CODEWITHSUNDEEP
javascriptnode.jswebsocket
spaceman12
spaceman12

Reputation: 1109

Decode continuation frame in websocket

How do you decode a continuation frame in websocket? Can someone please give me a useful insights into this? Decoding a continuation frame as text frame results in error.

I'm sending a large text-string to the server and I can manage to decode only the first incoming text frame and fails after that.

Here is a simple function in nodejs that handles the text frame decoding -

function decodeWS(data)
{
    var dl = data[1] & 127;
    var ifm = 2;
    if (dl == 126) 
    {
        ifm = 4;
    } else if (dl == 127) 
    {
        ifm = 10;
    }
    var i = ifm + 4;
    var masks = data.slice(ifm,i);
    var index = 0;
    var output = "";
    var l=data.length;
    while (i < l) 
    {
        output += String.fromCharCode(data[i++] ^ masks[index++ % 4]);
    }
    return output;
}

Upvotes: 2

Views: 2951

Answers (1)

idoodler
idoodler

Reputation: 3545

Get specific Frame bits

Fin Bit

bool fin = (data[0] & 128) == 128;

OpCode

int opCode = data[0] & 15;

IsMasked

bool isMasked = (data[1] & 128) == 128;

DataLength

int dataLength = data[1] & 127;

Websocket frame overview

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)     |             (16/64)           |
|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 ...                |
+---------------------------------------------------------------+

General Info on how to get specific bits of a byte

Example: Getting the FIN Bit

First of all write down the binary representation of the first byte, lets pretend the first bit is 130, so 1000 0010 in Binary or more understandable Fin bit set and opCode 2 for Binary Data. If you want the get the most significant bit you need to use the logical AND (&) operator with 128 (1000 0000 in Binary).

So basically 1 and 1 will be 1, everything else will be 0.

1000 0010 -> First Byte
1000 0000 -> Our Masking Byte => 128 in Decimal
---------
1000 0000 -> Resulting Byte => 128 in Decimal (Fin bit set)

Another Example Fin bit not set and Text Data.

0000 0001 -> First Byte
1000 0000 -> Our Masking Byte -> 128 in Decimal
---------
0000 0000 -> Resulting Byte => 0 in Decimal (Fin bit not set)

Example: Getting OpCode (OpCode tells you what this frame is used for)

The OpCode can be gathered from the first 4 bits of the first byte. Pretend the FIN bit is set and the OpCode is Text.

1000 0001 -> First Byte
0000 1111 -> Our Masking Byte => 15 in Decimal
---------
0000 0001 -> Resulting Byte => 1 in Decimal (Text OpCode)

Upvotes: 6

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