Exponent 3 damage spreads...

[Bill Frantz]

jamesd at echeque.com (James A. Donald) on Thursday, September 14, 2006 wrote:

>Obviously we do need a standard for describing structured data, and we 
>need a standard that leads to that structured data being expressed 
>concisely and compactly, but seems to me that ASN.1 is causing a lot of 
>grief.
>
>What is wrong with it, what alternatives are there to it, or how can it 
>be fixed?

In SPKI we used S-Expressions.  They have the advantage of being simple,
perhaps even too simple.

In describing interfaces in the KeyKOS design document
<http://www.cis.upenn.edu/~KeyKOS/agorics/KeyKos/Gnosis/keywelcome.html>
we used a notation similar to S-Expressions which was:

(length, data)

These could be combined into a structure: e.g. (4, len), (len, data) for
data proceeded by a four byte length field.  If you standardize that the
data is always right justified in a field of length "len", and that
binary data is encoded with a standard encoding (hexadecimal,
6-bit/character, decimal etc.), most of the problems I have seen
described in this thread should "just go away".

Some might object that having a specific number of bits for the length
field limits future expansion of this approach.  Indeed, ASN.1 avoids
this issue by allowing the encoding of "infinite" length integers, and
XML does the same.  The cost of that flexibility is much more difficult
encoding and decoding.  If a length field length of 4 to 8 bytes (32 to
64 bits) is chosen, as a practical matter, any length data that is
transmittable in an exchange can be represented.  (A terabit/second is
10**12 bits/second.  32 bits can represent a million seconds at that
data rate.  64 bits can represent much longer data items.)

Cheers - Bill

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Bill Frantz        | gets() remains as a monument | Periwinkle 
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