Is the cancel operation already supported in RE23?
It seems so (playing around with it with python-ldap).
But supportedFeatures in root DSE does not contain 220.127.116.11.1.8.
RFC 3909 states:
Servers SHOULD indicate their support for this extended operation by
providing 18.104.22.168.1.8 as a value of the 'supportedExtension'
attribute type in their root DSE. A server MAY choose to advertise
this extension only when the client is authorized to use it.