# 1.4 A Day in the Life of a Packet

* Application: stream of data
* Transport: segments of data 
* Network: packets of data 

#### TCP Byte Stream 

* 3-way handshake 
  * Client sends a synchronize message to the server ("SYN")
  * Server responds synchronize message that also acknowledges the client's synchronize, or ("SYN/ACK")
  * Client responds by acknowledging the server's synchronize ("ACK") 
* Network layer: responsible for delivering packets to computers 
* Transport layer: responsible for delivering data to applications 
* To open a TCP stream, needs 
  * IP address: address that network layer uses to deliver packets to the computer 
  * TCP port: tell the computer's software which application to deliver data to 
    * Web server: TCP port 80 

![](/files/-MgXCLIqzOr-yvKJ23UC)

#### Inside the Stream

* Hop: link connecting two routers 
* A router can have many links connecting it. As packet arrives, a router decides which of its links to send it out on. 
* Routers have IP addresses 
  * Can deliver to its own software instead of forwarding it 

![](/files/-MgXChvuBoiylngLKzwz)

#### Inside each hop 

![](/files/-MgXCtiiu1NbJ1ccHpEr)

* "Best": most specific match (longest prefix match) 
* Default: least-specific pattern (matches every IP addresses) 
  * Useful in edge networks 
    * E.x. stanford example, if the destination address isn't stanford, then should send it out to larger internet 

#### Under the Hood 

![](/files/-MgXDPE_BLN3nH0XDHWb)

* 3-way handshakes 
* More packets for HTTP requests and response 
* tracerout:
  * Shows the hop that packet takes 
  * traceroute -w 1 [www.cs.brown.edu\&#x20](http://www.cs.brown.edu\&#x20);

![traceroute to brown  ](/files/-MgXEm98rPuo79Rspr5n)

![traceroute to MIT web server ](/files/-MgXF0QKZ2Eun41nwhS4)

* 20 hops, less than 90 ms 


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