ICAP

ICAP stands for Internet Content Adaptation Protocol. It is an application-layer protocol designed to offload content modification tasks from HTTP servers and gateways to dedicated ICAP servers. It is commonly used for content filtering, virus scanning, ad insertion, and transcoding in enterprise proxy architectures.

Overview: ICAP defines a simple request/response model in which an ICAP client, typically a caching proxy or gateway, communicates with an ICAP server. The server applies content adaptation to HTTP messages and returns either modified messages or an indication that no changes are required. ICAP commonly runs over TCP and uses HTTP-like syntax. The protocol supports several message types, most notably OPTIONS, REQMOD (modifies an HTTP request), and RESPMOD (modifies an HTTP response).

Operation: In normal operation, a client’s HTTP request may be sent to an ICAP server via REQMOD so the request can be altered before reaching the origin server. After a response is received, RESPMOD allows the ICAP server to modify the response before it reaches the client. The server may inspect headers, transform content, filter data, or insert additional data such as advertisements or sanitization results.

Deployment and impact: ICAP is widely deployed in corporate networks and filtering gateways to enforce policies and perform heavy-lifting content adaptations on specialized servers. This setup enables offloading of CPU-intensive tasks (for example, antivirus scanning or format transcoding) from origin servers.

History and status: ICAP was standardized by the IETF in RFC 3507. While widely used, it complements rather than replaces other content-filtering approaches and may be used alongside other security and optimization tools.