ICMP Type 8 Echo Request
ICMP — Internet Control Message Protocol
The message preceding an ICMP Type 0 Echo Reply.
Every host MUST implement an ICMP Echo server function that receives Echo Requests and sends corresponding Echo Replies.
Source RFC 1122.
Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
┌───────────────┬───────────────┬───────────────────────────────┐
│ Type = 8 │ Code = 0 │ Checksum │
├───────────────┴───────────────┼───────────────────────────────┤
│ Identifier │ Sequence Number │
├───────────────────────────────┴───────────────────────────────┤
│ Data │
└───────────────────────────────────────────────────────────────┘
Fields
| Field | Purpose |
|---|---|
| Identifier | Matches requests and replies to a ping process or session |
| Sequence Number | Matches each reply to a request and helps detect loss |
| Data | Payload that the receiver returns unchanged in the reply |
Ping sends Echo Requests to test IP reachability and measure round-trip time. A missing reply does not prove the destination is down; ICMP may be filtered or rate-limited.
References
RFC 792: Internet Control Message Protocol | RFC Editor