IT service requirements offer a seemingly classic Requirements Engineering (RE) problem. But, when attempting to solve it with RE methods, we are faced with difficulties. RE methods encourage us to identify the functional and non-functional requirements of a service. Industrial service-management frameworks, however, use a different vocabulary. ITIL, one of the most prominent service-management frameworks, refers to service utilities and service warranties. In this paper, we propose a method for modeling warranties as a function of the service constancy expected by stakeholders and the threats to this constancy. We identify four kinds of warranties: express, implied, tacit and pending. We thereby seek to bridge the gap between service-management frameworks and RE methods and to improve the practice of service management in organizations.