writing an irule to log all traffic

I did try a tcpdump but there is too much data ultimatley we just want to find out how long the transfer time to and from this external host is

I modified the rule to this:

  when HTTP_REQUEST {
   set flag 0
   if { [HTTP::host] eq "78.42.24.X"  } {
      set flag 1
      set http_request_time [clock clicks -milliseconds]
      set LogString "Client [IP::client_addr]:[TCP::client_port] -> [HTTP::host][HTTP::uri]"
   }
}

when HTTP_RESPONSE {
   if {$flag} {
     log local0. "$LogString (response) - pool info: [LB::server] - status: [HTTP::status] (request/response delta: [expr [clock clicks -milliseconds] - $http_request_time]ms)"
   }
}
 

and in the ltm file got the following:

Oct 19 14:02:33 sys-bip-01 mcpd[1726]: 01070151:3: Rule [Data_Cookie_Log] error: line 3: [wrong args] [HTTP::host equals "78.42.24.X"] line 12: [use curly braces to avoid double substitution] [[clock clicks -milliseconds]]