Deterministic Networking (DetNet): OAM Functions for The Service Sub-Layer

3.1. Illustrative example

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.¶

3.2. DetNet Service Sub-layer Specifics for OAM

Several DetNet Service Sub-layer specifics have to be considered for OAM.¶

1. The service sub-layer graph is segmented into multiple parts, as forwarding sub-layer paths are terminated at DetNet relay nodes.¶

2. These are particular characteristics of DetNet PW:¶

   1. PREOF acts as per-packet protection. PEF is a brand-new functionality at network layer, due to the per-packet merge action.¶
   2. All paths are active and forward traffic. These paths may have a different number of hops.¶
   3. Mandatory usage of a sequence number.¶

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).¶

3.3. Information Needed during DetNet OAM Packet Processing

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.¶

3.4. A Possible Format of DetNet Associated Channel Header (d-ACH)

[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].]¶

5.1. Overview

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:¶

1. DetNet PING originator node,¶
2. Intermediate DetNet service sub-layer node,¶
3. DetNet PING targeted node.¶

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).¶

5.2. OAM processing at the DetNet service sub-layer

Detailed OAM packet processing rules of various DetNet relay nodes are described in the following sections.¶

7.1. DetNet MPLS OAM Flags Registry

[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].]¶