[Live-devel] Unable to play RTP over TCP when RTT is large

[Arslan, Engin]

Hi,

I’m doing local experiments (1Gbps connection) with two servers: OnDemandServer and openRTSPClient. I want to test 4K video streaming (~15Mbps) when RTT is large. So I inject delay via Linux’s traffic controller (aka tc).
When an RTT that is larger certain value (I tested with 100ms mostly), RTP over TCP gets very slow and stops after few seconds. I know TCP slow start has impact on it but I don’t get it why TCP cannot catch after couple RTTs. I increased TCP buffer size to a large values (~10MB) on sending and receiving ends so it should not be buffer size issue. I checked tcpdump outputs and realized that RTSP server does not increase cwnd after certain value even sending/receiving congestion windows were not full and ACKs are received on time. So I’d appreciate if you tell me whether or not there is a tunable knob to handle this issue?


Thanks
Engin

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