Piik/docs/research/advanced-peer-distribution.md

Advanced Peer Distribution

• Last reviewed: 2026-08-27
• Scope: advanced low-server distribution candidates for sub-second interactive media
• Status: research comparison and falsifiable gates, not an accepted product route

Terms And Conservation

Let B be one complete stream bitrate, N the Viewer count, and r repair overhead. Headers, audio and retransmission are additional. Every Viewer still needs roughly B useful last-hop traffic, so a peer overlay redistributes about N*B; it does not eliminate it.

• Full-stream relay: forwards a complete representation to each child.
• Multiple trees: split a representation into stripes carried by different interior-node-disjoint trees; every receiver reassembles the stripes.
• SVC: one encoder creates dependent temporal or spatial layers. It permits selective forwarding but does not create a topology or transport.
• Network coding/FEC: adds repair symbols so some loss can be recovered without retransmission; it neither compresses video nor repairs a failed path.

Candidate Comparison

Candidate	Host upload	Relay encode	Potential benefit	Decisive no-go
Browser full-stream tree	up to C*B	one per child pipeline	deployable with standard WebRTC	relay CPU, generational loss or hop latency exceeds the small-room value
Native RTP relay	up to C*B	zero	removes relay re-encode	feedback, pacing or per-child congestion cannot remain isolated and bounded
Two encoded stripe trees	about (1+r)B	zero	lowers Host upload while retaining peer egress	endpoint upload remains near 2B, two decoders/compositor are required, or churn breaks assembly
Browser encoded objects	about C*B, later stripeable	zero	pure-Web single encode and forwarding	application framing, jitter, A/V sync or queues recreate an unsafe media stack
FEC/network coding	unchanged plus repair	unchanged	faster repair for bounded loss	clean-path bytes/CPU exceed benefit or whole-path loss still dominates
Central SFU/MoQ	about B	central forwarding	mature low-latency availability	central egress remains N*B without a measured compensating gain

C is an endpoint child bound, not a promise that the endpoint can encode or upload C good streams. Native shared encode can reduce compute, while useful network copies remain subject to conservation.

Candidate Gates

Native RTP Relay

A native relay may read encoded RTP from one receiver and write it to independent downstream PeerConnections without decoding. Each child still needs its own SSRC, pacing, RTCP feedback, DTLS-SRTP and ICE state.

Retain only if a 720p30, one-parent/two-child gate proves:

• zero relay video-encoder calls and at least 40% lower relay CPU than browser remote-track forwarding;
• no more than 25 ms added p95 relay latency and queue age below 100 ms;
• per-child bytes no more than 110% of input payload bitrate;
• isolated PLI, NACK/RTX and congestion behavior under clean and 1% loss; and
• a decodable first picture within one second after failure detection.

Reject if downstream feedback cannot be isolated, queue age is unbounded, or ordinary browser leaves lose interoperability. This candidate lowers relay compute, not total room upload.

Two Stripe Trees

The Host sends complementary stripes through two deterministic, interior-node-disjoint trees. An internal node forwards only its stripe; a Viewer needs both parents and a bounded assembler. SVC alone cannot supply this shape: its layers remain in one encoded RTP stream, and Encoded Transform does not move encoded frames between transports.

Start with 50/50 single-layer frame/object striping. Retain only if:

• Host and relay upload stay at or below 1.1B without repair;
• Host encodes once, relays encode zero times, and room useful bytes stay below 1.1*N*B;
• per-hop overhead is at most 20 ms p95 and depth-four glass-to-glass p95 is at most 350 ms;
• path failure restores a decodable picture within one second; and
• repair still saves at least 30% Host upload relative to duplicating 2B.

Reject if supported clients require unrelated decoders plus application-side picture composition, or if bounded churn cannot preserve stripe identity.

Browser Encoded Objects

WebCodecs exposes EncodedVideoChunk; DataChannel can carry framed objects over peer ICE. This replaces the browser RTP receiver with application pacing, framing, jitter buffering, keyframe requests, rendering and eventually A/V sync.

The first gate is only Host -> relay -> leaf, 720p30, chunks no larger than 16 KiB and a hard bufferedAmount ceiling. Retain only if:

• the Host encodes once and the relay does not encode;
• Host encode CPU falls at least 30% versus two browser send pipelines and relay CPU falls at least 40% versus remote-track forwarding;
• framing bytes are no more than 110% of the RTP baseline;
• two-hop glass-to-glass p95 is at most 250 ms, queues remain bounded, and first picture after reroute is below one second; and
• required mobile decoders pass and later A/V drift stays within 50 ms.

Reject rather than growing an application RTP replacement when those gates fail.

FEC And Network Coding

First replay captured RTP/object traces through a simulator: random loss at 0/1/3/5%, 20/50/100 ms bursts, and 20/40/80 ms RTT. Compare NACK/RTX with small XOR or Reed-Solomon blocks; consider sliding-window RLC only if simpler repair passes.

Retain only if 1-3% loss or 20-50 ms bursts show at least 50% lower residual loss/freeze, repair p95 below 100 ms and at least 25% faster than RTX, total media plus repair and residual RTX no more than 110% of baseline, generation buffering at most 16 ms, active repair overhead at most 15%, and coding CPU below about 5% of one reference core. Clean paths should keep repair absent or minimal.

MoQ

Browser WebTransport connects to a server; it does not turn a normal browser into an Internet-listening peer relay. A central MoQ service therefore retains the same egress class as an SFU. Keep it only if equal-quality measurements improve latency or server CPU by at least 20% at the same egress, and draft-version churn is isolated behind a small adapter.

Local Reconciliation Evidence

Narada, Overcast and NICE show that overlay measurements can improve tree shape; SplitStream adds multi-tree striping. BitTorrent, WebTorrent, CoolStreaming and P2P Media Loader can contact several suppliers because buffered chunks are interchangeable. None is a maintained drop-in controller for a live MediaStream with sub-second interaction.

ICE already races and paces address, interface and NAT candidate pairs inside one exact RTCPeerConnection. Trying another parent requires another offer/answer, ICE/DTLS/RTP state, reservation and potentially another encoder. Application-level all-pairs probing therefore multiplies real media work rather than extending ICE Happy Eyeballs.

For a small room, one to three child observations do not provide the independent sample volume needed for statistical host ejection or a stable weighted score. Current-edge stats can diagnose that edge; they cannot prove an unconnected parent is better.

The smallest general reconciliation model supported by this evidence is:

1. one authoritative graph and one event-driven desired/current loop;
2. hard eligibility from authenticated presence, bounded capacity, acyclicity and source reachability;
3. at most one child transition at a time, fenced to exact sessions, source and connection identity;
4. make-before-break when the old path remains usable, with the first newly decoded frame as commit proof; and
5. release on failure or deadline, while transport libraries retain ICE, consent, congestion control and transient reconnect ownership.

Reparenting a relay can retain its subtree, and several bad child edges can move independently. This changes traffic placement, not the N*B aggregate. Library retry values are implementation references, not universal application timers; any timeout must be measured against the target network and user-visible bound.

Sources And License Boundary

• WebRTC, WebRTC Stats, WebRTC SVC, Encoded Transform, WebCodecs, and WebTransport.
• ICE, RFC 8445, ICE Happy Eyeballs, RFC 8421, Trickle ICE, RFC 8838, Data Channels, RFC 8831, and Sliding-window RLC, RFC 8681.
• SplitStream, Narada, Overcast, NICE, and CoolStreaming/DONet.
• BitTorrent BEP 3, WebTorrent, and P2P Media Loader.
• Pion WebRTC and klauspost/reedsolomon are MIT; LiveKit is Apache-2.0.
• Jitsi P2P/JVB ownership is implementation evidence, not copied code.
• MOQT and moq-dev/moq are study references; the core uses MIT/Apache-2.0 while its OBS plugin is GPL-2.0-or-later.

No GPL/AGPL implementation code is copied or required by these candidates.