s the block and processes the next block. SWu Tunnel Establishment 7.1.2.2.1. SA block The SA block is used for the negotiation of the parameters of IKE and ESP/AH security association. An SA block can contain several proposals (P) ranked in order of preference. Each proposal defines a protocol (IKE, ESP or AH) and the value of the SPI. Each proposal includes several transformations (T), and each transformation includes one or more attributes (A). The transformation T involves the following operations: - the encryption algorithm ENCR: this operation is used for negotiation pertaining to the IKE and ESP; - the pseudo-random function PRF: this operation is used for negotiation pertaining to the IKE; - the integrity algorithm INTEG: this operation is used for negotiation pertaining to IKE, AH and (optionally) ESP; - the Diffie-Hellman (D-H) group: this operation is used for negotiation pertaining to IKE and (optionally) AH and ESP; - the extended sequence number ESN: this operation is used for negotiation pertaining to the AH and ESP. The attribute A specifies the length of the encryption algorithm key defined in the ENCR transformation. The other transformations, PRF, INTEG, D-H and ESN have no attributes. 7.1.2.2.2. KE block The key exchange (KE) block contains the public Diffie-Hellman value, enabling each end point (initiator and responder) to construct a shared secret. The block also mentions the D-H group defined in the SA block. 7.1.2.2.3. IDi and IDr blocks Identification initiator (IDi) and identification responder (IDr) blocks contain an identification of the initiator of the IKE message and the responder. This identification is based on an IPv4 or IPv6 address, a name, a messaging address or a group of bytes. Wi-Fi Integration to the 4G Mobile Network 7.1.2.2.4. CERT block The certificate (CERT) block provides a means of transporting a certificate or information pertaining to authentication. 7.1.2.2.5. CERTREQ block The certificate request (CERTREQ) block is a request pertaining to a certificate. It is us