Parallel lightpath provisioning under wavelength continuity constraint in multi-domain optical networks based on hierarchical path computation element

In recent years, Path Computation Element (PCE) architecture has been standardized as a suitable solution for path computation in multi-domain network scenarios. The Traffic Engineering Database (TED) of the PCE is updated with the information of the control plane. However, there is a delay from the PCE replies to a request, until the TED is updated with the network status information. This delay includes not only the control plane delay, but also Path Computation Element Protocol (PCEP) information exchange. This desynchronization in the TED and the real status information leads to an extra blocking situation when the Label Switch Router (LSR) tries to reserve a path, but it has been previously reserved by other LSR. To solve such problem, a recent draft is submitted to the IETF proposing new PCEP extensions for a pre-reservation of the computed path resources for a certain period. This work implements in C three multi-domain algorithms: Per-domain Path Computation, Backward-Recursive PCE-based Computation and Hierarchical Path Computation Element and it assesses their performance with and without mechanisms to reduce this extra blocking probability due to the uncertainty of the TED information.

Quality of Service-enabled applications and services rely on Traffic Engineering-based (TE) Label Switched Paths (LSP) established in core networks and controlled by the GMPLS control plane. Path computation process is crucial to achieve the desired TE objective. Its actual effectiveness depends on a number of factors. Mechanisms utilized to update topology and TE information, as well as the latency between path computation and resource reservation, which is typically distributed, may affect path computation efficiency. Moreover, TE visibility is limited in many network scenarios, such as multi-layer, multi-domain and multi-carrier networks, and it may negatively impact resource utilization. The Internet Engineering Task Force (IETF) has promoted the Path Computation Element (PCE) architecture, proposing a dedicated network entity devoted to path computation process. The PCE represents a flexible instrument to overcome visibility and distributed provisioning inefficiencies. Communications between path computation clients (PCC) and PCEs, realized through the PCE Protocol (PCEP), also enable inter-PCE communications offering an attractive way to perform TE-based path computation among cooperating PCEs in multi-layer/domain scenarios, while preserving scalability and confidentiality. This survey presents the state-of-the-art on the PCE architecture for GMPLS-controlled networks carried out by research and standardization community. In this work, packet (i.e., MPLS-TE and MPLS-TP) and wavelength/spectrum (i.e., WSON and SSON) switching capabilities are the considered technological platforms, in which the PCE is shown to achieve a number of evident benefits.

The control of multidomain elastic optical networks (EONs) is possible by combining Hierarchical Path Computation Element (H-PCE)-based computation, Border Gateway Protocol with Extensions for Traffic Engineering Link State Information (BGP-LS) topology discovery, remote instantiation via Path Computation Element Communication Protocol (PCEP), and signaling via Resource Reservation Protocol with Extensions for Traffic Engineering (RSVP-TE). Two evolutionary architectures are considered, one based on stateless H-PCE, PCEP instantiation, and end-to-end RSVP-TE signaling (SL-E2E), and a second one based on stateful active H-PCE with perdomain instantiation and stitching. This paper presents the first multiplatform demonstration that fully validates both control architectures achieving multiprotocol interoperability. SL-E2E leads to slightly faster provisioning but needs to keep the state of the stitching of the end-to-end label-switched paths in the parent PCE.

The path computation element (PCE) architecture was originally proposed with a stateless condition, i.e., considering only network reserved resources during constraint-based path computations. More recently, a stateful architecture was introduced to additionally maintain the state of computed and established label switch paths (LSPs). Furthermore, the PCE architecture evolved to active functionality, enabling the PCE to directly issue recommendations to the network. In this study, we present and discuss several use cases where the active stateful architecture can provide some benefits. They include impairment-aware path computations in the context of multirate optical networks, recovery solutions, global defragmentation, and dynamic LSP adaptations. The latter use case is then specifically demonstrated in a network testbed including a flexigrid optical network operated with a multicarrier 1 Tb/s transmission with coherent detection. Novel advanced digital signal processing (DSP) monitoring functionalities are introduced and experimentally demonstrated. These monitoring functionalities are utilized to trigger a new hitless dynamic adaptation technique operating on the applied low-density parity check (LDPC) transmitted coding. The technique has been successfully demonstrated to increase transmission robustness upon impairment degradation, such that no traffic disruption is experienced. Moreover, to accommodate the LSP coding adaptation, network reconfiguration has been performed, successfully driven by the PCE thanks to the active functionality.

Flexible optical networks, based on bandwidth-variable optical cross-connects (BV-OXCs) and novel flexible transponders, are expected to significantly improve the overall spectrum efficiency with respect to traditional networks where fixed frequency spacing is applied. Flexible optical networks will exploit the BV-OXC capability to dynamically configure the reserved bandwidth as a set of frequency slots. In addition, flexible transponders will be employed to dynamically configure transmission parameters, such as bit-rate and modulation format. To enable these new configuration capabilities, network operation enhancements need to be efficiently introduced and investigated. In this study, we focus for the first time on the Path Computation Element (PCE) architecture for flexible optical networks. PCE architecture and PCE communication protocol are enhanced to maximize the spectral efficiency and to provide indications also on the specific transmission parameters to configure. Experimental demonstration is provided through two different experiments, successfully showing the PCE capability to trigger dynamic rerouting with bit-rate or modulation format adaptation. In particular, the experiments demonstrate, in a real testbed, dynamic frequency slot assignment and format adaptation from DP-16QAM to DP-QPSK at 100 Gb/s, and bit-rate adaptation at DP-16QAM from 200 Gb/s to 100 Gb/s.

We examine various algorithms for calculating quality of service (QoS)-enabled paths spanning multiple autonomous systems (ASs) using the path computation element (PCE) architecture. The problem is divided into two parts. We first calculate an AS path, then the node-by-node path. Using extensive simulation, we compared various AS-path calculation algorithms based on border gateway protocol (BGP) and various AS-aggregation procedures, such as mesh, star and nodal aggregation. For node-to-node path calculation, we employed the per-domain backward algorithm and the per-domain backward tree algorithm (also known as backward recursive PCE-based computation). Results point to the fact that complex AS-path calculation algorithms do not perform significantly better than BGP. However, if the service quality provided by ASs varies greatly, either in time or space, then we expect a QoS-aware AS-path computation algorithm, e.g., static nodal aggregation, to outperform BGP. Although the per-domain backward tree algorithm generally performs better than the per-domain backward algorithm, using a persistent variant of the latter makes it outperform the per-domain backward tree algorithm. The cost is a negligible increase in computational complexity and a slightly increased connection setup delay.

For the past few years, Hierarchical Path Computation Element (h-PCE) is an architecture that has been promoted to handle packet routing in multi-domain networks. However, this architecture has a potential drawback of poor scalability with respect to the number of domains. In tackling this complicated problem, we focuses on Inter-Domain Path Computation problem under the Domain Uniqueness constraint (IDPC-DU), which is employed to improve h-PCE. The objective of IDPC-DU is to find the shortest path between two given nodes that traverses every domain at most once. Since the IDPC-DU belongs to NP-Hard class, this paper introduces a two-level approach, combining the advantages of two metaheuristic algorithms, Genetic Algorithm (GA) and Ant Colony Optimization (ACO). Specifically, the upper-level GA plays the role of navigating the path for ants at the lower level ACO. Furthermore, an anti-stuck strategy that helps ants avoid being stuck is also equipped. To analyze the effectiveness of the proposed algorithm, experiments and comparisons with other algorithms are conducted. The results demonstrated that the proposed algorithm outperforms all other comnared ones in most cases.

Differentiated services provision according to clients or applications is an added feature that network operators ambition to fully provide. One of the possible approaches is to map DiffServ into MPLS based network. However in an IP over WDM (Wavelength Division Multiplexing) network data flow traverse multiple layers and, therefore, Class-Type information from the MPLS DiffServ layer is eventually lost. Besides, with the implement of a Path Computation Element (PCE) on multi-layer and multi-domain networks, the bandwidth constraints used to differentiate services are no longer applied. This paper presents a proposal for a QoS DiffServ approach applied on PCE as a traffic grooming technique for WDM core networks and the problematic involved on the application of this technology on multilayer and multi-domain networks.

A dynamic domain-sequencing scheme is proposed for Backward Recursive Path Computation Element (PCE) -Based Computation (BRPC) in multi-domain WDM networks. Simulation evaluation shows that the proposed scheme is effective in inter-domain path computation with more efficient resource utilization and lower blocking probabilities.

The need to preserve information confidentiality among network providers has prevented the actual deployment of effective Traffic Engineering (TE) solutions for QoS-enabled end-to-end connectivity services in multi-domain multi-provider networks. The use of Path Computation Element (PCE) architecture can foster the effective implementation of TE through a centralized engine devoted to end-to-end path computations. However, despite authentication, authorization and encryption, confidentiality issues may arise due to abuses of information included in path computation replies to bogus requests issued by malicious PCEs. This paper first demonstrates the security leak allowing the exposure of intra-domain information in current inter-PCE path computation procedures. Then it proposes an anomaly-based approach, namely PCE Anomaly Detector (PAD) in order to detect malicious utilization of path computation services. The proposed PAD employs a novel double-step multi-dimensional formulation based on the Sequential Hypothesis Testing (SHT) statistical classification procedure, able to recognize a suspicious sequence of requests while aiming at inferring confidential information about other domains. PAD is extensively evaluated through simulations. Results show good performance in terms of detection capabilities while guaranteeing the trade-off between accuracy and responsiveness, minimizing false alarm occurrences. Robustness against smart attacks is also proved with respect to a comprehensive set of attack patterns and under different network load conditions. Finally, intra-domain information exposition is evaluated, showing the PAD ability to preserve confidentiality.

A Path Computation Element (PCE) is a computing device in a network capable of calculating paths. Once a PCE receives a path computation request, the PCE is suppose to provide a response within a reasonable short time. However, computing a feasible path is not a straightforward task, since the computational problem must consider several constraints in a short time. In this paper we propose an algorithm for light-paths computation for this purpose that is intended to comply with the PCE standard. Our algorithm considers optical impairments, wavelength conversion and nodes with limited switching capabilities. Optical impairments are estimated using a Q-factor. The routing problem is solved using Multi-Constrained Shortest Path exact algorithm.

A Path Computation Element (PCE) based architecture has been specified for the computation of Traffic Engineering (TE) Label Switched Paths (LSPs) in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks in the context of single or multiple domains (where a domain refers to a collection of network elements within a common sphere of address management or computational responsibility such as IGP areas and Autonomous Systems). Path Computation Clients (PCCs) send computation requests to PCEs, and these may forward the requests to and cooperate with other PCEs forming a path computation chain. In PCE-based environments, it is thus critical to monitor the state of the computation chain for troubleshooting and performance monitoring purposes: liveness of each element (PCE) involved in the PCE chain, detection of potential resource contention states and statistics in term of computation times are examples of such metrics of interest. This document specifies procedures and extensions to the Path Computation Element Protocol (PCEP) in order to gather such information.

An active stateful Path Computation Element (PCE) is capable of computing as well as controlling via Path Computation Element Communication Protocol (PCEP) Multiprotocol Label Switching Traffic Engineering (MPLS-TE) Label Switched Paths (LSPs). Furthermore, it is also possible for an active stateful PCE to create, maintain, and delete LSPs. This document defines the PCEP extension to associate two or more LSPs to provide end-to-end path protection.

In a many-to-many session, a participant distributes and receives traffic flows to/from all others in the same session. It is desirable to achieve both high resource utilization and low power consumption when such sessions are provisioned. In wavelength division multiplexing (WDM) optical networks, traffic grooming has been widely applied as the key technology for aggregating sessions to improve resource utilization. However, the current many-to-many grooming approaches, the lightpath circle and hubbed light-tree, do not make an effort to reduce the power consumption of many-to-many sessions. In this paper, we formulate the green routing and aggregation of many-to-many sessions problem and prove its NP-completeness. For problem solving in WDM optical networks, we apply a rational combined method of lightpaths. Both an integer linear programming-based approach and a novel graph-based heuristic are devised. We further provide an illustrative comparison between our heuristic and the existing benchmarks in terms of power consumption. Extensive simulation results demonstrate that, compared with the benchmarks, our heuristic improves the average power efficiency in the range of 13-19%.

We study the problem of providing real-time service to hard and soft real-time messages in wavelength division multiplexing (WDM) optical networks. We propose and evaluate a set of scheduling algorithms which schedule message transmissions in single-hop WDM passive star networks based on specific time constraints. We compare the performances of our algorithms with that of the typical WDM scheduling algorithm which does not consider the time constraint of the transmitted messages by discrete-event simulations and an analytical model. We find that the improvement on real-time performance can be attributed to our scheduling algorithms where the time constraint of messages has been taken into consideration. This study suggests that when scheduling real-time messages in WDM networks, one has to consider not only the problem of resources allocation in the network but also the problem of sequencing messages based on their time constraints.