Python socket receive - incoming packets always have a different size

Question

I m using the SocketServer module for a TCP server  I m experiencing some issue here with the recv   function  because the incoming packets always have a different size  so if I specify recv 1024   I tried with a bigger value  and smaller   it gets stuck after 2 or 3 requests because the packet length will be smaller  I think   and then the server gets stuck until a timeout   class Test SocketServer BaseRequestHandler    def handle self       print  From    self client address     while True            data   self request recv 1024       if not data  break       if data 4       x20                        self request sendall  hello        if data 4       x21          self request sendall  bye        else         print  unknow packet     self request close      print  Disconnected   self client address  launch   SocketServer ThreadingTCPServer      int sys argv 1    Test   launch allow reuse address  True   launch serve forever     If the client sends multiples requests over the same source port  but the server gets stuck  any help would be very appreciated  thank

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That s the nature of TCP  the protocol fills up packets  lower layer being IP packets  and sends them  You can have some degree of control over the MTU  Maximum Transfer Unit    In other words  you must devise a protocol that rides on top of TCP where your  payload delineation  is defined   By  payload delineation  I mean the way you extract the unit of message your protocol supports   This can be as simple as  every NULL terminated strings

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The answer by Larry Hastings has some great general advice about sockets  but there are a couple of mistakes as it pertains to how the recv bufsize  method works in the Python socket module   So  to clarify  since this may be confusing to others looking to this for help    The bufsize param for the recv bufsize  method is not optional   You ll get an error if you call recv    without the param   The bufferlen in recv bufsize  is a maximum size   The recv will happily return fewer bytes if there are fewer available    See the documentation for details   Now  if you re receiving data from a client and want to know when you ve received all of the data  you re probably going to have to add it to your protocol -- as Larry suggests   See this recipe for strategies for determining end of message   As that recipe points out  for some protocols  the client will simply disconnect when it s done sending data   In those cases  your while True loop should work fine   If the client does not disconnect  you ll need to figure out some way to signal your content length  delimit your messages  or implement a timeout   I d be happy to try to help further if you could post your exact client code and a description of your test protocol

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I know this is old  but I hope this helps someone    Using regular python sockets I found that you can send and receive information in packets using sendto and recvfrom    tcp echo server py import socket  ADDRESS      PORT   54321  connections      host   socket socket socket AF INET  socket SOCK STREAM  host setblocking 0  host bind  ADDRESS  PORT   host listen 10     10 is how many clients it accepts  def close socket connection       try          connection shutdown socket SHUT RDWR      except          pass     try          connection close       except          pass  def read        for i in reversed range len connections             try              data  sender   connections i  0  recvfrom 1500              return data         except  BlockingIOError  socket timeout  OSError               pass         except  ConnectionResetError  ConnectionAbortedError               close socket connections i  0               connections pop i      return b      return empty if no data found  def write data       for i in reversed range len connections             try              connections i  0  sendto data  connections i  1           except  BlockingIOError  socket timeout  OSError               pass         except  ConnectionResetError  ConnectionAbortedError               close socket connections i  0               connections pop i     Run the main loop while True      try          con  addr   host accept           connections append  con  addr       except BlockingIOError          pass      data   read       if data    b            print data          write b ECHO      data          if data    b exit               break    Close the sockets for i in reversed range len connections         close socket connections i  0       connections pop i  close socket host    The client is similar    tcp client py import socket  ADDRESS    localhost  PORT   54321  s   socket socket socket AF INET  socket SOCK STREAM  s connect  ADDRESS  PORT   s setblocking 0   def close socket connection       try          connection shutdown socket SHUT RDWR      except          pass     try          connection close       except          pass  def read           Read data and return the read bytes         try          data  sender   s recvfrom 1500          return data     except  BlockingIOError  socket timeout  AttributeError  OSError           return b       except  ConnectionResetError  ConnectionAbortedError  AttributeError           close socket s          return b    def write data       try          s sendto data   ADDRESS  PORT       except  ConnectionResetError  ConnectionAbortedError           close socket s   while True      msg   input  Enter a message         write msg encode  utf-8         data   read       if data    b            print  Message Received    data       if msg     exit           break  close socket s

User · Answer

You could try always sending the first 4 bytes of your data as data size and then read complete data in one shot  Use the below functions on both client and server-side to send and receive data  def send data conn  data       serialized data   pickle dumps data      conn sendall struct pack   gt I   len serialized data        conn sendall serialized data    def receive data conn       data size   struct unpack   gt I   conn recv 4   0      received payload   b quot  quot      reamining payload size   data size     while reamining payload size    0          received payload    conn recv reamining payload size          reamining payload size   data size - len received payload      data   pickle loads received payload       return data  you could find sample program at https   github com vijendra1125 Python-Socket-Programming git

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You can alternatively use recv x bytes  socket MSG WAITALL   which seems to work only on Unix  and will return exactly x bytes

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Note that exact reason why your code is frozen is not because you set too high request recv   buffer size  Here is explained What means buffer size in socket recv buffer size   This code will work until it ll receive an empty TCP message  if you d print this empty message  it d show b      while True        data   self request recv 1024    if not data  break   And note  that there is no way to send empty TCP message  socket send b    simply won t work    Why  Because empty message is sent only when you type socket close    so your script will loop as long as you won t close your connection   As Hans L pointed out here are some good methods to end message

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The network is always unpredictable   TCP makes a lot of this random behavior go away for you   One wonderful thing TCP does  it guarantees that the bytes will arrive in the same order   But   It does not guarantee that they will arrive chopped up in the same way   You simply cannot assume that every send   from one end of the connection will result in exactly one recv   on the far end with exactly the same number of bytes     When you say socket recv x   you re saying  don t return until you ve read x bytes from the socket    This is called  blocking I O   you will block  wait  until your request has been filled   If every message in your protocol was exactly 1024 bytes  calling socket recv 1024  would work great   But it sounds like that s not true   If your messages are a fixed number of bytes  just pass that number in to socket recv   and you re done   But what if your messages can be of different lengths   The first thing you need to do  stop calling socket recv   with an explicit number   Changing this   data   self request recv 1024    to this   data   self request recv     means recv   will always return whenever it gets new data   But now you have a new problem  how do you know when the sender has sent you a complete message   The answer is  you don t   You re going to have to make the length of the message an explicit part of your protocol   Here s the best way  prefix every message with a length  either as a fixed-size integer  converted to network byte order using socket ntohs   or socket ntohl   please   or as a string followed by some delimiter  like  123      This second approach often less efficient  but it s easier in Python   Once you ve added that to your protocol  you need to change your code to handle recv   returning arbitrary amounts of data at any time   Here s an example of how to do this   I tried writing it as pseudo-code  or with comments to tell you what to do  but it wasn t very clear   So I ve written it explicitly using the length prefix as a string of digits terminated by a colon   Here you go   length   None buffer      while True    data    self request recv     if not data      break   buffer    data   while True      if length is None        if     not in buffer          break         remove the length bytes from the front of buffer         leave any remaining bytes in the buffer        length str  ignored  buffer   buffer partition            length   int length str       if len buffer   lt  length        break       split off the full message from the remaining bytes       leave any remaining bytes in the buffer      message   buffer  length      buffer   buffer length       length   None       PROCESS MESSAGE HERE

[python] Python socket receive - incoming packets always have a different size

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