(아래있는 내용읽다가 생각한건데...-_-)
지금 우린 tcp/ip(protocol) 쓰는데...

그럼 telnet에서 vi 쓸때 remote host와 terminal 동기화를 위해 presentation layer에서 data를 formatting 하는걸로 알아들었거든요...
그럼 tcp/ip에서는 presentation layer가 없으니깐
 application layer에서 presentation layer의 기능을 하는건가?



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command		userThis layer doesn\'t mean the application itself, but the protocols that do the work for the application. Examples: HTTP for web browsers, SMTP/POP/IMAP for e-mail,
This layer is an example of the political processes mentioned above. The theory was that the application layer didn\'t need to format the data for transmission across the wire; that would be the job of an underlying layer. Furthermore, the idea was that a client and server would negotiate which format they wanted to use.

This actually made sense back in the late 1970s, because most networking involved dumb, character-mode terminals controlled by mainframes. An application wanted to deal with abstract concepts like database forms that a user filled out. Different terminals have different control codes to display this type of information. However, it makes virtually no sense nowadays. As a result, OSI protocols always negotiate a Presentation encoding, even though only one option is available.

In TCP/IP, the only protocol that really does his negotiation is Telnet. Character-mode applications such as \'<TT>vi</TT>\' go through a package called \"curses\". When you Telnet to a host, you exchange your terminal type with with the host. When an application such as \'vi\' wishes to clear the screen, the \'curses\' packages tells it how to do so for your particular terminal.

However, the concept of how data is formatted on the wire is extremely important to applications. So even though it doesn\'t exist as a real layer outside of OSI protocols, it is still an important component of all applications.

Like the Presentation layer, the Session layer is an artifact of the ISO political processes, but more so. In fact, as it turns out, the Session layer is completely useless. If you pick up a book on OSI and read up on the Session layer, you will end up reading lots of technical content about it but you still won\'t be able to answer the question: What is the session layer for? If you ask somebody what the Session layer does, they will tell you something like \"It establishes a session between two entities\". If you ask what a \"session\" is, the answer would be \"It\'s what the Session layer establishes\". If they give you a more detailed answer, they are probably confused and will really be describing a Transport layer \"connection\", which isn\'t the same thing.

There is a Session layer protocol defined by OSI, but the OSI hasn\'t defined any uses for it (that I know of). In other words, all the OSI applications establish both Transport layer \"connections\" and Session layer \"sessions\" when they start talking to each other, then tear down the connections/sessions together when they stop talking. Luckily, the OSI protocol has been defined in a fairly efficient manner such that both can be established in the same packet, and the Session layer only adds a couple bytes on average to every packet sent.

Now I could actually tell you what the Session layer really is for, but I\'m not going to. (Hint: it has to do with terminals talking to mainframes.) You should simply remember the concept that it really doesn\'t do anything. The 7-Layer model really just contains 6-layers (actually, 5-layers, because the Presentation layer should be thrown out as well).


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