CBDF Specification: Meta Section (QMail)

1. Overview

The Meta section is the first section of every CBDF document. It contains key/value pairs that describe the document's identity, routing, formatting options, and preview content.

The Meta section serves as the document's "envelope" -- an email client can read ONLY the Meta section (without downloading styles, text, or resources) and display the message in an inbox listing with subject, sender, date, preview text, and AI summary.

2. Meta Section Format

[Pair Count: 2 bytes LE] [Key/Value pairs...]

Pair Count: A 16-bit little-endian unsigned integer (0-65535) indicating the total number of key/value pairs that follow. This count includes repeated keys (e.g., multiple To recipients each count as one pair).

The Meta section ends when either:

• All pairs indicated by Pair Count have been read, OR
• A SECTION_SEP (0x1C) marker is encountered (start of Styles section), OR
• EOF is reached (for meta-only documents with the EOF flag).

3. Key/Value Pair Format

Each pair has the same structure:

This gives a maximum value size of 255 bytes per pair.

4. Meta Key Table

4A. Phase I Keys (Key IDs 1-29)

These keys are defined in Phase I and are fully backward-compatible. Phase II clients must support all Phase I keys.

Key IDs 3-11, 15-18, 20-24, 26-29 are reserved for future Phase I extensions. * = Required for all QMail documents.

4B. Phase II Keys (Key IDs 30-39)

These keys are new in Phase II. Phase I clients will ignore unknown keys (they skip Key ID + Value Length + Value bytes for any unrecognized key).

Key IDs 40-255 are reserved for future use.

Detailed Key Descriptions

KEY 30 - VERSION (required):

• 0 = Phase I (plain text only, no styles section)
• 1 = Phase II (styled text, resources, full CBDF)
• 2-255 = Future versions

A Phase I document has no Version key (the absence of key 30 implies version 0). Phase II documents MUST include this key.

KEY 31 - COMPRESSION TYPE:

If absent, compression type = 0 (none).

KEY 32 - DEFAULT STYLE SET: 0 = Explicit styles, 1 = Client default, 2-255 = Named style set IDs. If absent, default = 0.

KEY 33 - EOF FLAG: 1 = This document consists of ONLY the Meta section. No FS separators, no body sections follow. Use for ultra-compact messages.

KEY 34 - DOCUMENT TYPE: 0 = Email (QMail), 1 = Web page (QWeb), 2 = Attachment/embedded document, 3-255 = Reserved.

KEY 35 - AI SUMMARY: A short AI-generated summary of the document content. UTF-8, max 255 bytes. Used for inbox preview and as a fallback for semantic encoding.

KEY 36 - PREVIEW TEXT: The first ~100 characters of the document body. REQUIRED when semantic encoding is active. Optional otherwise but recommended.

KEY 37 - SUBJECT STYLE ID: Index into the Text Styles sub-table. Allows styled subjects in inbox listings without opening the email.

KEY 38 - SEMANTIC MODEL: 20 bytes total: [Model ID: 4 bytes] + [Version Hash: 16 bytes]. Only present when Compression Type = 5.

KEY 39 - SEMANTIC FLAGS: Bitfield: Bit 0 = Lossy OK, Bit 1 = Exact words matter, Bits 2-7 = Reserved (must be 0). Only meaningful when Compression Type = 5.

5. Mailbox Address Format

All mailbox addresses (To, CC, From) are exactly 7 bytes:

Example: CloudCoin denomination 2, serial number 147352:

06 00 02 98 3F 02 00
(Group=0x0006 LE, Denom=0x02, SN=0x00023F98 LE)

6. Backward Compatibility

Phase I documents (version 0):

• Have no Version key (key 30 absent).
• Have no Phase II keys.
• Phase II parsers detect version 0 by the absence of key 30.

Phase II documents (version 1):

• MUST include key 30 (Version = 1).
• SHOULD include key 34 (Document Type) for proper rendering.
• MAY include any other Phase II keys.
• Phase I parsers will encounter unknown keys (30+) and skip them safely because the KLV format is self-describing.

Key ID ordering: Keys MAY appear in any order. Parsers must not assume a specific ordering. Encoders SHOULD place Version (30) and EOF Flag (33) early for fast detection.

7. Worked Examples

7A. Phase I Plain Text Email: "Hello World!"

Two To recipients, no CC, no attachments.

07 00                               Pair count: 7

01 10                               Key 1 (QMail ID), length 16
BF 7B 94 B3 91 A2 46 B5            GUID bytes 0-7
8E 48 54 5D D8 F1 31 01            GUID bytes 8-15

02 0C                               Key 2 (Subject), length 12
48 65 6C 6C 6F 20 57 6F            "Hello Wo"
72 6C 64 21                         "rld!"

0C 01 00                            Key 12 (Attachments), length 1, value 0

0D 07                               Key 13 (To Mailbox), length 7
06 00 02 98 3F 02 00                Group=6, Denom=2, SN=147352

0D 07                               Key 13 (To Mailbox), length 7
06 00 02 2E 67 04 00                Group=6, Denom=2, SN=288558

13 07                               Key 19 (From Mailbox), length 7
06 00 02 A0 78 E8 03                Group=6, Denom=2, SN=65566880

19 04                               Key 25 (Timestamp), length 4
68 CF B6 AD                         Unix: 1758443181 (2025-09-21)

1C                                  FS (end of Meta)
1C                                  FS (end of Styles, empty)
02                                  STX
48 65 6C 6C 6F 20 57 6F            "Hello Wo"
72 6C 64 21                         "rld!"

Total: 79 bytes.

7B. Phase II Meta-Only Message: "Meeting at 3pm"

Ultra-compact notification, no body.

05 00                               Pair count: 5

1E 01 01                            Key 30 (Version), length 1, value 1
21 01 01                            Key 33 (EOF Flag), length 1, value 1

01 10                               Key 1 (QMail ID), length 16
AA BB CC DD EE FF 00 11            GUID bytes 0-7
22 33 44 55 66 77 88 99            GUID bytes 8-15

02 0E                               Key 2 (Subject), length 14
4D 65 65 74 69 6E 67 20            "Meeting "
61 74 20 33 70 6D                   "at 3pm"

13 07                               Key 19 (From), length 7
06 00 02 A0 78 E8 03                Group=6, Denom=2, SN=65566880

Total: 52 bytes. No FS markers, no sections beyond meta.

7C. Phase II Styled Email with Preview

Email with compression, preview text, and AI summary.

0A 00                               Pair count: 10

1E 01 01                            Key 30 (Version=1)
22 01 01                            Key 34 (Document Type=1, email)
1F 01 01                            Key 31 (Compression=1, zlib)

01 10                               Key 1 (QMail ID), length 16
[16 bytes GUID]

02 12                               Key 2 (Subject), length 18
"Quarterly Report Q4"

0C 01 03                            Key 12 (Attachments=3)

0D 07 [7 bytes]                     Key 13 (To)
13 07 [7 bytes]                     Key 19 (From)
19 04 [4 bytes]                     Key 25 (Timestamp)

23 32                               Key 35 (AI Summary), length 50
"Revenue up 15%, marketing leads at 23% growth."

24 64                               Key 36 (Preview Text), length 100
"The quarterly sales report shows a 15% increase in revenue across..."

After meta: FS + compressed(Styles + Text) + FS + Resources + FS

8. Implementation Notes (C)

The existing Phase I encoder (qmail_cbdf.c) uses these key IDs: Key 1 = GUID, Key 2 = Subject, Key 12 = Attachments, Key 13 = To, Key 14 = CC, Key 19 = From, Key 25 = Timestamp.

To extend for Phase II, the encoder should:

1. Add key 30 (Version) as the first pair.
2. Add key 34 (Document Type) for email vs web.
3. Optionally add keys 31, 32, 33, 35, 36, 37.
4. Keep all Phase I keys unchanged.

Parser key lookup:

static const char* cbdf_meta_key_name(uint8_t key_id) {
    switch (key_id) {
        case 1:  return "QMail ID";
        case 2:  return "Subject";
        case 12: return "Attachment Count";
        case 13: return "To Mailbox";
        case 14: return "CC Mailbox";
        case 19: return "From Mailbox";
        case 25: return "Timestamp";
        case 30: return "Version";
        case 31: return "Compression Type";
        case 32: return "Default Style Set";
        case 33: return "EOF Flag";
        case 34: return "Document Type";
        case 35: return "AI Summary";
        case 36: return "Preview Text";
        case 37: return "Subject Style ID";
        case 38: return "Semantic Model";
        case 39: return "Semantic Flags";
        default: return "Unknown";
    }
}

Key validation:

• Required keys (1, 12, 13, 19, 25, 30) must be present.
• Key 30 (Version) must be >= 1 for Phase II documents.
• Key 33 (EOF Flag) = 1 means no FS markers follow.
• Unknown keys (future extensions) must be silently skipped, not treated as errors.