DragonSpirit
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96 lines
3.3 KiB
Markdown
96 lines
3.3 KiB
Markdown
# NXST Wire Protocol
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## Overview
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NXST uses two sockets per transfer session:
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| Socket | Purpose |
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|--------|---------|
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| UDP multicast | Receiver advertisement (discovery) |
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| TCP | File data stream |
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Both sides must be on the same local network segment. The sender (Transfer mode) initiates; the receiver (Receive mode) listens.
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---
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## Discovery — UDP Multicast
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| Parameter | Value |
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|-----------|-------|
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| Group | `239.0.0.1` |
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| Port | `8081` |
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| Direction | sender → receiver |
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**Flow:**
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1. Receiver joins multicast group and binds `0.0.0.0:8081`.
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2. Sender sends `"DISCOVER_SERVER"` (15 bytes, no null terminator) to `239.0.0.1:8081`.
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3. Receiver replies `"SERVER_HERE"` (11 bytes) to the sender's source address.
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4. Sender extracts the receiver's IP from the reply source and closes the UDP socket.
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Sender polls in 100 ms slices for up to 3 seconds. Cancel is checked each slice.
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---
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## File Transfer — TCP
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| Parameter | Value |
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|-----------|-------|
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| Port | `8080` |
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| Direction | sender connects → receiver listens |
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| Buffer size | 65 536 bytes (`proto::kBufSize`) |
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**Connection:**
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1. Receiver listens on `0.0.0.0:8080` (started concurrently with multicast listener).
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2. Sender connects after receiving `"SERVER_HERE"`.
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**Wire layout — one file:**
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```
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┌─────────────────────────────────┐
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│ filename_len │ uint32_t LE │ 4 bytes
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├─────────────────────────────────┤
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│ filename │ filename_len │ bytes, no null terminator
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│ │ bytes │
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├─────────────────────────────────┤
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│ file_size │ uint64_t LE │ 8 bytes
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├─────────────────────────────────┤
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│ file_data │ file_size │ bytes
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│ │ bytes │
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└─────────────────────────────────┘
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```
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Files are sent sequentially. The stream ends with a sentinel frame:
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```
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filename_len == 0 (proto::kEofSentinel)
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```
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No `filename` or `file_size` field follows the sentinel.
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**Constraints:**
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- `filename_len > proto::kMaxFilename` (4 096) is treated as a protocol error; the receiver aborts.
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- Filenames are full paths as produced by `io::backup` (e.g. `/switch/NXST/<title>/<user>/...`).
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- On the receiver, the username path component is rewritten to match the local user's nickname before writing to disk.
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---
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## Post-Transfer
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After the TCP stream closes (sentinel received), the receiver calls `io::restore`:
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1. Mounts the title's save filesystem.
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2. Clears existing save data.
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3. Copies received files into `save:/`.
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4. Commits via `fsdevCommitDevice("save")`.
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The sender creates a local backup via `io::backup` before connecting, so the sender's own save is never at risk.
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---
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## Cancellation
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Either side can cancel at any time by closing the relevant socket (`shutdown` + `close`). The other side's blocking read/write will return an error and the transfer loop exits cleanly. The receiver's accept thread sets `receiver_state.done = true` regardless of how the connection ends, so the UI poll loop always terminates.
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