Framework of Operations, Administration and Maintenance (OAM) for Deterministic Networking (DetNet)

Introduction Deterministic Networking (DetNet) has proposed to provide a bounded end-to-end latency on top of the network infrastructure, comprising both Layer 2 bridged and Layer 3 routed segments. That work encompasses the data plane, OAM, time synchronization, management, control, and security aspects. Operations, Administration, and Maintenance (OAM) Tools are of primary importance for IP networks . DetNet OAM should provide a toolset for fault detection, localization, and performance measurement. This document's primary purpose is to detail the specific requirements of the OAM features recommended to maintain a deterministic/reliable network. Specifically, it investigates the requirements for a deterministic network, supporting critical flows. In this document, the term OAM will be used according to its definition specified in . DetNet expects to implement an OAM framework to maintain a real-time view of the network infrastructure, and its ability to respect the Service Level Objectives (SLOs), such as in-order packet delivery, packet delay, delay variation, and packet loss ratio, assigned to each DetNet flow. This document lists the functional requirements toward OAM for a DetNet domain. The list can further be used for gap analysis of available OAM tools to identify possible enhancements of existing or whether new OAM tools are required to support proactive and on-demand path monitoring and service validation.

Definitions This document uses definitions, particularly of a DetNet flow, provided in Section 2.1 of . The following terms are used throughout this document as defined below:

DetNet OAM domain: a DetNet network used by the monitored DetNet flow. A DetNet OAM domain (also referred to in this document as "OAM domain") may have MEPs on its edge and MIPs within.
DetNet OAM instance: a function that monitors a DetNet flow for defects and/or measures its performance metrics. Within this document, a shorter version, OAM instance, is used interchangeably.
Maintenance End Point (MEP): an OAM instance that is capable of generating OAM test packets in the particular sub-layer of the DetNet OAM domain.
Maintenance Intermediate Point (MIP): an OAM instance along the DetNet flow in the particular sub-layer of the DetNet OAM domain. An active MIP MUST respond to an OAM message generated by the MEP at its sub-layer of the same DetNet OAM domain.
Control and management plane: the control and management planes are used to configure and control the network. Relative to a DetNet flow, the control and/or management plane can be out-of-band.
Active measurement methods (as defined in ) modify a DetNet flow by injecting specially constructed test packets ).
Passive measurement methods infer information by observing unmodified existing flows.
Hybrid measurement methods is the combination of elements of both active and passive measurement methods.
In-band OAM is an active OAM 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.
Out-of-band OAM is an active OAM whose path through the DetNet domain is not topologically identical to the path of the monitored DetNet flow, or its test packets receive different QoS and/or PREOF treatment, or both.
On-path telemetry can be realized as a hybrid OAM method. The origination of the telemetry information is inherently in-band as packets in a DetNet flow are used as triggers. Collection of the on-path telemetry information can be performed using in-band or out-of-band OAM methods.

Terminology OAM: Operations, Administration, and Maintenance DetNet: Deterministic Networking PREOF: Packet Replication, Elimination and Ordering Functions SLO: Service Level Objective

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 and when, and only when, they appear in all capitals, as shown here. The requirements language is used in and applies to future implementations of DetNet OAM.

Role of OAM in DetNet DetNet networks expect to provide communications with predictable low packet delay, packet loss, and packet misordering. Most critical applications will define a set of SLOs to be required for the DetNet flows it generates. To respect strict guarantees, DetNet can use an orchestrator able to monitor and maintain the network. Typically, a Software-Defined Network controller places DetNet flows in the deployed network based on their SLOs. Thus, resources have to be provisioned a priori for the regular operation of the network. Many legacy OAM tools can be used in DetNet networks, but they are not able to cover all the aspects of deterministic networking. Fulfilling strict guarantees is essential for DetNet flows, resulting in new DetNet-specific functionalities that must be covered with OAM. Filling these gaps is inevitable and needs accurate consideration of DetNet specifics. Similar to DetNet flows themselves, their OAM needs careful end-to-end engineering as well. For example, appropriate placing of MEPs along the path of a DetNet flow is not always a trivial task and may require proper design, together with the design of the service component of a given DetNet flow. There are several DetNet-specific challenges for OAM. Bounded network characteristics (e.g., delay, loss) are inseparable service parameters; therefore, Performance Monitoring OAM is a key topic for DetNet. OAM tools are needed to monitor each SLO without impacting the DetNet flow characteristics. A further challenge is strict resource allocation. Resources used by OAM must be considered and allocated to avoid disturbing DetNet flows. The DetNet Working Group has defined two sub-layers:

DetNet service sub-layer, at which a DetNet service (e.g., service protection) is provided.
DetNet forwarding sub-layer, which optionally provides resource allocation for DetNet flows over paths provided by the underlying network.

OAM mechanisms exist for the DetNet forwarding sub-layer, but the service sub-layer requires new OAM procedures. These new OAM functions must allow, for example, to recognize/discover DetNet relay nodes, to get information about their configuration, and to check their operation or status. DetNet service sub-layer functions use a sequence number for PREOF. That creates a challenge for inserting OAM packets in the DetNet flow. Fault tolerance also assumes that multiple paths could be provisioned to maintain an end-to-end circuit by adapting to the existing conditions. The DetNet Controller Plane, e.g., central controller/orchestrator, controls the PREOF on a node. OAM is expected to support monitoring and troubleshooting PREOF on a particular node and within the domain. Note that a distributed architecture of the DetNet Control Plane can also control PREOF in those scenarios where DetNet solutions involve more than one single central controller. The DetNet forwarding sub-layer is based on preexisting technologies and has much better coverage regarding OAM. However, the forwarding sub-layer is terminated at DetNet relay nodes, so the end-to-end OAM state of forwarding may be created only based on the status of multiple forwarding sub-layer segments serving a given DetNet flow (e.g., in case of DetNet MPLS, there may be no end-to-end LSP below the DetNet Pseudowire).

Operation OAM features will enable DetNet with robust operation both for forwarding and routing purposes. It is worth noting that the test and data packets are expected to follow the same path, i.e., connectivity verification has to be conducted in-band without impacting data traffic. It is expected that test packets share fate with the monitored data traffic without introducing congestion in normal network conditions.

Information Collection Information about the state of the network can be collected using several mechanisms. Some protocols, e.g., Simple Network Management Protocol, send queries. Others, e.g., YANG-based data models, generate notifications based on the publish-subscribe method. In either way, information is collected and sent using the DetNet Controller Plane. Also, we can characterize methods of transporting OAM information relative to the path of data. For instance, OAM information may be transported in-band or out-of-band relative to the DetNet flow. In case of the former, the telemetry information uses resources allocated for the monitored DetNet flow. If an in-band method of transporting telemetry is used, the amount of generated information needs to be carefully analyzed, and additional resources must be reserved. defines the in-band transport mechanism where telemetry information is collected in the data packet on which information is generated. Two tracing methods are described - end-to-end, i.e., from the ingress and egress nodes, and hop-by-hop, i.e., like end-to-end with additional information from transit nodes. and are examples of out-of-band telemetry transport. In the former case, information is transported by each node traversed by the data packet of the monitored DetNet flow in a specially constructed packet. In the latter, information is collected in a sequence of follow-up packets that traverse the same path as the data packet of the monitored DetNet flow. In both methods, transport of the telemetry can avoid using resources allocated for the DetNet domain.

Continuity Check Continuity check is used to monitor the continuity of a path, i.e., that there exists a way to deliver packets between MEP A and MEP B. The continuity check detects a network failure in one direction, from the MEP transmitting test packets to the remote egress MEP. Continuity check in a DetNet OAM domain monitors the DetNet forwarding sub-layer and thus is not affected by a PREOF that operates at the DetNet service sub-layer (.

Connectivity Verification In addition to the Continuity Check, DetNet solutions have to verify connectivity. This verification considers an additional constraint, i.e, the absence of misconnection. The misconnection error state is entered after several consecutive test packets from other DetNet flows are received. The definition of the conditions for entry and exit of a misconnection error state is outside the scope of this document. Connectivity verification in a DetNet OAM domain monitors the DetNet forwarding sub-layer and thus is not affected by PREOF that operates at the DetNet service sub-layer (.

Route Tracing Ping and traceroute are two ubiquitous tools that help localize and characterize a failure in the network using an echo request/reply mechanism. They help to identify a subset of the routers in the path. However, to be predictable, resources are reserved per flow in DetNet. Thus, DetNet needs to define route tracing tools able to trace the route for a specific flow. Also, tracing can be used for the discovery of the Path Maximum Transmission Unit or location of elements of PREOF for the particular route in the DetNet domain. DetNet is not expected to use Equal-Cost Multipath (ECMP) . As the result, DetNet OAM in an ECMP environment is outside the scope of this document.

Fault Verification/Detection DetNet expects to operate fault-tolerant networks. Thus, mechanisms able to detect faults before they impact network performance are needed. The network has to detect when a fault has occurred, i.e., the network has deviated from its expected behavior. Fault detection can be based on proactive OAM protocols like continuity check or on-demand lmethods ike ping. While the network must report an alarm, the cause may not be identified precisely. Examples of such alarms are significant degradation of the end-to-end reliability, or a buffer overflow occurs.

Fault Localization and Characterization An ability to localize a network defect and provide its characterization are necessary elements of network operation.

Fault localization, a process of deducing the location of a network failure from a set of observed failure indications, might be achieved, for example, by tracing the route of the DetNet flow in which the network failure was detected. Another method of fault localization can correlate reports of failures from a set of interleaved sessions monitoring path continuity.
Fault characterization is a process of identifying the root cause of the problem. For instance, misconfiguration or malfunction of PREOF elements can be the cause of erroneous packet replication or extra packets being flooded in the DetNet domain.

Use of Hybrid OAM in DetNet Hybrid OAM methods are used in performance monitoring and defined in as:

Hybrid Methods are Methods of Measurement that use a combination of Active Methods and Passive Methods.

A hybrid measurement method can produce metrics as close to measured using a passive measurement method. The passive methods measure metrics closest to the network's actual conditions. A hybrid method, even if it alters something in a data packet, even if that is as little as the value of a designated field in the packet encapsulation, is considered an approximation of a passive measurement method. One example of such a hybrid measurement method is the Alternate Marking method (AMM) described in . As with all on-path telemetry methods, AMM in a DetNet domain with the IP data plane is natively in-band with respect to the monitored DetNet flow. Because the marking is applied to a data flow, measured metrics are directly applicable to the DetNet flow. AMM minimizes the additional load on the DetNet domain by using nodal collection and computation of performance metrics in combination with optionally using out-of-band telemetry collection for further network analysis.

Administration The ability to expose a collection of metrics to support an operator's decision-making is essential. The following performance metrics are useful:

Queuing Delay: the time elapsed between enqueuing a packet and its transmission to the next hop.
Buffer occupancy: the number of packets present in the buffer, for each of the existing flows.
Per DetNet flow, a metric reflecting end-to-end performance for a given flow. Each of the paths has to be isolated in a multipath routing environment.
Per path, detection of misbehaving path(s) when multiple paths are used for the service protection.
Per device, detection of a misbehaving device.

Collection of metrics It is important to optimize the volume and frequency of statistics/measurement collection, whether the mechanisms are distributed, centralized, or both. Periodic and event-triggered collection information characterizing the state of a network is an example of mechanisms to achieve the optimization.

Worst-case metrics DetNet aims to enable real-time communications on top of a heterogeneous multi-hop architecture. To make correct decisions, the DetNet Controller Plane needs timely information about packet losses/delays for each flow, and each hop of the paths. In other words, just the average end-to-end statistics are not enough. The collected information must be sufficient to allow a system to predict the worst-case scenario.

Maintenance Service protection (provided by the DetNet Service sub-layer) is designed to mitigate simple network failures more rapidly than the expected response time of the DetNet Controller Plane. In the face of events that impact network operation (e.g., link up/down, device crash/reboot, flows starting and ending), the DetNet Controller Plane needs to perform repair and re-optimization actions in order to permanently ensure SLOs of all active flows with minimal waste of resources. The Controller Plane is expected to be able to continuously retrieve the state of the network, to evaluate conditions and trends about the relevance of a reconfiguration, quantifying:

the cost of the sub-optimality: resources may not be used optimally (i.e., a better path exists).
the reconfiguration cost: the DetNet Controller Plane needs an ability to trigger some reconfigurations. For this transient period, resources may be twice reserved, and control packets have to be transmitted.

Thus, reconfiguration may only be triggered if the gain is significant.

Replication / Elimination When multiple paths are reserved between two MEPs, packet replication may be used to introduce redundancy and alleviate transmission errors and collisions. For instance, in , the source device S is transmitting a packet to devices A and B.

Packet Replication: S transmits twice the same data packet, to nodes A and B. (A) => (C) => (E) === // \\// \\// \\ source (S) //\\ //\\ (R) (root) \\ // \\ // \\ // ===> (B) => (D) => (F) === ]]>

Resource Reservation Because the quality of service criteria associated with a path may degrade, the network has to provision additional resources along the path.

Requirements According to , DetNet functionality is divided into forwarding and service sub-layers. The DetNet forwarding sub-layer includes DetNet transit nodes and may allocate resources for a DetNet flow over paths provided by the underlay network. The DetNet service sub-layer includes DetNet relay nodes and provides a DetNet service (e.g., service protection). This section lists general requirements for DetNet OAM as well as requirements in each of the DetNet sub-layers of a DetNet domain.

It MUST be possible to initiate a DetNet OAM session from a MEP located at a DetNet node towards MEP(s) downstream from that DetNet node within the given domain at a particular DetNet sub-layer.
It MUST be possible to initiate a DetNet OAM session from using any of DetNet Controller Plane solution, e.g., centralized controller.
DetNet OAM MUST support proactive OAM monitoring and measurement methods.
DetNet OAM MUST support on-demand OAM monitoring and measurement methods.
DetNet OAM MUST support unidirectional OAM methods, continuity check, connectivity verification, and performance measurement.
OAM methods MAY combine in-band monitoring or measurement in the forward direction and out-of-bound notification in the reverse direction, i.e., towards the ingress MEP.
DetNet OAM MUST support bi-directional DetNet flows.
DetNet OAM MAY support bi-directional OAM methods for bi-directional DetNet flows. OAM test packets used for monitoring and measurements MUST be in-band in both directions.
DetNet OAM MUST support proactive monitoring of a DetNet device reachability for a given DetNet flow.
DetNet OAM MUST support hybrid performance measurement methods.
Calculated performance metrics MUST include but are not limited to throughput, packet loss, out of order, delay, and delay variation metrics. provides detailed information on performance measurement and performance metrics.

DetNet OAM MUST support Path Maximum Transmission Unit discovery.
DetNet OAM MUST support Remote Defect Indication notification to the DetNet OAM instance performing continuity checking.
DetNet OAM MUST support monitoring levels of resources allocated for a particular DetNet flow. Such resources include but are not limited to buffer utilization and scheduler transmission calendar.
DetNet OAM MUST support monitoring any subset of paths traversed through the DetNet domain by a DetNet flow.

The OAM functions for the DetNet service sub-layer allow, for example, to recognize/discover DetNet relay nodes, to get information about their configuration, and to check their operation or status. The requirements on OAM for a DetNet relay node are:

DetNet OAM MUST provide OAM functions for the DetNet service sub-layer.
DetNet OAM MUST support the discovery of DetNet relay nodes in a DetNet network.
DetNet OAM MUST support the discovery of PREOF locations in the domain.
DetNet OAM MUST support the collection of the DetNet service sub-layer specific (configuration/operation/status) information from DetNet relay nodes.
DetNet OAM MUST support excercising functionality of PREOF in the domain.
DetNet OAM MUST work for DetNet data planes - MPLS and IP.
DetNet OAM MUST support a defect notification mechanism, like Alarm Indication Signal. Any DetNet relay node providing service for a given DetNet flow MAY originate a defect notification addressed to any subset of DetNet relay nodes along that flow.
DetNet OAM MUST be able to measure metrics (e.g. delay) inside a collection of OAM sessions, specially for complex DetNet flows, with PREOF features.

IANA Considerations This document has no actionable requirements for IANA. This section can be removed before publication.

Security Considerations This document lists the OAM requirements for a DetNet domain and does not raise any security concerns or issues in addition to ones common to networking and those specific to DetNet discussed in Section 9 of .

Acknowledgments The authors express their appreciation and gratitude to Pascal Thubert for the review, insightful questions, and helpful comments.