[e2e] Lost Layer?
jnc at mercury.lcs.mit.edu
Mon Feb 10 20:11:57 PST 2014
> From: Joe Touch <touch at ISI.EDU>
> I don't at all understand the difference between a "network layer" and
> an "internetwork layer".
ATM stuff, etc = 'network layer'. (Or, reaching further back, ARPANET stuff.)
IPvN = 'internetwork layer'.
> I have no idea what a 'network' layer is that is different from what we
> currently call the link layer.
The thing is that for more complex networks (like ATM, ARPANET, etc) you have
'link' layers which are lower/more-local than the system-wide 'network' layer.
For example, in the ARPANET example, ISTR that one of the interface modes
involved using an HDLC link to talk to the IMP. On top of that one ran 1822
headers (with the destination IMP/port, etc); the latter being the 'network'
Not all situations include a network layer. Most often (especially these
days), the internetwork layer runs directly on top of the link layer. But
sometimes there's a network layer sandwiched in there too.
Not so much any more, though, because with the increasing prevalence of
internetworking protocols, there's no real use for a network layer - it's
just replication of functionality, usually. So people just run the
internetwork layer directly on top of the link layer, and the functionality
that would usually be supplied by the network layer (e.g. path selection
across the 'subnetwork', in the older sense of that word - not to be confused
with IPvN 'subnets') is supplied by the internetwork layer instead.
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