# Enhanced Bit Repair IP Fast Reroute Mechanism for Rapid Network Recovery
## 1 Introduction
### Link Versus Node Protection  
The operation of FRR is based on two basic principles, which are the focus of the protection provided by the FRR: we distinguish between a protected interface or link and a protected node. A protected link is a line, where in the event of a failure, all communication that passed through the link will be redirected using FRR [34; 35; 36; 37; 38]. A protected node is a device on a network (such as a router) that will be protected by the FRR mechanism, including all its lines. Line protection helps to ensure that traffic from the router S (Figure 2, R1) passes through a particular line (or interface) on an alternative path to a next-hop router (or switch; Figure 2, R4), through which it will be further routed to the original next-hop router (Figure 2, R2). In other words, the device bypasses the failed link by an alternative path that includes the original next-hop router (Figure 2, green arrows).  
Node protection expands the possibilities of line protection. Node protection ensures that communication from the router passing through the next-hop router will be delivered even if the next-hop router fails. In other words, the FRR mechanism calculates an alternative path that bypasses the failure of the next-hop router in such a way that the original next-hop router for the specific destination D will not be part of the alternative path (Figure 2).  
Existing FRR mechanisms usually focus on scenarios where they provide FRR protection in situations when there is only one failure in a network.  
Figure 2: Line and node protection.  
In this paper, we present an FRR mechanism called the Enhanced Bit Repair (EB-REP) FRR mechanism (hereinafter EB-REP), which provides advanced reroute protection. EB-REP is an advanced version of the legacy Bit Repair (B-REP) [39] mechanism that removes its disadvantages and offers functions that enable IP FRR network protection even in situations with multiple outages, and besides, if necessary, for all flows within the protected domain. EB-REP can be compared to other well-known existing solutions, such as Loop-Free Alternates (LFA) or Remote Loop-Free Alternates (R-LFA). Moreover, as an advantage, EB-REP uses a standardized BIER (Bit Index Explicit Replication) header and efficient alternate route marking with Bit-String values, which allows us to define the entire alternative multi-hop FRR path. The EB-REP mechanism can provide full repair coverage. The EB-REP can be deployed in an IP network that uses link-state protocols, such as the Open Shortest Path First (OSPF) or Intermediate System to Intermediate System (IS-IS).  
The remainder of this paper is structured as follows: Section 2 contains an analysis of existing FRR solutions and discusses their problem areas. Section 3 describes the original B-REP FRR mechanism and its properties. Section 4 proposes the new Enhanced B-REP FRR mechanism. Section 5 focuses on the evaluation of the EB-REP mechanism and compares its features with other FRR solutions. Section 6 represents a discussion of the obtained results. Section 7 presents the conclusions of our work.