This document specifies a Return Routability Check (RRC) subprotocol for use in the context of the Connection ID (CID) construct for the Datagram Transport Layer Security (DTLS) protocol versions 1.2 and 1.3. Implementations offering the CID functionality described in RFCs 9146 and 9147 are encouraged to also provide the RRC functionality described in this document. For this reason, this document updates RFCs 9146 and 9147.