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 Open Access
Conflictfree rerouting scheme through flow splitting for virtual networks using switches
 Vianney Kengne Tchendji^{1},
 Yannick Florian Yankam^{1} and
 Jean Frédéric Myoupo^{2}Email author
https://doi.org/10.1186/s1317401800854
© The Author(s). 2018
Received: 13 October 2017
Accepted: 25 April 2018
Published: 2 July 2018
Abstract
The weaknesses of the Internet led to the creation of a new network paradigm – network virtualization. Virtualization is a very successful technique for sharing and reusing resources, which results in higher efficiency. Despite its advantages, including flexibility in network architecture, virtualization imposes many challenges, such as physical resource allocation to virtual devices. An efficient allocation strategy for these resources can ensure good Quality of Service (QoS) in virtual networks, whether in node or link failure events. This paper presents a conflictfree rerouting scheme with efficient additional capacity usage for link and node failure resilience in a virtual network using switches. Combining an IP Fast Rerouting approach and flowsplitting strategy, this scheme provides short reaction time, stable performance and low complexity because the rerouting calculation and configuration are performed in advance. We show that rerouting by traffic splitting based on the entering arc and destination is sufficient to address all linkfailure situations in the network, assuming that the network is twolink connected. After modelling the dimensioning problem as an Integer Linear Programme, we demonstrate through practical implementation of our rerouting scheme on different networks that the scheme can substantially minimize the additional capacity draw on the substrate network. A solution using multiple virtual planes is also provided to solve several conflict problems in the case of simultaneous multiple link failures.
Keywords
1 Introduction
Since its creation, the Internet has brought innovations and success to industry, economic and research fields; however, deployment of any new, radically different technology and architecture is becoming highly difficult, a situation that cloud computing can mitigate. That effect is what we call Internet ossification [1]. To fend off ossification, studies have proposed rethinking the architecture of the actual Internet [1]. However, network virtualization is the most promising approach to addressing current limitations of the Internet and supporting new requirements [2–5]. Its principle is to implement multiple virtual routers on each physical machine and to interconnect them through substrate network architecture. This implementation allows virtual networks to have different logical topologies from that substrate network, and each of them behaves as a true network in which it is possible to implement different routing protocols and services. As in the substrate network, failures could occur in virtual networks; in this case, rerouting mechanisms can be implemented to forward traffic by using available resources in the virtual network or additional ones taken from the physical network. This additional resource could cause dysfunctional risks inside another virtual plane.
1.1 Previous work

For rerouting schemes using a single path and additional capacity [7, 8], the limits of the physical resources are quickly reached, which paralyses the network.

For loopfree alternative mechanismbased methods [9], we are not certain of rerouting traffic to all destinations; doing so only helps to reduce the number of lost packets in an IP network.

Notvia addressing [10] and tunnelling [11] mechanisms require encapsulation and deencapsulation of packets, whereas in a multiple routing configuration mechanism [12], the packets must carry configuration information. With the appearance of optic networks, methods that modify the packets are not recommended but can instead be used to optimize the usage of resources in network virtualization.

Multipath rerouting [13] using spare capacity in the network can induce a capacity saving of up to 11% in randomly generated networks, but lack of spare capacity due to the existence of multiple virtual planes on a substrate network can undermine this result in network virtualization.

Network modelling using only one rerouting path cannot significantly minimize additional capacity, which means that physical resource limits are quickly reached and the network is paralysed for a moment. Because the physical resource limits are quickly reached in [7, 8], the number of failures handled is also reduced.

Based on their rerouting model, the authors in [7, 8] claim that a conflictfree rerouting scheme for the multiplelinkfailure situation cannot be constructed even when the network remains connected.
The work in [21] presents a virtual network embedding model that allows a virtual link to map multiple substrate paths. This model can help to build a rerouting strategy using multiple planes.
1.2 Our contribution
We initially propose a new scheme to solve the link and node failure problems in a network of switches that avoids conflict on rerouting paths in contrast to protocols in [6, 7]. The new rerouting scheme we present here clearly uses not just one but multiple bridges, and it avoids conflict on rerouting paths. Our strategy uses a flowsplitting technique [22–25], extends the results in [7, 8] and can minimize the dimensioning cost of the network. Flow splitting is a method for restoring traffic from a failed link using multiple rerouting paths in the case of insufficient residual capacity. In this first contribution, we consider only one virtual plane. We propose a rerouting scheme that ensures that for any link or node failure, the traffic will be rerouted until it reaches the destination through an alternative path.
Given that there generally are no rerouting solutions that avoid conflicts in all network configurations, our second contribution is to avoid these situations. To reach this goal we use multiple planes to provide a rerouting strategy that avoids conflicts that could not be solved by using only one plane. Therefore, we introduce here a new scheme to solve the link and node failure problems in a network of switches. This new scheme negates the drawbacks of [7, 8] by showing that a conflictfree rerouting scheme for a multiplelinkfailure situation can be constructed even when the network remains connected. The rerouting scheme described in this paper uses filters in switches to determine the next hop for the incoming flow. We provide each virtual network a specific filter. Then, the controller sets the flow path by programming the switches in the form of quadruplets (S, N, O, F) in which S is the source port (node), N the current node, O the output ports because flow can be split and forwarded through different paths to the same destination depending upon the spare capacity needed, and F the filter, which indicates the destination.
More precisely, for an incoming flow from a neighbour and a given destination, the scheme will assign the potential output ports. In the case of failure, only the upstream node must react by directing the disturbed traffic to one or many of its neighbours. The traffic is routed according to the filter programmed in each node of the network. Traffic can be split anytime at the level of each node if needed. Hence, the proposed scheme needs only a local reaction, making its implementation particularly easy in distributed environments. This local reaction helps the network operate normally and can solve the problem of transient failures. A transient failure is a failure whose duration is short, less than 10 min, whereas the duration of a persistent failure is longer [7]. When the failure is determined to be persistent, the controller can recalculate the routing table for all nodes in the network. To avoid the rerouted traffic of a failure causing disturbances to another part of network, additional capacity is added to all arcs. Because this additional capacity is added in the physical network and is exploited by several virtual layers, it is necessary to minimize it. The mathematical model that we propose in this paper can calculate the rerouting paths and optimize the total additional capacity injected into the network.
To the best of our knowledge, presently our work is the only one that shows how to effectively solve simultaneous multilink failures using flow splitting methods, thus providing an improvement in QoS of computer networks.
The rest of this paper is organized as follows. The next section presents motivations for traffic splitting. Section 3 provides a full description of our restoration scheme for a link failure configuration. Our mathematical model is described in Section 4. Section 5 presents numerical results of implementations. Section 6 studies the application of our rerouting scheme to the singlenodefailure problem. Section 7 extends our work to simultaneous multiplelinkfailure situations by providing a solution for some conflict problems. Finally, Section 8 ends the paper.
2 Motivation for traffic splitting
2.1 Improvements of computer networks Qos

Reduction of transit delay and packet loss rate, because the flows are more able to reach their destination nodes, thus improving the network QoS;

improvement of the packets routing delays: since the original flow is split into several lightersized streams, they can be transported more quickly to the destination;

improvement of load balancing distribution [26], leading to prevent or decrease congestion risk across the network. This is a wellknown benefit of flow splitting in computer networks.

extension of the lifetime of a network by allowing more flexible and efficient resources allocation;

better economy of the substrate network resources supporting virtual networks. Since virtual networks are built on a physical network infrastructure, it is necessary to avoid an abuse of these resources with the risk of causing the hosted virtual networks malfunctions.
Assume that faulty link (5, 2) is replaced by path 5–31. Because links (5, 3) and (3, 1) have only 5 and 7 spare capacity each, both links need, respectively, 3 and 1 more spare capacity to make the restoration possible. This example proves that the use of traffic splitting in the link restoration scheme can result in lower spare capacity requirements but in the context of a virtual network, it could be very important to reduce additional capacities added to the links to avoid disturbances due to the rerouting scheme.
2.2 The packets reordering problem

Avoiding the packet reordering. When a stream is subdivided into the network, the different parts must be reassembled without losing no part (a potential TCP performance problem [27, 28]), making the use of flow splitting strategy very delicate. So, we should try to make reordering rare. Therefore avoiding this reordering until the packets reached the destination is crucial. However to overcome this problems some approaches were elaborated. Some strategies separate traffic at the level of flows. This approach removes the problem of reordering but at the cost of a restriction in the granularity with which we can split traffic [29]. Another one operates on bursts of packets (flowlets) carefully chosen to avoid reordering, but allowing a finer granularity of load balancing [30]. Some other algorithms [31, 32] minimize or eliminate reordering in some situations. But, some reordering problem should occur, and probably often enough to affect performance of IP networks;

the problem of reassembling packet segments inside the destination node. Due to various reasons, such as multipath routing, route fluttering, and retransmissions, packets belonging to the same flow may arrive out of order at a destination. The problem is how to know which packet comes before or after another one when we want to rebuild the original flow packets [27–29]. Some algorithms based on packet numbering in [29] can be used to at least minimize reordering.
In this work, flow splitting is implemented by building little flow of packets from an original one. To face reordering challenge, we use the numbering packets each time the flows are split, because this method does not modify significantly the packets headers.
3 New multipath link failure restoration scheme

Safeguarding of network resources by minimizing spare and additional capacity usage to manage more traffic

Possible rerouting, however, is impossible to do with only one path, as shown in [7].
A routing tree called a nominal routing tree is associated with a given destination; this tree is constructed using the shortest path tree criterion. We assume that the routing is provided. In the case of a failure of an arc or edge (both arcs are then concerned) and a lack of spare capacity in links, we reroute the traffic through one or more alternative paths. When there is only one path used, our rerouting scheme is similar to [7]. According to the routing scheme, for two independent failures, if two rerouting paths to a given destination have a common arc, they must merge after this arc. This requirement holds for both nominal and rerouting paths. If two paths do not satisfy this requirement, we say that they are in conflict. Any routing scheme satisfying this requirement is said to be without conflict. In this strategy, only the extremity failed link nodes will know about the failure. The upstream nodes initiate the traffic diversion, whereas all other nodes in the network apply the filter for each incoming flow without any difference between disturbed and nondisturbed flows. Because the disturbed traffic is rerouted on multiple alternative paths and should satisfy the nonconflict requirement, the cost in terms of resources and of computational time is expected to be higher compared with conventional schemes using single path rerouting.
These figures represent a network with 6 nodes and 8 links.
The original graph with thespare capacity of each link is shown in Fig. 3.
4 Mathematical formulation
4.1 Description of our model

The graph is assumed oriented and symmetric.

There are at least two disjoint arc paths between any two nodes of the graph.

There is only one link failure at a time.
To resolve the question of the existence of a rerouting solution without conflict, we have the following theorem:
Theorem 1. For any destination d, there is a rerouting plan without conflict using one or many alternative paths.
The formal mathematical proof can be found in the Appendix.

Rt^{ d }: set of arcs of the routing tree to destination d

Ac_{ ab }: additional capacity assigned to arc (a, b)

\( {Tr}_n^d \): total traffic for d that passes through node n. n is the node that detects the failure. In fact, the failure is characterized by a source n and a destination d because we use the routing tree for nominal routing. For a destination d, the failed arc is the one routing the traffic going to d and coming from n by nominal routing.

F_{ i }: indicates fictive nodes used to divert traffic in the case of failure. We introduce the fictive nodes F_{ i } that will be used for all failures. For a given failure (n, d), the traffic to d will be rerouted by i paths from F_{ i } to d starting with arc (F_{ i }, n), i = 1, 2...

\( {\mathrm{SRed}}_{\mathrm{n}}^{\mathrm{d}} \): \( {\mathrm{SRed}}_{\mathrm{n}}^{\mathrm{d}} \) subtree of sink n. Recall that in the case of failure, the tree is divided into two parts, the isolated part, that is the Red part, and the Blue part. Alternative paths will reroute traffic from the Red part to the Blue part.

\( {\mathrm{SBlue}}_{\mathrm{n}}^{\mathrm{d}} \): subtree of sink d, with \( {Rt}^d{SRed}_{\mathrm{n}}^{\mathrm{d}}{\mathrm{SRed}}_{\mathrm{n}}^{\mathrm{d}}{\mathrm{SRed}}_{\mathrm{n}}^{\mathrm{d}} \)

\( {\mathrm{y}}_{\mathrm{efgh}}^{\mathrm{dn}}{\mathrm{y}}_{\mathrm{efgh}}^{\mathrm{dn}} \): this binary variable indicates whether the e^{ th } alternative path to destination d for a given failure contains arcs (f, g) and (g, h); node n is the node that detects the failure.

\( {\mathrm{x}}_{\mathrm{efgh}}^{\mathrm{d}} \): \( {\mathrm{x}}_{\mathrm{efgh}}^{\mathrm{d}} \) this binary variable indicates the rerouting scheme to destination d. It takes value 1 if there is a failure in which the e^{ th } alternative path to destination d contains arcs (f, g) and (g, h). Therefore, the variable takes value 1 if there are n and e where \( {y}_{efgh}^{dn}{\mathrm{y}}_{\mathrm{efgh}}^{\mathrm{dn}} \) is equal to 1.

\( {\upalpha}_{\mathrm{eab}}^{\mathrm{dn}} \): \( {\upalpha}_{\mathrm{eab}}^{\mathrm{dn}} \) this binary coefficient equals 1 if arc (a, b) belongs to one of the paths in the nominal routing from n to d except the failed arc.

Quadruplet: All quadruplets (e, f, g, h) where e is the number of an alternative path in a rerouting scheme, f, g, h are nodes of the graph; f can be the fictive node, and (f, g) and (g, h) are two adjacent arcs, with. f ≠ h.

Arc: All arcs of the graph

L: Set of links

N: Set of nodes
The objective is to minimize the sum of additional capacity allocated to each arc; our objective function is provided by (1):
The objective function will allow us to evaluate the ratio between the additional capacity and the installed capacity.
Equation (2) is a constraint implying that there exist multiple paths resulting from flow splitting, which go from n to d for disturbed traffic.
Equation (3) ensures that there is no conflict in the rerouting, i.e., the incoming flows to node n to destination d must follow the same rerouting paths. If we use arcs (i, k_{1}), (i, k_{2}), (i, k_{3}), ... for rerouting to destination d, there is at most one output (k_{ s }, j_{ s }) for each.
To avoid loops and conflict problems, the alternative paths should not contain any arc of nominal routing in the red part of the network. Equation (4) ensures that condition.
Constraint (5) ensures that there will be no loop in the network. For a given destination and a given failure, an alternative path could contain a loop if the flows go from a node with a larger index number to another one with a smaller index. This constraint prohibits this type of problem.
Equations (6), (7), and (8) are the flow constraints for the continuity of the alternatives paths. Equation (6) is the constraint of flow conservation. Referring to (7), the total amount of entering traffic in n is equal to the total outgoing traffic of g; because of flow splitting being used for a given failure and a destination, we could have multiple incoming streams and possibly multiple outgoing streams.
Equation (8) ensures that in the blue part, if a path uses an arc of the nominal routing tree, it must continue until destination d.
Equation (9) is a constraint for the relationship between two rerouting paths that avoids a conflict (see the definitions of variables x and y). Because x and y are binary variables, with the same quadruplet (e, f, g, h) and same destination d, we can deduce from (9) that (x) will take the maximum value of (y). We use the sum of failures divided by the cardinal to reduce the number of constraints.
Equations (10), (11), and (12) are the capacity constraints. For each failure of edge (n, m), the constraint in (10) assumes rerouted paths for arcs (n, m) and (m, n), and only trees that contain the arc failure are involved. They also consider the released bandwidth on the initial routing paths. Equations (11) and (12) are special cases of (9) for the nodes that detect the failure, node n and node m. Finally, (13) and (14) indicate that the variables take binary values.
4.2 Convexity of our model
where C is the set Arc and f is a function over C giving the additional capacity needed for a chosen arc. The problem described by Eq. (15) is convex in the set C and the function f is convex.
Where f, g_{ 1 },…., g_{ m }: ℝ^{ n } → ℝ are convex functions and h_{ 1 },…., hp.: ℝ^{ n } → ℝ are affine functions. Each constraint of our model can be written under any of the forms of constraints of Eq. (16). This proves that our model is implicitly convex.
5 Implementation and simulation results
Restoration rate without conflicts
Networks  Number of nodes  Number of links  Number of failures  Restoration 

Network1  5  7  7  7 
Network2  10  18  18  14 
Network3  20  31  28  23 
Network4  60  81  70  61 
Restoration rate neglecting conflicts
Networks  Number of nodes  Number of links  Failures  Restoration 

Network1  5  7  7  7 
Network2  10  18  18  14 
Network3  20  31  28  25 
Network4  60  81  70  65 
Comparison of our rerouting scheme with [7] concerning additional capacity used
Netwowrks  Number of nodes  Number of links  Unused CA  Our used CA  Used by X 

Network1  5  7  3342  1012  1624 
Network2  10  18  7216  2433  2741 
Network3  20  31  14,000  3802  4524 
Network4  60  81  12,052  4110  5021 
6 Node failure problem
When node 4 fails, flows coming from nodes 1 and 7 must be rerouted. The failure of node 4 implies a simultaneous failure of links (1–4), (7–4) and (4–6). Therefore, we must reroute the two flows (1–4) and (7–4) to destination 6 without conflict. To solve this type of failure, two solutions are possible:
6.1 First solution resort to the controller
In the case of node failure, each switch that detects the failure sends a specific message called packetin message to the controller that sets the rerouting order for the link failures related to these nodes. The idea of this rerouting approach is to solve these link failures as cascading failures. This order can be built on a node’s label criteria. The nodes are labelled in a decreasing order as we approach the destination node. We could handle the failure detected by the node of a smaller label before another one with a larger label. Once the resolution order is fixed, the controller updates the routing tables of involved nodes as described in [7] by using another specific message called packetout message. After this update, our rerouting scheme can be used to solve each link failure. The reaction time of this solution is too long, due to proactivity; therefore, the principles of IPFRR are not satisfied with this solution.
6.2 Second solution: No resort to the controller
For each destination, we determine the nominal routing tree from each node towards this destination (see Fig. 10). The failure of node 3 generates simultaneous failures of links (5–3), (6–3) and (3–1) (dotted links). Nodes 5 and 6 will detect the breakdowns of links (5–3) and (6–3), i.e., we have two flows to reroute. These failures split the graph into two parts: the blue part and the red part (see Fig. 11).
Using our rerouting scheme, the flow coming from link (5–3) could be rerouted through arcs (5 → 2), (5 → 6) and (5 → 8). The flow of link (6–3) could follow arcs (6 → 8), (6 → 4) and (6 → 5). However, arc (5 → 6) can lead to node 3 through arc (6 → 3) or keep the rerouted flow in cycle 5–6–85 (see Fig. 12). Thus, arc (6 → 3) will be excluded from the list of potential paths for rerouting the flow coming from link (5–3) or node 3. If we consider the criteria related to management of the cycles, arc (5 → 6) will be considered in rerouting the paths of link (5–3), which will not be true of arc (6 → 5) (see Fig. 13). Similarly, for rerouting link (6–3), arc (5 → 6) could also be used rather than (6 → 5).
Consequently, the possible rerouting paths will be 5–21, 5–8–9741 and 5–6–41 for flow from the failure of link (5–3); concerning the flow from the failure of link (6–3), the possible rerouting paths could be 6–8–9741 and 6–41. We can conclude that local reaction required by IPFRR strategy can also be preserved when addressing simple node failure situations through our rerouting scheme.
To achieve the local connectivity recovery, there is a filter similar to an agent, running inside each switch (example of OpenFlow switches) used in network architecture like ours. This agent detects the port states and acts as needed. For classical switches, there are control mechanisms provided to check that ports status.
Multiple link failures studied in the case of simple node failure involve links adjacent to that node, but we also have cases of simultaneous multiple link failures not adjacent to the same node.
7 Simultaneous multilink failures
We speak about simultaneous multiple link failures when several links fail at the same time. The case considered in this section concerns nonadjacent links to the same single node. In this case, there are multiple nodes, each of which detects a link failure as in the simple node failure case. This type of failure can also be handled using either of two methods:
7.1 First method: Treat only one link failure at a time
In this approach, despite many link failures occurring at the same time, they are handled as nonsimultaneous link failures; therefore, failures are treated sequentially. This method is used in [7], in which a rerouting scheme is proposed to solve the problem for the case of two links failing simultaneously. As stated in Section 7 about the node failure problem, the limit of this strategy is its slowness in rerouting.
7.2 Second method: Treat all link failures at the same time
This approach is similar to the second one presented in Section 7 for the node failure problem, and it enables all nodes that detect a failure to initiate the rerouting process. Our rerouting scheme for node failure can also be used here. When several link failures occur simultaneously during the rerouting process, we can use flow splitting each time to find spare capacity lacking in the network.
The nominal routing tree is shown, and the destination node is labelled 1.
Consider the configuration example given by [7] in Fig. 17, in which the authors claim that there is no rerouting solution. Two simultaneous link failures situations are considered: first, we have simultaneous link failures (AD) and (CD). Second, we have simultaneous link failures (EC) and (ID).
According to [7], when (AD) fails, the only available rerouting path is ABHGFECD because if we choose path HGK, there would be a conflict at node G. When (CD) fails, the traffic that comes from failure (AD) will be rerouted by C. To reroute the traffic of failure (CD), node C must transfer the traffic back to G; then, there are two possibilities: use GHBAD as the rerouting path, or transfer the traffic through link (GK). We cannot use GHBA because the traffic would be transferred indefinitely between A and C. Therefore, we must use FGK as the rerouting path in this situation.
Concerning the second situation in which the two links (EC) and (ID) fail at the same time, when (ID) fails, using the same reasoning as in the previous case, the only available rerouting path is IJOKGFECD, according to [7]. Because we used FGK in the previous case of link (EC) ’s failure, we must also transfer the traffic through FGK for this case to avoid conflict. Because both failures occur at the same time, the traffic will be transferred indeterminately between C and I; therefore, the traffic cannot be rerouted in this situation. That property is why [7] affirms that there is no rerouting scheme without conflict for destination D in this configuration.
Let us transpose the topology of Fig. 17 into the physical plane as illustrated by Fig. 18.
Assuming that the nodes which detect failures are nodes E and I in the case of simultaneous link (EC) ’s and (ID) ’s failures and considering the topology’s heterogeneity in the virtual networks, node E detects that a traffic redirection through path FGKOJ will be deviated on node I and cause a cyclic problem. To solve that problem, we use another plan for traffic coming from both link failures. We will choose paths EECD’D and II’D’D for link (EC) ’s and (ID)’s failure restoration. Thus, our approach can solve unsolvable conflicts presented in [7] by making use of multiple planes.
8 Conclusion and future work
Our aim in this paper was to propose a rerouting approach to handle the single link node failure and simultaneous multiple link failure problems in a network of switches in the context of network virtualization. We proposed a conflictfree rerouting scheme that can ensure that, whatever the case of link or node failure, traffic will be rerouted to the destination. The proposed method is based on local reaction of nodes placed at the extremities of the failed link, whereas the other nodes need not know about the failure or take any particular action. Thus, the implementation is particularly easy. The flow splitting strategy used when there is insufficient spare capacity on links helps to reduce additional capacity added to the network. We proved that there exists a restoration scheme without conflict in the network and provide a mathematical model that permits calculation of the rerouting scheme with optimization of the sum of additional capacities needed for one virtual plane. We also proposed a rerouting solution using several planes to solve cases of potentially unsolvable conflicts when we use only one plane. Further work will address congestion management into the nodes implied in the rerouting and routing table updates without disturbing the network.
Declarations
Acknowledgements
We thank the anonymous reviewers whose valuable comments and suggestions have significantly improved the presentation and the readability of this work.
Authors’ contributions
JFM suggested this work. VKT and YFY carried out analysis and performed the experiments. VKT and YFY wrote the first draft of this work and worked for the revised version. JFM revised the first draft and worked on the revised version. In addition, all authors read and approved the work.
Authors’ information
VKT and YFY are with the department of Mathematics and Computer Science of the University of Dschang, Cameroon. JFM is with the Computer Science Lab. MIS of the university of Picardie Jules Verne, Amiens, France.
Competing interests
The authors declare that they have no competing interests.
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