Expand a named rule
01A rule names a pattern; following its parts tells you which complete strings fit.
ID:4 fails the complete example because the second required digit is absent.
Source for this drawing: RFC 5234 · Read RFC section 2 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
Rule names begin with A–Z or a–z; later characters can also be digits or hyphens. Rule-name case does not distinguish rules.
- Rule names begin with A–Z or a–z; later characters can also be digits or hyphens. Rule-name case does not distinguish rules.
- The equals sign defines a rule. A rule line ends in CRLF; continuation lines are indented.
- In this invented ASCII example, label = "id" ":" 2DIGIT needs exactly two digits. Quotes and rule-formatting spaces are not sent as data.
Examples are invented ASCII strings, not a complete application protocol. Matching syntax does not establish application meaning or authorization. Core rules are in RFC5234 Appendix B.1; external encodings are separate (§2.4).
Read the RFC section ↗