ICAP: the Internet Content Adaptation Protocol

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ICAP theInternet ContentAdaptation Protocol

• ICAP Forum Meeting
• February 14, 2000

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what is icap?

• ICAP lets clients send HTTP messages to servers
  for some sort of adaptation
• In essence, an RPC mechanism (Remote Procedure
  Call) for HTTP messages
• An adapted message might be a request
• Modify request method, URL being requested, etc.
• ...or, it might be a reply
• Change any aspect of delivered content

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why icap?distribute load, orintegrate 3rd party
components
Specialized Servers for Compute-Intensive Operatio
ns
3rd-Party Devices
Transcoder Content Filter
Virus Checker Translator
Web Server or Proxy
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why icap?make local content smarter
Congested, Slow, Distant, and/or Expensive Link
Content distribution network, or (transparent)
proxy
Clients
Local sources of content Better for everyone
(client, network, server)
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what icap isnt(for now)

• A way to specify adaptation policy
• A configuration protocol
• A protocol that establishes trust between
  previously unrelated parties
• In other wordsICAP defines the how, not who,
  when or why

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icap general design

• Simple, simple, simple CGI should be able to
  turn a web server to an ICAP server
• Based on HTTP (special headers)
• Three modes
• Modify a request
• Satisfy a request (like any other proxy)
• Modify a response

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request modification(typical)
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Proxy Cache (ICAP Client)
Origin Server
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ICAP Server
ICAP server modifies a request modified request
continues on its merry way
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request modification

• The request is passed to the ICAP server (almost)
  unmodified just like a proxy would
• Whatever method was used by the user is used by
  the ICAP client as well
• The ICAP server sends back a modified request,
  encapsulated in response headers
• Body, if any (e.g. POST), may also be modified

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request satisfaction(typical)
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1
Proxy Cache (ICAP Client)
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Origin Server
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ICAP Server
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ICAP server satisfies a request just like a
proxy origin server MAY be contacted by ICAP
server (or not)
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response modification(typical)
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2
1
Proxy Cache (ICAP Client)
Origin Server
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ICAP Server
ICAP server modifies a response from an origin
server might be once, as the object is cached,
or once per client served
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response modification

• ICAP client always uses POST to send body
• Encapsulated in POST headers may also be
• Headers used by user to request the object
• Headers used by origin server in its reply
• ICAP server replies with modified content

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infinite variations

• These examples were illustrative, not
  comprehensive -- this generic interface enables
  many different kinds of apps!
• Edge content sources can pass pages to ad servers
• Expensive operations can be offloaded
• Content filters can respond either with an
  unmodified request or HTML (Get Back to Work!)

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future work

• Discussion should be split in two parts
• Bug fixes How is the current standard broken?
• Standard is limited in scope compared to policy,
  but a critical first step
• Wish list What do we want for the future?
• Harder policy questions go here
• Implementation experience will inform this
  discussion