I saw an interesting statement in a book I was browsing recently: Ray Horak,
in Communications Systems and Networks, calls UDP a layer seven protocol
while TCP is a layer four protocol. When I saw that, I immediately went to
my Radcom world of Protocols poster, and read their definition of layer
four, which does indeed say "reliable" I have not seen it argued this way
before, but I understand where the author is coming from. If layer four is
reliable, and UDP is not, it cannot be layer four.
I know some folks warn about trying to crunch everything into the OSI model.
I believe the purpose of studying the model is 1) to provide a structured
view of data communications 2) a means of structuring a troubleshooting
methodology 3) so one can at least be conversant with the terminology when
listening to all the presentations about what hardware or software operates
at what layer. I myself have begun to look at the world using a variant of
the ip model- physical/data link, network, transport, everything else.
Someone else on the group once suggested a box with four sections -
connection, connectionless on one axis and reliable, unreliable on the
other. As his learning process he would place a protocol into each section
based on the intersection. TCP = connection, reliable; frame relay PVC =
connection, unreliable, and so on.
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Howard C. Berkowitz
Sent: Friday, May 19, 2000 8:42 PM
To: mikey; [EMAIL PROTECTED]
Subject: Re: Connection(less) services
>According to the OSI model, layer 3 is connectionless and layer 4 is
>connection oriented. However, one must remember that the OSI model is
>theoretical.
And the OSI model has been extended by ISO, considerably beyond the
simple seven layers. Unfortunately, the extensions, such as the
internal organization of the network layer, the object-oriented
refinements of the application layer, management, etc., are rarely
presented.
>Specific protocol implementations do not necessarily adhere to
>the OSI model.
Actually, the main OSI Reference Model defined in ISO 7498 was
connection-oriented at all levels. Connectionless operation was
described in one of the Annexes to 7498 -- can't tell you the
specific annex number from memory.
In the actual OSI protocol suite, CLNP was the connectionless network
layer protocol, and the connection-mode network protocol was a
particular X.25 subset.
>
>Example: TCP and UDP are both layer 4, one is connection oriented the other
>is connectionless. Also, take a look at the IPX stack, it does not map
>nicely to the OSI model.
Quite correct.
>
>mikey
>Oscar Rau <[EMAIL PROTECTED]> wrote in message
>[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > When you look at TCP and UDP applications, they are placed
> > in the Transport layer of the OSI model. Some of the Cisco
> > certification literature suggests that connection oriented
> > services and connectionless services are handled by Layer 3
> > which is the network layer.
> >
> > Which layer is primarily responsible for connection oriented
> > and connectionless services?
> >
> > Thank you in advance.
> > --
> >
> > Oscar Rau
> > [EMAIL PROTECTED]
> >
> > ___________________________________
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>
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