OSI Model Help

bmunroe

In a CCNA text I'm reading, it talks about the Application Layer (L7) ensuring the availability of a remote host, resources on said host, and negotiation of procedures related to data integrity, privacy, and error recovery.

I'm somewhat confused by this. It almost sounds like this negotiation occurs apart from the model by some separate means, however this doesn't seem possible given that the Seven Layers are all inclusive, and that any communication between hosts occurs via "same layer" interaction within the constraints of the model. So when the text states that Layer 7 "ensures" these things, how exactly does this happen? Does this simply mean that when the first data frames are sent from a source host, which contain the negotiation terms (i.e., desired resources, data integrity, privacy, error recovery), the receiving machine de-encapsulates the frame all the way up to the Application layer, at which point the terms are read, and then in turn it responds to the source host (in agreement or disagreement) via encapsulation all the way down the seven layers and onto the wire?

Any help to clarify this would be greatly appreciated.

Slowhand

Can you quote the text directly for us, so we can make sure we've got the wording and the meaning of the question correct?

techster79

OSI is a theoritical model. While some things can happen only in layer 7, in the real world most things classified in the application layer overflow into the presentation and session layer as well. Compare OSI and DOD TCP/IP model which has an application layer in place of layers 5-7 on OSI. The test will focus more heavily on the DOD model, which is what is found in the real world. The OSI model never really took off but is great for troubleshooting and what not.

Data integrity, privacy could be ensured by IPSEC which encompasses layers 5-7, leaving the port/IP addresses unencrypted to allow delivery of a packet. Error recovery can be performed by an application if for some reason it wasn't already weeded out by layer 2 error checking.

dynamik

I believe it's referring to things like encryption, authentication, signing, etc.

edit: Wow. No response for 15 minutes, then a flurry of them. I guess everyone just finished their lunches

bmunroe

The orginal text;

"The Application layer ensures that the remote communication partner is available, that the needed communication resources exist (a modem, for example), and that both ends of the communication agree on procedures involving data integrity, privacy, and error recovery."
-Source: Chris Bryant CCENT

Slowhand

bmunroe wrote:

The orginal text;

"The Application layer ensures that the remote communication partner is available, that the needed communication resources exist (a modem, for example), and that both ends of the communication agree on procedures involving data integrity, privacy, and error recovery."
-Source: Chris Bryant CCENT

Definitely sounds like he's talking about the DOD model, not the OSI model. He's describing upper-layer functions in a very broad way, which are seperated out according to the OSI model, but are part of one giant layer in the DOD version.

bmunroe

Thanks for your input.

-Brian