ARP was developed without reference to the OSI reference model. That's the
easiest way to think about it. ;-) Cisco books that coerce protocols into
the strict 7 layers are simplifying the truth. In many cases a protocol
doesn't fit into a linear stack. ARP lies in a control plane. Its
functionality is similar to call setup tasks in a WAN network.
To understand ARP it helps to use the ISO document called "The Internal
Organization of the Network Layer," which relaxes the rigid boundaries
between Layer 2 and Layer 3. ARP resides in the Subnetwork Dependent
Convergence Facility. Lots of words but if you think about them, they make
sense.
Howard has answered this question a ZILLION times. Check the archives. I'm
sure he says it much better than I do. ;-)
Priscilla
At 11:50 AM 6/13/01, John Neiberger wrote:
>This topic has come up a few times in the past and I don't think we ever
>came to a common agreement. Several people made good arguments on both
>sides. I don't recall the specific argument, but I believe someone even
>made a convincing argument that it was an application layer function.
>Perhaps someone here remembers that thread and could refresh our
>memories.
>
>When most people think of host-to-host communications they think of one
>layer on one device speaking to the corresponding layer of another
>device. In this case of ARP I personally feel that we have the network
>layer of one device speaking to the datalink layer of another. Even
>that point is a little shaky because at the destination the packet must
>reach the network layer to be recognized, but the information desired
>from the end station is layer two, not layer three.
>
>I would also suggest that we determine the layer at which a function
>resides by looking at the layer that originated the request for
>information. In this case, it's the desire of the network layer in one
>device to speak to the network layer of another device that initiates
>this entire process. An ARP is generated at the request of the network
>layer. This ARP seeks out the destination device, gathers the necessary
>information, and delivers that information to the network layer of the
>originating device.
>
>Because of those two arguments I'd say that the ARP function overlaps
>both the network and datalink layer. It is a datalink frame generated
>at the request of the network layer, and it just doesn't fit perfectly
>into either layer.
>
>Then again, I may be wrong. :-)
>
>Regards,
>John
>
> >>> "Dr Rita Puzmanova" 6/13/01 7:58:53 AM >>>
>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
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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