Network Working Group                                         Joe Tansey
Internet-Draft: draft-joetansey-alvc-schc-lpwan-00
Cisco
Intended status: Experimental                           August 28, 2025
Expires: February 27, 2026

        ALVC over LPWAN: SCHC Fragmentation, Priority, and
        Security

Abstract

   This document specifies a transport profile for carrying the
   Adaptive
   Layered Voice Codec (ALVC) over constrained Low-Power Wide-Area
   Networks (LPWANs) using Static Context Header Compression and
   fragmentation (SCHC). It defines an ALVC object model, fragment
   headers, priority scheduling, unequal error protection, receiver
   behavior, and a CoAP mapping supporting progressive playback under
   regional duty-cycle limitations.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF). Note that other groups may also distribute
   working
   documents as Internet-Drafts. The list of current Internet-Drafts
   is
   at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six
   months
   and may be updated, replaced, or obsoleted by other documents at
   any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 27, 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this
   document.
   Please review these documents carefully, as they describe your
   rights
   and restrictions with respect to this document. Code Components
   extracted from this document must include Simplified BSD License
   text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD
   License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  ALVC Object Model
   4.  ALVC Fragment Header
   5.  Priority and Scheduling
   6.  Unequal Error Protection
   7.  Loss Handling and Parity Slicing
   8.  Receiver Behavior and Progress Reporting
   9.  CoAP and OSCORE Mapping
   10. Congestion Control and Duty-Cycle
   11. Parameter Sets
   12. Security Considerations
   13. Privacy Considerations
   14. IANA Considerations
   15. Acknowledgments
   16. References
   Authors' Addresses

1.  Introduction

   LPWAN links have small MTUs, long airtimes, and strict duty
   cycles,
   which make continuous audio streaming impractical. This profile
   defines the carriage of ALVC objects over LPWAN using SCHC
   fragmentation, enabling store-and-forward voice with early base-
   layer
   playback and progressive refinement. Gateways may cache fragments
   and
   perform store-and-forward across backhaul links with heterogeneous
   reliability and delay.

   This document is a companion to the Adaptive Layered Voice
   Codec (ALVC) specification [ALVC-CODEC].

2.  Requirements Language

   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].

3.  ALVC Object Model

   An ALVC Message comprises: optional Transcript, mandatory Layer-0,
   and zero or more Enhancement layers. Each layer is divided into
   fragments sized to the transport payload. Messages may arrive out
   of
   order. Gateways SHOULD prioritize Layer-0 recovery before
   enhancements.

4.  ALVC Fragment Header

   Each SCHC payload carrying ALVC includes an inner ALVC header with
   fields: MsgID, LayerID, Sequence number, Total fragments, TTL,
   Flags,
   and optional CRC16. Flags include BASE, ENH, FEC present,
   encrypted.

5.  Priority and Scheduling

   Priority order is Transcript (highest), then Layer-0, then
   Enhancements. Retransmissions SHOULD favor missing Layer-0
   fragments.
   Senders SHOULD emit a burst of early Layer-0 fragments to reach
   playable audio, then interleave enhancements in a deficit-round-
   robin
   manner. Gateways MAY reorder to favor base recovery.

6.  Unequal Error Protection

   Layer-0 SHOULD use stronger redundancy (for example, plus 20-30%
   parity) using Reed-Solomon or fountain codes. Enhancements MAY use
   reduced FEC or none. A MIC at the SCHC level is RECOMMENDED. Inner
   CRC16 is OPTIONAL when OSCORE is used.

7.  Loss Handling and Parity Slicing

   CRC-only detection is not sufficient. This profile specifies
   parity
   slicing for Layer-0 fragments using systematic FEC. Enhancements
   MAY
   use best-effort delivery with optional light FEC. Gateways MUST
   prioritize Layer-0 recovery over Enhancements. A header flag
   indicates when FEC is present.

8.  Receiver Behavior and Progress Reporting

   Receivers MUST permit early playback once all Layer-0 fragments
   for
   the initial window are received. Enhancement fragments MUST be
   applied idempotently. Progress MAY be reported as weighted
   completion
   across layers. Decoders SHOULD persist partial messages across
   reboots when permitted by policy.

9.  CoAP and OSCORE Mapping

   ALVC groups MAY be exposed as CoAP resources and transferred using
   Block-Wise over SCHC. OSCORE [RFC8613] provides confidentiality,
   integrity, and replay protection. Gateways SHOULD minimize
   metadata
   leakage by padding or batching transmissions where feasible.

10. Congestion Control and Duty-Cycle

   Senders MUST respect regional duty-cycle rules. Gateways MAY
   throttle
   enhancement layers under congestion. ACK-on-Error for Layer-0 is
   RECOMMENDED to bound recovery latency without incurring per-
   fragment
   acknowledgments.

11. Parameter Sets

   Example (informative): LoRaWAN SF10, payload 80 bytes. Layer-0 at
   800 bps for 20 seconds requires about 25 fragments; one
   enhancement
   layer may require about 50 fragments. Total transfer time spreads
   over one to two minutes with duty-cycle limitations.

12. Security Considerations

   Threats include spoofing, replay, and privacy leakage via
   transcripts.
   Mitigations include OSCORE, AEAD, and transcript redaction
   policies.
   Implementations MUST zeroize keys on gateway reboot and SHOULD use
   hardware-backed keystores.

13. Privacy Considerations

   Voice content is sensitive. Systems SHOULD apply data minimization
   and retention limits. Transcript-only modes MAY be used where
   voice
   is not required.

14. IANA Considerations

   This document has no IANA actions.

15. Acknowledgments

   Thanks to reviewers for feedback on SCHC operation and voice
   layering.

16. References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March
              1997.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase Key Words", BCP 14,
              RFC 8174, May 2017.

   [RFC8724]  Minaburo, A., et al., "SCHC for LPWAN", RFC 8724,
              April 2020.

   [RFC9363]  Minaburo, A., et al., "SCHC over LoRaWAN", RFC 9363,
              November 2022.

   [RFC7252]  Shelby, Z., et al., "The Constrained Application
   Protocol
              (CoAP)", RFC 7252, June 2014.

   [RFC8613]  Selander, G., et al., "Object Security for Constrained
              RESTful Environments (OSCORE)", RFC 8613,
              July 2019.

   [ALVC-CODEC]  Tansey, J., "Adaptive Layered Voice Codec (ALVC) for
              LPWAN Store-and-Forward", Internet-Draft,
              draft-joetansey-alvc-codec-00 (work in progress),
              August 2025.

Authors' Addresses

   Joe Tansey
   Cisco Systems
   170 West Tasman Dr
   San Jose, CA 95134
   United States

   Email: joetanse@cisco.com