application layer functionality

someone please throw some light on this sentence in todd-lammle book regarding the functionality of the application layer:

"application layer is responsible for identifying and establishing the availability of intended connection partner and determines that whether the essential resources for the intended communication are available or not!"

i couldnt get it as how the application layer even has any idea regarding what the resources will be used at any lower layer!!!

please explain it to me.


thanks and regards.


  • What is the context?  Is this application layer in terms of the OSI model or the TCP/IP protocol stack?  The reason I ask is because in context to the OSI model the Application layer provides application and end user services to the network. This layer interacts with software applications that use the network.  Todds statement sounds quite a bit like the Wikipedia article, which eloborates on that statement:

    " Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. "


    As networking people we typically don't get involved at the Application layer.  For now, simply understand that the application layer provides an application with access to the network.  


    Hope that helps.


    Brandon Carroll

  • Brandon Carroll, the Brandon Carroll, @ine? Say its true. Where is the big announcement?

    This after I already completed my CCIE security. What a pity.

    Welcome aboard man.


  • thanks brandon !!

    but i still ain't getting what these lines actually want to say!!

    i mean how is it possible that application layer knows anything about the reaources that would be required down the line!

    as i have read , the top three layers are entirely dealing at the user's end and they downt know that the data they are forming into packets is to be send on any network or so , am i wrong??

    n yes he(todd) is talking about the OSI model.


    please elaborate.

  • I look at it from this angle.  The application has certain configurations that indicate the resources that need to be used.  For example.  If we are talking about email, when your client application sends it uses an SMTP server.  The SMTP server is configured in the application so the application layer already has information about who it needs to talk to.  When you send an email, the application requests a SMTP connection to the configured mail server.  This requires TCP, and TCP uses IP, and then it goes over Ethernet.  So each layer is responsible for something, however the application layer is the one that prompted the communication in the first place.  The layers below are just going throough the motions because of the applications request.  

    So can you see how the application would know about the resources it needs?  The layers below are responsible for setting them up, but how would TCP at Layer 4 know that email needed a TCP connection to the mail server unless the application layer requested it?


  • thanks sir!! :-) its coming like a good flash file infront of me[:)]

    n ofcourse the transport layer is setting the connection but it wont do it unless a connection is required at the very first!! that make it very clear how the application layer is concerned with the required resources,

     please tell me if i am infering wrong that --> if the connection required(which is a required resource) is not available due to any reason(say faulty network ) the process wont continue and the user would be prompted for another attempt hence the application layer at the very firat  defines the reaources required to make the process go successfull.. isn't it??


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