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authorJR Rivers <jrrivers@cumulusnetworks.com>2012-09-13 17:17:36 +0000
committerDavid Lamparter <equinox@opensourcerouting.org>2012-10-25 10:15:58 -0700
commitd8a4e42b7d19a87eacc00c825e913907a58f39ee (patch)
tree2d0777bfbef0e77ca1722fe0e7b2591ef89d4102 /init/.gitignore
parentb0145ddb4bf75d2c8853aa3d0bc7c522f4f17531 (diff)
lib: improve fletcher checksum validation
OVERVIEW The checksum used in OSPF (rfc2328) is specified in rc905 annex B. There is an sample implementation in rfc1008 which forms the basis of the quagga implementation. This algorithm works perfectly when generating a checksum; however, validation is a bit problematic. The following LSA (generated by a non-quagga implementation) is received by quagga and marked with an invalid checksum; however, it passes both the rfc905 and rfc1008 validation checks. static uint8_t lsa_10_121_233_29[] = { 0x0e, 0x10, 0x02, 0x03, 0x09, 0x00, 0x35, 0x40, 0x0a, 0x79, 0xe9, 0x1d, 0x80, 0x00, 0x00, 0x03, 0x00, 0x8a, 0x00, 0x1c, 0xff, 0xff, 0xff, 0xe0, 0x00, 0x00, 0x36, 0xb0 }; LS Type: Summary-LSA (IP network) LS Age: 3600 seconds Do Not Age: False Options: 0x02 (E) Link-State Advertisement Type: Summary-LSA (IP network) (3) Link State ID: 9.0.53.64 Advertising Router: 10.121.233.29 (10.121.233.29) LS Sequence Number: 0x80000003 LS Checksum: 0x008a Length: 28 Netmask: 255.255.255.224 Metric: 14000 You'll note that one byte of the checksum is 0x00; quagga would calculate the checksum as 0xff8a. It can be argued that the sourcing implementation generates an incorrect checksum; however, rfc905 indicates that, for 1's complement arithmetic, the value 255 shall be regarded as 0, thus either values are valid. EXPLANATION The quagga ospfd and ospf6d implementations operate by copying the PDU's existing checksum in a holding variable, calculating the checksum, and comparing the resulting checksum to the original. As a note, this implementation has the side effect of modifying the contents of the PDU. Evaluation of both rfc905 and rfc1008 shows that checksum validation should involve calculating the sum over the PDU and checking that both resulting C0 and C1 values are zero. This behavior is enacted in the rfc1008 implementation by calling encodecc with k = 0 (checksum offset); however, this functionality had been omitted from the quagga implementation. PATCH This patch adds the ability to call the quagga's fletcher_checksum() with a checksum offset value of 0xffff (aka FLETCHER_CHECKSUM_VALIDATE) which returns the sum over the buffer (a value of 0 indicates a valid checksum). This is similar to the mechanism in rfc1008 when called with k = 0. The patch also introduces ospf_lsa_checksum_valid(). ospf6d had it's own implementation of the fletcher checksum in ospf6_lsa_checksum(); it's the same algorithm as in fletcher_checksum(). This patch removes the local implementation in favor of the library's as well as creates and uses ospf6_lsa_checksum_valid(). quagga's ISIS implementation suffers from the same problem; however, I do not have the facilities to validate a fix to ISIS, thus this change has been left to the ISIS maintainers. The function iso_csum_verify() should be reduced to running the fletcher checksum over the buffer using an offset of 0. Signed-off-by: JR Rivers <jrrivers@cumulusnetworks.com> Reviewed-by: Scott Feldman <sfeldma@cumulusnetworks.com> Reviewed-by: Nolan Leake <nolan@cumulusnetworks.com> Reviewed-by: Ayan Banerjee <ayan@cumulusnetworks.com> Reviewed-by: Shrijeet Mukherjee <shm@cumulusnetworks.com> Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
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