Session Date/Time: 21 Sep 2023 14:00

MPLS

Summary

The MPLS Working Group interim meeting focused on two main areas: the status of the M&A Header Solution draft and an in-depth discussion on Readable Label Depth (RLD). Key topics for RLD included its definition, advertising scope (per-link vs. per-node), protocol extensions for IGP and BGP-LS, and the process for IANA allocation for a new IGP Maximum Segment Depth (MSD) type. A working definition for RLD was tentatively agreed upon, and a plan for the IANA allocation via the M&A framework document was outlined. The next interim meeting will discuss M&A use cases.

Key Discussion Points

Administrative: The meeting experienced low attendance, and issues with audio and slide sharing were noted early on. The Note Well was presented.
M&A Header Solution Draft Status:
- Carlos provided feedback on the M&A Header Solution draft, and authors are converging on changes. A new version is expected soon.
- Dong had provided comments on the draft. An author, Tony, indicated he would circulate his personal views on these comments to the author team, suggesting a potential voice call among authors to quickly resolve substantive issues.
IOD/RLD Definition and Scope:
- Discussion on the definition of "IOD" (Intermediate Operation Depth) or "RLD" (Readable Label Depth). Several options were considered, including "no performance impact," "no processor impact," "line rate processing," or "forwarding rate processing."
- Decision: The working definition for RLD/IOD will be "with no performance impact" for the time being, with a commitment to revisit it during the framework document's Working Group Last Call.
- A question was raised about defining "readable depth" more generally (e.g., in bytes, beyond the MPLS header) to encompass payload inspection capabilities, but the focus remained on label depth for RLD to avoid defining terms not currently referenced.
- The term "Readable Label Depth" was contrasted with "Writable Label Depth" for M&A, questioning if hardware capabilities for reading and writing deep into the stack are always identical. For current hardware, it was stated they are generally the same.
- Advertising Scope: The protocol must support both link-specific and node-wide advertisement for RLD.
  - Use cases for link advertisement include modular nodes with different line card capabilities or virtualized instances.
  - Per-node RLD would represent the smallest RLD on that node.
  - A point was raised that if RLD varies on a node, both per-node and per-link could be advertised to avoid signaling the minimum and provide accurate information. Jiao was asked to work with Tony on refining the text in the framework document.
  - A question was raised about using older terminology like "per-platform" and "per-interface" versus "per-node" and "per-link"; Jiao will check on this.
Advertising Protocols and IANA Allocation:
- IGP protocol extensions for RLD are considered simple, requiring a new IGP MSD type.
- The term "MSD" (Maximum Segment Depth) from Segment Routing was discussed, with concerns raised about overloading it for a generic "label depth." It was clarified that existing IANA registries for MSD types are used for various label depths, even in non-SR MPLS scenarios, and replicating mechanisms for new terminology would be wasteful.
- For BGP-LS, no new extensions are required; it would carry the IGP-defined TLVs as is.
- Placement of IANA Request: A key discussion point was whether the request for a new IGP MSD type for RLD should be included in the M&A framework draft or a separate, short LSR-specific draft.
  - The existing IANA procedure for IGP MSD types is "Expert Review" which does not necessarily require a Standards Track document or a dedicated draft, only expert agreement.
  - A sense of those present indicated that including the allocation request directly in the M&A framework draft is acceptable.
Relationship to Network Actions and Capabilities:
- RLD is intended to be a general capability, independent of specific network actions (e.g., flow ID processing) or the capability to perform those actions.
- The conflation of "readable depth" and "capability to process entropy labels" in ER_LD (Entropy Label Readable Depth) was identified as a historical issue ("bug") to be avoided with the new RLD definition. RLD should indicate how far a router can read, separate from what actions it can perform on that data.

Decisions and Action Items

Decision: The working definition for IOD/RLD is "with no performance impact" for now, subject to reconsideration during the M&A framework draft's Working Group Last Call.
Decision: The request for a new IGP MSD type for RLD will be incorporated into the M&A framework draft, rather than a separate, dedicated LSR draft.
Action Item: M&A framework draft authors (specifically Tony) will reach out to the LSR Working Group's IANA experts to discuss and obtain approval for the new IGP MSD type allocation.
Action Item: Jiao will work with Tony to clarify the text in the M&A framework draft regarding RLD advertising scope (per-node vs. per-link) to ensure accuracy and consistency.
Action Item: Jiao will check for the usage of "per-platform" terminology in relation to per-node/per-link.
Action Item: M&A header solution draft authors are to address outstanding comments (e.g., from Dong) and may schedule an author-specific voice call if needed to expedite convergence.

Next Steps

The next MPLS interim meeting will focus on discussing M&A use cases. A reminder email will be sent to authors and participants asking them to review relevant documents and prepare for the discussion.