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I treat the worldtube constraints which arise in the null-timelikeinitial-boundary value problem for the Bondi-Sachs formulation of Einstein'sequations. Boundary data on a worldtube and initial data on an outgoing nullhypersurface determine the exterior spacetime by integration along the outgoingnull geodsics. The worldtube constraints are a set of conservation laws whichimpose conditions on the integration constants. I show how these constraintslead to a well-posed initial value problem governing the extrinsic curvature ofthe worldtube, whose components are related to the integration constants.Possible applications to gravitational waveform extraction and to thewell-posedness of the null-timelike initial-boundary value problem arediscussed.

Worldtube conservation laws for the null-timelike evolution problem