  Given Krein and Hilbert spaces $\left( \mathcal{K},[.,.] \right)$ and $\left(
\mathcal{H}, \left( .,. \right) \right)$, respectively, the concept of the
boundary triple $\Pi =(\mathcal{H}, \Gamma _{0}, \Gamma_{1})$ is generalized
through the abstract Green's identity for the isometric relation $\Gamma$
between Krein spaces $\left( \mathcal{K}^{2}, \left[
.,.\right]_{\mathcal{K}^{2}} \right) $ and $\left(\mathcal{H}^{2}, \left[
.,.\right]_{\mathcal{H}^{2}} \right) $ without any conditions on $\dom\,
\Gamma$ and $\ran\, \Gamma$. This also means that we do not assume the
existence of a closed symmetric linear relation $S$ such that $\dom\,
\Gamma=S^{+}$, which is a standard assumptions in all previous research of
boundary triples. The main properties of such a general Green's boundary model
are proven. In the process, some useful properties of the isometric relation
$V$ between two Krein spaces $X$ and $Y$ are proven. Additionally, surprising
properties of the unitary relation $\Gamma : \mathcal{K}^{2}
\rightarrow\mathcal{H}^{2}$ and the self-adjoint main transformation
$\tilde{A}$ of $\Gamma$ are discovered. Then, two statements about generalized
Nevanlinna families are generalized using this Green's boundary model.
Furthermore, several previously known boundary triples involving a Hilbert
space $\mathcal{K}$ and reduction operator $\Gamma : \mathcal{K}^{2}
\rightarrow\mathcal{H}^{2}$, such as AB-generalized, B-generalized, ordinary,
isometric, unitary, quasi-boundary, and S-generalized boundary triples, have
been extended to a Krein space $\mathcal{K}$ and linear relation $\Gamma$ using
the Green's boundary model approach.
