ELVIS-PLUS This document specifies the use of variable-length output Pseudo- Random Functions (PRFs) in the Internet Key Exchange Protocol Version 2 (IKEv2). Current IKEv2 specification relies on traditional PRFs with fixed output length for key derivation and uses iterative application of a PRF (called "prf+") in cases when longer output is required. Appearance of PRFs that can output as much bits as requested allows to streamline the key derivation functions of IKEv2. This document updates RFCs 5723, 6617, 6631, 7296, 8784, 9370, 9838, 9867 for the cases when variable-length output Pseudo-Random Functions are used in IKEv2 and its extensions.