How do applications know which OSI protocol to use?

I would like to understand what happens when we type "google.com" in our browser wrt OSI model. What all protocols comes into picture AT EACH LAYER any how does they know whi开发者_JAVA技巧ch one to be used?

1. Also I would like to know, TCP/UDP which one to be used in Transport layer is decided for network/application? That is for a normal web page like google.com TCP must be used but for video streaming UDP. how is this conveyed to network?

2. Is the browser also part of this as it is helping. Should it also come under application layer?

3. Http comes under Application layer of OSI model. now for this particular example, what will come under Presentation and Session layers? Will the sessions be maintained in session layer or HTTP will be doing it on their behalf?

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It sounds like you would like a tutorial on TCP/IP. May I suggest a couple of good books

URLs in webpages have a prefix that tells the applications what to do with that link. For instance, ftp:// opens an ftp session, which uses tcp (ref RFC 959 - File Transfer Protocol). As a general rule, the URL prefixes correspond to a standardized protocol, which was specified via RFC. The RFC denotes which transport protocol is used.

EDIT

In the case of youtube, their videos have a http:// URL prefix, but after you make the request, it is redirected to a rtsp:// URL. RTSP can be streamed over UDP.

To answer questions like this, you may find that wireshark is very useful to investigate these things... keep in mind that wireshark works best on wired connections.

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Applications don't use any layer of the OSI protocol stack. They use TCP or UDP from the TCP/IP stack, and they 'know which' because of they are programmed to use one or the other or both. The OSI model is obsolete and certainly doesn't apply to TCP/IP.

EDIT: The OSI layer model is a Procrustean bed into which TCP/IP does not fit. You should forget about OSI immediately. It doesn't describe anything in the real world.