Guidelines for Characterizing "OAM"

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-switched 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 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.

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. Alternative terms and definitions are presented in this document for future use in IETF documents that refer to OAM. 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 can also be called Path-Incongruent OAM, and 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." 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 clarity.
Depends on 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. Examples of protocols classified under "Hybrid OAM" include and . Hybrid OAM can be implemented by piggybacking OAM-related information onto data packets, as seen in , or by utilizing reserved fields in the packet header or specific values of existing header fields, as proposed in . This document defines the term In-Packet OAM, which is a special case of Hybrid OAM:
The OAM information is carried in the packets that also carry the data traffic. This was sometimes referred to as "in-band". The MPLS echo request/reply messages 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 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". 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. 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 . For example, 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".