Echo Request
ICMP Type 8 Echo Request is the outbound half of the ping mechanism. A host sends Echo Requests to test reachability and measure round-trip latency to another host. The receiving host responds with ICMP Type 0 Echo Reply. Both the Identifier and Sequence Number fields are echoed back unchanged.
Description
ICMP Echo Request is generated by the ping command and similar network diagnostic tools. The sender includes an Identifier (typically the process ID) and an incrementing Sequence Number so replies can be matched to specific requests even when multiple pings are in flight.
The data payload is arbitrary and is echoed back verbatim by the receiver. Ping implementations typically fill the payload with a timestamp so the round-trip time can be computed from the reply alone without requiring synchronized clocks.
Many production firewalls block ICMP Echo Requests inbound on public interfaces to prevent reconnaissance. This creates a common false negative: a host is reachable on TCP port 80 or 443 but does not respond to ping, leading operators to conclude it is unreachable.
Common Causes
- –ping command sending reachability probes
- –Network monitoring systems performing availability checks
- –MTU probing tools sending Echo Requests of varying sizes
- –traceroute using ICMP mode (Windows default)