## 4 Innovation through SDN-Based Network Applications
### SDN Inter-Domain Routing  
Various efforts exist to integrate SDN and OpenFlow in carrier networks that often require network routing protocols. The authors of [61] integrate the routing suite, Quagga, into the NOX OpenFlow controller. This enhanced NOX controller is called QuagFlow. The authors highlight the advantages of reusing existing software rather than rewriting a routing suite in a centralized fashion for OpenFlow. Their main argument is that new software tends to have more bugs than an equivalent software that was already tested and applied in real networks. Their solution is based on virtualized routers that run the Quagga routing suite. The QuagFlow architecture ensures that routing control messages are correctly delivered from OpenFlow switches to virtual Quagga routers and _vice versa_. This avoids inconsistencies between the IP and the OpenFlow network. The RouteFlow architecture [62] is based on QuagFlow. In addition, RouteFlow is transparent with regard to the routing engine, e.g., Quagga can be replaced with any appropriate routing software. RouteFlow allows for flexible mapping and operation between virtual elements and physical switches. For instance, each virtual router may control a single or multiple OpenFlow switches. Both approaches, QuagFlow and RouteFlow, enable SDN to interact with IP networks and allow for incremental migration towards SDN-based networks.  
The authors of [63] discuss how SDN can perform inter-domain routing. They provide an inter-domain routing component to an SDN control plane. They implement inter-AS routing in the OpenFlow NOX controller. Finally, concepts of centralized BGP control platforms are re-evaluated again. Routing Control Platforms (RCP) [64] were discussed in the mid-2000s to implement a scalable and extensible control platform for BGP. However, RCP focused mainly on the BGP control plane and its integration in IP networks running Interior Gateway Protocols (IGPs), such as Open Shortest Path First (OSPF) or Intermediate System to Intermediate System (IS-IS). The integration in IP networks is enabled through an \"IGP Viewer\" component. The authors of [65] discuss the concept of RCP in combination with OpenFlow-based networks. They describe how RCP can be embedded as an SDN-based network application and discuss the advantages of this approach, as well as potential difficulties. A centralized BGP control plane can scale for a high number of exterior BGP sessions, as shown in the past. The authors discuss how a centralized BGP control plane simplifies the edge architecture by replacing expensive IP border routers with simplified SDN switches. The RCP architecture is also believed to provide more stability, security and improved policy management than common BGP architectures,such as route reflectors and confederations. However, the authors discuss scalability issues with OpenFlow-based RCP that are due to the flow table limitations, as well as the required backup flow installations. The availability of a centralized BGP control plane can also be problematic, and the authors discuss potential solutions based on BGP SHIM to avoid the single point of failure problem.