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+Frontend-Backend Protocol
+=========================
+
+This document describes how the frontend-backend protocol works. It is not
+contractual, and the protocol may change in incompatible ways without prior
+announcements.
+
+Definitions
+-----------
+
+'c' refers to the ASCII value of the given character encoded as a byte.
+
+MAC is a MAC address as a 6 byte binary value.
+
+status is a one-byte value that can be either 0 (available), 1
+(unavailable) or 2 (offline)).
+
+msgid is an unsigned 16 bit value that is used to identify targets of ACKs.
+You can treat it as more or less an opaque identifier that is valid at
+least as long the backend stays running.
+
+nick is the nick encoded as utf-8. It is allowed to have all valid Unicode
+code points other than C0 and C1 control characters and the characters
+U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR. nick can be at
+maximum 255 bytes long.
+
+message is the message encoded as utf-8. It is allowed to have all the
+codepoints that nick is, plus U+0A LINE FEED which is used to separate
+lines in a multi-line message. message can be at maximum 1494 bytes long.
+
+All multibyte integer values are big-endian a.k.a. network byte order.
+
+Commands
+--------
+
+Frontend sends commands to the backend.
+
+### Quit
+Format: 'q'
+
+Tells the backend to exit normally, doing cleanup.
+
+### Status request
+Format: 'r' + MAC
+
+Tells the backend to send an EtherMess status request message to the given
+MAC address. No caching will be used, and one status request command
+corresponds to one status request message sent.
+
+### Set status
+Format: 's' + status + nick length in bytes (u8) + nick
+
+Tells the backend to change its stored values of nick and status, and
+broadcast the new values to the network. A message will be sent even if you
+provide the same nick and status that are already set.
+
+### Send message
+Format: 'm' + MAC + message length in bytes (u16) + message
+
+Tells the backend to queue this message for sending.
+
+Note that only one message can be queued at once, so you should wait until
+you receive an event either telling you of a failure to send message (Msgid
+failed, ACK failed) or telling you it has been succesfully received (ACK)
+before sending another message.
+
+Events
+------
+
+Frontend receives events from the backend.
+
+### Status
+Format: 's' + MAC + status + nick length in bytes (u8) + nick
+
+Generated when an EtherMess status message is received by the backend.
+
+### Msgid
+Format: 'i' + msgid
+
+Generated when backend is able to give the queued message a msgid.
+
+### Msgid failed
+Format: 'I'
+
+Generated when backend is unable to give the queued message a msgid. You
+should consider the most recently queued message to have failed to send.
+
+### ACK
+Format: 'a' + MAC + msgid
+
+Generated when an EtherMess ACK is received by the backend.
+
+Make sure to pay attention to the msgid, because there is no guarantees it
+refers to the most recently sent message or to a message you have sent at
+all.
+
+### ACK failed
+Format: 'A'
+
+Generated when backend times out on waiting for ACK for the most recently
+queued message. You should consider the message to have failed to send.
+
+### Message received
+Format: 'm' + MAC + message length in bytes (u16) + message
+
+Generated when backend receives a valid message.