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Applications in OSI layers

How applications interact with the OSI model, real-world examples and the role of networking devices at each layer.

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By now, we all know what is OSI model, it layers and all that stuff we studied about in out college lectures. Right?

7 layers!! From 1 to 7, they go like Physical, Data Link, Network, Transport, Session, Presentation and Application Layer.

But, where is my application sitting with respect to this model???

The intuitive answer would be..."duh, my application is in the Application Layer!" But in some cases it is more interesting than that.

The Layers and what they do.

The 7 layers each describe a specific networking component.

• Layer 7: Application - HTTP/FTP/gRPC/etc
• Layer 6: Presentation - Encoding, Serialization
• Layer 5: Session - Connection establishment, TLS
• Layer 4: Transport - UDP/TCP
• Layer 3: Network - IP
• Layer 2: Data link - Frames, Mac address, Ethernet
• Layer 1: Physical - Electric signals, fiber or radio waves

Example

Lets say you have an application that sends a POST request to an HTTPS webpage.

Sender (the Client)

• Layer 7
  • POST request with JSON data from the client to HTTPS server
• Layer 6
  • That JSON data is serialized and encoded to flat byte strings.
  • This helps the data to become platform independent, and into a standardized, easy-to-transfer format.
• Layer 5
  • Request to establish TCP connection/TLS (Also sometimes we might not need to establish a session/connection, let's say in case of UDP)
• Layer 4
  • Sends SYN request target port 443 (default port for https)
  • SYN is a request to establish a TCP connection, which might not be needed for UDP
• Layer 3
  • SYN is placed an IP packet(s) and adds the source/dest IPs
• Layer 2
  • Each packet goes into a single frame and adds the source/destination MAC addresses
• Layer 1
  • Each frame becomes string of bits which converted into either a radio signal (wi-fi), electric signal (ethernet), or light (fiber)

But this structure is not set in stone. Few layers can inherit from other layers.

Receiver (the Server)

• Layer 1
  • Radio, electric or light is received and converted into digital bits
• Layer 2
  • The bits from Layer 1 is assembled into frames
• Layer 3
  • The frames from layer 2 are assembled into IP packet.
• Layer 4
  • The IP packets from layer 3 are assembled into TCP segments
  • Deals with Congestion control/flow control/retransmission in case Of TCP
  • If Segment is SYN we don't need to go further into more layers as we are still processing the connection request
• Layer 5
  • The connection session is established or identified
  • We only arrive at this layer when necessary (three way handshake is done)
• Layer 6
  • Deserialize flat byte strings back to JSON for the app to consume
• Layer 7
  • Application understands the JSON POST request and your express json or apache request receive event is triggered

Visualization

This image shows how the flow goes for a HTTP request (TCP) We start with the client, and once it reaches the session layer, and sees, oh we need to create a session, it keeps the serialized data, does a 3 way handshake first with SYN and ACK, and once the connection is established, the data is sent.

But what happens in real world?

Your client isn't connected to the server directly right? There's switches, routers, load balancers, proxies, VPNs, CDNs and many more things in between. Then what? What happens there?

Those things look into your data. Not literally but yes. For example:

• Switches look till the layer 2 (Data Link for the MAC addresses) to see which network to send to.
  • Wireshark (in raw capture mode) can capture Ethernet frames before IP decoding
• Routers look till the layer 3 (Network) to see which IP to 'route' the data to.
  • Cisco IOS / Juniper Junos routers – route IP packets via OSPF, BGP, etc.
• Firewalls look till the transport layer to see which IPs & ports to send the data to and which to block. (ISP uses this method to block certain websites)
• Load balancers/CDNs look till layer 7 (Application) to cache data/ or the data is sent to a different server based on the /xyz/abc path.
  • Cloudflare operates at L3–L7: DNS, Anycast routing, DDoS protection, HTTP proxying, TLS termination.
• VPNs are essentially a layer 3 (Network) tunneling mechanism, encapsulating your packets to provide privacy and security over public networks. (esssentially they put our packets and put them in a different IP packet and send it to the destination IP)

Shortcomings of OSI model

• OSI model has too many layers which can be hard to comprehend
• Hard to argue about which layers does what exactly
• Simpler to deal with layers 5-6-7 as just one layer (application) - TCP/IP model does just that, it is a protocol stack