Web Page Delivery: Proxies
CS6204: Java and the WWW
Pull Acceleration: Caching Proxy Servers
Caching automatically migrates document copies from servers to browsers
(But caching has limitations.)
How proxies work:
-
Client configures Web browser to use a proxy P either for caching
or as firewall
-
When client requests URL http://S/doc.html, request is sent to P
(not S).
-
If P has doc.html, and it's not expired, P returns doc.html to client.
-
Else if doc.html is expired, send GET-IF-MODIFIED with last modification
time to server S. S either returns new version of doc, or a return code
indicating P's copy is still up to date.
-
Else P doesn't have doc.html, and issues GET to server S.
Popular as proxy-only in US, essential as caches outside US
Caches can optimize one of three things:
1. Bandwidth: Bytes transferred over network perhaps retain large documents
2. Server hit rate perhaps retain popular documents
3. Time users wait to load document perhaps retain documents from other
countries
Caching proxies be used in four locations
Inter-cache protocol ICP allows caches to share data items.
Some popular proxies:
-
Harvest (outdated today, but introduced ICP)
-
Squid (public domain decendant of Harvest, used by most ISPs worldwide)
-
Cache3D (commercialized version of Harvest, owned by NetApp)
-
Netscape Proxy Server (department level proxy; comes with SuiteSpot)
-
Microsoft Proxy (department level proxy, and semi-firewall; comes with
BackOffice)
-
CERN Proxy
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Last modified on 30 May 1997.
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