How can I send and receive WebSocket messages on the server side

Question

How can I send and receive messages on the server side using WebSocket  as per the protocol  Why do I get seemingly random bytes at the server when I send data from the browser to the server  It the data encoded somehow  How does the framing work in both the server  rarr  client and client  rarr  server directions

User · Answer

C  Implementation  Browser -  Server      private String DecodeMessage Byte   bytes                String incomingData   String Empty          Byte secondByte   bytes 1           Int32 dataLength   secondByte  amp  127          Int32 indexFirstMask   2          if  dataLength    126              indexFirstMask   4          else if  dataLength    127              indexFirstMask   10           IEnumerable lt Byte gt  keys   bytes Skip indexFirstMask  Take 4           Int32 indexFirstDataByte   indexFirstMask   4           Byte   decoded   new Byte bytes Length - indexFirstDataByte           for  Int32 i   indexFirstDataByte  j   0  i  lt  bytes Length  i    j                          decoded j     Byte  bytes i    keys ElementAt j   4                       return incomingData   Encoding UTF8 GetString decoded  0  decoded Length           Server -  Browser      private static Byte   EncodeMessageToSend String message                Byte   response          Byte   bytesRaw   Encoding UTF8 GetBytes message           Byte   frame   new Byte 10            Int32 indexStartRawData   -1          Int32 length   bytesRaw Length           frame 0     Byte 129          if  length  lt   125                        frame 1     Byte length              indexStartRawData   2                    else if  length  gt   126  amp  amp  length  lt   65535                        frame 1     Byte 126              frame 2     Byte   length  gt  gt  8   amp  255               frame 3     Byte  length  amp  255               indexStartRawData   4                    else                       frame 1     Byte 127              frame 2     Byte   length  gt  gt  56   amp  255               frame 3     Byte   length  gt  gt  48   amp  255               frame 4     Byte   length  gt  gt  40   amp  255               frame 5     Byte   length  gt  gt  32   amp  255               frame 6     Byte   length  gt  gt  24   amp  255               frame 7     Byte   length  gt  gt  16   amp  255               frame 8     Byte   length  gt  gt  8   amp  255               frame 9     Byte  length  amp  255                indexStartRawData   10                     response   new Byte indexStartRawData   length            Int32 i  reponseIdx   0             Add the frame bytes to the reponse         for  i   0  i  lt  indexStartRawData  i                          response reponseIdx    frame i               reponseIdx                         Add the data bytes to the response         for  i   0  i  lt  length  i                          response reponseIdx    bytesRaw i               reponseIdx                       return response

User · Answer

C   Implementation  not by me  here  Note that when your bytes are over 65535  you need to shift with a long value as shown here

User · Answer

Note  This is some explanation and pseudocode as to how to implement a very trivial server that can handle incoming and outcoming WebSocket messages as per the definitive framing format  It does not include the handshaking process  Furthermore  this answer has been made for educational purposes  it is not a full-featured implementation   Specification  RFC 6455     Sending messages   In other words  server  rarr  browser   The frames you re sending need to be formatted according to the WebSocket framing format  For sending messages  this format is as follows    one byte which contains the type of data  and some additional info which is out of scope for a trivial server  one byte which contains the length either two or eight bytes if the length does not fit in the second byte  the second byte is then a code saying how many bytes are used for the length  the actual  raw  data   The first byte will be 1000 0001  or 129  for a text frame   The second byte has its first bit set to 0 because we re not encoding the data  encoding from server to client is not mandatory    It is necessary to determine the length of the raw data so as to send the length bytes correctly    if 0  lt   length  lt   125  you don t need additional bytes if 126  lt   length  lt   65535  you need two additional bytes and the second byte is 126 if length  gt   65536  you need eight additional bytes  and the second byte is 127   The length has to be sliced into separate bytes  which means you ll need to bit-shift to the right  with an amount of eight bits   and then only retain the last eight bits by doing AND 1111 1111  which is 255    After the length byte s  comes the raw data   This leads to the following pseudocode   bytesFormatted 0    129  indexStartRawData   -1    it doesn t matter what value is                           set here - it will be set now   if bytesRaw length  lt   125     bytesFormatted 1    bytesRaw length      indexStartRawData   2  else if bytesRaw length  gt   126 and bytesRaw length  lt   65535     bytesFormatted 1    126     bytesFormatted 2      bytesRaw length  gt  gt  8   AND 255     bytesFormatted 3      bytesRaw length        AND 255      indexStartRawData   4  else     bytesFormatted 1    127     bytesFormatted 2      bytesRaw length  gt  gt  56   AND 255     bytesFormatted 3      bytesRaw length  gt  gt  48   AND 255     bytesFormatted 4      bytesRaw length  gt  gt  40   AND 255     bytesFormatted 5      bytesRaw length  gt  gt  32   AND 255     bytesFormatted 6      bytesRaw length  gt  gt  24   AND 255     bytesFormatted 7      bytesRaw length  gt  gt  16   AND 255     bytesFormatted 8      bytesRaw length  gt  gt   8   AND 255     bytesFormatted 9      bytesRaw length         AND 255      indexStartRawData   10     put raw data at the correct index bytesFormatted put bytesRaw  indexStartRawData       now send bytesFormatted  e g  write it to the socket stream      Receiving messages   In other words  browser  rarr  server   The frames you obtain are in the following format    one byte which contains the type of data one byte which contains the length either two or eight additional bytes if the length did not fit in the second byte four bytes which are the masks    decoding keys  the actual data   The first byte usually does not matter - if you re just sending text you are only using the text type  It will be 1000 0001  or 129  in that case   The second byte and the additional two or eight bytes need some parsing  because you need to know how many bytes are used for the length  you need to know where the real data starts   The length itself is usually not necessary since you have the data already   The first bit of the second byte is always 1 which means the data is masked    encoded   Messages from the client to the server are always masked  You need to remove that first bit by doing secondByte AND 0111 1111  There are two cases in which the resulting byte does not represent the length because it did not fit in the second byte    a second byte of 0111 1110  or 126  means the following two bytes are used for the length a second byte of 0111 1111  or 127  means the following eight bytes are used for the length   The four mask bytes are used for decoding the actual data that has been sent  The algorithm for decoding is as follows   decodedByte   encodedByte XOR masks encodedByteIndex MOD 4    where encodedByte is the original byte in the data  encodedByteIndex is the index  offset  of the byte counting from the first byte of the real data  which has index 0  masks is an array containing of the four mask bytes    This leads to the following pseudocode for decoding   secondByte   bytes 1   length   secondByte AND 127    may not be the actual length in the two special cases  indexFirstMask   2             if not a special case  if length    126               if a special case  change indexFirstMask     indexFirstMask   4  else if length    127          ditto     indexFirstMask   10  masks   bytes slice indexFirstMask  4     four bytes starting from indexFirstMask  indexFirstDataByte   indexFirstMask   4    four bytes further  decoded   new array  decoded length   bytes length - indexFirstDataByte    length of real data  for i   indexFirstDataByte  j   0  i  lt  bytes length  i    j       decoded j    bytes i  XOR masks j MOD 4       now use  decoded  to interpret the received data

User · Answer

Thank you for the answer  i would like to add onto hfern s above  Python version to include the Sending function if any one is interested    def DecodedWebsockRecieve stringStreamIn       byteArray    stringStreamIn      datalength   byteArray 1   amp  127     indexFirstMask   2      if datalength    126          indexFirstMask   4     elif datalength    127          indexFirstMask   10     masks    m for m in byteArray indexFirstMask   indexFirstMask 4       indexFirstDataByte   indexFirstMask   4     decodedChars          i   indexFirstDataByte     j   0     while i  lt  len byteArray           decodedChars append  chr byteArray i    masks j   4             i    1         j    1     return    join decodedChars   def EncodeWebSockSend socket data       bytesFormatted          bytesFormatted append 129       bytesRaw   data encode       bytesLength   len bytesRaw      if bytesLength  lt   125           bytesFormatted append bytesLength      elif bytesLength  gt   126 and bytesLength  lt   65535           bytesFormatted append 126          bytesFormatted append    bytesLength  gt  gt  8    amp  255           bytesFormatted append  bytesLength  amp  255       else           bytesFormatted append  127           bytesFormatted append    bytesLength  gt  gt  56    amp  255           bytesFormatted append    bytesLength  gt  gt  48    amp  255           bytesFormatted append    bytesLength  gt  gt  40    amp  255           bytesFormatted append    bytesLength  gt  gt  32    amp  255           bytesFormatted append    bytesLength  gt  gt  24    amp  255           bytesFormatted append    bytesLength  gt  gt  16    amp  255           bytesFormatted append    bytesLength  gt  gt   8    amp  255           bytesFormatted append  bytesLength  amp  255        bytesFormatted   bytes bytesFormatted      bytesFormatted   bytesFormatted   bytesRaw     socket send bytesFormatted     Usage for reading   bufSize   1024      read   DecodedWebsockRecieve socket recv bufSize     Usage for writing   EncodeWebSockSend sock  hellooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

User · Answer

Clojure  the decode function assumes frame is sent as map of   data byte-array-buffer  size int-size-of-buffer   because the actual size may not be the same size as the byte-array depending on chunk size of your inputstream   Code posted here  https   gist github com viperscape 8918565   defn ws-decode  frame     decodes websocket frame     let  data   data frame          dlen  bit-and  second data  127          mstart  if     dlen 127  10  if     dlen 126  4 2           mask  drop 2  take    mstart 4  data           msg  make-array Byte TYPE  -   size frame     mstart 4         loop  i    mstart 4   j 0         aset-byte msg j  byte  bit-xor  nth data i   nth mask  mod j 4             if   lt  i  dec  size frame     recur  inc i   inc j         msg     defn ws-encode  data     takes in bytes  return websocket frame     let  len  count data          blen  if   gt  len 65535  10  if   gt  len 125  4 2           buf  make-array Byte TYPE    len blen              aset-byte buf 0 -127     bit-or  unchecked-byte 0x80                                               unchecked-byte 0x1             if    2 blen                aset-byte buf 1 len    mask 0  len              do                dorun map   aset-byte buf  1                         unchecked-byte  bit-and  bit-shift-right len    -  2 2  8                                                  255                           range 2 blen   into    range 2 blen                    aset-byte buf 1  if   gt  blen 4  127 126                System arraycopy data 0 buf blen len       buf

User · Answer

JavaScript implementation   function encodeWebSocket bytesRaw       var bytesFormatted   new Array        bytesFormatted 0    129      if  bytesRaw length  lt   125            bytesFormatted 1    bytesRaw length        else if  bytesRaw length  gt   126  amp  amp  bytesRaw length  lt   65535            bytesFormatted 1    126          bytesFormatted 2      bytesRaw length  gt  gt  8    amp  255          bytesFormatted 3      bytesRaw length         amp  255        else           bytesFormatted 1    127          bytesFormatted 2      bytesRaw length  gt  gt  56    amp  255          bytesFormatted 3      bytesRaw length  gt  gt  48    amp  255          bytesFormatted 4      bytesRaw length  gt  gt  40    amp  255          bytesFormatted 5      bytesRaw length  gt  gt  32    amp  255          bytesFormatted 6      bytesRaw length  gt  gt  24    amp  255          bytesFormatted 7      bytesRaw length  gt  gt  16    amp  255          bytesFormatted 8      bytesRaw length  gt  gt   8    amp  255          bytesFormatted 9      bytesRaw length          amp  255            for  var i   0  i  lt  bytesRaw length  i             bytesFormatted push bytesRaw charCodeAt i              return bytesFormatted     function decodeWebSocket  data       var datalength   data 1   amp  127      var indexFirstMask   2      if  datalength    126            indexFirstMask   4        else if  datalength    127            indexFirstMask   10            var masks   data slice indexFirstMask indexFirstMask   4       var i   indexFirstMask   4      var index   0      var output           while  i  lt  data length            output    String fromCharCode data i      masks index     4              return output

User · Answer

Implementation in Go  Encode part  server -  browser   func encode  message string   result   byte      rawBytes      byte message    var idxData int    length    byte len rawBytes     if len rawBytes   lt   125     one byte to store data length     result   make   byte  len rawBytes    2      result 1    length     idxData   2     else if len rawBytes   gt   126  amp  amp  len rawBytes   lt   65535     two bytes to store data length     result   make   byte  len rawBytes    4      result 1    126   extra storage needed     result 2      length  gt  gt  8    amp  255     result 3      length         amp  255     idxData   4     else       result   make   byte  len rawBytes    10      result 1    127     result 2      length  gt  gt  56    amp  255     result 3      length  gt  gt  48    amp  255     result 4      length  gt  gt  40    amp  255     result 5      length  gt  gt  32    amp  255     result 6      length  gt  gt  24    amp  255     result 7      length  gt  gt  16    amp  255     result 8      length  gt  gt   8    amp  255     result 9      length          amp  255     idxData   10        result 0    129   only text is supported       put raw data at the correct index   for i  b    range rawBytes       result idxData   i    b       return     Decode part  browser -  server   func decode  rawBytes   byte  string     var idxMask int   if rawBytes 1     126       idxMask   4     else if rawBytes 1     127       idxMask   10     else       idxMask   2        masks    rawBytes idxMask idxMask   4    data    rawBytes idxMask   4 len rawBytes     decoded    make   byte  len rawBytes  - idxMask   4     for i  b    range data       decoded i    b   masks i   4        return string decoded

User · Answer

Java implementation  if any one requires   Reading   Client to Server          int len   0                      byte   b   new byte buffLenth             rawIn is a Socket getInputStream            while true               len   rawIn read b               if len  -1                    byte rLength   0                  int rMaskIndex   2                  int rDataStart   0                    b 0  is always text in my case so no need to check                  byte data   b 1                   byte op    byte  127                  rLength    byte   data  amp  op                    if rLength   byte 126  rMaskIndex 4                  if rLength   byte 127  rMaskIndex 10                   byte   masks   new byte 4                    int j 0                  int i 0                  for i rMaskIndex i lt  rMaskIndex 4  i                         masks j    b i                       j                                       rDataStart   rMaskIndex   4                   int messLen   len - rDataStart                   byte   message   new byte messLen                    for i rDataStart  j 0  i lt len  i    j                         message j     byte   b i    masks j   4                                       parseMessage new String message                       parseMessage new String b                     b   new byte buffLenth                              Writing   Server to Client  public void brodcast String mess  throws IOException      byte   rawData   mess getBytes         int frameCount    0      byte   frame   new byte 10        frame 0     byte  129       if rawData length  lt   125           frame 1     byte  rawData length          frameCount   2       else if rawData length  gt   126  amp  amp  rawData length  lt   65535           frame 1     byte  126          int len   rawData length          frame 2     byte   len  gt  gt  8    amp   byte 255           frame 3     byte  len  amp   byte 255            frameCount   4       else          frame 1     byte  127          int len   rawData length          frame 2     byte   len  gt  gt  56    amp   byte 255           frame 3     byte   len  gt  gt  48    amp   byte 255           frame 4     byte   len  gt  gt  40    amp   byte 255           frame 5     byte   len  gt  gt  32    amp   byte 255           frame 6     byte   len  gt  gt  24    amp   byte 255           frame 7     byte   len  gt  gt  16    amp   byte 255           frame 8     byte   len  gt  gt  8    amp   byte 255           frame 9     byte  len  amp   byte 255           frameCount   10             int bLength   frameCount   rawData length       byte   reply   new byte bLength        int bLim   0      for int i 0  i lt frameCount i             reply bLim    frame i           bLim              for int i 0  i lt rawData length i             reply bLim    rawData i           bLim               out write reply       out flush

User · Answer

PHP Implementation   function encode  message         length   strlen  message         bytesHeader            bytesHeader 0    129     0x1 text frame  FIN   opcode       if   length  lt   125                 bytesHeader 1     length        else if   length  gt   126  amp  amp   length  lt   65535                 bytesHeader 1    126               bytesHeader 2       length  gt  gt  8    amp  255               bytesHeader 3       length         amp  255        else                bytesHeader 1    127               bytesHeader 2       length  gt  gt  56    amp  255               bytesHeader 3       length  gt  gt  48    amp  255               bytesHeader 4       length  gt  gt  40    amp  255               bytesHeader 5       length  gt  gt  32    amp  255               bytesHeader 6       length  gt  gt  24    amp  255               bytesHeader 7       length  gt  gt  16    amp  255               bytesHeader 8       length  gt  gt   8    amp  255               bytesHeader 9       length          amp  255              str   implode array map  chr    bytesHeader      message       return  str

User · Answer

pimvdb s answer implemented in python    def DecodedCharArrayFromByteStreamIn stringStreamIn        turn string values into opererable numeric byte values     byteArray    ord character  for character in stringStreamIn      datalength   byteArray 1   amp  127     indexFirstMask   2      if datalength    126          indexFirstMask   4     elif datalength    127          indexFirstMask   10     masks    m for m in byteArray indexFirstMask   indexFirstMask 4       indexFirstDataByte   indexFirstMask   4     decodedChars          i   indexFirstDataByte     j   0     while i  lt  len byteArray           decodedChars append  chr byteArray i    masks j   4             i    1         j    1     return decodedChars   An Example of usage   fromclient     x81 x8c xff xb8 xbd xbd xb7 xdd xd1 xd1 x90 x98 xea xd2 x8d xd4 xd9 x9c    this looks like     O      g           o    in unicode  received by server print DecodedCharArrayFromByteStreamIn fromclient      H    e    l    l    o         W    o    r    l    d

User · Answer

In addition to the PHP frame encoding function  here follows a decode function    function Decode  M        M   array map  ord   str split  M         L    M 1  AND 127       if   L    126           iFM   4      else if   L    127           iFM   10      else          iFM   2        Masks   array slice  M   iFM  4         Out           for   i    iFM   4   j   0   i  lt  count  M    i     j                Out    chr  M  i     Masks  j   4              return  Out      I ve implemented this and also other functions in an easy-to-use WebSocket PHP class here

[encoding] How can I send and receive WebSocket messages on the server side?

Examples related to encoding

Examples related to protocols

Examples related to websocket

Examples related to decoding