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>
2008-08-15 Paul Jakma <paul.jakma@sun.com>
* {ospf6d,ripngd}/*: Finish job of marking functions as static, or
exporting declarations for them, to quell warning noise with
Quagga's GCC default high-level of warning flags. Thus allowing
remaining, more useful warnings to be more easily seen.
* (global): Fix up list loops to match changes in lib/linklist,
and some basic auditing of usage.
* configure.ac: define QUAGGA_NO_DEPRECATED_INTERFACES
* HACKING: Add notes about deprecating interfaces and commands.
* lib/linklist.h: Add usage comments.
Rename getdata macro to listgetdata.
Rename nextnode to listnextnode and fix its odd behaviour to be
less dangerous.
Make listgetdata macro assert node is not null, NULL list entries
should be bug condition.
ALL_LIST_ELEMENTS, new macro, forward-referencing macro for use
with for loop, Suggested by Jim Carlson of Sun.
Add ALL_LIST_ELEMENTS_RO for cases which obviously do not need the
"safety" of previous macro.
LISTNODE_ADD and DELETE macros renamed to ATTACH, DETACH, to
distinguish from the similarly named functions, and reflect their
effect better.
Add a QUAGGA_NO_DEPRECATED_INTERFACES define guarded section
with the old defines which were modified above,
for backwards compatibility - guarded to prevent Quagga using it..
* lib/linklist.c: fix up for linklist.h changes.
* ospf6d/ospf6_abr.c: (ospf6_abr_examin_brouter) change to a single
scan of the area list, rather than scanning all areas first for
INTER_ROUTER and then again for INTER_NETWORK. According to
16.2, the scan should be area specific anyway, and further
ospf6d does not seem to implement 16.3 anyway.
* ospf6_flood.c: When adding a thread to retransmit an lsa after
rxmt_interval, the ospf6d used thread_add_event(). However,
thread_add_event() executes immediately and ignores the delay parameter.
This was causing unnecesary unicast LSU sends. Adding the thread has
been changed to use thread_add_timer() which will expire after delay.
After the fix results appear consistent with expectation.
