diff --git a/interface.go b/interface.go
index 0b75fe2b111..1d690b64b4b 100644
--- a/interface.go
+++ b/interface.go
@@ -211,15 +211,22 @@ type EarlyConnection interface {
 	NextConnection() Connection
 }
 
-// A ConnectionIDGenerator is a interface that allows clients to implement their own format
+// A ConnectionIDGenerator is an interface that allows clients to implement their own format
 // for the Connection IDs that servers/clients use as SrcConnectionID in QUIC packets.
+//
+// Connection IDs generated by an implementation should always produce IDs of constant size.
 type ConnectionIDGenerator interface {
 
 	// GenerateConnectionID generates a new ConnectionID.
+	// Generated ConnectionIDs should be unique and observers should not be able to correlate two ConnectionIDs.
 	GenerateConnectionID() ([]byte, error)
 
 	// ConnectionIDLen tells what is the length of the ConnectionIDs generated by the implementation of
 	// this interface.
+	// Effectively, this means that implementations of ConnectionIDGenerator must always return constant-size
+	// connection IDs. Valid lengths are between 0 and 20 and calls to GenerateConnectionID.
+	// 0-length ConnectionsIDs can be used when an endpoint (server or client) does not require multiplexing connections
+	// in the presence of a connection migration environment.
 	ConnectionIDLen() int
 }