if session layer responsible for establishing connection b/w devicez then why transport layer iz responsible for establishing too.. specialy if v talking about UDP dat it jst makez establish connection...
Okay so what your asking is:
If the session layer is responsible for establishing connections between devices then why is the transport layer responsibily for establishing connections as well? Specifically how does this apply to UDP connection establishment?
What you need to understand here is the session layer manages sessions for various applications and makes requests for network services to send data for applications. TPC and UDP in the transport layer basically facilitate these requests when TCP/IP is the chosen protocol.
So lets say for example your running 2 applications a big file transfer using FTP(file transfer protocol) and your watching some streaming video on CNN.com for all the late breaking news.
First lets make some notes of things:
The FTP file being transfered is 10 Gigs worth of data.
FTP uses TCP to insure reliable data transfer.
The streaming media is an unknown sized bit stream. (it lasts as long as the sender is transmitting)
Streaming uses UDP an unreliable data method because if frames in a video stream get dropped it doesn't matter because a live feed isn't going to wait on retransmission of an old video frame.
(just like if your phone loses connection for a second the connection is simply lost it doesn't try to resend the lost words because it would end up with you talking over yourself)
The network card on the computer is the point in which data will be exiting for both applications.
So at this point you have 2 applications trying to gain access to network resources to communicate something. Who gets to access the local ether and send data first and how much? Should the live feed be put on hold while the 10 gigs worth of data transfer over the network? Should the FTP be put on hold for the very important streaming media broadcast *of potentially limitless size*? Well the answer is a balance needs to be struck and in order to do that sessions need to be created. More the one application needs to use the outgoing network interface however load balancing of some sort must be done. A network interface will be responsible for deciding what packets get sent out based on a variety of different things including urgency (high urgency on streaming media and voip packets) and availiablity of bandwidth ammong other things. Internally to the computer if there is a bottleneck there is a backlog and something needs to manage what needs to be sent, what no longer needs to be sent and what priority the data to be sent has this something is a session.
For example lets say that the NIC did not give the media stream enough bandwidth to continue a live stream because it gave too much frame transmission time to the FTP transfer it would be great for the Session trying to send streaming media to not even attempt sending old data and just send the relevant live data. It is able to do this because the NPU (networking processing unit) managing the LOCAL session for that application knows the old data doesn't need to be sent on a live stream.
That being said you can think of the session layer a sort of que that sits near the application and makes what it thinks will be valid requests of network resources.
When these requests are made with TCP/IP as the protocol they made using TCP or UDP to manage how the layer 4 segments should be created for data handeling.
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dat --- >> that ;Use correct lettering on basic words
vry --- >> very ;Use correct lettering on basic words
b/w ;use "between" instead
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