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eclipse-wakaama / wakaama

Eclipse Wakaama is a C implementation of the Open Mobile Alliance's LightWeight M2M protocol (LWM2M).
BSD 3-Clause "New" or "Revised" License
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Client registration update timeout is broken for lifetime > 93.0s #613

Open mlasch opened 3 years ago

mlasch commented 3 years ago

For a client lifetime value less than 93s a registration update is sent to the server every lifetime/2 as expected. Above a value of 93.0, a registration update is send every timeout=lifetime-93.0. The number 93.0 is results from a definition COAP_MAX_TRANSMIT_WAIT as a float value.

This can be reproduced by starting the client with lwm2mclient -t 100. After initial registration an update is sent every 7s.

The following section seems to cause the problem: https://github.com/eclipse/wakaama/blob/a1281ab4abb2199200b406545f7a169b23da143c/core/registration.c#L2071-L2081

sbernard31 commented 3 years ago

Not directly linked but in Leshan we are using EXCHANGE_LIFETIME instead of MAX_TRANSMIT_WAIT (rfc7252#section-4.8.2) I'm not sure what is the best choice. :thinking:

We also think that lifetime is not the best way to configure a kind of communication period. For more details, see :

Maybe this thoughts could interest you.

mlasch commented 3 years ago

@sbernard31 thanks for providing more context.

The time between registration updates is defined via the lifetime resource in the LWM2M Server IPSO object /1/x/1. There is no RangeEnumeration specified there, so I would guess the whole positive Integer range (up to 64bit signed) in seconds is valid here up to 9,223,372,036,854,775,807.

As far as I understand, there should be no limitations by any CoAP timing parameters since this is a new (confirmed) request issued by the client?

Maybe not directly linked to this issue but, shouldn't we use some time shorter than lifetime/2 between registration updates. That would make the session more robust, because then the second update will arrive at the server with a safety margin and not just when the client expires. Ok, I think I got what is happening here. Wakaama uses COAP_MAX_TRANSMIT_WAIT as a safety margin which is subtracted from the lifetime as I mentioned above. For values less than COAP_MAX_TRANSMIT_WAIT the registration update timeout it set to lifetime/2. This results in the following behavior:

sbertin-telular commented 3 years ago

I would suggest making the check 2 * COAP_MAX_TRANSMIT_WAIT < nextUpdate. That will make it increase up to 93s which will then be half the lifetime.

Edited: I shouldn't respond before coffee. I had the wrong result after the limit.