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

John Day jeanjour at comcast.net
Wed May 29 09:04:41 PDT 2013

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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