  Exact field theory dualities can be implemented by duality domain walls such
that passing any operator through the interface maps it to the dual operator.
This paper describes the S-duality wall of four-dimensional ${\cal N}=2$ SU(N)
SQCD with 2N hypermultiplets in terms of fields on the defect, namely
three-dimensional ${\cal N}=2$ SQCD with gauge group U(N-1) and 2N flavours,
with a monopole superpotential. The theory is self-dual under a duality found
by Benini, Benvenuti and Pasquetti, in the same way that T[SU(N)] (the
S-duality wall of ${\cal N}=4$ super Yang-Mills) is self-mirror. The
domain-wall theory can also be realized as a limit of a USp(2N-2) gauge theory;
it reduces to known results for N=2. The theory is found through the AGT
correspondence by determining the braiding kernel of two semi-degenerate vertex
operators in Toda CFT.
