> For the complete documentation index, see [llms.txt](https://sliu583.gitbook.io/blog/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sliu583.gitbook.io/blog/networking/index/computer-networking-a-top-down-approach/stanford-cs144/chapter-1/1.2-the-4-layer-internet-model.md).

# 1.2 The 4-Layer Internet Model

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* Internet: end-hosts, link, and routers 
  * Data delivered hop-by-hop over each link in turn 
    * In packet: data, header (from, to) 
* **Link layer**: carry data over one link at a time 
  * E.x. ethernet, WiFi 
* **Network layer**: deliver packets end-to-end across the Internet from the source to the destination 
  * Network layer packets: datagrams 
  * Hands datagrams to the Link Layer, telling it to send the datagram over the first link 
    * Link layer provides service to the network layer 
  * Special 
    * Must use the Internet Protocol (IP) 
      * IP makes a best-effort attempt to deliver datagrams to the other end, but no promises 
      * IP datagrams can get lost, can be delivered out of order, and can be corrupted. No guarantees 

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* **Transport Layer** 
  * TCP: transmission Control Protocol 
    * Ensure orders
    * Provides service to application that guarantees *correct, in-order* delivery of the data running on top of the network layers 
  * UDP: user datagram protocol 
    * No delivery guarantees 
* **Application Layer** 
  * Typically, Bi-directional, reliable byte stream 

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* Numbering system: 
  * Network layer: layer 3 
  * Ethernet: layer 2 
  * Application: layer 7