Roadwarrior carol proposes to gateway moon the ESP cipher suite AES_CBC_128 / HMAC_SHA2_256_96 which uses 96 bit instead of the standard 128 bit truncation, allowing compatibility with Linux kernels older than 2.6.33 by defining esp=aes128-sha256_96-curve25519! in ipsec.conf. A ping from carol to alice successfully checks the established tunnel.