JPL, Calif. Inst. Of Technology This document describes Bundle-in-Bundle Encapsulation (BIBE), a Delay-Tolerant Networking (DTN) Bundle Protocol (BP) "convergence layer" protocol that tunnels BP "bundles" through encapsulating bundles. The services provided by the BIBE convergence-layer protocol adapter encapsulate an outbound BP "bundle" in a BIBE convergence-layer protocol data unit for transmission as the payload of a bundle. Security measures applied to the encapsulating bundle may augment those applied to the encapsulated bundle. The protocol includes a mechanism for recovery from loss of an encapsulating bundle, called "custody transfer". This mechanism is adapted from the custody transfer procedures described in the experimental Bundle Protocol specification developed by the Delay-Tolerant Networking Research Group of the Internet Research Task Force and documented in RFC 5050.