Plan and Document versus Agile requirements

The SRS serves as an agreement between customers and developers. But in many projects customers are unsure of the requirements, and their understanding evolves over time. The Agile perspective addresses this: working closely and interacting frequently with stakeholders to develop and refine requirements, building a working prototype and refining it by adding features in sprints/iterations (typically every 2 weeks). In Agile we are basically in maintenance mode as soon as we have some requirements. The Agile lifecycle begins with Behaviour Driven Design.