# US-029 — Outbound relay client: NAT/internet pipeline hops

Status: done
Priority: Critical
Stage: Implemented

## Context

Nodes behind NAT (WSL2 with 172.x.x.x addresses, 5G mobile, home routers) register with the
tracker and include a `relay_addr` (`wss://relay/rpc/{peer_id}`). When the head node needs to
forward activations to such a peer, it currently fails because the direct HTTP endpoint is
unreachable.

The relay server (US-017) is already running and the node already opens a persistent outbound
WebSocket (`RelayHttpBridge`). What is missing is the *outbound caller side*: given a `relay_addr`,
open a per-hop WebSocket to the relay's `/rpc/{peer_id}` endpoint and send the activation through it.

## Protocol

```
Node A  →  WS connect wss://relay/rpc/{peer_id_B}
       →  send JSON: {request_id, method, path, headers, body_base64}
Relay   →  forward as relay-http-request envelope to Node B's persistent WS
Node B  →  process /forward locally
       →  send relay-http-response envelope back
Relay   →  resolve future, send response JSON to Node A
Node A  ←  {request_id, status, headers, body_base64}
```

Binary activations (bfloat16) are base64-encoded. No precision loss.

## Acceptance criteria

- [ ] `_relay_hop(relay_addr, path, body, headers, timeout)` in `torch_server.py` — opens WS, sends, receives, returns `(status, headers_lower, body_bytes)`
- [ ] `_get_remaining_route` returns `list[dict]` with `relay_addr` field (was `list[tuple]`)
- [ ] `_run_downstream_pipeline` dispatches via `_relay_hop` when hop has `relay_addr`; falls back to direct HTTP if relay connection fails
- [ ] Tracker `_handle_proxy_chat` includes `relay_addr` in downstream hop dicts when node has one
- [ ] `relay_bridge._handle_request` decodes `body_base64`; response uses `body_base64` for `octet-stream` content
- [ ] All 157 tests pass (`python -m pytest`)
- [ ] QUICKSTART.md updated with relay NAT/internet architecture and test scenario

## WSL2 test scenario

Start two nodes in WSL2 pointing at the public tracker. Both get `172.x.x.x` endpoints (unreachable from outside). Both connect to relay automatically. Send an inference request through the tracker — activations flow via relay. No `--advertise-host` needed.