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

Bob Braden braden at isi.edu
Tue May 28 09:51:42 PDT 2013

On 5/25/2013 4:34 PM, Lachlan Andrew wrote:
> However, perhaps that isn't the link layer. I've always wondered why 
> switched ethernet (which does the ISO layer-3 tasks of addressing, 
> routing over multiple point-to-point links and buffering) is called a 
> "link layer" by the internet community... Cheers, Lachlan 

Ah, interesting point. Of course an Ethernet, whether switched or 
bussed, is a network. It has addressing, routing,
and flow control. Ethernet, or rather its predecessor X.25, was at the 
core of the OSI  7 layer model.
But the pesky internetwork crowd came along and said, we need a network 
of networks, so we need a new end-end
INTERnetwork protocol, let's call it IP."

So what are the poor OSI devotees, who believe in One Network to Rule 
them All, to do?

We IETFers are pragmatists, not layer model purists. (as illustrated by 
our lack of
embarassment about splitting the layers with MPLS, IPsec, TLS, etc.)  We 
became careless and smudged over
the network/internetwork distinction. So, IETFers generally refer to the 
Internetwork layer as the "network layer." Then the Internet
protocol stack sort of looks like the OSI stack, and there is an 
illusion that the OSI stack has something to do with reality.

If IP is the "network layer" (we are too lazy to say "internetwork 
layer"), then what is the Ethernet? In IP land, it is a subnet(work). But
for those who believe that IP is really our network layer, then the next 
layer below IP was dubbed  the Link Layer, because it
seems to correspond to the OSI Data Link Layer. That is the answer to 
your question.

Note that the Internet's Link Layer should not be (but often is) 
confused with the OSI Data Link Layer. It co ntains
"everything" in Vint's famous phrase " I  P  over everything".

Section 1.1.3 of Host Requirements RFC 1122 defines the internetwork 
layer model carefully.
And MAP was the only internet community member who bothered to 
straighten this out.
(You are an Internet pioneer iff you know who MAP was).

Bob Braden

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