feat(catalog): add m2tsEsPid field and ES-level carriage mode

Introduce ES-level carriage as a third mode alongside unmodified and
modified carriage. A track with `m2tsEsPid` present carries exactly one
elementary stream or signaling table per MOQT track, enabling per-ES
subscription and subscriber-side program recombination.

* Add optional `m2tsEsPid` Number field: identifies the single PID
  carried in the track; requires m2tsModified true; forbids m2tsMpts,
  m2tsPmtPid, m2tsSiPids, and m2tsScte35Pid
* Add §5.5.3 Modified ES-Level Carriage: defines track payload rules,
  PCR cross-track reference, Group boundary synchronization requirement,
  subscriber recombination obligations, and altGroup audio semantics
* Define MSF role values for SI-table and SCTE-35 ES-level tracks:
  "nit", "sdt", "eit", "tdt", "scte35"
* Update §5.5 intro to describe three carriage modes
* Update m2tsPmtPid, m2tsPcrPid, m2tsSiPids, m2tsScte35Pid definitions
  with ES-level constraints and cross-track PCR reference semantics
* Add ES-level subscriber processing paragraph to §8
* Add catalog example: video + two alternate audio tracks + EIT track

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gwendalsimon

draft-gregoire-moq-msfts.md

@@ -252,12 +252,16 @@ assuming every Object has the declared size.
 
 ## Source Handling and Carriage Modes {#mpts}
 
-A publisher carries a transport stream in one of two modes, signaled by the
-required `m2tsModified` field ({{m2ts-modified}}): unmodified carriage, where
-the source packets are forwarded as received, and modified carriage, where the
-publisher has changed the stream.  The `m2tsMpts` field ({{m2ts-mpts}})
-indicates whether the track carries a single program or a whole multiplex, and
-it alone controls the presence of the per-program fields.
+A publisher carries a transport stream in one of three modes, signaled by the
+required `m2tsModified` field ({{m2ts-modified}}) and the optional
+`m2tsEsPid` field ({{m2ts-es-pid}}).  When `m2tsModified` is false, the
+publisher forwards the source packets without modification (unmodified
+carriage).  When `m2tsModified` is true and `m2tsEsPid` is absent, the
+publisher has modified the stream at the program level (modified carriage).
+When `m2tsModified` is true and `m2tsEsPid` is present, the track carries a
+single elementary stream (ES-level carriage).  The `m2tsMpts` field
+({{m2ts-mpts}}) indicates whether the track carries a single program or a
+whole multiplex, and it alone controls the presence of the per-program fields.
 
 ### Unmodified Carriage {#unmodified-carriage}
 
@@ -336,6 +340,39 @@ tracks are derived from the same MPTS source, the publisher SHOULD use the MSF
 `altGroup` field if the programs are alternate renditions of the same content;
 programs that are independent services SHOULD be published as separate tracks.
 
+### Modified ES-Level Carriage {#es-level-carriage}
+
+When `m2tsEsPid` ({{m2ts-es-pid}}) is present, the track carries a single
+elementary stream or signaling table.  The track payload contains only TS
+packets for the PID identified by `m2tsEsPid`; PAT, PMT, and null packets are
+not included.  Publishers SHOULD use the MSF `initDataList` field to carry the
+PAT and PMT of the originating program so that subscribers can identify the
+program structure before processing elementary-stream packets.  When the
+publisher chooses to interleave PAT and PMT in the track payload for inline
+program initialization, `m2tsPsiInterval` SHOULD be declared; otherwise
+`m2tsPsiInterval` MUST be absent.
+
+When `m2tsPcrPid` equals `m2tsEsPid`, the track embeds the Program Clock
+Reference and provides the timing reference for the program.  When `m2tsPcrPid`
+identifies a different PID, that PID is carried by another track; a subscriber
+requiring PCR-based timing MUST subscribe to the track carrying that PID.
+
+A publisher producing multiple ES-level tracks for the same program MUST align
+Group boundaries across all those tracks so that matching Group numbers
+correspond to the same presentation position.  This alignment allows a
+subscriber to combine ES-level tracks reliably.  A subscriber combining
+multiple ES-level tracks into a single TS output MUST construct a PAT listing
+the carried program and a PMT listing the PIDs of all subscribed ES-level
+tracks, and MUST interleave packets from all tracks.  The subscriber sources
+PCR from the track where `m2tsPcrPid` equals `m2tsEsPid`.  When `m2tsMuxRate`
+is declared on the PCR-bearing track, the subscriber SHOULD use that value to
+restore the original mux rate in the combined output.
+
+Audio ES-level tracks for the same program MAY share an `altGroup` to indicate
+that they are alternate language renditions.  A subscriber MAY subscribe to
+more than one audio track within an alternate group and include all subscribed
+audio PIDs in the reconstructed PMT.
+
 ## PCR and Timing {#pcr-timing}
 
 The Program Clock Reference (PCR) is carried inside adaptation fields of
@@ -398,6 +435,7 @@ Table 1 lists the m2ts-specific fields defined within a track object.
 | M2TS random access            | m2tsRandomAccess        | {{m2ts-random-access}} |
 | M2TS timestamp mode           | m2tsTimestampMode       | {{m2ts-timestamp-mode}} |
 | M2TS SCTE-35 PID              | m2tsScte35Pid           | {{m2ts-scte35-pid}} |
+| M2TS ES PID                   | m2tsEsPid               | {{m2ts-es-pid}} |
 | M2TS MPTS                     | m2tsMpts                | {{m2ts-mpts}} |
 
 Use of the MSF `initRef` and `initDataList` fields by m2ts tracks is described
@@ -446,7 +484,8 @@ Required: Optional    JSON Type: Number    Location: Track Object
 The packet identifier carrying the Program Map Table for `m2tsProgramNumber`.
 This field is advisory and does not replace the Program Association Table or
 Program Map Table carried in the transport stream.  It MUST be absent when
-`m2tsMpts` is true.
+`m2tsMpts` is true.  It MUST be absent when `m2tsEsPid` is present, because
+an ES-level track does not carry a PMT in its payload.
 
 ## M2TS PCR PID {#m2ts-pcr-pid}
 
@@ -455,7 +494,9 @@ Required: Optional    JSON Type: Number    Location: Track Object
 The packet identifier carrying the Program Clock Reference for the program
 identified by `m2tsProgramNumber`.  This field is advisory and does not
 replace PCR signaling in the transport stream.  It MUST be absent when
-`m2tsMpts` is true.
+`m2tsMpts` is true.  When `m2tsEsPid` is present, this PID MAY be carried by
+a different track in the same session; a subscriber requiring PCR-based timing
+MUST subscribe to the track where `m2tsPcrPid` equals `m2tsEsPid`.
 
 ## M2TS PSI Interval {#m2ts-psi-interval}
 
@@ -486,7 +527,9 @@ The packet identifiers of SI tables retained in the filtered track, in addition
 to those listed in the Program Map Table.  This field is advisory.  Publishers
 SHOULD include this field when they retain DVB or ATSC SI tables.  Subscribers
 MAY use this list to verify which service information tables are present without
-inspecting the packet stream.
+inspecting the packet stream.  This field MUST be absent when `m2tsEsPid` is
+present; at ES-level granularity, each SI table is published as a separate
+track identified by `m2tsEsPid` and the MSF `role` field.
 
 ## M2TS Random Access {#m2ts-random-access}
 
@@ -513,9 +556,33 @@ Required: Optional    JSON Type: Number    Location: Track Object
 
 The PID carrying SCTE-35 splice_info_section() messages for this track.  This
 field is advisory; SCTE-35 messages are also discoverable via the PMT
-conditional access or registration descriptor.  When present, receivers MAY use this value to locate splice events
-without parsing PMT.  Publishers SHOULD include this field when the track carries
-SCTE-35 splice signaling.
+conditional access or registration descriptor.  When present, receivers MAY
+use this value to locate splice events without parsing the PMT.  Publishers
+SHOULD include this field when the track carries SCTE-35 splice signaling.
+This field MUST be absent when `m2tsEsPid` is present; when SCTE-35 is
+published as an ES-level track, the track's `m2tsEsPid` and `role` fields
+identify it.
+
+## M2TS ES PID {#m2ts-es-pid}
+
+Required: Optional    JSON Type: Number    Location: Track Object
+
+The Packet Identifier of the single elementary stream or signaling table
+carried by this track.  When present, the track carries only TS packets for
+this PID; it does not carry PAT, PMT, or null packets.  This field MUST be
+absent when `m2tsMpts` is true.  When `m2tsEsPid` is present, `m2tsModified`
+MUST be true, `m2tsPmtPid` MUST be absent, `m2tsSiPids` MUST be absent, and
+`m2tsScte35Pid` MUST be absent.
+
+For tracks carrying DVB or ATSC service information tables, publishers SHOULD
+set the MSF `role` field to one of the following values: `"nit"` for the
+Network Information Table (PID 0x0010), `"sdt"` for the Service Description
+Table and Bouquet Association Table (PID 0x0011), `"eit"` for the Event
+Information Table (PID 0x0012), and `"tdt"` for the Time and Date Table and
+Time Offset Table (PID 0x0014).  For tracks carrying SCTE-35 splice
+information, publishers SHOULD set `role` to `"scte35"`.  For media
+elementary streams, publishers SHOULD set `role` to the MSF-defined value for
+the stream type, for example `"video"` or `"audio"`.
 
 ## M2TS MPTS {#m2ts-mpts}
 
@@ -772,6 +839,90 @@ PAT and PMT on the new track before routing packets to a decoder.
 }
 ~~~
 
+## ES-Level Tracks - Per-Elementary-Stream Publishing {#example-es-level}
+
+This example shows a live program published as separate ES-level tracks: one
+video track carrying the PCR, two alternate audio tracks (English and Spanish),
+and one Event Information Table track.  The video and audio tracks MUST have
+synchronized Group boundaries.  A subscriber combining video and one audio
+track constructs a PAT and PMT listing the two subscribed PIDs and sources PCR
+from the video track (PID 257).
+
+~~~ json
+{
+  "version": "draft-01",
+  "generatedAt": 1746104606044,
+  "tracks": [
+    {
+      "name": "program-1-video",
+      "namespace": "live.example.com/channel/1",
+      "packaging": "m2ts",
+      "m2tsModified": true,
+      "isLive": true,
+      "targetLatency": 1000,
+      "role": "video",
+      "mimeType": "video/mp2t",
+      "bitrate": 5000000,
+      "m2tsPacketSize": 188,
+      "m2tsPacketsPerObject": 64,
+      "m2tsProgramNumber": 1,
+      "m2tsPcrPid": 257,
+      "m2tsEsPid": 257,
+      "m2tsRandomAccess": true
+    },
+    {
+      "name": "program-1-audio-en",
+      "namespace": "live.example.com/channel/1",
+      "packaging": "m2ts",
+      "m2tsModified": true,
+      "isLive": true,
+      "targetLatency": 1000,
+      "role": "audio",
+      "mimeType": "video/mp2t",
+      "bitrate": 128000,
+      "altGroup": 2,
+      "m2tsPacketSize": 188,
+      "m2tsPacketsPerObject": 32,
+      "m2tsProgramNumber": 1,
+      "m2tsPcrPid": 257,
+      "m2tsEsPid": 258,
+      "m2tsRandomAccess": true
+    },
+    {
+      "name": "program-1-audio-es",
+      "namespace": "live.example.com/channel/1",
+      "packaging": "m2ts",
+      "m2tsModified": true,
+      "isLive": true,
+      "targetLatency": 1000,
+      "role": "audio",
+      "mimeType": "video/mp2t",
+      "bitrate": 128000,
+      "altGroup": 2,
+      "m2tsPacketSize": 188,
+      "m2tsPacketsPerObject": 32,
+      "m2tsProgramNumber": 1,
+      "m2tsPcrPid": 257,
+      "m2tsEsPid": 259,
+      "m2tsRandomAccess": true
+    },
+    {
+      "name": "program-1-eit",
+      "namespace": "live.example.com/channel/1",
+      "packaging": "m2ts",
+      "m2tsModified": true,
+      "isLive": true,
+      "role": "eit",
+      "mimeType": "video/mp2t",
+      "m2tsPacketSize": 188,
+      "m2tsPacketsPerObject": 16,
+      "m2tsProgramNumber": 1,
+      "m2tsEsPid": 18
+    }
+  ]
+}
+~~~
+
 # Subscriber Processing {#subscriber-processing}
 
 A subscriber obtains the catalog using the MSF catalog workflow and subscribes
@@ -800,6 +951,16 @@ time bound to a concrete MOQT Group location for use with a Joining FETCH
 {{MOQTransport}}.  A subscriber MUST NOT begin media presentation until it has
 received a valid PAT and PMT for the program to be decoded.
 
+When a subscriber receives ES-level tracks ({{es-level-carriage}}), it MUST
+align on the same starting Group number across all subscribed ES-level tracks
+for the same program before combining them.  The subscriber constructs the combined TS
+output by building a PAT listing the carried program and a PMT listing the PIDs
+of all subscribed ES-level tracks, then interleaving packets from all tracks.
+PCR is sourced from the track where `m2tsPcrPid` equals `m2tsEsPid`.  A
+subscriber MUST NOT begin media presentation until it has received at least
+one Group from each subscribed ES-level track and has obtained the originating
+program's PAT and PMT, either from `initDataList` or from the packet stream.
+
 # Switching and Alternate Renditions {#switching}
 
 Tracks with `m2tsMpts` set to true MUST NOT be included in an `altGroup`,