SP3 is a network layer protocol that has been designed to provide security services for network service users. It is assumed that the network service users reside in different private internetworks and must traverse a public internetwork to communicate. It is further assumed that SP3 services are being provided by intermediate systems that reside at the internetwork boundaries. The problem is that, in this environment, the mapping from SP3 users to SP3 providers is not simple: multiple SP3 systems can serve the same user. This mapping problem is further compounded by characteristics of the mapping of SP3 service users to the set of SP3 service providers. A dynamic method of performing this mapping is offered by proposing a new network layer protocol and procedures to work in conjunction with interrouting domain routing procedures.>