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kixelated@gmail.com

wit moq Fragmented MP4 (fMP4) is widely used for live streaming, but the ISO Base Media File Format (ISOBMFF) box structure imposes significant per-fragment overhead. Each box header requires 8 bytes (4-byte size + 4-byte type), and payload fields use fixed-width integers (u32/u64). For low-latency streaming with single-frame fragments, this overhead can exceed the media payload itself. This document defines a compression scheme for ISO BMFF that replaces box headers with compact varint-encoded identifiers and sizes, and defines compressed variants of commonly used boxes with varint payload fields. The scheme reduces per-fragment overhead from ~100 bytes to ~20 bytes while preserving the full box hierarchy. Discussion Venues Discussion of this document takes place on the Media Over QUIC Working Group mailing list (moq@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here.

Introduction Fragmented MP4 (fMP4) is the dominant container format for low-latency live streaming. Each fragment consists of a moof (movie fragment) box followed by an mdat (media data) box. The moof box contains metadata describing the media samples in mdat. For a typical single-frame video fragment, the overhead looks like this:

Box	Header	Payload	Total
moof	8	0	8
mfhd	8	4	12
traf	8	0	8
tfhd	8	8	16
tfdt	8	12	20
trun	8	16	24
mdat	8	0	8
Total	56	40	96

This 96 bytes of overhead is substantial when a single encoded video frame might only be 100-500 bytes at low bitrates or high frame rates. Audio frames are even smaller, often 4-20 bytes for Opus at low bitrates, making the container overhead several times larger than the payload. This document defines two layers of compression:

1. Header Compression: A compression table (cmpd) maps varint IDs to box type names. All boxes after the moov use compressed headers: [varint ID][varint size] instead of [u32 size][4-char type].
2. Payload Compression: Compressed variants of common boxes (cmfh, cfhd, cfdt, crun) replace fixed-width payload fields with varints.

Together, these reduce the per-fragment overhead to approximately 20 bytes.

Variable-Length Integer Encoding This document uses the variable-length integer encoding from QUIC , Section 16. The first two bits of the first byte indicate the encoding length:

2MSB	Length	Usable Bits	Range
00	1	6	0-63
01	2	14	0-16383
10	4	30	0-1073741823
11	8	62	0-4611686018427387903

In the message formats below, fields marked with (i) use this variable-length integer encoding.

Compression Table (cmpd) The cmpd box is a standard ISO BMFF box placed inside the moov box. It defines a mapping from compact varint IDs to 4-character box type names. Count: The number of entries in the compression table. ID: A varint identifier assigned to this box type. IDs SHOULD be assigned starting from 0 to minimize encoding size. Name: The 4-character ISO BMFF box type name (e.g., moof, mdat, traf). The cmpd box itself uses a standard ISO BMFF header since it appears inside the moov before compressed encoding takes effect. A typical compression table for live video streaming:

ID	Name	Description
0	moof	Movie Fragment
1	mdat	Media Data
2	mfhd	Movie Fragment Header
3	traf	Track Fragment
4	tfhd	Track Fragment Header
5	tfdt	Track Fragment Decode Time
6	trun	Track Run

With 7 entries using IDs 0-6, each ID fits in a single varint byte. Header compression alone reduces the 56 bytes of box headers (7 boxes x 8 bytes) to 14 bytes (7 boxes x 2 bytes), saving 42 bytes per fragment.

Compressed Box Header The presence of a cmpd box in the moov signals that all top-level boxes following the moov use compressed box headers. A standard ISO BMFF box header is: This is replaced with: ID: The varint identifier from the compression table. The receiver looks up the corresponding 4-character box type name in the cmpd table. Size: A varint containing the size of the box payload in bytes. Unlike standard ISO BMFF where the size field includes the header itself, the compressed size field contains only the payload length. This avoids the need for extended size fields since varints natively handle large values. The box hierarchy (nesting) is preserved exactly as in standard ISO BMFF. Container boxes (e.g., moof, traf) contain nested boxes whose sizes sum to the parent's payload size. The receiver MUST be able to reconstruct the original uncompressed ISO BMFF structure by reversing the ID-to-name mapping and adjusting size fields.

Compressed Box Variants This section defines compressed variants of commonly used fMP4 boxes. These variants replace fixed-width integer fields with varints, further reducing overhead. An encoder MAY use the standard box (with a compressed header) OR the compressed variant for any given box. The compression table determines which box type is used.

cmfh — Compressed Movie Fragment Header Replaces mfhd (Movie Fragment Header). Sequence Number: The fragment sequence number (varint instead of u32). Standard mfhd uses 4 bytes for the sequence number. With cmfh, a sequence number under 64 requires only 1 byte.

cfhd — Compressed Track Fragment Header Replaces tfhd (Track Fragment Header). Track ID: Identifies the track (varint instead of u32). Flags: A varint encoding the optional field presence flags. The flag values match the standard tfhd tf_flags semantics but are renumbered for compact varint encoding:

Flag	Field
0x01	base-data-offset-present
0x02	sample-description-index-present
0x08	default-sample-duration-present
0x10	default-sample-size-present
0x20	default-sample-flags-present

Standard tfhd uses 4 bytes for version/flags and 4 bytes for track ID (minimum 8 bytes). With cfhd, a single-track stream with no optional fields requires as few as 2 bytes.

cfdt — Compressed Track Fragment Decode Time Replaces tfdt (Track Fragment Decode Time). Base Decode Time: The decode timestamp of the first sample in this fragment (varint instead of u32/u64). Standard tfdt uses 4 bytes for version/flags plus 4 or 8 bytes for the timestamp (8-12 bytes total). With cfdt, small timestamps require as few as 1 byte.

crun — Compressed Track Run Replaces trun (Track Run). Sample Count: The number of samples in this run (varint instead of u32). Flags: A varint encoding which optional fields are present. The flag values match the standard trun tr_flags semantics:

Flag	Field
0x001	data-offset-present
0x004	first-sample-flags-present
0x100	sample-duration-present
0x200	sample-size-present
0x400	sample-flags-present
0x800	sample-composition-time-offset-present

Standard trun uses 4 bytes for version/flags, 4 bytes for sample count, and 4 bytes per optional field. With crun, a single-sample run with only sample-size typically requires 4-5 bytes instead of 16.

Example This section provides a concrete byte-level comparison for a single-frame video fragment.

Standard fMP4 A typical single-frame fragment with sequence number 42, track ID 1, decode time 3840, and a 200-byte sample: ]]> Total overhead: 96 bytes (excluding media data).

Compressed fMP4 The same fragment using compressed encoding, with the compression table from the example in Section 4: ]]> Total overhead: ~21 bytes (excluding media data). This represents a 78% reduction in per-fragment overhead (from 96 bytes to ~21 bytes).

Security Considerations TODO Security

IANA Considerations This document registers the following ISO BMFF box types:

Box Type	Description
cmpd	Compression Table
cmfh	Compressed Movie Fragment Header
cfhd	Compressed Track Fragment Header
cfdt	Compressed Track Fragment Decode Time
crun	Compressed Track Run

References Normative References This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References

Acknowledgments This draft was generated with the assistance of AI (Claude).