Simple TCP Server

This is a project of my Computer Internet course a project inspired by my school homework.

Several days before I'm asked to write a project on TCP server-client communication, and I wondered why not build a simple chat service beyond that which would be much more fun?

Therefore here it is, a simple tcp server with several functions which makes it acts like a small chat app.

Functions

• User register / login / logout
• Remember login state for device (simple token approach though)
• Send messages via users
• Search users and add contacts (and accept them of course)
• Message sync via different devices
• Send message to offline server (a SMTP-like approach)
• File handling (transfer to and fetch from server)

Notice

• To support multilanguage, use base64Encode(utf8.encode(yourMessageHere)) before wrapping client messages in json object and sending that to the server (the serve will crash!)
• Always open a new TCP connection to fetch or send file

Compile and Deploy

To clone and run this project in command prompt, do the following under Windows environment with dart SDK configured.

git clone https://github.com/Linloir/Simple-TCP-Server.git
cd Simple-TCP-Server

# [+] In case you want to build an exe
# mkdir build
# dart compile exe ./bin/tcp_server.dart -o ./build/tcp_server.exe
# cd build
# tcp_server.exe [listen port]

dart run

Application Layer Protocol

Since I was not allowed to base my homework on an existing HTTP protocol, I create my private protocol for this project.

To communicate with the server, your data should conform the following formats:

Overall Request / Response Structure

Every request sent by any client to this server should and should only consist four regions:

• Request length region (4 bytes)
• Payload length region (4 bytes)
• Request JSON (variable length)
• Payload byte stream (variable length)

Each section should interconnect with each other with no byte insets.

Request JSON Region

The request JSON region is self-explanatory, which contains a encoded JSON string describing the request from the client.

Fields of Request JSON

To get use of the JSON object, all you need is the table below describing the supported fields in a JSON object and the possible combinations between them:

Field	Content	Meaning
request	STATE: Check for the current login state (also used to fetch a token for first used device) REGISTER: Register new user LOGIN: Login via username and password LOGOUT: Logout current device SENDMSG: Send message to user FETCHMSG: Fetch messages sent to or from the logged user that haven't been fetched by current device SEARCHUSR: Search user info via username ADDCONTACT: Add contact via userid FETCHCONTACT: Get the contact (Including both dual-sided and single-sided contacts) FETCHFILE: Fetch the attached file of a message	The type of the request
body	JSON object	The information needed for the request
tokenid	number | null	The identifier of a device

Body of Request JSON

The body field of the JSON object is the key part of a request; it contains the key information the server needs to perform the request command.

There are mainly four different types of a body:

The UserInfo body is used to describe the information of an arbitrary user:

• userid: The user ID
• username: The username
• avatar: The base64 encoded form of the user's avatar

The UserIdentity body is used as a credential of a specific user:

• username: The username
• passwd: The password of the user
• newPasswd: The modified password (Should only exists if a user is trying to modify his/hers password)

The Message body is used to describe a message:

• userid: The user ID of the sender
• targetid: The user ID of the reciever
• contenttype: The type of content attached to the message, should only contain values among plaintext, image and file
• content: The base64encoded utf8encoded string of the original message (Should contain filename if the content type is 'file')
• md5encoded: The calculated md5 value of the encoded content string
• filemd5: The attached file's md5 value (Calculated at client side before sending the request; should only exist if the content type is 'file')

The MessageIdentifier is the identifier for a client to fetch a file for a message, it contains only the necessary part to identify a message:

• msgmd5: The md5 of the message

The UserName or UserID is self-explanatory, which contains only the username of a user in order to search the full info of the user:

• username: The provided username Or userid: The provided user ID

The usage of different body parts for different request types is described below:

Request Type	Body Part Contents
STATE	NONE
REGISTER	UserIdentity
LOGIN	UserIdentity
LOGOUT	NONE
SENDMSG	Message
FETCHMSG	NONE
FETCHFILE	MessageIdentifier
SEARCHUSR	UserName
ADDCONTACT	UserID
FETCHCONTACT	NONE

Response JSON Region

Fields of Response JSON

The fields of a response JSON is similar to that of a request JSON, excludes for the tokenid field.

The response JSON also offers extra fields to describe the state of a performed command:

Field	Content	Meaning
status	OK: The request completes with no err ERR: The request completes with at least one err	The completion status of the request
info	String | null	Description of error (Only exists if the status is 'ERR')

Body of Response JSON

The body of a response JSON object contains all possible types of a request JSON object, with the addition of two special types below.

The TokenInfo body is self-explanatory, which carries the info of a token. This kind of response body only appears in an extra response preceding the original responsse of the first request from a new client device, which offers the client device a token number:

• tokenid: The allocated token ID

The MessageList body is also self-explanatory, which is a list of Message objects, this kind of response body exists in a FETCHMSG response:

• messages: A list of Message objects