# 1.2 The 4-Layer Internet Model

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* Internet: end-hosts, link, and routers&#x20;
  * Data delivered hop-by-hop over each link in turn&#x20;
    * In packet: data, header (from, to)&#x20;
* **Link layer**: carry data over one link at a time&#x20;
  * E.x. ethernet, WiFi&#x20;
* **Network layer**: deliver packets end-to-end across the Internet from the source to the destination&#x20;
  * Network layer packets: datagrams&#x20;
  * Hands datagrams to the Link Layer, telling it to send the datagram over the first link&#x20;
    * Link layer provides service to the network layer&#x20;
  * Special&#x20;
    * Must use the Internet Protocol (IP)&#x20;
      * IP makes a best-effort attempt to deliver datagrams to the other end, but no promises&#x20;
      * IP datagrams can get lost, can be delivered out of order, and can be corrupted. No guarantees&#x20;

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* **Transport Layer**&#x20;
  * TCP: transmission Control Protocol&#x20;
    * Ensure orders
    * Provides service to application that guarantees *correct, in-order* delivery of the data running on top of the network layers&#x20;
  * UDP: user datagram protocol&#x20;
    * No delivery guarantees&#x20;
* **Application Layer**&#x20;
  * Typically, Bi-directional, reliable byte stream&#x20;

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* Numbering system:&#x20;
  * Network layer: layer 3&#x20;
  * Ethernet: layer 2&#x20;
  * Application: layer 7
