Hello Rodferna,
Perhaps the best comparison of TCP/IP with the ISO model (with
illustrations and explanations for level assignments) is at
http://home.att.net/~s.k.vincent/2502_7.htm
This is one of several class notes on a course in computer networks.
Note that TCP and IP themselves only address the lower levels of the
ISO model. I will assume your question really means to address
"Internet Protocols" in general - not just TCP and IP.
User applications could use all three of the higher level protocol
layers, but traditionally only use the top layer. There are a number
of reasons for this.
Looking at
http://www.polonica.org.au/polonet/osi.html
which describes the ISO model in more detail (again with mapping to
internet protocols), I will refer to descriptions of the session and
presentation level protocols.
The session layer describes "logging into the network", something that
is most often omitted in operating at TCP/IP network. It is not a
necessary step prior to establishing an application level interface. A
number of application level interfaces implemented on internet
protocols require no authentication or they do so at that level
instead.
The presentation layer describes transformations of data between well
known formats. Text compression is mentioned as an example. However,
most applications define and do this at the application level instead
of depending on separate presentation level packages. For example, the
rsync program
http://www.die.net/doc/linux/man/man1/rsync.1.html
can optionally compress data being transferred between systems.
However, selection of this, which files will be compressed (and those
not compressed), and the specific technique is controlled by the
application.
Part of the "where they went" answer also gets into the historical
sequence of events. The ISO seven level model was defined after the
basic internet protocols were defined. The internet protocols were
also implemented as a result of a "Request for Comment" process that
required two separate implementations prior to broad adoption on the
ARPAnet. Compliance with the ISO model was not a consideration in the
design of those protocols.
Performance also is a consideration. Going through a seven level
interface is usually more costly than going through a four or five
level interface. Generally this is due to the need to copy data in
buffers. I can expand on this if desired - ask in a clarification
request.
Further searches can be done using terms such as
iso protocol stack presentation
iso protocol stack session
iso protocol stack tcp/ip comparison
and so on.
--Maniac |
