use constant IPv4 => 0x0800; # Regular TCP/IP
use constant IPv6 => 0x86dd; # New and improved!
use constant TCP  => 0x06;   # As opposed to UDP or ICMP
sub filter {
  my $type = unpack 'x12 n1', $_;
  my $proto;
  if ( $type == IPv6 ) {
    $proto = unpack 'x20 C1', $_;
  elsif ( $type == IPv4 ) {
    $proto = unpack 'x23 C1', $_;
  else {
    return;
  }
  return 1 if $proto == TCP;
  return;
}