alkgit/docs/architecture/transport.md

---
status: draft
last_updated: 2026-09-25
---

# alkgit: Git Smart Protocol (wire layer)

## What it is

The wire half of the `alkgit` protocol crate: pkt-line session substrates,
protocol V2 state machines (advertisement, ls-refs, fetch, receive-pack),
and the substrate types that keep doors and handlers from touching
`futures_io` or raw packetline APIs. Depends on alkcall and — only under
the `gix` feature — the backend implementations (ADR-010). The wire layer
itself is backend-trait-only, which is what makes `default-features =
false` compile without gix.

## Substrate layer (ADR-005)

Two session entry points over one state-machine core. ADR-005 owns the
full decision (what each substrate owns and why); the surface is:

- **Duplex session** (the `alk/git` ALPN producer, channels-opened git
  sessions, embedder stream doors) — input: (peer identity, resolved repo
  id, duplex stream, `Limits`). Encapsulates the split/compat/packetline
  bridge, the request reader (delim-aware parsing, `reset()` discipline,
  break-on-error), and the sideband writer.
- **Stateless session** (smart-http doors, e.g. alkhttp's future `git`
  feature) — input: (peer identity, resolved repo id, request-reader,
  response-writer, `Limits`) per http POST; adds the http-framing rules
  (capability-dump skip, flush-only responses, probe handling). IO-abstract:
  the door supplies reader/writer; see [doors.md](doors.md) for the
  mounting.

Both substrates feed the same V2 state machines; statelessness is a
substrate property (per-request state), not a protocol fork.

## Protocol core (ADR-003: V2-first)

### Advertisement

- Emitted once per duplex session (never between commands — real git
  hangs on re-advertisement); per-request-set on http (stateless: the
  client re-sends the dump).
- Honest capability list: exactly what we serve (`ls-refs=unborn`,
  `fetch=wait-for-done` — the ack loop needs no capability change, the
  acknowledgments section is grammar not capability (ADR-014);
  `object-format=sha1`). Unimplemented features are declined by omission
  (validated against real git, POC-1). `git-upload-archive` is not
  served (fixed refusal — ADR-008's never-execute rule, ssh analog in
  doors.md).

### ls-refs

- Parse `command=ls-refs` (peel, symrefs, ref-prefix), stream ref lines
  from the backend's listing, flush. `ref-prefix` filtering is
  client-driven.

### fetch

- Parse wants/haves/done/args; object-format check (reject mismatches —
  the object-format line is validated against the advertisement's
  `object-format` on every command; ADR-013 pins the push-side check to
  the same rule).
- Negotiation policy: the full ack loop (ADR-014) — no-`done` rounds
  get an `acknowledgments` section (`ACK <oid>` per recognized have via
  the backend's `common_haves`, `NAK` when none, flush; never `ready`),
  the `done` round generates closure(wants) − closure(haves) via
  `GitPackGen`. No cross-round state on either substrate (the client
  re-sends wants + commons each round — negotiation-captures.md).
  Advertisement text is unchanged: `fetch=wait-for-done`.
- Pack generation via `GitPackGen` (ADR-004), streamed over sideband on
  duplex / sideband-in-response on http; generation runs on
  `spawn_blocking` with the owned handle moved in (POC-2's shape: store
  shared, handle per session, generation on blocking threads).
- Round/haves budgets enforced here (ADR-009); an empty resulting pack
  (client already has everything) is a valid zero-object packfile.

### receive-pack (push)

V0-framed by upstream design (no version negotiation on the push path —
ADR-013); shapes are capture-grounded (`push-captures.md`), not
grammar-inferred.

- **Advertisement**: V0-shaped ref advertisement — caps NUL-attached on
  the first ref line, `capabilities^{}` sentinel only for empty repos;
  served set `report-status report-status-v2 delete-refs side-band-64k
  atomic ofs-delta object-format=sha1` (+ `push-options` under config
  gate); ACL before the first ref line (ADR-007).
- **Request**: command lines (`<old> <new> <ref>`), shallow lines
  rejected up front for v1 (symmetric with fetch's decline, ADR-013 §4),
  flush, optional push-options section, then the pack stream — which is
  always expected (missing pack errors at unpack). An immediate flush is
  a client-side nothing-to-do (reply flush, no report).
- **Ingestion** via `GitPackIngest` (ADR-004): thin packs accepted with
  bases from the server odb (default client behavior, no capability);
  `Bundle::write_to_directory_eagerly` with the repo's pack dir, `.keep`
  guard; `gix-fsck` connectivity per new tip; missing objects →
  `unpack ng`.
- **CAS timing**: unpack-first, then per-ref checks (name via
  `gix_validate::reference::name` + reserved deny-list, CAS via `gix-ref`
  transactions, policy), atomic rollback, report — the observed upstream
  order (ADR-013 §7). One transaction per push.
- **Status report**: band-1 pkt-line-framed (`unpack ok|ng`, per-ref
  `ok|ng <ref> <reason>`, inner flush, outer flush) when sideband was
  selected; bare pkt-lines otherwise. Substrate owns the framing
  (unwrapped reports abort real clients).
- **Http framing**: Content-Length or chunked request (probe POST
  answered 200-empty above the client's postBuffer), Content-Type
  `application/x-git-receive-pack-request` / `...-result`, response ends
  at flush (ADR-005 unchanged).
- Push-options parsing and `atomic` rollback semantics per ADR-013
  §10–11.

### Error taxonomy

- io errors are terminal (session ends); protocol errors map to pkt-line
  error bands (duplex) or http status + body (stateless). Substrate-level
  `thiserror` enum; no panics in library code (convention 2).

## Limits

Every session carries `Limits` (ADR-009): negotiation rounds, haves per
round, receive-pack max size, wall clock, sideband chunk size (fixed
65000), advertisement ref cap. Missing `Limits` is a type-level error.

## Public API surface (v1)

Crate-root re-exports (alktty pattern; the full list in
[backend.md](backend.md) §public API): `GitAdapter` + `register_openable`,
`GitSession`, substrate types, `Limits`, the backend traits (ADR-010's
seam), protocol error enums; gix-feature types under the feature.
Publish-freeze point: **OQ-03**.

## Design Decisions

| ADR | Decision | Summary |
|---|---|---|
| [002](decisions/002-front-door-blind-core.md) | Session boundary | duplex + stateless entry points |
| [003](decisions/003-protocol-v2-first.md) | V2-first | honest advertisement; V0/V1 declined on fetch, push V0-framed (ADR-013) |
| [004](decisions/004-pack-pipeline.md) | Pack pipeline | generation on blocking threads, O(counts) |
| [005](decisions/005-session-substrate-types.md) | Substrate types | request reader, sideband sink, http framing rules |
| [009](decisions/009-bounded-resources-budget.md) | Budgets | `Limits` in every session tuple |
| [010](decisions/010-pure-protocol-crate.md) | Pure protocol crate | wire layer is backend-trait-only |
| [013](decisions/013-receive-pack-state-machine.md) | receive-pack | V0-framed push machine, thin-pack acceptance, unpack-first CAS |
| [014](decisions/014-v2-negotiation-ack-loop.md) | Negotiation | ack loop, no `ready`, wait-for-done stays |

## Open Questions

- **OQ-03**: publish/API freeze (partially resolved — single-crate shape
  settled by ADR-010).
- **OQ-05**: sha256 policy (deferred(scope)).
- OQ-02 resolved (ADR-014 — ack loop design).
- OQ-04 resolved (ADR-013 — receive-pack state machine).

## References

- `docs/research/poc-1-findings.md`, `docs/research/poc2-findings.md`,
  `docs/research/poc3-findings.md` (the normative wire behavior —
  observed against real git, not docs' grammar)
- `docs/research/push-captures.md` (the push-path normative record —
  ADR-013's basis)
- `docs/research/negotiation-captures.md` (the negotiation normative
  record — ADR-014's basis)
- `docs/research/git-protocol.md` (inventory + observed corrections)
- `docs/research/gitoxide.md` §"Wire format" (packetline contracts)
- alktty `wire.rs`/`session.rs`/`adapter.rs` (the template's half shapes)