A Framework for Deterministic Networking (DetNet) Controller Plane

DetNet (Deterministic Networking) provides the ability to carry specified unicast or multicast data flows for real-time applications with extremely low packet loss rates and assured maximum end-to-end delivery latency. A description of the general background and concepts of DetNet can be found in . The DetNet data plane is defined in a set of documents that are anchored by the DetNet Data Plane Framework (and the associated DetNet MPLS defined in and DetNet IP defined in and other data plane specifications defined in , , , and ). Note that in the DetNet overall architecture, the controller plane includes what are more traditionally considered separate control and management planes (see section 4.4.2 of ). Traditionally, the management plane is primarily involved with fault management, configuration management and performance management (sometimes accounting management and security management is also considered in the management plane (see section 4.2 of ), but not in the scope of this document), while the control plane is primarily responsible for the instantiation and maintenance of flows, MPLS label allocation and distribution, and active in-band or out-of-band signaling to support DetNet functions. In the DetNet architecture, all of this functionality is combined into a single controller plane. See Section 4.4.2 of and the aggregation of control and management planes in for further details. While the DetNet Architecture and Data Plane documents are primarily concerned with data plane operations, they do contain some requirements, and considerations for functions that would be required in order to automate DetNet service provisioning and monitoring via a DetNet controller plane (e.g., section 4 of ). The purpose of this document is to take these requirements and considerations into a single document and extend and discuss how various possible DetNet controller plane architectures could be used to satisfy these requirements, while not providing the protocol details for a DetNet controller plane solution. Such controller plane protocol solutions will be the subject of subsequent documents. Therefore, this document should be considered as the authoritative reference to be considered if/when protocol work on the DetNet controller plane starts.

Other DetNet documents, including , , and , among others, contain requirements for the Controller Plane. For convenience, these requirements have been compiled here. These requirements have been organized into 3 groups requirements primarily applicable to the control plane, requirements primarily applicable to the management plane and requirements applicable to both planes. In addition, security requirements for the DetNet Controller Plane have been discussed in , and a summary of those requirements is provided in Section 2.4. For the sake of clarity, when applicable, the document where the requirements originally appears is referenced.

The primary requirements for the DetNet Control Plane include: Support the dynamic instantiation, modification, and deletion of DetNet flows. This may include some or all of explicit path determination, link bandwidth reservations, restricting flows to specific links (e.g., IEEE 802.1 Time-Sensitive Networking (TSN) links), node buffer and other resource reservations, specification of required queuing disciplines along the path, ability to manage bidirectional flows, etc., as needed for a flow . Support DetNet flow aggregation and de-aggregation via the ability to dynamically create and delete flow aggregates (FAs), and be able to modify existing FAs by adding or deleting participating flows . Allow flow instantiation requests to originate in an end application (via an Application Programming Interface (API), via static provisioning, or via a dynamic control plane, such as an SDN (Software-Defined Networking). See for further discussion of these options. In the case of the DetNet MPLS data plane, manage DetNet Service Label (S-Label), Forwarding Label (F-Label), and Aggregation Label (A-Label) allocation and distribution . Also in the case of the DetNet MPLS data plane, support the DetNet service sub-layer, which provides DetNet service functions such as protection and reordering through the use of packet replication, elimination, and ordering functions (PREOF) . Support queue control techniques defined in Section 4.5 of and that require time synchronization among network data plane nodes. Advertise static and dynamic node and link characteristics such as capabilities and adjacencies to other network nodes (for dynamic signaling approaches) or to network controllers (for centralized approaches) . Scale to handle the number of DetNet flows expected in a domain (which may require per-flow signaling or provisioning) . Provision flow identification information at each of the nodes along the path. Flow identification may differ depending on the location in the network and the DetNet functionality (e.g., transit node vs. relay node) .

The primary requirements of the DetNet management plane are that it must be able to: Monitor the performance of DetNet flows and nodes to ensure that they are meeting required objectives, both proactively and on-demand . Support DetNet flow continuity check and connectivity verification functions . Support testing and monitoring of packet replication, duplicate elimination, and packet ordering functionality in the DetNet domain .

The following requirements apply to both the DetNet control and management planes: Operate in a converged network domain that contains both DetNet and non-DetNet flows . Adapt to DetNet domain topology changes such as links or nodes failures (fault recovery/restoration), additions and removals .

In addition to the above, the DetNet controller Plane should also satisfy security requirements derived from , which defines the security framework for DetNet. The following requirements are especially relevant: Integrity and authenticity of control/signaling packets: The controller plane should ensure that signaling and control messages cannot be modified or injected by unauthorized entities and prevent spoofing and segmentation attacks. Protection against controller compromise: Mechanisms should exist to verify the legitimacy of controllers and prevent unauthorized components from impersonating them. System-wide security design: The architecture must account for the possibility of compromised nodes or controllers, ensuring resilience so that the failure or subversion of a single component does not cause catastrophic impact. Timely delivery of control plane messages: The controller plane should ensure control and signaling messages are delivered without undue delay to prevent disruption of DetNet services without resource leakage.

As noted in the Introduction, the DetNet control plane is responsible for the instantiation and maintenance of flows, allocation and distribution of flow related information (e.g., MPLS label), and active in-band or out-of-band information distribution to support these functions. The following sections define three types of DetNet control plane architectures: a fully distributed control plane utilizing dynamic signaling protocols, a fully centralized SDN-like control plane, and a hybrid control plane containing both distributed protocols and centralized controlling. This document describes the various information exchanges between entities in the network for each type of these architectures and the corresponding advantages and disadvantages. In each of the following sections, there are examples to illustrate possible mechanisms that could be used in each type of the architectures. They are not meant to be exhaustive or to preclude any other possible mechanism that could be used in place of those used in the examples.

In a fully distributed configuration model, User-to-Network Interface (UNI) information is transmitted over a DetNet UNI protocol from the user side to the network side. Then UNI and network configuration information propagate in the network via distributed control plane signaling protocols. Such a DetNet UNI protocol is not necessary in the case that the End-systems are DetNet capable. Taking an RSVP-TE MPLS network as an example, where end systems are not part of the DetNet domain: Network nodes collect topology information and DetNet capabilities of the network nodes through IGP; The ingress edge node receives a flow establishment request from the UNI and calculates one or more valid path(s); The ingress node sends a PATH message with an explicit route through RSVP-TE. After receiving the PATH message, the egress edge node sends a RESV message with the distributed label and resource reservation request. In this example, both IGP and RSVP-TE may require extensions for DetNet.

In the fully SDN/centralized configuration model, flow/UNI information is transmitted from a centralized user controller or from applications via an API/ northbound interface to a centralized controller. Network node configurations for DetNet flows are performed by the controller using a protocol such as NETCONF /YANG DetNet YANG or PCE-CC . Take the following case as an example: A centralized controller collects topology information and DetNet capabilities of the network nodes via NETCONF/YANG; The controller receives a flow establishment request from a UNI and calculates one or more valid path(s) through the network; The controller chooses the optimal path and configures the devices along that path for DetNet flow transmission via PCE-CC. Protocols in the above example may require extensions for DetNet.

In the hybrid model, controller and control plane protocols work together to provide DetNet services, and there are a number of possible combinations. In the following case, RSVP-TE and controller are used together: A controller collects topology information and DetNet capabilities of the network nodes via an IGP and/or BGP-LS ; A controller receives a flow establishment request through API and calculates one or more valid path(s) through the network; Based on the calculation result, the controller distributes flow path information to the ingress edge node and configures network nodes along the path with necessary DetNet information (e.g., for replication/duplicate elimination) Using RSVP-TE, the ingress edge node sends a PATH message with an explicit route. After receiving the PATH message, the egress edge node sends a RESV message with the distributed label and resource reservation request. There are many other variations that could be included in a hybrid control plane. The requested DetNet extensions for a protocol in each possible case is for future work.

This section discusses requested control plane features for DetNet mechanisms as defined in , including explicit path, resource reservation, service protection (PREOF). Different DetNet services may implement any or all of them based on the requirements.

Explicit paths are required in DetNet to provide a stable forwarding service and guarantee that DetNet service is not impacted when the network topology changes. The following features are necessary in the control plane to implement explicit paths in DetNet: Path computation: DetNet explicit paths need to meet the SLA (Service Level Agreement) requirements of the application, which include bandwidth, maximum end- to-end delay, maximum end-to-end delay variation, maximum loss ratio, etc. In a distributed network system, an IGP with CSPF (Constrained Shortest Path First) may be used to compute a set of feasible paths for a DetNet service. In a centralized network system, a controller can compute paths satisfying the requirements of DetNet based on the network information collected from the DetNet domain. Path establishment: The computed path for the DetNet service has to be sent/configured/signaled to the network device, so the corresponding DetNet flow could pass through the network domain following the specified path.

DetNet flows are supposed to be protected from congestion, so sufficient resource reservation for a DetNet service could protect services from congestion. There are multiple types of resources in the network that could be allocated to DetNet flows, e.g., packet processing resources, buffer resources, and bandwidth of the output port. The network resource requested by a specified DetNet service is determined by the SLA requirements and network capability. Resource Allocation: Port bandwidth is one of the basic attributes of a network device which is easy to obtain or calculate. In current traffic engineering implementations, network resource allocation is synonymous with bandwidth allocation. A DetNet flow is characterized with a traffic specification as defined in , including attributes such as Interval, Maximum Packets Per Interval, and Maximum Payload Size. The traffic specification describes the worst case, rather than the average case, for the traffic, to ensure that sufficient bandwidth and buffering resources are reserved to satisfy the traffic specification. However, in the case of DetNet, resource allocation is more than simple bandwidth reservation. For example, allocation of buffers and required queuing disciplines during forwarding may be required as well. Furthermore, resources must be ensured to execute DetNet service sub-layer functions on the node, such as protection and reordering through the use of packet replication, duplicate elimination, and packet ordering functions (PREOF). Device configuration with or without flow discrimination: The resource allocation can be guaranteed by device configuration. For example, an output port bandwidth reservation can be configured as a parameter of queue management and the port scheduling algorithm. When DetNet flows are aggregated, a group of DetNet flows share the allocated resource in the network device. When the DetNet flows are treated independently, the device should maintain a mapping relationship between a DetNet flow and its corresponding resources.

DetNet path redundancy is supported via packet replication, duplicate elimination, and packet ordering functions (PREOF). A DetNet flow is replicated and goes through multiple networks paths to avoid packet loss caused by device or link failures. In general, current control plane mechanisms that can be used to establish an explicit path, whether distributed or centralized, support point-to-point (P2P) and point-to-multipoint (P2MP) path establishment. PREOF requires the ability to compute and establish a set of multiple paths (e.g., multiple LSP segments in an MPLS network) from the point(s) of packet replication to the point(s) of packet merging and ordering. Mapping of DetNet (member) flows to explicit path segments has to be ensured as well. Protocol extensions will be required to support these new features. Terminology will also be required to refer to this coordinated set of path segments (such as an 'LSP graph' in the case of the DetNet MPLS data plane).

For the purposes of this document, 'traditional MPLS' is defined as MPLS without the use of segment routing (see for a discussion of MPLS with segment routing) or MPLS-TP . In traditional MPLS domains, a dynamic control plane using distributed signaling protocols is typically used for the distribution of MPLS labels used for forwarding MPLS packets. The dynamic signaling protocols most commonly used for label distribution are LDP , RSVP-TE, and BGP (which enables BGP/MPLS-based Layer 3 VPNs Layer 2 VPNs and EVPN ). Any of these protocols could be used to distribute DetNet Service Labels (S-Labels) and Aggregation Labels (A-Labels) . As discussed in , S-Labels are similar to other MPLS service labels, such as pseudowire, L3 VPN, and L2 VPN labels, and could be distributed in a similar manner, such as through the use of targeted LDP or BGP. If these were to be used for DetNet, they would require extensions to support DetNet-specific features such as PREOF, aggregation (A-Labels), node resource allocation, and queue placement.

For the purposes of this document, 'traditional IP' is defined as IP without the use of segment routing (see for a discussion of IP with segment routing). This section will discuss possible protocol extensions to existing IP routing protocols. It should be noted that a DetNet IP data plane is simpler than a DetNet MPLS data plane , and doesn't support PREOF, so only one path per flow or flow aggregate is required.

Segment Routing is a scalable approach to building network domains that provides explicit routing via source routing encoded in packet headers and it is combined with centralized network control to compute paths through the network. Forwarding paths are distributed with associated policy to network edge nodes for use in packet headers. Segment Routing reduces the amount of network signaling associated with distributed signaling protocols such as RSVP-TE, and also reduces the amount of state in core nodes compared with that required for traditional MPLS and IP routing, as the state is now in the packets rather than in the routers. This could be useful for DetNet, where a very large number of flows through a network domain are expected, which would otherwise require the instantiation of state for each flow traversing each node in the network. Note that the DetNet MPLS and IP data planes described in and were constructed to be compatible with both types of segment routing, SR-MPLS and SRv6 .

To effectively manage DetNet flows, the controller plane will need have a clear understanding of the encapsulation and metadata capabilities of the underlying network nodes. This will require a control mechanism that can discover, configure, and manage these parameters for each flow. The controller plane needs to understand and manage the encapsulation and metadata capabilities of the network nodes to provision DetNet flows effectively. This process might need a discovery phase, in which the controller discovers which encapsulation types (e.g., MPLS, IP) and metadata schemes (e.g., sequencing, timestamping) each node supports. After discovery, the controller might instruct nodes on the specific encapsulation and companion metadata to apply for a given flow. This ensures that DetNet packets are handled consistently across the network. For example, the controller might instruct a node to use an MPLS header and add a sequence number for a particular flow.

The management plane includes the ability to statically provision network nodes and to use OAM to monitor DetNet performance and detect outages or other issues at the DetNet layer.

This document covers the general considerations for OAM.

Active PM is performed by injecting OAM packets into the network to estimate the performance of the network by measuring the performance of the OAM packets. Adding extra traffic can affect the delay and throughput performance of the network, and for this reason active PM is not recommended for use in operational DetNet domains. However, it is a useful test tool when commissioning a new network or during troubleshooting.

Passive PM, such as IOAM , monitors the actual service traffic in a network domain in order to measure its performance without having a detrimental effect on the network. As compared to Active PM, Passive PM is much preferred for use in DetNet domains.

The detailed requirements for connectivity and fault/defect detection and management in DetNet IP domain and DetNet MPLS domain are defined in and , respectively.

When there are multiple domains involved, one or multiple controller plane functions (CPF) would have to collaborate to implement the requests received from the flow management entity (FME, as defined in ) as per-flow, per-hop behaviors installed in the DetNet nodes for each individual flow. Adding multi-domain support might require some support at the CPF. For example, CPFs of different domains, e.g., PCEs need to discover each other, authenticate and negotiate per-hop behaviors. Furthermore, in the case of wireless domains, the per-domain RAW specific functions like the PLR (Point of Local Repair)s have to be also considered, e.g., in addition to the PCEs. Depending on the multi-domain support provided by the application plane, the controller plane might be relieved from some responsibilities (e.g., if the application plane takes care of splitting what needs to be provided by each domain).

This document has no actions for IANA. Note to RFC Editor: this section may be removed on publication as an RFC.

This document provides a framework for the DetNet controller plane, and does not include any protocol specifications. Any future specification that is defined to support the DetNet controller plane is expected to include appropriate security considerations. For overall security considerations of DetNet see and

Thanks to Jim Guichard, Donald Eastlake, and Stewart Bryant for their review comments. Authors would also like to thank Deb Cooley, Mike Bishop, Mohamed Boucadair and Dave Thaler for their comments during the different directorate and IESG reviews.

Fengwei Qin China Mobile Email: qinfengwei@chinamobile.com