Subject: [[email-masked]: Re: Encoding of multiple levels]

From: Tony Li <[email-masked]>
Date: Tue, 4 May 1999 01:14:44 -0700 (PDT)

By way of a reminder...  now that we have spam filtering enabled, one must
be directly on the root mailing list or mail to the list will bounce back
to my mailbox.

Tony

p.s. My guess is that Dave waved his hands rapidly and said the magic
words.  ;-) ;-) ;-)

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From: Mike Shand <[email-masked]>

At 22:51 30/04/99 -0700, Tony Li wrote:
>
>|  Also, does anyone remember why version is there twice in the fixed part
>|  of the header? I got to make a joke about it in the book, which is nice,
>|  but how did it happen? Was one field intended as a major version number and
>|  the other a minor version number? That would have been useful. I think
>|  it was that way in DECnet Phase V (major/minor version
>|  number, where minor was bumped for compatible changes). Doesn't matter, 
>but I'm 
>|  just curious.
>
>
>My guess was that one field was always intended as the protocol version
>number, the other was the PDU format version number.
>
>Well, at least that sorta made sense.  ;-)

Yes, sorta :-) Its really much uglier than that though. You have to
remember that its really two protocols merged. The normal ISO format is to
have the protocol ID field at the front of the header, c.f. 8473 (CLNP) and
9542 (ES-IS), so that's why that one is there. But IS-IS was pretty much
designed by Radia as a DECnet protocol, before we decided to get it
ISOised. Standard practice for DECnet was to have the triple
VERSION/ECO/USER-ECO, which pretty much corresponds to major/minor/who
cares (I never DID understand what the user-ECO field  was for). In the
early days we were not really sure whether we were going to succeed, so we
kept our options open - we could run either in an ISO format -using ISO
protocol IDs and LSAPs etc., or in a pure DECnet format (there are even
some other vestiges of this in the spec, since the max PDU size is 1492,
which allows room for a SNAP SAP!

As with most things like this, by the time we were confident 10589 was
going ahead, it was really too late to tidy up the mess. But the user-ECO
field did get stolen to be the "Maximum Area Addresses when not equal to
zero" field (I think - I don't have a spec to hand). And the ECO field was
made a reserved field, thus disguising the DECnet origins.

Surprisingly  nobody ever asked why there were two version fields during
the standardization process - just goes to show how careful people are :-)
(or maybe they did and Dave just invented a plausible reason :-)

	Mike (Network archeologist)
>Tony
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