alkcall/docs/architecture/decisions/042-hub-relay-translate-not-forward.md

ADR-042: Hub Relay — Translate, Not Transparently Forward

Status

Accepted

Context

The hub is the architectural role (ADR-024, ADR-034) that bridges peers and browsers. With channels, the hub holds one channels connection per leg (browser↔hub, hub↔spoke) and relays channels between them. The phase-0 research (docs/research/alknet-channels/phase-0-findings.md §OQ-CH-11, §The hub relay) identified the key question: does the hub translate channel/open (terminate channel 0 on both legs, re-issue the open on the spoke leg) or transparently forward (pass the call operation through unchanged)?

This is the most under-specified part of the research for something that is the primary motivation for the channels crate (§Hub Motivation: the multi-transport collapse). The research said "Phase 1 must specify whether the hub translates or transparently forwards, and how the channel_id mapping is maintained."

The answer is derivable from the existing machinery:

• The hub terminates channel 0 on both legs (it runs its own CallAdapter per leg — ADR-036).
• The hub's CallAdapter receives the browser's channel/open as a call operation, runs AccessControl::check with the browser's identity, then forwards via from_call to the spoke (the hub as caller, the browser as forwarded_for — ADR-026 §3).
• The spoke allocates its channel_id and returns it; the hub maps browser-id ↔ spoke-id.

Transparent forwarding (passing the channel/open call operation through without the hub's CallAdapter terminating it) would bypass the hub's AccessControl::check and the forwarded_for auth chain — the hub would not authenticate the open, and the spoke would see the browser as the direct caller (not the hub), breaking the ADR-026/ADR-011 auth model. Translation is the only option that preserves the auth model.

Decision

The hub translates, not transparently forwards

The hub's relay has two layers:

1. Call-protocol layer (channel 0): translate. The hub terminates channel 0 on both legs. A channel/open from the browser is received by the hub's CallAdapter, which:

   1. Runs AccessControl::check on channel/open with the browser's identity (bearer token resolved per ADR-034). If denied → channel:forbidden to the browser.
   2. Issues a new channel/open on the spoke's channel 0 via from_call, with the hub as caller and the browser as forwarded_for (ADR-026 §3). The spoke's AccessControl::check sees the hub as the direct peer (authorized per ADR-011) and the browser as forwarded_for.
   3. The spoke allocates its channel_id and returns it.
   4. The hub opens a matching channel on the browser's side (the hub is now the responder for the browser leg, initiator for the spoke leg) and records the channel_id mapping: browser_id ↔ spoke_id.

2. Data-channel layer: byte-forward with channel_id rewrite. Once the mapping is established, the relay reads chunks for browser_id off the browser's channels connection, rewrites the channel_id field to spoke_id, and writes them onto the spoke's channels connection — and vice versa. The relay does not parse the payload; it does not know if the bytes are TTY chunks, SSH frames, or tunnel data. The channels layer on each end does the chunk↔stream conversion; the relay just moves bytes between two AsyncRead + AsyncWrite pairs with a 4-byte header rewrite.

channel_id mapping

The hub maintains a HashMap<channel_id, channel_id> per (browser, spoke) pair — the relay map. On channel/open (translated), the mapping is inserted. On channel/close (translated the same way), the mapping is removed. The relay task per channel reads the map to determine the rewrite target.

channel/control operations on channel 0 carry channel_id in their JSON payload (not in the chunk header). The hub's CallAdapter translates these too: the browser's channel/control for browser_id is re-issued on the spoke leg with spoke_id in the payload. The relay does not touch channel/control — it's a call operation, translated by the hub's CallAdapter, not byte-forwarded.

What the hub runs

Leg	What the hub runs
Browser leg	ChannelsAdapter (the relay's read/demux) + CallAdapter (channel 0, for the hub's own ops + translating the browser's ops)
Spoke leg	ChannelsAdapter + CallAdapter (same)
Relay	Per-channel byte-forward tasks with channel_id rewrite

The hub never runs a handler for alk/tty, alk/ssh, or alk/tunnel. It runs alk/channels (the relay) and alk/call (for its own hub-level operations + translation). The endpoints at each end do the protocol work.

What the hub still owns (unchanged from phase-0 §What the hub does still own)

• Routing: which spoke serves container:abc123? The hub's resource registry / ownership store (ADR-011), queried via call operations on channel 0. Channels doesn't touch this.
• ACL at the hub: does this browser's identity have channel:open scope for alk/ssh to spoke-X? AccessControl::check on channel/open, run by the hub's CallAdapter before it forwards. Channels doesn't touch this.
• Relay lifecycle: when a browser disconnects, the hub tears down the spoke-side channels (and vice versa). channel/close on each channel, or a transport-level close the channels layer observes (REQ-CH-02).

Scope note: this is a hub-crate concern, not a channels-crate concern

This ADR defines the relay contract (translate channel 0, byte-forward data channels with ID rewrite) so the channels crate's ChannelManager exposes the interface the relay needs (open_channel_stream(channel_id) -> BiStream for the byte-forward pumps). The relay implementation lives in alknet-hub (or a downstream hub like alkapi), not in alknet-channels. The channels crate is ALPN-blind and does not know it is being relayed. The channel_id rewrite is a 4-byte field rewrite within the 8-byte header (per ADR-035); the relay does not parse the payload.

Consequences

Positive:

• The auth model reuses cleanly: the hub's AccessControl::check + forwarded_for (ADR-026) is the existing machinery, not a new one. The spoke sees the hub as caller, the browser as forwarded_for — the kernel/user-land + forwarded-for model from ADR-011.
• The relay is one pump function per channel, not per (protocol × transport) cell. The hub's complexity is O(channels), not O(protocols × transports × spokes).
• The hub never runs protocol-specific handlers — it doesn't parse TTY chunks, SSH frames, or tunnel data. It moves bytes and translates call operations.
• channel/resources/subscribe (ADR-037) gives the hub a live view of each spoke's resources, which the hub aggregates and exposes to the browser.

Negative:

• The hub maintains a channel_id mapping per (browser, spoke) pair. This is per-channel state, not per-connection — a hub with many concurrent browser sessions each with multiple channels has a non-trivial map. The map is HashMap<u32, u32> per pair — cheap per entry, but the entry count is (browsers × channels-per-browser). Bounded by max_channels (ADR-040) per connection.
• The translate path adds one channel/open round-trip per relayed channel (browser→hub, hub→spoke). This is the same cost as any hub-relayed call operation and is not avoidable without transparent forwarding, which breaks the auth model.
• channel/control translation requires the hub's CallAdapter to rewrite channel_id in the JSON payload. This is a small but real translation step — the hub is not a pure byte relay for channel 0.

Door type

One-way. The translate-vs-forward decision is structural: transparent forwarding would bypass the hub's AccessControl::check and the forwarded_for chain, breaking the auth model. Reversing to transparent forwarding after deployments exist would require re-architecting the hub's auth path. The channel_id mapping strategy (HashMap per pair) is two-way — an implementation detail that can change without breaking the contract.

References

• ADR-024: peer-graph routing model (the hub's role)
• ADR-026: forwarded-for identity (the auth chain the translate path uses)
• ADR-034: outgoing-only X.509 and the three peer roles (browser identity resolution)
• ADR-011: dynamic resource ownership (the ownership store the hub queries)
• ADR-036: channel 0 is pre-negotiated alk/call (what the hub terminates on each leg)
• ADR-037: channel lifecycle operations (what the hub translates)
• ADR-039: ChannelsAdapter and ChannelManager (the interface the relay uses)
• ADR-035: channels pure channel multiplexing (the 8-byte header the relay reads/writes; the 4-byte channel_id rewrite; the BiStream-yielding open_channel_stream interface)
• docs/research/alknet-channels/phase-0-findings.md §Hub Motivation, §The hub relay, §OQ-CH-11
• docs/architecture/crates/hub/README.md — the hub crate (the relay implementation's home)