Clarification of Behavior when a Bootstrap Server Account is replaced

[lunar-seal]

Is behavior of an LwM2M Client well defined by the LwM2M Core Specification if the LwM2M Bootstrap Server Account is replaced during Client Initiated Bootstrap?
I think the LwM2M Client should try to bootstrap with that account, but I didn't find any specification for that.
Also, there seems to be a distinction between initial bootstrap and subsequent bootstraps, as all bootstrap information is optional in that case. (6.1.3.3. Client Initiated Bootstrap)
So if someone wants to just change the LwM2M Bootstrap Server Account during initial bootstrap, they would technically need to also add an LwM2M Server Account?

[mkgillmore]

Request noted and added to the 12/1/2025 agenda to discuss

[sbernard31]

(I'm not OMA author)

@likeazir

Also, there seems to be a distinction between initial bootstrap and subsequent bootstraps, as all bootstrap information is optional in that case. (6.1.3.3. Client Initiated Bootstrap)

Yep during Factory bootstrap having a LwM2M Server Account seems optional where for Client Initiated Bootstrap it seems mandatory.

So if someone wants to just change the LwM2M Bootstrap Server Account during initial bootstrap, they would technically need to also add an LwM2M Server Account?

Yes, I also understand that...
So if you have a Bootstrap Server Account from Factory Bootstrap and you want that bootstrap server just redirect to another bootstrap server, then you need to replace the Bootstrap Server Account but also add a fake LW2M2M Server Account.
Then client will try to register, then failed (as it is fake server), then fall back to new bootstrap server.

That's a bit strange but this is my understanding. 🤷

I think the LwM2M Client should try to bootstrap with that account, but I didn't find any specification for that.

Not sure to see what you mean.

[sbernard31]

Some source about that ☝️

3. If the LwM2M Client has any LwM2M Server Object Instances from the previous steps, the LwM2M Client tries to register to the LwM2M Server(s) configured in the LwM2M Server Object Instance(s).

4. If the LwM2M Client fails to register to all the LwM2M Servers or the Client doesn't have any LwM2M Server Object Instances, the LwM2M Client performs the Client Initiated Bootstrap.

5. A Server Initiated Bootstrap attempt (e.g. for updating a LwM2M Server Account) remains possible, but only if the LwM2M Client retains the corresponding LwM2M Bootstrap-Server Account.

(source : LWM2M-v1.2.2@core§6.1.4. Bootstrap Sequence

And

The LwM2M Client MAY be configured to use one or more LwM2M Server Account(s).

The LwM2M Client MUST have at most one LwM2M Bootstrap-Server Account.

.... ....

(*) The LwM2M Client MUST have at least one LwM2M Server Account after completion of Bootstrap Sequence specified in Section 6.1.4. Bootstrap Sequence.

(source : LWM2M-v1.2.2@core§6.1.2. Bootstrap Information

[sbernard31]

Not directly linked but in this 👉 #522 issue we talk a bit about problem around changing/deleting bootstrap server.

[lunar-seal]

Wow, thank you for your detailed reply @sbernard31 !
I fully agree with your comments.

About the very unclear part I wrote:

I think the LwM2M Client should try to bootstrap with that account, but I didn't find any specification for that.

What I mean is
"My expectation was the LwM2M Client starts the bootstrap sequence after the Bootstrap Server has been replaced (or there's a way to force an lwm2m client to enter the bootstrap sequence with the bootstrap server)"
But I could not find anything in LwM2M that specifies any behavior of the LwM2M client after the Bootstrap Server has been replaced.

As I understand it now:

What I missed is the "Bootstrap on Registration Failure" Resource (default true) in an LwM2M Server Object.

If set to true, this indicates that the LwM2M Client should re-bootstrap when either registration is explicitly rejected by the LwM2M Server or registration is considered as failing as dictated by the other resource settings. If set to false, the LwM2M Client will continue with the registration attempts as dictated by the other resource settings.
So by making sure there is only a Fake Server on an LwM2M client, we can force an LwM2M Client to start the Bootstrap Sequence after it has failed to register to the Fake Server.

If there is already a successful Registration to a LwM2M Server, it could also be used for Server Initiated Bootstrap. But it is implementation defined whether the LwM2M Client keeps registrations active during the Bootstrap Sequence. 6.1.3.3. Client Initiated Bootstrap:

Before entering the "Client Initiated Bootstrap" mode, the LwM2M Client may be registered to LwM2M Servers. Whether or not the LwM2M Client keeps these registrations active during the Client Initiated Bootstrap is implementation dependent.
So this is not a reliable way to force an LwM2M Client to re-enter the Bootstrap Sequence.

It can of course also be implementation defined behavior that an LwM2M Client triggers the Bootstrap Sequence. (Source: note in 6.1.3.4. Server Initiated Bootstrap)

Did I miss anything?

[sbernard31]

it is implementation defined whether the LwM2M Client keeps registrations active during the Bootstrap Sequence

I would not count on this.
This could lead to interoperability issue and also I bet most client should stop communicate with their LWM2M Server during bootstrap... at least that seems safer to me .

---

If there is use case where a bootstrap server just want to redirect a a client to another bootstrap server, then maybe that 👇 constraint is too strict :

The LwM2M Client MUST have at least one LwM2M Server Account after completion of Bootstrap Sequence specified in Section 6.1.4. Bootstrap Sequence

Maybe better something like that 👇 ?

The LwM2M Client MUST have at least one LwM2M Server Account OR one LWM2M Bootstrap Server Account after completion of Bootstrap Sequence specified in Section 6.1.4. Bootstrap Sequence

[lunar-seal]

I think your modification is a good idea to adress the issue of redirecting an LwM2M client to another bootstrap server during initial bootstrap.

---

Just to clarify, do you agree that the behavior of an LwM2M client after the bootstrap server has been changed is not specified?
Also, I think it is not specified what happens if an LwM2M Client only has an LwM2M Bootstrap Server and no LwM2M Server.
(The client may restart the bootstrap sequence or not)

[sbernard31]

Just to clarify, do you agree that the behavior of an LwM2M client after the bootstrap server has been changed is not specified?

I think this is specified. 🤔
After a valid bootstrap session, client MUST have at least on register server.
So it tries to register and if it fails it generally goes to bootstrap following chart at LWM2M-v1.2.2@core§6.2.1.1. Bootstrap and LwM2M Server Registration Mechanisms

I'm not sure I get which part is missing for you ?

Also, I think it is not specified what happens if an LwM2M Client only has an LwM2M Bootstrap Server and no LwM2M Server. (The client may restart the bootstrap sequence or not)

I think it should restart the bootstrap sequence.

4. If the LwM2M Client fails to register to all the LwM2M Servers or the Client doesn't have any LwM2M Server Object Instances, the LwM2M Client performs the Client Initiated Bootstrap.

[lunar-seal]

Ah, thank you. I misunderstood something. I thought of the Bootstrap Sequence as a literal series of steps.

Your suggestion in #594 (comment) fully fixes my issue.

[sbernard31]

(Keep in mind I'm not member of OMA or one of LWM2M authors, so we will see what will be the answer of working group #594 (comment))

[svdwalt007]

Analysis: Post-Bootstrap-Finish Evaluation Logic Needs Explicit Definition

The spec should explicitly address this in §5.2.7 (Bootstrap-Finish). After receiving Bootstrap-Finish, the client evaluates its complete Security Object state. The expected behaviour:

1. If Server accounts exist (Security Object instances with Bootstrap-Server=false) → register with those servers.
2. If only a Bootstrap-Server account exists (no Server accounts provisioned) → the client MUST initiate Client-Initiated Bootstrap using the current BS credentials.
3. If the BS account was itself replaced → the new BS credentials apply immediately for any subsequent bootstrap.

For the initial bootstrap case: changing the BS account alone without adding a Server account is a valid (if unusual) provisioning step — it effectively redirects future bootstraps to a different Bootstrap-Server. The client should NOT attempt registration until a Server account is provisioned.

The distinction between initial and subsequent bootstraps matters here: during initial bootstrap, the Bootstrap-Server is expected to provision at least one LwM2M Server account (Security + Server Object instances). If it only replaces the BS account, the client has no registration target and must re-bootstrap with the new credentials.

Recommended spec fix: A note in §5.2.7 clarifying the post-Bootstrap-Finish evaluation logic — enumerate the three cases above explicitly.

---

This comment was automated using the new Best-LwM2M-Agentic-Skills that Sean van der Walt has developed (available for your use). Feedback on these skills' use would be highly appreciated.