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Re: [mosquitto-dev] keep-alive in Mosquitto and in MQTT v5/v3.1.1
To my knowledge... (after reading the docs again)...

- Normally, Mosquitto should accept a connection request from a client asking for a zero keep alive. In that case, it won't use a keep alive for _that_ client. That is compliant with the spec.

- Mosquitto can also be configured with "max_keepalive=0" which will effectively turn off keep alive for all clients. I believe any client connection request will be rejected if they specify a non-zero keep alive. So all clients have to connect with zero. For v5, I believe this is compliant, because the broker is allowed to specify the "accepted" keep alive in the CONNACK. But this would be non-compliant for v3.

- Mosquitto's "keepalive_interval" configuration parameter does appear to be limited to >=5 sec, but this is only for when it is being configured to act as a _bridge_ to another broker; meaning when Mosquitto is acting as a _client_. So this is not what it will accept, but rather what it can request as a client. In this case, it has nothing to do with the spec, but rather what the author(s) feel would be appropriate behavior for a client.

Frank

On 9/25/23 12:16, Tobias Assarsson via mosquitto-dev wrote:
Hello!

Mosquitto should support a keep alive interval of 0, but, recently (I don't know the exact version) one has to configure it to allow a value of 0.


See `max_keepalive` parameter


On Mon, Sep 25, 2023 at 5:41 PM Lemburg, Martin via mosquitto-dev <mosquitto-dev@xxxxxxxxxxx> wrote:

Hi Frank,

 

Thanks for your “immediate” response!

 

Sorry, I was too unspecific in my question.

 

If I understood it correctly, a keep-alive value of zero, would switch off the broker-sided test for receiving PINGREQ packets to decide, if a connection is alive or should be closed.

 

To switch of the PINGREQ test for connection closure at the broker, the client needs to specify the keep-alive interval to be zero.

But the Mosquitto API allows only a keep-alive interval of five or more seconds.

 

I’m with you and I don’t think, that it mostly makes no sense to specify values lower then five, but zero is valid and as a semantic!

 

That’s why I’m asking.

 

Best regards

Martin

 

From: mosquitto-dev <mosquitto-dev-bounces@xxxxxxxxxxx> On Behalf Of Frank Pagliughi via mosquitto-dev
Sent: Monday, September 25, 2023 4:31 PM
To: mosquitto-dev@xxxxxxxxxxx
Cc: Frank Pagliughi <fpagliughi@xxxxxxxxxxxxxx>
Subject: Re: [mosquitto-dev] keep-alive in Mosquitto and in MQTT v5/v3.1.1

 

Keep in mind that the value of zero for Keep Alive is a special indicator to turn it off (i.e. don't use Keep Alive).

So don't think of the specs as saying you can "go down to zero". Technically, I guess, a 1-sec Keep Alive should be supported, but practically, this would induce a huge overhead on the broker. If you had ten thousand clients pinging at that rate, it would be hitting the broker with 10k msgs/sec just to maintain otherwise quiet connections.

So you will find that many clients won't let you set a Keep Alive of less than 5 or 10, and default to something like 30, 45, or 60.

Whether a _client_ is required to support all values 1-65535 is debatable. Technically, a _broker_ probably should, to be 100% compliant, but allowing really low values would let some clients put a large burden on the broker.

Frank

On 9/25/23 09:06, Lemburg, Martin via mosquitto-dev wrote:

Hello,

 

May be a naive question, but why I can only connect with Mosquitto to a broker with a keep-alive greater or equal to 5, while the protocols allow a zero keep-alive?

 

That’s all for the beginning.

 

Best regards

Martin



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