'*********************************
'  Very basic IP implementation  '
'   by john.timmins@link.co.nz   '
' This file is IP_ICMP_Reply.BAS '
'  Please let me know the bugs!  '
'*********************************
'
'
'ICMP / PING reply
'Responds well to 32-byte pings
'Or other even byte datagrams in the 20-38 byte range.
'Needs more work to see why larger datagrams don't get a successful reply
'
Icmp_reply:
'Copt RX frame to TX frame as they are very similar.
For Temp16 = 0 To Frame_length
    'This gets an extra word, but that's OK. See ICMP checksum calculation below.
   Value16 = Rx_packet(temp16)
   Tx_packet(temp16) = Value16
Next Temp16
'Modify the Ethernet header for return frame
'Get source MAC
Srcmac1 = Tx_packet(offset_frame_src_mac_1)
Srcmac2 = Tx_packet(offset_frame_src_mac_2)
Srcmac3 = Tx_packet(offset_frame_src_mac_3)
'Copy DEST MAC to SRC MAC
Temp16 = Tx_packet(offset_frame_dst_mac_1)
Tx_packet(offset_frame_src_mac_1) = Temp16
Temp16 = Tx_packet(offset_frame_dst_mac_2)
Tx_packet(offset_frame_src_mac_2) = Temp16
Temp16 = Tx_packet(offset_frame_dst_mac_3)
Tx_packet(offset_frame_src_mac_3) = Temp16
'Copy SRC MAC to DEST MAC
Tx_packet(offset_frame_dst_mac_1) = Srcmac1
Tx_packet(offset_frame_dst_mac_2) = Srcmac2
Tx_packet(offset_frame_dst_mac_3) = Srcmac3
'Now set up the IP frame
'Save source IP Address
Srcip1 = Tx_packet(offset_ip_srcadd_1)
Srcip2 = Tx_packet(offset_ip_srcadd_2)
'Copy dest IP to source IP
Temp16 = Tx_packet(offset_ip_dstadd_1)
Tx_packet(offset_ip_srcadd_1) = Temp16
Temp16 = Tx_packet(offset_ip_dstadd_2)
Tx_packet(offset_ip_srcadd_2) = Temp16
'Copy source IP to dest IP
Tx_packet(offset_ip_dstadd_1) = Srcip1
Tx_packet(offset_ip_dstadd_2) = Srcip2
'Now do the ICMP (data) area of IP frame
'Change Type to '0' (reply)
Value16 = Tx_packet(offset_icmp_typecode)                          'Get TYPE/CODE word
Value16 = Value16 And &B0000000011111111                           'Change ICMP type to '0' (reply)
Tx_packet(offset_icmp_typecode) = Value16
'Now calculate new ICMP checksum
Tx_packet(offset_icmp_checksum) = 0                                'Make checksum Zero (for including in calculation)
Ip_data_length = Tx_packet(offset_ip_len)                          'Get IP data length (in bytes)
Ip_data_length = Ip_data_length / 2                                'Adjust for Word transfers
Ip_data_length = Ip_data_length + Offset_icmp_typecode             'Point to start of IP data
Checksum = 0                                                       'Reset checksum accumulator...
For Word_count = Offset_icmp_typecode To Ip_data_length            'Loop through IP Data
    Value16 = Tx_packet(word_count)                                'Get data word
'    Value16 = Not Value16                                          '1's Complement the data
    Checksum = Checksum + Value16                                  'Add to checksum
Next Word_count
'Now add the carry-over from the 16-bit addition
Checksum16 = Loww(checksum)                                        'Get sum
Temp16 = Highw(checksum)                                           'Get carry's
Checksum16 = Checksum16 + Temp16                                   'Add carry's
Checksum16 = Not Checksum16                                        '1's Complement the Checksum
Tx_packet(offset_icmp_checksum) = Checksum16
Value16 = Frame_length                                             'Reply is the same length as request
Transmit_frame                                                     'Send it!
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