I. Introduction  
A. IMS  
B. Cloud Computing and NFV  
C. Requirements  

III. Approaches That Deal with the Entire IMS  
A. Virtualized IMS  
B. IMS as a Service  

IV. Approaches That Deal with Specific IMS Entities  
A. HSS  
B. Presence Service  

V. Research Directions  
A. Reconsidering the Granularity Level of IMS  
B. Towards a PaaS for IMS  

VI. Conclusions