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

Joe Touch touch at isi.edu
Tue Feb 11 17:05:44 PST 2014

On 2/11/2014 11:50 AM, John Day wrote:
> Apparently, one must repeat explanations to rectify misconceptions about
> the past.  The accounts today exhibit the same errors that they did 7
> months ago.

Yes, in that running over X.25 and talking about the service X.25 
provides to IP relative to Internet terminology still has nothing to do 
with the terminology used within either X.25 or OSI.


>> 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,
>> John
>> 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.
>>> Joe

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