ES.REQUEST_START:
          activeRequests++;
          if (activeRequests > ACTIVE_REQUEST_TOLERANCE) {
            networkQuietWindowStart = undefined;
          }
          break;
        case ENDPOINT_TYPES.REQUEST_END:
          activeRequests--;
          if (activeRequests === ACTIVE_REQUEST_TOLERANCE) {
            // Just became network 2-quiet.
            networkQuietWindowStart = endpoint.time;
          }
          break;
        default:
          // This should never happen.
          throw n