OAM Packet and Behavior in the Network Service Header (NSH)

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Table of Contents

1. Introduction

This document clarifies an ambiguity related to the definition of Operations, Administration, and Maintenance (OAM) packet discussed in [RFC8300].¶

The processing of the O bit in the Network Service Header (NSH) must follow the updated behavior specified in Section 3.¶

2. Terminology

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 document makes use of the terms defined in [RFC7665] and [RFC8300].¶

The document defines the following terms:¶

SFC data plane element:

refers to SFC-aware SF, SFF, SFC Proxy, or Classifier as defined in the SFC data plane architecture [RFC7665] and further refined in [RFC8300].¶

OAM control element:

an NSH-aware element that is capable of generating NSH OAM packets. An SFC data plane element may behave as an OAM control element.¶

SFC OAM data:

refers to an OAM request (e.g., Connectivity Verification and Continuity Checks [RFC7276]), any data that influences how to execute a companion OAM request (e.g., identity of a terminating Service Function (SF)), the output data of an OAM request, and any combination thereof.¶

User data:

refers to user packets cited in Section 5.7 of [RFC7665].¶

3. An Update to RFC8300

This document updates Section 2.2 of [RFC8300] as follows:¶

OLD:

   O bit:  Setting this bit indicates an OAM packet (see [RFC6291]).
      The actual format and processing of SFC OAM packets is outside the
      scope of this specification (for example, see [SFC-OAM-FRAMEWORK]
      for one approach).

      The O bit MUST be set for OAM packets and MUST NOT be set for
      non-OAM packets.  The O bit MUST NOT be modified along the SFP.

      SF/SFF/SFC Proxy/Classifier implementations that do not support
      SFC OAM procedures SHOULD discard packets with O bit set, but MAY
      support a configurable parameter to enable forwarding received SFC
      OAM packets unmodified to the next element in the chain.
      Forwarding OAM packets unmodified by SFC elements that do not
      support SFC OAM procedures may be acceptable for a subset of OAM
      functions, but it can result in unexpected outcomes for others;
      thus, it is recommended to analyze the impact of forwarding an OAM
      packet for all OAM functions prior to enabling this behavior.  The
      configurable parameter MUST be disabled by default.

NEW:

O bit:

Setting this bit indicates an NSH OAM packet. Such a packet is any NSH-encapsulated packet that exclusively includes SFC OAM data. SFC OAM data can be included in the Fixed-Length Context Header, optional Context Headers, and/or the inner packet.¶

The O bit is typically set by an OAM controller or a final destination of an NSH OAM packet that triggers a response (e.g., a specific SFC-aware SF, the last SFF of an SFP).¶

The O bit MUST be set for NSH OAM packets and MUST NOT be set for non-OAM packets. The O bit MUST NOT be modified along the SFP.¶

NSH-encapsulated packets that include user data are not considered as NSH OAM packets even if some SFC OAM data (e.g., record route) is also supplied in the packet.¶

When SFC OAM data is included in the inner packet, the Next Protocol field is set to reflect the structure of that inner OAM packet. The setting and processing of the O bit neither assumes nor expects detailed analysis of the content of any inner IP packet carried by the NSH. As such, SFFs, SFC-aware SFs, and SFC Proxies SHOULD discard any NSH packets with the O bit set and Next Protocol set to something that is not itself an OAM protocol. This includes discarding the packet when the O bit is set and the Next Protocol is set to 0x01 (IPv4), 0x02 (IPv6), 0x03 (MPLS), or 0x05 (Ethernet).¶

An NSH OAM packet MAY include optional Context Headers (e.g., a subscriber identifier [RFC8979] or a flow identifier [I-D.ietf-sfc-nsh-tlv]) that are used to influence the processing of the packet by SFC data plane elements.¶

An NSH OAM packet MAY include SFC OAM data in both Context Headers and the inner packet. The processing (including the order) of the SFC OAM data SHOULD be specified in the relevant OAM or Context Header specification.¶

SFC-aware SF/SFF/SFC Proxy/Classifier implementations that do not support SFC OAM procedures SHOULD discard packets with O bit set, but MAY support a configurable parameter to enable forwarding received NSH OAM packets unmodified to the next element in the chain. Forwarding NSH OAM packets unmodified by SFC data plane elements that do not support SFC OAM procedures may be acceptable for a subset of OAM functions, but it can result in unexpected outcomes for others; thus, it is recommended to analyze the impact of forwarding an NSH OAM packet for all OAM functions prior to enabling this behavior. The configurable parameter MUST be disabled by default.¶

The actual format and additional processing of NSH OAM packets is outside the scope of this specification.¶

5. Security Considerations

Data plane SFC-related security considerations, including privacy, are discussed in Section 6 of [RFC7665] and Section 8 of [RFC8300]. Additional security considerations related to SFC OAM are discussed in Section 9 of [RFC8924].¶

Any data included in an NSH OAM packet SHOULD be integrity-protected [RFC9145].¶

6. Acknowledgments

Thanks to Jim Guichard, Greg Mirsky, Joel Halpern, Christian Jacquenet, Dirk von-Hugo, Carlos Pignataro, and Frank Brockners for the comments.¶

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