Packet Structure
All messages sent over the air take the form of a packet. Packets encapsulate the actual user data payload with information required for the network to operate. They are similar to their 802.15.4 counterparts, but extended to allow larger payloads.
Headers
These go at the start of the packet, before the user payload. Going from outermost to innermost header:
Synchronization
While this is not technically part of the data, it's used by the underlying radio subsystem to recognize the frame. The sync header is the very first part of the packet to be transmitted, and is always the same. It's first a 32-bit preamble sequence (generated and specified by the radio PHY) followed by a fixed synchronization word – this is specified by the network, with a default value of 0xF6.
PHY
It is 2 bytes, and is used by the PHY layer to know how large of a packet it should receive. The length counter is a 10 bit quantity, meaning packets up to 1024 bytes (0 byte packets are not permitted) are allowed.
| Byte 0 | Byte 1 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Reserved (must be set to 0) | Length Counter | ||||||||||||||
MAC
Next, a variable length header is used to indicate the source and destination address of the packet, as well as some additional flags and information about encryption, if used. At a minimum, the header will be 6 bytes in length, and contains the following information:
| 0 | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Flags | Sequence number (tag) | Source Address | Destination Address | ||
Source and destination address are short addresses, which are assigned to the nodes when they associate to the network. These addresses may change between associations, but are guaranteed to be constant for a node as long as it's associated to the network. (Higher level protocols should implement a form of address resolution to convert more convenient, permanent logical addresses to these network addresses, similar to IPv4 ARP.)
The sequence number is a monotonically increasing counter. It is used to identify the packet for acknowledgement, and may be used as an input to security schemes (but may NOT be used as a key.) Devices can use this to drop duplicate packets, though they must pay attention to correctly implement counter roll-over.
Flags
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|
| Reserved (must be set to 0) |
Endpoint | Ack request | Data pending | Security Enabled | |||
When "ack request" is set, the recipient of the message should generate an acknowledgement response, and send it to the source of this message, once it's been processed. Data pending is set if the source device has additional buffered packets for the recipient. Security enabled indicates the MAC header is followed by a security header, containing information on how to authenticate and decrypt the packet.
Lastly, the endpoint value specifies how the payload of the packet is to be interpreted. The following values are assigned:
- 0b000: Network control (used for network management, such as association requests)
- 0b001: Acknowledgement response
- 0b010: User data
All other endpoint values are reserved, and packets containing such values will be discarded.
Security
If the MAC header indicates the packet has security, this variable length (5 byte minimum) security header will be added immediately after the MAC header, but before the payload. The first part of the header indicates the type of security scheme used, as well as a unique nonce for the message. (TODO: define nonce overflow handling better)
| 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Security Type | Frame Counter | |||
The frame counter is a unique monotonic counter, used as a nonce for message authentication, encryption and anti-replay protection.
Security type is an enumeration that defines if there are any additional security header data, as well as the crypto scheme in use to protect the message. The following values are defined; messages containing any other security type will be discarded by the protocol layer:
- 0x00: No encryption or security, only anti-replay via frame counter
- 0x01: AES-CCM-128
- Authentication + encryption (hardware accelerated)
- 0x02: AES-CTR-128
- Encryption only (hardware accelerated)
- 0x03: ChaCha20-Poly1305
- Authentication + encryption (hardware accelerated)
Footers
Additional information needed to validate packets goes after the payload data.
Checksum
Contains a 16-bit checksum over all headers (starting with the PHY header) and payload data.
This checksum is to be a 16-bit CRC, using the CCITT polynomial with a seed of 0xFFFF. CRCs are calculated LSB first, and output most significant byte (and bit in the byte) first.
The radio layer shall discard all received packets where the checksum footer's value does not match the computed packet checksum. Packet headers shall not be interpreted until the checksum has been validated, as they may have been corrupted otherwise.