alkgit/docs/architecture/transport.md

---
status: reviewed
last_updated: 2026-09-30
---

# 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, service, authorized-repo marker, duplex stream, `Limits`). The
  marker is a type the door/adapter constructs only after the ADR-007
  check passes (skipping the check is a type error — ADR-007; D-3). The
  service is selected at establishment on every native path (ADR-016:
  the channels open-op params `{repo, service}`, or the in-band
  git-daemon request line the direct-ALPN `GitAdapter` parses,
  POC-1-verbatim), and selects the state machine: the V2 capability
  advertisement for fetch, the V0 ref advertisement for push — both are
  server-emitted firsts, so the service must precede the stream.
  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, service,
  authorized-repo marker, request-reader, response-writer, `Limits`) per
  http POST. The door's route still selects the service (doors.md), but
  the substrate input carries it explicitly — nothing in a POST body
  distinguishes an upload-pack POST from a receive-pack POST before
  parsing (ADR-016). 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.
- **Unborn-HEAD rider** (review 001 N-5): `ls-refs=unborn` is advertised
  (POC-1 validated the *capability token* was accepted, but serving an
  unborn HEAD's symref line was never exercised against real git — the
  only advertised promise without capture evidence). Implementation-phase
  verification: an ls-refs round against an unborn repo, real client,
  both substrates (the unborn repo fixture in the POC set has refs —
  a new one without any). If it cannot be served correctly, the honest
  move per ADR-003 is dropping the token.

### 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`, with the boundary set the *recognized* subset (request
  haves filtered through `common_haves` — the same honest-boundary rule
  as the ack rounds; never honor an unverified have; ADR-014 §2). 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; the call is async-trait, with
  the pipeline-concurrency permit acquired around the call by the wire
  layer (ADR-009's enforcement point — backend.md concurrency model;
  the gix impl's internal `spawn_blocking` is its own detail).
- 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).
- At the wire-mapping point (review 001 N-2), `RegistryError::NotFound`
  and an authorization failure collapse to the same wire error — the
  unknown-repo ≡ unauthorized indistinguishability rule (ADR-007/ADR-008)
  holds at the variant→wire mapping, so no variant leaks an existence
  oracle to the client.

## 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.
The advertisement ref cap is fail-closed like every other budget
(review 001 N-1): breach is a session error, never a silent truncation —
a partial ref list is the worst failure mode an advertisement can have
(clones appear to succeed).

## Public API surface (v1)

Crate-root re-exports (alktty pattern; the full list in
[backend.md](backend.md) §public API): `GitAdapter` + `register_openable`,
`GitSession` (the consumer half — ADR-017's typed client:
`ls_refs`/`fetch`/`push` over the ADR-016 preamble; fetch via
gix-protocol's client machinery over a custom alkcall Transport impl,
push hand-rolled to ADR-013's shapes), substrate types (including the
service selector and the authorized-repo marker), `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 |
| [016](decisions/016-native-session-preamble.md) | Native session preamble | `{repo, service}` open-op params, request-line preamble, service in the tuple |
| [017](decisions/017-consumer-half-git-session.md) | Consumer half | `GitSession` typed client (`ls_refs`/`fetch`/`push`), custom alkcall Transport + gix-protocol, hand-rolled push |
| [018](decisions/018-backend-trait-signatures-and-storage-error-model.md) | Trait signatures + storage errors | the seam shapes transport calls (`generate`/`prepare`/`apply_updates` signatures, `StorageError`, boxed-stream ownership) |

## Open Questions

- **OQ-03**: publish/API freeze (partially resolved — single-crate shape
  settled by ADR-010; the native-path preamble shapes entered the freeze
  inventory via ADR-016).
- **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; the git:// request-line framing ADR-016 adopts)
- `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)