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docs: Add serial transport message framing documentation #544

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24 changes: 24 additions & 0 deletions docs/transports.md
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Expand Up @@ -66,6 +66,30 @@ be used to *summon* the transport:
When no url is provided (i.e. `zcm_create(NULL)`), the `ZCM_DEFAULT_URL` environment variable is
queried for a valid url.

### Serial Transport Message Framing

When using the `serial://` transport, ZCM uses a specific framing protocol to identify and parse
messages in the serial byte stream. This is handled automatically by the `generic_serial_transport`
implementation (see `zcm/transport/generic_serial_transport.h`).

The frame structure is as follows:

| Field | Size | Description |
|-------|------|-------------|
| Sync bytes | 2 bytes | `0xCC 0x00` - Start of frame marker |
| `chan_len` | 1 byte | Length of the channel name string |
| `data_len` | 4 bytes | Length of the message data |
| `*chan` | `chan_len` bytes | Channel name string |
| `*data` | `data_len` bytes | Serialized message data |
| `sum1` | 1 byte | Checksum byte 1 |
| `sum2` | 1 byte | Checksum byte 2 |

This framing allows the receiver to:
1. Synchronize to the start of a message by looking for `0xCC 0x00`
2. Determine the channel name to route the message appropriately
3. Extract the exact message payload
4. Verify message integrity via the checksum

## Custom Transports

While these built-in transports are enough for many applications, there are many situations
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