This field specifies the data encryption scheme for en(de)crypting PPP encapsulated multi-protocol datagrams. Setting to mppe-40(2), mppe-128(3) or mppe-56(7) the Microsoft Point to Point Encryption Protocol (MPPE) will be enabled, using a 40 bit, 128 bit respectively 56 bit session key for initializing encryption tables. Setting to mppev2-40(8), mppev2-56(9) or mppev2-128(10) the old-style Microsoft Point to Point Encryption Protocol (MPPE) 'stateless mode' will be enabled, using a 40 bit, 56 bit respectively 128 bit session key. Setting to mppev2-40-rfc3078(12), mppev2-56-rfc3078(13) or mppev2-128-rfc3078(14) the Microsoft Point to Point Encryption Protocol (MPPE) 'stateless mode' in conformance with RFC 3078 will be enabled, using a 40 bit, 56 bit respectively 128 bit session key. Setting to mppev1-40-fixed(15), mppev1-56-fixed(16) or mppev1=128-fixed(17) only the 'old' MPPE V1 (stateful) mode will be negotiated and used. Setting to mppev1-128-ms-compat(18) 'old' MPPE V1 (stateful) 128 bit mode negotiation will be enforced and MS-CHAP V1 NT password hashing non-conform to RFC 3079 but compatible with Microsoft and Cisco products will be used. Setting to mppev2-128-ms-compat(19) 'old' MPPE V2 (stateless) 128 bit mode will be offered first and MS-CHAP V1 NT hash non-conform to RFC 3079 but compatible with Microsoft and Cisco products will be used.