The UI node type has two hooks, upstream and downstream. Packets received on downstream must have 0x03 (indicating unnumbered information) as their first byte; if not the packet is dropped. This byte is then stripped and the remainder of the packet sent out on upstream.
Conversely, packets received on upstream will have a 0x03 byte prepended to them before being forwarded out on the downstream hook.
This node type supports the following hooks: