Deterministic Networking (DetNet) Data Plane: IEEE 802.1 Time-Sensitive Networking over MPLS
Extracted elements (20)
Due to intentional similarities between DetNet PREOF and TSN FRER functions, service protection interworking is conceptually possible between the TSN and DetNet domains, potentially requiring copying of sequence number fields from TSN (L2) to PW (MPLS) encapsulation. However, such interworking is explicitly left out of scope for further study.
The basic approach builds on existing PW encapsulations for the DetNet service sub-layer and existing MPLS-TE encapsulations for the forwarding sub-layer. This reuse of established mechanisms is intentional, as edge node functions are described as 'functionally similar to PW T-PE nodes which use MPLS-TE LSPs.'
TSN Streams are treated as App-flows from the DetNet MPLS perspective, meaning a single DetNet MPLS flow can aggregate multiple TSN Streams. The whole DetNet domain behaves as a TSN relay node for the TSN Streams, with service proxies acting as ports of that relay node.
From the TSN network perspective, the DetNet domain is a single hop (single TSN relay node), so some parameters such as delay may differ significantly between the DetNet segment and native TSN segments. The bandwidth of the DetNet network must be set large enough to handle all TSN flows it supports.
MPLS DetNet edge nodes participate in both the DetNet domain and the connected TSN network. Management plane techniques share similarities, but control plane protocols differ substantially — for example, RSVP-TE (DetNet) and MSRP (TSN) behave differently, making control plane design a critical aspect of TSN-DetNet interworking.
Configuration information MUST be provisioned per DetNet flow, covering TSN-related configuration per IEEE 802.1Q/CB/CBdb, DetNet MPLS configuration per RFC 8964, and App-flow identification information to map TSN Streams to DetNet flows.
If the PE decides to forward a packet, it MUST be forwarded according to the TSN-Stream-specific configuration. If there are no TSN-Stream-specific forwarding configurations, the PE MUST flood the packet to other locally attached CEs and to the DetNet service proxy. If the administrative policy does not allow flooding, the PE MUST drop the packet.
Implementations SHALL use management and control information to ensure TSN-specific functions of the edge node according to the expectations of the connected TSN network.
N:1 mapping (aggregating multiple TSN Streams in a single DetNet flow) SHALL be supported. The management or control function that provisions flow mapping SHALL ensure that adequate resources are allocated and configured to fulfill the service requirements of the mapped flows.
Network fragmentation for DetNet MUST be avoided because it is inconsistent with the packet delivery times needed for DetNet. IPv6 fragmentation MUST NOT be used, and IPv4 packets MUST be sent with the DF bit set.
The network operator MUST ensure that all the DetNet encapsulation overhead plus the maximum TSN App-flow frame size does not exceed the DetNet network's MTU, as fragmentation is prohibited.
When a DetNet service proxy receives a packet from the TSN entity, it MUST check whether such an App-flow is present in its mapping table. If present, it MUST forward the packet to the DetNet service and forwarding sub-layers; if no match is found, it MUST drop the packet.
When a PE receives a packet from a CE and no Stream ID is matched and no other VPN service is configured for the AC, the packet MUST be dropped.
When a PE receives a packet from the service proxy function, it MUST handle the packet as defined in RFC 8964. When a PE receives an MPLS packet from a remote PE and the top label is a DetNet S-Label advertised by this node, the PE MUST forward the packet according to the configured DetNet service and forwarding sub-layer rules.
DetNet edge nodes (T-PE) sit at the boundary of the DetNet domain and are responsible for mapping non-DetNet-aware L2 TSN traffic to DetNet services, as well as imposing and disposing the required DetNet encapsulation. They function similarly to PW T-PE nodes using MPLS-TE LSPs.
PREOF (Packet Replication, Elimination and Ordering Functions) are available only within the DetNet domain, as DetNet flow IDs and sequence numbers are not valid outside the DetNet network. MPLS edge nodes terminate all related information elements encoded in MPLS labels.
The DetNet MPLS data plane uses an MPLS service label (S-Label) to identify both DetNet flow identity and MPLS payload type, forwarding labels (F-Labels) for the forwarding sub-layer, and a DetNet control word (d-CW) carrying the sequence number and Data/OAM discriminator.
The DetNet service proxy function maps between App-flows (TSN Streams) and DetNet flows. It must support TSN Stream identification functions as defined in IEEE 802.1CB and P802.1CBdb to recognize packets related to the App-flow.
Security considerations for DetNet are described in detail in the DetNet Security Considerations document and in RFC 8655. Considerations specific to the DetNet MPLS data plane, including TSN Stream application flows, are covered by the DetNet MPLS data plane security considerations in RFC 8964; this document introduces no additional security considerations beyond that scope.
TSN over DetNet MPLS encapsulation treats TSN Streams as the MPLS App-flow payload. The encapsulation stack (from top to bottom) comprises: MPLS Labels, d-CW, TSN-L2 frame, and optionally IP/UDP sub-network headers. Two sequence number sizes are supported in the d-CW: 16-bit and 28-bit.