Re: [tsvwg] New Version Notification for draft-touch-tsvwg-udp-options-05.txt
Gorry Fairhurst <gorry@erg.abdn.ac.uk> Mon, 27 March 2017 16:02 UTC
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Date: Mon, 27 Mar 2017 11:02:18 -0500
From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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To: Joe Touch <touch@isi.edu>
CC: "C. M. Heard" <heard@pobox.com>, Mark Smith <markzzzsmith@gmail.com>, tsvwg <tsvwg@ietf.org>
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Subject: Re: [tsvwg] New Version Notification for draft-touch-tsvwg-udp-options-05.txt
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I'd think CRC-16 is likely to be a good proposal. 32b alignment is always good. Gorry On 27/03/2017, 10:37, Joe Touch wrote: > So do we want to recommend a 16-bit value or jump to the 32-bit one? > > IMO, for UDP, anything reasonably stronger than the Internet checksum > should be fine. I was hoping for a 16-bit selection to leave the result > 32-bit aligned... > > Joe > > > On 3/25/2017 7:37 PM, C. M. Heard wrote: >> On Sat, Mar 25, 2017 at 3:06 PM, Joe Touch<touch@isi.edu> wrote: >>> On 3/25/2017 6:26 AM, C. M. Heard wrote: >>>> On Wed, Mar 22, 2017 at 10:43 PM, Joe Touch<touch@isi.edu> wrote: >>>>>> In section 5.4, was a decision made as to what the CRC16 is? Details >>>>>> will be needed in order to ensure interoperability. >>>>> That's on my to-do list (I was a bit distracted by these other issues). >>>>> There are three obvious possibilities: >>>>> >>>>> CRC-16-CCITT used by Bluetooth, X.25, HDLC (4 terms - 0x1021) >>>>> CRC-16-IBM used by USB (4 terms -- 0x8005) >>>>> CRC-16-CDMA2000 used by CDMA mobile nets (8 terms - 0xC867) >>>>> >>>>> There are other analyses that point to other polynomials: >>>>> https://users.ece.cmu.edu/~koopman/crc/ >>>>> >>>>> Any suggestions? >>>> Both the CRC-16-CCITT and CRC-16-IBM polynomials factor into the product >>>> of x+1 times a primitive polynomial of degree 15 (*op in Koopman's notation) >>>> and are in a sense optimal for random error patterns. They detect all triple >>>> errors (and all error patterns of odd weight) for data lengths of 4093 bytes >>>> or less. The CRC-16-CDMA2000 has a single factor, which is a primitive >>>> degree 16 polynomial (*p in Koopman's notation), and it will detect all >>>> double errors for data lengths of 8189 bytes or less. By data length I >>>> mean of course the length of the data protected by the CRC (not >>>> including the CRC itself). >>>> >>>> There are generic fast table lookup algorithms for all CRC-16 polynomials, >>>> including automated methods for generating the lookup tables, so that is >>>> not really a factor in choosing a polynomial. >>> I that case should we go with CRC-16-CDMA2000? >>> >>> Or is there a better/stronger one from the table that's more useful? >> Actually, CRC-16-CCITT or CRC-16-IBM (which are theoretically equivalent) >> are ***stronger** than CRC-16-CDMA2000 for datagram payloads up to 32751 >> bits (i.e., CRC + data length = 32767 bits), when errors are random. For >> datagrams of that exact length It can be proven (again for random errors) >> that the undetected error rate is no worse than 1/65536, and for lesser >> lengths simulations bear this out (see, for example, >> http://doc.utwente.nl/64267/1/schiphorst.pdf). The only reason I can see >> to choose CRC-16-CDMA2000 would be to provide protection for datagrams >> longer than 4K bytes, in which case a better choice would be a 32-bit CRC. >> The standard Autodin/Ethernet/ADCCP/HDLC CRC-32 polynomial is >> >> x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+1 >> >> Although it is a primitive polynomial (without a factor of x+1) it will >> protect against all triple error patters for datagram lengths of 11450 >> bytes, according to simulations that I ran about 14 years ago, and >> its undetected error rate (for random errors) can be expected to be >> under 2^-32. So that's what I would be inclined to recommend. >> >> Regards, >> >> Mike Heard
- [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Gorry Fairhurst
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Gorry Fairhurst
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Tom Herbert
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- Re: [tsvwg] Regarding DTLS and UDP options Joe Touch
- [tsvwg] Fwd: New Version Notification for draft-t… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Derek Fawcus
- Re: [tsvwg] New Version Notification for draft-to… C. M. Heard
- Re: [tsvwg] Fwd: New Version Notification for dra… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Tom Herbert
- Re: [tsvwg] New Version Notification for draft-to… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Derek Fawcus
- Re: [tsvwg] Fwd: New Version Notification for dra… Tom Herbert
- Re: [tsvwg] Fwd: New Version Notification for dra… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Derek Fawcus
- Re: [tsvwg] Fwd: New Version Notification for dra… Derek Fawcus
- Re: [tsvwg] Fwd: New Version Notification for dra… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Tom Herbert
- Re: [tsvwg] Fwd: New Version Notification for dra… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Joe Touch
- [tsvwg] summary of issues for draft-touch-tsvwg-u… Joe Touch
- Re: [tsvwg] Fwd: New Version Notification for dra… Tom Herbert
- Re: [tsvwg] New Version Notification for draft-to… C. M. Heard
- Re: [tsvwg] Fwd: New Version Notification for dra… Joe Touch
- Re: [tsvwg] New Version Notification for draft-to… Joe Touch
- Re: [tsvwg] New Version Notification for draft-to… C. M. Heard
- Re: [tsvwg] New Version Notification for draft-to… Joe Touch
- Re: [tsvwg] New Version Notification for draft-to… C. M. Heard
- Re: [tsvwg] New Version Notification for draft-to… Joe Touch
- Re: [tsvwg] New Version Notification for draft-to… Gorry Fairhurst
- Re: [tsvwg] New Version Notification for draft-to… C. M. Heard
- Re: [tsvwg] New Version Notification for draft-to… Joe Touch
- Re: [tsvwg] New Version Notification for draft-to… C. M. Heard
- Re: [tsvwg] New Version Notification for draft-to… Joe Touch
- Re: [tsvwg] New Version Notification for draft-to… Gorry Fairhurst
- Re: [tsvwg] New Version Notification for draft-to… C. M. Heard
- Re: [tsvwg] New Version Notification for draft-to… Joe Touch