# Resilience Evaluation of Multi-Path Routing against Network Attacks and Failures
## 1 Introduction
### Multi-Path Routing Approaches  
Many approaches have been proposed to solve the problem from network attacks and failures using concepts of multiple paths. A link failure can occur in several ways, such as through natural hazards or by a DDoS attack. In multihoming [9], a network is connected to multiple providers and uses its own range of addresses. If one of the links fails, the protocol recognizes the failure and reconfigures its routing tables. Although multihoming provides a way to connect multiple providers, it is not a multi-path routing but rather an SP routing because we cannot select the connection to the providers for every communication instantly. Resilient overlay networks (RON) [10] are a remedy for potential problems, such as BGP's fault or path outages, because BGP hides information about traffic conditions and topological details in the interests of scalability and policy enforcement. RON can often find paths between its nodes, even while wide-area Internet routing protocols, such as BGP, cannot. This RON approach is an overlay network that selects different waypoints, but it cannot select the actual network path. Multi-path TCP (MPTCP) is the de-facto standard multi-path transport protocol [11]. MPTCT can only make use of a single path per local network interface and, therefore, does not permit path choice as in MMP.  
Software Defined WANs (SD-WAN) is a multi-path approach that can mitigate these problems by providing multiple routing paths from the first hop. However, if network congestion or a link failure occurs in other hops through a hazard or adversary that a victim AS cannot control, the SD-WAN will be unable to deal with the problem as it the SD-WAN has no control over other such hops, which can be described as the first type of multi-path routing, first-hop multi-path (FMP) routing. Figure 1 contrasts fully disjoint and partially disjoint paths. In Figure 1a, source AS _Src_ has two links--one to next AS \(A_{1}\), and the other to AS \(A_{2}\). The AS following \(A_{1}\) and \(A_{2}\) is AS \(B\). Source AS _Src_ has two paths to destination AS _Dst_. However, the two paths use the same links between AS \(B\) to AS _Dst_. These paths can be disconnected by only one link, that is, link \(B\)-C or \(C\)-_Dst_. Figure 1b shows the non-overlapping paths from _Src_ to _Dst_. As we can see, we cannot guarantee that the given paths are non-overlapped if the paths are established only by source AS's link selection. The second type of multi-path routing is multi-hop multi-path (MMP) routing, which can take advantage of controlling the paths available between a source and a destination. MMP can change to an alternative path to mitigate the problem, if hazards or attacks occur in other hops that a victim AS cannot control. However, to implement MMP on the current Internet architecture, a redesign of the Internet architecture is required.  
Figure 1: Source AS _Src_ has two links to next hop ASes \(A_{1}\) or \(A_{2}\) but (**a**) shows partially disjoint paths that the multi-paths between _Src_ and _Dst_ uses same links from AS \(B\) to AS _Dst_, which can be disconnected by a single link failure, whereas (**b**) shows fully disjoint paths that cannot be disconnected by a single link failure.