TCP plaintext chat

TCPChat.md

TCP Chat

Should be plaintext and relatively usable with netcat, telnet or similar utilities when used with terminal emulator having UNIX newlines (\n)

No authentication or authorization is implied.

Server

• Listens on port 20000/tcp
• Accepts connections from clients
• Relays message of each client to other clients
• Notifies all clients when other client joins or leaves
• Maintains timeouts for inactive users, disconnecting them
• Maintains history of messages and join/part events in memory, supplying new client with history of past 10 messages

Protocol

Protocol data

Partial message

• Any UTF-8 encoded payload of TCP packet sent by client

Complete message

• A UTF-8 encoded string in format "[$TIME] $AUTHOR $BODY\n" sent by server
• $TIME is time of day in format "$0_padded_24_hour:$0_padded_minute:$0_padded_second" at moment of receiving first non-empty partial message or event generation
• $AUTHOR is in format "<$IDENTITY>" of message belongs to user with identity $IDENTITY, or "**SERVER**" if message sent due to server-generated event.
• $BODY is the body of complete message
• \n being a UNIX newline

Protocol events

PARTIAL_MESSAGE event

• Happens whenever client sends data over TCP connection, packet payload being message content
• Raw partial message could not be longer than 2048 bytes (likely less than 1500 due to IP fragmentation).
• Complete control characters are filtered out of message content.
• Empty (after filtering) messages are not kept, but do prevent client from timing out just as any other.
• Non-empty partial messages are buffered
• Up to 8 messages are buffered.

COMPLETE_MESSAGE event

• Happens whenever last non-empty message in partial message buffer gets older than 100ms since receiving it.
• Happens whenever partial message buffer is filled full with messages.
• All partial messages in buffer are concatenated with following rules:
  • Starting with empty $complete string buffer and empty $reminder byte buffer
  • Current partial message being referenced ad $i_partial
  • Copy all valid UTF-8 sequences from concatenation of ($reminder + $i_partial), while skipping all invalid UTF-8 sequences and control characters, to $complete buffer
  • If $i_partial ends with invalid UTF-8 sequence, put that last invalid UTF-8 sequence in $reminder buffer, otherwise empty $reminder buffer
• $complete should be filtered out of any control characters that was split into different partial messages.
• Complete message with body $complete, identity of sending client and timestamp of first partial message then gets relayed to all clients
• Server could generate complete messages on it's own on other events.

JOIN event

• Happens whenever client establishes TCP connnection.
• Client IDENTITY = sha1($IP + ':' + $PORT).
• $IP is IP-address of connected user
• $PORT is TCP port of connected user
• Just joined client receives 10 last messages from server history
• All clients (including just joined one) receive complete message with body "Client $IDENTITY has joined"

PART event

• Happens whenever client closes TCP connection or fails to maintain activity within timeout of 60 seconds
• All remaining clients receive complete message with body "Client $IDENTITY has $ACTION"
• If user gracefully terminated TCP connection, ACTION = "left"
• If user were disconnected due to time out, ACTION = "timed out"

KEEPALIVE event

• Happens whenever cleint sends TCP packet to server, regardless of it's payload
• Prevents client from timing out