docs: int getAssembledLengthIfKnown() {
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) SibTr.entry(this, tc, "getAssembledLengthIfKnown");
      int flatSize = -1;
      if (jmfPart instanceof JMFMessage) {
        flatSize = ((JMFMessage)jmfPart).originalFrame();
      }
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) SibTr.exit(this, tc, "getAssembledLengthIfKnown",  flatSize);
      return flatSize;
    }