We investigate in the paper general (not necessarily definite) canonical
systems of differential equation in the framework of extension theory of
symmetric linear relations. For this aim we first introduce the new notion of a
boundary relation $\G:\gH^2\to\HH$ for $A^*$, where $\gH$ is a Hilbert space,
$A$ is a symmetric linear relation in $\gH, \cH_0$ is a boundary Hilbert space
and $\cH_1$ is a subspace in $\cH_0$.
Let $l[y]$ be a formally selfadjoint differential expression of an even order
on the interval $[0,b> \;(b\leq \infty)$ and let $L_0$ be the corresponding
minimal operator. By using the concept of a decomposing boundary triplet we
consider the boundary problem formed by the equation $l[y]-\l y=f\;(f\in
L_2[0,b>)$ and the Nevanlinna $\l$-depending boundary conditions with constant
values at the regular endpoint 0.
The main objects of our considerations are differential operators generated
by a formally selfadjoint differential expression of an even order on the
interval $[0,b> (b\leq \infty)$ with operator valued coefficients. We
complement and develop the known Shtraus' results on generalized resolvents and
characteristic matrices of the minimal operator $L_0$.