# A Survey on Efforts to Evolve the Control Plane of Inter-Domain Routing
## 6 Further Discussion and Lessons Learned
### Inter-Domain Routing Limitations  
The BGP will never select two, or more, next hops per prefix. It always chooses a unique best hop per prefix. Different from internal routing protocols (e.g., the ECMP) where multiple destinations are allowed to a given prefix for load balancing proposes, the BGP standard protocol does not incorporate this behavior. Hence, to network, operators maximize the utilization of external links, and they have to split the traffic using a subnet of the prefix, which has the side effect of increasing the global routing table (Figure 3).  
The use of multiple links may have the consequence of improving path diversity and network reliability [74]. The BGP can filter routes by matching prefix, prefix-length and/or based on different path attributes that are associated with each BGP route and are part of an update message. However, the forwarding data plane elements can only forward the packet based on destination prefixes, once the BGP follows a destination-based forwarding paradigm [6]. In other words, routers can only apply their policies and forward decision based on the destination address of the IP packet.  
Routing systems that use source addresses, policy-based routing or are based on application requirements cannot be deployed on the current Internet as this is not supported by the current BGP logic of control. Furthermore, novel network applications may require some other fields in the packet header to optimize the inter-domain routing path or simply to distinguish routing by source addresses [60]. Those requirements are not available with the use of the BGP.  
Moreover, it has long been known that the Internet architecture has several issues, despite the Internet's unparalleled success. Highly rigid and static, the traditional networking infrastructure was initially designed to operate for a particular type of traffic (text-based content), and that does not satisfy today's increasingly demanding users that request interactive and dynamic multimedia streams. Along with the multimedia trends, the recent emergence of new and innovate technologies, e.g., IoT, has been pressuring the inter-domain infrastructure to support those new types of network demands.  
Previous works have attempted to solve these architectural deficiencies with brand new designs or incremental improvements, which have had limited success [87]. On the one hand, the resistance to these new efforts stems from the requirement that new solutions must be mature enough and have obvious benefits. In other words, an AS will not risk its business existence in adopting technologies with unproven scalability, reliability, interoperability and consistency. Moreover, the current Internet's infrastructure took years to reach its current size, and a myriad of ASs is still expecting to reach the Return on Investment (ROI) from that.  
On the other hand, many ASs are searching for ways to increase their profit: be more agile to address business requirement changes, provide a network that reduces cost, simplifies operations and supports new and innovative products and services. Thereby, the next generation control plane needs to be more suitable to their business needs, and it is expected that automation [88] and intelligent network traffic control systems [89] will play an important role in evolving the inter-domain routing.  
Therefore, to bridge the gap between what is expected and desirable on architectural evolvability and what is feasibly deployable has to include, among other ideas, modularity solutions with BGP compatibility and extensive support of automation.