[e2e] Fwd: Re: Historical question: Link layer flow control / silent discard

John Day jeanjour at comcast.net
Tue Feb 11 11:50:55 PST 2014

Apparently, one must repeat explanations to rectify misconceptions 
about the past.  The accounts today exhibit the same errors that they 
did 7 months ago.

>X-CAA-SPAM: 00000
>Date: Wed, 29 May 2013 12:04:41 -0400
>To: Joe Touch <touch at isi.edu>, John Day <jeanjour at comcast.net>
>From: John Day <jeanjour at comcast.net>
>Subject: Re: [e2e] Historical question: Link layer flow control / silent
>  discard
>Cc: braden at isi.edu, end2end-interest at postel.org
>OSI divided the Network Layer into 3 sub-layers (not all of which 
>were present for all networks):  3a Subnet Access, 3b Subnet 
>Dependent, and 3c Subnet Independent.  (see the Internal 
>Organization of the Network Layer, ISO 8648).
>X.25 was (according to its title) 3a  Subnet Access.  The PTTs had 
>the "foresight" ;-) to call it a Data-Terminating-Equipment (DTE) to 
>Data Communicating Equipment (DCE) interface.  (Don't you love the 
>nomenclature!)  X.25 was just at the boundary of the network.  In 
>other words, Host to Network protocol, the equivalent of BBN1822! 
>;-) So OSI took them at their word.  ;-) If the network had X.25, 
>then it was at 3a.
>Whether a PTT used X.25 internal to its network was its business and 
>not within the purview of CCITT.  I believe most X.25 networks at 
>the time heavily modified it beyond what the Recommendation said. 
>(CCITT's habit of defining its recommendations as the interfaces 
>between boxes is why I refer to this as the beads-on-a-string model! 
>boxes strung together with a wire!)
>With X.25, LAPB (also known as HDLC) was the Data Link Layer.
>CLNP was 3c, Subnet Independent.
>One can think of 3a/3b as a traditional network layer for networks 
>that had that; and 3c/Transport as the Internet Layer.    3c 
>addresses were global, while addresses in 3a/3b were only 
>unambiguous within the network.  Think of 3a/3b as points of 
>attachment addresses, and 3c as node addresses.  (see the Saltzer 
>paper RFC 1498 for background on this)
>Take care,
>At 8:31 AM -0700 5/29/13, Joe Touch wrote:
>>On 5/28/2013 2:02 PM, John Day wrote:
>>>Just for the record and then I will let this discussion go on, but X.25
>>>was not at the core of the OSI Model.
>>FWIW, there was an implementation of ISO - ISODE (the ISO 
>>development environment). UPenn was snail-mailing out 9-track tapes 
>>and 8mm cassettes back in the early 90's when I was there. I still 
>>have one of the enamel pins.
>>It implemented layers 3-6, and could be configured to run over X.25 
>>- thus the possible confusion that X.25 was its L2.

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