Session Date/Time: 25 Jun 2024 14:00

SCHC

Summary

The SCHC working group held an interim meeting to discuss the status of current drafts and prepare for the upcoming IETF 120 meeting in Vancouver. Key topics included the potential adoption of draft-ietf-schc-zero-energy-devices, an update on draft-ietf-schc-protocol-numbers, and a detailed presentation on the draft-ietf-schc-transmission-over-15-4 (6Lo) draft, focusing on aligning it with SCHC architecture concepts, multihop communication modes, and terminology clarifications.

Key Discussion Points

Meeting Logistics: There were initial technical issues with slide upload and an audio problem for one of the chairs (Alex). The IETF Note Well and patent disclosure BCP 79 were reviewed.
IETF 120 Vancouver Meeting: The WG secured a two-hour slot on Monday, July 22nd (first day), which accommodates European participants. This extended slot allows for presentations and discussions on all active drafts.
draft-ietf-schc-zero-energy-devices Status:
- The Call for Adoption ends tomorrow (July 9th, evening).
- Until now, only positive reviews have been received, making adoption highly likely.
- Discussion arose regarding the draft's name and scope. While currently focused on "Zero Energy Devices," future cellular (3GPP LPWA) changes might broaden its applicability. A sense of those present indicated a preference to adopt the draft as-is and allow for renaming and scope enlargement later, if needed. Edgar (author) agreed.
draft-ietf-schc-protocol-numbers Status: This draft has been published as draft-ietf-schc-protocol-numbers-00. The website update may take a day or two to reflect this.
Milestones: The working group's milestones are outdated and unclear. Given the current uncertainty, the suggestion is to leave the existing milestone dates as-is and operate on a "best effort" basis until better clarity can be achieved through AD consultation.
draft-ietf-schc-transmission-over-15-4 (6Lo) Presentation (Carles and Anna):
- The goal is to update the 6Lo draft to align with SCHC architecture v02 concepts.
- Multihop Communication Modes:
  - Straightforward Route Over (SRO): All intermediate routers store rules. Lowest overhead but scalability challenges. Pascal noted that SRO is very applicable in homogeneous networks where rules are network-wide (e.g., Wi-SUN) or if only a few bits of the IP address vary.
  - Tunnel-Based Route Over (TBO): Utilizes tunnels (e.g., RPL non-storing mode). Compressed packets are tunneled to the 6LBR/root. Pascal highlighted RFC 8138 for IP-in-IP tunnel compression.
  - Pointer-Based Route Over (PBO): Avoids intermediate routers storing all rules by prepending a "pointer" to the SCHC compressed packet, indicating how to locate the compression residue for the IPv6 destination address. Pascal raised security concerns, noting that if a pointer can be tampered with, it could lead to incorrect routing, though Laurent pointed out that tampering with the pointer is not inherently different from tampering with other packet parts (e.g., destination address or residue). The authors will add a security discussion.
- Network Types and SCHC Header Compression:
  - Single Instance Networks: All devices use a single SCHC packet instance. The SCHC header can be fully compressed (zero bits over the air). Rule IDs and corresponding rules must be unique network-wide (SRO, TBO) or, for PBO/Mesh And Register (Mander), disjoint pairs of endpoints may use the same Rule ID even for different rules.
  - Multiple Instance Networks: At least some devices use more than one SCHC packet instance. The SCHC header cannot generally be fully compressed, as extra information is needed to distinguish which instance's rule to apply (e.g., "black" vs. "blue" rules). An exception was noted for Mander when the source can be identified from the mesh header, allowing full compression in that specific scenario.
- Hybrid Modes: Pascal suggested a hybrid approach where an "access router" might act as a PBO endpoint for attached routing-unaware leaves, while the rest of the network's core operates using tunnels (no SCHC processing in the core). This could optimize the first/last hop.
- Terminology Clarification ("SCHC Header"):
  - Confusion exists between "SCHC header" as defined in the architecture draft (v02) (which can be zero bits) and the "Rule ID + compression residue" traditionally referred to as the SCHC header.
  - Pascal suggested using "Stratum Header" or "SCHC Stratum Header" to differentiate and reduce ambiguity.
  - Anna raised a question about whether a separate SCHC header is needed for each stratum (e.g., UDP and CoAP) if they are compressed within the same rule set. The consensus was that there is one SCHC header per stratum, but a rule can compress multiple headers (e.g., IPv6, UDP, CoAP jointly).
  - Alex noted the importance of clarifying this terminology in the SCHC architecture draft (v03) first.
- Frame Formats: Examples were shown for how the SCHC header would be inserted into 6LoWPAN, PRO, and Mander frame formats. For PRO, the SCHC header is placed within the PRO header to avoid complicating the bit pointer field.
- Application of Architecture Concepts: The presentation summarized how concepts like SCHC stratum (layer 2.5), discriminator (SCHC Dispatch/Pointer Dispatch), and packet instances apply to single and multiple instance networks.

Decisions and Action Items

Decision: The draft-ietf-schc-zero-energy-devices will likely be adopted by the WG. The initial scope and name will be retained, with flexibility for future changes as the cellular LPWA landscape evolves.
Decision: draft-ietf-schc-protocol-numbers has been published as a WG document.
Action Item: Working Group Chairs to continue working with ADs on clarifying and updating SCHC WG milestones, but for now, operate on a "best effort" basis with existing dates.
Action Item: Carles and Anna to update draft-ietf-schc-transmission-over-15-4 to:
- Include discussion on the applicability of Straightforward Route Over (SRO) in homogeneous networks.
- Add a security discussion for Pointer-Based Route Over (PBO).
- Explore and document hybrid multihop modes (e.g., PBO in access, tunneling in core).
- Clarify terminology related to the "SCHC Header" versus "Rule ID + residue", potentially using "Stratum Header" where appropriate, in alignment with future architecture draft updates.
Action Item: Authors and Chairs of the SCHC Architecture draft to prioritize clarifying the terminology around "SCHC Header," "Stratum," and related concepts in the upcoming v03 version, ideally before the IETF 120 cutoff.

Next Steps

Continue the adoption process for draft-ietf-schc-zero-energy-devices.
Finalize updates to the draft-ietf-schc-transmission-over-15-4 draft.
Work on clarifying core terminology within the SCHC architecture draft (v03).
Prepare presentations for all active drafts for the IETF 120 meeting in Vancouver.