RFC 5050 defines the Bundle Protocol (BP), the wire format for Delay- and Disruption-Tolerant Networking (DTN). Where RFC 4838 laid out the DTN architecture, RFC 5050 is the actual protocol you implement — the one that has flown on the International Space Station, Mars orbiters, and a handful of undersea and Arctic experiments.

The problem. TCP/IP quietly assumes a lot: end-to-end paths exist right now, round-trip times are bounded (seconds, not hours), packet loss is a signal rather than the default, and both endpoints are simultaneously reachable. None of those hold on an interplanetary link, a submarine that surfaces twice a day, a sensor network that duty-cycles to save battery, or a village connected by a motorcycle carrying a USB stick. Retransmission timers that expect milliseconds are catastrophic when the RTT is 40 minutes to Mars and back.

The design. BP is a store-and-forward overlay. Applications hand it self-contained bundles — variable-sized messages with source, destination, lifetime, and payload — and BP nodes hop them toward the destination whenever a suitable contact becomes available, potentially holding bundles on disk for hours or days in between. Key decisions:

• Custody transfer. A node can accept "custody" of a bundle, promising to keep retrying until it either forwards custody onward or the bundle expires. Reliability is moved from end-to-end (impractical over 40-minute RTT) to hop-by-hop between willing custodians.
• Late binding via URIs. Endpoints are named with Endpoint IDs (URIs like dtn://mars-rover-3/telemetry), and the mapping to a routable address can happen anywhere along the path — critical when the destination may not have existed when the bundle was sent.
• Self-Delimiting Numeric Values (SDNVs). A variable-length integer encoding (later swapped for CBOR in BPv7) that keeps headers compact for tiny sensor bundles but grows for huge scientific payloads.
• Convergence layers. BP doesn't define how bytes cross a link — it rides on top of TCP, UDP, Bluetooth, LTP (Licklider Transmission Protocol, designed for one-way space links), or literally sneakernet. Each is a "convergence layer adapter."
• Fragmentation, both proactive and reactive. If a contact window ends mid-transfer, the receiver keeps what it got and only the remainder is forwarded.

The backstory. BP grew out of Vint Cerf's InterPlaNetary Internet work at JPL in the late 1990s — the same Vint Cerf who co-authored TCP. He explicitly wanted to design the network layer for a solar-system-scale internet before humans needed it, so the standards wouldn't be improvised under deadline pressure. Scott Burleigh (JPL) built the reference implementation, ION, which now runs on spacecraft. NASA demonstrated BP over the "Deep Impact Networking" experiment in 2008, bouncing bundles off a spacecraft 20 million miles away.

Why it still matters. BPv7 (RFC 9171, 2022) obsoletes 5050 but preserves the architecture — and the questions BP was designed to answer keep resurfacing in terrestrial contexts: LoRa mesh networks, disaster-response comms when cell towers are down, Arctic and maritime IoT, and increasingly, low-Earth-orbit constellations where satellites need to cache traffic for a ground station that isn't overhead yet. Any time your network's assumption of "always on, low latency" breaks, BP's design decisions — custody, late binding, convergence layers — are the vocabulary the field uses to reason about it.