| Internet-Draft | OAM for DetNet over MPLS | April 2023 |
| Mirsky, et al. | Expires 22 October 2023 | [Page] |
This document defines format and use principles of the Deterministic Network (DetNet) service Associated Channel (ACH) over a DetNet network with the MPLS data plane. The DetNet service ACH can be used to carry test packets of active Operations, Administration, and Maintenance protocols that are used to detect DetNet failures and measure performance metrics.¶
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[RFC8655] introduces and explains Deterministic Networks (DetNet) architecture and how the Packet Replication, Elimination, and Ordering functions (PREOF) can be used to ensure a low packet drop ratio in a DetNet domain.¶
Operations, Administration, and Maintenance (OAM) protocols are used to detect and localize network defects, and to monitor network performance. Some OAM functions (e.g., failure detection) are usually performed proactively in the network, while others (e.g., defect localization) are typically performed on demand. These tasks can be achieved through a combination of active and hybrid, as defined in [RFC7799], OAM methods.¶
Also, this document defines format and use principles of the DetNet service Associated Channel over a DetNet network with the MPLS data plane [RFC8964].¶
The term "DetNet OAM" used in this document interchangeably with longer version "set of OAM protocols, methods and tools for Deterministic Networks".¶
CW Control Word¶
DetNet Deterministic Network¶
d-ACH DetNet Associated Channel Header¶
d-CW DetNet Control Word¶
GAL Generic Associated Channel Label¶
G-ACh Generic Associated Channel¶
OAM: Operations, Administration, and Maintenance¶
PREOF Packet Replication, Elimination, and Ordering Functions¶
PW Pseudowire¶
E2E End-to-end¶
BFD Bidirectional Forwarding Detection¶
TSN IEEE 802.1 Time-Sensitive Networking¶
LSR Label Switching Router¶
F-Label A Detnet "forwarding" label. The F-Label identifies the LSP used to forward a DetNet flow across an MPLS PSN, e.g., a hop-by-hop label used between label switching routers (LSR).¶
S-Label A DetNet "service" label. An S-Label is used between DetNet nodes that implement also the DetNet service sub-layer functions. An S-Label is also used to identify a DetNet flow at DetNet service sub-layer.¶
Underlay Network or Underlay Layer: The network that provides connectivity between the DetNet nodes. One example of an underlay layer is an MPLS network that provides LSP connectivity between DetNet nodes.¶
DetNet Node - a node that is an actor in the DetNet domain. Examples of DetNet nodes include DetNet domain Edge nodes, and DetNet nodes that perform PREOF within the DetNet domain.¶
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.¶
OAM protocols and mechanisms act within the data plane of the particular networking layer, thus it is critical that the data plane encapsulation supports OAM mechanisms that comply with the OAM requirements listed in [I-D.ietf-detnet-oam-framework]. One such example that requires special consideration is requirement #5:¶
Operation of a DetNet data plane with an MPLS underlay network is specified in [RFC8964]. Within the MPLS underlay network, DetNet flows are to be encapsulated analogous to pseudowires as specified in [RFC3985], [RFC4385]. For reference, the Generic PW MPLS CW (as defined in [RFC4385] and used with DetNet) is reproduced in Figure 1.¶
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PREOF in the DetNet domain is composed of a combination of nodes that perform replication and elimination functions. The Elimination sub-function always uses the S-Label in conjunction with the packet sequencing information (i.e., the Sequence Number encoded in the d-CW). The Replication sub-function uses the S-Label information only. An example of a PREOF sequence of operations for data packets in a DetNet domain is shown in Figure 2.¶
1111 11111111 111111 112212 112212 132213
CE1----EN1--------R1-------R2-------R3--------EN2----CE2
\2 22222/ 3 /
\2222222 /----+ 3 /
+------R4------------------------+
333333333333333333333333
DetNet OAM, like PW OAM, uses PW Associated Channel Header defined in [RFC4385]. At the same time, a DetNet PW can be viewed as a Multi-Segment PW, where DetNet service sub-layer functions are at the segment endpoints. However, DetNet service sub-layer functions operate per packet level (not per segment level). These per-packet level characteristics of PREOF require additional fields for proper OAM packet processing. Encapsulation of a DetNet MPLS [RFC8964] active OAM packet is shown in Figure 3.¶
+---------------------------------+
| |
| DetNet OAM Packet |
| |
+---------------------------------+ <--\
| DetNet Associated Channel Header| |
+---------------------------------+ +--> DetNet active OAM
| S-Label | | MPLS encapsulation
+---------------------------------+ |
| [ F-Label(s) ] | |
+---------------------------------+ <--/
| Data-Link |
+---------------------------------+
| Physical |
+---------------------------------+
Figure 4 displays encapsulation of a test packet of an active DetNet OAM protocol in case of MPLS-over-UDP/IP [RFC9025].¶
+---------------------------------+
| |
| DetNet OAM Packet |
| |
+---------------------------------+ <--\
| DetNet Associated Channel Header| |
+---------------------------------+ +--> DetNet active OAM
| S-Label | | MPLS encapsulation
+---------------------------------+ |
| [ F-label(s) ] | |
+---------------------------------+ <--+
| UDP Header | |
+---------------------------------+ +--> DetNet data plane
| IP Header | | IP encapsulation
+---------------------------------+ <--/
| Data-Link |
+---------------------------------+
| Physical |
+---------------------------------+
Figure 5 displays the format of the DetNet Associated Channel Header (d-ACH).¶
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version|Sequence Number| Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Node ID |Level| Flags |Session|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The d-ACH encodes the following fields:¶
0 1 2 3 4
+-+-+-+-+-+
|U|U|U|U|U|
+-+-+-+-+-+
U: Unused and for future use. MUST be 0 on transmission and ignored on receipt.¶
The DetNet flow, according to [RFC8964], is identified by the S-label that MUST be at the bottom of the stack. An Active OAM packet MUST include d-ACH immediately following the S-label.¶
At the DetNet service sub-layer, special functions (notably PREOF) MAY be applied to the particular DetNet flow to potentially lower packet loss, improve the probability of on-time packet delivery, and ensure in-order packet delivery. PREOF relies on sequencing information in the DetNet service sub-layer. For a DetNet active OAM packet, PREOF MUST use the bit string from bit 4 through bit 31 inclusive of the first 32-bit word of the d-ACH, i.e., the concatenation of Version, Sequence Number, and Channel Type fields, as the source of this sequencing information.¶
Interworking of two OAM domains that utilize different networking technology can be realized either by a peering or a tunneling model. In a peering model, OAM domains are within the corresponding network domain. When using the peering model, state changes that are detected by a Fault Management OAM protocol can be mapped from one OAM domain into another or a notification, e.g., an alarm, can be sent to a central controller. In the tunneling model of OAM interworking, usually, only one active OAM protocol is used. Its test packets are tunneled through another domain along with the data flow, thus ensuring the fate sharing among test and data packets.¶
Active DetNet OAM can be used to provide the E2E fault management and performance monitoring for a DetNet flow. In the case of DetNet with an MPLS data plane and a TSN underlay network, this implies interworking of DetNet active OAM with TSN OAM, which is specified in [RFC9037].¶
When the peering model is used in CFM OAM, then the node that borders both TSN and DetNet MPLS domains MUST support [RFC7023]. [RFC7023] specifies the mapping of defect states between Ethernet Attachment Circuits and associated Ethernet PWs that are part of an E2E emulated Ethernet service, and are also applicable to E2E OAM across DetNet MPLS and TSN domains. The Connectivity Fault Management protocol [IEEE.CFM] or in [ITU.Y1731] can provide fast detection of a failure in the TSN segment of the DetNet service. In the DetNet MPLS domain BFD (Bidirectional Forwarding Detection), specified in [RFC5880] and [RFC5885], can be used. To provide E2E failure detection, the TSN and DetNet MPLS segments could be treated as concatenated such that the diagnostic codes (see Section 6.8.17 of [RFC5880]) MAY be used to inform the upstream DetNet MPLS node of a failure of the TSN segment. Performance monitoring can be supported by [RFC6374] in the DetNet MPLS and [ITU.Y1731] in the TSN domains, respectively. Performance objectives for each domain should refer to metrics that additive or be defined for each domain separately.¶
The following considerations apply when using the tunneling model of OAM interworking between DetNet MPLS and TSN domains based on general principles described in Section 4 [RFC9037]:¶
Note that the tunneling model of the OAM interworking requires that the remote peer of the E2E OAM domain supports the active OAM protocol selected on the ingress endpoint. For example, if BFD is used for proactive path continuity monitoring in the DetNet MPLS domain, BFD support (as defined in [RFC5885]) is necessary at any TSN endpoint of the DetNet service.¶
Interworking between active OAM segments in DetNet MPLS and DetNet IP domains can also be realized using either the peering or the tunneling model, as discussed in Section 4.1. Using the same protocol, e.g., BFD, over both segments, simplifies the mapping of errors in the peering model. To provide performance monitoring over a DetNet IP domain, STAMP [RFC8762] and its extensions [RFC8972] can be used.¶
This document describes a new IANA-managed registry to identify DetNet Associated Channel Header Flags bits. The registration procedure is "IETF Review" [RFC8126]. The registry name is "DetNet Associated Channel Header Flags". IANA should treat "DetNet Associated Channel Header Flags" as the name of the registry group. There are five flags in the five-bit Flags field, defined as in Table 1.¶
| Bit | Description | Reference |
|---|---|---|
| 0-4 | Unassigned | This document |
Security considerations discussed in DetNet specifications [RFC8655], [RFC9055], [RFC8964] are applicable to this document. Security concerns and issues related to MPLS OAM tools like LSP Ping [RFC8029], BFD over PW [RFC5885] also apply to this specification.¶
Authors extend their appreciation to Pascal Thubert for his insightful comments and productive discussion that helped to improve the document. The authors are enormously grateful to Janos Farkas for his detailed comments and the inspiring discussion that made this document clearer and stronger. The authors recognize helpful reviews and suggestions from Andrew Malis, David Black, Tianran Zhou, and Kiran Makhijani. And special thanks are addressed to Ethan Grossman for his fantastic help in improving the document.¶