RFC 792 · September 1981 · J. Postel
Internet Control Message Protocol
Index fetched 2026-10-19 from rfc-editor.org
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Built by hand from RFC 792. Every arrow, step and field cites the RFC sentence it comes from, and an automated test checks each quote against the RFC text. Provisional: not yet reviewed by a maintainer. Help verify this visual
RFC 792 The data received in the echo message must be returned in the echo reply message.
1. Host A → Host B: echo message
Host A sends an echo message (Type 8) to Host B. It may carry an identifier, a sequence number and some data.
RFC 792The address of the source in an echo message will be the destination of the echo reply message.
2. Host B → Host A: echo reply message
Host B answers with an echo reply: it swaps the source and destination addresses, changes the type to 0, recomputes the checksum, and returns the same data, identifier and sequence number.
RFC 792To form an echo reply message, the source and destination addresses are simply reversed, the type code changed to 0, and the checksum recomputed.
Quoted text consists of unmodified excerpts from RFC 792, published by the IETF (line breaks reflowed). Copyright in the RFC text belongs to the IETF Trust and the persons identified as the document authors, as stated in the RFC. The diagrams and plain-English explanations are rfc-visual's own and do not replace the RFC.
Not shown: The IP header fields used to carry ICMP messages; Time Exceeded, Parameter Problem, Source Quench, Redirect, Timestamp and Information Request/Reply messages; A Destination Unreachable exchange (when gateways and hosts send it).
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