>Hi all,
>
>Trivial yet fundamental question. I have seen ARP described as part of
>the network (internet) layer so many times that I have started to
>believe it belongs there (although I know well that it operates "as if"
>the Layer 2 protocol - as per OSI RM). Now I have eventually come across
>Doug Comer's statement: "It's part of the network interface layer."
>
>I should not ask where the truth is but still I will. That would mean
>quite a lot of books are incorrect in this (including Cisco materials).
>
>Rita
Lots of books are indeed wrong, when judged against the ISO Internal
Organization of the Network Layer document. Stripping to a minimum
of OSI speak, what we think of the network layer actually has three
sublayers, one of which arguably extends into the data link layer or
at least overlaps it.
The Subnetwork Independent Convergence Layer is the truly
link-independent part such as IP or CLNP.
The Subnetwork Access Protocol is the interface to intelligent layer
2 protocols that have payload identification -- X.25, ATM, LLC, etc.
The Subnetwork Dependent Convergence Facility maps between the
Subnetwork Independent and Subnetwork Access parts. ARP goes here.
Going a step farther, it's also worth considering the functional as
well as layering models of B-ISDN/ATM, which identify the U(ser),
C(ontrol)/signaling and M(anagement) planes at each layer. ARP is a
C plane protocol between the end host and the ingress relay, much as
is Q.931 and Q.2931. IP routing protocols, PNNI and SS7 are
relay-to-relay M plane protocols.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8355&t=8335
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