ICAP Message - REQMOD
The ICAP server returns a modified version of the request, an HTTP response, or, if the client indicates it supports 204 responses, an indication that no modification is required.
ICAP Message - RESPMOD
ICAP client sends an origin server's HTTP response to an ICAP server and, if available, the original client request that caused the response. Similar to the Request Modification method, the response from the ICAP server can be an adapted HTTP response, an error, or a 204 response code indicating that no adaptation is required.
ICAP Message - OPTIONS
The OPTIONS method MUST be implemented by all ICAP servers. All other methods are optional and MAY be implemented. The ICAP OPTIONS method is used by the ICAP client to retrieve configuration information from the ICAP server. With this method, the ICAP client sends a request addressed to a specific ICAP resource and receives a response with options specific to the service named by the URI. All OPTIONS requests MAY also return options that are global to the server (i.e., apply to all services).
ICAP Message with payload
ICAP messages with encapsulated HTTP data MUST use chunked encoding. These messages can contain encapsulated HTTP request and response data for manipulation. The encapsulated data is always wrapped in this chunked encoding and may not contain all the original data if a preview header is present.