[unspecified]

> FFF #176 - Belts optimization

The last bit of this post mentions UDP packets not making it through ISPs if their checksum is 0x0000 or 0xFFFF...what is THAT all about? This would have been in February of 2017.

> If you remember from the previous FFF, our map downloader was having some extremely rare problems with some mysterious packets that would always get filtered over the Internet. We already had a fix for it, but I was curious what was going on. Thanks to the investigative power of the Factorio community, we found out that all those mysterious packets, before NAT, had a checksum of 0xFFFF. Admalledd from the forum sent some hand-crafted packets through his Internet connection and surprise, all packets would go through, except those with a checksum of 0xFFFF or 0x0000. At this point I would just assume this ISP(and some other few ISPs around the world) have some faulty hardware or software that do not handle these special cases of UDP checksums.

[zamadatix]

If the checksum comes out as all 0's you're supposed to make it all 1's (0 in a UDP checksum means no checksum) so really it's related to that singular value and, if you've ever seen the quality of some of the dirt cheap hardware with ancient firmware some carriers use, it's really not that surprising for it to be bugged as described as they really don't care about dropping the occasional UDP packet when it's probably dropping more than that of general traffic due to how cheap and overrun it is anyways.