I. INTRODUCTION

II. BASIC FORMULATION AND OPTIMIZATION STRATEGIES
A. Physical network
B. Virtualized Network Functions (VNFs)
C. Service requests
D. SFC Resource Allocation
E. Optimization approaches

III. RELATED PROBLEMS
A. Virtual machine placement
B. Virtual network embedding

IV. VARIANTS OF SFC-RA PROBLEM
A. Basic
B. Dynamic
C. Online
D. Multiple providers
E. Schedule
F. Mobile network

V. FUTURE RESEARCH DIRECTIONS
A. Resiliency
B. Distributed SFC-RA
C. SFC-RA in data center

VI. CONCLUSION