Chapter 2. Application Layer

All of my writings are based on Computer Networking: A Top-Down Approach, 8th edition by James F. Kurose and Keith W. Ross.

 

2.2.4 User-Server Interaction: Cookies

As HTTP server is stateless, we need Cookies to identify users, either to restrict user access or serve content as a function of the user identity.

 

Keeping user state with cookies

Four components of Cookie technology

1. HTTP response message
2. HTTP request message
3. the user's browser
4. a back-end database at the Web site

How the Cookie works

1. User visit "something site" - HTTP request message
2. The "something site" web server sends HTTP response message
   -> Set-cookie : something
3. The user's browser set cookie
4. User revisit "something site" use cookie 
5. "something site" remembers the user

2.2.5 Web Caching

Web cache (= proxy server): network entity that satisfies HTTP requests on the behalf of an origin Web server. 

Clients requesting objects through a Web cache

Sequence of Web Cache

1. browser establishes a TCP connection to the Web cache, and sends an HTTP request for the object to the Web cache.
2. The Web cache checks if it has a copy, if it has, the Web cache returns the object within an HTTP response message
3. If it doesn't have the copy, the Web cache opens a TCP connection to the origin server.
4. Web cache sends an HTTP request for the object into the cache-to-server TCP connection. When the server receives this request, the origin server sends the object within an HTTP response to the Web cache.
5. Web cache receives the object, stores a copy, and sends a copy within an HTTP response message(using the TCP connection between the client browser and the Web cache)

 

The reason Web caching has been deployed on the Internet

1. Web cache can substantially reduce the response time for a client request.
   -> bottleneck bandwidth(client - origin server) VS bottleneck bandwidth(client -  cache) 
   particularly, if bottleneck bandwidth (client - origin) is much less than the other one
2. Web caches can substantially reduce Web traffic on an institution's access link to the Internet.
   -> Does not have to upgrade bandwidth - reduce cost
   substantially reduce Web traffic on the Internet whole, improve performance for all applications