Fast reroute (FRR) is among the fastest survivable routing approaches in packet-switched networks, because the routers are equipped with a resilient routing table in advance such that the packets can be rerouted instantly upon failures solely relying on local information, i.e., without notification messages. However, designing the routing algorithm for FRR is challenging as the number of possible sets of failed network links can be extremely high, while the algorithm should keep track of which routers are aware of the failure. Therefore, FRR methods often rely on spanning arborescences, which provide multiple disjoint failover paths up to the global connectivity of the network. In this paper, we propose a generic algorithmic framework that theoretically increases the number of failover paths to the local connectivity between each node and the root by extending an efficient connectivity preserving operation from graph theory – called edge splitting-off – to decompose the network topology node-by-node, and use Integer Linear Programs (ILPs) on these partial subproblems to build routing arborescences in the reverse direction for the original topology. Although our practical implementation cannot reach the local connectivity in all instances, we demonstrate through simulations that it still outperforms the state-of-the-art FRR mechanisms and provides better resilience with shorter paths in the arborescences.

Fast reroute (FRR) mechanisms that can instantly handle network failures in the data plane are gaining attention in packet-switched networks. In FRR no notification messages are required as the nodes adjacent to the failure are prepared with a routing table such that the packets are re-routed only based on local information. However, designing the routing algorithm for FRR is challenging because the number of possible sets of failed network links and nodes can be extremely high, while the algorithm should keep track of which nodes are aware of the failure. In this paper, we propose a generic algorithmic framework that combines the benefits of Integer Linear Programming (ILP) and an effective approach from graph theory related to constructive graph characterization of k-connected graphs, i.e., edge splitting-off. We illustrate these benefits through arborescence design for FRR and show that (i) due to the ILP we have great flexibility in defining the routing problem, while (ii) the problem can still be solved very fast. We demonstrate through simulations that our framework outperforms state-of-the-art FRR mechanisms andvprovides better resilience with shorter paths in the arborescences.

With the advent of 6G and its mission-critical and tactile Internet applications running in a virtualized environment on the same physical infrastructure, even the shortest service disruptions have severe consequences for thousands of users. Therefore, the network hypervisors, which enable such virtualization, should tolerate failures or be able to adapt to sudden traffic fluctuations instantaneously, i.e., should be well-prepared for such unpredictable environmental changes. In this paper, we propose a latency-aware dual hypervisor placement and control path design method, which protects against single-link and hypervisor failures and is ready for unknown future changes. We prove that finding the minimum number of hypervisors is not only NP-hard, but also hard to approximate. We propose optimal and heuristic algorithms to solve the problem. We conduct thorough simulations to demonstrate the efficiency of our method on real-world optical topologies, and show that with an appropriately selected representative set of possible future requests, we are not only able to approach the maximum possible acceptance ratio but also able to mitigate the need of frequent hypervisor migrations for most realistic latency constraints.

Communication networks are evolving towards a more adaptive and reconfigurable nature due to the evergrowing demands they face. A framework for measuring network flexibility has been proposed recently, but the cost of rendering communication networks more flexible has not yet been mathematically modeled. As new technologies such as software-defined networking (SDN), network function virtualization (NFV), or network virtualization (NV) emerge to provide network flexibility, a way to estimate and compare the cost of different implementation options is needed. In this paper, we present a comprehensive model of the cost of a flexible network that takes into account its transient and stationary phases. This allows network researchers and operators to not only qualitatively argue about their new flexible network solutions, but also to analyze their cost for the first time in a quantitative way.

Control plane survivability in virtual software-defined networks (vSDN) – where multiple tenants share the same physical infrastructure – is even more critical than in normal SDN networks. A reliable communication channel from the switches through the network hypervisor to the virtual controller is inevitable in order to avoid state inconsistencies, tenant isolation and security issues on the virtual switches. Although reliable controller placement and control plane design was thoroughly investigated in SDNs, there was a lack of attention for resilient hypervisor placement and control path design for vSDNs. Therefore, in this paper we make a two-fold contribution towards a survivable vSDN control plane. First, we propose an approximation algorithm for (hypervisor) placement which – in contrast with traditional approaches which minimize the average latency to the hypervisors as an objective function – focuses on finding the appropriate number of hypervisor instances to satisfy the control path length constraints declared in the service level agreements, leaving enough options open for self-driving network designs and intelligent algorithms. Second, we propose a general dynamic program that calculates minimum length paths traversing specific type of nodes in a given order, and apply it to find control paths from the virtual controller of the slice to the virtual switches traversing the corresponding hypervisor location. We conduct thorough simulations on real-world topologies to demonstrate the effectiveness of our approaches in no failure and single link failure scenarios.

In the field of networking research, increased flexibility of new system architecture proposals, protocols, or algorithms is often stated to be a competitive advantage over its existing counterparts. However, this advantage is usually claimed only on an argumentative level and neither formally supported nor thoroughly investigated due to the lack of a unified flexibility framework. As we will show in this paper, the flexibility achieved by a system implementation can be measured, which consequently can be used to make different networking solutions quantitatively comparable with each other. The idea behind our mathematical model is to relate network flexibility to the achievable subset of the set of all possible demand changes, and to use measure theory to quantify it. As increased flexibility might come with additional system complexity and cost, our framework provides a cost model which measures how expensive it is to operate a flexible system. The introduced flexibility framework contains different normalization strategies to provide intuitive meaning to the network flexibility value as well, and also provides guidelines for generating demand changes with (non-)uniform demand utilities. Finally, our network flexibility framework is applied on two different use-cases, and the benefits of a quantitative flexibility analysis compared to pure intuitive arguments are demonstrated.

Generalized diversity coding is a promising proactive recovery scheme against single edge failures for unicast connections in transport networks. At the source node, the user data is split into two parts, and their bitwise XOR is computed as a third redundancy sub-flow. In order to guarantee instantaneous failure recovery without costly node upgrades, the network must ensure that any two of the three sub-flows reach the destination node in case of a single edge failure only by allowing flow duplication or merging identical flows, and avoiding any coding operation in the core network. In this paper, we investigate the corresponding routing problem to calculate capacity-efficient routes for these sub-flows. We propose a polynomial-time algorithm for topologies without capacity constraints on the links and without capability limitations of the nodes. We show that with node limitations the presented algorithm (as well as a minimum cost disjoint path-pair) provides a 4/3-approximation for the routing problem. Furthermore, we formulate an integer linear program to provide a minimum cost solution with arbitrary constraints in general graphs and we propose a polynomial-time algorithm in directed acyclic graphs. Our simulation results suggest that with upgrading only a small set of core network nodes with flow duplication and merging capabilities most of the benefits of generalized diversity coding can be achieved.

For communication networks research, flexibility of network design and networking solutions is considered a competitive advantage. However, this advantage is typically only claimed on an argumentative level and neither formally supported nor thoroughly investigated. To support the claim of flexibility and to make the flexibility of different solutions comparable, its degree must be quantified and thus made measurable. In this work, we propose a mathematical basis to quantify a degree of flexibility achieved by communication networks. We motivate that flexibility can be cast to the “size” of a set of achievable demand changes. Consequently, we propose the use of mathematical measure theory to quantify achieved networking flexibility. We derive several implications on the basic structure of flexibility, extend the insights towards a utility of flexibility and develop systematic approaches for both analytical and empirical measurement of flexibility. We apply the insights to several use-cases, showing that flexibility is in fact not as straight-forward to argue as it seems at first glance.

Nowadays, a majority of the Internet service providers are either piloting or migrating to software-defined networking (SDN) in their networks. In an SDN architecture a central network controller has a top-down view of the network and can directly configure each of their physical switches. It opens up several fundamental unsolved challenges, such as deploying efficient multipath routing that can provide disjoint end-to-end paths, each one satisfying specific operational goals (e.g., shortest possible), without overwhelming the data plane with a prohibitive amount of forwarding state. In this paper, we study the problem of finding a pair of shortest (node- or edge-) disjoint paths that can be represented by only two forwarding table entries per destination. Building on prior work on minimum length redundant trees, we show that the complexity of the underlying mathematical problem is NP-complete and we present fast heuristic algorithms. By extensive simulations, we find that it is possible to very closely attain the absolute optimal path length with our algorithms (the gap is just 1%-5%), eventually opening the door for wide-scale multipath routing deployments. Finally, we show that even if a primary tree is already given it remains NP-complete to find a minimum length secondary tree concerning this primary tree.

In this article, we investigate the Advanced Global Neighborhood Failure Localization (AG‐NFL) monitoring trail (m‐trail) approach, which provides ultra‐fast all‐optical restoration for any shared protection scheme. In contrast with its previous counterparts, AG‐NFL separates the management tasks of protection switching and link maintenance, and focuses on the identification of the proper switching actions in a timely manner rather than unambiguously localizing link failures. We form switching link groups at each node, that is, links whose failures do not have to be distinguished from each other, for example, because their corresponding switching actions can be performed at the same time. Forbidden link‐pairs are introduced to identify the minimal set of conflicting switching actions, which minimizes the number of switching link groups. Furthermore, in order to minimize the number of m‐trail reconfigurations upon dynamic traffic, we analyze the AG‐NFL performance in four different m‐trail design scenarios with decreasing dependency on the data plane. We prove that AG‐NFL is NP‐complete, and we propose an efficient heuristic to solve it. We demonstrate through simulations that unambiguous localization of switching link groups instead of single link failures leads to a significantly improved m‐trail performance both in wavelength resources and the number of required transponders, while signaling‐free restoration is still provided. © 2017 Wiley Periodicals, Inc. NETWORKS, Vol. 70(4), 327–341 2017

In this paper, we propose a new proactive recovery scheme against single edge failures for unicast connections in transport networks. The new scheme is a generalization of diversity coding where the source data AB are split into two parts A and B and three data flows A, B, and their exclusive OR (XOR) A⊕B are sent along the network between the source and the destination node of the connection. By ensuring that two data flows out of the three always operate even if a single edge fails, the source data can be instantaneously recovered at the destination node. In contrast with diversity coding, we do not require the three data flows to be routed along three disjoint paths; however, in our scheme, a data flow is allowed to split into two parallel segments and later merge back. Thus, our generalized diversity coding (GDC) scheme can be used in sparse but still two-connected network topologies. Our proof improves an earlier result of network coding, by using purely graph theoretical tool set instead of algebraic argument. In particular, we show that when the source data are divided into two parts, robust intra-session network coding against single edge failures is always possible without any in-network algebraic operation. We present linear-time robust code construction algorithms for this practical special case in minimal coding graphs. We further characterize this question, and show that by increasing the number of edge failures and source data parts, we lose these desired properties.

Network survivability has been recognized as an issue of major importance in terms of security, stability and prosperity. A crucial research problem in this context is the identification of suitable pairs of disjoint paths. Here, “disjointness” can be considered in terms of either nodes or links. Accordingly, several studies have focused on finding pairs of either link or node disjoint paths with a minimum sum of link weights. In this study, we investigate the gap between the optimal node-disjoint and link-disjoint solutions. Specifically, we formalize several optimization problems that aim at finding minimum-weight link-disjoint paths while restricting the number of its common nodes. We establish that some of these variants are computationally intractable, while for other variants we establish polynomial-time algorithmic solutions. Finally, through extensive simulations, we show that, by allowing link-disjoint paths share a few common nodes, a major improvement is obtained in terms of the quality (i.e., total weight) of the solution.

A fundamental unsolved challenge in multipath routing is to provide disjoint end-to-end paths, each one satisfying certain operational goals (e.g., shortest possible), without overwhelming the data plane with prohibitive amount of forwarding state. In this paper, we study the problem of finding a pair of shortest disjoint paths that can be represented by only two forwarding table entries per destination. Building on prior work on minimum length redundant trees, we show that the underlying mathematical problem is NP-complete and we present heuristic algorithms that improve the known complexity bounds from cubic to the order of a single shortest path search. Finally, by extensive simulations we find that it is possible to very closely attain the absolute optimal path length with our algorithms (the gap is just 1 -- 5%), eventually opening the door for wide-scale multipath routing deployments.

Survivable routing methods have been thoroughly investigated in the past decades in transport networks. However, the proposed approaches suffered either from slow recovery time, poor bandwidth utilization, high computational or operational complexity, and could not really provide an alternative to the widely deployed single edge failure resilient dedicated 1 + 1 protection approach. Diversity coding is a candidate to overcome these difficulties with a relatively simple technique: dividing the connection data into two parts, and adding some redundancy at the source node. However, a missing link to make diversity coding a real alternative to 1+1 in transport networks is finding its minimum cost survivable routing, even in sparse topologies, where previous approaches may fail. In this paper we propose a polynomial-time algorithm with O(|V||E| log |V|) complexity for this routing problem. On the other hand, we show that the same routing problem turns to be NP-hard as soon as we limit the forwarding capabilities of some nodes and the capacities of some links of the network.

Large-scale information dissemination in multicast communications has been increasingly attracting attention, be it through uptake in new services or through recent research efforts. In these, the core issues are supporting increased forwarding speed, avoiding state in the forwarding elements, and scaling in terms of the multicast tree size. This paper addresses all these challenges-which are crucial for any scalable multicast scheme to be successful-by revisiting the idea of in-packet Bloom filters and source routing. As opposed to the traditional in-packet Bloom filter concept, we build our Bloom filter by enclosing limited information about the structure of the tree. Analytical investigation is conducted and approximation formulas are provided for optimal-length Bloom filters, in which we got rid of typical Bloom filter illnesses such as false-positive forwarding. These filters can be used in several multicast implementations, which are demonstrated through a prototype. Thorough simulations are conducted to demonstrate the scalability of the proposed Bloom filters compared to its counterparts.

Shared protection, such as failure-dependent protection (FDP), is well recognized for its outstanding capacity efficiency in all-optical mesh networks, at the expense of lengthy restoration time due to multihop signaling mechanisms for failure localization, notification, and device configuration. This paper investigates a novel monitoring trail (m-trail) scenario, called Global Neighborhood Failure Localization (G-NFL), that aims to enable any shared protection scheme, including FDP, for achieving all-optical and ultra-fast failure restoration. We first define the neighborhood of a node, which is a set of links whose failure states should be known to the node in restoration of the corresponding working lightpaths (W-LPs). By assuming every node can obtain the on-off status of traversing m-trails and W-LPs via lambda monitoring, the proposed G-NFL problem routes a set of m-trails such that each node can localize any failure in its neighborhood. Bound analysis is performed on the minimum bandwidth required for m-trails under the proposed G-NFL problem. Then, a simple yet efficient heuristic approach is presented. Extensive simulation is conducted to verify the proposed G-NFL scenario under a number of different definitions of nodal neighborhood that concern the extent of dependency between the monitoring plane and data plane. The effect of reusing the spare capacity by FDP for supporting m-trails is examined. We conclude that the proposed G-NFL scenario enables a general shared protection scheme, toward signaling-free and ultra-fast failure restoration like p-Cycle, while achieving optimal capacity efficiency as FDP.

In this paper we close the gap between end-to-end diversity coding and intra-session network coding for unicast connections resilient against single link failures. In particular, we show that coding operations are sufficient to perform at the source and receiver if the user data can be split into at most two parts over the filed GF(2). Our proof is purely combinatorial and based on standard graph and network flow techniques. It is a linear time construction that defines the route of subflows A, B and A ⊕ B between the source and destination nodes. The proposed resilient flow decomposition method generalizes the 1+1 protection and the end-to-end diversity coding approaches while keeping both of their benefits. It provides a simple yet resource efficient protection method feasible in 2-connected backbone topologies. Since the core switches do not need to be modified, this result can bring benefits to current transport networks.

Failure dependent protection (FDP) is known to achieve optimal capacity efficiency among all types of protection, at the expense of longer recovery time and more complicated signaling overhead. This particularly hinders the usage of FDP in all-optical mesh networks. As a remedy, this paper investigates a new restoration framework that enables all-optical fault management and device configuration via state-of-the-art failure localization techniques, such as the FDP restoration process. It can be implemented without relying on any control plane signaling. With the proposed restoration framework, a novel spare capacity allocation problem is defined and is further analyzed on circulant topologies for any single link failure, aiming to gain a solid understanding of the problem. By allowing reuse of monitoring resources for restoration capacity, we are particularly interested in the monitoring resource hidden property, where less or even no monitoring resources are consumed as more working traffic is in place. To deal with general topologies, we introduce a novel heuristic approach to the proposed spare capacity allocation problem, which comprises a generic FDP survivable routing scheme followed by a novel monitoring resource allocation method. Extensive simulation is conducted to examine the proposed scheme and verify the proposed restoration framework.

The paper investigates a novel monitoring trail (m-trail) scenario that can enable any shared protection scheme for achieving all-optical and ultra-fast failure restoration. Given a set of working (W-LPs) and protection (P-LPs) lightpaths, we firstly define the neighborhood of a node, which is a set of links whose failure states should be known to the node in restoration of the corresponding W-LPs. A set of m-trails is routed such that each node can localize any failure in its neighborhood according to the ON-OFF status of the traversing m-trails. Bound analysis is performed on the minimum bandwidth required for the m-trails. Extensive simulation is conducted to verify the proposed scheme.

In this comment, we present some remarks on the availability evaluation of overlap dedicated segment protection (o-DSP) method in Tornatore et al., 2010, ãAvailability Formulations for Segment Protection" . We show how to correctly apply the pivotal decomposition availability-evaluation method in directed graphs (which was claimed to inapplicable in ), such that it gives the same (exact) connection availability value as the generating function method proposed in the original paper.

Large-scale information distribution has been increasingly attracting attention, be it through uptake in new services or through recent research efforts in fields like information-centric networking. The core issue to be addressed is the more efficient distribution of information to a large set of receivers. Avoiding state in the forwarding elements is crucial for any scheme to be successful. This paper addresses this challenge by revisiting the idea of in-packet Bloom filters and source routing. As opposed to the traditional in-packet Bloom filter concept which represent the trees flatly as sets, we build our filter by enclosing limited information about the structure of the tree, namely its stage decomposition, which helps to get rid of typical Bloom filter illnesses as infinite loops and false positive forwarding. Our analytical and simulation results show that by using this information we obtain more succinct tree representation while still maintaining forwarding efficiency.

Survivable routing serves as a key role in connection-oriented communication networks for achieving desired service availability for each connection. This is particularly critical for the success of all-optical mesh networks where each lightpath carries a huge amount of data. Currently, 1+1 dedicated path protection appears to be the most widely deployed network resilience mechanism because it offers instantaneous recovery from network failures. However, 1+1 protection consumes almost twice as much capacity as required, which imposes a stringent constraint on network resource utilization. In addition, finding an SRLG-disjoint path is essential for 1+1 protection, which is nonetheless subject to non-trivial computation complexity and may fail in some SRLG scenarios. To address these problems, we introduce a novel framework of 1+1 protection, called Generalized Dedicated Protection (GDP), for achieving instantaneous recovery from any SRLG failure event. It is demonstrated, that finding a non-bifurcated optimal solution for GDP is NP-complete. Thus, the paper presents a novel scheme applying Generalized Dedicated Protection and Network Coding (GDP-NC) to ensure both optimal resource utilization among dedicated protection approaches and instantaneous recovery for single unicast flows, which can be split into multiple parts in all-optical networks. We demonstrate that the proposed GDP-NC survivable routing problem is polynomial-time solvable, owing to the ability to bifurcate flows. This flexibility comes at the expense of additional hardware for linear combination operations for the optical flows.

Communication networks have to provide a high level of resilience in order to ensure sufficient Quality of Service for mission-critical services. Currently, dedicated 1+1 path protection is implemented in backbone networks to provide the necessary resilience. On the other hand, there are several possible realization strategies for 1+1 path protection functionality (1PPF), utilizing both diversity- and network coding. In this paper we consider the cost aspects of the different realization strategies. We evaluate the cost of providing 1PPF both analytically and empirically in realistic network topologies. Our results show that both diversity and network coding can provide 1PPF with reduced cost compared to traditional 1+1 path protection, even in case of short paths and strict coding restrictions. Specifically, the network coding scheme could be used as a cost-efficient and potentially all-optical realization of 1PPF.

Being reported as the most general monitoring structure for out-of-band failure localization approach, the monitoring trail (m-trail) framework has been witnessed with great efficiency and promises to serve in the future Internet backbone with all-optical mesh wavelength division multiplex (WDM) networks. Motivated by its potential and significance, this paper investigates failure localization in all-optical mesh networks using m-trails. By considering shared risk link groups (SRLGs) with up to all adjacent links of any node in the network, a novel algorithm of m-trail allocation for achieving unambiguous failure localization (UFL) of any single SRLG failure is developed. The proposed algorithm aims to minimize the number of required m-trails and can achieve superb performance with respect to the computation efficiency. We claim that among all the previously reported counterparts, this paper has considered one of the most applicable scenarios to the design of network backbone, and the proposed method can be easily extended to the case of node failure localization. Extensive simulation is conducted to verify the proposed algorithm in comparison to its existing counterparts.

We study SRLG (Shared Risk Link Group) failure monitoring and localization in all-optical WDM (Wavelength Division Multiplexing) networks. All links in each SRLG are logically grouped as a whole, and they fail at the same time when the SRLG failure event occurs. To achieve fast SRLG failure localization, monitoring is carried out at the optical layer using the recently proposed monitoring trail (m-trail) structure. By formulating an ILP (Integer Linear Program), we optimally solve the m-trail allocation problem to achieve unambiguous SRLG failure localization with the minimum monitoring cost. We claim that our work provides the first study in optimally allocating free-routed m-trails for achieving fast and unambiguous SRLG failure localization, with flexible tradeoff between the monitor cost and the bandwidth cost (i.e., supervisory wavelength-links).