# A Survey on Efforts to Evolve the Control Plane of Inter-Domain Routing
## 4 Issues in the Evolution of the Inter-Domain Routing Control Plane
### Traffic Engineering in the Inter-Domain  
The traffic engineering inside an AS is performed using intra-domain routing systems, which are the processes of routing inside any single AS with the purpose of delivering the packets as efficiently as possible. To manage intra-domain routing, there are a couple of routing protocols, for example OSPF. Each protocol has its own direct mechanisms to reach the goal of applying traffic engineering techniques. However, inter-domain routing that fulfills traffic engineering requirements (e.g., QoS) is more tricky.  
The major design focus of the BGP was a routing protocol capable of applying routing policies and scale when exchanging connectivity information, once scalability is a paramount requirement for inter-domain routing protocols. The BGP has proven its scalability and efficiency to impose routing policies over years of deployment. Nonetheless, ASs that use exclusively the BGP for inter-domain routing cannot receive other types of routing information other than connectivity. Besides, due to the fact that the BGP is designed to use only one best route to each destination entry (see Section 3.2), the control plane has limited connectivity information for routing in a more optimized route selection when it is possible.  
For example, due to the lack of direct mechanisms to explore the path diversity of the Internet, ASs have been seeking to improve resilience requirements for their domains, and a common technique to reach that goal is the ASs splitting their prefixes and advertising them for different paths. The side effect of this technique is the Internet routing table growth.  
Figure 3 presents the Internet routing table size growth over almost the last two decades. As is depicted in Figure 4, the Internet is composed mostly of stub ASs, and the exhaustion of IPv4 (re-use), provider-independence addressing and load balancing and fail-over techniques for multi-homing ASs are stressing the current inter-domain routing scheme [19]. The exponentially-growing table size on the Internet is a scalability issue that threatens the future of the Internet, and the consequences are, for example, expensive router devices and increased delays for convergence time [22].  
On the one hand, BGP does not transport network metrics, such as capacity, bandwidth or cost, to create the topology state of its path-vector algorithm. Having those types of information is crucial to apply traffic engineering optimization tasks over network traffic. On the other hand, flooding inter-domain routing with AS internal network metrics can impose an overhead in the communication between domains, revealing sensitive operational information (for example, bandwidth utilization in inter-domain links) or detailing their view of the network status. Therefore, exploring the full potential of Internet path diversity, avoiding the side-effects of TE techniques and having appropriate tools for managing traffic engineering tasks in the inter-domain environment with security and in alignment with the ASs' business requirements are the trade-offs for the next generation of inter-domain routing proposals.