All right so what about Layers 5 and 6? Well in the OSI Model the job of Layer 5 is called the session layer is to provide long-lived sessions so for example if a transport connection fails then it will establish a new transport connection and the session can continue over that new connection it's like automatic redial on the telephone and layer 5 potentially could multiplex data across multiple transport connections so if you need more bandwidth than a single transport layer connection will provide then you can multiplex data across multiple transport connections and at layer 6 the presentation layer is all about data reformatting so the concept there is that it would translate data from one format to another to make it suitable for the application to use so it might translate from one character set to another character set now these layers exist in the osi model but they do not exist in the tcp suite so these features if required the application itself would be responsible for doing them so if the application needs to translate from one character set to another the application has to do it and if the if the application wants to handle disconnections and reconnections the application itself has to take care of it the tcpip stack basically has empty layers at those points but we keep them in the model because some networks do provide these functions different technologies provide these functions and it's just a reference model that was useful for us at the very top we have layer seven layer seven is the application layer and layer seven is the actual job you wanted to do across the network so for example you wanted to view a web page and at layer 7 there will be a different application protocol specific to each type of job you want to do so for example for web browsing the protocol is called http hypertext transport protocol and it runs over tcp atlas at layer 4 but http has its own way of formatting an http request and headers and an http response and its headers another example would be email so smtp the simple mail transfer protocol is used by one email server to deliver an email message to another email server and again that has its own series of messages that it uses to set up a connection to give the envelope for a message give the body of a message and request its delivery and return error codes and so on all of those are application layer specific activities and they're part of the application protocol and i'm sure you can think of many others so in email there's POP3 and IMAP for retrieving email from a server for example and so on.

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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