Generic MPLS Information Elements for Ordered Export in IPFIX

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Table of Contents

1. Introduction

The IPFIX protocol [RFC7011] allows the same Information Element (IE) to appear multiple times in a Template Record. [I-D.claise-opsawg-ipfix-ordered-ie] defines Ordered Template Sets and Ordered Options Template Sets that provide an explicit, end-to-end ordering guarantee: the Metering Process records repeated IE values in observation order, the Exporting Process preserves and signals that order, and the Collecting Process interprets the n-th occurrence as the n-th observation at the Observation Point.¶

MPLS label stacks are one of the primary use cases for Ordered Export. As described in Section 3.2 of [I-D.claise-opsawg-ipfix-ordered-ie], the absence of an ordering guarantee has led to the definition of ten positional MPLS label stack Information Elements (mplsTopLabelStackSection through mplsLabelStackSection10, IPFIX IE 70 through IE 79), plus a cluster of MPLS label attributes.¶

This document defines new generic MPLS Information Elements that, when repeated in an Ordered Template Record, can be used instead of the positional MPLS IEs. The positional IEs are not deprecated; see Section 6. The ordering mechanism, and the general rules for repeated Information Elements are specified in [I-D.claise-opsawg-ipfix-ordered-ie].¶

2. Terminology and Definitions

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.¶

The terminology defined in [RFC7011] applies to this document.¶

The terms "Ordered Template Record", "Ordered Options Template Record", "Ordered Template Set" and "Ordered Options Template Set" are used as defined in [I-D.claise-opsawg-ipfix-ordered-ie].¶

Ordered Export: The export of repeated Information Elements using Ordered Template Sets or Ordered Options Template Sets as defined in [I-D.claise-opsawg-ipfix-ordered-ie].¶

3. Existing IEs for MPLS

The IPFIX Information Elements registry [IANA.IPFIX] currently defines ten positional label stack Information Elements, originally specified in [RFC5102]:¶

• mplsTopLabelStackSection (IPFIX IE 70)¶
• mplsLabelStackSection2 (IPFIX IE 71)¶
• mplsLabelStackSection3 (IPFIX IE 72)¶
• mplsLabelStackSection4 (IPFIX IE 73)¶
• mplsLabelStackSection5 (IPFIX IE 74)¶
• mplsLabelStackSection6 (IPFIX IE 75)¶
• mplsLabelStackSection7 (IPFIX IE 76)¶
• mplsLabelStackSection8 (IPFIX IE 77)¶
• mplsLabelStackSection9 (IPFIX IE 78)¶
• mplsLabelStackSection10 (IPFIX IE 79)¶

In addition, a cluster of Information Elements exists for attributes of the top-of-stack label only: mplsTopLabelType (IPFIX IE 46), mplsTopLabelIPv4Address (IPFIX IE 47), mplsTopLabelPrefixLength (IPFIX IE 91), mplsTopLabelIPv6Address (IPFIX IE 140), mplsTopLabelTTL (IPFIX IE 200), mplsTopLabelExp (IPFIX IE 203), and postMplsTopLabelExp (IPFIX IE 237).¶

With the Ordered Export mechanism defined in [I-D.claise-opsawg-ipfix-ordered-ie], the Exporting Process can use a single generic Information Element repeated N times in an Ordered Template Record and signal that the sequence reflects the top-to- bottom order observed by the Metering Process.¶

4. Generic MPLS Information Elements

The five Information Elements defined in this section are intended for use in Ordered Template Records and Ordered Options Template Records as defined in [I-D.claise-opsawg-ipfix-ordered-ie]. The ordering, occurrence, and observation rules defined in that document apply to these Information Elements. This section only defines the element-specific semantics.¶

5. Use with Ordered Export

The generic MPLS Information Elements defined in this document are intended for use in Ordered Template Records and Ordered Options Template Records as defined in [I-D.claise-opsawg-ipfix-ordered-ie]. The Exporting Process includes the same generic IE multiple times in an Ordered Template Record; the Collecting Process interprets the n-th occurrence as the n-th observation by the Metering Process at the Observation Point, as specified in [I-D.claise-opsawg-ipfix-ordered-ie].¶

These Information Elements can also be used in a Template Record of a Template Set or an Options Template Set (Set ID 2 or 3). A single occurrence then describes stack position 1, the top of the stack. If an Information Element is repeated in such a Template Record, the order of the Information Elements in the Template Record is not guaranteed (see Sections 4.2 and 4.3 of [RFC7011]), and the n-th occurrence therefore does not correspond to stack position n.¶

For example, an Ordered Template Record may contain mplsLabelStackEntry repeated N times to export an N-entry label stack, with the first occurrence representing the top of the stack and the N-th occurrence representing the bottom. Similarly, mplsLabelType, mplsLabelIPv4Address, mplsLabelPrefixLength, or mplsLabelIPv6Address may be repeated to export the corresponding attribute for each stack position at which that attribute is available.¶

The ordering guarantee applies independently to each Information Element, as specified in Section 4.3 of [I-D.claise-opsawg-ipfix-ordered-ie], which defines no relative ordering between occurrences of different Information Elements. The correspondence between the Information Elements defined in this document comes from their definitions in Section 4: for every one of them, the n-th occurrence corresponds to stack position n. The n-th occurrence of mplsLabelStackEntry and the n-th occurrence of mplsLabelType therefore describe the same stack position, because each is defined that way, and not because of any ordering between the two Information Elements.¶

This document relies on the dense, consecutive export model defined in [I-D.claise-opsawg-ipfix-ordered-ie]. The cases where only a sparse subset of stack positions is exported are out of the scope of this document.¶

6. Coexistence and Migration

The generic MPLS Information Elements defined in this document do not immediately obsolete the existing positional IPFIX IEs. Collecting Processes that do not support Ordered Export continue to require the positional IPFIX IEs. An Exporting Process that needs to serve such Collecting Processes continues to export the positional IEs, and can export the generic IEs toward Collecting Processes that support Ordered Export.¶

Once Ordered Export is widely deployed, the positional MPLS IPFIX IEs listed in Appendix A may be deprecated in a future document. This document does not request deprecation of those IEs.¶

Appendix A provides the migration mapping from positional MPLS IEs to the generic IEs defined in this document when used with Ordered Export.¶

7. Operational Considerations

Ordered Export is additive, as described in [I-D.claise-opsawg-ipfix-ordered-ie]. Operators SHOULD enable Ordered Export of MPLS Information Elements only toward Collecting Processes that are known to support the Ordered Template Sets or Ordered Options Template Sets defined in [I-D.claise-opsawg-ipfix-ordered-ie]. Deployment against unaware Collecting Processes may result in silent loss of entire Ordered Sets, as discussed in Section 5.4 of [I-D.claise-opsawg-ipfix-ordered-ie].¶

The new mplsLabelStackEntry IE is 4 octets, whereas the existing positional label stack IEs are 3 octets. Deployments that do not require TTL information may observe a small increase in per-label export size when using the generic IE.¶

Verification of correct operation requires confirming that the Exporting Process preserves the Metering Process observation order and that the Collecting Process interprets the n-th occurrence as stack position n. Correct interpretation can be verified by comparing collected data against known MPLS label stacks in a test environment.¶

8. Security Considerations

This document does not change the trust model of the IPFIX protocol. The security considerations of [RFC7011] and [I-D.claise-opsawg-ipfix-ordered-ie] apply.¶

An Exporting Process that uses an Ordered Template Set but fails to preserve the Metering Process observation order will cause the Collecting Process to misinterpret MPLS label stack positions. Operators SHOULD ensure that the Exporting Process and Metering Process are correctly coordinated before using Ordered Export for MPLS Information Elements.¶

9. IANA Considerations

This document requests IANA to allocate the Information Elements defined in Section 4 in the "IPFIX Information Elements" registry [IANA.IPFIX]:¶

Appendix A. MPLS Positional-to-Generic Migration

This appendix is informative. It provides the migration mapping from positional MPLS IPFIX IEs to the generic IEs defined in this document when used with Ordered Export as defined in [I-D.claise-opsawg-ipfix-ordered-ie].¶

The "Generic IE" column identifies the Information Element that can be used in place of the positional IE when Ordered Export is used. It does not indicate that the positional IE is deprecated or obsolete.¶

The positional label stack IEs (IPFIX IE 70 through IE 79) are 3 octets long and carry the Label, Exp, and S fields. The generic mplsLabelStackEntry is 4 octets long and carries the complete label stack entry of Section 2.1 of [RFC3032], which adds the TTL field as the fourth octet. The first three octets of mplsLabelStackEntry have the same layout as the positional IEs. A Collecting Process therefore cannot decode the two with the same field length. It has to account for the extra octet, and the TTL is available only with the generic IE.¶

Acknowledgements

TBD.¶

Authors' Addresses