Spell node identifiers and quote their boundaries
03The for and by values use a node identifier: an address, unknown, or an underscore-prefixed opaque label, with an optional port.
for=unknown reports that the preceding identity is not known. for=_req7 reports a generated opaque identifier. The underscore is mandatory for the obfuscated form; neither authenticates a client.
Source for this drawing: RFC 7239 · Read RFC section 6 ↗
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.
Distinguish an address from an unknown or obfuscated node.
- IPv4 uses address syntax from RFC 3986. IPv6 is enclosed in square brackets and SHOULD follow RFC 5952 text recommendations.
- An IPv6 value and any node with a port MUST be quoted: colon and brackets are not token characters. Port syntax is one to five digits, or a valid underscore-prefixed obfuscated port.
- Obfuscated node and port identifiers MUST start with underscore, followed by one or more letters, digits, dot, underscore or hyphen. unknown means the preceding identity is not known; it is not an IP address.
The examples are separate valid alternatives, not duplicate for parameters in one element. Syntax alone does not establish a reachable endpoint or trusted identity. unknown can even have a port identifier; the adding proxy’s owner defines its interpretation. Address disclosure is chosen here, not the default recommendation.
Read the RFC section ↗