compression.md

Compression (v1.0)

Status: Frozen policy.

Flag

Bit	Name	Who sets
3	TRUNCATED	Latch / any producer omitting optional TLVs
4	COMPRESSED	Gateways only (zstd of entire payload)

Devices MUST NOT set bit 4 unless they implement the same zstd profile as Relay.

Gateway zstd (bit 4)

1. Start from plain envelope (flags 0 or TRUNCATED only).
2. zstd-compress the payload bytes.
3. Set FlagCompressed, update payload_length and CRCs.
4. Optionally Seal afterward.

Receive: Open → if COMPRESSED → zstd decompress → TLV parse.

MCU helpers (Latch)

RLE / varint / delta codecs exist in-library for field encoding; they are not the header COMPRESSED bit.

Limits

Decompressed size MUST be ≤ configured max envelope. Reject compression bombs.