And really ironically, the one IP example that fits well into the 7-layer
model is the one that many Cisco documents and books get wrong. That's the
SUN Open Network Computing (ONC) protocols:
Application Network File System (NFS)
Prentation External Data Representation (XDR)
Session Remote Procedure Call (RPC)
Transport UDP
Network IP
Many Cisco documents put NFS and RPC in the wrong place and don't mention
XDR. It probably doesn't matter that much, I suppose. It's just irritating.
I think the main thing that the model helps one understand is that protocol
design is modular. Each layer operates semi-independently and offers a
service interface for a layer above it. Because the layers are independent
we can stack protocols in creative ways. For example, you may see SNA
encapsulated in LLC2 and Token Ring and then also encapsulated in TCP/IP
and various data links for routing across an IP network. Or, don't be
surprised to see Apple Filing Protocol (AFP) on top of TCP/IP. Or Server
Message Block on top of AppleTalk Session Protocol.
When the concept of independent layers isn't followed, we have problems.
Take the case of FTP which has the 32-bit IP address encoded into the
application layer, which causes challenges for NAT and other implementations.
An understanding of encapsulation really does help with troubleshooting, as
Marty mentioned.
Be wary of the books that count up the number of encapsulations and use the
number to assign an OSI layer number. That may be the explanation for so
many books that put RTMP at the transport layer. (Well, Inside AppleTalk
started it, but that was the editor's fault from what I've heard.) I have
also seen books that put IP routing protocols at various strange levels,
depending on the number of encapsulations. RIP, OSPF, BGP, and EIGRP are
all definitely network-layer protocols. They have varying number of layers
below them, though.
Instead of counting encapsulations to determine which layer a protocol
resides at, it's much more helpful to consider the functionality of the
protocol. Even a newbie CCNA knows the basic functionality of each layer
and should easily be able to put routing protocols in their proper place.
And if you can't put a protocol, such as ARP, into an inelastic stack, just
remember that the stack boundaries aren't as unbending as you probably
learned they are. I liked the way that Howard said, "The Internal
Organization of the Network Layer" relaxes the rigid boundaries between
Layer 2 and Layer 3.
Priscilla
At 09:28 AM 6/14/01, Howard C. Berkowitz wrote:
>I'm simultaneously amused and confused by some of the debates on the
>list, especially with respect to protocol architecture. True, in many
>cases, it is important to know what Cisco is looking for in tests,
>which is not necessarily the same as what the protocol designers had
>in mind. But a lot of the discussions have the flavor of the sort
>of sports debates: "Would Muhammad Ali have beaten Joe Louis?"
>
>OSI, in its _basic_ 7 layer form, is a useful tool for
>conceptualizing and educating. That's it. Certain concepts, like the
>generic relationship between layers, protocol encapsulation, etc.,
>are generally useful. But I assure you, from personal experience in
>ISO, IETF, CCITT/ITU-T, and ANSI, nobody spends more than a few
>seconds thinking about what layer something goes into.
>
>Indeed, some of the layers are there for political reasons,
>especially the session layer. The major reason the session layer was
>separated out is to give an existing CCITT committee from the
>teletext work something to do. The functions of session quite
>reasonably could be given to transport and application, which is
>usually the case in IP stacks. Ironically, one of the few stacks I
>know of that truly has seven layers, NFS, comes out of the IP, not
>OSI, world.
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8558&t=8335
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