last-N-mode configuration question

Install & Config
1

Hi, Jitsi Team
Recently I notice the “Last-N-Mode” concept about Jitsi,
I am confused about that two different kinds of configuration file include similar configuration parameter:

  1. jitsi-meet configuration file: /etc/jitsi/meet/domain-config.js
    // Default value for the channel “last N” attribute. -1 for unlimited.
    channelLastN: -1,

  2. jvb configuration file: /etc/jitsi/videobridge/jvb.conf
    videobridge {
    …
    cc{
    …
    jvb-last-n = -1
    }

Would you please share the difference and relation about the usage of these parameters in these two files?

Thanks

2

The former configures the value the client (aka web app) will request, and the latter the one the backend (videobridge) will provide.

Someone may want to limit last N on the backend side to save some bandwidth for example.

3

Hi saghul, Thanks a lot for your kind information as always:)

There are three further questions about the backend parameters in jvb config file:

  1. If there is no cc.jvb-last-n parameter provided in config file, what is the default behavior of jvb?
  2. If I set the value of cc.jvb-last-n, I wonder whether I also need to add the section of “load-management” in jvb config file as well
  3. I notice there is another related parameter: “load-management.load-reducers.last-n.maximum-enforced-last-n-value” in jvb config file, I wonder the relationship between them.

Btw, If anyone can share how to configure these last-N-mode related parameters with an example will be great. Such as, if the value of cc.jvb-last-n set to 10 and meanwhile, set load-management.load-reducers.last-n.maximum-enforced-last-n-value to 20, what will happen to jvb

4
5

Hi emrah,
thank you for your response.
Actually, I already went through the reference you mentioned here and still have my above three questions.
I am not sure whether it is what i think as below:
Q1 - if there is no cc.jvb-last-n parameter in jvb conf, the default behavior is same as the value of “-1”
Q2 - Once I set the value of cc.jvb-last-n, I also need add “load-management” section in jvb config as the reference file
Q3 - After went through the reference file, I still have no idea about the question: if the value of cc.jvb-last-n set to 10 and meanwhile, set load-management.load-reducers.last-n.maximum-enforced-last-n-value to 20, what will happen to jvb

Thanks and wait for your further guide on this.

6

AFAIK the default values come from reference.conf

1 Like
7

Correct, it will come from the reference file.

Not really. The main config file is merged with the reference one.

I don’t know that one. @Boris_Grozev can help perhaps?

8

saghul and emrah, thanks a lot for your confirmation about Q1 and Q2.

Hi, @Boris_Grozev , would you please help to share the light on my Q3
Q3 - After went through the reference file, I still have no idea about the question: if the value of cc.jvb-last-n set to 10 and meanwhile, set load-management.load-reducers.last-n.maximum-enforced-last-n-value to 20, what will happen to jvb

9

Sorry for the late response, I just see this now.

To clarify the semantics a bit: the effective maximum number of streams that the bridge will forward to a receiver is the lowest of the following values:

  • The value requesed from the client (what’s configured in config.js plus any change the client decides to make, e.g. use last-n=0 for audio-only mode)
  • The static value configured in cc.jvb-last-n (and also ssrc-limit.video)
  • The value based on conference size configured in load-management.conference-last-n-limits
  • The value computed by the load manager, if load-management.reducer-enabled is true

The value -1 is interpreted as “unlimited”.

The load reducer is optional. If enabled, it monitors the global load of the JVB and lowers the last-n limit in order to reduce the load.

Q3: if cc.jvb-last-n is 10, the bridge will never forward more than 10 streams. Since maximum-enforced-last-n-value is more than that, it will never have any effect.

2 Likes
10

@Boris_Grozev ,
Thanks a lot for your clear explanation on last-N mode. It is very helpful.

Just double confirm below two cases:

  1. If the value requested from client is lowest value among above parameters: but one client A is 5, another client B is 3, I wonder whether the JVB will forward the maximum number of streams to client A is 5 and forward the maximum number of streams to client B is 3.

  2. If I do not set the cc.jvb-last-n in JVB config file, as my understanding, the default value of cc.jvb-last-n is “-1”, I wonder whether “-1” is considered as the lowest value.

11

That value is per client. So if will forward 5 to A and 3 to B.

-1 means unlimited.

2 Likes
12

Thanks a lot for your kind confirmation, I am clear now, thanks all :slight_smile:

1 Like