Re: [ippm] [OPSAWG] New I-D -> Guidelines for Charactering "OAM"

[Adrian Farrel <[e-mail]>]

Hi Greg,

 

Thanks for your thoughtful inspection of our draft. 

 

Carlos and I wanted to be sure that all of the discussions of this draft were indexed on one list, and we wanted to avoid multiple copies going to people who are subscribed to multiple lists. So we asked that follow-ups went only to OPSAWG. I have moved IPPM, MPLS, SPRING, and DetNet to BCC on this email.

 

It was certainly not our intent to disparage any work that was being done in any other working group, and I understand the effort that has gone into the DetNet OAM Framework to ensure that the terminology is clear and unencumbered in the DetNet context.

 

Our concern was, however, that different contexts are applying different definitions of the terms “in-band” and “out-of-band”. Those definitions are (often) clear and precise, but they are not consistent across contexts. Thus, a water-tight definition in the DetNet context is not universally applicable, and a reader coming to DetNet from another context may bring with them their own understanding of the terms.

 

Our intent, therefore, is to select a finer-grained set of terms that have universal applicability and that can be selected within a context without loss of generality.

 

This is a tricky little subject and I know that Carlos and I expected it to generate more than a little discussion. If we end up with “everything is OK and nothing needs to change” that will be OK with us. If we discover that some work is using terms too generally, while others already have perfect definitions, that will lead to something similar to this document to bring the good into the light.

 

Further comments in line…

 

 

From: Greg Mirsky <[e-mail]> 
Sent: 12 January 2024 00:09
To: Carlos Pignataro <[e-mail]>; Adrian Farrel <[e-mail]>
Cc: Ops Area WG <[e-mail]>; IETF IPPM WG <[e-mail]>; mpls <[e-mail]>; spring <[e-mail]>; DetNet WG <[e-mail]>
Subject: Re: [OPSAWG] New I-D -> Guidelines for Charactering "OAM"

 

*	Hi Carlos and Adrian,

thank you for starting this work. I believe that having a common dictionary helps in having more productive discussions. I took the liberty of inviting all the WGs which you invited to review the document and added DetNet WG. Please feel free to trim the distribution list.

I've read the document and have several general questions to start our discussion:

*	It seems that the motivation of this document is the assumption that "in-band OAM" and "out-of-band OAM" are not representative or cannot be understood or correctly attributed, interpreted by the IETF community. Is that correct?

I think the wording here would be “cannot be reliably understood consistently”. That is, without looking at a context-specific definition (such as that which you supply in the DetNet OAM Framework), the use of the terms may be misinterpreted.

This is an assertion, but one (we think) is founded on observation of recent conversations on mailing lists, and also of witnessing many years of people talking passed each other.

*	As we discuss and try to establish (change) the IETF dictionary, it is important to analyze the terminology used by other SDOs. I believe that it is beneficial to maintain consistent terminology which will minimize misunderstandings among experts with different experiences of working at different centers of technological expertise.

This is a good point. It is certainly true that if other SDOs working with packet networks have established terminology that we can agree with and which is not, itself, subject to context-specific definitions, then there is no reason to choose other terms. Do you have any suggested sources?

It is notable that the ITU-T has long worked with non-packet transport networks and has used the terms in-band and out-of-band. But even there we see some fragmentation with terms such as “in-fibre, but out-of-band” becoming necessary.

*	I that DetNet OAM Framework <https://datatracker.ietf.org/doc/draft-ietf-detnet-oam-framework/>  provides sufficiently clear interpretation of terms that can be generalized for non-DetNet networks:

   *  In-band OAM is an active OAM that is in-band within the monitored

      DetNet OAM domain when it traverses the same set of links and

      interfaces receiving the same QoS and Packet Replication,

      Elimination, and Ordering Functions (PREOF) treatment as the

      monitored DetNet flow.

 

This, of course, does not distinguish between “in-packet” (such as IOAM), and having its own packet (such as ping).

 

   *  Out-of-band OAM is an active OAM whose path through the DetNet

      domain is not topologically identical to the path of the monitored

      DetNet flow, or its test packets receive different QoS and/or

      PREOF treatment, or both.

As can be seen, the interpretation of "in-band" accepted by the DetNet WG includes not only topological equivalence between the monitored data flow and path traversed by active OAM but also QoS equivalence between them. I believe that is essential in differentiating in-band OAM from out-of-band OAM.

 

Right. But is there terminology to talk about a packet that does follow the topology, but does not receive the same QoS treatment?

 

It is perhaps a little strong to say this, but what you have done is define two classes of OAM (both good things and meaningful in the DetNet context) and then assigned existing names to those classes. What we are suggesting is that you have some finer granularity categorisation of OAM available, and that for the purposes of your DetNet work, you are collecting those granularities into two different sets.

*	I find the use of "path congruence" in inherently meshed packet-switched networks confusing if not misleading. (Note that RFC 6669 <https://www.rfc-editor.org/rfc/rfc6669>  explains congruence by using in-band term.) Is there evidence of the term being used besides a single case in RFC 6669?

Well, I would say that 6669 is an example of how “in-band” has been used, and I’d point out that it does not match your DetNet OAM Framework definition (as there is no mention of identical treatment). Note that the text from RFC 6669 is replicated into RFC 7276 (same authors, same subject).

You don’t say what you find misleading or confusing. Is it that, in a meshed packet network, each individual packet might be forwarded differently so that congruence cannot be guaranteed? That could be true, but we hope for greater stability than that, I think.

If “path congruence” was a new term (with only one previous use) that might make it a really neat term to use (because it would lack all previous meanings). However, it is not. It has been used (to mean the same set of links, ports, and nodes) in our more path-oriented work such as RFC 5828, RFC 6373, right up to RFC 9059.

Perhaps “congruent” is overloaded given that we are not talking about “topological congruence”, a term that is also quite widely used (e.g., RFC 2796, RFC 4258, RFC 5059, RFC 6549, RFC 8795)

*	Similarly, "in-packet" vs. "dedicated packet". I believe that RFC 7799 <https://www.rfc-editor.org/rfc/rfc7799>  has that addressed by using "active", "passive", and "hybrid" terminology. Although these terms applied to measurement methods, i.e., performance monitoring component of OAM, but, in my opinion, can be extended to fault management OAM.

Well, we agree that RFC 7799 can be used to generate the terms "active OAM", "passive OAM", and "hybrid OAM". Although we think, for the benefit of clarity, the reader should not be left to examine RFC 7799 and project meaning from performance monitoring to OAM in general: they should be presented with a clear set of definitions (per our section 3). 

It is further our belief that the definitions of active and passive OAM do not match with “in-packet” and “dedicated-packet”. Indeed, possibly, the closest is that “active OAM” is “dedicated-packet”, and “hybrid OAM” is “in-packet” leaving “passive OAM” to be just observation.

*	It seems like the definition of Compound/Combined misses the point that RFC 7799 already defines a hybrid measurement method (not OAM but measurement methods) as a method in which elements of active and passive measurement methods are used. Hence, hybrid is already a combination of active and passive measurement methodologies and the introduction of compound or combined terms is unnecessary, a duplication of the existing and accepted terminology (at least in IPPM WG). And "Active-Hybrid-Passive OAM" is the result of that omission because, according to the definition in RFC 7799, Active-Passive is Hybrid. Thus, Active-Hybrid-Passive is nothing else but Hybrid-Hybrid. Does that make sense?

I should certainly have preferred it had RFC 7799 not used the term “hybrid” to actually refer to a third category that is not a hybrid of the first two categories. For the definitions of active OAM and passive OAM, I don’t think the combination matches the definition of hybrid OAM.

So, perhaps, let’s stop referring to RFC 7799’s definitions of not-actually-OAM-packets, and nail down our own definitions. That will tell us whether we need two, three, four, or more terms.

*	I cannot agree that RFC 7799 "adds to the confusion" by pointing that

   Passive performance metrics are assessed independently of the packets

   or traffic flows, and solely through observation.  Some refer to such

   assessments as "out of band".

Indeed, passive measurement methods are not required to use packets that are in-band with the monitored data flow. Usually, the management plane protocol is used to collect, to perform the observation function. In some cases, in-band active OAM packets may be used, e.g., direct loss measurement in ETH-OAM.

 

Yes, but where is this “in-band with the monitored data flow” defined for a packet network? And you say “are not required to” rather than “MUST NOT”. That means that the passive methods might send their packets with the monitored data flow or might not. 

We live in a world where there is not necessarily a distinction between the MCN and DCN. 

 

I find that throw-away sentence in RFC 7799 both helpful and unhelpful. It is helpful to know that some people call this “out of band”. It is unhelpful to refer to an assessment method as “out of band” as there is no message or packet involved to be in or out of band.

 

FWIW, I believe that RFC 7799 and DetNet OAM Requirements already provide a clear terminology for OAM in general and its elements, i.e., Fault Management and Performance Monitoring.

 

OK. I suspect that we are going to have to come up with a set of OAM techniques and ask you to categorise them according to your terminology to see whether all bases are covered.

 

But I am also going to have to review your text from the DetNet OAM Framework because it contains phrases that are not clear (to me)…

 

      In-band OAM is an active OAM that is in-band within the monitored

      DetNet OAM domain when it traverses the same set of links and

      interfaces receiving the same QoS and Packet Replication,

      Elimination, and Ordering Functions (PREOF) treatment as the

      monitored DetNet flow.

 

There is something broken here. Maybe too many words. Perhaps you mean…

 

      In-band OAM is an active OAM that traverses the same set of links and

      interfaces receiving the same QoS and Packet Replication,

      Elimination, and Ordering Functions (PREOF) treatment as the

      monitored DetNet flow within the monitored DetNet OAM domain

 

…and…

 

      Out-of-band OAM is an active OAM whose path through the DetNet

      domain is not topologically identical to the path of the monitored

      DetNet flow, or its test packets receive different QoS and/or

      PREOF treatment, or both.

 

As noted before, this leaves a few gaps.

*	Active OAM that follows the same path, but does not receive the same QoS treatment
*	There is no distinction between instrumentation of data packets and dedicated instrumentation packets

 

Cheers,

Adrian

 

On Fri, Jan 5, 2024 at 12:39 PM Carlos Pignataro <[e-mail] <mailto:[e-mail]> > wrote:

Hi, Ops Area WG,

 

Every now and again, there are discussions on how to best characterize or qualify a particular kind of "OAM", as well as misunderstandings due to having different definitions and contexts for a given term. A case in point is "in-band" or "out-of-band" OAM, as recently surfaced at https://mailarchive.ietf.org/arch/msg/opsawg/jREEH1sFOZ-uxZNky-RTggpxkuk/.

 

To alleviate this issue, Adrian and I wrote a short I-D to provide forward-looking guidance on "foobar OAM".

 

We would appreciate feedback and input on this position, which aims at updating the guidelines for the "OAM" acronym, with unambiguous guidelines for their modifiers.

 

Guidelines for Charactering "OAM":

https://datatracker.ietf.org/doc/draft-pignataro-opsawg-oam-whaaat-question-mark/

 

Look forward to input and comments to make this more clear and effective!

 

Adrian & Carlos.

 

 

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