Packet format

2007-12-16 18:18:47 +0000

The format of the packets in the empeg’s TCP/IP protocol owe a lot to the fact that the empeg mk1 communicated over a serial link. This means that there’s a certain amount of cruft still in the protocol.

Most of this information is in the emptool sources, but I’m going to explain some of it in more detail here.

As revealed by Wireshark, the first thing that emplode asks for is the player’s type. Let’s take a quick look at the request packet:

02 04 00 07 00 95 00 81 80 07 00 00 00 FC 43

This breaks down into a header, payload and trailer:

02 04 00 07 00 95 00 81 80 07 00 00 00 FC 43

All packets start with 0x02. The emptool source code calls this PSOH. This is left over from the serial protocol. It’s a pseudo-SOH (start-of-header). It signals the start of the packet.

The next two bytes (04 00) are the size of the payload. Note that the empeg protocol is little-endian. This is because the x86 is little-endian, and Linux on the empeg uses the ARM in little-endian mode. It would have been wasteful to convert to network byte order (big-endian) and back again, so everything’s in little-endian.

With that in mind, we can see that the payload is 4 bytes.

The next byte (07) is the opcode. 0x07 is OP_STATFID, which is used to discover the size of a file on the empeg, so that we can allocate enough memory, and so that we can report progress.

The next byte (00) is the optype. 0x00 is OPTYPE_REQUEST, so this is a request packet.

The last 4 bytes (95 00 81 80) are the packet ID. I’ll talk about packet IDs in the future.

Payload

02 04 00 07 00 95 00 81 80 07 00 00 00 FC 43

We know from looking at the header that the payload size is 4 bytes. It’s little endian, so it’s a seven.

Trailer

02 04 00 07 00 95 00 81 80 07 00 00 00 FC 43

The trailer is the CRC16 of the packet, starting with the opcode (i.e. opcode, optype, packet id and payload).