Many times, such as on the website describing the WebSocket diagram here I see "frame" diagrams (at least that is what I think they are called) like the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-------+-+-------------+-------------------------------+
|F|R|R|R| opcode|M| Payload len | Extended payload length |
|I|S|S|S| (4) |A| (7) | (16/63) |
|N|V|V|V| |S| | (if payload len==126/127) |
| |1|2|3| |K| | |
+-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - +
| Extended payload length continued, if payload len == 127 |
+ - - - - - - - - - - - - - - - +-------------------------------+
| |Masking-key, if MASK set to 1 |
+-------------------------------+-------------------------------+
| Masking-key (continued) | Payload Data |
+-------------------------------- - - - - - - - - - - - - - - - +
: Payload Data continued ... :
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
| Payload Data continued ... |
+---------------------------------------------------------------+
Can someone explain to me how to read such a diagram? The way I interpret it would be that the 0 1 2 3 on the top would be the bytes that arrive in a packet, and the 0-9 repeating would be the individual bits. However this doesn't make sense as there are only 8 bits in a byte.
Further more:
What are fin rsv opcode and mask?
What exactly is the Payload Data.
Is this entire frame one packet, or are there multiple frames in a packet?
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
means
| 1st byte | 2nd byte | 3rd byte | 4th byte |
+-----------------+-----------------+-----------------+-----------------+
| 0 0 0 0 0 0 0 0 | 0 0 1 1 1 1 1 1 | 1 1 1 1 2 2 2 2 | 2 2 2 2 2 2 3 3 |
| 0 1 2 3 4 5 6 7 | 8 9 0 1 2 3 4 5 | 6 7 8 9 0 1 2 3 | 4 5 6 7 8 9 0 1 |
That is, the table is 32-bit (= 4-byte) wide.
Descriptions about fin, rsv, opcode and mask are written right after the table you excerpted from RFC 6455.
Payload Data is a byte array. It is application-specific data.
The table represents the structure of one frame. A message consists of either one frame or multiple frames.