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166 lines
8.2 KiB
Python
166 lines
8.2 KiB
Python
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# Calculation for UDP checksum
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#
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# This module calculates the UDP checksum in case of IPv6 usage.
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#
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from helpers import showAsHex
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# A valid ethernet frame, containing IPv6 and UDP, to verify the checksum algorithm.
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testethernetframe = [
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0x33 , 0x33 , 0x00 , 0x00 , 0x00 , 0x01,
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0x04 , 0x65 , 0x65 , 0x00 , 0x64 , 0xc3 ,
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0x86 , 0xdd ,
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0x60 , 0x00
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, 0x00 , 0x00 , 0x00 , 0x12 , 0x11 , 0x0a , 0xfe , 0x80 , 0x00 , 0x00 , 0x00 , 0x00 , 0x00 , 0x00 , 0x06 , 0x65
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, 0x65 , 0xff , 0xfe , 0x00 , 0x64 , 0xc3 , 0xff , 0x02 , 0x00 , 0x00 , 0x00 , 0x00 , 0x00 , 0x00 , 0x00 , 0x00
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, 0x00 , 0x00 , 0x00 , 0x00 , 0x00 , 0x01 , 0xcc , 0xaf , 0x3b , 0x0e , 0x00 , 0x12 , 0x89 , 0x5e , 0x01 , 0xfe
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, 0x90 , 0x00 , 0x00 , 0x00 , 0x00 , 0x02 , 0x10 , 0x00 ]
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def calculateUdpChecksumForIPv6(udpframe, ipv6source, ipv6dest):
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# Parameters:
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# udpframe: the udp frame including udp header and udppayload
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# ipv6source: the 16 byte IPv6 source address. Must be the same, which is used later for the transmission.
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# ipv6source: the 16 byte IPv6 destination address. Must be the same, which is used later for the transmission.
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udpframe[6] = 0 # at the beginning, set the checksum in the udp header to 00 00.
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udpframe[7] = 0
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# construct an array, consisting of a 40-byte-pseudo-ipv6-header, and the udp frame (consisting of udp header and udppayload)
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bufferlen = 40+len(udpframe)
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if ((bufferlen & 1)!=0):
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# if we have an odd buffer length, we need to add a padding byte in the end, because the sum calculation
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# will need 16-bit-aligned data.
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bufferlen+=1
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buffer = bytearray(bufferlen)
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for i in range(0, len(buffer)):
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buffer[i] = 0 # everything 0 for clean initialization
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# fill the pseudo-ipv6-header
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for i in range(0, 16): # copy 16 bytes IPv6 addresses
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buffer[i] = ipv6source[i] # IPv6 source address
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buffer[16+i] = ipv6dest[i] # IPv6 destination address
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udplen = len(udpframe)
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nxt = 0x11 # should be 0x11 in case of udp
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buffer[32] = 0 # high byte of the FOUR byte length is always 0
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buffer[33] = 0 # 2nd byte of the FOUR byte length is always 0
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buffer[34] = udplen >> 8 # 3rd
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buffer[35] = udplen & 0xFF # low byte of the FOUR byte length
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buffer[36] = 0 # 3 padding bytes with 0x00
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buffer[37] = 0
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buffer[38] = 0
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buffer[39] = nxt # the nxt is at the end of the pseudo header
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# pseudo-ipv6-header finished. Now lets put the udpframe into the buffer. (Containing udp header and udppayload)
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for i in range(0, len(udpframe)):
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buffer[40+i] = udpframe[i]
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# showAsHex(buffer, "buffer ")
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# buffer is prepared. Run the checksum over the complete buffer.
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totalSum = 0
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for i in range(0, len(buffer)>>1): # running through the complete buffer, in 2-byte-steps
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value16 = buffer[2*i] * 256 + buffer[2*i+1] # take the current 16-bit-word
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totalSum += value16 # we start with a normal addition of the value to the totalSum
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# But we do not want normal addition, we want a 16 bit one's complement sum,
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# see https://en.wikipedia.org/wiki/User_Datagram_Protocol
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if (totalSum>=65536): # On each addition, if a carry-out (17th bit) is produced,
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totalSum-=65536 # swing that 17th carry bit around
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totalSum+=1 # and add it to the least significant bit of the running total.
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# Finally, the sum is then one's complemented to yield the value of the UDP checksum field.
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checksum = totalSum ^ 0xffff
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return checksum
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if __name__ == "__main__":
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print("Testing the udp checksum calculation...")
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showAsHex(testethernetframe, "testethernetframe ")
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ipv6frame = bytearray(len(testethernetframe)-6-6-2) # without the ethernet header (MAC, MAC, ethertype)
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for i in range(0, len(ipv6frame)):
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ipv6frame[i] = testethernetframe[14+i]
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showAsHex(ipv6frame, "ipv6frame ")
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# checksum calculation see https://en.wikipedia.org/wiki/User_Datagram_Protocol
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# We want to calculate the UDP checksum. This needs to include also some data from "lower level" IP header, so we need the complete IP frame, not
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# only the UDP part.
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#
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# From wikipedia.org: Checksum is the 16-bit one's complement
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# of the one's complement sum of a pseudo header of information from
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# * the IP header,
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# * the UDP header,
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# * and the data,
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# padded with zero octets at the end (if necessary) to make a multiple of two octets.
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# The pseudoIPv6 header for checksum calculation has a DIFFERENT format then a normal IPv6 header. So we cannot use the original frame,
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# instead, we copy the relevant information out of it into a dedicated buffer.
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# We have IPv6, so the IP header is
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ipv6pseudoheader = bytearray(40)
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for i in range(0, 16):
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ipv6pseudoheader[i] = ipv6frame[8+i] # IPv6 source address
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ipv6pseudoheader[16+i] = ipv6frame[24+i] # IPv6 destination address
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udplen = ipv6frame[4]*256 + ipv6frame[5] # the real IPv6 header has a two byte length info
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nxt = ipv6frame[6] # the real IPv6 header next-protocol information (should be 0x11 in case of udp)
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print("udplen=" + str(udplen))
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print("nxt=" + str(nxt))
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ipv6pseudoheader[32] = 0 # high byte of the FOUR byte length is always 0
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ipv6pseudoheader[33] = 0 # 2nd byte of the FOUR byte length is always 0
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ipv6pseudoheader[34] = udplen >> 8 # 3rd
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ipv6pseudoheader[35] = udplen & 0xFF # low byte of the FOUR byte length
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ipv6pseudoheader[36] = 0 # 3 padding bytes with 0x00
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ipv6pseudoheader[37] = 0
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ipv6pseudoheader[38] = 0
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ipv6pseudoheader[39] = nxt # the nxt is at the end of the pseudo header
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showAsHex(ipv6pseudoheader, "ipv6pseudoheader ")
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udpHeader = bytearray(8)
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for i in range(0, 8):
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udpHeader[i] = ipv6frame[40+i] # in the real IPv6, we have also 40 byte IPv6 header, and afterwards the 8 byte UDP header
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showAsHex(udpHeader, "udpHeader ")
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udpPayload = bytearray(udplen-8) # payload size is the announced udp size minus udpHeaderSize
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for i in range(0, len(udpPayload)):
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udpPayload[i] = ipv6frame[40+8+i]
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showAsHex(udpPayload, "udpPayload ")
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transmittedChecksum = udpHeader[6] * 256 + udpHeader[7]
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print("The transmitted checksum is " + hex(transmittedChecksum))
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# Checksum algorithm:
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# 1. Set the checksum in the udp header to 00 00
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udpHeader[6] = 0
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udpHeader[7] = 0
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runningTotal = 0 # startvalue zero
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for i in range(0, len(ipv6pseudoheader)>>1):
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value16 = ipv6pseudoheader[2*i] * 256 + ipv6pseudoheader[2*i+1]
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runningTotal += value16
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if (runningTotal>=65536): # On each addition, if a carry-out (17th bit) is produced,
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runningTotal-=65536 # swing that 17th carry bit around
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runningTotal+=1 # and add it to the least significant bit of the running total.
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for i in range(0, len(udpHeader)>>1):
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value16 = udpHeader[2*i] * 256 + udpHeader[2*i+1]
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runningTotal += value16
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if (runningTotal>=65536): # On each addition, if a carry-out (17th bit) is produced,
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runningTotal-=65536 # swing that 17th carry bit around
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runningTotal+=1 # and add it to the least significant bit of the running total.
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for i in range(0, len(udpPayload)>>1):
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value16 = udpPayload[2*i] * 256 + udpPayload[2*i+1]
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runningTotal += value16
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if (runningTotal>=65536): # On each addition, if a carry-out (17th bit) is produced,
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runningTotal-=65536 # swing that 17th carry bit around
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runningTotal+=1 # and add it to the least significant bit of the running total.
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# Finally, the sum is then one's complemented to yield the value of the UDP checksum field.
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checksum = runningTotal ^ 0xffff
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print("calculated checksum=" + hex(checksum))
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# Same test, using the function
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myUdpFrame = bytearray(udplen) # payload size is the announced udp size
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for i in range(0, len(myUdpFrame)):
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myUdpFrame[i] = ipv6frame[40+i]
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myIpv6Source = bytearray(16)
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myIpv6Dest = bytearray(16)
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for i in range(0, 16):
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myIpv6Source[i] = ipv6frame[8+i] # IPv6 source address
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myIpv6Dest[i] = ipv6frame[24+i] # IPv6 destination address
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print("calculated with the function call: " + hex(calculateUdpChecksumForIPv6(myUdpFrame, myIpv6Source, myIpv6Dest))) |