1. A method comprising: carrying out offline functionality-based co-ranking and clustering on a knowledge base, said knowledge base characterizing a heterogeneous information technology services network comprising a plurality of services, a plurality of providers, and a plurality of attributes; storing results of said functionality-based co-ranking and clustering as annotations of said services and said providers in said knowledge base, to obtain an annotated knowledge base; obtaining a service requirement from a customer requiring information technology services; querying said annotated knowledge base, based on said service requirement; and returning to said customer an ordered list of at least given ones of said services, based on said querying, wherein said step of carrying out offline functionality-based co-ranking and clustering on said knowledge base comprises: calculating an initial service clustering for said heterogeneous information technology services network; classifying said providers into a plurality of service clusters; within each of said service clusters, ranking each of said providers; integrating ranking and clustering distances between said providers into a uniform provider distance to obtain updated provider similarity; classifying said services into a plurality of provider clusters; within each of said provider clusters, ranking each of said services; and integrating ranking and clustering distances between said services into a uniform service distance to obtain updated service similarity.