Are associations from Boundary to Boundary or Control to Control allowed?

I have a design class diagram using the boundary, control, entity pattern. The diagram contains 5 boundary classes, 1 control class, and 2 entity classes.

Are the following links allowed:

• boundary to boundary ?
• control to control?

Are there rules about allowing these relations?

The ECB pattern is meant to ease implementation of use-cases. In its philosophy, controls implement use cases, boundaries the relation of use cases with external world, and entities, the business data that is managed by the use cases.

So only the following links should be allowed:

             Entity     Boundary   Control
Entity         X                      X
Boundary                              X
Control        X           X          X

You see that boundary to boundary is not allowed. Boundaries can only be linked to actors or to controls.

Control to control is allowed, but you haven't any.

An additional question is the kind of link that should be represented. Is it really plain association (i.e. structural, one end is a property of the other) ? Or is it a usage dependency (i.e. a dashed line with the stereotype <<use>> would be more appropriate) ? I'd opt for the first alternative between entities, but the second with controls and boundaries.