Re: [hackathon] Formal Languages at the Hackathon

"Timothy B. Terriberry" <tterriberry@mozilla.com> Wed, 06 November 2019 20:46 UTC

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From: "Timothy B. Terriberry" <tterriberry@mozilla.com>
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To: Stephen McQuistin <sm@smcquistin.uk>, hackathon@ietf.org
Cc: Marc Petit-Huguenin <marc@petit-huguenin.org>, Colin Perkins <csp@csperkins.org>
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Date: Wed, 6 Nov 2019 12:46:45 -0800
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Subject: Re: [hackathon] Formal Languages at the Hackathon
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Stephen McQuistin wrote:
> Please join us: we are particularly interested in examples of packet 
> header diagrams that might be challenging to write
> in the format our draft describes, and in how our tooling might fit into 
> existing workflows.

I haven't reviewed the full draft, but one thing that stands out is the 
limitation, "A packet can contain only one unspecified length field, to 
ensure there is no ambiguity" in Section 4.1.

As a counter-example, I offer "code 1 packets" from RFC 6716 Section 
3.2.3: https://tools.ietf.org/html/rfc6716#section-3.2.3

These packets have two fields where the length is determined by the 
total size of the packet, under the restriction that they must be equal.

I didn't see a lot of detail on _how_ "the size of a field is ... 
derived from the value of some previous field", and if there are any 
restrictions on this derivation, but in general for Opus this can be 
somewhat involved, so I am certainly curious about structured text that 
allows for the parsing process to be fully specified.