Re: [dtn] DTN BPv7 test bundles and CRCs

[Brian Sipos <[e-mail]>]

All,
Thank you for the pointers to other implementations. I was able to reconcile the CRCs listed in the BPv7 spec with the 16-bit CRC listed at [1] and the 32-bit CRC listed at [2], both of these references have "check" results for test input.

To Scott B.,
I think the current citation in [3] is not accurate, the ITU-T V.42 spec is *not* the same as the CRC-32C identified in BPv7. I would suggest citing the same reference as was done in [4] and it would be extremely helpful to have CRC examples as are included in [5].
I was being very careful to cross-check my CRC use, but a naive implementation which only checks bundles generated by itself could easily use the wrong CRC parameterization and end up not being interoperable and not know it.

Thanks,
Brian S.

[1]: http://reveng.sourceforge.net/crc-catalogue/16.htm#crc.cat.crc-16-ibm-sdlc
[2]: http://reveng.sourceforge.net/crc-catalogue/17plus.htm#crc.cat.crc-32-castagnoli
[3]: https://tools.ietf.org/html/draft-ietf-dtn-bpbis-12#ref-CRC
[4]: https://tools.ietf.org/html/rfc7143#ref-Castagnoli93
[5]: https://tools.ietf.org/html/rfc7143#appendix-A.4<https://tools.ietf.org/html/rfc7143>


________________________________
From: dtn <[e-mail]> on behalf of Brian Sipos <[e-mail]>
Sent: Friday, March 22, 2019 11:45
To: [e-mail]
Subject: [dtn] DTN BPv7 test bundles and CRCs

To anyone who has been prototyping Bundle Protocol version 7 (BPbis) agents,
I'm spending a little time on a Wireshark dissector to examine and troubleshoot bundles and the only trouble I've run into is in verifying the CRCs. The BPbis spec is not very helpful in determining exactly what CRC implementation to use only because the ITU specification itself doesn't provide CRC test vectors.

I'm doing prototyping in Python and dissecting in C language, so there is even a disparity between what CRC algorithms are available and how well they are documented. This seems like such an easy use/build decision to use COTS CRC function but I'm struggling to find implementations to match what BPbis requires.

I think there would be a lot of value to have some test/representative bundles in an appendix which include blocks with different CRC types and different block sizes. Some CRC implementations pad out octet-sequences to a multiple of the CRC size and some apparently do something else. The problem is in the CRC implementations not being very clear about their parameters but having a few encoded test bundles to work with would go a long way to ensuring compatibility of implementations.

Thanks for any advice,
Brian S.