University:
York University
Course:
AP/ITEC 1610 | Introduction to Computer Programming
Academic year:

2023
Views:

151

Pages:

2
Author:

Nick T.

Key Points: Cookies maintain some state between transactions. Components: Cookie header line in HTTP response message Cookie header line in next HTTP request message Cookie file kept on user's host, managed by the user's browser Backend database at the website Use Cases: Authorization Shopping carts Recommendations User session state Conditional GET: Don't send the object if the cache has an up-to-date cached version. Client specifies data of the cached copy it received: If-Modified-Since: Server: Response contains no object if the cached copy is up-to-date: HTTP/1.0 304 Not Modified Key Points: Satisfy client requests without involving the origin server. Browser points to a local web cache. If the object isn't in the cache, the cache requests the object from the origin server, receives it, and sends it back to the client. Why use web caches? Reduced response time Reduced traffic on the access link Enables "poor" content providers to deliver content effectively Scenario: Access link rate: 1.54 Mbps RTT from institutional router to server: 2 sec Web object size: 100K bits Avg. data rate from browser to origin servers: 16/sec Avg. request rate from browsers: 1.50 Mbps Performance: Access link utilization: 0.97 LAN utilization: 0.0015 End-end delay: Internet delay + access link delay + LAN delay = 2 sec + minutes + microseconds Improvement with cache: Hit rate: 0.4 (40% serviced by cache, 60% satisfied by origin) Rate to browsers over access link = 0.6 * 1.50 Mbps = 0.9 Mbps Access link utilization with cache = 0.9 / 1.54 = 0.58 (low queueing delay) Avg. end-end delay = 0.6 * origin server delay + 0.4 * cache delay = 0.6 * 2.0 + 0.4 * (milliseconds) ≈ 1.2 secs