Deterministic Networking (DetNet): DetNet PREOF via MPLS over UDP/IP

The DetNet Working Group has defined Packet Replication (PRF), Packet Elimination (PEF) and Packet Ordering (POF) functions to provide service protection by the DetNet service sub-layer . The PREOF service protection method relies on copies of the same packet sent over multiple maximally disjoint paths and uses sequencing information to eliminate duplicates. A possible implementation of the PRF and PEF functions is described in and the related YANG data model is defined in . A possible implementation of POF function is described in . shows a DetNet flow on which PREOF functions are applied during forwarding from the source to the destination.

In general, the use of PREOF functions require sequencing information to be included in the packets of a DetNet compound flow. This can be done by adding a sequence number or time stamp as part of DetNet encapsulation. Sequencing information is typically added once, at or close to the source. The DetNet MPLS data plane specifies how sequencing information is encoded in the MPLS header. However, the DetNet IP data plane described in does not specify how sequencing information can be encoded in the IP header. This documents provides sequencing information to DetNet IP nodes by re-using the DetNet MPLS over UDP/IP data plane with the restriction of using zero F-labels.

This document uses the terminology established in the DetNet architecture , and the reader is assumed to be familiar with that document and its terminology.

The following abbreviations are used in this document: Deterministic Networking. Packet Elimination Function. Packet Ordering Function. Packet Replication, Elimination and Ordering Functions. Packet Replication Function.

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.

The requirements for adding PREOF to DetNet IP are: to reuse existing DetNet data plane solutions (e.g., , ). to allow the DetNet service sub-layer for IP packet switched networks with minimal implementation effort. The described solution practically gains from MPLS header fields without requiring the support of the MPLS forwarding plane.

The DetNet IP encapsulation supporting DetNet Service sub-layer is based on the "UDP tunneling" concept. The solution creates a set of underlay UDP/IP tunnels between an overlay set of DetNet relay nodes. At the edge of a PREOF capable DetNet IP domain the DetNet flow is encapsulated in an UDP packet containing the sequence number used by PREOF functions within the domain. This solution maintains the 6-tuple-based DetNet flow identification in DetNet transit nodes, which operate at the DetNet forwarding sub-layer between the DetNet service sub-layer nodes; therefore, it is compatible with . shows how the PREOF capable DetNet IP data plane fits into the DetNet sub-layers.

The PREOF capable DetNet IP encapsulation builds on encapsulating DetNet PW directly over UDP. That is, it combines DetNet MPLS with DetNet MPLS-in-UDP , without using any F-Labels as shown in . DetNet flows are identified at the receiving DetNet service sub-layer processing node via the S-Label and/or the UDP/IP header information. Sequencing information for PREOF is provided by the DetNet Control Word (d-CW) as per . The S-label is used to identify both the DetNet flow and the DetNet App-flow type. The UDP tunnel is used to direct the packet across the DetNet domain to the next DetNet service sub-layer processing node.

IP ingress and egress nodes of the PREOF capable DetNet IP domain add and remove a DetNet service-specific d-CW and Service-ID (i.e., S-Label). Relay nodes can change Service-ID values when processing a DetNet flow, i.e., incoming and outgoing Service-IDs of a DetNet flow can be different. Service-ID values are provisioned per DetNet service via configuration, i.e., via the Controller Plane described in . In some PREOF topologies, the node performing replication sends the packets to multiple nodes performing e.g., PEF or POF and the replication node can use different Service-ID values for the different member flows for the same DetNet service. Note, that Service-IDs is a local ID on the receiver side providing identification of the DetNet flow at the downstream DetNet service sub-layer receiver.

Two methods can be used for flow aggregation: aggregation using same UDP tunnel, aggregating DetNet flows as a new DetNet flow. In the first case, the different DetNet PWs use the same UDP tunnel, so they are treated as a single (aggregated) flow at the forwarding sub-layer. At the service sub-layer, each flow uses a different Service ID. For the second option, an additional hierarchy is created thanks to an additional Service-ID and d-CW tuple added to the encapsulation. The Aggregate-ID is a special case of a Service-ID, whose properties are known only at the aggregation and de-aggregation end points. It is a property of the Aggregate-ID that it is followed by a d-CW followed by an Service-ID/d-CW tuple. shows the encapsulation in case of aggregation.

A node operating on a received DetNet flow at the DetNet service sub-layer uses the local context associated with a received Service-ID to determine which local DetNet operation(s) are applied to received packet. A Service-ID can be allocated to be unique and enabling DetNet flow identification regardless of which input interface or UDP tunnel the packet is received. It is important to note that Service-ID values are driven by the receiver, not the sender. The DetNet forwarding sub-layer is supported by the UDP tunnel and is responsible for providing resource allocation and explicit routes. The outgoing PREOF encapsulation and processing can be implemented via the provisioning of UDP and IP header information. Note, when PRF is performed at the DetNet service sub-layer, there are multiple member flows, and each member flow requires their own Service-ID, UDP and IP header information. The headers for each outgoing packet are formatted according to the configuration information, and the UDP Source Port value is set to uniquely identify the DetNet flow. The packet is then handled as a PREOF capable DetNet IP packet. The incoming PREOF processing can be implemented via the provisioning of received Service-ID, UDP and IP header information. The provisioned information is used to identify incoming app-flows based on the combination of Service-ID and/or incoming encapsulation header information.

shows using PREOF in a PREOF capable DetNet IP network.

______ ____ / \__ ____ / \__/ \____________ +----+ __/ \____/ \ +----+ |src |_____/ \___| dst| +----+ \_______ DetNet network __________/ +----+ \_______ _/ \ __ __/ \_______/ \___/ +------------+ +---------------E1---+ | | +----+ | | +---R3---+ | +----+ |src |------R1 +---+ | E3----O----+ dst| +----+ | | E2-------+ +----+ +----------R2 | +-----------------+ ]]>

The information needed to identify individual and aggregated DetNet flows is summarized as follows: Service-ID information to be mapped to UDP/IP flows. Note that, for example, a single Service-ID can map to multiple sets of UDP/IP information when PREOF is used. IPv4 or IPv6 source address field. IPv4 or IPv6 source address prefix length, where a zero (0) value effectively means that the address field is ignored. IPv4 or IPv6 destination address field. IPv4 or IPv6 destination address prefix length, where a zero (0) effectively means that the address field is ignored. IPv6 flow label field. IPv4 protocol field set to "UDP". IPv6 next header field set to "UDP". For the IPv4 Type of Service and IPv6 Traffic Class Fields: Whether or not the DSCP field is used in flow identification as the use of the DSCP field for flow identification is optional. If the DSCP field is used to identify a flow, then the flow identification information (for that flow) includes a list of DSCPs used by the given DetNet flow. UDP Source Port. Support for both exact and wildcard matching is required. Port ranges can optionally be used. UDP Destination Port. Support for both exact and wildcard matching is required. Port ranges can optionally be used. For end systems, an optional maximum IP packet size that should be used for that outgoing DetNet IP flow. This information is provisioned per DetNet flow via configuration, e.g., via the controller plane. Ordering of the set of information used to identify an individual DetNet flow can, for example, be used to provide a DetNet service for a specific UDP flow, with unique Source and Destination Port field values, while providing a different service for the aggregate of all other flows with that same UDP Destination Port value. The minimum set of information for the configuration of the DetNet service sub-layer is summarized as follows: App-flow identification information. Sequence number length. PREOF + related Service-ID(s). Associated forwarding sub-layer information. Service aggregation information. The minimum set of information for the configuration of the DetNet forwarding sub-layer is summarized as follows: UDP tunnel specific information. Traffic parameters. Note: this document focuses on the use of MPLS over UDP/IP encapsulation throughout an entire DetNet IP network, making MPLS-based DetNet OAM techniques applicable. Using the described encapsulation only for a portion of a DetNet IP network that handles the PREOF functionality would complicate OAM.

There are no new DetNet related security considerations introduced by this solution.

This document makes no IANA requests.

Authors extend their appreciation to Stewart Bryant, Pascal Thubert, David Black, Shirley Yangfan and Greg Mirsky for their insightful comments and productive discussion that helped to improve the document.