Several approaches to having messages routed through the Federation Substrate are possible. These routing methods can most easily be classified based on the mechanism for technical trust that is used. The choice of technical trust mechanism constrains how rules determination is implemented. Regardless of what deployment strategy is chosen, it is important that the technical trust mechanism be able to validate the identities of both parties to the exchange. The trust mechanism must ensure that the entity acting as the IdP for a given NAI is permitted to be the IdP for that realm and that any service name claimed by the RP is permitted to be claimed by that entity. Here are the categories of technical trust determination: