Encrypted ESP Echo Protocol

This document defines the Encrypted ESP Echo Function, a mechanism to assess the reachability of IP Security (IPsec) network paths using Encapsulating Security Payload (ESP) packets. It detects end-to-end path status by exchanging only encrypted ESP packets between IPsec peers. The Encrypted Echo message can either use existing congestion control payloads from RFC9347 or a new message format defined here, with an option to specify a preferred return path when there is more than one pair of IPsec SAs between the same set of IPsec peers. A peer can announce support using a new IKEv2 Status Notification ENCRYPTED_PING_SUPPORTED.