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These network architectures with decoupled control planes open many unanswered questions regarding reliability, scalability, and performance when compared to more traditional purely distributed systems. This paper opens the investigation by focusing on two specific questions: given a topology, how many controllers are needed, and where should they go? To answer these questions, we examine fundamental limits to control plane propagation latency on an upcoming Internet2 production deployment, then expand our scope to over 100 publicly available WAN topologies. As expected, the answers depend on the topology. More surprisingly,\n                    <jats:italic>one<\/jats:italic>\n                    controller location is often sufficient to meet existing reaction-time requirements (though certainly not fault tolerance requirements).\n                  <\/jats:p>","DOI":"10.1145\/2377677.2377767","type":"journal-article","created":{"date-parts":[[2012,9,25]],"date-time":"2012-09-25T19:48:08Z","timestamp":1348602488000},"page":"473-478","update-policy":"https:\/\/doi.org\/10.1145\/crossmark-policy","source":"Crossref","is-referenced-by-count":279,"title":["The controller placement problem"],"prefix":"10.1145","volume":"42","author":[{"given":"Brandon","family":"Heller","sequence":"first","affiliation":[{"name":"Stanford University, Stanford, CA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Rob","family":"Sherwood","sequence":"additional","affiliation":[{"name":"Big Switch Networks, Palo Alto, CA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nick","family":"McKeown","sequence":"additional","affiliation":[{"name":"Stanford University, Stanford, CA, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"320","published-online":{"date-parts":[[2012,9,24]]},"reference":[{"key":"e_1_2_1_1_1","unstructured":"Ansi t1.tr.68-2001 enhanced network survivability performance.  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Aster*x: Load-balancing as a network primitive. 9th GENI Engineering Conference (Plenary), November 2010."},{"key":"e_1_2_1_14_1","doi-asserted-by":"publisher","DOI":"10.1145\/261342.571216"},{"key":"e_1_2_1_15_1","unstructured":"S. Knight H. Nguyen N. Falkner R. Bowden and M. Roughan. The internet topology zoo.  S. Knight H. Nguyen N. Falkner R. Bowden and M. Roughan. The internet topology zoo."},{"key":"e_1_2_1_16_1","volume-title":"OSDI. USENIX","author":"Koponen T.","year":"2010","unstructured":"T. Koponen , M. Casado, and et al. Onix: A distributed control platform for large-scale production networks . In OSDI. USENIX , 2010 . T. Koponen, M. Casado, and et al. Onix: A distributed control platform for large-scale production networks. In OSDI. USENIX, 2010."},{"key":"e_1_2_1_17_1","doi-asserted-by":"publisher","DOI":"10.1145\/964725.633047"},{"key":"e_1_2_1_18_1","unstructured":"The openflow switch. http:\/\/www.openflowswitch.org.  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