A frame is defined simply as the concatenation of
| Octets: | 1 | 1-3 | n |
|---|---|---|---|
| Fields: | HEADER | CMD | CMD_PAYLOAD |
The header byte is broken down as follows:
0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | FLG | NLI | TID | +---+---+---+---+---+---+---+---+
The flag field of the header byte (FLG) is always set to the value two (or 10 in binary). Any frame received with these bits set to any other value else MUST NOT be considered a Spinel frame.
This convention allows Spinel to be line compatible with BTLE HCI. By defining the first two bit in this way we can disambiguate between Spinel frames and HCI frames (which always start with either 0x01 or 0x04) without any additional framing overhead.
The Network Link Identifier (NLI) is a number between 0 and 3, which is associated by the OS with one of up to four IPv6 zone indices corresponding to conceptual IPv6 interfaces on the NCP. This allows the protocol to support IPv6 nodes connecting simultaneously to more than one IPv6 network link using a single NCP instance. The first Network Link Identifier (0) MUST refer to a distinguished conceptual interface provided by the NCP for its IPv6 link type. The other three Network Link Identifiers (1, 2 and 3) MAY be dissociated from any conceptual interface.
The least significant bits of the header represent the Transaction Identifier(TID). The TID is used for correlating responses to the commands which generated them.
When a command is sent from the host, any reply to that command sent by the NCP will use the same value for the TID. When the host receives a frame that matches the TID of the command it sent, it can easily recognize that frame as the actual response to that command.
The TID value of zero (0) is used for commands to which a correlated response is not expected or needed, such as for unsolicited update commands sent to the host from the NCP.
The command identifier is a 21-bit unsigned integer encoded in up to three bytes using the packed unsigned integer format described in (#packed-unsigned-integer). This encoding allows for up to 2,097,152 individual commands, with the first 127 commands represented as a single byte. Command identifiers larger than 2,097,151 are explicitly forbidden.
| CID Range | Description |
|---|---|
| 0 - 63 | Reserved for core commands |
| 64 - 15,359 | UNALLOCATED |
| 15,360 - 16,383 | Vendor-specific |
| 16,384 - 1,999,999 | UNALLOCATED |
| 2,000,000 - 2,097,151 | Experimental use only |
Depending on the semantics of the command in question, a payload MAY be included in the frame. The exact composition and length of the payload is defined by the command identifier.