Guidelines for Characterizing "OAM"

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 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. Additionally, this document recommends avoiding the creation and use of extended abbreviation for the qualifiers of "OAM". For example, the first "O" in "OOAM" could mean out-of-band, overlay, or something else. This document updates by adding to the guidelines for the use of the term "OAM". It does not modify any other part of . Note that defines terms for active and passive performance assessments through metrics and methods. That RFC does not substantially discuss OAM, and although the concepts are similar, this document does not modify the definitions in .

Historically, the terms "in-band" and "out-of-band" were used extensively in radio communications as well as in telephony signaling . 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 switch networks (PSNs) which do not have a "band" per se, and, in fact, have multiple "somethings" that OAM 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 redefinitions of these terms cannot be generalized and can be confused by participants from other contexts. 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.

The guidance in this document is to avoid the terms "in-band" and "out-of-band" when refering to OAM, and instead find finer-granularity descriptive terms. The definitions presented in this document are for use in all future IETF documents that refer to OAM, and the terms "in-band OAM" and "out-of-band OAM" are not to be used in future documents. OAM in relation to a path.
The OAM information follows the exact same path as the observed data traffic. This was sometimes referred to as "in-band".
The OAM information does not follow the exact same path as the observed data traffic. This was sometimes referred to as "out-of-band". , Section 2 fourth bullet, gives an example of "Path-Congruent OAM", and further describes that the OAM Packets "share their fate with data packets." OAM in relation to a user data packet.
The OAM information is carried in the packets that also carry the data traffic. This was sometimes referred to as "in-band".
The OAM information is carried in its own OAM packets, separate from data traffic. This was sometimes referred to as "out-of-band". The MPLS echo request/reply messages are an example of "Dedicated-Packet OAM", since they are described as "An MPLS echo request/reply is a (possibly MPLS-labeled) IPv4 or IPv6 UDP packet". In situ OAM is an example of "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.' Initially, "in situ OAM" was also referred to as "In-band OAM", but was renamed due to the overloaded meaning of "in-band OAM". Further, also intertwines the terms "in-band" with "in situ", though settled on using "in Situ". Other similar uses, including and , still use variations of "in-band", "in band", or "inband". It is noteworthy that In-Packet OAM cannot be Non-Path-Congruent OAM. OAM in relation to the forwarding treatment of user data packets, as for example QoS treatment.
The OAM packets receive the same forwarding (e.g., QoS) treatment as user data packets. This was sometimes referred to as "in-band".
The OAM packets receive different forwarding (e.g., QoS) treatment as user data packets. This was sometimes referred to as "out-of-band". For a case of either "Non-Path-Congruent OAM" or "Different-Forwarding-Treatment OAM", says "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." uses similar text. OAM in relation to multiple criteria. For example, in relation to both topological congruence and packet treatment.

uses Combined OAM when it says "In-band OAM: an active OAM method 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." uses similar text.

There are many examples of "in-band OAM" and "out-of-band OAM" in published RFCs. While interpreting those, 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. For example, says "as in-band traffic with the PW's data, or out-of-band", and "in-band (i.e., following the same data-plane faith as PW data)". Hence, in that specific case, the term "band" refers to the "Pseudowire data".

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 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 use in all future IETF documents that refer to OAM.
Depends on injected, dedicated OAM packets.
Depends solely on the observation of one or more existing data packet streams and does not use dedicated OAM packets.
Uses instrumentation or modification of data packets themselves. and are examples labeled "Hybrid OAM" under this definition.
Uses a combination of at least two of Active OAM, Passive OAM, and Hybrid OAM (i.e., a combination of atomic OAM packets, data packet modification for OAM, and no explicit OAM). Note that Section 3.7 of also uses the term "Hybrid" to refer to metric types in-between active and passive, for OAM there are no in-betweens per se, only active, passive, hybrid, or a compound combination.

Compound OAM can be characterized in a more explicit way, for nuanced use-cases, and named according to the OAM types that are combined: Active-Passive OAM. Active-Hybrid OAM. Hybrid-Passive OAM. Active-Hybrid-Passive OAM.
When two or more OAM types are combined, it is simply the case that both OAM types are used at once. For example, in Active-Passive OAM both specific OAM packets and the observation of data packets may be used to provide information about the data flow in the network. Each component may supplement the total data. Note that describes "passive methods" as "out of band" which is contrary to the concept of "Passive OAM" as defined here because there are no OAM packets to be in-band or out-of-band. Following the guidelines of this document, OAM may be qualified according to the terms described in Sections and of this document, and the term "out of band OAM" is not to be used in future documents.

This document recommends avoiding the creation and use of extended abbreviations for the qualifiers of "OAM". For example, the first "O" in "OOAM" could mean out-of-band, overlay, or something else. and other dependent documents currently uses the abbreviations "IOAM" for In situ Operations, Administration, and Maintenance (IOAM). While this document does not obsolete that abbreviation, it still recommends that the expanded "in situ OAM" is used instead to avoid potential ambiguity.

There are multiple processing capabilities that nodes processing OAM packets can utilize. Some of those capabilities are explained in for in situ OAM and are further generalized in this document. Depending on the Type of OAM processing, nodes are categorized as follows. Please note that this characterization exists within the context of a particular OAM protocol instance, and a given node can support multiple types. Hybrid OAM instruments or modifies data packet themselves. Consequently:
Adds OAM information to data packets.
May process OAM information in data packets.
Does not process or even notice OAM information in data packets.
Removes OAM information from data packets. Active OAM uses dedicated OAM packets, separate from data packets. Consequently:
Creates and injects OAM packets into a flow.
Processes and removes OAM packets from a flow. A node could be an OAM Source Node and an OAM Sink Node for Active OAM packets simultaneously. In some use-cases, such as in situ OAM described in , Compound OAM is used. In the forward direction, Hybrid OAM is used with a single Encapsulating Node. Multiple Transit Nodes may process the OAM information, and this may trigger them to act as OAM Source Nodes for Active OAM sent back to the Encapsulating Node which serves as an OAM Sink Node.

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, and Roni Even for their thorough review and useful feedback comments that greatly improved this document.