Protocol
The Direct connect protocol is a text-based computer protocol, in which commands and their information are sent in clear text, without encryption in original Neo-Modus software (encryption is available as protocol extension). As clients connect to a central source of distribution (the hub) of information, the hub is required to have a substantial amount of upload bandwidth available.
There is no official specification of the protocol. This means that every client and hub besides the original Neo-modus client and hub has been forced to reverse engineer the information. As such, any protocol specification this article may reference is likely inaccurate and/or incomplete.
The client-server (as well in client-client, where one acts as "server") aspect of the protocol stipulates that the server speak first when a connection has been made. For example, when a client connect to a hub's socket, the hub is first to talk to the client.
The protocol don't have specified default character encoding for clients or hubs. In original client and hub is use Ascii encoding of Operating system. This allow to move to UTF-8 encoding in newer software.
Port 411 is the default port for hubs, and 412 for client-to-client connections. If either of these ports is already in use, the next higher port is used. For example, if 411, 412 and 413 are in use, then port 414 will be used.
Hub addresses are in the following form: dchub://example.com, where 411 is an optional port.
There is no global identification scheme; users are identified with their nickname on a hub-to-hub basis.
An incoming request for a client-client connection cannot be linked with an actual connection.
A search result cannot be linked with a particular search.
Supported by the protocol is the ability to kick or move (redirect) a user to another hub. If a user is kicked, the hub isn't required to give the user a specific reason and there is no restriction on where a user might be redirected. However, if another client in power instructs the hub to kick, that client may send out a notification message before doing so. Redirecting a user must be accompanied by a reason. There is no HTTP referer equivalent.
Hubs may send out user commands to clients. These commands are only raw protocol commands, and are used mostly for making a particular task simpler. For example, the hub cannot send a user command that will trigger the default browser to visit a website. It can however add the command "+rules" (where '+' indicates to the hub that it's a command - this may vary) to display the hub's rules.
The peer-to-peer part of the protocol is based on a concept of "slots" (similar to number of open positions for a job). These slots denote the number of people that are allowed to download from a user at any one time. These slots are controlled by the client.
In client-to-client connections, the parties negotiate a random number to see who should be allowed to download first. The client with the highest number wins.
Transporting downloads and connecting to the hub requires TCP. While active searches use UDP.
There are two kinds of modes a user can be in, either "active" or "passive" mode. Clients using active mode can download from anyone else on the network. Clients using passive mode users can only download from active users. In NeoModus Direct Connect, passive mode users receive other passive mode users' search results, while the user will not be able to download anything. In DC++, users will not receive those search results. In NeoModus Direct Connect, all users will be sent at most five search results per query. If a user has searched, DC++ will respond with ten search results when the user is in active mode, or five, when the user is in passive mode. Passive clients will be sent search results through the hub, while active clients will receive the results directly.
Protocol delimiters are '$', '|' and ' ' (space). Protocol have for them (and few others) escape sequence and most software use them correctly in login (Lock to Key) sequence. For some reason that escape sequence was ignored by DC++ developers and they use HTML equivalent if these characters are to be viewed by the user.
Continued interest exists in features such as ratings and language packs. However, the authors of DC++ have been actively working on a complete replacement of the Direct connect protocol called Advanced Direct Connect.
One example of an added feature to the protocol, in comparison with the original protocol, is the broadcasting of Tiger-Tree Hashing of shared files (TTH). The advantages of this include verifying that a file is downloaded correctly, and the ability to find files independently of their names.
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