---
title: "Stream Module Architecture"
description: "Internal architecture of the Nym Stream subsystem: wire protocol, multiplexing, router task, and how concurrent byte channels share a single MixnetClient."
schemaType: "TechArticle"
section: "Developers"
lastUpdated: "2026-03-15"
---

# Stream Architecture

import { Callout } from 'nextra/components'

{/* Canonical source: sdk/rust/nym-sdk/src/mixnet/stream/ARCHITECTURE.md */}

## Overview

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.

```mermaid
---
config:
  theme: neo-dark
---
flowchart TD
    subgraph MixnetClient
        SA["MixnetStream A"] -->|writes| CI["Client input channel"]
        SB["MixnetStream B"] -->|writes| CI
        CI --> MX["── Mixnet ──"]
        MX --> RT["Router task"]
        RT -->|Open messages| ML["MixnetListener.accept()"]
        RT -->|Data messages| SM["Stream routing table"]
        SM --> SA
        SM --> SB
    end
```

## Wire protocol

Every stream message has a fixed 16-byte LP frame header prepended to the payload:

```
[LpFrameKind: 2 bytes LE][StreamId: 8 bytes BE][MsgType: 1 byte][SequenceNum: 4 bytes BE][Reserved: 1 byte][payload ...]
```

- **LpFrameKind:** `3` (SphinxStream). Distinguishes stream traffic from other LP frame types (Opaque, Registration, Forward).
- **StreamId:** random `u64` generated by the opener, used to multiplex streams.
- **MsgType:** `Open` (0) or `Data` (1).
- **SequenceNum:** `u32` counter, incremented per write. Used by the receiver's per-stream reorder buffer to deliver data in the correct order.
- **Reserved:** must be `0x00`.

There is no `Close` message type; see [Known Limitations](#known-limitations) for why.

## Stream mode

Stream mode is activated lazily on the first call to `open_stream()` or `listener()`. This is a **one-way transition**:

1. The client's message receiver is handed off to a background router task
2. `stream_mode` flag is set to `true`
3. Message-based methods (`send_plain_message`, `wait_for_messages`) are disabled and return errors

There is no switching back without disconnecting and creating a new client.

## Opening and accepting streams

**Opening (outbound):**
1. `open_stream(recipient, surbs)` generates a random `StreamId`
2. An `Open` message is sent through the Mixnet to the recipient
3. A `MixnetStream` is returned, ready for writing and reading

**Accepting (inbound):**
1. `listener.accept()` waits for an `Open` message from a remote peer
2. A `MixnetStream` is created with the opener's `sender_tag` for anonymous replies
3. The stream is ready for bidirectional I/O

## Cleanup

- **On `drop`:** the stream deregisters from the routing table. No close message is sent over the wire.
- **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).

## Known limitations

<Callout type="info">
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.

- **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.
- **Duplicates:** messages with a sequence number below the next expected are dropped.
- There is no `Close` message type, since a close could race ahead of in-flight data.
</Callout>

## Internal details

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.
