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TCP/IP Model

The layering the real internet uses — application / transport / network / link / physical.

TCPIPLayeringBasics
核心 · Key Idea

In one line: The TCP/IP model is what today's internet actually runs on: application / transport / network / link / physical (sometimes "four layers" by merging link and physical). It grew up from real protocols, not from a design document.

Five layers at a glance#

+---------------------+
| Application          | HTTP / DNS / SSH / SMTP / gRPC
+---------------------+
| Transport            | TCP / UDP / QUIC
+---------------------+
| Network              | IP / ICMP / routing protocols
+---------------------+
| Link                 | Ethernet / Wi-Fi / PPP
+---------------------+
| Physical             | Copper / fibre / radio
+---------------------+

Why not seven#

OSI defines application / presentation / session / transport / network / link / physical. In TCP/IP:

  • No standalone session or presentation layer — their work is split between application (HTTP / TLS) and transport.
  • Link and physical are conceptually distinct, but in practice are often spoken of together (one NIC).

Analogy#

打个比方 · Analogy
  • OSI is like theoretical physics: symmetrical, beautiful.
  • TCP/IP is like engineering physics: gets the spaceship flying — fewer layers, actually used.

Key concepts#

ApplicationApplication
Business semantics. HTTP / DNS / gRPC / SSH — protocols you call directly.
TransportTransport
End-to-end 'reliable' (TCP) or 'unreliable but fast' (UDP).
NetworkNetwork / Internet
Inter-subnet routing. IP labels packets 'from where to where'.
LinkLink
Frames over a single physical link.
PhysicalPhysical
Bits and signals.

How it works#

Each layer adds its header → receiver peels them off → hands to the application.

Practical notes#

  • Beginners only need TCP/IP's five layers: physical / link / network / transport / application.
  • L4 / L7 are everyday terms: load balancers route by L4 (ports) or L7 (HTTP) — a common interview topic.
  • Wireshark / tcpdump capture is broken down by these layers.
  • Where does TLS sit? Strictly "between application and transport"; engineering-wise easiest to think of as part of the application layer.

Easy confusions#

TCP/IP 5 layers
Engineering reality: physical / link / network / transport / application.
Sufficient and matches actual protocols.
OSI 7 layers
Teaching model: presentation / session added.
Not separately implemented.

Further reading#