| Internet-Draft | Characterizing OAM | June 2025 |
| Pignataro, et al. | Expires 26 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.¶
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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.¶
The motivation for Equal-Forwarding-Treatment OAM lies in the desire to ensure that OAM packets experience the same network conditions as the user data they are intended to monitor. This includes not only traversing the same topological path but also receiving identical Quality of Service (QoS) treatment, such as queuing, scheduling, and traffic shaping. When both topological and forwarding treatment equivalence are achieved, the OAM packets are said to exhibit fate-sharing [RFC7276] with the data traffic. Fate-sharing ensures that any impairments or anomalies affecting the user traffic are also reflected in the behavior of the OAM packets, thereby making the results of the OAM observations more operationally meaningful and actionable. Without such equivalence, discrepancies in treatment could lead to misleading measurements or diagnostics, reducing the utility of the OAM mechanism for performance monitoring and fault detection.¶
An example of "Equal-Forwarding-Treatment OAM" is presented 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, enabling the construction of metrics and methods to be described as either "Active" or "Passive". Even though [RFC7799] does not explicitly use these terms as modifiers of "OAM", they are widely used in practice and are included here for clarity. The terms "Active", "Passive" and "Hybrid", as described below, are consistent with [RFC7799].¶
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".¶
OAM protocols and tools can be classified according to the three criteria that were described in the previous sections. However, not all criteria are applicable to all OAM protocols, and not all combinations are necessarily possible. For example:¶
Passive OAM relies solely on observing existing data traffic and does not generate dedicated OAM packets. As such, the forwarding treatment criterion is not relevant, since there are no separate OAM packets to receive differentiated treatment.¶
In-Packet OAM, a subset of hybrid OAM, inherently implies path congruence and equal treatment, as the OAM data is embedded within the data packets themselves.¶
When defining a new OAM protocol or analyzing an existing one, it is recommended to explicitly consider which of these criteria are applicable and to describe the protocol accordingly. For example:¶
IP Ping, which uses ICMP Echo messages, can be classified as active OAM, but since it is not guaranteed to be forwarded through the same path or receive the same treatment as user data packets, it can be classified as non-path-congruent and different-forwarding-treatment.¶
When IOAM [RFC9197] is incorporated in data packets it can be classified as in-packet, path-congruent and equal-forwarding-treatment.¶
VCCV [RFC5085], as discussed above, is classified as active, path-congruent and different-forwarding-treatment.¶
MPLS inferred loss measurement [RFC6374] uses specially generated test messages, and therefore can be classified as active. It is also path-congruent, and can be deployed either as equal- or different-forwarding-treatment OAM. MPLS direct loss measurement [RFC6374] uses OAM messages that exchange counters that count user data traffic. Hence, it is classified as hybrid OAM, and as in the inferred mode, it is path-congruent, and can be either equal- or different-forwarding-treatment OAM.¶
This multi-dimensional classification enables a more precise and consistent understanding of OAM mechanisms.¶
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, Tim Chown, Marcus Ihlar, Med Boucadair, and Benoit Claise for their thorough review and useful feedback comments that greatly improved this document.¶