SWIP-061: BPS multihop — FCFS multicast tree - #105
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Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Per SWIP-65 (PR #106): a delivery gap on a self-indexed cohort is detectable and recoverable from storage, so the contract is satisfied by deliver-or-recover. The second parent becomes a per-subscriber latency/cost choice - single-parented nodes were already well-formed, and a single-parented relay endangers only itself. Conformance items 5-6 bind only nodes that maintain a second parent. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Amendment pushed (1ca57ae), companion to SWIP-65 (#106):
🤖 Generated with Claude Code |
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Pull request overview
This PR introduces SWIP-61, extending the BPS (Broadcast Pub/Sub) family beyond the SWIP-60 single-hop audience ceiling by specifying a multihop, FCFS multicast tree join/control plane with subtree probing and dual-parent attachment for churn masking.
Changes:
- Adds the SWIP-61 specification document describing probe-based joining, two-parent resilience, and make-before-break reparenting.
- Adds the SWIP-61 wire specification (
bps.proto) definingReparent,Probe,Candidates, andAck.candidates.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.
| File | Description |
|---|---|
SWIPs/swip-61.md |
New SWIP-61 spec describing multihop join/probe behavior, dual-parent resilience, and reparenting semantics. |
SWIPs/assets/swip-61/bps.proto |
Proto definitions for SWIP-61 control frames and the Ack extension carrying attachment candidates. |
Suppressed comments (3)
SWIPs/swip-61.md:381
- Inconsistent acronym casing:
dCUTrhere should match theDCUtRspelling used earlier in the document.
dCUTr: [bee#5355](https://github.com/ethersphere/bee/issues/5355)
SWIPs/assets/swip-61/bps.proto:84
- The
Subscribecomment is broker-specific, but in multihop a joiner canSubscribeto any known cohort node (broker or relay). Keeping the comment broker-only is misleading for implementers.
// Joiner -> broker: names the topic — nothing more. Subscribers carry no
// cohort metadata; auth is present iff the joiner publishes (publishers
// connect directly to the broker).
SWIPs/assets/swip-61/bps.proto:96
- The
Ackcomment is broker-specific, but in multihop relays also answerSubscribewithAck. Adjust wording to cover any cohort node responding on the stream.
// Broker -> peer, answering Open or Subscribe. The echoed CohortSpec lets a
// subscriber verify every message end-to-end against the topic binding.
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| status: Draft | ||
| type: Standards Track (Networking) | ||
| created: 2026-08-04 | ||
| requires: 60 |
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| Reaching a NAT'd relay is the transport's business, not this protocol's: libp2p circuit | ||
| relay + DCUtR hole punching (the dcutr work item, bee | ||
| [#5355](https://github.com/ethersphere/bee/issues/5355)). BPS carries no signalling for |
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| // Stream establishment, stream name "pubsub/1.0.0" — one stream per (peer, topic). | ||
| // The first message on a fresh stream is Open (fixes a new cohort) or | ||
| // Subscribe (joins an existing one); the broker answers with Ack. | ||
| // --------------------------------------------------------------------------- |
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| // Spec: swip-bps-singlehop.md (SWIP-60, base protocol) and | ||
| // swip-bps-multihop.md (SWIP-61, multihop control plane). |
| Two cases: | ||
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| - **graceful leave** — a departing relay `Reparent`s each child toward a replacement | ||
| (its own parents, or spread over its children **(?)**) before closing; the child's |
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Extends SWIP-60 (#104): removes the singlehop audience ceiling. A full node at capacity answers
Subscribeby probing its own subtree over existing streams and returning the two shallowest attachment points — the joiner attaches to both, so every node keeps two parents (typically sisters): churn is masked, not suffered, self-healing is re-running the join, and withholding shows by comparing feeds. Join is ideally a two-step process regardless of tree size.Key properties: forwarders are subscribers (accepting your first child = becoming a relay); no standing control state — idle cohorts are silent, probes carry depth in the frame itself; no depth bound (per-hop pricing steers joins rootward; closed cohorts are structurally singlehop);
CohortSpecuntouched; three control frames (Reparent,Probe,Candidates) fill the reservedBroadcastrange; single newAckfield. Unmodified SWIP-60 clients attach as conformant (single-parented) leaves.Deliberately out of scope (own SWIPs): bandwidth incentives, message history (pre-subscription delivery), broker discovery (SWIP-59, #103), NAT traversal (transport: circuit relay + DCUtR).
Wire:
assets/swip-61/bps.proto— the SWIP-60 proto with the multihop control plane filled in. Implementation groundwork: bee ethersphere/bee#5435.🤖 Generated with Claude Code