The TTLSHeader control block extends the SIOCTTLSCTL ioctl. You can use Get requests to obtain additional information about the AT-TLS connection. You can use Set requests to change the AT-TLS behavior for the connection. TTLS Version 2 is required to use the TTLSHeader control block. The TTLSHeader control block includes a fixed section and a variable length self-defining section that contains Set and Get requests followed by the set and get data area. The set data area is not overlaid with the Get request data when the SIOCTTLSCTL returns.