The big complex beast that turns an infrastructure of volatile packets into a magic tunnel where things actually come out the way they went in.
PSYC supports both TCP and UDP. UDP is used by default when only a _notice is about to be sent, and a circuit hasn't been created yet.
The circuit concept of PSYC puts an abstraction layer on top of TCP to ensure things work even when TCP doesn't.
Techie details on Wikipedia:Transmission Control Protocol.
In situations of lossy links, like a flaky wireless LAN, a file transfer using a TCP socket can become very tedious: Every few seconds a packet is missing and the entire transfer is halted for recovery. It would probably be more efficient if a protocol was being used that has a suitable transfer rate to get most data through, but doesn't halt on dropped packets - merely resends them later. The receiving side needs the ability to reconstruct the file from its fragments in a technique like Gnutella or BitTorrent. This means essentially re-inventing TCP minus just a little impractical thing.