CDNET: An Optional High-Layer Protocol for CDBUS

Protocol status: Stable [Version 1.0]

The CDBUS frame containing a CDNET package is as follows:
[src, dst, len] + [CDNET package] + [crc_l, crc_h]
[src, dst, len] + [CDNET header, payload] + [crc_l, crc_h]

CDNET Levels

CDNET protocol offers two levels, determined by bit7 of the first header byte:

Bit7	DESCRIPTION
0	Level 0: Simplified communication
1	Level 1: Standard communication

You can select one or more levels for your application as required.

The CDNET is little endian.

Level 0 Format

Request

A single-byte header:

FIELD	DESCRIPTION
[7]	Always 0: Level 0
[6]	Always 0: request
[5:0]	dst_port, range 0 ~ 0x3f

Note: The port is analogous to UDP port and can be thought of simply as command.

Reply

A single-byte header:

FIELD	DESCRIPTION
[7]	Always 0: Level 0
[6]	Always 1: reply
[5:0]	Low bits of first payload byte

The first payload byte omits bits [7:6], which were intended to be b10.

E.g.: Header 0x42 implies 0x82 as the first byte of the payload.

Level 1 Format

The first byte of the header:

FIELD	DESCRIPTION
[7]	Always 1
[5]	MULTI_NET
[4]	MULTICAST
[2:1]	PORT_SIZE
[6][3][0]	Reserved as 0

MULTI_NET & MULTICAST

MULTI_NET	MULTICAST	DESCRIPTION
0	0	Local net: append 0 byte
0	1	Local net multicast: append 2 bytes [mh, ml]
1	0	Cross net: append 4 bytes: [src_net, src_mac, dst_net, dst_mac]
1	1	Cross net multicast: append 4 bytes: [src_net, src_mac, mh, ml]

Notes:

• mh + ml: multicast_id, ml is mapped to mac layer multicast address (h: high byte, l: low byte);
• Could simply use MULTI_NET = 0 and MULTICAST = 0 for local net multicast and broadcast.
• Implementations of MULTI_NET and MULTICAST are optional.

PORT_SIZE:

Bit2	Bit1	SRC_PORT	DST_PORT
0	0	Default port	1-byte
0	1	1-byte	Default port
1	0	1-byte	1-byte
1	1	2-byte	2-byte

Notes:

• The default port is 0xcdcd for convention and doesn't consume space.
• Append bytes for src_port before dst_port.
• Implementation of 2-byte port is optional.

Recommendation for first byte of payload

For Request and Report, the first byte definition:

FIELD	DESCRIPTION
[7]	is_reply, always 0
[6]	not_reply
[5:0]	sub command

For Reply, the first byte definition:

FIELD	DESCRIPTION
[7]	is_reply, always 1
[6:0]	status, 0 means no error

Port Allocation Recommendation

Ports 0 to 0xf are recommended for general purposes. Specifically, port 1 is designated for device information queries. While all ports in this section are optional, it is advisable to implement the basic function of port 1 (read device_info).

Port 1

Provide device info.

Types:
 - mac_start and mac_end: uint8_t
 - max_time: uint16_t (unit: ms)
 - "string": variable-length string, including empty string
   (excluding the terminating null byte '\0')


Read device_info string:
  Write [0x00]
  Return [0x80, "device_info"]

Search devices by filters (for resolving mac conflicts):
  Write [0x10, max_time, mac_start, mac_end, "string"]
  Return [0x80, "device_info"] after a random time in the range [0, max_time]
    only if "device_info" contains "string" (always true for an empty string)
    and the current mac address is in the range [mac_start, mac_end] (inclusive)
  Not return otherwise
    and reject any subsequent modify mac (or save config) commands

Example of device_info:
M: model; S: serial id; HW: hardware version; SW: software version ...
Field order is not important; it is recommended to include the model field at least.

CDNET Address String Formats

           localhost      local link     unique local    multicast
            10:00:00
level0:                    00:NN:MM
level1:                    80:NN:MM        a0:NN:MM       f0:MH:ML

Notes:

• NN: net_id, MM: mac_addr, MH+ML: multicast_id (H: high byte, L: low byte).
• The string address is analogous to IPv6 address.

Examples

Request device info:
(Local network, requester's mac address: 0x0c, target's mac address: 0x0d)

Reply the device info string: "M: c1; S: 1234", expressed in hexadecimal: [0x4d, 0x3a, 0x20, 0x63, 0x31, 0x3b, 0x20, 0x53, 0x3a, 0x20, 0x31, 0x32, 0x33, 0x34].

The Level 0 Format:

• Request:
  • CDNET socket: [00:00:0c]:0xcdcd -> [00:00:0d]:1: 0x00 (net id: 0)
  • CDNET packet: [0x01, 0x00] (dst_port: 0x01)
  • CDBUS frame: [0x0c, 0x0d, 0x02, 0x01, 0x00, crc_l, crc_h]
• Reply:
  • CDNET socket: [00:00:0d]:1 -> [00:00:0c]:0xcdcd: 0x80 + "M: c1; S: 1234"
  • CDNET packet: [0x40, 0x4d, 0x3a, 0x20 ... 0x34] (first 0x40 implies payload byte 0x80)
  • CDBUS frame: [0x0d, 0x0c, 0x0f, 0x40, 0x4d, 0x3a, 0x20 ... 0x34, crc_l, crc_h]

The Level 1 Format:

• Request:
  • CDNET socket: [80:00:0c]:0xcdcd -> [80:00:0d]:1: 0x00
  • CDNET packet: [0x80, 0x01, 0x00] (src_port: default, dst_port: 0x01)
  • CDBUS frame: [0x0c, 0x0d, 0x03, 0x80, 0x01, 0x00, crc_l, crc_h]
• Reply:
  • CDNET socket: [80:00:0d]:1 -> [80:00:0c]:0xcdcd: 0x80 + "M: c1; S: 1234"
  • CDNET packet: [0x82, 0x01, 0x80, 0x4d, 0x3a, 0x20 ... 0x34] (src_port: 0x01, dst_port: default)
  • CDBUS frame: [0x0d, 0x0c, 0x11, 0x82, 0x01, 0x80, 0x4d, 0x3a, 0x20 ... 0x34, crc_l, crc_h]

Sequence & Flow Control

For transferring large data, such as transmitting a jpg image in the cdcam project, the sub command is defined as follows:

[5:4] FRAGMENT: 00: not fragment, 01: first, 10: more, 11: last, [3:0]: cnt

Here, the cnt value corresponds to 0 for the first packet, and increments by 1 for each subsequent packet.

If flow control is required, for example, we can group every 12 packets as a transmission set, where only the last packet in each set has not_reply set to 0. Start by transmitting 2 sets of packets, wait for a reply regarding the last packet in the first set, and then append another set of packets.

For sub commands that do not have a cnt definition like the one mentioned above, if we want to ensure the uniqueness of a command, such as ensuring that a command can only be executed once, we can also achieve this by cyclically sending commands through multiple source port numbers.

This way, if a command does not receive a reply, resend the command. The receiving port checks whether the source port number of the received command matches that of the previous command; if they match, the command is not executed again.