Echo Reply
ICMP Type 0 Echo Reply is sent in response to an Echo Request (Type 8). It is the reply half of the ping mechanism. The receiving host copies the Identifier and Sequence Number fields from the request into the reply so the sender can match responses to requests.
Description
ICMP Echo Reply is the response to ICMP Echo Request (Type 8). When a host receives an Echo Request, it sends an Echo Reply with the same Identifier and Sequence Number fields. The round-trip time between sending the request and receiving the reply is what tools like ping report.
The Echo Reply carries the same data payload as the Echo Request. This allows basic reachability testing and latency measurement between any two IP-connected hosts. Routers do not generate Echo Replies; only end hosts respond.
Firewall rules that block ICMP Type 0 inbound while allowing Type 8 outbound cause asymmetric behavior: pings appear to time out from the originating host even though the remote host received them and replied.
Common Causes
- –Response to a ping (ICMP Echo Request Type 8)
- –Reachability probe response in network monitoring tools
- –traceroute response from final destination hop