Warhammer: Vermintide

[PSA] Connectivity errors are not the game fault, it is UPC Poland ISP dropping IPv4 datagrams of certain size.

chivalry medieval warfare 01 1024x576 - [PSA] Connectivity errors are not the game fault, it is UPC Poland ISP dropping IPv4 datagrams of certain size.

tl;dr UPC Poland for customers with enabled IPv6 drops outgoing IPv4 datagrams with size of 445 Bytes. Which happens to be breaking Vermintide 2 login(?) function, preventing the game from staring.


I noticed there is a lot confusion and misinformation about the connectivity issues. No one identified the root cause of the issue, so I figured I will share what I've found some time ago, in completely different scenario and noticed recently that it affects Vermintide 2. Note that it is only applicable to UPC Poland customers. Any other issues are not related. I figured I will explain the current situation so the newcomers can find the accurate information about the problem.


Backstory: youdon'treallyneedtoreadthis

Since few months I noticed that sometimes webpages won't load or load not fully. I always blamed intermittent connection issues or server side issues. It was hard to catch because HTTP requests are changed constantly depending on cookies you have, cache state, browser user-agent and so on. But one day I found consistent steps to reproduce the issue, I knew it is not server issues yet resource wouldn't load. I initially blamed Firefox, because only in this browser I was able to reproduce. Fast-forward some investigations it turned out that my ISP (UPC Poland) drops all outgoing IPv4 datagrams with exactly 445 Bytes. I was quite surprised that such error is made by one of the biggest ISPs in Poland. After some more digging, I've found that the issue affects only users with IPv6 enabled and not all of them. Here is why. Some time after I posted a thread about the issue on official UPC forum I had "network grid modernization in my area", whatever they did, now the issue is gone for me. So they can certainly make it work. And few day after that I bought the game and my friend told me that he can't play because Vermintide 2 "doesn't work with UPC"… this was a long shot, but I checked network traffic and guess what… there is a TCP packet with 405B TLS payload (whole IPv4 datagram has 445B with headers). I asked him to run one command and yes he is still affected. While I am not, so the fix is possible, but probably need CMTS changes near user. Dunno really and it doesn't matter, it is UPC's job. In all this I found the timing little bit funny, because all those events: discovering the issue, fixing it (in fact they have not fixed it on my request), discovering that the game is affected are slightly not related, yet they happened close in time to each other. I hope you get what I mean :>




All outgoing IPv4 datagrams with exact size of 445 Bytes are dropped and never reach destination. It does not matter what is encapsulated in the datagram as long as it has 445 Bytes it will fail to reach destination. You can test if you are affected with the following command: ping -f -l 417 upc.pl, you can also change address to whatever you want, only size matters. To make it clear, it affect only outgoing datagrams, only IPv4 and only 445B. Still it is quite a big problem.



Some programs does not work if they relies on network communication with affected packet sizes. The most common issue is that some pages or resources will not load in internet browser, but you probably have not noticed it as reloading the page changes the HTTP request (cache-control header for example) therefore changing its size making it work. So it will look like it does not work "once in a while". This is actually how I found the issue in the first place. Vermintide 2 is one of the affected programs (fails with RECV_ERROR, 1123, GOT_NOTHING errors).


Possible solutions:

  • On the game side, adding 1B of padding to the payload to make the IPv4 datagram bigger would make it work and completely workaround (aka. hide) the issue. Providing IPv6 server would work too.
  • On the customers side, requesting UPC to disable IPv6 is also good workaround as the issue does not occur in this configuration.
  • On the UPC side, fixing their network hardware/software to properly forward datagrams of size 445B. They can do that, because like I said they did for me. But as users we need to put pressure on them so they recognize the issue. Likely they will just disable IPv6 for you regardless, unfortunately.


References: Polish UPC forum thread with the issue https://forum.upc.eu/t5/connectbox/wychodzace-datagramy-ipv4-o-rozmiarze-445-bajtow-sa-gubione/m-p/15046 I marked it as resolved, before I noticed that it still affects broader customer base. Fell free to reply if you are affected.

P.S The issue is quite simple and I fell I have written too many words about it.

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