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Re: [dtn] BPbis - time units

"Birrane, Edward J." <Edward.Birrane@jhuapl.edu> Wed, 29 July 2020 16:25 UTC

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From: "Birrane, Edward J." <Edward.Birrane@jhuapl.edu>
To: "Burleigh, Scott C (US 312B)" <scott.c.burleigh@jpl.nasa.gov>, "Taylor, Rick" <rick.taylor@airbus.com>, "Jeremy.Mayer@dlr.de" <Jeremy.Mayer@dlr.de>, "cabo@tzi.org" <cabo@tzi.org>
CC: "dtn@ietf.org" <dtn@ietf.org>
Thread-Topic: [dtn] BPbis - time units
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Date: Wed, 29 Jul 2020 16:25:08 +0000
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Subject: Re: [dtn] BPbis - time units
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Comments inline.

Edward J. Birrane, III, Ph.D.
Embedded Applications Group Supervisor
Principal Staff, Space Exploration Sector
Johns Hopkins Applied Physics Laboratory
(W) 443-778-7423<tel:(443)%20778-7423> / (F) 443-228-3839<tel:(443)%20228-3839>

We all agree that the primary block must be immutable end-to-end, because otherwise its authenticity can't be protected by a BIB.  But does "immutable" mean "none of the bits in the block must ever change" or does it mean "the canonicalized representation of the block must always be the same"?  I can imagine a free-spirited implementation of BP inserting some leading zeroes into the representation of, say, the source EID's scheme code number; has immutability been violated in that event?

[EJB] Agree that BPAs may want to "harmlessly recode" values especially to reduce processing downstream for higher rate applications. Which seems like a use case to consider while talking about millisecond and microsecond timing precision.

I suspect it has not, which suggests that maybe we need some language in the BPbis specification to say so.  If so, maybe the canonicalization language in BPsec needs to migrate to BPbis?  And if that is the case, then perhaps that general language addresses Ed's point?

[EJB] I think this is the issue - semantic immutability versus syntactic immutability.  Specifying a canonical form for the primary block and extension block headers would be one way to solve this. Prohibiting recoding would be another.

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