[SR-Users] About tm retrans time on ser or kamailio 3.1?

HI I did the trace using ngrep: ngrep -T -W byline -d any port 5060 And I am confused with tm retran. (1) case one tm module, the| retr_timer1| default is 500 ms. #U +0.001824 xxx.17:5060 -> xxx.16:5060 INVITE sip:224@172.16.8.49:2057;line=lnzlkxu5 SIP/2.0. #U +0.000012 xxx.17:5060 -> xxx.16:5060 INVITE sip:224@172.16.8.49:2057;line=lnzlkxu5 SIP/2.0. the ngrep show the delta between packet, the second invite is only 12 ms behind? is my understanding correct? (2) change retr_timer1 to 2000 ms modparam("tm", "retr_timer1", 1000) #U +0.001936 xxx.17:5060 -> xxx.16:5060 INVITE sip:224@172.16.8.49:2057;line=lnzlkxu5 SIP/2.0. #U +0.000013 xxx.17:5060 -> xxx.16:5060 INVITE sip:224@172.16.8.49:2057;line=lnzlkxu5 SIP/2.0. the delta still 13 ms behind, so similar to the case 1, The question is: is the second invite the re-trans by tm? If so why it is around 12/13 ms? not the 500 ms or as configured 2000 ms? thanks. Kind regards / Mit freundlichen Grüßen Min Wang