Protocol

Important

• UDP MTU
• Don't create packages with a size of (bit-size mod 8) != 0. It makes it hard on the receiver side to interpret those!

General field descriptions

Type [4 Bit]:
0000 => Client-Hello-Handshake
0001 => Server-Hello-Handshake
0010 => File-Creation
0011 => File-Transfer
0100 => File-Status
0101 => ACK
0110 => Ping
0111 => Transfer-Ended
1000 => Auth-Request
1001 => Auth-Result

Client ID [32 Bit]:
An unique client id generated by the client on first contact.
E.g. A random int

Checksum [32 Bit]:
CRC32 Algorithm Wiki Link

Sequence Number [32 bit]:
Like TCP.

FID Length [64 Bit]:
The length of the FID field in bytes.

FID [Defined in the FID Length filed]: The relative path to the file. Includes the file name e.g. folder/file.txt.

FID Part Number [32 Bit]:
The file part number.

Pub Key [32 Bit]:
The client public key for the Diffie Hellman encryption.

Client-Hello-Handshake

The initial connection message that gets send by the client.

0      4       8      24          56             88              120        152
+------+-------+------+-----------+--------------+----------------+---------+
| Type | Flags | Port | Client ID | Prime Number | Primitive Root | Pub Key |
+------+-------+------+-----------+--------------+----------------+---------+
152        184               216                 1000
+----------+-----------------+----------+--------+
| Checksum | Username Length | Username | UNUSED |
+----------+-----------------+----------+--------+

Flags [4 Bit]:

0000
||||
|||+-> Connect requested
||+--> Reconnect
|+---> *UNUSED*
+----> *UNUSED*

Port [16 Bit]:
The port on which the client listens to server messages.

Prime Number [32 Bit]:
The client prime number for the Diffie Hellman encryption.

Primitive Root [32 Bit]:
The client primitive root for the Diffie Hellman encryption.

Username Length [32 Bit]:
Describes how long the the following Username is in byte.

Username [X Byte]:
Defined via the Username Length.

UNUSED [816-X Bit]:
To "prevent" DoS attacks. Ensures the package is a least 1000 Bit long.

Server-Hello-Handshake

Once the server received a Client-Hello-Handshake message he should reply with this message.

0      4       8           40                72            88        120        152
+------+-------+-----------+-----------------+-------------+---------+----------+
| Type | Flags | Client ID | Sequence Number | Upload-port | Pub Key | Checksum |
+------+-------+-----------+-----------------+-------------+---------+----------+

Upload-port [16 Bit]:
The Port where the client should send all following messages to.

Flags [4 Bit]:

0000
||||
|||+-> Client accepted
||+--> Too many clients - connection revoked
|+---> Client ID taken - connection revoked
+----> Invalid username - connection revoked

File-Creation

Marks the start of a file transfer. Tells the server to create the given file with the given path.

0      4           36                68          72         328        360          424
+------+-----------+-----------------+-----------+----------+----------+------------+-----+
| Type | Client ID | Sequence Number | File Type | File MD5 | Checksum | FID Length | FID |
+------+-----------+-----------------+-----------+----------+----------+------------+-----+

File Type [4 Bit]:

0000
||||
|||+-> Folder
||+--> Delete folder
|+---> File
+----> Delete file

File MD5 hash [256 Bit]:
The file MD5 hash to check if the file was transmitted correctly. Unused for folders. Wiki Link

File-Transfer

The actual file transfer message containing the file content.

0      4       8           40                72                104
+------+-------+-----------+-----------------+-----------------+
| Type | Flags | Client ID | Sequence Number | FID Part Number |
+------+-------+-----------+-----------------+-----------------+

104        136              200
+----------+----------------+---------+
| Checksum | Content Length | Content |
+----------+----------------+---------+

Flags [4 Bit]:

0000
||||
|||+-> First package for the given file
||+--> File content
|+---> Delta Sync
+----> Last package for the file

Content Length [max 900 Bit]:
The length of the Content field.

Content [defined in "Content Length" in Bit]:
The actual file content.

File-Status

Used for requesting and responding the current file status bevor a file gets transfered.

0      4       8           40                72                     104
+------+-------+-----------+-----------------+----------------------+
| Type | Flags | Client ID | Sequence Number | Last FID Part Number |
+------+-------+-----------+-----------------+----------------------+

104        136          200
+----------+------------+-----+
| Checksum | FID Length | FID |
+----------+------------+-----+

Flags [4 Bit]:

0000
||||
|||+-> Request status of FID
||+--> FID status response
|+---> Restart sending file system
+----> File = 0/Folder = 1

Last FID Part Number [32 Bit]: The last received FID Part Number. Ignored if Request status of FID is set.

ACK

For acknowledging Ping, File-Creation and File-Transfer messages.

0      4                     36         68      72
+------+---------------------+----------+--------+
| Type | ACK Sequence Number | Checksum | UNUSED |
+------+---------------------+----------+--------+

ACK Sequence Number [32 Bit]:
The acknowledged Sequence Number or Ping Sequence Number.

Transfer-Ended

Gets send by the client once he wants to end the transfer/close the connection.

0      4       8           40         72
+------+-------+-----------+----------+
| Type | Flags | Client ID | Checksum |
+------+-------+-----------+----------+

Flags [4 Bit]:

0000
||||
|||+-> Transfer finished
||+--> Cancelled by user
|+---> Error
+----> *UNUSED*

Ping

This message is used for ensuring the opponent is still there. The opponent should acknowledge each received Ping message with an Server-ACK.Should get send by each side if there was no message exchange for more than 2 seconds.
It also can be used for package loss and throughput tests with a modified Payload Length.

0      4                      36         68          100      104              136
+------+----------------------+-----------+----------+--------+----------------+---------+
| Type | Ping Sequence Number | Client ID | Checksum | Unused | Payload Length | Payload |
+------+----------------------+-----------+----------+--------+----------------+---------+

Ping Sequence Number [32 Bit]
An unique number for identifying each ping.

Payload Length [32 Bit]:
Describes how long the the following payload is in byte.

Payload [X Byte]:
Defined via the Payload Length.

Auth-Request

Send by the client to authentificate at the server.

0      4        8           40                                       72         104
+------+--------+-----------+----------------------------------------+----------+
| Type | UNUSED | Client ID | Server-Hello-Handshake Sequence Number | Checksum |
+------+--------+-----------+----------------------------------------+----------+
104               136
+-----------------+----------+
| Password Length | Password |
+-----------------+----------+

Server-Hello-Handshake Sequence Number [32 Bit]:
The sequence number of the Server-Hello-Handshake message.

Password Length [32 Bit]:
Describes how long the the following password is in byte.

Password [X Byte]:
Defined via the Password Length.

Auth-Result

Send by the server with the authentification result.

0      4       8           40                             72         104
+------+-------+-----------+------------------------------+----------+
| Type | Flags | Client ID | Auth-Request Sequence Number | Checksum |
+------+-------+-----------+------------------------------+----------+

Flags [4 Bit]:

0000
||||
|||+-> Authentification successfull
||+--> *UNUSED*
|+---> *UNUSED*
+----> *UNUSED*

Auth-Request Sequence Number [32 Bit]:
The Sequence number of the received ````Auth-Request``` message.

State charts

FileClient

FileServer

The server it selfe is stateless but it has FileServerClient objects with the following state chart:

Process example

Client				  Server
  |	Client-Hello-Handshake	    |
  | ------------------------------> | The clients starts the connection on the default port
  |				    | and tells the server the port on which he listens for answers.
  |				    | It also contains ```ClientStartConnection``` key exchange data.
  |	Server-Hello-Handshake      |
  | <------------------------------ | The server responds with an upload port and the
  |				    | ```onServerReceive``` key exchange data.
  |	Auth-Request		    |
  | ------------------------------> | If the client got accepted he sends his password via the
  |				    | now encrypted connection.
  |	Auth-Result		    |
  | <------------------------------ | The server answers with the result of the authentification.
  |				    | Now the connection is established.
  |	File-Status		    |
  | ------------------------------> | The client requests the file status.
  |				    |
  |	Auth-Result		    |
  | <------------------------------ | The server responds with the current file status.
  |				    |
  |	File-Creation		    |
  | ------------------------------> | The client sends this message if the server does not has
  |				    | the file yet.
  |	Server-ACK  		    |
  | <------------------------------ | The server acks the ```File-Creation```.
  |				    |
  |	File-Transfer		    |
  | ------------------------------> | The client starts sending the file in chunks.
  |				    |
  |	Server-ACK  		    |
  | <------------------------------ | The server sends an ACK message for each message
  |				    | it received from the client
  |	File-Transfer		    |
  | ------------------------------> | The client sets the ```Last package for the file``` flag
  |				    | to inform the server, it is the last file part.
  |	Server-ACK  		    |
  | <------------------------------ |
  |				    |
  |	Transfer-Ended		    |
  | ------------------------------> | The client tells the server that he liks to close the connection.