Network Class 6

1.  User-server state : cookies

 

many Web sites use cookies four components:

 

     1) cookie header line of HTTP response message

     2) cookie header line in next HTTP request message

     3) cookie file kept on user's host, managed by user's browser

     4) back-end database at Web site

 

example :

     - Susan always access Internet from PC

     - visits specific e-commerce site for first time

     - when initial HTTP requests arrives at site, site creates:

             ‣ unique ID

             ‣ entry in backend database for ID

 

what cookies can be used for :

     - authorization

     - shopping carts

     - recommendations

     - user session state (Web e-mail)

 

2.  Web caches (proxy server)

 

• goal : satisfy client request without involving origin server

 

- user sets browser : Web accesses via cache

- browser sends all HTTP requests to cache

          ‣ object in cache : cache returns object

          ‣ else cache requests object from origin server, then returns object to client

 

More about Web caching

• cache acts as both client and server

• typically cache is installed by ISP(university, company..)

 

why Web caching?

• reduce response time for client request

• reduce traffic on an institution's access link

• Internet dense with caches : enable 'poor' content providers to effectively deliver content

 

 

3.  Conditional GET

 

• Goal : don't send object if cache has up-to-date cached version

 

• cache : specify date of cached copy in HTTP request

• server : response contains no object if cached copy is up-to-date

 

 

4.  Electronic mail

 

• User Agent

       - composing, editing, reading mail messages

       - e.g) Outlook, iPhone mail client

 

• Mail Servers

       - outgoing, incoming messages stored on server

       - mailbox contains incoming messages for user

       - message queue of outgoing mail messages

 

• Mail Protocols

       - simple mail transfer protocol(SMTP)

       - POP3, IMAP, HTTP

 

 

• SMTP protocol

       - from user agent to mail server to send email message

       - SMTP uses persistent connections

       - uses TCP to reliably transfer email message from client to server, port 25

       - command/response interaction

                  ‣ commands : ASCII text

                  ‣ response : status code and phrase

 

✓ comparison with HTTP: 

       - HTTP : pull, each object encapsulated in its own response msg

       - SMTP : push, multiple objects sent in multipart msg

       - both have ASCII command/response interaction, status codes