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Change order of complement & filtering

master
Nico Schottelius 3 years ago
parent
commit
26c27cefa8
  1. 4
      bin/checksum_delta_diff_test.py
  2. 10
      doc/plan.org
  3. 24
      p4src/actions_delta_checksum.p4

4
bin/checksum_delta_diff_test.py

@ -13,16 +13,20 @@ def checksum_scapy(pkt):
if len(pkt) % 2 == 1:
pkt += b"\0"
# P4: ABOVE not needed (always even)
# array: create an array of 16 bit values from the input
# and then sum it up -> this might be sligthly/much higher
# than 16 bit (additions!)
s = sum(array.array("H", pkt))
# P4: summing manually into 32bit
# add the (right shift 16) and the 16 right bits
# basically: assuming 32 bit (?): add the two 16 bit "words"
# together
# This might still exceed 16 bit!
s = (s >> 16) + (s & 0xffff)
# P4:
# right shift 16 -> zero least significant bits,
# and add the upper bits to it

10
doc/plan.org

@ -5245,6 +5245,16 @@ likely this is the cause for the main error:
**** go through all steps again and try to understand why it (silently) fails later
*** 2019-07-13: fix overflow error
*** 2019-07-15: fix off-by-one-sometimes
- scapy shift code / return does not work
**** scapy code results
Using meta.v6sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff)
;
we get on the wire:
sum 0x324a (incorrect -> 0x5429),
** The NetPFGA saga
Problems encountered:
- The logfile for a compile run is 10k+ lines

24
p4src/actions_delta_checksum.p4

@ -13,13 +13,12 @@ action v4sum() {
tmp = tmp + (bit<32>) hdr.ipv4.totalLen -20; // 16 bit
tmp = tmp + (bit<32>) hdr.ipv4.protocol; // 8 bit
carryover = tmp >> 16; /* maximum is 6*(2**16) >> 16 == 6 */
tmp = (tmp & 0xffff) + carryover; /* Now tmp contains at maximum 65541 */
carryover = tmp >> 16; /* Now carryover contains at maximum 1 */
tmp = tmp + carryover; /* No overrun possible anymore */
/* filtering code copied from scapy */
meta.v4sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff) ;
tmp = (tmp >> 16) + (tmp & 0xffff);
tmp = tmp + (tmp >> 16);
tmp = ~tmp;
meta.v4sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff);
}
action v6sum() {
@ -47,15 +46,14 @@ action v6sum() {
tmp = tmp + (bit<32>) hdr.ipv6.payload_length; // 16 bit
tmp = tmp + (bit<32>) hdr.ipv6.next_header; // 8 bit
carryover = tmp >> 16; /* maximum is 18*(2**16) >> 16 == 18 */
tmp = (tmp & 0xffff) + carryover; /* Now tmp contains at maximum 65554*/
carryover = tmp >> 16; /* Now carryover contains at maximum 1 */
tmp = tmp + carryover; /* No overrun possible anymore */
/* filtering code copied from scapy */
tmp = (tmp >> 16) + (tmp & 0xffff);
tmp = tmp + (tmp >> 16);
tmp = ~tmp;
meta.v6sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff) ;
}
action delta_prepare()
{
v4sum();
@ -70,13 +68,13 @@ action delta_prepare()
action delta_udp_from_v4_to_v6()
{
delta_prepare();
hdr.udp.checksum = hdr.udp.checksum + ~meta.headerdiff;
hdr.udp.checksum = hdr.udp.checksum + meta.headerdiff;
}
action delta_tcp_from_v4_to_v6()
{
delta_prepare();
hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff;
hdr.tcp.checksum = hdr.tcp.checksum + meta.headerdiff;
}
action delta_ipv4_from_v6_to_v4()

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