This document outlines the experimental protocol for Delay Tolerant Networking (DTN) known as the Bundle Protocol (BP). BP is designed to facilitate communication in highly stressed environments, such as those with intermittent connectivity, large and variable delays, and high bit error rates. The protocol operates at the application layer of the Internet model, using native internet protocols for communication within constituent internets. Key capabilities of BP include custody-based retransmission, handling intermittent connectivity, and leveraging scheduled, predicted, and opportunistic connectivity. The document details the format of protocol data units (bundles), which consist of a sequence of blocks, including primary bundle blocks, payload blocks, and extension blocks. It also describes the processing of bundles, including generation of administrative records, bundle transmission, dispatching, forwarding, and handling of status reports and custody signals. The document emphasizes the importance of implementation architectures and provides examples of how BP can be implemented in different scenarios, such as application servers, peer application nodes, sensor networks, and dedicated bundle routers. The document is produced by the IRTF's Delay Tolerant Networking Research Group (DTNRG) and represents the consensus of active contributors. It is intended for experimental use and does not specify an Internet standard. Readers are advised to exercise caution in evaluating its value for implementation and deployment.This document outlines the experimental protocol for Delay Tolerant Networking (DTN) known as the Bundle Protocol (BP). BP is designed to facilitate communication in highly stressed environments, such as those with intermittent connectivity, large and variable delays, and high bit error rates. The protocol operates at the application layer of the Internet model, using native internet protocols for communication within constituent internets. Key capabilities of BP include custody-based retransmission, handling intermittent connectivity, and leveraging scheduled, predicted, and opportunistic connectivity. The document details the format of protocol data units (bundles), which consist of a sequence of blocks, including primary bundle blocks, payload blocks, and extension blocks. It also describes the processing of bundles, including generation of administrative records, bundle transmission, dispatching, forwarding, and handling of status reports and custody signals. The document emphasizes the importance of implementation architectures and provides examples of how BP can be implemented in different scenarios, such as application servers, peer application nodes, sensor networks, and dedicated bundle routers. The document is produced by the IRTF's Delay Tolerant Networking Research Group (DTNRG) and represents the consensus of active contributors. It is intended for experimental use and does not specify an Internet standard. Readers are advised to exercise caution in evaluating its value for implementation and deployment.