  The Grothendieck--Serre conjecture predicts that every generically trivial
torsor under a reductive group scheme $G$ over a regular local ring $R$ is
trivial. We settle it in the case when $G$ is quasi-split and $R$ is
unramified. Some of the techniques that allow us to overcome obstacles that
have so far kept the mixed characteristic case out of reach include a version
of Noether normalization over discrete valuation rings, as well as a suitable
presentation lemma for smooth relative curves in mixed characteristic that
facilitates passage to the relative affine line via excision and patching.
