switching (OPS) [2] are respectively a medium and a longer term networking solution, promising more flexibility and efficiency in bandwidth usage combined with the ability to support diverse services [3]. Research activities on OPS have mainly concentrated on issues at the single node level, studying architectural and algorithmic solutions to provide satisfying levels of performance [4]. In particular, the problem of congestion resolution has been extensively studied, showing that smart policies able to effectively exploit both the time and the wavelength domains can make a huge difference in terms of average packet loss and latency [5]. The aim of this paper is to introduce and discuss about some of the problems arising when the focus is moved from the single node perspective toward a network-wide scope, with an overview of adaptive, multi-path routing techniques that may be adopted in an optical packet-switched backbone. Also different routing and contention resolution strategies for service differentiation can be proposed and analyzed in a QoSaware scenario based on the DiffServ model [6]. However, the number of QoS classes must be kept as small as possible in order to minimize operational efforts, since complex scheduling algorithms may not be applicable because of the peculiarity of queuing in the optical domain [7]. Multi-path routing strategies may also be exploited in order to provide reliability to the optical packet-switched network. In particular, when a single link failure occurs, packets previously routed on that link may be transmitted on alternative paths, according to the multi-path routing strategy adopted. Additionally, an important issue that should be taken into account is the impact of wavelength management and multi-path routing strategies on the packet sequence, which can be easily broken, for instance, when the difference in terms of latency along different routing paths is not negligible. The paper is structured as follows: first, the behavior assumed at the single node level is addressed in section II; then, an overview of multi-path, adaptive routing techniques that may be deployed in an OPS network is presented in section III, with particular attention to traffic differentiation in section IV; the application of adaptive routing strategies to network recovery in case of single link failure is discussed in section V, while a methodology for the evaluation of the effects of dynamic wavelength and routing management on packet sequence is introduced in section VI; finally, section VII concludes the work. II. SINGLE NODE BEHAVIOR The reference scenario considers a packet-switched optical network switching asynchronous, variable-length packets, statistically multiplexed over multi-wavelength links. A general switching node with N input and N output fibers, carrying W wavelengths each, is considered. Each node is equipped with Fiber Delay Line (FDL) buffers, which are able to delay optical packets for an amount of time multiple of a given time unit D (called buffer granularity). The buffer size is related to the maximum available delay D MAX which depends on the number B of delay lines used. Contentions due to contemporary packet arrivals at a given output port are resolved by means of load balancing techniques that exploit both the wavelength and time dimensions. In general, after the output fiber has been determined for a given packet by the routing strategy, the switch control logic must face a two-dimensional scheduling problem: the choice of the wavelength to transmit the packet on and, in case of contention, the delay to be assigned to that packet. This problem can be referred to as the Wavelength and Delay Selection (WDS) problem [5].

2 Two classes of WDS algorithms, characterized by a very similar computational complexity, are the following: delay oriented algorithms (D-type), that aim at minimizing the waiting time of a queued packet and therefore act according to the principle that, when a packet has to be queued, it will join the shortest available queue (the shortest delay provided by the FDL buffer); gap oriented algorithms (G-type), that aim at minimizing the gaps (caused by the discrete number of delays available) between packets and, consequently, maximizing the throughput of the switching matrix, acting according to the principle that, when a packet has to be queued, it will be sent to the delay that is closest to the transmission end of the preceding packet. λ λ 2 λ 3 λ 4 Fig.. G-type D-type t 0 t 0 +D t 0 +2D t 0 +3D t 0 +4D Different node-level packet scheduling policies When considering traffic differentiation, incoming packets are assumed to belong to two classes, namely High Priority (HP) and Low Priority (LP) optical packets. Since FDL buffers cannot allow a full random access to the queue [7], conventional preemptive or priority queuing techniques are not easily applicable here. Thus QoS management must mainly rely upon mechanisms based on differentiated scheduling and a-priori access control to the optical buffer, by adopting, for instance, either a time-based or a wavelength-based resource partitioning technique [9]. Considering the limitations of today s optical technology, a partitioning policy relying on the wavelength domain may be more effective and provide more flexibility than a partitioning policy exploiting the time domain. In the following, a wavelength is considered congested when the corresponding FDL buffer is full and no more packets are allowed into it. Under this assumption, the shared wavelength partitioning strategy reserves any K wavelengths to HP traffic based on the actual buffer occupancy; namely when more than K wavelengths are not congested, both LP and HP packets may be transmitted, otherwise when only K or less wavelengths are congested (whichever they are), only HP packets can be transmitted. Figure 3 shows an example for K = 2,W = 4,B = 4. LP HP The two approaches are briefly sketched in Fig., for the case of an output fiber with W = 4 wavelengths and B = 4 delays (D, 2D, 3D, 4D). Other policies based on the voidfilling principle proposed in [8] are not considered here due to their higher computational complexity. A G-type buffer policy will be assumed in the following, since it proves to realize a good trade-off between complexity and performance. In fact, this choice results in a lower packet loss rate compared to a D-type wavelength assignment policy, as shown in Fig. 2 for a reference case with N = 4,W = 6,B = 7. λ λ 2 λ 3 λ 4 t 0 t 0 +D t 0 +2D t 0 +3D t 0 +4D LP HP.0e- Packet Loss Probability λ λ 2 λ 3 λ 4.0e-2.0e-3 t 0 t 0 +D t 0 +2D t 0 +3D t 0 +4D.0e-4 D-type G-type.0e-5.0e D (normalized to the average packet length) Fig. 2. Performance of different node-level packet scheduling policies Fig. 3. Wavelength-based partitioning policy for QoS differentiation III. ADAPTIVE ROUTING IN OPS NETWORKS Generally speaking, routing algorithms can be either static or adaptive, i.e. dynamic. The former define static routing tables once and for all, whereas the latter route traffic by exploiting information regarding the state of the network. The

3 adaptive algorithms may be further specialized depending on the number and cost of the paths that are considered in order to take the routing decision [0]. This paper assumes a meshed network topology with WDM links. The basic idea is to combine the flexibility of adaptive routing with the efficiency of packet multiplexing over a large set of wavelengths by means of an effective WDS policy. At each node, traffic is normally forwarded along the shortest path but alternative paths of equal or higher hop count are also identified and may be used. Therefore, two possible routing strategies may be defined []: Shortest-Path Routing (SPR), based on minimum hop count and not using any alternative path, i.e. a static choice; Multi-Path Routing (MPR), including alternative paths that are dynamically used by the network nodes when the link along the shortest path (also called the default link) becomes congested. The first decision to be taken concerns how many alternative paths, among those possible, should be taken into account for load balancing in MPR. Once the alternative routes have been defined, each node tries to send the incoming packets on the default link and, in case this one is congested, i.e. there is no wavelength available and the buffer is full, one of the alternative paths is used, picking it up from one or more alternative sets. In case also the alternative paths do not have available wavelengths and/or delays, the packet is dropped. The following options may be applied in the definition of the alternative sets of routes: no alternative path: this is the case of SPR; shortest alternative paths (SAP): beside the default link an alternative set of routes is considered, which includes any other shortest path different from the default one, i.e. any other path with the same hop count as the shortest one; depending on the network connectivity, such an alternative set may or may not exist; n-shortest alternative paths (n-sap): besides the default link, n alternative sets of routes are considered, where the i-th set includes every path with i hops more than the shortest one; obviously, the first set (i = ) does not include the default shortest path itself; An additional, more dynamic approach to the load balancing policy consists into applying the WDS policy not on a single link (either default or alternative) but on an entire set of links, which can be defined as follows: shared shortest paths (SSP): WDS is performed directly taking into account all the wavelengths on any shortest path link, including the default one; n-shared shortest paths (n-ssp): the WDS is performed directly taking into account all the wavelengths on any link belonging to paths with up to n hops more than the shortest one. A comparison of the behavior of the aforementioned MPR strategies in terms of average packet loss rate is shown in Fig. 4, evaluated over the European optical network topology Packet Loss Probability e-03 e-04 e-05 e-06 SPR SAP 2-SAP SSP 2-SSP Fig. 4. Performance of different MPR strategies discussed in []. The figure shows that a MPR approach starts to be effective when also non-shortest paths are taken into account (only the case for paths with up to a single hop more than the shortest one have been considered). IV. QOS SUPPORT When the incoming traffic is classified into priority classes, QoS differentiation can be achieved by differentiating the concept of congestion and/or providing different alternatives to LP and HP traffic. As an example, congestion may be defined according to the wavelength availability on the default link, adopting a strategy similar to the shared wavelength partitioning used at the single node level. The value of K may be different for different classes of service, giving the capability to decide which class of traffic makes use of the alternative paths more frequently. However, the use of nonshortest, alternative paths may cause the packets to stay longer in the network and the transmission delay to become much higher than the SPR case. Therefore, in the following it is assumed that MPR strategy may be used for LP traffic only, while HP traffic is always routed according to the SPR strategy, aiming at preserving the HP traffic stream as intact as possible. An evaluation of such strategies over a simple 5-nodes meshed topology, under the assumptions considered in [2], is presented in Fig. 5. The plot shows the good level of differentiation between losses of HP traffic (20% of the total) and LP traffic (80%), when only the LP traffic is re-routed on alternative paths in case of congestion on the default link with a 2-SAP strategy (curves labeled MPR) or when the MPR in not applied at all (curves labeled SPR). As expected, the higher the number of reserved wavelengths K, the higher the gain obtained by the HP traffic, while the performance of the LP traffic is barely affected in the range considered here. Furthermore, HP traffic is also not affected by the different routing strategies applied to LP packets. On the other hand, according to Fig. 6 where K was set to 3, for a very low percentage of HP traffic a good level of performance may be achieved. Of course, when the percentage of HP traffic grows over the 20%, the service differentiation strategy becomes less effective, although the LP traffic seems

4 0 2 3 Packet loss probability e-0 e-02 e-03 e-04 e-05 e-06 SPR HP SPR LP MPR LP e-07 MPR HP Fig. 7. Network topology with average node degree E = 3.25 e No. of reserved wavelenghts (K) Fig. 5. Performance of QoS differentiation by means of routing and resource partitioning techniques Packet loss probability e-0 e-02 e-03 e-04 e-05 e-06 e-07 e-08 SPR LP 0 SPR HP MPR HP Percentage of HP traffic MPR LP Fig. 6. Performance of QoS differentiation by means of routing and resource partitioning techniques to be still slightly affected. It follows that in case a maximum value of the loss probability is required by HP packets, the admission to the network has to be kept under control in order to avoid performance degradation due to the limited resources reserved to HP traffic. Larger network topologies are shown in Figs. 7 and 8, where each vertex in each graph represents an OPS node, while each edge represents a pair of fiber links, transmitting packets in opposite direction. Due to this assumption, the number of links in the network is always twice the number of edges in the corresponding graph. The two topologies present a different connectivity, which can be quantified using the average node degree E, defined as the average number of edges connected to a node. Evaluations on the above topologies have been performed under the following assumptions: the traffic matrix is uniform, i.e. the input traffic at the ingress of the network is assumed to be the same on each node and equally distributed towards all the other nodes; Fig. 8. Network topology with average node degree E = 5.75 this choice leads to different values of the average load per link, according to the network topology; the load distribution on the wavelength is balanced, i.e. the number of wavelengths on every single link is chosen in order to obtain the same load on each wavelength (i.e. 80%); the amount of input traffic is the same for both the topologies; 25% of wavelengths on each link are reserved to HP packets; 20% of the traffic is HP; LP traffic is re-routed with MPR strategy. Link loss rate is shown in Figs. 9 and 0. A good level of service differentiation has been obtained, although different links experience a heavily unfair behavior. It is worth to notice that the higher the topology connectivity, the higher the packet loss rate. This is a consequence of the redistribution of the load among a higher number of links, leading to a smaller number of wavelengths per link and less effective wavelength multiplexing. As a measure of the impact of the routing strategy on the network latency, the distribution of the number of hops experienced by a packet is shown next. Figure clearly shows that for the less connected topology the use of the MPR strategy involves typically alternative paths of the same length as the shortest one. This is related to the structure of the topology itself (similar to a Manhattan-Street Network), which often includes several shortest paths between pairs of nodes. The situation is quite different for the other topology, that is much more connected. In this case the diagonal links between

5 Loss rate.e00.e-0.e-02.e-03.e-04.e-05.e Link LP loss HP loss nodes lead to the presence of several alternative paths crossing one hop more than the shortest one. Furthermore, as shown in Fig., the highly congested state of several links, due to the small number of wavelengths available, causes LP packets to be often re-routed on alternative paths. Such packets keep traveling inside the network, trying to follow less congested paths before ending up to the destination node. In some cases, they enter in temporary loops and experience a quite large number of hops, a typical behavior of congestion-based deflection routing techniques. A network design procedure aiming at finding a good tradeoff between performance guarantees and network cost in terms of number of wavelengths per link is also illustrated and discussed in [2]. Fig. 9. Link loss rate for the topology of Fig. 7 with uniform traffic matrix and balanced load distribution Loss rate.e00.e-0.e-02.e-03.e-04.e-05.e Link LP loss HP loss Fig. 0. Link loss rate for the topology of Fig. 8 with uniform traffic matrix and balanced load distribution Distribution.E+00.E-0.E-02.E-03.E-04.E-05.E-06.E-07.E No. of hops E = 3.25 E = 5.75 Fig.. Distribution of the number of hops for the topologies of Fig. 7 (E = 3.25) and Fig. 8 (E = 5.75), with uniform traffic matrix and balanced load distribution V. LINK FAILURE ISSUES Reliability in optical networks has been a widely discussed research topic, being a very important requirement for the next generation optical networks [3]. Different kinds of failure can happen even at the same time and the network must be capable to recover from them. This must be done in an efficient way by detecting the failure as quickly as possible, so that a recovery procedure can be called immediately. Information loss needs to be limited during the failure detection time, as well as when recovery is taking place. To this purpose a smart and efficient recovery algorithm is required. Adaptive routing techniques could be employed in OPS networks in the presence of optical link failures due to accidental fiber cuts or device malfunctioning. The application of these techniques to the OPS scenario represents a completely different approach compared to protection and restoration techniques traditionally adopted in wavelength-routed networks [4]. The MPR strategies described above may be exploited in order to provide reliability to the network. In fact, when a link failure occurs, packets previously routed on that link are transmitted on alternative paths, depending on the MPR strategy adopted. In this case, a key parameter is the time d required to detect the failure and start re-routing the packets: obviously, the shorter the fault detection delay, the smaller the number of packets lost due to the link unavailability. Figure 2 shows the time behavior of the packet loss related to the failed link. Between the failure instant t f and the detection instant t f +d the loss rate reaches a level that depends on the value of the failure detection time d. This behavior can be easily described by the analytical model presented in [5]. The use of MPR strategies for packet re-routing after a link failure detection affects the traffic distribution within the network. This is a consequence of the higher traffic delivered to links adjacent to the one which failed. Therefore, the same amount of overall input traffic as before is spread over a network that is now lacking one fiber. This causes the average load per link to be higher and, when the network works with high loads (e.g. 0.7 Erlang/wavelength) and a failure occurs, some links may become overloaded. In this case, a simple

6 Packet Loss Rate loss rate when d = d3 loss rate when d = d2 loss rate when d = d 0. loss rate without link failure 0.0 d d2 failure time tf 0.06 d Time Fig. 2. Time diagram of the packet loss rate on a failed link for different failure detection times 3.55e+2 Network Throughput [bps] 3.50e e+2 before failure 3.40e+2 during failure detection 3.35e e+2 +73% protection resources after failure detection no protection resources 3.25e Simulation Time [sec] Fig. 3. Throughput behavior in case of failure, with and without a protection scheme adaptive routing approach is no longer reliable since it cannot bring the network back to the original performance level. This leads to the need for a specific protection scheme based on shared resources [5], which can be realized by dimensioning the network in two main steps. First of all, the network is dimensioned for a given average load per wavelength (e.g. 0.7) with relation to the input traffic matrix. Then, for each node, further wavelengths are added to each fiber going out of that node until it sees all its output links with the same capacity. Simulation results for a reference case showed that with this approach the additional cost due to protection is 73% of the initial cost in terms of number of wavelengths. Figure 3 shows the trend of the throughput before and after a failure. It can be seen that the performance level of the failure-free scenario is almost completely recovered. When failure occurs performance drops drastically. However, without the protection scheme, the throughput goes even worse after failure detection, whereas when protection is applied the original throughput is practically restored. VI. E FFECTS ON THE PACKET SEQUENCE In a datagram-based communication network, packet loss as well as out-of-order packet delivery and delay variations af- fects end-to-end protocols behavior and may cause throughput impairments [6], [7]. When considering TCP-based traffic it is well known that these phenomena influence the typical congestion control mechanisms adopted by the protocol [8] and may result in a reduction of the transmission window size and consequently in bandwidth under-utilization. Another example is delay-sensitive UDP-based traffic, such as realtime traffic. In this case, because of the timing constraints, re-transmission of lost packets is not possible and therefore a high percentage of lost packets may result in a significant degradation of the quality of the conversation. Moreover unordered packets may arrive too late and/or the delay required to reorder several out-of-sequence packets may be too high with respect to the timing requirements of the application. These brief and simple examples make evident the need to limit the percentage of lost packets as well as the number of unordered packets. In general the former event is due to congestion while the latter is typically caused by the fact that packets belonging to the same flow of information can take different paths through the network and then can experience different latency. However, in an OPS network using the wavelength domain for congestion resolution, packets traveling along the same network path may use different wavelengths in order to exploit wavelength multiplexing for congestion resolution purposes, according to a given WDS policy. Therefore it may happen that packets of the same flow are delivered out of sequence, even though still following the same network path. To prove this, the first thing to do is to provide a clear definition of out-of-sequence and a framework to evaluate the delay jitter experienced by packets when crossing a node. A formal definition of such a framework, that is briefly recalled here, can be found in [9]. For a generic couple of subsequent ordered packets Pn and Pn+ incoming on a given OPS node, let Jn be the jitter between them, defined as the packet offset variation after they have crossed the node. Since the two packets may experience different delays while crossing the node, seven different alternatives may happen, according to the cases shown in Fig. 4: ) the packet sequence is always guaranteed since Pn+ experiences more delay than Pn ; 2) the node is transparent and Pn and Pn+ have the same offset at the input and output (i.e. Jn = 0); 3) Pn+ experiences less delay than Pn but at the output it is still behind the tail of Pn ; 4) the head of Pn+ partially overlaps the tail of Pn ; 5) Pn+ completely overlaps Pn ; 6) Pn+ has overtaken Pn but they are partially overlapping; 7) Pn+ has completely overtaken Pn. The previous formalization allows to evaluate the delay jitter distribution as well as the amount of out-of-order packets, that depends on the specific definition of packet sequence. For instance, in case overlapping packets are not considered in sequence, then the in-sequence regions will be, 2, and 3. If

7 INPUT P n P n+ OUTPUT ❶ ❷ ❸ ❹ ❺ ❻ ❼ Jitter distribution Fig. 4. Examples of jitter between subsequent packets Region 0 - Fig. 6. Delay jitter distribution for a G-type WDS algorithm with the sequence constraint over the different regions shown in Fig. 4 Jitter distribution Region Packet loss probability with sequence constraint without sequence constraint Fig. 5. Delay jitter distribution for a G-type WDS algorithm over the different regions shown in Fig. 4 some overlapping is allowed, then the sequence is guaranteed also in region 4. The same for region 5, in case packets arriving at the same time are not considered out-of-order. The effects of a G-type WDS on packet sequence are shown in Fig. 5, where the jitter distribution over the different regions defined above is presented. As expected, the WDS policy considered here causes some packets to get out of the node unordered, since it does not take into account the correct packet sequence when wavelength and delay are assigned to a packet. However, the most frequent behavior is the one related to region 2, which means that congestion happens rarely and the packets are often transmitted transparently across the node. In order to take into account the correct packet sequence at the WDS stage, the G-type algorithm can be modified as discussed in [20], introducing the additional constraint that the current packet must not overtake the previous one. In this case the jitter distribution is illustrated in Fig. 6. Obviously, the trade-off behind this approach is a performance impairment, as shown in Fig. 7 for N = 4,W = 6,B = 3. The evaluations presented above are related only to the packet jitter introduced by a single node due to the WDS policy adopted. A similar approach can be applied at the whole network level, in order to take into account also the effects on the packet sequence of multi-path routing, whose impact may be much stronger when the difference of the propagation times D (normalized to the average packet length) Fig. 7. Impact of the sequence constraint on packet loss probability for G-type WDS algorithm over different paths is not negligible. The analysis of such a scenario and the related consequences at the transport layer are currently under investigation. VII. CONCLUSION This paper presented an overview on some of the problems arising when WDM optical packet-switched networks are evaluated from a network-wide point of view. The effects of resource partitioning as well as multi-path routing have been shown by applying dynamic wavelength and buffer management (WDS) on each link jointly with dynamic routing strategies, both in undifferentiated and differentiated traffic cases. The effectiveness of the proposed strategies has been demonstrated, leading to the conclusion that the wavelength domain is confirmed to be the key factor to achieve performance optimization. The application of adaptive routing strategies to network recovery in case of single link failure was also discussed, showing again the importance of the wavelength domain to realize an effective protection scheme. Finally, the problem of how the packet sequence may be affected by dynamic multi-wavelength management and multipath routing was discussed.

8 REFERENCES [] C. Qiao, M. Yoo, Optical burst switching: A new paradigm for an optical internet, Journal of High Speed Networks, vol. 8, no., pp , January 999. [2] M. J. O Mahony, D. Simeonidou, D. K. Hunter, A. Tzanakaki, The application of optical packet switching in future communication networks, IEEE Communications Magazine, vol. 39, no. 3, pp.28-35, March 200. [3] W. Vanderbauwhede, D. A. Harle, Design and Modeling of an Asynchronous Optical Packet Switch for DiffServ Traffic, Proc. ONDM 2004, Gent, Belgium, pp. 9-35, February [4] L. Dittmann, et al., The European IST project DAVID: A viable approach towards optical packet switching, IEEE Journal on Selected Areas in Communications, vol. 2, no. 7, pp , September [5] F. Callegati, W. Cerroni, G. Corazza, Optimization of Wavelength Allocation in WDM Optical Buffers, Optical Networks Magazine, vol. 2, no. 6, pp.66-72, November 200. [6] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, W. Weiss, An architecture for differentiated services, IETF RFC 2475, December 998. [7] D. K. Hunter, M. C. Chia, I. Andonovic, Buffering in Optical Packet Switching, IEEE/OSA Journal of Lightwave Technology, vol. 6, no. 0, pp , December 998. [8] L. Tancevski et al., Optical routing of asynchronous, variable length packets, IEEE Journal on Selected Areas in Communications, vol. 8, no. 0, pp , October [9] F. Callegati, W. Cerroni, C. Raffaelli, P. Zaffoni, Wavelength and time domain exploitation for QoS management in optical packet switches, Computer Networks, vol. 44, no. 4, pp , 5 March [0] R. Ramamurthy, B. Mukherjee, Fixed-alternate routing and wavelength conversion in wavelength-routed optical networks, IEEE/ACM Transactions on Networking, vol. 0, no. 3, pp , June [] F. Callegati, W. Cerroni, G. Muretto, C. Raffaelli, P. Zaffoni, Adaptive routing in DWDM optical packet switched network, Proc. ONDM 2004, Gent, Belgium, pp. 7-86, February [2] F. Callegati, W. Cerroni, C. Raffaelli and M. Savi, QoS differentiation in optical packet-switched networks, to appear on Computer Communications, Elsevier, [3] J. P. Vasseur, M. Pickavet, P. Demeester, Network Recovery: Protection and Restoration of Optical, SONET-SDH, IP and MPLS, Morgan Kaufmann Publishers, [4] A. Fumagalli, L. Valcarenghi, IP restoration vs. WDM protection: is there an optimal choice?, IEEE Network, vol. 4, no. 6, pp. 34-4, November-December [5] F. Callegati, W. Cerroni, G. Muretto, C. Raffaelli, Exploitation of dynamic routing for optical packet-switched network reliability, Proc. NOC 2005, London, UK, July [6] J. C. R. Bennett, C. Patridge, Packet reordering is not a pathological network behavior, IEEE/ACM Transactions on Networking, vol. 7, no. 6, pp , December 999. [7] M. Laor, L. Gendel, The effect of packet reordering in a backbone link on application throughput, IEEE Network, vol. 6, no. 5, pp , September/October [8] M. Allman, V. Paxson, W. Stevens, TCP congestion control IETF RFC 258, April 999. [9] F. Callegati, W. Cerroni, G. Muretto, C. Raffaelli, P. Zaffoni, A framework for performance evaluation of OPS congestion resolution, Proc. ONDM 2005, Milan, Italy, pp , February [20] F. Callegati, D. Careglio, W. Cerroni, G. Muretto, C. Raffaelli, J. Solé Pareta, P. Zaffoni, Keeping the packet sequence in optical packetswitched networks, Optical Switching and Networking, vol. 2, no. 3, pp , November 2005.