echo, echo-reply and unreachable in ASA

Hi Any one can explain the difference among following three highlighted in ASA. Thank you

hostname(config)# icmp permit host 172.16.2.15 outside
hostname(config)# icmp permit host 172.16.2.15 echo outside
hostname(config)# icmp permit host 172.16.2.15 echo-reply outside
hostname(config)# icmp permit any unreachable outside
eemoonAsked:
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Paul SimmonsCommented:
The three lines are permitting 3 different ICMP messages. They basically say allow ping, affirmative ping response, and destination unreachable response.
eemoonAuthor Commented:
Thank you so much for your fast reply. What is difference among them ? and command " icmp permit host 172.16.2.15 outside" also is allowed to ping. why does asa has other three commands ?
Paul SimmonsCommented:
The first three rules allow outbound ICMP traffic from one specific host (172.16.2.15); and because rule 1 has no restriction defined, rules 2 and 3 are probably unnecessary. Rule 4 has no source host restriction (any), but only allows the destination unreachable message to pass.

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