| Internet-Draft | Characterizing OAM | June 2025 |
| Pignataro, et al. | Expires 13 December 2025 | [Page] |
As the IETF continues to produce and standardize different Operations, Administration, and Maintenance (OAM) protocols and technologies, various qualifiers and modifiers are prepended to the OAM abbreviation. While, at first glance, the most used appear to be well understood, the same qualifier may be interpreted differently in different contexts. A case in point is the qualifiers "in-band" and "out-of-band" which have their origins in the radio lexicon, and which have been extrapolated into other communication networks.¶
This document considers some common qualifiers and modifiers that are prepended, within the context of packet networks, to the OAM abbreviation and lays out guidelines for their use in future IETF work.¶
This document updates RFC 6291 by adding to the guidelines for the use of the term "OAM". It does not modify any other part of RFC 6291.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 13 December 2025.¶
Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
It is not uncommon for historical and popular terms to have nuances in how they are interpreted or understood. This was, for example, the case with the abbreviation for Operations, Administration, and Maintenance, "OAM", and [RFC6291] provided guidelines for its use as well as definitions of its constituent parts.¶
Characterizations or qualifiers for "OAM" within packet networks often encounter similar problems of interpretation, such as with the adjective phrases "in-band" and "out-of-band". This document considers some common qualifiers and modifiers that are prepended to the OAM abbreviation, and lays out guidelines for their use in future IETF work to achieve consistent and unambiguous characterization.¶
This document updates [RFC6291] by adding to the guidelines for the use of the term "OAM". It does not modify any other part of [RFC6291].¶
Historically, the terms "in-band" and "out-of-band" were used extensively in radio communications as well as in telephony signaling [RFC4733]. In both these cases, there is an actual "Band" (i.e., a "Channel" or "Frequency") to be within or outside.¶
While those terms, useful in their simplicity, continued to be broadly used to mean "within something" and "outside something", a challenge is presented for IP communications and packet-switched networks (PSNs) which do not have a "band" per se, and, in fact, have multiple "somethings" that OAM traffic can be carried within or outside. A frequently encountered case is the use of "in-band" to mean either in-packet or on-path.¶
Within the IETF, the terms "in-band" and "out-of-band" cannot be reliably understood consistently and unambiguously. Context-specific definitions of these terms are inconsistent and therefore cannot be generalized. More importantly, the terms are not self-defining to any further extent and cannot be understood by someone exposed to them for the first time, since there is no "band" in IP.¶
There are many examples of "in-band OAM" and "out-of-band OAM" in published RFCs. For instance, the term "in-band" appears in both [RFC5085] and [RFC9551]. While the context in each of these documents is clear, the term carries different meanings in each case. These two examples, as well as other examples of uses of the term "in-band" in previous documents are described throughout Section 3.¶
While interpreting existing documents, it is important to understand the semantics of what "band" is a proxy for, and to be more explicit if those documents are updated. This document does not change the meaning of any terms in any prior RFCs.¶
This document recommends avoiding the terms "in-band" and "out-of-band" when referring to OAM. Instead, it encourages the use of more fine-grained and descriptive terminology. The document also presents alternative terms and definitions for use in future IETF documents referencing OAM, without precluding the use of other precise, descriptive terms that do not rely on the "-band" convention.¶
The terminology presented in this section classifies OAM according to three criteria: whether it follows the same path as data traffic; whether it receives the same treatment as data traffic; and whether it operates in an active, passive, or hybrid mode.¶
In this document, the term "path-congruent packets" describes packets that follow the exact same path (i.e., traverse the same nodes and links) within a network. Note that this definition does not describe how the packets are treated in queues within the nodes on the path. A further concept, "equal-forwarding-treatment" describes how path-congruent packets receive the same forwarding treatment (e.g., Quality of Service (QoS)).¶
An example of "Path-Congruent OAM" is the Virtual Circuit Connectivity Verification (VCCV), described is [RFC5085] as "The VCCV message travels in-band with the Session and follows the exact same path as the user data for the session". Thus, the term "in-band" in [RFC5085] refers to using the same path as the user data. This term is also used in Section 2 of [RFC6669] with the same meaning, and the word "congruent" is mentioned as synonymous.¶
As an example, the behavior of "Equal-Forwarding-Treatment OAM" is described in [RFC9551] as "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 is classified in [RFC9551] as "In-band OAM". Similarly, the property of "Different-Forwarding-Treatment OAM" can be found in the following definition in [RFC9551]: "Out-of-band OAM: an active OAM method whose path through the DetNet domain may not be topologically identical to the path of the monitored DetNet flow, its test packets may receive different QoS and/or PREOF treatment, or both." [I-D.ietf-raw-architecture] uses similar text.¶
[RFC7799] provides clear definitions for active and passive performance assessment such that the construction of metrics and methods can be described as either "Active" or "Passive". Even though [RFC7799] does not include the specific terms "Active", "Passive", or "Hybrid" as modifiers of "OAM", the following terms are used in many RFCs and are provided here for clarity.¶
This document defines the term In-Packet OAM as a more specific and narrowly scoped instance within the broader category of Hybrid OAM.¶
The MPLS echo request/reply messages [RFC8029] are an example of "Active OAM", since they are described as "An MPLS echo request/reply is a (possibly MPLS-labeled) IPv4 or IPv6 UDP packet".¶
In situ OAM [RFC9197] is an example of "Hybrid OAM" that is also "In-Packet OAM", given that it: '...records OAM information within the packet while the packet traverses a particular network domain. The term "in situ" refers to the fact that the OAM data is added to the data packets rather than being sent within packets specifically dedicated to OAM.'¶
An example of "Hybrid OAM" which is not classified as "In-Packet OAM" is Direct loss measurement [RFC6374].¶
Initially, "In situ OAM" [RFC9197] was also referred to as "In-band OAM", but was renamed due to the overloaded meaning of "In-band OAM". Further, [RFC9232] also intertwines the terms "in-band" with "in situ", though [I-D.song-opsawg-ifit-framework] settled on using "in Situ". Other similar uses, including [P4-INT-2.1] and [I-D.kumar-ippm-ifa], still use variations of "in-band", "in band", or "inband".¶
Note that OAM can be classified in relation to multiple criteria, e.g., relating to both topological congruence and packet treatment, as well as other criteria, such as active, passive or hybrid [RFC7799]. For example, [RFC9551] classifies OAM both in terms of whether it is active or passive as well as in terms of whether it receives the same treatment as the user traffic.¶
Security is improved when terms are used with precision, and their definitions are unambiguous.¶
This document has no IANA actions.¶
The creation of this document was triggered when observing one of many on-mailing-list discussions of what these terms mean, and how to abbreviate them. Participants on that mailing thread include, alphabetically: Adrian Farrel, Alexander Vainshtein, Florian Kauer, Frank Brockners, Greg Mirsky, Italo Busi, Loa Andersson, Med Boucadair, Michael Richardson, Quan Xiong, Stewart Bryant, Tom Petch, Eduard Vasilenko, and Xiao Min.¶
The authors wish to thank, chronologically, Hesham Elbakoury, Michael Richardson, Stewart Bryant, Greg Mirsky, Med Boucadair, Loa Andersson, Thomas Graf, Alex Huang Feng, Xiao Min, Dhruv Dhody, Henk Birkholz, Alex Huang Feng, Tom Petch, Roni Even, and Tim Chown for their thorough review and useful feedback comments that greatly improved this document.¶