Operations, Administration, and Maintenance (OAM) for Deterministic Networking (DetNet) with the MPLS Data Plane
Extracted elements (26)
d-ACH introduces per-packet-level fields (Sequence Number, Node ID, Level, Session ID) beyond the standard PW Associated Channel Header because DetNet service sub-layer PREOF functions operate per packet rather than per segment, requiring OAM packets to carry sequencing information for PREOF processing.
In the tunneling model of OAM interworking, usually only one active OAM protocol is used, with its test packets tunneled through other domains along with the data flow, ensuring fate sharing between test and data packets — a key property for meaningful OAM measurements.
Mapping between a DetNet flow and a TSN Stream is performed only on the edge node of the TSN sub-network; intermediate TSN nodes do not need to recognize the S-Label. This design, using passive and active stream identification functions at the edge, simplifies intermediate node forwarding by keeping identification within the Ethernet header.
The Sequence Number field in d-ACH is only 8 bits (range 0–255) because the rate of DetNet active OAM packets is significantly lower than that of DetNet app-flow packets; therefore, wrapping around within the circular sequence space is not a practical concern.
DetNet MPLS OAM packets use the same S-Label as DetNet flow data packets, so the TSN edge mapping function treats them as data packets, achieving fate sharing of OAM and data packets within the TSN sub-network.
For interworking between DetNet MPLS and DetNet IP OAM domains, using the same protocol (e.g., BFD) over both segments simplifies error mapping in the peering model; respective BFD sessions can be in a concatenated relationship per RFC 5880 Section 6.8.17.
Performance monitoring across DetNet MPLS and DetNet IP domains can use STAMP (RFC 8762) and its extensions (RFC 8972) to measure packet loss and delay, enabling composable metrics (RFC 6049) that reflect end-to-end DetNet service performance.
The tunneling model of OAM interworking requires that the remote peer of the E2E OAM domain supports the active OAM protocol selected at the ingress endpoint. For example, if BFD is used in the DetNet MPLS domain, BFD (RFC 5885) must be supported at TSN endpoints of the DetNet service.
A DetNet node MUST be provisioned with a Node ID that is unique within the DetNet domain. The 20-bit Node ID field identifies the originating DetNet node.
An active OAM packet MUST include d-ACH immediately following the S-Label. The S-Label that identifies a DetNet flow MUST be at the bottom of the MPLS label stack.
Bits 0–3 of the d-ACH MUST be set to 0b0001. This distinguishes the packet from an IP packet (per RFC 4928) and from a DetNet data packet (per RFC 8964).
For DetNet active OAM packets, PREOF MUST use the Sequence Number field in d-ACH as the source of sequencing information; RFC 8964 Section 4.2.1 (which governs the generic Control Word Sequence Number) does not apply.
The Channel Type field (16 bits) MUST be one of the values listed in the IANA 'MPLS Generalized Associated Channel (G-ACh) Types (including Pseudowire Associated Channel Types)' registry.
The originator DetNet node MUST set the Sequence Number field before transmitting a packet. The initial value SHOULD be random (unpredictable), and the originator SHOULD increment it by 1 for each active OAM packet.
Unused flag bits (U) in the d-ACH Flags field MUST be set to 0 on transmission and MUST be ignored on receipt.
When using the peering model for CFM OAM interworking between TSN and DetNet MPLS domains, the border node MUST support RFC 7023, which specifies the mapping of defect states between Ethernet Attachment Circuits and associated Ethernet PWs.
When using the tunneling model for OAM interworking between DetNet MPLS and TSN, active OAM test packets MUST be processed in the TSN domain based on their S-Label and Class of Service marking (the Traffic Class field value).
DetNet flows are encapsulated within the MPLS underlay analogous to pseudowires as specified in RFC 3985 and RFC 4385, using the Generic PW MPLS Control Word (4 bits set to zero followed by a 28-bit Sequence Number).
The d-ACH Sequence Number is an 8-bit unsigned circular field with a sequence space of 0–255. App-flow and OAM use separate, independent sequence number spaces; PREOF algorithms execute with respect to the sequence number space identified by the flow's characteristic information.
The d-ACH Version field is 4 bits wide. This document specifies version 0 as the only currently defined version.
The Level field is a 3-bit field semantically analogous to the Maintenance Domain Level in IEEE 802.1Q. It is used to create hierarchical relationships between OAM domains to cope with the 'all active path forwarding' characteristics of PREOF.
The Session ID is a 4-bit field that distinguishes multiple simultaneously active OAM sessions originating from the same DetNet node at a given Level. A single Maintenance End Point may have multiple concurrent sessions.
IANA created the 'DetNet Associated Channel Header (d-ACH) Flags' registry within a new 'DetNet Associated Channel Header (d-ACH) Flags' registry group. The registration procedure is IETF Review. The 5-bit Flags field (bits 0–4) is initially Unassigned.
Security considerations from the DetNet architecture (RFC 8655), MPLS data plane (RFC 8964), DetNet security considerations (RFC 9055), and the DetNet OAM Framework apply to this specification. Security concerns related to MPLS OAM tools such as LSP Ping (RFC 8029) and BFD over PW (RFC 5885) also apply.
DetNet active OAM MPLS encapsulation stacks layers as: DetNet OAM Packet / DetNet Associated Channel Header (d-ACH) / S-Label / F-Label(s) / Data-Link / Physical. For MPLS over UDP/IP, a UDP Header and IP Header are inserted below the F-Labels.
The DetNet Associated Channel Header (d-ACH) is a 64-bit structure with fields: bits 0–3 fixed to 0b0001, Version (4 bits), Sequence Number (8 bits unsigned circular), Channel Type (16 bits), Node ID (20 bits unsigned), Level (3 bits), Flags (5 bits), and Session ID (4 bits).