Question

I’m makinga Java server for the last version of the draft. I managed to make the connection, and that’s working great.

The problem is that I don’t understand how the data is encoded, I’ve been trying to find some example of how it has to be done but I couldn’t find anything. so I’m trying to do it by myself but need some help.

Here is an image of the frame.

But I don’t understand where the payload begins. What is:

Extended payload length (16/63) (if payload len==126/127)

That’s the place where my payload should be?

Can someone give some help because as you may see, I’m completely lost…

Answer 1

The problem is that the length does not always fit in 7 bits (you can only express the numbers 0 to 127 with 7 bits), and in that case either the following 2 or 8 bytes will be used to make the length fit:

• 126 means the following 2 bytes are used for the length
• 127 means the following 8 bytes are used for the length

So the payload starts at either index 2, 4 or 10, if not encoded. When encoded, it starts at either 6, 8 or 14 (because there are 4 mask bytes).

I previously posted some pseudocode about decoding the payload data.

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To actually get the length as a “real number” (instead of separate bytes), you can use bitwise shift operators as follows (in case there are two bytes for the length):

var length = (bytes[2] << 8) | (bytes[3] << 0);

This will calculate it like this:

Suppose:

• bytes[2] is 01101001 (105 in base 10)
• bytes[3] is 10100101 (165 in base 10)

Then << will be doing:

01101001 00000000   // moved 8 places to the left, filled with zeroes
         10100101   // moved 0 places (nothing really happening, you can eliminate '<< 0')

| is basically adding them:

01101001 00000000
         10100101
-----------------  |
01101001 10100101      (in base 10 that's 27045)

So if you have the bytes 105 and 165, then they represent a length of 27045.