Ad Creative Signaling over the MSF Event Timeline

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Table of Contents

1. Introduction

SCTE-35 messages [SCTE35] carried on an MSF Event Timeline track [SCTE35-MOQ] give Media over QUIC deployments the function SCTE-35 was built for: signaling placement opportunities -- break windows and splice conditioning -- on a shared media timeline, using the Event Timeline mechanism of the MOQT Streaming Format (Section 8.1 of [MSF]). What that layer deliberately does not carry is a description of what plays inside those windows.¶

On HLS and DASH, that gap is filled at the manifest: a packager or stitcher decorates each session's playlist with creative identity and tracking metadata. On a one-to-many publish/subscribe transport [MOQT] there is no per-session manifest to decorate -- every subscriber receives the same cached objects -- so anything a client needs at render time (which creative is playing, which tracking resources to request at which quartiles, which verification script to load) must be delivered in band, on the shared timeline, ahead of the splice point.¶

This need is not incidental. Accurate measurement of server-side ad insertion increasingly means client-requested tracking -- per-device accounting at render time rather than server-side firing at publish time, which measures the wrong clock -- and verification means initializing a measurement script [OMSDK] before the slot renders. SVTA 2053-1 [SVTA2053] already standardizes exactly this payload: a Pod/Slot/TrackingEvent/Verification data model (Section 4 of [SVTA2053]), explicitly agnostic to client-side, server-side, and server-guided insertion, with carriage bindings defined per stream format -- DASH EventStream, HLS EXT-X-DATERANGE, HLS Interstitials -- and none for MOQT.¶

This document fills that gap using the structures [MSF] and [SCTE35-MOQ] already put in place: it defines an Event Timeline event class whose records carry SVTA 2053-1 Version 2 carriage envelopes, specifies how those records are timed and correlated with splice signaling, and gives operational guidance for tracking on one-to-many delivery. It changes nothing in the SVTA 2053-1 data model; payload normativity remains with [SVTA2053], and this binding could additionally be published as a carriage section in a future revision of that specification.¶

On status: [MSF] is an adopted work item of the MoQ Working Group, and its Event Timeline mechanism is the substrate here; this document and [SCTE35-MOQ] are individual submissions layered on that substrate, and this one is written to track [MSF] as it evolves under WG change control -- including following its registry, wherever that registry ultimately lands.¶

2. 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.¶

This document uses the terms Track, Group, Object, Event Timeline, and the index reference fields t, l, and m as defined in [MSF], and the terms Pod, Slot, Tracking Event, Verification, and carriage envelope as defined in Sections 4 and 5 of [SVTA2053].¶

Additionally:¶

Distributor:

The party operating the publisher that emits the media and signaling tracks (typically the party performing ad insertion).¶

Placement opportunity:

A window on the media timeline, typically signaled by SCTE-35, in which advertising content plays.¶

Splice signaling track:

An Event Timeline track carrying SCTE-35 records per [SCTE35-MOQ].¶

Capability URL:

A URL whose knowledge alone grants the ability to perform a narrow action, used here as a short-lived, opaque reference to a distributor-operated tracking endpoint [CAP-URLS].¶

3. Track Properties

An MSF track carrying ad creative signaling:¶

• MUST declare a packaging value of "eventtimeline";¶

• MUST declare an eventType value of "urn:svta:advertising-wg:ad-creative-signaling". This is the data scheme URI defined in Section 5 of [SVTA2053], used unversioned: as in the DASH binding, the payload version is carried inside the payload itself (Section 4.3);¶

• MUST declare a depends attribute naming the media track or tracks whose timeline its records index. When the track accompanies a splice signaling track, it SHOULD name the same media track(s) as that track.¶

Because eventType is uniform for a track, and because implementations do not mix payload flavors within one Event Timeline track (see Section 4.3 of [SCTE35-MOQ] for the same rule applied to SCTE-35), ad creative signaling is carried on a sibling Event Timeline track alongside any splice signaling track, not interleaved within it.¶

A splice signaling track is not required. Ad creative signaling describes what plays inside a placement opportunity; it does not depend on how the boundaries of that opportunity were signaled. A deployment MAY carry ad creative signaling with no SCTE-35 records present, leaving break detection to another mechanism entirely -- in-band timed metadata such as ID3, an emsg box, or the timing carried by the creative records themselves. Where a splice signaling track is present, the alignment rules in Section 5.2 let the two be correlated; where it is absent, those rules simply do not apply.¶

4. Record Format

{ "version": 2, "type": "...", "payload": [ ... ],
  "features": { ... } }

5. Timing and Correlation

6. Worked Example

An SCTE-35 record on the splice signaling track (reproduced from Section 5.1 of [SCTE35-MOQ]) announces a splice at media time 480500 ms:¶

{
  "m": 480500,
  "data": {
    "scte35_payload":
      "/DAhAAAAAAAAAP/wFAUAAArXf+/+AAAAAH4AARSyAAAAAA=="
  }
}

The companion record on the ad creative signaling track, published at least one Group earlier (Section 5.3), carries the same index. It describes a 45.045-second pod with two slots: explicit quartile tracking and a verification resource on slot 1 (no offset on the quartile events, so timing is driven by event-type semantics per Section 4.4.5 of [SVTA2053]), and lazily resolved tracking on slot 2 via $remote (Section 7):¶

{
  "m": 480500,
  "data": {
    "version": 2,
    "type": "pod",
    "payload": [{
      "duration": 45.045,
      "slots": [
        {
          "type": "linear",
          "start": 0.0,
          "duration": 30.03,
          "identifiers": [{
            "scheme":
      "urn:smpte:ul:060E2B34.01040101.01200900.00000000",
            "value": "ABCD1234567H"
          }],
          "tracking": [
            { "type": "impression", "urls": [
              "https://beacons.example.net/c/7f3a92/imp"
            ] },
            { "type": "firstQuartile", "urls": [
              "https://beacons.example.net/c/7f3a92/q1"
            ] },
            { "type": "midpoint", "urls": [
              "https://beacons.example.net/c/7f3a92/q2"
            ] },
            { "type": "thirdQuartile", "urls": [
              "https://beacons.example.net/c/7f3a92/q3"
            ] },
            { "type": "complete", "urls": [
              "https://beacons.example.net/c/7f3a92/done"
            ] }
          ],
          "verifications": [{
            "vendor": "vendor.example-omid",
            "parameters": "key=value",
            "resource":
              "https://cdn.vendor.example/omid-verify.js"
          }]
        },
        {
          "type": "linear",
          "start": 30.03,
          "duration": 15.015,
          "identifiers": [{
            "scheme": "urn:com:example:ads:id",
            "value":
              "972c79e1-2363-403e-9287-a0fa4323c389"
          }],
          "$remote": {
            "tracking":
              "https://beacons.example.net/c/9d41c7/trk"
          }
        }
      ],
      "tracking": [
        { "type": "podStart", "urls": [
          "https://beacons.example.net/c/p8842/start"
        ] },
        { "type": "podEnd", "urls": [
          "https://beacons.example.net/c/p8842/end"
        ] }
      ]
    }],
    "features": { "remoteFields": true }
  }
}

7. Tracking Indirection and Capability URLs

Section 5.2.2.2 of [SVTA2053] already says interpreters SHOULD defer $remote resolution until the containing object is about to become active. That deferral maps naturally onto broadcast fan-out, with one addition: on a shared timeline, the URLs in tracking and $remote fields SHOULD be short-lived capability URLs [CAP-URLS] referencing a distributor-operated tracking endpoint (a "beacon proxy"), rather than raw measurement-vendor URLs.¶

At activation, the client requests the capability URL (or resolves $remote from it), appending a client identifier (Section 7.1). The endpoint performs macro substitution per Section 6 of [VAST] (as Section 5.2.3 of [SVTA2053] provides) and requests the corresponding vendor resources with the calling device's context. Two properties fall out for free on one-to-many delivery:¶

• Payload size: shared-timeline records stay small even for heavy vendor tracking lists, with late binding after the pod record ships.¶

• Privacy and cache-safety: no per-user macro values and no vendor endpoint inventory appear in bytes that every subscriber and relay cache can read.¶

8. Per-Session Insertion Plans

Everything above is deliberately identical bytes for every subscriber -- that is what makes it relay-cacheable. Server-guided insertion, per-session capability URLs, and entitlement-scoped fields are per-session by definition and cannot ride the shared timeline. The authors believe the standardization target for that layer is a separate per-session insertion plan mechanism: a unicast-scoped metadata track (or equivalent side channel) carrying distributor session metadata -- the session's pod assignment for each shared-timeline placement opportunity, its capability URLs, and policy fields -- which composes with the shared events defined here (the shared timeline announces placement geometry and the broadcast/default pod; the insertion plan overlays the per-session decision).¶

That mechanism is out of scope for this document and deserves its own. It is named here only as a design constraint: the event class defined in this document is intended to be overridden or parameterized by such a layer without being redefined by it.¶

9. Open Issues

This section lists issues to be resolved through working group discussion; it is to be removed before publication.¶

1. Granularity: one pod envelope per placement opportunity (the RECOMMENDED default), or slot/trackingEvent envelopes per creative? Redelivery deduplication is simpler at pod granularity.¶

2. Update semantics for live: pods change close to air. Is a later record at the same index a full supersede (last-writer-wins), or is a patch rule needed? [SVTA2053] has no cancel/supersede concept; SCTE-35 inherits cancellation from its payload semantics.¶

3. Deduplication and late joiners: the DASH/HLS bindings give each event a manifest-unique id for dedup on re-encounter; MSF records carry no id. Is (eventType, index, payload) a sufficient dedup key under relay redelivery and FETCH catch-up -- and which past records MUST a mid-break joiner fetch?¶

4. Switching sets: when ladder rungs are separate tracks, does one creative signaling track depends on all of them? Splice conditioning can land at slightly different media times per rung; container-relative interior offsets survive that, but a single m may not. Sender-side track switching under discussion in the MOQ Working Group sharpens this: if relays select the delivered rung per subscriber and switch at Group boundaries, (a) these records must remain valid for every rung of a switching set, (b) splice conditioning must keep switching sets coherent at placement boundaries, and (c) the delivered rendition is known only at the client, so rendition-level measurement is necessarily client-observed.¶

5. Common-format scope: is the Event Timeline binding the whole story, or is a container-level mapping (the envelope in CMAF emsg) also wanted so one payload survives packager transit across HLS/DASH/MOQT boundaries?¶

6. Feature negotiation on one-to-many: [SVTA2053] interpreters SHOULD error on unsupported flagged features -- sensible unicast behavior, but a broadcast publisher cannot negotiate per subscriber. Should the catalog advertise which features a track uses so clients subscribe eyes-open?¶

7. Record integrity: do these records need payload signing (JWS or COSE over the envelope, or a catalog-pinned key) beyond transport authentication? See Section 11.¶

10. Implementation Status

(This section follows [RFC7942] and is to be removed before publication as an RFC.)¶

The author operates a continuously running server-side ad insertion deployment over MoQ. This section states what that deployment does and does not implement, because the two are easy to conflate: the mechanism this document specifies is NOT yet what the deployment publishes.¶

Running today:¶

• Fixed-timeline linear channels, publisher to relay to browser, with ad creatives spliced server-side into one shared broadcast -- so every subscriber receives the same media objects, which is the condition that motivates this document (Section 1).¶

• Placement opportunities detected from inbound in-band SCTE-35 (emsg), parsed to a break window and duration. The deployment consumes SCTE-35; it does not publish SCTE-35 records, and it does not publish a splice signaling track.¶

• Creative identity and per-creative tracking published in band, ahead of the splice, on a control track the subscriber reads before the first event renders. That validates the delivery model in Section 5.3. The track's schema is the deployment's own and is NOT the event class defined here.¶

• Tracking indirection exactly as described in Section 7: a proxy holds vendor beacon URLs and returns compact per-event capability URLs, which devices fire with their own client identifier, with per-{uuid, event, client} deduplication so a shared broadcast yields per-device counts. This is the part of the document with the most operational experience behind it.¶

• An MSF catalog, published automatically by the MoQ implementation in use and verified against an interop profile by a tool in the same codebase. A capture from the live deployment validates clean, so the transport and catalog layers this document builds on are exercised, not assumed.¶

NOT yet implemented:¶

• The event class this document defines. No track advertises eventType urn:svta:advertising-wg:ad-creative-signaling, and no record carries an SVTA 2053-1 Version 2 carriage envelope (Section 4.3). Migrating the control track to the envelope form is in progress.¶

• The IANA registration in Section 12, which this document requests.¶

• Correlation with a co-published splice signaling track (Section 5.2), since no such track is published.¶

Consequently the timing and lead-time rules in Section 5 and Section 5.3 derive from running code, while the record format in Section 4 and the payload constraint in Section 4.3 are specified but unexercised. The JSON Schema and test vectors in Appendix "JSON Schema and Test Vectors" are conformance examples written against this specification, not captured output; the rules they express are executed in the implementation's own test suite, so they are internally consistent, but they have not yet been produced by a publisher.¶

11. Security Considerations

12. IANA Considerations

This document requests registration of the following entry in the "MSF Event Timeline Types" registry established by Section 14.2 of [MSF]. [SCTE35-MOQ] registers entries in that same registry; this document follows the same pattern.¶

Editor's note (to be resolved before publication): the URN is defined and owned by the SVTA Advertising Working Group; this registration is made in coordination with that group.¶

13. References

Acknowledgments

This document deliberately reuses the carriage pattern established by Will Law and Suhas Nandakumar in [SCTE35-MOQ], and is designed to compose with it. The payload data model is the work of the SVTA Advertising Working Group [SVTA2053].¶

JSON Schema and Test Vectors

A JSON Schema for the record shape and a set of test vectors are published alongside this document at https://riedlse.github.io/draft-riedl-moq-ad-creative-signaling/.¶

The schema encodes exactly the two requirements of Section 4.1 — one index reference field, and a data member holding one Version 2 envelope — and nothing further; the envelope's own contents are governed by [SVTA2053], not by this document.¶

The vectors come in two sets, and both matter: six that MUST validate, and six that MUST NOT. A validator that accepts everything passes the first set alone. The rejecting cases pin the constraints most likely to be implemented loosely:¶

A minimal conforming record, one pod envelope describing a single 30 s creative with its quartile chain (timings taken from a running deployment, so the values are the real ones a 30 s transcode produces -- note 30030, not 30000):¶

{
  "m": 60000,
  "data": {
    "version": 2,
    "type": "pod",
    "payload": [
      {
        "id": "brk-947",
        "startTime": 60000,
        "duration": 30030,
        "slots": [
          {
            "id": "ad-32600921",
            "index": 0,
            "startTime": 60000,
            "duration": 30030,
            "trackingEvents": [
              { "event": "start",         "startTime": 0,
                "urls": ["https://aip.example/v2/impression/9f2c1e84-.../start"] },
              { "event": "firstQuartile", "startTime": 7507,
                "urls": ["https://aip.example/v2/impression/9f2c1e84-.../firstQuartile"] },
              { "event": "midpoint",      "startTime": 15015,
                "urls": ["https://aip.example/v2/impression/9f2c1e84-.../midpoint"] },
              { "event": "thirdQuartile", "startTime": 22522,
                "urls": ["https://aip.example/v2/impression/9f2c1e84-.../thirdQuartile"] },
              { "event": "complete",      "startTime": 30030,
                "urls": ["https://aip.example/v2/impression/9f2c1e84-.../complete"] }
            ]
          }
        ]
      }
    ],
    "features": { "capabilityUrls": true, "perDeviceDedup": true }
  }
}

The urls values are capability URLs (Section 7), abbreviated here.¶

Two notes on provenance, so implementers know what these are and are not.¶

The vectors are conformance examples authored against this specification, not captured output. The reference implementation publishes its break timeline on a non-standard control track today; migrating it to the envelope form described here is in progress, and vectors captured from it will replace these once that lands. Their timings, identifiers and quartile offsets are taken from a live deployment, so they are realistic, but the record framing is this document's.¶

The rules the schema expresses are executable and run in that implementation's CI, so the published vectors cannot silently drift from the prose: the valid set is asserted to pass and the invalid set to fail, each for the specific reason its name claims.¶

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