Guidelines for Characterizing the Term "OAM"

Terminology and Guidance

Recommendation 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 that discuss OAM (including a reference to this document), 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 operates in an active, passive, or hybrid mode (); whether it follows the same path as data traffic (); and whether it receives the same treatment as data traffic ().

Active, Passive, and Hybrid OAM 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 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 . This document does not update or change the terms of .

Active OAM:: Uses dedicated OAM packets.
Passive OAM:: Relies on the observation of one or more existing data packet streams and does not use dedicated OAM packets and does not modify data packets.
Hybrid OAM:: Uses a combination of Active Methods and Passive Methods, which may include augmentation or modification of the stream of interest. makes a distinction between Hybrid Type I, referring to a single stream of interest, and Hybrid Type II, referring to two or more streams of interest.

This document defines the term "In-Data-Packet OAM" as a more specific and narrowly scoped instance within the broader category of Hybrid OAM. This new term allows for a more fine-grained classification of OAM mechanisms, as the broad category of Hybrid OAM includes a diverse set of possible OAM methods.

In-Data-Packet OAM:: OAM-related information is carried in the packets that also carry the data traffic. This is a specific case of Hybrid OAM. It was sometimes referred to as "in-band".

Note that In-Data-Packet OAM is a specific case of Hybrid Type I, as it is applied to a single stream of interest. The following examples illustrate the terms Active, Passive, Hybrid, and In-Data-Packet OAM:

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".
Monitoring a packet stream by maintaining counters for the packets within the stream is an example of "Passive OAM".
An example of "Hybrid Type I OAM" that is also "In-Data-Packet OAM", is an IOAM (In Situ OAM) trace option that is incorporated into data packets of a single stream of interest. According to , IOAM '...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.'
Another example of "Hybrid Type I OAM" that is also "In-Data-Packet OAM" is Alternate Marking , when applied to data packets of a single stream. In this case, a small number of bits in the packet header is used for marking a subset of packets in a flow.
An example of "Hybrid Type I OAM" that is not classified as "In-Data-Packet OAM" is Direct Loss Measurement , in which user packets are not modified by the protocol. Instead, OAM packets are used for carrying information about observed network characteristics -- namely, user packet counter values that allow for packet loss computation.
Another example of "Hybrid Type I OAM" that is not "In-Data-Packet OAM" is the case where a packet stream is (actively) generated while an existing stream of interest is (passively) observed. This example was introduced in as a Hybrid Type I method. Extending this example, if the packets of the active stream include an IOAM trace option, the method is characterized by the more general term, Hybrid Type I.

Path-Congruent OAM

Path-Congruent OAM:: The OAM information follows the exact same forwarding path as the observed data traffic.
Non-Path-Congruent OAM:: The OAM information is not guaranteed to follow the exact same forwarding path as the observed data traffic.

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. An example of "Path-Congruent OAM" is the Virtual Circuit Connectivity Verification (VCCV) Type 1 (), which was also referred to as "In-Band VCCV". The term "congruent" also appears in in the context of path sharing.

Packet-Forwarding-Treatment OAM

Equal-Forwarding-Treatment OAM:: The OAM packets receive the same forwarding treatment (e.g., QoS) as user data packets.
Different-Forwarding-Treatment OAM:: The OAM packets might receive different forwarding treatment (e.g., QoS) than user data packets.

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 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, and even inadequate corrective actions, reducing the utility of the OAM mechanism for performance monitoring, fault detection, and fault mitigation. An example of "Equal-Forwarding-Treatment OAM" is presented in in the context of Deterministic Networking (DetNet) OAM: "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". (The property of "Equal-Forwarding-Treatment" is referred to in as "In-band OAM".)

Using Multiple Criteria 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 path congruence and forwarding treatment criteria are not relevant, because no dedicated OAM packets are exchanged between the measurement points.
Non-Path-Congruent OAM, by nature, cannot be Equal-Forwarding-Treatment.

When defining a new OAM mechanism or analyzing an existing one, it is recommended to explicitly consider which of these criteria are applicable and to describe the mechanism accordingly. As a first step, all OAM mechanisms can be classified according to the first criterion, as Active, Passive, or Hybrid/In-Data-Packet. Further classification according to the other two criteria should be considered on a case-by-case basis. A few examples of OAM classification according to the three criteria are presented below:

IP Ping, which uses ICMP Echo messages, can be classified as Active OAM. Since it is not guaranteed to follow the same path or receive the same treatment as user data packets, it is classified as Non-Path-Congruent and, consequently, as Different-Forwarding-Treatment.
When an IOAM trace option is incorporated in data packets, it can be classified as In-Data-Packet, Path-Congruent, and Equal-Forwarding-Treatment.
VCCV , as discussed above, is classified as Active, Path-Congruent, and Different-Forwarding-Treatment.
MPLS Inferred Loss Measurement (ILM) () 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 (DLM) () uses OAM messages that carry counters that count user data traffic. Hence, it is classified as Hybrid Type I OAM, and as in the Inferred Loss Measurement, it is Path-Congruent and can be either Equal- or Different-Forwarding-Treatment OAM.

In measurement protocols, accurate results depend on Path-Congruence and Equal-Forwarding-Treatment. In contrast, these properties are not always required in other OAM protocols. For example, Bidirectional Forwarding Detection (BFD) control packets are often sent with the highest priority, which means they do not adhere to the Equal-Forwarding-Treatment property. This multidimensional classification enables a more precise and consistent understanding of OAM mechanisms.

Summary of Terms This section summarizes the terminology.

Active OAM:: Uses dedicated OAM packets.
Passive OAM:: Relies on the observation of one or more existing data packet streams and does not use dedicated OAM packets and does not modify data packets.
Hybrid OAM:: Uses a combination of Active Methods and Passive Methods, which may include augmentation or modification of the stream of interest. makes a distinction between Hybrid Type I, referring to a single stream of interest, and Hybrid Type II, referring to two or more streams of interest.
In-Data-Packet OAM:: OAM-related information is carried in the packets that also carry the data traffic. This is a specific case of Hybrid OAM. It was sometimes referred to as "in-band".
Path-Congruent OAM:: The OAM information follows the exact same forwarding path as the observed data traffic.
Non-Path-Congruent OAM:: The OAM information is not guaranteed to follow the exact same forwarding path as the observed data traffic.
Equal-Forwarding-Treatment OAM:: The OAM packets receive the same forwarding treatment (e.g., QoS) as user data packets.
Different-Forwarding-Treatment OAM:: The OAM packets might receive different forwarding treatment (e.g., QoS) than user data packets.

Applicability and Conformance Statement The definitions here SHOULD be used by IETF documents qualifying the term "OAM". IETF documents that explicitly want to create different characterizations beyond the definitions of the terms in this document, can introduce such terms provided they are different from the ones defined in this document for clarity's sake. See also . Authors who follow the terms as defined in this document SHOULD incorporate the following in the document (typically in the terminology section): OAM terms [INSERT TERMS] are to be interpreted as described in [RFC10014]. ]]>