The paper presents a delay-tolerant network (DTN) architecture for challenging environments where traditional Internet protocols like TCP/IP may not perform well. These environments include mobile networks, exotic media networks, military ad-hoc networks, and sensor/actuator networks, which often experience long delays, frequent disconnections, and limited resources. The DTN architecture is designed to provide reliable message delivery and interoperability between these diverse networks, which often use specialized protocols and do not rely on IP. The architecture operates as an overlay above the transport layers of the networks it connects, offering services such as in-network data storage, retransmission, interoperable naming, authenticated forwarding, and a coarse-grained class of service. It is based on the concept of message switching, where messages are stored and forwarded when necessary, rather than relying on end-to-end connectivity. The architecture uses name tuples for routing, which consist of a region name and an entity name, allowing for flexible and scalable naming across different networks. The DTN architecture also incorporates a postal class of service, inspired by the US Postal Service, to manage message delivery priorities. It includes mechanisms for custody transfer, where responsibility for message delivery is passed between nodes, and retransmission strategies that account for the high latency and potential disconnections in challenging environments. The architecture also addresses issues such as path selection, scheduling, time synchronization, and security, ensuring reliable and efficient message delivery across diverse and challenging networks. The paper argues that traditional approaches like link repair and proxies are insufficient for these environments and that a general-purpose message-oriented overlay architecture is necessary to achieve interoperability and reliable communication.The paper presents a delay-tolerant network (DTN) architecture for challenging environments where traditional Internet protocols like TCP/IP may not perform well. These environments include mobile networks, exotic media networks, military ad-hoc networks, and sensor/actuator networks, which often experience long delays, frequent disconnections, and limited resources. The DTN architecture is designed to provide reliable message delivery and interoperability between these diverse networks, which often use specialized protocols and do not rely on IP. The architecture operates as an overlay above the transport layers of the networks it connects, offering services such as in-network data storage, retransmission, interoperable naming, authenticated forwarding, and a coarse-grained class of service. It is based on the concept of message switching, where messages are stored and forwarded when necessary, rather than relying on end-to-end connectivity. The architecture uses name tuples for routing, which consist of a region name and an entity name, allowing for flexible and scalable naming across different networks. The DTN architecture also incorporates a postal class of service, inspired by the US Postal Service, to manage message delivery priorities. It includes mechanisms for custody transfer, where responsibility for message delivery is passed between nodes, and retransmission strategies that account for the high latency and potential disconnections in challenging environments. The architecture also addresses issues such as path selection, scheduling, time synchronization, and security, ensuring reliable and efficient message delivery across diverse and challenging networks. The paper argues that traditional approaches like link repair and proxies are insufficient for these environments and that a general-purpose message-oriented overlay architecture is necessary to achieve interoperability and reliable communication.