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Internet Content Adaptation Protocol (ICAP) is a lightweight protocol providing simple object-based content vectoring for HTTP services. ICAP is used to extend transparent proxy servers. This frees up resources and standardizes the implementation of new features. It uses a cache to proxy all client transactions and process the transactions using ICAP Web servers, which are designed for specific functions such as virus scanning, content translation, content filtering or ad insertion.
ICAP performs content manipulation as a value added service for the appropriate client HTTP request or HTTP response. Thus the name "content adaptation."
This term is also known as Internet Content Adaption Protocol.
Internet Content Adaptation Protocol was proposed in 1999 by Danzig and Schuster of Network Appliance. Don Gillies enhanced the protocol in 2000 allowing pipelined ICAP servers. All three encapsulations permitted by HTTP 1.1 are supported. He also produced training materials for vendors about 2005.
ICAP leverages caches and proxies to aid in producing value-added services. The value-added services can be off-loaded from Web servers to ICAP servers. Then, Web servers can be scaled using raw HTTP throughput.
Despite the similarity, ICAP is not HTTP. And it is not an application running over HTTP.