while (1) {
    my $hdr;
    my $len = read $handle, $hdr, $PKT_HDR_LEN;
    die "read failed: $!\n" if not defined $len;
    return undef if $len == 0;
    $TotalPkts++;
    my @vals = unpack "LLLL", $hdr;
    $pkt{secs} = $vals[0];
    $pkt{usecs} = $vals[1];
    $pkt{stamp} = ($vals[0] * 1.0) + ($vals[1] / 1000000);
    $pkt{len} = $vals[3];
    $len = read $handle, $data, $vals[2];
    die "read failed: $!\n" if not defined $len;
    die "file truncated: $!\n" if $len < $vals[2];
    my $ipdata = eth_strip($data);
    $ip = NetPacket::IP->decode($ipdata);
    if ($$ip{proto} == 47) {
      $is_ce = ($$ip{tos} & 3 == 3);
      $ipdata = NetPacket::IP::strip($ipdata);
      $ipdata = substr $ipdata, 4;
      my $gre = NetPacket::IP->decode($ipdata);
      next if $$gre{src_ip} ne $ip1 && $$gre{src_ip} ne $ip2;
      $ip = $gre;
    }