LWM2M + DTLS + NBIoT proper implementation

[fastfox]

Hi,

I am trying to implement energy efficient(batt powered) application for nrf9160, using UDP/DTLS/LWM2M as comm stack. Got "hello world" working, but now I am really confused about many details. Hope you can help me.

Question 1: Operator NAT/firewall rules and NRF9160 DTLS implementation

It looks like operator networks loose UDP port rules about after 1 minute of silence. This essentially means that UDP port number changes between consecutive packets sent from device to server if communications happens for example in every 10 minutes. This again means that DTLS connection would need to be resumed for it to work. It looks like resumption does not work in NRF or is not implemented at all? Effect is dramatic, you have to either ping server in <60s intervals or go through connection failure+re-register cycle every time you send anything.

Am I missing something here or how do people in general make things work? This ruins the whole idea of power save modes and LWM2M benefits. Is the only workaround to use MBED TLS stack instead?

Question 2: It looks like DTLS connection(to LWM2M bootstrap server) always ends to TLS ALERT sent by nrf91. Any idea why? Is this a feature? Other clients do not do that.

Question 3: Zephyr does not support QUEUE mode. If I understood the LWM2M spec correctly, QUEUE mode is exactly what low power NB-IoT application needs, but zephyr LWM2M stack does not support it. Do you have any idea how Nordic users tend to solve this? Implement queues as server side application on top of LWM2M? Use different LWM2M client?

Probably not the easiest questions and not all directly in Nordics domain, but I hope that you have some ideas how to make actual production quality app using NRF91. Thanks.

[fastfox]

Hi, I already tried to take modem traces, but failed to figure out exactly how to do that in my custom PCB. I'll give it another try or rewrite the SW to evaluation board.

Didrik Rokhaug said:

Are you sure that you have configured the client and the server to use TLS?

Where the data encrypted?

Or, could it be that you simply missed the client hello?

Yes, I  am sure. But it is DTSL to be exact.

Yes, data is encrypted

Possible of course, but very unlikely since I have tested this tens of times. I'll attach two pcaps here. From there(90s) you can see that packet #37 at time 102 is application data. This is the registration update. With a working client(leshan_client) the same registration update starts with client hello + resumptionip_port_changes_90s.pcapngip_port_changes_leshan_client.pcapng

[Didrik Rokhaug]

Do you know what the DTLS timeout on the server is?

The modem assumes that the connection is valid, and will not initiate a new DTLS session unless the server rejects the old one.

So what might have happened is that the modem tries to use the "old" session, which is still valid. That means that a new DTLS handshake is unnecessary, hence no client hello.

[fastfox]

That is a good question, I need to investigate that.

I thought that it is up to the client to decide that, but it might very well be that it is somehow negotiated in initial handshake. Is there any default values for this timeout in modem or can it be adjusted somehow?

I am not even sure what is the term to look for here. I think RFCs are talking about "idle timeout" and "session timeout" both meaning that session is no longer valid. But what we need is "do resumption timeout", which is network depended and always smaller than "idle timeout"

[jp]

Hi Didrik, how can the server reject the DTLS session when the communication is coming from a different IP address, and it doesn't recognise the packet?

Is there some way for the nRF9160 to invalidate the DTLS session?

Thanks,
John

[fastfox]

I've tried to read all the related RFCs bacwards and fowards, but can't really find any detail spec/timer/something about "When should client do session resumption" There are some hints e.g. in https://tools.ietf.org/html/rfc7925

For the case where the IP address and port number
changes, it is necessary to recreate the record layer using
session resumption.

and LWM2M spec itself http://openmobilealliance.org/release/LightweightM2M/V1_0_2-20180209-A/OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf

Note: During
the time the LwM2M Client has been sleeping the IP address assigned to it may have been released and / or existing
NAT bindings may have been released. If this is the case, then the client needs to re-run the DTLS handshake with
the LwM2M Server since an IP address and/or port number change will destroy the existing security context. For
performance and efficiency reasons it is RECOMMENDED to utilize the DTLS session resumption.

So my conclusion is that it is CLIENT side implementation detail about when to do resumption. And since client can't really know when IP address changes, I would assume that NRF91 has some internal logic to start the resumption. Whether it is a hardcoded timeout, configurable timeout or something else, I hope that Didrik Rokhaug can shed some more light here