RFC 4443 / START HERE
ICMPv6, before the details
ICMPv6 means Internet Control Message Protocol for Internet Protocol version 6 (IPv6). It carries control messages inside IPv6 packets, not through a TCP or UDP port.
A host or router reports a packet-processing problem back to its source, or answers a diagnostic Echo Request used by ping. Error messages include part of the packet that caused the problem.
The idea in one picture
Source host
Router feedback
Error context
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
IPv6 needs readable feedback about unreachable destinations, oversized packets, exhausted Hop Limits and bad headers. RFC 4443 defines the base error and Echo rules; Neighbor Discovery and complete Path MTU Discovery procedures live in other documents. RFC 4884 updates selected error layouts.
A familiar example
For example, a fictional router cannot forward a 1600-byte packet over a link with MTU 1500. It reports Packet Too Big to the sender, subject to error-suppression and rate rules. Ping replies can show an Echo exchange worked; they do not authenticate a peer or prove an application works.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Read the control messageICMPv6 is the Internet Control Message Protocol for IPv6. Its messages report packet-processing problems and support diagnostics. They are IPv6 payloads, not TCP or UDP messages on a port.→
- Return an error with contextAn error goes back toward the original packet’s source and quotes the beginning of that packet. The quote helps the source identify which upper-layer protocol and process should hear about the failure.→
- Recognize the four base errorsThe first four ICMPv6 Types describe different failures: unreachable destination, an oversized packet, an expired Hop Limit or reassembly timer, and a bad header field. Their bodies carry different evidence.→
- Suppress and limit errorsUnrestricted error replies can create loops and amplify traffic. General suppression rules and a required rate limit take precedence over the per-message rules for originating errors.→
- Echo and its limitsEcho Request and Echo Reply carry matching identifiers, sequence numbers and unchanged data. They test an ICMPv6 exchange; they do not prove that every application is available or that a peer is authenticated.→
This introduction explains RFC 4443 (2006). Read the original RFC ↗