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Evgeny @ SimpleX Chat
39 lines
2.5 KiB
Markdown
39 lines
2.5 KiB
Markdown
# Simplex.Messaging.Notifications.Transport
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> NTF protocol version negotiation, TLS handshake, and transport handle setup.
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**Source**: [`Notifications/Transport.hs`](../../../../../src/Simplex/Messaging/Notifications/Transport.hs)
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## Non-obvious behavior
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### 1. ALPN-dependent version range
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`ntfServerHandshake` advertises `legacyServerNTFVRange` (v1 only) when ALPN is not available (`getSessionALPN` returns `Nothing`). When ALPN is present, it advertises the caller-provided `ntfVRange`. This is the backward-compatibility mechanism for pre-ALPN clients that cannot negotiate newer protocol features.
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### 2. Version-gated features
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Two feature gates exist in the NTF protocol:
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| Version | Feature | Effect |
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|---------|---------|--------|
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| v2 (`authBatchCmdsNTFVersion`) | Auth key exchange + batching | `authPubKey` sent in handshake, `implySessId` and `batch` enabled |
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| v3 (`invalidReasonNTFVersion`) | Token invalid reasons | `NTInvalid` responses include the reason enum |
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Pre-v2 connections have no command encryption or batching — commands are sent in plaintext within TLS.
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### 3. Unused Protocol typeclass parameters
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`ntfClientHandshake` accepts `_proxyServer` and `_serviceKeys` parameters that are ignored. These are passed through from the `Protocol` typeclass's `protocolClientHandshake` method for consistency with SMP. A third parameter (`Maybe C.KeyPairX25519` for key agreement) is discarded at the Protocol instance wrapper level. The NTF protocol does not support proxy routing or service authentication.
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### 4. Block size
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NTF uses a 512-byte block size (`ntfBlockSize`), significantly smaller than SMP. This is sufficient because NTF protocol commands (TNEW, SNEW, TCHK, etc.) and their responses are short. `PNMessageData` (which contains encrypted message metadata) is not sent over the NTF transport — it is delivered via APNS push notifications.
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### 5. Initial THandle has version 0
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`ntfTHandle` creates a THandle with `thVersion = VersionNTF 0` — a version that no real protocol supports. This is a placeholder value that gets overwritten during version negotiation. All feature gates check `v >= authBatchCmdsNTFVersion` (v2), so the v0 placeholder disables all optional features.
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### 6. Server handshake always sends authPubKey
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`ntfServerHandshake` always includes `authPubKey = Just sk` in the server handshake, regardless of the advertised version range. The encoding functions (`encodeAuthEncryptCmds`) then decide whether to actually serialize it based on the max version. This means the key is computed even when it won't be sent.
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