How GTM/DNS detect the round trip time of LDNS?

Hi Team,

 

I got a pretty simple configuration on my Bigip DNS. A pool was set for a wide IP and in that pool, Round Trip Time ( RTT ) is selected as load balancing method for pool member selection. I choose ICMP as the protocol of metrics collection:

 

tmsh list gtm global-settings metrics metrics-collection-protocols

gtm global-settings metrics { metrics-collection-protocols { icmp } }

 

What's interesting to me is when I monitoring DNS behavior of getting RTT time to a specific LDNS, bigip used tcp instead of icmp to detect the LDNS on its port 53? Is this an expected behavior? For my understanding, won't bigip only use ICMP to determine ldns's RTT? Any idea why bigip to poke LDNS's port 53? Does "metric collection" has any undocumented internal details to determine which protocol to be used for detecting a LDNS?

 

pcap examples: (IP masked)

 

BIGIP: a.a.a.a LDNS: b.b.b.b

 

15:52:40.580506 IP a.a.a.a > b.b.b.b: ICMP echo request, id 3840, seq 0, length 44 out slot1/tmm0 lis= 15:52:40.583850 IP b.b.b.b > a.a.a.a: ICMP echo reply, id 3840, seq 0, length 44 in slot1/tmm0 lis= 15:52:40.584441 IP a.a.a.a > b.b.b.b: ICMP echo request, id 3840, seq 256, length 44 out slot1/tmm0 lis= 15:52:40.588375 IP b.b.b.b > a.a.a.a: ICMP echo reply, id 3840, seq 256, length 44 in slot1/tmm0 lis= 15:52:40.588987 IP a.a.a.a > b.b.b.b: ICMP echo request, id 3840, seq 512, length 44 out slot1/tmm0 lis= 15:52:40.592167 IP b.b.b.b > a.a.a.a: ICMP echo reply, id 3840, seq 512, length 44 in slot1/tmm0 lis=

 

15:54:05.772630 IP a.a.a.a.1075 > b.b.b.b.53: Flags [S], seq 185599154, win 2048, length 0 out slot1/tmm0 lis= 15:54:05.776398 IP b.b.b.b.53 > a.a.a.a.1075: Flags [R.], seq 0, ack 185599155, win 0, length 0 in slot1/tmm0 lis= 15:54:06.523840 IP a.a.a.a.1076 > b.b.b.b.53: Flags [S], seq 1382293689, win 2048, length 0 out slot1/tmm1 lis= 15:54:06.527986 IP b.b.b.b.53 > a.a.a.a.1076: Flags [R.], seq 0, ack 1382293690, win 0, length 0 in slot1/tmm1 lis= 15:54:07.273797 IP a.a.a.a.1077 > b.b.b.b.53: Flags [S], seq 3604873194, win 2048, length 0 out slot1/tmm0 lis= 15:54:07.277753 IP b.b.b.b.53 > a.a.a.a.1077: Flags [R.], seq 0, ack 3604873195, win 0, length 0 in slot1/tmm0 lis=