URI schemes DCU proxy servers

The Web

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HTTP client

Web browser

Uses MIME types.
(a) Plug-in - Runs inside browser process.
(b) Helper application - Separate process.

• MIME types
• Data formats
• Web browsers

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HTTP server

Doesn't make separate disk access for every file request - too slow.
Instead maintains cache in memory of frequently accessed files.
Multi-threaded.
Site spread over multiple disks to help many reads going on at once.

For high-demand sites: Multiple copies of entire site - "server farm" - front end routes requests to different CPUs.

Problem: OK to have all (small size) requests come in through one front end and get routed to searching nodes.
Not OK to have all (large size) replies go back through one front end - bottleneck.
Solution: TCP handoff - trick to have the searching node reply directly in a manner that is invisible to client.
The reply load is therefore distributed over all the nodes.

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URI schemes

Some URL formats.

URI schemes listed above:

• http, https
• ftp (no password) hardly used any more - pre-web system
  ftp (with password) still important - now secure ftp - but not done through clicking a link
• file: very useful (may not need prefix)
• news: usenet - not as important as used to be
  1. read usenet on Google Groups
  2. so many other places to talk now - discussion websites, blogs
• gopher - port 70 - not used any more - pre-web system
• mailto: very useful - but spammers search for these
• telnet: telnet and ssh still important - but not done through clicking a link

Others:

• mms - old streaming media
• rtsp - streaming media (old RTE streaming)
• rtmp - streaming media (new RTE streaming, also TG4)
• callto - launch phone call
• wtai - WAP
• view-source:URL - can make a link to view source (does not work on all browsers)

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HTTP and HTML reference

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How to write a web page

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Keeping state

Relating one client-server stateless request with other client-server requests.

Identify user (pay-to-view, register, personalisation). Shopping carts.

• cookies

• PHP cookie handling
• PHP sessions are more secure - avoid users spoofing someone else's cookie (e.g. for user login)

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Performance (client-side)

Caching

• Browser maintains cache.

• Site-wide (or ISP-wide) cache via proxy server.

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DCU proxy servers

In DCU, communication through proxy apparently no longer required. Though proxies all still seem to be up and working.

1. wwwproxy.computing.dcu.ie = 136.206.11.243 (forwards requests through 136.206.11.249)
   • port: 8000

2. proxy.dcu.ie = alternates between returning a few different IP addresses (for load balancing)
   • port: 8080 or 3128
   • lookup shows it alternates between:
     1. proxy1.dcu.ie = 136.206.1.20
     2. proxy2.dcu.ie = 136.206.1.49
     3. proxy3.dcu.ie = 136.206.1.17

To set proxy, something like:

1. Firefox - Tools - Options - Advanced - Network - Settings
2. IE - Tools - Options - Connections - LAN settings

You may use a proxy auto-config (PAC) file:

1. http://www.computing.dcu.ie/proxy.pac
2. http://proxy.dcu.ie/proxy.pac

Test the IP address other sites see:

• DCU
• Outside DCU (just looks at REMOTE_ADDR)
• Outside DCU (looks at HTTP_X_FORWARDED_FOR)