With protocol objects, managing protocol dependencies is not only possible during the design and implementation phases (between protocol classes), but also at runtime (between protocol objects). This is partly due to the fact that protocol objects can execute more than one protocol at a time. In this context, trying to compose protocols comes down to answering the question ``How are protocol layers assembled and how do they cooperate?''.
Figure 2 (a) presents a runtime snapshot of
aCSSObject, some protocol object of class CSSObject which
implements an algorithm for solving the distributed consensus problem. The
consensus problem is defined on some set 
of distributed objects as
follows: all correct objects in propose an initial value and must
reach agreement on one of the proposed values (the decision) [3].
Class CSSObject defines operations propose() and
decide(), which mark the beginning and the termination of the protocol
respectively [2]. Besides consensus, protocol object
aCSSObject is also capable of executing any protocol inherited by its
class, e.g., reliable point-to-point communications and reliable
multicasts, as well as failure detections. In
Figure 2 (a), aCSSObject is concurrently managing
five different protocol stacks for the application layer, and issuing
low-level calls to the transport layer. Focusing on the consensus stack,
protocol composition means here to assemble various layers, each being
necessary to execute the consensus protocol, into the protocol stack
pictured in Figure 2 (b). The assembling occurs at
runtime and creates a new stack each time the application invokes operation
propose().
Figure 2: Protocol layers and protocol objects