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For a long time, I have this kind of silly question: How we can say some network protocols are working in X layer of the OSI model?

For example, HTTP working in L7 of OSI, OSPF is L3, ICMP is L4, TCP and UDP are L4.

Can anyone suggest how I can understand easily?

since we can say BGP is a routing protocol, but it is using TCP for L4 as a transport protocol. So, is BGP L3 or L4?

RIP is a L3 protocol, but it is using UDP for transport.

DHCP uses both UDP and BOOTP header, so in which layer is DHCP working?

Kindly give me your suggestion to understand better.

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In addition to what others have said regarding the services each layer provides, different layers typically also differ in terms of how far they are encapsulated within packets (and in what order they are performed at each sender/receiver along the way). For example, a common scenario is where application layer (HTTP layer 7) header and data has transport layer (TCP - layer 4) bits added to the beginning of it, which has IP (layer 3) bits added to the beginning, which has Ethernet (layer 2) data added to it. The complete message would look something like this:

[ Ethernet bits [ IP bits [ TCP bits [ HTTP bits [ App payload ] ] ] ] ]

This makes sense, because lower layer functions need to be performed last at the sender, and first at the receiver. At each hop a networking device (host, router, switch) will look at the lower (outer) layers first, and only move inward as far as necessary.

This may not be 100% true all the time, but it's another way I like to think about it.

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The OSI model is not 100% conformed to. It is a guide line for understanding and organizing the various functions required to create a network, or an inter-network, or even the Internet.

So not everything falls perfectly within a single OSI layer. However, understanding the primary purpose of each layer will help categorize different protocols into different layers.

Layer 1 is primarily responsible for providing a mechanism or medium to move bits (1s and 0s) from one device to another. Twisted Pair wiring, Serial cables, Fiber Optics are all considered Layer 1 technologies.

Wifi, though it doesn't have a physical medium, is also considered a Layer 1 technology since it has the ability to move 1s and 0s from one device to the next.

Anecdotally, you could consider the string between two cans a Layer 1 "technology", since it has the ability to move 1s and 0s from one device to another.

Layer 2 is concerned with what I like to call Hop to Hop delivery. It is responsible for putting the 1s and 0s on whatever L1 technology is being used, as well as retrieving the 1s and 0s from whatever L1 technology is being used.

The Network Interface Card (NIC) is an example of a Layer 2 technology.

To facilitate "hop to hop" delivery, Layer 2 uses a MAC address -- which is effectively the identification of a particular device's NIC.

Layer 3 is responsible for what I like to call End to End delivery. This is where protocols like IP exist.

Between two end points on either side of the Internet, there might be 10s or 100s of NICs that a packet must pass through. Layer 2 will get it from one NIC to the next. But Layer 3 will determine where the final destination is.

Layer 2 and Layer 3 work together to move Data from one end point to another through each individual "NIC to NIC" hops between the end points.

Layer 4 is responsible for segregating Network Streams. All the 1s and 0s carried by L1 arrive on your PC on the same NIC. Something has to distinguish which 1s and 0s belong to your Internet Browser, or your Music Streaming application, or your Chat program, or Operating System updates, or a plethora of other applications that send or receive data from the Internet.

Layer 5, 6, and 7 all blend together. I wrote a potential example of the initial intention of each of those layers on reddit, but understand that was mostly an example, not a description of how it actually works:

L5 - Session - this could be something like HTTP cookies, keeping individual browser sessions different from the next. It is a way identify and keep track of different set of bits, within the same L4 TCP/UDP connection.

L6 - Presentation - The bits show up as 1s and 0s. Something needs to deterimine how those 1s and 0s should be interpretted. For example, should bits be grouped into sets of 8 and converted into letters according to ASCII. Or should the bits be converted to large numbers and used to decrypt the content.

L7 - Application - Now that L6 has turned the bits into letters, we need to turn those letters into something meaningful. For example, the letters GET /index.html HTTP/1.1 mean something in the HTTP application, they are a command to fetch the index.html page from the server.

Source


Given the brief description of each Layer, you can then apply that knowledge to determine what layer a particular protocol operates at. For example...

  • HTTP is not a function to move 1s and 0s across a wire.
  • HTTP is not a function that puts and pulls 1s and 0s on/off a wire
  • HTTP is not a function that enables end to end delivery of data
  • HTTP is not a function that segregates all the data streams from a wire.

As such, HTTP can be considered as a L5/L6/L7 protocol. I've seen this abbreviated as a L5 protocol, or L5+, or L7. It all means the same thing.

But again the OSI model is not perfect, and everything doesn't perfectly conform to a particular layer. For example...

  • ICMP is not a function to move 1s and 0s across a wire.
  • ICMP is not a function that puts and pulls 1s and 0s on/off a wire
  • ICMP is not a function that enables end to end delivery of data
  • ICMP is not a function that segregates all the data streams from a wire.

Despite all that, ICMP is considered a L3 protocol, mainly because its primary usage (ping) tests if a packet can get from one end of the Internet to the other. AKA, it is primarily associated with end to end delivery.

As for the rest of the protocols you listed, this is where they typically are associated with:

  • L3 - OSPF, ICMP, BGP, RIP
  • L4 - TCP, UDP
  • L5/L6/L7 - HTTP

DHCP is a tricky one. On one hand, it is primarily concerned with providing end hosts with IP addresses, but on the other its communication does not happen over IP. Personally, despite that, I like to consider DHCP a L3 protocol.

ARP is also the other often debated tricky one. It communicates without an IP (L3) header, but its purpose is to resolve IP addresses to MAC addresses -- and it is only used by L3 devices. It is sometimes considered both layers, or in between both layers. Or sometimes simply identified as a L2 protocol.

It doesn't really matter where you land on the debate, so long as you understand the view points from both sides.


Hopefully that will help you distinguish or place protocols in a particular layer. Again, keep in mind, it never works out perfectly. The OSI model is a conceptual model, not an explicit standard everyone strictly adheres to.

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The OSI model is just a model, one of many, and it reflects an ideal. The real world doesn't fit so neatly into the models, and there is much controversy about which protocols operate in which layers of the OSI model.

OSes have never really adopted the OSI model. Application programmers tend to lump those layers 5, 6, and 7 into the application because OSes don't provide the separate services. If you think about it, web browser applications use all three of those layers: session, presentation, and application.

When it comes to routing protocols, you can distinguish the routing part from the communication part. Routing happens at layer-3 because packets are switched based on the layer-3 protocol (IPv4, IPX, IPv6, etc.) address. The communication between instances of a routing protocol on separate routers can conceptually be an application-to-application function. BGP uses TCP for this, but other routing protocols use other protocols to communicate. You could say that BGP routes at layer-3 and transfers prefixes using layer-4.

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Protocols within a suite are defined with a specific purpose in order to make the design and operation easier.

These protocols inside the protocol stack communicate with other layers based on either type of media( bits, frames, packets, segments etc ) or transport ( TCP or UDP ) or the kind of desired application ( this is dependent on the software implementation, but it might require use of additional suites like IPsec ).

We can always tell which protocol works on which layer, if knowing that the protocol conforms the purpose of that particular layer.

E.g TCP works at the Transport layer because although it complements the function of IP, it provides a method of transportation between two hosts.

Hope that helps :)

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First of all, this is not a silly question to ask which protocol works on which x layer .

APPLICATION LAYER: BOOTP, DHCP, DNS, FTP, HTTP, HTTPS, IMAP4, PING, POP3, NSLOOKUP, NTP, SFTP, SMTP, Telnet, TFTP

PRESENTATION LAYER MIME, SSL, TLS

SESSION LAYER: RTP, SIP

TRANSPORT LAYER: TCP, UDP

NETWORK Layer: ARP, ICMP, IGMP, IP, IPSec, RARP

DATA LINK Layer: L2TP, PPP, PPTP, SLIP

PHYSICAL LAYER: IEEE 802.3 (Ethernet)802.5 (Token Ring), 802.11 (Wi-Fi)

Here is this infographic on this link that might help you which protocol works on which layer of OSI layer model.

OSI Layer Model Infographics

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