header.py

'''
    #Utility functions: 1) to create a packet of 1472 bytes with header (12 bytes) (sequence number, acknowledgement number,
    #flags and receiver window) and applicaton data (1460 bytes), and 2) to parse
    # the extracted header from the application data. 

'''

from struct import *
import struct


# I integer (unsigned long) = 4bytes and H (unsigned short integer 2 bytes)
# see the struct official page for more info

header_format = '!IIHH'

#print the header size: total = 12



def create_packet(seq, ack, flags, win, data):
    #creates a packet with header information and application data
    #the input arguments are sequence number, acknowledgment number
    #flags (we only use 4 bits),  receiver window and application data 
    #struct.pack returns a bytes object containing the header values
    #packed according to the header_format !IIHH
    header = pack (header_format, seq, ack, flags, win)

    #once we create a header, we add the application data to create a packet
    #of 1472 bytes
    packet = header + data
    #print (f'packet containing header + data of size {len(packet)}') #just to show the length of the packet
    return packet


def parse_header(header):
    #taks a header of 12 bytes as an argument,
    #unpacks the value based on the specified header_format
    #and return a tuple with the values
    header_from_msg = unpack(header_format, header)
    #parse_flags(flags)
    return header_from_msg

"""def parse_header(data):
    if len(data) < 12:
        raise ValueError("Data is too short to parse the header.")
    header = data[:12]
    header_from_msg = unpack(header_format, header)
    return header_from_msg"""
    
    

def parse_flags(flags):
    #we only parse the first 3 fields because we're not 
    #using rst in our implementation
    syn = flags & (1 << 3)
    ack = flags & (1 << 2)
    fin = flags & (1 << 1)
    return syn, ack, fin



if __name__ == "__main__":
    print (f'size of the header = {calcsize(header_format)}')

    #now let's create a packet with sequence number 1
    print ('\n\ncreating a packet')

    data = b'0' * 1460
    print (f'app data for size ={len(data)}')

    sequence_number = 1
    acknowledgment_number = 0
    window = 0 # window value should always be sent from the receiver-side
    flags = 0 # we are not going to set any flags when we send a data packet

    #msg now holds a packet, including our custom header and data
    msg = create_packet(sequence_number, acknowledgment_number, flags, window, data)

    #now let's look at the header
    #we already know that the header is in the first 12 bytes

    header_from_msg = msg[:12]
    print(len(header_from_msg))

    #now we get the header from the parse_header function
    #which unpacks the values based on the header_format that 
    #we specified
    seq, ack, flags, win = parse_header (header_from_msg)
    print(f'seq={seq}, ack={ack}, flags={flags}, recevier-window={win}')

    #let's extract the data_from_msg that holds
    #the application data of 1460 bytes
    data_from_msg = msg[12:]
    print (len(data_from_msg))


    #let's mimic an acknowledgment packet from the receiver-end
    #now let's create a packet with acknowledgement number 1
    #an acknowledgment packet from the receiver should have no data
    #only the header with acknowledgment number, ack_flag=1, win=6400
    data = b'' 
    print('\n\nCreating an acknowledgment packet:')
    print (f'this is an empty packet with no data ={len(data)}')

    sequence_number = 0
    acknowledgment_number = 1   #an ack for the last sequnce
    window = 0 # window value should always be sent from the receiver-side

    # let's look at the last 4 bits:  S A F R
    # 0 0 0 0 represents no flags
    # 0 1 0 0  ack flag set, and the decimal equivalent is 4
    flags = 4 

    msg = create_packet(sequence_number, acknowledgment_number, flags, window, data)
    print (f'this is an acknowledgment packet of header size={len(msg)}')

    #let's parse the header
    seq, ack, flags, win = parse_header (msg) #it's an ack message with only the header
    print(f'seq={seq}, ack={ack}, flags={flags}, receiver-window={win}')

    #now let's parse the flag field
    syn, ack, fin = parse_flags(flags)
    print (f'syn_flag = {syn}, fin_flag={fin}, and ack_flag={ack}')