[hackathon] Re: reserved fields: must be zero

Jeffrey Haas <jhaas@pfrc.org> Tue, 07 May 2024 11:26 UTC

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From: Jeffrey Haas <jhaas@pfrc.org>
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To: Toerless Eckert <tte@cs.fau.de>
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Subject: [hackathon] Re: reserved fields: must be zero
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Date: Tue, 07 May 2024 11:26:30 -0000
X-Original-Date: Tue, 7 May 2024 07:26:16 -0400


> On May 7, 2024, at 12:40 AM, Toerless Eckert <tte@cs.fau.de> wrote:
> 
> Why do you think that running a checksum or hash over a header and including reserved
> fields would turn those fields into MBZ ? That is not the case. The originator
> can set them as it sees fit. The receiver simply validates the header including
> the reserved field against the hash/checksum - but otherwise ignores the reserved field.

It does mean being clear about what "ignore" may mean since it may be contextual.

Offering an example, RFC 8955 BGP flowspec §4.2.1.1/4.2.1.2 has some MUST/MUST zero bits.  In the context of the operator they're defined in the middle of, the ignore semantic is clear.

The protocol, however, will do bit-wise comparisons over those bits for purposes of a bit-string comparator. Thus, the value of the bits isn't ignored even if the meaning is ignored.

The same would be true of checksums as it has been since the beginning.

-- Jeff (agreeing)