# 1.1 A Day in the Life of an Application

* Connectivity: connect to one another and exchange data 

#### Network Application 

* Read and write data over network 
* Dominant network: bidirectional, reliable byte stream connection
  * One side reads what the other writes 
  * Operates in both directions 
  * Reliable (unless connection breaks) 

#### Byte Stream Model  

![](/files/-MgWkGqfIBVlC-qCQVf9)

* Either side can close the connection 

#### World Wide Web (HTTP) 

* Hypertext transfer protocol 

![](/files/-MgWkgZstzUoZr766b6J)

* HTTP: document centric 
  * Human readable 
  * Request 
    * "Get / HTTP / 1.1"
  * Response 
    * "HTTP/1.1 200 OK" 

#### BitTorrent 

* Share and exchange large files 
* Client request documents from other clients 
* Swarms 
* When the client wants to download a file, it first has to find something called a torrent file 
  * Find it using the www 
  * Download it using HTTP 
* torrent file describes some information about:
  * data file you want to download 
  * tracker: keeps track of what clients are members of the swarm  
* Dense graphs of connections between clients, dynamically exchanging data 

![](/files/-MgWm4Db7QD7J_afHS7B)

#### Skype 

![](/files/-MgWmWAi9DkA82dov_dE)

* NAT: network address translator 
  * Behind a NAT: can open connections out to the Internet, but other nodes in the Internet can't easily open the connections to you 
  * Red-green area 

![](/files/-MgWmsIXZON4IJ_czGHe)

![](/files/-MgWn3yVlw659qSIeUIV)

* Reverse connection (from B to A)
* This happens in Skype
  * Skype client: personal machine (behind NAT, for security) 

![](/files/-MgWn_paTp0DYceADeCN)

![](/files/-MgWncfqzxrAmJwmWPQ1)


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