| Internet-Draft | DetNet Service-Sub-layer OAM | July 2022 |
| Varga, et al. | Expires 26 January 2023 | [Page] |
Operation, Administration, and Maintenance (OAM) tools are essential for a deterministic network. The DetNet architecture [RFC8655] has defined two sub-layers: (1) DetNet service sub-layer and (2) DetNet forwarding sub-layer. OAM mechanisms exist for the DetNet forwarding sub-layer. Nonetheless, OAM for the service sub-layer might require new extensions to the existing OAM protocols. This draft presents an analysis of OAM procedures for the DetNet service sub-layer functions.¶
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The DetNet Working Group has defined two sub-layers: (1) DetNet service sub-layer, at which a DetNet service (e.g., service protection) is provided and (2) DetNet forwarding sub-layer, which optionally provides resource allocation for DetNet flows over paths provided by the underlying network. In [RFC8655] new DetNet-specific functions have been defined for the DetNet service sub-layer, namely PREOF (a collective name for Packet Replication, Elimination, and Ordering Functions).¶
Framework of Operations, Administration and Maintenance (OAM) for Deterministic Networking (DetNet) is described in [I-D.ietf-detnet-oam-framework]. OAM for the DetNet MPLS data plane is described in [I-D.ietf-detnet-mpls-oam] and OAM for the DetNet IP data plane is described in [I-D.ietf-detnet-mpls-oam].¶
This draft has been submitted as an individual contribution to OAM discussions, in particular, to kick-off Working Group discussions on introducing OAM functions for the DetNet service sub-layer. It is also up to the Working Group discussions to which draft parts of this contribution may go, if any.¶
The OAM functions for the DetNet service sub-layer allow, for example, to recognize/discover DetNet relay nodes, to get information about their configuration, and to check their operation or status. Furthermore, the OAM functions for the DetNet service sub-layer need to meet new challenges (see section Section 3) and requirements (see section Section 4) introduced by PREOF.¶
An approach described in this draft introduces a new OAM shim layer to achieve OAM for the DetNet service sub-layer. In the rest of the draft, this approach is referred to as "DetNet PING", which is an in-band OAM approach, i.e., the OAM packets follow precisely the same path as the data packets of the corresponding DetNet flow(s) The OAM packets provide DetNet service sub-layer specific information, like:¶
DetNet PING applies both to IP and MPLS DetNet data planes.¶
This document uses the terminology established in the DetNet architecture [RFC8655], and the reader is assumed to be familiar with that document and its terminology.¶
The following abbreviations are used in this document:¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This section introduces an example that is used to explain the DetNet Service Sub-layer OAM challenges. Figure 1 shows a DetNet flow on which PREOF functions are applied during forwarding from source to destination.¶
<------------------------ App Flow ----------------------->
/-------------- DetNet domain -----------------/
<-------------- DetNet flow ----------------->
P6
P1 P4 +------------+
+--------------E1----+ | P7 |
+----+ | | +---R3---+ | P9 +----+
|src |------R1 +---+ | E3----O1---+ dst|
+----+ | P2 | P3 E2-------+ +----+
+----------R2 P5 | P8
+-----------------+
<----- P1 ----> <- P4 -> <--- P6 ----> <-P9->
<-- P2 --> <P3> <- P4 -> <P7> <- P8 -> <-P9->
<-- P2 --> <----- P5 ------> <- P8 -> <-P9->
|------------ G1 DetNet graph ---------------->
R: replication point (PRF) P: forwarding sub-layer path
E: elimination point (PEF) G: service sub-layer graph
O: ordering function (POF)
DetNet service sub-layer nodes are interconnected by DetNet forwarding sub-layer paths. DetNet forwarding sub-layer path (e.g., P1 = R1->E1 path, P4 = E1->R3 path) may contain multiple transit nodes. A DetNet forwarding sub-layer path is used by a member flow and terminated by relay nodes (see [RFC8655] for relay node definition).¶
A DetNet service sub-layer graph includes all relay nodes and the interconnecting forwarding sub-layer paths. This graph can be also called as "PREOF graph" and it describes the compound flow as a whole.¶
Several DetNet Service Sub-layer specifics have to be considered for OAM.¶
These are particular characteristics of DetNet PW:¶
The above specifics have to be considered in combination with the requirement that DetNet OAM and DetNet data flows MUST receive the same treatment. OAM packets MUST follow precisely the same graph as the monitored DetNet flow(s).¶
This section collects some questions that have been already discussed by the DetNet WG and/or require further discussions by the WG. The section is structured in the form of a question list.¶
Question-1: Injecting OAM traffic in a DetNet flow? A DetNet data flow has a continuous Sequence Number. In order not to spoil that, the injected OAM packets require OAM-specific Sequence Number added. (See also Section Section 5.)¶
Question-2: How to process OAM packets by DetNet service sub-layer nodes? In order to cover the DetNet forwarding graph by OAM, PREOF has to be executed in an OAM specific manner (i.e., PREOF uses a separate SeqNum space for OAM. See details in Section 5.¶
Note: the question list is non-exhaustive.¶
[Editor's note: The content of this section has been discussed and the outcome of the discussion has been documented in [I-D.ietf-detnet-mpls-oam].]¶
[Editor's note: The content of this section has been discussed and the outcome of the discussion has been documented in [I-D.ietf-detnet-oam-framework].]¶
The "DetNet PING" approach uses two types of OAM packets: (1) DetNet-Echo-Request and (2) DetNet-Echo-Reply. Their encapsulation is identical to that of the corresponding DetNet data flow, so they follow precisely the same path as the packets of the corresponding DetNet data flow. They target DetNet service sub-layer entities, so they may not be recognized as OAM packets by entities not implementing DetNet service sub-layer for a packet flow (e.g., transit nodes). Other entities treat them as packets belonging to the corresponding DetNet data flow.¶
The following relay node roles can be distinguished:¶
An originator node sends (generates) DetNet-Echo-Request packet(s). DetNet-Echo-Request packet contains an OAM specific "PINGSeqNum", which can be used by the DetNet service sub-layer of relay nodes. Note that "PINGSeqNum" is originator specific.¶
An intermediate DetNet service sub-layer node executes DetNet flow-specific service sub-layer functionality. Packet processing may be done in an OAM specific manner (see details in Section 5.2).¶
A targeted node answers with DetNet-Echo-Reply packet for each DetNet-Echo-Request. DetNet-Echo-Reply packet provides DetNet service sub-layer specific information on (i) identities of DetNet service sub-layer node (e.g., Node-ID, local Flow-ID), (ii) ingress/egress flow related configuration (e.g., in/out member flow specific information (including forwarding sub-layer specifics)), and (iii) status of service sub-layer function (e.g., local PxF-ID, Action-Type=x, operational status, value of key state variable(s), function related counters).¶
Detailed OAM packet processing rules of various DetNet relay nodes are described in the following sections.¶
A DetNet relay node with PRF processes DetNet OAM packets in a stateless manner.¶
If the relay node with PRF is the target of a DetNet-Echo-Request packet, then the DetNet-Echo-Request packet MUST NOT be further forwarded, and a DetNet Echo-Reply packet MUST be generated. If the relay node with PRF is not the target of a DetNet Echo-Request packet, then the DetNet Echo-Request packet MUST be sent over all DetNet flow specific member flow paths (i.e., it is replicated).¶
A DetNet Echo-Reply packet MUST contain the following information:¶
A DetNet Echo-Reply packet MAY contain the following information:¶
A DetNet relay node with PEF processes DetNet OAM packets in a stateful manner.¶
If the relay node with PEF is the target of DetNet-Echo-Request packet, then the DetNet Echo-Request packet MUST NOT be further forwarded and an DetNet Echo-Reply packet MUST be generated. If the relay node with PEF is not the target of DetNet Echo-Request packet, then elimination MUST be executed on the DetNet Echo-Request packet(s) using the OAM specific "PINGSeqNum" in the packet. So only a single DetNet Echo-Request packet is forwarded and all further replicas (having the same originator's sequence number) MUST be discarded.¶
Note, PEF MAY use a simplified elimination algorithm for DetNet Echo-Request packets (e.g., "MatchRecoveryAlgorithm" in [IEEE8021CB]) as OAM is a slow protocol.¶
A DetNet-Echo-Reply packet MUST contain the following information:¶
A DetNet Echo-Reply packet MAY contain the following information:¶
A DetNet relay node with POF processes DetNet OAM packets in a stateless manner.¶
If the relay node with POF is the target of DetNet Echo-Request packet, then the DetNet Echo-Request packet MUST NOT be further forwarded and a DetNet Echo-Reply packet MUST be generated. If the relay node with POF is not the target of DetNet-Echo-Request packet, then the DetNet Echo-Request packet(s) MUST be forwarded without any POF-specific action.¶
A DetNet Echo-Reply packet MUST contain the following information:¶
A DetNet Echo-Reply packet MAY contain the following information:¶
A DetNet relay node without PREOF processes DetNet OAM packets in a stateless manner.¶
If the relay node without PREOF is the target of DetNet Echo-Request packet, then the DetNet Echo-Request packet MUST NOT be further forwarded and an DetNet Echo-Reply packet MUST be generated. If the relay node without PREOF is not the target of DetNet-Echo-Request packet, then the DetNet-Echo-Request packet(s) MUST be forwarded (as any data packets of the related DetNet flow).¶
A DetNet Echo-Reply packet MUST contain the following information:¶
Tbd.¶
[Editor's note: The content of this section has been discussed and the outcome of the discussion has been documented in [I-D.ietf-detnet-mpls-oam].]¶
Authors extend their appreciation to Janos Szabo and Gyorgy Miklos for their insightful comments and productive discussion that helped to improve the document.¶