s between different policies? 4. Interworking between legacy and SDN/NFV technologies: As highlighted in Section 15.4, there are significant challenges involved in introducing new technologies into large-scale, operational legacy networks. Such technology integration must be done seamlessly, with- out customer impact. How should legacy and SDN/NFV technologies interwork? How can we most effectively and seamlessly migrate from legacy technologies to SDN/NFV? To what extent can legacy technologies be retired? ACKNOWLEDGMENTS The authors wish to acknowledge Taso Devetzis, Juan Flores, Mark Francis, Mohammad Islam, and Mike Paradise for the insight and wisdom contributed to this chapter. REFERENCES [3GPP_KPIS] 3GPP TS 32.450 V9.1.0 (2010-06)-Key Performance Indicators (KPI) for Evolved Universal Terrestrial Radio Access Network (E-UTRAN): Definitions (Release 9), ETSI, 2010. [CELLULAR_QOS] G. Gomez and R. Sanchez, End-to-End Quality of Service over Cellular Networks, John Wiley & Sons, 2005. [CORRELATION] https://en.wikipedia.org/wiki/Event_correlation. [MERCURY] A. Mahimkar, Z. Ge, J. Wang, J. Yates, Y. Zhang, J. Emmons, B. Huntley and M. Stockert, Rapid detection of maintenance induced changes in service performance, Proceedings of the Seventh Conference on emerging Networking Experiments and Technologies, Article No. 13, Tokyo, Japan, December 6-9, 2011. [LT_MERCURY_1] A. Mahimkar, H. H. Song, Z. Ge, A. Shaikh, J. Wang, J. Yates, Y. Zhang and J. Emmons, Detecting the performance impact of upgrades in large operational networks, Proceedings of the ACM SIGCOMM 2010 conference, pp. 303-314, New Delhi, India, August 30-September 3, 2010. [LT_MERCURY_2] A. Mahimkar, Z. Ge, J. Yates, C. Hristov, V. Cordaro, S. Smith, J. Xu and M. Stockert, Robust assessment of changes in cellular networks, Proceedings of the ninth ACM conference on Emerging networking experiments and technologies, pp. 175-186, Santa Barbara, California, December 9-12, 2013. [ARGUS] H. Yan, A. Flavel, Z. Ge, A. Gerber, D. Massey, C. Papadopoulos, H. Shah