  We study three dimensional topologically massive gravity (TMG) in presence of
a generic codimension one null boundary. The existence of the boundary is
accounted for by enlarging the Hilbert space of the theory by degrees of
freedom which only reside at the boundary, the boundary degrees of freedom. The
solution phase space of this theory in addition to bulk massive chiral
gravitons of the TMG, involves boundary modes which are labeled by surface
charges associated with large diffeomorphisms. We show boundary degrees of
freedom obey a local thermodynamic description over the solution phase space,
null surface thermodynamics, described by a local version of the first law, a
local Gibbs-Duhem equation, and local zeroth law. Due to the expansion of the
boundary and also the passage of the bulk mode through the boundary, our null
surface thermodynamics describes an open boundary system that is generically
out of thermal equilibrium.
