A Few Questions About the OSI Model

I'm learing about the OSI reference model in my networking class, and the layers and examples all seems a little vague to me.

For example, I've learned that HTTP is one protocol that operates at the application layer. I know in definition what HTTP is, and why it is needed. But where is it located? Take the firefox application for example. I can find where it installs to, and know that firefox.exe is the main program. But what about protocols? Are they files that programs use? Do programs install their own specific protocols? Where are they?

Hint: programs implement protocols. (And so do humans!)

So the programmers who wrote the part of firefox that communicates with web servers to comply with HTTP standards? Is that fair to call the HTTP protocol a standard? Am I correct when I say that HTTP is a specific implementation of communication that defines how a program sends and receives data? Do they use APIs for this?

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Originally Posted by jrahhali

So the programmers who wrote the part of firefox that communicates with web servers to comply with HTTP standards? Is that fair to call the HTTP protocol a standard? Am I correct when I say that HTTP is a specific implementation of communication that defines how a program sends and receives data? Do they use APIs for this?

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Yes
Yes
Sort of
Generally the OS supplies much of the API

HTTP is a protocol (that's what the 'P' stands for).
Just like FTP, SMTP, RTP.

All the internet protocol standards can be found here -> RFC-Editor Webpage

> Do they use APIs for this?
It depends how abstract you want to get.

Berkeley sockets - Wikipedia, the free encyclopedia are somewhere between bashing bits directly in the hardware of your NIC, and having some high level function which takes a URL and gives you back a DOM tree of the HTML page in question.

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Originally Posted by jrahhali

Is that fair to call the HTTP protocol a standard?

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Yes. WRT to the OSI model, HTTP is considered part of the "presentation" and "application" layers.

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Am I correct when I say that HTTP is a specific implementation of communication that defines how a program sends and receives data?

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More specifically then, HTTP is an application/presentation layer protocol, which is only part of what "defines how a program sends and receives data". The other parts on the web are the TCP transport layer protocol, the IP network layer protocol, and the ethernet data-link layer protocol, all of which require a software implementation. There is no distinct "session layer" on the internet -- the application/presentation layer works directly with the transport layer, as it does for all TCP/IP networking. This means the internet is really composed of only 4 software layers: datalink->network->transport->application/presentation.

Great. Thanks everyone who replied.