A new burst assembly and dropping scheme for service differentiation in optical burst switched networks+ Zhizhong Zhang*a,b, Fang Chenga, Jianxin Wangb, Jiangtao Luob, Qingji Zengb, Xuelei Xuanb a Chongqing Univ. of Posts & Telecommunications, Chongqing, China, 400065; b R&D Center for Broadband Optical Net. Tech., Shanghai Jiaotong Univ., Shanghai, China, 200030 ABSTRACT It has been proved that through burst segmenting, and thus dropping the segment of one burst that is overlapped with another burst, the packet loss probability will be significantly improved for optical burst switched networks. Based upon this, two main segment dropping strategies, head- and tail-dropping, can be used to resolve contention. In this paper, we point out that tail-dropping policy, which is adopted in most literatures, may not be a feasible solution, while head-dropping policy, even though it may make the packets to arrive out of order, is in deed a solution which can resolve the contention effectively. Moreover, with respect to tail-dropping policy, the increase of the disorder of packet arrivals for head-dropping policy is trivial. To support service differentiation and decrease the disorder of packet arrivals generated by contention resolution, we further propose a new threshold-based hybrid-assembly scheme. The most striking characteristic of the hybrid-assembly scheme is that the low and high packet classes are aggregated into one burst simultaneously. Once contention occurs, head-dropping policy is adopted to drop the overlapping packets. We describe the concrete implementation of QoS supporting, and the corresponding dropping strategy improved head-dropping policy, which aims at guaranteeing a better QoS support and a feasible implementation, is also detailed. Simulation results demonstrate that the proposed burst assembly scheme, together with the head-dropping policy, perform well in terms of performance metrics such as the average packet loss probability and service differentiation. Keywords: Optical burst switching, segmentation, hybrid-assembly, head-dropping, service differentiation
1. INTRODUCTION In an effort to eliminate the electronic bottleneck, new optical switches/routers (hardware) are being built for the next-generation optical Internet in which IP traffic runs over an all-optical WDM layer [1,2,4]. However, important issues yet to be addressed in terms of protocols (software) are how to develop a new paradigm that does not require any buffer at the WDM layer, as in circuit switching, and elimination of any layers between which exist mainly due to historical reasons [1]. At the same time, such a paradigm should also efficiently support bursty traffic with high resource utilization as in packet switching. Optical burst switching (OBS), with the idea of decoupling the data-path from the control-path, is such a promising mechanisms for the next generation optical Internet to support IP over WDM. For OBS networks, one of the challenging issues is how to support fast resource provisioning, asynchronous transmission (of variable size packets, e.g., IP packets), and a high degree of statistical resource sharing for efficient handling of bursty traffic, all without requiring buffers at the WDM layer since there is no optical form of random
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This paper is supported by the Natural Science Foundation of China under Contract 69990540. *Email:
[email protected]; Tel: 86-21-62932166; Fax: 86-21-62932166.
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access memory (RAM) available today. Therefore, any all-optical transport methods must avoid optical buffering as much as possible. On the other hand, contention for resources between bursts is inevitable. Therefore, appropriate protocols must be adopted to minimize the losses of packets, especially for the traffic with high class of quality of service (QoS) requirement. Conventionally, Methods used for contention resolution can essentially be classified according to the domain of the techniques belonging to time domain, wavelength domain, and space domain [4]. Each has particular advantages and disadvantages. These resolutions are all to find an idle output port so as to pass through the whole burst, and the implementation of these techniques involves in fiber delay-lines (FDLs) and wavelength converters, which may not be feasible concerning system size and the cost. Recently, an untraditional approach of contention resolution burst segmentation, is proposed in the literatures [4,5,11], which only discards one of the overlapping parts of the bursts in order to reserve the wavelength as soon as the wavelength becomes free on the output port, the main advantage of this technique is that it can resolve the contention efficiently, while the FDLs and converters can be eliminated. In this paper, we also adopt burst segmenting technique to handle contention, with the objective to optimize the packets losses rather than bursts losses. Moreover, to have a better support of service differentiation, we propose a new burst assembly scheme and correspondingly, a segment dropping strategy, which aims at guaranteeing a better QoS support and a feasible signaling implementation, is also presented. Simulation results demonstrate that (1) with respect to the conventional methods, which aim at resolving contentions from time domain or wavelength domain and thus minimize the bursts losses, burst segmenting technique can, however, minimize the packet losses rather than the bursts losses, (2) the burst assembly scheme proposed in this paper, together with the proposed segment dropping policy, perform well in terms of performance metrics such as the average packet loss probability and service differentiation.
2. TAIL-DROPPING OR HEAD-DROPPING? In an OBS networks, contention will occur when two or more incoming bursts are directed to the same output port at the same time. If burst segmentation technique is adopted, the overlapping packets partial of the burst, can be dropped to ensure an improved packet loss probability, note that a single burst contains a set of packets, so using the packet loss probability will be more effective to evaluate the network’s performance than that of the burst loss probability. Dropped packets Burst 1 Burst 2
Dropped packets Contention region
Burst 1 Burst 2
Contention region time
(a) Head-dropping
time
(b) Tail-dropping
Fig.1 An illustrative examples of contention and the corresponding segment dropping policy
As far as dropping policies of the overlapping packets is concerned, as shown in Fig.1, two main schemes, headdropping and tail-dropping strategies, can be used. While, at the best knowledge of the authors, segment dropping policies adopted in most literatures all adopt tail-dropping mechanisms [5-8,11]. As illustrated in Fig.1(b), the
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overlapping packets contained in the first arriving burst, burst 1, will be dropped if the burst which arrives later (burst 2) contending with it. The main reason to do so, according to [5-8], is that there will be a better chance to keep the packets to arrive at the destination in right order, i.e., the tail-dropping policy will not disorder the right-hand packets contained in burst 1. As a matter of fact, a serious limitation of tail-dropping solution has been neglected. As we know, a key and fundamental characteristic of the OBS technique, is that the burst payload and its associated header are transmitted separately. Each time a burst will be transferred, its associated control packet will be transmitted firstly on a different (control) channel with the objective to reserve the wavelength resources, more accurately, if the wavelength resources have been reserved for a specified burst, these resources can not be released or difficult to be changed even if the partial of the burst contained in the burst have been lost. Therefore, downstream nodes can not know that the burst has been truncated, and it is possible that the previously truncated tail segments will continue contending with other bursts, even though these tail segments have already been dropped at a previous node. Obviously, this will result in unnecessary packet losses. To have a better understanding of the limitation associated with tail-dropping policy, in the following we will give an example to illustrate the infeasibility of this scheme. Without loss of generality, we consider two bursts, A and B, will contend with each other at a particular node, C. The following assumptions are supposed for the convenience of the following description: • Bursts A and B are both originally composed of two parts, in which A=(A1+A2) and B=(B1+ B2), A1 and B1 are, respectively, in the foreside of their associated bursts; • Bursts A and B are all wish to pass through the node C on the same output port; • The tail of burst A (as shown in Fig.2, the shaded area A2), has been discarded at previous node(s); • tos, toe: The starting, ending time of the resources reserved for original burst by control packets; • tcs, tce: The starting, ending time of the resources reserved for contending burst by control packets; • t′ce, t′oe: The actual ending time of the contending, original burst.
B2
Contending burst B t
tce= t′ce
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Original burst B Contending burst A
B1 toe
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t
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B2 A2
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tce toe= t′oe t′ce
tcs
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(b) t′ce< toe≤ tce
Fig.2 An illustration of the limitations associated with the tail-dropping policy
According to the definitions described above, we can see that if one burst has not lost any of its packets, then its associated ending time of the resources reserved by control packets will equal to its actual ending time of the burst transmission. For example, if no contentions have been occurred for original burst, then t′oe = toe. Now we can find two limitations associated with tail-dropping policy may be generated at node C. A. Nominal contention
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As shown in Fig.2(a), suppose burst A will arrive at node C in advance of burst B, and node C has reserved the output port according to the control packets of burst A. Note that resource reservation includes the duration and the starting time of the occupation. Now we consider a general case of the arrival of burst B. If the arriving time of burst B, tcs, is just overlapped with the tail of burst A (A2), i.e., t′oe< tcs< toe, then a nominal contention will occur, and node C has to resolve this contention. In fact, this kind of contention is nonexistent at all, because the truncated burst A will have been transmitted over before the arrival of burst B. Note that this contention can not be avoided at node C due to the fact that the occupying duration of the output port has been determined and node C can not obtain any information occurred at the previous node(s), thus node C can not adjust the size of the duration of the occupation (readers with interests can be further referred to Just-Enough-Time protocol described in [9] for example). Of course, as shown in [5], if a trailing control message associated with burst A is retransmitted to node C to indicate that there has occurred a contention and this message demand that node C should adjust the ending time of the reservation before the arrival of burst A, this nominal contention can be avoided. However, this contradicts with the original intention of the OBS technology, i.e., simplicity (also see Just-Enough-Time protocol as an example), moreover, the nominal contention will continue occurring if the trailing control message arrives at node C behind the arrival of the control packets of burst B. B. Deterioration of the packet loss probability Although the nominal contention illustrated in Fig.2(a) make the scheduler to spend extra time to handle contention, it does not worsen the packet loss probability, because the contention occurred at node C does not in factually result in the packet losses. However, another example shown in Fig.2(b) can be very different, i.e., it can worsen the packet loss probability. This is explained as follows. As can be seen from Fig.2(b), in this case burst B will arrive at node C in advance of burst A, and node C has reserved the output port according to the control packets of burst B. When burst A arrives at node C, if a contention occurs such that t′ce< toe≤ tce (Note that in this case burst A is the contending burst), then according to the tail-dropping policy, node C will discard the overlapping packets contained in burst B, specifically, if measured in time scale, the dropping packets of burst B are from tcs to toe. In fact, as illustrated in Fig.2(b), the overlapping part of both bursts are from tcs to t′ce, wherein t′ce < toe, so an extra part of burst B has to be discarded, which worsen the packet loss probability. Based on the above-mentioned analysis, we can see that tail-dropping policy may not be a feasible solution concerning contention resolution, moreover, besides the extra consumption of the scheduling time, it may even worsen the packet loss probability. For head-dropping policy, the situation is very different. Because at a particular upstream node, the resource reservation message sent to the downstream nodes can be sent right after the processing of the burst at this upstream node, or, head-dropping policy has enough time to update the control message already sent to the downstream nodes before the burst is transmitted to the downstream nodes. In one word, the control message received at the downstream nodes is accurate, thus the limitations associated with the tail-dropping policy are no longer in existence. As for the disorder of the arriving packet, as packets (bursts) with same source/destination may reach the destination node through different paths with different transmitting latency, any destination node can not avoid the operation of
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sorting all the received packets. Moreover, the packet number contained in a burst is usually only several hundreds of bytes, so, with respect to tail-dropping policy, the increase of the disorder of the packet arrivals for head-dropping policy is trivial. Therefore, the disorder of the packet arrivals should not be a criterion for choosing the dropping policy. In addition, as mentioned below, we can also adopt the untraditional assembly scheme to reduce the disorder of packet arrivals.
3. HYBRID-ASSEMBLY AND IMPROVED HEAD-DROPPING WITH SERVICE DIFFERENTIATION 3.1 Hybrid-assembly scheme The most common burst assembly techniques, which perform similarly, are timer-based and threshold-based assembly policies [5,8,10]. In timer-based scenario, a burst, which may have variable length, is created and injected into the network at periodic time intervals. While in threshold-based approaches, a limit is placed on the maximum number of packets contained in each burst. Therefore, fixed-size burst will be generated at the network edge. At the knowledge of the authors, burst assembly policies in most literature under QoS support scenario assume that the packets are electronically aggregated at the ingress edge nodes into bursts, according to their destination and class of services [2]. In the following of this section, we will present a new kind of burst assembly scheme. We assume bursts are generated at the network edge with fixed-size, according to their destinations. In other word, packets with different QoS types but with the same destination can be aggregated into a burst simultaneously, hereafter we call this hybrid-assembly scheme. Fig.3 shows the corresponding assembling results of different threshold-based assembly schemes, where, three kinds of packet classes are considered, and class A, B and C are corresponding to the high, medium and low classes of packets. We can see the bursts generated in two assembly schemes are very different, i.e., for a considered source-destination pair, packets with different QoS types in traditional assembly scheme are aggregated into three kinds of bursts, while the proposed assembly scheme, different packet classes are aggregated into one burst simultaneously, thus just one type of burst is generated. In addition, each class of packets are grouped into a segment with separate correlative information of it’s own, including the length, number of packets, location of packets and the check sum etc. The number of segments in a burst is the priority number that OBS network supports. For hybrid-assembly scheme, the priority of the segment is corresponding to the priority of the packets. All segments in a burst are initially transmitted as a single unit. In a burst, segments are arranged in ascending order from burst’s head to tail, i.e., the high priority segment is placed at the tail part, while the low priority segment is placed at the head part. In Fig.3, notation L denote the length of the burst, and LA, LB and LC, denote in hybrid-assembly scenario, the corresponding proportion of high, medium and low packet classes, wherein the proportion of each packet class is proportional to its associated traffic load. Recall L is in fixed size at each ingress edge node. Obviously, with respect to the conventional assembly scheme, the waiting time of the hybrid-assembly scheme will be reduced and the burst utilization will be higher due to the fact that in hybrid-assembly scenario, the average packets arriving rate for a single burst is much higher than that of the conventional counterpart, and a burst will leave less space to be filled by useless packets. The improvements are much distinctive for low traffic load.
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L Assembler
Assembler
C B
Incoming packet
A
(a) Traditional assembly scheme
Incoming packet
A
B
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LA
LB
LC
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Fig.3 A comparison of different threshold-based burst assembly schemes (
,
,
: Corresponding to high, medium and low packet classes respectively)
In addition, for hybrid-assembly scheme, as a single burst is grouped, according to the QoS level, into several relatively independent segments. Each segment has its own assembly information, thus if the head of the burst is dropped, the other packets that do not belong to the associated segment will not lose their assembly information, thus the disorder of the packet arrivals can be minimized. 3.2 Improved head-dropping policy Based on the description and analysis of the hybrid-assembly scheme, it is natural that head-dropping policy must be used to guarantee the service differentiation. However, to have a simplified implementation and a better support of QoS, the concrete implementation of head-dropping policy adopted in this paper should be different from the method introduced in [11], i.e., dropping scheme needs to be further detailed to coincide well with the assembly scheme. Here we first would like to make clear that in OBS networks, a burst that reserve the resources earlier might not imply this burst will arrive at the specified node earlier, because control packets are separated from the burst payload, so, a burst whose control packets reserve the resources later may arrive at the specified node earlier than those of the bursts whose reservation take earlier. So, in the following analysis, the contentions are corresponding to the concrete bursts, not the associated control packets. Without loss of generality, we just illustrate the case that two bursts are involved in the considered contention, and, the switching time of the switch matrix is ignored. For the convenience of the following analysis, we define: • ∆c: The overlapping length between two bursts; • Lc: The total length of the contending burst; • tos: The arriving time of the original burst. • tcs: The arriving time of the contending burst. Concrete implementation of contention resolution: Step 1: Compare the overlapping length (∆c) with the total length of the contending burst (Lc), If ∆ctcs, contention is resolved by dropping the head part of the original burst (Fig.4(b)), note that the truncated original burst will continue to be forwarded after the output port has been released by the contending burst. Step 4: Update the control packets of the burst which has been truncated. ∆c
∆c
Original burst
Original burst
Contending burst
Contending burst
∆c=Lc
Original burst Contending burst
tos t
Discarded
tcs
tos
(a) ∆c1) data wavelengths for each input/output fiber, the packet loss probability corresponding to low and high packet classes will be much lower than the results shown in Fig.7. As the main purpose of this paper is to study the validity of our QoS-based hybrid-assembly and dropping scheme, so we would like to verify it by the switch assumed before, in which there is a much higher probability to occur contention for the same traffic intensity; (2) with respect to the traditional burst assembly techniques, hybrid-assembly scheme does not require extra cost, i.e., under the same complexity, hybrid-assembly scheme is help to support service differentiation.
5. SUMMARY In this paper, we propose a new threshold-based hybrid-assembly and segment dropping scheme with service differentiation in optical burst switched networks, the main characteristics of the developed technique can be summarized into: (1) burst assembled at the ingress edge node contain the low and high packet classes simultaneously, and a single burst is grouped, according to the QoS level, into several relatively independent segments. Each segment has its own assembly information; (2) the packets with a relatively high level of QoS requirement are assembled at the relatively backside of the burst; (3) once contention occurs, an improved head-dropping policy is adopted to drop the overlapping packets. As the developed scheme can always guarantee the downstream nodes to receive the correct control packets, which is inherently difficult to be achieved in tail-dropping scheme, thus it can, effectively, eliminate the nominal contentions and improve the packet loss probability. We demonstrate that the proposed scheme can, to some degree, alleviate the disorder of packet arrivals, and, with respect to tail-dropping scheme, the increase of the disorder of packet arrivals is trivial. Simulation results show that the proposed scheme performs well in terms of performance metrics such as the average packet loss probability and the support of service differentiation. Suggested future work is to investigate timer or combining both timer and threshold-based assembly techniques to provide end-to-end QoS.
REFERENCES 1.
M.Yoo, C.Qiao, and S.Dixit, “Optical burst switching for service differentiation in the next-generation optical Internet”, IEEE
2.
M.Yoo, C.Qiao, and S.Dixit, “QoS performance of optical burst switching in IP-over-WDM networks”, IEEE J. Select. Areas
Commun. Mag., 39, pp.98-104, 2001.
Commun., 18, pp. 2062 –2071, 2000. 3.
M.C. Yuang, et al., “QoS Burstification for optical burst switched networks” OFC’2002, California, ThGG118, Mar 2002.
4.
Yao Shun, B.Mukherjee, and S.Dixit, “Advances in photonic packet switching: an overview” IEEE Commun. Mag., 38, pp. 84-94, 2000.
5.
V.M.Vokkarane, J.P.Jue, and S.Sitaraman, “Burst segmentation: An approach for reducing packet loss in optical burst switched networks”, IEEE, ICC 2002, New York, NY, pp.2673-2677, April 2002.
6.
A.Detti, V.Eramo, and M.Listanti, “Performance evaluation of a new technique for IP support in a WDM optical network: Optical composite burst switching (OCBS)”, IEEE J. of Lightwave Techno., 20, pp.154-165, 2002.
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Proc. of SPIE Vol. 5281
7.
M.Neuts, Z.Rosberg, H.L.Vu, and J.White, “Performance Analysis of Optical Composite Burst Switching”, IEEE
8.
V.M.Vokkarane,K.Haridoss,and J.P.Jue, “Threshold-based burst assembly policies for QoS support in optical burst–switched
Communications Letters, 6, pp.346-348, 2002.
networks”, SPIE OptiComm 2002, Boston, MA, July 2002. 9.
C. Qiao, and M. Yoo, “Optical burst switching (OBS)-a new paradigm for an optical Internet”, Journal High Speed Networks, 8, pp.69-84, 1999.
10. A.Ge, F.Callegati, and L.S.Tamil, “On optical burst switching and self-similar traffic”, IEEE Commun. Lett., 4, pp.98-100, 2000. 11. C.Gauger, et al., “Contention resolution in optical burst switching networks” Advanced Infrastructures for Photonic Networks: WG2 Intermediate Report, pp.62-82, 2002.
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