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TCP over OLS Networks: Real-time TCP Management Scheme for QoS Provision Yassine Khlifi and Noureddine Boudriga CN&S Research Lab., University of November 7th at Carthage, Tunisia Corresponding author:
[email protected] ABSTRACT Optical Label Switching (OLS) has been proposed as a promising technology for providing resource provisioning and quality of service (QoS) support. Since TCP is (and may remain) the most popular protocol in the Internet, TCP implementation became a major issue in OLS network engineering. To take full advantage of OLS networks and integrate some TCP functions in the optical domain, the implementation of TCP over OLS is addressed to provide QoS control, resources provisioning and, finer granularity analysis. In this paper, we introduce a novel technique for packet retransmission based on the real-time management of TCP parameters, network resources, and traffic requirements. The technique handles activities such as signaling, switching, and contention resolution. Our approach is scalable packet retransmission and is based on buffering technique and contention resolution. It also provides a formal model to support QoS provision. Finally, simulation activity is performed in order to validate the proposed approach. Keywords: OLS, TCP, QoS provision, contention resolution, packet retransmission, optical buffering. 1. INTRODUCTION The fast growing of the Internet traffic and the emergence of new applications has introduced new requirements in terms of resources utilization and QoS support. Optical Label Switching (OLS) has been proposed as a new paradigm to fully utilize the benefits of wavelength division multiplexing (WDM) and provide fast switching capability, resource provisioning, and QoS support [1, 3]. In such a network, the traffic is transmitted using a set of labels created during an out-of-band signaling scheme, which uses a 2-way signaling scheme to build the label switching path (LSP) and update the switching tables on the LSP-cores. QoS control and resource provision are critical tasks in OLS networks, since resources management and efficient data transmission costs are essential design criteria in networking. While TCP is the dominant transport protocol today and likely to be adopted in future optical networks, TCP implementation becomes an attractive topic that can be addressed for integrating several of fundamental functions at the optical layer (e.g., QoS control), providing finer granularity analysis, and offering efficient data transmission costs. Currently, the TCP implementation over optical networks is an important design issue, especially in OLS networks. To our knowledge, there is no previous work related to TCP implementation over OLS networks integrating buffer processing and contention resolution. In most of existing works, authors have attempted to implement some of TCP functions over optical burst switching (OBS) networks using the just-enough-time (JET) signaling scheme to resolve false time-out detection and false congestion detection problems. These methods provided a static approach using time-out and congestion detection parameters [4]. Other works (e.g. [5, 6]) have addressed burst delay and end-to-end TCP throughput using a core node architecture with no Optical Delay Lines (ODL) and a limited wavelength conversion capability. All these methods do not take into account ODL-based buffering due to the contention and the real-time variation of system parameters, and cannot achieve a suitable decision for high level resources provisioning and QoS control. In this work, we address the issue of TCP implementation over OLS networks based on a dynamic real-time management of TCP parameters and network resources. We propose a scalable packet retransmission approach and formalize it using the available network resources in terms of transmission and buffering capabilities. We first extend the existing OLS signaling protocol to take care of two concepts: resource management and dynamic TCP parameters control. The proposed approach considers a real-time supervision of resources and wavelengths, as well as traffic characteristics, such as the real-time variation of time-out of the incoming traffic and buffering capacity at each core node belonging to the built lightpath. We also develop an analytic model for formulating our approach by means of a conservation law and queuing network model [3]. Finally, a simulation experiments has been conducted in order to validate the efficiency of the proposed approach. The remaining part of this paper is organized as follows. Section 2 briefly studies the basic concepts of the OLS core node and network architectures, and signaling protocols. It also presents the adopted contention resolution scheme. Section 3 presents in detail the proposed packet retransmission protocol and its algorithm. Section 4 develops a theoretical model that helps the design and formulation of the proposed approach using a conservation law and queuing network model. Section 5 discusses the validation of our proposed approach through a simulation work. Section 6 concludes the paper.
978-1-4244-1639-4/07/$25.00 ©2007 IEEE
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2. OLS NETWORK ARCHITECTURE AND SCHEMES In this section, we aim to provide a brief description of core node and network architectures, signaling, and contention resolution schemes that we use to design an efficient packet retransmission protocol. 2.1 OLS core node and network architectures An OLS network consists of a collection of edge and core nodes. The network uses the OLS core node architecture that we have proposed and discussed in [2]. The core nodes are interconnected with DWDM links. Each node transfers incoming IP traffic from an input port to its destination based on optical switching label [3] containing traffic requirements and TCP parameters. Such switch performs signaling protocol, and adopts dynamic contention resolution based on wavelength converters and ODL buffers. The core node architecture is typically composed of N input and N output ports. Each port consists of several channels which can handle with w wavelengths and a set of multiplexers and demultiplexers. k wavelengths are dedicated to data traffic transport. The remaining part of the wavelength is used for signaling and survivability schemes. For a complete description of this architecture, the reader can refer to [2]. The main components of this node architecture are: the input processing unit (IPU), switch control unit (SCU), switch fabric unit (SFU), waiting unit (WU), and output processing unit (OPU). In the sequel, we describe briefly the key components of the node architecture used during the design of our proposal, such as SCU and WU. • The SCU is used to supervise the SFU functioning. It supervises the switching process of new label framework with the contended packets when using wavelength conversion or ODL buffering. • The WU is composed of a set of shared multi-wavelengths ODL buffers. The ODL length is equal to the packet slot duration and the required buffering delay for a contended packet. WU uses feed-backward ODL buffers, which increase the buffering capability. The ODL allows a packet emerging from an ODL buffer to be (re)buffered, in case of successive contentions. 2.2 OLS signaling scheme In OLS network, the transmission process is preceded by the lightpath, label switching path (LSP), setup that defines the route between the ingress node and egress node. In this work, we assume that each TCP segment is contained in an IP packet. For this, we extend the signaling protocol proposed in [3] by introducing two new label frameworks: request label and mapping label. The first label is created by the ingress edge and sent to the egress node. Its structure contains the traffic requirements in terms of delay and TCP parameters. The second label is built and sent back to the ingress node to confirm LSP establishment. In this case, the traffic is transmitted over the network core using the established LSP. IP packets have a variable-length and the switching label, which insures that the switching process is completed ahead of the incoming traffic packets. This label carries switching information, traffic requirements, and TCP parameters values in terms of retransmission delay and number processed along the related LSP. 2.3 Contention resolution scheme Contention occurs if multiple packets arriving at different input ports are destined to the same output port and wavelength at same moment. The contention resolution scheme is made based on wavelength conversion and optical buffering techniques (in that order). In the contention case, the choice of the packet to convert or delay is taken based on traffic parameters and the available resources on the entire network. For a complete description of the contention resolution, the reader can refer to [3]. In this work, when contention occurs, the node compares the difference between TCP real parameters in terms of packet retransmission number and delay of contended packets. The authorized packet is directed to its original output port whereas the contending packet is converted to an alternative wavelength to avoid conflict. If no wavelength is available, the packet is stored in one among the available ODL buffers. If no ODL is available, the node discards the packet and requests its retransmission. If contention is still occurring, a packet may be buffered several times without exceeding the authorized delay. If the delay is exceeded the packet is retransmitted using the pre-established LSP. 3. OLS PACKET RETRANSMISSION ALGORITHM QoS control and resource provisioning is essentially based on the development of a packet retransmission protocol referred to as DRT protocol. The proposed scheme is mainly based on the exchange, during data transmission, of control labels that carry the information needed to provide the available resources and TCP parameters along a transmission LSP. The implementation of DRT protocol, needs the extension of the adopted signaling scheme [3] to carry the required information including packet retransmission number (PRTN), maximum packet blocking delay (MPBD) and packet retransmission delay (PRTD) as well as the incoming traffic time out (TO). The first control information is needed to determine the retransmission number for each packet, which is needed for the implementation of the above presented contention resolution scheme. The second control information is used for contention resolution when the conflicting packets have the same retransmission number constraints. Also, the third and forth information are used to supervise packet retransmission events.
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The implementation of our protocol needs that each core node on the packet lightpath should know the maximum blocking delay and online blocking delay. In this case, when contention occurs (between two packets Pi and Pj), the core node compares the retransmission number of the conflicting packets (PRTNi and PRTNj) and switches the packet that has the higher transmission number to its original path. The packet which has the lower packet retransmission number is switched to WU, where it can be buffered if an available ODL buffer exits. In case that they are equal, the core node compares the difference of maximum and online blocking delay of the conflicting packets (MPBDi - OPBDi and MPBDj - OPBDj). The packet which has the lower difference in terms of packet blocking delay is privileged. When the contending packet is buffered at a core node, a control label is created and sent to all remainder nodes to update the online measured packet blocking delay. At the same time, the core node updates the packet retransmission number by adding 1 (i.e., PRTN+1) and computes packet retransmission delay (PRTD) which depends on the core node location in the built LSP. If no available ODL in WU is found, the node discards the contending packet, updates the system parameters and sends a control label in which it requests the retransmission of dropped packet. Following a packet dropping at the node, a control label is created and sent to the ingress node in which the node requests the retransmission of the dropped packet. The ingress edge node, upon receiving the control label, creates a switching label which contains the updated packet retransmission number and delay, and retransmits the packet using the same established lightpath. Finally, the packet is dropped definitely, when the measured online retransmission delay exceeds the packet time out constraints. As it has been mentioned, the proposed scheme is performed using an extended signaling scheme, where new control label frameworks are used to carry the needed information for providing dynamically the available resources and TCP parameters. In case of contention, each core node performs DRT algorithm to compute dynamically the different system parameters in order to undertake the suitable decision on the retransmission process of the contended packet which can be retransmitted several times without exceeding the time out (see following algorithm). Before presenting the proposed algorithm, let us consider the following notations: – n: Core node number of the related LSP, – MPBD: Maximum packet blocking delay of traffic – OPBD: Online packet blocking delay of traffic – PRTN: Packet retransmission number – PRTD: Packet retransmission delay – TO: Time out of the accepted packet – PT: Propagation time between two adjacent core nodes. OLS-PRA algorithm( ) Receive Label switching, Identify MPBD,TO, switching information For k=1 to n do (from the 1st core node to the last core node of the established LSP) If (contention occurs) Then if (RTNi < RTNj ) (of contending packets Pi and Pj) Then Switch Pj to it original path, Switch Pi to WU of the core node If (available ODL exists) Then OPBDi= OPBDi+1 Else If (TO < PRTDi) Then Retransmit Pi, PRTNi= PRTNi +1, PRTDi= PRTDi +2.k.PT Else Delete Pi } Else if (RTNi > RTNj ) Then Switch Pi to its original path, Switch Pj to WU of the core node If (available ODL exists) Then OPBDj= OPBDj+1 Else If ( TO < PRTDj ) Then {Retransmit Pj, PRTNj= PRTNj +1, PRTDj= PRTDj +2.k.PT Else Delete Pj } Else If (MPBDi – OPBDi ) < ( MPBDj – 0PBDj ) Then Switch Pi to its original path, Switch Pj to WU of the core node If (available ODL exists) Then OPBDj= OPBDj+1 Else If TO < PRTDj Then Retransmit Pj, PRTNj= PRTNj +1,PRTDj= PRTDj +2.k.PT Else Delete Pj } Else Switch Pj to its original path, Switch Pi to WU of the core node If (available ODL exists) Then OPBDi= OPBDi+1 Else If (TO < PRTDi) Then Retransmit Pi, PRTNi= PRTNi +1, PRTDi= PRTDi +2.k.PT } Else Delete Pi }
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4. PERFORMANCE ANALYSIS To design suitably the proposed scheme, it is necessary to develop the fundamental parameters of implementation. To do so, the core node needs to maintain traffic characteristics, such as TO, MPBD, OPBD, PRTN, and PRTD as it they are presented previously. In addition, the node needs to maintain the information related to the available resources in terms of transmission and buffering capabilities. We assume a core node with N traffic types labeled 0,1,…N-1 that are composed of variable-length traffic packets. Let T, d, and k denote mean packet length, ODL buffer number and wavelengths number, respectively. The accepted traffic is assumed to require a maximum packet blocking delay, MPBD. We specify the end-to-end packet delay constraints as an integer number, m (ODL buffering number a long a transmission LSP), which is used to resolve contention. Let PT,j, for 0≤j≤m, represents the traffic sub-type consisting of packets that have been buffered j times in the WU. PT0 represents new arriving packets at the node. Moreover, we assume that arriving packets that are addressed to a specific output port of a node follow Poisson process with rate λ (initial rate λ0). Optical packet addressed to a given output port are transmitted with needed blocking delay, status of online blocking delay and integrate the adequate packet sub-types. Finally, PBj and PF,j denote the blocking probabilities due to the lack of wavelengths at output port and the blocking probability due to the lack of available ODLs in WU, 0≤j≤m for the traffic sub-type PT,j. Based on the collected information, the node can compute the packet dropping probability at node i, PDPi is given by the following expression: m −1 ⎛ k −1 ⎞⎛ k −1 ⎞ ⎛ m ⎞⎛ m −1 ⎞ PDP i = BP0i ⋅ FP0i + ∑ ⎜ ∏ BPji ⎟⎜ ∏ (1 − FPji ) BPji ⎟ + ⎜ ∏ BPji ⎟⎜ ∏ (1 − FPji ) ⎟ k =1 ⎝ j =1 ⎠⎝ j =1 ⎠ ⎝ j =1 ⎠⎝ j =1 ⎠
(1)
The above expression can be stated as follows: Let us recall that BPji , and FPji , denote the blocking probabilities due to the lack of wavelengths and ODL, respectively, in the ith node for the packet of sub-type PTj. the analysis of the blocking probabilities can be established based on the conservation law and the queuing network model (as it is discussed in [3]). For more details, the interested reader can refer to [2, 3]. Based on the conservation law, the blocking probabilities of the traffic of sub-type PT0, PB0 and BF0 are given by:
ρ0k BP0 =
k!
∑
k t =0
ρ 0d BF0 =
(2)
ρ0t i!
d!
∑
d t =0
ρ 0t
(3)
i!
Where ρ0 denotes the traffic intensity of the initial traffic sub-types PT,0, 0≤j≤m. To analyze the blocking probabilities of the different packet sub-types, we follow the analysis based on the conservation law and queuing network model [2, 3]. Let now λj and ρj denote the arrival rate and the traffic intensity of the traffic sub-types PT,j, 0≤j≤m. Then, λj and ρj are given by the following expressions: j −1
λ j = λ0 ⋅ ∏ PBt (1 − PFt )
(4)
t =0
j −1
ρ j = λ j ⋅ T = ρ0 ⋅ ∏ PBt (1 − PFt )
(5)
t =0
Based on the above expressions, we can establish the blocking probabilities of the different traffic sub-types, PBj and PFj are given by the following expressions:
(ρ ⋅∏ 0
BPj =
∑
(ρ ⋅∏ 0
k t =0
(ρ ⋅∏ 0
BFj =
∑
j −1 l =0
d t =0
(
j −1 l =0
PBl (1 − PFl ) k! j −1 l =0
)
k
PBl (1 − PFl ) i!
PBl (1 − PFl ) d! j −1
)
)
k
(6)
k
ρ0 ⋅ ∏ l = 0 PBl (1 − PFl )
)
t
(7)
t! Using the analysis of the blocking probabilities, we can formulate the dropping probability of the initial traffic sub-type PT0 at the i th node, PDPi is given by the following expression:
PDP i = BP0i ⋅ FP0i
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(8)
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By computing the dropping probability iteratively from the sub-type PTj to PTm and by summing the computed probability of each sub-type, we can derive the expression of PDPi as it is shown in expression (1). The proposed scheme also incorporates the formulation of the average packet retransmission time which is denoted by APRT and can be easily given by the following expression: j −1 n ⎡ ⎤ APRT = 2 PT ⋅ ⎢ PDP1 + ∑ j ⋅ PDP j ∏ (1 − PDP k ) ⎥ j =2 k =2 ⎣ ⎦
(9)
The above expression can be stated as follows: Let’s recall that PDP1 denote the packet dropping probability in the 1st node. By computing the average packet retransmission time at this node, we can obtain the following expression: APRT = 2 PT ⋅ PDP1 (10) In the proposed approach, we assume that the same propagation time is assigned between two neighboring core nodes related to the built LSP. Based on the previous assumption and using an iterative computation of the average packet retransmission time from the 2nd node to the nth node, we can derive the expression of the average packet retransmission time, APRT as presented in expression (9). In this case, the packet can be retransmitted, if TO verify the following condition (TO ≥ APRT*TRT), where TRT denotes the Round-trip time which is equal to (2PT*n).
5. SIMULATION AND NUMERIC RESULTS We present here the implemented simulation model and discuss some of the most important numerical results. 5.1 Simulation model We consider hereinafter an OLS network, where each built lightpath contains several core nodes. The configuration of every node assumes two input and output channels capable of a transmission capacity equal to 40 Gbps. A node is equipped with a WU insuring buffering task in the contention case. The traffic generated by an input channel is composed of IP packets with variable length. Packet lengths are generated uniformly distributed in the interval [250, 1500] bytes. The inter-arrival time between two successive packets is assumed to be exponentially distributed. The traffic generated by an input channel is assumed to be uniformly distributed between the different output channels. In our approach, the time out of each accepted packet is fixed to 150µs. The following performance metrics have been chosen to validate our approach: the time-out triggered and average packet mean rate. Two input parameters have been considered: the inter-arrival mean time and traffic load (packet number per traffic). We have found it interesting to extend the simulation work by comparing the obtained results using the proposed scheme and static retransmission approach (say SRT scheme), where the contending packet is retransmitted without exceeding a limited time out which is equal to 50 µs. 5.2 Numeric results We present hereinafter the numerical results obtained for above configuration to show how the input parameters affect the output parameters. All simulations have been performed for a period long enough to reach a steady state in which each core node can handle a large number of packets. For all simulation experiments, we have considered an LSP with 2 edge nodes (ingress and egress edge nodes) and 4 core nodes. Also, the propagation time between two adjacent nodes is fixed to (PT) 5µs, ODL buffering length is equal to 1 µs and the maximum packet blocking delay (MPBD) in terms of buffering number is equal to 5. Figures 1-2 depict the time out triggered and packets-loss mean rate versus the impact of the traffic load.
Figure 1. Time out triggered versus packet number.
Figure 2. Packet loss versus packet number.
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These figures illustrate that, for the two schemes, the increase of the input parameter increases the output parameters. This can be explained by the fact that the growth of the packet number increases the traffic charge, which induces the increase of the risk of contention, and so the output parameters. Figures 3-4 plot the time out triggered and packets-loss mean rate versus the impact of packet inter-arrival mean time. We notice that, for the two schemes, the increase of packet inter-arrival mean time decreases the output parameters. This is because the increase of packet inter-arrival mean time increases the time separating two successive packets, which decreases the risk of contention and the considered output parameters.
Figure 3. Time out vs. packet inter-arrival mean time.
Figure 4. Packet loss vs. packet inter-arrival mean time.
In the previous figures, we observe that the DRT scheme performs better than SRT scheme and offers a high level of resource utilization and provides better traffic management and acceptable level of QoS satisfaction. This can be explained by the fact that the proposed approach provides dynamically the real-time status of system parameters and traffic characteristics. Based on the collected information, the contending packets have more opportunity to be retransmitted before reaching the considered time out. This can help each LSP node to reach a suitable decision on packet retransmission mechanism triggering for providing an enhanced level of traffic control and resources provisioning. 6. CONCLUSION In this paper, we mainly addressed the issue of TCP implementation over OLS networks. We introduced a new technique for packet retransmission based on a dynamic real-time management of TCP parameters and lightpath resources. We first extended the existing OLS signaling protocol to take care of two concepts: resource management and dynamic TCP parameters control. The proposed approach considers a buffering aspect due to the contention as well as the management of traffic characteristics including real-time variation of delay and time-out. We have developed an analytic model for formulating the proposed approach by means of a conservation law and queuing network model. The simulation experiments show that the proposed approach can effectively better reduce the loss and time-out triggered for the incoming traffic compared to the SRT scheme. REFERENCES [1] S.J.B. Yoo et al., “Optical-label switching based packet routing system with contention resolution capabilities in wavelength, time, and space domains.” OFC’2002, Anaheim, Mar. 2002. [2] Y. Khlifi, N. Boudriga, M.S. Obaidat, “A QoS-based scheme for planning and dimensioning of optical label switched networks”, ICC’07, Glasgow, Scotland, June 2007. [3] Y. Khlifi, N. Boudriga, M.S Obaidat, “Performance analysis of a dynamic QoS in optical label switched networks”, SPECT’06, Calgary, Canada, pp. 450-457, Aug. 2006. [4] X. Cao, J. Li, Y. Chen, C. Qiao “Assembling TCP/IP packets in optical burst switched networks”, GLOBECOM, Taipei, Taiwan, R.O.C., pp. 2808-2812, Nov. 2002. [5] Q. Zhang, V.M. Vokkarane, Y. Wang, J.P. Jue “Analysis of TCP over optical burst-switched networks with burst retransmission”, GLOBECOM, St. Louis, MO, pp. 124-129, Dec. 2005. [6] X. Yu, C. Qiao, Yong Liu, D. Towsley “Performance evaluation of TCP implementations in OBS networks”, Journal of Lightwave Technology, vol. 22, no. 12, pp. 2722-2737, Dec. 2004.
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