Hey Meaton, I didn't make it clear, the size we want to control is "client TCP window size", rather than the "server TCP window size" or "H2". We used libCurl to test and get an initial 64KB client TCP window size which is much bigger than "BackgroundDownloadTask".

Hey Eskimo, The server is Akamai CDN. We think we find the root reason, looks like the TCP window size for the request is just 2KB, it's not able to hold the data from server, then the error occurs. The small window size introduces two problems for us: 1: The error 2: The download speed is not ideal ( We captured the net traffic, looks like the window size increases very slow, 2KB -> 4KB -> 6KB, we get maximum size 400KB. But for libCurl, it provides an initial 64KB window size and bump to 128KB immediately, the max size reaches to 1700KB ). Are we able to adjust the TCP windows for background download task? I found you've answered the question 5 years ago, https://developer.apple.com/forums/thread/93736 It was not able to be controlled by developer, but not sure whether it's been enhanced right now. Thanks