Subject: [AVTCORE] Discussion: RTP/SDP Signaling for Opus Multistream (multi‑Opus)
From: Sun Shin <[email-masked]>
To: "[email-masked]" <[email-masked]>
Date: Mon, 20 Oct 2025 17:57:56 +0000
List-Id: Audio/Video Transport Core Maintenance <avt.ietf.org>

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 <[email-masked]>
GitHub link: https://github.com/xingri/multi-opus