sip invite x a request message 600 the first sip invite data record 602 is then added together with the first timestamp t 1 to a first active cache of the first host terminal 102 indexed by a call id of the sip invite x a request message 600 a first ipv6 destination options header is also generated and inserted between the payload and ipv6 header of the sip invite x a request message 600 the first ipv6 destination options header being encoded as a tlv object comprising the first timestamp t 1 the tlv object constituting the first destinations options header is identifiable as containing data relating to a measure of sip invite transactions the sp invite x a request message 600 is then sent 606 to the proxy server 104 the sip invite x a request message 600 is received 608 by the proxy server 104 the presence of the first destination options header in the sip invite x a request message 600 triggering a second sip al invite module in the proxy server 104 to generate a second sip invite data record 610 equivalent to the first sip invite data record 602 a second timestamp t 2 is also computed 612 corresponding to the reception time of the sip invite x a request message 600 and the second sip invite data record 610 is added together with the first timestamp t 2 extracted from the tlv object and the second timestamp t 2 to a second active cache not shown of the proxy server 104 again indexed by the call.