Compressed MP4
draft-lcurley-compressed-mp4-00

moq                                                            L. Curley
Internet-Draft                                             17 March 2026
Intended status: Informational                                          
Expires: 18 September 2026

                             Compressed MP4
                    draft-lcurley-compressed-mp4-00

Abstract

   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

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Media Over QUIC
   Working Group mailing list (moq@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/moq/.

   Source for this draft and an issue tracker can be found at
   https://github.com/kixelated/moq-drafts.

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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 18 September 2026.

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   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Variable-Length Integer Encoding  . . . . . . . . . . . . . .   4
   4.  Compression Table (cmpd)  . . . . . . . . . . . . . . . . . .   4
   5.  Compressed Box Header . . . . . . . . . . . . . . . . . . . .   6
   6.  Compressed Box Variants . . . . . . . . . . . . . . . . . . .   6
     6.1.  cmfh — Compressed Movie Fragment Header . . . . . . . . .   7
     6.2.  cfhd — Compressed Track Fragment Header . . . . . . . . .   7
     6.3.  cfdt — Compressed Track Fragment Decode Time  . . . . . .   8
     6.4.  crun — Compressed Track Run . . . . . . . . . . . . . . .   8
   7.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Standard fMP4 . . . . . . . . . . . . . . . . . . . . . .  10
     7.2.  Compressed fMP4 . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  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* |
                  +---------+--------+---------+-------+

                                 Table 1

   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:

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   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.

3.  Variable-Length Integer Encoding

   This document uses the variable-length integer encoding from QUIC
   [RFC9000], 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 |
          +------+--------+-------------+-----------------------+

                                  Table 2

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

4.  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.

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   cmpd {
     Count (i),
     Compressed Box Entry (..) ...,
   }

   Compressed Box Entry {
     ID (i),
     Name (32),
   }

   *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                  |
                +----+------+----------------------------+

                                 Table 3

   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.

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5.  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:

   Standard Box Header {
     Size (32),
     Type (32),
   }

   This is replaced with:

   Compressed Box Header {
     ID (i),
     Size (i),
   }

   *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.

6.  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.

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6.1.  cmfh — Compressed Movie Fragment Header

   Replaces mfhd (Movie Fragment Header).

   cmfh {
     Sequence Number (i),
   }

   *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.

6.2.  cfhd — Compressed Track Fragment Header

   Replaces tfhd (Track Fragment Header).

   cfhd {
     Track ID (i),
     Flags (i),
     Base Data Offset (i),              ; present if flags & 0x01
     Sample Description Index (i),      ; present if flags & 0x02
     Default Sample Duration (i),       ; present if flags & 0x08
     Default Sample Size (i),           ; present if flags & 0x10
     Default Sample Flags (i),          ; present if flags & 0x20
   }

   *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:

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                +======+==================================+
                | 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     |
                +------+----------------------------------+

                                  Table 4

   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.

6.3.  cfdt — Compressed Track Fragment Decode Time

   Replaces tfdt (Track Fragment Decode Time).

   cfdt {
     Base Decode Time (i),
   }

   *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.

6.4.  crun — Compressed Track Run

   Replaces trun (Track Run).

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   crun {
     Sample Count (i),
     Flags (i),
     Data Offset (i),                   ; present if flags & 0x01
     First Sample Flags (i),            ; present if flags & 0x04
     Per-Sample Fields (..) ...,
   }

   Per-Sample Fields {
     Sample Duration (i),               ; present if flags & 0x100
     Sample Size (i),                   ; present if flags & 0x200
     Sample Flags (i),                  ; present if flags & 0x400
     Sample Composition Time Offset (i),; present if flags & 0x800
   }

   *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 |
            +-------+----------------------------------------+

                                 Table 5

   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.

7.  Example

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

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7.1.  Standard fMP4

   A typical single-frame fragment with sequence number 42, track ID 1,
   decode time 3840, and a 200-byte sample:

   moof (size=80)                           8 bytes
     mfhd (size=16)                         8 bytes
       version=0, flags=0                   4 bytes
       sequence_number=42                   4 bytes
     traf (size=56)                         8 bytes
       tfhd (size=16)                       8 bytes
         version=0, flags=0x020000          4 bytes
         track_id=1                         4 bytes
       tfdt (size=20)                       8 bytes
         version=1, flags=0                 4 bytes
         base_decode_time=3840              8 bytes
       trun (size=20)                       8 bytes
         version=0, flags=0x000200          4 bytes
         sample_count=1                     4 bytes
         sample_size=200                    4 bytes
   mdat (size=208)                          8 bytes
     <200 bytes of media data>

   *Total overhead: 96 bytes* (excluding media data).

7.2.  Compressed fMP4

   The same fragment using compressed encoding, with the compression
   table from the example in Section 4:

   moof (id=0, size=11)                     2 bytes
     cmfh (id=2, size=1)                    2 bytes
       sequence_number=42                   1 byte
     traf (id=3, size=6)                    2 bytes
       cfhd (id=4, size=1)                  2 bytes
         track_id=1                         1 byte
         flags=0                            0 bytes (no optional fields)
       cfdt (id=5, size=2)                  2 bytes
         base_decode_time=3840              2 bytes
       crun (id=6, size=3)                  2 bytes
         sample_count=1                     1 byte
         flags=0x200                        2 bytes
         sample_size=200                    2 bytes
   mdat (id=1, size=200)                    2 bytes
     <200 bytes of media data>

   *Total overhead: ~21 bytes* (excluding media data).

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   This represents a *78% reduction* in per-fragment overhead (from 96
   bytes to ~21 bytes).

8.  Security Considerations

   TODO Security

9.  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                  |
           +----------+---------------------------------------+

                                 Table 6

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

10.2.  Informative References

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   [ISOBMFF]  "Information technology — Coding of audio-visual objects —
              Part 12: ISO base media file format", 2022,
              <https://www.iso.org/standard/83102.html>.

Acknowledgments

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

Author's Address

   Luke Curley
   Email: kixelated@gmail.com

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