In some regions (including New York and London), a subset of Steam's content servers will fail to allow a connection via IPv6 about 50% of the time. The practical result is that when starting a download, Steam will sit at the "Starting Download" step for a long time, possibly over a minute if you're unlucky. This is completely unnecessary and only happens because one or more content servers has failed to accept the Steam client's IPv6 connection, and the client waits until the connection attempt times out before trying another server.

Note that this seems to be an IPv6-only problem, if your ISP or system only uses IPv4 then it should be unaffected.

This can be avoided by setting another region's content servers in Steam's Download settings. For example if you're in the UK and you change your download region to "France - Paris" you should never get stuck at "Starting Download" again, because the Paris servers seem unaffected by this problem.


Taking London as an example, there are 14 Steam content servers currently active. They are named cacheX-lhr1.steamcontent.com, where X is a number in the ranges 1-2 or 5-16. Of those 14 servers, 6 servers will fail to accept an IPv6 connection about 50% of the time, while the other 8 seem to work fine and accept an IPv6 connection 100% of the time.

(At the time of writing the "bad" servers are 1, 5, 6, 10, 13 and 16 and the "good" ones are 2, 7, 8, 9, 11, 12, 14 and 15. This may change over time.)

When Steam starts a download, it will choose one of those servers to retrieve the manifest for each depot that needs to be downloaded. If it chooses a "bad" server then there's 50% change that the IPv6 connection won't be accepted, but the Steam client won't try another server until the connection attempt times out, which takes about 20 seconds. If it hits multiple bad servers then it can take over a minute before it retrieves all of the required depot manifests and can actually start the content download.

You can see this happening in the Steam content log, which is usually located at: C:\Program Files (x86)\Steam\logs\content_log.txt

When a download gets stuck at "Starting Download" you will see "Received 0 (Invalid) HTTP response for depot..." which is the server failing to accept the connection and then the client eventually timing out before trying another server.

It's easy to prove that this is the problem by repeatedly attempting a socket connection to one of the content servers and seeing how many attempt succeed. Here's a Python script that compares the results for London servers 1 (bad) and 2 (good), over 100 IPv6 connection attempts.


Example output:


And this is an example comparing good/bad New York servers:


And this is testing every Paris server, showing they all work as they should, with no connection attempts failing:


I have reproduced the issue from networks other than my own home network (including a Google Cloud VM) so I know it's a problem with my own internet connection, it's a Steam server issue. Not sure if this will make it to anyone at Valve who can do anything about it but it's worth a try!