Communication Protocol

We adopt an XML-like syntax for the client/server communication protocol. We use the same extended BNF notation as in Appendix A to describe the syntax of protocol messages. The $\langle$ name$\rangle$ and $\langle$ number$\rangle$ terminals are defined in exactly the same way as for PPDDL. An $\langle$ integer$\rangle$ is a nonempty string of numeric characters. A $\langle$ message$\rangle$ is an arbitrary character string, possibly empty.

Figure 13 shows the expected sequence of messages. A session starts by the client sending a $\langle$ session-request$\rangle$ message to the server. The server replies with a $\langle$ session-init$\rangle$ message, which tells the client the number of evaluation rounds that will be run. To start an evaluation round, the client sends a $\langle$ round-request$\rangle$ message, to which the server replies with a $\langle$ round-init$\rangle$ message. At this point the evaluation round starts. The server sends a $\langle$ turn-response$\rangle$ message to the client, which can be a $\langle$ state$\rangle$ message or an $\langle$ end-round$\rangle$ message. For every $\langle$ state$\rangle$ message that the client receives, it sends an $\langle$ action spec$\rangle$ message in return. Once the client receives an $\langle$ end-round$\rangle$ message, it ends the current evaluation round. The client then starts a new evaluation round with a $\langle$ round-request$\rangle$ message to the server, or waits for an $\langle$ end-session$\rangle$ message from the server in case all rounds have already been run. The server sends an $\langle$ error$\rangle$ message to the client if an error occurs, for example if the server receives an unexpected message from the client.

Figure 13: Successful communication session.