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