Operations, Administration and Maintenance (OAM) features for RAW

Reliable and Available Wireless (RAW) is an effort that extends DetNet to approach end-to-end deterministic performances over a network that includes scheduled wireless segments. In wired networks, many approaches to Quality of Service (QoS) tried to implement traffic differentiation so that routers handle differently each type of packets. However, this differentiated treatment was expensive for most applications. 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. Their work encompasses the data plane, OAM, time synchronization, management, control, and security aspects. However, wireless networks create specific challenges. First of all, radio bandwdidth is significantly lower than for wired networks. In these conditions, the volume of signaling messages has to be very limited. Even worse, wireless links are lossy: a layer 2 transmission may or may not be decoded correctly by the receiver, depending on a large set of parameters. Thus, providing high reliability through only wireless segments only is particularly challenging. Last but not least, radio links present very unstable characteristics. If the wireless networks use an unlicensed band, packet losses are not anymore temporally and spatially independent. Typically, links may exhibit a very bursty characteristic, where several consecutive packets may be dropped. Thus, providing availability and reliability on top of the wireless infrastructure requires specific layer 3 mechanisms to counteract these bursty losses. Operations, Administration, and Maintenance (OAM) Tools are of primary importance for IP networks . It defines a toolset for fault detection and isolation, and for performance measurement. The main purpose of this document is to detail the specific requirements of the OAM features recommended to construct a predictable communication infrastructure on top of a collection of wireless segments. This document describes the benefits, problems, and trade-offs for using OAM in wireless networks to provide availability and predictability. In this document, the term OAM will be used according to its definition specified in . We expect to implement an OAM framework in RAW networks to maintain a real-time view of the network infrastructure, and its ability to respect the Service Level Objectives (SLO), such as delay and reliability, assigned to each data flow.

OAM entity: a data flow to be controlled; Maintenance End Point (MEP): OAM devices crossed when entering/exiting the network. In RAW, it corresponds mostly to the source or destination of a data flow. OAM message can be exchanges between two MEPs; Maintenance Intermediate end Point (MIP): OAM devices along the flow; OAM messages can be exchanged between a MEP and a MIP; Defect: a temporary change in the network (e.g. a radio link which is broken due to a mobile obstacle); Fault: a definite change which may affect the network performance, e.g. a node runs out of energy.

OAM Operations, Administration, and Maintanence DetNet Deterministic Networking SLO Service Level Objective QoS Quality of Service SNMP Simple Network Management Protocol SDN Software Defined Network

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 when, and only when, they appear in all capitals, as shown here.

RAW networks expect to make the communications reliable and predictable on top of a wireless network infrastructure. Most critical applications will define an SLO to be required for the data flows it generates. RAW considers network plane protocol elements such as OAM to improve the RAW operation at the service and the forwarding sub-layers. To respect strict guarantees, RAW relies on an orchestrator able to monitor and maintain the network. Typically, a Software Defined Network (SDN) controller is in charge of scheduling the transmissions in the deployed network, based on the radio link characteristics, SLO of the flows, the number of packets to forward. Thus, resources have to be provisioned a priori to handle any defect. OAM represents the core of the over provisioning process, and maintains the network operational by updating the schedule dynamically. Fault-tolerance also assumes that multiple paths have to be provisioned so that an end-to-end circuit keeps on existing whatever the conditions. The replication/elimination processes (PREOF) on a node is typically controlled by the central controller/orchestrator. OAM is in charge of controlling that PREOF is working properly on a node and within the domain. To be energy-efficient, reserving some dedicated out-of-band resources for OAM seems idealistic, and only in-band solutions are considered here. RAW supports both proactive and on-demand troubleshooting.

OAM features will enable RAW with robust operation both for forwarding and routing purposes.

Several solutions (e.g., Simple Network Management Protocol (SNMP), YANG-based data models) are already in charge of collecting the statistics. That way, we can encapsulate these statistics in specific monitoring packets, to send them to the controller.

We need to verify that two endpoints are connected. In other words, there exists "one" way to deliver the packets between two endpoints A and B.

Additionally, to the Continuity Check, we have to verify the connectivity. This verification considers additional constraints, i.e., the absence of misconnection. In particular, the resources have to be reserved by a given flow, and no packets from other flows steal the corresponding resources. Similarly, the destination does not receive packets from different flows through its interface. It is worth noting that the control and data packets may not follow the same path, and the connectivity verification has to be conducted in-band without impacting the data traffic. Test packets must share the fate with the monitored data traffic without introducing congestion in normal network conditions.

Ping and traceroute are two very common tools for diagnostic. They help to identify a subset of the list of routers in the route. However, to be predictable, resources are reserved per flow in RAW. Thus, we need to define route tracing tools able to track the route for a specific flow. Wireless networks are meshed by nature: we have many redundant radio links. These meshed networks are both an asset and a drawback: while several paths exist between two endpoints, we should choose the most efficient one(s), concerning specifically the reliability, and the delay. Thus, multipath routing can be considered to make the network fault-tolerant. Even better, we can exploit the broadcast nature of wireless networks to exploit meshed multipath routing: we may have multiple Maintenance Intermediate Endpoints for each hop in the path. In that way, each Maintenance Intermediate Endpoint has several possible next hops in the forwarding plane. Thus, all the possible paths between two maintenance endpoints should be retrieved.

RAW expects to operate fault-tolerant networks. Thus, we need mechanisms able to detect faults, before they impact the network performance. The network has to detect when a fault occurred, i.e., the network has deviated from its expected behavior. While the network must report an alarm, the cause may not be identified precisely. For instance, the end-to-end reliability has decreased significantly, or a buffer overflow occurs.

The network has isolated and identified the cause of the fault. For instance, the quality of a specific link has decreased, requiring more retransmissions, or the level of external interference has locally increased.

The network has to expose a collection of metrics to support an operator making proper decisions, including: Packet losses: the time-window average and maximum values of the number of packet losses have to be measured. Many critical applications stop to work if a few consecutive packets are dropped; Received Signal Strength Indicator (RSSI) is a very common metric in wireless to denote the link quality. The radio chipset is in charge of translating a received signal strength into a normalized quality indicator; Delay: the time elapsed between a packet generation / enqueuing and its reception by the next hop; Buffer occupancy: the number of packets present in the buffer, for each of the existing flows. These metrics should be collected: per virtual circuit to measure the end-to-end performance for a given flow. Each of the paths has to be isolated in multipath routing strategies; per radio channel to measure, e.g., the level of external interference, and to be able to apply counter-measures (e.g. blacklisting) per device to detect misbehaving node, when it relays the packets of several flows.

We have to minimize the number of statistics / measurements to exchange: energy efficiency: low-power devices have to limit the volume of monitoring information since every bit consumes energy. bandwidth: wireless networks exhibit a bandwidth significantly lower than wired, best-effort networks. per-packet cost: it is often more expensive to send several packets instead of combining them in a single link-layer frame. Thus, localized and centralized mechanisms have to be combined together, and additional control packets have to be triggered only after a fault detection.

RAW aims to enable real-time communications on top of a heterogeneous architecture. Since wireless networks are known to be lossy, RAW has to implement strategies to improve reliability on top of unreliable links. Hybrid Automatic Repeat reQuest (ARQ) has typically to enable retransmissions based on the end-to-end reliability and latency requirements. To make correct decisions, the controller needs to know the distribution of packet losses for each flow, and each hop of the paths. In other words, the average end-to-end statistics are not enough. They must allow the controller to predict the worst-case.

RAW targets also low-power wireless networks, where energy represents a key constraint. Thus, we have to take care of power and bandwidth consumption. The following techniques aim to reduce the cost of such maintenance: piggybacking: some control information are inserted in the data packets if they do not fragment the packet (i.e., the MTU is not exceeded). Information Elements represent a standardized way to handle such information; flags/fields: we have to set-up flags in the packets to monitor to be able to monitor the forwarding process accurately. A sequence number field may help to detect packet losses. Similarly, path inference tools such as insert additional information in the headers to identify the path followed by a packet a posteriori.

RAW needs to implement a self-healing and self-optimization approach. The network must continuously retrieve the state of the network, to judge about the relevance of a reconfiguration, quantifying: the cost of the sub-optimality: resources may not be used optimally (e.g., a better path exists); the reconfiguration cost: the controller needs 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.

To be fault-tolerant, several paths can be reserved between two maintenance endpoints. They must be node-disjoint so that a path can be available at any time.

When multiple paths are reserved between two maintenance endpoints, they may decide to replicate the packets to introduce redundancy, and thus to alleviate transmission errors and collisions. For instance, in , the source node S is transmitting the packet to both parents, nodes A and B. Each maintenance endpoint will decide to trigger the replication/elimination process when a set of metrics passes through a threshold value.

(A) => (C) => (E) === // \\// \\// \\ source (S) //\\ //\\ (R) (root) \\ // \\ // \\ // ===> (B) => (D) => (F) === ]]>

Because the QoS criteria associated with a path may degrade, the network has to provision additional resources along the path. We need to provide mechanisms to patch a schedule (changing the channel offset, allocating more timeslots, changing the path, etc.).

Since RAW expects to support real-time flows, we have to support soft-reconfiguration, where the novel resources are reserved before the ancient ones are released. Some mechanisms have to be proposed so that packets are forwarded through the novel track only when the resources are ready to be used, while maintaining the global state consistent (no packet reordering, duplication, etc.)

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

This section will be expanded in future versions of the draft.

TBD