Teaches the relay pair to route direct-call signaling across an
existing federation link. Alice on Relay A can now place a direct
call to Bob on Relay B if A and B are federation peers — the
wire protocol, call registry, and signal dispatch all learn to
track and route the cross-relay flow.
Phase 3.5's dual-path QUIC race then carries the media directly
peer-to-peer using the advertised reflex addrs, with zero
changes needed on the client side.
## Wire protocol (wzp-proto)
New `SignalMessage::FederatedSignalForward { inner, origin_relay_fp }`
envelope variant, appended at end of enum — JSON serde is
name-tagged so pre-Phase-4 relays just log "unknown variant" and
drop it. 2 new roundtrip tests (any-inner nesting + single
DirectCallOffer case).
## Call registry (wzp-relay)
`DirectCall.peer_relay_fp: Option<String>` — federation TLS fp
of the peer relay that forwarded the offer/answer for this call.
`None` on local calls, `Some` on cross-relay. Used by the answer
path to route the reply back through the same federation link
instead of trying (and failing) to deliver via local signal_hub.
New `set_peer_relay_fp` setter + 1 new unit test.
## FederationManager (wzp-relay)
Three new methods:
- `local_tls_fp()` — exposes the relay's own federation TLS fp
so main.rs can build `origin_relay_fp` fields.
- `broadcast_signal(msg) -> usize` — fan out any signal message
(in practice `FederatedSignalForward`) to every active peer
link, returning the reach count. Used when Relay A doesn't
know which peer has the target fingerprint.
- `send_signal_to_peer(fp, msg)` — targeted send for the reply
path where the registry already knows which peer relay to
hit.
Plus a new `cross_relay_signal_tx: Mutex<Option<Sender<...>>>`
field that `set_cross_relay_tx()` wires at startup so the
federation `handle_signal` can push unwrapped inner messages
into the main signal dispatcher.
## Federation handle_signal (wzp-relay)
New match arm for `FederatedSignalForward`:
- Loop prevention: drops forwards whose `origin_relay_fp` equals
this relay's own fp (prevents A→B→A echo loops without needing
TTL yet).
- Otherwise pulls the inner message out and pushes it through
`cross_relay_signal_tx` so the main loop's dispatcher task
handles it as if it had arrived locally.
## Main signal loop (wzp-relay)
### DirectCallOffer when target not local
Before falling through to Hangup, try the federation path:
- Wrap the offer in `FederatedSignalForward` with
`origin_relay_fp = this relay's tls_fp`
- `fm.broadcast_signal(forward)` — returns peer count
- If any peers reached, stash the call in local registry with
`caller_reflexive_addr` set, `peer_relay_fp` still None
(broadcast — the answer-side will identify itself when it
replies)
- Send `CallRinging` to caller immediately for UX feedback
- Only if no federation or no peers → legacy Hangup path
### DirectCallAnswer when peer is remote
- Registry lookup now reads both `peer_fingerprint` and
`peer_relay_fp` in one acquisition
- If `peer_relay_fp.is_some()`:
* Reject → forward a `Hangup` over federation via
`send_signal_to_peer` instead of local signal_hub
* Accept → wrap the raw answer in `FederatedSignalForward`,
route to the specific origin peer, then emit the LOCAL
CallSetup to our callee with `peer_direct_addr =
caller_reflexive_addr` (caller is remote; this side only
has the callee)
- If `peer_relay_fp.is_none()` → existing Phase 3 same-relay
path with both CallSetups (caller + callee)
### Cross-relay signal dispatcher task
New long-running task reading `(inner, origin_relay_fp)` from
`cross_relay_rx`. In Phase 4 MVP handles:
- `DirectCallOffer` — if target is local, create the call in
the registry with `peer_relay_fp = origin_relay_fp`, stash
caller addr, deliver offer to local callee. If target isn't
local, drop (no multi-hop in Phase 4 MVP).
- `DirectCallAnswer` — look up local caller by call_id, stash
callee addr, forward raw answer to local caller via
signal_hub, emit local CallSetup with `peer_direct_addr =
callee_reflexive_addr` (peer is local now; this side only
has the caller).
- `CallRinging` — best-effort forward to local caller for UX.
- `Hangup` — logged for now; Phase 4.1 will target by call_id.
## Integration tests
`crates/wzp-relay/tests/cross_relay_direct_call.rs` — 3 tests
that reproduce the main.rs cross-relay dispatcher logic inline
and assert the invariants without spinning up real binaries:
1. `cross_relay_offer_forwards_and_stashes_peer_relay_fp` —
Relay A gets Alice's offer, broadcasts. Relay B's dispatcher
creates the call with `peer_relay_fp = relay_a_tls_fp`.
2. `cross_relay_answer_crosswires_peer_direct_addrs` — full
round trip; both CallSetups (one on each relay) carry the
OTHER party's reflex addr.
3. `cross_relay_loop_prevention_drops_self_sourced_forward` —
explicit loop-prevention check.
Full workspace test goes from 413 → 419 passing. Clippy clean
on touched files.
## Non-goals (deferred to Phase 4.1+)
- Relay-mediated media fallback across federation — if P2P
direct fails (symmetric NAT on either side), the call errors
out with "no media path". Making the existing federation
media pipeline carry ephemeral call-<id> rooms is the Phase
4.1 lift.
- Multi-hop federation (A → B → C). Phase 4 MVP supports a
direct federation link between A and B only.
- Fingerprint → peer-relay routing gossip.
PRD: .taskmaster/docs/prd_phase4_cross_relay_p2p.txt
Tasks: 70-78 all completed
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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