Network OSI layer question NFS/SMB

[jaapio]

Dear community,

I'm prepping for CISSP and overall I managed to understand the OSI layer logic and memorized what layer can be found where.

What I still don't get is why NFS is considered to be a layer 5 protocol and SMB a layer 7 protocol. From a users perspective these two protocols both enable sharing of files, only the native platform it does this for differs.

Does anyone understand and is willing to share the logic of this categorization?

[wimremes]

Differentiating between Layer 5 - 7 isn't very productive when analyzing specific services. NFS effectively leverages technologies on all layers 5 through 7 to offer its full set of capabilities as documented here (the TCP/IP Model effectively collapses layer 5 through 7 into one single Application layer):

http://www.tcpipguide.com/free/t_NFSArchitectureandComponents.htm#:~:text=NFS%20Procedures%20and%20O...

In terms of answering questions on tests, you'll just have to memorize that the authors of the CBK believe SMB is at layer 7 while NFS is at layer 5 without diving into the rabbithole of trying to prove them wrong ... and then hope that the exam question developers agreed with that. 

[gidyn]

To be honest, you'll be very unlucky if any exam question hinges on such technical trivia. They want to know if you can do your job, not split OSI hairs.

[jaapio]

I don't assume the exam can be covered by knowing only the protocols and where they reside in the OSI model, though it seems to be one of the highlighted exam essentials, so I beg to differ on the importance. Even in the context of doing a good job I think it is fairly important, as without this knowledge the network protocols look like a spaghetti in which it's difficult to understand why you need multiple to build proper network connections with the desired attributes.

This said, I have to admit NFS is the only protocol that seemed spot on in a TCP/IP model context, but a bit oddly placed in an OSI context, the others looked quite logical in the way they've been placed. While still hoping for someone to share an explanation (as my way of studying is linking theory to stories or practical experience) I think just cramming this odd one seems doable to me.

[dcontesti]

I think Wim provided the best explanation but maybe this pic will assist.

 The three subprotocols work at the application layer of the TCP model and therefore it could be confusing to some that it works at layer 5.

SMB works typically at layer 6 and 7 of the OSI model.

However, I doubt that the exam will go to this level of understanding but I could be mistaken.

For your reference, here are some decent resources

https://mac.eltima.com/what-is-smb.html

https://www.tcpipguide.com/

https://docs.microsoft.com/en-us/windows/win32/fileio/microsoft-smb-protocol-and-cifs-protocol-overv...

Regards

d

[JoePete]

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@jaapio wrote:

What I still don't get is why NFS is considered to be a layer 5 protocol and SMB a layer 7 protocol. From a users perspective these two protocols both enable sharing of files, only the native platform it does this for differs.

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My networking is a bit rusty, but I'll have a go at it. One of the things NFS gets out of layer 5 is Remote Procedure Call to set up the mounting of remote directories. While in the end, NFS and SMB may be sharing files, NFS can be more efficient about it (thanks to RPC). But that may not be much of a distinction. Or put another way, it says something that under the TCP/IP model, the application layer is the same as OSI's 5-7. A crude analogy: Let's say you are a big online retailer. To deliver a package, you can use existing delivery services that have their own rules and tracking, and that can make things easier on you (NFS). But you could also have your own delivery fleet (SMB). To the user it might not matter, but to your infrastructure it does.