ICAP is the acronym for Internet Content Adaptation Protocol. The purpose of the feature is to offload work that would normally take place on the firewall to a separate server specifically set up for the specialized processing of the incoming traffic. This takes some of the resource strain off of the FortiGate firewall leaving it to concentrate its resources on things that only it can do.
Offloading value-added services from Web servers to ICAP servers allows those same web servers to be scaled according to raw HTTP throughput versus having to handle these extra tasks.
ICAP servers are focused on a specific function, for example:
l Ad insertion l Virus scanning l Content translation l HTTP header or URL manipulation l Language translation l Content filtering
The following topics are included in this section:
Offloading using ICAP
Example ICAP sequence
Example ICAP scenario
ICAP is an Application layer protocol; its specifications are set out in RFC 3507. It is, in essence, a lightweight protocol for executing a “remote procedure call” on HTTP messages and is a member of the member of the TCP/IP suite of protocols.
The default TCP that is assigned to it is 1344. Its purpose is to support HTTP content adaptation by providing simple object-based content vectoring for HTTP services. ICAP is usually used to implement virus scanning and content filters in transparent HTTP proxy caches. Content adaptation refers to performing the particular value added service, or content manipulation, for an associated client request/response.
Essentially it allows an ICAP client, in this case the FortiGate firewall, to pass HTTP messages to an ICAP server like a remote procedure call for the purposes of some sort of transformation or other processing adaptation. Once the ICAP server has finished processing the content, the modified content is sent back to the client.
The messages going back and forth between the client and server are typically HTTP requests or HTTP responses. While ICAP is a request/response protocol similar in semantics and usage to HTTP/1.1 it is not HTTP nor does it run over HTTP, as such it cannot be treated as if it were HTTP. For instance, ICAP messages can not be forwarded by HTTP surrogates.
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