This file contains material useful for client writers. Protocol ======== The protocol between Empire client and server is plain text. It is simple enough that you could play using nothing more than telnet. That's a feature. A session uses either ASCII or UTF-8, controlled by session option utf-8. See below for session options. The session always starts in ASCII. Client-server communication is line-oriented. The server sends lines of output. Each output line starts with an identification string, followed by a space, then arbitrary text, then a line feed. Identification strings encode small integers called output IDs as base 36 numbers. Characters '0' to '9' represent digits 0 to 9, and 'a' to 'z' represent 10..35, as do 'A' to 'Z'. Symbolic names for IDs are defined in proto.h. empire-client versions before version 4.3.11 parse large output IDs incorrectly. Such IDs do not currently occur. Clients shall be able to safely handle output lines of arbitrary length. Naturally, a client may not be able to handle a sufficiently large line without loss of functionality, but safety must be ensured. When using ASCII, the most significant bit in text characters indicates highlighting. The following control characters have a defined meaning: ASCII name decimal meaning ---------------------------------------------------- bell 7 ring the bell horizontal tab 9 move to next tab stop(1) line feed 10 end of line shift out 14 begin highlighting(2) shift in 15 end highlighting(2) (1) Tab stops are every eight columns. (2) Only if session uses UTF-8 Other ASCII control characters should not occur and may be ignored by clients. Likewise, overlong or malformed UTF-8 sequences should not occur and may be ignored. The server prompts for input. Each prompt `consumes' one line of input (except for C_EXECUTE, and when a command is aborted, as described below). Input lines are arbitrary text, terminated by line feed, which is optionally preceded by carriage return (decimal 13). Lines should not contain ASCII control characters other than horizontal tab. Clients should not send overlong or malformed UTF-8 sequences. A client is called synchronous if it waits for a prompt before it sends another line of input. Else it is called asynchronous. Asynchronous clients must take care to avoid blocking on sending input. If the client process blocks that way, it can't receive server output until the server reads more input. That may never happen, because the server may well block on output, which then deadlocks the session. An Empire session consists of three phases: login, playing, and shutdown. empire-client is synchronous in the login phase, and asynchronous afterwards. Versions before 4.3.11 could deadlock as described above. Login phase ----------- In the login phase, the server prompts for login commands. The server starts with a C_INIT prompt. The login phase ends when the server sends another C_INIT prompt, which starts the playing phase, or when it enters the shutdown phase, as described below The server replies to a login command with another prompt. Except as noted below, the server replies C_BADCMD for syntax errors, C_CMDERR for other errors, and C_CMDOK on success. In any case, the prompt text contains further information intended for humans. Login commands are: * client CLIENT-ID... Identify the client. This is optional. If given, version information should be included. * coun COUNTRY Set country name to COUNTRY. * kill If another connection for this country is in the playing phase, force it into the shutdown phase, else do nothing. Country must be authenticated. Reply is C_EXIT regardless of success. * options OPTION[=VALUE]... Negotiate session options. Each argument requests setting OPTION to VALUE. The string VALUE is interpreted in an option-specific way. The form without the `=' sets it to an option-specific implied value. The server accepts the request by sending C_CMDOK. If the server encounters an unknown option, it rejects the request by sending C_BADCMD. It rejects unsupported values by sending C_CMDERR. It may or may not process valid parts of rejected requests. If there are no arguments, the server lists its options. For each option, it sends a C_DATA line with OPTION=VALUE as text, and finally a C_CMDOK. If it supports no options at all, it may reply with C_BADCMD instead. See below for supported session options. * pass PASSWORD Authenticate. Country name must be set already. * play [USER COUNTRY PASSWORD] Start playing. If no arguments are given, the country must be authenticated already. Else, argument USER sets the user name, COUNTRY sets the country name, and PASSWORD authenticates. Some error conditions result in a C_EXIT reply. Clients should treat it just like C_CMDERR. On success, the server sends C_INIT. The text is the protocol version number in decimal, currently 2. The client should terminate on protocol versions it doesn't know how to handle. The protocol version is not expected to change often. In fact, it hasn't changed since the oldest known versions of Empire. Unlike the first C_INIT, the second one is not a prompt, i.e the server does not consume a line of input for it. The session then proceeds to the playing phase. * quit Terminate the session. The server replies with C_EXIT and enters the shutdown phase. * sanc List all countries that are still in sanctuary. The output consists of a number of C_DATA lines with human-readable text. This command is only recognized if option BLITZ is enabled. * user NAME Set the user name. This is optional and defaults to "". If the login phase takes more than login_grace_time (default 120s), the server enters the shutdown phase. Playing phase ------------- In the playing phase, the server sends data, control and prompt lines. Clients signal `EOF' by sending a line "ctld\n", and `interrupt' by sending "aborted\n". The server treats these conditions specially, as described below. Anything else is either a command or input for a command, depending on how the server prompts for the line. empire-client signals `EOF' when it encounters an end-of-file condition while reading player input. It signals `interrupt' when it catches SIGINT, which is normally triggered by Ctrl-C. The following IDs occur: * Command prompt C_PROMPT The server consumes a line of input. On EOF, the server enters the shutdown phase. Interrupt is ignored. Anything else is interpreted as Empire command. Text is minutes used, space, BTUs left. Both numbers are in decimal. Clients shall ignore additional text separated by another space. empire-client prints this prompt using format "[%d:%d] Command : ". Clients with a tty-like user interface are advised to use a similar format, to minimize differences to the examples in info. * Argument prompt C_FLUSH The server consumes a line of input and passes it to the currently executing command. The server aborts the command on interrupt and EOF. Any argument prompts it may send before the next command prompt do not consume input. On EOF, the server then enters the shutdown phase (but see C_EXECUTE for an exception). If an update runs while the server waits for the line of input to arrive, the current command is aborted. Whether the server consumes a line of input for this argument prompt is unpredictable. Any argument prompts it may send before the next command prompt do not consume input. Text is a human-readable prompt supplied by the command. empire-client prints the text verbatim. * Data C_DATA Text is human-readable server output. empire-client prints the text verbatim. * Control C_EXECUTE Request execution of a batch file. The text is whatever was on the line of input after the execute command. Its syntax and semantics are left to the client. empire-client interprets the first word (sequence of non-space characters) in the text as file name, and sends the contents of that file. The security considerations on C_PIPE (below) apply to C_EXECUTE as well. Note that servers before 4.3.11 sent a copy of the execute command's first argument as text. This made it hard for clients to ensure that the text is identical to what was sent, because the server strips funny characters and interprets and strips '"' characters when splitting input lines into command and arguments. empire-client gets confused when old servers mangle the text that way. The client shall mark the end of the batch file by signalling EOF as described above. This does not terminate the session. It may signal interrupt if it is unable or unwilling to send the complete batch file. While executing the batch file, the server rejects redirections and execute commands, and sends no C_PROMPT command prompts. It still sends C_FLUSH argument prompts. Protocol flaw: not sending C_PROMPT here screws up redirections: they'd apply until the next C_PROMPT, i.e. from start of redirected command until end of containing batch file. That's why the server rejects redirections in batch files. Certain bad failures make the server ignore the rest of the batch file. This feature is too hard to predict to be really useful. Protocol flaw: strictly asynchronous clients cannot support C_EXECUTE correctly. By the time C_EXECUTE arrives, the client may have sent more input already. That input `overtakes' the contents of the batch file in the input stream, and is interpreted as part of the batch file. Because this is almost certain to happen when the execute comes from a batch file, the server rejects execute commands there. empire-client has this problem. Clients are not required to support C_EXECUTE. Clients are encouraged to offer alternative means for scripting. * Control C_FLASH Asynchronous message. The client should display the text immediately. empire-client prints the text verbatim, prepended by a line feed. This is clearly sub-optimal, because it can be inserted in the middle of user input. Clients wishing to display asynchronous messages together with normal I/O should insert them before the current prompt. Although asynchronous messages can be switched off with the toggle command, client support for C_FLASH is not really optional, because a C_FLASH could arrive before the client manages to switch them off. And if the client lets the user send arbitrary commands, the user can switch it back on at any time. A session option controlling C_FLASH would make more sense. Since all popular clients support C_FLASH, it's not worth the trouble. * Control C_INFORM Notification that the number of unread telegrams changed. The text is the notification in human-readable form. empire-client prints the last received C_INFORM text right before each prompt. It also repeats the last prompt when a C_INFORM arrives. This is sub-optimal just like its treatment of C_FLASH. The user can switch these off with the toggle command. Client support is not really optional for the same reasons as for C_FLASH. * Control C_PIPE When a command is redirected to a pipeline, its output is preceded by a C_PIPE line. The text is a copy of the redirection, starting with '|'. Syntax and semantics of the text after the '|' are left to the client. empire-client executes text after '|' as shell command, with standard input connected to the Empire command's output. The redirection applies to a single command, i.e. until the next C_PROMPT. For obvious security reasons, clients supporting pipes shall ensure that the text is identical to whatever was sent to the server. Note that the server recognizes redirection only in command lines, not argument lines. Asynchronous clients cannot distinguish the two. empire-client prepares for redirections being recognized in any line, and copes with only some of them being actually recognized. * Control C_REDIR When a command is redirected to a file, its output is preceded by a C_REDIR line. The text is a copy of the redirection, starting with '>', optionally followed by '>' or '!'. Syntax and semantics of the remaining text are left to the client. empire-client interprets the first word (sequence of non-space characters) in the remaining text as file name, and redirects the command's output to that file. The use of the first word is unfortunate, as it arbitrarily limits the user's choice of file names. If the text starts with '>!', it silently overwrites any existing file, with '>>' it appends to the file, and with just '>' it refuses to overwrite an existing file. The redirection applies to a single command, i.e. until the next C_PROMPT. The security considerations on C_PIPE apply to C_REDIR as well. * Other IDs Other IDs do not occur currently. Clients shall deal gracefully with IDs they don't know. empire-client treats unknown IDs like C_DATA. Versions before 4.3.11 prepend "Aborted\n" to C_ABORT lines, and "Error; " to C_CMDERR and C_BADCMD lines for historical reasons. Shutdown phase -------------- In the shutdown phase, the server first sends a C_EXIT message. Its text is a human-readable farewell. empire-client prints this text prepended with "Exit: ". The server then waits for the output queue to drain, then shuts down the output direction of the connection. This makes the client detect an end-of-file condition after it received all output. The client should then close its end of the connection. The server continues to read and ignore input until it detects an end-of-file condition. It then closes its end of the connection, and the session terminates. If the server closed its end of the connection without waiting for the client to close the other end first, asynchronous clients could lose output: an attempt to send input after the server closed its end would fail with a connection reset error even when some output is still in flight. Versions before 4.3.30 could misbehave like that in certain circumstances. If the shutdown phase doesn't complete within login_grace_time (default 120s), the server closes the connection immediately. Output may be lost. Session Options =============== Session options control client preferences. They are not persistent; each session starts with the same default session options. The only session option so far is utf-8. It controls whether to use ASCII (disabled) or UTF-8 (enabled). Initial value is disabled. Setting it to 0 disables, setting it to 1 enables, and the implied value is enabled. Session options should not be confused with command toggle, which controls player preferences. Clients should leave those accessible to their users. Commands Useful for Clients =========================== Traditional dumps ----------------- A number of commands are available for clients that wish to maintain their own game state. These commands are called dumps. dump - Dump sector information ldump - Dump land unit information lost - Report lost items ndump - Dump nuclear stockpile information pdump - Dump plane information sdump - Dump ship information See the various info pages on these for complete documentation on how they work and how you can use them to help improve your clients. Each of the above commands prints a timestamp, which is a decimal number. This together with the timestamp selector enables you to dump incrementally, i.e. retrieve only what changed since another dump. For instance, if `dump *' printed timestamp 855544597, `dump * ?timestamp>855544596' dumps everything changed since. Note that the condition compares with the timestamp value minus one. That's because timestamps have a noticeable granularity: things may change between a dump and the next timestamp increase. Timestamp values are currently seconds since the epoch, but this might change, and clients are advised not to rely on it. Extended dump ------------- Traditional dumps have a number of shortcomings. They cover only the most important part of the game state (sectors, ships, planes, land units, nukes), but not game configuration, loans, news, and so forth. They are not quite complete even for what they attempt to cover. Finally, their output is harder to parse than necessary. The new `xdump' command is designed to overcome these deficiencies. See doc/xdump for the full story. Advice on parsing human-readable command output =============================================== To be written.