This specification defines the “topic-discovery” ENR entry, which signals that a node implements the TopDisc capability for discv5 and is willing to participate in topic service tables, registrations, and lookups.
Entry Format
entry-key = "topic-discovery"
entry-value = versionWhere version is an unsigned integer identifying the supported TopDisc protocol version.
A node implementing the version described in the TopDisc document sets:
topic-discovery = 1Semantics
A node MUST publish the topic-discovery entry in its ENR before it can be selected as a
registrar or as a target of topic-discovery queries by other nodes. Nodes whose ENR does
not contain topic-discovery, or whose topic-discovery value is not understood by the
local implementation, MUST NOT be inserted into local TopDisc service tables and MUST NOT
be selected for topic registration or lookup requests.
The entry does not by itself indicate which services or topics a node advertises. Service membership is established through TopDisc registrations and observed at runtime; the ENR entry only signals capability and protocol version compatibility.
Future, non-backwards-compatible revisions of the topic discovery capability MUST bump the
version value. Implementations that encounter an unknown version SHOULD treat the node
as if it did not publish the entry at all. Implementations MAY support multiple versions
simultaneously; the matching rule between local and remote versions is defined by local
policy.
Change Log
Initial version (2026)
The initial version of the “topic-discovery” entry is proposed in this document, alongside the TopDisc specification.