Generating a UDP message in python with a header and payload in python3-CodePudding

I am new to Networking and trying to implement a network calculator using python3 where the client's responsibility is to send operands and operators and the server will calculate the result and send it back to the client. Communication is through UDP messages and I am working on client side. Each message is comprised of a header and a payload and they are described as shown in the below figures.

UDP header:

UDP payload:

I am familiar with sending string messages using sockets but having a hard-time with how to make a message with both header and payload and how to assign the bits for various attributes or how to generate message/client id's in the header and If there is any way to automatically generate the Id's. Any help or suggestions will be highly appreciated.

Thanks in advance

CodePudding user response：

I will only do a portion of your homework.
I hope it will help you to find energy to work on missing parts.

import struct
import socket

CPROTO_ECODE_REQUEST, CPROTO_ECODE_SUCCESS, CPROTO_ECODE_FAIL = (0,1,2)

ver = 1  # version of protocol
mid = 0  # initial value
cid = 99 # client Id (arbitrary)

sock = socket.socket( ...) # to be customized

def sendRecv( num1, op, num2):
  global mid
  ocs = (" ", "-", "*", "/").index( op)
  byte0 = ver   (ocs << 3)   (CPROTO_ECODE_REQUEST << 6)
  hdr = struct.pack( "!BBH", byte0, mid, cid)
  parts1 = (b'0000'   num1.encode()   b'0000').split(b'.')
  parts2 = (b'0000'   num2.encode()   b'0000').split(b'.')
  msg = hdr   parts1[0][-4:]   parts1[1][:4]   parts2[0][-4:]   parts2[1][:4]
  
  socket.send( msg)         # send request
  bufr = socket.recv( 512)  # get answer
  # to do:
  #   complete socket_send and socket.recv
  #   unpack bufr into: verr,ecr,opr,value_i, value_f
  #   verify that verr, ecr, opr, are appropriate
  #   combine value_i and value_f into answer
  mid  = 1
  return answer

result = sendRecv( '2.47', ' ', '46.234')

There are many elements that haven't be specified by your teacher:

what should be the byte-ordering on the network (bigEndian or littleEndian)? The above example suppose it's bigEndian but you can easily modify the 'pack' statement to use littleEndian.
What should the program do if the received packet header is invalid?
What should the program do if there's no answer from server?
Payload: how should we interpret "4 most significant digits of fraction"? Does that mean that the value is in ASCII? That's not specified.
Payload: assuming the fraction is in ASCII, should it be right-justified or left-justified in the packet?
Payload: same question for integer portion.
Payload: if the values are in binary, are they signed or unsigned. It will have an affect on the unpacking statement.

In the program above, I assumed that:

values are positive and in ASCII (without sign)
integer portion is right-justified
fractional portion is left justified

Have fun!