Function build_tcp_ipv4 

fn build_tcp_ipv4(
    src_ip: [u8; 4],
    src_port: u16,
    dst_ip: [u8; 4],
    dst_port: u16,
    seq: u32,
    ack: u32,
    flags: u8,
    payload: &[u8],
    out: &mut [u8],
) -> Result<usize, &'static str>