This document describes version 6 of the Delay Tolerant Networking (DTN) "bundle" protocol. It defines the end-to-end protocol, block formats, and abstract service description for message exchange in DTN. The protocol is experimental and produced within the IRTF's DTNRG. It is not an Internet standard and is not reviewed for security, congestion control, or protocol interaction. The protocol sits at the application layer of multiple internets, forming a store-and-forward overlay network. Key capabilities include custody-based retransmission, handling intermittent connectivity, and using scheduled connectivity. The protocol defines bundle formats, processing, and services for bundle nodes. It includes definitions for bundle, payload, fragment, endpoint, and other terms. The protocol specifies the format of bundles, including primary and payload blocks, and describes how bundles are processed, transmitted, and delivered. It also defines administrative records, custody transfer, and other services. The protocol includes details on bundle processing, including transmission, dispatching, forwarding, expiration, and delivery. It defines the structure of bundle blocks, including SDNV encoding, processing control flags, and endpoint IDs. The protocol also outlines the services required of the convergence layer and includes security considerations, IANA considerations, and references. The document provides a detailed specification of the bundle protocol for DTN.This document describes version 6 of the Delay Tolerant Networking (DTN) "bundle" protocol. It defines the end-to-end protocol, block formats, and abstract service description for message exchange in DTN. The protocol is experimental and produced within the IRTF's DTNRG. It is not an Internet standard and is not reviewed for security, congestion control, or protocol interaction. The protocol sits at the application layer of multiple internets, forming a store-and-forward overlay network. Key capabilities include custody-based retransmission, handling intermittent connectivity, and using scheduled connectivity. The protocol defines bundle formats, processing, and services for bundle nodes. It includes definitions for bundle, payload, fragment, endpoint, and other terms. The protocol specifies the format of bundles, including primary and payload blocks, and describes how bundles are processed, transmitted, and delivered. It also defines administrative records, custody transfer, and other services. The protocol includes details on bundle processing, including transmission, dispatching, forwarding, expiration, and delivery. It defines the structure of bundle blocks, including SDNV encoding, processing control flags, and endpoint IDs. The protocol also outlines the services required of the convergence layer and includes security considerations, IANA considerations, and references. The document provides a detailed specification of the bundle protocol for DTN.