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93 lines
4.1 KiB
Plaintext
93 lines
4.1 KiB
Plaintext
---
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title: "Stream Module Architecture"
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description: "Internal architecture of the Nym Stream subsystem: wire protocol, multiplexing, router task, and how concurrent byte channels share a single MixnetClient."
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schemaType: "TechArticle"
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section: "Developers"
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lastUpdated: "2026-03-15"
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---
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# Stream Architecture
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import { Callout } from 'nextra/components'
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{/* Canonical source: sdk/rust/nym-sdk/src/mixnet/stream/ARCHITECTURE.md */}
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## Overview
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The stream subsystem gives each `MixnetClient` the ability to hold many concurrent byte channels (`AsyncRead + AsyncWrite`) to different remote peers, multiplexed over a single client connection.
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```mermaid
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---
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config:
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theme: neo-dark
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---
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flowchart TD
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subgraph MixnetClient
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SA["MixnetStream A"] -->|writes| CI["Client input channel"]
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SB["MixnetStream B"] -->|writes| CI
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CI --> MX["── Mixnet ──"]
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MX --> RT["Router task"]
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RT -->|Open messages| ML["MixnetListener.accept()"]
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RT -->|Data messages| SM["Stream routing table"]
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SM --> SA
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SM --> SB
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end
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```
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## Wire protocol
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Every stream message has a fixed 16-byte LP frame header prepended to the payload:
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```
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[LpFrameKind: 2 bytes LE][StreamId: 8 bytes BE][MsgType: 1 byte][SequenceNum: 4 bytes BE][Reserved: 1 byte][payload ...]
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```
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- **LpFrameKind:** `3` (SphinxStream). Distinguishes stream traffic from other LP frame types (Opaque, Registration, Forward).
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- **StreamId:** random `u64` generated by the opener, used to multiplex streams.
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- **MsgType:** `Open` (0) or `Data` (1).
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- **SequenceNum:** `u32` counter, incremented per write. Used by the receiver's per-stream reorder buffer to deliver data in the correct order.
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- **Reserved:** must be `0x00`.
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There is no `Close` message type; see [Known Limitations](#known-limitations) for why.
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## Stream mode
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Stream mode is activated lazily on the first call to `open_stream()` or `listener()`. This is a **one-way transition**:
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1. The client's message receiver is handed off to a background router task
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2. `stream_mode` flag is set to `true`
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3. Message-based methods (`send_plain_message`, `wait_for_messages`) are disabled and return errors
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There is no switching back without disconnecting and creating a new client.
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## Opening and accepting streams
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**Opening (outbound):**
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1. `open_stream(recipient, surbs)` generates a random `StreamId`
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2. An `Open` message is sent through the Mixnet to the recipient
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3. A `MixnetStream` is returned, ready for writing and reading
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**Accepting (inbound):**
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1. `listener.accept()` waits for an `Open` message from a remote peer
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2. A `MixnetStream` is created with the opener's `sender_tag` for anonymous replies
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3. The stream is ready for bidirectional I/O
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## Cleanup
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- **On `drop`:** the stream deregisters from the routing table. No close message is sent over the wire.
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- **Idle timeout:** streams idle for longer than the configured timeout (default: 30 minutes) are automatically cleaned up. Configure with [`MixnetClientBuilder::with_stream_idle_timeout()`](https://docs.rs/nym-sdk/latest/nym_sdk/mixnet/struct.MixnetClientBuilder.html).
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## Known limitations
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<Callout type="info">
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The Mixnet does not guarantee message ordering at the transport level, but each stream write includes a `sequence_num` in the LP frame header. The receiver maintains a per-stream reorder buffer (BTreeMap keyed by sequence number) that buffers out-of-order messages and drains them in sequence, so protocols that depend on byte ordering (HTTP, TLS, protobuf) work correctly over streams.
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- **Buffer cap:** 256 messages per stream. If the buffer fills (e.g. a large gap in sequence numbers), the receiver skips ahead to the lowest buffered sequence.
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- **Duplicates:** messages with a sequence number below the next expected are dropped.
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- There is no `Close` message type, since a close could race ahead of in-flight data.
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</Callout>
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## Internal details
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For the full implementation details (router task, `StreamMap`, `PollSender` usage, base-client type rationale), see the `ARCHITECTURE.md` file next to the module source code, or the [docs.rs](https://docs.rs/nym-sdk/latest/nym_sdk/) API reference.
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