SPIE Proceedings [SPIE Asia Pacific Optical Communications - Wuhan, China (Thursday 1 November 2007)] Optical Transmission, Switching, and Subsystems V - Group scheduling based on

Group Scheduling Based on Control-packet Batch Processing in Optical Burst Switched Networks

Yuan Chi, Li Zhengbin, He Yongqi, and Xu Anshi

State Key Laboratory of Advanced Optical Communication Systems & Networks School of Electronics Engineering and Computer Science, Peking University, Beijing, 100871, China

Email: [email protected], [email protected]

Abstract: Optical burst switching (OBS) is proposed as a high-speed, flexible, and transparent technology. It is thought

to be the best way to adapt the bursty IP traffic over optical wavelength division multiplexing (WDM) networks. OBS technology facilitates the efficient integration of both IP and WDM. It provides statistical multiplexing gains and avoids long end to end setup time of traditional virtual circuit configuration. However, there are still a lot of challenges, one of which is burst contention. Owing to the fact that random access memory like buffering is not available in the optical domain at present, there exists a real possibility that bursts may contend with one another at a switching node. Many contention resolutions are proposed. The major contention resolutions in literature are wavelength conversion, fiber delay lines, and deflecting routing. In this paper, a new data burst scheduling scheme, called group scheduling based on control-packet batch processing (GSCBP) was proposed to reduce burst contention. Like transmission control protocol, GSCBP has a batch processing window. Control packets which located in the batch processing window are batch processed. A heuristic scheduling algorithm arranges the relevant bursts’ route based on the processing result and the network resource. A new node architecture supporting group scheduling was presented. The GSCBP algorithm is combined with wavelength converter and/or fiber delay lines which is shared by some data channels. Meanwhile, an extended open shortest path first (E-OSPF) routing strategy was proposed for OBS. Both GSCBP and E-OSPF are introduced into 14-node national science foundation network by means of simulations. The ETE delay, burst blocking probability, as well as burst dropping probability were attained. Results show that the GSBCP lead to the higher-priority traffic drop rate decrease one order of magnitude, if drop rate and ETE delay of lower priority traffic is sacrificed.

Key words: Group Scheduling based on Control-packet Batch Processing (GSCBP), Optical Burst Switching (OBS), Contention Resolution (CR), Extended Open Shortest Path First (E-OSPF), Quality of Service (QoS)

I. Introduction The rapid explosion of Internet traffic, especially the continuous increasing peer-to-peer traffic [1] has brought

about an acute need for high-performance networks, and the amount of bandwidth available on a single fiber has increased dramatically by the wavelength division multiplexing (WDM) technology [2]. It strengthens the demand for high-performance switches/routers than ever before. It is significant to design switching and multiplexing schemes to enhance bandwidth efficiently. All optical networks promise to be able to satisfy the ever-increasing traffic requirements. Meanwhile, it is envisioned that the majority of traffic carried on the network will be Internet Protocol (IP) carried directly over optical WDM. Photonic/Optical switching in optical WDM networks offers high speed, flexible and transparent transmission. It can also support different types of traffic agilely. However, optical signal processing to recognize the address requires optical to electrical conversion, which cause a technological limitation in the optical domain.

Optical burst switching (OBS) [3-4] technology is proposed as a high-speed, flexible and transparent transmission technology. It is proposed for IP networks that may be able to efficiently utilize extremely high-capacity links without optical buffering or optical-electronic conversions at the core node of an OBS network. OBS technologies combining the best of coarse-grained optical circuit switching and fine-grained optical packet switching, and it facilitate the efficient integration of both bursty IP and WDM. It is proposed to avoid long end-

Optical Transmission, Switching, and Subsystems V,edited by Dominique Chiaroni, Wanyi Gu, Ken-ichi Kitayama, Chang-Soo Park,

Proc. of SPIE Vol. 6783, 67830W, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.743726

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to-end (ETE) setup time of traditional virtual circuit configuration. Using a one-way signalling scheme, OBS networks can decrease the ETE delay. Meanwhile, its dynamic characteristic leads to high network adaptability and scalability, which makes OBS quite suitable for the transmission of bursty IP traffic. Therefore, it can provide statistical multiplexing gains at sub-wavelength granularity while having the advantage of simplicity in deployment. However, under the current and foreseeable limitations of optical technology, performance of OBS network is mainly hampered at the network nodes by resource contention between bursts, which from different input ports wanting to be forwarded to the same output link, on the same wavelength, at the same time. Burst contention is inescapable in OBS for optical random-access memory (RAM) is not available at present.

Due to the fact that the RAM-like buffering is not available in the optical domain, the most important design goal in OBS networks is to reduce burst dropping probability resulting from resource contention. Contention will occur when two or more bursts from different input ports wanting to be forwarded on the same wavelength, on the same output port, at the same time. If not enough valid output ports for these bursts, burst dropping is inescapable in this instance. This feature is quite different from traditional packet switching networks. Typically, contention in traditional electronic packet switching networks is handled through buffering. However, store-and-forward is not available in OBS for the absence of optical buffers. Therefore, contention resolution in OBS networks has attracted much attention recently. To guarantee burst delivery, to reduce packet dropping probability, some contention resolution methods have been proposed in literature. Existing work addresses this issue mostly in the wavelength domain (using wavelength conversion [5]), the time domain (using fiber delay lines, FDLs [6]), the space domains (using deflection routing [7]), the burst domain (using burst segmentation), and the soft domain (using different resolution algorithms).

In this paper, a new data burst scheduling scheme, called group scheduling based on control-packet batch processing (GSCBP) was proposed to reduce burst contention. Like transmission control protocol (TCP), GSCBP has a batch processing window. Control packets which located in the batch processing window are batch processed. A heuristic scheduling algorithm arranges the relevant bursts’ route based on the processing result and the network resource. The GSCBP algorithm is combined with wavelength converter and/or FDLs which is shared by some data channels. Meanwhile, an extended open shortest path first (E-OSPF) routing strategy was proposed for OBS. Not only the shortest path but also the minimum hops path is considered in this strategy. Both GSCBP and E-OSPF are introduced into 14-node national science foundation network (NSFnet) by means of simulations. The ETE delay, burst blocking probability, as well as burst dropping probability were attained. The simulation results show that the E-OSPF can decrease about 15% burst blocking and dropping probability at moderate loads. The GSCBP combines with wavelength converter and/or FDLs scheme is effectively resolve burst contention and achieve good network performance.

The rest of the paper is organized as follows. Section II introduces the burst contention resolutions and burst scheduling schemes in OBS networks. Section III presents the soft contention resolution method and new node architecture. Section IV presents the simulation results and some analysis. Finally, Section V contains the conclusions.

II. Burst Contention and Resolutions in OBS Networks In this section, first, we introduce the OBS network architecture. Then we discuss burst contention and

contention resolutions in OBS networks. Finally, we review burst scheduling schemes in OBS networks.

A. OBS Network Architecture An OBS network is assumed that the network utilizes all-optical switching elements and ultra-long-reach all-

optical transport. Basic components of an OBS network include some edge nodes, some core nodes, and some WDM links connecting these nodes together. Fig. 1 shows the OBS network architecture. The edge node is in charge of traffic aggregation, and the core node is in charge of burst switching and routing. The traffic from multiple access networks is accumulated at the ingress edge node and transmitted through high-capacity WDM links over the core node. The edge node provides legacy interfaces such as Gigabit Ethernet, ATM and IP interfaces, and are responsible for data burst assembly and disassembly. Data burst assembly refers to the process by which incoming packets are aggregated into data bursts according to some assembly algorithm. The reverse operation is referred to as the disassembly process. An ingress edge node is referred to the source edge node, and an egress edge node is referred to the destination edge node.



Edge NodeAccess

Core Node

In an OBS network, incoming packets (such as IP packets) from the access networks are gathered and sorted at the ingress edge node according to their destination and quality-of-service (QoS). According to a preselect assembly algorithm, these packets are assembled into variable sized elementary switching and processing entities, namely burst data packet (BDP, throughout this paper, the terms BDP and burst are used interchangeably). Once a data burst has been created and ready to be transmitted by the edge node and based on the information of burst assembly and control information, a control packet (CP) will be generated synchronously by the edge node. To reserve network resource for its burst, a CP will be transmitted as soon as it is created. The burst is transmitted on dedicated data channels after an offset time. Commonly, the offset time includes basic offset time and QoS offset time. The former allows for the CP to be processed at the intermediate node in the electrical domain and the network resource reservation, and the latter allows for the burst be transmitted with differentiated QoS. All CPs are transmission on the control channel, which may be separated from the data channels. All CPs are processed by intermediate core nodes in the electrical domain. According to the processing information (such as burst length, offset-time and QoS, etc.), the intermediate core node reserves network resources orderly for this burst’s transmission. The burst is switched all-optically without waiting for acknowledgment of light path setup [3-4, 8]. It follows its CP and cut-through the OBS network without optical-electrical-optical conversion.

Fig. 1 Optical burst switched network architecture

B. Burst Contention and Contention Resolution OBS is designed to avoid the long end-to-end setup time of conventional virtual circuit configuration with no

need for buffers at the core node. Owing to the fact that optical logic and RAM-like buffering is not available in the optical domain, transmission of bursts will fail when network resource reservation fails for two or more bursts competing for the same resource simultaneously, knowing as burst contention. That is, contention will occur when two or more bursts from different input ports wanting to be forwarded on the same wavelength, on the same output port, at the same time. Contention is inherent to OBS technology, which basically assumes that the network is buffer-less and connectionless. This feature is quite different from traditional packet switching networks. Typically, contention in traditional electronic packet switching networks is handled through buffering. However store-and-forward is not available in OBS networks. To avoid burst dropping, some contention resolution methods have been proposed in the literature.

Obviously, most contention resolution methods in the literature so far can be classified as reactive and proactive approaches. Reactive approaches are provoked after contention occurs. Examples of reactive contention resolution schemes are the space domain (as deflection routing [7]), the time domain (buffering by using FDLs [9]), the wavelength domain (as wavelength conversion [9]), the burst domain (as segmentation [10]), and the control domain (as look-ahead contention resolution [11] and shortest-drop policy [12]). Although wavelength conversion is a good way jointed with optical buffering, but the technology is not mature at present. Use of FDLs will increase data latency and implement complexity and is not commercially viable. Segmentation will increase complexity of control and implement action. Deflection routing will add burst latency and a burst can loop in the network and waste resources in case of dropped burst and invalidation.

In proactive contention resolution approaches, traffic management policies are invoked to prevent the network entering the congestion state. Such schemes can be classified as non feedback-based and feedback-based. In a



non feedback-based scheme, the ingress edge nodes have no knowledge of the network state and they cannot respond to changes of the network load. This can be achieved through traffic load balancing or data burst assembly. In a feedback-based scheme, contention avoidance is achieved by dynamically varying the data burst flows at the ingress edge node to match the latest status of the network and its available resources. One way to achieve this is to reroute some of the traffic from heavily loaded paths to under-utilized paths [13]. A similar approach has also been introduced by [14] where the authors consider balancing the data burst traffic between predefined alternative paths. In [15] a global load-balancing contention resolution scheme is proposed and its performance is examined for both dynamic and static traffic.

Another way to avoid contention is to implement a burst transfer control protocol (BTCP) with a congestion avoidance mechanism to regulate the burst transmission rate [16-18]. In this approach, the ingress edge node receives BTCP acknowledgment (ACK) packets from the egress edge node, calculate the most congested links, and reroute their traffic accordingly. In [19], burst copying is proposed as an effective way to reduce data burst loss. The basic idea in burst copying is to replicate a burst at an appropriate node and send duplicated copies of the burst through the network simultaneously. In [20], a feedback-based OBS network is proposed by using explicit feedback signaling to each source, the required data burst flow rate going to congested links is controlled. Clearly, a major concern with having feedback-based proactive contention resolution schemes is additional signaling overhead and control message processing. Hence, it is critical to design signaling protocols which are simple to implement and require minimum overhead.

Reference [9] shows an ideal network, where the core node uses a number of FDLs in the time domain and uses wavelength converter in the wavelength domain to reduce contention. But contention still occurs at a switching node when the load gets higher. It indicates that these methods may reduce the burst contention probability, but they are very vulnerable to network load and may suffer from severe data loss in case of heavy traffic load. Even at moderate traffic load, contention caused by using these methods lead to burst blocking and data dropping probabilities have a long way to go to commercially viable. Undoubtedly, a combination of mentioned above methods can be very effective. Therefore, to improve the performance of OBS networks, we proposed a novel burst scheduling algorithm. This unique contention resolution approach can be used in conjunction with a differentiated Quality of Service (QoS) scheme to reduce the burst dropping probability.

C. Burst Scheduling Burst scheduling uses information contained in CPs to reserve output ports for the corresponding data bursts

in the intermediate node. Several scheduling algorithms have been proposed for burst scheduling in OBS networks. Earlier proposals were mainly based on the Horizon scheduling [9], where the scheduler in Horizon maintains a time horizon for each outgoing wavelength channel. The scheduler processes a CP and attempts to reserve wavelength for a data burst immediately after the processing. A just-in-time (JIT) burst scheduling scheme is proposed in [8]. A just-enough-time (JET) burst scheduling scheme is proposed in [3]. The main difference of JIT and JET schemes is the reservation way. The former uses an immediate reservation to reserve a wavelength for a burst at the time of the CP arrival time, while the latter uses a delay reservation to reserve a wavelength for a burst at the time of the burst arrival time. To reduce the burst dropping probability, a randomized offset time scheme is proposed in [21].

Recently, several optimization burst scheduling schemes have been proposed in order to reduce the burst dropping probability. A class queue based burst scheduling algorithm is proposed in [22] to support differentiated services. A burst scheduling scheme, named differentiated scheduling with identical priority offset time (DSIPO), is proposed in [23] to support differentiated services in OBS networks. In [11], a new scheduling scheme called look-ahead window contention resolution is proposed to reduce burst contention and support service differentiation. An OBS group scheduling approach is introduced in [24] to optimizing resource utilization. A new buffering and scheduling algorithm, named burst delay scheduling algorithm is proposed in [25]. In this schemes, when burst contention occurring, the low-priority class burst is buffered with FDL in order to forward the high-priority burst as early as possible, and the reserved wavelength channel for the low-priority burst be re-assigned to the contending high-priority burst by send a new CP. This scheme can obviously reduce burst dropping probability and ETE delay for high-priority class burst.

III. Group Scheduling based on Control-packet Batch Processing (GSCBP) A. GSCBP



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Due to the fact that ORAM is not available, there exists a real possibility that bursts may contend with one another at a switching node. Contention in traditional electronic packet-switched networks is resolved through the store-and-forward technique. However, store-and-forward is unavailable in optical domain at present. Another significant issue for optical burst switched networks is the quality of service (QoS) provision. Literature [26] proposed an offset-time-based scheme which uses an extra offset time instead of buffer to isolate classes of traffic. In this scheme, an extra offset time is given to higher-priority bursts. Therefore, network resources can be reserved further in advance of the burst’s arrival, consequently increasing the probability of a successful reservation. This scheme can efficiently support basic QoS in OBS due to its simplicity, scalability, and effectiveness.

Like transmission control protocol (TCP), GSCBP has a batch processing window. In GSCBP, the control packet which arrive on a core node in advance of bursts are batch processed, a heuristic scheduling algorithm arrange these bursts’ route based on the processing results and the network resource. The GSCBP algorithm can be divided into four basic steps: (a) collects all control packets contained in the batch processing window; (b) batch processes these control packets; (c) decides the contention domain and contending bursts; and (d) determines which of the contending burst should be invoked by some contention resolutions or must be discarded according to the scheduling algorithm and the network resource. The GSCBP is illustrated introduced as Fig. 2. In Fig. 2 (a), there is a slide window. There are two high-priority bursts contend with four low-priority bursts in current GSCBP window. From Fig. 2 (b), we can see that the contention is resolved, if the burst B3 be converted into W2 and the burst B2, B4 be delayed a proper time. Under the assumption of the node configured limited WCs and FDLs, the group scheduling result is shown in Fig. 2 (c) and (d).

To reduce the higher-priority burst dropping probability, GSCBP is combined considered with offset-time-based QoS scheme. Undoubtedly, a combination of mentioned above methods can be very effective. Therefore, the GSCBP algorithm is combined with WC and FDL devices in this paper. These devices are shared by some wavelength channels. The new node architecture using optical switches and shared WC and FDL is presented in Fig. 3. The node has 2m mXm optical switches, m tunable WC and m FDL. Both the WC and FDL are shared by m channels. The node connects with m input and m output fibers. In GSCBP, a high-priority burst is assigned an extra QoS offset time than a low priority burst. Therefore, a high-priority burst has a longer offset time and a longer ETE delay than a low priority burst. The high-priority burst has a better chance to obtain a wavelength.

Fig.2 GSCBP algorithm with wavelength converter and FDL shared by W1, W2 wavelength channel

B. Extended Open Shortest Path First (E-OSPF)



Open shortest path first (OSPF) is a routing protocol developed for IP networks by Internet Engineering Task Force (IETF). OSPF protocol has two primary characteristics. The first principal characteristic is that the protocol is open. The second principal characteristic is that OSPF protocol is based on the shortest path first (SPF) algorithm. OSPF protocol uses the SPF algorithm to calculate the shortest path to each node. For load balance, an extended open shortest path first (E-OSPF) routing strategy was proposed. In this routing strategy, the shortest path and the least hop path are found and considered simultaneously. The cost of a route is described by a single dimensionless metric which calculated as:

Metric= [K1 × Length + K2 × Hops] (1) K1and K2 are coefficients with default value 0.005, 8 respectively in this paper. The lower the cost, the more

likely the route is selected to be used to forward data traffic. When several equal-cost routes to a destination exist, the data traffic is distributed equally among them. For example, the least-hop cost is 38 and the shortest path cost is 58.5 between node 3 and node 8 in Fig 4. The path from node 8 traverse node 1 to node 3 is selected.

IV. Performance Evaluation In this section, some numerical results were presented. First, the simulation conditions, such as the selected

topology and some assumptions were introduced. Second, the simulation results of GSCBP algorithm and E-OSPF were attained through extensive simulations.

GSCBP is introduced into the 14-node NSFnet as Fig. 4 by simulation. For comparison, we considered three scenarios: the baseline, E-OSPF, and GSCBP scenarios. The baseline scenario simulate the conventional OBS network with OSPF routing protocol, the E-OSPF scenario simulate the OBS network with E-OSPF routing strategy, and the GSCBP simulate the GSCBP in OBS networks with E-OSPF routing strategy. They are use identical burst assembly algorithm: data-length time-lag product assembly algorithm [27], which uses the product of assembled data length and its assembly time-lag as burst assembly threshold. In this paper, the threshold is select 9 Mbms (Megabit millisecond). In the 14-node NSFnet, each node is connected with an edge node (locally generated and terminated bursts and CPs) that has four local add ports to inject traffic into the OBS network. For each local add port, there is a traffic source to generate IP traffic packets. The ETE delay, burst blocking probability, and burst dropping probability have been investigated in these three scenarios in this network. In the following figures, the notation GSCBP means a curve for the OBS network with GSCBP algorithm, E-OSPF for the OBS network with E-OSPF routing strategy, and Baseline for the conventional OBS network.

The following assumptions are made in the simulations: - The link has four data wavelength channels and each at 10 Gigabit per second transmission rate. - IP packets arrive at the edge node according to a Poisson process. - IP packet’s length is exponential distribution and average IP packet length is 1250 Bytes. - The QoS-based Offset time is set 1 millisecond and the basic Offset time is set 1 millisecond. - Switching and processing time of the CP in each node is set to 5 microseconds. - Propagation delay between two connected nodes is determined by their distance. - Higher-priority traffic account for 40 percent of the total. - The slide window is selected as 25 microseconds. - Input traffic is uniformly distributed to all nodes. - Static shortest path routing is used between nodes. - All nodes use first-in first out (FIFO) scheduling. - Just enough time (JET) [2] OBS based signaling.



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Fig. 4 E-OSPF in the national science foundation (NSF) network

Fig. 5 shows the average ETE delay varying with offered load. It can be easily observed that the baseline has a minimal ETE delay. The GSCBP scenario gets the highest ETE delay which is about 4% higher than that of the baseline. The following reasons might give you a clear picture about the difference. First, the E-OSPF may select the least hop path. Second, there is an extra QoS-based offset time for the higher-priority traffic. Last but not least, the remarkable different burst dropping probability shown in Fig. 7 leads to the difference of ETE delay.

Fig. 6 displays the burst blocking probabilities varying with offered load. Blocking probability means the failure probability of reserve wavelength. In Fig. 6, all curves have the same increasing tendency varying with load. And there is a small difference between these three scenarios. The GSBCP with E-OSPF gets the lowest burst blocking probability, and the baseline scenario gets the highest burst blocking probability. Fig. 7 exhibits the burst dropping probabilities varying with offered load. Here we considered both burst blocking probability and burst dropping probability for the OBS network has a variable length burst. In Fig. 6, there is an obvious difference between these curves. The higher-priority traffic in GSBCP scenario gets a burst dropping probability about two orders of magnitude less than those of the others when offered load less than 0.15. When offered load excess 0.15, the decrease is larger than one order of magnitude. The burst dropping probability of lower-priority traffic only gets 20% higher than that of baseline. The E-OSPF gets about 15% less than that of the baseline. These results indicate that the high-priority burst has a better chance to transmit success.



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V. Conclusions In this paper, a novel soft contention resolution method, GSCBP was proposed for OBS netwroks. The E-

OSPF routing strategy and a new node architecture supporting group scheduling are presented. The proposed techniques are investigated through extensive simulations. The simulation results indicate that the GSBCP lead to the higher-priority traffic drop rate decrease one order of magnitude, if performance of the lower-priority traffic is sacrificed.

Acknowledgment This work was supported by National Hi-tech Research and Development Program of China (863 Program)

(No. 2006AA01z249), Project of Ministry of Education of China (No. 105002), and National Natural Scientific Foundation of China No. 60372025.

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SPIE Proceedings [SPIE Asia Pacific Optical Communications - Wuhan, China (Thursday 1 November 2007)] Optical Transmission, Switching, and Subsystems V - Group scheduling based on