Does initial-last-n actually works? about jitsi-videobridge HOT 10 CLOSED

Do you see any jvb log messages containing "initialLastN"?

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@bgrozev yes, it has

Setting initialLastN = 1

Also I try to disable sending ReceiverVideoConstraints message in lib-jitsi-meet. In this case I see the same behaivour. It seems that JVB always sends ForwardedSources message with all endpoints.

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That's strange and I can't reproduce it. Do you use SCTP or WS for the data channel?

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WebSockets, the screenshots above were taken from Chrome DevTools

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I think I see how this happens. The WS must have been established before the bridge received the initial-last-n from jicofo. In this case the bridge will send the initial forwarded sources message (with lastN=-1), then send the updated on (with lastN=1) once it receives initial-last-n.

I'm surprised it happens often, because the WS requires at least an extra RTT for the TCP handshake, while the initial-last-n is just a packet sent to jicofo which immediately updates the bridge via an existing connection.

For your use case, would it work if we add a config option to the bridge setting a default initial lastN value?

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@bgrozev yes, you are right. It's a race condition.

In my case I see the problem exist in 9 of 10 attempts. May be initial-last-n should be a parameter when jicofo allocate user session on JVB to prevent this race condition ?

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Jicofo doesn't have that information about a participant when it does the allocation. It only has the set of capabilities (i.e. just booleans) and presence. We want to avoid adding it in presence because it's broadcast to everyone.

Longer term using SCTP where the race doesn't exist might be the best option. That's why I'm looking for some easy workaround instead of a proper solution which would require more time that we can't invest. If you control your environment and want initial-last-n=1 for everyone I'm happy to add a configurable default in the bridge.

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@bgrozev I'd love to have initial-last-n as a configurable option of JVB

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It's a very simple change, so I'll assume it works. Re-open if necessary.

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@bgrozev yes, it works! Thanks you

PS: I think this condition appears when RTT between end user and bridge less than RTT between user and jicofo

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