Limits on fine‑grained cancellation in Waku v2 Filter Protocol

Answer to the core questions

1. Add an explicit unsubscribe message? Yes. A dedicated unsubscribe that carries the filter_id lets a peer stop forwarding a single filter without tearing down the entire session. It reduces bandwidth waste and simplifies per‑filter lifecycle management.

2. Add a deadline or timeout to the subscription? No. A deadline only provides automatic expiration; it does not allow an on‑demand cancel. A timeout field would still require the client to drop the whole session or rely on local logic, so it does not solve the fine‑grained cancellation problem.

3. Implications of keeping only session‑level close? All active subscriptions are dropped when a session closes. In multi‑subscription scenarios this forces the client to re‑establish the session to restore the remaining filters, causing extra traffic, state re‑synchronization, and a higher chance of missed messages during reconnection.

Why an unsubscribe message is preferable

• Targets a single filter, preserving other active subscriptions.
• Allows the publisher to acknowledge receipt, giving a clean stop‑signal.
• Keeps the session alive, so peers can continue exchanging other messages (e.g., keep‑alive, other filter updates).
• Reduces unnecessary data flow and memory usage on both sides.

Why a deadline field is insufficient

• It only triggers after a fixed period; it cannot be invoked on demand.
• Clients still need to maintain the session for other filters.
• It does not provide a reliable way to stop a filter if the peer misbehaves or the network stalls.

Practical impact on multi‑subscription sessions

• Without unsubscribe, cancelling one filter forces a full session teardown and rebuild.
• Rebuilding may miss late messages for the remaining filters.
• Bandwidth and CPU overhead increase because the publisher must re‑apply all active filters after reconnection.
• Clients need to implement more state‑management logic to remember which filters were active across reconnections.

Recommendation

Extend the Waku v2 Filter Protocol with an unsubscribe message containing the filter_id. Keep the existing session‑level close for graceful shutdown, but provide the unsubscribe to enable fine‑grained cancellation. A deadline field can be added later for automatic expiration, but it should complement, not replace, the unsubscribe mechanism.