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6. Protocol reference

Under some circumstances, it may be preferable to communicate directly with BrlAPI's server.avoid rather than using BrlAPI's library. Here are the needed details to be able to do this. This chapter is also of interest if a precise understanding of how the communication stuff works is desired, to be sure to understand how to write multithreaded clients, for instance.

In all the following, integer will mean an unsigned 32 bits integer in network byte order (ie most significant bytes first).

6.1 Reliable packet transmission channel

The protocol between BrlAPI's server and clients is based on exchanges of packets. So as to avoid locks due to packet loss, these exchanges are supposed reliable, and ordering must be preserved, thus BrlAPI needs a reliable packet transmission channel.

To achieve this, BrlAPI uses a TCP-based connection, on which packets are transmitted this way:

The size does not include the { size, type } header, so that packets which don't need any data have a size of 0 byte. The type of the packet can be either of BRLPACKET_* constants defined in brlapi.h. Each type of packet will be further discussed below.

BrlAPI's library ships two functions to achieve packets sending and receiving using this protocol: brlapi_writePacket and brlapi_readPacket. It is a good idea to use instead of rewriting them, since this protocol might change one day in favor of a real reliable packet transmission protocol such as the experimental RDP.

6.2 Responses from the server

As described below, many packets are `acknowledged'. It means that upon reception, the server sends either:

Some other packets need some information as a response. Upon reception, the server will send either:

6.3 Operating modes

The connection between the client and the server can be in either of the four following modes:

6.4 Details for each type of packet

Here is described the semantics of each type of packet. Most of them are directly linked to some of BrlAPI's library's functions. Reading their online manual page as well will hence be of good help for understanding.

BRLPACKET_AUTHKEY (see brlapi_loadAuthKey())

This must be the first packet ever transmitted from the client to the server. It lets the client authenticate itself to the server. Data is the authentication key itself.

If the authentication key matches the servers', it is acknowledged, and other types of packets might be used, other BRLPACKET_AUTHKEY shouldn't be sent by the client.

If the authentication key doesn't match, the server sends a BRLERR_CONNREFUSED and closes the connection.

BRLPACKET_BYE (see brlapi_closeConnection())

This should be sent by the client when it doesn't need server's services any more, just before disconnecting from it. The server will acknowledge this packet and close the connection. It will also clean up things like the tty which was got by the application, as well as raw mode, by sending a special reset event to the braille device, if the brltty driver implements it. But this shouldn't prevent clients from cleanly leaving ttys and raw mode before issuing BRLPACKET_BYE !

BRLPACKET_GETDRIVERID (see brlapi_getDriverId())

This should be sent by the client when it needs the 2-char identifier of the current brltty driver. The returned string is \0 terminated.

BRLPACKET_GETDRIVERNAME (see brlapi_getDriverName())

This should be sent by the client when it needs the full name of the current brltty driver. The returned string is \0 terminated.

BRLPACKET_GETDISPLAYSIZE (see brlapi_getDisplaySize())

This should be sent by the client when it needs to know the braille display size. The returned data are two integers: width and then height.

BRLPACKET_GETTTY (see brlapi_getTty())

This should be sent by the client to get control of a tty. Sent data are two integers: the number of the tty to get control of (it mustn't be 0), and how key presses should be sent: either BRLKEYCODES or BRLCOMMANDS. This packet is then acknowledged by the server.

BRLPACKET_KEY and BRLPACKET_COMMAND (see brlapi_readKey()and brlapi_readCommand())

As soon as the client got a tty, it must be prepared to handle either BRLPACKET_KEY or BRLPACKET_COMMAND incoming packets (depending on the parameter given in the BRLPACKET_GETTTY packet), at any time (as soon as the key was pressed on the braille terminal, hopefuly). The data holds the key press code as an integer, either a raw key code in BRLPACKET_KEY packets, or a brltty command code in BRLPACKET_COMMAND packets.

BRLPACKET_LEAVETTY (see brlapi_leaveTty())

This should be sent to free the tty and masked keys lists. This is acknowledged by the server.

BRLPACKET_MASKKEYS and BRLPACKET_UNMASKKEYS (seebrlapi_ignoreKeys() and brlapi_unignoreKeys())

If the client doesn't want every key press to be signaled to it, but some of them to be given to brltty for normal processing, it can send BRLPACKET_MASKKEYS packets to tell ranges of key codes which shouldn't be sent to it, but given to brltty, and BRLPACKET_UNMASKKEYS packets to tell ranges of key codes which should be sent to it, and not given to brltty. The server keeps a dynamic list of ranges, so that arbitrary sequences of such packets can be sent. Data are 2 integers: the lower and the upper boundaries; lower and upper must be equal to tell one key, for instance.

BRLPACKET_WRITE (see brlapi_writeBrl())

To display text on the braille terminal and set the position of the cursor, the client can send a BRLPACKET_WRITE packet. This packet holds the cursor position as an integer, and then the text to display, one byte per caracter. The text must exactly fit the braille display, ie hold height*width bytes, where height and width must be get by sending a BRLPACKET_GETDISPLAYSIZE packet. Moreover, characters are expected to be encoded in latin-1. This packet is acknowledged by the server.

BRLPACKET_WRITEDOTS (see brlapi_writeBrlDots())

Is used the same way as BRLPACKET_WRITE, the only difference being that characters are encoded in braille dots.

BRLPACKET_STATWRITE

Some braille terminals have a special status display, whose text is independant from normal display's. To display text on it, the client can send a BRLPACKET_STATWRITE packet, just like BRLPACKET_WRITE packets.

BRLPACKET_STATWRITEDOTS

Is used the same way as BRLPACKET_STATWRITE, the only difference being that characters are encoded in braille dots.

BRLPACKET_GETRAW (see brlapi_getRaw())

To enter raw mode, the client must send a BRLPACKET_GETRAW packet, which is acknowledged. Once in raw mode, no other packet than BRLPACKET_LEAVERAW or BRLPACKET_PACKET will be accepted. The data must hold the special value BRLRAW_MAGIC: 0xdeadbeef, to avoid erroneous raw mode activating.

BRLPACKET_LEAVERAW (see brlapi_leaveRaw())

To leave raw mode, the client must send a BRLPACKET_LEAVERAW packet, which is acknowledged.

BRLPACKET_PACKET (see brlapi_sendRaw() andbrlapi_recvRaw())

While in raw mode, only BRLPACKET_PACKET packets can be exchanged between the client and the server: to send a packet to the braille terminal, the client merely sends a BRLPACKET_PACKET packet, its data being the packet to send to the terminal. Whenever its receives a packet from the terminal, the server does exactly the same, so that packet exchanges between the terminal and the server are exactly reproduced between the server and the client.

Packets' content depend on the braille driver, so that the client should check for its id or name thanks to a BRLPACKET_GETDRIVERID packet or a BRLPACKET_GETDRIVERNAME packet, prior to sending any BRLPACKET_GETRAW packet.


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