How to measure the delay of packets using timestamp annotation?

Hi, all,
        I want to measure the delay of each packet. So I select
SetTimestamp and StoreTimeStamp(tail) as my elements and set system time
when each packet arriving at SetTimestamp element to timestamp annotation,
then store it on the end of each packet using StoreTimestamp element.  So I
want to know how to get the value of timestamp annotation and timestamp
stored on the end of packet.
Best wishes.
messi,