[AVTCORE] Re: Discussion: RTP/SDP Signaling for Opus Multistream (multi‑Opus)

[Marius Kleidl <[e-mail]>]

Hi Sun,

thank you for proposing this draft. I'll add you to the agenda for IETF 124, so you can present this to the working group.

Best regards,
Marius Kleidl

On Mon, Oct 20, 2025, at 19:57, Sun Shin wrote:
> Hi AVTCORE,
> I’d like to initiate a discussion and request feedback on standardizing RTP/SDP signaling for Opus multistream (“multi‑Opus”) to support multichannel layouts (e.g., 5.1, 7.1) in real‑time communications like WebRTC <https://webrtc-review.googlesource.com/c/src/+/129768>.
> *Problem*
>  • RFC 7587 <https://www.rfc-editor.org/rfc/rfc7587> defines mono/stereo SDP signaling and excludes multistream.
>  • RFC 6716 <https://www.rfc-editor.org/rfc/rfc6716> supports multistream (up to 255 channels) but lacks RTP/SDP signaling.
>  • RFC 7845 <https://www.rfc-editor.org/rfc/rfc7845> defines channel mapping families for Ogg but not for RTP/SDP.
>  • Non‑standard SDP usage (e.g., in libwebrtc <https://webrtc-review.googlesource.com/c/src/+/129768>, GStreamer <https://gstreamer.freedesktop.org/documentation/rtp/rtpopuspay.html?gi-language=c>) exists but lacks interoperability.
> *Proposal Scope* Define an RTP/SDP profile for Opus multistream that:
>  • Uses a distinct encoding name (`multiopus/48000/<channels>`).
>  • Specifies `fmtp` parameters: `num_streams`, ` coupled_streams`, ` channel_mapping`, and optionally ` mapping_family`.
>  • Reuses RFC 7845 <https://www.rfc-editor.org/rfc/rfc7845> mapping semantics.
>  • Defines Offer/Answer behavior and provides examples (e.g., 5.1, 7.1).
>  • Offers backward compatibility guidance with ` opus/48000/2`.
> *Relationship Summary*
>  • RFC 6716 <https://www.rfc-editor.org/rfc/rfc6716>: codec spec (multistream capability).
>  • RFC 7845 <https://www.rfc-editor.org/rfc/rfc7845>: Ogg encapsulation (mapping families).
>  • RFC 7587 <https://www.rfc-editor.org/rfc/rfc7587>: RTP payload (mono/stereo only).
>  • This draft: RTP/SDP signaling for multistream, reusing RFC 7845  <https://www.rfc-editor.org/rfc/rfc7845>semantics.
> *Use Cases*
>  • Cloud gaming, immersive media: negotiate multichannel audio without hacks.
>  • Interop: align existing “multiopus” deployments under a standard.
> *Draft* : https://datatracker.ietf.org/doc/draft-shin-avtcore-rtp-multi-opus/
> *Title:* RTP/SDP Signaling for Opus Multistream
> *Questions for the WG*
>  1. Should we use a distinct encoding name (“multiopus”) or extend “opus” with new ` fmtp`?
>  2. Should `mapping_family` be registered via IANA for well-known layouts?
>  3. How should this interact with existing Opus ` fmtp` (e.g., stereo/sprop-stereo) and downmix guidance?
> *Next Steps* If there’s interest, I’d like to present this in an upcoming AVTCORE session and work toward WG adoption.
> Best regards,
> Sun Shin
> NVIDIA <[e-mail]>
> GitHub link: https://github.com/xingri/multi-opus 
> 
> _______________________________________________
> Audio/Video Transport Core Maintenance
> To unsubscribe send an email to [e-mail]
>