Editing Routing (section)

==Centralized routing==
In networks where a logically centralized control is available over the forwarding state, for example, using [[software-defined networking]], routing techniques can be used that aim to optimize global and network-wide performance metrics. This has been used by large internet companies that operate many data centers in different geographical locations attached using private optical links, examples of which include Microsoft's Global WAN,<ref>{{cite web |url=https://azure.microsoft.com/en-us/blog/how-microsoft-builds-its-fast-and-reliable-global-network/ |title=How Microsoft builds its fast and reliable global network |last=Khalidi |first=Yousef |date=March 15, 2017}}</ref> Facebook's Express Backbone,<ref>{{cite web |url=https://code.fb.com/data-center-engineering/building-express-backbone-facebook-s-new-long-haul-network/ |title=Building Express Backbone: Facebook's new long-haul network |date=May 1, 2017}}</ref> and Google's B4.<ref>{{cite web |url=https://www.networkcomputing.com/networking/inside-googles-software-defined-network/512240144 |title=Inside Google's Software-Defined Network |work=Network Computing |date=May 14, 2017 |access-date=October 8, 2018 |archive-date=December 8, 2018 |archive-url=https://web.archive.org/web/20181208111113/https://www.networkcomputing.com/networking/inside-googles-software-defined-network/512240144 |url-status=dead }}</ref> 

Global performance metrics to optimize include maximizing network utilization, minimizing traffic flow completion times, maximizing the traffic delivered prior to specific deadlines and reducing the completion times of flows.<ref>{{cite journal|last1=Noormohammadpour|first1=Mohammad|last2=Raghavendra|first2=Cauligi|title=Datacenter Traffic Control: Understanding Techniques and Tradeoffs|journal=IEEE Communications Surveys and Tutorials|date=16 July 2018|volume=20|issue=2|pages=1492–1525|doi=10.1109/COMST.2017.2782753|arxiv=1712.03530|s2cid=28143006}}</ref> Work on the later over private WAN discusses modeling routing as a graph optimization problem by pushing all the queuing to the end-points. The authors also propose a heuristic to solve the problem efficiently while sacrificing negligible performance.<ref>{{cite journal|last1=Noormohammadpour|first1=Mohammad|last2=Srivastava|first2=Ajitesh|last3=Raghavendra|first3=Cauligi|title=On Minimizing the Completion Times of Long Flows over Inter-Datacenter WAN|journal=IEEE Communications Letters|volume=22|issue=12|pages=2475–2478|date=2018|url=https://www.researchgate.net/publication/328008697|doi=10.1109/LCOMM.2018.2872980|arxiv=1810.00169|bibcode=2018arXiv181000169N|s2cid=52898719}}</ref>