n to the 4G Mobile Network Security parameters index (SPI): this field, coded on four bytes, contains a value pertaining to the previously negotiated security association. Sequence Number: this field, coded on four bytes, contains a value increased by one unit for each IPv4 or IPv6 packet transmitted. This field enables protection against replay. This field has a value of 1 for the first packet transmitted. When the counter reaches the maximum value, a new security association must be negotiated in order to avoid the start of a new cycle. An extended sequence number (ESN), coded on eight bytes, constitutes an option, making it possible for the lifetime of the security association to be prolonged. In order to preserve the structure of the extension, the 32 least significant bits are transmitted in the sequence number field. However, the seal is calculated on all 64 bits. Integrity check value (ICV): this field is coded on a multiple of four bytes and contains the seal of the data, ensuring authentication and integrity checking. 7.1.1.2. ESP extension The presence of the ESP extension (Figure 7.2) is indicated by the Next Header (in IPv6) or Protocol (in IPv4) field of the previous header, with a value of 50. 01234567890123456789012345678901 Security Parameters Index (SPI) Sequence Number Payload Data (variable) Padding (0-255 bytes) Pad Length Next Header Integrity Check Value- - ICV (variable) Figure 7.2. ESP extension format SWu Tunnel Establishment The ESP extension contains the same fields as the AH extension. It starts with the SPI and Sequence Number fields. After these fields come the encapsulated data, which may contain synchronization data (initialization vector) of the encryptor. Following the encapsulated data, the extension ends with the following fields: Padding, Pad, Length, Next Header and optionally ICV (authentication, data). The Padding field is necessary when block encryption is used, and the block must be of a certain size, to align the packet size with a multiple of four bytes. 7.1.1.3. Transp