Does the TCP/IP model have 4 or 5 layers?

Question

In some sites , It is given that the TCP/IP model have 4 layers; network access, internet, transport and application. But in other references , the model is shown to have 5 layers (instead of network access, physical and link are used).

What is the standard actually?

Answer 1

It seems like a combination of circumstances here has led to some confusion. Just so everyone is clear, The TCP/IP Model has a slightly different set of layers when compared to the OSI Model.

To be clear, the OSI Model's top three layers - that is: Application, Presentation, and Session - are essentially collapsed into the Application layer in TCP/IP. Additionally, the bottom two layers - Physical and Data Link - are combined into the Network Access layer for TCP/IP.

Therefore, there are 4 layers in the TCP/IP Model. Specifically, they are the Network Access Layer, Internet Layer, Transport Layer, and Application Layer.

You can find some (Difficult to read) technical information in RFC 1122, and some better-presented educational information provided by the University of Pittsburgh.

Answer 2

I believe you are conflating the DOD\DARPA stack for IP with the OSI model.

See here for how the layers of each model map to one another.

http://netcert.tripod.com/ccna/internetworking/dod.html

Keep in mind, there are aspects of protocol modeling that are purely conceptual, and don't necessarily mimic analog reality. For instance, in modern networking, the OSI L1 and L2 are implemented in the network card's circuitry and in its driver code, but there's no good way to tell where one ends and the other begins. After all, the task of sending a media-compliant frame across the network cannot be separated from the task of constructing a 802.3 frame with all the correct data structures.

The layers are not necessarily concrete (especially when viewed from app perspectives), and as such there may not be a "correct" answer. A protocol can have as many layers as as you choose to slice it up into, regardless of the architecture of the code and the circuitry.

Models are often just how you think about a problem, and in this case, both models are valid for different analytic purposes.

Answer 3

Not sure if this helps but.

The original TCP/IP Network Model started with 4 Layers

1. Link Layer
2. Internet
3. Transport
4. Application

The second version of TCP/IP became 5 layers: changed the name of Internet Layer to Network layer and divided the link Layer to 2 layers

1. Physical
2. Data-Link
3. Network
4. Transport
5. Application

References

Daou, I. (n.d.). Why we should understand TCP/IP and OSI Models? Retrieved from CCNA Hub: http://www.ccnahub.com/ip-fundamentals/understanding-tcp-ip-and-osi-models/

Answer 4

It can be argued that there are 4 layers, even in the 5-layer model, as it seems that in the 5-layer model they are merely splitting the Link Layer into 2 layers: hardware and link.

The TCP/IP model (4-layer model) that is being talked about is the RFC 1122, which aligns with Cisco Academy: the important player here

Here is a picture of the various layered models, I'm unable to embed it yet.

reference: Internet protocol suite - Wikipedia

Answer 5

Maybe a bit late to answer to this question, but actually it's simple:

These are the traditional TCP/IP 4 layers as defined by RFCs 1122 and 1123:

• 4. Application
• 3. Transport
• 2. Internet
• 1. Link

I found articles referring to the "Internet" Layer as "Internetwork" Layer and the "Link" Layer as "Network Access" Layer. And a lot of other articles also referring to a 5-layers model, but these are as far as I know, not referenced in any RFC.

So, the 5-layers model can indeed help to make the comparison with the OSI model a bit smoother, and help in education, but it's not the official model.

However, the 5-layers model is the modern model that everyone uses today. Read this article to understand why. You can also see a clear figure there.

Answer 6

I was taught they were 4 in TCP/IP mapped as follows over "iso/osi" layers, example of protocols between square brackets:

ISO/OSI          TCP/IP (extended)       Data unit's name (ISO-OSI)
Application   -> Application [telnet]        => Packet   (APDU)
presentation  -> Application                 => Packet   (PPDU)
Session       -> Application (Security) [TLS]=> Packet   (SPDU)
Transport     -> Transport [TCP/UDP]         => Segment  (TPDU)
----------Above this line the communication is end to end-------------
Network       -> Network [IP]                => Datagram (Packet)
DataLink      -> Host To Network             => Frame    (Frame)
Physical      -> Host To Network                         (Bit)

Answering your question the TCP/IP model has 5 layers: Application, tansport, network and host-to-network. But I'd add the 6th layer as now more and more common TLS security because it has it's own identifiers and is a layer between TCP and application.

I can remember this by finding the "identifiers" for each data unit relatively to more used connections:

 Application has application defined ID  
(TLS         has [implicit] message sequence number)  
 TCP         has segment sequence number  
 Network     has IP address  
 H2N         has MAC address

This makes, in my opinion, HTTPS as an example of "session" level (I'd call it "security" level because there is where most security operations could be bundeled) because each TLS connection has its own identifiers and is incapsulated into TCP segments.