Delta checksum approach
This commit is contained in:
Nico Schottelius
parent
e7769abe31
commit
408dd6b782
2 changed files with 117 additions and 2 deletions
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@ -2,6 +2,8 @@
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# Code copied from /usr/lib/python3/dist-packages/scapy/utils.py
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import struct
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import array
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if struct.pack("H",1) == b"\x00\x01": # big endian
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def checksum(pkt):
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@ -14,10 +16,52 @@ if struct.pack("H",1) == b"\x00\x01": # big endian
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return s & 0xffff
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else:
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def checksum(pkt):
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# padding to even length
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if len(pkt) % 2 == 1:
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pkt += b"\0"
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# create array with unsigned shorts,
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# sum it up -> results in 16 bit uint (?)
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s = sum(array.array("H", pkt))
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# (s & 0xffff) -> mask for 16 bit, truncate rest
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# (s >> 16) right shift by 16 bits
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# >>> 2**17 >> 16
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# 2
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# Basically:
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# add higher 16 bits to lower 16 bits
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s = (s >> 16) + (s & 0xffff)
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# Now we add the remaining possible 16 bits
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# i.e. above we take bits 0..15 + 16..31
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# now we add [0..15 + 16..31] + "32..47"
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# I assume this is for catching the carry
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# over bits from the previous calculation
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s += s >> 16
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# according to https://stackoverflow.com/questions/8305199/the-tilde-operator-in-python
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# ~ is the bitwise complement operator in python
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# which essentially calculates -x - 1
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# 2 complement
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s = ~s
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# (s>>8)&0xff) -> filter bits 8..15
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# | = bitwise OR
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# s<<8 : left shift everything in original s
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# | s<<8: ?
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# & 0xffff: filter / allow only 16 bits
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# Seems like the idea is to filter correctly for
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# 16 bit - unclear why "s & 0xffff" does not do the job
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return (((s>>8)&0xff)|s<<8) & 0xffff
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if __name__ == '__main__':
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import sys
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pkt = sys.argv[1]
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pkt_bytes = pkt.encode('utf-8')
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print(pkt_bytes)
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print(checksum(pkt_bytes))
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75
doc/plan.org
75
doc/plan.org
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@ -2946,6 +2946,62 @@ SimpleSumeSwitch(
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TopDeparser()
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) main;
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**** Understand the different switch models (?)
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*** TODO Find out whether delta based checksumming is feasible [in P4]
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**** DONE Analysing scapy
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CLOSED: [2019-06-23 Sun 14:18]
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#+BEGIN_CENTER
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[13:51] line:~% dpkg -L python3-scapy | grep inet
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/usr/lib/python3/dist-packages/scapy/layers/inet.py
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/usr/lib/python3/dist-packages/scapy/layers/inet6.py
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if self.chksum is None:
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ck = checksum(p)
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p = p[:10]+chb(ck>>8)+chb(ck&0xff)+p[12:]
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return p+pay
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[13:55] line:~% grep checksum -r /usr/lib/python3/dist-packages/scapy/ | grep def
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/usr/lib/python3/dist-packages/scapy/contrib/isis.py: def checksum_info(self, hdrlen):
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/usr/lib/python3/dist-packages/scapy/contrib/isis.py: def checksum_info(self, hdrlen):
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/usr/lib/python3/dist-packages/scapy/contrib/ospf.py:def ospf_lsa_checksum(lsa):
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/usr/lib/python3/dist-packages/scapy/utils.py: def checksum(pkt):
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/usr/lib/python3/dist-packages/scapy/utils.py: def checksum(pkt):
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/usr/lib/python3/dist-packages/scapy/utils.py:def fletcher16_checksum(binbuf):
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/usr/lib/python3/dist-packages/scapy/layers/sctp.py:def sctp_checksum(buf):
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[13:55] line:~%
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if struct.pack("H",1) == b"\x00\x01": # big endian
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def checksum(pkt):
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if len(pkt) % 2 == 1:
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pkt += b"\0"
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s = sum(array.array("H", pkt))
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s = (s >> 16) + (s & 0xffff)
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s += s >> 16
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s = ~s
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return s & 0xffff
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else:
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def checksum(pkt):
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if len(pkt) % 2 == 1:
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pkt += b"\0"
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s = sum(array.array("H", pkt))
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s = (s >> 16) + (s & 0xffff)
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s += s >> 16
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s = ~s
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return (((s>>8)&0xff)|s<<8) & 0xffff
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#+END_CENTER
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**** TODO Trying to create a delta diff: AAAA vs. BBAA
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#+BEGIN_CENTER
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[14:17] line:bin% python3 checksum_from_scapy.py AAAA
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b'AAAA'
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32125
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[14:18] line:bin% python3 checksum_from_scapy.py BBAA
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b'BBAA'
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31868
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#+END_CENTER
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*** TODO Get ANY p4 program to successfully run on netpfga
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**** TODO mirroring ethernet
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***** no packets seen on source interface
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@ -3061,6 +3117,9 @@ ERROR: failed to convert p.ethernet.dstAddr of type <type 'str'> to an integer
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** Additional features queue (to be discussed)
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*** TODO Offset based translation (v4->v6) -> same as range (?)
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*** TODO IP address learning (v6/v4) for real life switch? How do hosts find it?
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** Netpfga cabling
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| eth2 <--> nf0 |
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| eth1 <--> nf3 |
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* Thesis documentation
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** Introduction
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*** Related work
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@ -4685,9 +4744,21 @@ SerialException: device reports readiness to read but returned no data (device d
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"The checksum field is the 16 bit one's complement of the one's
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complement sum of all 16-bit words in the header and text."
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https://en.wikipedia.org/wiki/Transmission_Control_Protocol#Checksum_computation
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*** 2019-06-21: create table entries on new card
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#+BEGIN_CENTER
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>> table_cam_add_entry lookup_table send_to_port1 ff:ff:ff:ff:ff:ff =>
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CAM_Init_ValidateContext() - done
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WROTE 0x44020050 = 0xffffffff
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WROTE 0x44020054 = 0xffff
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WROTE 0x44020080 = 0x0003
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READ 0x44020044 = 0x0001
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WROTE 0x44020040 = 0x0001
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READ 0x44020044 = 0x0001
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READ 0x44020044 = 0x0001
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success
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>>
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#+END_CENTER
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** References / Follow up
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*** RFC 791 IPv4 https://tools.ietf.org/html/rfc791
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*** RFC 792 ICMP https://tools.ietf.org/html/rfc792
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