OSI Model.. is the Cisco book wrong?

sentimetal

Heyo. I just started learning network fundamentals with the Cisco Network Academy via a community college. We're currently going over the OSI model; the first chapter vaguely going over each layer and comparing TCP/IP to it, etc. and the next chapter going over the application layers. This week in class, we did a review of the test for last week's chapter (the vague overview of OSI) and one of the questions was "In which layer is data segmented?" - which I answered the transport layer... which is what the book originally said. The answer key, which the instructor went over, says the Network Layer provides data segmentation. I can't complain too much, as I received a 97% on the test... but I can't help but wonder why they're so contradictory.

I've read many different sources, including http://www.techexams.net/technotes/ccna/osimodel.shtml and they all indicate that the Transport layer provides data segmentation/desegmentation. I

Now, keep in mind I have no networking knowledge prior to the three weeks of class I've taken so far (I come from an accounting background), so pardon the elementary question. I have, however, been studying 8 hours+ a week for each chapter, and gobble up any new information I can that pertains to my class without going too far ahead.

If I'm correct, and it is the Transport layer, I'm a bit concerned that my instructor didn't catch the error.

Anyone care to elaborate?

CrapMasterZero

sentimetal wrote: »

Heyo. I just started learning network fundamentals with the Cisco Network Academy via a community college. We're currently going over the OSI model; the first chapter vaguely going over each layer and comparing TCP/IP to it, etc. and the next chapter going over the application layers. This week in class, we did a review of the test for last week's chapter (the vague overview of OSI) and one of the questions was "In which layer is data segmented?" - which I answered the transport layer... which is what the book originally said. The answer key, which the instructor went over, says the Network Layer provides data segmentation. I can't complain too much, as I received a 97% on the test... but I can't help but wonder why they're so contradictory.

I've read many different sources, including http://www.techexams.net/technotes/ccna/osimodel.shtml and they all indicate that the Transport layer provides data segmentation/desegmentation. I

Now, keep in mind I have no networking knowledge prior to the three weeks of class I've taken so far (I come from an accounting background), so pardon the elementary question. I have, however, been studying 8 hours+ a week for each chapter, and gobble up any new information I can that pertains to my class without going too far ahead.

If I'm correct, and it is the Transport layer, I'm a bit concerned that my instructor didn't catch the error.

Anyone care to elaborate?

What I learned is that Transport Layer does segmentation....in fact PDUs in Transport Layer are called "segments", Network Layer it's called "packets" while Data Link Layer is called "frames"

badboyeee

CrapMasterZero wrote: »

What I learned is that Transport Layer does segmentation....in fact PDUs in Transport Layer are called "segments", Network Layer it's called "packets" while Data Link Layer is called "frames"

This is what I learned too. I had Chapter 4 Transport Layer exam last weekend got a 100%. Hope I score that for CCNA too

docrice

The transport layer (4) does produce "segments." The network layer (3) produces fragments if an MTU in the path to the destination is exceeded (and assuming the Don't Fragment bit in the IP header isn't set). Seemingly the same concept, and I'm guessing the instructor got these confused after a long day. The term "segments" in reference to the OSI model does refer to layer 4.

sentimetal

Another thing I'm having trouble wrapping my head around are application layer protocols such as SMTP and POP3. In the book, it illustrates the MUA sending the email to the MTA/MDA using SMTP and then delivering the message via POP3... but in my mind, this is completely replacing the OSI model and the TCP/IP stack. If the SMPT and POP3 protocols are instructions/rules, etc for the application layer... how exactly is mail being "delivered and retrieved using SMPT and POP3"?

docrice

The app-layer protocols add application-specific information in the payload headers. Then layer 7 sends this down to layer 6, where it might add its headers. Layer 6 sends this to layer 5 after adding its headers, and so on. In many cases, you don't necessarily have layers 5 and 6 headers as this depends on what protocols are involved. This is kind of the reason why in the TCP/IP four-layer model we combine the top three layers of the OSI into one bucket called Application.

Keep in mind that the OSI model is just an abstract way of viewing the information flow and the encapsulation / decapsulation process.

Mail is relayed between servers via SMTP. A client who wants to read his inbox will connect to the mail server via POP3 to retrieve it (or log into a web GUI, use IMAP, etc.). The SMTP-to-SMTP and client-to-mail-server-via-POP3 are separate, independent connections.

chmorin

sentimetal wrote: »

but in my mind, this is completely replacing the OSI model and the TCP/IP stack.

You seem to be thinking about it in the wrong fashion. Once something reaches the application layer, it allows the applications to handle the data that was passed up the stack the way it chooses to. In order for this information to be usable by the application, it must 'speak the same language' as the other endpoint.

Think of it as the OSI/TCP stack's are individuals 'talking'. Now, the language they speak determine if they two actually understand what they are 'saying'. Protocols are the language, and the rest of the OSI/TCP stack is the means of communication. I can 'speak'(Transmit data through the OSI Stack) to you in 'German'(Protocol) but that does not mean you can understand it unless you 'speak' in 'German' too.

Savvy?

sentimetal

I can understand that analogy well, thank you. But what gets me is... why, then, are emails being sent over a network referred to as "sent via SMTP"? I understand the data (including the text-based SMTP protocol), gets segmented, addressed, etc and shipped over the network and then is translated by the app on the other side, but wouldn't it make more sense to say "sent to an mail server as SMTP protocol"?

Perhaps I am also /overthinking/ this as well.

/edit: yes, I am overthinking it. Because if I can' understand how/why web page data is sent from a web server to a client "via http" then surely I can understand that email is sent via SMTP to an MTA/MDA and is delivered/pulled via POP3 or IMAP.

Sorry... 7 hours of studying after 9 hours of work will do this to you, apparently.