squid34 doesn't send ICP ping request at all on RHEL6 or RHEL7

Issue

The squid 3.4 in the customer's environment doesn't send ICP ping request at all.

The squid has two parent proxy servers:

cache_peer <ip address of parenet proxy 1> parent 8080 3130 background-ping weighted-round-robin basetime=3000
cache_peer <ip address of parenet proxy 2> parent 8080 3130 background-ping weighted-round-robin basetime=0

acl InternetService dstdomain "/etc/squid/InternetService.list"

cache_peer_access <ip address of parenet proxy 1> allow InternetService localnet
cache_peer_access <ip address of parenet proxy 1> allow InternetService localhost
cache_peer_access <ip address of parenet proxy 1> deny all

cache_peer_access <ip address of parenet proxy 2> allow InternetService localnet
cache_peer_access <ip address of parenet proxy 2> allow InternetService localhost
cache_peer_access <ip address of parenet proxy 2> deny all

http_port 8080
icp_port 3130

They did the following steps to check whether ICP packets were sent from the squid.

1) Gathered tcpdump on the machine where squid was runnging.
2) Accessed to the domain which was listed in the InternetService.list (list of domains which should be forwarded to the parent proxy) from a browser.
3) tcpdump showed that no ICP packet/request was sent to the parent proxy servers.

However, it sends ICP request if a client send HTTP request (which should be forwarded to the cache peer(s)).