Acknowledgments serve a purpose of ensuring that packets arrived at destination.

PSYC currently provides end-to-end echoes, which is a simple acknowledgment mechanism that only ensures arrival of rather important messages, not minor notices. For the end user echoes are very useful, as they are visible.

PSYC also has a plan for packet ids maintaining a count on how many messages a context has generated. This is an end-to-end strategy on the multicast level allowing for decentralized recovery of multicasts.

Acknowledgments on a circuit instead are considered useless, since properly set-up TCP will inform you of network errors. If not, pinging is sufficient to trigger such an error.

XMPP defines ACKs in http://xmpp.org/extensions/xep-0198.html (Stream Management). This is a circuit-oriented strategy.