IMT Atlantique A packet can cross an IPv6 segment whose properties - long delay, scheduled contacts, a strict size budget - defeat the feedback an endpoint normally relies on. This document defines ICMPv6-SEC (SECure Segment Treatment), an ICMPv6 message that a segment boundary sends to a packet's source when the packet is not prepared for such a segment. The message carries a COSE-signed Segment Descriptor: a compact CBOR object stating which segment is meant, how long the statement is valid, and what treatment future packets require. A receiver applies a descriptor only after verifying the signature, the validity window, and that the signer is authorized for the segment.