After stumbling upon this buffering issue myself on an iPhone after a recent in-place upgrade from Server 2016 to 2019, I did some digging. Looking at a packet capture in Wireshark of a buffering stream, I noticed frequent TCP Retransmissions during that time and the RTO was 20ms.
After some research, I came across the Get-NetTCPSetting PowerShell command and found a very familiar number there. (Hint: It’s 20)
PS C:\WINDOWS\system32> Get-NetTCPSetting | ft -AutoSize
SettingName CongestionProvider MinRto(ms) InitialRto(ms) CwndRestart DelayedAckTimeout DelayedAckFrequency AutoTuningLevelEffective
----------- ------------------ ---------- -------------- ----------- ----------------- ------------------- ------------------------
Automatic
InternetCustom CUBIC 300 3000 False 40 2 Local
DatacenterCustom CUBIC 20 3000 False 10 2 Local
Compat NewReno 300 3000 False 200 2 Local
Datacenter CUBIC 20 3000 False 10 2 Local
Internet CUBIC 300 3000 False 40 2 Local
From what I understand, by default, Windows classifies TCP connections automatically based on the initial RTT of the TCP handshake. You can read more about this at https://techcommunity.microsoft.com/t5/networking-blog/tcp-templates-for-windows-server-2019-8211-how-to-tune-your/ba-p/339795. For an iPhone and server on the same LAN, connections will typically follow the Datacenter setting. You can see this with the below command, substituting your Plex server port and client IP:
PS C:\WINDOWS\system32> Get-NetTCPConnection -LocalPort 32400 -RemoteAddress 192.168.0.150
LocalAddress LocalPort RemoteAddress RemotePort State AppliedSetting OwningProcess
------------ --------- ------------- ---------- ----- -------------- -------------
192.168.0.149 32400 192.168.0.150 59518 Established Datacenter 6140
192.168.0.149 32400 192.168.0.150 50007 Established Datacenter 6140
192.168.0.149 32400 192.168.0.150 49926 Established Datacenter 6140
Now that familiar number of 20 appears under the Datacenter config as MinRto which is the minimum Retransmission timeout. So I tried increasing it to the max value of 300 using:
PS C:\WINDOWS\system32> Set-NetTCPSetting -SettingName "DatacenterCustom" -MinRtoMs 300
PS C:\WINDOWS\system32> Set-NetTCPSetting -SettingName "Datacenter" -MinRtoMs 300
After making this change and restarting my Plex client app on the iPhone, I no longer came across buffering nor TCP retransmissions after a few minutes of playback. Perhaps there are other parameters that can be tweaked, but this seems like a simple fix. Let me know how it goes for you.
PS, if you want to fully reset your TCP settings, you can run netsh int tcp reset.
