  This is the first of a series of papers discussing canonical aspects of the
two-dimensional non-linear sigma model in the presence of conformal defects on
the world-sheet in the framework of gerbe theory. In the paper, the basic tools
of the state-space analysis are introduced, such as the symplectic structure on
the state space of the sigma model and the pre-quantum bundle over it, and a
relation between the defects and dualities of the sigma model is established.
Also, a state-space description of the splitting-joining interaction of the
string across the defect is presented, leading to an interpretation of the
geometric data associated to junctions of defect lines in terms of intertwiners
between the incoming and outgoing sectors of the theory in interaction.
