Re: [netmod] system configuration sync mechanism

["maqiufang (A)" <[e-mail]>]

Hi, Kent,
Please see my reply inline.

From: Kent Watsen [mailto:[e-mail]]
Sent: Friday, July 16, 2021 2:55 AM
To: maqiufang (A) <[e-mail]>
Cc: [e-mail]
Subject: Re: [netmod] system configuration sync mechanism

Hi Qiufang,

            [snip]
The question is if the server implementation prunes dangling/unused objects when <intended> is applied, updating <operational>.  My assumption is that the server will discard any object that doesn’t actually impact the running configuration of the system (i.e., values are consumed by the underlying operating system, drivers, etc.).  Thusly, it is my opinion that only the referenced objects are applied.  Hence why, to answer your last question, I wrote that these configurations (manufacturer-defined objects) are not applied immediately but only after they are referenced.  Makes sense?
[Qiufang Ma] Yes, try to sum up our discussion about the categories of the system configuration:

·         Physical-resource-dependent--> whether this sort of system configuration exists in <system> dependents on if the physical resource is present(e.g., physical interface).

·         Physical-resource-independent-->which is provided by the device system

o   Further classification from the perspective of “applied” time(dependents on whether the system configuration impacts the running of the system)

§  Config that is applied immediately(e.g., the loopback, the predefined minimum length of password…)

§  Config that is applied only after being referenced by other configs(e.g, definitions for applications ftp/tftp…)

o   Further classification from the perspective of generation time

§  Config that is generated unconditionally at each boot time(e.g, loopback, predefined minimum length of password, ftp/tftp…)

§  Config that is generated conditionally during the device running(e.g., system-generated local-port and remote-port for a new established BGP connection)

Keep in mind that what is described above is just one aspect of what can be in <system>.  In addition to defining reference-able objects, <system> can also define/apply configuration immediately (e.g., the loopback interface).  That is, configuration not does not have to be referenced in order to become activated.
[Qiufang Ma] Noted.





Note that, <running> by itself would not pass validation, due to missing leafrefs.  Thankfully, NMDA never says that validation runs on <running>.  But once <running> and <system> have been merged, to become <intended>, the result does pass validation.

 [Qiufang Ma] The referenced instance must also exist for the data to be valid since the require-instance defaults to true if not present. Is this what you had in your mind? Yes, NMDA says that it is <intended> which is subject to validation. But I also notice that In section 5.1.3 of the NMDA:”<running> MUST always be a valid configuration data tree, as defined  in Section 8.1 of [RFC7950]. ” So my thought here is that <running> should also conform to the YANG model constraints and that’s to say, a referenced system-defined data item should also exists in <running>.

Therefore, if system configurations do not exist in <running>, they still need to be  configured in <running> manually in order for being referenced. In this case, the original purpose of predefining some system configurations for user convenience is lost. This is the reason why we would like to define some mechanism here to synchronize <system> into <running>.


I see in RFC 8342 "<running> MUST always be a valid configuration data tree, as defined in Section 8.1 of [RFC7950]”.

But the question remains if it is possible for the system is able to validate <running> without, e.g., expanding templates.  There may be a 'leafref' or ‘must’ expression somewhere that will fail because the evaluation occurs without expanding a template that supplies the missing parts.
[Qiufang Ma] On condition that <running> should be valid, the operators will need to retrieve from the <intended> or <operational> to get the template-expanded configurations and then create them in the <running>, right?
My feeling is that it loses the meaning of predefining and seems no differences between operator-defined configurations if operators have to create system configures in <running> before they use them. So I am beginning to think, if it’s possible to expand the system-defined template during the copying between <system> and <running>?

If this draft “updates” RFC 8342 (NMDA), then it can supply a clarifying statement about what it means that "<running> MUST always be a valid configuration data tree”.  Either that, or an Errata if it’s determined that the statement isn’t correct.

You make a good technical point, but I think that we should *want* to avoid having to copy <system> (or <operational>) configuration into <running> if we can avoid it.  Agreed?
[Qiufang Ma] tend to agree. Maybe we should try to avoid it, unless we have to.

FWIW, also in RFC 8342, Section 5.1.4.:


   <intended> is tightly coupled to <running>.  Whenever data is written

   to <running>, the server MUST also immediately update and validate

   <intended>.



   <intended> MAY also be updated independently of <running> if the

   effect of a configuration transformation changes, but <intended> MUST

   always be a valid configuration data tree, as defined in Section 8.1<https://datatracker.ietf.org/doc/html/rfc7950#section-8.1>

   of [RFC7950]<https://datatracker.ietf.org/doc/html/rfc7950#section-8.1>.







> I am wondering if these configuration will present in the <operational> (which contains all the configuration actually used by the device) before they’re referenced.



I think that it would depend in the specific server’s behavior, regarding if *unused* predefine objects are present in <operational>.  Certainly the unused objects would not have to be present in <operational>.  If I were implementing the server, the unused objects would NOT be present in <operational>.

[Qiufang Ma] Yes, if the predefined system configurations is unused, then I also tend to agree that they would not be present in <operational> but may also depend on the vendor implementation.


Yup, this is the same conclusion is in my response above.
[Qiufang Ma] :)




> It would be good if we could determine if there are any other "resource-independent” configuration categories here.

> [Qiufang Ma] Do you think there exists conditional system configuration (if the preceding configurations you mentioned above is not)? For example, if SSH is enabled on a device, SSH-related keys are automatically generated. Such configurations are generated at the moment when a special functionality is enabled.



I’m unsure what you mean in general by "conditional configuration”, but I can speak to your specific example. Though I must preface my comments that I imagine there are a number of ways servers might go about enabling `sshd`.  What follows is my personal view, forged by being around systems for awhile  ;)



In general:



         - `sshd` is NOT enabled by default.

         - `sshd` is enabled via a configuration knob.

         - the SSH host key is dynamically generated the first time `sshd` is enabled.

         - the SSH host key itself is in <operational> (not <running>)



This view is consistent with the first paragraph in Section 3 of the “keystore” draft (reproduced below):



         3.  Support for Built-in Keys



            In some implementations, a server may support built-in keys.  Built-

            in keys MAY be set during the manufacturing process or be dynamically

            generated the first time the server is booted or a particular service

            (e.g., SSH) is enabled.



As a closing thought, this model (which I stated upfront may not be universal) would have no presence-in or interaction-with <system>…though, perhaps, there may be some predefined values for what key-algorithms and/or key-lengths to use when generating the SSH host key...

[Qiufang Ma] By “conditional system configuration”, I was meaning some of the system configurations are not generated immediately after the device is powered on. Instead, they are generated when a specific condition is satisfied during the device running(e.g., a functionality is enabled due to some client configurations). I am not sure if it really exists, maybe not, just try to explore the possibilities of various resource-independent system configurations.:)

To this point I agree..as does RFC 8342 (NMDA), Section 5.3.3.:


   Sometimes, resources are controlled by the device and the

   corresponding system-controlled data appears in (and disappears from)

   <operational> dynamically.  If a system-controlled resource has

   matching configuration in <intended> when it appears, the system will

   try to apply the configuration; this causes the configuration to

   appear in <operational> eventually (if application of the

   configuration was successful).

[Qiufang Ma] See above, as I summarized as the system configurations that is generated conditionally during the device running.



Firstly, I again have to preface my comment that there are likely many ways that templating mechanisms can be defined.    But, in general, once a templating mechanism has been defined, then it stands to reason that templates could be defined either in <running> (by operators) or in <system> (by the manufacturer).  In one implementation I’m familiar with, the templates are objects that are referenced/parameterized by other parts of the configuration.  (Same as with the predefined objects discussion above.)



To answer your questions:



1) Yes, it is my opinion that *activated* templates in <system> will be expanded and present in <intended>.



2) I would never suggest that the system-defined templates are present in <running>, though they may be referenced/parameterized by config in <running>.



3) if a config-template is configured in <running> (i.e., it is operator-defined) then, yes, the expanded configuration in <intended> is "client configuration” (note, "client configuration” is not a formal term).  That said, it seems fair to say that a template defined in <system> and then referenced by "client configuration” in <running> is also expanded as "client configuration” in <intended>.



4) I don’t not understand your last sentence, that the expansion of <system> templates are only present in <operational>.  Maybe you’re saying something subtle, e.g., that servers currently don’t support GET on <intended>.  But, in theory, the expansion of <system> templates should (IMO) be present in <intended>, so that they may be subject to validation.   Of course, all the <intended> configuration (whether originating in <running> or <system>) that is successfully “applied” will also be present in <operational>.

[Qiufang Ma] Assume that there is no <system> and this work, the expansion of system templates are only present in <operational>. Because this is compatible with system configuration definition in NMDA.

But if system configurations are only present in <operational>, the predefined system configurations still need to be retrieved and created into <running> explicitly when being referenced. I think we’ve reached an agreement on the need for <system> to exist, and our main point of disagreement is whether <system> should be copied into <running>. Your point is that being merged into <intended> is enough to make sure a success validation. But my understanding is that the referenced system configuration data item must also exist in the <running> to obey the model constraints.

Yes, I believe that you provided an accurate description of the difference in our opinions.  Per my earlier response, you make a valid technical point, my goal is to waive that interpretation to the side so that a simpler solution can emerge.  It would be good to get other opinions on list, otherwise we’ll take it into the meeting.
[Qiufang Ma] OK.  Hopefully someone else would share some opinions here. Otherwise let’s take this into the IETF meeting.

[BTW, in keeping with this thread moving from the NETCONF to the NETMOD mailing lists, would it make sense to move the IETF 111 presentation slot from NETCONF to NETMOD too?  I think it does and, further, it would help with scheduling (NETCONF is over, NETMOD is under).  Would you be okay with this?   AD Rob and the NETCONF chairs discussed this morning, and think it's okay, but would still need to confirm with the NETMOD chairs.]
[Qiufang Ma] I am happy with the proposal, if it’s also okay for NETMOD chairs:-). I have sent an email to the NETMOD chairs to request to move this presentation slot from NETCONF to NETMOD.
A new version of the draft will also be submitted to NETMOD when the draft-submitting window reopens.



 <big snip>

> I’m beginning to think that:

> ·         auto-copying into <running> is likely never a good idea, because it violates the definition of <running>

> [Qiufang Ma] I am quite aware that different datastores in NMDA represents different views of data nodes.  And <running> represents a configuration datastore holding the current configuration of the device.

> Should we consider system configuration also be part of current configuration of the device? From my perspective, the difference between system configuration and  client-configuration lies only in who provides it.



<running> holds the current *operator-specified* configuration of the device.  System-provided configuration is NOT specified by operators (though system-defined objects may be referenced by operator-specified config in <running>).   I believe that this arrangement is consistent with the definition of <running>.  Agreed?

[Qiufang Ma] Yes. Actually we are not trying to violate the principles of NMDA and the definition of <running>.  The issue we try to resolve here is that system configurations cannot be used(referenced or overwritten) by the operators directly and need to be created into <running> explicitly. This actually loses the meaning of “predefining and bringing convenience”. If auto-copying is not a good idea, what do you think about defining an RPC operation for the operators to do the copy(which is also what Rob suggests at the meeting)?

If we have to copy into <running>, then I think that I agree an RPC (<edit-config>?) would be better.
[Qiufang Ma] From my perspective, <edit-config> is feasible but not efficient because operators still need to retrieve <system>/<operational> firstly. If we could define a RPC to copy the entire <system> into <running>, it seems more convenient for operators. However, some system configurations which are not going to be referenced or modified may also be copied into <running>. I don't have a strong feeling about which one is preferred. Anyway, we need to figure out whether it would be fine for <running> to missing referenced system configurations.

You mention “overwritten” by the operators?  Why wouldn’t the operators just define their own?  For instance, if they don’t like the vendor’s “vendor-foobar” object, they could copy/paste/edit their own “my-foobar” object with the values needed, yes?
[Qiufang Ma] Yes, defining their own would be okay. By overwriting, I mean sometimes the operators would like to modify the specific system configuration, e.g., the MTU value of a specified interface(identified by its name).
If the operators want to modify the system configurations, there is no way but redefine them in <running>.


Best Regards,
Qiufang Ma








> ·         having in <operational> doesn’t make sense, since the tweaks wouldn’t go thru <running> --> <intended> validation.

>

> I’m wondering if a model like below would work for everyone - thoughts?

> [Qiufang Ma] <intended> represents the configuration after all configuration transformations to <running> have been performed, so I think it is only coupled to <running>.

> Anyway, the <system> should also interacts with <operational>.  Agreed?



I don’t agree that <intended> must only be coupled to <running>.  Specifically, I think that it is okay (compatible with NMDA) to define a <system> that also impacts <intended>.   This is the only (IMO) sane approach, as it enables the combination <running> + <system> to be validated.

[Qiufang Ma] Please see above. If <running> is OK to miss referenced system configuration, your proposal makes sense to me.

Ack.




 Best Regards,

Qiufang Ma


Kent // contributor