Online impairments-aware routing within a path computation element

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.

Inter-domain path provision under a wavelength continuity constraint (WCC), including path computation and resource reservation procedures, is a major challenge in multi-domain wavelength division multiplexing (WDM) optical networks, because of the requirement for confidentiality between the domains. Based on the hierarchical path computation element (PCE) architecture, this work proposed a path establishment scheme under a WCC to provide a fast inter-domain path establishment solution and decrease the influence of the WCC. The hierarchical PCE architecture consists of a single parent PCE serving as the arbitrator and multiple child PCEs responsible for the child domains. By cooperation between the parent PCE and the child PCEs, both inter-domain path computation and inter-domain wavelength reservation can be processed in parallel to accelerate and optimize WCC lightpath provisioning and thus to reduce the blocking probability. The effectiveness of this solution has been evaluated through simulations. The simulation results show that the proposed solution can achieve lower computation and reservation delays and thus lead to lower blocking probabilities on the multi-domain WCC paths.

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.

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) 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.

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.

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.

Emerging services, such as high-definition Internet Protocol TV (IP-TV) or data center migration, are going to increase the amount of multicast traffic in the Internet. The support of multicast directly in the optical domain, instead of at the IP layer, is a target for reducing the amount of optical–electronic–optical conversions (thus, the network operational and capital expenditure) and energy consumption. In parallel, flex-grid technology (e.g., bandwidth variable wavelength selective switches) is emerging as a candidate solution to be adopted in future optical transport networks given its capacity of improving spectrum efficiency. This paper is focused on optical multicast in flex-grid optical networks and on its control through the Path Computation Element (PCE). First, we present two node architectures supporting optical multicast. The first node architecture achieves optical multicast through passive light split and requires that the multicast connection satisfies the spectrum continuity constraint. The second node architecture achieves optical multicast with frequency conversion. In particular, a specific implementation of the second architecture is proposed in this paper exploiting a periodically poled lithium niobate (PPLN) waveguide. Then, a PCE architecture to control optical multicast (with and without frequency conversion) is proposed. Optical multicasting, based on the proposed node architectures, at 100 and 200  Gb/s is experimentally demonstrated in a flex-grid network testbed. In particular, multicasting is demonstrated with 112  Gb/s polarization multiplexing 16 quadrature amplitude modulation (PM-16QAM) and polarization multiplexing quadrature phase shift keying (PM-QPSK), and with 224  Gb/s PM-16QAM considering the light-split node architecture. Moreover, optical multicast with two frequency conversions, achieved in a single PPLN device, is demonstrated for the first time with a 224  Gb/s PM-16QAM signal. The testbed also includes the PCE, which is extended to control optical multicast in flex-grid optical networks.

This paper provides an overview of the recent developments in research and the IETF standardization body on using a path computation element in complex telecom networking scenarios such as the multi-layer and multi-domain cases. Implications on the control plane when using such element are addressed as well as the required protocol extensions. The emerging impairment-aware routing and wavelength assignment problem is extensively treated for networks using lightpaths.

In the existing three layers multi-granularity OCDM switching system (TLMG-OCDMSS), F-LSP, L-LSP and OC-LSP can be bundled as switching granularity. For CPU-intensive network, the node not only needs to compute the path but also needs to bundle the switching granularity so that the load of single node is heavy. The node will paralyze when the traffic of the node is too heavy, which will impact the performance of the whole network seriously. The introduction of stateful PCE(S-PCE) will effectively solve these problems. PCE is composed of two parts, namely, the path computation element and the database (TED and LSPDB), and returns the result of path computation to PCC (path computation clients) after PCC sends the path computation request to it. In this way, the pressure of the distributed path computation in each node is reduced. In this paper, we propose the concept of Learning PCE (L-PCE), which uses the existing LSPDB as the data source of PCE’s learning. By this means, we can simplify the path computation and reduce the network delay, as a result, improving the performance of network.

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.

New trends and emerging requirements have driven the development of extensions to the path computation element (PCE) architecture beyond the computation of a set of constrained routes and associated resources between endpoints, given a network topology. Such extensions involve the use of a PCE for the control of network services, in which deploying a PCE as a centralized network controller facilitates the adoption of software-defined networking (SDN) principles while allowing a progressive migration of already existing deployments. A key requirement for the adoption of centralized control solutions is the ability to deploy a resilient, secure, dynamically configurable, adaptive, and highly available (virtualized) infrastructure supporting end-to-end services, including critical and vertical ones. Part of this infrastructure is the control plane functional elements (e.g., controllers), and the use of network function virtualization (NFV) is a enabler for the high availability of such elements while additionally reducing OPEX and CAPEX. NFV provides a feature-complete framework for the replication of software components that is a straightforward and commonly adopted approach to address the aforementioned requirement, but it implies the need for timely synchronization of databases between replicas. In this paper we present, implement, and validate an architecture for PCE and SDN control high availability, combining the virtualization of the control function by means of dynamic replication and the timely synchronization of their internal state using the PCEP and BGP-LS protocols. We experimentally validate the approach with a testbed, including a GMPLS/PCE control plane, and a replica management system implemented following the ETSI NFV framework, using the OpenStack cloud management software.

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 consider the problem of finding end-to-end shortest disjoint paths for a given sequence of domains in multi-domain networks. A Path Computation Element (PCE) serves as a computing entity in each domain, specializing in path computation and optimization. We propose a novel PCE-based scheme that computes the shortest path over multiple domains in the forward direction and computes the disjoint path in the backward direction. The proposed scheme has linear-time computation and message overhead in regard to the number of border nodes. Simulation results show that our proposed scheme can result in an optimal solution, while significantly reducing computation time compared to existing algorithms.

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.