Identification of Overlay Operations, Administration, and Maintenance (OAM)
draft-mirsky-rtgwg-oam-identify-04

Abstract

This document analyzes how the presence of Operations, Administration, and Maintenance (OAM) control command and/or special data is identified in some overlay networks and an impact on the choice of identification may have on OAM functionality.

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Table of Contents

• 1. Introduction
• 2. Conventions used in this document
• 2.1. Terminology
• 2.2. Keywords
• 3. A Control Channel in an Overlay Network
• 4. Overlay Network Encapsulations
• 4.1. Encapsulations with Meta-data
• 4.1.1. Available Solutions
• 4.2. Fixed-size Encapsulations
• 4.3. Source Information Availability
• 4.4. On-path OAM
• 5. Conclusions
• 6. IANA Considerations
• 7. Security Considerations
• 8. Acknowledgment
• 9. References
• 9.1. Normative References
• 9.2. Informational References
• Author's Address

1. Introduction

Operations, Administration, and Maintenance (OAM) protocols are used to detect, localize defects in the network, and monitor network performance. Some OAM functions, e.g., failure detection, work in the network proactively, while others, e.g., defect localization, usually performed on-demand. These tasks achieved by a combination of active, passive, and hybrid OAM methods, as defined in [RFC7799].

This document analyzes how the presence of Operations, Administration, and Maintenance (OAM) control command and/or special data, i.e., OAM packet, is identified in some overlay networks, and an impact the choice of identification may have on OAM functionality of active and hybrid OAM methods for the respective overlay network encapsulation.

2. Conventions used in this document

2.1. Terminology

AMM Alternate Marking method

BIER Bit Indexed Explicit Replication

DetNet Deterministic Networks

GUE Generic UDP Encapsulation

HTS Hybrid Two-step

NSH Network Service Header

NVO3 Network Virtualization Overlays

OAM Operations, Administration and Maintenance

SFC Service Function Chaining

TLV Type-Length-Value

VXLAN-GPE Generic Protocol Extension for VXLAN

ACH Associated Channed Header

Underlay Network or Underlay Layer: The network that provides connectivity between the DetNet nodes. MPLS network that provides LSP connectivity between DetNet nodes is an example of an underlay layer.

2.2. Keywords

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.

3. A Control Channel in an Overlay Network

There's a need for a general control channel between the endpoints of an overlay network for OAM protocols that can be used for fault detection, diagnostics, maintenance, and other functions. Such a control tunnel is dedicated to carrying only control and management data between tunnel endpoints. In other words, the control channel of an overlay network SHOULD NOT carry the client's data. And the endpoint node SHOULD NOT forward a packet received over the control channel. The identification of the control channel might be using different methods. For example, Virtual Network Identifier might be used to identify the control channel in VXLAN and Geneve.

4. Overlay Network Encapsulations

New overlay network encapsulations analyzed in two groups:

• encapsulations that support optional meta-data;
• fixed-size encapsulations.

4.1. Encapsulations with Meta-data

Number of the new encapsulation protocols (e.g., Geneve [I-D.ietf-nvo3-geneve], GUE [I-D.ietf-intarea-gue], and SFC NSH [RFC8300]) support use of Type-Length-Value (TLV) encoding to include optional information into the header. The identification of OAM in these protocols is as the following:

• Geneve:
  • O (1 bit): after the WGLC discussion, the interpretation of the O field has changed. The O field now identifies a control packet. This packet contains a control message. Control messages are sent between tunnel endpoints. Tunnel Endpoints MUST NOT forward the payload and transit devices MUST NOT attempt to interpret it. Since these are infrequent control messages, it is RECOMMENDED that tunnel endpoints direct these packets to a high priority control queue (for example, to direct the packet to a general purpose CPU from a forwarding ASIC or to separate out control traffic on a NIC). Transit devices MUST NOT alter forwarding behavior on the basis of this bit, such as ECMP link selection.
  • [I-D.mmbb-nvo3-geneve-oam] defines the Geneve encapsulation for active OAM. Initially, four options have been presented:
    • with IP/UDP header demultiplexing active OAM protocols, e.g., Fault Management and Performance Monitoring, can be done using the destination UDP port number.
    • demultiplex active OAM protocols by the value of the Protocol Type field in the Geneve header.
    • with using MPLS Generic Associated Channel Label [RFC5586] and Associated Channel Header (ACH) [RFC4385]. Active OAM protocols are demultiplexed using the value of the Channel Type field.
    • using the new EtherType to identify Geneve OAM and the ACH. Active OAM protocols will be demultiplexed based on the Channel Type field's value.

• GUE:
  • C-bit provides the separate namespace to carry formatted data that are implicitly addressed to the decapsulator to monitor or control the state or behavior of a tunnel. The payload is interpreted as a control message with the type specified in the proto/ctype field. The format and contents of the control message are indicated by the type and can be variable length.

• SFC NSH:
  • O bit: Setting this bit indicates an OAM packet.

Common between Geneve and NSH is the use of the dedicated flag to identify the OAM packet and, at the same time, the presence of the field that identifies the protocol of the payload that immediately follows after the encapsulation header. [RFC8393] points out that if the value of that field interpreted as none, i.e., no payload follows the header, then OAM may be included in TLVs, thus creating an active OAM packet. The problem with this mechanism to support active OAM methods may be a limitation of the size of data that can be included in a TLV. For example, the maximum size of data in an NSH Meta-data Type 2, as defined in section 2.5.1 [RFC8300], is 512 octets. The maximum length of data in Geneve Option, per section 3.5 [I-D.ietf-nvo3-geneve], is 128 octets. Thus, using one TLV as active OAM packet, would not allow creating test packets of larger size, which is useful when measuring packet loss and latency with synthetic traffic as part of the service activation procedure.

[I-D.ietf-sfc-oam-framework] suggests that the O bit used to identify OAM packet and the Next Protocol field identifies the OAM function:

• While the presence of OAM marker in the overlay header (e.g., O bit in the NSH header) indicates it as OAM packet, it is not sufficient to signal for which OAM function the packet is intended.

At the same time, some of in-situ OAM proposals, e.g., [I-D.ietf-sfc-ioam-nsh], suggest using TLV to communicate hybrid OAM commands and data. The proposed resolution of using the combination of O bit and the Next Protocol field:

• ... the O bit MUST NOT be set for regular customer traffic which also carries IOAM data and the O bit MUST be set for OAM packets which carry only IOAM data without any regular data payload.

implies that the O bit only identifies the active OAM packet and not set when hybrid OAM methods used.

4.1.1. Available Solutions

One of the possible solutions for encapsulations with meta-data has been specified in [I-D.ietf-sfc-multi-layer-oam]:

To identify the active OAM message the value on the Next Protocol field MUST be set to Active SFC OAM. The rules of interpreting the values of O bit and the Next Protocol field are as follows:

• O bit set and the Next Protocol value is not one of identifying active or hybrid OAM protocol (per [RFC7799] definitions), e.g., defined in this specification Active SFC OAM - a Fixed-Length Context Header or Variable-Length Context Header(s) contain OAM command or data and the type of payload determined by the Next Protocol field;
• O bit set and the Next Protocol value is one of identifying active or hybrid OAM protocol - the payload that immediately follows SFC NSH contains OAM command or data;
• O bit is clear - no OAM in a Fixed-Length Context Header or Variable-Length Context Header(s) and the payload determined by the value of the Next Protocol field;
• O bit is clear, and the Next Protocol value is one of identifying active or hybrid OAM protocol MUST be identified and reported as the erroneous combination. An implementation MAY have control to enable processing of the OAM payload.

From the above-listed rules follows the recommendation to avoid the combination of OAM in a Fixed-Length Context Header or Variable- Length Context Header(s) and in the payload immediately following the SFC NSH because there is no unambiguous way to identify such combination using the O bit and the Next Protocol field.

4.2. Fixed-size Encapsulations

Number of the new encapsulation protocols (e.g., VXLAN-GPE [I-D.ietf-nvo3-vxlan-gpe], BIER [RFC8296]) suse fixed-size header. The identification of OAM in these protocols is as the following:

• VXLAN-GPE:
  • OAM Flag Bit (O bit): The O bit is set to indicate that the packet is an OAM packet.

• BIER:
  • OAM packet identified by the value of the Next Protocol field. IANA BIER Next Protocol Identifiers registry includes the identifier for OAM (5).

The use of a combination of OAM Flag Bit and the Next Protocol field in VXLAN-GPE requires clarification of the header interpretation when the OAM Flag Bit is set, and the value of the Next Protocol field is one of defined in section 3.2 of [I-D.ietf-nvo3-vxlan-gpe].

BIER encapsulation, defined in [RFC8296], identifies OAM message immediately following the BIER header by the value of the Next Protocol field.

4.3. Source Information Availability

Availability of the packet originator's source information is required for active two-way OAM, e.g., echo request/reply. In cases when the underlay network is IPv4/IPv6 the source information will be derived from the underlay. But when using MPLS underlay network encapsulation of an active OAM packet have to follow specific rules:

• if available, use Sender ID in the overlay domain (example BFIR ID in BIER [RFC8296];
• use IP/UDP encapsulation of an OAM packet in the overlay (similar to Section 4.3 [RFC8029]).

4.4. On-path OAM

In addition to active methods, OAM toolset may include methods that don't use specially constructed and injected in the network test packets. [RFC7799] defines OAM methods that are neither entirely active nor passive but are a combination of both as hybrid methods.

One of the examples of the hybrid OAM methods, in-situ OAM, mentioned in Section 4.1. Another example, Alternate Marking method (AMM) [RFC8321], enables on-path OAM functions, e.g., delay and loss measurements, using the data traffic. Because AMM impact on the network can be minimized, measured metrics can be correlated to the network conditions experienced by the specific service. Of all listed in Section 4, BIER allocated the field that may be used for AMM, as discussed in [I-D.ietf-bier-pmmm-oam]. Applicability of AMM to other overlay protocols, i.e., SFC NSH discussed in [I-D.mirsky-sfc-pmamm], Geneve [I-D.fmm-nvo3-pm-alt-mark], and in IPv6 networks [I-D.fioccola-v6ops-ipv6-alt-mark], been actively discussed.

Hybrid Two-step (HTS), defined in [I-D.mirsky-ippm-hybrid-two-step], provides on-path collection and transport of the telemetry information. HTS enables accurate and consistent measurements by separating the measurement action from the transporting data while ensuring that the follow-up packet that carries the telemetry information does follow the data packet that had triggered the measurement.

5. Conclusions

OAM control commands and data may be present as part of the overlay encapsulation header or as a payload that follows the overlay network header. The recommendations:

• OAM in the overlay header, if supported by the overlay network, identified by the dedicated flag. Use of this method as active OAM is possible, but functionality is limited.
• OAM that follows the overlay header identified as payload type, e.g., by the value of the Next Protocol field.

6. IANA Considerations

This document does not propose any IANA consideration. This section may be removed.

7. Security Considerations

This document lists the OAM requirements for a DetNet domain and does not raise any security concerns or issues in addition to ones common to networking.

8. Acknowledgment

TBD

9. References

9.1. Normative References

9.2. Informational References

Author's Address