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Re: [OPSAWG] draft-gharris-opsawg-pcap.txt --- FCS length description

Michael Richardson <mcr+ietf@sandelman.ca> Tue, 22 December 2020 04:13 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
To: Carsten Bormann <cabo@tzi.org>
cc: Pcap-ng file format <pcap-ng-format@winpcap.org>, tcpdump-workers <tcpdump-workers@lists.tcpdump.org>, opsawg@ietf.org
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Date: Mon, 21 Dec 2020 23:13:32 -0500
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Subject: Re: [OPSAWG] draft-gharris-opsawg-pcap.txt --- FCS length description
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Carsten Bormann <cabo@tzi.org> wrote:
    > On 2020-12-22, at 01:31, Michael Richardson <mcr+ietf@sandelman.ca> wrote:
    >>
    >> #define LT_FCS_LENGTH(x)		(((x) & 0xF0000000) >> 28)
    >> #define LT_FCS_DATALINK_EXT(x)		((((x) & 0xF) << 28) | 0x04000000)
    >>
    >> this suggests that the FCS length is really only 3 bits

    > 4 bits, really (assuming 32-bit unsigned integers).

The LT_FCS_LENGTH(x) (bit 28) is treated as a flag to say that the thing is there.
The DATALINK_EXT(x) might be borked here, only accepting odd values, which
would really be a problem.  I suspect there are bugs here.

- --
Michael Richardson <mcr+IETF@sandelman.ca>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide




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