pub fn build_pkt<'a>(collector: &Collector, packet: &'a mut [u8]) -> TcpPacket<'a> {
    const IPV4_HEADER_LEN: usize = 20;
    {
        // construct IP header
        let mut ip_header = MutableIpv4Packet::new(packet).unwrap();
        ip_header.set_version(4);
        ip_header.set_header_length(5);
        // Miss: type of service
        // Miss: total length
        ip_header.set_identification(random::<u16>());
        ip_header.set_flags(Ipv4Flags::DontFragment);
        // Miss: fragment offset
        ip_header.set_ttl(128);
        ip_header.set_next_level_protocol(Tcp);
        ip_header.set_source(collector.src_ip);
        ip_header.set_destination(collector.dst_ip);
        let checksum = checksum(&ip_header.to_immutable());
        ip_header.set_checksum(checksum);
    }
    {
        // construct TCP header
        let mut tcp_header =
            MutableTcpPacket::new(&mut packet[IPV4_HEADER_LEN..]).unwrap();
        tcp_header.set_source(collector.src_port);
        tcp_header.set_destination(collector.dst_port);
        tcp_header.set_sequence(random::<u32>());
        // Miss: ACK
        tcp_header.set_data_offset(5);
        // Miss: Reserved
        tcp_header.set_flags(TcpFlags::SYN);
        tcp_header.set_window(29200);
        tcp_header.set_urgent_ptr(0);
        let checksum = ipv4_checksum(
            &tcp_header.to_immutable(),
            &collector.src_ip,
            &collector.dst_ip,
        );
        tcp_header.set_checksum(checksum);
    }
    TcpPacket::new(&'a packet).unwrap()
}