Because having only an ICAP server is boring because it is doing nothing. So you should write your first Service.

So lets start with a little template:

# load the icap server
require 'icaprb/server'

# create a class that inherits the base of (all) ICAP services in ICAPrb
class SampleService < ::ICAPrb::Server::Services::ServiceBase
  # initialize the service: our module does only RESPMOD
  # (which means it has no support to edit responses which are
  # called REQMOD in the protocol and request_mod in ICAPrb)
  def initialize
    # the first parameter here is the service name used in the icap headers (not the URL)
    super('name',[:response_mod])
  end

  # you need to override this method to do the work of your module
  def process_request(icap_server,ip,socket,data)
    # get the logger from the server to use it for sending data to the log
    # use it for errors in your module
    # logger = icap_server.logger
    # this creates a new ICAP response to be sent to the ICAP client
    response = ::ICAPrb::Server::Response.new
    # if you change the data, you should use 200.
    # If the data is unchanged and the client accepts 204, use it here
    # as it is shorter (less network traffic, better performance).
    response.icap_status_code = 200
    http_resp_header = data[:http_response_header]
    http_resp_body = data[:http_response_body]

    # do your modification here

    # modified is a variable you must set
    if modified
      # this will update the HTTP Content-Length header because the length may have changed
      # (for example if you removed ad tags, the content is shorter now)
      http_resp_header['Content-Length'] = http_resp_body.length.to_s
      # this is required by the interface of ICAPrb::Server
      # it detects that this string is an ICAP response body by checking the type (class)
      http_resp_body = ::ICAPrb::Server::ResponseBody.new(http_resp_body)
      # add the components to your response - order does not matter because it will find it by the type - other types are ignored.
      # If you have an error here, your response may be malformed and your ICAP service is broken - make sure you are not using a wrong type like string.
      response.components << http_resp_header
      response.components << http_resp_body
      # this method sends only the headers to the socket, not the content itself - if your 
      response.write_headers_to_socket socket
      # send everything in one chunk - you could use multiple but you are not required to - on large downloads, this may make sense
      socket.write(http_resp_body.to_chunk)
      # this methods sends the last chunk of your connection - after that, the connection may be terminated
      ::ICAPrb::Server::Response.send_last_chunk(socket,false)
    else
      # Just send a 204 message (we do not check if it is supported here because this is only a short sample).
      response.icap_status_code = 204
      response.write_headers_to_socket socket
    end
  end
end

The next step is using it in your ICAPrb server. It is really simple. If s contains the instance of your server, you only need to attach it. 

# this makes your service available as service.
# For example, your URL may look like icap://[::1]:1344/service
s.services['service'] = SampleService.new