The Yinrih Ansible Protocol (YAP) is used to communicate between ansibles over the Underlay.
YAP addresses are 27 trits long. A YAP frame consists of an inter-frame flag, a topology trit, a destination address, a source address, a 6-trit payload info field, the payload itself whose length can vary, a checksum, and then the flag for the following frame.
The topology trit says whether there are only two ansibles communicating point to point, or whether multiple ansibles are on the same link. Because of the physics behind tailstone and the Underlay, the link is a shared medium, meaning it's half duplex and only one ansible can talk at a time, so the topology trit dictates when an ansible can talk and for how long.
The source and destination addresses are self-explanatory for the most part.
The payload info field says what protocol the payload uses. It also dictates how big the payload is.
The checksum helps insure the message was received correctly.
The inter-frame flag separates one frame from the next.